Solid State Communlcations,Vol. 16, pp. 663—665, 1975.
Printed in Great Britain
INELASTIC ELECTRON TUNNELLING SPECTROSCOPY EVIDENCE FOR ASURFACE CHEMICAL REACTION D.G. Wahnsley, I.W.N. McMorris and N.M.D. Brown School ofPhysical Sciences, New University of Ulster, Coleraine, Northern Ireland (Received 7 October 1974 by C. W. McCombie)
The inelastic electron tunnel effect has been used to show the occurrence of a surface chemical reaction between an organic molecule (benzoyl chloride) dopant and the tunnel barrier of a junction (Al—oxide—Pb). SINCE Jaklevic and Lambe1’2 first demonstrated that electron tunnelling can display the vibrational modes of organic molecules there have been several instances 8—8 of the method being applied to the study of adsorbates. Here we report a tunnelling spectrum which can be interpreted in terms ofa chemical reaction between the dopant molecule and host oxide.
some experimentation to get the best combination of oxidation and doping conditions. The second harmonic characteristic or spectrum from a nominally clean junction is shown in Fig. 1(a). It is sensibly free of organic contamination but there are present two features which have been reported by other workers.16’8 The peak at 943 cm1 (117 meV) may be associated with Al—O—H surface moieties and the other peak at 3660 cm’ (454 meV) as the stretching mode of the (weakly) bound OH; both are thought to be accidentally produced by the residual water vapour in the preparation chamber. The absence of any bands at energies around 1600—1650 cm~shows9 that no surface adsorbed H 20 molecules are present.
The aluminium—oxide—lead tunnel junctions wese fabricated in an Edwards EI2E evaporator. To minimise hydrocarbon contamination the silicone oil in the diffusion pump was replaced by SANTOVAC 5 which has a lower vapour pressure; cryosorption pumps were used for roughing out the system. Initial clean-up before junction manufacture involved flushing with helium and nitrogen and this was followed by extended oxygen plasma discharge. All stages of junction fabrication were completed within the evaporator without any exposure of the system to atmosphere until the junction was complete. The oxide was grown in the 7 and the dopant plasma of antooxygen d.c. discharge introduced the chamber at the appropriate stage through a valved port.
Fig. 1(b) is a tunnel spectrum from a junction doped with 2-phenylethylamine, C6H5 CH2 CH2 NH2. There is good correlation with the[Fig.2(a)]. infra-red (i.r.) spectrum of the same compound Specifically we have five C—H stretch peaks in the tunnel spectrum at 3065, 3045, 2904, 2854 and 2743 cm1 while the i.r. peaks are at 3040,3020,2940,2860 and 2750 cm1. Other related peaks in the two spectra occur at lower energy. The choice of features to be compared is somewhat arbitrary and it is clear that the intensities and line shapes are not always in complete agreement but overall there is a marked similarity in the important features of both spectra. In particular, all prominent features in the i.r. spectrum
In the measurements, ac modulation of 2 mY p—p was applied at 50 kHz and second harmonic signals were detected with a Brookdeal lock-in amplifier, preceded by a circuit of the type used by Lambe and Jaldevic2 and Lewis et a!. r, The circuit operates most satisfactorily for samples with resistances in the range 10—100 ~l. It was therefore necessary to do 663
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E$JERGY (cm~) FIG. 1. (a) Tunnel spectrum at 4.2 K of clean Al—I—Pb junction. (b) Tunnel spectrum at 2.5 K of Al—I—Pb junction doped with 2-phenylethylamine. (c) Tunnel spectrum at 4.2 K of Al—I—Pb junction doped with benzoyl chloride, have their counterpart in the tunnel spectrum. The i.r. spectrum is taken from liquid film sample while the tunnel spectrum is from an adsorbed layer. It is still an open question to what extent crystal-field smearing, adsorptive line shifts and differences in transition selection rules are responsible for the differences in such spectra. We have observed comparable correlations between i.r. and tunnel spectra of several other aromatic and aliphatic adsorbates but an outstanding exception has been with the more reactive molecule, benzoyl chloride, C6H5COC1. The tunnel spectrum [Fig.1(c)] has a very sharp intense band at —, 1603 cm’ and there is no counterpart to the broad i.r.Essentially peak in theidentical region 1650—1850 cm’ [Fig.2(b)]. tunnel spectra have been obtainedin differentsamples made at different times with a range of other adsorbates having been used in the evaporator in the meantime. We regard this as conclusive evidence that the cleanlug procedure between the use of different dopants is adequate and that the benzoyl chloride spectrum is in fact reproducible,
2. (a) I.R. spectrum of 2-phenylethylamine. (b) I.R. spectrum of benzoyl chloride. There is much less correlation overall between
the peaks in the tunnel spectrum and the i.r. spectrum of liquid benzoyl chloride than between those of other dopants which we have studied. It is also notable that while the 3660 cm’ band is of the same strength and width in the 2-phenylethylamine doped junction as in a clean junction the peak height is reduced by about a factor of 2 in the adsorbed benzoyl chloride case while the peak width remains unchanged. The interpretation we put on the benzoyl chloride tunnel spectrum is as follows. The dopant reacts with the surface hydroxyl groups present on the aluminium oxide to form aluminium benzoate and HU which then pumps away. Thus the decrease in the OH band at 3660 cm’ is explained. It is known that there are C = 0 stretch modes at 1780 and 1730 cnf’ and C —C ring stretch modes at 1600 and 1582 cm4 in the i.r. spectrum of benzoyl chloride [Fig.2(b)]. But in th&complex benzoyl chloride: aluminium trichioride (C 6H5COC1: AlCl3) the higher energy C = 0 modes move down to 1590 and 1570 cm’ where they are superimposed on the C~-~-C 11 peaks inmode.’° i.r. at Aluminium (benzoate)3 itself has 1600 and 1560 cm’. We conclude that the signal at 1600 cm1 in our spectrum consists of the CC background and a sharp peak corresponding to shifted C = 0 on top of it. There is no strong signal in the tunnel spectrum at 877 cm”1 where the C—Cl is expected in the unreacted species but this is only a necessary and not a sufficient condition to justify the disappearance of the chloride; it could simply have shifted as a result ofadsorption. Indeed there is a peak at 943 cnf” superimposed on the background
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that The theresharpness is of the peak atof1603 cm~implies and thus the molecules arelittle on acrystal-field unique typesmearing site. This is a further indication that a reaction of high specific energy rather than an absorptive process of relatively low energy has occurred. Since the signal shows a single sharp mode it is suggested that the final configuration may be as shown in Fig. 3(a) or (b) rather than that in
Fig. 3(c) which, if present should give rise to a symmetric—antisymmetric pair of stretching modes at 1400 ±20Cm”’ and 1510 ±2Ocm’1 respectively.12
G~<~ NP —
(c) FIG. 3. (a) Benzoate covalently bound to aluminium, (b) Benzoate oxygen inserted interstitially. (c) Benzoate ionically bound to aluminium, of the tunnel spectrum and the C—Cl mode is known to shift to 945 cm’ in co-ordinated benzoyl chloride. This may be argued to support simple CO-oi ~ination rather than reaction and loss of HC1. However, we look upon the reduction in OH at 3660 cm”’ as strong evidence in favour of at least some substitution. Since also it would be very unlikely that the shift of the C = 0 peak would be identical for both adsorbed (hence perturbed) C 6H5COC1 and the surface reacted “benzoate” forming species, we take the view that most of the adsorbate has reacted.
Apart from the central consideration of this paper, it is interesting to note that the various C—H stretching modes associated with both alkyl and aryl systems are distinguishable, for example that at 2750 cm’ is absent from Fig. 1(c) but is apparent, as expected, in Fig. 1(b). We conclude that inelastic tunnelling spectroscopy shows a reaction between benzoyl chloride and a freshly prepared aluminium oxide substrate. From the spectrum it has been possible to decide that the reacted dopant occupies a unique type of site. Acknowledgements The authors grateful to R.C. Jakievic and J. Lambe for veryare helpful discussions —
and to R.B. Floyd for technical assistance. One of us (I.W.N. McMorris) was supported on a Studentship from the Government of Northern Ireland.
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