Anal. Chem. 1996, 68, 185R-230R
Scanning Probe Microscopy Lawrence A. Bottomley,†,* Joseph E. Coury,† and Phillip N. First‡
School of Chemistry and Biochemistry, School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30032 Review Contents
Table 1. List of Acronyms
Books and Reviews Technical Advances in SPM Tips and Cantilevers Calibration and Metrology Force Measurement and Physical Properties Chemical Identification Variable Temperature New Probe Techniques Friction Force Microscopy Scanning Thermal Microscopy Magnetic Resonance Force Microscopy Magnetic Force Microscopy Photon Scanning Tunneling Microscopy Near-Field Scanning Optical Microscopy Scanning Capacitance Microscopy Ballistic Electron Emission Microscopy Other Probe Microscopies Scanning Electrochemical Microscopy Applications Electrochemical Scanning Probe Microscopy Alkanethiols Langmuir-Blodgett Films Biology Nanoparticles Liquid Crystals Crystal Growth Polymers Nanotechnology Porous Silicon References
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Scanning probe microscopy is a family of techniques which provide images of the surface topography and, in some cases surface properties, on the atomic scale. Since its inception in 1982, the number of papers devoted to technical advances in SPM (see Table 1 for a list of acronyms), the development of new scanned probe techniques, and the applications of SPM have risen at a dramatic rate (see Figure 1). The exponential growth since 1986 can be attributed to the realization that high-resolution images can be acquired under a myriad of sample conditions (i.e., in air, under vacuum, under liquids, etc.) and to the availability of highquality commercial instruments. This review covers papers published during the period from October 1, 1993 through January 1, 1996. Because of the sheer number of publications (∼6000 since the last review of this kind) and the available space in the journal, this review cannot be all inclusive. The articles cited illustrate only some of the many directions research in scanning probe microscopy is currently taking. For example, this review does not address the application of SPM to the characterization of semiconductor and supercon† ‡
School of Chemistry and Biochemistry. School of Physics.
S0003-2700(96)00008-X CCC: $25.00
© 1996 American Chemical Society
BEEM CDW CVD DNA ECSFM ECSPM ECSTM EQCM FFM FMM HOPG HREM IRAS LB LFM LS MIS MFM MOCVD MOKE MOS MOSFET MRFM NSOM PMMA PSTM RICM RIE RNA SAM SCM SECM SEM SFM SPM SPR SQUID ssDNA SSPM STEM SThM STM STOM STPM STS TCNQ TEM TTF UFM UHV UV XPS YBCO
ballistic electron emission microscopy charge density wave chemical vapor deposition deoxyribonucleic acid electrochemical scanning force microscopy/microscope electrochemical scanning probe microscopy/microscope electrochemical scanning tunneling microscopy/microscope electrochemical quartz crystal microbalance friction force microscopy/microscope force modulation microscopy/microscope highly ordered pyrolitic graphite high-resolution electron microscopy/microscope infrared reflection absorption spectroscopy Langmuir-Blodgett lateral force microscopy/microscope Langmuir-Schaefer metal-insulator-semiconductor magnetic force microscopy/microscope metal oxide chemical vapor deposition magnetooptic Kerr effect metal-oxide-semiconductor metal oxide semiconductor field effect transistor magnetic resonance force microscopy/microscope near-field scanning optical microscopy/microscope poly(methyl methacrylate) photon scanning tunneling microscopy/microscope reflection interference contrast microscopy reactive ion etching ribonucleic acid self-assembled monolayer scanning capacitance microscopy/microscope scanning electrochemical microscopy/microscope scanning electron microscopy/microscope scanning force microscopy/microscope scanning probe microscopy/microscope scanning plasmon resonance microscopy/microscope superconducting quantum interference device single-stranded DNA scanning surface potential microscopy/microscope scanning transmission electron microscopy/microscope scanning thermal microscopy/microscope scanning tunneling microscopy/microscope scanning tunneling optical microscopy scanning thermopower microscopy scanning tunneling spectroscopy tetracyanoquinodimethane transmission electron microscope tetrathiafulvalene ultrasonic force microscopy ultrahigh vacuum ultraviolet X-ray photoelectron spectroscopy yttrium barium copper oxide
ductor materials. Just these two areas accounted for ∼2000 publications during the period of this review. We assert that proper coverage of the contribution of SPM to these areas requires a separate review. The citations in this review were downloaded from the STN International Data Base. We paraphrased claims of the author(s) from the abstracts without consideration for their scientific validity. A number of tables are located throughout this work with additional references not specifically mentioned in the text. Analytical Chemistry, Vol. 68, No. 12, June 15, 1996 185R
Figure 1. Number of papers published concerning SPM since the development of STM in 1982.
Table 2. Reviews of SPM-Based Applications review area
ref
general organic films and adsorbates/carbon semiconductors/silicon superconductors catalysts
A7-A20 A21-A25 A26-A33 A34, A35 A36, A37
Table 3. Sharpening Procedures for Metallic Tips tip material
technique
ref
W W Ag W, Ir Pd W(111) W, Pt, Ir, Au Pt, Ir, Au, Pd, Rh W, Ni, Mo, Si Re Pt-Ir
electrochemical etching etching, vacuum anneal electrochemical etchng Ga+ ion beam milling electrochemical etching oxygen processing electrochemical etching electrochemical etching focused ion beam milling electrochemical etching electrochemical etching
B1 B2 B3 B4 B5 B6 B7-B9 B10 B11 B12, B13 B14
BOOKS AND REVIEWS A number of books have been published concerning SPM on different levels. The general principles of STM along with descriptions of some applications have been covered in detail (A1-A3). Scanning force microscopy and its applications to magnetic and atomic forces are detailed in Sarid’s book (A4). A collection of papers devoted to SFM and presented at the 67th ACS Colloid and Surface Science Symposium can be found in a special book issue (A5). The application of STM and SFM to the study of organic molecules is detailed in Bunshi’s book (A6). Review articles not mentioned in the text can be found listed in Table 2. Generally, these reviews contain at least 20 references and review the work of several research groups. TECHNICAL ADVANCES IN SPM Tips and Cantilevers. A great deal of effort continues to be put into improving the quality, reproducibility, and characterization of SPM tips and cantilevers. Publications in this category ranged from new etching methods and tip materials to new techniques based on special tips or cantilevers. Table 3 summarizes those publications related to sharpening procedures for metallic STM tips. In addition, there has been at least one review of tip and sample preparation techniques for STM experiments (B15). Several novel STM techniques based on unique tip materials or configurations were introduced or improved during the review period. Alvarado (B16) and Wiesendanger (B17-B19) discussed spin-polarized tunneling experiments using ferromagnetic STM 186R
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tips. Others demonstrated that several hundred picoamps of photoinduced tunnel current could be obtained from GaAs tips (B20). This may also be useful for obtaining magnetic contrast in the STM since spin-polarized electrons can be optically pumped into the GaAs conduction band. Sueoka et al. (B21) performed the inverse experiment. They optically pumped a GaAs sample and used a Ni tip to show that spin-polarized tunnel current was generated. GaAs tips have also been used as local photodetectors in a STM configuration (B22, B23). Using these tips, a magnetooptical imaging technique was developed with 250-nm resolution. It was demonstrated also that highly doped GaAs could be used directly for STM imaging (B24), and a MOCVD technique for GaAs tip fabrication was developed (B25). Other semiconducting STM tips were fabricated and tested. Diamondcoated tips were fabricated via plasma CVD (B26, B27) and shown to have enough conductivity for STM. Atomic resolution images of Si(111) 7 × 7 were obtained with SiC tips (B28). The presence of a band gap in the tip opens the way for some new spectroscopies. Single-crystal whiskers of ZnO have also been shown to function as STM tips (B29). Fullerene-coated STM tips were found to be stable and to produce enhanced corrugation on graphite samples (B30). A method was developed for ultrafast time resolution in the STM using magnetostrictive tips to directly gate the tunnel current (B31). Hoerber et al. (B32) arranged a double-tip STM to measure the influence of lateral electric fields on the ordering of liquid crystals. No interference between the two tunnel currents was observed to tip separations of 2 µm. This implies that surface currents are not a major contributor to the imaging mechanism on nonconductive molecules. Fabrication techniques for the specialized tips used in ECSTM were published. Chen et al. made ECSTM tips of 1-µm radius and faradaic leakage current less than 0.1 nA in 1 M NaCl (B33). Techniques for the fabrication of a new type of potentiometric ECSTM tip were described by Toth et al. (B34). Zhang and Wang have fabricated ECSTM tips with apparent electrochemical radii as small as 0.4 µm (B35). Advances in tips for photon-based techniques such as PSTM and NSOM were also made during the past two years. One review of tip preparation for PSTM was given (B36). The conversion of evanescent waves into propagating modes in Si and SiN tips was studied both experimentally and theoretically (B37-39). Some new photon-based techniques were also introduced. Davis et al. (B40) invented a submicrometer photodiode probe for detection of subwavelength optical intensity variations in the near-field of an illuminated device. A scanning force microscope with a fiberoptic tip was constructed for the study of ferroelectrics (B41). The SFM provides topographic information, while optical harmonics give information on the piezoelectric and electrostrictive properties of the sample. An optical near-field detection scheme was developed based on a modulation of the image force due to the surface photovoltage at a semiconducting SFM tip (B42). Table 4 presents publications related to the fabrication of tips for force microscopy. Modification of the adhesive properties was also studied by some groups. In general, it was determined that reducing the adhesive forces resulted in fewer imaging artifacts and better resolution. Adhesive forces were reduced by coating SFM tips with hydrophobic layers of octadecyltrichlorosilane (B53) or by treating in UV-ozone or oxygen-plasma (B54). A simple method for rendering tips either hydrophilic or hydrophobic was introduced and applied to the study of LB films (B55).
Table 4. Fabrication of Tips for SFM
Table 5. Probe Tip Self-Imaging Methods
notes
ref
Si-based micromachining of tips, etc. ultrahigh-resolution, single-domain MFM tips photoetchable glasses for high aspect ratio tips tips/cantilevers via single-mask process; few nanometer radii RIE of Si-on-insulator wafers; high aspect ratio “hopping mode” with ZnO whisker as high aspect tip RIE for sharp, high aspect ratio tips e-beam carbon contamination tips e-beam-deposited tips
B43 B44, B45 B46 B47 B48 B49 B50 B51 B52
Force sensing technology continued to advance over the last two years. Micromachining techniques were used to fabricate cantilevers integrated with various other components (B56). Piezoelectric force sensors were demonstrated (B57-B59), as was a sensor based on polymer thin-film technology (B60). Polymer cantilevers were shown to be 100 times softer than Si. A polyimide diaphragm was also found to be suitable as a replacement for the usual SFM cantilever (B61). A unique force sensor employed a carbon fiber “string” (B62). Changes in the tip-sample interaction force modify the string tension and hence the resonance frequency of the string. Kawakatsu et al. demonstrated a dual optical lever detection system for SFM with 10-pm sensitivity (B63), while Sader and White gave a theoretical analysis of the static deflection of cantilevers (B64). Improved or new techniques based on novel SFM tips or cantilevers include SFM with a conductive tip for monitoring conductance (B65, B66) or for direct writing applications (B67, B68). Two different voltage probes with picosecond time resolution were developed based on SFM designs (B69, B70). A strategy has been introduced for performing highly localized chemical catalysis on the surface groups of self-assembled monolayers via SPM (B71). SFM tips in the shape of a boot were used for imaging the sidewalls of vertical and near-vertical features (B72). A special servo mechanism was developed for sidewall measurements. SFM cantilevers by themselves have proven to be extremely versatile sensors, even without scanning capabilities. In one recent study (B73), cantilevers were used to perform surface stress measurements for Au and Pt while their surface free energies were varied by immersion in aqueous electrolyte solution and a variable potential was applied. The cantilever bending radius is a direct measure of the surface stress. Others have proposed that adsorption-induced surface stress on a cantilever could provide the basis for sensitive chemical sensors (B74, B75). The nature of a sensitizing overlayer on the cantilever determines the adsorbed species. Examples of micromechanical sensors based on this approach include detectors of Hg vapor, humidity, and optical irradiance (B76). The effect of tip shape on the acquired image is a pervasive issue in all SPM measurements. Theoretical approaches to this problem have been reviewed by Ciraci (B77) and by Landman and Luedtke (B78). For SFM it has been demonstrated by simulations that qualitatively different images are obtained by changing the size of the SPM tip (B79). SFM images reflect not only the geometrical structure of the surface but also various microscopic properites of the tip and the surface (B80). STM images have a large contribution from the local electronic structure, as shown by calculations for benzene on Pt(111) (B81). First-principles calculations of STM and STS have been carried out (B82-B84), including the effect of tip shape on the imaging.
self-imaging method SFM SFM SFM SFM SFM SFM SFM SFM SFM SFM SFM SFM SFM SFM STM
sharp structures on MOCVD-grown Cu films tip arrays via e-beam lithography and ion beam etching spherical particles submicrometer 3D features patterned in Si sharp sides of holes in polymer films profiles of monatomic steps on Au(111) aerosol deposition of metal particle columns + plasma etching latex balls stepped (305) surface of SrTiO3 sharp edges of MgO and NaCl crystals chemically textured alumite with micropillars colloidal gold particles colloidal gold particles 10-nm gold spheres Si “nanoneedles” from negative high-voltage pulses
ref B85 B86 B87 B88 B89 B90 B91 B92 B93, B94 B95 B96 B97 B98 B99 B100
It was found that a single atom can easily determine the tunnel current. Several groups devised means to determine experimentally the tip shape for SFM or STM probes. These techniques consist generally of imaging a surface with protrusions sharper than the SPM tip, the so-called “self-imaging” technique. Table 5 summarizes self-imaging methods for both SFM and STM. Methods for removing the effect of the SFM tip shape from experimental images were discussed by many authors, including some of those listed in Table 5. A new method of “envelope image analysis” was developed (B101). The method is essentially equivalent to existing methods but does not require numerical derivatives. An algorithm for removal of the probe tip shape from images of calibration grids was also presented (B102, B103). Nagase et al. (B104) determined the dimensions of Si nanostructures by expressing the SFM image profile as a modeling equation that includes the critical dimensions of the sample and tip. Refinement techniques for the removal of instrumental effects in SPM were discussed by Williams et al. (B105). Three papers demonstrated morphological restoration of SPM images (B99, B106, B107), given some information about the tip shape from either a standard sample or sharp features within an image. The effect of tip shape on measurements and the limits of SPM imaging were also the subject of several papers. Arai and Fujihara determined the effect of tip shape on SFM force-distance curves in aqueous electrolytes (B108). Experimental results were compared to the authors’ calculations. The influence of the SFM tip on the limits of measurement for rough surfaces was discussed by several authors (B109-B111), and the identification of tip artifacts continues to be of practical concern (B112-B114). The effect of tip deformation is also an issue for high aspect ratio tips, such as those grown by electron beam deposition (B115). Finally, questions have been raised concerning the ability of the SFM to image neighboring atoms in condensed structures. The nature and interpretation of atomic-scale imaging by the SFM was discussed by Lin and Meier (B116). For STM imaging, the effect of tip structure and its characterization has been addressed experimentally (B100, B117). A method for calculating STM images was also presented and applied to experimental images of sulfur adsorbed on Re(0001) (B118). Calibration and Metrology. Accurate, quantitative determinations of probe tip displacements and applied forces are essential for many measurements. Atomically resolved experiAnalytical Chemistry, Vol. 68, No. 12, June 15, 1996
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ments often rely on the underlying periodicity of the crystal lattice to calibrate distances. However, for most SPM techniques, the calibration procedures require a separate measurement standard. Calibration requirements and performance limitations for SPM have been reviewed by Stedman (C1). Under this topic, papers can be divided roughly into three categories: lateral displacement calibration, vertical displacement calibration, and force calibration. Nonlinearity and hysteresis in the piezoelectric actuators must be characterized periodically to assure accurate measurements. Joergensen et al. (C2) and Fu (C3) have developed techniques for extracting information on these aspects from hysteresis observed in SFM images of smooth, tilted surfaces. Stoll (C4) has also discussed the correction of geometrical distortions due to piezohysteresis and nonlinear feedback, and an automated method for estimation of lateral scan calibration factors was developed by others (C5). It requires that a periodic structure be imaged, however. Algorithms for drift and slope correction were also given by Yurov and Klimov (C6). Displacement calibration by external means was reported by several authors. SEM (C7) and a coupled-cavity laser diode sensor (C8) were used to determine the displacement versus voltage characteristics of tube-shaped piezoscanners. Patterned substrates are useful for calibration purposes, but Turner et al. (C9) took this a step further and designed a patterned substrate that can be used for relocating submicrometer features even after removal of the sample and subsequent replacement in the SPM head. For angstrom-scale distance measurements by SFM, Snetivy and Vancso (C10) have discussed the effect of specimen height on the lateral resolution. Height standards consisting of latex or SiO2 balls were used to calibrate SFM sensors (C11). Nagahara et al. developed a method for etching standard height steps on mica (C12). A method was also devised for in situ measurements of the large tip-sample displacements used in resonant atomic force microscopy (C13). Burnham has analyzed the effect of different SFM imaging modes on the apparent height of surface features (C14). For nanoindentation measurements, the effects of hysteresis and creep were reduced by replacing the standard PZT (leadzirconium-titanate) actuators with Pb[Mg,Nb]O3 electrostrictive actuators (C15). Accurate measurements of interatomic forces require calibrated force sensors. Hutter and Bechhoefer (C16) have developed ways to calibrate the cantilever spring constant and sharpness of SFM tips. They showed that SFM can be used for local force measurements with accuracy approaching that of a surface force apparatus. Other methods for force calibration were given by Scholl et al. (C17), who used a capacitance force sensor in situ, and by Smith and Howard (C18), whose technique attains a resolution better than 70 nN/Hz1/2. A fast and nondestructive method for the evaluation of spring constants was proposed by Sader et al. (C19). Instruments and methods for quantitative measurements of lengths and surface roughness were developed. A large-range tunneling profilometer was constructed by Liu et al. (C20). The instrument has 50-mm lateral motion and 15-µm vertical displacement. Resolution of a few nanometers was demonstrated, and the positioning repeatability was 0.2 nm over 1.4-mm traces. Procedures for extracting roughness parameters from SFM data were discussed by Brown (C21), and by Yoshinobu et al. (C22). Both procedures rely on scaling properties to characterize the surface roughness. 188R
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Table 6. Force Measurements by SFM measurement of forces between
ref
two metal surfaces two polymer surfaces similar and dissimilar oxide surfaces metal and semiconductor surfaces Si3N4 tip and oxide substrate Si3N4 tip and chemically modified surface metal and chemically modified surface polymer sphere and silicon substrate
D12-D13 D14-D17 D18-D22 D23-D25 D26 D27-D29 D30 D31-D32
Force Measurement and Physical Properties. SFM has been acclaimed as a quantitative probe of surface forces such as van der Waals, capillary, electrostatic, capacitive, double-layer, friction, magnetic, or adhesive forces. Its advantages are that virtually any surface of interest can be investigated and that measurements are relatively fast and easy to perform. The interacting areas are small and need only to be smooth and homogeneous on a small scale. A plot of the force interaction between a tip mounted on a cantilever beam and a flat surface as a function of relative tip-sample separation is commonly referred to as a force curve. Burnham et al. described how to interpret force curves so as to gain information about (i) the instant of tipsample contact, (ii) the magnitude and functional dependence of adhesive and long-range attractive forces between different tips and samples, (iii) mechanisms of long-range force interaction, (iv) tip-sample contact area, (v) the elastic modulus and plasticity of thin and thick films, and (vi) how the pull-off force varies as a function of the maximum load (D1). Butt and colleagues as well as Nakagawa and co-workers have reviewed measurement of surface forces and their impact on the behavior and properties of colloidal materials including biological molecules, micelles, and membranes (D2, D3). Measurement of attractive forces between individual colloidal particles and surfaces is hampered by particle movement preventing acquisition of equilibrium force-vs-distance curves. Butt showed that this problem can be overcome by analyzing force curves recorded during particle movement using Newton’s equation of motion (D4). Siedle and Butt have identified artifacts in force measurements made with the SFM associated with digitization (D5). Fujisawa et al. devised a method to calibrate sensitivity for the three-dimensional displacement of X, Y, and Z directions by using a lateral force curve (D6). Aime et al. showed that the contribution of the force of friction lying in the tangent contact plane between the tip and the object depends on the geometry of the object being scanned (D7). Noteworthy applications of force microscopy include Eastman and Zhu’s demonstration that with the strong capillary forces between a tip and a solid surface covered with a thin-layer liquid, the interfacial melting phenomenon can be detected with a SFM (D8). Luckham and Costello devised a method that can be used in air, electrolyte, or solvent for the direct measurement of the forces between colloidal sized particles (D9). Electrical doublelayer forces between monolayers with different terminal groups were measured by Ishino and co-workers using gold-coated SFM tips while electrochemically controlling surface potential of the cantilever (D10). Ott and Mizes used a SFM to measure the adhesion of surface-modified toner particles to various surfaces of relevance to xerography (D11). Table 6 presents additional references detailing the measurement of forces between different types of surfaces.
Several interesting biological applications of force measurements were published during the period of this review. Radmacher et al. simultaneously measured the topography, adhesion forces, elasticity, van der Waals, and electrostatic interactions between the SFM tip and thin metal films, aggregates of lysozyme, and single molecules of DNA (D33). Generally, observed discontinuities in the SFM force-displacement curves are attributed to the breaking of discrete bonds. Stuart and Hlady demonstrated, using surface-immobilized antifluorescyl IgG molecules and SFM probe-bound fluorescein ligands, that similar intermittent discontinuities in the SFM force-displacement curves may in fact be largely due to nonspecific discrete interactions between the protein and the SFM probe (D34). The enumerated factors that may cause anomalous behavior in a specific ligand-protein system and misinterpretation of SFM adhesion measurements were discussed. Moy and co-workers developed a tip functionalized with specific receptors for imaging complex specimens such as the cell membrane (D35). Binding was observed between the tips and ligands cross-linked to agarose beads by SFM-based force measurements. Specificity was confirmed by competitive blockage of both the ligands and the receptors. Dammer et al. used SFM to measure the binding strength between molecules of the cell adhesion proteoglycan of the marine sponge Microciona prolifera to assess the contribution of these molecules to the maintenance of the anatomical integrity of multicellular organisms (D36). The results indicated that under optimum conditions a single pair of adhesion proteoglycan molecules could hold the weight of 1600 cells! SPM techniques have been used to measure a number of physical properties that can be related to displacement sensing or modulation of mechanical resonances. The majority of these measurements are concerned with surface roughness, friction, and wear. Synopses of various aspects of these types of measurements have been given (D37-D42). One group has related SFM measurements of surface roughness to those obtained via an optical technique (D43). Plastic deformation in atomic-scale Pb contacts has been studied with combined SFM/STM (D44). The authors were able to perform simultaneous measurements of load and contact area. For glassy polymers, Young’s modulus was successfully measured via contact-mode SFM (D45). Nanoindentation measurements of hardness have been performed with a diamond SFM tip (D46). Hardness measurements can be made on surface monolayers and ultrathin films as a consequence of the shallow (1 nm) indentation depth. Others have examined the effect of tip blunting on nanohardness measurements (D47). Methods were also developed to determine the mechanical properties of polymer surfaces (D48). There have been several studies focusing on wear and friction in various materials. “Triboscopic” techniques have been developed for understanding local wear processes (D49). Combined SFM and FFM studies were used to correlate friction changes with variations in the local surface slope (D50). Friction and adhesive forces have been studied as a function of humidity using FFM (D51). Both decrease substantially with increasing humidity, implying lubrication by adsorbed water. The tribology of C60 and AgBr thin films has also been studied using UHV-SFM (D52). Regimes of wearless friction were found and the initial stages of wear were discussed. The effects of nitrogen (D53) and hydrogen (D54) incorporation on the tribological properties of carbon films
has been studied by SPM techniques. Similar studies have been made on magnetic tapes (D55, D56) and PET films (D57). Changes in surface morphology or mean surface height can be induced by various physical processes. A discussion of the quantitative determination of the crystallographic Miller indices via SFM measurements of facet angles has been given (D58). A STM was used to monitor the expansion and surface morphology changes of 1-2-mm-diameter Pd spheres exposed to hydrogen gas (D59). Initially flat surfaces were found to buckle into 3050-nm-high features. The expansion behavior was found to be consistent with a proposed model. The swelling and shrinking of polyaniline films during oxidation and reduction has been measured with SFM and STM (D60). SFM measurements agree with ellipsometry, while the STM gives much larger thickness changes. Morphological changes were also observed in polypropylene during stretching (D61), in yttria films due to water penetration (D62), and in metal films under stress (D63). Magnetostriction has been investigated using STM (D64), as has ferroelectric behavior (D65, D66). It was found that the ferroelectric domain structure could be modified, and in the latter study, the switching of individual crystallites was detected. The piezoelectric constant of thin films was also measured using STM (D67). In a unique experiment, the polarization force between an electrically charged SFM tip and the substrate was used to follow the process of condensation and evaporation of a monolayer of water on mica (D68). Cai et al. (D69) observed the reversible adsorption of molecular species on the surface of a layered clay by in situ SFM. Etch pits on graphite were used by Patrick et al. (D70) as “molecule corrals” to isolate ensembles of molecules for study by STM. The nucleation rate constant was extracted from images acquired during self-assembly of a molecular film. A method was introduced for studying thin films of adsorbates under various stages of compression (D71). The film is adsorbed onto a Hg sessile drop that can be expanded and contracted using a digitally controlled microliter syringe. This allows precise control of the drop volume and surface area. Campos et al. (D72) demonstrated that the cleaved surface of an organic salt can present at least two possible surfaces: a complete metallic cation layer or a complete insulating anion layer. Finally, an interesting recent development is the introduction of picosecond-resolved measurements of photothermal displacement, using the SPM to resolve the rise time of the surface expansion (D73). Chemical Identification. STM is unsurpassed as a technique for obtaining surface structural information at the atomic level. However, it is often impossible to identify the elements present at the surface exclusively from a STM image (E1). Although several research teams are actively addressing this problem, success in this endeavor has been limited (E2, E3). Using very low tunneling resistances, Biedermann et al. imaged surface ordering of a Pt25Ni75(111) crystal by chemical contrast between the alloy components and both the substrate lattice and carbon atoms of carbon superstructures on a Pt10Ni90(100) surface with STM (E4). Ruan and co-workers achieved atomically resolved discrimination of the oxygen or metal atoms of reconstructed (110) surfaces of Cu and Ni by reversibly manipulating the apex of the STM tip (E5). These findings reinforce the notion that STM can be used to discriminate between chemically different elements on metal surfaces only when they are in a well-ordered arrangement and an extremely sharp tip is used. Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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Table 7. New Designs in Variable-Temperature SPM temp (K)
ref
1.3-4.2 4.2 4.2, 77, 300 5-50 5-300 6-77 7-300 20-700 120-300 125-400 300-670 300-750
F1 F2 F3 F4 F5 F6 F7 F8 F9 F10 F11 F12
notes magnetic fields to 9.5 T UHV; detects photon emission UHV BEEM capabilities UHV; magnetic fields to 8 T UHV UHV UHV UHV high scan rates
Mo and Himpsel distinguished monolayer Cu stripes attached to the steps of W(110) from W terraces by STM with inverse photoemission spectroscopy from a Cu-induced empty state (E6). Usually, the contrast between Cu and W atoms is dominated by the difference of the atomic sizes. The contrast can be reversed with an applied bias that affords tunneling into the Cu empty state. This effect provides an unambiguous means for identifying the different metal atoms but is not widely applicable. In contrast, significant advances in the use of SFM for identifying the chemical nature of adsorbates has been made during the period of this review. Several groups measured adhesion and friction forces between probe tips and substrates covalently modified with self-assembled monolayers that terminate in distinct functional groups (E7-E10). Probe tips were prepared by coating commercial silicon or Si3N4 cantilever/tip assemblies with a thin layer of Au followed by immersion in a solution of a functionalized thiol. The spring constants and radii of the chemically modified cantilever/tip assemblies must be characterized to allow for quantitative friction and adhesion measurements. Au-coated Si and Si substrates have been treated with functionalized thiols and silanes, respectively, to produce substrate coatings terminated with different functional groups. Measured adhesive forces were found to agree well with predictions of the Johnson, Kendall, and Roberts theory of adhesive contact and correlate with the surface free energy of the molecular groups in EtOH. Friction forces between different chemical functional groups correlated directly with the adhesion forces between these same groups. High friction was observed between groups that adhere strongly, while low friction was observed between weakly interacting functional groups. The dependence of friction forces on the tip and sample functionality was shown to be the basis for chemical force microscopy in which lateral force images can be interpreted in terms of the strength of both adhesive and frictional interactions between different functional groups (E11). Variable Temperature. Variable-temperature SPM designs introduced during the review period were predominantly scanning tunneling microscopes. Papers that presented new designs are summarized in Table 7. Most can be classified as either low temperature (LT; below 300 K) or high temperature (HT; above 300 K), but at least one wide temperature range (20-700 K) instrument was demonstrated (F8). Applications of variabletemperature microscopes included studies of superconductors and CDW materials, Coulomb charging effects, equilibrium structures and fluctuations, and growth kinetics among others. Superconductors remained a popular focus for LT-STM work. Behler et al. (F13) found that a ferromagnetic STM tip generates extra vortices in NbSe2, the density of which provides a measure 190R
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of the tip stray magnetic field. For lead-bismuth alloys, STM investigations of the vortex state were carried out at 80 mK (F14). Spectroscopy of the superconducting gap was performed on highTc materials (F15, F16) and on the fullerene superconductor Rb3C60 (F17). Spectroscopic measurements also demonstrated the semiconducting nature of the laser-ablated YBCO surface (F18, F19). Other LT conductance spectra from YBCO samples showed zero-bias peaks, whose magnetic behavior was found to be consistent with the Appelbaum-Anderson tunneling model (F20). The proximity effect was also studied in high-Tc materials (F21) and conventional superconductors (F22). For CDW materials, among the most interesting results was the discovery of a Mott localization gap in 1T-TaS2 (F23). Significant charge transfer from CDW trough to crest was observed as a result of Mott-type electron localization. Dai and Lieber (F24) have published a review of STM work on the CDW in TaS2. STM studies of organic conductors also revealed CDW modulation consistent with appreciable interstack Coulomb interactions and spin density wave structure (F25). Coulomb charging effects were once more observed at low temperatures in small Au clusters (F26, F27), in cluster molecules (F28), and in granular metal films (F29, F30). The latter experiments found evidence for tunneling through three junctions. Dynamic observations of charge trapping and detrapping on Au and In clusters were made by Dubois et al. (F31, F32). It was found that the electric field from the STM tip could induce these events. Discrepancies with the “orthodox” model of the Coulomb staircase were found by Schonenberger et al. (F33) and attributed to the relaxation of polarization charge in their double-junction geometry. A number of variable-temperature STM studies were carried out to determine equilibrium surface structures or to derive activation barriers from equilibrium fluctuations. Low-temperature experiments included a determination of hydrogen-induced structural changes in Pd(110) (F34), a study of reconstructions on the W(100) surface (F35), and images showing the stablilization of asymmetric dimers on Si(100) (F36). Bucher et al. studied the morphology of self-assembled hexane- and octadecanethiol monolayers by variable-temperature STM (F37) and the approach to equilibrium (F38). Depressions observed were identified as substrate vacancy islands generated by chemical erosion during the self-assembly process. At high temperatures, metastable structures were imaged on the Si(111) surface after quenching (F39), and Ge(110) surfaces were observed in situ by STM during cooling from 720 °C (F40). Lateral diffusion of surface boron dopants was observed on Si(111) for temperatures up to 600 °C (F41). These observations also showed the importance of dimerization for strain relief. The time fluctuations of steps on Cu(11n) surfaces was investigated by Giesen-Seibert et al. (F42). Monte Carlo simulations were used to confirm a transition from uncorrelated kink motion to correlated kink motion. Activation energies were extracted for the dominant processes ocurring at step edges on the Si(100) 2 × 1 surface by three different groups (F43-F45). Nucleation and growth has been studied on an atomic scale for Ag on Pt(111) (F46, F47). The statistical data were compared with rate equations from nucleation theory. The growth of Ag on Si(111) was also studied in UHV between 80 and 100 K (F48). A STM study of thiophene adsorption on Ag(111) at 120 K showed that all molecules of a given identity appear to orient in
the same direction with respect to step edges (F49). At liquid helium temperatures, the adsorption of Xe on Pt(111) was studied from the arrival of the very first atoms up to completion of a monolayer (F50). It was found that Xe preferentially binds to a low coordination site at the upper edge of Pt steps. For the growth of Si on Si(111) surfaces, both step-flow and island growth were observed by HT-STM (F51). HT-STM was also used to monitor the growth of SiC hillocks on Si(110) (F52, F53). Feltz et al. used HT-STM to observe oxide growth on Si(111) (F54) and chlorine etching of the Si(111) surface (F55). At least one HT-SFM study was published during the review period. Surface self-diffusion on polycrystalline Au was measured at 300-773 K using an SFM and the so-called single-surface scratch method (F56). LT-SFM was used to image monomeric type I collagen (F57). Length distributions were significantly narrower than those derived from TEM. LT-SFM and LT-STM were also used to measure the internal friction in fullerene films (F58). Among other variable-temperature investigations, there has been a low-temperature measurement of STM-induced photon emission (F59). In this work, the 1 × 2 reconstruction of Au(110) was resolved in the map of emission intensity. It was the first observation of atomic resolution in STM-induced photon emission. 1/f noise in the STM tunnel current has been studied as a function of temperature (F60). Most other papers were concerned with technical advances in SPM components or characterization of materials used in fabrication of variabletemperature instruments. These publications include the description of a detachable thermocouple contact for the measurement of SPM sample temperature in UHV (F61), an inexpensive sample heater stage for 300-400 K SFM studies (F62), and measurements of the thermal properties of SPM materials (F63, F64). NEW PROBE TECHNIQUES Friction Force Microscopy. Nanotechnology is concerned with mechanical structures and their properties in the nanometer range. This scale is below the resolution capability of classical tribometers. SFM and FFM, when operated as tribometers, are suitable tools for investigating nanotribological properties. The SFM and FFM simultaneously measures forces normal and parallel to the sample surface with atomic or near-atomic resolution. The theory of atomic-scale friction has been reviewed (G1). During the period of this review, several interesting applications of FFM have been reported (G2-G5). Frictional forces between a self-assembled dialkylammonium surfactant monolayer on mica and a silicon tip were measured under dynamic shear LFM and compared with those for bare substrate mica under similar conditions (G6). At a fixed shear velocity, friction increased linearly with applied load for both substrates. The microscopic frictional properties of barium arachidate and behenic acid LB films were investigated with FFM (G7). Direct observation of phase separation in LB films composed of polyamic acid long alkylamine salts by FFM was reported by Yuba and coworkers (G8). Overney and co-workers presented a fundamental study of FFM on an organic bilayer assembly (G9). Frictional domains were observed. A localized frictional anisotropy was found and shown to originate in different 2D crystal orientations. Stick-slip motion on a molecular scale was also observed. Local elastic compliance was measured simultaneously with a FFM on thin films of phase-separated mixtures of fluorocarbons and
hydrocarbons (G10). Higher friction and lower Young’s modulus were found on fluorocarbon domains. Variations in pH during sample preparation lead to differences in film formation, i.e., topology, elasticity, and friction. On increasing pH, both the Young’s modulus and friction force were shown to be inversely proportional to pH. Fujihira and Takano reviewed the characterization of the frictional properties of LB films by FFM (G11). Lamellar crystals of poly(oxymethylene), grown from dilute bromobenzene solution, were studied with an FFM (G12). Frictional force anisotropy was observed, indicative of molecular loops arising from chain folds at the folded surface, oriented predominantly parallel with the edges of the crystals in the different chain fold domains. SFM, LFM, and FMM was used to determine the viscoelastic properties of tow-toughened isotactic polypropylene/ethylene-propylene copolymer resins (G13). The surface morphology of films prepared by compression molding of two incompatible polymers, polypropylene and poly(ethylene terephthalate), was studied using SEM, time-of-flight secondary ion mass spectrometry, and LFM (G14). FFM of photoreactive polyacrylamide thin films suitable for medical devices is presented (G15). Dynamic contact angles and frictional coefficients depend upon the thin films’ structure and thickness. Overney reviewed nanotribological studies on polymers focusing on indentation, nanoscratching, failure of adhesion of pretreated polymer surfaces, stretched and stained polymers on the surface, molecular lubrication, and wearless friction on the molecular scale (G16). Cross sections of semiconductor heterostructures and multiple quantum wells were imaged in air with LFM and SFM. Differences in the dynamic frictional coefficient enabled differentiation of these features with a lateral resolution of ∼4 nm (G17). Bhushan and Ruan reviewed useful applications of FFM for determining the tribological properties of magnetic recording media (G18). Nanoindentation studies provide measures of hardness uniformity of the magnetic coating material. Nanoscratch experiments revealed deformation and displacement of tape surface material under light load. Friction force and surface topological profile measurements afford localized values of friction and surface roughness. Noteworthy improvements in instrumentation or methods have also appeared. Binggeli and co-workers described applications of an electrochemical FFM that enabled frictional force measurements during potentiostatic control of a sample immersed in electrolytes (G19). Goeddenhenrich and co-workers presented a lateral modulation technique for simultaneous friction and topography measurements with the SFM (G20). Recording topographical and lateral force data simultaneously allows one to quantify the contribution of friction to topographical images and the kinetic friction coefficient of the Si3N4 tip on the mica sample (G21). The twisting spring constants of the cantilever have been determined by computer statistical analysis (G22). These values, even though they are essential for the quantitative determination of frictional coefficients, are not provided by the manufacturer. An approach to imaging friction force distribution on the nanometer scale was presented by Yamanaka and Tomita (G23). The sample is laterally vibrated and both the amplitude and the phase of resultant torsion vibration of the cantiliver are employed for imaging. A friction force curve was proposed in which the friction force amplitude and phase are recorded simultaneously with the normal force, as a function of the tip-sample distance. The Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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usefulness of this curve was illustrated in the characterization of slip and deformation in the tip-sample interaction. Contributions to other areas of SPM resulted from in-depth evaluations of frictional force profiles. A particularly noteworthy example involves measurement and evaluation of force curves determined by SFM. Hoh and Engel investigated the contribution of friction on force curves because of their growing importance in the study of intermolecular forces and surface properties (G24). They showed that friction contributes to the hysteresis in the contact part of the force curve and creates an offset in the contact line. In addition, sudden onset of the friction at contact can cause force curve discontinuity which may be misinterpreted as a shortrange attractive interaction. Scanning Thermal Microscopy. Several variations of SThM were implemented or improved during the review period. Nakabeppu et al. presented designs of thermocouple cantilever probes for SThM (H1, H2). They found that the dominant mode of heat transfer and a major source of image distortion is gas conduction. Proper probe design is helpful, but operation in a vacuum eliminates the problem. Thermocouple probes were used earlier to image temperature variations in operating semiconductor devices (H3). Methods for producing heat flow across the tip-sample junction included resistively heated tips (H4, H5) and modulated laser irradiation (H6, H7). The latter method relies on a STM to measure modulations in the tip-sample distance due to local thermal expansion. Molecular contrast observed on liquid crystals was presumed to be due to variations in the local barrier height. In the SThM image, contrast depends on the temperature gradient at the tunnel junction (H8). Surface roughness and topogographic variations of the underlying substrate are not convoluted with the signal due to the feed-forward acquisition method of this technique. The review period also saw SThM and other techniques applied to studies of CVD diamond films (H9). This study related electrical, thermal, and structural properties of the films to CVD growth conditions. Finally, one theoretical investigation of heat transfer across a tunnel gap was carried out (H10). It was determined that heat transfer across the vacuum gap through nonpropagating electromagnetic modes is insufficient to account for the observed rates of heat transfer. Magnetic Resonance Force Microscopy. Within the review period, the first experiments. in MRFM were conducted by Sidles and co-workers (I1, I2). In these experiments, a force microscope cantilever was used to detect the magnetic force exerted by electrons and nuclei in a sample. The magnetization of the sample was modulated at the resonant frequency of the cantilever, using standard magnetic resonance techniques. The resulting excitation of the cantilever was detected optically. The potential for singlespin detection by MRFM and potential applications in biomolecular imaging make this new technique extremely exciting. In a similar technique, Wellstood and co-workers have developed scanning probe magnetic microscopes which use dc SQUIDs to obtain images of the magnetic field above a sample surface (I3). In these instruments, the SQUID is held fixed and the sample is scanned in a raster pattern by a computer-controlled cryogenic positioning mechanism. The authors record the output of the SQUID as a function of sample position and use this to construct gray scale or false color images of the magnetic field above the sample. Spatial resolution of ∼20-80 µm and a 192R
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magnetic field resolution of ∼20-200 pT for a 1-s average were reported. Magnetic Force Microscopy. MFM imaging methods and probes have been reviewed recently by Babcock et al. (J1). Over the last two years, advances were made in tip fabrication, instrumentation, MFM techniques, and the understanding of tipsample interactions. MFM was often applied to studies of magnetic recording media, including thin films and small particles, though some other interesting applications were found for the method. Because the stray magnetic field of the probe tip determines most of the MFM imaging characteristics as well as the tipsample interaction, it is extremely important to use tips apropriate to the sample under study. The optimization of thin-film tips for MFM has been discussed by Babcock et al. (J2), and Matteucci et al. have applied electron holography to the characterization of MFM probe tips (J3). Results from the latter experiments were in good qualitative agreement with calculations based on a macroscopic dipole model for the sensor tips. Thin magnetic films deposited on SFM probes were popular choices for MFM tips. Probes of this sort included e-beam deposited carbon tips overcoated with magnetic films (J4), electrodeposited thin-film tips (J5), CoPt-Cr as a minimally interacting coating (J6), and FeNi bilayer tips (J7). Using Fe-Ni bilayer tips or unmagnetized, single-layer Fe tips, the image contrast showed domains or domain walls, respectively. The last two years has seen its share of new or improved instrumentation for MFM. A novel MFM utilizing a vertically cantilevered probe tip was developed (J8). This new geometry provides maximum sensitivity while inhibiting uncontrolled vertical deflections. A “just-on-surface” MFM for less than 10-nm spatial resolution was proposed (J9), and a low-temperature MFM using contact and noncontact forces was demonstrated (J10). Another low-temperature MFM was developed using piezoresistive cantilevers (J11). The authors suggest that the microscope is ideally suited for characterizing thin films of high-Tc superconductors. A MFM was combined with a SEM in vacuum in order to facilitate selection of the MFM imaging field and improve MFM performance (J12). MFM technical advances include the further development of tunneling-stabilized MFM (J13-J16), a fast technique that routinely achieves submicrometer resolution. MFM performed in liquids offers many benefits, including the simulation of real environments for technological and biological process. Giles et al. (J17) demonstrated MFM of recorded magnetic bits in both air and liquid. The theory was worked out for the force acting on a magnetic dipole above a type-II superconductor (J18); the results are applicable to low-temperature imaging of vortices. The understanding of tip-sample interactions benefited from micromagnetic calculations performed by Mueller-Pfeiffer et al. (J19). The simulations revealed that domain walls in thin Fe films are polarized by the stray field of the tip. Contrast from the polarized walls differed drastically from that expected from undisturbed walls but agreed well with experimental observations. Experiments by others studied distortions caused by the magnetic tip (J20) and reversible and irreversible magnetization changes induced by the tip (J21). The image contrast was shown to depend on the geometry of the imaged structure (J22). A theoretical study by Ness and Gautier also addressed the problem of magnetic tip-surface interactions (J23).
Clearly the most popular industrial application of MFM is the study of magnetic recording media. This includes both thin-film and particulate materials. Among the most significant advances was a technique for submicrometer measurements of the coercivity of perpendicular recording media using MFM and an electromagnet (J24). In other work, MFM and spin-polarized SEM were used to study the relation between recorded magnetization structures and reproduced noise properties in CoCr alloys (J25, J26). In CoCrTa/Cr thin-film recording media, the effects of different film thicknesses and Cr concentration were determined (J27). The erasure of magnetic bits was studied by Proksch et al. (J28) by incorporating a high-field magnet into a commercial SPM. Domain wall structures were studied in magnetite and Fe thin films (J29, J30), in sputter-deposited Co-Pd multilayers (J21), and in Ni/Cu/Si(001) thin-film samples (J31). The last study was the first to perform MFM on epitaxial thin films. In this work, the domain structure was shown to have a sharp transition to a finer length scale above a Ni thickness of 9 nm. Bochi et al. have also investigated the magnetic anisotropy in epitaxial Ni/Cu1-xNix/Cu/Si(001) thin films as a function of Ni thickness and alloy composition (J32). Magnetization reversal mechanisms in small particles (barium ferrite) have been studied by Chang et al. (J33, J34), and by Luo and Zhu (Fe) (J35). Among other interesting applications of MFM published during the review period was the observation of single vortices in thin films of high-Tc superconductors at 77 K (J36). A disordered vortex arrangement was observed, with the vortices strongly pinned. In another study, the stray field of the MFM tip was used to locally nucleate vortex bundles (J37). Xu et al. proposed that the MFM could be used to accurately measure the levitation force acting on a magnet placed above a type-II superconductor (J38). The microscopic size of the MFM tip enables one to obtain the intrinsic temperature-dependent penetration depth of a single grain. Local topographic modifications induced by STM were found by MFM to change the local stray-field distribution in Permalloy thin films (J39). Finally, MFM of amorphous, compressed Terfenol-D films on both Si and glassy silica showed that their ferromagnetic domain structure is 1D and periodic (J40). All observations indicated that the domain morphology is determined by elastic contributions to the domain wall and film/ substrate interaction energies. Photon Scanning Tunneling Microscopy. Goudonnet and co-workers reviewed applications of PSTM and spectroscopy since 1988 (K1). The authors demonstrate the ability of this microscope to produce images of the surface of transparent samples with a resolution below the Rayleigh limit for both a visible and an IR source. Kaupp has reviewed the applications of SFM, STM, and PSTM in photochemistry (K2). The history of PSTM has been reviewed by Wu (K3). Many of the applications reported during the past two years have centered on the characterization of planar optical waveguides. PSTM has been used to determine the mode-cutoff wavelengths (K4) and refractive index profiles (K5) for optical waveguides (K6). Barchiesi and Labeke (K7) showed that it is feasible to distinguish index defects near the surface of the sample and geometric defects of the profile with either the STOM or PSTM provided that the three experimental parameters of detection distance, angle of incidence, and light polarization are taken into account. Adam et al. used an incoherent polychromatic light source to image submicrometer structures in the constant intensity mode. This
new method for operating a PSTM offers a wide range of applications including determination of local indexes of refraction or local spectroscopies (K8). PSTM is a powerful tool for measuring the spectroscopic properties of condensed matter. It functions as a localized excitation probe and enables spatially resolved investigations into coherently excited quantum-mechanical states, such as an exciton, or controlled radiation fields. Saiki and Gonokami utilized PSTM to observe potential structure in quantum wells (K9). Highly localized, laser-excited optical modes of silver colloid fractal clusters were observed by Tsai et al. using PSTM (K10). Their results verified the main concepts of the recently developed resonant optical theory of fractal objects. Adam and co-workers (K11) determined the spatial extension of the surface plasmon evanescent field of a silver film with a PSTM and showed that, for tip-to-sample distances smaller than half the wavelength of the incoming light, the collected intensity curves are identical in any area of the sample. Krenn et al. used a PSTM with probe-sample distance controlled by electron tunneling to probe the localized surface plasmon fields of individual nanometric silver particles (K12). They found that the strength and spatial localization of the surface plasmon fields strongly depended on the excitation wavelength. Dawson et al. (K13) directly imaged surface plasmon propagation on thin silver films. They found that the surface plasmon remains tightly confined in the original launch direction with insignificant scattering to other momentum states. They exploited the spectroscopic capability of PSTM to directly study the launch and propagation of surface plasmons on thin silver films (K14). Using two input laser beams of differing wavelengths to excite surface plasmons at the Ag-air interface, light from both wavelengths was coupled into the fiber probe via the respective surface plasmon evanescent fields. The high asymmetry in images acquired at the probe wavelength afforded a means to measure surface plasmon propagation. Excitation of localized scattering centers on rough Ag films enabled measurement of the launch of delocalized surface plasmons. Fillard and colleagues espoused the use of PSTM as a nondestructive, submicrometer-scale imaging method for microelectronic applications (K15). They placed emphasis on the ability of PSTM to provide images of layers on semiconductor substrates. Lee investigated the possibility of using a PSTM for laser lithography (K16). A contrast enhancement material was coated onto a sample slide and coupled to the prism of a PSTM. Direct patterns were produced by varying the exposure time and the shape of the probe tip. A highly localized evanescent light from a fiber probe with a 100-nm-diameter aperture of a PSTM was used by Jiang et al. (K17) to carry out localized photochemical transformations in LB films of photochromic material. Ohtsu reviewed the status of PSTM and its applications in ultrafine device fabrication drawing mainly from his group’s recent contributions in this area (K18, K19). New developments in PSTM instrumentation have been reported during the last two years. Fornel presented the first images obtained in the near-infrared region with the PSTM on quartz and silicon oxide substrates (K20). Moers et al. used silicon nitride cantilevers as near-field optical probes in a PSTM (K21). Vertical forces and torsion were detected simultaneously with the optical near field, allowing a comparison between topography and the optical signal. Images of a thin indium tin Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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oxide film and a LB layer of 10,12-pentacosadiynoic acid showed absorption contrast with a lateral resolution of ∼30 nm. Scanning tunneling-induced emission from surfaces has also been investigated. Berndt and co-workers observed STM-induced emission from small particles on a Cu single crystal surface (K22) as well as intense emission from monolayer films of C60 fullerenes on Au(110) surfaces when the STM tip was placed above an individual molecule (K23). Sivel et al. (K24) demonstrated that the photon emission by STM in air can be controlled, i.e., enhanced or suppressed, by the manner in which the tunneling experiment is performed. Berndt and Gimzewski (K25) asserted that STM-induced light emission from noble metal surfaces arose from inelastic tunneling excitation of tip-induced plasmon modes. They have used the STM tip-induced photon emission to determine the role of local dielectric properties and the strength of the electromagnetic coupling between metallic objects in nanometer proximity (K26). They have reviewed previous work dealing with photon intensity emitted from the tunneling gap of a STM (K27). By careful analysis of the maps of the integral photon intensity, they elucidated the effects of adsorbates and structures created with the STM on the local photon emission properties. McKinnon and co-workers (K28) described simultaneous photon emission and tunneling spectroscopy measurements on polycrystalline Ag surfaces. They showed that the photon emission process and the intergranular contrast in the photon maps depend upon interactions between individual grains in the Ag film and that certain grains behaved as isolated entities while others radiate in a more collective manner. They concluded that, in Ag films, the photon emission behavior is strongly affected by grain-boundary effects. Bischoff (K29) attributed the observed luminescence from the surfaces of semiconductors or thin metallic films to the recombination of minority carriers injected into a semiconductor and to the induction of surface plasmons by electrons tunneling to a metallic surface. Emitted photons originate from the decay of surface plasmons. Horn (K30) determined the dependence of dopant type, dopant concentration, and surface quality on STMinduced emission from p and n doped GaAs (100) surfaces. Areas with mechanically induced crystal defects showed drastically reduced luminescence. Ito (K31) found a correlation between the light emission spectra and the surface topography of surface microstructures on granular Au films. STM light emission spectroscopy of surface microstructure has been reviewed by Uehara and Ushioda (K32). Majlis and colleagues (K33) presented a convenient way to determine localized optical constants of a magnetic sample using photon emission STM. Their approach combined theoretical analysis that included the influence of the sample-tip geometry and the anisotropic response of the ferromagnetic substrate with measurements of the circular dichroism of the emitted light. Van Labeke (K34) and co-workers experimented with a new kind of scanning near-field microscope in which the tip is used in emission mode and where detection is mediated via homogeneous or evanescent waves. Near-Field Scanning Optical Microscopy. Several articles describing the technique and reviewing recent progress in the field of NSOM have been published during the past two years (L1-L3). The design and theory of operation of a new form of nearfield polarizing optical microscope was presented by Vaez-Iravani 194R
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and Toledo-Crow (L4). The system uses electrooptic premodulation of light to generate two simultaneous complementary images of samples, which generates a final output signal that is a linear representation of the sample birefringence and is independent of the sample transmissivity/reflectivity. These authors used this microscope in the characterization of thin sections of Kevlar fibers and polymer-dispersed liquid crystals (L5). Polarization contrast was presented in fluorescence images of a LB monolayer obtained with a NSOM operated in reflection. A tapered optical fiber was used both to excite and to collect the fluorescence. Jalocha and van Hulst showed that the fluorescence is polarized along the molecular orientation in the LB monolayer (L6). Raman spectroscopy in conjunction with NSOM was first used by Jahncke et al. to image Rb-doped KTiOPO4 at high spatial resolution (L7). Differences between near- and far-field Raman measurements were presented and discussed. Kim and coworkers invented a new NSOM technique based on the surface plasmon resonance (L8). Enhanced fields are localized at individual surface irregularities by the scattering of plasmons which produce a conical radiation. Interactions between a probe tip and the enhanced fields were recorded and related to surface topography. The authors compared this technique to SFM and STM and discussed its potential advantages. Bielefeldt and co-workers imaged opaque samples in reflection mode (L9). A quartz glass fiber tip was used both to illuminate and to collect light locally reflected from or emitted by the surface. The collected light was directed to a monochromator for spectral analysis at each sampled point. Boehm and co-workers demonstrated that a NSOM combined with a picosecond laser sampling system is capable of performing function and failure analysis of microwave devices at electrical signals up to 7.5 GHz (L10). Hsu and co-workers presented a nonoptical shear force feedback method to regulate tip-sample distance in NSOM (L11). In the shear force setup, the dither piezo and the tip form an electromechanical system, whose power dissipation on resonance is sensitive to the change in damping force as the tip approaches and interacts with the sample. The change in power dissipation is proportional to the change in the electrical impedance of the dither piezo. Tip-sample distance feedback control was achieved by measuring this change in dither piezo impedance. Leong and Williams presented a new approach to shear force detection based upon capacitance sensing (L12). The design, operation, and performance of the capacitance detection using W and fiber tips are presented. Shear force topographical images of hard and soft surfaces were shown. Safarov and co-workers developed a NSOM specifically for the study of thin magnetic films (L13). The magnetooptical effect in the evanescent mode was measured through a lock-in amplifier by modulating the magnetic field produced by a coil surrounding the tip. The authors used this system to study two films exhibiting perpendicular magnetization and found that the intensity of the evanescent mode was strongly enhanced at resonance. The interaction of the light electric field with the Au surface plasmon yielded an amplification of the magnetooptical effects in the evanescent mode. Silva and Schultz developed a NSOM for magnetooptic Kerr imaging of magnetic domains with 10-nm resolution (L14). Ag particles were used as probes for MOKEsensitive imaging since these small metal particles exhibit a localized plasmon resonance in the visible range, which greatly enhances their optical scattering cross section. The separation
of the probe and sample was regulated by a Newton ring interferometer. The polarization rotation of the scattered light due to near-field magnetooptic interactions was measured. They presented a two-dimensional image of thermomagnetically recorded magnetic domains in a perpendicularly magnetized Co/Pt multilayer material with an imaging resolution of
