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Imaging and Modification of Nonconducting Polymer Films by the Scanning Tunneling Microscope? James R. Sheats Hewlett-Packard Laboratories, 3500 Deer Creek Road, Palo Alto, California 94304 Received November 22, 1993. I n Final Form: March 4, 1994@
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Images of films (500-1100 thick) of several nonconducting polymers, spin cast on Au-coated silicon wafers, have been obtained with a scanning tunneling microscope (STM). Imaging was not possible if films were cast on Si02 and depends on scan rate: when a critical value was exceeded, the image became grossly degraded and a damaged spot under the tip was optically visible. The damage threshold was generally higher for Pt/Ir tips than for ion-implanteddiamond. Damage was not observed when stable, repeatable images were obtained;these showed mean roughness of -5 A, and 10-50-a undulations 100-300 A wide. Atomic force microscopy (AF'M) images were also obtained and confirm the STM data. Grooves several nanometers deep were formed and imaged with the same tip (ion-implanted diamond), without tip degradation. The mechanism of imaging is postulated to involve a combination of field-assistedelectron transfer and pressure-induced effects.
Introduction The scanning tunneling microscope (STM)is expected to be severely limited in its application to organic materials,l since it requires electrical conductivity. Nevertheless, ultrathin layers (monolayers or small multiples thereof) of a variety of molecules have been imaged successfully in molecular detail,2-9despite a lack of clear understanding of the process. A much smaller number of reports exist of imaging of thicker layers. The first, from Michel et a1.,I0describes the imaging of films (up to 100 nm thick) of solid alkanes on gold or graphite substrates, although the images published (and apparently the best-resolved ones) were somewhat thinner, in the range of -2-20 nm. Very recently Tang and co-workers reported imaging a polyhydroxycellulose film several hundred angstroms thick.'] We have found that the surfaces of several chemically distinct polymers (cast as films -500-1000 thick on metal substrates) can be imaged in the STM; these include poly(methy1 methacrylate) (PMMA), polystyrene (PS), polydiphenylsiloxane (PDPS), and a commercial novolak photoresist. These images do not show molecular details, but the lateral resolution of -1-5 nm is capable of revealing significant features of surface morphology. Comparison to atomic force microscopic (AFM) images establishes that these features are real. There is an approximately thresholdlike dependence of the imaging
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f Dedicated to Prof. Harden M. McConnell on the occasion ofhis 65th birthday. Apreliminary report ofthis work was first presented publicly on April 4, 1992, at Stanford University in a symposium celebrating Professor McConnell's birthday. Abstract published in Advance A C S Abstracts, J u n e 15,1994. (1)Magonov, S.N.Appl. Spectrosc. Rev. 1993,28,1-121. (2)Lindsay, S.M.; Sankey, 0. F.; Li, Y.; Herbst, C.; Rupprecht, A. J . Phys. Chem. 1990,94,4655-4660 and references cited therein. (3)Breen, J. J.;Flynn, G. W. J . Phys. Chem. 1992,96,6825-6829. (4)Albrecht, T.R.; Dovek, M. M.; Lang, C. A,; Grutter, P.; Quate, C. F.; Kuan, S. W. J.; Frank, C . W.; Pease, R. F. W. J . Appl. Phys. 1988, 64,1178-1184. (5)Hawley, M.E.;Benicewicz, B. C. J . Vue. Sei. Technol. 1991,B9, 1141-1147. (6)Alves, C. A.;Smith, E. L.; Porter, M. D. J.Am. Chem. SOC.1992, 114,1222-1227. 131, 59-68. (7)Swalen, J. D. Colloids Surf. 1989,38,71-77. (8) Mitzutani, W.; Shigeno, M.; Ono, M.; Kajimura, K. Appl. Phys. Lett. 1990,56,1974-1976. (9)Edinger, K.; Gijlzhauser, A.; Demota, K.; Wo11, Ch.; Grunze, M. Langmuir 1993,9,4-8. (10)Michel, B.; Travaglini, G.; Rohrer, H.; Joachim, C.; Amrein, M. 2.Phvs. B : Condens. Mater. 1989.76. 99-105. (ll") Tang, S.-L.; McGhie, A. J.; Suna, A. Phys. Rev. B , 1993,47, 3850-3856. @
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quality on scan rate: beyond some level the image quality suddenly and irreversibly degrades. The value of this threshold depends on the tip material, and the nature of the good images is also affected by the tip material.
Experimental Section Films were prepared by spin casting from appropriate solutions, made from commercial materials (typical research grade) without further purification: PMMA and PS (Aldrich),and PDPS (Petrarch) (all in chlorobenzene). Thicknesses (measured optically,by a Nanometrics Nanospec) after a 60 s, 105 "C hotplate bake were 1230 h 8, 598 & 13, and 550 & 9 A,respectively (& limits are l a for nine data). The resist film (AZ5214E, AZ Photoresist Products) was measured by AFM on a patterned feature to be 526 h 10 (average of five scans over one edge). The substrates were oxidized Si wafers (-1100 A thermal oxide) sputter coated with -200 A of Au (for PS, PDPS, and AZ)or -3300 A of Cu (PMMA). All films were baked for 60 s at 105 "C; the AZ film was hardbaked at 185 "C for 15min. Contact angles for water were estimated under a low-power standard microscope. Polymer analysis for metals was done by Curtis and Tompkins, Ltd. (Berkely, CA) by inductively coupled plasma spectroscopy or atomic absorption. A Digital Instruments Nanoscope I1 and I11 was used for all experiments. STM tips included both etched PVIr and ion-implanted diamond, with tip biased negatively. Currents were usually set to around 200 PA, however, for PMMA 1nA was used. Cantilever force was minimized with PS (roughly N), and with PDPS good images were obtained without manual force adjustment. Minimization attempts with the resist film led to erratic behavior of the cantilever. A 5 x 5 median filter, in which each pixel is replaced by the median of the 5 x 5 pixel array centered on the pixel, is used for computation ofR,, and a low-pass filter has been used for some of the photos.
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Results Figure 1is an STM image of the Au film, showing the degree of roughness and topography of the substrate on which the films are cast. The average roughness R, (deviation from mean plane) over the whole image is 3.5 A,and the distance R, from the highest to the lowest point in the image is 33 A. The rms noise level, obtained either by disabling they scan or by averaging over flat or nearly flat regions at most a few nanometers long, is about 0.5 A. (The noise levels quoted in this paper have not themselves been systematically averaged, for example over
0 1994 American Chemical Society
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Figure 1. STM image of the bare Au substrate (Ptnr tip). V = 501 mV; I = 200 PA, scan rate 4.7 Hz. This and all other images consist of 256 x 256 pixels.
Figure 2. STM image of PDPS film with diamond tip. V = 500 mV; I = 200 PA; scan rate 3.3 Hz; low-pass filtered. In this and Figures 3-8, the image has been rotated 90"from the original, so the scan direction ( x ) is approximately parallel to the sides of the page.
many different parts of an image; they should be taken as representative but not precise.) Figures 2-4 show images of PDPS with the two M e r e n t tip materials for the STM and with AFM; roughness and noise statistics are summarized in Table 1. These images are in general quite invariant to scan repetition, size, or rate over wide ranges, except that for diamond, very slow scans (50 nm, 0.5 Hz) are significantly noiser than faster ones. (1 Hz means one complete x scan cycle: a 100 nm wide area a t 1Hz is actually scanned at 200 n d s . ) The topography is qualitatively similar. (Ourpresent equipment does not allow us to image the same precise region of the sample with two different tips.) No difference was observed while varying the bias from 75-1000 mV. We will comment further on scan rate dependence below.
Generally similar images and statistics were obtained for AZ resist (Figures 5 and 6) and PS (Figure 7). PS images were not fully reproducible from scan to scan with the AFM; it appeared that the tip was pushing the polymer around despite force minimization. STM scans for all of the polymers were fully reproducible within the limits of the pixel noise (again, more noise was sometimes noted for the slowest scans). Unlike the case of PDPS, the images of PS with a Pt/Ir tip differed significantly from that with diamond (cf. Table 1): the Pt/l[r gave a much smoother image. AFM images of AZ resist closely resembled the STWdiamond images and were reproducible, although some uncertainty in fine details arise because they had to be filtered more extensively since there was a serious periodic tip oscillation on a few nanometer scale in the
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Figure 3. STM image of PDPS film with P a r tip. V = 500 mV; I = 200 PA, scan rate 3.9 Hz.
Figure 4. AFM image of PDPS film.
scan direction (probably attributable to the tip sticking to the surface). In summary, both techniques reveal surfaces with undulations of -10-50-A heights and -100-300-A widths, and local roughness of a few to several angstroms. Although the surfaces appear similar (not surprising since these are amorphous polymers whose surface topography should be determined primarily by viscosity, surface tension and solvent evaporation rates during film casting), there are noteworthy differences in behavior in the STM. If the scan size for PS is increased to 700 nm, holding the rate constant (0.5 Hz), the image soon (within one scan) deteriorates grossly, in the sense that extreme topography (several hundred nanometers, much greater than the film thickness) appears. It is not possible a t this point to say to what extent this is real topography or spatial electronic variations, but the appearance is very similar to what is obtained by attempting to image bare Si (i.e., with just the native oxide). After a few scans under such conditions, if the tip is withdrawn one can see (under a
45x monocular microscope)a damaged spot (i.e., the film
is no longer smooth at that point). It is noteworthy that these images are repeatable except in fine details. The maximum scan rate to avoid this damage is between 300 and 700 n d s . PDPS,on the other hand, never showed any damage for scan rates up to 12000 n d s (3.3Hz,2 pm) with the Pt/Ir tip. With diamond, 900 n d s (8.7 Hz, 50 nm) was achieved (without testing further) a t the end of one series of increasing rates. In another case, however, failure occurred at 400 n d s ; this tendency of the diamond tip to be somewhat less successful than the FWr was found in most of the polymers. AZ resist behaved in a different and more erratic fashion. With the diamond tip, the usual sharp degradation occurred a t 1300 n d s ; for rates in the 100-350 n d s regime, parts of the image would degrade while other regions remained stable. Consistently, the smaller the rate the larger the region that remained good, and the greater the number of scans before any degrada-
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Figure 5. STM image of AZ resist film with diamond tip. V = 500 mV; I = 200 PA; scan rate 1.0 Hz; low-pass filtered. Table 1. Surface Roughness of Polymer Eilmso STM diamond WIr AFM Polystyrene6 Ra 2.1 f 0.8 9.5 f 2.7 Rm 11 f 4 65 f 13c noise 2 6 Polydiphenylsiloxand RIl 6.4 4.0 6.3 f 0.7e Rm 48 39 47 f 5 noise 2 0.6 5 Novolak Photoresist (AZ5214E)f Ra 4.5 0.85 6.8 f 1.1 Rm 68 4.8 46 f 9 noise 1 0.6 3 a Alldatainangstroms. Raandnoisearegivenonlyforunfiltered data;the change with filteringis small and typically not significant. Rm is with a 5 x 5 median filter (see Experimental Section). R, and R, for AFM were obtained from a 100-nmscan; four 50-nm regions are averaged and listed with the standard deviations. The STM data are from a 300-nm scan, with eight 50-nm regions averaged. (Nodata were taken with the PVIr tip on PS.) Averaged over three regions; the fourth has Rm = 195 even after filtering. The AFM scan was 116 nm, and the STM data are from a single 50-nmscan. e With the 5 x 5 filter, 4.8 f 0.6. f As in Table 2, except the AFM scan was 200 nm,and nine 50-nm regions were averaged.
tion appeared (100d s was generally acceptable). With the Pt/Ir tip, the same behavior was seen in one case at 1800 n d s . However, in another series of scans, from 2000 to 5000d s (allat 3.4 Hz), there was no irreversible degradation. PMMA (Figure 8) shows a remarkable fine structure that may be related to the imaging mechanism (vide infra): there appear quasi-periodic ridges approximately aligned along the scan direction (albeit not perfectly so). These ridges are seen to varying extents in all of the STM images, although they are the most pronounced with PMMA, and with the diamond tip (they are hardly evident with PDPS and the Wr tip). (The AFM also tends to produce some striations due to the tipsticking propensity, but these have a different character.) While we cannot a t this time exclude the possibility that these are real features arising from a hydrodynamic instability during
spin casting, it seems unlikely due to the extremely short distance scale. Figures 9 and 10show what happens when the STM tip is deliberately lowered into the film. Starting with a PMMA film as in Figure 8, the diamond tip was lowered (at the middle of each x scan) into the film 20 nm for 100 ,us while scanningat 2.48 Hz. Agrmveis produced (Figure 9) which has approximatelythe expected dimensionsand shape. If however the tip is lowered 10 nm, the result is a roughened surface (Figure lo), but no regular groove, and no depression anywhere near 10 nm deep. If the tip lowering is 5 nm, there is little roughening, and with 2 nm the subsequent image is identical to the initial. Thus the tip can penetrate elastically a few nanometers into the film, but beyond around 5 nm irreversible changes occur; this provides powerful further evidence that the surface (and not the metal substrate) is being imaged, and suggests that tip-substrate contact may indeed occur during normal imaging. Because of the possibility that ionic impurities might be involved in the imaging process (cf. Discussion), an analysis was carried out for 24 different metals. Unfortunately, at the time this experiment was considered,there were insufficient amounts available of the polymers actually used to do such an extensive analysis. A sample of poly(ethy1methacrylate), also from Aldrich, of the same grade, was therefore used as a representative case which should be very similar to the PMMA. In the following list, each element is followed by the measured concentration (ND = not detected), and the minimum detection limit, in parts per billion: Ag (ND, lo), Al (ND, 200), As (ND, 5), Ba (ND, lo), Be (ND,21, Ca (1500,500), Cd (ND, 51, Co (ND, 20), Cr (ND,lo), Cu (8.4,5), Fe (ND, 1001, Hg (ND, 0.2), K (ND,500), Mg (ND, 500), Mn (ND, lo), Mo (ND,20), Na (2800,500), Ni (ND, 20), Pb (ND, 3), Sb (ND, 60), Se (ND, 5), Sn (ND, 40), Ti (ND,lo), T1 (ND,51, V (ND,lo), Zn (ND, 20). Nothing is present in concentrations greater than 1ppm except Ca (1.5ppm) and Na (2.8 ppm), and the largest amount that could be present from sublimit elementsis 1.57 ppm. While the specific numbers cannot be assumed to apply to the imaged polymers, it is highly unlikely that the impurity concentrations in the latter exceeded 10 ppm. In the case of AZ resist, the
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Figure 6. AFM image of AZ film.
Figure 7. STM image of PS film with diamond tip. V = 641 mV; I = 198 PA; scan rate 0.5 Hz;low-pass filtered.
manufacturer specifies 2 ppm of inorganic impurities (Na,
K, Fe) and typically observes less than 250 ppb. Discussion The central result is that stable, reproducible images with nanometer resolution can be obtained of the surfaces formed by a wide variety of electrically insulating polymers. The images do not resemble that of the substrate. The possibility that the tip might simply penetrate the polymer to the metal substrate (and then give an image which does not resemble the substrate because of interference from the polymer) is rigorously eliminated by visual observations: we can see such damage optically, and we do not see it when the images are good. For example, a polymer film on bare Si gives an “image”that is not reproducible from scan to scan and shows “topography’’ of many hundreds or thousands of angstroms, and looks essentially the same as the result of scanning the Si surface without any polymer; a damaged spot is easily
visible under the 45x microscope used for tip approach control, as would be expected if an approximately micrometer-sized spot of polymer had been scraped aside. Thus it is clear that in this case the tip has dug through both polymer and native oxide to establish electrical contact. A damaged spot can also be seen in the polymer on metal if the scan rate is increased too high, and the image quality is grossly degraded in a similar way. On a normal metal surface, image fidelty is lost gradually as a function of increasing scan rate, and no damage spot is seen. For the images described above, the film appears unchanged in the 45 x microscope after withdrawing the tip. In the case of PDPS,scans 2 pm wide were taken, still with no visually observable damage. We note that the four polymers behave quite differently, and the hardest polymer gives the best images, contrary to expectation if tip penetration were responsible. The tip-dependent differences are not seen with metals. Finally, it is quite implausible that one should obtain
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Figure 8. STM image of PMMA with diamond tip. V = 707 mV; I = 1 nA, scan rate 1.34 Hz.
Figure 9. STM image of groove formed by 20-nm-deep diamond tip penetration in PMMA. V = 707 mV; I Hz. Original surface as in Figure 8.
= 1 nA, scan rate 2.48
images that are consistent from scan to scan of one sample, but differ between materials, if the substrate were being imaged by the tip plowing through solid material. Imaging of these surfaces is not possible within the context of conventional STM theory;l the thicknesses are much too great for electrons to tunnel through directly. Only two reports of imaging comparably thick films by STM have appeared in the prior literature.lOJ1 We are aware of three mechanisms that might account for these observations. First, tunneling might occur to a layer of adsorbed water12(possiblywith some dissolved ions) on the surface. Thisis ruled out by the experimentsusing Si as a substrate just described, as well as oxidized Si (-1000 thermal
Si02): if surface conductivitywere reponsible, the polymer should be imageable on an insulator. With the polymer cast on Si02, however, no current ever flows. We also note that these polymers are all hydrophobic materials that are not wetted by water. We do not have the equipment to measure contact angles accurately, but they are in the vicinity of 90" or greater, and PDPS is more hydrophobic than PMMA (albeit less than PS). Other types of surface contamination of polymers are generally less important than for inorganic surfaces because of the low surface energy, although the composition may indeed be different from the bulk.13 Second, tunneling into molecular states might be made possible by the pressure-induced resonance mechanism
(12) Yuan, J.-Y.; Shao, Z.; Gao, C . Phys. Reu. Lett. 1991,67,863-
(13)Ratner, B. D.; Castner, D. G.; Horbett, T. A.; Lenk,T.J.; Lewis, K. B.;Rapoza, R. J. J . Vac. Sci. Technol. A 1990,8, 2306-2317.
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Figure 10. STM image of film roughening by 10 nm deep diamond tip penetration in PMMA. Same conditions as Figure 9.
described by Lindsay et aL2 In order for this mechanism to apply, it is necessary for there to be continuous intermolecular orbital overlap connecting the molecules through the thickness of the film, and the strength of these overlaps should lead to transfer integrals comparable to the through-bond connections. It seems doubtful, although not impossible, that this should be the case. In addition, for the chemically complex system of a compressed polymer film, the simple model of Lindsay et aL2 would have to be modified substantially, and the strong resonant tunneling that they calculate would probably be diminished at best. A third possibility, suggested by Tang et aZ.,l1on the basis of imaging of poly(hydroxypropylcellulose) is that there is a dielectricbreakdown between the tip and surface. The current increases rapidly (on a nanosecond time scale) to a large value (many nanoamperes) and then is shut down entirely as the feedback responds by pulling the tip back; the tip then approaches again and the cycle is repeated. This occurs on a time scale that is not noticeable in ordinary practice, and the transient currents do not deposit enough energy to damage the polymer. They have observed this process with an oscilloscope. Michel et aZ.,l0 who have imaged highly nonpolar crystalline materials, discuss their results in the context of possible mediation by defect-induced surface states. The specific nature of these as well as the feasibility of their accounting quantitatively for the conductivity remains unexplored; however, the proposed mechanism for assisting tunneling is qualitatively similar to the Lindsay model. We believe that the observations both of Lindsay et al. and Tanget al. have relevance to ours, but there are several additional features that must be accomodated. The ridges in Figure 8 are suggestive of an oscillatory imaging mechanism (although we note that Tang et al. did not see periodic structures in their images that were not intrinsic to the sample). The variations in the maximum tolerable scan rate that differ between polymers and tip materials clearly indicate dependence on some kinetic time scale, and could be related to the elastic relaxation rate of the polymer, the kinetics of dielectric breakdown, or some other process.. (The greater noisiness seen at very slow scan rates might be due to charge accumulation.) For
“conventional” STM imaging, the work function of the tip simply scales the distance dependence of the current, but does not affectthe image. The present data suggest rather that the injection rate depends sensitively on the electronic characteristics of the tip. The model of Tang et al. predicts that the voltage for current injection should be inversely proportional to the square of the polymer dielectric constant e and that the process is mediated by ionizable (inorganic) impurities. For PMMA and PS E is only ~2.5,’~ and so a screening reduction by ‘/6 of a typical impurity ionization energy of several electron volts is implied; our biases are (at least in some cases) substantially lower (75mV). In addition, the chemical analysis is quite different for our materials. Tanget al. obtained some evidence for inorganic impurities (a few percent of “Si-related contaminants” detected by XPS, and 1-3 ppm of Cu and Si in the water used as solvent). Certainly it is plausible to expect a significant ionic content in a water-soluble polymer. Our chemical analysis shows an extremely low metal content, and it seems quite improbable that the few parts per million of ions (less for AZ) present could lead to the observed conductivity and resolution. Organic materials that are deformed by high pressure, however, might have sufficiently low energies to participate in charge transfer. We have shown that the tip can penetrate up to 2 nm into PMMA without permanent changes, and so some penetration may occur during normal imaging; this will certainly produce large pressures2J5J6as envisioned by Lindsay et al. Rather than shifting molecular energy levels in the manner seen with isotropic (hydrostatic) pressure, however, the effect may be a stress predominantly normal to the surface, resulting in bond stretching. We can crudely assess the potential of this mechanism by considering the force constant of a C-C bond: the vibrational energy of 3000 cm-l corresponds to 30 Nkm. Thus, a stretching force of -3 x N would suffice to break a bond; forces of the order (14) Polymer Handbook,2nd ed.; Brandrup, J., Immergut, E. H., Eds.;Wiley: New York, 1975. (15) Soler,J. M.; Baro, A. M.; Garcia,N.; Rohrer, H. Phys. Rev. Lett. 1986,57,444-447. (16) Tang, S . L., Bokor, J.; Storz, R. H. Appl. Phys. Lett. 1988,52, 188-190.
Letters of have been measured under STM tips.16 Of course, the force applied by the tip is not exclusively along any one polymer chain, and the foregoing calculation is intended not to prove anything other than that the hypothesis is not unreasonable. Nevertheless, in a polymer where an entire molecule cannot easily move aside under the influence of a n anisotropic force, it seems possible that bonds may be either broken or stretched to the point of being highly polarized; the resulting charged entities, partially screened according to the mechanism described by Tang et al., would very likely be capable of charge transfer a t low bias. At the same time, different tip materials will have different charge-transfer characteristics (and, presuming direct contact, the work function will be differently affected by the organic material). In this context, it is noteworthy that the saturated hydrocarbons used by Michel et al.lo are unlikely to have significant inorganic content. They refer to the possibility of surface states with energies in the middle of the gap, but the presence of significant densities of such states in simple organic molecular systems has not been established. Sufficientlyhigh pressure could,however, produce electron-accepting states in their molecules just as in the polymers studied here. If pressures resultingin surface deformation are present in the STM, one may question why the images of PS were repeatable while they were not with the AFM (due, presumably, to pressure). However, the oscillatory imaging process described by Tang et al. is in fact a sort of “tapping mode” in which there is little or no tendency for the tip to drag laterally. Despite the lack of fundamental understanding, it has been demonstrated that meaningful images of surfaces of relatively thick polymer films can be obtained by STM. We emphasize that they are meaningful in the sense that they are reproducible, obtainable by standard procedures with no special preparation, and statistically similar to AFM images. The images of lithographically modified films are in close correspondence with the expected trench dimensions and shape. Nevertheless, we do not assert that all of the details reported by the STM represent “real” topographic features (and indeed they almost certainly do not: the tip-dependent differences are a clear counterexample). Clearly the physical interpretation of the STM signals remains open to discussion. More detailed comparison to the AFM would help, although ultrahigh resolution with the AFM on “soft” materials is not easily obtained.17 Despite advances in this area,18the STM still may have an advantage in resolution, because of lateral (17)Radmacher, M.; Tillmann, R. W.; Fritz, M.; Gaub, H. E. Science 1992,257,1900-1905. (18)Ohnesorge, F.; Binnig, G. Science 1993,260,1451-1456.
Langmuir, Vol. 10,No. 7,1994 2051 forces on ultrasharp AFM tip^.'^,^^ The ability to compare STM and AFM images is of course desirable precisely because of the differences in interaction. Finally, these results suggest an additional level of caution in interpreting STM images of much thinner films in terms of a conventional tunneling mechanism.
Conclusions Each of the three polymers that were compared with both techniques exhibited different characteristics. The best AFM images were obtained with PDPS, which is relatively nonpolar and has a high TP2l Novolak resist was next: images were reproducible but tip sticking was a problem. The film was baked to cross-link and harden it, but it still should have a heavily oxygenated (primarily hydroxyl) surface. PS has a relatively low Tgof about 105 “C. The sensitivities of the STM were similar: PDPS gave the best images (especially the ability to scan large areas at relatively high rates), AZ resist was intermediate, while PS tolerated only very slow scans over small areas. (PMMA, however, with about the same Tgas PS, did quite well.) Our results suggest that, at least in some cases, the imaging of thick organic films by STM occurs via charge injection facilitated by a combination of pressure-induced changes, electronic interaction of tip and substrate, and field-inducedelectron transfer. There are both similarities as well as differences in these results with respect to previous reportslOJ1that highlight the complexity of the processes in these systems. However, the presently available information suggests that both electronic and topographic information may be extracted with further research. Acknowledgment. I thank Sau-Lan Tang for communicating her findings to me prior to their publication (at a time when these results were without any very plausible explanation),and Stuart Lindsay for his helpful comments. I am indebted to Mark Ratner for some very insightful discussions and for drawing my attention to the paper of Michel et al. Finally, it is a pleasure to acknowledge the contributions to this work of Harden McConnell, who stimulated my interest in physical chemistry and gave me the confidence and opportunity to pursue it. It is exemplary of the incisiveness of his scientific career that his work on superexchange in 196122 is relevant to the issues discussed in the present paper. (19)Vasile, M. J.;Grigg, D.; Griffith, J. E.; Fitzgerald, E.; Russell, P. E. J . Vm. Sci. Technol. B 1991,9,3569-3752. (20)GrifKth, J. E.;Grigg, D. A.;Vasile, M. J.;Russell, P. E.; Fitzgerald, E.A.J . VUC.Sci. Technol. B 1991,9,3586-3589. (21)Mi, Y.; Stern, S. A. J . Polym. Sci. B , Polym. Phys. 1991,29, 389-393. (22)McConnell, H.M. J. Chem. Phys. 1961,35,508-515.
