Ultramicroscopy North-Holland

33 (1990) 107-116

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ELEclWXHEMICAL DEPOSITION OF MOLECULAR FOR IN SITU SCANNING PROBE MICROSCOPY L.A. NAGAHARA, Department

T. THUNDAT,

of Physics, Arizona State University,

P.I. ODEN,

SM.

ADSORBATES

LINDSAY

*

Tempe, AZ 85287, USA

and R.L. RILL Department

of Chemistry and Institute of Molecular Biology, Florida State University,

Tallahassee,

FL 32306, USA

Received 13 February 1990; in final form 8 March 1990

We have studied gold and graphite electrodes in an electrochemistry cell under various solutions using the scanning tunneling microscope @TM). The gold (111) surface yields quite reproducible images and cyclic voltammograms. In situ voltammograms show that, under certain conditions, nanomolar quantities of DNA fragments can suppress the adsorption of a buffer salt of millimolar concentration. When the DNA concentration is reduced below that required for a monolayer coverage, the salt adsorption is restored. We show images of bare gold, gold covered with an adsorbate produced by the buffer salt, and gold prepared with a concentration of DNA fragments close to that required for monolayer coverage added to the buffer. Under these conditions, the surface is found to be uniformly covered with a characteristic structure.

1. lntroductlon The microscopy of complex materials, such as biological samples, is limited by the uniformity of the molecular adsorbate on the substrate. A heterogeneous specimen can be searched for the “desired” image, a questionable procedure with new techniques, such as scanning tunneling microscopy (STM) or atomic force microscopy (AFM). We have been using STM and AFM to study DNA under water [l-5] using electrochemistry to deposit samples onto a substrate and to control the interface during scanning (a number of groups have studied DNA dried onto graphite, beginning with the pioneering work of Binnig and Rohrer [6], but we are unaware of any other work with biopolymers in an electrochemistry cell). We have presented images that show single complete molecules clearly [1,3,5]. Although highly reproducible,

* To whom correspondence 0304-3991/90/$03.50

should be sent.

0 1990 - Elsevier Science Publishers

these images were selected from inhomogeneous patches (containing, for example, large aggregates). Under these conditions, control experiments are of limited validity, although more convincing than experiments on uncontrolled surfaces (in air, for example). However, one of the great advantages of the discovery of microscopies that work in water [7,8] is that electrochemistry should, in principle, allow the preparation of uniform molecular adsorbates and allow them to be maintained during imaging. Furthermore, macroscopic electrochemical measurements on the cell can aid interpretation of the images. We have studied gold and graphite substrates at a number of potentials, and under a number of solutions. We will illustrate our procedure with a description of one preparation on gold substrates that gives particularly uniform coverage, and we will provide full details of the experimental methods. We will show a virgin surface, one prepared under buffer solution alone, and one prepared in a buffer solution containing DNA. It is not our goal in this paper to give a

B.V. (North-Holland)

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detailed interpretation of these images; our main point here is to show that each surface yields images that are different in a characteristic way that correlates with our macroscopic electrochemical observations, and to show that the surface texture can be homogeneous at the nanometer level over millimeter distances.

2. Electrochemical

considerations

All dielectric interfaces are charged; at a metal-solution interface, the solution chemical potential is matched to the Fermi level by concentration of a diffuse cloud of ionic charge in a layer immediately adjacent to the metal. In the simplest mean-field description, most of the corresponding potential drop occurs in distance L, (the Debye length) given approximately by 0.3 nm/[ C]i’2 where [C] is the normal concentration of ions (molar concentration of monovalent ions for this particular formula) in the bulk solution [9]. In reality, the finite size of ions and correlations (due to both charge and size) can lead to complex structures at the interface. The most concentrated region of ion packing is not diffuse, but has a, structure dominated by (strong) repulsive interactions; this is called the Stern layer. We believe that the tip interacts strongly with the adsorbate, so that it is most likely that we image molecules well packed in the Stem layer. Even in the simplest picture of an “ideal gas” diffuse ion cloud, the contribution of a poly-valent ion (charge Z) to the free energy varies as exp(Ze&&T) where \c10is the potential at the surface (relative to the bulk solution) so that a small concentration of highly charged ions can dominate the structure at an electrochemical interface. Obviously, these interface structures are also influenced by carrier concentration and the distribution of current density, so pH and electrode geometry are important parameters. In the face of such complexity, electrochemists have devised the three-electrode cell to control at least one interface [lo]. The potential drop between the solution and the electrode of interest (the working electrode) is monitored by a high impedance contact constructed so as to minimize polarization at its surface (the reference elec-

deposition of molecular aakorbates

trode). A servo circuit drives a low impedance contact (the counter electrode) to the voltage required to maintain the desired potential difference between the reference and working electrodes. We are particularly interested in imaging DNA molecules, and this imposes some constraints on the electrochemistry. If a monolayer coverage of non-interacting molecules is desired, the extreme anisotropy of DNA dictates a low initial concentration if an efficient means of accumulating it onto the interface is used. We have been working with the 146 base-pair frtgment extracted from the nucleosome [ll], a 20 A diameter molecule of about 500 A length. If a millimeter depth of solution contributes molecules to the substrate surface (19 mm2 in area), then concentrations of the order of 0.1 pg per ml are required (about 1nM of the complete fragments). Concentrations in excess of - 10 pg/rnl will cover the surface. In addition to the salt required to stabilize DNA, a buffer is needed if the pH is not to be dominated by impurities. Even near their pK,, mM concentrations are required for proper buffering action, so that one can only hope to image DNA clearly at potentials where the nanomolar DNA is preferentially adsorbed over the mM buffer salt. We have found the most uniform adsorption at low cell currents (- PA); so, at mM monovalent salt, the ohmic cell field (outside the double layers) is very small indeed (less than a V/m) [lo]. The mobility of DNA is of the order of low4 cm2 per second per volt [12], so the cell current must be maintained for times of the order of a minute. The solutions used in this work have the following composition: the buffer is 3mM tris(hydroxymethyl)amino methane (referred to as Tris), 0.3mM EDTA, 0.3mM NaN, and 0.3mM cacodylic acid dissolved in 18 MS2 water and adjusted to pH 7.3 with HCl. The DNA solutions consist of the above buffer with nucleosomal DNA fragments dissolved into it to a final concentration between 10 and 200 pg/rnl (30nM to 600nM of total fragment, as measured by the absorbance at 260 nm). The other minor components are standard preservatives, and alter the cyclic voltammograms but do not appear to dominate the nature of the images in the same way that other factors

LA. Nagahara et al. / Electrochemical

(like pH or DNA concentration) do. The adsorption of DNA and the organic bases has been studied on mercury [13] and graphite [14] electrodes, but we are unaware of any studies of the system we have developed here. We have carried out studies of DNA/Iris solutions using gold electrodes both ex situ and in the STM. We find that DNA can have a dramatic effect on the Tris peaks in cyclic voltammograms.

3. Experimental

procedure

We use the TAK 2.0 STM from Angstrom Technology, Inc. (1815 W. 1st Ave, Mesa, AZ 85202). This instrument has a potentiostat/ galvanostat built into the STM base, and a transputer-based operating system which permits simultaneous STM operation and sample control. We have replaced the electrochemistry cell with one of our own design shown in fig. 1. A glass tube (5 mm 0) is polished so as to seal against the substrate. The tube is fixed with epoxy into a thin steel plate which holds the assembly onto the STM stage in a manner that allows the STM tip and substrate to be viewed with an optical microscope. The counter electrode consists of a 4.5 mm diameter loop made from two 0.1 mm Pt wires twisted together. It rests about 1.5 mm above the substrate. The reference electrode is made from a 1 mm diameter capillary which contains a filament so that the tube can be filled by capillary

Fig. 1. Side (a) and top (b) views of the STM electrochemistry cell; (1) is the substrate which is sealed onto the polished end of a glass tube (2). The outside of the tube is glued into a stainless steel disc (3) which is bolted onto the STM stage, holding the substrate in place and forming an electrical contact to it. A glass reference electrode (4) dips into the solution. Tbe counter electrode (5) is a loop of 4.5 mm diameter made from two 0.1 mm 0 platinum wires twisted together and it is located 1.5 mm above the substrate.

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action (World Precision Instruments, 375 Quinnipiac Ave, New Haven, CT 60513). It is pulled so as to form a narrow orifice at the end of a rightangle bend. Typical contact resistances for the solutions we use here are - 10 MSL The tube is filled with 3.5M KCl, and a 0.1 mm Ag/AgCl wire is glued into the open end. The Ag/AgCl interface is prepared by anodic oxidation of, Ag to form insoluble AgCl [15]. These electrodes give readings within 1 mV of a standard. The STM tip is inserted into the cell through the counter electrode loop, and imaging normally takes place within a mm of the center of the cell. The tips are fabricated and characterized as described elsewhere [16]. Substrate preparation is critical both for reproducible imaging and for cyclic voltammetry. Graphite substrates were made by cleaving pyrolytic graphite (ZYA grade from Union Carbide, Cleveland, OH 44101) in air. Their electrochemistry was somewhat variable. Gold gives much better results if produced so as to yield surfaces that are nearly atomically flat and without much contamination visible in STM images. Scratch-free green mica, sold as 0.003 to 0.005 inch thick sheet (AshvilleSchoonmaker, P.O. Box 318, Newport News, VA 21607) is cleaved in a laminar flow hood, and mounted on a heated substrate in a UHV evaporator. It is baked at 500°C until the pressure at the substrate is - lop8 Torr. It is cooled to the deposition temperature (300 to 400 o C at which point the pressure is - :O-’ Torr), and gold (99.999%) is evaporated at 1 A/s for a total film thickness of 2500 A. The pressure during evaporation is - lo-’ Torr. The substrate temperature is then raised 30°C and the films annealed for half an hour. We have only recently begun evaporations in these clean conditions, and are not yet certain of the optimum procedure; however, our best results have been obtained with films deposited near 400°C. The substrates are kept under argon until required. They are exposed to ambient conditions for times which may reach half an hour in the worst cases, resulting in some clear deterioration of the STM images. The substrates are loaded onto the STM stage and surveyed under ambient conditions. About 60 ~1 of the appropriate solution is then deposited

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into the cell, and the potentiostat set to the desired potential. The counter electrode is then connected for one minute (for the images shown here). The cell charging current is monitored on a storage oscilloscope (a typical trace is shown as the inset in fig. 2). The counter electrode is then disconnected, and the STM tip introduced to the cell at the appropriate potential for imaging ( - 100 mV for this work). A high-impedance probe connected to the counter electrode shows that its potential changes slowly (- 50 mV in 10 min), so the double-layer must change, but the STM images suggest that the structure in the Stern layer is much more stable. Many experiments on both gold and graphite show that the images are characteristic of the potential at which deposition was carried out, and not a function of the STM tip bias (provided a well insulated tip is used [16]). We do notice alterations that are presumably a consequence of the charging or discharging due to the tip. These occur over minutes in the case of graphite, and over tens of minutes to hours in the case of gold. We have also imaged the substrates while the cell remains under potentiostatic control. We find results that are similar to those obtained with the counter electrode disconnected, but the images are marred by transient noise which suggests that particles are more likely to become attached to the tip under these conditions. The images shown here were taken with a tip bias of -100 mV and a tunneling current of 1 nA. Leakage due to the solution was too small to measure ( < 10 PA). The solutions are also characterized in the STM cell using cyclic voltammetry. The deposition is dependent on the sample history, so the results presented here pertain to the first use of a fresh substrate. For this reason, cyclic voltammetry cannot be carried out on the same samples that are imaged (there are clear changes both in voltammograms and in STM images that result from previous cycling of the cell). We carry out parallel experiments in which a substrate similar to that used for imaging is subjected to cyclic voltammetry in the STM cell used for imaging. The small cell yields voltammograms that are a little noisier than those collected in a conventional cell ex situ but that are otherwise identical. We show some

deposition of molecuhr

aakorbates

voltammograms obtained in situ in fig. 2 (these are traced from a print of the screen of the TAK 2.0). The entire scan is shown to illustrate the capabilities of the microscope. The top trace (fig. 2a) is for the buffer alone, and shows a peak near 0.6 V (the scan rate was 0.1 V/s). Studies in the ex situ cell show that this major peak is due to Tris, and is not changed much by the addition of the EDTA, NaN, or cacodylic acid (this is not the case on graphite where addition of these components suppresses the Tris peaks). However, when DNA is added, the voltammogram is changed dramatically (fig. 2b). For concentrations above - 40 pg/ml the Tris peak is suppressed. The same

1 r+80uA

Fig. 2. Cyclic voltammograms recorded in the STM cell on fresh gold substrates (scanning 0.1 V/s). (a) is for the buffer alone, the peak near 0.6 V (marked with an arrow) being characteristic of Tris on gold. (b) is as above, but with DNA added to a concentration of 40 pg/ml. The Tris peak appears to be suppressed. (c) is recorded under the same conditions, but with the DNA content reduced to 10 ag/ml. The Tris peak reappears. The inset (d) shows the polarization charging current of the STM cell over 50 seconds for the buffer solution, with the substrate at + 1 V (Ag/AgCl) and the counter electrode COMeCted at zero time.

LA. Nagahara et al. / Electrochemical deposition of molecular aakorbates

effect is seen in a conventional ex situ cell. These DNA concentrations are far too small to yield a measurable adsorption peak for DNA, but adding DNA clearly blocks the Tris peak. When the DNA concentration is reduced below that which would yield a monolayer (- 10 pg/ml for a 60 ~1 loading of our cell) the Tris peak is restored (fig. 2~). The DNA probably complexes with some of the other minor components of the buffer; the effect of adding DNA is much less dramatic when these are not present.

4. STM images Electrochemical control of the graphite results in much greater uniformity than can be obtained in uncontrolled experiments, but the results were not promising for the development of microscopy and will not be discussed in detail in this paper. We have surveyed potentials between + 1 and - 1 V (Ag/AgCl) and note the following general points: evidence of DNA adsorption is seen at all potentials, except near +0.2 V (Ag/AgCl). Sec-

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ondly, most of the adsorption appears to occur at steps. Indeed, the steps are so reactive (compared to the basal plane) that they easily become decorated with very small amounts of contaminant, and it is not easy to distinguish a polymer-like feature in an image from such an artifact. Gold has yielded much more promising results. We begin with the bare substrates imaged in air. Two preparations are shown in fig. 3: in (A) a 4 pm square is.shown which has been exposed to air for about half an hour. The surface is remarkably flat and well ordered (note the extent of the terraces crossing the image). It contains small holes (of - 50 A depth) separated by about 0.5 pm. Particles of dirt litter the surface; a particularly large cluster is located in the lower left comer. Contamination like this continues to accumulate if the sample is left exposed, but much of it is removed if the surface is covered with clean water, and the images remain stable thereafter. Fig. 3B shows a substrate made at a lower deposition temperature. The holes are closer together (- 0.2 pm) but the overall morphology is similar. This image is shown at higher magnification to clarify the atomic steps

Fig. 3. STM images of typical substrates in air. The sample in (A) had been exposed for about half an hour, and shows obvious signs of contamination. The sample in (B) is cleaner, but has more holes (it was deposited at a lower temperature). (B) is shown at higher magnification to show the atomic-scale steps on the surface. On these, and the following images, the apparent height scale for each image is shown directly beneath it.

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that are seen on the surface (the majority of steps visible in this image are two atoms high, but some single-atom steps can be found). After deposition of the buffer alone at + 1 V (Ag/AgCl) as described in the last section (the corresponding charging curve is shown in fig. 2d) the surface is modified considerably. A 4 pm square is shown in fig. 4A. The following images (4B-4D) are obtained by scanning part of the

deposiition of molecular odrorbares

preceding area at double the resolution. The surface is quite rough; the distribution of holes is similar to what was observed on the bare substrate, but there is no sign of the terraces at the 60 o angles characteristic of the gold (111) surface. At the highest magnifications, an underlying fine texture of about 75 A size can be seen. This series of images was selected as representative from amongst several different runs in which the surface

Fig. 4. A se-ties of scans at increasing magnification over a substrate onto which buffer has been deposited as described in the text. They are obtained with the sample under the buffer solution. The surface shows no sign of the underlying gold structure and is quite rough. It has a characteristic fine grainy structure visible in (C) and (D). The arrowheads in (D) point to one grain.

LA. Nagahara et al. / Eiectrochemical deposition ofmolecularadsorbates

was surveyed over macroscopic distances (hundreds of pm) in each case. Other structures, such as finger-like cohunns, are seen in about 10% of the images. Fig. 5 shows a series of images obtained in conditions identical to the buffer run just de-

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scribed, but with the nucleosomal DNA added to a concentration of 10 pg/rnl. Figs. 5B-5D (and 5E) are images obtained by successive scans at increasing magnification of the area shown in the preceding image. The images show that the surfa? is uniformly covered with “fluff” of about 500 A

Fig. 5. A series of scans over a substrate onto which buffer containing DNA has been deposited as described in the text. The surface is ima,ged under solution. The surface texture is very different from that obtained with the buffer alone. The underlying gold structure appears quite clearly in (A) and (B). In this series, (B), (C) and (D) are obtained by scanning the upper left comer of the preceding I at higher magnification. (E) is taken from the same region as (D) but shown for tip scans from left to right (as opposed to right : for the preceding images). The arrowheads point to the same set of small features in each image, showing that the sample does not move much under the tip as it is scanned.

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dimension. Note that in this case, the underlying gold structure is clearly observed as the 60 o triangles visible in figs. 5A and 5B (there is some indication of the underlying hole distribution too, particularly in the dark region near the center of 5B). The “fluff’ coverage is remarkably uniform; Figs. 6A-6C show three areas selected at random

deposition of molecular aakorbates

from a survey in which the tip was brought down and a 1 pm square imaged after the STM head had been moved by about 100 pm using the mechanical positioning micrometers. In this partitular experiment, all of the substrate that we surveyed was covered in this manner. Such uniform coverage is very unusual; increasing or de-

distances apart on a substrate onto whi ch a Fig. 6. (A), (B) and (C) show some representative scans taken at macroscopic bufl Eer-DNA solution has been deposited as for fig. 5. Each image is randomly selected from scans in which the STM head WBS mo14 to a spot located about 100 pm from the previous scan. The characteristic 500 A bundles appear to cover the entire electi rode. (D) shows a scan (at lower magnification) taken under identical conditions, except that the buffer pH was adjusted to 8.0 rather than 7.3. The texture of the surface is very different, and far less homogeneous.

LA. Nagahara et al. / Electrochemical

creasing the DNA concentration results in more heterogeneity. It is also a sensitive function of pH. Fig. 6D shows a 2 pm square scan made over a substrate prepared exactly as described for the images shown in fig. 5, but with the solution pH adjusted to 8.0 (the coverage was much more heterogeneous, so this image illustrates just one of a number of morphologies observed). The STM tip probably interacts with the adsorbates rather strongly [17,lgj%nd the stability of the adsorbate is important. We have shown elsewhere how images of the same group of molecules scanned in different directions can be very different, indicating gross sample movement [1,2,19]. The images we show here are quite stable: The images in figs. 5D and 5E are cut from larger scans of the same area of the substrate in which the tip was scanned from left to right (5D) and right to left (5E) for data acquisition. They are almost identical. The arrowheads point to three small features separated by - 50 A. They appear in the same position in both scans and are almost identical, indicating that sample movement must have been small.

5. Discussion Interpretation of these experiments is predicated upon the possibility of tunneling through thick dielectric films. Many experiments in our laboratory and elsewhere suggest that this occurs. We have recently constructed a tight binding model for the STM imaging of organic adsorbates, finding that such tunneling is possible if molecular orbitals in the sample are brought into resonance with the metallic Fermi level as a consequence of the pressure in the tunnel gap induced by the action of the STM servo [18]. We have studied the electronic properties of DNA at pressure through its UV adsorption, finding large shifts which could indeed bring about the required resonance [18]. However, if our theory is correct, the contrast in STM images of organic materials will not reflect the real height of the adsorbate in any simple way (it is mostly a function of the energy difference between the Fermi level and the energy of the molecular orbital that provides the needed reso-

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nance [18]). The salt peak (fig. 2a) shows that a macroscopic amount of material is probably adsorbed onto the substrate when the adsorption is not suppressed by DNA, which would explain the lack of gold-like features in the images shown in fig. 4. The single most striking aspect of this work is the homogeneity of the images at a DNA concentration that coincides with the reappearance of the Tris peaks in the voltammograms. This transition occurs close to the concentration at which a simple calculation indicates a monolayer coverage of DNA. The most obvious interpretation is that the DNA is preferentially adsorbed onto the gold electrode (presumably because of its high charge) thus blocking Tris adsorption. The gold is exposed for adsorption when the DNA concentration falls below that required for a monolayer coverage. At higher concentrations, the gold surface shows a heterogeneous coverage consisting of both aggregates and what appear to be isolated molecules [5]. At lower concentration, the image is again complicated by a mixture of Tris-induced structures and DNA. When the coverage is close to the point at which the Tris peaks reappear, it is presumably constrained to be rather uniform by packing forces. We do see features that we might reasonably interpret as individual DNA fragments, but the central point of this paper is the remarkable uniformity of the images at the concentration that gives a monolayer coverage. Indeed, since the Tris adsorption peak has reappeared at this concentration of DNA, much of the structure must be due to Tris (possibly complexed with other components of the buffer).

6. Conclusions This work is in its infancy, and many factors are poorly controlled; for example, atmospheric contamination of the substrates must be important, and the run-to-run variation in substrate morphology and electrode placement must introduce large uncertainties into ‘our measurements and calculations of coverage. We have not identified the role of the minor components of the buffer. However, we do see clear effects in both

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cyclic voltammograms and in STM images when monolayer amounts of DNA are added to the buffer. This suggests that it may be possible to prepare uniform molecular adsorbates for microscopy in a semiquantitative manner. We believe that this will prove valuable in the development of microscopes that can examine these surfaces.

Acknowledgements Jim DeRose helped us in the lab. Uwe Knipping developed all the electrochemistry software. Roger Wartell suggested many interesting experiments. Rich Satterlie helped us develop the reference electrode. The staff of Angstrom Technology (particularly John Alexander) assisted us with instrumentation. Daphne Yariv made helpful comments on the manuscript. We received financial support from the NSF (BBS8615653), the office of the vice president for research at ASU and Angstrom Technology, Inc.

References [l] B. Barris, U. Knipping, S.M. Lindsay, L. Nagahara and T. Thundat, Biopolymers 27 (1988) 1691. [2] SM. Lindsay, T. Thundat and L. Nagahara, J. Microscopy 152 (1988) 213.

&position

of molecular adsorbates

[3] SM. Lindsay, L.A. Nagahara, T. Thundat, U. Knipping,

R.L. RiIl, B. Drake, C.B. Prater, A.L. Weisenhom, S.A.C. Gould and P.K. Hansma, J. Biomol. Struct. Dynamics 7 (1989) 279. [S] SM. Lindsay, L.A. Nagahara, T. Thundat and P. Oden, J. Biomol. Struct. Dynamics 7 (1989) 289. [5] S.M. Lindsay, T. Thundat, L. Nagahara, U. Knipping and RL. RiII, Science. 244 (1989) 1063. [6] G. Binnig and H. Rohrer, in: Trends in Physics, Eds. J. Janta and J. Pantofhcek (European Physical Society, Bristol, 1984) p. 38. (71 R. Sonnenfeld and P.K. Hansma, Science 232 (1986) 211. [8] B. Drake, C.B. Prater, A.L. Weþ, S.A.C. Gould and P.K. Hansma, Science. 243 (1989) 1586. [9] J.N. Israelachvilh, Intermolecular and Surface Forces (Academic Press, N.Y., 1985). [lo] A.J. Bard and L.R. FauIkner, Electrochemical Methods, Fundamentals and Applications (Plenum, New York, 1980). [ll] T.E. Strzelecka and R.L. Rill, J. Am. Chem. Sot. 109 (1987) 4513. [12] P.D. Ross and R.L. Scruggs, Biopolymers 2 (1964) 231. [13] P. Valenta, H. Numberg and P. KIahre, Bioelectrochem. Bioenerg. 1 (1974) 487. [14] D. Th&enot, J. EIectroanaI. Chem. 46 (1973) 89. [15] D.G.J. Ives and G.J. Janz, Reference EIectrodes, Theory and Practice (Academic Press, New York, 1989). [16] L.A. Nagahara, T. Thundat and S.M. Lindsay, Rev. Sci. Instr. 60 (1989) 3128. [17] J.C.H. Spence, W. Lo and M. Kuwabara, Ultramicroscopy 33 (1990) 69. [18] S.M. Lindsay, O.F. Sankey, Y. Li, C. Herbst and A. Rupprecht, J. Phys. Chem., in press. [19] SM. Lindsay, T. Thundat and L. Nagahara, in: Biological and Artificial Intelligence Systems, Eds. E. clementi and S. Chin (ESCOM, Leiden, 1988).

Electrochemical deposition of molecular adsorbates for in situ scanning probe microscopy.

We have studied gold and graphite electrodes in an electrochemistry cell under various solutions using the scanning tunneling microscope (STM). The go...
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