c
a-
'AnUpdate on Scanning Force Microscopies
I
Simple, lowmaintenance instmmentation can be used to image virtually atty material at the microscopic and nanoscopic level
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canning probe microscopiesare quickly becoming routine methdds 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 (I). 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 0 1995 American Chemical Society
microscope functionsby 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.
Analytical Chemistry, Vol. 67, No. 9, May 1, 1995 297 A
Figure I.Typical atomic force microscope.
Instrumentation
AFM instrumentation is relatively simple; it requires only a tip, a flexible cantilever, a deflection (or force) detection system, a positioning system for the tip or sample, and a data system. Figure 1 shows a common AFM setup. The movement system is a piezoelectric ceramic tube scanner situated to move the sample under the tip in the x, y, and z directions. 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 discussed briefly in the box on page 302 A. Laser beam deflection is the simplest and most widely used of these force detection methods. Cantilevers are usually made by lithographic methods using material that can easily be fabricated en masse, such as silicon, 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 A,and are blunter than tips made from silicon or silicon oxide; silicon tips generally have a radius of 10-20 HL Tip shape is very important in AFM because a blunt tip can interfere with the imaging 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 image will be a convolution that takes into account both tip and sample geometries. Modes of operation
The topography of a surface is imaged by either an attractive or a repulsive interaction of the tip with the surface. This interaction can be described by the LennardJones potential (6)
U
=
( A / d 2 )- (B/r6)
where A and B are constants determined
by the tip and 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 arid 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,usually one term dominates depending on how the microscope is operated. The two modes of operation used in normal force microscopy are contact (repulsive) and noncontact (attractive). Each has its own advantages and disadvantages, and the choice for imaging is usually determined by the sample characteristics. 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 cantilever images the sample as the tip scans the sample at this constant height. This d e flection also can be held constant by applying a voltage to the sample piezo. In general, the tip can scan the sample much faster when it is held at a constant height because of the finite time constant of the electromechanicalfeedback system. The resolution in contact AFM is determined by the area of contact between the tip and sample. With commonly used tips, this area has been estimated to be about 25 Hi2 (8).Although this contact area seems too large for achieving atomic
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.
Analytical Chemistry, Vol. 67, No. 9,May I, 1995
resolution, atomically resolved lattice structures have been imaged by AFM. The process by which atoms are imaged is not well understood, but it is b e lieved that the atomic image is a convolution of the tip and sample atoms. For example, with layered materials such as MoS,, it is thought that a piece of the surface under the tip breaks off and is carried along under the tip (7). Atoms are imaged via stick-slip interactions between rows of atoms of the moving piece and the stationary atoms of the sample. This sliding 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 using a very sharp tip and imaging under liquid, 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 N. When higher forces were used, the steps disappeared,leaving what seemed to be an atomically smooth surface. As a r e sult, the van der Waals forces were reduced by an order of magnitude because of the screening effect of the liquid between 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 curvature in the sharp tip smaller, but the m e niscus is also smaller because of the smaller area of the tip that is near the surface. For a sharp tip, if the adsorbed liquid is assumed to be water, the force will be 9 x lo-' N. Forces of this magnitude are measured routinely in force microscopy 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 x N. In both cases the force is too large for true atomic-scale imaging because for an area of contact of 25 A,, the force per unit area is 3.6 x 10'' N/m2, which exceeds the shear force of diamond. Only by submersing the tip and sample in liquid or by placing them in a vacuum can the forces
-
-
I ip on suri ittraction by L
3
uer Waals torce
moving up (tip off sur
ff surface
Figure 3. An idealized force curu-. Tip-sample forces can be measured using the force constant of the cantilever and the sensitivity of the z piezo. The difference between V, and V, is the voltage that must be applied to the z 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 cantileverdeflection versus tipsample separation, also called the force curve, the magnitude and range of influence of these forces can be measured. This method of measuring the force exerted on the tip as a function 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 z piezo, which causes the tip and sample to approach and withdraw repeatedly. When the sample approaches the tip, the cantilever 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 deflection of the cantileverbefore a large u p ward deflection caused by repulsive contact. Figure 3 shows an idealized force curve that takes into account only the normal attractive and repulsive forces b e tween tip and sample. Some samples, such as biological materials and soft polymers, are damaged by the larger forces of contact mode operation. By using noncontact AFM, these forces can be substantially reduced; howf the larger tipsample s e p aration, there is a loss of resolution.
ntact AFM technique held above the sample 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 because the force constant of the cantilever is modified by the force gradient between the tip and sample. The van der Waals
methods. In interferometrywas used to meas es 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 A above the surface to prevent capture by the surface water layer, which can dampen the oscillation. Operation in a vacuum can circumventthe 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 cantilevernear the surface and close to
Analytical Chemistry, Vol. 67, No. 9, May 1, 1995 299 A
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, imN
b
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. 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 ofthe SLlrfaCe 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(methy1 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 N
Figure 4. MoS, film on mica. (a) Topographic image, (b)frictional force image, and (c) elasticity image.
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300 A AnalyticalChemistry, Vol. 67, No. 9,May 1, 1995
Figure 5m Patterns etched into SnSe, 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 MoS, deposited on mica. Figure 4a shows a topographic image of MoS, deposited on mica, and Figure 4b shows a friction force image of the same area. The contrast between the two images demonstrates that the MoS, layer causes less friction than the mica substrate. This fact is not surprising because MoS,, 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 MoS, 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 obTo make a small diode from this pattained simultaneously, demonstrating the tern, three different layers would be amount of information that a single scan- needed. The bottom layer would be an inning probe experiment can provide. sulating substrate, the middle layer a Frisbie et al. (14)used AFM to proconducting material such as NbSe, or duce a map of the interactions between TaS,, and the top layer a semiconducting material such as MoS, or WSe,. Contact various combinations of hydrophobic and hydrophilic groups on a tip and patwith the top layer could be accomplished terned on a sample surface. Gold-coated using a conductive force probe. tips and samples were functionalized by The atomic force microscope can be self-assemblyof thiol monolayers, which used in situ in combination with a potenwere terminated with either COOH or CH, tiostat and a liquid cell to study electrogroups protruding up from the surface or chemical reactions such as underpotendown from the tip. The greatest attractive tial deposition (UPD). UPD involves the interaction between the tip and the surdeposition of a monolayer or submonoface occurred when like groups were on layer of metal on an electrode surface at the tip and surface. The attractive force be- a potential positive of its bulk reduction potween the COOH groups was the greattential. This deposited layer forms a new est because of hydrogen bond formation, atomic lattice, called an ad lattice, on top of and the attraction between COOH and ps was the lowest, as would be of hydrophilic and hydrophobic groups. Manipulation of the surface is also possible using the force microscope. Patterns can be etched into many different surfaces by the force of the interaction p and the substrate. One le is that of pataces of twodimensional layered dichalcogenides by scanning a tip over their surfaces in contact mode (15).These materials are composed of layers of chalcogenide and metal chalcogenide weakly held together by van der Waals forces. The rate of etching the gold lattice. AFM can provide images was proportional to the force between the of the ad lattice with atomic resolution. tip and the substrate and also was depenChen and Gewirth (17) have used dent on the number of missing atoms on AFM to study the atomic structure of bisthe surface, which act as nucleation sites muth underpotentially deposited on a for the removal of surface atoms. gold electrode surface. This system shows catalytic activity for the electroreduction Figure 5 shows patterns formed on a SnSe, surface by scanning the tip with a of H,O, dependent on the bismuth dehigh force and rotating the substrate to position. They knew that the catalytic acachieve different line angles. The lines in tivity was higher for submonolayers than Figure 5a are 20 nm wide and one unitfor complete monolayers of bismuth, but cell (0.61 nm) deep. Figure 5b shows a de- the structure of the ad lattices was unvice that might be patterned in a subknown. strate grown using van der Waals epitaxy, The different ad lattices formed by the which is a technique for making multibismuth could be imaged by observing the structure of the electrode surface at difnsional materials withstraints caused by inter- ferent potentials using AFM. At potentials layer covalent bonding (16).This subpositive of the potential for UPD, only strate is a single material with a mesa the gold 5 x 5 lattice with a spacing of 0.29 measuring about 50 x 100 x 0.6 nm, isonm is imaged (Figure 6a). When the polated from the rest of the top layer but still tential is moved to between 250 and 190 connected to the next layer. mV versus E%/', the hexagonal 2 x 2 bis-
muth lattice is formed with a distance of 0.57 0.02 nm between bismuth atoms (Figure 6b). If the potential is moved negative to 100 mV, a new rectangular lattice with an average distance of 0.34 f 0.02 nm is formed, replacing the hexagonal configuration (Figure 6c). Electroreduction of H,O, was found to be greatest at potentials corresponding to the formation of the 2 x 2 bismuth surface structure. Because more gold is exposed in this configuration, Chen and Gewirth proposed that a heterobimetallic formed by the peroxide moleeen the bismuth and gold ating the 0-0 bond, allowing
bismuth on Au( 111). (a) Gold lattice with 0.29 spacing before UPD, (b) 2 x 2 bismuth ad lattice with 0.57 nm spacing at 200 mV versus E$'', and (c) rectangular bismuth ad lattice with 0.34 nm spacing at 100 mV versus Eg31+/0.(Adapted from Reference 17.)
Analytical Chemistry, Vol. 67, No. 9, May 1, 1995 301 A
reaction occurring on its surface. Oscillabe measured simultaneously,thus allowing identification of areas of power dissipa- tions were induced by differential thermal expansion of the silicon cantilever coated tion in the electronic device. A metal semiconductor field effect transistor made with an aluminum layer. They were able to measure the reaction of ndoped GaAs and having a gold drain, gate, and source contacts was imaged therUsing the atomic force ZH, + 0, -+2H,O (9) microscopeas a sensor mally and topographically. The temperaRecent experiments used a modification of ture of the GaAs was higher than that of on a platinum catalyst deposited on the an atomic force microscope as a heat sen- the metal gate, source, and drain when a aluminum overlayer of the cantilever. sor for small electronic devices (18). Two bias of 4 V was applied between the source The microscope was placed in a highwires of chrome1and alumelwere epoxied and gate. vacuum chamber, and the reactant gases together, etched to create a thermocouwere allowed to flow into the chamber. AFM can also be used as a sensor for ple junction with a sharp tip, and then bent chemical phenomena. For example, n occurred, the platinum down to form a cantilever. A piece of aluheat to the aluminum Gimzewski et al. (19)used a modified minum foil was glued to the cantilever for atomic force microscope probe and its decantileverto oscillate laser deflection detection. The topograwith increasing amplitude until the gases flection detection system to measure the phy and temperature of the sample could of the chamber. Local oscillations induced in the cantileverby a es were estimated to d heats of reaction as small d be measured. The reForce detection tulate that an artificial nose Many different methods, described below, are used to detect the deflectionoi -__ could be produced using pattern recognicantileverproduced by the interaction of the sample with the probe tip. An ideal tion with an array of these tips having difdetectinn cvctem i c e a c v tn set tin m m n a r t cencitkre tn x r e n r cmQllrleflertinnc sts deposited on them. ctablc. et al. (20) used AFM to sense disg LU crete interactions between a mica surface ovement of the tip. An STM tip is placed above the back 01 111etalcantilevel, dlid the feedback circuit is used to maintain a constant tunneling current. The vol of biotinylated bovine serum alage applied to either the z piezo or the sample piezo produces an image of the surSA) coating.They functionalface. The tunneling current itself can also be used for imaging by mc-muring the roscope tip by attaching glass actual deflection of the cantilever. spheres to it and then coating the Capacitance. If the cantilever on an atc c force microscope one plate es with BBSA. This arrangement the capacitor and another plate is mounted above this plate, the capacitance b reen the two plates changes relative to the deflection of the cantilever. An in that could interact when the bican be formed either by moving the cantilever or the sample to maintain a cor onal streptavidinwas present. Force stant capacitance by monitoring the change in capacitance as the sample is rves taken with both blocked and unanr din present in the soluInterferometry. Interferometry uses a laser split into two beams: one focuse tip and sample surface rethe back of the cantilever and the other directed into a photodiode. The beam f ed that a greater force was required [sed on the cantileveris reflected back into the photodiode. The coherent beam the tip off of the surface when univel different pathlengths and produce an interference pattern, which changes d streptavidinwas present in soluas the cantilevermoves up and down. The photodiode registers this change as a tion, Measurement of this force provided &age signal, which is used to control the feedback of the instrument. an estimate of the rupture force of the Laser diode feedback. If light from a single-modediode laser is fo,,.,ed streptavidin-biotin bonds. the back of a cantilever and there are only a few micrometers between the lasl. A similar technique has been applied and cantilever, the laser is reflected back into the laser cavity, causing a change i to DNA base pairs (21).Oligonucleotides _--egain of the laser. The change in laser gain can be registered by a photodiode were attached to silica spheres on a sur2nd turned into a voltage signal to control the feedback of the microscope. face and attached to a microscope tip. Optical deflection. In this method a diode laser is focused onto the ba,., When the base pairs were oriented so ,.e cantilever and reflected onto a split photodiode.The diffP-n- in -1t2ges that complementarypairs could interact, +-reen the two halves of the photodiode is then monitored. the interchain forces could be measured Piemresistive detection. A silicon cantileverwith a resisLul UI I [;;51;5Lul II between molecules on the tip and the wark patterned into it is used. As the cantileverbends, the resistance of the si surface. The force required to pull apart gle silicon crystal changes because of the stress on the crystal. Measuring thi' the tip and surface depended on the numsistance change provides the signal for the feedback control. The advantage o ber of pairings between bases. When this method is that no external deflection rr------ement system is needed. noncomplementary strands were used, much lower forces were measured.
easier cleavage. Only this configuration has enough gold atoms exposed for the formation of the heterobimetallicintermediate.
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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(1) and multiple connections between the tip and surface. Cuture 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 the 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 ( 1O-l’ 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) Sarid, D. Scanning Force Microscopy, 1st ed.; 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) Grif6th, J . E.; Grigg, D. A.J. Appl. Phys. 1993, 74, R83. (6) Atkins, P. W. In Physical Chemistry, 3rd ed.; Oxford University Press: Oxford, U.K., 1986. (7) Bumham, N. A; Colton, R J. In Scanning Tunneling Microscopy and Spectroscopy Theory, Techniques,and Applications; Bonnell, D. A., Ed.; VCH: New York, 1993. Ch. 7. (8) Hutter, J. L.; Bechhoefer, J . J. 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. 1 9 8 7 , 61, 4723. (11) Hansma, P. K.; Cleveland, J . P.; Radmacher, M.; et al. Appl. Phys. Lett. 1 9 9 4 , 64, 1738. (12) Stem, J. E.; Tenis, B. 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, A.; Wrighton, M. S.; Lieber, C. M. Science 1994,265,2071. (15) Delawski, E.; Parkinson, B. A. J. Am.
Chem. SOC.1992,114,1161. (16) Ueno, R;Koma, A; Ohuchi, F. S.; Parkinson, B. A. Appl. Phys. Lett. 1991,58,472. (17) Chen, C-H.; Gewirth, A. A J 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. A.; Colton, R J . Langmuir 1994,10,354. (21) Lee, G. U.; Chrisey, L. A.; Colton, R J . Science 1994,266,771. (22) Deng, G.; Wu, R Methods Enzymol. 1983, 100,96. (23) Sidles, J . A; Garbini, J . L.; Drobny, G. P. Rev. Sci. Znstrum. 1992,63,3881. (24) Sidles, J. A.; Rugar, D. Phys. Rev. Lett. 1 9 9 3 , 70,3506.
Darrell R. Louder, a doctoral candidate at Colorado State University, is studying doping and defects in layered dichalcogenides by STM and SFM. Bruce A. Parkinson, professor of chemistry at CSU, conducts research in photo electrochemistry, scanning probe microscopies, and su$ace and materials 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. F.; Gerber, C. Phys. Rev. Lett. 1986,56,930.
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