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Imaging Porphyrin-Based Molecules on a Gold Substrate in Ambient Conditions D. K. Luttrul1,t J. Graham,? J. A. DeRose,n D. Gust,**+T. A. Moore,*,+and S. M. Lindsay*gs Department of Chemistry, STM Industrial Associates Program, and Department of Physics, Arizona State University, Tempe, Arizona 85287 Received September 10, 1991. In Final Form: December 12, 1991
Covalently tethering porphyrin-based molecules to an otherwise inert gold substrate allows them to be imaged in an ambient environment using scanning tunneling microscopy. Interpretation of the data is aided by comparing images of mono- and diporphyrin molecules taken with the same tip. At low coverage we obtain a markedly larger average particle size (396 A2) for diporphyrins than for monoporphyrins (121 A2). The particle size distributions are very broad, but the images show a characteristic variation of contrast with tip bias, suggesting that the heterogeneity is due to different molecular conformationsrather than contamination. We find evidence of ordered structures at high coverage. The scanning tunneling microscope (STM) is now applied to the study of molecules adsorbed onto metal surfaces in ultrahigh vacuum (UHV) almost routinely.' Ita applications in an ambient environment are more limited. Liquid crystal molecules have been imaged.2They form ordered arrays that are easy to distinguish from contamination and they can be imaged under the parent liquid which helps reduce contamination. Reliable imaging of arbitrary distributions of molecules in ambient conditions is a much more difficult problem. This is because even an "inert" surface becomes contaminated very rapidly3 making identification of features very difficult. However, the study of "real" (dirty) interfaces is of enormous practical importance. Here, we illustrate the application of the STM to one such practical problem using porphyrin-based molecules. These are relatives of molecules we have prepared to mimic the photoinitiated charge separation and energy storage found in photosynthetic organism^.^ Development of molecular electronic devices based on these molecules is greatly faciiitated when their attachment to a metal electrode can be studied directly. The STM tip exerts large forces on the substrate as it scans5 so that molecular adsorbates must be tethered if they are to be imaged. We use Au(ll1) that has been grown epitaxially on mica to produce large atomically flat faces? Molecules (Figure 1D) made from one porphyrin + Department of Chemistry and Biochemistry, Center for the Study of Early Events in Photosynthesis. STM Industrial Associates Program. S Department of Physics. (1)Ohtani, M.; Wilson, R. J.; Chiang, S.; Mate, C. M. Phys. Reu. Lett. 1988,60,2398. Chiang, S.;Wilson, R. J.; Mate, C. M.; Ohtani, M. J . Microsc. 1988,152,567.Lippel, P. H.; Wilson, R. J.; Miller, M. D.; Wo11, Ch.; Chiang, S. Phys. Reu. Lett. 1989,62,171.Hallmark, V.M.; Chiang, S.; Brown, J. K.; WB11, C. H. Phys. Reu. Lett. 1991,66, 48. (2)Foster, J. S.;Frommer, J. E. Nature 1988,333,542.Smith, D. P. E.; HBrber, H., Gerber, Ch. G.; Binnig, G. Science 1989,245,43.Spong, J. K.;Mizes, H. A.; LaComb, L. J., Jr.; Dovek, M. M.; Frommer, J. E.; Foster, J. S. Nature 1989,338,137. (3) Smith, T. J. Colloid Interface Sei. 1980,75,51. (4)Gust, D.; Moore, T. A. Aduances in Photochemistry 1991,16,1. Gust, D.; Moore, T. A. Science 1989,244,35. Gust, D.; Moore, T. A.; Moore, A. L.; Lee, S.-J.;Bittersmann, E.; Luttrull, D. K.; Rehms, A. A., DeGraziano, J. M.; Ma, X. C.; Gao, F.; Belford, R. E.; Trier, T. T. Science 1990,248,199. (5)Durig, U.; Gimzewski,J. K.; Pohl, D. W. Phys. Reo. Lett. 1986,57, 2403. Spence,J. C. H.; Lo, W.; Kuwabara, M. Ultramicroscopy 1990,33, 69.
*
ring (1) or two porphyrin rings (2) are prepared with pendant isocyano7groups that react with the gold to hold the molecule in place.8 The molecules are dissolved in toluene, and the solution is placed on a freshly prepared substrate. After the attachment reaction the substrate is rinsed several times with clean solvent, dried, and transferred to a Nanoscope I1 STM for imaging. The images we show here were obtained in the "constant-height" mode which produces better contrast than the usual constantcurrent mode (but we have also reproduced the general features of all our experiments in constant-current mode images). The constant-height mode suffers from the drawback that the "height" data are not calibrated (current variations are measured), but it should be recalled that, in the absence of a theoryfor molecular contrast, constantcurrent mode images do not give absolute indications of the surface topography either. Gold exposed to toluene alonee appears unaltered. A typical image of such an unmodified substrate is shown in Figure 1A. Features due to atomic steps between gold terraces are seen (their appearance is a consequence of imaging in the constant-height mode with some residual current servo gainlo), but the terraces themselves appear clean. Gold exposed to toluene containing the reactive ~~
(6)DeRose, J. A.; Thundat, T.; Nagahara, L. A.; Lindsay, S. M. Surf. Sci. 1991 256,102-108. (7)Singleton, E.;Oosthuizen, H. E. Ado. Organomet. Chem. 1983,22, 209. Hickman, J. J.; Zua, C.; Ofar, D.; Harvey, P. D.; Wrighton, M. J. Am. Chem. SOC.1989,111, 7271. Rubinstein, I. N.;Steinberg, S.;Tur, Y.; Shauzer, A.; Sagiv, J. Nature 1988,332,426. (8)5-(4-Isocyanophenyl)-10,15,20-tris(4-methylphenyl)porphyrin (1) was synthesized from 5-(4-aminophenyl)-l0,15,20-tris(4-methylphenyl)porphyrin using the phase transfer Hofmann carbylamine reaction. Diporphyrin 2 was synthesized using 5-(4-is0cyanophenyl)-lO-(4-aminophenyl)-15,20-bis(4-methylphenyl)porphyrinand 5-(4-carboxYphenyl)-10,15,20tris(4-methylpheny1)porphyrin.Details are given in D. Luttrull, Ph.D. Thesis, Arizona State University, 1991. (9)The solvent is reagent grade toluene that has been redistilled. The porphyrins were purified by thin-layer chromatography (silicagel-toluene) just before use. (10)The STM images shown in this paper were taken in a nominal constant-height mode where current variation is recorded as the tip is rastered over the surface a constant distance from it. However, in order to compensate for drift and gross surface topography, a little integral gain is left in the constant-current servo. The result is that features such as steps are differentiated and appear as light (Figure 1)or dark (Figure 3)stripes depending upon the direction of approach. The magnitude of the current variations is also affected by this residual servo gain, and this introduces considerable uncertainty into the absolute range of currents recorded.
0 1992 American Chemical Society
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766 Langmuir, Vol. 8,No. 3, 1992
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Area (A ) Figure 1. Clean gold (A), gold treated with compound 1 (B), and gold treated with compound 2 (C) (the latter two images are from different areas of the same substrate obtained with the same tip). Images are obtained in the constant-height mode at a tip bias of -1.2 V. The arrows labeled S point to features associated with steps on the gold while the numbered arrows (1and 2) point to features associated with compounds 1 and 2. The structures of compounds 1 and 2 are shown in (D). Graphs E and F show particle size distributions obtained from runs over the side of the substrate treated with 1 (E) or the side treated with 2 (F).12 The solid lines are Gaussian fits. Scattered particles occurred in the range 1200-1600 Hi2for 2. These particles were included in the Gaussian fits, but are not shown on the histograms. The average tunnel current was 1nA, and the whitest points on the images correspond to -100 nA.
molecules is modified in a way that correlates with the total dose (concentration X exposure) and the particular molecules (1 or 2) used. We illustrate this correlation with the following experiment: Half of a substrate was allowed to reactll with 1 only, the other half with 2 only, and each side was imaged alternately by translating the sample under the STM tip. Thus, the same tip was used for forming images of both types of molecule. Typical images from this experiment are shown in parts B (1) and C (2) of Figure 1. After (11) Submonolayer coverage was obtained by dipping the substrate into a -3 x 10-7 M toluene solution of the molecule for 2 s followed by three 5-9 rinses in clean toluene.
reaction with the porphyrin the surface is littered with “blobs”. These blobs are, on the average, larger on the side of the substrate treated with 2 when compared with the images obtained from the side treated with 1. In contrast to images of ordered arrays (like liquid crystals) there is considerable variation in the images. We have quantified the differences in the images from the two regions of the substrate by using a computer program to generate a distribution of particle sizes. We rejected particles smaller than 10A2(becauseof noise) and counted 80-100 particles on each side of the substrate using the public domain NIH The me shown in Figure 1E (for the half of the substrate treated
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Figure 2. A series of images taken at different tip bias over the same area of substrate treated with 2. Steps on the gold appear as dark bands (one is labeled S). The tip bias was -2 V (A), -1.5 V (B), -1.2 V (C), -0.4 V (D), -50 mV (E), and then back to -1.2 V (F). The same contrast scale has been used in each image. The average tunnel current was 1nA, and the whitest points on the images correspond to -100 nA.
with 1) and Figure 1F (for the half of the substrate treated with 2). These histograms quantify the degree to which the larger molecules produce larger images. Many of the molecular images appear to have rather characteristic features. Porphyrin 1 often gives rise to elliptical images while 2 often gives images that show two blobs, one often “higher” than the other. If we assume that the molecular electronic states that enhancetunneling are confined to an area similar to the van der Waals dimensions of the molecules, then the observed mean particle sizes (121 A2for 1 and 396 A2 for 2) are consistent with the dimensions of molecular models of 1bound edgeon (area of top view 120 A2) and 2 binding with one ring edge-on and the other flat on the surface (area of top view 380 A2). However, both distributions are very broad (with standard deviations of 98 Hi2 for 1 and 252 Hi2 for 2). This heterogeneity may be genuine (reflecting a variety of conformations on the surface) or spurious (due to contamination). Experiments described below favor the former explanation. Images of the substrate alone do not depend on tip bias because the density of states does not vary much for small excursions about the Fermi level of the metal. The molecular images, on the other hand, show a strong dependence on bias. We illustrate this in Figure 2 with a series of images taken over the same area of the substrate (the black lines are due to gold steps; the S marks a fixed point on the substrate). The images are taken with the tip negative with respect to the substrate a t 2 V (A), 1.5 V (B), 1.2 V (C), 0.4 V (D), and 50 mV (E). The molecular image contrast is highest near 1.2 V and has gone altogether at 50 mV. The Nanoscope does not permit the current servo to be turned off altogether,1° so the tip will have been driven into the surface during these experiments, knocking molecules away from the scanned region. However, this (12) The particle size distribution depends upon the height contour used to define the boundary. In the absence of a theoretical basis for interpreting image contrast, the choice is arbitrary. A value was chosen in the middle of a range where the resulting size distribution was fairly insensitive to the choice of boundary contour, and the same value was used for 1 and 2.
does not explain the loss of contrast, for when the bias is returned to 1.2 V, the image contrast returns (some molecules can be found a t the same locations in (F) and (D)). Similar effects have been seen in images of liquid crystals14 and alkane chains.15 Mizutani et aI.l4 have attributed this behavior to resonant tunnelingl3J6 as the electrons tunnel from filled statesin the tip to empty states in the substratevia the lowest unoccupied molecular orbital (LUMO) of the molecule. However, such a direct electronic process is unlikely in this case because there is a significant time delay ( N 10 s) between adjustment of the tip bias and the corresponding change in contrast. Whatever the mechanism, one might expect differences when the tip is positive with respect to the substrate if molecular states are implicated, especially as the highest occupied molecular orbital (HOMO)might well be involved in the latter case.l6 Although we find that the molecular image contrast is lost if the bias is reversed, this is likely because the adsorbate is destabilized (the isonitrile attachment should be less strong a t a negatively charged gold surface). When a large area is scanned after a smaller scan made with the tip positive, a square hole is observed where the substrate has been swept clean. Thus, while these data suggest that molecular states are involved in some way, they do not support a simple resonant tunneling model. These experiments help to clarify the origin of the heterogeneity discussed above. This is because images of a contaminated surface (e.g., exposed to laboratory air for several hours) show features with a random bias dependence. Note that when the surface is systematically reacted with porphyrin-based molecules, the contrast changes with bias are similar over the whole surface (Figure 2). Recall (13) Lindsay, S. M.; Sankey, 0. F.; Li, Y.; Herbst, C.; Rupprecht, A. J . Phys. Chem. 1990,94,4655. (14) Mizutani, W.; Shigeno, M.; Ohmi, M.; Kajimura, K. Appl. Phys. Lett. 1989,56, 1974. (15) McGonigal, G. C.; Bernhardt, R. H.; Yeo, Y. H.; Thompson, D. J. In Scanned Probe Microscopies: STM and Beyond; Wickramasinghe, H. K., Ed.; AIP New York, 1991; in press. (16) Lindsay, S. M. In Scanning Tunneling Microscopy: Theory, Techniques and Applications; Bonnell, D., Ed.; V C H New York, 1991; in press.
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Figure 3. Scans over densely covered regions of 1 (A) and 2 (B). Arrows point to the periodic furrow structure. (A) was obtained at -0.2 V where there is considerablevariation from scan to scan;this image became disordered in three scans. (B) was taken at 4 . 7 V. The average tunnel current was 0.5 nA, and the whitest points on the images correspond to -4 nA.
that control experiments with toluene alone produced no features. Therefore, the heterogeneity of the images must be due to a variety of conformations of the molecule on the surface, and/or a variety of tip-molecule interactions. We went to some lengths to find reaction conditions that produced these submonolayercoverages.1° It is very difficult to control the coverage precisely, and extended exposure to the reactive molecules usually produces a dense layer that does not often yield recognizable images. However, we have noticed a remarkable effect when such dense layers are imaged at low biases (-0.2 V so that the tip “scrapes off“ the top surface). An underlying ordered pattern is almost always observed. Continued scanning at a low bias causes this structure to be disrupted, but if the bias is taken to a high value (>0.5V), the ordered structure often persists. Although interaction with the tip is usually required to first “expose” the structures, they show no obvious correlation with the scan direction and their appearance depends on whether 1 or 2 is used. Thus, we believe that these patterns reflect an ordered layer that forms a t the interface at high coverage. We show some examples of these patterns in Figure 3. M 1 or 2 for 48 h. Figure Substrates were exposed to 3A was obtained on the first scan over a layer of 1 after the tip bias was lowered to 0.2 V and the pattern persisted for three scans after which it appeared disordered. This pattern of 60-A “furrows” is found in every experiment all over substrates with a high coverage of 1. Figure 3B shows a scan over a substrate covered with 2. In this case, one scan was made at low bias, and the bias was then increased
to 0.7 V for this image. Furrows of -30 A are observed. However, other runs have yielded furrows with periods of 30,40, and 60 when molecule 2 was used. Thus, both 1 and 2 form ordered arrays at high coverage, but the arrangements of 2 are more varied. We do not understand the origin of these periods, but note that electrochemical adsorbates on this surface tend to align with the underlying 23Xd3 reconstruction of Au(ll1) which has a period of -65 A (N.-J. Tao and S. M. Lindsay, unpublished data). These data provide convincing evidence that molecules can be bound to a surface strongly enough to permit them to be imaged in an ambient environmentwhile more loosely bound contamination is swept away by the STM tip. The images are of use in determining the coverage, mechanism of attachment, and to some extent, electronic properties of a molecular adsorbate in an environment appropriate for the operation of practical molecular electronic devices. They also show how a number of ordered arrangements of the molecules are formed when the coverage is high.
Acknowledgment. This work was supported by grants from the NSF (DIR 89-20053) and ONR (N00014-90-J1455) to S.M.L. and the DOE (DE-FG02-87ER13791)to D.G. and T.A.M. This is Publication No. 98 from the Arizona State University Center for the Study of Early Events in Photosynthesis. The Center is funded by DOE Grant DE-FG07-88ER13969 as part of the U.S. Department of Agriculture-Department of Energy-National Science Foundation Plant Science Center Program. We thank Professor Mark Wrighton for helpful discussions.
