Langmuir 1991, 7, 1981-1987
1981
Reversible Oxidative Roughening of Au( 11 1) in Aqueous Salt Solutions Erwin Holland-Moritz,? Joseph Gordon 11, K. Kanazawa, and Richard Sonnenfeld' IBM Almaden Research Center, 650 Harry Road, Sun Jose, California 95120-6099 Received October 1, 1990.In Final Form: February 20, 1991 The commercial Nanoscope I was modified to work with a scanning tunneling microscope (STM) in an open electrochemical cell under potential control. The gold electrode samples were grown epitaxially on green mica. We can simultaneously image the sampe and scan the electrochemical potential. We have studied the Au(ll1) surface in the following solutions: 0.01 M KCl, 0.01 M, 0.03 M, and 0.05 M AgN03; 0.01 M NaN03; 0.01 M and 0.05 M AgC104. Scanning the potential toward the oxidative range of gold roughens the surface. We found that the roughening was completely reversed when the potential was reduced. In fact, single atom steps on the original surface reappeared unchanged upon completion of an oxidation-reduction cycle. However,this complete reversibilityheld only so long as a certainelectrochemical potential in the solution was not exceeded. In our STM studies we also found a dependence of the roughness amplitude on solution concentration. On samples of the same batch the final roughness was about 40 and 100 i% in 0.01 and 0.05 M AgNO3, respectively. This roughness is much too high to be explained by a monolayer of oxide and therefore it is rather surprising, especially in the case of nitrate and perchlorate electrolytes. The irreversible roughening previously mentioned 0.9 V vs Ag/AgCI in KCl can be explained by dissolution of Au chloride ions, while the reversible part may be understood by creation of neutral adsorbed AuCl. We believe that in general the reversible roughening we observed is probably due to slow adsorption processes that take several minutes to reach equilibrium;without STM images simultaneous with voltammograms, this process would likely have continued to go undetected.
Introduction Since it was learned that the scanning tunneling microscope (STM) could provide high-resolution images of surfaces immersed in electrolytes,lI2rapid progress has been made in the use of STM to study electrochemical processes in Particularly exciting has been the observation of metal (lead? silverg) deposition onto Au(111) surfaces in solution by a STM as well as the observation of Au atoms in aqueous solution with an atomic force microscope (AFM).'O Because of their lack of surface oxides, Au(ll1) surfaces are ideal model systems for STM studies in general and for deposition investigations of metallic species from an electrolyte in particular. There are two reasonable ways of preparing Au(ll1) surfaces: sputtering and annealing single crystals under vacuumll and epitaxial deposition onto green mica.12 Although the surfaces of bulk crystals are usually more perfect than those of films, metal films are considerably more convenient for the experimentalist. t Visiting scientistfrom the 11.Phys.Institut, Universitlt zu Kdln, Zulpicherstr. 77, D-5000Kdn, F.R.G. (1)Sonnenfeld, R.; Hansma, P. K. Science 1986, 232, 211. (2)Liu, H. Y.;Fan, F.-R. F.; Lin, C. W.; Bard, A. J. J . Am. Chem. SOC.
1986,108, 3838.
(3)Lustenberger, P.; Rohrer, H.; Christoph, R.; Siegenthaler,H.; Quate, C. F. J. Electroanal. Chem. Interfacial Electrochem. 1988,243, 225. (4) Gewirth, A. A.; Craston, D. H.; Bard, A. J. J. Electroanal. Chem.
Interfacial Electrochem. 1989, 261, 477. (5) Heben, M. J.; Penner, R. M.; Lewis, N. S.; Dovek, M. M.; Quate, C. F. Appl. Phys. Lett. 1989,54, 1421. (6)Fan, F.-R. F.; Bard, A. J. J . Electrochem. SOC.1989, 136, 3216. (7) Sonnenfeld, Richard; Schneir, Jason; Hansma, Paul K. Modern Aspects of Electrochemistry; edited by White, R. E., Bockris, J. O'M., Conway, B. E., Eds.; Plenum Press: New York, 1990; Vol. 21. (8)Green, M. P.;Richter, M.; Xing, X.; Scherson, D.; Hanson, K. J.; Ross, P. N.; Carr, R.; Lindau, I. J. Microsc. 1988, 152, 823. (9)Magnussen, 0.M.; Hotlos, J.; Nicols, R. J.; Behm, R. J.; Kolb, D. M. Phys. Rev. Lett. 1990, 64, 2929. (10)Manna, S.;Butt, H. J.; Gould, S. A. C.; Hansma, P. K. Bull. Am. Phys. SOC. 1990, 36, (3),759. (11)Chiang, S.;Wilson, R. J.; Gerber, Ch.; Hallmark, V. M. J . Vac. Sci. Technol., A 1988,6, 386. (12)Holland-Moritz, E.;Gordon, J.; Borges,G.; Sonnenfeld, R. Langmuir 1991, 7, 301.
Films are easier to pattern and shape to fit the needs of an experiment, which is an important advantage especially for the use as electrodes in electrochemical experiments. As far as metal films go, gold and silver on mica are unique. When deposition conditions are properly tuned, the surfaces can be ideal, that means nearly atomically flat over a few square micrometers, consisting of flat terraces about 1000A wide, separated by monoatomic steps of 2.36 A height. A very clean preparation system is a requisite for getting such ideal stable surface structures, i.e. no step motion effects,12 which is important for studies in electrolytes. There are a few "in situ" STM investigations on metal surfaces in solutions under potential control. Trevor et al. have shown reversible roughening of Au(ll1) in dilute perchloric acid.13 However, they have imaged their surface only near the rest potential before and after cycling to oxidative potentials. Morita et al.14 presented some preliminary results of their roughness investigations on silver and gold as function of potential in 0.1 M KC1 on polycrystalline samples. Besides these STM studies there is microbalance and coulometric work done by Schumacher et al.15 on Au in perchlorate solutions. The authors of that work concluded that there is a roughening of gold on the order of 40-60 i% at oxidative potentials. In this paper we report on a reversible roughening, that means even the monoatomic step structure is restored, so long as the potential does not cross a peak visible in the corresponding voltammograms. As we will show, there does not exist a sharp peak in the voltammograms which corresponds to the roughening. However, a small positive current is observed at the potentials where the roughening occurs. This roughening process seems to be correlated (13)Trevor, D. J.; Chidsey, C. E. D.; Loiacono,D. N. Phys. Rev. Lett.
1989, 62, 929.
(14)Morita, S.;Okada, T.; Mikoshiba, M. Jpn. J. Appl. Phys. 1989,
28, 535.
(15)Schumacher, R.; Borges, G.; Kanazawa, K. Surf. Sci. 19815,163, L621.
0743-7463/91/2407-1981$02.50/0 0 1991 American Chemical Society
1982 Langmuir, Vol. 7, No.9, 1991
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PROGRAMMER
TUNNELING
I t'ELECTRMF
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0
-
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m$ SCOPE
IL
STM/POTENTIOSTAT SCHEMATIC
Figure 2. Schematic block diagram demonstrating the inteVbm 0
0
-Working
Figure 1. Potentiostat for the STM electrochemicalcell showing differences from the standard Wenking potentiostat (right of the dashed line) by an additional difference amplifier (gain 1). This allows a floating measurement of the voltage between working and reference electrode. That is necessary because the working electrode must be held a t the tunneling bias potential.
to a reaction which is unknown to date. We believe that this unknown process is probably a very slow adsorption process.
Experimental Section For our studies we used a Nanoscope I with a homebuilt scan head. The microscope was modified to permit investigations in an electrolyteunder potential control. Both workingand auxiliary electrodes were evaporated on a green mica disk of half an inch diameter, as described earlier.I2 One part of Figure 1shows their concentric shape with the counter electrode partially encircling the working electrode. The reference electrode was fixed to the xyz scan tube in a holder similar to that which holds the tip. To keep the electrochemical background current through the tip smaller than the typical tunneling current of 1to 6 nA, we used commercial glass-coated Pt/Ir tips.ls (Typical background currents were less than 1%of the current in the electrochemical STM cell.) A drop of electrolyte was placed on this assembly. Up to 0.2 mL of solution could be confined on the sample disk by painting a rim of paraffin around the electrodes (see Figure 1). Thus, evaporation was not a seriousproblem; a working period of about 2 h could easily be reached. The commercial Nanoscope retracts the tip, when the "tunneling" current through the tip exceeds 10 nA. As we thought that in general it is more desirable to work with a lower limit of 3 nA, some extra care was necessary for working in solutions. Proper grounding of the head was important. In the Nanoscope, the tunneling voltage is applied to the entire base unit. In humid environments, this voltage is modulated by the X and Z piezo voltages. The only way we could operate reliably in solution was to apply the tunnel voltage directly to the working electrode (sample) and to ground the rest of the instrument. Moreover, we put a quartz wafer with a grounded copper foil below the piezo tube to screen the electric field of the piezo electrodes from the electrolyte. We also found it critical to disable the absolute value amplifier in the base of the Nanoscope for work in solution. For in situ imaging of electrodes in an electrochemical cell, a further important step is the control of the potential between the reference electrode and the working electrode. In order to interface with the Nanoscope tunneling current preamplifier without modifications, we run the working electrode a t the tunneling potential Vv,, instead of ground as is usual in an electrochemical cell. Accordingly, our potentiostat (Figure 1) was designed to take a floating tunneling bias on the working electrode into account as shown by the functional block diagram in Figure 1. The circuitry to the right of the dashed line is identical with a standard Wenking style potentiostat except for the polarity (16) Produced by Longreach Scientific Enterprises.
gration of the electrochemical cell into the Nanaoscope I. It shows also the concentric shape of working and counter electrode and thedifference amplifier to keep the voltage between reference and working electrode equal to the input voltage (see also Figure 1). of the diff-amplifier of the output current. The additional diffamplifier with gain 1 to the left of the dashed line keeps the potential difference between working and reference electrode constant a t V , in the presence of a floating tunneling potential Vb,,. The potential on the reference electrode is then Vb,, - Vw: (seeFigure 1). The polarities of the twodiff-amplifiersare chosen to be in agreement with electrochemical convention. As in the standard Wenking potentiostat, the electrochemical current is measured in the counter rather than in the working electrode. In our case that is still justified on the grounds that the current to the tunneling tip is 3 orders smaller than the total electrochemical current and can be neglected. The whole experimental setup of the electrochemical STM is shown schematically by the block diagram in Figure 2. We have investigated the Au(ll1) surface in the following solutions: 0.01 M KC1,O.Ol M, 0.03 M, and 0.05 M AgN03; 0.01 M NaNOs; 0.01 M and 0.05 M AgClO,. For the experiments in Ag salt solutions we simply used a Ag wire as reference electrode, while a Ag wire coated with AgCl just before starting the STM experiment was a convenient reference in the case of KC1. The AgCl coating was done by holding the Ag wire in NaCl and applying a voltage of 6 V (positive pole to the Ag wire) for a few seconds. Both kinds of reference electrodes were immediately immersed in the solution. When we quote voltages in the text and figures, they are all related to the reference electrode used in the given experiment. The difference is 0.45 V between Ag/AgCl and a Ag wire (more positive for Ag/AgCl) as indicated in Figure 9. The scan rate for all experiments was 2 mV/s. Our working electrode area was 0.07 cm2.
Results and Discussion Figure 3 showsan typical example for 0.01 M KCl. There is a lateral drift due to thermal effects, which was corrected between the images taken at 1413 and 14:14and just before the last image shown. First at 0.2 V (vs Ag/AgCl) the Au(ll1) surface is atomically flat, showing some monoatomic steps. (The steps appear as faint curved lines in the images, which are presented on a rather coarse scale in order to show the roughness which will grow on the oxidized surface.) At 0.5 V the first indications of a rougheningoccur. (The large hole is a preexisting structure that has drifted into the field of view; the roughness which we discuss begins as the several white dots appearing on the right-hand side and center of the image.) A t 0.6 V one can clearly see the roughness growing in time over 3 min, from 14:14 to 14:17. The thermal drift at 14:13 was corrected by 14:14, so that 14:09 and 14:14 show identical fields of view. The small defect which shows as a dark crescent two-thirds of the way to the right a t the very top of the image is visible in both the 14:09 and 1414 data, and one can see that the growing roughness has rendered the underlying step structure indiscernible. Somewhat
Reversible Oxidative Roughening of Au(ll1)
Langmuir, Vol. 7, No. 9, 1991 1983
Figure 3. Reversible roughening of Au(ll1) in 0.01 M KC1. Potentials are given with respect to a Ag/AgCl reference. Note that the curved lines, which are atomic steps, are visible in the first, second, and sixth images and how roughness increased over 3 min at fixed potential.
surprisingly, the original step structure is observed nearly unchanged after the roughening is reversed by decreasing the potential to 0.2 V. One can compare the image a t 14:32to those at 14:09 or 14:13to see that the original step structure is restored. This effect is highly reproducible. Images below 0.2 V in KC1remain unchanged for indefinite periods, but the surface can be roughened a t any time by bringing the potential oxidative to 0.5 or 0.6 V. In fact, the image a t 14:32 was obtained after a second potential cycle during which the surface was again roughened at 0.9 V and restored a t 0.2 V. Our second example (Figure 4) shows almost the same behavior for 0.01 M AgN03. At 0.2 V (now vs Ag wire) the Au(ll1) surface shows a monoatomic step structure and at 0.4 V one can again watch the growth of roughness vs time. In this example the roughening starts at the step edges, but that seems not to be a general rule. In the image at 13:17, which shows the highest roughness, there is evidence of a multiple tip effect, but nevertheless the occurrence of a high structure must be real. Decreasing the potential to 0.2 V smoothed the surface but did not completely restore it. In this examplewe could only restore the original step structure by scanning down to 0 V and up again to 0.2 V. That is not generally the case: we also found examples where the roughening starts a t 0.55 V, i.e. at higher potentials, and where the old surface structure could be restored by simply decreasing the potential to 0.2 V (see Figure 6). Finally we consider our third electrolyte AgC104. The results of one of these runs are shown in Figure 5. This is one of a few examples where we could observe the same area before and after adding the electrolyte. The image on the top of the left side of Figure 5 is an area of 3800 by 3800 A imaged in air. The following images in solution were taken over an area of 2000 by 2000 A. (They are a subset of the area imaged in air.) Furthermore, these
images show that the roughening begins a t a potential of 0.2 V (vs Ag wire). We could stop the roughening and smooth the surface somewhat by decreasing the potential to 0.1 V, but the surface does not return to its original state. After the last image of Figure 5, we roughened the surface further by increasing the potential again to 0.2 V. The above examples are not single occurrences, in each of several experiments with each type of solution we found the same behavior. The exact voltage a t which roughening began varies over a range of 200 mV in different experiments with the same solution. There appears to be a dependence on the sample or even on the site of the sample one is watching with the STM. However, there is a clear correlation: the lower the potential is when roughening starts,the lower is also the potential to restore a flat surface. Thus in the above example of AgClO4 the surface never gets flat again, while we have other examples in the same solution with a roughening starting point of about 0.3 V where we obtained a completely flat surface a t 0.1 V after scanning the potential through 0 V (vs Ag wire). In the case of the Ag salt solution the zero potential is a natural limit for studyingthe roughening processes in the oxidative range, because at negative potentials the Ag bulk deposition starts. There are three more important points. We have verified that we are watchingthe same area of the substrate throughout the experiment. Some deep structure like a hole or a groove is more advantageous than a flat step structure, as it is a good natural marker. Such a case is shown in Figure 6 for 0.01 M AgN03. The roughening is visible beside some deep structures, which we believe to be grain boundaries. Secondlythis roughness is a real hill structure and not a hole structure. That is demonstrated in Figure 7. In this series of images in 0.03 M AgN03, one can clearly see how small hills merge together to wider hills of a slightly larger height. Finally, the roughness
1984 Langmuir, Vol. 7, No. 9, 1991
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Figure 4. Reversible roughening of Au(ll1) in AgNOs (here 0.01 M). Potentials are given with respect to a Ag-wire reference. Note that roughening starts a t atomic steps.
Figure 5. Reversible roughening of Au(ll1) in 0.01 M AgC104. Although the C104 ion is more inert than either the NO3 or C1 ions, oxidative roughening starts at 0.2 V (vs Ag wire).
grows above a threshold potential (as already discussed), and after several minutes above this potential, the growth stops. The surfaces shown in Figure 8 were stable for 15 min at the potentials indicated. Note that this final roughness in Figure 8 depends on solution concentration. One notices easily that the total volume below the hills
generated in AgN03 gets larger with increasing concentration. The same effect was also found in AgC104 and was not investigated for KCI. Although there may be an uncertainty in the absolute value of the roughness height due to some anomalous tipsurface forces, we do not believe that this uncertainty will
Langmuir, Vol. 7, No. 9,1991 1985
Reversible Oxidative Roughening of Au(ll1)
Figure 6. Roughness of Au( 111)as a function of potential in 0.01 M AgNOs. While atomic steps are not visible in these lower resolution images, one sees the same deep grooves in all images throughout the redox process. This proves that we are imaging the same area throughout the roughening process. Potentials are with respect to a Ag-wire reference.
exceed more than 10 A. Moreover the literature contains microbalance and coulometric investigations on Au( 111) in perchlorate solutions that indicate a roughness of about 40-60 k 1 5 In this work also SEM images of roughened surfaces are shown. From these images one cannot extract the height but can obtain an average diameter of the hills of about 300 A,which is, again, in good agreement to our STM results. As all these early studies were done after removing the surface from solution at potentials where the surface should be rough, we did a similar experiment with the STM. We removed the surface from 0.03 M AgN03 at 0.55 V (vs Ag wire) after seeing the roughening "in situ" and saw that the roughness persisted in air. To complete the experiment, a new solution was added with 0.2 V potential applied to the potentiostat. The surface once again smoothed itself. As mentioned above we believe that the local properties of the surface determined the exact start potential for roughening, but there seems to be a lower limit for that potential which can be extracted from corresponding voltammograms. We want to reinforce one obvious, but significant, difference between our voltammograms and STM images. The STM images show a very local feature, which is not necessarily a uniform property on the whole surface, while the voltammogram is an average measure of the whole surface. Figure 9 presents two voltammograms for each type of solution discussed above. On the right column a rather wide voltage range is shown, while the voltammograms in the left column side correspond to the voltage range used during the STM experiments. The KC1 voltammograms are shifted on the page by 0.45 V to lower potentials with respect to the ones in Ag-salt solutions. That shift takes into account the potential difference between the different reference electrodes used in the two cases (VAg/AgCl- V A g = 0.45V, where we ignore
the 30 mV shift between a 0.01 M and 0.05 M solution as negligible). Thus one can readily see by the dashed line on the far right of Figure 9 that the current increase due to oxygen evolution occurs a t the same potential relative to a standard reference potential (0.95V vs Ag wire and 1.4 V vs Ag/AgCl). In contrast to the Ag salt solutions, KC1 shows a remarkable peak around 1 V (vs Ag/AgCl). This peak is well-known and due to the formation of soluble AuIC12and Au111C14-at 0.9-1.0 V.I7 This dissolution process is stopped at about 1.3 V due to the formation of a passivating oxide. If the potential in 0.01 M KCl is scanned only from 0 to 0.4 V (vs Ag/AgCl), the current is close to zero (left on top of Figure 9). However, there is a very small positive current above 0.4 V which is more than 2 orders of magnitude smaller than that in the main peak around 1V. This structure occurs at the same potentials as the roughening observed by the STM studies. Keeping in mind that the voltammogramgives an average measure of the whole surface, it is not surprising that in some examples (as mentioned above) the local roughening measured by STM starts at a potential slightly higher than 0.4 V (vs Ag/AgCl). However, we did not find any example where the roughness starts a t a potential lower than 0.4 V. That means that the onset of a positive current in the voltammogramsgives a lower limit for the potential at which local roughening starts. Due to the high electrochemical current in the dissolution peak around 1V, the background current through the tip becomes so large that imaging is no longer possible above 0.9 V. If the potential is kept a t 1 V for 1 min or longer, the roughening observed by STM is no longer (17) Heumann, T.;Panesar, H. S. 2.Phys. Chem. (Leipzig) 1965,229, 84.
1986 Langmuir, Vol. 7, No. 9, 1991
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Figure 7. Wire frame presentation of data taken in 0.03 M AgNOBas function of time and potential clearly showing how a wide hill grows by the merging of smaller hills. Potentials are with respect to a Ag-wire reference.
reversible. That is easily understandable, because gold is taken out of the surface to the solution and the reduction process will not bring the gold atoms back to the original sites. This implies that the reversible roughening which we observe below the dissolution peak (0.9 V) does not involvedissolution of Au atoms. We believe that the small current between 0.4 and 0.8 V is due to a (previously undetected) process like the adsorption of C1-ions, perhaps combined with the formation of neutral gold chloride, which remains pinned on the surface. Let us now turn to the voltammograms of the Ag salt solutions. A dissolution peak like that in KCl does not occur in either of the two Ag salt solutions. However, there is again a small positive current (like the one in KCl above 0.4 V) and this is also the lower limit of the potential at which roughening is observed by STM. Figure 9 shows that this small positive oxidative current begins at a lower potential in perchlorate (0.05 V) than in nitrate (0.15 V). This correlates with our STM observations that the roughening on perchlorates starts earlier than in the nitrates. The large roughness amplitude that we have seen in every solution studied cannot be understood by a monolayer oxide (and in fact the roughening occurs at lower potentials than one would expect for forming gold oxide, e.g. =1.3 V vs Ag/AgCl in 0.01 M KCl). We do not know what type of reaction is going on, but we know that it is a slow process and that this process produces only a very small current in the cell. Thus it should be an adsorption taking place very slowly (over minutes). We were rather surprised by the amplitude of the roughening upon which we report here, as well as by its reversibility. A skeptic could in principle claim that the
roughening in the KC1 case is due to the formation and subsequent dissolution of gold chloride. (We actually believe that this is the case for the irreversible part of the roughening in KCl but that a t lower potentials there is some other phenomenon, as it seems very unlikely that gold dissolution would be fully reversible.) This same skeptic could also claim that the phenomena we saw in silver nitrate and silver perchlorate were nothing more than the deposition and subsequent stripping of silver. To refute this “virtual” skeptic we make the following points: 1. Our voltammograms clearly show the potential at which bulk silver deposition occurs. We are well above that potential for these experiments. 2. We have tried these experiments also in sodium nitrate; the roughening still occurs at roughly 0.5 V vs Ag/ AgC1.
Summary We have observed roughening of Au(ll1) surfaces at oxidative potentials by “in situ” STM studies in an electrochemical cell. We have observed this roughening many times, each on surfaces immersed in KC1, AgN03, AgC104 and NaN03. The roughening persists after the surface is removed from solution but can be removed by reimmersion and application of appropriate potential. Our results (Figure 6 in particular) show that material is added to the solid surface during the roughening, as opposed to merely being redistributed. This roughening is reversible except if the potential is scanned into a region where dissolution of the surface takes place (as it does via the formation of AuClz and AuC14- in 0.01 M KCl). For the
Reversible Oxidative Roughening of A u ( l l 1 )
Langmuir, Vol. 7, No. 9, 1991 1987
0.05 M AgNOJ
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Figure 9. Voltammograms taken in the electrochemical STM
cell for all three salt solutions investigated in this work by STM technique. The voltammogramms for KCl are shifted on the page with respect to the Ag salt solutions to correct the potential difference of the two reference electrodes used. The voltages a t which small positive currents appear correspond to the voltages a t which roughening was observed with the STM. A current of 10 p A corresponds to 150 pA/cm2.
Figure 8. Concentration dependence of roughness. Final roughness developed increases with increasing concentration demonstrated here for the example of AgN03. Potentials are with respect to a Ag-wire reference.
reversible reaction the restoration of the original surface is so perfect that even the monoatomic step structures which have been observed before roughening are visible nearly unchanged (Figure 3). The absolute height of the rough structure is 40-100 A, and we found in AgN03 that the roughness generated increases with increasing electrolyte concentration. The chemical process leading to the roughening is still unknown, but we can correlate the voltage for the onset of roughening in the STM images with the voltage in the voltammogramsat which the current
becomes oxidative. We believe that this is the first observation that more reactions take place on the surface than clearly indicated by cyclic voltammetry. This roughening process takes several minutes and could be observed very nicely with the STM as a function of time.
Acknowledgment. We thank G. Borges for preparing the samples and other technical help, M. Lazaga for help with electronics, D. Miller and E. Schuler for performing ESCA measurements, and P. Alexopolous, J. Behm, G. Blackman, A. Gewirth, R. Hoyt, M. Philpott, J. Salem, and G. Singh for useful discussions. Partial support for E.H.M. was provided by IBM Germany. Paul Hansma provided the Nanoscope prototype scanner that we used in this study, purchased under his NSF Grant DMR8613486 with additional support from the Office of Naval Research.
