STM-Induced Desorption of Polydiacetylene Nanowires and

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6161

2007, 111, 6161-6166 Published on Web 04/06/2007

STM-Induced Desorption of Polydiacetylene Nanowires and Reordering via Molecular Cascades Rajiv Giridharagopal and Kevin F. Kelly* Department of Electrical and Computer Engineering and Rice Quantum Institute Rice UniVersity, Houston, Texas 77005 ReceiVed: February 5, 2007; In Final Form: March 14, 2007

The desorption of polydiacetylene nanowires has been studied using scanning tunneling microscopy at both solid-air and solid-vacuum interfaces. Langmuir-Schaefer films of 10,12-pentacosadiyonic acid on highly ordered pyrolytic graphite can be polymerized to form well-ordered polydiacetylene nanowires in ambient and ultrahigh vacuum conditions. These nanowires can either fully desorb or be cut into shorter segments depending on the strength of the interaction with the scanning tunneling microscope (STM) tip. A single disruption usually causes the nanowire to completely desorb. In both desorption cases, the surrounding monolayer order is fully restored within a single line acquisition (approximately 100 ms). The subsequent monolayer reordering, though expected in a solid-liquid interface experiment, is unexpected at solid-air and solid-vacuum interfaces and may be due to a molecular cascade of the diacetylene derivative molecules. Desorption has been observed over numerous sample imaging conditions on both blue-phase and red-phase polydiacetylene nanowires. The results reported here shed light on the relative interaction strengths between the scanning tip, the polymer backbone, and the graphite substrate. Observations of desorption also imply that polydiacetylene nanowires are sensitive to perturbation by the local scanning probe, which is a critical factor to consider in evaluating tunneling spectroscopy data.

Increasing scientific and industrial interest in lower-dimensional electronic structures has encouraged investigations into molecular electronics. Molecular electronic device operation has been reported in novel systems using various experimental measurement methods.1-3 To enhance their broader applicability, molecular electronic circuits require interconnects so that different switch elements can function as an integrated logical unit as has been demonstrated using a crossbar architecture.4 The material used as an interconnect would necessarily have to meet several criteria to be of physical and chemical utility in molecular circuits. The interconnect would have to be highly conducting, able to be controllably employed, and stable. In an effort to find such a material, conducting polymers such as poly(3-hexylthiophene),5,6 polyaniline, polypyrrole,7 and polydiacetylene8 have been studied as potential low-dimensional interconnecting nanowires. Polydiacetylene (PDA) is ideal for interconnects given its linear, π-conjugated structure9-11 and high conductivity when doped with iodine.12,13 Recently, a double-tip scanning tunneling microscope was used to analyze iodine-doped PDA thin films and measured conductivities as high as 10-3 S/cm.14 A unique advantage of PDA over most other conducting polymers is the ability to controllably form nanowires from an ordered film. Diacetylene derivative molecules can be polymerized using the scanning tunneling microscope to form linear nanowires at both solid-air15-17 and solid-liquid18,19 interfaces. By applying voltage pulses at predetermined points during the scanning * Corresponding author. E-mail: [email protected].

10.1021/jp070998l CCC: $37.00

process, PDA nanowires can be formed where needed. PDA thus seemingly satisfies the first two criteria for a molecular interconnect given its potential for high conductivity and ability to be controllably formed on a surface. The issue of stability in polydiacetylene nanowires, however, has not been investigated sufficiently at the level of detail afforded by scanning tunneling microscopy. Elucidation of the structural integrity of these nanowires is important for practical applications, such as recently demonstrated PDA monolayer transistors,20 and also further clarifies the physical and chemical nature of the wires on the surface. Although a number of papers have reported on the phase change from blue- to red-phase polydiacetylene,21-24 little attention has been placed on surface effects such as polymer desorption. Here we report the scanning tunneling microscope (STM) tip-induced desorption of polydiacetylene nanowires controllably and uncontrollably formed in a monolayer film of 10,12-pentacosadiynoic acid (PCDA), CH3(CH2)11CtC-CtC(CH2)8COOH, in both ambient and ultrahigh vacuum (UHV) conditions. The STM has been used to provide a nanoscale view of desorption at these solid-air and solidvacuum interfaces. Desorption at the liquid-solid interface has been discussed briefly before, but this is the first extensive report of nanowire desorption and also the first to investigate desorption at solid-air and solid-vacuum interfaces. This is also the first report of successful STM tip-induced polymerization in UHV. Prior to this report, all tip-induced polymerization has been performed in ambient conditions or in a liquid environment. Two types of tip-induced desorption effects can occur: either the nanowire fully desorbs or is cut into a shorter segment; the © 2007 American Chemical Society

6162 J. Phys. Chem. C, Vol. 111, No. 17, 2007 latter effect is rarely observed in ambient conditions and never observed in UHV conditions. The interaction between the polymer backbone and the STM tip causes desorption, and the strength of the interaction may determine which effect occurs. In either case, however, the PCDA molecules restore the original monolayer ordering on the surface. Monolayer films of PCDA (Sigma-Aldrich) were prepared by a Langmuir-Schaefer or horizontal lifting technique using a commercial Kibron Microtrough S Langmuir-Blodgett apparatus.25 The PCDA molecules were dissolved in chloroform in a 2 mM solution (0.75 mg/mL), and 20 to 40 µL of the solution was deposited on a subphase of ultrapure water. All preparation was completed in UV-filtered environments to prevent extraneous polymerization of the PCDA molecules. Films were deposited onto highly ordered pyrolytic graphite (HOPG) and molybdenum disulfide (MoS2)26 and subsequently polymerized using either the STM tip or through UV irradiation applied by a pen lamp.27 Images were acquired in ambient conditions using a custom-built STM28 and commercial RHK electronics.29 Images in UHV were acquired with a commercial variable-temperature STM and electronics from Omicron30 with main chamber pressures on the order of 10-10 Torr. All images were acquired in the constant current mode of operation, and bias voltages given refer to the sample bias voltage.31 Considerable effort was taken to try and minimize possible direct tip interactions or tip crashes that would physically disrupt the nanowire. Specifically, images here were acquired using lower tunneling currents, constant current imaging, and slower scan speeds than those used by Sullivan et al. in discussing liquid-solid interface desorption.18 The tunneling current was set to less than 30 pA in ambient scans and less than 110 pA in UHV scans, and the bias voltage was -1.2 V or less (in magnitude). In UHV, the gap resistance was approximately 10 GΩ. In ambient conditions, the gap resistances was approximately 100 GΩ , an order of magnitude difference in tipsample gap, yet in both environments desorption was observed. Also, the scanning here was not in the “constant height” mode of operation, where the tunneling current, not the tip height, varies during imaging. Constant height imaging eliminates the feedback mechanism, thus causing the tip to potentially disrupt the surface at raised structures like a PDA nanowire. In constant height mode, it is possible that the nanowire is not so much “desorbing” due to an interaction force as it is being physically disrupted by contact with the STM tip. Desorption observed under these circumstances is due to interactive forces between the tip and sample. In constant current mode, as used here, the feedback loop ensures that the tip moves in response to the nanowire in order to maintain a constant tunneling current. Also, the scan speeds used were slow, generally 1-2 min/image. Faster scanning is clearly likely to interfere with surface structures given that the feedback circuit must be much more responsive as the scan rate is increased, which would pose a problem at discontinuous features such as the nanowire on the surface. Desorption occurs within an STM scan, as noted by the series of consecutive images in Figure 1, taken in UHV. Figure 1a shows a typical UHV STM image of a PCDA monolayer with a nanowire visible as a bright line on the surface. The nanowire in Figure 1a was formed through UV polymerization prior to scanning. At the point indicated in the image in Figure 1b, a voltage pulse was applied to initiate controlled polymerization, creating a nanowire mid-scan. However, Figure 1b shows the nanowire in UHV being polymerized controllably and then disrupted within the same scan. The nanowire was abruptly

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Figure 1. STM topography images of polymerization and desorption in UHV (all 130 × 130 nm2, -1.0 V, 100 pA). (a) The film area prior to initiating polymerization. The wire in the image was formed by UV polymerization. (b) A voltage pulse was applied (-4 V, 10 µs) at the point indicated by the white circle. Polymerization occurred, and the wire was immediately imaged during the same scan, but it was subsequently disrupted due to an interaction with the STM tip at the black circle near the bottom of the image. The PCDA molecules restored the original monolayer ordering within a single line scan replacing the desorbed nanowire as seen in c. The dashed circle in c indicates a disruption in the wire from a. Insets in a-c represent an atomicresolution image of PCDA ordering (6.2 × 1.2 nm2, -1.3 V, 3.5 pA), a schematic showing polymerization of PCDA to form PDA nanowires, and a high-resolution image of a PDA nanowire (8.9 × 1.0 nm2, -1.0 V, 10 pA), respectively. The last inset was high-pass filtered to account for the height difference between the PDA nanowire and surrounding monolayer.

terminated due to an interaction with the STM tip, but it was replaced by well-ordered PCDA molecules in the same image within the time between subsequent line scans, typically no more than 100 ms. In Figure 1c, it is evident that the tip-induced nanowire has completely desorbed despite only a single tippolymer interaction. The disruptive effect shown in Figure 1c on the original UV-polymerized nanowire from Figure 1a also results in it desorbing at the point indicated, implying that tipinduced desorption can occur on nanowires formed through either polymerization method.

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Figure 2. Successive STM topography images of polymerization and desorption of a wire in air (all images 47.3 × 90.3 nm2, -1.0 V, 10 pA). (a) Before and (b) after images of the same area where a voltage pulse (-4 V, 8 µs) was applied at the point indicated by the dashed circle, resulting in a formed polydiacetylene wire. The other two wires were formed by UV polymerization. In c, one of the UV-formed nanowires was disrupted, causing it to be terminated mid-scan and appear shorter in length compared to the formed nanowire below it. In d, the nanowire fully desorbed, leaving the STM tip-induced nanowire unaltered at 54 nm in length throughout b-d.

As in UHV, nanowires in ambient conditions will also desorb due to a single STM tip-nanowire interaction. The images in Figure 2, taken from a larger set of images over the same area, compare the lengths of a UV-polymerized and tip-induced nanowire over consecutive scans. A nanowire was generated through STM tip-induced polymerization of the monolayer at the point indicated in the first image, thus creating a nanowire parallel to the original, UV-polymerized nanowire above it. In this way, the two nanowires’ lengths could be compared to provide a simple visual depiction of the tip-cutting process. The tip-induced nanowire was terminated at the right by a monolayer domain boundary and on the left by a defect site. The length of the UV-formed wire compared to the unaltered length of the created nanowire is shown in Figure 2b, where the UV-formed nanowire is approximately 42.7 nm and the tip-induced nanowire is 54.1 nm. In Figure 2c, the UV-formed nanowire was disrupted by an interaction with the STM tip and thus appears shorter in length (approximately 33.2 nm) relative to the stable tip-induced nanowire in the image. By the subsequent scan, the nanowire fully desorbed, as was seen in UHV in Figure 1. The tip-induced nanowire was unaltered throughout and was approximately 54 nm long in each image. In ambient conditions, the STM tip can sometimes cut the longer wire into shorter oligomer segments, an effect that was never witnessed in any of the UHV scans. This tip-induced nanowire cutting is not the typical desorption effect. Figure 3 illustrates this effect, where the images are from a larger set of images of the same wire. The nanowire was interrupted by the STM tip at the point indicated by the dashed circle in Figure 3b, thus cutting the wire and causing the remaining part of the wire to desorb. Unlike the case of UHV, though, the entire nanowire has not desorbed; in fact, the subsequent scan in Figure 3c indicates that the nanowire remained, though shorter in length. Another STM tip-wire interaction occurred at the point indicated in the third image, and the nanowire was again cut shorter. The monomer order was again restored immediately. By the final image in Figure 3d, the nanowire is no longer visible. It is possible that the nanowire fully desorbed or simply is not visible within the scanned area due to drift that occurs during STM imaging. If the nanowire did fully desorb, then that implies that both desorption effects are visible on the same wire. However, it is more typical for STM tip interactions with polydiacetylene nanowires to cause them to fully desorb upon a single disruption.

Figure 3. STM topography images in air showing explicit cutting of a polydiacetylene wire by an STM tip (63.6 × 63.6 nm2, -1.0 V, 20 pA). (a) The image was taken from a set of images where the two nanowires were formed by UV irradiation. (b) The circle indicates where the STM tip interrupted the wire, and it was immediately replaced with well-ordered PCDA molecules. The nanowire was still present in c, indicating that the nanowire was cut shorter and did not fully desorb upon interaction with the STM tip in b, unlike the previous cases above. An additional disruption in the nanowire is denoted by the dashed circle. (d) The nanowire is no longer present in the image, implying that it has either desorbed or drifted beyond the area shown.

Several nanowires can simultaneously desorb between scans as well. Figure 4 shows several nanowires desorbing between consecutive scans of the same area. In Figure 4a, an ordered PCDA monolayer is visible in the background, but this monolayer is disordered after the desorption event. Such mass desorption has been observed in other samples as well. Also, the straight, well-ordered nanowires in Figure 4a give way to disordered wires. A few observations can be made based on this result. First, there seems to have been no discrimination between which type of desorption occurs over which nanowire. Some of the nanowires that desorbed, particularly those that fully desorbed, did not seem to exhibit any sign of an STM tip-sample interaction mid-scan as was seen in the earlier figures. Additionally, it is possible that the nanowires in Figure 4 are in the so-called “red phase” rather than the pristine “blue phase,” which may be more sensitive to tip-induced desorption effects. Blue-phase polydiacetylene is well-ordered and linear, but it is known to be less stable than the disordered red-phase form. In Figure 4a, the presence of the monolayer implies that these nanowires may be in the blue phase. However, the disordered nature of the wires in Figure 4b implies that the desorption event caused this transition. The more disordered red-phase polydiacetylene probably disrupts the surrounding PCDA monolayer ordering. The nanowires in Figures 1-3 are most likely in the “blue phase” given the well-ordered surrounding monolayer.21-23 It is not possible to directly confirm which chromatic phase is present with the STM, though, so this conclusion is tentative. Figure 4 also shows how the disordered liquid area of the monolayer can spread. In Figure 4a and c, the PCDA monolayer is present. By Figure 4b and d the liquid area has expanded, and, in the case of b, the PCDA monolayer is no longer visible. Thus, reordering may not be the only possible result after

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Figure 4. Desorption of multiple nanowires occurring between consecutive scans at the same scan conditions (-1.0 V, 10 pA, 198 × 198 nm2), scanned in air. (a) STM topography image of numerous nanowires on the surface. An ordered monolayer is barely visible around some of the nanowires, but not all, which may be indicative of red-phase polydiacetylene nanowires. (b) A large percentage of the nanowires desorbed; unlike the previous figures, a number of nanowires simultaneously desorbed. There is no indication of ordered PCDA molecules on the surface after the desorption event. (c) Before and (d) after images of the same area on a different sample that show the expansion of disordered liquid area of the film (-0.75 V, 20 pA, 72 × 72 nm2). In both images, the white area is an upper step on the graphite surface, but the contrast has been adjusted to emphasize the cascade data on the lower step. Between c and d, the nanowire present desorbed.

Figure 5. Nanowire desorption on MoS2 (138 × 58 nm2, -1.23 V, 5 pA). (a) Before and (b) after images of the same area, indicating that three nanowires desorbed upon interaction with the STM tip. The PCDA monolayer again filled in to restore the ordering on the surface.

nanowires have desorbed; the PCDA molecules’ collective motion may play a role in whether and how reordering occurs. This is discussed further below. Desorption has also been observed on MoS2 substrates, indicating that nanowire desorption is not necessarily related to the interaction with the HOPG substrate, as shown in Figure 5. As was observed on HOPG, the STM tip caused the nanowire on MoS2 to desorb. However, none of the nanowires on MoS2

Letters were cut into shorter segments. This may be an issue concerning the number of samples. As noted earlier, tip-induced cutting is relatively rare, and there were far fewer MoS2 samples than HOPG samples. Polydiacetylene desorption has been observed at constant bias voltages and when the sample bias voltage is progressively decreased or increased. This indicates that the voltage bias is not a substantial factor in the desorption process. The effect was observed multiple times in air and UHV under various scanning conditions. In air, desorption has been observed on over 100 different wires, and the surrounding PCDA monolayer is restored immediately upon desorption except in cases of multiple desorption as in Figure 4. Reordering takes place within 100 ms, which is the time to acquire a line in the STM image. The fact that the monolayer is reordered at all is surprising, and the chemical process involved in monolayer order restoration is obfuscated by the successful observation of desorption in both UHV and air environments. Reordering is expected in a liquid environment given the seemingly infinite reservoir of surrounding material available to fill in for a desorbed wire. However, at the solid-air/-vacuum interface this result is unexpected given that there should only be a single layer of molecules on the surface in an LS film deposited with the surface pressure and film area used here, though regions of multilayered molecules cannot be excluded. Upon initial observation of the monomer reordering effect, it was assumed that the STM tip was in fact piercing a secondary layer of molecules and imaging the molecules at the interface, similar to how the STM tip pierces a very large drop of material and images molecules at the liquid-solid interface.18,19,32 However, the images here should be indicative of a single-layer and not a multiple-layer effect. A bilayer, though expected in a typical LB film deposition technique,33 is not expected to form on the surface in an LS film. The formation of a bilayer is what is assumed to be the case when a noticeably disordered film is observed, as was shown in some of the earliest work concerning scanning tunneling microscopy of ordered LS films.34 This is consistent with our observations that the use of LB-deposited PCDA films often results in highly disordered films with no discernible structure. One possible explanation is that, contrary to initial assumptions, the molecules do not have any reservoir of material to restore the order but rather the molecules at the solid-air interface exhibit a molecular cascade effect on the surface. Here the term “molecular cascade” is used to describe a correlated motion of many molecules similar to reports of benzene molecular cascades on gold.35 In this case, the correlated motion is two-dimensional in order to reorder the film in both directions. A molecular cascade view implies that the PCDA molecules have sufficient mobility to fill in and reorder upon desorption of the PDA nanowire, a reasonable assertion given the relatively weak van de Waals force between the PCDA molecules and the HOPG and MoS2 substrates that should allow for an increased mobility of PCDA molecules on the surface. The fact that the PCDA monolayer is stable over continued scanning while the PDA wire can be induced to desorb is attributable to the relative interaction strengths involved. Previous work on LB films on HOPG have shown evidence of interaction with the tip, which is a result of the low binding strength of the molecules to the substrate.36 Even though the PCDA molecules are weakly bound to the surface, it is possible that the molecules are less likely to interact with the STM tip than with the substrate, supporting the idea that molecular cascades can occur. The PCDA-substrate binding strength may

Letters thus be high enough to avoid the effects of STM tip-induced disruption yet low enough to allow reordering. PDA nanowire desorption can thus be explained by comparing three relevant interactions in the system, specifically the tip-PDA, PDA-substrate, and tip-PCDA interactions. The nature of these interactions is a complicated set of factors that minimally involve van der Waals forces between the tip and substrate, implying a deeper mechanism.37 STM tip-induced desorption of nanowires is due to the relative interaction strengths between the polymer and the monomers, even at high gap resistances of 100 GΩ or more. This is because the weak PDA nanowire-substrate interaction is comparable to or less than the strength of the STM tip-polymer interaction,18 similar to the effect observed in STM of carbon nanotubes38 and polymer chains.39 The PCDA monolayer is not disrupted or affected by the STM tip, however, because the tip-polymer interaction is stronger than the tip-PCDA interaction in both UHV and ambient conditions. In the work reported here, gap resistances of at least 100 GΩ in air and at least 10 GΩ in UHV40 were used. However, in both instances interaction with the PCDA monolayer was negligible, indicating that higher currents (lower gap resistances) in UHV cause less of an STM tip-PCDA monolayer interaction than equivalently high tunneling currents in ambient scanning. This should not be construed to mean that the tip-polymer interaction is necessarily weaker in UHV. Accordingly, the PCDA monolayer is not restricted by the STM tip in either environment and thus is free to reorder upon nanowire desorption. Molecule-molecule interactions, while relevant in determining whether the PCDA monolayer is in a disordered liquid phase or well-ordered solid phase, are not likely to affect whether the nanowire desorbs or not. The strength of the tip-polymer interaction may also determine which particular desorption effect occurs. The nanowire may be more likely to fully desorb as the strength of the tip-polymer interaction increases. This is consistent with the observation that nanowires were never cut shorter in UHV. The tip-polymer interaction may be stronger in UHV because of the lower gap resistance. The effect of vacuum may also play a role, though to what extent that role is remains uncertain at this point. Likewise, condensation effects in ambient STM may also play a role in nanowire desorption. It should also be noted that PDA nanowire desorption effects observed here can interfere with the observation of scanning tunneling spectroscopy (STS) data. In STS work on PDA nanowires, we have observed that the spectroscopy process in ambient conditions can cause a disruption in the nanowire on the surface. Akai-Kasaya et al. remarked upon how STS measurements can disrupt a 10,12-nonacosadiyonoic acid monolayer if the tunneling current is too high.8 The results here extend that observation to PDA nanowires as well. Taking spectra without a concurrent image may lead to erroneous data due to interactions with the STM tip and should be treated with caution. This explains why the monolayer is stable over continued scanning yet the PDA wire can be disrupted. Previous work on LB films on HOPG have shown evidence of interaction with the tip, which is a result of the low binding strength of the molecules to the substrate.36 Even though the PCDA molecules are weakly bound to the surface, it is possible that the unpolymerized molecules are less attracted to the STM tip than the substrate. The PCDA-substrate binding strength may thus be high enough to avoid the effects of STM tip-induced desorption, yet low enough to allow reordering. We have observed polydiacetylene nanowire desorption in ambient and ultrahigh vacuum conditions using scanning

J. Phys. Chem. C, Vol. 111, No. 17, 2007 6165 tunneling microscopy. Desorption has been observed over nanowires formed by UV irradiation or STM tip-induced polymerization. The surrounding PCDA molecules restore order immediately upon desorption, implying a mobility of the monomers on the surface that is consistent with a molecular cascade effect. This is significant in evaluating the stability of polydiacetylene for molecular electronics applications and also in evaluating scanning tunneling spectroscopy data on polydiacetylene nanowires. Because these results were at room temperature, thermal effects may be involved as well; further experiments at low temperatures may better explain desorption. The results here may be applicable to other conducting polymer systems as well. Acknowledgment. We thank Prof. J. H. Hafner at Rice University for provision of Langmuir-Blodgett trough equipment and Y. Zhou for experimental assistance. R.G. is supported by a National Science Foundation Graduate Research Fellowship. This work is funded by the Rochester MURI on Nanoscale Subsurface Spectroscopy and Tomography (F49620-031-0379), administered by the Air Force Office of Scientific Research and the National Science Foundation (ECS0601303). References and Notes (1) Donhauser, Z. J.; Mantooth, B. A.; Kelly, K. F.; Bumm, L. A.; Monnell, J. D.; Stapleton, J. J.; Price, D. W., Jr.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 2001, 292, 2303. (2) Keane, Z. K.; Ciszek, J. W.; Tour, J. M.; Natelson, D. Nano Lett. 2006, 6, 1518. (3) Reed, M. A.; Zhou, C.; Muller, C. J.; Burgin, T. P.; Tour, J. M. Science 1997, 278, 252. (4) Kuekes, P. J.; Stewart, D. R.; Williams, R. S. J. Appl. Phys. 2005, 97, 034301. (5) Sakaguchi, H.; Matsumura, H.; Gong, H. Nat. Mater. 2004, 3, 551. (6) Sakaguchi, H.; Matsumura, H.; Gong, H.; Abouelwafa, A. M. Science 2005, 310, 1002. (7) He, H. X.; Li, X. L.; Tao, N. J.; Nagahara, L. A.; Amlani, I.; Tsui, R. Phys. ReV. B 2003, 68, 045302. (8) Akai-Kasaya, M.; Shimizu, K.; Saito, A.; Aono, M.; Kuwahara, Y. Phys. ReV. Lett. 2003, 91, 255501. (9) Bre´das, J. L.; Change, R. R.; Silbey, R.; Nicholas, G.; Durand, P. J. Chem. Phys. 1981, 75, 255. (10) Graja, A. Low-Dimensional Organic Conductors; World Scientific: Singapore, 1992. (11) Sebastian, L.; Weiser, G. Phys. ReV. Lett. 1981, 46, 1156. (12) Sakamoto, M.; Wasserman, B.; Dresselhaus, M. S.; Wnek, G. E.; Elman, B. S.; Sandman, D. J. J. Appl. Phys. 1986, 60, 2788. (13) Day, D. R.; Lando, J. B. J. Appl. Polym. Sci. 1981, 26, 1605. (14) Takami, K.; Kuwahara, Y.; Ishii, T.; Akai-Kasaya, M.; Saito, A.; Aono, M. Surf. Sci. 2005, 591, L273. (15) Okawa, Y.; Aono, M. Nature 2001, 409, 683. (16) Okawa, Y.; Aono, M. J. Chem. Phys. 2001, 115, 2317. (17) Miura, A.; De Feyter, S.; Abdel-Mottaleb, M. M. S.; Gesauire`re, P.; Grim, P. C. M.; Moessner, G.; Sieffert, M.; Klapper, M.; Mu¨llen, K.; De Schryver, F. C. Langmuir 2003, 19, 6474. (18) Sullivan, S. P.; Schnieders, A.; Mbugua, S. K.; Beebe, T. P., Jr. Langmuir 2005, 21, 1322. (19) Nishio, S.; I-i, D.; Matsuda, H.; Yoshidome, M.; Uji-i, H.; Fukumura, H. Jpn. J. Appl. Phys. 2005, 44, 5417. (20) Scott, J. C.; Samuel, J. D. J.; Hou, J. H.; Rettner, C. T.; Miller, R. D. Nano Lett. 2006, 6, 2916. (21) Saito, A.; Urai, Y.; Itoh, K. Langmuir 1996, 12, 3938. (22) Tieke, B.; Lieser, G.; Wegner, G. J. Polym. Sci.: Polym. Chem. Ed. 1979, 17, 1631. (23) Lio, A.; Reichart, A.; Nagy, J. O.; Salmeron, M.; Charych, D. H. J. Vac. Sci. Technol., B 1996, 14, 1481. (24) Sasaki, D. Y.; Carpick, R. W.; Burns, A. R. J. Colloid Interface Sci. 2000, 229, 490. (25) Kibron, Inc., http://www.kibron.com. (26) Both purchased from SPI. (27) Model Spectroline 11SC-2, Sigma-Aldrich. The UV lamp was held from 3 to 8 cm above each sample, and the irradiation times varied from 1 to 10 min. (28) Kelly, K. F.; Sarkar, D.; Prato, S.; Resh, J. S.; Hale, G. D.; Halas, N. J. J. Vac. Sci. Technol., B 1996, 14, 593. (29) RHK, http://www.rhk-tech.com.

6166 J. Phys. Chem. C, Vol. 111, No. 17, 2007 (30) Omicron GmbH, http://www.omicron.de. (31) Images were plane and offset-subtracted using WSxM (Nanotec Electronica, http://www.nanotec.es) and XPMPro (RHK, http://www.rhktech.com) to correct for piezo drift and were then converted to grayscale TIFF images using Adobe Photoshop Elements and Adobe Photoshop. No other processing was performed on the images unless stated otherwise. (32) Grim, P. C. M.; De Feyter, S.; Gesquie`re, A.; Vanoppen, P.; Ru¨cker, M.; Valiyaveettil, S.; Moessner, G.; Mu¨llen, K.; De Schryver, F. C. Angew. Chem., Int. Ed. 1997, 26, 2601. (33) Hann, R. A. Molecular Structure and Monolayer Properties. In Langmuir-Blodgett Films; Roberts, G. G., Ed.; Plenum Press: New York, 1990; p 17. (34) Kuroda, R.; Kishi, E.; Yamano, A.; Hatanaka, K.; Matsuda, H.; Eguchi, K.; Nakagiri, T. J. Vac. Sci. Technol., B 1991, 9, 1180.

Letters (35) Han, P.; Mantooth, B. A.; Sykes, E. C. H.; Donhauser, Z. J.; Weiss, P. S. J. Am. Chem. Soc. 2004, 126, 10787. (36) Chiang, S. Molecular Imaging by STM. In Scanning Tunneling Microscopy, 2nd ed.; Gu¨ntherodt, H.-J., Wiesendanger, R., Eds.; SpringerVerlag Press: Berlin, 1994; Vol. I, p 181. (37) Ciraci, S. Theory of Tip-Sample Interaction. In Scanning Tunneling Microscopy; Wiesendanger, R., Gu¨ntherodt, H.-J., Eds.; Spring-Verlag Press: Berlin, 1996; Vol. III, p 179. (38) Curran, S.; Carroll, D. L.; Ajayan, P. M.; Redlich, P.; Roth, S.; Ru¨hle, M.; Blau, W. AdV. Mater. 1998, 3, 311. (39) Hua, Z. Y.; Xu, W. J. Vac. Sci. Technol., B 1997, 15, 1353. (40) Hardware limitations in the Omicron system prevented the use of lower tunneling currents (higher gap resistances) when scanning in UHV.