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Investigation and modification of free and adsorbate-covered surfaces by scanning tunneling microscopy
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Materials Science and Engineering, A 139 ( 1991 ) 230-238
Investigation and modification of free and adsorbate-covered surfaces by scanning tunnelling microscopy L. M. Eng and H. Fuchs BASF A G., Polymer Research Laboratory, ZKLJ 543, D-6 700 Ludwigshafen (F.R.G.)
Abstract An overview is given of the different analytical aspects of scanning tunnelling microscopy (STM) and its applications in the fields of basic research and technology. Surface investigations under vacuum conditions are dealt with as well as experiments in air and electrolytes. Examples are given demonstrating that STM can be used as a nanometre size tool for manipulating matter on the mesoscopic and the (near) atomic scale.
1. Introduction Scanning tunnelling microscopy (STM) is a unique surface-sensitive technique allowing investigations of atomic scale features in real space [1, 2]. Since its invention by Binnig and Rohrer [3], an intensive evolution has occurred [4, 5] resulting in a variety of proximal probe derivatives which are sensitive to various physical properties. Nevertheless, they all have in common the near-field sensitivity and the raster-like scan motion of the probing stylus. Besides probing tunnelling currents, specific surface sensors have been developed for the local detection of forces [6, 7], (chemical) potential barriers [8], thermal profiles [9], capacitances [10], photons, noise signals [ 11 ], etc. In the following we will concentrate on some aspects of conventional STM in the field of surface inspection and modification. STM not only probes the atomic arrangement of surfaces but also provides information on the local surface chemistry and surface-induced chemical reactions. It is therefore an interesting instrument for use in electrochemistry and biology. Furthermore, local electronic and mechanical processes (i.e. forces and elastic moduli) as well as structural transitions on the sample surface may be induced and detected. STM is thus an excellent tool for local surface modification down to the atomic scale [12, 13]. In contrast to conventional surface analytical techniques, STM is not limited to special environ0921-5093/91/$3.50
mental conditions such as ultrahigh vacuum (UHV). Investigations in ambient air and gaseous phases have been reported together with operation in liquid environments (i.e. oil and electrolytes) at room temperature. Apart from basic research in surface science, chemistry and biology, STM is thus a very useful instrument for the routine inspection of technical surfaces on the mesoscopic scale (e.g. surfaces after ion impact, machined surfaces or adsorbate-covered systems).
2. Experimental and instrumental STM is based on the quantum mechanical effect of electron tunnelling between two electrodes separated by a few fingstr6ms only (d ~ 2 nm) [14]. At a first approximation the tunnelling probability depends exponentially on the electrode separation d. A small bias voltage Vt between the two electrodes (with I v t l < ~ / e ~ 4 V) induces a tunnelling current I, flowing across the metal-insulator-metal (MIM)junction. In a one-dimensional model (Fig. 1) [15, 16], electron tunnelling at small bias voltages ( V, ~ 4p/e, k B T/e) can be expressed by lt~c Vt exp( - A d ~
~/2)
with A ~ 1.025 A -l eV -~/2, k B the factor, T the absolute temperature local barrier height. Owing to the decay of the wavefunctions outside
(1) Boltzmann and • the exponential the sample
© Elsevier Sequoia/Printedin The Netherlands
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d Fig. 1. Energy diagram illustrating the tunnelling barrier in STM. Owing to the voltage difference I't applied between thc two electrodes, a distinct current (the tunnelling current It) l]ows. It is a sensitive measure of the local density of d e c tronic states 1){12, rl near the Fermi energy E I directly above lhe samplc surface, el/, denotes the difference in energy between the two Fermi levels I-LP and E~' respectively. T h e local barrier height ',1), of both the tip and sample material is indicated to bc the difference bctween E,:,,' and F L' (with i = p,samplcl, sltip)l.
surface, this method permits the measurement of
the local density of states (LDOS) near the sample surface with a vertical resolution below 0.1 A ] 15-17]. Similarly, the high lateral resolution (approximately 1 A) of the instrument depends on the shape of the tip apex, since essentially only those atoms of the tip closest to the surface contribute to the tunnelling current. By scanning the tip line by line across the sample surface, an image is obtained of the atomic structure of the sample surface. In contrast to conventional electron microscopy, this image originates from bound electrons. The process of imaging is therefore not limited to vacuum applications but rather relies on a finite potential barrier across the tunnelling gap. Therefore it may consist of a gaseous or liquid medium as well. On the other hand, bound electrons reflect the electronic states on the surface and may easily be probed with modulation techniques. Variation of In(/,) vs. 6d (eqn. (1)) provides information on • (with d ln(lt)/ddoc on~2), whereas a derivation after 6 Vt provides information on the local conductivity o(E, r) or density of states D(E, r) near E v (as follows from a microscopic expression of I t in the transfer hamiltonian approach [16]: 6 ln(lt)/dVtoc I)(E. r)oc o(E, r)). Experimentally, the movement of the tunnelling tip is controlled by piezoceramic transducers driven by the inverse piezoeffect. Originally, "tripod scanners" constructed from three linear transducers mounted perpendicularly to each other were used. More recently, scanning tunnel-
ling microscopes have been equipped with singletube scanners [18]. Their higher mechanical eigenfrequencies permit imaging at a higher scan speed. Again, this is mandatory for the investigation of a lot of soft materials such as biological l 19] and synthetic organic [2] adsorbate layers. So far, little has been done with respect to the preparation of well-defined tunnelling tips [20]. Usually, tips are prepared by mechanical grinding, (electro-) chemical etching or simply by pinching off tips from inert metal rods such as Pt-Ir and gold. While all these primitive preparation techniques provide excellent results on atomically flat surfaces, severe problems may occur in STM investigations on rough surfaces exhibiting steep craters or trenches with diameters of approximately twice the tip radius. In this contribution we present some of our recent results obtained with different types of scanning tunnelling microscopes used for the investigation of structural and electronic phenomena on both atomically fiat and rough surfaces under different environmental conditions.
3. Layered materials Highly oriented pyrolytic graphite (HOPG) is one of the materials most often investigated with the tunnelling microscope [21]. Owing to its chemical inertness and its atomically flat surface over thousands of fingstr6ms, atomic resolution is easily obtained in air [22, 23]. This semimetal therefore provides an appropriate substrate for the deposition of organic and inorganic adsorbates as well as a good reference surface lot piezocalibration purposes. The relative ease of measurement on the (0001 ) flakes is based on the electronic [24, 25] and even more on the elastic [26} response of this material due to the proximity of the probing stylus. Figure 2 presents an image of the graphite surface as recorded with a tunnelling tip pinched off from a Pt-Ir wire. The sixfold symmetry originates from the alternate A-B stacking of subsequent graphite layers in the [0001] direction. The inequivalence [27] occurs because fl site atoms Jack a neighbouring atom in every second layer whereas a site atoms are weakly bound to the adjacent layer [25]. As a consequence, /3 site atoms occur as protrusions while a site atoms represent saddle points. At high tunnelling currents (i.e. several
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Fig. 2. Highly oriented pyrolytic graphite provides an atomically flat surface suitable for piezocalibration (shown over an area of 74 A × 79 A,). Owing to its flatness, it is a good substrate for the deposition of organic adsorbates. The threefold symmetry of the fl site atoms (hexagonal packing) is seen. Lateral spacings between bright spots correspond to the 2.46 A separation of fl site atoms. Using a tungsten tip biased at V~= - 50 mV, a z corrugation of about 1.0 A is measured.
nanoamps) and small bias voltages (i.e. a few millivolts) the tip comes very close to the HOPG surface (d~< 2.5 A) because of the low density of states (DOS) near the Fermi level E v. Consequently, the sample will be loaded mechanically with the tunnelling tip exerting a force of up to 10 -6 N on the HOPG surface [28]. This yields a local surface pressure of 1 TPa (!) given an effective tip area of 100 A2. Evidently, these strong forces may be used for surface modification on a nanometre scale. Although under these conditions a slight point contact between tip and sample is obvious, tunnelling tips normally do not destroy the HOPG surface. Owing to the very low elastic coefficient in the [0001] direction (c33 = 36.5 GPa) [29], elastic deformation with penetration depths of some tenths of a nanometre has been measured [7]. By contrast, stiffer materials, e.g. crystalline silicon, exhibit brittle behaviour. Tip indentations on this material will thus result in a persistent local surface destruction [30]. The various methods for which STM is used to modify surfaces are excellently reviewed in ref. 12. The published data all refer to the modification of bare sample surfaces resulting in local deterioration. Until recently, direct surface modification with preserved atomic order in the modified areas could not be demonstrated. However, by using samples of transition metal dichalcogen-
Fig. 3. (a) Atomically resolved W S e 2 surface spot (83 A × 83 A) monitored at V~= 100 mV with a tunnelling current of 1 nA. Bright spots indicate high current densities above tungsten atoms separated by 3.3 +_0.1 A. (b) Atomically resolved WSez surface (over an area of 72 A × 95/k) after a mechanical tip indentation. A 5 V voltage pulse applied to the z piezo produced a protrusion on the sample surface. Astonishingly, the atomic lattice of tungsten atoms is preserved over the indentation spot as well. This indicates that the indentation process induced subsurface modifications rather than atomic defects at the sample surface.
ides, we have found that on this class of layered materials, plastic deformation on the nanometre scale is in fact possible without destroying the lateral atomic order of the topmost layer [31]. Evidently, this kind of surface modification may be relevant for technological applications (e.g. in storage devices) and ~also when investigating plastic deformations and relaxation processes on the (sub-) nanometre scale. Figure 3 depicts an image of a W S e 2 surface before (Fig. 3(a)) and after (Fig. 3(b)) tip indenta-
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tion. Brighter spots indicate the positions of tungsten atoms [32] with the lateral registry being preserved even over the spot of the mechanical tip-sample contact. Most interestingly, the local deformation did not change the lateral position of atoms in the topmost layer but rather resulted in a reorientation of atoms in the bulk material. Besides these specific mechanical properties of the transition metal dichalcogenides, some of them exhibit an intriguing electronic effect known as charge density waves (CDWs). Their unit cell (either prismatic or orthorhombic)[33] additionally shows a periodic lattice distortion [34] with the ions being shifted for about 0.1-0.3 A from their equilibrium position [35]. As a consequence, the charge distribution of conducting electrons will be modulated. In the case of 1TTaSe~ a commensurate 13 ~/2 x 13J/2 bulk superstructure is formed at room temperature. Since STM is sensitive to electronic surface states, it will thus probe this charge modulation at the sample surface. Figure 4 presents the constantcurrent image acquired when scanning Pt-Ir tip over the surface of a freshly cleaved 1T-TaSe~ sample. In addition to the atomic lattice (with an interatomic spacing of about 3.4+_0.1 A), the 13~/2 x 13 ~/2 CDW superstructure (with a periodicity of 12.5 _+0.1 A) is clearly revealed. This experiment was repeated under silicone oil. As a result, both the atomic and CDW spacings were measured with an enhanced signal-tonoise ratio over the former measurement in air (Fig. 4). This is primarily due to the absence of field-induced instabilities arising in the tunnelling junction in the presence of air. Furthermore, polarization of oil molecules or other polar liquids located directly under the tunnelling tip modifies the electrical properties of the surface such as local barrier height and surface electronic states. 4. Adsorbate-covered surfaces
Heterogeneous surfaces are of great interest in many different technological areas, e.g. vacuum or electrochemical deposition of metallic films on substrates, surface finishing and organic coatings. In all these cases an understanding of the microscopic behaviour of the interface between substrate and adsorbate is essential for tailoring macroscopic properties. Some of the questions which arise concern the first stages of film formation, epitaxial growth, adhesion and electronic
Fig. 4. Constant-current image of TaSe~ imaged in air at room temperature. Both the atomic lattice as well as the c o m m e n s u r a t e periodicity of the charge density wave arc monitored over the 200 A x 200 A surface area. Lattice spacings are m e a s u r e d to be 3.4_+0.1 and 12.5_+0.1 A for the atomic and supcrlattice respectively. Imaging parameters were l, n A and Vt = - 2 6 3 mV.
properties at the interface. Tunnelling microscopy on these systems thus provides a direct investigation in real space. Below we describe some recent results in these fields. 4.1. Electrochemically deposited copper on HOPG
STM has opened up the possibility of in situ investigations on electrochemical processes at electrodes [36]. It thus excellently complements conventional voltanogrammic method which provide integral information only. Experimental problems arising from the ionic leakage current 1~on superimposed on the tunnelling current I t have been solved using coated tips and by applying an appropriate voltage bias between tip and sample relative to the surrounding electrolyte [37]. Besides imaging the adsorbed layer after deposition, STM is also able to directly monitor the electrochemical deposition process in situ. As an example (Fig. 5), copper was deposited on HOPG out o f a 0.1 M CuSO 4 solution with an ionic current of 1 mA for 1 min. Deposition was carried out with an additional platinum counterelectrode immersed in the liquid. To check reproducibility, the deposition process was stopped and a series of pictures was collected. Obviously, new features appear within this image
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Fig. 5. Copper electrochemically deposited on HOPG with a charge dose of 60 inC. Copper preferentially adheres to the graphite surface step which crosses the picture from top to bottom. The copper grows in a fairly linear manner away from the surface step at an angle of about 45 °. A cluster size approximately 5 A in height and 25 A. in width is measured indicating this anisotropic growth process. Imaging of the 500 A x 500 A surface spot was carried out with a tunnelling current I t of 1 nA and a bias voltage V~set to - 20 mV (with respect to the tunnelling tip).
(Fig. 5) which formerly had not been observed on the bare graphite substrate. A slightly curved monatomic step in the graphite surface is seen crossing the image from top to bottom. Height differences between left and right indicate a 3.5/k value, in fairly good agreement with half the unit cell height of H O P G (3.35 A). Copper deposition preferentially occurs at surface steps presumably because of the increased activation barrier for diffusion. Once a copper cluster physisorbs at the substrate, additional copper atoms deposited from the electrolytic solution preferentially agglomerate to the existing nucleus. The cluster increases in size, forming in this case a linear chain in a direction about 45 ° off from the graphite surface step (indicated with an arrow in Fig. 5). It was even possible to deposit copper clusters on top of extended H O P G terraces. We believe that impurities and local (sub-) surface defects are responsible for both pinning and nucleation of the clusters. However, the adhesive forces are very weak. When a certain size is exceeded, some clusters can be moved on the HOPG surface using the tunnelling tip as "nanotweezers".
4.2. Langmuir-Blodgett films Langmuir-Blodgett (LB) films [38] have attracted considerable technological interest owing to their high degree of structural order and anisotropy [39]. Their tailor-made macroscopic
Fig. 6. A monolayer film of 2,2-tricosenoic acid deposited on WSe, was imaged at 1 nA and a tip bias of 600 mV over an 80 A-× 64 A surface area [40]. The superlattice of the LB film varies between 3.6 and 6.5 A and is significantly larger than the substrate lattice spacing of 3.3 A (see Fig. 3(a)).
properties such as the (piezo-) electrical and nonlinear optical properties partly exceed those of inorganic materials and may be widely varied by appropriate chemical functionalization. Organic films may also be synthesized for size-selective membranes and represent an ideal system for the modelling of biological membranes. To optimize these properties, it is necessary to correlate the local structure of the films and their defect structure with macroscopic physical properties. Furthermore, an understanding of their local electronic and transport properties as well as the way interfaces are built with solid substrates is a prerequisite for their application in any kind of molecular electronic devices. It is thus appealing to use the scanning tunnelling microscope for direct imaging of the local structure and the electronic properties of ordered organic films on solid substrates. Transition metal dichalcogenides (e.g. WSe2) have proven to be good substrates for LB film deposition (Fig. 6) [40]. In contrast to previous experiments on H O P G [23], the molecular constitution of a 2,2tricosenoic acid (CHz~CH--(CH)20--COOH) monolayer LB film was imaged reproducibly in air over large surface areas (16 nm × 16 nm) and long observation times (over 2 months). This indicates a stronger adhesion of organic material to polar layered semiconductors such as WSe 2. Both lateral lattice constants as well as the measured corrugation were significantly different for the LB film and the WSe: substrate respectively (film spacings, 3.6-6.5 A, WS% spacings,
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3.3+_0.1 A; film z corrugation; 4-6 A; WSe2 z corrugation, 2-3 A). The observed variation of intermolecular spacings indicates a mechanical and/or electrostatic interaction of the tip with the soft organic film. Nevertheless, this LB film was surprisingly stable; apart from a bending deformation of film molecules, the tunnelling tip obviously did not destroy the film structure.
4.3. Properties of thin gold and silverfilms Material parameters of metallic films, such as local electrical conductivity, adhesion properties and electromigration phenomena, are of great technological importance. Our investigations on thin adsorbate films of gold and silver deposited on flat substrates (i.e. HOPG and mica) revealed some interesting physical aspects of these systems. In contrast to polycrystalline films, single crystals as well as epitaxially grown films provide flat surface spots with defined crystallographic orientations [41]. Most often thin films reconstruct in the (l 11 ) crystallographic surface orientation yielding the closest-packed surface arrangement of adsorbed atoms. Figures 7(a) and 7(b) present the STM data obtained on gold and silver films 100 nm thick evaporated on mica. Both images show the stepped metallic surface over an 850 nm x 850 nm surface spot. The smallest surface steps shown are one atomic unit in height (approximately 2.4 A). The whole silver film in Fig. 7(a) is very smooth and continuous while the gold surface in Fig. 7(b) seems to consist of a patchwork of individual gold islands. It was found that their shape may change with time owing to the selfdiffusion of gold [42]. This process leads to drastic surface deformations resulting in the filling of holes as well as on purpose-produced craters on the gold surface [43, 44]. Gold diffusion preferentially occurs along surface steps but also across flat surface areas. Besides the presence of a large number of bulk interstitial atoms in the vicinity of island boundaries and holes, the rate-determining process for hole filling and island growth is the capture of atoms arriving from remote regions of the gold surface. Additionally, gold diffusion is found to be enhanced by the tunnelling process itself owing to the amount of energy deposited in the thin metallic film. The surface-diffusive effect is much more dramatic for the silver coverage owing to the increased mobility of silver on Ag( 111 ) compared
Fig. 7. (a) Silver film 100 n m thick evaporated on mica, T h e 850 n m x 850 n m surface spot shows a continuous silver film consisting of extended terraces. Surface steps down to the atomic unit height (2.4 A) separate these surface planes. (b) Gold film 100 n m thick on mica monitored over the same area of 850 n m x 850 nm. Flat gold islands are clustered together.
with gold [45]. Thus silver films form more readily continuous films via the Franck-van der Merve growth mechanism. Figure 8 demonstrates the effect of exposing a certain part of the silve~ surface to an increased electron dose emitted from the tunnelling tip. In the middle of this image ( 1.2/Jm x 1.2/~m in size) we see a broad strip extending 120 nm in the y direction and 1.2 ~m in x. The surface was exposed to a charge dose D of about 10/~C nm-2 for 1 h. Enhanced surface self-diffusion results in surface roughening.
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Fig. 8. Surface modification on a thin silver film after exposure to an increased tunnelling current density. Scanning for 1 h at an electron dose of about 10 p C nm-2 leads to the roughening of a strip (measuring 120 nm in width) in the middle of the 1.2 p m x 1.2/am image, probably due to silver self-diffusion.
No roughening was observed when imaging the surface after 1 h with the tip intermediately having been retracted. Once a surface spot was roughened, time-dependent relaxation was observed with the surface roughness continuously diminishing with time. Imaging, however, is difficult over longer periods because surface roughening occurs for tunnelling currents down to 50 pA. Imaging should therefore be performed at even smaller tunnelling currents, which, however, were not available in our instrumentation. To reduce surface exposure to high current densities, the scan frequency was increased for imaging (corresponding to a total electron dose D < 1 nC nm- 2). The epitaxial growth of gold on H O P G most often allows STM to probe the atomic arrangement of the adsorbate film in real space. When evaporating at a slow speed, the gold surface will end up fully relaxed with single-crystalline domains spread all over the substrate. The closest arrangement of atoms within a grain will then be the dense (111) packing with a bulk interatomic spacing of 2.885 A [46]. Compared to the same orientation on a gold single crystal [5, 44], thin films do not show the phenomenon of "buckling" [47], which is a one-dimensional effect yielding a 23×31/2 reconstruction of a macroscopically stressed crystal. Thin gold films evaporated
Fig. 9. Atomic resolution image of a gold film evaporated on HOPG. The Au(111) closest packing of the reconstructed film is shown with an atomic spacing of 2.95 _+0.1 A. When passing the centre of the image from bottom to top, the scan width was reduced by a factor of 2 to check reality. Current imaging was performed at a sample bias voltage of 59 mV with a tunnelling current of 1 nA.
epitaxially on flat substrates lack this superstructure and are thus fully relaxed [48]. Figure 9 depicts the atomic view of a gold surface at two different scan widths within the same picture. To test the reality of the measured atomic corrugation, the scan width (of both the x and y piezoscanner) was reduced by a factor of 2 just after having passed the centre of the image from bottom to top. After accommodation to the new conditions, atomic resolution in the same brightness as in the lower part was recorded. The measured interatomic distances of 2.95+0.1 A agree reasonably well with the bulk interatomic spacing of gold. STM investigations on gold films revealing the atomic structure were somewhat surprising [49] when first observed on gold films evaporated on mica [44, 50]. Atomic resolution images on this metallic system obviously stood in contradiction to the common interpretation assuming STM contour plots to be an image of the total charge density, even for small tunnelling gaps (this would result in a corrugation of less than 0.01 A on metals only). Doubts became even more serious when atomic resolution on other close-packed metallic surfaces was reported as well: AI(111) [51] and Cu(111)[52]. To explain these observations, different theoretical models were proposed including electronic effects [15, 50], influence of the applied electric field [53], effects of forces [7, 37] as well as tip-induced localized states (T1LSs) [54] and contamination- [55] or impurity- [56] enhanced resolution. Also, a tip model with an
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attached elastic cluster [56, 57] was proposed to explain the observed atomic corrugations of about 0.2 A in amplitude. For the Au( 111 ) surface, force effects are negligible [58]. Because similar results were obtained in various environmental conditions (i.e. UHV, air, oil) using scanning tunnelling microscopes equipped with different tunnelling tips, contamination-mediated amplification of the tunnelling current seems to be unlikely as well. The most favoured picture thus explains the measured corrugation amplitude by electronic surface states localized close to the Fermi energy EF. These results clearly show (as in the case of HOPG) that tunnelling phenomena sometimes may not be explained by first-order perturbation theory; rather, the whole tunnelling junction including tip and sample has to be treated as the new system. 5. Conclusions Since its invention 10 years ago, STM has entered many fields of surface analysis and inspection. Owing to its sensitivity to structural and electronic properties of surfaces on the atomic scale, STM is not only a valuable complementary method to conventional surface analytical techniques but also permits, for instance, the analysis of atomic scale features on non-periodic substrates and adsorbates. Although instrumentation has been extensively developed over the last 5 years, it is still considered to be in its infancy. Also, the application of STM as a tool for surface modification has rarely been explored. When the challenging results reported from the numerous STM groups (which are still increasing in number) are reviewed, it is easy to predict a fascinating future for this field of research. Acknowledgments We would like to thank G. Durstberger, K. Graf, R. Laschinski and R. Sander for technical assistance as well as F. Levy (Ecole Polytechniquc F~d6rale de Lausanne) for providing the 1TTaSe2. Furthermore, one of us (L.E.) expresses his gratitude to H.-J. Gtintherodt at the University of Basel for support by a research project of the "Schweizerischer Nationalfond" during the measurements on electrochemically deposited copper and the atomically resolved pictures of gold. We gratefully acknowledge financial support
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38 K. B. Blodgett and I. Langmuir, Phys. Rev., 51 (1937) 964. G. L. Gaines Jr., Insoluble Monolayers at the Liquid-Gas Interface, Wiley-lnterscience, New York, 1966. 39 J. D. Swalen, D. L. Allara, J. D. Andrade, E. A. Chandross, S. Garoff, J. Israelachvili, T. J. McCarthy, R. Murray, R. F. Pease, J. F. Rabolt, K. J. Wynne and H. Yu, Langmuir, 3 (1987) 932. H. Fuchs, H. Ohst and W. Prass, Adv. Mater., 3 119911 10. 41/ H. Fuchs, S. Akari and K. Dransfeld, Z. I'hys. B--Condensed Matter, 80 ( 199111389. 41 St. Buchholz, H. Fuchs and J. P. Rabe, J. Vac. Sci. Technol. A, (1991). 42 N. L. Petersom Difli~sion, American Society for Metals, Metals Park, OH, 1973, Chap. 3. D. N. Duhl, K. Hirano and M. Cohen, Acta Metall., 11 (1963) 1. 43 R.C. Jaclevic and L. Elie, Phys. Rev. Lett., 60 (1988) 120. 44 R. Emch, J. Nogami, M. M. Dovek, C. A. Lang and C. E Quate, J. Appl. Phys., 65 (1989) 79. 45 K. Reichelt and H. O. Lutz, J. Cryst. Growth, 10 (1971) 103. D. Stark, Surf Sci., 189-190(198711111. 46 N. W. Ashcroft and N. D. Mermin, in D. G. Crane (ed.), Solid State Physics, Holt-Saunders, Tokyo, 1976, p. 70. 47 Ch. W611, S. Chiang, R. J. Wilson and P. H. Lippel, l'hys. Rev. B, 39(1989) 7988. R. J. Behm, STM studies of metal surfaces, paper presented at Int. School NATO Advanced Study Institute, Erice, April 17-29, 1989. 48 G. Santoro, A. Franchini, V. Bortolani, U. Harten, J. P. Toennes and Ch. W611. Surf. Sci., 183 (1987) 1811. 49 S. Chiang, R. J. Wilson, Ch. Gerber and V. M. Hallmark, J. Vac. Sci. Teehnol. A, 6(1988) 386. 50 V. M. Hallmark, S. Chiang, J. F. Rabolt, J. D. Swalen and R. J. Wilson, Phys. Rev. Lett., 59 (1987) 2879. 51 J. Wintterlin, J. Wiechers, T. Gritsch, H. H6fer and R. J. Behm, J. Microsc., 152 (1988) 423. J. Wintterlin, J. Wiechers, H. Brune and T. Gritsch, Phys. Rev. Lett., 62 (1988) 59. 52 A. Brodde, St. Tosch and H. Neddermeyer, J. Microsc., 15211988)441. 53 J. E. Inglesfield, Surf Sci., 188(1987) L701. 54 E. Tckmann and S. Ciraci, I'hys. Rev. B, 40 (1989) 111286. 55 H. J. Mamin, E. Ganz, D. W. Abraham, R. E. Thomson and J. Clark, Phys. Rev. B, 34 (1986) 9015. 56 N. J. Zheng and I. S. T. Tsong, Phys. Rev. B, 41 11990) 2671. 57 J. Wintterlin, Ph.1). Thesis', Freie Universit~it Berlin, 1988. 58 J. deLaunay, Solid State Phys., 2 (1956) 220.
Materials Science and Engineering, A 139 ( 1991 ) 230-238
Investigation and modification of free and adsorbate-covered surfaces by scanning tunnelling microscopy L. M. Eng and H. Fuchs BASF A G., Polymer Research Laboratory, ZKLJ 543, D-6 700 Ludwigshafen (F.R.G.)
Abstract An overview is given of the different analytical aspects of scanning tunnelling microscopy (STM) and its applications in the fields of basic research and technology. Surface investigations under vacuum conditions are dealt with as well as experiments in air and electrolytes. Examples are given demonstrating that STM can be used as a nanometre size tool for manipulating matter on the mesoscopic and the (near) atomic scale.
1. Introduction Scanning tunnelling microscopy (STM) is a unique surface-sensitive technique allowing investigations of atomic scale features in real space [1, 2]. Since its invention by Binnig and Rohrer [3], an intensive evolution has occurred [4, 5] resulting in a variety of proximal probe derivatives which are sensitive to various physical properties. Nevertheless, they all have in common the near-field sensitivity and the raster-like scan motion of the probing stylus. Besides probing tunnelling currents, specific surface sensors have been developed for the local detection of forces [6, 7], (chemical) potential barriers [8], thermal profiles [9], capacitances [10], photons, noise signals [ 11 ], etc. In the following we will concentrate on some aspects of conventional STM in the field of surface inspection and modification. STM not only probes the atomic arrangement of surfaces but also provides information on the local surface chemistry and surface-induced chemical reactions. It is therefore an interesting instrument for use in electrochemistry and biology. Furthermore, local electronic and mechanical processes (i.e. forces and elastic moduli) as well as structural transitions on the sample surface may be induced and detected. STM is thus an excellent tool for local surface modification down to the atomic scale [12, 13]. In contrast to conventional surface analytical techniques, STM is not limited to special environ0921-5093/91/$3.50
mental conditions such as ultrahigh vacuum (UHV). Investigations in ambient air and gaseous phases have been reported together with operation in liquid environments (i.e. oil and electrolytes) at room temperature. Apart from basic research in surface science, chemistry and biology, STM is thus a very useful instrument for the routine inspection of technical surfaces on the mesoscopic scale (e.g. surfaces after ion impact, machined surfaces or adsorbate-covered systems).
2. Experimental and instrumental STM is based on the quantum mechanical effect of electron tunnelling between two electrodes separated by a few fingstr6ms only (d ~ 2 nm) [14]. At a first approximation the tunnelling probability depends exponentially on the electrode separation d. A small bias voltage Vt between the two electrodes (with I v t l < ~ / e ~ 4 V) induces a tunnelling current I, flowing across the metal-insulator-metal (MIM)junction. In a one-dimensional model (Fig. 1) [15, 16], electron tunnelling at small bias voltages ( V, ~ 4p/e, k B T/e) can be expressed by lt~c Vt exp( - A d ~
~/2)
with A ~ 1.025 A -l eV -~/2, k B the factor, T the absolute temperature local barrier height. Owing to the decay of the wavefunctions outside
(1) Boltzmann and • the exponential the sample
© Elsevier Sequoia/Printedin The Netherlands
231
.
E P
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.
,)
.
.
.
.
.
.
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d Fig. 1. Energy diagram illustrating the tunnelling barrier in STM. Owing to the voltage difference I't applied between thc two electrodes, a distinct current (the tunnelling current It) l]ows. It is a sensitive measure of the local density of d e c tronic states 1){12, rl near the Fermi energy E I directly above lhe samplc surface, el/, denotes the difference in energy between the two Fermi levels I-LP and E~' respectively. T h e local barrier height ',1), of both the tip and sample material is indicated to bc the difference bctween E,:,,' and F L' (with i = p,samplcl, sltip)l.
surface, this method permits the measurement of
the local density of states (LDOS) near the sample surface with a vertical resolution below 0.1 A ] 15-17]. Similarly, the high lateral resolution (approximately 1 A) of the instrument depends on the shape of the tip apex, since essentially only those atoms of the tip closest to the surface contribute to the tunnelling current. By scanning the tip line by line across the sample surface, an image is obtained of the atomic structure of the sample surface. In contrast to conventional electron microscopy, this image originates from bound electrons. The process of imaging is therefore not limited to vacuum applications but rather relies on a finite potential barrier across the tunnelling gap. Therefore it may consist of a gaseous or liquid medium as well. On the other hand, bound electrons reflect the electronic states on the surface and may easily be probed with modulation techniques. Variation of In(/,) vs. 6d (eqn. (1)) provides information on • (with d ln(lt)/ddoc on~2), whereas a derivation after 6 Vt provides information on the local conductivity o(E, r) or density of states D(E, r) near E v (as follows from a microscopic expression of I t in the transfer hamiltonian approach [16]: 6 ln(lt)/dVtoc I)(E. r)oc o(E, r)). Experimentally, the movement of the tunnelling tip is controlled by piezoceramic transducers driven by the inverse piezoeffect. Originally, "tripod scanners" constructed from three linear transducers mounted perpendicularly to each other were used. More recently, scanning tunnel-
ling microscopes have been equipped with singletube scanners [18]. Their higher mechanical eigenfrequencies permit imaging at a higher scan speed. Again, this is mandatory for the investigation of a lot of soft materials such as biological l 19] and synthetic organic [2] adsorbate layers. So far, little has been done with respect to the preparation of well-defined tunnelling tips [20]. Usually, tips are prepared by mechanical grinding, (electro-) chemical etching or simply by pinching off tips from inert metal rods such as Pt-Ir and gold. While all these primitive preparation techniques provide excellent results on atomically flat surfaces, severe problems may occur in STM investigations on rough surfaces exhibiting steep craters or trenches with diameters of approximately twice the tip radius. In this contribution we present some of our recent results obtained with different types of scanning tunnelling microscopes used for the investigation of structural and electronic phenomena on both atomically fiat and rough surfaces under different environmental conditions.
3. Layered materials Highly oriented pyrolytic graphite (HOPG) is one of the materials most often investigated with the tunnelling microscope [21]. Owing to its chemical inertness and its atomically flat surface over thousands of fingstr6ms, atomic resolution is easily obtained in air [22, 23]. This semimetal therefore provides an appropriate substrate for the deposition of organic and inorganic adsorbates as well as a good reference surface lot piezocalibration purposes. The relative ease of measurement on the (0001 ) flakes is based on the electronic [24, 25] and even more on the elastic [26} response of this material due to the proximity of the probing stylus. Figure 2 presents an image of the graphite surface as recorded with a tunnelling tip pinched off from a Pt-Ir wire. The sixfold symmetry originates from the alternate A-B stacking of subsequent graphite layers in the [0001] direction. The inequivalence [27] occurs because fl site atoms Jack a neighbouring atom in every second layer whereas a site atoms are weakly bound to the adjacent layer [25]. As a consequence, /3 site atoms occur as protrusions while a site atoms represent saddle points. At high tunnelling currents (i.e. several
232
Fig. 2. Highly oriented pyrolytic graphite provides an atomically flat surface suitable for piezocalibration (shown over an area of 74 A × 79 A,). Owing to its flatness, it is a good substrate for the deposition of organic adsorbates. The threefold symmetry of the fl site atoms (hexagonal packing) is seen. Lateral spacings between bright spots correspond to the 2.46 A separation of fl site atoms. Using a tungsten tip biased at V~= - 50 mV, a z corrugation of about 1.0 A is measured.
nanoamps) and small bias voltages (i.e. a few millivolts) the tip comes very close to the HOPG surface (d~< 2.5 A) because of the low density of states (DOS) near the Fermi level E v. Consequently, the sample will be loaded mechanically with the tunnelling tip exerting a force of up to 10 -6 N on the HOPG surface [28]. This yields a local surface pressure of 1 TPa (!) given an effective tip area of 100 A2. Evidently, these strong forces may be used for surface modification on a nanometre scale. Although under these conditions a slight point contact between tip and sample is obvious, tunnelling tips normally do not destroy the HOPG surface. Owing to the very low elastic coefficient in the [0001] direction (c33 = 36.5 GPa) [29], elastic deformation with penetration depths of some tenths of a nanometre has been measured [7]. By contrast, stiffer materials, e.g. crystalline silicon, exhibit brittle behaviour. Tip indentations on this material will thus result in a persistent local surface destruction [30]. The various methods for which STM is used to modify surfaces are excellently reviewed in ref. 12. The published data all refer to the modification of bare sample surfaces resulting in local deterioration. Until recently, direct surface modification with preserved atomic order in the modified areas could not be demonstrated. However, by using samples of transition metal dichalcogen-
Fig. 3. (a) Atomically resolved W S e 2 surface spot (83 A × 83 A) monitored at V~= 100 mV with a tunnelling current of 1 nA. Bright spots indicate high current densities above tungsten atoms separated by 3.3 +_0.1 A. (b) Atomically resolved WSez surface (over an area of 72 A × 95/k) after a mechanical tip indentation. A 5 V voltage pulse applied to the z piezo produced a protrusion on the sample surface. Astonishingly, the atomic lattice of tungsten atoms is preserved over the indentation spot as well. This indicates that the indentation process induced subsurface modifications rather than atomic defects at the sample surface.
ides, we have found that on this class of layered materials, plastic deformation on the nanometre scale is in fact possible without destroying the lateral atomic order of the topmost layer [31]. Evidently, this kind of surface modification may be relevant for technological applications (e.g. in storage devices) and ~also when investigating plastic deformations and relaxation processes on the (sub-) nanometre scale. Figure 3 depicts an image of a W S e 2 surface before (Fig. 3(a)) and after (Fig. 3(b)) tip indenta-
233
tion. Brighter spots indicate the positions of tungsten atoms [32] with the lateral registry being preserved even over the spot of the mechanical tip-sample contact. Most interestingly, the local deformation did not change the lateral position of atoms in the topmost layer but rather resulted in a reorientation of atoms in the bulk material. Besides these specific mechanical properties of the transition metal dichalcogenides, some of them exhibit an intriguing electronic effect known as charge density waves (CDWs). Their unit cell (either prismatic or orthorhombic)[33] additionally shows a periodic lattice distortion [34] with the ions being shifted for about 0.1-0.3 A from their equilibrium position [35]. As a consequence, the charge distribution of conducting electrons will be modulated. In the case of 1TTaSe~ a commensurate 13 ~/2 x 13J/2 bulk superstructure is formed at room temperature. Since STM is sensitive to electronic surface states, it will thus probe this charge modulation at the sample surface. Figure 4 presents the constantcurrent image acquired when scanning Pt-Ir tip over the surface of a freshly cleaved 1T-TaSe~ sample. In addition to the atomic lattice (with an interatomic spacing of about 3.4+_0.1 A), the 13~/2 x 13 ~/2 CDW superstructure (with a periodicity of 12.5 _+0.1 A) is clearly revealed. This experiment was repeated under silicone oil. As a result, both the atomic and CDW spacings were measured with an enhanced signal-tonoise ratio over the former measurement in air (Fig. 4). This is primarily due to the absence of field-induced instabilities arising in the tunnelling junction in the presence of air. Furthermore, polarization of oil molecules or other polar liquids located directly under the tunnelling tip modifies the electrical properties of the surface such as local barrier height and surface electronic states. 4. Adsorbate-covered surfaces
Heterogeneous surfaces are of great interest in many different technological areas, e.g. vacuum or electrochemical deposition of metallic films on substrates, surface finishing and organic coatings. In all these cases an understanding of the microscopic behaviour of the interface between substrate and adsorbate is essential for tailoring macroscopic properties. Some of the questions which arise concern the first stages of film formation, epitaxial growth, adhesion and electronic
Fig. 4. Constant-current image of TaSe~ imaged in air at room temperature. Both the atomic lattice as well as the c o m m e n s u r a t e periodicity of the charge density wave arc monitored over the 200 A x 200 A surface area. Lattice spacings are m e a s u r e d to be 3.4_+0.1 and 12.5_+0.1 A for the atomic and supcrlattice respectively. Imaging parameters were l, n A and Vt = - 2 6 3 mV.
properties at the interface. Tunnelling microscopy on these systems thus provides a direct investigation in real space. Below we describe some recent results in these fields. 4.1. Electrochemically deposited copper on HOPG
STM has opened up the possibility of in situ investigations on electrochemical processes at electrodes [36]. It thus excellently complements conventional voltanogrammic method which provide integral information only. Experimental problems arising from the ionic leakage current 1~on superimposed on the tunnelling current I t have been solved using coated tips and by applying an appropriate voltage bias between tip and sample relative to the surrounding electrolyte [37]. Besides imaging the adsorbed layer after deposition, STM is also able to directly monitor the electrochemical deposition process in situ. As an example (Fig. 5), copper was deposited on HOPG out o f a 0.1 M CuSO 4 solution with an ionic current of 1 mA for 1 min. Deposition was carried out with an additional platinum counterelectrode immersed in the liquid. To check reproducibility, the deposition process was stopped and a series of pictures was collected. Obviously, new features appear within this image
234
Fig. 5. Copper electrochemically deposited on HOPG with a charge dose of 60 inC. Copper preferentially adheres to the graphite surface step which crosses the picture from top to bottom. The copper grows in a fairly linear manner away from the surface step at an angle of about 45 °. A cluster size approximately 5 A in height and 25 A. in width is measured indicating this anisotropic growth process. Imaging of the 500 A x 500 A surface spot was carried out with a tunnelling current I t of 1 nA and a bias voltage V~set to - 20 mV (with respect to the tunnelling tip).
(Fig. 5) which formerly had not been observed on the bare graphite substrate. A slightly curved monatomic step in the graphite surface is seen crossing the image from top to bottom. Height differences between left and right indicate a 3.5/k value, in fairly good agreement with half the unit cell height of H O P G (3.35 A). Copper deposition preferentially occurs at surface steps presumably because of the increased activation barrier for diffusion. Once a copper cluster physisorbs at the substrate, additional copper atoms deposited from the electrolytic solution preferentially agglomerate to the existing nucleus. The cluster increases in size, forming in this case a linear chain in a direction about 45 ° off from the graphite surface step (indicated with an arrow in Fig. 5). It was even possible to deposit copper clusters on top of extended H O P G terraces. We believe that impurities and local (sub-) surface defects are responsible for both pinning and nucleation of the clusters. However, the adhesive forces are very weak. When a certain size is exceeded, some clusters can be moved on the HOPG surface using the tunnelling tip as "nanotweezers".
4.2. Langmuir-Blodgett films Langmuir-Blodgett (LB) films [38] have attracted considerable technological interest owing to their high degree of structural order and anisotropy [39]. Their tailor-made macroscopic
Fig. 6. A monolayer film of 2,2-tricosenoic acid deposited on WSe, was imaged at 1 nA and a tip bias of 600 mV over an 80 A-× 64 A surface area [40]. The superlattice of the LB film varies between 3.6 and 6.5 A and is significantly larger than the substrate lattice spacing of 3.3 A (see Fig. 3(a)).
properties such as the (piezo-) electrical and nonlinear optical properties partly exceed those of inorganic materials and may be widely varied by appropriate chemical functionalization. Organic films may also be synthesized for size-selective membranes and represent an ideal system for the modelling of biological membranes. To optimize these properties, it is necessary to correlate the local structure of the films and their defect structure with macroscopic physical properties. Furthermore, an understanding of their local electronic and transport properties as well as the way interfaces are built with solid substrates is a prerequisite for their application in any kind of molecular electronic devices. It is thus appealing to use the scanning tunnelling microscope for direct imaging of the local structure and the electronic properties of ordered organic films on solid substrates. Transition metal dichalcogenides (e.g. WSe2) have proven to be good substrates for LB film deposition (Fig. 6) [40]. In contrast to previous experiments on H O P G [23], the molecular constitution of a 2,2tricosenoic acid (CHz~CH--(CH)20--COOH) monolayer LB film was imaged reproducibly in air over large surface areas (16 nm × 16 nm) and long observation times (over 2 months). This indicates a stronger adhesion of organic material to polar layered semiconductors such as WSe 2. Both lateral lattice constants as well as the measured corrugation were significantly different for the LB film and the WSe: substrate respectively (film spacings, 3.6-6.5 A, WS% spacings,
235
3.3+_0.1 A; film z corrugation; 4-6 A; WSe2 z corrugation, 2-3 A). The observed variation of intermolecular spacings indicates a mechanical and/or electrostatic interaction of the tip with the soft organic film. Nevertheless, this LB film was surprisingly stable; apart from a bending deformation of film molecules, the tunnelling tip obviously did not destroy the film structure.
4.3. Properties of thin gold and silverfilms Material parameters of metallic films, such as local electrical conductivity, adhesion properties and electromigration phenomena, are of great technological importance. Our investigations on thin adsorbate films of gold and silver deposited on flat substrates (i.e. HOPG and mica) revealed some interesting physical aspects of these systems. In contrast to polycrystalline films, single crystals as well as epitaxially grown films provide flat surface spots with defined crystallographic orientations [41]. Most often thin films reconstruct in the (l 11 ) crystallographic surface orientation yielding the closest-packed surface arrangement of adsorbed atoms. Figures 7(a) and 7(b) present the STM data obtained on gold and silver films 100 nm thick evaporated on mica. Both images show the stepped metallic surface over an 850 nm x 850 nm surface spot. The smallest surface steps shown are one atomic unit in height (approximately 2.4 A). The whole silver film in Fig. 7(a) is very smooth and continuous while the gold surface in Fig. 7(b) seems to consist of a patchwork of individual gold islands. It was found that their shape may change with time owing to the selfdiffusion of gold [42]. This process leads to drastic surface deformations resulting in the filling of holes as well as on purpose-produced craters on the gold surface [43, 44]. Gold diffusion preferentially occurs along surface steps but also across flat surface areas. Besides the presence of a large number of bulk interstitial atoms in the vicinity of island boundaries and holes, the rate-determining process for hole filling and island growth is the capture of atoms arriving from remote regions of the gold surface. Additionally, gold diffusion is found to be enhanced by the tunnelling process itself owing to the amount of energy deposited in the thin metallic film. The surface-diffusive effect is much more dramatic for the silver coverage owing to the increased mobility of silver on Ag( 111 ) compared
Fig. 7. (a) Silver film 100 n m thick evaporated on mica, T h e 850 n m x 850 n m surface spot shows a continuous silver film consisting of extended terraces. Surface steps down to the atomic unit height (2.4 A) separate these surface planes. (b) Gold film 100 n m thick on mica monitored over the same area of 850 n m x 850 nm. Flat gold islands are clustered together.
with gold [45]. Thus silver films form more readily continuous films via the Franck-van der Merve growth mechanism. Figure 8 demonstrates the effect of exposing a certain part of the silve~ surface to an increased electron dose emitted from the tunnelling tip. In the middle of this image ( 1.2/Jm x 1.2/~m in size) we see a broad strip extending 120 nm in the y direction and 1.2 ~m in x. The surface was exposed to a charge dose D of about 10/~C nm-2 for 1 h. Enhanced surface self-diffusion results in surface roughening.
236
Fig. 8. Surface modification on a thin silver film after exposure to an increased tunnelling current density. Scanning for 1 h at an electron dose of about 10 p C nm-2 leads to the roughening of a strip (measuring 120 nm in width) in the middle of the 1.2 p m x 1.2/am image, probably due to silver self-diffusion.
No roughening was observed when imaging the surface after 1 h with the tip intermediately having been retracted. Once a surface spot was roughened, time-dependent relaxation was observed with the surface roughness continuously diminishing with time. Imaging, however, is difficult over longer periods because surface roughening occurs for tunnelling currents down to 50 pA. Imaging should therefore be performed at even smaller tunnelling currents, which, however, were not available in our instrumentation. To reduce surface exposure to high current densities, the scan frequency was increased for imaging (corresponding to a total electron dose D < 1 nC nm- 2). The epitaxial growth of gold on H O P G most often allows STM to probe the atomic arrangement of the adsorbate film in real space. When evaporating at a slow speed, the gold surface will end up fully relaxed with single-crystalline domains spread all over the substrate. The closest arrangement of atoms within a grain will then be the dense (111) packing with a bulk interatomic spacing of 2.885 A [46]. Compared to the same orientation on a gold single crystal [5, 44], thin films do not show the phenomenon of "buckling" [47], which is a one-dimensional effect yielding a 23×31/2 reconstruction of a macroscopically stressed crystal. Thin gold films evaporated
Fig. 9. Atomic resolution image of a gold film evaporated on HOPG. The Au(111) closest packing of the reconstructed film is shown with an atomic spacing of 2.95 _+0.1 A. When passing the centre of the image from bottom to top, the scan width was reduced by a factor of 2 to check reality. Current imaging was performed at a sample bias voltage of 59 mV with a tunnelling current of 1 nA.
epitaxially on flat substrates lack this superstructure and are thus fully relaxed [48]. Figure 9 depicts the atomic view of a gold surface at two different scan widths within the same picture. To test the reality of the measured atomic corrugation, the scan width (of both the x and y piezoscanner) was reduced by a factor of 2 just after having passed the centre of the image from bottom to top. After accommodation to the new conditions, atomic resolution in the same brightness as in the lower part was recorded. The measured interatomic distances of 2.95+0.1 A agree reasonably well with the bulk interatomic spacing of gold. STM investigations on gold films revealing the atomic structure were somewhat surprising [49] when first observed on gold films evaporated on mica [44, 50]. Atomic resolution images on this metallic system obviously stood in contradiction to the common interpretation assuming STM contour plots to be an image of the total charge density, even for small tunnelling gaps (this would result in a corrugation of less than 0.01 A on metals only). Doubts became even more serious when atomic resolution on other close-packed metallic surfaces was reported as well: AI(111) [51] and Cu(111)[52]. To explain these observations, different theoretical models were proposed including electronic effects [15, 50], influence of the applied electric field [53], effects of forces [7, 37] as well as tip-induced localized states (T1LSs) [54] and contamination- [55] or impurity- [56] enhanced resolution. Also, a tip model with an
237
attached elastic cluster [56, 57] was proposed to explain the observed atomic corrugations of about 0.2 A in amplitude. For the Au( 111 ) surface, force effects are negligible [58]. Because similar results were obtained in various environmental conditions (i.e. UHV, air, oil) using scanning tunnelling microscopes equipped with different tunnelling tips, contamination-mediated amplification of the tunnelling current seems to be unlikely as well. The most favoured picture thus explains the measured corrugation amplitude by electronic surface states localized close to the Fermi energy EF. These results clearly show (as in the case of HOPG) that tunnelling phenomena sometimes may not be explained by first-order perturbation theory; rather, the whole tunnelling junction including tip and sample has to be treated as the new system. 5. Conclusions Since its invention 10 years ago, STM has entered many fields of surface analysis and inspection. Owing to its sensitivity to structural and electronic properties of surfaces on the atomic scale, STM is not only a valuable complementary method to conventional surface analytical techniques but also permits, for instance, the analysis of atomic scale features on non-periodic substrates and adsorbates. Although instrumentation has been extensively developed over the last 5 years, it is still considered to be in its infancy. Also, the application of STM as a tool for surface modification has rarely been explored. When the challenging results reported from the numerous STM groups (which are still increasing in number) are reviewed, it is easy to predict a fascinating future for this field of research. Acknowledgments We would like to thank G. Durstberger, K. Graf, R. Laschinski and R. Sander for technical assistance as well as F. Levy (Ecole Polytechniquc F~d6rale de Lausanne) for providing the 1TTaSe2. Furthermore, one of us (L.E.) expresses his gratitude to H.-J. Gtintherodt at the University of Basel for support by a research project of the "Schweizerischer Nationalfond" during the measurements on electrochemically deposited copper and the atomically resolved pictures of gold. We gratefully acknowledge financial support
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