Inelastic electron tunneling

Inelastic electron tunneling

INELASTIC ELECTRON TUNNELING Paul K. HANSMA Department of Physics, University of California, Santa Barbara, California 93106, USA NORTH-HOLLAND PUBL...

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INELASTIC ELECTRON TUNNELING

Paul K. HANSMA Department of Physics, University of California, Santa Barbara, California 93106, USA

NORTH-HOLLAND PUBLISHING COMPANY



AMSTERDAM

PHYSICS REPORTS (Section C of PHYSICS LETTERS) 30, No.2 (1977) 145—206. NORTH-HOLLAND PUBLISHING COMPANY

INELASTIC ELECTRON TUNNELING* Paul K. HANSMA* Department of Physics, University of California, Santa Barbara, California 93106, U.S.A. Received August 1976 Abstract: Inelastic Electron Tunneling Spectroscopy (lETS) is a new technique for measuring the vibrational spectrum of minute quantities of organic compounds. Sensitivity is its key advantage over the conventional techniques of infrared and Raman spectroscopy. This article wiil first discuss the technique itself: its theoretical basis, selection rules, sensitivity, vibrational mode shifts due to surface interactions and superconductivity, and sample preparation. Then it will discuss applications of the technique to problems in biology, radiation physics, surface physics, and catalysis.

Contents: I. Introduction 2. Discovery 3. Theory 3.1. General remarks 3.2. Peak position 3.3. Peak width 3.4. Peak intensity 4. Experimental methods 4.1. General remarks 4.2. Sample preparation 4.3. Cryogenics 4.4. Sample evaluation

147 148 150 150 151 152 154 162 162 162 165 166

4.5. Taking tunneling spectra 5. Experimental results 5.1. Sensitivity 5.2. Selection rules 5.3. Peak shifts due to the oxide layer 5.4. Peak shifts due to the top metal electrode 5.5. Electronic transitions 6. Applications 6.1. Biochemistry 6.2. Radiation damage 6.3. Surface studies and catalysis References

Single ordersfor this issue PHYSICS REPORTS (Section C of PHYSICS LEUERS) 30, No. 2 (1977) 145—206. Copies of this issue may be obtained at the price given below. All orders should be sent directly to the Publisher. Orders must be accompanied by check. Single issue price Dfl. 25.00, postage included.

*Work supported by the National Science Foundation. *Alfred P. Sloan Foundation Fellow (1975—1977).

167 169 169 172 174 175 180 182 182 188 193 204

P. K. Hansma, Inelastic electron tunneling

147

1. Introduction Inelastic electron tunneling spectroscopy (lETS) measures the vibrational spectrum of a minute quantity of organic molecules on the insulator of a metal—insulator—metal junction. A vibrational mode of energy hi.’ (h is Planck’s constant, and z.’ is the frequency) is detected as a slight change in the electrical characteristics of the junction at a voltage V such that e V, the energy of the most energetic tunneling electrons, equals hi.’. Specifically, a tunneling spectrum, d2 V/d12 versus voltage, has a peak corresponding to the vibrational frequency ~ at a voltage V = hv/e. This is shown in the idealized view at the top of fig. 1. 1. The bottom half of fig. 1. 1 shows the actual situation. The metal—insulator—metal junctions are usually fabricated on glass slides. Most commonly an aluminium electrode is evaporated first. It is then oxidized either in a glow discharge or in air to form the necessary insulating layer. Then it is doped, usually with organic molecules. Finally, it is completed with an evaporated top metal electrode, usually Pb. The metal—insulator—metal junctions of the idealized picture are formed at the intersections of the crossed metal electrodes. Since the typical organic molecule has a large number of vibrational modes, there are a large number of peaks in an actual inelastic tunneling spectra as shown on the right. This particular one is for the dopant molecule, benzoic acid. We will come back to a detailed discussion of this particular molecule later. The fundamental advantage of tunneling spectroscopy over the conventional optical techniques IDEALIZED d2V

1

d12

eV~_ INSULATOR

_______ 7/

NETAL//’/ I hv

V

e ACTUAL d2V d 12

~

~IL1~

Fig. sample ized 1.1. Idealized represents and one actual of views the many of the vibrational samples and modes spectra of anoforganic inelastic molecule electronintunneling. the actualThe sample. harmonic oscillator in the ideal-

148

P. K. Hansma, Inelastic electron tunneling

for vibrational spectroscopy, infrared and Raman, is its greater sensitivity. One monolayer is the sample thickness that is optimal for most tunneling experiments. It is this sensitivity which is of key importance to most of the applications of tunneling spectroscopy. This review begins with a brief discussion of the discovery of tunneling spectroscopy less than ten years ago. Then it presents a theoretical treatment of peak position, width, and intensity. Since this is the first comprehensive review of tunneling spectroscopy of molecular vibrations, it includes a fairly detailed section on the experimental methods involved, especially in the area of sample preparation. A section on general experimental results discusses the sensitivity of the technique, selection rules, and peak shifts due to interactions with the oxide layer and the top metal electrode. It should be emphasized that, at least at present, tunneling spectroscopy is not the easiest way to obtain molecular vibration spectra. Infrared and Raman spectroscopy are both easier, primarily because commercial instruments are available and cryogenic temperatures are not required. Tunneling spectroscopy is, however, orders of magnitude more sensitive. It is this sensitivity (not resolution) that makes tunneling spectroscopy so promising in a number of applications: (1) Its application to biological systems will be illustrated by spectra of a wide variety of biochemicals and by recent results on the identification of nucleotides. (2) Its application to radiation damage will be illustrated by results on molecular degradation from electron beam exposure. (3) Its application to surface studies and catalysis will be illustrated by data on the identification of a number of chemisorbed surface species.

2. Discovery The study of inelastic electron tunneling began in 1966 when Jaklevic and Lambe studied some tunnel junctions at higher voltages than had been used previously [1, 21. They hoped to observe band structure effects due to the metal electrodes. As so often happens in science, they found something exciting, but not what they were looking for.* Using a second derivative technique in order to see very small changes in the electrical properties in their junctions as a function of voltage, they found peaks that occurred at voltages characteristic of molecular vibrations. For example, fig. 2. 1 shows their spectrum for a clean junction, one with negligible molecular impurities. The slowly rising background is due to the slowly increasing elastic conductance that is characteristic of aluminium oxide tunneling barriers. Fig. 2.2 shows their spectrum for a junction doped with a large molecular weight hydrocarbon (Convoil-20) evaporated from a tantalum boat. Note that, in addition to the background peaks of the clean junction, there are peaks near 175 and 360 mV. This figure also shows an infrared spectrum of the same material for comparison. The comparison strongly suggests that the new peak near 360 mV in the tunneling spectrum is associated with the C—H stretching vibrations of the hydrocarbon and that the peak near 175 mV in the tunneling spectrum is associated with the C—H bending vibrations. This is a simple example of how almost all vibrational identifications in lETS have been made: by referring to the huge literature of assigned infrared and Raman vibrational spectra. *ln fact, they found what they were originally looking for several years later. A recent review that discusses this work has been written by Wolf [3].

P. K. Hansma, Inelastic electron tunneling

Al-Pb

CLEAN

149

STRETCH

V (VOLTS) —~ Fig. 2.1. Tunneling spectrum of an undoped Al—oxide—Pb junction from the pioneering work of Lambe and Jakievic [21. The peak near 0.12 V is now believed to be predominately due to aluminum oxide phorions, with 0—H bending modes as a shoulder on the low voltage side.

LARGE MOL WT HYDROCARBON PUMP 01LI

4 2°K (0)

t

2I

d

~AB~TION

ROCK

0

BEND

STRETCH

2 V.

I -~

(VOLTS)

I

—~

Fig. 2.2. (a) Tunneling spectrum of a large molecular weight hydrocarbon (Convoil-20) evaporated onto the oxide of an Al—oxide— Pb junction. Note the additional structure over the clean junction spectrum (fig. 2.1) near 0.360, 0.175 and 0.090 V. (b) Assigned infrared absorption spectrum of the same material for comparison. Figure from Lambe and Jaklevic [2].

P. K. Hansma, Inelastic electron tunneling

150

ACE~CACIO~/~

DEUTERATED

0

.1

.2

.3

.4

.5

V(VOLTS)-.-

Fig. 2.3. Tunneling spectra of acetic acid (CH

3COOH) and deuterated acetic acid (CD3COOD) evaporated onto the oxide of an Al—oxide—Pb junction. Note C—D stretching mode shifted down by roughly 1/\/~from the C—H stretching mode. Figure from Lambe and Jaklevic [2).

Lambe and Jaklevic established beyond doubt that they were indeed seeing vibrational spectra with deuteration experiments. For example, fig. 2.3 shows their spectra for normal and deuterated acetic acid. Note in particular the peaks near 275 mV for the deuterated sample. This position is, as we would expect, approximately 1 /..,/~ times the position of the C—H stretching vibration in the undeuterated sample. Scalapino and Marcus [4] quickly provided a theoretical framework for understanding the experimental observations. Rapid progress has been made in the ten years since the discovery of lETS. Some highlights have been the observation of electronic transitions (discussed in section 5.5), the development of new techniques to investigate biological compounds and other macromolecules (sections 4.2 and 6.1), and the study of chemical reactions on surfaces (section 6.3).

3. Theory 3.1. General remarks The theory of LETS must predict the position, widths, and intensities of the peaks in tunneling spectra. Fortunately the positions and widths can be predicted on very general grounds, indepen-

P. K. Hansma, Inelastic electron tunneling

151

dent of the details of the electron—molecule interaction. These will be presented in the following two subsections. The peak intensities are much more difficult to predict because they do depend on the details of the interaction. The final subsection will describe the pioneering work of Scalapino and Marcus [4] and subsequent approaches to modeling the interaction and predicting intensities. Throughout this section we will assume that both metal electrodes are not superconducting. The additional effects due to superconductivity are not, in practice, dramatic (a slight sharpening and shift of peaks). They will be discussed in section 5.4. All of the theoretical results of this section will be for d21/dV2 because it is easiest to calculate. Unfortunately, though d21/d V2 can be measured with modern bridge circuits [5], d~V/d12 can be measured more easily; almost all experimental results have been for d2V/d12. This is not as serious as it sounds because d2V/d12 = —(dV/df)3(d21/dV2) and dV/dlis relatively constant over the voltage range of interest, at least for aluminum oxide and magnesium oxide junctions. Furthermore, d V/dJ has no sharp structure in the voltage range of interest so that peaks shapes are essentially identical for the two second derivatives. 3.2. Peak position Fig. 3.2.1 shows an energy diagram of the two metal electrodes of a tunnel junction separated by a thin insulating layer. The shaded regions indicate filled electron states up to the Fermi level in each metal electrode. If a voltage V is applied between the metals, the Fermi levels will differ in energy by e V as shown in the figure. The top of the potential barrier region labeled I corre-

ELASTIC -

FERMI LEVEL

PETAL A

__ I

I

eV

IvETAL B

0

hi, e

V>

Fig. 3.2.1. Tunneling electrons can excite a molecular vibration of energy hv only if eV > hi.’. For smaller voltages, V, there are no vacant final states for the electron to tunnel into. Thus the inelastic current has a threshold at V hv/e. The increase in conductance at this threshold is typically ~ 1%. A standard tunneling spectrum, d21/dV2 versus V, accentuates this small increase; the step in dI/dVbecomes a peak in d21/dV2.

=

152

P. K. Hansma, Inelastic electron tunneling

sponds to the conduction band edge of the insulator. Elastic electron tunneling involves a transfer of electrons from the filled states of metal A to adjacent empty states in metal B. This elastic current increases linearly with applied voltage as shown by the initial portion of the current—voltage curve on the top right of the figure [6]. In addition to this elastic conduction path, there may be an inelastic electron conduction path in which electrons from metal A lose an energy hi.’ to excite a vibrational mode of a molecule in the insulating layer and continue on into an empty state of metal B. The key point is that this inelastic conduction path opens up only for e V ~ hv; otherwise an electron from metal A cannot lose an energy hi.’ and still tunnel into an empty state of the metal B. The current due to this inelastic conduction path is indicated by the dotted line in the voltage curve. Thus the total current, shown by a solid line, has a kink at V = hz.’/e. The middle curve shows the conductance versus voltage curve in which the kink becomes a step. Unfortunately, only a small fraction of the electrons (typically 1% or less) tunnel inelastically. Thus, the height of the conductance increase due to the onset of the inelastic tunneling channel is too small to be conveniently observed. In practice, researchers electronically differentiate to obtain the bottom curve in which the step becomes a peak. Thus the position of the peak corresponding to a vibrational mode of energy hi.’ is V = hi.’/e. This conclusion is based on energy conservation and is independent of the mechanism for the electron—molecule interaction. Small shifts, of order 1% or smaller, due to superconductivity and interactions with the top metal electrode will be discussed in section 5.4. 3.3. Peak width In LETS there are two contributions to vibrational peak width in addition to the natural width: thermal broadening and modulation voltage broadening. The effect of thermal broadening was first discussed by Lambe and Jaklevic [21. Following their treatment, let us assume that we have a vibrational mode of negligible natural width at an energy hi.’. The inelastic electron current due to the corresponding conductance channel can be written as: IjCf

dEf(E)(1 —f(E+eV—hp)).

(3.1)

Here we have hidden all the details of the electron—molecule coupling in the parameter C and displayed only the integral over the Fermi functions for the two normal metal electrodes. This integral simply expresses analytically the condition we discussed qualitatively in the previous subsection: electrons must tunnel from a filled state in the one electrode into an empty state in the other electrode. This integral can be easily evaluated =

cf

~((l

=C(eV—hi.’)

+

exp(E/kfl)) (i ex

ex_1



I

+

exp{(E+eV— hv)/kT})

(3.2)

P.K. Hansma, Inelastic electron tunneling

153

where x

(eV— hv)/kT.

(3.3)

Taking derivatives, we obtain (3.4) kT eX (x_2)ex (ex +(x+2) dV2 This function is plotted in fig. 3.3. 1. Note that the predicted linewidth at half maximum is 5.4 kT/e. This was confirmed in the experimental work of Jennings and Merrill [7]. A final point is that eq. (3. 1) implicitly assumed that the energy dependence of the effective tunneling density of states was negligible. This is certainly true for normal metal electrodes over the millivolt energy range around hv for which eq. (3.4) is appreciable [6]. The effect of modulation voltage broadening was first discussed analytically by Klein et al. [8]. Following their treatment, let us assume that the modulation voltage is e V,~.’cos wt and that f”(eV) is the exact second derivative (with no modulation voltage broadening). We can write the current as —

I—f(eV 0 +eV~coswt)

(3.5)

where V0 is the slowing changing bias voltage. 21/dV2 we compute ‘2W~ the current at the second harmonic frequency: To obtain 2 d = _fy(eVo +eJ/Wcoswt)cos2wtdr. T

(3.6)

T

Two partial integrations give:

f

e V~

‘2w

-eV~

f”(eV

2 E2)312 dE —

0

+

(3.7)

E)((eV~)

where E e V~cos wz’. Thus, ‘2~ is a convolution of the exact second derivative with a function that is proportional to ((eV~)2 E2)3’2 for [El < eV~and zero for IEI > eV~.This function is also shown in fig. 3.3. 1. It would, of course, be the complete peak shape for a vibrational mode of negligible natural width in the absence of thermal broadening. It is plotted in terms of = 0.707 V~,the rms value of the modulation voltage, since this is usually the experimentally measured quantity. Of these two contributions to peak width, the modulation voltage broadening is perhaps the more serious. The thermal broadening can obviously be reduced by lowering the temperature (or, not so obviously, by using superconducting electrodes). But, in order to make the modulation voltage broadening comparable to the thermal broadening at 1 K (even with normal electrodes) requires ~ 0.27 mV. Even at .~V 0.7 mV, the smallest modulation voltage used for any of the traces reported in this review, the second harmonic signal is so small (< 50 mV for even the largest peaks) that it takes overnight to obtain a complete tunneling spectrum. Since the second harmonic signal varies roughly with the square of the modulation voltage, and since signal to —

154

P. K. Hansma, Inelastic electron tunneling

(~iIIIIiIIIII)

—I.41AV

0

+I.4I~V

v

0

V

Fig. 3.3.1. The line shapes due to modulation voltage broadening and thermal broadening. The observed peak width in a real tunneling spectrum is a convolution of these two functions with the natural peak width. Redrawn from Klein et al. [8].

noise improvement varies with the square root of averaging time, it would take over two weeks to obtain a spectrum with the same signal to noise ratio with ~V 0.27 mV. Thus, little is to be gained in general by taking spectra below 1 K (or minimizing the thermal broadening with superconducting electrodes) since modulation broadening will dominate, at least with current experimental techniques. (Superconducting electrodes decrease modulation broadening only slightly. See section 5.4.) 3.4. Peak intensity The first theory for lETS intensities was developed by Scalapino and Marcus [4]. It approximated the electron—molecule interaction as a Coulomb interaction between the electron and the dipole moment of the molecule. Thus the interaction potential for an electron at z and r1 (coordinates defined by fig. 3.4.1) with a molecule at the origin with z component of dipole moment, was assumed to be 2ep~z U1~~(z) = 2 + r~)312 (3.8) (z Here the factor of 2 arises from the doubling of the effective z component of the dipole moment by the image dipole in the metal electrode. (The image dipole nearly cancels p 1, which is thus neglected.) Scalapino and Marcus then treated U111~as a perturbation on the barrier potential U(z), which they assumed to be rectangular: U(z) = U if 0 z ~ 1 and zero otherwise. If the inelastic energy loss is neglected, then the WKB approximation for the electron part of the tunneling matrix element is [9, 10] .

‘~

1

I cx exp

_[

r2m thL~~ (U(z)

+

U1~~(z) (E —

1



E1))j

1/2

(3.9)

where E is the total energy of the electron and E1 is the kinetic energy associated with its motion perpendicular to the z direction. In this expression ~ still contains the operator p~.To com-

P. K. Hansma, Inelastic electron tunneling

155

r~ • METAL

INSULATOR

METAL

Fig. 3.4.1. An electron at position (z, r

1) interacts with a molecular dipole moment at the origin. The induced image dipole in the metal electrode approximately cancels the perpendicular component of the molecular dipole moment, p1, and enhances the z component, p~,by a factor of 2. Thus the effective interaction is with a dipole of magnitude 2p~in the z direction.

plete evaluation of this tunneling matrix element we must later take it between the initial and final states of the molecule. To find the lowest order effect of U1~5(z),we approximate U(z) — (E — E1) 1, a constant, and expand to lowest order in the small parameter U~/I obtaining: 1M1,21

cx

2m 1/2 ep~ r1 [1 +(—~_.) -~-g(--~-)] exp

2)1”2.

2m~

—(—-—-—)

1/2

1

(3.10)

whereg(x) l/xa good 1/(1deal +x of mathematics and proceed quickly to the final result by making a We can avoid few observations at this point. (1) What we want as a final result is (dJi/dV)/(dle/dV), the ratio of the inelastic conductance for a given vibrational mode to the elastic conductance. (2) Current and thus conductance in tunneling is proportional to the square of the relevant matrix element. (This comes from the golden rule.) (3) The relevant matrix element for inelastic electron tunneling is the second term of eq. (3. 10) taken between the ground state I 0) and the first excited state (11 of energy hw 1 of the molecule. (At the temperatures of interest essentially all molecules are in the ground state for all vibrations.) (4) The relevant matrix element for elastic electron tunneling is the first term of eq. (3.10). (5) If, following Scalapino and Marcus, we assume that the density of states of the metal electrodes are independent of energy (approximately true for normal metal electrodes [9]) and that transverse electron momentum is conserved for inelastic tunneling, just as for elastic tunneling, then the rest of the quantities multiplying square of the matrix element are the same for the inelastic and elastic current. Thus the ratio of the inelastic conductance a distance r1 from an impurity to the elastic conductance is simply the square of the matrix element of the second term in parenthesis of eq. (3.10), —

156

P. K. Hansma, Inelastic electron tunneling

(d11(r1)/dV) (dle/dV)

2m

e

~

r1

2

hw1

IUIp~lWI2g2(7)0(V_~~~__)

(3.11)

where we have included the 0 function (0(x) = 1 for x 0. 0 otherwise) to express the energy conservation discussed in section 3.2: electrons cannot tunnel inelastically unless they have enough energy to excite the vibration, eV> flwo.* To obtain the total inelastic conductance due to one vibrational mode of one impurity we must integrate eq. (3.11) over r1 using: ~‘

2~f dr1r1g2(~)

2~l2ln~.

(3.12)

Note that in order to perform this integration we had to introduce the cutoff r0. Fortunately it only enters in a logarithm so its exact magnitude is not critical. Nevertheless, it is a weakness in the treatment that will preclude quantitative accuracy. A more recent theory avoids cutoffs [11]. Multiplying our integrated eq. (3. 11) by N impurities per unit area and summing over all the post sible vibrations of each impurity, we obtain the final result: 2 1 / hWm\ 2 OIV— (3.13) d11/dV 4irme in — ~ I(mIp dJ~/dV 4h2 r 2IO)1 0 m e / where (ml denotes the first excited state of the mth vibrational mode and hWm denotes its energy. Two observations are in order: (1) The calculated magnitude is reasonable: about 1% for typical parameters with the assumption 1/30of~the 1 Adipole [4]. (2) The element size of aof conductance increasevibrais 2,the r0 square matrix the corresponding proportional to I(mIp~I0)I tional mode. This is the same quantity that determines the integrated intensity of peaks in an infrared absorption spectrum. Thus the theory predicts that intensities in a tunneling spectrum should be the same as in an infrared spectrum. Experimentally it is found that although big peaks in infrared spectra usually correspond to big peaks in tunneling spectra, the proportionality is not exact. Further, peaks appear in tunneling spectra that are completely absent from infrared spectra. Lambe and Jaklevic pointed out that there are other mechanisms for electron—molecule interaction [2]. They generalized the Scalapino and Marcus theory to include a particular one: interaction through the polarizability of the molecule. The electron induces a dipole moment in the molecule and interacts with the induced dipole. The interaction energy, including effects from the nearest image of the dipole, is U~t(a)_4eactz2/(z2

+r~)3.

(3.14)

Here a is the polarizability of the molecule, and the superscript R on U 1~~ is for Raman since vibrational modes that have a polarizability associated with them are observed in Raman spectroscopy and are thus called “Raman active modes”. (The vibrational modes, considered by *The abrupt onset at V= 11w0/e expressed by eq. (3.11) is characteristic of T= 0. At non-zero temperature the onset will be broadened as discussed in section 3.3. Here we are only concerned with the magnitude of the conductance increase (which is the integrated intensity of a peak in a tunneling spectrum).

P. K. Han sma, Inelastic electron tunneling

157

Scalapino and Marcus, that are associated with a dipole moment are observed in infrared spectroscopy and are thus called “infrared active modes”.) If we insert this new interaction into eq. (3.9) for the tunneling matrix element and expand to lowest order in U~/I as before, we obtain: ‘2m’ 1/2 ae 2 frL\-l 12mc1’ 1/2 IM~2Icx II+(—) 3 \IIJ \ h2 I 1 —tI—Ilexp —(--—--—-—I L \F/ 4/il where r

t(x) =

I —

r I

l——x2

1 +

x2 [(1 +x2)2



x

(3.15)

/1 tan’ I—

At this point we can proceed exactly as we did from eq. (3.10) to eq. (3.13) and write: dI~/dV

I4irme2\

e2

)

(~6)

2 dIe/dV~

~h

I

f

r 1 t2(_)

hWm

r~dr 1~

I(mla 10)12 0

(v_ ~_).

(3.16)

Unfortunately t(x) is strongly divergent for small x with rapid oscillations from the tan’ ( l/x). It must be estimated numerically and is very dependent on the value of the cutoff r0. Nevertheless, estimates for reasonable values of parameters give inelastic conductance changes of nearly the same order of magnitude as for the electron—dipole moment interaction that we considered previously. Specifically, Jaklevic and Lambe [10] calculate conductance changes of (0. 1 to 0.5%) for this interaction as opposed to 1% for the electron—dipole interaction. We can conclude that it is reasonable to expect both infrared active and Raman active vibrational modes in tunneling spectra. There is no question that the theory of Scalapino and Marcus as extended by Lambe and Jaklevic was successful. It predicted magnitudes in the range found experimentally with a simple formalism. It has, however, several difficulties: (I) It assumes that the transverse electron momentum is conserved. In fact, inelastic scattering by an impurity usually changes the transverse electron momentum. The inclusion of the “off-axis scattering” substantially modifies selection rules [111. (2) It does not include the effect of the electron energy lost in exciting a molecular vibration on the tunneling probability of that electron. (3) It depends on a cutoff r0 which is difficult to determine. How would we actually calculate relative intensities for the various vibrational modes of a relatively complex molecule? Is r0 the same for each mode? (4) Finally, it implicitly assumes localized tunneling electrons by first computing 11(r1) and then integrating over r~. It is natural to turn to the transfer-Hamiltonian formalism originated by Bardeen [12] to formulate a “second generation theory”. The transfer-Hamiltonian formalism has been successfully used to explain many tunneling phenomena. Cohen, Faiicov and Phillips [13] used it to explain the presence of energy gaps in tunnel junctions with a superconducting electrode. Josephson won the Nobel Prize for using it to predict the Josephson effect [14]. It can be used for inelastic electron tunneling. The transfer-Hamiltonian formalism begins with the assumption that the Hamiltonian for a Metal—Insurator—Metal junction can be written as a sum of three terms: H=HL+HR+HT

(3.17)

158

P. K. Hansma, Inelastic electron tunneling

where HL and HR are the Hamiltonians that describe the electrons in the left and right metal electrodes. HT is the transfer Hamiltonian that describes the tunneling of electrons from one electrode to the other. The wavefunctions for the electrons in each metal electrode are solutions toHL and HR. Consider the process of tunneling from an initial state in the left electrode to a final state in the right electrode. In the usual, free-electron approximation we can use the WKB approximation and write the wavefunction for the electrons in the initial state as [12]: cx

exp{i(k~x+ k~y)} sin(k2z

+

y),

z < 0 (in the metal)

cx exp{i(k~x+ k~y)}exp(—IK~Iz),

where, for a rectangular barrier (U(z) j,272

i2m / ‘~“z\I \ 2mIJ IKJ=I—2 IU——--——Il

0< z < l(in the oxide)

(3.18)

U for 0 < z < 1, 0 otherwise):

1/2

.

L/i

(3.19)

The electrons in the right metal electrode are described by a similar wavefunction (see fig. 3.4.2). Note that these are wavefunctions of the unperturbed system: the effect of the electron—molecule interaction on the wavefunctions is neglected within the transfer-Hamiltonian formalism. The probability per unit time that H 1 will induce a transfer of an electron from an initial state, i/i1, to a set of final states, ~ is given by the golden rule [15]: Ffj=—~

2f I(H.~.)1,1I 1(l —f~)~(E~—E1)

(3.20)

3r’T’~’HT ‘I’s. The Fermi functions,f(x) (exp(~3x)— l)’, result from the rewhere (HT)f 1 f d quirement that the initial state be occupied and that the final state be vacant. The delta function insures conservation of total energy. For inelastic electron tunneling the total energy includes both the electron energy and the molecular vibration energy because the states, ‘I’~and ‘I’s,, include both the electron wavefunction and the molecular vibration wavefunction. The current is obtained by integrating ~‘t’j over the initial and final states and multiplying by

2e. (The two comes from the spin sum.) If the transfer-Hamiltonian is a sum of terms, Me describing elastic and M describing inelastic 2 1because these two tunneling, cross terms appear final in the matrix (Mefor + M1)~1I types of tunneling sharewillnonot common states; theelement final states inelastic electron tunneling include an excited molecular vibration that is not present for elastic electron tunneling. Thus we can separately calculate the elastic current cx I(Me)fiI2 and the inelastic current cx I(M 2 Bennett, Duke and Silverstein [16] were the first to apply the transfer-Hamiltonian1)f~I formalism to inelastic electron tunneling. Unfortunately for this review, they were interested in collective excitations of the barrier, not molecular excitations. Their electron—excitation interaction is independent of position. It cannot describe a localized molecular vibration. In his book Duke outlines a procedure appropriate to localized molecular vibrations [17]. He does not, however, specify an electron—molecule interaction and calculate results. Recently Kirtley, Scalapino and Hansma [11] have put a new electron—molecule interaction into the transfer-Hamiltonian formalism. Rather than making the dipole approximation, they assumed that the charge distribution within the molecule can be broken up into partial charges,

P. K. Hansma, Inelastic electron tunneling

METAL

INSULATE~

159

METAL

Fig. 3.4.2. Schematic illustration of the initial and final states of the transfer-Hamiltonian formalism. The rate of transfer of electrons from the initial to the final state is proportional to the square of the matrix element of the transfer-Hansiltonian between these states.

with each partial charge localized on a particular atom. The partial charges arise from an uneven sharing of the electrons involved in bonding. This allowed them to describe the interaction at distances comparable to interatomic lengths, distances within which the dipole approximation breaks down. The interaction is: V1(r)

2Z ~

—e

1/Ir—R11

(3.21)

where R1 is the position of the jth atom, Z1e is its partial charge, and r is the position of the electron. In order to connect initial and final electronic states of different energy, they separated out the component of the total interaction potential which oscillates at the frequency of the vibrational mode of interest. Expanding the R1’s as~ R1 = R1(0) + 5R1

(3.22)

they find: V~(r,w)~

_e2ZISRJ.VJ(1

R(0)I)

(3.23)

Finally, taking into account the images of the partial charges in the two electrodes they find: 2Z.~R.V. 1 (3.24) —e / / ‘ __________ / C Ir—R 1—2n111 Ir—R1—(2nl—2a1)~I —

where 1 is the thickness of the barrier, E is the dielectric constant of the oxide, and a1 is the distance of the jth atom from one metal electrode. With this interaction in the transfer-Hamiltonian formalism they obtain agreement with experiment for the percentage conductance change for the OH stretch mode [11]. Specifically they find 0.5% while a typical experimental value is 0.4%. Whether or not the conductance changes for more complex molecular vibrations can be calculated with this accuracy remains to be seen.

160

P. K. Hansma, Inelastic electron tunneling A(A(O~BENZOICACID Pb

eVKmeV) 2 VI d12 ) versus eV for an Al—oxide—benzoidacid—Pb tunneling juncFig. 3.4.3. Plots of the first and second derivatives (dV/dI, d tion with Al biased negative (upper curves) and positive (lower curves) with respect to Pb. Note that the peaks are higher for Al biased negative (consequently this is the usual bias direction for tunneling spectra). The ratios of the integrated intensities of d21/d(e V)2 (proportional to (d V/d[)2) are compared to theoretical predictions in fig. 3.4.4. Figure from Kirtley et al. [ill.

They also calculate the ratio of conductance changes for tunneling in opposite bias directions. Crudely, in one bias direction the electron tunnels and then excites the molecule (Alneg in fig. 3.4.3). In the other it excites the molecule and then tunnels (Al~ 0~ in fig. 3.4.3). Since tunneling probability increases with electron energy, the first process is favored. Though this crude, twostep model gives physical insight into the asymmetry, it overestimates its magnitude. The dots in fig. 3.4.4 show experimental results based on fig. 3.4.3 for the ratio of conductance changes in the two bias directions. The lowest curve shows the prediction of the two-step model [18]. The problem is that the range of the electron—molecule interaction is of the same order of magnitude as the barrier thickness. This is taken into account naturally in the theory of Kirtley et al. which predicts the upper solid curve assuming no barrier asymmetry. An easy, though only approximate, correction for barrier asymmetry is to multiply the ratio of the inelastic conductance changes by the ratio of the measured elastic conductance at each bias voltage. This results in the dashed curve of fig. 3.3.4. They show that the inclusion of tunneling processes in which the electron changes transverse momentum results in coupling to infrared inactive modes as well as infrared active modes even —

P. K. Hanxma, Inelastic electron tunneling

0

200

16 1

400

eV (meV) Fig. 3.4.4. Plot of the ratio of the lETS integrated intensities for Al biased positive divided by Al biased negative, ~(dj/d(e V), )/~(dj/d(eV)_), versus e V. The lower solid curve is based on a two-step model proposed by Yanson et al. [18). The upper solid curve is based upon the theory of Kirtley et al. [11]. The dashed curve includes an approximate correction for barrier asymmetry in that theory. Figure from Kirtley et al. [11].

without the inclusion of any polarizability interaction. In fact, it results in coupling even to modes that are both infrared and Raman inactive. Such modes have not yet been reported experimentally. Finally, they discuss orientation effects for a number of model systems. For example, they find the ratio of conductance changes for a CO2 molecule oriented perpendicular to the metal versus parallel to the metal is about 1 6. It is too early to tell if this theory will be successful in actually predicting the intensities in the measured tunneling spectra of complex molecules. Perhaps a more sophisticated interaction will be required. Perhaps the transfer-Hamiltonian approach will not be good enough. The initial results, however, are very promising. Brailsford and Davis [19] and Davis [20] pointed out that there are avoidable approximations in both the Scalapino and Marcus theory and the transfer-Hamiltonian approach. They first derived a one-electron theory based on orthogonal current-carrying states which are eigenfunctions of the entire noninteracting system. The coupling to the impurity was treated without the adiabatic approximation implied in the WKB approach. Davis then extended this one-electron theory to a many-electron theory and compared his results to those of the WKB and transfer-Hamiltonian theories [20]. Perhaps the most important result of this work was to verify that the simpler transferHamiltonian theory is appropriate and accurate if the vibrator is far enough inside the2/2mcfi] tunneling 1/2 barrier. The relevant length scale is the decay length of the electron wavefunction, [h 1 A. Though this condition may be met in tunneling experiments, it is not met by a wide margin. For example, the effective vibrator—electrode distance determined recent experiment for an OH group directly covered by a Pb electrode was 0.8 A [21]. Other molecular vibrations should have, in general, a larger effective distance — particularly internal vibrations of large molecules. The strongest confirmation that typical vibrators are far enough inside the barrier comes from consideration of the predicted peak shapes if they were not. In the near limit, Davis finds logarith-

162

P. K. Hansma, Inelastic electron tunneling

mic singularities and step functions in the elastic conductance. The effect of these would be to modify the line shapes in tunneling spectra in a way that is not experimentally observed.* Three other formulations that avoid the transfer-Hamiltonian approximation have been made by Caroli et al. [22], and by Feuchtwang [23], and by Duke et al. [24]. The first two are based on the nonequilibrium formalism of Keldysch [25]. The third is based on constructing extended basis functions of the entire junction (in contrast to the left metal functions and right metal functions of the transfer-Hamiltonian model). None of these has yet included an electron—molecule interaction appropriate to inelastic electron tunneling. Unfortunately, this section must end on an unhappy (but challenging?) note. No one has yet calculated even the relative intensities for all the peaks in any measured tunneling spectrum.

4. Experimental methods 4.1. General remarks The experimental methods used in inelastic electron tunneling are similar to those used in all of electron tunneling. Two reviews of tunneling in general with a great deal of relevant information on experimental methods have been written by Coleman et al. [26] and by McMillan and Rowell [27]. Since, however, inelastic electron tunneling is a new (less than ten years old) and relatively small field (fewer than ten laboratories worldwide), it is probably worth taking some time to detail the most important aspects of making and running samples. 4.2. Sample preparation The thin film junctions are fabricated on clean, insulating substrates. Some researchers have used silica substrates, but ordinary glass microscope slides work well and have been used by most. Cleaning the substrates is regarded by all as a very important part of sample preparation. Though the number of recipes for this cleaning is roughly equal to the number of researchers in the field, most recipes include washing in laboratory detergent and water followed by very thorough rinsing in tap water and then distilled or deionized water. Some recipes also include rinses with various organic solvents before or after this detergent and water wash.** The samples are made in a high vacuum evaporator. It is absolutely essential that this evaporator be kept free of organic contaminants. Most researchers have found it necessary to have their own evaporator; it is difficult to maintain the required cleanliness in a common facility. On the more positive side, however, most evaporation systems can be used. Although very elaborate sorptionroughed, ion-pumped systems have been used, perfectly good results have also been obtained with ordinary mechanically-roughed, oil-diffusion-pumped systems. The important thing is to keep the system free from outside organic contamination.*** *Compare fig. 3 of ref. [20] to any of the spectra in this review. **ln our laboratory we wash several staining racks full of slides at one time (twenty-five slides per rack). The washed racks of slides are then stored under ethanol. Before use, the slides are removed from the ethanol and blown dry with a heat gun. our laboratory we (1) are careful to use gloves whenever touching anything that is to go into the vacuum system, (2) use detergent and water for cleaning everything that is put into the vacuum system and try to avoid use of organic solvents, and (3) use high intensity (greater than 0.1 A) glow discharge cleaning between every sample.

P. K. Hansma, Inelastic electron tunneling

163

The first metal electrode is evaporated onto the substrate in the evaporator at a pressure less than I 0~torr (below l0~torr is desirable). The shape of the electrode is determined by a metal mask close to the substrate. This mask can be machined, electroformed, or etched from a variety of metals, usually aluminum or stainless steel (brass should be avoided because of possible zinc sublimation). In general, it is desirable to have the electrodes be narrow in the region of the junetion to minimize the possibility of dust specks or other imperfections in the junction area. A very common width is 200 p, because this corresponds to the width of the narrowest saw usually available in machine shops. The thickness of the electrode is not critical. Most researchers make it between 1000 and 3000 A. The most common first metal electrode is aluminum. It can be evaporated either from wire wrapped around a tungsten filament or with an electron beam gun. Lead, tin and magnesium have also been used successfully, but they are somewhat more difficult to work with; they are generally used only when aluminum is unsuitable for some reason. After the evaporation of the first metal electrode comes a critical part of junction fabrication: the formation of the insulating layer. The reason this step is critical can easily be understood. The resistance of a tunnel junction is proportional to exp(+t/t0) where t is the thickness of the insulating layer, and ~0 h/’~/8mV0 1 A. Thus, the resistance of the tunnel junction changes by an order of magnitude if the thickness of the insulating layer changes by only a few A’s. Furthermore, since the tunneling depends exponentially on thickness, the barrier must be reasonably uniform. Almost all of the current would flow through one pinhole or thin spot if it were present. The difficulty of this requirement can be appreciated when it is realized that if the junction area were scaled up to the size of a typical room’s floor, the barrier would still be less than a tenth of a millimeter thick. This illustration suggests why no one has yet developed a reliable method for evaporating the insulating layer of a tunnel junction. Fortunately, the formation of an insulating barrier by the oxidation of the bottom metal electrode can be made routine and reliable. Here again there are as many recipes as there are laboratories. But these recipes divide into two general classes: glow discharge oxidation and thermal oxidation. In glow discharge oxidation [28, 29] the parameters are sensitive to the details of electrode and fixturing geometry. For aluminum, however, typical recipes involve dry oxygen at a pressure of order 0.05 torr with a glow discharge current of order 10 mA for times of order ten minutes. In thermal oxidation [30] the parameters are sensitive to atmospheric conditions, especially humidity. But in practice we have found (for liquid doped samples) that the minute or so that elapses between venting of the vacuum system and liquid doping of the sample is sufficient. For other types of samples, oxidation at temperatures up to 120°Cfor times up to many hours have been used. A detailed study of the techniques used to form oxide barriers on aluminum has recently been completed by Magno and Adler [31]. They have investigated both glow discharge and thermal oxidation in a number of gases and gas mixtures. One of their most interesting findings is that the impurity bands of thermally oxidized aluminum can be reduced by a glow discharge in argon. The next step, the doping of the insulating layer with the desired molecules, is in practice the most difficult step injunction fabrication. Jaklevic and Lambe vapor doped liquids (e.g., acetic acid) by breaking a small capillary containing the liquid inside the evacuated vacuum chamber after oxidation of the aluminum electrode [2]. They vapor doped less volatile materials (e.g., cyanoacetic acid) with a resistance heater. Lewis, Moseman and Weinberg [32] have done careful studies of the spectra of some volatile liquids as a function of the exposure over the range l0~

164

P.K. Hansma, Inelastic electron tunneling

through 10 ton seconds. Simonsen and Coleman [33, 34] carried the technique of vapor doping of less volatile molecules to the present state of the art with their carefully monitored temperature controlled evaporation source in a special doping chamber connected to the main vacuum chamber via a liquid-nitrogen-trapped, valved tunnel. Problems with vapor doping are, (1) since vapor pressure is a rapid function of temperature, it is difficult to control exposure, especially for larger molecules, (2) cross contamination is difficult to control, and (3) the technique is, of course, limited to molecules that can be vaporized without decomposition. In collaboration with Coleman, we have developed a liquid doping technique [35, 36]. After oxidation the substrates are placed face up on a spinner similar to those used for applying photoresist on integrated circuit chips. A solution of the desired dopant is simply dropped onto the oxidized aluminum strips, and the excess spun off. Water is the solvent of choice, but benzene, methanol, and some other organic solvents can be used. Water and benzene leave no detectable residue. Methanol and other organic solvents typically leave some residue which must be taken into account. The amount of dopant in the junction is controlled by varying the concentration of the dopant solution. In practice the concentration of the dopant solution varies between approximately 1/1 0 and 1 g/liter. The correct concentration is determined within this range by trial and error guided, of course, by previous experience with any related compounds. A problem with liquid doping is that samples must be exposed to the atmosphere and thus to possible atmospheric contamination. This may be severe in areas with significant air pollution. In practice, Coleman has not found it to be a problem at the University of Virginia, and we have not found it to be a problem at the University of California, Santa Barbara. It may, however, prove necessary to have the evaporator and spinner in an enclosure with filtered air in larger urban areas. Another type of liquid doping was developed independently by Skarlatos, Barker, Hailer and Yellon [37]. They simply dipped their oxidized aluminum strips into aqueous solutions of organic molecules. Though this dipping technique does not yet seem to yield spectra of the quality of the spinning technique, it is promising as a sensitive method for detecting and identifying organic impurities in solutions it has sensitivity down to the parts per million level. We will return to a discussion of this in section 6. 1. A quite different type of liquid doping was used by Bogatina, Yanson, Verkin and Batrak [38] for obtaining spectra of organic solvents. In this case the problem is that the solvent molecules won’t ordinarily stick to the oxide layer. So they first cooled their oxidized strips to around 200°K in a nitrogen atmosphere, then placed a drop of the dopant on the strip, and finally, evaporated off excess solvent in a vacuum of 10-6 torr while keeping the substrate cold. In this way they obtained spectra of a number of compounds, including acetone and benzene. Unfortunately, some of their spectra are difficult to interpret, possibly because of condensed impurities. After doping, the junction is completed with an evaporated second metal electrode. Again it is generally desirable to keep this electrode narrow, and again the thickness of this electrode is not critical. Thicknesses of 1 000 to 10 000 A are typical. The most widely used metal is lead. Lead, as will be discussed below, causes the smallest perturbation of the dopant’s vibrational spectrum arid the least damage to it. Why lead works better than any other metal is something of a mystery. Researchers have speculated that it may be the large ionic radius which minimizes diffusion of the lead into the oxide layer, that lead is chemically rather inert, and that lead evaporates at a very low temperature which minimizes sample heating during the lead evaporation. Whichever one or combination of these is in fact important, the simple fact is that lead works best of any metal that has been tried to date. —

P. K. Hansma, Inelastic electron tunneling

165

Other metals have, however, been successfully used: for example, Sn, In, Ag, Cu, Zn and Au [39]. These are listed roughly in order of increasing difficulty. All require liquid nitrogen cooling of the substrate to minimize damage to the dopant molecules. Without liquid nitrogen cooling of the substrate, the spectra are usually irreproducible and noisy. The mechanism of molecular degradation by the second metal electrode is not, however, understood. Finally, the junctions are evaluated with a low power ohm-meter [401. For the case of liquid doped junctions, the center junction of five junctions on a slide is typically left undoped and should have a resistance of order 1—1 0 ohms for a 200 p X 200 p junction. Our experience has shown that the best spectra are obtained for doped junctions with resistances approximately two orders of magnitude higher than this. In general, two junctions at one end of the slide are doped with a different solution concentration from the two junctions at the other end of the slide. If none of the doped junctions fall within the desred range (50—2000 ohms), new junctions are made with the solution concentration appropriately altered: increased if the junction resistances are too low and vice versa. For the case of vapor doped junctions, the oxidation parameters are first found to produce the desired control resistances. Then other slides are vapor doped, and the resistances measured. Unfortunately, it is difficult to make an undoped control junction on the same slide with the doped junctions. 4.3. Cryogenics Tunneling spectra can only be obtained at liquid helium temperatures (4.2 K and below). The finished samples are mounted on a dewar insert, electrical leads are attached, and the samples are immersed into a dewar. Figure 4.3.1 shows two different dewar inserts. The left one holds 1” X 3” microscope slides for insertion into a 5 cm ID, 3 liter liquid helium research dewar. The right one holds ~ X 2 cm substrates cut from a standard microscope slide for insertion down the 1.3 cm diameter neck of a standard liquid helium storage dewar. The tricky part of this step is attaching the electrical leads. Three methods are: (I) Mechanical pressure. Either screw clamps as shown in fig. 4.3.2, or spring contacts cut from printed circuit-board connectors. Surprisingly enough, the simple brass screw clamps neither crack the slide nor fall off with no special preparation of the brass screw tips. They are growing in acceptance and may well become standard [41]. (2) Indium solder. At present the majority of researchers in tunneling attach leads with indium solder. Pure indium sticks to a wide variety of metals and forms reliable bonds. Some workers have claimed that these connections are of lower noise than any other type, though we have observed no differences in our laboratory. One problem with indium solder that is especially severe for inelastic electron tunneling is sample heating during the soldering which can cause molecular degradation. This can be minimized by soldering under a bath of liquid nitrogen or by using pressed-on, cold-flow indium contacts. (3) Silver paint or paste. The upper sample holder of fig. 4.3. 1 has leads attached with silver paint. Wires are soldered to reusable copper disks which are in turn fastened to the substrate with silver paint.

166

P.K. Hansma, Inelastic electron tunneling

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Fig. 4.3.2. A drawing of the tip of the sample holder shown in fig. 4.3.1. Note the brass screw clamps that make electrical contact with the evaporated films. No special preparation of the films or screw tips is necessary.

long.

4.4. Sample evaluation The first step in sample evaluation is a four terminal measurement of sample resistance. One possible circuit for obtaining this measurement is shown in fig. 4.4.1. The function generator applies a sawtooth voltage of frequency of order 100 Hz, to the series combination of Rseries, the

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junction, and Rmeas. Rseries is greater than 1000 times the junction resistance and Rmeas is of order the junction resistance. Voltage across Rmeas, which is proportional to the current through the sample, is displayed on the y-axis of an x—y oscilloscope. The voltage across the junction is amplified with a differential dc amplifier and applied to the x-axis of the same oscilloscope. (The ac oscillator, lock-in amplifier, and x—y chart recorder are not used for these current versus voltage measurements.) After the resistance is measured at room temperature from the slope of the current-voltage curve, the sample is cooled to 4.2 K or below, and the current-voltage curve is remeasured. The resistance at the low temperature must be greater than or equal to the resistance at room temperature or the sample is discarded; any decrease in resistance is indicative of metallic shorts through the insulating layer. In the usual case of an Al—I—Pb junction there is another, more sensitive, test of junction quality. This is the shape of the current voltage curve in the voltage range of a few millivolts. Good tunneling junctions at 4.2 K have a resistance near zero voltage greater than or about equal to 7 times their normal state resistance due to the superconducting energy gap of the lead [42] . A smaller ratio is indicative of nontunneling current through the insulating layer. A more complete discussion of this criteria and others for judging junction quality has been given by McMillan and Rowell [271. 4.5. Taking tunneling spectra A tunneling spectra is a plot of d2 V/d.P versus V, the second derivative of voltage with respect to current as a function of voltage. There are numerous circuits for obtaining this curve [5, 32, 43—45]. A block diagram of one of the simplest is shown in fig. 4.4. 1. For this measurement Rmeas is set to zero, and the x—y oscilloscope is not used. A slow current sweep (sweep times are typically between 10 minutes and 10 hours) is applied to the junction with a function generator through Rseries. Rseries is kept as large as possible consistent with sweeping the junction out to the desired voltage (~V). In addition to this slow sweep, a small amplitude ac current is applied to the junction with a stable low distortion ac oscillator through Rmod. The frequency is typically 1—100 kHz. Rmod must be greater than 1000 times the junction resistance. It and the ac oscillator

1 68

1~K. Hansrna. Inelastic electrO,? tunneling

0

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Fig. 4.5.1. Electronics and cryogenics for tunneling ~pectroscopy. Note tile sansple holder from fig. 4.3.1 inserted down the neck of the liquid helium storage dewar. Tue photograph was taken through the door of a copper-screened room that minimizes electrical interference. Such a room is not essential, but desirable.

P. K. Hansrna, Inelastic electron tunneling

169

amplitude are adjusted to obtain the desired voltage modulation across the junction (usually 0.5—2 rnV rms). The dc voltage across the junction is applied through a differential dc amplifier to the x-axis of an x—y chart recorder. The voltage at the second harmonic frequency, which is proportional to d2 V/dP, is measured with a lock-in amplifier and applied to the y-axis of the chart recorder. Its magnitude is generally in the range 0. 1 through 2 pV. Many researchers use a bridge circuit [5, 43, 44] to minimize the first harmonic signal at the lock-in amplifier’s input. However, some modern lock-in amplifiers [461 have enough dynamic range to reject the first harmonic voltage without the use of a bridge circuit, especially when they are used with tuned internal filters (a notch at the first harmonic or a band pass at the second harmonic). Because of the low level of voltages that must be measured, a good deal of care with shielding and grounding is important. Though it would be inappropriate to give a lengthy description of shielding and ground techniques* in this review, a few general remarks would possibly be helpful: (1) Wherever possible, leads should be in twisted pairs with electrostatic cable shielding.* (2) All circuitry must be designed to minimize electrical transients which can burn out junctions. (3) A screened room, though not essential, is very desirable. A photograph of an actual experimental setup inside a shielded room is shown in fig. 4.5. 1. Despite the apparent complexity, this circuitry has the basic schematic diagram shown in fig. 4.4. 1.

5. Experimental results 5.]. Sensitivity The most important advantage of inelastic electron tunneling over infrared and Raman spectroscopy is its sensitivity; electrons couple more strongly to molecular vibrations than do photons. Workers in the field ever since Jaklevic and Lambe have speculated that the dopant layers involved for inelastic electron tunneling spectroscopy (lETS) are of the order of one monolayer, and there is a good deal of indirect evidence for this. Specifically, many compounds react with the oxide layer to form new surface species. The tunneling spectra for these compounds usually show no trace of the unreacted compound [48]. Thus, one can infer that only a monolayer or less is present in the junction. Recent quantitative measurements on one test compound, benzoic acid, have confirmed this qualitative conclusion and yielded a good deal of additional information [49]. Fig. 5.1.1 shows spectra obtained by liquid doping two different concentration solutions of benzoic acid in water onto oxidized aluminum. Note that, as one would expect, the peak heights decrease as the concentration of the dopant solution is decreased. The lower curve on fig. 5. 1.2 summarizes the data from many runs on the peak height versus dopant. Note that the peak height rises roughly linearly with dopant solution concentration up to a plateau for solution concentrations >0.2 mg/mi. The upper curve of fig. 5.1.2 shows the actual surface concentration as a function of dopant solution concentration. The surface concentration was determined by using dopant solutions of radioactively labeled benzoic acid and measuring their activity with a scintillation counter. The *Morrison 147] has written a clear, useful book on this subject. *We have found that attaching all cable shields to ground through a 10 ohm resistor at each end works well.

P. K. Hansma, Inelastic electron tunneling

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conversion between activity and surface concentration was determined by a calibration run in which measured volumes of benzoic acid solutions of known specific activity were dropped onto glass substrates and evaporated to dryness. Note that the surface concentration also rises roughly linearly to a plateau and that the plateau occurs at a surface concentration of about 7 X l0” molecules/cm2 about 1 molecule per 15 A2. This, of course, explains why the tunneling peak height flattens out above approximately 0.2 mg/mI. These curves show that coverages down to on the order of 1/30 of a monolayer can be detected with inelastic electron tunneling. In a 200 micron square junction, such as we generally use, this corresponds to roughly 10’°atoms or 2 picograms. It is this extreme sensitivity that is of key importance to many of the applications to be discussed later in this review. These curves also suggest that peak height decreases more rapidly than the surface concentration below saturation coverage. For example, as the surface concentration decreases by a factor

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of 10 from 5 X l0’~to 5 X 1013 molecules/cm2, the peak height decreases by a factor of 40 (see fig. 5.1.2). We believe that this result can be explained qualitatively as follows. Junctions doped with a monolayer of the benzoate ion typically have resistances two orders of magnitude higher than undoped junctions. Thus, the tunneling conductance through the dopant plus insulating layer is much lower than through the insulating layer alone. As the surface concentration falls below a monolayer, microscope regions in which the top electrode is in direct contact with the insulating layer develop. Since the tunneling conductance per unit area is higher for these regions, the tunneling current density will be higher in them. Thus, the fraction of current flowing through the insulating layer plus benzoate ion will fall more rapidly than the fraction of the surface covered by the ion. Making the reasonable assumption that the fraction of the current flowing through the insulating layer plus the benzoate ion is primarily responsible for inelastic electron excitation of the benzoate ion, we can then conclude that the fraction of inelastic tunneling events (and thus the measured peak height) will fall more rapidly than the surface coverage. Following these simple arguments one step further, we would hypothesize that the degree of nonlinearily would be dependent on the particular dopant molecule used. The trend should be that the nonlinearity is larger for larger molecules since the difference in tunneling conductance between doped and undoped areas varies with thickness of the dopant layer. Specifically, from elementary quantum mechanical arguments [501, the difference in conductance per unit area will be of order exp(—t/t 0) where t is the thickness of the dopant layer and to llRf8m V0 1 A where m is the electron mass and V0 is the effective barrier height of the dopant layer. Finally, it is of interest to note that a dopant solution concentration of 0.5 mg/mI was decided upon on the basis of the preliminary evaluation technique described in the section on sample pre-

172

P. K. Hansma, Inelastic electron tunneling

paration (i.e. looking for the dopant solution concentration that gives a two order of magnitude resistance increase). Thus the sample evaluation technique selected the dopant solution concentration just sufficient to give saturation surface coverage. 5.2. Selection rules The experimental data on selection rules is scanty and easy to summarize: there is no evidence for any strong selection rules except for orientational selection rules. This is consistent with the theoretical predictions of section 3.4. The first attempt to investigate whether both infrared and Rarnan active modes appeared in tunneling spectra was made by Bogatina, Yanson, Verkin and Batrak [38]. As a part of their investigation of the spectra of organic solvents they examined the spectrum of benzene in Pb—oxide—Pb junctions. Bogatina [511 discussed this spectrum using group theory. This was the first molecule studied that had enough symmetry to have separate Raman and infrared modes. (The dipole moment of a molecule oscillates during an infrared active vibration. The polarizability oscillates during a Raman active vibration. In a molecule without sufficient symmetry there are no vibrational modes that are only Raman or only infrared active: both the dipole moment and the polarizability oscillate during its vibrations.) Their results were puzzling because many vibrational modes were not present (particularly between 200 and 300 meV). Nevertheless they found no evidence that infrared active modes differed in intensity from Raman active modes in any systematic way. Some of the strongest modes in the tunneling spectrum were infrared active; others were Raman active. Fig. 5.2.1 shows the tunneling spectrum of a larger molecule that has a center of inversion symmetry: anthracene [361. The arrows are drawn in the positions of prominent infrared and Raman active vibrational modes. Note that the tunneling spectrum contains both and with comparable intensity. The lack of the usual optical selection rules is shown dramatically in the recent work of Cass, Strauss and Hansma [52] on the tunneling spectra of long chain fatty acids. The “chain vibrations” (as opposed to “end group vibrations”) can be assigned k-values, where the vibrations of adjacent methylene groups on the chain of N groups are out of phase by approximately kir/(N + 1). In infrared or Raman spectra the intensities of bands characterized by different k-values decrease as k increases. In the tunneling spectra the intensities are roughly independent of k. The point is that optical selection rules depend on the long wavelength of the electromagnetic radiation used. The electric field is roughly uniform over the spatial extent of the molecule. Thus, for example, there is cancellation of the 0—C and the C—O dipole moments during the vibrational mode of CO2 in which both oxygen atoms move toward and away from the carbon atom in phase. This mode is consequently infrared inactive. The same kind of cancellation is responsible for the decrease in intensity of the higher k-value modes in long chain fatty acids. For the tunneling electron, on the other hand, this cancellation is minimized since the tunneling electron—molecule interaction varies spatially over distances comparable to the size of molecules (see section 3.4). Orientational selection rules have most probably been observed. Fig. 5.2.2 shows a comparison of infrared [531 and tunneling [361 spectra for benzaldehyde adsorbed on alumina. A detailed comparison of peak positions over the entire vibrational spectrum will be discussed in section 6.3. Here let me merely statet. that agreement in isposition is to within a weak few tenths of a by percent for One the mode, however, either anomolously or shifted an anommodes below 2000 cm olously large amount: the COO- asymmetric mode at 1560 cm~.

P. K. Hansma, Inelastic electron tunneling

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A possible explanation for the absence of this mode in the tunneling spectrum is that the benzoate ion is oriented such that the dipole moment of the asymmetric mode oscillates parallel to the tunneling barrier: the oxygen atoms are in equivalent positions, equidistant from the alumina. This conclusion is supported by the energies of the modes; the values 1430 and 1560 cm’ are consistent with the symmetric and asymmetric vibrations of a COO group in which the

174

P. K. Han sma, Inelastic electron tunneling

I ETS

JJf\. 1435 cm~1 ~

L

560 cm~ ~t’ 0 C

o___~ SYMMETRIC

~ ASYMMETRIC

Fig. 5.2.2. A comparison of infrared 153] and tunneling [36] spectra for benzaldehyde adsorbed on alumina over the region 1300—1700 cm~.The absence of the asymmetric C00 mode from the tunneling spectra is suggestive that the C00 group is oriented such that the dipole moment of the asymmetric mode oscillates parallel to the alumina. Tunneling electrons couple most strongly to dipole oscillations perpendicular to the alumina.

oxygen atoms are in equivalent sites (if the sites were inequivalent, the values would be more widely spaced) [55]. This general feature can be seen, though not as dramatically, in several of the other spectra in this review. For example, fig. 5.2.3 shows the spectra of formic acid adsorbed on alumina [81. Again the asymmetric C00 mode is lower in intensity than the symmetric COO mode despite the fact that they have comparable intensities in the infrared [56]. The only thing that keeps these observations from being conclusive is the possibility that this one mode is more strongly temperature dependent than all other modes. The tunneling measurements were made at 4.2 K or below. Clearly, more experimental work is needed on selection rules in lETS. 5.3. Peak shifts due to the oxide layer

A disappointing feature of the early experimental results was that the vibrational spectrum of molecules as measured by tunneling was not the same as the vibrational spectrum measured by optical techniques. For example, the tunneling spectrum of formic acid had similarities to the infrared spectrum: the C—H stretch and C—H bending vibrations were there and in approximately the right energy range. Further, these vibrations shifted as expected with deuteration [2], see fig. 2.3. But there were also differences: for example, the carboxylate stretch near 220 meV was totally missing from the tunneling spectrum, and most of the rest of the peak were somewhat shifted. There are two reasons for these discrepancies: (1) molecules may react with the oxide layer on which they are adsorbed, and (2) the second metal electrode, which is evaporated on top of the

/

P. K. Hansma, Inelastic electron tunneling

175

2V d d 12

VCO

VC-H

— —



ACID BAND SALT BAND



I~SA 8

____ ~

0

JRSPECTRI)~ ~QFHC00No /.

pil

I

I

100

200

_J”~

300

400 meV

Fig. 5.2.3. Tunneling spectrum of an Al—oxide—Pb junction vapor doped with formic acid. Comparison with infrared data shows that the formic acid reacts with the oxide to form a formate ion (HCOO) on the surface. Note the lower intensity of the C00 mode labeled “A” (for asymmetric) relative to the one labeled “5” (for symmetric). Figure from J. Klein et al. [8].

molecules, may shift their vibrational frequencies. In this section we will discuss the first of these. As will be seen below, it is usually the dominant cause of discrepancy; shifts due to the second metal electrode, which will be discussed in the next section, are generally much smaller. The first researchers to fully take into account the effects of adsorption on the oxide layer were Klein, Leger and coworkers [8]. They realized that the absence of the carboxylate frequency near 220 meV in the spectrum of formic acid was indicative of a chemical reaction between the formic acid and the aluminum oxide. Specifically, that the formic acid had reacted to give a formate surface species. When they compared their tunneling spectra, not to the infrared spectra of formic acid, but rather to the infrared spectra of a metallic formate, they found excellent agreement (see fig. 5.2.3). It turns out that aluminum oxide in the presence of very small amounts of water becomes covered with 0—H groups [57]. Even the partial pressure of water left in most good high vacuum systems is enough to saturate the aluminum oxide. The 0—H groups on the surface make the aluminum oxide basic. It is believed that the mechanism of interaction with an acid is for one of the 0—H groups on the surface to accept the proton from the acid and leave the surface as water [57]. The resultant ion is ionically bound to the oxide surface. The reactivity of the aluminum oxide can be decreased by lowering its temperature. Alternately, a less reactive oxide, such as lead oxide, can be used [38]. However, as we will see below, it is precisely the reactivity of the aluminum oxide which is of importance and interest in many applications of inelastic electron tunneling: especially its application to the study of surface physics and catalysis. 5.4. Peakshifts due to the top metal electrode The effect of the second metal electrode on vibrational mode positions is more subtle. No one

176

P.K. Hansma, Inelastic electron tunneling



50

__________________________ I

OPb xSn

I

A

E(meV) Fig. 5.4.1. Comparison of the tunneling spectra for magnesium--ocide—metal junctions doped with Water vapor and deuterium-oxide vapor, for lead versus gold top metal deetrodes. Both the OH stretch mode at ‘-450 meV and the OD stretch mode at 320 meV are visible. The bands are shifted down and broadened in energy for gold with respect to lead top metal electrodes. Figure from Kirtley and Hansma [39].

430

440

E(meV)

Ag

450

Fig. 5.4.2. Plot of peak positions versus bandwidth (full width at half-maximum) for the OH stretch mode on aluminum oxide for four different top metal electrodes. Larger frequency shifts (to lower peak energies) are correlated with larger bandwidths. Figure from Kistley and Hansma [391.

has yet reported any evidence of a reaction between the molecules and the evaporated second metal electrode. Nevertheless, it does have some effects. Figure 5.4. 1 compares the tunneling spectra for magnesium—oxide—metal junctions with lead and gold second metal electrodes [39]. The oxide was vapor doped with D2O, and both the 0—H stretch mode at about 450 meV and the O—D stretch mode at about 320 meV are clearly visible. The figure shows that the vibrational frequencies are shifted down and broadened for gold with respect to lead top metal electrodes. The size of the shifts is approximately 5%. An image dipole effect can explain both the shifts and broadening of the 0—H and O—D stretching modes [58]. The physical idea is simple: an oscillating charge outside of a plane metal surface induces an image charge inside the metal. The attractive interaction potential between the charge and its image is proportional to l/d, where d is the separation between the charge and the surface of the metal. This attractive potential shifts the resonance frequency down. The correlation between peak shifts and broadening can be explained by assuming that the 0—H ions occupy inequivalent sites on the surface of the oxide. There is then a distribution in effective d’s, with some of the ions being shifted more than others. The top metals that produce larger overall peak shifts can be expected to have larger distribution in peak shifts and thus larger bandwidths. It follows that the peak shifts will be about the same size as the bandwidths, as can be seen from fig.

5.4.2. The data can be analyzed from the point of view: what effective d is necessary to give the experimentally observed peak shifts? It turns out to be of order 1 A [58]. Thus, physically reasonable values of d will account for the observed mode shifts. Furthermore, the effective d seems to corre-

177

P.K. Hansma, Inelastic electron tunneling I I

I

I

I

98.3

30-

85.0

-

2

0

100

200

300

400

eV(meV) Fig. 5.4.3. Inelastic electron tunneling second (top) and third (bottom) harmonic spectra for an Al—oxide—Pb tunneling junction liquid doped with benzoic acid, taken at 1.1°K with a 1-mV modulation voltage. The labeled peaks are those chosen for study and discussed in the text. The second harmonic spectra corresponds to a second derivative of the current-voltage characteristic, and the third harmonic corresponds to a third derivative. Notice that a peak maximum in the second harmonic signal corresponds to a zero crossing in the third harmonic. The third harmonic, while smaller in amplitude than the second, has little background and enables a greater accuracy in mode energy measurements. Figure from Kirtley and Hansma [21].

late well with the atomic radius of the metal. This suggests that metals with smaller radii pack more tightly around molecules adsorbed on the oxide surface and thus have smaller effective d’s. Finally, we can compare the positions of the 0—H stretching mode on aluminum oxide for various top metal electrodes to infrared results with no top metal electrode. For the series lead, tin, silver, gold, the energies are 446, 444, 438 and 433 meV, respectively. With no top metal

P. K. Hansma, Inelastic electron tunneling

178

electrode the energy is approximately 455 ± 3 meV for various aluminas [60]. Thus, the tunneling results with a lead top electrode are the least perturbed with a shift of order 2%. The other metals have larger shifts which, as discussed above, are correlated with the smaller atomic radii. But even the shift with the lead top metal electrode is large enough to reek havoc on a vibrational mode analysis. It would be difficult to correct for shifts as large as 2% in the region below 200 meV where typical organic molecules have a large number of modes. Fortunately, the mode shifts seem to be much smaller for these modes. Fig. 5.4.3 shows the tunneling spectrum for benzoic acid adsorbed on alumina [21]. This acid undergoes the reaction discussed above forming a benzoic surface species. It was chosen for a study of peak shifts due to the top metal electrode because detailed infrared and Raman results were available for comparison. The modes below 200 meV can be divided into two general classes; those that involve CH bending motion, and those that don’t. Let us first turn our attention to the modes that do involve CH bending. The major difference, from the standpoint of the image dipole theory, between the CH and OH modes is that the CH dipole derivative is smaller: approximately 1/3 of the OH dipole derivative [61]. The image dipole effect scales as the square of the dipole derivative. Thus we would expect shifts of order tenths of a percent rather than a few percent as for OH. Fig. 5.4.4 plots the tip of the sharp peak near 200 meV of fig. 5.4.3 with an expanded voltage scale. Note that the difference between the peak position with a normal Pb versus an Ag top electrode is 0.4 meV, 0.2%. The position as measured by infrared, with no top metal electrode, is 0.2 meV, 0.1%, above the normal Pb result [53]. Thus our expectation that peak shifts will be of order tenths of a percent is fulfilled. Finally, let us consider the modes that do not involve CH motion (for example, ring deform a-

9.

80.4 meV

—.

> 0 X7.

3 > Cu

6Ag TOP METAL ELECTRODE Pb TOP METAL ELECTRODE B FIELD ON a Pb TOP METAL ELECTRODE B FIELD OFF 0

A

5

-

197

198

99

200

eV ~meV)

Fig. 5.4.4. A magnified view of the tip of the 198.3 mV peak of the spectrum shown in fig. 5.4.3. Note the small shift due to different metals both in the normal.state: the peak is at lower energy with an Ag top electrode than with a normal Pb top electrode. Note also the shift due to superconducting n the electrodes: this shift is not by simply the sum of the half gaps of the electrodes (1.45 meV, in this case). Figure from Kirtley and Hansma [211.

P. K. Hansma, Inelastic electron tunneling

179

tions). The major difference between these modes and the CH and OH modes is that d should be larger. These modes primarily involve carbon atoms within the ring. If we supposed that the appropriate d was thus increased by of order 1 A (CH bond length), the l/d3 dependence of the mode shifts would reduce these shifts by a factor of roughly 0. 1. Furthermore, the effective mass is larger by a factor of 6. The dipole derivatives are comparable to those for CH modes. Thus we would expect the ratio of these shifts to the OH shifts to be of order 5 X l0~ that is, shifts less than 0.01 mV. This is well below our resolution of 0.2 mV. Indeed, we observed no shifts to within our resolution. This is gratifying, since such modes are extremely useful for compound identification by vibrational analysis. Fig. 5.4.4 also shows the difference between the top of sharp 198.3 mV peak at 1.1 K with and without a magnetic field to quench the superconductivity in the Pb and Al electrodes.* The first and perhaps most important, feature to note is that superconducting electrodes do not dramatically increase resolution. As discussed in section 3.3, the primary contribution to peak width at 1.1°K in all published spectra is modulation voltage broadening(L~V~ 0~1 mV in this case). This source of broadening is not appreciably reduced with superconducting electrodes. A second feature to note is that the shift in the peak position, 1.05 mV, is less than the combined gap ~A1 + i~Pb~1 .45 meV, as measured from the elastic I—V characteristic of the junction. This effect, which is also due to ~Vmod~ can be understood qualitatively as follows: the conductance of a junction with a superconducting electrode well below its transition temperature is small until V = ~/e where it abruptly rises to well above the normal state conductance (the conductance if the superconducting electrode is made normal) [27]. It then decreases and asymptotically approaches the normal statepeak conductance onfollowed a voltageby scale of a fewundershoot mV. The second 21/d 172, thus has a sharp at V = z~/e a negative which derivative, d approaches 0. If this is smeared by a modulation voltage broadening ~Vmod the asymptotically negative undershoot will pull down the high voltage side of the peak, thus shifting the peak’s maximum down in voltage. The same thing occurs, shifted out by hv, for the inelastic tunneling peaks. Since typical modulation voltages are comparable to gap energies, the observed shifts are not negligible when compared to gap energies. This effect can be quantitatively modeled using a formalism similar to that of section 3.3 [211. Fig. 5.4.5 compares the experimental and theoretical lineshapes for the 198.3 mV peak for a benzoic acid doped Al—I—Pb junction at 1.1°K for three different modulation voltages. Note that for larger modulation voltages the peaks get larger, broader, and are shifted to lower energies. The agreement between the experiment and computer generated curves is in general satisfying except that the computer generated curves have undershoots on the high voltage side which are absent in the experimental curves. This experimental absence of undershoots is almost universal (e.g., see any spectrum in this review. One exception is the OD stretching peak on magnesium oxide that was observed and fit by Klein et al. [8]). The lack of undershoots in general is not presently understood. Finally, fig. 5.4.6 shows computer generated theoretical curves for the peak shifts due to superconductivity as a function of modulation voltage eV~/~ (Vs, = 1.41 ~ is the peak modulation voltage, ~ + ~AI for an Al—I—Pb junction at 4.2°Kand 1.1°K). For comparison, the points are experimental measurements of the shifts produced in the 198.3 mY peak of a *Klein et al. (81 were first to look carefully, both experimentally and theoretically, at the effect of superconducting electrodes. Their fig. 11 and accompanying discussion display most of the features to be discussed in this subsection.

180

P. K. Hansma, Inelastic electron tunneling >

~2O EXPERIMENT

~

19

,t~y

-3

202 V(mV)

)

6eV—hv (meV)

Fig. 5.4.5. Comparison of experimental and computer generated line shapes for an Al—oxide—Pb junction at 1.1°Kfor the second harmonic signals with three different modulation voltages. The vertical line in the experimental curve corresponds to the measured mode energy when the superconductivity was quenched with a magnet. The squares correspond to a 0.5-mV modulation, the triangles to a 1-mV modulation, and the circles to a 2-mV modulation. For larger modulation voltages the signals get larger and broader and are shifted to lower energies. Note that although the computed and experimental curves match quite well in general the predicted undershoots are not present in the experimental plots. Figure from Kirtley and Hansma [211.

benzoic acid doped Al—I—Pb junction when superconductivity was quenched with a magnetic field. Note that as the modulation voltage is decreased, the shifts increase toward the value they would have in the absence of all broadening, i~. This figure can be used to estimate the shift due to superconductivity in high resolution measurements of peak position. Note that for typical parameters: L~V 2meV, T 4.2 K, the error in simply assuming that the shift is ~ is more than 3 cm~. .0

-

4.2°K 0

0

I.I°K

eo

I

I

I

I

2

3

eV(~)/~~

0

0

I

I

I

I

2

3

eV~/~

Fig. 5.4.6. Plot of predicted and experimental peak shifts (8(eV~~

5k)/AI versus modulation voltage [eVJ~] for Al—oxide—Pb junctions at 1.1 and 4.2°K. The solid lines were computer generated as described in the text. Here ~ ‘~Al+ ~Pb was measured from the elastic I— V characteristics of the junctions. For small modulation voltages the shift approaches ~.. Figure from Kirtley and Hansma [21].

5.5.

Electronic transitions

~

Since the discovery of lETS there has been discussion on the possibility of observing electronic excitations in addition to the vibrational excitations commonly observed. Such electronic transitions are not routinely observed because of three significant difficulties: (1) Typical oxide barriers irreversibly break down at voltages smaller than the energies of typical electronic transitions. (Typical electronic transitions appear above 2 eV.)

P. K. Hansma, Inelastic electron tunneling

181

(2) Even if the junction does not break down, its elastic conductance increases rapidly in the voltage range of even low lying electronic transitions. This rapid increase in the elastic conductance makes it difficult to observe a small conductance increase due to inelastic tunneling. (3) A tunneling electron which loses enough energy to excite an electronic transition has to tunnel through a significantly higher barrier than the electrons that tunnel elastically at the same bias voltage. This difficulty, which is, of course, closely related to difficulty (2), makes it more difficult to observe high energy transitions than low energy transitions with lETS. Despite these difficulties, Leger, Klein, Belin and Defourneau succeeded in making the first observation of an electronic transition with lETS in 1972 [641. Fig. 5.5.1 shows recent results [65] for copper phtalocyanine vapor doped onto an aluminum—oxide—lead junction, the system on which the first observations were made. The large peak near 1. 15 V corresponds to an optically forbidden singlet—triplet (S—I) electronic tran~,ition.The change in conductance can only be approximated because of the difficulties mentioned above, but it is of order L~a/a= 5%: an order of magnitude greater than for typical vibrational transitions. Note that structure is present both for aluminum positive and aluminum negative with respect to the lead. This S—I transition is optically forbidden. Thus the selection rules for tunneling are different than the selection rules for optical transitions in the case of electronic transitions just as in the case of vibrational transitions. JETS can also see the optically allowed singlet—singlet (S—S) transitions. Fig. 5.5.2 shows the results of de Cheveigne [661 for laser dye tetracyanin liquid doped on a magnesium—oxide—lead junction. Since this transition is optically allowed, the tunneling results can be compared to optical results shown on the figure. This comparison reveals that the tunneling peak is in roughly the same position as the optical peak but is much broader. It is of interest to note that some of the molecules were deposited by vapor doping while others were deposited by liquid doping. The authors emphasized that the two methods each have their own advantages. The primary advantage of the vapor doping is the freedom from possible atmospheric contamination during the liquid doping step. The primary advantage of the liquid doping is that molecules that cannot be evaporated can be deposited. Four points can be made about the future of electronic spectroscopy with lETS: AI—0i (PtCu)-Pk

/

~

/

~

/

~

T~4~K

!‘~

~I +

VmOd

2OmV

/

I

~

I

I

I

i’sv’

~II+ I V

Fig. 5.5.1. Plots of (l/a)(dq/dV) and (d/dV)(l/a)(da/dV) versus V for copper phtalocyanine vapor doped onto the oxide of an Al— 21/dV2 versus V. The unusual derivatives were chosen to oxide—Pb junction.in Note that this is not tunneling spectrum, minimize changes the background due atousual elastic tunneling and thusd enhance the structure at 1.15 V, which is attributed to a singlet—triplet electronic transition. Figure from de Cheveigne, Leger and Klein [65].

P. K. Hansma, In elastic electron tunneling

182

do

0~dV

Mg O~_T,tr~cyanInePb V~od~3OmVpp

~2~C~e

0

0.5

1

5

.~Ctq~e

2

Fig. 5.5.2. Plot of (1/o)(dci/dV) versus V for the laser dye tetracyanin liquid doped onto the oxide of a Mg—oxide—Pb junction. Note the broad peak in the tunneling curves that may correspond to a singlet—singlet electronic excitation. The narrower, optical spectrum of this excitation is shown in the insert. Figure from de Cheveign~[661.

(1) It would be highly desirable to find an insulating barrier with a greater barrier height than that of aluminum oxide and magnesium oxide. This would minimize the difficulties listed above. (2) Broad structure from the metal electrodes themselves must be carefully taken into account before making positive identifications [67]. (3) The resolution of lETS for electronic transitions is poorer than that of optical techniques. Work on the mechanism of this broadening and its minimization would be a significant contribution. (4) The ability to see electronic transitions in addition to vibrational transitions may be important in some of the applications of lETS, in particular in its application to surface studies and catalysis.

6. Applications 6.1. Biochemistry The first systematic attempt to obtain spectra of a variety of molecules of biological interest was made by Simonsen and Coleman [34, 35]. They used a sophisticated vacuum system with a provision for vapor doping in a special isolated chamber to minimize cross contamination. Fig. 6. 1. 1 shows their spectrum of a typical undoped tunneling junction for use in distinguishing the

P. K. Hansma, Inelastic electron tunneling

183

10

-

9-

t —~

— (fl

z

z

a.

z

0

-

C

0 0 I

0

-

a-



0

I

-e0



4.2°K

-

r

0

I

‘-‘

R_

“I



N

>4-

-

~0

-

N 0

2—

0 0

z -

1—

o 0.0



I

I

I

800 I 0.1

1600

2400 I 0.3

I

0.2



3200 I 0.4

4000 0.5

I 4800 I 0.6

cm1 eV

Fig. 6.1.1. Tunneling spectrum of an undoped junction. Except for the small, impurity C—H stretch peak near 2900 cm’, the peaks in this spectrum are background peaks present for any (doped or undoped) Al—oxide—Pb junction. They are due to the aluminum electrode, the aluminum oxide insulating layer, and the OH groups that typically cover any aluminum oxide surface that has not been dehydrated at high temperature [57]. Figure based on Simonsen and Coleman [34] ,with assignment changes (68].

background peaks in the large number of spectra to be presented in this and subsequent sections. Fig. 6.1.2 shows their spectrum for glycerol in an aluminum—oxide—lead junction. Note the excellent agreement with the Raman and infrared spectra of this same compound as shown on the figure. At low energies the CH bend and CH—OH stretch modes are resolved. Though the tunneling curves do not have as high resolution as the optical results, they do represent an accurate envelope of those results. The resolution of the tunneling curves can, of course, be improved by lowering the temperature to 1 K. Simonsen and Coleman state that at the lower temperature, for example, the doublet structure of the CH stretching peak can be resolved. One significant but not presently understood feature of the spectrum is the huge strength of the CH stretch near 2900 cm’ relative to the OH stretch near 2900 cm’. Though this OH peak is present in the tunneling spectra, it is suprisingly small in this spectrum and, for that matter, in all reported tunneling spectra in which it is present. Possibly a reaction takes place between these OH groups and the oxide surface as found for phenol [32, 70, 72]. This will be further discussed in section 6.3. Fig. 6.1.3 shows their spectrum for 7,7,8,8-tetracyanoquinodimethan (TCNQ). Though this is not really a biochemical, it is of enough current interest to warrant its reproduction here. Note, in particular, the strong CseN stretch mode near 2160 cm~. Perhaps the most interesting spectra in the work of Simonsen and Coleman are those of amino acids. For example, fig. 6. 1.4 shows their spectrum for phenylalanine. This spectrum contains,

P. K. Hansma, Inelastic electron tunneling

184

10 9

GLYCEROL H H H I I I H—C—C—C—H III

-

TUNNELING SPECTRUM -~

0



£

-



OHOHOH

-

-

2-

~

i

~ ~

RAMAN —

II~ O

800

I 1600

I 2400

I’ 3200

I 4000

I 4800

0.0

I 0.1

I 0.2

I 0.3

I 0.4

I 0.5

I 0.6

2400

3200





cm~ eV

0~ 20

-

4°60

~



.

o 0.0

800

I

1600

I

I

0.1

0.2

.

I

0.3

0.4

4000 •

0.5

4800 cm~ •

I

0.6

eV

Fig. 6.1.2. Tunneling spectrum of glycerol vapor doped onto the oxide of an Al—oxide—Pb junction. For comparison the Raman spectra is given as a series of lines on the same graph, and the infrared spectra is given below. Figure from Simonsen and Coleman [34].

in addition to the usual hydrocarbon bands, an NH stretch at 2200 cm1, an OH bend at 13 10 a CO stretch at 1170 cm’, and an OH deformation at 590 cm1. In addition, there are peaks arising from the six-membered aromatic ring which constitutes the characteristic side group of phenylalanine. These modes are an aromatic CH stretch at 3040 cm1, a ring deformation at 380 cm’, and a ring double bond stretch at 1600 cm’. Curiously, the structure observed in the infrared spectrum in the 2000—2600 cm1 range is very weak in the tunneling spectrum. These

P. K. Hansma, Inelastic electron tunneling

185

0 TUNNELING SPECTRUM

9-

IL

-

uJ 0

H

8-

-

I

D

2

-

NEC~

I

-

NaC~

C

0

800 I

0.0

0.1

I 600

TCNQ 4.21< —



I 2400

3200

I 0.3

I 0.4





~-CaN

-

~C~N

-

I 4000

I 4800 cm

I 0.2

I

I

I

0.5

I 0.6

eV

Fig. 6.1.3. Tunneling spectrum of 7,7,8,8-tetracyanoquinodimethan (TCNQ) vapor doped onto the oxide of an Al—oxide—Pb t. Figure from Simonsen and Coleman [34]. junction. Note the strong CEN stretch at 2160 cm

spectra are representative of the spectra obtained by Simonsen and Coleman. Together they represent the most extensive work with the vapor doping technique to date. As mentioned in section 4, however, they experienced several difficulties in obtaining the spectra. Especially serious were problems of controlling the quantity of deposited material and of decomposition during vaporization. These difficulties can be overcome with the liquid doping technique discussed in section 4. For example, fig. 6.1.5 shows the spectrum of phenylalanine obtained by Hansma and Coleman [35] with the liquid doping technique for comparison with fig. 6.1.4 obtained with the vapor doping technique. Note that the same peaks are present in both spectra, but that the resolution is clearly superior in the liquid doped spectrum allowing more peaks to be identified. In general, all the simple amino acids can be easily distinguished from each other with lET. For example, fig. 6. 1.6 shows the spectrum obtained by Simonsen et al. [36] of the simplest amino acid, glycine, for comparison with the phenylalanine spectrum. In addition to the very different (and reproducible) envelopes, note particularly the absence of the three ring modes from this spectrum. There is really no problem with applying larger molecules containing many amino acids. For example, fig. 6. 1.7 shows the spectrum of the protein hemoglobin. Unfortunately, however, the resolution of lETS is not sufficient to resolve any more than the envelope of the huge number of adjacent modes for a molecule this large. As further examples of the application of JETS to biologically important molecules, figs. 6.1.8 through 6.1.10 show the spectra of various components of nucleic acids. As before, identifications have been made by comparison to infrared and Raman

186

P. K. Hansma, Inelastic electron tunneling

TUNNELING SPECTRUM

10

-

-

9-

-

E

8-

-

~‘

7-

(I)

0 .~

~

>

-

a:

6-

i— U, 0

5

-

~4 ~

4.2K

Z

0

PHENYLALANINE -

HO

Ow

4

w 0 0I~.I WO 001

1 -0 0 I~

CH2-C-C0H

-

NH2

-

ol

4

0 I~

~

-

H

-

C

I

I

I

~20z 40

II

~) I z

olt

2-~~4 I-

0 HZ~ II)~ 0I0~

I

I

I -

L- PHENYLALANINE

-

I

I

I

I

.0

800

1600

2400

3200

00

I 01

I 0.2

~

I

-

NUJOL MULL

4000

4800 cm

I

I 0.3

I

0.4

05

0.6

eV

Fig. 6.1.4. Tunneling spectrum of phenylalanine vapor doped onto the oxide of an Al—oxide—Pb junction. The infrared spectrum is given for comparison. Figure from Simonsen and Coleman [34].

spectra. Rather than give a complete discussion of these identifications, let us just make a few important observations. In particular, note the large differences between the spectra of cytosine and adenine and between cytosine and its derivative, cytidine-5’-diphosphate~disodium. Again the spectra of macromolecules such as DNA shown in fig. 6.1.11 can be easily obtained, though the spectra represents only an envelope of the huge number of individual peaks for these complex systems. Recently, Clark and Coleman [731 have investigated the tunneling spectra of a series of adenine derivatives. They have demonstrated that the spectra of a large number of slightly different derivatives can be distinguished from each other. Fig. 6.1.12 shows the relatively large differences that result from the replacement of the CH2OH group of adenosine with a phosphonoester group, CH2OPO3H2. They have shown that much more subtle differences can also be distinguished: for example,

187

P. K. Hansma, Inelastic electron tunneling

~L-PHENYLALANINE

(~1CH2~.C0H

0

H —

~

-

Ii =

~

00

II

4.21<

+

-

W 0

oO

~

-

~

I

0 I

0

I U

~j~O

~.

I 800

I 600

I 0.1

I 0.2

I 2400 I 0.3

I

-

I 3200

I 4000

I 0.4

I 0.5

I ~ I

eV

Fig. 6.1.5. Tunneling spectrum of phenylalanine liquid doped onto the oxide of an Al—oxide—Pb junction. Note the improved resolution and peak height over the vapor doped spectrum (fig. 6.1.4). Figure from Coleman, unpublished, is similar to the one in [35] ,but has higher resolution.

even the position of the group on the some ring. Fig. .13 only showsbythe energy t) phosphomonoester for various derivatives including that6.1 differ thelower position region (below 600 cm of the phosphomonoester group (the 2’, 3’, and 5’ positions are shown). The reason that only the low energy region is shown is that the differences between spectra are more pronounced there: the spectra are very similar in the higher energy region. The key points are: (1) The tunneling spectra can be obtained from only micrograms of sample. (2) Slightly different derivatives can be more easily distinguished with lETS than with either infrared or Raman spectroscopy. I would like to close this section by mentioning another possible application: the identification of biochemical pollutants in water and air. Skarlatos, Barker, Hailer and Yelon [37] have dipped their oxidized aluminum strips into water with known concentrations of acetic, formic and cynoacetic acid in the 1 to l0~ppm range. They found that the intensity of the CH peak at 360 mV for strips dipped into solutions of cynoacetic acid, CNCH 2COOH, saturated for concentrations ~ 10 ppm. Curiously, the intensity of the CseN peak at 280 mV did not saturate until much higher concentrations,? 1000 ppm. They also investigated other methods of sampling the dilute solutions: for example, exposing

188

P. K. Hanxma, Inelastic electron tunneling



GLYCINE

-

a:



-

H-C-COOH

-

NH2 Z

-

4.2K U)

I

0

I

0

800 I

01

1600 I

I

0.2

2400 I

I

0.3

3200 I

I

0.4

-~

4000 I 0.5

cm I

I

eV

Fig. 6.1.6. Tunneling spectrum of glycine liquid doped onto the oxide of an Al—oxide—Pb junction. This spectrum can be easily distinguished from that of other amino acids. (See, for example, fig. 6.1.5.) Figure from Simonsen et al. 136].

an oxidized aluminum strip to the vapor over the solution. They conclude, however, that dipping is the most sensitive method. It would be interesting to compare results of their dipping method with the spinning method used by Hansma and Coleman [351. More recently Skarlatos et al. [75] have used the dipping method to study the ozonation of phenol in dilute water solutions (100 ppm). They conclude from their tunneling spectra that the product after ozonation is acetic acid. Further, they compare their tunneling spectra of partially ozonated phenol to what would be expected from two different ozonation mechanisms. They conclude that their spectra are consistent with the one that does not involve quinone and catechol intermediates, which they do not find. Surprisingly, no one has yet reported a systematic study of the detection and identification of air pollutants, though it is all too clear to workers in the field that junctions are sensitive detectors; junctions exposed to polluted air rapidly develop large impurity peaks. 6.2. Radiation damage Understanding radiation damage by fast electrons on condensed molecular films is of importance to several areas:

P.K. Hansma, Inelastic electron tunneling

189

HE MOG LOB IN

42K

0 fl

o

800 I

I

oi

1600

2400

0.2

I 0.3

I

I 4000

3200 I

I 04

-

I

cm~ I

I

I

05

eV

Fig. 6.1.7. Tunneling spectrum of the protein hemoglobin liquid doped onto the oxide of an Al—oxide—Pb junction. Figure from Simonsen et al. [36].

(1) This damage currently limits the attainable resolution in the electron microscopy of biological molecules. The Inolecules are dissociated by the electron beam before they can be imaged at the instrumental resolution (~1 A in the best microscopes) [76]. (2) This damage also limits the information attainable from surface techniques such as LEED and Auger spectroscopy on adsorbed organic molecules [77, 781. Despite its importance, relatively little is known about this damage. Energy loss spectroscopy [79] and mass loss spectroscopy [80] have provided some information about changes in molecular specimens under the electron beam. But changes in the physical structure of the molecules are difficult, if not impossible, to ascertain from such studies. Vibrational spectroscopy is potentially a powerful tool for damage studies. The breaking of various bonds can be monitored by the decrease in intensity of the corresponding vibrational peaks. Similarly, the formation of new bonds can be monitored by the growth of new vibrational peaks. A number of groups have used infrared vibrational spectroscopy for damage studies. For example, a recent study by Marcus and Corelli [81] examined damage to DNA bases and their derivatives. Infrared studies cannot, however, study damage to monolayers because of insufficient sensitivity. Parikh [82] has suggested the use of lETS because of its greater sensitivity. The first measure-

P. K. Hans,na, Inelastic electron tunneling

190

ADENINE

-

NH2

I

-

-

~

N~~C~\

I-

-

C-H H~C~N~C~N



>—

-

-

ID



_Jz

0

Ifl<

4.2K •i:

~ H

0__~w -

Zc~~

a:~ u~ on a:



1w



0

~O

z w m

~

I

0

-

Z

ID

IZ

800

1600 I

0

-

0.1

0.2

2400 I

I

03

3200

‘I’ 4000

cm~

I

I

0.4

0.5

1

eV

Fig. 6.1.8. Tunneling spectrum of adenine liquid doped onto the oxide of an Al—oxide—Pb junction. Figure from Simonsen et al. [36].

ments using lETS were reported by Hansma and Parikh [83] for the simple sugar, fructose. The samples were prepared by liquid doping a solution of 13-D-fructose in water (0.5 mg/ml). Four or five junctions were prepared very closely spaced to insure uniformity of the doping. (The outermost junctions were approximately 2 mm apart.) Electron irradiations of the junctions (and thus of the sandwiched molecule) were carried out in a scanning electron microscope. The exposed area was slightly larger than the junction area, thereby ensuring the even exposure of the entire junction. In most cases different junctions were given different electron fluences (fluence is the correct term for what is commonly called exposure or dose, namely the time integral of the irradiation current per unit area). Typical irradiation parameters were: electron beam voltage, 30 keV; beam current, 0.1 to 30 nA; exposure time, 2 to 10 minutes; and pressure, l0~to 10-6 torr. Figure 6.2.1 shows results for four different electron fluences. Since 1 m coulomb/cm2 = 0.625 electrons/A2, these curves show that appreciable damage has occurred at fluences of less than 10 electrons/A2. Thus, for example, damage should be expected in a LEED experiment (typical parameters: 1~tAelectron beam current in a 1 mm spit size) in times of order 100 seconds for this particular molecule. But, perhaps more important, these spectra contain detailed information on the mechanism for the damage. The decrease in intensity of peaks can be correlated with damage

P. K. Hansma, Inelastic electron tunneling

191

Uw 0

a: 10

-

TUNNELING

SPECTRUM

I (00

0

‘,~

a: ~ E

-

I-

8-

I

-

~J\ \

CYTOSINE 4,2K

w

H\/H

a

~

5 >

~-

N

,noà

-~

0 0 ~-

ID

I

~

4

I



Z

a:

H

2-

c

~I

I

~

I

I

-

k1~~~V:O~INE,cromato, pure

80

lao I

0

800 I

0.0

I

01



600



I

0.2



1

NUJOL MULL I I

2400

3200

4000

I

I

I

0.3

0.4

0.5



I

4800

cm~



I

0.6

eV Fig. 6.1.9. Tunneling spectrum of cytosine liquid doped onto the oxide of an Al—oxide—Pb junction. Figure from Simonsen et al.

[361. to the corresponding functional groups. Similarly the growth of new peaks can be correlated with the formation of new functional groups. For example, the COH functional groups in the fructose molecule can be expected to have three strong vibrational bands at positions close to those found for this functional group in primary and secondary alcohols. These are a CO stretching vibration in the range 1050 through 1100 cm’, an OH bending vibration in the range 1260 through 1300 cm’, and an OH stretching vibration above 3600 cm’. Fig. 6.2.1 shows that there are three strong bands at these wave numbers. These are labeled A at 1100 cm1, v at 1261 cm1, and• at 3580 cm’. The effect of the electron irradiation is to produce comparable large decreases in the intensities of these bands; thus Hansma and Parikh deduced that one of the primary effects of electron irradiation is to disrupt the COH functional group in fructose.

P. K. Hansma, Inelastic electron tunneling

192

-

.

OH OH HO-~-O-P-O-CH

CYTIDINE- 5- DIPHOSHATE.

DISODIUM

~

10

0

0

2

CYTOSINE

~H’~

.2N4

OHOH

.~

42K

—H

— —

00

~

~

z•

0 -

~—

4

N

04

~

~

H I

I

I

I

0

800

1600

2400

3200

0

0.1

I

I

0.2

I

0.3

I

I

04

I

4000 0,5

Cm~ eV

I

Fig. 6.1.10. Tunneling spectrum of a cytosine derivative, eytidine-5’-diphosphate disodium liquid doped onto the oxide of an Al—oxide—Pb junction. Figure from Simonsen et al. [36].

1 (labeled ~) can be unambiguously associated with CH The large broad peak at 2900 cm 2 and CH stretching modes. The peak at 1460 cm’ (labeled V) can be tentatively associated with CH2 deformation [usual frequency range, 1465 ± 15 cm’] although other modes may contribute in this frequency range. Both of these modes decrease in intensity with irradiation at comparable rates (for example, by approximately percent at a functional fluence ofgroups. 30 2), a both resulthave that decreased supports their identification 40 with the same mcoulomb/ cm The growth of a peak at 1638 cm’ (labeled 0), a position characteristic of C =C stretching vibrations, suggests that, as the OH group and the peripheral protons are removed by electron action, double bonds are formed between C atoms. Parikh et al. [84] have worked toward deriving cross sections by: (1) deconvoluting the spectra to obtain integrated intensities for individual peaks, and (2) theoretically determining the electron energy distribution (including both primary and secondary electrons) at the molecular layer as a function of the incident electron energy distribution. Recently, Hall et al. [85] have studied the damage to a series of molecules with comparable size but different structure and conclude that for monolayer, as for bulk, straight chains damage more readily than conjugated chains, which in turn damage more readily than aromatic rings. They obtain damage cross sections that differ by more than an order of magnitude through this series. They also conclude that cross linking as a result of irradiation is less common in their adsorbed monolayers than in bulk. Though this work has exposed the potential of JETS for studying molecular degradation of monolayers, clearly much remains to be done. For example, an ultrahigh-vacuum electron mi-

P.K. Hansma, Inelastic electron tunneling

193

-

(00

-

:~

l.I~K

IN H20

I

U

I

I.~ ~

>

CALF THYMUS DNA

I

~“ 140

I—

I

I— ~ z~0 >-<—



//

-

0~I

I

I

0

800

600

I 0

I 0.1

I 0.2

I

I

I

2400

3200

4000

I 0.3

I 0.4

I 0.5

—I Cm

eV

I

Fig. 6.1.11. Tunneling spectrum of a macromolecule, DNA, liquid doped onto the oxide of an Al—oxide—Pb junction. Figure from Simonsen et al. [36].

croscope (or an electron probe), equipped with a stage cooled to liquid He temperatures and the necessary electronics, would make possible the measurement of successive tunneling spectra of the same junction, continuously and during irradiation. Studies in an electron irradiation instrument that has provisions for in situ evaporation of metal would yield information on the effect of the overlayer of metal as well as on the evaporation of fragments. 6.3. Surface studies and catalysis As discussed in the section of peak shifts, very early in the history of tunneling Jaklevic and Lambe noted discrepancies between the lETS results for simple organic acids and the optical spectra of those acids [2]. Klein, Léger et al. [8] explained these discrepancies by observing that the JETS spectrum, though it didn’t correspond to the optical spectrum of the free acid, corresponded beautifully to the optical spectrum of a metallic salt of the acid. Thus they showed that the acid reacted with the alumina to form a salt. See fig. 5.2.3. This observation marks the beginning of the study of surface physics and chemistry with lETS. Lewis, Mosesman and Weinberg [321 extended the study of the irreversible chemisorption of simple organic acids on alumina. By admitting controlled pressures of formic acid for controlled

194

P. K. Hansma, Inelastic electron tunneling 1111111

1111111

,‘.

01

~

nI

I

I

~I

ADENOSINE 3 5’- CYCLIC PHOS. Till III

__________________________

~

—~ (/)

ADP 1111111

01 ¼)

0¼ VI

1 H

}_

(

¼)I’~

p..

~~V1

I~

VII

>-

~ U, I—

~

___________

H —

ADENINE

0b00

3’-1111111 DEOXYADENOSINE 01

___________ 111111 AMP-S

OHOH “I~

I~ I

01

II

P..

II

i—I

I

¼)

01

0101

I— 42K

> (NJ

ADENIOSINE Till III I

0

I0

N

AMP-3’ II..JI III 0)01

01

0)01

0)

IL

I

011 01¼) OH

~

~

AMP ADENINE

~

I

01

OH OH

,01’~

-

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-

~

¼)I

1

1J/

~

,

4.2K I

0

800

I

I

I

I

1111111

o

o.i

I

2- ~AMP- 3

I

1600 2400 3200 4000

0.2

liii

0.3

0.4

0.5 eV

Fig. 6.1.12. Tunneling spectrum of adenosine and its denyative AMP, adenosine-5’-monophosphate, liquid doped onto the oxide of Al—oxide—Pb junctions. The numbers above the peaks were used by Clark and Coleman [73] (from whom this figure was obtained) for a detailed comparison with the Raman results of Lord and Thomas 1741.

I

0

I

I

I

400

I

OMP-

I

I

0 cm-’

I

I

I

2 I

400

Fig. 6.1.13. Tunneling spectra of the low voltage region for a number of adenine derivatives liquid doped onto the oxide of Al—oxide—Pb junctions. Note that even derivatives that differ only in the position of a phosphomonoester group around the ring (AMP-5’, AMP-3’ and AMP-2’) can be distinguished. The complete spectra for two of these derivatives are given in fig. 6.1.12. Figure from Clark and Coleman [73].

P. K. Hansma, Inelastic electron tunneling

$-D-FRUCTOSE

~

EXPOSURE 2) (mC/cm >—

195



OAVV

0

800

1600

2400

3200 4000

cm~ Fig. 6.2.1. Tunneling spectra for #-D-fructose liquid led onto the oxide of an Al—oxide—Pb junction and irradiated with 30 keV electrons in a scanning electron microscope. The curves are labeled by exposure, or equivalently, electron fluence and are offset for clarity. Note that some peaks decrease more rapidly than others; moreover, a new peak grows at 1638 cm’ as a result of the formation of C’~Cbonds upon irradiation. Figure from Hansma and Parikh [831.

times they found information about the kinetics of adsorption. Fig. 6.3. 1 shows their results for exposures of 10 torr/sec, 2 X l0~ torr/sec, and l0~torr/sec. By assuming that the integrated peak intensity in the tunneling spectrum was proportional to the fractional surface coverage O,~’they obtained the curve shown in fig. 6.3.2 for 0 as a function of e, the exposure. Further, they used this data to derive the sticking probability S as a function of 0. This curve is also shown in the figure; the straight line through the data points is for 5(0) = (5.5 X l0~)/0.Finally, they reported small shifts (~ 1/2%) in some vibrational frequencies as a function of surface coverage. They related these shifts to variations in adsorbate—adsorbent and adsorbate—adsorbate interactions. Lewis, Bowser, Horn, Luu and Weinberg [861 have examined the irreversible chemisorption of phenol (C 6HSOH) and hydroquinone (1 ,4-C6H4(OH)2) on alumina, see for example fig. 6.3.3. They found from an analysis of tunneling peak positions that the phenol adsorbed predominantly as C6H5O and that the hydroquinone adsorbed predominately as C6H4(OH)0. It is interesting to note that this is just what is observed to happen for the adsorption of simple acid: the compound loses a proton, and the resultant ion binds to the alumina. In addition to these primary ad*In light of the discussion in the section on sensitivity pointing out the nonlinearity between surface coverage and tunneling peak height (the integrated peak intensity is proportional to the peak height since the width is roughly independent of coverage), these quantitative conclusions can not be viewed as definitive. However, the formic acid used in these experiments is a smaller molecule than the benzoic acid used in the peak height versus concentration experiments. Thus, as discussed in the section on sensitivity, the nonlinearity is probably less pronounced for the formic acid. Clearly experiments on a number of different sized compounds similar to the experiments for benzoic acid would be a valuable contribution to the field of lETS. Hopefully these experiments would yield at least an empirical rule for determining surface coverage as a function of peak intensity.

P. K. Hansma, Inelastic electron tunneling

196

AI/AI 2O3/Pb

HCOOH 4.2~K

(a) (0

C

0

00

200

300

400

V, meV Fig. 6.3.1. Tunneling spectra for acetic acid, HCOOH, vapor doped onto the oxide of Al—oxide—Pb junctions. The exposure to the acid was varied from 10 torn sec in (a) to 2 X i0~ torn sec in (b) to 10~torr sec in (c). The highest exposure saturated the surface (0 1). The surface coverages for (b) and (c) were estimated from peak heights as 0 0.4 and 0 0.08 respectively. Figure from Lewis et al. [32].

=

=

sorbed species, Lewis et al. also found evidence for small amounts of other species. They observed a long low energy tail on the OH stretching mode and interpreted this as evidence for extensive hydrogen bonding. They found that hydroquinone adsorbed both as the single ion, C6 H4 (OH)O, and as the di-ion, C6H4O~.Weinberg, Bowser and Lewis [72] extended the measurements to catechol, 1,2-C6 H4(OH)2, and resorcinol, 1,3-C6H4 (OH)2, and found that both adsorbed primarily as the di-ion, C6H4O~. Very recently McMorris et al. [70] have repeated the measurements on phenol and analyzed the results with group theory. They agree with the conclusion of Lewis et al. [86] that the phenol is adsorbed as a C6HS.0 ion based on the absence of phenol OH modes from the spectrum. They have extended the measurements to eight phenol derivatives. Comparison of the peak positions between phenol and its derivatives and between related derivatives, together with their group theoretical analysis, allows them to make complete assignments. At this point I would like to anticipate a key question. Why use inelastic electron tunneling for these adsorption studies? Why not use infrared or Raman spectroscopy? In fact, researchers have been using infrared and Raman spectroscopy for many years in attempts — both successful and unsucessful — to identify surface species. There has even been a

P.K. Hansma, Inelastic electron tunneling

4

I

IO

197

I0~

~

I I

I

1

I

e IO1~

-

0.21

I

I

III I

III I

I

I04

torr-seC

E,

Fig. 6.3.2. The fractional surface coverage, 0, as a function of the exposure, e, for vapor doped acetic acid. The data points are based on an analysis of tunneling spectra such as shown in fig. 6.3.1. Also shown is the sticking probability S(0) (dO/dt)/(F/n 5), where Fis the flux of impinging acetic acid atoms and ns is the density of adsorption sites. Figure from Lewis et al. [32].

=

book written on the subject [57]. A serious problem for Raman studies is fluorescence. Most catalysts and catalyst supports fluoresce strongly: typically 1 to 3 orders of magnitude more intensely than a Raman spectrum of an adsorbed species [87]. Though there are techniques for reducing this fluorescence, Raman spectroscopy is still difficult because of low sensitivity. Even though infrared is often more sensitive, large regions of spectra are typically obscured by broad absorption bands of the catalyst itself. Further, the sensitivity is still poor. Basically, a path length of at least 500 A is required to obtain a good infrared spectra [57]. This is, of course, very short on the scale of a liquid cell, but it unfortunately corresponds to hundreds of monolayers for most compounds. There are a number of methods to get around this I

I

I

I

I

I

AI/A12O3/Pb

-

C6H5OH

0

IOO

200

300

400

V. meV Fig. 6.3.3. Tunneling spectrum of phenol, C6H5OH, vapor doped onto the oxide of an Al—oxide—Pb junction. Figure from Lewisetal. [86].

P. K. Hansma, Inelastic electron tunneling

198 I

I

I

I

I

I

I

I

Ill

111111

INFRARED

~II

I~I~

000 I

I500 I

11”I

I

I

2000

I

I

I

I

4000cm’

3000 I

I

I

I

IIIT~II)II

1500

3200 3000

I I I INELASTIC ELECTRON TUNNELING

000

500cm’

I

I

~ 0

800

I600

2400

3200

cm~

Fig. 6.3.4. A comparison of infrared [53], Raman [53] and tunneling [36] results for the same system: benzaldehyde adsorbed on alumina. Note that the tunneling spectrum has as good resolution as the infrared in the region where the infrared is the best, 1, but extends this resolution over a much wider range. The agreement between peak positions determined by the 1300—1700 crn three spectroscopies is excellent in the regions where they overlap. Figure from Hansma [54].

fundamental difficulty; the most common is to use a powdered sample of substrate and depend on multiple scattering. Unfortunately, this introduces problems of its own; but rather than discuss in detail the problems with optical techniques, let me just show you some typical results for comparison with tunneling results. Alumina is a commonly studied material by researchers in catalysis. A search of recent issues of the Journal of Catalysis revealed a study of the adsorption of benzaldehyde on alumina [53]. The infrared and Raman spectra from that study are shown in fig. 6.3.4. These spectra are typical of what a well-equipped modern laboratory can do with these techniques. Note the good resolution in the infrared spectrum between roughly 1300 and 1700 cm’, but note also that resolution

199

P. K. Hansma, inelastic electron tunneling

Table 1 Assignment

Aluminum benzoate

Benzaldehyde on Al

2 03

Inelastic tunneling in

Raman

v(CH) v(CH)

3070(w) 3030(x)

3080(m)

3065(m) 3025(mw) 2888(w)

? v(CC)

1620(sh) 1600(s)

1602(2)

1597(vs)

0 asym v(C~ —)

1560(s)

v(CC) v(CC)

1495(m) 1450(sh)

1498(m) 1460(sh)

1491(mw) 1450(sh)

1435(s)

1430(mw)

1433(m)

1170(Vw) 1070(w) 1020(w) 1

1160(m)

1153(m) 1068(Vw) l021(Vw) 988(m) 9 36(mw) 847(m) 823(m) 723(m) 686(s) 6l7(mw) 440(m) 406(m) 307(w)

sym v(CN p(=CH)X a(=CH) v(CC)

—)

v(=CH)

(sh?)

1026(w) 1005(vs) 858(m)

unavailable

I I

B(CCC)

617(m)

t alumina absorption beand sensitivity degrade of The this limited Below comes too severe to seeoutside anything. Raman region. spectrum picks1000 out acm few more modes. The lower spectrum in fig. 6.3.4 shows the tunneling results. We simply put a drop of benzaldehyde on the oxidized aluminum strip and spin off the excess. Note that this spectrum has resolution equal to the optical resolution in the region where the optical resolution is the best: 1300—1700 cm’. The tunneling spectrum, however, has this resolution throughout the entire range 200—4000 cm1. Note particularly all of the modes below 1000 cm1: a region inaccessible to infrared spectroscopy of an alumina surface. Table 1 shows a detailed comparison of tunneling peak positions with infrared and Raman peak positions for the compound aluminum benzoate. Note that the comparison is not with the optical results for benzaldehyde. A surface reaction takes place on the alumina surface; the resulting surface species has the vibrational modes characteristic of aluminum benzoate. Interestingly enough, Walmsley et al. have recently observed that benzoyl chloride reacts with the alumina to give exactly the same surface species [88,89]. And so, for that matter, does benzoic acid [90]. All give the benzoate surface species. At any rate, the point is that the agreement between optical and tunneling results is very good considering that the uncertainties in both the tunneling and optical data are of order ±5 cm~. As another example: in the work by Lewis et al. [86] there is a comparison of tunneling versus

P. K. Hanxma, Inelastic electron tunneling

200

PYRIDINE

~

~

I

FLUOR,/P~,Itt

~

0

800

1600 2400 cm~

3200 4(2)0

Fig. 6.3.5. Tunneling spectrum of pynidine vapor doped onto the oxide of Al—oxide—Pb junctions. For the upper trace the oxidized aluminum had been pretreated with a solution of ammonium fluoride in water spun on in the usual manner. Both the fluonided and unfluorided oxidized aluminum strips were returned to a high vacuum system, baked for 60 sec with a quartz iodide lamp in high vacuum, and then exposed to 1.0 torr of pyridine vapor for several minutes. The junctions were completed with the usual evaporated lead electrodes. Figure from Hansma [54].

infrared data [91] for phenol adsorbed on alumina. They found good agreement with the infrared results but emphasize that the tunneling results had much higher resolution: above 150 meV fifteen vibrational modes can be resolved in the tunneling spectrum versus five in the infrared. In our laboratory we have shown that the nature of the absorption of a molecule can be changed by a liquid doping pretreatment. The literature of catalysis suggests that chemisorption and catalytic activity can be enhanced by fluoriding an oxide surface [92]. This can be easily accomplished on the oxidized aluminum strips by liquid doping with a solution of ammonium fluoride in water. Figure 6.3.5 compares the adsorption of pyridine on a fluorided and an unfluorided surface [54, 93]. Why pyridine? Because pyridine is a weak base which is extensively used by surface chemists to determine the strength of acid sites on surfaces [94, 92, 57]. As can be seen from the figure, the tunneling spectrum of the pyridine is markedly different on the fluorided versus the nonfluorided alumina surface. The spectrum on the nonfluorided surface most closely resembles the infrared spectrum of pure pyridine [95]. The spectrum on the fluorided surface most closely resembles the infrared spectrum of pyridine coordinately bound to something. That is, pyridine that has donated an electron to an electron acceptor (for example, borane) [95, 96]. To obtain these data it was necessary to heat the substrate with a quartz iodide lamp. Presumably this was necessary to drive off the OH groups from some of the surface acid sites. Though the spectrum of pyridine on fluorided alumina more closely resembles the optical spectrum of pyridine bound to a Lewis acid site (a site that will accept an electron), there are still some discrepancies. These may be due to photochemical reactions due to the quartz iodide lamp.

P. K. Hansma, inelastic electron tunneling

201

a

1000 A

Fig. 6.3.6. Electron micrograph of small Al particles formed by the evaporation of Al metal in argon at a pressure of 10 torn. The particles were oxidized in air and studied with standard catalyst evaluation techniques to characterize the oxide that grows on Al metal. Photograph from Hickson et al. [93].

A new method for heating the oxidized aluminum strips has been developed by Bowser and Weinberg [69]. This method is to simply pass a current through the aluminum strip in the vacuum chamber. The amount of heating can be determined by simultaneously measuring the resistance of the strip since the strip resistance is a function of temperature. This method of heating has the advantages over the quartz iodide lamp method that (I) there is no ultraviolet light to induce possible photochemical reactions and, perhaps even more important, (2) only the aluminum strip itself is heated, which minimizes impurity desorption from the vacuum chamber walls. They have used this technique for studying the insulator formed on aluminum films in an H2O plasma discharge as a function of temperature [69]. They found that an aluminum hydroxide

202

P.K. Hansma, Inelastic electron tunneling

was formed at low temperatures. At higher temperatures 200°C) an aluminum oxide was formed that was indistinguishable in its tunneling spectrum from that formed by oxidation in air or by glow discharge in oxygen. This raises the important question: What is the nature of the alumina formed by the oxidation of aluminum in air or by glow discharge oxygen? Is it related to the aluminas used in conventional catalysis research? A partial answer to these questions can be found in the adsorption studies discussed above. In every case for which comparisons could be made, the adsorption on oxidized aluminum films as studied by JETS resulted in the same surface species as adsorption on standard ‘y-alumina as studied by optical techniques. Another approach to answering the questions is to prepare a large enough surface area of oxidized aluminum to do conventional catalyst evaluation techniques. This can be done by preparing and then oxidizing small aluminum particles. Small metallic particles can be produced by evaporating a metal in a gas at a pressure of order 0.1—10 torr [97]. Fig. 6.3.6 shows an electron micrograph of aluminum particles produced by evaporating aluminum from a W filament in argon at a pressure of approximately 10 torr. The particles were oxidized after formation by exposure to air just as the aluminum electrode of a tunnel junction is often oxidized. The surface area, as measured by BET nitrogen adsorption, was 13.6 mt2/gm [93]. This corresponds to what would be expected from fig. 6.3.6: an aluminum sphere of radius 800 A has a surface area of 13.6 mt2/gm. This is clearly the size range of the particles. The measured mass vs temperature curve (or thermo-gravimetric analysis as it is known to surface chemists) shows no significant mass loss between 200°and 400°C indicating that the oxide is not significantly hydrated; it is an aluminum oxide, not an aluminum hydrate [93]. (There are, of course, OH groups on the surface which show up clearly in tunneling spectra and would not be expected to desorb at these low temperatures.) Above 400°C there is a rapid mass increase presumably due to further oxidation of the particles. Another standard catalyst evaluation test is butene isomerization: specifically the % conversion of 1 butene to 2 butene at given temperature and flow rate. In this test the oxidized aluminum particles had somewhat higher, but comparable, activity per unit surface area to that of standard ‘y aluminas [93]. These measurements on particles strengthen the conclusion that was drawn from the adsorption studies: the oxidized aluminum surface studied by lETS is very similar in its properties to standard y aluminas. Finally, I would like to close this section by mentioning an exciting new possibility: the study of supported metal catalysts. The point is that a small quantity of metal, on the order of a monolayer, can be evaporated onto the oxidized aluminum. Then gases can be adsorbed onto this alumina supported metal and studied with JETS. Supported metal catalysts are of enormous economic importance. Examples of their application are as auto-exhaust catalysts and in the reaction CO + 3H 2 —* CH4 + H20 to make methane from the primary product of coal gasification. Unfortunately, these supported metal catalysts are very difficult to study with optical techniques: the metal—carbon vibration has never been observed for any gas adsorbed on any supported metal! For example, the optical studies that have been done on the adsorption of CO on supported metals [98, 99, 100] have concentrated on the CO stretching region (1800—2100 cm’). From observations of the position of the CO stretching mode, researchers have tried to deduce informa(~i

203

P. K. Hansma, Inelastic electron tunneling

CO ON ALUMINA SUPPORTED Rh

~

Rh-4A/~

JNA~

~IIIIIII

800

600

2400

3200

4000

C •

0

0.1

I

0.2



eV

I

0.3





0.4

I

0.5

Fig. 6.3.7. Tunneling spectra ofCO adsorbed on various thickness ofrhodium evaporated onto the oxide of an Al—oxide—Pb junction. Note that new peaks appear as the thickness of rhodium increases implying new types of bonding. For all these spectra the CO exposure was ~ i0~Langmuir (10~totr for 100 see).

tion on the bonding of the CO to the metal (e.g., Is it linear or bridging?). Clearly the direct observation of the metal—carbon modes would be extremely useful, but they are obscured by strong absorption of the support (usually alumina, silica, or zeolite). Fig. 6.3.7 shows the recent tunneling results of Hansma, Kaska and Lame [101] on the adsorption of CO on rhodium supported on alumina. At low rhodium coverage (0—1 A) vibrational modes occur near 420 and 1925 cm1. At higher coverage (2—4 A) new modes appear near 510, 585 and 1 720 cm’ indicating the presence of new surface species. A possible explanation of these results is that as more rhodium is evaporated onto the surface, larger clusters of rhodium atoms form. These larger clusters have new types of bonding sites (e.g., sites for bridging CO). Similar experiments on nickel yield only one peak in the nickel—carbon stretching region, at 400 ± 5 cm~[101]. In contrast to the rhodium experiments no new peaks form as the coverage is increased. This work is only a start, but it illustrates the potential of tunneling spectroscopy for the study of chemisorbed species on supported metals. The investigation of catalytic reactions on supported metals should be exciting.

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P. K. Hansma, Inelastic electron tunneling

Acknowledgments I want to thank Professors R.V. Coleman, D.G. Walmsley and W.H. Weinberg and Drs. Jaklevic and Lambe for supplying original figures, for preprints of unpublished work, and for many useful discussions. Special thanks goes to Professor D.J. Scalapino and Drs. J. Kirtley and J. Hall for reading and helping me improve sections of the manuscript. Note added in proof Since completing this review, I have seen two others: 1. N.M. Brown and D.G. Walmsley, Chemistry in Britain 12 (1976) 92. 2. R.G. Keil, T.P. Graham and K.P. Roenker, Applied Spectroscopy 30 (1976) 1. The first is a short, populat account. The second is longer, with an excellent section on the theoretical basis of the experimental techniques for measuring spectra.

References [1] [2] [3] [4]

R.C. Jaklevic andJ. Lambe, Phys. Rev. Lett. 17(1966)1139. J. Lambe and P.C. Jakievic, Phys. Rev. 165 (1968) 821. E.L. Wolf, Solid State Physics V. 30, eds. H. Ehrenriech, F. Seitz and D. Turnbull (academic Press, New York, 1975) p. 1. D.J. Scalapino and S.M. Marcus, Phys. Rev. Lett. 18 (1967) 459. 15] A.F. Hebard and P.W. Shumate, Rev. Sci. Instrum. 45 (1974) 529. [6] Tunneling Phenomena in Solids, eds. F. Burstein and S. Lundqvist (Plenum Press, 1969) contains several useful articles, including two by Jaklevic and Lambe. [7] R.J. Jennings and J.R. Merrifi, J. Phys. Chem. Solids 33 (1972) 1261. [8] J. Klein, A. L~ger,M. Belin, D. Défourneau and M.J.L Sangster, Phys. Rev. B 7 (1973) 2336. [9] W.A. Harrison, Phys. Rev. 123 (1961) 85. [10] R.C. Jaklevic and J. Lambe, Tunneling Phenomena in Solids (Plenum Press, 1969) Chap. 18. This chapter and the preceding one by the same authors reviews inelastic electron tunneling shortly after its discovery. [11] J. Kirtley, D.J. Scalapino and P.K. Hansma, Phys. Rev. B14 (1976) 3177. [12] J. Bardeen, Phys. Rev. Lett. 6 (1961) 57. [13] M.H. Cohen, LM. Falicov and J.C. Phillips, Phys. Rev. Lett. 8 (1962) 316. [14] B.D. Josephson, Phys. Letters 1(1962) 251; Adv. Phys. 14 (1965) 419. [15] See, for example, LI. Schiff, Quantum Mechanics (McGraw Hill, 3rd ed. 1968) p. 314. [16] A.J. Bennet, C.B. Duke and S.D. Silverstein, Phys. Rev. 176 (1968) 969. [17] C.B. Duke, Tunneling in Solids (Academic Press, 1969) p. 290. [18] I.K. Yanson, N.I. Bogatina, B.!. Verkin and 0.!. Shklyarevskii, JETP 35 (1973) 540 first presented the two-step model and hypothesized that the deviation of the experimental points from the model prediction was due to tunneling via excited vibrational states. This seems unlikely since the typical time between vibrational excitations in a tunnel junction is of order 3 see, and vibrational relaxation times should be many order of magnitude shorter (~ 10—6 see). lIT [19] A.D. Brailsford and L.C. Davis, Phys. Rev. B 2 (1970) 1708. [20] L.C. Davis, Phys. Rev. B 2 (1970) 1714. [21] J. Kirtley and P.K. Hansma, Phys. Rev. B 13 (1976) 2910. [22] C. Caroli, R. Combescot, P. Nozieres and D. Saint-James, J. Phys. CS (1972) 21; 4 (1971) 916. [23]T.E. Feuchtwang, Phys. Rev. B 13 (1976) 517; Phys. Rev. B 10 (1974) 4135; Phys. Rev. B 10 (1974) 4121. [24]C.B. Duke, G.G. Kleiman and T.E. Stakelon, Phys. Rev. B 6 (1972) 2389. [25]LV. Keldysh, Zh. Eksp. Theor. Fiz. 47 (1964) 1515 [Soy. Phys. JETP 20 (1965) 1018]. [26] LV. Coleman, P.C. Morris and J.E. Christopher, Methods of Experimental Physics VII. Solid State Physics, ed. R.V. Coleman (Academic Press, 1974). [27] W.L McMillan and J.M. Rowell, in: Superconductivity, ed. R. Parks (Marcel Dekker, 1969) Chap. 11. [28] J.L. Miles and P.R Smith, J. Electrochem. Soc. 110 (1963) 1240. [29] J.F. O’Hanlon, J. Vacuum Sci. Technol. 7 (1970) 330.

P. K. Hansma, Inelastic electron tunneling [30] [31] [32] [33] [34] [35] [36] [37] [38]

205

Recipes for a number of metals are presented in ref. [26] section 1.5.1.2. R. Magno and J.G. Adler, Phys. Rev. B 13 (1976) 2262. B.F. Lewis, M. Mosesman and W.11. Weinberg, Surface Science 41(1974)142. M.G. Simonsen and LV. Coleman, Nature 244 (1973) 218. M.G. Simonsen and LV. Coleman, Phys. Rev. B 8 (1973) 5875. P.K. Hansma and LV. Coleman, Science 184 (1974) 1369. M.G. Simonsen, R.V. Coleman and P.K. Hansma, J. Chem. Phys. 61(1974) 3789. Y. Skarlatos, R.C. Barker, G.L. Hailer and A. Yelon, Surface Science 43 (1974) 353. N.J. Bogatina, 1.K. Yanson, B.I. Verkin and A.G. Batrak, zh. Eksp. Teor. Fiz. 65 (1973) 2327 (Soy. Phys. JETP 38 (1974) 1162]. [39] J.R. Kirtley and P.K. Hansma, Phys. Rev. B 12 (1975) 531. [40] B.D. Wallace, Electronic Design 14 (1974) 110. [41]1 am grateful to LV. Coleman for introducing me to these clamps. [42] This ratio can be obtained from a number of sources. For example, D.H. Douglass and L.M. Falicov, Progress in Low Temperature Physics 4, ed. C.J. Gorter (North-Holland, 1964) Chap. III, fig. 4.3. [43] J.G. Adler and I.E. Jackson, Rev. Sci. Instr. 37 (1966) 1049. [44] D.E. Thomas and J.M. Rowell, Rev. Sci. lnstr. 36 (1965) 1301. [45] W.R. Patterson and I. Shewchun, Rev. Sci. Instr. 35 (1964) 1704. [46] For example, the Princeton Applied Research model 124A on model l28A with tuned filter option. [47] It Morrison, Grounding and Shielding Techniques in Instrumentation (John Wiley and Sons, Inc., 1967). [48] See, for example, the discussion of the adsorption of formic acid in [8]. [49] J.D. Langan and P.K. Hansma, Surface Science 52(1975)211. [50] The probability that a particle of mass m will penetrate a barrier of height V 0 and thickness t shown in most elementary texts on quantum mechanics to be proportional to exp(_2..J(2,nVo/h2)t) in the limit where the energy of the particle is small relative to V0 and the probability itself is small. See, for example, R. Eisberg and R. Resnick, Quantum Physics of Atoms, Molecules, Solids, Nuclei and Particles (John Wiley & Sons, Inc., 1974) Chap. 6. [51] N.l. Bogatina, Opt. Spectrosc. 38(1974)43. [52] D.A. Cass, H.L Strauss and P.K. Hansma, Science 192 (1976) 1128. [53] A.E.T. Kuiper, J. Medema and J.J.G.M. Van Bokhoven, J. Catalysis 29 (1973) 40. [54] P.K. Hansma, Proc. 14th IntL Conf. Low Temp. Phys., Otaniemi, Finland V.5 (1975) 264. [55] See, for example, the discussion of the C00 attachment group frequencies of cynoacetic acid in [37]. [56] J.J.F. Scholten, P. Mars, P.G. Menon and P. Van Hardeveld, Proc. Third Intl. Cong. on Catalysis (1965) 881. [57] LH. Little, Infrared Spectra of Adsorbed Species (Academic Press, 1966). [58] The simplest possible image dipole model was used in [39]. A more sophisticated one, based on a theory by Morowitz [59], was used in [21]. Both models have similar qualitative predictions, but the quantitative predictions of [211 should be more accurate. [59] H. Monawitz, Phys. Rev. 187 (1969) 1792. [60] J. Schwartz, private communication based on several years of measuring infrared spectra of catalysts in the OH stretch region. [61] The 0—H dipole derivative is 0.32e [62]. A C—H dipole derivative is 0.le [63]. [62] P.E. Cade, J. Chem. Phys. 17 (1967) 2390. [63] H. Spedding and D.H. Whiffen, Proc. Roy. Soc. A238 (1956) 245. [64] A. Léger, J. Klein, M. Belin and D. Défourneau, Solid State Commun. 11(1972)1331. [65] 5. de Cheveign~,A. Léger and J. Klein, Proc. 14th Intl. Conf. Low Temp. Phys., Otaniemi, Finland V.3 (North-Holland, 1975) p. 491. 0 voltage data is in [65]. [66]S. [67]J. Lambe de Cheveigné, and R.C. thesis Jaklevic, University pnivateofcommunication Paris VII (1975). based A similar on their figure work reporting reviewedonly in [3]. the Mg [681 LV. Coleman, private communication. The highest background peak in tunneling spectra of Al—oxide—Pb junctions, near 118 meV, was originally believed to be due to 0—H bend [2]. Comparison with Mg—oxide—Pb data [8], and examination of recent spectra with varying amounts of 0—H [691,has convinced most researchers that the highest peak is in fact due to the aluminum oxide itself. This conclusion was reached independently by McMornis et al. [70], based on examination of infrared data [71] on thin aluminum oxide films. [69] W.M. Bowser and W.H. Weinberg, Rev. Sci. Instr. 47 (1976) 583. [70] I.W.N. McMornis, N.M.D. Brown and D.G. Walmsley, preprint. [71] A.J. Maeland, It Rittenhouse, W. Lahar and P.V. Romano, Thin Solid Films 21(1974)67. [72] W.H. Weinberg, W.M. Bowser and B.F. Lewis, Japan. J. Appl. Phys. SuppL 2, Pt. 2 (1974) p. 863. [73] J.M. Clark and LV. Coleman, Proc. NatL Acad. 73 (1976) 1598. [74] P.C. Lord and G.J. Thomas Jr., Spectrochim. Acta A23 (1967) 2551. [75] Y. Skariatos, R.C. Barker and G.L. Haller, J. Phys. Chem. 79 (1975) 2587.

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Hansma, Inelastic electron tunneling

[76] Physical Aspects of Electron Microscopy and Microbeam Analysis, eds. B. Siegel and D. Beaman (John Wiley & Sons, 1975). [77]W.J. Claffey and D.F. Parsons, Phil. Mag. 25 (1972) 637. (78] M.A. Chesters, B.J. Hopkins, A.R. Jones and It Nathan, J. Phys. C 7 (1974) 4486. [79] M. Isaacson, D. Johnson and A.V. Crewe, Rad. Res. 55 (1973) 205. [801 G.F. Bahr, F.B. Johnson and F. Zeitler, Lab. Inv. 14 (1965) 1115. [81] M.A. Marcus and J.C. Corelli, Radiation Research 57 (1974) 20. [82] M. Parikh, 32nd Annual Meeting of Electron Microscope Society of America (Claitor Press, Baton Rouge, Louisiana, 1974) p. 382. [83] P.K. Hansma and M. Parikh, Science 188 (1975) 1304. [84]M. Panikh, J. Hall and P. Hansma, Phys. Rev. A14 (1976) 1437. [85] J. Hall, P. Hansma and M, Parikh, manuscript in preparation for submission to Surface Science. [86] B.F. Lewis, W.M. Bowser, J.L. Horn Jr., T. Luu and W.H. Weinberg, J. Vac. Sci. Technol. 11(1974)262. [87] T.A. Egerton, A.H. Hardin, Y. Kozirovski and N. Sheppard, J. Catalysis 32 (1974) 343. [881 D.G. Walmsley, I.W.N. McMornis and N.M.D. Brown, Solid State Commun. 16 (1975) 663. [89] D.G. Walmsley, N.M.D. Brown, I.W.N. McMorris and R.B. Floyd, Proc. 14th Intl. Conf. Low Temp. Phys., Otaniemi, Finland V.3 (North-Holland, 1975) p. 495. [90] Compare, for example, fig. 5.1.1 with fig. 6.3.4. [91] D.R. Taylor and K.H. Ludlum, J. Phys. Chem. 76 (1972) 2882. [92] T.R. Hughes, H.M. White and R.J. White, J. Catalysis 13 (1969) 58. [93] D. Hickson, J. Schwartz and P.K. Hansma, submitted to J. Catalysis. [94] E.P. Parry, J. Catalysis 2 (1963) 371. [951 A.T. Balaban, G.D. Mateescu and M. Elian, Tetrahedron 18 (1962) 1083. [961N.S. Gill, RH. Nuttal, D.E. Scaife and D.W. Sharp, J. Inorg. Nucl. Chem. 18 (1961) 79. [97] This technique for forming small particles has been used by many groups. One of the earliest studies was by K. Kimoto, Y. Kamiya, M. Nonoyama and R. Uyeda, Jap. J. Appl. Phys. 2 (1963) 702. [981One of the earliest studies was by R.D. Eischens, W.A. Pliskin and S.A. Francis, I. Chem. Phys. 22 (1954) 1786. The work of many others was reviewed by Ford [99] and by Mills and Steffzen [100]. [99] R.P. Ford, Adv. in Catalysis 10 (1970) 51. [100] G.A. Mills and F.A. Steffzen, Catalysis Rev. 8 (1973) 159. [101] P. Hansma, W. Kaska and R. Laine, J. Am. Chem. Soc. 98 (1976) 6064.