Scanning tunneling microscopy of thin organic films on conducting substrates

Scanning tunneling microscopy of thin organic films on conducting substrates

Surface Science 232 (1990) 339-345 North-Holland 339 SCANNfNG TUNNELING MICROSCOPY OF THIN ORGANIC FILMS ON CONDUCTING SUBSm-I.ES P. DIETS and K.-H...

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Surface Science 232 (1990) 339-345 North-Holland

339

SCANNfNG TUNNELING MICROSCOPY OF THIN ORGANIC FILMS ON CONDUCTING SUBSm-I.ES

P. DIETS and K.-H. HERRMANN htitut

[email protected] Angewandte

Physik,

Uniuersiriit TCbingen, Auf c&r ~urg~~stei~e

12, 7400 Tiibingen, Fed. Rep. of Germany

Received 13 October 1989; accepted for publication 7 February 1990

We have tested the capability of the scanning tunneling microscope @TM) to characterize thin organic films on conducting substrates. Graphite turned out to be the favourite substrate material because of its large atomically flat regions and its typical fine structure. As a simple and well-known organic “model structure” perforated Formvar films with a thickness of 35 A were used. The edges of holes and details in the structure of the foil could be imaged. The correspondence between measured and prepared thickness indicates that a special ~oRduct~on mechanism through the fitm must exist. Formvar could be distinguished from graphite in fine structure and work function. We discuss the displacement of structures by the tip. A short description of the STM used is given.

1, Iut.wduction

Nan-dest~ctive hip-resolution imaging of organic structures, from single molecules like phthal~yanine, DNA filaments or proteins up to very complex systems like viruses or cell-sheaths, is important for the understanding of many biochemical processes. Moreover, organic materials play an increasing rGle in various technological applications, e.g. membranes for the separation of gases or fluids. Conventional methods like the transmission electron microscope (TEM) are handicapped by considerable disadvantages: For unstained organic probes the contrast is very low and atomic resolution is possible only in few exceptional cases; radiation damage by high-energy electron as well as the special preparation for vacuum requirements can destroy the original structures. Since its development in 1982 [IJ] the STM has become a powerful tool for surface physics with a surprising high lateral resolution even at atmospheric pressure or in aqueous solution. Because of its non-destructive nature combined with a high environmental flexibility the STM appears to be an ideal instrument for organic or biological probes. Unfortunately, an obvious problem is the ~39~02~/~/S~3.~#

low intrinsic conductivity of most organic materials. Preliminary experiments indicate that for very thin organic structures (20-50 A) on conducting substrates [3,4] direct imaging with the STM should be possible. In many cases the distinction between real organic structures and artefacts or structures of the substrate is still an inherent problem. In the present paper we describe a more systematical approach to that problem by investigating a very simple and well-known organic “model structure”. We present criteria fur suitable substrates and methods for distinguishing between real structures and artefacts. Furthermore, the interaction of the organic material with the substrate and possible conduction mechanisms will be discussed.

2. The microscope

The whole STM unit is shown in fig. 1; a description of details has been given elsewhere [5,6]. The main features are a double-lever system for coarse positioning of the tip, a piezoelectric tripod (PZT .5A, Vernitron) for scanning and a special specimen manipulator with f 1 mm dis-

0 1990 - Efsevier Science Publishers B.V. ~North-Holland~

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/ STM

of thin organic fllrns on conductrng

placement in both directions. The selection of particular spots on the surface without interrupting the experiment turned out to be very useful, especially for extended organic films. Tip and sample are housed in a rigid stainless-steel frame, directly connected to an UHV flange. All manipulations can be done by rotary feedthroughs. Working on a particular base which is separated from the laboratory floor, atomic resolution is achieved without further vibration screening. The vertical 0 stability is better than 0.1 A. The maximum scanning range is 4000 X 4000 A2, applying 1000 V to the bars. The thermal drift is measured to be less than 4 A/min in all directions. Tunneling tips are cut from a tungsten wire (0.4 mm) and etched to a radius of less than 0.1 pm [7]. For high-resolution STM pictures an ad-

Fig. 1. Photograph

of the complete

substrcltes

ditional treatment of the tip with fine sandpaper was often necessary, creating new nanotips in the course of this procedure [S]. The electronics for the feedback system are our own development with the purpose to get a good frequency response and an optimum adjustment to the tunneling unit. The experiment is controlled by a personal computer. The tunneling signals from the STM are fed directly into a frame store (256 X 256 pixel, 8 bit deep), which is connected to the PC. The images were presented on a blackand-white monitor and can be stored on a 30 MB hard disc. The instrument works either in the constant current (topographic) or in the constant height mode. Work function mapping is also possible by measuring d(ln I)/ds with a lock-in amplifier. Image processing routines like subtrac-

STM unit (a), the rigid x,,v.z-piezo-tripod (b) and the specimen selection of particular spots on the probe.

manipulator

(c). used for the

P. Dietz, K.-H. Herrmnnn

/ STM of thin organic films on conducting

tion of a sloping background, contrast amplification, contour lines, Fourier and median filtering are available.

3. Substrate materials

An important element of the preparation of thin organic structures for microscopy is the selection of a suitable substrate. For the STM, the most desirable characteristics of a substrate are: (1) large atomically flat regions, so that any deposited material can be easily identified; (2) good electrical conductivity; (3) easy and fast preparation of fresh layers; (4) inertness and absence of contamination for a longer period of time; (5) the adhesion of the deposited material must be strong enough to prevent a displacement of structures by the tip; (6) a well-known fine structure to distinguish between substrate and organic material; (7) high stability to prevent deformations in the course of the preparation. We have tested three possible substrates [5]. Epitaxial gold films on mica show a characteristic grain structure. Single grains are a few 100 A in diameter and the corrugation is about 50 A. Flat regions on top of the grains are not very extended, so that these films are only suitable substrates for very small organic structures up to 100 A. Other easily prepared substrates are single-crystal globules of platinum or gold [9]. They show flat facets with atomic steps and terraces [6], but the facets are too tiny (50-100 pm) for the deposition of an organic foil. For our purpose graphite turned out to be the favourite material because of its extended utilizable surface with large atomically flat areas (larger than the maximum scanning range of the STM) and its typical fine structure. Easy preparation with an adhesive tape and inertness over a long time are additional advantages. Gold or platinum surfaces seem to be contaminated already after a short exposure to air and atomic resolution was difficult to obtain [5].

substrates

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4. Thin organic films on graphite In order to test the capability of the STM to image very thin organic structures, it is not only necessary to select the right substrate, but also to find a suitable thin organic “model structure”. Important characteristics of such a specimen should be: (1) preparability with a defined thickness, (2) good mechanical stability to remain intact during the preparation procedure, (3) characterization in detail by other methods like TEM or SEM, (4) simple and easily detectable structures, which can be found with the limited scanning range of the STM. In our experiments we have employed perforated films of polyvinylformal (Formvar) [lo]. These films have been successfully used in TEM for many years because they are very stable and their structure is quite uniform. The thickness of these films can be exactly adjusted by the preparation conditions. Simple structures were produced by exposing the film to a jet of steam resulting in perforations, whose size and number are adjustable over a wide range [lo]. The floating film was directly deposited on the graphite substrate by dipping it into the emulsion. In order to get optimal adhesion the foil covers the whole graphite surface. Before we started our experiments with the STM, one of the samples was imaged with the scanning electron microscope (SEM) (fig. 2a). We found hole diameters between 0.4 and 2 pm, most of them larger than the maximum scanning range of our STM. The hole density is very high, so that regions of interest can easily be found. In figs. 2b-2d and 3 we present STM images of perforation edges at different sample positions obtained in air. In each picture two regions with different heights are clearly visible. From fig. 3, the z-displacement of the tip after scanning over the edge was measured to be 32 A. This value agrees quite well with the prepared film thickness (35 A) and indicates that a special conduction mechanism through the thin film must exist. Structures along the edges (fig. 2c) are supposed to be due to mechanical strain, but they may also originate from a multiple tip. At higher magnification (fig. 2d) the regular fine structure of graphite in the

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P. Die&z,K-H. Herrmann / STM

of thinorganic films on conducting subsrrates

Fig. 2. Perforated Formvar films on graphite: (a) SEM image to show size distribution and density of perforations. The maximum scanning range of our STM is represented in the lower right corner. (b-d) STM images of perforation edges. Two regions with a difference in height of about 30 A are clearly visible. The current is 1.5 nA, the voltage 120 mV and the line-scan frequency is 1.5 Hz. In (b) steps on the substrate surface (running perpendicular to the edge) are also visible in the area of the foil. Typical structures in the form of stripes along the edges were often observed (c). (d) Higher magnification of the boundary between HOPG and Focmvar in (c).

upper left part of the image differs obviously from the long molecule chains in the area of the foil. In order to assure that the STM images the organic film and not any artefacts of the substrate, we have almost simultaneously studied two small regions (20 x 20 A2) on graphite and on the foil. The tip has been switched several times from HOPG to the film and back again to exclude

effects like tip deformation or contamination of the tip by film material, which can also modify the fine structure. In the area of the foil no graphitic character was observed, the surface was much rougher, while on the graphite substrate the hexagonal structure was visible (fig. 4). Another method of distinguishing between the two materials is to compare the work function. In fig. 5 we have

Q. Dietz, K.-H. Hewmann / ST&i of thin organicfibas on conducting substrates

Fig, 3. Perspective view of a perforation edge, obtained with the x, y-recorder. The average thickness of the film is 32 A. Tunneling current is I.5 nA, v&age 120 mV and line-scan frequency 0.2 Hz.

recorded d(fn f}/ds (and simultaneously the to~a~aphic image) while scanning the tip over the boundary between substrate and film. A work function difference of about 0.1 eV was measured. In the region of the boundary, where the motion of the tip is not perpendicular to the surface, the work function is in~uenced by topographic ef-

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Fig. 5. Simultaneous measurement of topographic and work function image at the same sample position. At the edge of t&e foil the work function information is strong& inRuenced by topography. Scanning parameters as in fig. 3. For work &metion mapping, the tip-to-sample distance was modulated by 0.2 A with a frequency of 1 kHz.

fects, resulting in a lower value. This indicates that only for flat surfaces work function measurements are reliable, In fig. 6 we show regions where parts of the foil (or of the graphite) are ripped out lying just on

Fig. 4. Comparison of very small scanning areas on the substrate (a) and on the Formvar film (b). In (a) the typical hexagonal structure of graphite is visible, in (b) no fine structure can be detected. Pictures are taken in the constant heigbt mode. The current is IO nA, the v&age 50 mV and the fine-scan frequency 20 Hz.

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ftlms on conducting

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Fig. 6. STM images of regions where the organic foil is highly deformed. Small parts were ripped out lying either on the film (a,b) or on the substrate (c,d). One of the marked fragments ( 1) in (c) is displaced by about 700 A in scanning direction in (d), the other ( T) was fully removed from the scanning area. Scanning parameters as in fig. 2.

top of the foil or on the substrate, which happened probably during the preparation. For small fragments up to 200 A in diameter we often observed displacements by the tip of several 100 A (figs. 6c and 6d). Other pieces were fully removed from the scanning area. In most cases a deformation of the tip was simultaneously observed (fig. 6d).

5. Conclusion We suggest that it is possible to image perforated Formvar films on graphite with the STM because: (a) comparison with SEM or TEM pictures shows very similar structures; (b) the measured thickness of the films agrees quite well with

P. Die&, K.-H. Herrmann

/ STh4 of thin organic films on conducting substrates

the value determined by the preparation conditions; (c) graphite and adsorbed material show different fine structures and (d) different work functions. It appears unlikely that tunneling alone could account for the high conductivity we observed. For a current of 1 nA, a voltage of 100 mV and a tunneling area of 100 A2 the resistivity of a 30 A thick film is 300 &?. cm. Similar results were obtained for 50 A lipid bilayers [ll]. Typical insulators show resistivities in the range from lo9 to 1015 Q. cm. This indicates that tunneling takes place only from the tip to the Formvar surface followed by a low resistance transport to the conducting substrate. Possible conduction mechanisms are: (1) Schottky emission into the conduction band of the insulator by thermionic or field emission [12]. For a voltage of 0.1 V and a distance of 5 A the electric field between tip and sample is about 2 X lo6 V/cm. Moreover, the special geometry of the tunneling region must be taken into account. (2) Bulk conduction of the hopping type, as it was proposed for LB films [13]. In this case effects like inelastic or resonant tunneling would be important, which can increase the tunneling probability up to a factor lo4 [14]. (3) Free ions and electrons could be generated in the strong electric field between tip and sample by field ionization or by cracking weak chemical bonds. Impurities and sorbed water may play a very important role [15]. (4) A positive charge on the organic foil acting through the thin film can produce a high gradient. In the presence of a strong external field emission of electrons can occur (Malter effect) [16]. From the observed displacement of small Formvar (or graphite) fragments we conclude that the deposited material does not have a strong interaction with the HOPG surface. For smaller areas of contact the adhesion decreases and attractive forces between tip and fragment can cause the observed displacements. This may explain the difficulties that arise when the STM is used to image small organic objects like amino acids [15] or

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viruses on graphite. For this reason it would be useful to search for alternative substrates; large unperforated Formvar films on graphite are possible candidates. Finally, experiments with thin organic films prove that the STM offers a considerable potential for the characterization of unshadowed organic materials on conducting substrates. Further systematic investigations with films of variable thickness and with different organic materials are in progress. Theoretical work is urgently needed to understand the mechanisms of electron transport in such systems.

Acknowledgement We are grateful to K. Kasper who developed the hard- and software for image registration and image processing.

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