Self-assembly of molecular superstructures studied by in situ scanning

J. Phys. Chem. 1993, 97, 910-919

910

Self-Assembly of Molecular Superstructures Studied by in Situ Scanning Tunneling Microscopy: DNA Bases on Au( 11 1) N. J. Tao, J. A. &Rose,

and S. M. Lindsay' Department of Physics, Arizona State University. Tempe, Arizona 85287- 1504 Received: July 2, 1992; In Final Form: October 5. I992

We have studied the DNA bases adenine, thymine, guanine, and cytosine adsorbed onto Au( 11 1) using in situ scanning tunneling microscopy (STM), atomic force microscopy (AFM), and cyclic voltammetry. Adenine, guanine, and cytosine adsorbed spontaneously onto the electrode with the counter electrode disconnected and yielded stable images over a range of electrode potentials. Thymine was not adsorbed until an electron-transfer reaction occurred at +0.4 V (SCE). Adenine and guanine formed polymeric aggregates in which the bases stacked with repeat distances of 3.4 i 0.2 A (adenine) and 3.3 f 0.3 A (guanine). Adenine aggregates aligned along the Au[lTO] directions and also formed a complex that decorated the path of the p X .\/3 (p Y 23) reconstruction. Guanine aggregates were disordered with respect to the underlying gold. Cytosine formed an oblique lattice with a = 10.5 f 0.2 A, b = 9.5 f 0.2 A, and y = 103 f 3 O , oriented at 30° to the Au[liO] directions. It is fitted well by a model in which every other molecule inverted to bond 0 2 to N4 and N1 to N3, followed by N4 to 0 2 and N 3 to N1. Thymine also formed an oblique lattice with a = 6.5 f 0.5 A, b = 7.1 i 0.5 A, and y = 105 i 5 O . The STM corrugation measured over adenine, cytosine, and guanine adsorbates was 1 A but only -0.1 A over thymine. Thymine contrast also changed with the electrochemical potential of the electrode. We discuss the possibility of a connection between electrochemical electron transfer and STM contrast.

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1. Introduction

Spontaneous and charge-induced formation of monolayer adsorbatesof organicmolecules on solid surfaceshave been widely studied for both fundamental' and practical reasons.2 The scanningtunnelingmicroscope(STM) makes molecular resolution of these adsorbates possible for the first time. It has been used to study organic adsorbates on metal surfaces in v a ~ u u m in ,~ liquids,'-l2 and in ambient conditions in air.I3-l6 Rather special conditions are required to image such adsorbates, particularly outside an ultrahigh-vacuum environment. The molecules must form a stablelayer which resists the forcesencounteredin scanning probe microscopy17J*while all contamination must be so weakly bound that it is not imaged. Liquid crystals often satisfy these criteria,particularlywhen the imaging is carried out in a protective layer of the liquid phase.'-" Another approach is to carry out a chemical reaction which binds the moleculeto the substrate,'3-16 and it relies on the strong tip-substrate interaction to %weepn unwanted molecules away. In general, these experiments are quite difficult to carry out in a reproducible way. Even if the adsorbate remains stable, it is unlikely that the tip will do so in an uncontrolled environment. We have been exploring electrochemical methods for control of molecular adsorbates for STM imaging.19 Modern electrochemical experimental methods permit a remarkable degree of control and characterization of interfacial properties.20J We have recently shown that DNA molecules can be reversibly adsorbed and imaged in situ on gold electrodes.22-24 The use of electrochemical methods in scanning probe microscopy has several advantages. Firstly, contamination can be controlled in a way that is not possible in ambient conditions in air. Secondly, control of the surface potential permits the interfacial free energy to be varied over a large range. Finally, by studying the electrochemical characteristics of the sample in situ as STM imaging is carried out, it is possible to study both the macroscopic and microscopic aspects of electron transfer in the same experiment. The DNA bases are a particularlyinterestingsubject for study. Pure preparations are available, and their properties have been 0022-3654/58/2097-0910$04.00/0

studied extensi~ely.~~ A rapid method for sequencingDNA based on direct imaging of its composition would have important technological and medical implications.26 This might appear to be a hopeless goal because the structural differences between thymine and cytosine, for example, are minimal (see insets in Figure 3). However, the differences in chemistry25 and, inter alia, electronicpr0perties2~.2* are significant,a point we illustrate with images of individual molecules in this paper. Several groups have used the STM to image the DNA bases, usually adsorbed onto graphite2e31(although Heckl et aI.3O have also used Ma92 as a substrate). Graphite can be difficult to work with because of a number of substrate artifacts that are poorly understood at ~resent.'~-3~ These add to the difficulty of interpreting images, especially those obtained in ambient conditions in air. Here, we report a study of the bases adsorbed onto clean Au( 11 1) substrates under electrochemical potential control. In these earlierSTM studies29J'Jofthe bases,droplets of adenine or guanine solutionswere placed onto graphiteor MaSzsubstrates which were baked before imaging. Water moleculesplay a critical role in the structures that are formed by these molecules. For example, it is well-known that the purines will spontaneously assemble into long base stacked chains in aqueous solution as a consequence of their hydrophobicity and dispersion interactions between the heterocyclic r i n g ~ . * ~The J ~ ,adsorbates ~~ studied to date do not demonstrate this effect,formingordered planar arrays in which the molecules lie flat on the substrate.29-3' This is, perhaps, not surprising in experiments in which the bases were dried onto the s ~ b s t r a t e s . ~ Srinivasan ~ . ~ ~ et al." have, independently, developed an approach similar to ours. They imaged guanine that was adsorbed onto graphite and scanned in situ under potentialcontrol using NaCl electrolyte. The guanine was found to adsorb into an ordered array in which the molecules lay flat on the graphite basal plane. The results we report here for the Au( 11 1) surface are quite different. We find that the purines formed aggregates that are composed of the base stacked Ypolymersw known to exist in solution.36 The pyrimidines (where base-stacking interactions are ~maller25.~~) form planar arrays in which the molecules appear to lie flat on the substrate. Our work has been confined to monolayers formed near the potential for Q 1993 American Chemical Society

Self-Assembly of Molecular Superstructures

The Journal of Physical Chemistry, Vol. 97, No. 4, 1993 911

P

Figure 1. Typical appearance of Au( 1 1 1) substrates in NaC104 electrolyte alone: (A) shows single atom steps (height is 2.5 A); (B) shows "stripes" owing to the p X f i reconstruction. The height of the stripes is 0.1 A. (C) shows hexagonal packing of atoms: the lattice constant is 2.9 A and the corrugation is -0.05 A. All images are raw data, obtained at a substrate potential (V,) of 0.24 V (SCE) with a tip bias (V,)of -100 mV and tunnel currents (It) of 0.1 nA (A, B) and 1 nA (C).

zero change (pzc) for our electrode/electrolytesystem. We have also carried out some limited studies of the effects of sweeping surface potential. Multilayers are often invisible to the STM2) because the tip will push away insulating adsorbates in order to tunnel to the substrate. For this reason, we have undertaken some parallel studies using an atomic force microscope (AFM). Most electrochemical studies of the adsorption of the bases have been carried out on the dropping mercury electrode and are consequentlyrestricted to negatively charged s ~ r f a c e s . 3Some ~~ studies have been carried out on graphite electrodes which can be operated at quite positive potential^.^^.^^ We have developed methods for fabricating clean and flat Au(ll1) surfaces43which we have characterized in phosphatebuffers for DNA adsorption st~dies.~31~~ This procedure is complicated by adsorption of the P042- ion so, in this study, we use perchlorate electrolytes, for which anion adsorption is smaller. Au( 11 1) has been extensively studied in HC10444-46 and, to a lesser extent, in NaC104.47 The Au( 111) surface is unique among the close-packed noble-metal surfaces in that it reconstructs into a p X 6 (p 23) struct ~ r only e ~ recently ~ observed by STM in ultrahigh vacuum ( U ~ V ) . ~ The ~,~O same structure has also been observed in electrolytes using STM51.52and AFM,53and the dynamicsof the transition between the p X 6 reconstruction and the 1 X 1 phase have been studied by STM.54 The adsorption of cationss5.56 and anions23~57~58 on this surface has also been studied by STM and AFM.23 We have studied thecyclicvoltammetryof adenineand guanine in water as part of our STM studies of DNA.23 Unfortunately, voltammetryof these bases was carried out without a supporting electrolyte. We have therefore studied the bases adsorbed onto Au( 111) p X 6using a NaC104 supporting electrolyte. 2. ExperimentalProcedmes 2.1. Microscopy and SamplePreparation. Our procedure has been described in detail elsewhere.23 Briefly, we prepare Au(1 11) substrates by epitaxial growth of Au on heated mica under uhv conditi0ns.~3 Extended baking of the mica is required prior to deposition of gold, and in this work we have reduced the temperature for this step to 325 K. This results in smaller atomically flat terraces (-1000 A in extent) but eliminates contamination caused by mica degradation. Substrates are transferred to a glass container for storage under argon with a few minutes of exposure to laboratory air. This undoubtedly contaminates them,59 but we have not been able to discern any effects from such exposure as long as it is less than about 20 min. The STM base (from a modified NanoScope I1 from Digital InstrumentsInc.) is loaded under a laminar flow hood using the

cell we have described elsewhere.23 Further exposureof substrates to laboratory air prior to being covered with electrolyteis about a minute. We find that essentially all of the substrate is covered with atomically flat terraces of the sort illustrated in Figure 1A. Therefore, the effective area (for electrochemical purposes) and the measured macroscopic area are equal to within a percent or so. Furthermore, almost all of the terraces have the p X 6 structure that is found on Au( 111) in uhvSoas long as they are examined negativeof the potential of zero charge (PZC).~~V~O This is illustrated in Figure 1B. The corrugationof this reconstructed surfaceis only 0.1 A, so any bound contaminationwould be obvious in the images. In experiments, we find that adsorbates tend to lift the recon~truction.~3~5~~5~ Weakly bound insulating adsorbates may not be imaged with the STM23 so we also studied these surfaces by AFM, finding that most terraces are reconstructed at, or negative of, the pzc.53 We use pfo.81r0.2tips coated with Apezion wax and tested as described e l ~ e w h e r e . ~These ~ . ~ ~tips give no measurable leakage current on the Nanoscope display, even when the cell is driven into gas evolution (in excess of 1.5 V on the SCE scale). Typical operating conditions for the STM were currents of 50-100 pA with the tip biased 0.1 V negative with respect to the substrate, but these were optimized for particular images as described in the figurecaptions. Wealtemated between tipcontrolwith respect to the substrateand control with respect to the reference electrode, finding no systematic difference in images. A freshly prepared set of electrodes, new substrate, and new tip were used for each experimental run. We use a silver wire as a quasi-reference electrode.23 It is tested ex situ in all the solutions used in this work (both aerated and deaerated) against an Ag/AgCl/KCl reference and is found to be stable and reproducible to within about 20 mV. For the solutions used here, 0 V on the silver wire quasi-reference scale is equal to 0.24 f 0.01 V on the SCE scale. The SCE scale is used throughout this paper. Solutions are made with water from a Bioresearch Grade purification system from Bamstead fed with campus distilled water. They are sparged for 3 h with 99.999%pure N2 gas prior to use. The small volume of the sample cell makes it difficult to control dissolved gases after the sample is transferred into the cell. There is always some indication of them in voltammetry carried out in the STM cell (see below). In STM experiments, we used both pure water and NaC104 solutions. NaC104 was purchased from Johnson Mathey and had a maximum C1 concentrationof 30ppm. Adenine, thymine, guanine, and cytosine were crystalline Sigma Graded bases purchased from Sigma Chemical Corp. and were used without further purification. Cytosineand thyminewere prepared as 10mM solutionsin water.

Tao et al.

912 The Journal of Physical Chemistry, Vol. 97, No. 4, 1993

Guanine and adenineare very sparingly solubleand were prepared as saturated solutions. The concentrationsof these solutionswere determined to be 3 mM (adenine) and 0.1 mM (guanine) from measurements of optical density. The substrates were surveyed in water or 0.1 M NaC104with the cell half full (40pL) in order to determine that the surfaces of the electrodes were of the quality shown in Figure 1. A further 40 pL of the stock solution of the appropriate base was added to the cell, and the substrate was brought to the desired potential and imaged. We alsocarried out experiments in which the surface was imaged continuously as the solutions of the bases were added. Adenine, guanine, and cytosine adsorb on contact with the substrate with the cell open circuit. Thymine does not adsorb until the substrate is brought above the pcz (which is around 0.24 V vs SCE for clean Au( 11l)62-63).Thymine adsorbates formed in this way remained stable when the substrate potential was lowered to the pzc, desorbing when the potential was taken negative of the pzc. (Throughout this paper pzc means the potential which is the pzc for the clean surface.) The concentrations of adenine, cytosine, and thymine were varied by about a factor of 5 with no observable change in the STM images. Atomic force microscopy was carried out in the fluid cell supplied by Digital Instruments, Inc. We used cantilevers of 0.12 N/m and minimized contact force to between 1 and 10 nN. The AFM cell is harder to load, resulting in greater exposure of the substrate and electrolyte to air. The quality of the images was generally much poorer than those obtained by STM. For this reason we show only STM images in this paper, discussing the general features of AFM images where appropriate. We have chosen not to present data with an accompanying height scale because this can be very misleading. Instead, we quote the values measured for certain features (by taking a trace acrossthe imageusing the NanoScope software) in the appropriate figure captions. We discuss this point further when we analyze the corrugation of images of the bases. 2.2. Electrochemistry. Characterization of the samples was carried out in situ in the STM cell using a freshly prepared set of electrodesand a new substrate for each run. Our goal was to study the electrochemistry under the same conditions in which STM imaging was carried out. As mentioned above, the small volume of the STM cell makes it difficult to avoid contamination by dissolved gases (mainly 02).We used measurements of the oxidation and reduction of the gold electrodeas a sensitivemethod for characterizing the severity of the problem.4s47 Data for oxidation reactions are taken after multiple sweeps when the voltammograms are stable. STM images and interfacial capacitance data are taken on the first sweep only, working near the pzc. Near the pzc, and on the first part of the first cycle of the cell potential, most of the surface is reconstructed into the p X fistructure. The p X f i to 1 X 1 transition is hysteretic,s4 and because we work on surfaces that have been taken from uhv and are reconstructed to begin with, this means that we usually work with the reconstructed surface up to much higher potentials than would be the case if a electrochemical pretreatment of the surface is used. It is usual practice to pretreat bulk crystals by repeated oxidation and reduction, and this lifts the reconstruction, which is only restored if the surface is taken negative of the pzc,52,54,60 We first examined the current-voltage characteristics of the cell at potentialsbelow the potential for OH- bindi11g~~945 or rapid liftingof the recon~truction.5~ Dissolved gases limited the extent of the negative part of the cycle, and most data were taken positive ofthe pzc and into the region wherecharge transfer from adsorbed C104Interfacial capacitance was measured by varying thesweepratefrom -10to -1000mV/s. Wehavealsocollected some data using ac analysis of the cell impedance,23finding reasonable agreement with the results of the simpler procedure.

‘i F ‘

Figure 2. Cyclic voltammograms obtained in the STM cell for 0.1 M NaC104 on Au(l11). Data are taken at 50 mV/s scanning between -1 and + 1.4 V (SCE) for (solid line) solution deaerated prior to loading and (short-dashedline) nondeaeratcdsolution. Long-dashed line shows data taken at 500 mV/s prior to taking substrate above 0.5 V (SCE).See text for discussion of peaks.

3. Results 3.1. Electrochemicnl Aspects of Adsorption. Cyclic voltammograms (sweep rate = 50 mV/s) for 0.1 M NaC104are shown in Figure 2. Experimental conditions are listed in the caption. An example of data obtained (at a sweep rate of 500 mV/s) prior to sweeping above 0.5 V (SCE) is shown as the large-dashed curve. Similar data are shown for solutions of the bases in 0.05 M NaC104in Figure 3. We have also obtained data (not shown) for various concentrations of HC104. 3 . 1 ~ .Sodium Perchlorate on A u ( l l 1 ) in the STM Cell. Voltammetric data for 0.1 M NaC104 on Au( 111) have been published by Hamelin et aI.4’ The results are almost identical to those obtained in the well-studied case of HC104,434Swhen plotted on a constant potential scale (RHE). One exception is the first major cathodic peak (owing to reduction of the AuO2 surface) which is shifted about 0.34 V negative of its relative position in the acid en~ironment.~’Data obtained in our STM cell are almost identical to those published by Hamelin et al.47 for NaC104 with the exception of the potential at which this reduction occurs. The anodic peaks (labeled 1 and 2 in Figure 2)correspond to the two steps in the oxidation of the gold surface (complicated by C104-adsorption&). The cathodic peak labeled R occurs at the same potential (and corresponds to the same charge transfer) as the peak found in HC104 (i.e., it is shifted 0.34 V positive of its position in deaerated solution^^^). Although the measured pH of our NaC104 was near 7, the solution is unbuffered. This might exaggerate the effect of dissolved COz on the pH near the electrode surface. This view is supported by comparing the voltammogram of a solution which has been deaerated before being placed in the STM cell (solid curve, Figure 2) with that of one that has not (short-dashed curve in Figure 2). The peak labeled R is smaller for the solution that had been deaerated, and a new peak (labeled 3) is found where the oxide reduction occurs in fully deaerated NaC104.47 The large-dashed curve showsdata taken by sweepingat 500 mV/s (prior to raising the potential into the OH- adsorption region near 0.5 V). Averaging data from the full range of sweep rates yields a value for the double-layer capacitance of 46 f 15 pF/cm2, consistent with the results of Hamelin et al.47 The small peaks (labeled 4 and 5) are believed to be caused by an electron-transfer process involving adsorbed C104-.& 3.16. Solutions of the Bases. Voltammograms for solutions of the bases that had been deaerated prior to injection into the STM cell are shown in Figure 3 (solid curves). These data were obtained at 50 mV/s and recorded after they became stable on repeated cycling. All samples were scanned from -1 to +1.4 V (SCE) and are plotted on the same current-density and voltage scales. Data are also shown for a rapid (500 mV/s) sweep before the first cycle of the cell into the oxidation region (dashed curves). Similar curves were obtained for each solution for sweep rates between 10 and 1000 mV/s and showed (in the limited potential


Figure 3. Cyclic voltammograms for (a) 1.5 mM adenine, (b) 0.05 mM guanine, (c) 5 mM cytosine, and (d) 5 mM thymine in 0.05 M NaC104, deaerated prior to insertion in STM cell. Solid curves are for stable cycles between -1 and +1.4 V vs SCE (50 mV/s). Dashed curves are scanning at 500 mV/s prior to raising potential above 0.5 V. Shaded areaon (d) shows integration used toestimatecharge transfer. Structures with atom labeling convention are inset to left of voltammograms. region scanned) that the current was proportional to the scan rate, as expected for capacitative behavior (Le., double-layer capacitance or rapid electron-transfer processes). The corresponding currents at sweep rates of 50 mV/s were an order of magnitude smaller than those shown as the dashed lines in Figure 3. Comparisons of the two sets of data show that major changes occur after cycling to high potentials. Capacitance data based on runs taken at various sweep rates are given in Table I along with values for the potential for the main reduction peak (labeled R in Figure 3) and for the charge transferred in that peak. The adenine data (Figure 3a) are similar to those obtained in the supporting electrolyte alone (Figure 2) although the capacitance in the double-layer region (28 f 3 pF/cm2) is significantly reduced, indicating adsorption of adenine. (The decreased capacitance results from the smaller dielectric constant of the organic moleculecompared to that of water.) The charge passed in the reduction peak (labeled R) is similar to that found in the supporting electrolyte alone, although a comparison of Figures 2 and 3a indicates increased oxidation current at the positive extreme. It appears that, although the adenine is adsorbed, it is easily displaced by small ions at potentialswhere reactionsoccur, so that the surface behaves much like the gold in NaC104alone. Although the "R" peak is found at similar potentialsin cytosine and thymine solutions (Figure 3c,d), much more charge is passed than for adenine, exceeding (in the "R" peak) that required for a two-electron reduction on Au( 111) (444pC/cm2). This implies that some soluble species is formed on the positive going sweep where large anodic current densities are observed. Prior to these reactions, the double-layer capacitance in the presence of cytosine is similar to that found for adenine solutions,once again implying adsorption.

The Journal of Physical Chemistry, Vol. 97, No. 4, 1993 913 The "double-layer" capacitance for thymine is anomalously large, and inspection of the rapid sweep in Figure 3d shows that this is because of a partly reversible electron-transfer peak at about 0.4 V (SCE). There is a background which showshysteresis characteristic of a nonreversible Faradaic process, and this complicates the estimation of the charge transfer. We have integrated the areasof both the anodicand cathodic peaks (shaded in Figure 3d) after subtracting the small true double-layer contribution (-40 pF/cm2 X dV/dt) and a background that is assumed to be linear (as shown in Figure 3d). Analysis of data taken between 10 and lo00 mV/s yields a charge transfer of 54 i 25 pC/cm2 (where the range is mainly due to the difference between the anodic and cathodic peaks). Our STM data (see below) indicate a thymine coverage such that this corresponds to approximately one electron transferred per adsorbed thymine molecule. Guanine shows a number of changes between the first (not shown) and subsequent cycles of the potential. The first cycle is similar to the data shown for adenine (Figure 3a). The "R" peak drops down to 0.4 V after the first time that the cell is taken negative, and extra structure appears throughout the voltammogram (Figure 3b). The capacitance (prior to sweeping the potential above 0.5 V) is similar to that found for the supporting electrolytealone. The STM images (Figure 5 ) indicate that the surface is not fully covered, consistent with this observation. The guanine concentration is low enough (0.05 mM) that a monolayer is unlikely to diffuse onto the interface in a reasonable time. 3.2. STM Images. 3 . 2 ~ .The A u ( l l 1 ) Surface in 0.1 M NaC10,. Images of a typical electrode surface at 0.24 V (SCE) were shown in Figure 1. Figure 1B showsthe characteristicstripes due to the stacking faults associated with the p X reconstruction. They occur in pairs separated by about 44 A and are repeated with a period of -65 A. The corrugation is 0.14.2 A. This surface is maintained up to about 0.4 V (SCE) although a gradual lifting of the reconstruction is observed if the substrate is left above the pzc for long enough.s4 Rapid lifting of the reconstruction tends to leavethe excess gold atoms (1/22 or about 5%) nucleated into islands that decorate the path of the reconstruction.54 Scanning at increased resolution showsthe closepacked hexagonalatomicstructureoftheAu( 111)surface(Figure 1C). The measured lattice constant is 2.9 i 0.3 A where the uncertainty arises mainly from drift. 3.2b. Adenine on Au(111). Dramatic changes occur when a droplet of adenine solution is added to a cell containing water or 0.1 M NaC104. These changes occur spontaneously with the counter electrode disconnected when the adenine is injected. Similar changes are seen when the adenine is injected with the cell under potential control near the pzc. The pattern owing to the reconstruction appears to remain intact, but its height corrugation is enhanced -20-fold. (The "height" of the stripes is -2 A compared with 0.1 A prior to addition of adenine to the cell.) This is illustrated in Figure 4A where the data are taken with the counter electrode disconnected. Data look similar with the cell under potential control up to about 0.4 V (SCE). A higher magnification image (Figure 4B) shows that the reconstruction stripes have become rows of bright spots that decorate the path that the reconstruction followed prior to adenine adsorption. (We will discuss these features later.) This image also shows that the surface is covered with wormlike featyes that tend to aling along the three equivalent directions, [ l lo], [loll, and [Oli]. Still higher magnification (Figure 4C) shows that the "worms" consist of a periodic structure with a repeat of 3.4 f 0.2 A. The "worms" pack together with a spacing of 7 i 0.5 A.

We have repeated this experiment many times using both the AFM and the STM. We always observe somewormlikestructure. However, we have not observed the 3.4-A fine structure with the AFM. It is not observed with the STM in euery experiment,


Tao et al.

Figure 4. STM images taken after spontaneous adsorption of adenine onto Au( 1 1 1). At low magnification (A), the "reconstruction stripes" stand out due to "decoration" by the adenine adsorbate (bright spots, lower left of B). Higher magnification (B) shows that the surface is covered with "wormlikeo features aligned along gold [ 1TO] directions (indexed by previous scans at atomic resolution and direction of the reconstruction stripes). Still higher magnification ( C ) shows 3.4-A base stacking repeat. All images were taken at the rest potential with V, = -100 mV and It = 0.24 nA and are unprocessed.

being seen clearly in about one experiment in three (depending on the perseveranceof the operator and the number of tip changes). Resolutionof the fine structureappearstodependon thecondition of the tip. (Resolution can almost always be lost by crashing the tip.) This is not at all surprising, because similarly high resolution in uhv conditions is not obtained in every experiment. In the case of theobservationof gold atoms (as shown in Figure lC), CherP has argued that the accidental Occurrence of a d, state on the end of the scanning tip is required. However, on those occasions when high resolution is obtained, images such as those shown in Figure 4 are quite typical. We note that stable images could not be obtained with tunnel currents above 300 PA. The other adsorbates studied in this work had to imaged at even smaller tunnel currents. The data shown in Figure 4 were taken in pure water. The lack of a supporting electrolyte is not important near the pzc when no current is flowing, but potential control will be lost if even a microampere flows. We have repeated the experiments with 0.05 M NaC104as a supportingelectrolyte, obtainingresults that are essentially identical. We have also swept the potential as images are taken. Sweeping in the positive direction resulted in streaky images, perhaps owing to the formation of multilayers. The adsorbatedestabilized when the substratewas made negative of 0.0 V (SCE), revealing the underlyinggold. The gold revealed after adenine desorption appeared flat, showing neither the reconstruction stripes nor the pattem of islands sometimesfound after the reconstruction is lifted ra~idly.5~ Scans with an AFM showed some evidence for multilayers at positive potentials. Although some polymeric ("wormlike") features could be discerned in the AFM images, the bright "stripes" (Figure 4A) or "dots" (Figure 4B) were not observed in a run carried out with the AFM. 3 . 2 ~ .Guanine on Au( I 2 I ) . The "decorated" reconstruction is not seen when guanineis added to the cell, but wormlike features cover the surface, as shown in Figure 5. Once again, adsorption occursspontaneously,even if the guanineis added with thecounter electrodedisconnected. The images shown in Figure 5 were taken with the cell open circuitafter a dropof saturated guaninesolution had been added to the cell. The surface appeared similar to Figure 1prior to addition of guanine. Imagessuch as those shown in Figure5 werealsoobtainedwith thecell under potential control up to 0.4 V (SCE). In contrast to adenine, the "worms" do not line up along obvioussymmetry directionson the Au( 111) surface. Figure 5B shows a scan at higher magnification (not necessarily higher resolution;the fine structurealong the "worms" is somewhat clearer in Figure 5A). The fine structure of the "worms" is not resolved as frequently as in the adenine experiments, but it is reproducible when observed. The periodicity of the fine structure

Figure 5. Guanine spontaneouslyadsorbed onto Au( 1 1 1) and imaged at the rest potential (A, medium,and B, high magnification). Basestacking (3.3 A) can be seen in (A) (arrows). Images are unprocessed and taken at V, = -300 mV and It = 50 PA.

Figure 6. Cytosine spontaneously adsorbed onto Au( 111) and imaged at the rest potential with V, = -100 mV and I, = 150 PA. (A) shows a region of adsorbate adjoining uncovered gold (top of image). Rows of gold atoms are just visible in the unfiltered image (white arrows) but are clearer in filtered image (Figure 12C). Zoom-in (B)shows structure in unit cell (outlined in black).

is 3.3 f 0.3 A, and the distance between adjacent "worms" is 7 f 0.5 A. Resolution of the 3.3-A fine structure is obtained in about one run in five, again dependingon the perseveranceof the operator and the number of tip changes. 3.2d. Cytosine on Au(lI1). Cytosine also adsorbs spontaneously onto Au( 111)with the cell open circuit or under potential control positiveof the pzc. However, theimagesarequitedifferent from those obtained with adenine and guanine. They show large areas of a highly ordered oblique lattice. Figure 6A shows an image obtained with the cell open circuit in which the boundary can be seen between an area of uncovered gold (upper quarter of Figure 6A) and the cytosine adsorbate (lower three quarters of Figure 6A). Both the gold lattice and the cytosine lattice can be seen in the same image. (This is easier to see when the data

The Journal of Physical Chemistry, Vol. 97, No. 4,1993 915


m

Figure 7. Thymine adsorption process: In (A) the substrate is at -160 mV (SCE) in 0.5 m M thymine in 0.05 M NaCIOd. The reconstruction has been lifted by cycling the cell, and the surface is featureless. In (B), the potential has been raised to 640 mV (SCE) while scanning, and the structure is now seen. Moving the scan to an adjacent area (C) shows area scanned in (B) and adsorbate on surrounding surface. All images are unprocessed data taken at V,= -145 mV and 1, = 50 PA.

Figureb High-magnification scan over region like upper right of Figure 7C showing thymine adsorbate. Directions of unit cell vectors shown as white arrows, upper left. The lattice structure is clearer in Fourier transform shown in upper right where the outermost spots give a = 6.5 A, b = 7.1 A, and y = 105'.

arc filtmd as in Figure 12C.) The cytosine lattice has dimensions Q = 10.5 f 0.2 A and b = 9.5 f 0.2 A with 7 = 102 f 3 O , and it is oriented at 30f 3' with respect to the gold lattice (see Figure 12C). Zooming in on the cytosine lattice shows a complex fine structure (Figure 6B). Each unit cell has a central bright spot, a large "hole", and sharesits vertices with four other bright spots. Images which showed resolution of this fine structure were obtained in about one run in three. Again, similar images were obtained with the cell under potential control up to 0.4 V (SCE). 3.2e. Thymine on Au(ZZ2). Thymine does not adsorb spontaneously. In order to form an adsorbate, it is necessary to raise the substrate potential above 0.4 V (SCE). The contrast of the images of the adsorbates is poor and, in contrast to the other bases, depends quite strongly on substrate potential. This makes the thymine images a little less convincing than in the previous three cases (where transformation of the surface was observed spontaneously after adding bases). For this reason, we show the progress of a typical adsorption experiment in Figure 7. Figure 7A showsthegold surface in a solution of 5 mM thymine at -160 mV (SCE). The substrate had been cycled once through the thymine adsorption, so the reconstruction (Figure 1B) was not observed. There is no sign of any adsorbate. The substrate potential was then raised to +640 mV and the surface reimaged

in the same region (Figure 7B). The surface appearedsomewhat roughened. The scan was then displaced and anotherimagetaken (Figure 7C,also at +640 mV (SCE)). A square "hole" was seen in the region where the scan shown in Figure 7B was taken, The area surrounding the scan was clearly covered with an adsorbate (Figure 7C). The adsorption was hysteretic, and the adsorbate could be imaged with the substrate at the pzc, although contrast diminished as the potential was lowered. The adsorbate was removed when the substratewas brought negativeof the pzc, and images like that shown in Figure 7A were obtained again. Adlayers made in this way show some evidence of molecularscale structure. An example is shown in Figure 8. An oblique latticecan be seen covering the surface. (Its alignmentis indicated by the arrows in the upper left of Figure 8.) The images are too noisy to permit determination of the lattice constants, but spots from this lattice are easily seen in the Fourier transform (insert, upper right of Figure 8). The outer set of spots show that the adsorbate forms an oblique lattice with a = 6.5 f 0.5 A,b = 7.1 f 0.5 A, and 7 = 105 f 5". The inner spots correspond to a longer range periodicity that is not always seen. The contrast improved as the substrate potential was increased, but images became streaky above about 0.4 V (SCE). The image shown in Figure 8 was obtained at +300 mV (SCE). 4. Discussion 4.1. Interpretation of STM Contrast, The interpretation of STM (and even AFMS3)images on an atomic or molocular scale is far from simple. It has only recently been established that true singleatom resolution can be obtained by STM on metal surfaces, and proper interpretation of the contrast requires not only complete knowledge of the tip wave function but also calculation of the structure as distorted by the interaction forces inherent in t~nneling.~-~S A clean metal tip tunneling into a clean metal surface operates in a regime where interactionsare attractive,so that single atom to atom interactions are possible.64 This may not be the case for contact-mode AFM image^.^^.^ The image contrastformolecularadsorbatesmustbemuchmorecomplicated. It is clear that full quantum mechanical complexity of the problem must bedealt with and that simplepictures based on modification of vacuum barriers are not always appropriate? Furthermore, the role of interaction forces in distorting the gap and modifying molecular states is not yet understood.18 We have described some aspects of the problem using a tight bindingmodel.I8 However, this is only appropriateif thecoupling between tip, molecule, and substrate is relatively strong so that "charging times"6' are small. In weakly coupled systems,

916

The Journal of Physical Chemistry, Vol. 97, No. 4. 1993

Tao et al.

on molecule 2 and N6 on molecule 1 to N1 on molecule 2 for which the spacing is 7.8 A (Figure 9C). Another possibility is van der Waals packing, for which the closest spacing is 7.4 A (Figure 9D) where we have used 1.2, 1.7, and 1.5 A for the van der Waals radii of H, C, and N,respecti~ely.~~ The most compact planar structure has a periodicity which is almost double the observed 3.4 A. On the other hand, this period is in excellent agreement with the base stacking distance of 3.4 A found in crystals of adenine.35 These stacks also form in solution as a consequence of the hydrophobicity of the bases and attractive dispersion forces between them.2s,35v36Evidently, interactions with the Au( 111) surface in the presence of water are not strong enough todisrupt thesestructures. Theobserveddistancebetween adjacent "worms" of about 7 A is consistent with either van der Waals packing (Figure 9D) or hydrogen bonding as shown in Figure 9B. Similar constraints apply to the packing of guanine for which wealsocannot find a planar structureconsistent with theobserved periodicityof3.3A(Figure5). However, this periodisinexcellent agreement with the base stacking distance of 3.3 A observed in crystals of guaninej5so that we believe that, again, the molecules (c) (d) form polymeric arrays of stacked bases. Figure 9. Some possible hydrogen-bonding schemes for adenine and the The alignment of the adenine stacks along the [IT01 and corresponding center-to-center distance (a, b, and c). Closest van der Waals packing is illustrated in (d). equivalent directions is clearly a consequence of an interaction with the substrate. There is no obvious structural difference blockading due to ~harging,6~,6~ between adenine and guanine that would account for the observed dephasing of the wave and relaxation70~80will complicate the transfer process considdifference in the ordering of the stacks on thesubstrate (compare erably. Clearly, simple-minded interpretation of a high point in Figures 4 and 5 ) . On the other hand, adenine interacts in some an image as an atom or molecule is not justified. Our approach way with siteson thep X f i reconstruction (see below). It may to solving structures is based upon interpretation of spatial be that this interaction is highly specific, causing the alignment frequencies in the images. of the neighboring stacks via packing interactions. The reproducibility of the images is a key issue. Unlike The images of the cytosine adsorbate (Figure 6) suggest a experiments performed in ambient conditions in air, we find that planar hydrogen-bonded lattice. Such networks have been found the images shown here are remarkably reproducible, prouiding in cytosine monohydrate ~ r y s t a l sand ~ ~ in . ~crystals ~ made from molecular resolution is obtained in thefirstplace. This condition its derivatives.73 Of the many possibilities we have examined, suggests that, as stated before, a particular tip state is required the model illustrated in Figure 10 fits the periods seen in the for molecular resolution, just as is the case for experiments done images well. It consists of chains of hydrogen-bonded molecules in ~ h v . ~ ~ in which every other molecule is inverted about the CN axis and An experimental complication deserves comment: Images then rotated clockwise by 60" to form an alternating pattern of taken at high resolution are often scanned rather quickly in order hydrogen bonding; 0 2 to N4 and N1 to N3 followed by 0 2 to to minimize the effects of drift (typically 20-30 lines per second N4andN1 toN3. Using thedimensionsandbondlengthsquoted in this work). The error in the servo correction varies approxearlier, we obtain a unit cell which forms an oblique lattice with is the temporal frequency associated imately as &/fp where a = 10.0 A, b = 9.5 A, and y = 102O, in excellent agreement with with a particular spatial frequency in the image and fp is the the dimensions of the observed structure. The unit cell contains frequency of the dominant pole in the servo. We estimate that two molecules and a large "hole" (see box in Figure 10). Ignoring this error in height could be quite large (1040% for the finest our earlier caveats about interpreting STM contrast, it is details in the images). This effect will be compounded by the interesting to note that, if bright spots are taken to be cytosine frequency dependence of the piezoelectric transducer hysteresis. molecules, the proposed structure would account for the observed These effects are manifested in the relative change of contrast image (Figure 6B): a bright spot at the center, a hole, and vertices for molecular features recorded over large scans and small scans. shared at four bright spots. The contrast range in each part of Figure 4B,C is about 2 A, but The contrast of the thymine images is too poor to allow us to the "worms" have much less contrast in the larger-area scan solve the structure. The unit cell dimensions, as determined from (Figure 4B). the Fourier transform, are a = 6.5 A, b = 7.1 A, and y = 102O. 4.2. Adsorbate Structures. The images of purines (Figures 4 This is much smaller than the unit cell obtained for the cytosine and 5 ) aresuggestive of the polymeric "stacks" found in crystals3s lattice so it is unlikely that it contains more than one molecule and solutions.36 This is incompletecontrastto earlierSTM studies per unit cell. In this case, the density of molecules in a monolayer carried out on graphite or MoS2 s~bstrates~~-jl in which planar is 2.4 X 1014/cm2. A single electron-transfer reaction from this 2-dimensional hydrogen-bonded arrays were observed. We have monolayer corresponds to a charge transfer of -40 pC/cm2, considered various possible planar structures in which the within the range measured (53 f 25 pC/cm2). molecules lie flat on the substrate in an attempt to account for our images of the purines. These are illustrated for the case of We have imaged these adsorbates with and without supporting adenine in Figure9. We have usedvaluesof 2.9 A for the hydrogen electrolyte. Near the pzc, the salt should make little difference bond N-H--N.2s Three hydrogen-bonded structures are the to the adsorption of the organic molecules,74and we see no following: (1) N6 on molecule 1 to N7 on molecule 2 and N7 differencesin the images. Sweeping the potential away from the on molecule 1 to N6 on molecule 2 (Figure 9A; the atom labeling pzc causes many changes (which are not reproducible in the scheme is illustrated in Figure 3). This gives a center-to-center absence of supporting electrolyte). Broadly, images become spacing of 5.8 A. (2) N6 on molecule 1 to N1 on molecule 2 and streaky at higher potentials. Initial runs with an AFM on the N7 on molecule 1 toN6 on molecule 2 (Figure 9B). The resulting same systems indicate that, in the case of adenine, at least, center-to-center spacing is 7.3 A. (3) N1 on molecule 1 to N6 multilayers may be forming on the electrode. Sweeping the



Figure 11. Structure in the vicinity of the bright spots that decorate the path of the reconstruction after adenine is adsorbed. (A) shows raw data, and (B) shows samedata after high pass filtering toenhancecontrast for the background adsorbate. In many places the "worms" appear to pass right through the bright spots.

Figure 10. Proposed packing of cytosine on Au( 1 1 1) with the 02-N4 and N 1-N3 hydrogen bonds shown as dashed lines. The unit cell (shown in thecenter) is in excellent agreement with theexperimentally measured unit cell (Figure 6B).

potential negative generally leads to desorption of the adsorbates and the reappearanceof the underlyinggold (although it appears as the 1 X 1 surface without copious island formation along the path of the reconstruction). Since the organic molecules must displace water in the inner layer, one expects that adsorption at high potentials would be unfavorable because of the reduced dielectric constant when organics are present in the inner layer. Desorption of adenine appears to occur more rapidly when the dissolved gas content is higher, so dissolved gases play a role in destabilizing this adsorbate. 4.3. Decorationof the Reconstructionby Adenine. Other STM studies have shown that adsorbates bind preferentially at sites along the path of the stacking faults associated with the p X fireconstruction, both in uhv7sand in electr~lytes.~~ We have found that adsorbatescan lift the reconstruction, leaving behind gold islands which decorate the path of the reconstruction preferentially.23*56The island coverage correspondsto the extra 5% atom density contained in the topmost reconstructed layer. There are two plausible explanations for the rows of bright dots shown in Figure 4. One is that the binding of the adenine lifts the reconstruction, leaving islands of gold trapped underneath the adsorbate. This view is supported by the fact that the height of the bright dots is about 2.5 A, about the size of a step on the (111) surface. On the other hand, these islands are not seen when the adenine is desorbed by cycling the substrateto negative potentials. Furthermore, AFM images do not detect them (although the AFM can resolve the gold reconstruction quite readilys3). These observations suggest that the features might be electronic in origin. This view is supported by high pass filtering,and images showing both the bright dots and the"worms" are shown in Figure 11. Inspection of these images shows that the worms pass right "through" the bright dots. It seems unlikely that base stacking would be maintained over the top of gold islands. This observation suggests that the bright spots are not islands, but regions of enhanced tunnel current due to a localized electronic state on the surface with an energy near the Fermi energy.18 4.4. ElectronicEffects in Contrast andAdsorption. It appears that thymine gives images that are very different in contrast, pointing to a distinct electronic difference in the states on the surface near the Fermi energy. It is quite difficult to quantify this effect because of the run-to-run variations in contrast that we have discussed elsewhere in this paper and, also, because of the noise on the thymine images. We define a measure of the contrast for each molecule by the following procedure: We first

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Figure 12. Imagesof adsorbates that have been Fourier filtered toremove noise and allow comparison of corrugation owing to adsorbate unit cell. Images are taken under conditions listed for the molecular resolution images shown in Figures 4-6 and 8. (A) is adenine, (B) is guanine, (C) is cytosine, and (D) is thymine. White lines show path for cross sections displayed below images. Black bars on left are height calibrations as marked.

select those images (taken for similar scan speeds over similar areas) that display molecular resolution. We then use a Fourier filter to remove all spatial frequencies except those associated with the repeat pattern on the surface. (In the case of guanine, this is a ring in the Fourier transform corresponding to spatial periods of 7 A.) We then take a series of cross sections of such images and, from these, obtain an average value for the peakto-peak corrugation for each image. This process is illustrated with the filtered images shown in Figure 12. The average values for peak-to-peak contrast are listed in Table I. The variation from run to run (i.e., tip to tip that gave molecular resolution) is reflected in the uncertainty quoted. The contrastof the thymine images depended upon the electrochemical potential of the substrate(a point we discuss below), and thecorrugationisquoted for the optimum conditions. Contrast for the other bases is not noticeably sensitive to the electrochemical potential of the substrate. Somebaseto-basevariationin contrastcan be expected simply on the basis of topography (e.g., the "hole" in the cytosine lattice). However, it seems unlikely that the order of magnitude difference between thymine and the other bases can be accounted for by topography alone.

Tao et al.


TABLE I: Some Properties of the BLpeB on Au( 111). solution “R”peak (V (SCE)) “R”charge (pC/cm2) capacitance (pF/cm2) 46 f 5 0.1 M NaC104 0.7 12w adenine guanine cytosine thymine

0.7 0.4 0.6 0.6

28 f 3 50 f 5? 30f 5 170 f 40 (280 f

-200 -600 -690 500

-

HOMO (eV) NA -8.503 -7.926 -8.968 -9.516

LUMO (eV) NA -1.031 -0.823 -1.675 -1.838

contrast (A peak/peak) NA 1.3 f 0.3 0.9 i0.3 0.7 f 0.3 0.1 i0.05

a The first three columns list electrochemical properties measured in the STM cell using 0.05 M NaCI04 supporting electrolyte. Capacitance data are for the pzc (0.24 V vs SCE). Maximum value for charge transfer in nondeaerated solutions. Close to value for electrolyte alone, probably because electrode is not covered. Data for adsorption pcak a t 0.4 V (SCE). Electronic levels (HOMO and LUMO) are from ref 28. STM contrast is for best imaging conditions and is determind as outlined in the text.

Why is thymine so different? Understanding this may provide clues to a procedure for identifying molecules by their STM images, so it is an important problem. Calculations of the electronic properties of the bases28,78 show nothing distinctive about thymine. All bases have filled HOMOs (Le., an even number of total valanceelectrons) and rather similar gaps between HOMO and LUMO. (Data are summarized in Table I; these calculated HOMOs are in good agreement with measured values for ionization energy.28) The work function for the hydrated Au( 111) surface is about 5.5 eV,63which lies in the HOMOLUMO gap for the free molecules. Unfortunately, no model is available for the many processes that will modify these energies as the molecules a n hydrated and brought to the electrode surface.42 One possibility lies in the chemistry of uracil (which is the same as thymine but with a proton replacing the methyl group at C5). The bases do not readily form anions, but some uracil derivatives are unusual in that they become deprotonated at N3 at high pH.81 We cannot find any electrochemical evidence for this process in the literature because most work has been carried out at negative electrodes. It would provide a pathway for electron transfer into the gold: OH- T- H+ H20 + T e-. We have observed a somewhat similar phenomenon in images of iodine adsorbates on Au(l1 l).s7 Iodine (which spontaneously transfers an electron to gold) is almost invisible when imaged with the tipnear thegold Fermi energy (i.e.,at small tip-substrate bias). Images of bromine, which transfers much less charge on adsorption (electrovalency 0.4), do not “disappear” when the tip bias is brought near the substrate Fermi level. This unexpected loss of contrast in conditions in which electron transfer occurs can be examined qualitativelyusing a tight binding model of the tip-molcculc-subtrate complex. We have discussed this model extensively We illustrate how STM contrast might be lost on electron transfer in Figure 13, which shows electron transmission versus electron energy for a model of a molecule in a tunnel gap. (Details can be found in ref 76.) In this calculation, the coupling between one atom of the molecule and the substrate ( T L in Figure 13) is increased until an electron is transferred into the substrate (case of T L = 1). When this happens, resonant enhancement of the tunnel current associated with the state owing to the electron that was transferred becomes lost (because the state is no longer localized on the molecule but becomes a Bloch wave in the metal). 5. Conclusions Electrochemical controlof the deposition of organic adsorbates has been shown to be a reliable technique for imaging these adsorbates with an STM. Structures have been proposed for adenine, guanine, and cytosine superstructures adsorbed onto Au( 111) near the pzc. Cytosineforms a planar hydrogen-bonded array of molecules on the substrate, similar to that observed for other bases in STM experiments carried out in ambient conditions in air or in electrolyte on a hydrophobic substrate. The purines show strikingly different behavior, adsorbing as the stacked polymeric chains known to exist in solution. These resultssuggest that it might be possible to obtain singlebase resolution of singlestranded DNA in which the bases may be stacked in a similar manner.79

+

+ +

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.2

$1

Y I

- 1 5

.I

- 0 5

0

0 5

1

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2

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Figure 13. Electron transmission versus electron energy for a tight binding model of a molecule in a tunnel gap. Open circles to right and left are semiinfinite rows corresponding to tip and substrate metals modeled with an on-site energy of 0 and a hopping matrix element of 1 (giving the conduction band from -2 to +2). The molecule has two atoms of on-site energy 4(black) and two atoms of on-site energy 0 (open) leading to the two resonanm in the band near 0. The coupling between ‘tip” and molecule ( T R ) is kept constant (0.1) while the coupling between the substrate and the molecule is increased from 0.1 to 1.0 in the series of curves shown. Figure is taken from ref 76.

Interfacial capacitance measurements show evidence of the spontaneous adsorption of adenine and cytosine into the inner layer. In contrast, thymine is not adsorbed until it undergoes a partly reversibleelectron-transferreaction with the substrate near 0.4 V (SCE). This may be accounted for by a shift of the energy levelsof the adsorbed, hydrated molecules, placing the gold Fermi energy just above the thymine HOMO at the rest potential. The most striking feature in these images is cawed by “decoration” of the path of the p X & by adenine adsorption. The effect may be electronic,owing to the formation of a localized electronicstate at certain points where adenine forms a complex with sites on the reconstructed surface (although we cannot rule out the possibility of trapped gold islands under the adenine). Thymine is a second examples7of an adsorbatein which electron transfer at the Fermi level is accompanied by loss of contrast in an STM image. We suggest that this might be accounted for by loss of resonant enhancement of quantum transmission as the state near the Fermi energy becomesdelocalized. Theobservation that electrochemicalpropertiesand STM contrast are correlated suggest that it might be possible to identify adsorbed species by combining electrochemical and STM observations. Thus, while cytosine and thyminearealmost identicalin structure, they might be distinguished in STM images because of their different electronic properties. Ack~tonledlpnent.We thank Rick Oden, Yinquan Li, Jin Pan, and Hongjian Song for help in the laboratoryand Michael Weaver, Otto Sankey, and Jim Lewis for useful discussions and advice. This work was supported by Grant DIR-89-20053 from the NSF and Grant N00014-90-5-1455 from the ONR.



References mod Notes

(38)Sridharan, R.; DeLevie, R. J. Electroanal. Chem. 1986,201, 133. (39)Brabec, V. Eiolelectrochem. Eioenerg. 1983. 11, 245. (40) Elving, P. J.; Pace, S.J.; OReilly, J. E. J. Am. Chem. Soc. 1973,95, 647. (41)Thtvenot, D. Electroanal. Chem. Interfacial Electrochem. 1973,46, 89. (42)Dryhurst, G.; Elving, P. J. Talanta 1969,16, 855. (43) DeRose, J. A.; Thundat, T.; Nagahara, L. A.; Lindsay, S.M. Sur/. Sci. 1991,256, 102. (44) Angerstein-Kozlowska, H.; Conway. 9. E.; Hamelin, A.; Stoicoviciu, L. J . Electroanal. Chem. 1987,228, 429. (45) Angerstein-Kozlowska, H.; Conway, 9. E.; Hamelin, A.; Stoicoviciu, L. Electrochim. Acta 1986,31, 1051. (46) Hamelin, A.; Weaver, M. J. J. Electroanal. Chem. 1987,223, 171. (47) Hamelin, A.; Sottomayor, M. J.; Silva, F.; Si-Chung Chang; Weaver, M. J. J. Electroanal. Chem. 1990,295,291. (48) Harten, U.;Lahee. A. M.; Toennies, J. P.; Woll, Ch. Phys. Rev. Lett. 1985,54, 2619. (49) Woll,Ch.;Chaing,S.; Wilson, R. J.;Lippel,P. H.Phys. Reuv.B1989, 49,7988. (50) ljarth, J. V.; Brune, H.; Ertl, G.; Behm, R. J. Phys. Reu. B 1990,42, 9307. (51) Tao, N. J.; Lindsay, S . M. J . Appl. Phys. 1991, 70, 5141. (52) Gao, X.;Hamelin, A.; Weaver, M. J. J . Chem. Phys. 1991,95,6993. (53) Oden,P. I.;Tao,N. J.;Lindsay,S. M.J. Vac.Sci. Technol.,submitted. (54) Tao, N. J.; Lindsay, S . M. Surf, Sci. Lett. 1992,274,L454. (55) Green, M. P.; Hanson, K. J.; Carr, R.; Lindau, I. J . Electrochem. SOC.1990. 137. 3493. (56) Tao, N. J.; Pan, J.; DeRose, J. A.; Oden, P. I.; Lindsay, S . M. Surf, Sci. Lett. 1992,271,L338. (57) Tao, N. J.; Lindsay, S . M. J . Phys. Chem. 1992,96, 5213. (58) Gao, X.P.; Weaver, M. J. J . Am. Chem. Soc., in press. (59) Smith. T. J. Colloid Interface Sci. 1980. 75. 51. (60)Wang, J.; Davenport, A. J i Isaacs, H. S.;'hko, B. M. Science 1992, 255, 1416. (61) Nagahara, L. A.; Thundat, T.; Lindsay, S . M. Reu. Sci. Insrrum. 1989,60,3128. (62) Hamelin, A.; Vitanov, T.; Sevstyanov, E.; Popov, A. J. Electroanal. Chem. 1983,145, 225. (63) Ross, P. N.; DAgostino, A. T. Electrochim. Acta 1992,37, 515. (64) Chen, J. C. Introduction toscanning TunnelingMicroscopy;Oxford University Press: New York, in press. (65) Doyen, G.; Drakova, D.; Scheffler, M. Phys. Reu. B, in press. (66) Burnham, N. A.; Colton, R. J. In Scanning Tunneling Microscopy, Theory, Techniques and Applications; Bonnel, D., Ed.; VCH: New York, 1992;Chapter 7. (67) Lindsay, S.M.; Sankey, 0. F.; Schmidt, K. E. Comments Mol. Cell. Eiophys. 1991,A7, 109. (68) Stockman, M. I.; Murtov, L. S.;Pandey, L. N.; George, T. F. Phys. Rev. E , in press. (69) Mullen, K.; Ben-Jacob, E.; Jaklevic, R. C.; Schuss, Z. Phys. Reu. B 1988,37,9810. (70) Drickamer, H. G.; Frank, C. W.; Schlichter. C. P. Proc. Narl. Acad. Sci. (U.S.A) 1972,69, 993. (71) Weast, R. C., Ed. Handbook of Chemistry and Physics; CRC Press: Cleveland, OH, 1980; p D194. (72) Jeffry, G. A.; Kinoshita, Y. Acta Crystallogr. 1963,16,20. (73) Voet, D.; Rich, A. Prog. Nucleic Acid Res. Mol. Eiol. 1970,IO, 183. (74) Bockris, J. OM.; Argade, S.D.; Gileadi, E. Electrochim. Acta 1969, 14, 1259. (75) Dovek, M. M.; Lang, C. A.; Nogami, J.; Quate, C. F. Phys. Rm. E 1989,40,11973. (76) Lindsay, S.M.; Sankey, 0. F. In Scanned Probe Microscopies,STM and Beyond; Wickramasinghe, H. K., Ed.; American Physical Society: New York, 1991, pp 125-135. (77) Lindsay, S.M. In Scanning Tunneling Microscopy. Theory, Techniques and Applications; Bonnel, D., Ed.; VCH: New York, 1992;Chapter 8.3. (78) Lewis: J. P.; Sankey, 0. F. Bull. Am. Phys. Soc. 1992,37,549. (79) Hansma, H.G.; Weisenhorn, A. L.; Gould, S.A. C.;Sinsheimer, R. L.; Gaub, H. E.; Stucky, G. D.; Zaremba, C. M.; Hansma, P. K. J. Vac. Sci. Technol. 1991,E9, 1282. (80) Kuznetsov, A.; Sommer-Larsen, D.; Ulstrup, J. Surf. Sci. 1992,275, 52. (81) Taylor, R.; Kennard, 0. J . Mol. Struct. 1982. 78, 1.

(1) Buess-Herman, C. In Trends in Interfacial Electrochemistry;Silva, A. F., Ed.; D. Reidel Publishing: Dordrecht, 1986;p 205. (2) Rubinstein, I.; Steinberg, S.;Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988,332,426. (3) Chaing, S. In Scanning Tunneling Microscopy 1; Giintherodt, H., Weisendanger, R., Us.; Springer-Verlag: Berlin, 1991;Chapter 7. (4) Spong, J.; LaComb, L.; Dovek, M.; Frommer, J.; Foster, J. J. Phys. (Paris) 1989,50, 2139. (5) Smith, D.; HBrber, H.; Binnig, G.; Nejoh, H. Nature 1990,344,641. (6) Spong, J.; Mizes. H.; LaComb, L.; Dovek, M.; Frommer, J.; Foster, J. Nature 1989,338, 137. (7). Mizutani, W.; Shigano, M.; Sakakibara, Y.; Kajimura, K.; Ono, M.; Tanishima. S.:Ohno. K.: Toshima. N. J. Vac. Sci. Technol. 1990.A8.675. (8) McMaster, T.;Carr, H.;Miles, M.; Cairns, P.; Morris, V. J: Vac. Sci. Technol. 1990,A8, 672. (9) Mizutani, W.; Shigeno, M.; Ono, M.; Kajimura, K. Appl. Phys. Lett. 1990.56, 1974. (10) Smith. D.; Hdrber. H.: Binnia. G. Science 1989. 245. 43. (11) Brand, S.;DeLella, D.; Colt&, R. J . Vac. Sci. Technol. 1991,A8, 672. (12) Mcgonigal, G.; Bernhardt, R.; Thompson, D. Appl. Phys. Lett. 1990, 57,28. (13) Luttrull, D. K.; Graham, J.; DeRose, J. A.; Gust, D.; Moore, T. A,; Lindsay, S. M. Lungmuir 1992,8,765. (14) Htckl, W. M.; Kallury, K. M. R.; Thompson, M.; Gerber, C.; Horber, H. J. K.: Binnie. G. Lunemuir 1989.5. 1433. (15) Lyubcikko, Y.-L.; Lindsay, S . M.;DeRose, J. A.; Thundat, T. J. Vac. Sci. Technol. 1991, 89, 1288. (16) Allison, D. P.; Bottomley,L. A.;Thundat,T.; Brown, G. M.; Woychik, R. P.; Schrick, J. J.; Jacobsen, K. B.; Warmack, R. J. Proc. Natl. Acad. Sci. (U.S.A.),in press. (17) Spence, J. C. H.; Lo, W.; Kuwabara, M. Ultramicroscopy 1990,33, 69. (18) Lindsay, S.M.; Sankey, 0. F.; Li, Y.; Herbst, C.; Rupprecht, A. J . Phys. Chem. 1990,94,4655. (19) Lindsay, S.M.; Barris, 9. J . Vac. Sci. Technol. 1988,A6, 544. (201 Bard. A. J.; Faulkner, L. R. Electrochemical Methods. Techniaues and Applications; Plenum: New York, 1980. (21) Bockris, J. OM.; Reddy, A. K. N. Modern Electrochemistry; Plenum: New York, 1977. (22) Tao, N. J.; DeRose, J. A.; Oden, P. I.; Lindsay, S . M. In Proceedings of the 49th Annual Meeting of EMSA; Bailey, G. W., Ed.; San Francisco Press: San Francisco, 1991;pp 376-377. (23) Lindsay, S.M.;Tao, N. J.; DeRose, J. A.; Oden, P. I.; Lyubchenko, Y. L.; Harrington, R. E.; Shlyakhtenko, L. S . Eiophys. J . 1992, 61, 1570. (24) Lindsay, S.M.; Tao, N. J. In STM and SFM in Biology; Amrein, M., Marti, O., Eds.; Academic Press: London, 1992. (25)Saenger, W. Principles of Nucleic Acid Structure; Springer: New York, 1984;Chapters 4-6. (26) Lindsay, S. M.; Philipp, M. Gene Anal. Tech. 1991,8,8 . (27) Pullman, A. In Structure and Conformation of Nucleic Acids and Prorein-Nucleic Acid Interactions; Sundaralingam, M., Rao, S. T., Eds.; University Park Press: Baltimore, 1975;pp 457-484. (28) Nagata, C.; Imamura, A.; Fujita, H. In Aduances in Biophysics; Kotani, M., Ed.; University of Tokyo Press: Tokyo, 1973;Vol. 4,pp 1-69. (29) Allen, M.; Balhorn, R.; Seikhaus, W. J.; Tench, R. J.; Subbiah, S . In Scanning Probe Microscopy; Wickramasinghe, H. K., Ed.; American Physical Society: New York, 1991;p 176. (30) Heckl, W. M.;Smith, D. P.; Binnig, G.; Klagges, H.; HBnsch, T. W.; Maddocks, J. Proc. Natl. Acad. Sei. (U.S.A.) 1991.88,8003. (31) Srinivasan, R.; Murphy, . . J. C.; Fainchtein, R. J. Electroanal. Chem. 1991,312, 293. (32) Clemmer, C. R.; Beebe, T. P. Science 1991, 251,640. 133) Heckl. W.: Binnie. G. Ultramicroscoov 1992. 42-44. 1073. (34j Oden,'P. I.'; Thunldat, T.; Nagahara, i.:A.; Lindsay, S. M.; Adams, G. 9.;Sankey, 0. F. Surf. Sci. Lett. 1991, 254, L454. (35) Bugg, C. E.;Thomas, J. M.; Sundaralingam, M.; Rao, S. T. Eiopolyme&~l971, 10, 175. (36) Broom, A. D.; Schweizer, M. P.; Ts'O, P. 0. P. J . Am. Chem. SOC. 1967.89.3612. Ts'0. P. 0.P.: Kond0.N. S.:Robbins. R. K.: 9room.A. D. J . Am. Chem. Soc. l%9, 91,5625. (37) Mitchell, P. R.; Sigel, H. Eur. J . Eiochem. 1978,88, 149. '

Self-assembly of molecular superstructures studied by in situ scanning

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