Local Scanning Probe Polymerization of an Organic Monolayer

Sep 13, 2012 - Joon Sung Lee†, Young Shik Chi†, Insung S. Choi‡, and Jinhee Kim*†. † Korea Research Institute of Standards and Science (KRIS...
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Local Scanning Probe Polymerization of an Organic Monolayer Covalently Grafted on Silicon Joon Sung Lee,† Young Shik Chi,† Insung S. Choi,‡ and Jinhee Kim*,† †

Korea Research Institute of Standards and Science (KRISS), Daejeon 305-340, Republic of Korea Molecular-Level Interface Research Center, Department of Chemistry, KAIST, Daejeon 305-701, Republic of Korea

‡

ABSTRACT: The possibility of lateral extension of conjugation within a covalently grafted molecular layer by a scanning probe-based method was tested. A molecular layer derived from ω-(N-pyrrolyl)propanol was formed on n-type Si(111) surface. Application of large sample biases greater than ±4 V during conductive atomic force microscope (AFM) scans under vacuum resulted in changes of mechanical and electrical characteristics of the molecular layer: the tip−sample conductance was increased greatly, the friction was reduced significantly, and the surface potential of the scanned area was increased. The reduction in friction could be attributed to molecular linking formed within the layer. The increased conductance suggested extended conjugation among the pyrrolyl end groups. Therefore, it was inferred that the biased AFM scan successfully induced local polymerization/oligomerization within the covalently grafted molecular layer.

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film formed on highly oriented pyrolytic graphite (HOPG) substrate.8 Lateral chain polymerization induced by STM pulses applied on self-assembled monolayers of diacetylene-containing compounds on HOPG was studied by Okawa and Aono9 and Miura et al.10 separately. Reversible bond formation and breaking among C60 molecules in physisorbed single- and multilayers on Si(111)-(7 × 7) or HOPG substrates under vacuum has also been studied by several groups.11−13 These works done using STM in a dry environment showed very fine control of oligomerization/polymerization patterns, typically down to single-molecule-level resolution. However, the above examples had one common attribute which might be undesirable for certain applications: the molecules, either polymerized or unreacted, were not covalently bonded to the substrates. Molecular layers weakly bonded to substrates can be easily damaged during subsequent manufacturing processes or under usage. Weak adhesion of the polymers to the substrates has been one of the issues in this field. Usage of surface-grafted molecular layer to be merged into the polymer can be the method of choice to deal with the adhesion problem. Jiang et al. have reported fabrication of 25 nm thick polypyrrole patterns defined by e-beam lithography on a pyrrole-containing molecular monolayer covalently grafted on a SiO2 /Si substrate.14 Beyond the adhesion issue, thinning the polymer layer down to monolayer scale may provide us new insights and applications. Lateral polymerization in self-assembled mono-

INTRODUCTION Electrical conductors and semiconductors based on organic polymers have been intensely studied from technological interests for the past decades. One of the most highlighted topics in the field is nanoscale formation of conducting polymer patterns on solid substrates,1 which has the potential to be applied to various electronic devices and sensors via incorporation into the current solid state electronics technology. Methods for the polymer pattern formation reported so far use microcontact printing,2 nanoimprint lithography,3 scanning probe lithography,4 and traditional methods for microelectronics such as e-beam and photolithography. Among these, scanning probe lithography has the great advantage that it does not require a master pattern formed by separate processes. It also offers high lateral resolution and the capability for in-situ examination after processing, which is an excellent feature for research purposes. Studies on nanoscale polymer pattern formation using scanning probe-based methods have been done under various environments. Some works carried out with the dip-pen5 and the nanoshaving6 methods rely on the presence of water as a medium for both precursor transfer and chemical reaction. There also have been researches which do not involve on-thefly transfer of precursors near the probe tip. For example, Jang et al. have reported a high-speed electrochemical oxidative nanolithography7 on an insulating polymer precursor film using a conductive AFM tip in an electrochemical cell. Numerous other works on scanning probe-induced polymerization in preformed precursor layers have been done using scanning tunneling microscopy (STM). Ma et al. showed an STM pulseinduced spot polymerization in a nitrile-containing monomer © 2012 American Chemical Society

Received: June 22, 2012 Revised: August 17, 2012 Published: September 13, 2012 14496

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ultrasonication in toluene, dichloromethane, or ethanol. The choice of solvent for the ultrasonication influenced the quantity of physisorbed excess molecules remaining on the covalently grafted molecular layer. It was found that ultrasonication in ethanol removed most of the physisorbed molecules from the sample surface. The heated reaction solution was later checked by alumina thin layer chromatography (TLC) for possible oligomerization among the molecules. Comparison between the TLC results from the pre- and the postreaction solutions confirmed that there was no dimerization/oligomerization reaction among the PyC3OH molecules in the solution. Sample Characterization and Modification. Sample patterning and measurements were carried out using a UHV-AFM from Omicron GmbH. Thickness of the molecular layer was determined by contactmode topographic profiling done after scraping out a patch of the molecular layer using a Si noncontact AFM probe (30 N/m, 300 kHz). The removal of the molecular layer was done either by contact-mode scans with μN-level high normal forces or by STM-mode scans with low biases which also resulted in μN-level high normal forces on the tip. Noncontact topography scans on the pristine molecular layers were done using a diamond-coated force modulation probe (3 N/m, 70 kHz). The rest of the patterning and measurements were carried out using a Pt/Cr-coated force modulation probe (3 N/m, 70 kHz). The rectangular patterning on the samples was done by contact scans with a scan speed of 600 nm/s and a line-to-line distance of 2 nm. After contact mode tip engagement, the z-feedback was turned off and the scanner was retracted by a few nanometers before starting the biased scans, to compensate for the large capacitive force between the substrate and the tip and to prevent tip height fluctuation during the patterning scans. After the patterning, topograph and friction mapping of the samples were carried out with contact-mode scans. Friction of the sample surface with respect to the tip was obtained by subtracting the lateral force image of the reverse scan from that of the forward scan. The zero-bias contact-mode scans were done before the surface potential measurements to provide a channel of dissipation for any trapped charges formed within or near the molecular layer during the biased scans. The surface potential distribution of the patterned samples was measured by Kelvin probe force microscopy using the method of Sommerhalter et al.22 The tip−sample current mapping was carried out by contact-mode scans with a small positive bias to prevent any further modification in the molecular layer. I−V spectroscopic measurements on the pristine and the biasscanned area were done by a method so-called grid spectroscopy during a contact-mode scan on the area which contained both kinds of regions. The scan was interrupted at vertices of a virtual square grid on the whole scan area, and then the bias was ramped while the current was being recorded with the z-feedback off. Since a large number of the individual I−V curves were quite noisy, more than 10 well-behaved curves were picked, avoiding both of the extremes (high/low conductance), and averaged to represent typical I−V characteristics for each area.

layers (SAMs) of thiophene- and pyrrole-terminated molecules grafted on an insulating substrate has been reported by Ogawa et al.15,16 However, in their works, the lateral polymerization was driven electrochemically without any lithographical measures for pattern formation. To our knowledge, nanoscale patterning by lateral polymerization in covalently grafted precursor monolayers using scanning probes still remains as a novel challenge in the field of molecular electronics. Our previous attempt at lateral polymerization in pyrrolylterminated alkanethiol SAM on Au(111) using STM17 has not been successful. Electrochemical polymerization in pyrrolylterminated alkanethiol SAM on Au has been extensively studied by McCarley and Willicut.18 For the polymerization process to occur, electrons should be first removed from the pyrrolyl groups to make radicals, which can react with each other to form conjugated bonds at 2- and 5-carbons. Thus, a strong electric field or other kinds of stimulation are required for the polymerization to begin and proceed. Our previous failure was due to the weak metallic bonding between the molecule and the substrate, which was easily ruptured by the strong electric field applied between the substrate and the tip. This time, we used silicon, the substrate of choice for molecular electronics,19 to resolve that issue. In this work, we tested lateral polymerization in a pyrrolylterminated molecular layer grafted on silicon by a scanningprobe-based method. A molecular layer derived from ω-(Npyrrolyl)propanol (PyC3OH) was formed on n-type Si(111) substrate. Modification in the properties of the molecular layer sample was observed after contact-mode conductive AFM scans with biases greater than ±4 V under vacuum; the tip−sample electrical conductance was increased greatly, the friction between the tip and the sample surface was decreased significantly, and the surface potential of the modified area showed a small positive shift. The changes in the conductance and the friction suggested that lateral extension of conjugation through oligomerization/polymerization among the pyrrolyl groups in the molecular layer took place as the result of the biased AFM scans.

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EXPERIMENTAL SECTION

Sample Preparation. The molecular layer of PyC3O−Si was formed on hydrogen-passivated n-type Si(111) substrates.20 To make samples more adequate for the scanning probe-based experiment, the substrates were prepared to have large flat terraces by eliminating dissolved oxygen in the final etching solution. Rectangularly cut pieces from a Si(111) wafer (P-doped, 1−10 Ω·cm) were first cleansed by ultrasonication in acetone. Surface oxide was formed on the Si pieces using oxygen plasma and then was etched by 2% HF solution. The oxidation/etching process was repeated at least three times. After that, the hydrogen-terminated substrates were further etched in 1% water solution of ammonium sulfite for 90 min for flattening,21 which resulted in formation of a few hundred nanometers wide terraces on the substrates. The substrate preparation was done just before the formation of the molecular layer. ω-(N-Pyrrolyl)propanol (MW 125.17, d. 1.04, bp 231 °C (743 mmHg)) was purchased from TCI and degassed by N2 bubbling at 80 °C for about 1 h before use. 0.1 M PyC3OH solution in anhydrous toluene was prepared and degassed again by dry N2 bubbling in the reaction vessel for 1 h prior to the molecular layer formation. Freshly prepared H−Si(111) substrates were inserted into the solution, the reaction vessel was purged by dry N2 for 30 min, and then the vessel was sealed and heated to 100 °C in an oil bath. During this heating period, a PyC3O−Si layer was formed on the substrates. The reaction vessel was kept dark during the heating. After being kept at 100 °C for 19 h, the samples were removed from the solution and washed by

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RESULTS AND DISCUSSION Figure 1a is an STM image of the hydrogen-terminated Si(111) substrate before the molecular layer formation, which shows the 0.314 nm high silicon bilayer terrace structure of the substrate. This wide and flat terrace helps to grasp the structure of the molecular layers shown below more clearly. Figure 1b is a noncontact AFM topograph of the PyC3O−Si sample finalized by ultrasonication in ethanol. The corrugation height of the molecular layer surface is larger than the substrate terrace step height. The protrusions on the surface appear to be laterally spaced by a few nanometers, typically. This surface morphology indicates that the grafted molecules are not in a densely packed formation20 but in a disordered formation, suggesting a low molecular coverage. The average molecular layer thickness is less than 0.7 nm according to the height profiling done after scarping out a patch of the molecular layer. The protrusions are 14497

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Figure 1. (a) An STM image of the flattened H−Si(111) substrate (Vs = −1.5 V, It = 0.1 nA). (b−d) Noncontact AFM topographs of the PyC3O−Si samples finalized by ultrasonication in ethanol, dichloromethane, and toluene, respectively. All four images are drawn in the same height−color scale: 3.0 nm from minimum to maximum.

thought to be mostly local coalescence of the pyrrolyl terminal groups of the grafted molecules on the substrate. Figures 1c and 1d are noncontact AFM topographs from the PyC3O−Si samples finalized by ultrasonication in dichloromethane and in toluene, respectively. It can be seen that the base morphology of the grafted molecular layer is the same throughout in Figure 1b−d. The difference is in the physisorbed PyC3OH molecules remaining after the final sonication step. On the toluene-sonicated sample (Figure 1d), bunches of physisorbed molecules cover a major portion of the grafted molecular layer surface. The average thickness of the molecular layer including the physisorbed molecules is 1.5−2 nm for this sample. On the dichloromethane-sonicated sample (Figure 1c), the physisorbed molecules are more dispersed and significantly thinner than those on the toluene-sonicated sample. Because of the low molecular coverage, lateral oligomerization/polymerization among the pyrrolyl groups by biased AFM scans was hard to achieve in the ethanol-sonicated sample. We chose the dichloromethane-sonicated sample for the following demonstration and characterization of the scaninduced polymerization. The biased AFM scans were conducted on an array of 400 × 400 nm2 square patches of the PyC3O−Si sample under UHV, with a sample bias Vs = +5.5 V. Figures 2a and 2b are contactmode topographs of the sample obtained before and after the biased scans, respectively. In Figure 2b, it can be seen that the bias-scanned areas are a little bit lower and smoother than the surrounding area, while agglomeration of the physisorbed molecules plowed out to the scan boundaries is not significant. This suggests that a large portion of the physisorbed PyC3OH molecules were incorporated into the substrate-grafted molecular layer during the biased scans, probably via formation of chemical bonding. A hypothetical schematic of the sample modification by the biased scans is shown in Figure 2f, in which the incorporation of the physisorbed molecules and extension of conjugation among the pyrrolyl groups are depicted. Figure 2c is a tip-to-sample current image of the same patterned area, recorded by a contact scan with Vs = +1.5 V. Local high conductance regions are found to be scattered as

Figure 2. (a) Contact-mode AFM topograph of the dichloromethanesonicated PyC3O−Si sample taken before patterning by biased scans. (b) Topograph of the same area obtained after the biased-scan patterning with Vs = +5.5 V. The same height scale is used with (a): 4.4 nm from minimum to maximum brightness. (c) Tip-to-sample current image from the same area measured with Vs = +1.5 V. The scale maximum is set to 50 pA. (d) An image of the friction between the tip and the sample surface obtained simultaneously with (c). The bias-scanned area shows ∼30% lower friction than that of the unmodified surrounding area. (e) Surface potential map of the same area. The surface potential is higher by ∼80 mV on the bias-scanned area. (f) A hypothetical schematic of the sample modification by biased scans, representing scan-induced oligomerization/polymerization.

small dots of varying conductance within the modified square areas. This inhomogeneity of the conductance enhancement is thought to be partially caused by the effective tip area being as wide as a few tens of nanometers. Since the tip-to-sample current tends to be concentrated near some point under the tip during the sample modification, it is probable that a large portion of the current is carried via this “hot spot” while the tip travels over the width of itself. This concentration of current can be intensified as the conjugation among the pyrrolyl groups proceeds near this hot spot. When the tip is disconnected from this hot spot as the scan proceeds, another hot spot newly develops under the current tip position, and so on. In this way, high conductance regions can be formed as localized spots laterally spaced by distances that roughly correspond to, or are 14498

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larger than, the tip size. This observation also implies that the electrical current is likely the direct cause of the conductance enhancement, not the electric field. If the high electric field between the tip and the sample were the direct cause of the conductance enhancement, then the conductance enhancement should occur more evenly under the whole effective tip area, which would cause more homogeneous conductance enhancement than is shown in Figure 2c. Figure 2d is an image of the friction between the sample surface and the tip, obtained by subtraction between the lateral force signals from the forward and the reverse contact-mode scans. The areas modified with the biased scan show ∼30% lower friction than that of the unmodified area. There are a few horizontal streaking lines within the modified squares that show a frictional force similar to that of the unmodified area (marked with black triangles in Figure 2d). The positions of the streaking lines coincide with the lines of low conductance marked with yellow triangles in Figure 2c. Therefore, the conductance enhancement is a necessary condition for the reduction of friction, and vice versa in this experiment. On a closer inspection of Figure 2b, it can be seen that the height of the molecular layer in the bias-scanned area is slightly thicker where the low-conductance/high-friction streaking lines are (also marked with black triangles). We suppose that the streaking lines were caused by an incomplete modification of the molecular layer due to temporary excessive accumulation of physisorbed PyC3OH molecules under the tip. Such excessive accumulation of molecules increases the distance between the tip and the substrate and thus is expected to reduce the electric field and the current between the tip and the sample below the values required to modify the molecular layer sufficiently. Figure 2e is a surface potential map of the same area. The modified regions had a surface potential higher than that of the unmodified area by ∼80 mV. It is clear that the surface modification increased the surface potential, but the cause of this potential increase is not clear yet.23 It is possible that the increase of the surface potential was caused by an enhanced charge transfer from the conjugated pyrrolyl groups to the substrate, facilitated by the decrease of HOMO−LUMO gap24 following the oligomerization/polymerization. However, there are other possible causes such as elimination of the physisorbed molecules or change of attitude of the substrate-grafted molecules, which may also change the average dipole field in the molecular layer. For a control study, the region modified by a biased scan was compared with the region scanned in contact mode with a strong normal force enough to displace the physisorbed PyC3OH molecules. Figures 3a and 3b are topographs of the area taken before and after the modifying scans, respectively. In Figure 3b, it can be seen that the physisorbed molecules were displaced from the force-scanned area (bounded by the blue dotted rectangle in Figure 3a) to the scan boundary. As the result, the molecular layer within the force-scanned area became slightly lower and flatter, whereas the height of the bias-scanned area (bounded by the green dashed rectangle) remained similar to that of the surrounding area. Those molecules agglomerated at the scan boundary produced larger friction with respect to the tip as shown in Figure 3d. Figure 3c is a current image of the same area, recorded by a contact scan with Vs = +1.0 V. The conductance enhancement is evident in the bias-scanned area, whereas the force-scanned area devoid of the physisorbed molecules shows no significant increase of conductance. This result confirms that the

Figure 3. (a) Contact-mode AFM topograph of the dichloromethanesonicated PyC3O−Si sample taken before patterning. The rectangle marked with yellow solid lines corresponds to the area displayed in (b)−(e). The green dashed rectangle marks the area later scanpatterned with Vs = +5.0 V, and the blue dotted rectangle marks the area later scanned with a normal force FN = 160 nN to mechanically displace the physisorbed PyC3OH molecules away from the scan area. (b) Contact mode topograph after the comparative patterning. The height scale in (a) and (b) is 4.0 nm from minimum to maximum brightness. (c) A current image from the same area measured with Vs = +1.0 V. The scale max is set to 50 pA. (d) A friction image obtained simultaneously with (b). The bias-scanned area shows ∼20% lower friction, whereas the force-scanned area shows the same friction as the unmodified area. (e) A surface potential mapping of the same area. The surface potential of the bias-scanned area is higher by ∼40 mV than that of the unmodified area, whereas the force-scanned area is higher by ∼120 mV.

conductance enhancement is not caused by a simple removal of the molecules on the substrate. The high conductance region was also formed as small dots, but the distance between the dots was much shorter than that in Figure 2d. This was due to the tip size used for the biased scan in Figure 3 being smaller than that used for the biased scans in Figure 2. Figure 3d is a friction image of the sample obtained simultaneously with the topograph. The bias-scanned area shows ∼20% lower friction, whereas the force-scanned area shows a friction essentially the same with that of the unmodified area. This result confirms that the reduction of friction is not related to removal of the physisorbed molecules but caused by a chemical modification in the molecular layer induced by the biased scan. It is highly probable that this reduction of friction is caused by intermolecular linking among the pyrrolyl groups. The increased rigidity of the molecular layer caused by the molecular linking is expected to reduce the friction by decreasing the number of routes for dissipation of energy from the plowing motion of the tip.25 Figure 3e is a surface potential image of the area. The biasscanned area shows an increase of ∼40 mV in the surface potential. However, the force-scanned area shows a much larger 14499

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increase of ∼120 mV. From other separate tests, a surface potential increase of ∼0.5 V was observed after removal of the substrate-grafted molecular layer (data not shown). Therefore, the unexpectedly large potential increase in the force-scanned area of Figure 3e is probably due to partial detachment of the substrate-grafted molecules caused by the large pressure during the forced scan. Shown in Figure 4 are the current−voltage relations obtained before and after the sample modification by biased scans. The

for the chemical modifications done in air. For our case under vacuum, desorption of the surface-grafted molecules was observed near Vs ∼ −6 V and beyond. In addition, scan modification of our sample showed poor controllability even within the proper bias range when negative biases were used. It was because the local field and current conditions were apt to be affected heavily even by slight irregularities in the molecular layer between the tip and the substrate. Sample modification by positive Vs yielded more predictable results, since the irregularities had less impact on the modification process due to the depletion region within the substrate working as a series resistance/capacitance. It is most likely that the conductance enhancement induced by the biased scans was due to lateral extension of conjugation among the pyrrolyl groups in the molecular layer, of which the major part had been grafted to the Si substrate by alkoxy linkers while others were supplied from the physisorbed layer. It is also possible that some of the pyrrolyl moieties were directly bonded to Si atoms28 on the substrate while retaining the aromaticity, which might also result in the enhancement of the local electrical conductance. However, the above results cannot be explained solely by this direct bonding because in that case the thickness of the modified molecular layer should have become consistently smaller than that of the pristine molecular layer, and the conductance enhancement should have been laterally more homogeneous.

Figure 4. Current−voltage relation obtained from areas scanned with (a) Vs = 0 V (dotted curve), +8 V (dashed curve), −4 V (solid curve), and (b) Vs = −6 V. The toluene-sonicated sample was used here. Contact mode scans with Vs = −6 V resulted in almost total detachment of the grafted molecular layer, revealing the flat terrace structure of the underlying Si(111) substrate. The I−V curve in (b) shows characteristics of a small barrier height Schottky diode of an ntype semiconductor.

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CONCLUSION A covalently grafted molecular layer derived from ω-(Npyrrolyl)propanol was formed on hydrogen-terminated n-type Si(111) substrate. It was found that contact-mode AFM scans on the sample with biases over ±4 V under vacuum resulted in changes of mechanical and electrical characteristics of the molecular layer. While the thickness of the molecular layer did not change significantly, the tip−sample electrical conductance was increased greatly after the biased scans. This increase in electrical conductance suggested a lateral extension of conjugation through oligomerization/polymerization among the pyrrolyl groups. Reduction of friction between the tip and the sample was also observed to occur with the conductance enhancement. This reduction in friction could be attributed to a decrease in the mechanical energy dissipation which was caused by the same formation of chemical bonds among the pyrrolyl end groups. In addition to the changes in the electrical and the mechanical properties, various factors affecting the modification process were discussed in the analysis of the results. Nanoscale patterning by lateral polymerization in molecular monolayers using scanning probes has been an important challenge in the field of molecular electronics. There have been various works that showed precise control of oligomerization/ polymerization by use of scanning probes. However, previous scanning-probe-based studies on fabrication of molecularly thin polymer patterns had used precursor layers only physically adsorbed on substrates and thus were not free from the issue of adhesion. Our work is a novel example of oligomerization/ polymerization in a molecularly thin layer of precursors that are covalently grafted on a substrate, induced by electrical stimulation via local probes in vacuum. Molecularly thin pattern of high conductance oligomer/polymer that was strongly bonded to the substrate could be formed freely using conducting AFM. The covalent bonds between the precursor moieties and the substrate were strong enough to

toluene-sonicated sample was used here because it showed I−V characteristics that were less dependent on the measurement position after the modification. Figure 4a shows I−V curves obtained from the areas scanned with Vs = −4, +8, and 0 V. The unmodified area (scanned with Vs = 0 V) conducted negligible current within the bias range of |Vs| ≤ 3 V, whereas the areas modified with biased scans conducted much larger current for smaller biases. From the curves, two things can be noticed: (1) the area modified with a negative bias shows much higher conductance with smaller threshold voltages, and (2) the conductance is larger in the negative bias branch. Here, the first is related to the bias polarity dependence of the modification efficiency, and the second is mostly related to the conduction property of the substrate. While the direction of the electric field within the molecular layer may have a small influence on the efficiency of the sample modification, it is the doping type of the Si substrate that plays a major part in the polarity dependence described above. Figure 4b is the I−V relation measured in the area where the molecular layer had been largely removed by a biased scan with Vs = −6 V. The curve shows characteristics of a low-barrier Schottky diode made of an n-type semiconductor: almost constant low conductance for positive sample biases and steeply increasing conductance for negative sample biases. When a positive Vs is applied during the biased scan, the depletion region which is widened within the substrate reduces the current and the electric field below the tip, thereby limiting the efficiency of the sample modification. The same phenomenon also occurs during I−V measurements, which leads to lower conductance in the positive bias branch as shown in Figure 4a. There have been reports of chemical modification in selfassembled monolayers formed on Si that require biases larger than several volts.26,27 It also has been observed that the chemical modification can be achieved within a small range of bias above the threshold value in general. Application of biases beyond the range has resulted in oxidation of the Si substrate 14500

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(13) Nakaya, M.; Tsukamoto, S.; Kuwahara, Y.; Aono, M.; Nakayama, T. Molecular scale control of unbound and bound C60 for topochemical ultradense data storage in an ultrathin C60 film. Adv. Mater. 2010, 22, 1622−1625. (14) Jiang, L.; Yinghui, S.; Peng, H.; Li, L.-J.; Wu, T.; Ma, J.; Chiang Boey, F. Y.; Chen, X.; Chi, L. Enhanced electrical conductivity of individual conducting polymer nanobelts. Small 2011, 7, 1949−1953. (15) Yamamoto, S. I.; Ogawa, K. Surface modification through chemically adsorbed monolayer of thiophene molecules. Jpn. J. Appl. Phys. 2008, 47, 6142−6145. (16) Yamamoto, S. I.; Ogawa, K. Conductivity measurements of pyrrole molecules incorporated into chemically adsorbed monolayer by conducting probe technique in atomic force microscope. Jpn. J. Appl. Phys. 2006, 45, 2026−2032. (17) Lee, J. S.; Chi, Y. S.; Kim, J.; Yun, W. S.; Choi, I. S. Disorderorder phase change of ω-(N-pyrrolyl)alkanethiol self-assembled monolayers on gold induced by STM scans and thermal activation. Phys. Chem. Chem. Phys. 2008, 10, 3138−3149. (18) McCarley, R. L.; Willicut, R. J. Tethered monolayers of poly((N-pyrrolyl)alkanethiol) on Au. J. Am. Chem. Soc. 1998, 120, 9296−9304. (19) Aswal, D. K.; Lenfant, S.; Guerin, D.; Yakhmi, J. V.; Vuillaume, D. Self assembled monolayers on silicon for molecular electronics. Anal. Chim. Acta 2006, 568, 84−108. (20) Zharnikov, M.; Küller, A.; Shaporenko, A.; Schmidt, E.; Eck, W. Aromatic self-assembled monolayers on hydrogenated silicon. Langmuir 2003, 19, 4682−4687. (21) Fukidome, H.; Matsumura, M. Very simple method of flattening Si(111) surface at an atomic level using oxygen-free water. Jpn. J. Appl. Phys. 1999, 38, L1085−L1086. (22) Sommerhalter, C.; Matthes, T. W.; Glatzel, T.; Jäger-Waldau, A.; Lux-Steiner, M. C. High-sensitivity quantitative Kelvin probe microscopy by noncontact ultra-high-vacuum atomic force microscopy. Appl. Phys. Lett. 1999, 75, 286−288. (23) Palermo, V.; Palma, M.; Samorì, P. Electronic characterization of organic thin films by Kelvin probe force microscopy. Adv. Mater. 2006, 18, 145−164. (24) Brédas, J. L.; Silbey, R.; Boudreaux, D. S.; Chance, R. R. Chainlength dependence of electronic and electrochemical properties of conjugated systems: Polyacetylene, polyphenylene, polythiophene, and polypyrrole. J. Am. Chem. Soc. 1983, 105, 6555−6559. (25) Leggett, G. J. Friction force microscopy of self-assembled monolayers: Probing molecular organisation at the nanometre scale. Anal. Chim. Acta 2003, 479, 17−38. (26) Pignataro, B.; Licciardello, A.; Cataldo, S.; Marlette, G. SPM and TOF-SIMS investigation of the physical and chemical modification induced by tip writing of self-assembled monolayers. Mater. Sci. Eng., C 2003, 23, 7−12. (27) Yang, M.; Wouters, D.; Giesbers, M.; Schubert, U. S.; Zuilhof, H. Local probe oxidation of self-assembled monolayers on hydrogenterminated silicon. ACS Nano 2009, 3, 2887−2900. (28) Cao, X.; Coulter, S. K.; Ellison, M. D.; Liu, H.; Liu, J.; Hamers, R. J. Bonding of nitrogen-containing organic molecules to the silicon(001) surface: The role of aromaticity. J. Phys. Chem. B 2001, 105, 3759−3768.

withstand contact-mode AFM scans with very high normal forces. While this work may qualify as a progress in the field of molecular electronics in its own right, it is somewhat less interesting from the technological aspect. Usage of electrically conducting substrates would not only suppress the extent of lateral polymerization in the process but also pose a limitation on the future application of the scheme. Local probe-induced formation of a monolayer-thick conducting polymer channel grafted on an insulating substrate between electrodes can be naturally conceived as a more practical goal to be pursued. Our finding on the role of the electrical current in the scan-induced polymerization suggests that usage of a current-biased probe rather than voltage-biased ones might be the key to such a goal.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NRF (Contract number: 20110019180). REFERENCES

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dx.doi.org/10.1021/la302526t | Langmuir 2012, 28, 14496−14501