In Situ STM Imaging of Fused Thienothiopene Molecules Adsorbed on

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In Situ STM Imaging of Fused Thienothiopene Molecules Adsorbed on Au(111) Electrode YaHua Hsu, ShuehLin Yau,* Yu-Jou Lin, Peng-Yi Huang, and Ming-Chou Chen* Department of Chemistry, National Central University, Jhongli, Taiwan 320

Po-Yuan Lo Industrial Technology Research Institute, Hsincho300, Taiwan Received May 21, 2010. Revised Manuscript Received June 25, 2010 In situ scanning tunneling microscopy (STM) was used to reveal the structures of dithieno[2,3-b:3,2-d]thiophene diphenyl (DTT) molecules deposited onto Au(111) electrode from a dosing solution made of dichlorobenzene and 50 μM DTT. Potential control was proven to be of prime importance in guiding the arrangement of DTT admolecules on Au(111) in 0.1 M HClO4, as√disorder DTT adlayer seen at E > 0.3 V (vs reversible hydrogen electrode) was transformed into a highly ordered (2  7 3)rect -2DTT structure when the potential was made to 0.05 to 0.2 V. The ordered structure was stable for hours between 0.05 and 0.2 V. √However, switching the potential further negative to 0 V resulted in slow melting of the ordered structure. The (2  7 3)rect-DTT ordered adlattices recuperated when the potential was made positive to 0.2 V. Internal molecular functionalities of the thienothiophene and benzene in DTT admolecules were √ clearly discerned, from which the lateral structure for the (2  7 3)rect-2DTT structure and registries of admolecules were deduced. The dynamics of the DTT adlattices on the Au(111) electrode surface was examined by real-time STM imaging, showing reorientation of as many as 150 DTT admolecules to join a neighboring ordered array within minutes.

Introduction Organic thin-film transistors (OTFTs) are well received as the fundamentals of modern molecular electronics, used to construct flexible displays, inexpensive electronic papers, RFID components, smart textiles, and smart memory/sensor elements in the automotive and transportation industries.1 Among the welldeveloped organic semiconductor classes, pentacene (PEN),2 anthradithiophene (ADT),3,4 and fused-thiophene derivatives5-8 are typical small-molecule semiconductors. Both pentacene and ADT derivatives suffer from poor stability against photo-oxidation at the positions of C6/C13 for PEN or C5/C11 for ADT.9,10 Subsequently, highly stable materials based on fused thiophene *Corresponding authors. E-mail: [email protected] (S.Y.), [email protected] (M.-C.C.). (1) Shaw, J. M.; Seidler, P. F. IBM J. Res. Dev. 2001, 45, 3. (2) Kim, C.; Quinn, J. R.; Facchetti, A.; Marks, T. J. Adv. Mater. 2010, 22, 342– 346. (3) Chen, M.-C.; Kim, C.; Chen, S.-Y.; Chiang, Y.-J.; Chung, M.-C.; Facchetti, A.; Marks, T. J. J. Mater. Chem. 2008, 18, 1029–1036. (4) Laquindanum, J. G.; Katz, H. E.; Lovinger, A. J. J. Am. Chem. Soc. 1998, 120, 664–672. (5) Sun, Y. M.; Ma, Y. Q.; Liu, Y. Q.; Lin, Y. Y.; Wang, Z. Y.; Wang, Y.; Di, C. A.; Xiao, K.; Chen, X. M.; Qiu, W. F.; Zhang, B.; Yu, G.; Hu, W. P.; Zhu, D. B. Adv. Funct. Mater. 2006, 16, 426–432. (6) Mauldin, C. E.; Puntambekar, K.; Murphy, A. R.; Liao, F.; Subramanian, V.; Frechet, J. M. J.; DeLongchamp, D. M.; Fischer, D. A.; Toney, M. F. Chem. Mater. 2009, 21, 1927–1938. (7) Takimiya, K.; Kunugi, Y.; Konda, Y.; Ebata, H.; Toyoshima, Y.; Otsubo, T. J. Am. Chem. Soc. 2006, 128, 3044–3050. (8) Xiao, K.; Liu, Y.; Qi, T.; Zhang, W.; Wang, F.; Gao, J.; Qiu, W.; Ma, Y.; Cui, G.; Chen, S.; Zhan, X.; Yu, G.; Qin, J.; Hu, W.; Zhu, D. J. Am. Chem. Soc. 2005, 127, 13281–13286. (9) Li, Y.; Wu, Y.; Liu, P.; Prostran, Z.; Gardner, S.; Ong, B. S. Chem. Mater. 2006, 19, 418–423. (10) Kim, C.; Huang, P.-Y.; Jhuang, J.-W.; Chen, M.-C.; Ho, J.-C.; Hu, T.-S.; Yan, J.-Y.; Chen, L.-H.; Lee, G.-H.; Facchetti, A.; Marks, T. J. Org. Electron. 2010, 11, 1363–1375. (11) Li, J.; Qin, F.; Li, C. M.; Bao, Q.; Chan-Park, M. B.; Zhang, W.; Qin, J.; Ong, B. S. Chem. Mater. 2008, 20, 2057–2059.

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emerged as one of the most promising organic semiconductors used to produce electronic and optical devices.11,12 Several p-channel materials based on these fused thiophene derivatives with hole mobilities of 0.1 to 0.6 cm2/(V s)5-8,11-13 and n-channel molecules with a mobility as high as 0.07 cm2/(V s) have been reported.14,15 Among them, the dithieno[2,3-b:3,2-d]thiophene (DTT) diphenyl derivative has drawn much attention because of its accessibility, higher stability, and excellent p-channel transport with a hole mobility of 0.42 cm2/(V s).5 Meanwhile, the relevance between the electronic conductivity of OFETs and the packing of organic molecules such as pentacene is established.16,17 In addition to the traditional X-ray techniques for structural characterization,18 scanning probes of scanning tunneling microscope (STM) and atomic force microscope (AFM) have also been applied to gain structural details in thin films.19-26 The strength of STM lies in its subnanometer resolution, affording (12) McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; MacDonald, I.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W.; Chabinyc, M. L.; Kline, R. J.; McGehee, M. D.; Toney, M. F. Nat. Mater. 2006, 5, 328–333. (13) Liu, Y.; Wang, Y.; Wu, W.; Liu, Y.; Xi, H.; Wang, L.; Qiu, W.; Lu, K.; Du, C.; Yu, G. Adv. Funct. Mater. 2009, 19, 772–778. (14) Kim, C.; Chen, M.-C.; Chiang, Y.-J.; Guo, Y.-J.; Youn, J.; Huang, H.; Liang, Y.-J.; Lin, Y.-J.; Huang, Y.-W.; Hu, T.-S.; Lee, G.-H.; Facchetti, A.; Marks, T. J. Org. Electron. 2010, 11, 801–813. (15) Chen, M.-C.; Chiang, Y.-J.; Kim, C.; Guo, Y.-J.; Chen, S.-Y.; Liang, Y.-J.; Huang, Y.-W.; Hu, T.-S.; Lee, G.-H.; Facchetti, A.; Marks, T. J. Chem. Commun. 2009, 1846–1848. (16) Dimitrakopoulos, C. D.; Mascaro, D. J. IBM J. Res. Dev. 2001, 45, 11. (17) Dimitrakopoulos, C. D.; Brown, A. R.; Pomp, A. J. Appl. Phys. 1996, 80, 2501–2508. (18) Fritz, S. E.; Martin, S. M.; Frisbie, C. D.; Ward, M. D.; Toney, M. F. J. Am. Chem. Soc. 2004, 126, 4084–4085. (19) Pong, I.; Yau, S.; Huang, P.-Y.; Chen, M.-C.; Hu, T.-S.; Yang, Y.; Lee, Y.-L. Langmuir 2009, 25, 9887–9893. (20) Dougherty, D. B.; Jin, W.; Cullen, W. G.; Reutt-Robey, J. E.; Robey, S. W. J. Phys. Chem. C 2008, 112, 20334–20339. (21) Bavdek, G.; Cossaro, A.; Cvetko, D.; Africh, C.; Blasetti, C.; Esch, F.; Morgante, A.; Floreano, L. Langmuir 2008, 24, 767–772. (22) Satta, M.; Iacobucci, S.; Larciprete, R. Phys. Rev. B 2007, 75, 155401.

Published on Web 07/21/2010

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molecular level details on how pentacene molecules are adsorbed on a metal or semiconductor substrates and subsequently grow into thin films.20,21,24,25,27-33 Furthermore, in situ STM imaging, operated in an electrochemical environment, enables exploration of the effect of potential control on the adsorption strength, coverage, and arrangement of admolecules.19,30,34 Although in situ STM is operated under conditions that differ from the practical operational conditions of OFET, the use of in situ STM could elucidate the effect of electric field on the arrangement of organic molecules, as reported for pentacene admolecules deposited on Au(111) electrode.19 Given the potential uses of DTT to fabricate OFET, it is worthwhile to explore how DTT molecules were adsorbed on Au(111) and how molecular arrangement would be affected by electrochemical potential. High-quality molecular images were obtained to shed insight into the adsorption configuration and dynamics of molecular arrangement of DTT admolecules on Au(111). This information can be important to the understanding of electronic conduction in molecular thin films.35,36

Experimental Section We made the Au(111) single-crystal electrodes used for voltammetric and STM experiments by melting a Au wire (φ = 0.8 mm) with a hydrogen torch, as previously reported.37-40 After being annealed and quenched, the Au(111) bead electrode was rinsed with acetone and blown dry with nitrogen stream, followed by immersion in a dosing solution made of o-dichlorobenzene and 50 μM DTT for 30 s. The dosed Au(111) electrode was blown with nitrogen again to dry before it was mounted on the STM base. The temperature of the DTT dosing solution was ∼18 C. All electrochemical experiments were performed with the conventional hanging meniscus method in a three-electrode cell. A reversible hydrogen electrode (RHE) and Pt wire acted as the reference and the counter electrode, respectively. The potentiostat used was a CHI 703. The supporting electrolyte was typically 0.1 M HClO4 diluted from concentrated perchloric acid purchased from Merck (Darmstadt, DFG), whereas dichlorobenzene obtained from Showa chemicals (Tokyo, Japan) was used as received. Triple-distilled Millipore water (resistivity 18.3 MΩ) was used to prepare all solutions. The procedure for preparing DTT was reported.14 (23) Sato, K.; Sawaguchi, T.; Sakata, M.; Itaya, K. Langmuir 2007, 23, 12788– 12790. (24) Jaeckel, B.; Sambur, J. B.; Parkinson, B. A. Langmuir 2007, 23, 11366– 11368. (25) Parisse, P.; Passacantando, M.; Ottaviano, L. Appl. Surf. Sci. 2006, 252, 7469–7472. (26) France, C. B.; Schroeder, P. G.; Forsythe, J. C.; Parkinson, B. A. Langmuir 2003, 19, 1274–1281. (27) Soe, W. H.; Manzano, C.; Sarkar, A. D.; Chandrasekhar, N.; Joachim, C. Phys. Rev. Lett. 2009, 102, 176102. (28) Rui, H.; Nancy, G. T.; Graciela, B. B.; Aron, P. Appl. Phys. Lett. 2009, 94, 223310. (29) Conrad, B. R.; Cullen, W. G.; Riddick, B. C.; Williams, E. D. Surf. Sci. 2009, 603, L27–L30. (30) Yang, Y.-C.; Chang, C.-H.; Lee, Y.-L. Chem. Mater. 2007, 19, 6126–6130. (31) McDonald, O.; Cafolla, A. A.; Carty, D.; Sheerin, G.; Hughes, G. Surf. Sci. 2006, 600, 3217–3225. (32) Fanetti, M.; Gavioli, L.; Sancrotti, M.; Betti, M. G. Appl. Surf. Sci. 2006, 252, 5568–5571. (33) Joo, H. K.; Zhu, X. Y. Appl. Phys. Lett. 2003, 82, 3248–3250. (34) Yoshimoto, S.; Tsutsumi, E.; Narita, R.; Murata, Y.; Murata, M.; Fujiwara, K.; Komatsu, K.; Ito, O.; Itaya, K. J. Am. Chem. Soc. 2007, 129, 4366–4376. (35) Gundlach, D. J.; Lin, Y. Y.; Jackson, T. N.; Nelson, S. F.; Schlom, D. G. IEEE Electron Device Lett. 1997, 18, 87–89. (36) Lin, Y. Y.; Gundlach, D. J.; Nelson, S. F.; Jackson, T. N. IEEE Electron Device Lett. 1997, 18, 606–608. (37) Hamelin, A. J. Electroanal. Chem. 1996, 407, 1–11. (38) Hamelin, A.; Martins, A. M. J. Electroanal. Chem. 1996, 407, 13–21. (39) Liu, G. Z.; Ou Yang, L. Y.; Shue, C. H.; Ma, H. I.; Yau, S. L.; Chen, S. H. Surf. Sci. 2007, 601, 247–254. (40) Chang, C.-C.; Yau, S.-L.; Tu, J.-W.; Yang, J.-S. Surf. Sci. 2003, 523, 59–67.

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Figure 1. Cyclic voltammograms recorded at 50 mV/s with bare (inset) and DTT-coated Au(111) in 0.1 M HClO4. The DTT admolecules appear to be stable within the potential window of 0 and 0.9 V. The STM used in this study was a Nanoscope E (Digital Instruments, Santa Barbara, CA) with a single tube scanner (high-resolution A-head, maximal scan area ca. 600 nm2). Tungsten tips (φ 0.3 mm) prepared by electrochemical etching in 2 M KOH were used throughout this study. The tip was water-rinsed, dried by acetone washing, and finally painted with nail polish for insulation. The electrochemical cell used for STM measurements had a three-electrode configuration equipped with a RHE and Pt counter electrode. The use of STM in studying electrified interface is reviewed.41-43

Results and Discussion Cyclic Voltammetry. Figure 1 shows the cyclic voltammograms (CVs) recorded at 50 mV/s with bare (inset) and DTTmodified Au(111) electrodes in 0.1 M HClO4. The former contains a pair of broad peaks near 0.5 V, attributable to the phase √ transition between the reconstructed (22  3) and the ideal (1  1) structure as the potential was scanned positively from -0.05 to 0.9 V.44-47 In comparison, Au(111) electrode modified with DTT molecules produced a featureless CV between 0 and 0.9 V, whose current density is one-half of that of bare Au(111). Evidently, DTT admolecules were deposited on the Au(111) electrode by the simple dipping process, as also noted for Au(111) and Pt(111) coated with alkanethiol molecules.48-50 Furthermore, it is safe to state that DTT molecules were mostly stable against redox reactions between -0.05 and 0.9 V. The featureless CV profile however does not guarantee a molecular adlayer structurally stable against the modulation of potential. In fact, the in situ STM results later described elucidate marked (41) Itaya, K. Prog. Surf. Sci. 1998, 58, 121–247. (42) Magnussen, O. M. Chem. Rev. 2002, 102, 679–726. (43) Gewirth, A. A.; Niece, B. K. Chem. Rev. 1997, 97, 1129–1162. (44) Robinson, K. M.; Robinson, I. K.; O’Grady, W. E. Surf. Sci. 1992, 262, 387–394. (45) Wang, J.; Ocko, B. M.; Davenport, A. J.; Isaacs, H. S. Phys. Rev. B 1992, 46, 10321. (46) Wang, J. I. A.; Davenport, A. J.; Isaacs, H. S.; Ocko, B. M. Science 1992, 255, 1416–1418. (47) Structure of Electrified Interfaces; Lipkowski, J. R., Ross, P. N., Ed.; VCH Publishers: New York, 1993. (48) Poirier, G. E. Langmuir 1997, 13, 2019–2026. (49) Yang, Y.-C.; Lee, Y.-L.; Yang, L.-Y. O.; Yau, S.-L. Langmuir 2006, 22, 5189–5195. (50) Yang, Y.-C.; Yen, Y.-P.; Ou Yang, L.-Y.; Yau, S.-L.; Itaya, K. Langmuir 2004, 20, 10030–10037.

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Figure 2. In situ STM images recorded with Au(111) electrode precoated with a monolayer of DTT at (a-c) 0.3 and (d) 0.1 V in 0.1 M HClO4. The highlighted areas were sites with ordered molecular arrays. The two dotted lines in part b mark a section of step ledge at which a string of DTT admolecules were found. Nucleation of ordered structures occurred preferentially at terrace sites.

changes of the arrangement of DTT molecules with the potential. The precipitous increase in current starting at -0.05 V in the CV profile shows that DTT admolecules could be desorbed from the Au(111) electrode coupled to the evolution of hydrogen. In Situ STM Imaging of DTT Adsorbed on Au(111). Shown in Figure 2a is an STM topography scan obtained at 0.4 V in 0.1 M HClO4 with the typical imaging conditions of 100 mV in bias voltage and 1 nA in feedback current. This DTT-modified Au(111) electrode exhibited monatomic-height islands imaged as bright spots distributed randomly on a 200 nm wide terrace. These morphologic features are indicative of the lift of the reconstructed √ (22  3) by the adsorption of DTT molecules. These irregularly positioned bumps could limit the extent of ordering of the DTT molecular adlattice on the Au(111) substrate. A few patches of ordered arrays, attributed to DTT admolecules, were discerned amid a largely disordered adlayer. These locally ordered DTT adlattices were not seen until the potential was changed from the open-circuit potential (0.8 V) to 0.35 V. These local molecular arrays were all found on terrace sites rather than at step ledges. A higher resolution STM scan shown in Figure 2b provides a close-up view of one of the ordered domains formed near a step line. This ordered array consisted of roughly 42 DTT molecules, with each molecule exhibiting elongated shape. Also, about 10 DTT molecules were adsorbed at the lower side of a section of the step ledge on the left, as marked by two dotted lines. These DTT admolecules were adsorbed with their longer molecular axis aligned perpendicular to the step line, which contrasts with the parallel adsorption configuration observed with thiophene and TCNQ molecules adsorbed on Cu(111) surface at low temperature.51,52 These results are suggestive of the preferential nucleation of ordered molecular arrays on terrace, which could be further substantiated by the STM result (Figure 2c) obtained at 0.3 V. (51) Frank, E. R.; Chen, X. X.; Hamers, R. J. Surf. Sci. 1995, 334, L709–L714. (52) Kamna, M. M.; Graham, T. M.; Love, J. C.; Weiss, P. S. Surf. Sci. 1998, 419, 12–23.

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Figure 3. Molecular resolution STM images showing (a) the ordered packing and √ (b) the internal molecular structure of the Au(111)-(2  7 3)rect adlattice of DTT. √ The rectangle marked in part b denotes the unit cell of (2  7 3)rect, θ = 0.071, which can be represented by a corresponding ball model shown in part d. The cross-section profiles along a, b, and c are shown in part c. The fused thienothiophene group was imaged as trio.

Four step lines running vertically in the image were seen to separate five terraces. Ordered arrays imaged as parallel line patterns are seen mostly on wider terraces located on the two sides of this image. The two narrow terraces located in the middle of the image were essentially devoid of any ordered DTT structure. Because step sites were relatively unimportant in guiding molecular ordering, DTT admolecules were not preferentially aligned with respect to steps; the linear patterns of ordered molecular adlattices could be parallel to or intersect step line at 60 of others, as shown by the STM image in Figure 2d. Figure 3a, a 16  16 nm STM scan, shows a highly ordered DTT array with admolecules aligned diagonally in the image. Aided by an atomic resolution STM image of the Au(111) substrate (not shown here), we find that molecular rows run parallel to one of the main axes of the Au(111) substrate. On average, a molecular row spans ∼30 nm, consisting of 50 similarly arranged DTT molecules. Occasionally, with a highly ordered Au(111) substrate, molecular rows could span up to 100 nm thanks to the attraction among one another in directions parallel to the short and long molecular axes. The strength of intermolecular interaction represents another crucial factor in determining the extent of ordering. In typical, the dimension of an ordered domain in the longer molecular axis was roughly three times longer than that in the shorter axis, which directly reflects the 3:1 ratio of a DTT molecule. It appears then that the strength of intermolecular interactions in the short and long axes of the molecule was comparable. A further higher resolution STM scan shown in Figure 3b reveals the internal molecular features of DTT, a prominent trio joined by two weaker spots on the sides, corresponding to the three fused thiophene units and the two terminal benzene rings in the DTT molecule, respectively. DTT admolecules were adsorbed with their long and short molecular axis oriented in the Æ121æ and Æ110æ directions, respectively. The intermolecular distances along a molecular row and between two neighboring rows are 0.6 and 1.9 nm, respectively. These values are comparable to the physical DOI: 10.1021/la102058e

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Figure 4. (a,c) In situ molecular-resolution STM images showing √ two rotational domains of (2  7 3)rect structure of DTT at 0.2 V in 0.1 M HClO4. (b,d) These ordered adlattices are reconciled by the ball models. (e) DTT molecules in the ordered arrays have their longest molecular axis aligned in parallel to the step ledges.

dimensions of DTT molecule, suggesting that DTT admolecules were adsorbed in a close packed arrangement with their molecular planes lying flat on the Au(111) surface. The corrugation heights embedded in Figure 3b are measured, revealing that the fused thiophene units (or the trios) are 0.015 nm higher than the two terminal benzene rings (two dimer spots on the sides). The two benzene rings were equally bright. To decipher the corrugation patterns quantitatively, three section profiles along the directions of a, b, and c are shown in Figure 3c. Because the STM appearance of an adsorbate (shape and corrugation height) depends on its adsorption configuration and registry, the essentially identical STM appearances seen with the fused thiophene units and the terminal benzene rings suggest that DTT molecules were adsorbed with the same configurations and probably at the same type of sites on Au(111). Viewing along the Æ110æ directions, all DTT admolecules were adsorbed in the same manner, but DTT admolecules in two neighboring rows were rotated in-plane by 180 from each other. One can think of DTT admolecule as an eclipsed moon; rows of DTT molecules adopted the configurations of waxing and waning crescent moon alternatively. Subsequently, molecular orientation repeated itself every other row along the Æ121æ direction. The marked rectangular unit cell maps out the unit cell, whose two edges are aligned in the Æ110æ and Æ121æ directions and measured to be√ 0.6 and 3.5 nm in length. These results conform a (2  7 3)rect, θ = 2/28 = 0.0714 structure as two DTT admolecules covering an area of 28 gold atoms. Note √ that the crystal structure of DTT was reported, and the (2  7 3)rect structure seen here is essentially identical to one plane of the single crystal of DTT.5 This close correlation between the structure of adlayer and bulk crystal suggests the importance of intermolecular interaction in guiding the adsorption of molecules. √ A tentative ball model for the (2  7 3)rect structure is proposed in Figure 3d, where all DTT admolecules are presumed to lie parallel at identical surface sites. The similar STM appearance of DTT admolecules attests this assignment. The fused thiophene units sit on asymmetric sites; whereas two terminal benzene rings occupy three-fold hollow sites. In addition to the difference in chemical identity, this difference in registry could account for the fact that the former appeared 0.015 nm higher than the latter. 13356 DOI: 10.1021/la102058e

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According to experimental results and theoretical modeling of aromatic molecules adsorbed on metal surface, corrugation of benzene admolecule descends in the order of atop > two-fold bridge > three-fold hollow.53,54 Our assignment of the ordered structure is in line with this rule of thumb, although the registry of thiophene molecule on metal surface has been largely overlooked. It is worthwhile noting that thiophene admolecules on Au(111) can reorient from parallel to upright with surface coverage and potential.55 The structure seen in√ Figure 3 represents one of three ordered domains of the (2  7 3)rect structure. Two other rotational domains were also found, as revealed by the molecular-resolution STM images shown in Figure 4a,b. Similarly to Figure 3b, the internal molecular structure consisting of central trios and two terminal dimer spots is apparent. The corrugation profile illustrated by Figure 3c is also observed. These three ordered arrays distributed statistically; they were seen simultaneously on terrace spanning >100 nm. DTT admolecules adsorbed on narrow terraces (width 100 nm wide terrace) of these images, all three rotational domains of √ the (2  7 3)rect structure were observed and labeled as I, II, and III. The most prominent domain I had admolecules aligned in parallel to the nearby step ledges. To the right of domain I lay a less important domain II, segregated from domain I by boundaries imaged as depressed trenches. A patch of domain II consisting of ∼150 admolecules seen in Figure 5a reorganized and joined domain I in 3 min. The predominant domain I continued expanding eastward, seemingly forcing out another domain II. The rate of this transformation process was slow, as only onetenth of domain II restructured in 1 min. The perimeter of domain II seen in part b is superimposed on part c to illustrate changes in the ordered adlayer. These results indicate that a more prominent ordered adlattice tended to press admolecules in its neighboring, less important domains to restructure into one of its own. Ultimately, the tendency to remove lattice strain drove this restructuring event. The rate of this process seemed to increase with the difference in size of two domains. Unfortunately, no STM result was obtained after part c, so we are not sure if domain II was eventually transformed into I. Strings of admolecules were frequently disrupted by discontinuities imaged as depressed trenches in Figure 6a. A high-resolution STM scan was collected (Figure 6b) to provide more insight into√these defects, which were produced by two neighboring (2  7 3)rect adlattices with molecular rows in these two domains misaligned by 0.3 nm. This kind of mismatching defect was constantly observed, irrespective of the imaging conditions. Therefore, it is likely that these defects were characteristics of the adlattice rather than results made by the STM tip. Melting of the Ordered DTT Adlayer. The effect of potential on the structure of the Au(111)-supported DTT adlayer by in 0.1 M HClO4 is then described. The STM imaging experiment started 0.8 V, followed by stepwise negative shifts by 50 mV to -0.1 V. The DTT adlayer was always disordered at 0.8 V, and (53) Yau, S.-L.; Kim, Y.-G.; Itaya, K. J. Am. Chem. Soc. 1996, 118, 7795–7803. (54) Sautet, P.; Bocquet, M. L. Phys. Rev. B 1996, 53, 4910. (55) Su, G.-J.; Zhang, H.-M.; Wan, L.-J.; Bai, C.-L. Surf. Sci. 2003, 531, L363– L368.

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Figure 5. Time-dependent in situ STM images obtained with Au(111) coated with a monolayer of DTT admolecules at√0.1 V in 0.1 M HClO4. The time difference between (a) and (b) is 3 min, and it is 1 min between (b) and (c). Three rotational domains of (2  7 3)rect were identified as I, II, and III. The dotted line in parts b and c mark the perimeter of domain II seen in part b. Domain II appears to shrink with time.

Figure 6. In situ STM images showing the discontinuities seen within an ordered domain. Rows of DTT admolecules aligned in the Æ110æ directions of the Au(111) substrate on average spanned 25 nm in length. Panel b highlights one of the mismatching domain boundaries.

local ordering was not seen until 0.4 V, as revealed by the STM image in Figure 2a. Because the as-prepared DTT adlayer when imaged in the air was also disordered, it seems that ordered DTT adlattice was not formed spontaneously on Au(111) in the dosing solution. The adsorption of anions, ClO4- in this study, was expected to be too weak to have some major effects here. Prolonged potential holding at 0.3 V for hours yielded only local arrays, but the ordered adlattices of DTT expanded substantially, albeit slowly, to finish the conversion upon the shifting potential from 0.3 to 0.2 V, as shown in Figure 7a. The patchy appearance of ordered arrays was apparent on the 200 nm wide terrace, which could √ be ascribed to the three rotational domains of the (2  7 3)rect ordered adlattice. Also, given the uniform appearance of the surface, it is safe to state that the Au(111) electrode assumed a √ (1  1) structure rather than the (22  3) reconstruction favored at 0.2 V in acidic solution. The adsorption of DTT could stabilize the unreconstructed (1  1) structure. The long-range ordering of the DTT adlayer could be stable for hours before the potential was made to -0.1 V, where the DTT adlayer underwent dramatic changes, as illustrated by the timedependent STM images shown in Figure 7b,c. It appears that ordered structures located near domain boundaries were melting first, followed by creeping into main bodies of ordered domains until all ordered structures were removed. Changes in surface structure revealed by Figure 7 indicate that the adsorption of cation such as protons became sufficiently favorable to displace DTT adsorbates at E < -0.05 V. It is emphasized that this orderto-disorder transition was so slow that it took more than 2 h to complete, as revealed by Figure 7b,c. Clearly, DTT admolecules could restructure on the Au(111) electrode with the potential, which could result from competitive adsorption of cation and Langmuir 2010, 26(16), 13353–13358

Figure 7. Potential-dependent in situ STM images acquired with DTT-coated Au(111) electrode. A full monolayer of DTT found at 0.1 V was displaced slowly by hydrogen adatoms at -0.1 V. Protruded islands due to aggregated of gold atoms injected onto the surface upon the lift of surface reconstruction. All linear segments seen in the images were aligned in the Æ110æ directions of the Au(111) substrate indicated by arrows. These images were acquired in a period of 2 h. The STM image in part d highlights a protruded island (∼144 nm2 area), on which DTT molecules already formed an ordered array.

anions or alternation of surface bonds and intermolecular interactions. These STM results revealed an intriguing phenomenon, the effect of potential on the ordering of molecular adlayer, where DTT adlayer was highly ordered only between 0 and 0.2 V. We contend that this organization-potential correlation originated from the dependence of adsorption strength of DTT molecule on the electrochemical potential. According to present STM results, DTT admolecules lay flat on the Au(111) substrate, meaning that DTT adsorbate bonded covalently with gold atoms by donating its π-electrons to the sp orbitals of the Au(111) substrate, and there was a back-donation of electrons from the gold substrate to some electronic states at adsorbed DTT molecule. It is conceivable that negative charges at the gold electrode could enhance the adsorbate-substrate interaction to an extent that stabilized the adsorption of DTT molecules on the Au(111) substrate. DOI: 10.1021/la102058e

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Finally, the adsorption of DTT molecules with dodecyloxy side chains on highly oriented pyrolytic graphite (HOPG) is reported, showing the formation of highly organized adlattices.56 However, the lateral arrangement seen in this study differs substantially from those found in this study. Because the DTT molecules could interact with HOPG via van der Waals force, as opposed to the formation of chemical bond in the present DTT/Au(111) system, it seems then the adsorbate-substrate interaction plays an important role in guiding the organization of admolecules. We are exploring the effect of dosing [DTT] on the adsorption configuration and lateral structure of DTT admolecules on Au(111). Details are to be presented.

Conclusions In situ STM imaging has resulted in details of the structure and defects in the DTT adlayer supported by Au(111) electrode in 0.1 M HClO4. Potential control dominates the arrangement of DTT admolecules; only holding the potential between 0.05 and 0.2 V could safely render DTT admolecules arranged in highly √ ordered structure, determined as (2  7 3)rect by molecular resolution STM images. This prominent effect of potential on the (56) Wang, L.; Chen, Q.; Pan, G.-B.; Wan, L.-J.; Zhang, S.; Zhan, X.; Northrop, B. H.; Stang, P. J. J. Am. Chem. Soc. 2008, 130, 13433–13441.

13358 DOI: 10.1021/la102058e

Hsu et al.

structure of DTT adlayer might reflect the effect of potential on the strength of surface bond between DTT and Au(111). In particular, DTT molecules could be adsorbed more strongly at more negative potentials and thereby facilitated the formation of ordered DTT adlattices. Internal molecular structures of DTT admolecules (the thienothiophene and two terminal phenyl groups) were discerned by the STM, indicating that DTT molecules lay flat on the Au(111) substrate. This parallel molecular adsorption configuration implies the involvement of π-electrons of DTT admolecules and Au(111) substrate. Notwithstanding, DTT admolecules were mobile on the Au(111) substrate, even at 0.1 V; as many as 150 molecules could reorient themselves to join a neighboring more important ordered array. On average,the molecular row of DTT spans 30 nm, punctuated by mismatching defects where rows in two neighboring domains are laterally shifted by 0.3 nm. These sorts of defect could result from a low energy barrier for DTT admolecules to hop into neighboring sites. Acknowledgment. We acknowledge technical help from Prof. C. C. Su (Institute of Organic and Polymeric Materials, National Taipei University of Technology). The financial support is provided by the National Science Council of Taiwan (NSC 982113-M-008-001 and NSC98-2628-M-008-003) and partially provided by Industrial Technology Research Institute of Taiwan.

Langmuir 2010, 26(16), 13353–13358