Report
An Update on Scanning Force Microscopies Simple, low-maintenance instrumentation can be used to image virtually any material at the microscopic and nanoscopic level
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canning probe microscopies are quickly becoming routine methods in many laboratories. The ability to probe the microscopic and nanoscopic structure of surfaces in a variety of ambient conditions with a lowmaintenance instrument that sits on a benchtop has contributed to the popularity of these techniques. Scanning tunneling microscopy (STM), invented in 1982, was the first technique capable of directly imaging surface atoms in real space (Ï). Although this instrument has been miniaturized, multiplexed, and adapted to many environments, there have been few major innovations since its invention. The same cannot be said of atomic force microscopy (AFM), invented in 1986 (2). Whereas the scanning tunneling
Darrell R. Louder Bruce A. Parkinson Colorado State University 0003-2700/95/0367-297A/$09.00/0 © 1995 American Chemical Society
microscope functions by scanning a sharp metal tip over the surface of a conducting or semiconducting sample, a force microscope does not require a tip or a sample to be conductive. Therefore, virtually all materials can be imaged using variations of force microscopy. Instead of using a tunneling current to sense the proximity of the scanning tip to the surface, force microscopies take advantage of the variety of short- and long-range forces between two masses (e.g., van der Waals, magnetic, and electrostatic forces). Force microscopes image a sample by scanning a probe mounted on a cantilever across a sample surface and then detecting the changes in the forces between the tip and the surface by measuring the deflections of the cantilever. In this article we will describe the evolution of the atomic force microscope and its many variations, and survey applications in chemistry, physics, and biology.
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Image
Sample control and data acquisition
Force detection Cantilever
Holder Tip
Sample Sample piezo [(χ, y, z) control]
Feedback Figure 1 . Typical atomic force microscope.
Instrumentation
AFM instrumentation is relatively simple; it requires only a tip, a flexible cantile ver, a deflection (or force) detection sys tem, a positioning system for the tip or sample, and a data system. Figure 1 shows a common AFM setup. The move ment system is a piezoelectric ceramic tube scanner situated to move the sam ple under the tip in the x, y, and ζ direc tions. A wide range of deflection detection systems, such as tunneling, capacitance, interferometry, laser diode feedback and laser beam deflection, and piezoresistive detection (2,3), can be used and are dis cussed briefly in the box on page 302 A. Laser beam deflection is the simplest and most widely used of these force detec tion methods. Cantilevers are usually made by litho graphic methods using material that can easily be fabricated en masse, such as sili con, silicon nitride, or silicon oxide (4). The tip can be made of the same material or of a metal if magnetic or electrostatic forces are to be measured. Tips made of silicon nitride are usually pyramid shaped, have a radius of 300 Â, and are blunter than tips made from silicon or silicon oxide; silicon tips generally have a radius of 10-20 λ Tip shape is very important in AFM be cause a blunt tip can interfere with the im aging of sharp features and steps on the surface of a sample, as shown in Figure 2. 298 A
A very thin tip can be influenced by lateral forces next to steep features (5). The microscopist must remember that a force im age will be a convolution that takes into account both tip and sample geometries. M o d e s of o p e r a t i o n
The topography of a surface is imaged by either an attractive or a repulsive inter action of the tip with the surface. This in teraction can be described by the LennardJones potential (6) U = (A/r12) - (B/r6)
(1)
where A and Β are constants determined
by thetipand sample materials and r is the distance between the sample and the tip. A/r12 refers to the short-range repulsive force between electrons on the surface and the tip; B/r6 is the attractive term, commonly known as the van der Waals force, and is a result of the attraction between dipoles in the tip and the sample (7). Although both terms contribute to the image of the sample's topography, usu ally one term dominates depending on how the microscope is operated. The two modes of operation used in normal force microscopy are contact (re pulsive) and noncontact (attractive). Each has its own advantages and disadvan tages, and the choice for imaging is usu ally determined by the sample characteris tics. In contact mode, the tip approaches the sample until the first upward deflection of the cantilever is sensed as the tip touches the surface. The deflection of the cantile ver images the sample as the tip scans the sample at this constant height. This de flection also can be held constant by apply ing a voltage to the sample piezo. In gen eral, the tip can scan the sample much faster when it is held at a constant height because of the finite time constant of the electromechanical feedback system. The resolution in contact AFM is deter mined by the area of contact between the tip and sample. With commonly used tips, this area has been estimated to be about 25 Â2 (8). Although this contact area seems too large for achieving atomic
Tip and sample
Image
Scan direction
Figure 2. Convolution of tip and sample geometries. The side of the tip contacts the step edge before the end of the tip, resulting in an image of the step edge that reflects the angle of the tip and not the true step edge angle. A sharper tip reduces this effect.
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resolution, atomically resolved lattice structures have been imaged by AFM. The process by which atoms are im aged is not well understood, but it is be lieved that the atomic image is a convolu tion of the tip and sample atoms. For ex ample, with layered materials such as MoS2, it is thought that a piece of the sur face under the tip breaks off and is car ried along under the tip (7). Atoms are im aged via stick-slip interactions between rows of atoms of the moving piece and the stationary atoms of the sample. This slid ing piece can rotate and cause the sample lattice to appear distorted. Until recently no atomic-scale defects such as missing atoms or atomic steps had been detected by AFM. However, by us ing a very sharp tip and imaging under liq uid, Ohnesorge and Binnig (9) were able to image atomic- scale kinks in steps on calcite crystals. The forces between the tip and sample were measured at ΙΟ"11 Ν. When higher forces were used, the steps disappeared, leaving what seemed to be an atomically smooth surface. As a re sult, the van der Waals forces were re duced by an order of magnitude because of the screening effect of the liquid be tween the tip and surface. In the calcite example described above, the attractive capillary forces normally found in experiments performed in air were also eliminated by scanning under bulk liquid. The capillary force is caused by water or contaminants found in the air condensing between the tip and surface of the sample. Not only is the radius of cur vature in the sharp tip smaller, but the me niscus is also smaller because of the smaller area of the tip that is near the sur face. For a sharp tip, if the adsorbed liq uid is assumed to be water, the force will be ~ 9 χ ΙΟ"9 Ν. Forces of this magnitude are measured routinely in force micros copy performed in air with a sharp silicon probe. For a blunt probe of silicon nitride, which has a radius of curvature 10 times larger, the force would be ~ 9 χ 10~8 Ν. In both cases the force is too large for true atomic-scale imaging because for an area of contact of 25 A2, the force per unit area is 3.6 χ 1010 N/m2, which exceeds the shear force of diamond. Only by submers ing the tip and sample in liquid or by placing them in a vacuum can the forces
Tip on surface Attraction by van der Waals force Surface moving up (tip off surface) Cantilever deflection voltage
Surface moving down (lip on surface)
Tip snaps off surface
V, V2 ζ Piezo voltage Figure 3. An idealized force curve. Tip-sample forces can be measured using the force constant of the cantilever and the sensitivity of the ζ piezo. The difference between V2 and V, is the voltage that must be applied to the ζ piezo to pull the tip off the surface. This voltage is multiplied by the sensitivity of the piezo and the force constant of the cantilever, giving a value for the force applied to the sample.
acting on them be reduced sufficiently for accurate atomic-scale imaging. By examining a plot of cantilever de flection versus tip-sample separation, also called the force curve, the magnitude and range of influence of these forces can be measured. This method of measur ing the force exerted on the tip as a func tion of separation from the surface is often called force spectroscopy. To measure the force curve, a voltage signal in the form of a sawtooth wave is sent to the ζ piezo, which causes the tip and sample to approach and withdraw repeatedly. When the sample approaches the tip, the canti lever remains undeflected until the tip and sample are close enough to interact via the longer range attractive van der Waals force, which appears in the force curve as a small dip, indicating a downward deflec tion of the cantilever before a large up ward deflection caused by repulsive con tact. Figure 3 shows an idealized force curve that takes into account only the normal attractive and repulsive forces be tween tip and sample. Some samples, such as biological mate rials and soft polymers, are damaged by the larger forces of contact mode opera tion. By using noncontact AFM, these forces can be substantially reduced; how ever, because of the larger tip-sample sep aration, there is a loss of resolution.
A common noncontact AFM technique uses a stiff cantilever held above the sam ple surface and oscillated at a frequency close to its resonance frequency. When the tip is brought close to the surface, the resonance frequency of the cantilever changes in a measurable fashion be cause the force constant of the cantilever is modified by the force gradient between the tip and sample. The van der Waals force extends far enough above the sam ple to influence the tip without the tip actu ally touching the surface. The change in the oscillation of the cantilever can be measured using most deflection detection methods. In one case, interferometry was used to measure changes in the amplitude of the oscillation while imaging (10). Again, the spatial resolution was less than it was in contact mode because the tip was held 50 Â above the surface to prevent capture by the surface water layer, which can dampen the oscillation. Operation in a vacuum can circumvent the water layer problem, increase sensitivity (because softer cantilevers can be used in a vacuum), and lower oscillation amplitudes. In general, the resolution is no better than the tipsample separation. A hybrid of noncontact and contact AFM (tapping mode) involves vibrating the cantilever near the surface and close to
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Report its resonance frequency at an amplitude of ~ 20 nm. The amplitude of the oscillation is reduced when the tip interacts with the surface, and the reduced amplitude is used for feedback control and imaging. Lateral resolution of 5 A and a vertical resolution of 0.5 A have been achieved using this method. Resolution is dependent on tip radius, just as it is in contact mode, but the sample is not subjected to shear forces because the tip is not dragged across the surface. The frequency of vibration is usually several hundred kilohertz, which allows the tip to interact many times with the surface at each sampled point. Recently, plasmid DNA samples were imaged using a slight variation of tapping mode (11). Instead of oscillating the tip up and down, the sample was oscillated. Although the oscillation frequency used (17 kHz) was much lower than normal, im-
(a)
300 nm (b)
(c)
Figure 4 . M o S 2 f i l m on m i c a . (a) Topographic image, (b) frictional force image, and (c) elasticity image. 300 A
age quality was significantly improved. When scanned with normal contact mode AFM, the plasmid DNA appeared to be 18 nm wide and was "smeared out" by the scanning process. Tapping mode images of the same sample gave a width of 5 nm with no smearing.
(a)
Applications
Because AFM is so versatile, it has been applied to many different problems in chemistry, physics, and biology. With a modified tip, forces other than those described by the Lennard-Jones potential can be imaged. One of the most common applications is imaging magnetic domains by magnetic force microscopy (MFM), which uses a tip made of magnetic material or a regular force probe coated with magnetic material. MFM is a noncontact AFM mode; because magnetic forces are typically stronger and have a longer range than van der Waals forces, larger vibration amplitudes and tip-sample separations are used. Electrostatic forces may also be imaged by choosing the appropriate tip materials. Charged regions of the surface induce an equal and opposite charge on the tip, resulting in an attraction between the tip and surface. The resulting piezo movement causes the charge to appear as a hill in the image, for example, when charges were deposited on the surface of poly (methyl methacrylate) by corona discharge from the tip (12). The deposited surface charges (~ 1200 electrons) are stable and can be imaged with or without applying a bias (reversing the charge on the tip) to the sample. Application of an appropriate bias can make the charged areas appear as a hill or as a depression. Surface topography measurements are relatively routine for scanning probe microscopes, but chemical identification of surface species has only recently been undertaken. Frictional force microscopy (FFM) is one method that may yield this type of information. In FFM, the lateral deflection or twisting of the cantilever operating in contact mode is measured in addition to its vertical deflection. Different areas of a sample can cause more or less cantilever twisting because of changing frictional forces between the tip and the surface. Use of a four-section photodiode in the optical
Analytical Chemistry, Vol. 67, No. 9, May 1, 1995
(b)
Figure 5. Patterns etched into SnSe 2 using contact-mode AFM. (a) Lines are etched 20 nm wide and 0.61 nm deep, and (b) an example of a device pattern.
beam deflection technique allows the detection of both upward and downward cantilever deflection as well as the torsional movement caused by sticking and slipping of the cantilever as it moves across the surface of the sample. This method can be applied to organic materials deposited on a substrate, such as Langmuir-Blodgett films, or it can be applied to inorganic materials, such as MoS2 deposited on mica. Figure 4a shows a topographic image of MoS2 deposited on mica, and Figure 4b shows a friction force image of the same area. The contrast between the two images demonstrates that the MoS2 layer causes less friction than the mica substrate. This fact is not surprising because MoS2, like graphite, is used as a solid lubricant. A force microscope also can use elasticity to image a sample. Figure 4c shows an image of the elasticity of MoS2 on mica. Elasticity measurements are made by vibrating the cantilever and contacting the surface. The vibrational amplitude change is recorded as the surface is compressed by the tip; softer areas cause more damping
(23). All three of these images were ob tained simultaneously, demonstrating the amount of information that a single scan ning probe experiment can provide. Frisbie et al. (14) used AFM to pro duce a map of the interactions between various combinations of hydrophobic and hydrophilic groups on a tip and pat terned on a sample surface. Gold-coated tips and samples were functionalized by self-assembly of thiol monolayers, which were terminated with either COOH or CH3 groups protruding up from the surface or down from the tip. The greatest attractive interaction between the tip and the sur face occurred when like groups were on the tip and surface. The attractive force be tween the COOH groups was the great est because of hydrogen bond formation, and the attraction between COOH and CH3 groups was the lowest, as would be expected of hydrophilic and hydrophobic groups. Manipulation of the surface is also pos sible using the force microscope. Patterns can be etched into many different sur faces by the force of the interaction between the tip and the substrate. One particularly graphic example is that of pat terns etched in the surfaces of twodimensional layered dichalcogenides by scanning a tip over their surfaces in con tact mode (15). These materials are com posed of layers of chalcogenide and metal chalcogenide weakly held together by van der Waals forces. The rate of etching was proportional to the force between the tip and the substrate and also was depen dent on the number of missing atoms on the surface, which act as nucleation sites for the removal of surface atoms. Figure 5 shows patterns formed on a SnSe2 surface by scanning the tip with a high force and rotating the substrate to achieve different line angles. The lines in Figure 5a are 20 nm wide and one unitcell (0.61 nm) deep. Figure 5b shows a de vice that might be patterned in a sub strate grown using van der Waals epitaxy, which is a technique for making multi layers of two-dimensional materials with out the lattice constraints caused by interlayer covalent bonding (16). This sub strate is a single material with a mesa measuring about 50 χ 100 χ 0.6 nm, iso lated from the rest of the top layer but still connected to the next layer.
To make a small diode from this pat tern, three different layers would be needed. The bottom layer would be an in sulating substrate, the middle layer a conducting material such as NbSe2 or TaS2, and the top layer a semiconducting material such as MoS2 or WSe2. Contact with the top layer could be accomplished using a conductive force probe. The atomic force microscope can be used in situ in combination with a potentiostat and a liquid cell to study electro chemical reactions such as underpotential deposition (UPD). UPD involves the deposition of a monolayer or submonolayer of metal on an electrode surface at a potential positive of its bulk reduction po tential. This deposited layer forms a new atomic lattice, called an ad lattice, on top of
The increasing availability of force microscopes and their potential applications should result in additional vanations of the technique. the gold lattice. AFM can provide images of the ad lattice with atomic resolution. Chen and Gewirth (17) have used AFM to study the atomic structure of bis muth underpotentially deposited on a gold electrode surface. This system shows catalytic activity for the electroreduction of H202 dependent on the bismuth de position. They knew that the catalytic ac tivity was higher for submonolayers than for complete monolayers of bismuth, but the structure of the ad lattices was un known. The different ad lattices formed by the bismuth could be imaged by observing the structure of the electrode surface at dif ferent potentials using AFM. At potentials positive of the potential for UPD, only the gold 5x5 lattice with a spacing of 0.29 nm is imaged (Figure 6a). When the po tential is moved to between 250 and 190 mV versus E^/°, the hexagonal 2x2 bis
muth lattice is formed with a distance of 0.57 ± 0.02 nm between bismuth atoms (Figure 6b). If the potential is moved nega tive to 100 mV, a new rectangular lattice with an average distance of 0.34 ± 0.02 nm is formed, replacing the hexagonal con figuration (Figure 6c). Electroreduction of H202 was found to be greatest at potentials corresponding to the formation of the 2 χ 2 bismuth sur face structure. Because more gold is ex posed in this configuration, Chen and Gewirth proposed that a heterobimetallic bridge was formed by the peroxide mole cules between the bismuth and gold at oms polarizing the 0-0 bond, allowing
(a)
(b)
(c)
Figure 6. Images ( 5 x 5 nm) of bismuth on A U ( 1 11). (a) Gold lattice with 0.29 spacing before UPD, (b) 2 χ 2 bismuth ad lattice with 0.57 nm spacing at 200 mV versus £|*'°, and (c) rectangular bismuth ad lattice with 0.34 nm spacing at 100 mV versus E|*'0. (Adapted from Reference 17.)
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easier cleavage. Only this configuration has enough gold atoms exposed for the formation of the heterobimetallic interme diate. Using the atomic force microscope as a sensor
Recent experiments used a modification of an atomic force microscope as a heat sen sor for small electronic devices (18). Two wires of chromel and alumel were epoxied together, etched to create a thermocou ple junction with a sharp tip, and then bent down to form a cantilever. A piece of alu minum foil was glued to the cantilever for laser deflection detection. The topogra phy and temperature of the sample could
be measured simultaneously, thus allow ing identification of areas of power dissipa tion in the electronic device. A metal semiconductorfieldeffect transistor made of η-doped GaAs and having a gold drain, gate, and source contacts was imaged ther mally and topographically. The tempera ture of the GaAs was higher than that of the metal gate, source, and drain when a bias of 4 V was applied between the source and gate. AFM can also be used as a sensor for chemical phenomena. For example, Gimzewski et al. (19) used a modified atomic force microscope probe and its de flection detection system to measure the oscillations induced in the cantilever by a
Force detection Many different methods, described below, are used to detect the deflection of the cantilever produced by the interaction of the sample with the probe tip. An ideal detection system is easy to set up, compact, sensitive to very small deflections, and stable. Tunneling. The first atomic force microscope used tunneling to detect the movement of thetip.An STMtipis placed above the back of a metal cantilever, and the feedback circuit is used to maintain a constant tunneling current The volt age applied to either the ζ piezo or the sample piezo produces an image of the sur face. The tunneling current itself can also be used for imaging by measuring the actual deflection of the cantilever. Capacitance. If the cantilever on an atomic force microscope is one plate of the capacitor and another plate is mounted above this plate, the capacitance be tween the two plates changes relative to the deflection of the cantilever. An image can be formed either by moving the cantilever or the sample to maintain a con stant capacitance or by monitoring the change in capacitance as the sample is scanned. Interferometry. Interferometry uses a laser split into two beams: one focused on the back of the cantilever and the other directed into a photodiode. The beam fo cused on the cantilever is reflected back into the photodiode. The coherent beams travel different pathlengths and produce an interference pattern, which changes as the cantilever moves up and down. The photodiode registers this change as a voltage signal, which is used to control the feedback of the instrument Laser diode feedback. If light from a single-mode diode laser is focused on the back of a cantilever and there are only a few micrometers between the laser and cantilever, the laser is reflected back into the laser cavity, causing a change in the gain of the laser. The change in laser gain can be registered by a photodiode and turned into a voltage signal to control the feedback of the microscope. Optical deflection. In this method a diode laser is focused onto the back of the cantilever and reflected onto a split photodiode. The difference in voltages be tween the two halves of the photodiode is then monitored. Piezoresistive detection. A silicon cantilever with a resistor or resistor net work patterned into it is used. As the cantilever bends, the resistance of the sin gle silicon crystal changes because of the stress on the crystal. Measuring this re sistance change provides the signal for the feedback control. The advantage of this method is that no external deflection measurement system is needed.
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reaction occurring on its surface. Oscilla tions were induced by differential thermal expansion of the silicon cantilever coated with an aluminum layer. They were able to measure the reaction 2H2 + 0 2 -» 2H20 (g) on a platinum catalyst deposited on the aluminum overlayer of the cantilever. The microscope was placed in a highvacuum chamber, and the reactant gases were allowed to flow into the chamber. When a reaction occurred, the platinum catalyst released heat to the aluminum layer, causing the cantilever to oscillate with increasing amplitude until the gases were pumped out of the chamber. Local temperature changes were estimated to be ~ 1(Γ5 Κ and heats of reaction as small as ~ 1 pj could be measured. The re searchers postulate that an artificial nose could be produced using pattern recogni tion with an array of these tips having dif ferent catalysts deposited on them. Lee et al. (20) used AFM to sense dis crete interactions between a mica surface and a glass sphere, both of which have a monolayer of biotinylated bovine serum al bumin (BBSA) coating. They functionalized the microscope tip by attaching glass microspheres to it and then coating the spheres with BBSA. This arrangement provided two BBSA-functionalized sur faces that could interact when the bifunctional streptavidin was present. Force curves taken with both blocked and un blocked streptavidin present in the solu tion between the tip and sample surface re vealed that a greater force was required to pull the tip off of the surface when un blocked streptavidin was present in solu tion. Measurement of this force provided an estimate of the rupture force of the streptavidin-biotin bonds. A similar technique has been applied to DNAbase pairs (21). Oligonucleotides were attached to silica spheres on a sur face and attached to a microscope tip. When the base pairs were oriented so that complementary pairs could interact, the interchain forces could be measured between molecules on the tip and the surface. The force required to pull apart the tip and surface depended on the num ber of pairings between bases. When noncomplementary strands were used, much lower forces were measured.
This same study also reported the force required to rupture bonds in oligomers of inosine (I) averaging 160 bases in length (22). Cytosine-functionalized silica spheres on a surface and tip provided tethering sites for the oligomers and considerable force was needed to pull apart the tip and the surface, presumably because of the snapping of the oligomer chain. In some cases multiple snapping points could be seen in the force curve. These multiple snapping points are believed to be caused by the variation of the chain length of the poly (I) and multiple connections between the tip and surface. Future applications An unproven but intriguing application of the force microscope has been proposed by Sidles and co-workers (23, 28), who suggest using the cantilever as a detector for making magnetic resonance measurements. A strong magnetic field gradient is created by a magnetic particle on the end of a cantilever. When the sample is brought close to this particle and oscillated from side to side, it can induce an oscillation in the cantilever because of the coupling of the magnetic moments of the surface atoms and die magnet on the cantilever. Because the distance between the sample and cantilever is small, high resolution should be possible. The forces that are generated will be extremely small (~ 10"19 N), so advances in cantilever and force detection technology are needed for this experiment to be viable. A wide variety of forces have been detected and imaged with many variations of the force microscope, and the measurement of other molecular properties, such as individual electron and nuclear spins, may be possible. The increasing availability of force microscopes as well as their many potential applications should result in additional variations of force microscopy in the near future.
(3) Sand, D. Scanning Force Microscopy, 1st éd.; Oxford University Press: New York, 1991. (4) Albrecht, T. S.; Akamine, S.; Carver, T. E.; Quate, C. F.J. Vac. Sci. Technol. A 1990, 8, 3386. (5) Griffith, J. E.; Grigg, D. A. J. Appl. Phys. 1993, 74, R83. (6) Atkins, P. W. In Physical Chemistry, 3rd éd.; Oxford University Press: Oxford, U.K., 1986. (7) Burnham, Ν. Α.; Colton, R. J. In Scanning Tunneling Microscopy and Spectroscopy Theory, Techniques, and Applications; Bun nell, D. A., Ed.; VCH: New York, 1993. Ch.7. (8) Hutter, J. L; Bechhoefer, ].]. Appl Phys. 1993, 73,4123. (9) Ohnesorge, F.; Binnig, G. Science 1993, 260,1451. (10) Martin, Y.; Williams, C. C; Wickramasinghe, H. K.J. Appl. Phys. 1987, 61, 4723. (11) Hansma, P. K.; Cleveland, J. P.; Radmacher, M.; et al. Appl. Phys. Lett. 1994, 64, 1738. (12) Stem, J. E.; Terris, Β. D.; Mamin, H. J.; Rugar, D. Appl. Phys. Lett. 1988,53, 2717. (13) Scandella, L.; Schumacher, A; Kruse, N.; et al. Thin Solid Films 1994,240,101. (14) Frisbie, C. D.; Rozsnyai, L. F.; Noy, Α.; Wrighton, M. S.; Lieber, C. M. Science 1994,265,2071. (15) Delawski, E.; Parkinson, B. A . / Am.
Chem. Soc. 1992,114,1161. (16) Ueno, Κ.; Koma, Α.; Ohuchi, F. S.; Parkin son, Β. Α. Appl. Phys. Lett. 1991, 58,472. (17) Chen, C-H.; Gewirth, A. A / . Am. Chem. Soc. 1992,114,5439. (18) Majumdar, A; Carrejo, J. P.; Jai, J. Appl. Phys. Lett. 1993, 62, 2501. (19) Gimzewski, J. K.; Gerber, C; Meyer, E.; Schlitter, R. Chem. Phys. Lett. 1994,217, 589. (20) Lee, G. U.; Kidwell, D. Α.; Colton, R. J. Langmuir 1994,10, 354. (21) Lee, G. U.; Chrisey, L. A; Colton, R J. Sci ence 1994,266,771. (22) Deng, G.; Wu, R Methods Enzymol. 1983, 100,96. (23) Sidles, J. A; Garbini, J. L.; Drobny, G. P. Rev. Set. Instrum. 1992, 63,3881. (24) Sidles, J. Α.; Rugar, D. Phys. Rev. Lett. 1993, 70, 3506.
Darrell R. Louder, a doctoral candidate at Colorado State University, is studying dop ing and defects in layered dichalcogenides by STM and SFM. Bruce A. Parkinson, pro fessor of chemistry at CSU, conducts re search in photoelectrochemistry, scanning probe microscopies, and surface and materi als chemistry. Address correspondence to Parkinson at the Dept. of Chemistry, CSU, Fort Collins, CO 80523.
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We thank Jane Frommer and Andrew Gewirth for providing some of the images used in this article and the U.S. Department of Energy, Division of Chemical Sciences, for support.
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References (1) Binnig, G.; Roher, H.; Gerber, C; Weibel, E. Phys. Rev. Lett. 1982, 49, 57. (2) Binnig, G.; Quate, C. R; Gerber, C. Phys. Rev. Lett. 1986, 56,930.
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