Dan E. Barlow and K. W. Hipps*

Transcription

1 J. Phys. Chem. B 2000, 104, A Scanning Tunneling Microscopy and Spectroscopy Study of Vanadyl Phthalocyanine on Au(111): the Effect of Oxygen Binding and Orbital Mediated Tunneling on the Apparent Corrugation Dan E. Barlow and K. W. Hipps* Department of Chemistry and Materials Science Program, Washington State UniVersity, Pullman, Washington ReceiVed: February 3, 2000; In Final Form: April 6, 2000 Submolecular resolution STM images of vanadyl phthalocyanine (VOPc) are reported. Unlike previously studied metal phthalocyanines, the image has neither a simple dark or bright spot in the center. Rather, it appears to have nodal planes running through the center of the molecule. The dark central region is attributed to the oxygen atom reducing the number of available states near the Fermi energy. The apparent height of the Pc ring is very bias dependent having a value of 0.1 nm between -1.5 and 0.2 V (sample) bias. The apparent height increases rapidly to a plateau of about 0.55 nm at +1.0 V sample bias. I - V and di/dv data taken over VOPc islands are very different from the same measurements made on exposed Au(111) substrate adjacent to the islands. Strong peaks in di/dv are seen at and -1.8 V sample bias. The V feature is associated with orbital mediated tunneling via the π* LUMO of the Pc ring. Occasionally, the tip-surface interaction generates anomalous dark areas in the VOPc islands. Introduction The topography observed in Scanning Tunneling Microscopy (STM) has long been known to be due more to electronic effects than to actual atomic or molecular size, as understood by conventional X-ray or electron diffraction methods. For example, adatoms of relatively large size chemisorbed on metal surfaces can appear as depressions in the metal surface, whereas certain functional groups physisorbed on metal surfaces can appear taller than expected from their van der Waals radii. Why these differences arise and how they should be interpreted, especially in the context of chemical sensitivity of STM, is still very much a subject of ongoing research. Although the literature is filled with hundreds of papers on this subject, we will give a brief sampling to show the intensity and duration of experimental and theoretical studies relating to this issue. Tersoff and Hamann were among the first theorists to associate the STM constant current image with the local density of states of the surface near the Fermi energy. 1 On semiconductor surfaces, it has long been known that the images observed for the native surface atoms and for adatoms depend strongly on the applied bias voltage used in acquiring the STM image. 2,3 Lang has made many contributions to our understanding of the STM images of adatoms on metals. These include showing theoretically that chemisorbed Na should appear larger than S 4 and that O chemisorbed on a metal should appear as a depression. 5 Sautet and Joachim used electron scattering quantum theory to predict the STM images of adatoms on Pt(111). 6 They found that adatom corrugation generally decreased with increasing electronegativity. Sautet also performed calculations for O and S on Pd(111) and showed that O would appear as a depression in the STM image of the Pd(111) surface. 7 Kopatzki showed experimentally that STM gave a negative corrugation for O on Ni(111). 8 Chemisorbed small molecules (such as * To whom correspondence should be addressed. hipps@ wsu.edu. benzene) on active metal surfaces show overall symmetry and structure that reflects the interaction with underlying metal surface electronic structure. 9 The images seen for physisorbed molecular species on surfaces offer different challenges to our understanding. Lindsay et al. documented early efforts to image biomolecules by STM and provided a theoretical framework for discussing pressure and resonance effects in STM imaging of molecules. 10 Claypool et al. studied functionalized alkanes adsorbed on graphite. 11 They say that the apparent corrugations (STM measured heights) do not correlate with polarizability or with size, but they do seem to correlate with ionization potentials. However, this correlation fails for the bromide ion. Cyr et al. studied monolayer films of terminally monosubstituted hydrocarbons on graphite by STM. 12 NH 2, Br, SH, and I groups were observed as bright spots, whereas OH and Cl were indistinguishable from the alkyl chain. A model, based on the earlier work of Sponge, 13 was proposed for incorporating polarizability, electronic structure, and molecular orientation to account for the STM imaging mechanism. The authors make a very nice argument for bias insensitivity in these systems based on the LDOS and an analogy with Raman. For cases in which the density of frontier orbitals is large but far from E f (as it is in these hydrocarbons) the weak orbital mixing and molecule-surface interaction broadening should lead to bias insensitive structures (as in normal Raman). As the frontier orbital energies approach the Fermi energy, resonance interactions lead to pronounced increases in current and in definite bias dependence (as in resonance Raman). Electroactive species imaged in the STM should provide the resonance-like interactions expected when frontier orbital energies approach the Fermi energy of the substrate. Hipps and coworkers demonstrated experimentally that the d-orbital occupation of metal phthalocyanines (MPcs) had a dramatic effect on the apparent height of the central metal ion Under UHV conditions, the central metal ion in FePc, CoPc, NiPc, and CuPc /jp CCC: $ American Chemical Society Published on Web 06/07/2000

2 5994 J. Phys. Chem. B, Vol. 104, No. 25, 2000 Barlow and Hipps appears as either a hill or a dip, relative to the surrounding phthalocyanine ring, depending upon the occupation of the d z2 orbital. Tao studied electron transfer through redox-active molecules adsorbed on a conductive substrate with STM in aqueous solution. 17 By adjusting the substrate potential, the Fermi levels of the substrate and tip were shifted relative to the energy levels of the molecules. Aligning the Fermi levels to a molecular energy level produced a 10-fold increase in the tunneling current that flows between the substrate and the STM tip. No submolecular resolution was obtained. Lindsay and coworkers studied the STM of a series of metalloporphyrins tethered to Au(111). 18 Both images and I - V curves were obtained under an oxygen free solution. The contrast of the reducible molecules (at the metal ion) changed strongly with bias, and the corresponding I - V curves were highly asymmetric. The di/dv curves had a Gaussian shaped peak at a voltage characteristic of the compound, although local measurements showed that there was considerable variation in this value from molecule to molecule of a given compound. These authors conclude that, Our results demonstrate the link between tunneling and redox properties first proposed by Schmickler 19 on theoretical grounds, and Mazur and Hipps 20 on experimental grounds. One driving force in all of these studies is the search for simple models of the molecule-electron interaction that can be used as tools both for interpreting STM images and for providing molecular identification. In the present work, we will extend our previous efforts to conceptualize tunneling through redox-active molecules by chemically modifying the central metal ion in an MPc to eliminate any surface d-orbital density close to the Fermi energy. The STM results presented here for VOPc can be easily understood in terms of a local (submolecular) conductivity model, in which the p z orbitals of the phthalocyanine ring carry charge from the substrate to the tip, whereas the covalently bonded oxygen inhibits charge transfer through the central region of the VOPc. Through the use of current imaging tunneling spectroscopy (CITS) and scanning tunneling spectroscopy (STS), we observe that the C(p z ) tunneling is strongly energy dependent and that the LUMO (π*) is very effective in orbital mediated tunneling. Experimental Section Samples used for these STM studies were prepared as follows. The VOPc was purchased from Alfa-Aesar and was sublimed for further purification. The substrates were freshly prepared Au(111) epitaxially grown on mica. These substrates were grown in a cryopumped UHV chamber that routinely achieves Torr base pressure without baking (mostly He gas). Freshly cleaved mica (Ted Pella, Inc., 1 4 cm strips, catalog #54) was placed in an oxygen-free copper heating block and heated under vacuum to 500 C for 24 h to dehydrate the mica surface. The temperature was then reduced to 365 C and approximately 1 µm of gold was deposited at a rate of <0.05 nm/sec. These Au/mica substrates were allowed to cool to <30 C before the chamber was opened, and the substrates were rapidly transferred into the UHV chamber housing a McAllister STM and a fixture for depositing VOPc. The VOPc was deposited on the Au(111) surface to a thickness of about 0.2 nm as determined by a thin-film monitor. The sample was then placed on the STM stage. Both etched W and cut PtIr (Pt 0.9 Ir 0.1 and Pt 0.8 Ir 0.2 alloys) tips were used. These tips were cleaned by electron bombardment from a hot tantalum filament (typically 1kV bias and 3mA current between filament and tip for 30 s). The imaging and CITS were performed using Digital Instruments Nanoscope III software (version 3.31slater versions do not perform CITS correctly) and control electronics. All of the STM data were acquired at room temperature. I - V curves were extracted from the CITS data set using Nanoview version I - V curves were averaged over the majority of a VOPc covered region to produce a single I - V curve representative of the compound. Similarly, I - V curves were averaged over the majority of an adjacent clean Au surface to produce the I - V curve representative of the uncovered surface. Note that many STM images and CITS data sets were taken of several samples. A total of more than 700 separate images and more than 5 separate samples were studied. The data presented in this paper are the results of three separate trials, each involving a new set of nine tips, chamber bake-out, gold preparation, and VOPc deposition. The images, spectra, and I - V curves were all reproducible. Results The vanadyl ion (VO 2+ ) occupies an asymmetric site in the phthalocyanine ring, 22 as shown by the stick model in Figure 1. On the basis of van der Waals (vdw) radii of O and C, 23 one would expect the top of the oxygen atom to lay about 0.2 nm above the tops of the carbon atoms in the Pc ring. Highresolution STM images of VOPc monolayers and submonolayers, on the other hand, show a marked depression in the central region of the molecule, as is also shown in Figure 1. This image was acquired with a W tip at settings of 300 pa, 600 mv (sample bias), and 2.9 Hz scan rate. To show exactly how the STM intensities correspond to molecular features, we prepared Figure 2, a composite of experimental constant current data and molecular model placement. The STM constant current data used in Figure 2 were acquired at 300 mv sample bias and 1.0 na. The image was 3 3 median filtered, Fourier filtered, and converted to a contour plot using Image SXM version 1.62, and then a small segment was selected for display. Thus, the contour lines of Figure 2 represent the actual experimental data. Then, using the X-ray structure of VOPc to determine atom centers, 22 CPK molecular models were constructed, drawn to the same scale, and hand placed on the contour plot of the data. The fit is exceptionally good, clearly showing that, in the case of VOPc, only the isoindole rings have significant height. To the STM, the VOPc carbon atoms are the only elements present to a first approximation and the primary conduction path is through the carbon p z orbitals. The unit cell appears to be square, and the unit cell vectors are 1.42 nm in length. We might note that due to thermal drift and piezoelectric creep, the unit cells often appear somewhat distorted (as in Figure 3). Figure 3 contrasts high-resolution images of CoPc, CuPc, and VOPc on Au(111). Note that all three systems have distinctly different STM topographies. The CoPc molecule has a large hill at the center, ascribed to the half-filled d z2 orbital of the Co ,15 The CuPc molecule has a well-defined hole in which the filled d z2 orbital is located. VOPc, on the other hand, has two pronounced nodal planes running through the center of the molecule. Another interesting feature of Figure 3 is the fact that the isoindole rings of all three molecules are about the same size. The chemical variation at the center of the molecule is felt only weakly in the conjugated carbon structure. Note that the apparent corrugation of the VOPc layer (Figure 3B) is less than the height relative to the clean Au layer because the tip was too large to completely penetrate the spaces between the packed structure.

3 STM of Vanadyl Phthalocyanine on Au(111) J. Phys. Chem. B, Vol. 104, No. 25, Figure 1. High resolution STM image of a VOPc monolayer on Au(111). Also shown is a model of the VOPc molecule. Note that the vanadium ion sits slightly above the plane and that the van der Waals radius of the O atom extends more than 0.2 nm above top of the isoindole rings. This image has been plane-fit and low-pass filtered only. symmetry lattice that announces its presence through the existence of MPc islands that are symmetrically related by It was observed that the apparent height of the VOPc molecules depended critically on the applied bias. To quantify this, consider Figure 4, in which a large region ( nm 2 ) of a VOPc island surrounded by bare gold is shown. A series of sectional heights were measured (along the dark line in the image) at different sample bias voltages, and the results are shown in the lower half of Figure 4. Note first the gold step at the right edge of the figure. The measured height of this feature was (as expected) independent of the bias voltage and served as an internal calibration. The apparent height of the VOPc island, however, changed dramatically with applied voltage, rising to roughly twice the expected height, based on the van der Waals radius of carbon. Figure 5 is a graph of the apparent height versus applied voltage. The smooth curve is there to guide to the eye and is given by erf (3.08(V - 0.6)) (1) Figure 2. Contour plot of STM data. Constant current data acquired at 300 mv sample bias and 1.0 na. The image was median and Fourier filtered and converted to a contour plot using Image SXM version The VOPc molecular models are CPK models drawn to the same scale using the reported X-ray structure of VOPc and hand placed on the contour plot of the data. The unit cell appears to be square and 1.42 nm on a side (center to center distance). Both the CoPc and the VOPc lattices are approximately square and have 2 possible rotational packing patterns that are energetically equivalent. This is shown by the surface layers in Figure 3, parts A and B, which are related by a mirror plane, giving a left- and right-hand structure. These cannot be brought into equivalence by rotation but can be produced (conceptually) by flipping the MPc layer over. In addition, there is another where erf (x) ) ( π) 2 x 0 exp(-t 2 )dt (?) This function has no particular theoretical significance; but, it does provide a good fit to the data and should be useful for evaluating theoretical models. Note that for sample bias settings less than zero, the apparent molecular height ( 0.1 nm) is significantly less than the vdw diameter of carbon (0.31 nm). 23 It is certainly less than the predicted vdw distance across the thickest portion of the molecule (0.51 nm including O). As the sample bias voltage increases to above 1.0 V, the apparent height of the Pc ring increases to about 0.53 nm, roughly twice the vdw predicted thickness of the Pc ring.

4 5996 J. Phys. Chem. B, Vol. 104, No. 25, 2000 Barlow and Hipps Figure 3. STM images of two different phthalocyanine systems. 3A is of a mixed composition island of CoPc and CuPc; the arrow points to a CuPc molecule. 3B is of a monolayer island of VOPc. Both images are on the same 0.4 nm gray scales. 3A was acquired with 200 mv sample bias and 3nA current, whereas 3B was taken at 800 mv and 300 pa. Figure 5. Variation of apparent height of VOPc with applied bias voltage. Figure 4. Constant current image of a VOPc island on Au(111), top, and apparent constant current heights as a function of bias voltage, lower. A very useful method for determining the energetics of the STM image is current imaging tunneling spectroscopy, or CITS. 3 Figure 6 shows a constant current image of an island of VOPc on Au(111) as well as two of the 256 current images obtained via CITS. The +1.5 V image gives clear chemical specificity for the VOPc and is completely independent of topographic changes in the gold surface. The -1.5 V image, on the other hand, provides no selectivity within the (poor) signal-to-noise of the CITS measurement. To obtain high quality I - V curves, and from these di/dv curves, a spatial averaging technique was used. A CITS image containing both bare gold and a monolayer of VOPc (32 32 I - V curves of 256 data points each) was first collected. Then, the average of all I - V curves over VOPc was computed and is shown as data points in the upper half of Figure 7. A similar procedure was performed over the bare gold area, and that average curve is also shown in the upper half of Figure 7. For later purposes, we also fit a polynomial to the VOPc I - V data, and that polynomial is the smooth curve in the upper half of Figure 7. Numerical differentiation of the I - V data produces relatively noisy results, so two different methods were explored for providing cleaner di/dv curves. The lower portion of Figure

5 STM of Vanadyl Phthalocyanine on Au(111) J. Phys. Chem. B, Vol. 104, No. 25, Figure 6. Constant current height image, top, and constant-bias current images obtained at -1.5 and +1.5 V sample bias. 7 shows the di/dv curves for the clean gold and for VOPc obtained using a Gaussian averaging function of width <5kT (0.12 V). The di/dv curve for the VOPc region is offset upward for ease of viewing. Also shown in Figure 7 is the direct derivative of the polynomial fit to the data, as shown in the upper half of Figure 7. The small differences in the VOPc di/ dv curves are probably due to noise in the original data. The strong peaks at and V, however, are real features directly related to the electronic structure of the VOPc layer. Although hundreds of images similar to those shown in Figures 1, 2, 3, 4, and 6 were obtained, we very occasionally observed a scan dependent change in the molecular image. Figure 8 shows such an image. On the left side of Figure 8 is the image obtained after several scans at 1.14 V and 775 pa. The dark regions grew in as the number of scans increased. The boxed region in the left image was digitally expanded to produce the right-hand image in Figure 8. The crosses in the right-hand figure are placed to aid the eye in tracking the molecular order across the dark region. It was also observed that these dark regions sometimes returned to bright molecules. CITS data in the bias region of (1.5V were taken over these dark regions, and the resulting I - V curves were similar to those of bare gold. Discussion Although there has been considerable work on the STM of phthalocyanines, relatively little has been done on the nonplanar species. STM images of the closely related TiOPc have been reported but only on thick films and over large (0.25 µm) areas. 25 PbPc is also nonplanar (the Pb 2+ ion is too large to fit into the Pc pocket), and a few studies of this complex have appeared It is interesting to note that based on vdw radii, Pb protrudes above the Pc ring in PbPc about the same distance as O does in VOPc. This can be seen from the solvent volume models (solvent radius ) 0.2 nm) of VOPc and PbPc generated by WebLab Viewer and shown in Figure 9. Note that the hydrogen atoms are not included in this figure. Of the available PbPc STM studies, the one by Strohmaier et al. is by far the most complete and provides submolecular resolution. 26 Strohmaier deposited PbPc onto MoS 2 and observed two types of molecular images. Some had depressions in the center, whereas others appeared more or less flat across the top. Presumably, the flat topped molecules had Pb up, whereas the hollow centered ones had Pb down. In video sequences, the transition from one state to the other for individual molecules was sometimes observed. After aging for a few days, only the dark center molecules were observed. On a sulfide surface, it is intuitively pleasing that the energetically favorable orientation would be that with Pb down and bonded to S. They also noted that the application of (2V for 1 s resulted in a change in the image where some of the flat topped molecules were observed in an area previously only having dark centers. On the basis of atomic radii and the results reported for PbPc, one might argue that VOPc is sitting on Au(111) with the oxygen end pointed toward the surface, and thus, a depression is seen at the molecular center. This is not our preferred assignment. Rather, we suggest that the oxygen is actually pointed up and that the lack of states near the Fermi energy at the oxygen produces the apparent dip. This assignment draws a parallel between the bonding of O to a metal in a complex

6 5998 J. Phys. Chem. B, Vol. 104, No. 25, 2000 Barlow and Hipps Figure 7. Area averaged I - V curves and generated di/dv. The I - V curve obtained over VOPc was fit by a polynomial, and the exact derivative of the polynomial was taken. Alternatively, the raw data was smoothed and then a numerical derivative was taken. This curve has been artificially offset to aid viewing. and the bonding of O to a metal surface where it has been clearly shown that O appears as a depression (see introduction). Our arguments for this conclusion are as follows: (1) The apparent height of the Pc ring is of the order of the height observed in planar CoPc and CuPc systems (Setpoint: 300pA, sample bias between ( 500 mv). Because this height is determined by orbital mediated tunneling through the π and π* orbitals of the Pc, any significant change in ring height with respect to the Au surface would produce a significant change in the conductivity and, therefore, the apparent height. Thus, we believe that the Pc ring in VOPc is in close contact with the Au surface. (2) VOPc appears to have the same surface structure as planar MPcs, suggesting that the Pc ring-au(111) interactions dominate. (3) The position of the OMT band in the di/dv curve (+1.38 V) corresponds nicely with the predicted position based on metal-insulator-metal tunneling data, in which the metal spacing is more than 2 nm. 29 (4) In no case do we see VOPc molecules with bright centers. The rare deviation from the image shown in Figure 1 is represented by the case portrayed in Figure 8. The dark areas seen (Figure 8) may be interpreted either as vacancies or as occupied by a modified VOPc. Viewed as vacancies, one interprets the dark area formation as being due to molecules being swept away by the tip. Consideration of images such as that on the right of Figure 8, suggests to us that these dark areas are not simple vacancies. The ordering of the lattice continues across these dark regions, even for the case where a single VOPc has no bright nearest neighbor. One possible (but controversial) interpretation is that the dark regions are VOPc oriented with the oxygen end down. In this scenario, the increased distance between Au and the Pc ring dramatically reduces the tunneling probability and thereby results in dark VOPc units. The reduced coupling between the ring p z orbitals and the Au might also result in a dramatic decrease in resonance tunneling and thereby produce CITS images similar to Au. This is the inverse of the PbPc results, but the interaction of oxygen with gold is considerably weaker than lead with sulfur. The more conventional assignment would be that the dark areas are simply regions in which the molecules have been removed by the tip. If this is the case, then only one face of the VOPc is ever observed. The di/dv curves can be interpreted in terms of orbital mediated tunneling. 20,29 Assuming the Pc ring is in contact with the Au(111) surface, the peak near +1.4 V may be related to tunneling mediated by the π* LUMO of the Pc ring. 29 Taking the work function of Au(111) to be 5.3 ev, 30 the electron affinity of VOPc is then ) 3.9 ev in good agreement with values obtained for other phthalocyanines. 31 The peak in di/dv at -1.8 V is more difficult to assign. The literature abounds with assignments for the Pc ring band gap in phthalocyanines of the order of 2 ev, placing the HOMO (2a 1u ) at about Figure 8. Constant current image acquired after several scans of this region. A Pt-Ir tip was used with at 1.1 V sample bias and 775 pa setpoint. The left-hand image is the original data. The right-hand image is a digitally expanded view of the area in the box to the left.

7 STM of Vanadyl Phthalocyanine on Au(111) J. Phys. Chem. B, Vol. 104, No. 25, Figure 9. Solvent surface images of VOPc and PbPc (hydrogen atoms not shown) assuming a solvent radius of 0.2 nm V bias. Thus, on the basis of conventional assignments, the peak at -1.8 V must be due to a lower occupied orbital. The density functional calculations of Rosa and Baerends for MgPc, CoPc, NiPc, and CuPc place the next lowest π orbital, the 3b 1u, an average of 1.23 ev below the HOMO. This would place the 3b 1u near ( ev) -1.8 ev sample bias. The agreement with the actual peak position is almost too good to believe, and we have no explanation as to why the 3b 1u would be active but the 2a 1u would be inactive. Nevertheless, this assignment appears to be the only reasonable one, given a 2 ev band gap. An alternative assignment of the di/dv data would result if one assumes that the O end is down. Calculations for O and OH on Au(111) indicate that the distance from the metal surface plane to the center of the O atom increases as the effective charge of the O atom becomes more negative. 35 The oxygen atom center of O is 0.15 nm above the Au plane for the (most stable) hollow site but the O atom of OH is 0.18 nm above the same plane. Koper 35 also shows that for Ag(111), the difference in oxygen height between on-top and 3-fold hollow sites is only about 0.05 nm. The oxygen in the vanadyl ion (formally O 2- ) is therefore expected to see the gold surface as a nearly flat plane and should lay about one vdw radius above the top of the gold surface. Thus, we take the median plane of the Pc ring to be about 0.35 nm above the Au(111) surface. If the tip to Au surface distance is L, then the effective voltage at the ring will be [1 - (0.35/L)]V + when the sample is biased positively and -[0.35/L]V - when it is negatively biased. Thus, if the resonance state energy relative to E f is E r, two resonances will occur through the same LUMO, one when E r ) [1 - (0.35/ L)]V + and one when E r )-[0.35/L]V -. Thus, we will see peaks in di/dv when V - ) V + (1 - (L/0.35)). Taking V + ) 1.38 and V - )-1.80, then L ) 0.81 nm and E r ) 0.78 V. In this picture, there is only one resonance state in the entire window of the di/dv scan, and it is an unoccupied state about 0.8 ev above the Fermi level. This is still consistent with our assignment of LUMO mediated tunneling, but the tip would need to be very close to the surface (L ) 0.8 nm). However, the set point used, 300 pa and 850 mv, suggests otherwise. Experimentally, the tip retracts 0.5 nm as it passes from the Au surface to the VOPc surface at the setpoint used for the I - V date (see Figure 5), leaving only about 0.3 nm clearance between tip and Au layer when the tip is over the gold surface. It is noteworthy that in this model, reducing the tip-au distance to 0.7 nm would eventually cause the I - V curves to become symmetric. Although we have not performed I - V curves over the entire 4 V bias range for a wide variety of set points, we have done so over the (1.5 V range (for example, see Figure 6). In every case, these curves are highly asymmetrical with the maximum current occurring at large positive sample bias. Thus, our preferred assignment is again the O up orientation. It should also be noted that in the O-down picture, the molecule does not see the full (2 ev bias scan. Instead, the scan range would be only about (1 V. This effective reduction by 2 in the potential would place the HOMO (about 2 ev below the LUMO ) at the extreme edge, or perhaps outside, of the bias scan window. A final caution is that the analysis presented here is based on classical electrostatics and is certainly flawed. At this scale, a proper understanding of this effect probably requires a quantum mechanical calculation. Attempts to relate the height-bias data (Figure 5) to a single decay parameter and the I - V data (Figure 7) failed. Although this procedure produced curves qualitatively similar to Figure 5, they did not show a significant height rise until about 1 V bias. A determination of which of these two analyses of the di/dv data is correct can be assisted by measuring the di/dv curve for a planar MPc over the same substrate and energy region. Such experiments are currently under way. Conclusions VOPc adsorbs on Au(111) in a nearly square structure with a 1.42 nm lattice spacing similar to other planar MPcs on the same substrate. Submolecular resolution images of VOPc show that it has an STM topography uniquely different from other metal phthalocyanines investigated to dates there appears to be two nodes running through the center of the molecule. Both height versus sample bias and current versus sample bias curves are very asymmetrical, indicating that LUMO tunneling is the primary enhancement mechanism at low bias. We tentatively assign the stable form of the VOPc molecule as oriented with the oxygen up with the O atom blocking conduction from the surface, thereby appearing as a depression in the molecule. In this view, the alternative orientation (O-down) may be present as a defect or result from tip interactions and thus appear as a dark molecule. The dark areas observed may also be due to molecules being swept from the surface by the tip. The possibility that the O-down orientation is the stable form is also considered, and the consequences are discussed. VOPc may be acting as a molecular switch, where changing the surface orientation blocks (or turns on) the local electrical conductivity. Acknowledgment. We thank the National Science foundation for support in the form of Grant Nos. 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