In Situ STM Imaging of the Structures of Pentacene Molecules

Jun 11, 2009 - Industrial Technology Research Institute, Hsincho 300, Taiwan. YawChia Yang and Yuh-Lang Lee*. Department of Chemical Engineering, ...
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In Situ STM Imaging of the Structures of Pentacene Molecules Adsorbed on Au(111) IFan Pong, Shuehlin Yau,* Peng-Yi Huang, and Ming-Chou Chen* Department of Chemistry, National Central University, JhongLi, Taiwan 320

Tarng-Shiang Hu Industrial Technology Research Institute, Hsincho 300, Taiwan

YawChia Yang and Yuh-Lang Lee* Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan Received March 23, 2009. Revised Manuscript Received May 18, 2009 In situ scanning tunneling microscope (STM) was used to examine the spatial structures of pentacene molecules adsorbed onto a Au(111) single-crystal electrode from a benzene dosing solution containing 16-400 μM pentacene. Molecular-resolution STM imaging conducted √ √ √ √ √ in 0.1 M HClO4 revealed highly ordered pentacene structures of ( 31 31)R8.9, (310), ( 3110), and ( 72 7)R19.1 adsorbed on the reconstructed Au(111) electrode dosed with different pentacene solutions. These pentacene structures and the reconstructed Au(111) substrate were stable between 0.2 and 0.8 V [vs reversible hydrogen electrode, RHE]. Increasing the potential to E > 0.8 V lifted the reconstructed Au(111) surface and disrupted the ordered pentacene adlattices simultaneously. Ordered pentacene structures could be restored by applying potentials negative enough to reinforce the reconstructed Au(111). At potentials negative of 0.2 V, the adsorption of protons became increasingly important to displace adsorbed pentacene admolecules. Although the reconstructed Au(111) structure was not essential to produce ordered pentacene adlayers, it seemed to help the adsorption of pentacene molecules in a long-range ordered pattern. At room temperature (25 C), ∼100 pentacene molecules seen in STM images could rotate and align themselves to a neighboring domain in 10 s, suggesting that pentacene admolecules could be mobile on Au(111) under the STM imaging conditions of -150 mV in bias voltage and 1 nA in feedback current.

Introduction Pentacene, renowned for its high mobility of charge carrier, has been used to fabricate novel thin film transistors and solar cells.1-6 Because gold metal frequently serves as the source and drain electrodes in pentacene - based transistors, the structure of pentacene/gold interface can influence the mechanism of charge transport and the performance of transistors. Studies reported thus far have revealed the lateral structures and the electronic property of pentacene thin films on copper,7,8 silver,9,10 and gold.11-14 *Corresponding author: Shuehlin Yau. E-mail: [email protected]. (1) Yoo, S.; Domercq, B.; Kippelen, B. Appl. Phys. Lett. 2004, 85, 5427. (2) Sch€on, J. H.; Kloc, C.; Bucher, E.; Batlogg, B. Synth. Met. 2000, 115, 177. (3) Roland, S.; Marcus, A.; Heinz von, S. J. Appl. Phys. 2005, 98, 084511. (4) Kazuhito, T.; Iwao, Y.; Kunji, S.; Keiichi, Y.; Jun, T.; Yoshinobu, A. Appl. Phys. Lett. 2005, 87, 183502. (5) Abe, Y.; Hasegawa, T.; Takahashi, Y.; Yamada, T.; Tokura, Y. Appl. Phys. Lett. 2005, 87, 153506. (6) Koch, N.; Elschner, A.; Schwartz, J.; Kahn, A. Appl. Phys. Lett. 2003, 82, 2281. (7) Chen, Q.; McDowall, A. J.; Richardson, N. V. Langmuir 2003, 19, 10164. (8) Lukas, S.; Witte, G.; W€oll, C. Phys. Rev. Lett. 2001, 88, 028301. (9) Dougherty, D. B.; Jin, W.; Cullen, W. G.; Reutt-Robey, J. E.; Robey, S. W. J. Phys. Chem. C 2008, 112, 20334. (10) Eremtchenko, M.; Temirov, R.; Bauer, D.; Schaefer, J. A.; Tautz, F. S. Phys. Rev. B 2005, 72, 115430. (11) Zheng, Y.; Qi, D.; Chandrasekhar, N.; Gao, X.; Troadec, C.; Wee, A. T. S. Langmuir 2007, 23, 8336. (12) France, C. B.; Schroeder, P. G.; Parkinson, B. A. Nano Lett. 2002, 2, 693. (13) Schroeder, P. G.; France, C. B.; Park, J. B.; Parkinson, B. A. J. Appl. Phys. 2002, 91, 3010. (14) France, C. B.; Schroeder, P. G.; Forsythe, J. C.; Parkinson, B. A. Langmuir 2003, 19, 1274.

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Scanning probes have been very useful in probing the organization of pentacene adlayers with thickness ranging from monolayer to hundreds of nanometer on metallic and SiO2 supports.15-17 For pentacene adsorbed on Au(111), molecular resolution STM imaging in vacuum √ reveals√a series √ of highly √ ordered √ structures, (6  127), (3 3  19), (2 7  37), √ including √ and ( 21  43) at a coverage of 0.25 monolayer equivaand annealing lents.12,14 Raising the dosage to about a monolayer √ √ to 60 C for ∼15 min results in (2  2 7), and (2  31).14 Meanwhile, it is shown that arene molecules carried by benzene can be deposited onto gold surface. By immersing gold sample into a benzene dosing solution, coronene, pentacene, etc. are adsorbed spontaneously onto Au(111) in highly ordered arrays.18-22 (15) Fritz, S. E.; Martin, S. M.; Frisbie, C. D.; Ward, M. D.; Toney, M. F. J. Am. Chem. Soc. 2004, 126, 4084. (16) Conrad, B. R.; Cullen, W. G.; Riddick, B. C.; Williams, E. D. Surf. Sci. 2009, 603, L27. (17) Hagen, K.; Marcus, H.; Ute, Z.; Gunter, S.; Wolfgang, R.; Werner, W. J. Appl. Phys. 2002, 92, 5259. (18) Yoshimoto, S.; Narita, R.; Wakisaka, M.; Itaya, K. J. Electroanal. Chem. 2002, 532, 331. (19) Yang, Y.-C.; Chang, C.-H.; Lee, Y.-L. Chem. Mater. 2007, 19, 6126. (20) Uemura, S.; Taniguchi, I.; Sakata, M.; Kunitake, M. J. Electroanal. Chem. 2008, 623, 1. (21) Uemura, S.; Sakata, M.; Taniguchi, I.; Hirayama, C.; Kunitake, M. Thin Solid Films 2002, 409, 206. (22) 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.

Published on Web 06/11/2009

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This solution in two pentacene arrays, √ dosing √ method resulted √ identified as ( 13 31) and (6 31), which were not observed in vacuum.19 This difference in pentacene structure may result from different dosing methods and different imaging environment. We were motivated to conduct a more thorough study to resolve this inconsistency by performing in situ STM imaging with Au(111) samples dosed with pentacene from solution. The concentration of pentacene dissolved in benzene varied from 16 to 400 μM. STM results obtained √ in this √ study reveal√hitherto unidentified structures, namely ( 31 31), (310), ( 3110), and √ √ ( 72 7) as the coverage of pentacene increased from 0.0645 to 0.0714 (molecule/gold atom). None of these structures has been observed in vacuum.12,14 It is thought that this difference in lateral structure resulted from different exposure of pentacene at the Au (111) surface via the vapor and solution phases. Furthermore, STM imaging of pentacene-coated Au(111) electrode was carried out in 0.1 M HClO4 under potential control to elucidate the effect of potential on the spatial arrangement of pentacene admolecules. It is shown that the electrochemical potential controlled the organization of pentacene admolecules. Similar to the results observed in vacuum,12,14 ordered pentacene structures were observed mostly on the reconstructed Au(111) within the potential range between 0.2 and 0.7 V (vs reversible hydrogen electrode). In 0.1 M HClO4 the reconstructed Au(111) was lifted at E > 0.7 V and protons became adsorbed at E < 0.2 V. Both processes disrupted ordered pentacene arrays. These results suggest that the pentacene-Au(111) interface could be altered drastically if ubiquitous moisture and salt in the ambient find their ways into the system. In addition, despite the binding strength of pentacene on Au(111) is reported to be ∼110 kJ/mol,14 in situ STM results obtained here show that pentacene admolecules could be mobile on Au(111), as roughly one hundred pentacene molecules shuttled laterally between two neighboring arrays in 10 s at room temperature.

Figure 1. Cyclic voltammograms obtained with Au(111) with (solid trace) and without (dotted trace) a pentacene adlayer in 0.1 M HClO4. The scan rates of potential were 50 mV/s.

Experimental Section The Au(111) single crystal electrodes used for voltammetric and STM experiments were made by melting a Au wire (φ=0.8 mm) with a hydrogen torch, as reported earlier.23,24 The pretreatment of Au(111) electrode involved annealing by a hydrogen flame, followed by quenching in hydrogen-saturated Millipore water. This procedure is√shown to produce a highly ordered, reconstructed Au(111)-(22  3) surface.25,26 After removing from Millipore water, the Au(111) electrode was rinsed with acetone, and dried by blowing nitrogen. The as-prepared Au(111) electrode was then immersed for 30 s in the pentacene dosing solution made of benzene. The concentration of pentacene varied between 16 and 400 μM. Due to the rather short dosing time of 30 s, the asprepared pentacene adlayer might not be the equilibrium structures on Au(111) . However, according to STM results obtained in this study, this method reveals rich structural information of pentacene adlayers as a function of concentration and potential on Au(111). All electrochemical experiments were performed with the conventional hanging meniscus method in a three-electrode cell equipped with a RHE reference electrode and a Pt counter electrode. The potentiostat was a CHI 703 (Austin, TX). The supporting electrolyte used in voltammetric and STM experiments was 0.1 M HClO4. Ultrapure HClO4 was purchased (23) Chang, C.-C.; Yau, S.-L.; Tu, J.-W.; Yang, J.-S. Surf. Sci. 2003, 523, 59. (24) Liu, G. Z.; Ou Yang, L. Y.; Shue, C. H.; Ma, H. I.; Yau, S. L.; Chen, S. H. Surf. Sci. 2007, 601, 247. (25) Robinson, K. M.; Robinson, I. K.; O’Grady, W. E. Surf. Sci. 1992, 262, 387. (26) Wandlowski, T.; Ocko, B. M.; Magnussen, O. M.; Wu, S.; Lipkowski, J. J. Electroanal. Chem. 1996, 409, 155.

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Figure 2. In situ STM images with successively finer resolution, acquired with pentacene-coated Au(111) immersed in 0.1 M HClO4. The pentacene adlayer was made by immersion Au(111) in 16 μM pentacene carried by benzene. The potential of Au(111) was held at 0.5 V. The bias voltage and feedback current used to obtain these images were -0.2 V and 1 nA. This penta√ √ cene structure is ( 31  31)R8.9, as outlined by the rhombus marked in (c). A corresponding model of this structure is depicted in (d). from Merck (Darmstadt, DFG). Pentacene obtained from TCI Chemicals (Tokyo, Japan) was used without further purification. Benzene was obtained from Riedel-deHaen (Seelze, FRG). Triple-distilled Millipore water (resistivity 18.3 MΩ) was used to prepare 0.1 M HClO4. 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 ∼600 nm). The piezo scanner was calibrated against HOPG. 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, and finally painted with nail polish for insulation. Typically, intermolecular spacings between pentacene admolecules measured by the STM have errors of (3%, due to the difficulties in mechanic stability of the STM and the inaccuracy in the calibration of piezo scanner. The use of STM in Langmuir 2009, 25(17), 9887–9893

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√

√

Figure 3. In situ STM images showing the ( 72 7)R19.1 and (310) structures of pentacene adsorbed on Au(111) at 0.5 V in 0.1 M

HClO4. This Au(111) sample was made by immersing in 80 μM pentacene dosing solution for 30 s. The corresponding ball models of these two structures are shown in (d) and (e), respectively.

studying electrified interface is well-established and has been reviewed.27-29

Results and Discussion Cyclic Voltammetry (CV). Figure 1 shows the CVs recorded at 50 mV/s for Au(111) electrodes with (solid trace) and without (dotted trace) predeposition of a pentacene adlayer. The dotted trace reveals a gradual increase of anodic current at E > 0.5 V, which is ascribed to charging √ of the Au(111) interface. It is shown that the Au(111)-(22  3) reconstruction is lifted to a (1  1) structure as the potential is made positive of 0.6 V.26,30 A pentacene adlayer was deposited onto the Au(111) electrode by immersing in 50 μM pentacene dosing solution for 30 s. The resultant j-E profile is mostly featureless between 0.1 and 0.7 V, except a rather small peak at 0.3 V. Because its peak current is less than 2 μA/cm2, it is attributed to some insignificant local processes. This CV result indicates a strongly adsorbed pentacene adlayer on Au(111) which remained largely unchanged in the course of potential sweep at 50 mV/s between 0.1 and 0.7 V. Anodic current increased gradually E > 0.7 V, which signaled restructuring of the pentacene adlayer and the reconstructed Au (111) substrate, as revealed by in situ STM (see below). In situ STM. Effect of Concentration on the Arrangement of Pentacene Molecule on Au(111). The followings are in situ STM results obtained with Au(111) coated with pentacene molecules from benzene solutions containing 16-400 μM pentacene. We show that pentacene molecules were irreversibly adsorbed on Au(111), and their spatial structures varied with [pentacene] when all other experimental variables, such as electrochemical potential, dosing time, and temperature, were kept the same. Shown in Figure 2a is a typical STM topography scan, revealing the general surface morphology of a pentacene-coated Au(111) electrode. The Au(111) electrode was predosed with (27) Itaya, K. Prog. Surf. Sci. 1998, 58, 121. (28) Gewirth, A. A.; Niece, B. K. Chem. Rev. 1997, 97, 1129. (29) Magnussen, O. M. Chem. Rev. 2002, 102, 679. (30) Wu, S.; Lipkowski, J.; Magnussen, O. M.; Ocko, B. M.; Wandlowski, T. J. Electroanal. Chem. 1998, 446, 67.

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16 μM pentacene and potentiostated at 0.5 V. The bias voltage and set point current used for STM imaging were -200 mV and 1 nA, respectively. Terrains spanning 100 nm and monatomic steps were apparent. Close examination of this STM image reveals striated, corrugated lines attributed to the herringbone features of the reconstructed Au(111).31,32 Also observed were monatomicheight islands. Although these features could stem from multilayer of pentacene, the fact that less corrugated lines seen in regions near those islands suggests that they were clusters of gold atoms ejected from the uppermost layer upon the partial lifting of 32 Since the pristine Au(111) surface had the ideal reconstruction. √ (22 3) structure, the adsorption of pentacene could induce local lifts of reconstruction. This result however was not observed on Au(111) dosed with pentacene vapor in vacuum.12,14 The higher-resolution STM scan shown Figure 2b yielded the internal structure of pentacene admolecules. A single ordered domain could span 50 nm, depending on the quality of Au(111) surface and the cleanliness of the electrochemical environment. Within this ordered domain all pentacene molecules were aligned themselves with their long and short molecular axis in the directions 9 rotated from the Æ110æ azimuth of the Au(111) surface. Two neighboring pentacene molecules were separated by 1.64 and 0.82 nm (measured√center-to-center). These intermolecular spacings can equal to 31√ times √ the size of gold atom (diameter=0.295 nm), indicating a ( 31 31)R8.9 structure. Each unit cell marked in the close-up STM scan shown in Figure 2c would contain two pentacene molecules, which results in a coverage of θ=2/31=0.0645. A ball√model √ is depicted in Figure 2d to account for the Au (111)-( 31  31)R8.9 structure. The intermolecular spacing √ along the long axis is 1.64 nm ( 31 times 0.295 nm), which equals to the van der Waals diameter of pentacene molecule. In the direction of the short axis two nearest pentacene molecules were separated by 0.82 nm, which exceeds the dimension of pentacene by 17%. These dimensions suggest that pentacene admolecules could lie horizontally on Au(111) without experiencing strain. (31) Barth, J. V.; Brune, H.; Ertl, G.; Behm, R. J. Phys. Rev. B 1990, 42, 9307. (32) Structure of Electrified Interfaces; Lipkowski, J. R. P. N., Ed.; VCH Publishers, Inc.: New York, 1993.

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Figure 4. In situ STM images showing the adsorption of pentacene admolecules on Au(111) at 0.5 V (a), and 0.3 V (b and c) in 0.1 M HClO4. The pentacene adlayer √ was √ made by soaking Au(111) in 400 μM dosing solution for 30 s. The pentacene was apparently disorder at 0.5 V, but transformed to a ( 72 7)R19.1 structure, highlighted by the inset in (b). Two rotational domains of this structure imaged in (c).

They could experience attractive interaction from neighboring molecules in the direction of long axis. This view agrees with those concluded in previous studies.11,14,33 Also, given the trilobe STM appearance seen with all pentacene molecules, we decide to assign all pentacene molecules to the same type of sites. If only symmetric type of surface sites are allowed, all pentacene admolecules should have their centers placed on 2-fold bridge sites. This adsorption configuration yields near 3-fold coordination for the terminal benzene rings of pentacene molecules, which manifested in their lower corrugation heights seen in STM image (Figure 2c). Following the same experimental procedure but raising the pentacene dosing concentration by five times to 80 μM, we identified different pentacene structures. Two of the most important structures were observed simultaneously at 0.5 V with a bias voltage of -150 mV and a feedback current of 1 nA, as revealed by Figure 3a.√The ordered array occupying the left-hand √ side is identified as ( 7  2 7)R19.1 by using high resolution scan shown in Figure 3b. The coverage is 1/14=0.0714. The high resolution scan shown in Figure 3c was acquired by zooming in onto the right-hand side of Figure 3a. This structure has an elongated unit cell as marked. The two unit vectors were found to run parallel to the Æ110æ directions of the Au(111) substrate and were measured 0.876 and 2.95 nm in length. Thus, this ordered array is (3  10) with two pentacene molecules per cell or an equivalent coverage √ of√ 2/30 = 0.0666. It is slightly (7%) less populated than ( 72 7)R19.1. Increasing [pentacene] from 16 to 80 μM thus resulted in an increase of surface coverage from 0.0645 to 0.0666 and 0.0714, thereby producing different pentacene structures. √The ball √ models depicted in Figure 3d and e account for the ( 7  2 7)R19.1 and (3  10) structures, respectively. As the surface coverage of pentacene increased, intermolecular √ spacing √ shrank along the short axis, from 0.82 nm seen with ( 31 31) √ √ R8.9 to 0.78 nm measured for ( 72 7)R19.1. It was reduced from 1.64 to 1.57 nm along the long axis. These intermolecular distances are essentially identical to the van√der Waals √ dimensions of a pentacene molecule, suggesting that ( 72 7)R19.1 could be the most densely packed adlattice of pentacene on Au(111). The corresponding model shown in Figure 3e reveals that all pentacene admolecules occupy identical type of sites with their long axis rotated slightly away from the Æ110æ direction of Au (111). This adjustment might be necessary to avert a repulsive interaction caused by the end-to-end configuration between pentacene admolecules. In the past the adsorption sites of adsorbates such as iodine and benzene are extrapolated from their molecular resolution STM (33) Bavdek, G.; Cossaro, A.; Cvetko, D.; Africh, C.; Blasetti, C.; Esch, F.; Morgante, A.; Floreano, L. Langmuir 2008, 24, 767.

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images which exhibit well-defined corrugation patterns.34,35 This seems to be a difficult task in the present case, because pentacene admolecules, although adsorbed in ordered structures, sat on a corrugated surface of the reconstructed Au(111). Pentacene admolecules could exhibit different corrugation heights in the STM images, but this resulted largely from the undulation in the substrate, rather than the differences in registries. Thus, we could not determine the adsorption sites of pentacene admolecules from STM images, and the registries of pentacene admolecules proposed in the ball models are arbitrary. The highest dosing concentration of pentacene used in this study was 400 μM, yielding STM results presented in Figure 4. Here, the potential of Au(111) was 0.5 V. STM imaging revealed a largely disorder pentacene adlayer (Figure 4a). However, shifting the potential negatively to 0.3 V rendered transformation of this disorder adlayer into an ordered one, as indicated by Figure 4b. It appears that the 400 μM pentacene concentration used here could be too high to enable an ordered adlayer near the open-circuit potential (ca. 0.6 V). However, lowering the potential to 0.3 V could result in desorption of pentacene admolecules, reducing the coverage to a value suitable for the formation of ordered structure. Ordered arrays were found locally within 5 min after the potential was changed, and the breath of ordered domain increased with time. The ordered pentacene shown as √ structure √ the inset of Figure 4b is identified as ( 7  2 7)R19.1 whose rotational domains were responsible for the patches √ seen√on terraces (Figure 4b). Two rotational domains of the ( 72 7) R19.1 structure are observed in Figure 4c. It appears that this was the most densely packed ordered structure that could be produced by using the √ solution dosing method in this study. It is similar to the (2  2 7) structure observed in vacuum.9,14 This molecular arrangement resembles that of the (100) plane of pentacene crystal,36 indicating the increasing importance of intermolecular interaction in organizing admolecules at higher coverage. In Situ STM Observation of Restructuring of Pentacene Adlayer. We have acquired time-dependent STM images to show that pentacene admolecules were mobile on the Au(111) electrode in 0.1 M HClO4. The potential of Au(111) was held at 0.5 V and imaging conditions were -150 mV in bias voltage and 1 nA in feedback current. √These √ordered pentacene arrays are determined to be the ( 31  31)R8.9 structure described earlier. Figure 5a √ √reveals two ordered arrays due to rotational domains of ( 31 31)R8.9. Domain I occupied an area ten times larger than that of domain II. The immediately following scans acquired (34) Schardt, B. C.; Yau, S.-L.; Rinaldi, F. Science 1989, 243, 1050. (35) Yau, S.-L.; Kim, Y.-G.; Itaya, K. J. Am. Chem. Soc. 1996, 118, 7795. (36) Mattheus, C. C. D.; Dros, A. B.; Baas, J.; Meetsma, A.; Boer, J. L.; Palstra, T. T. M. Acta Crystallogr., Sec.C: Cryst. Struct. Commun. 2001, 57, 939.

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Figure 5. Time-dependent in situ STM images showing the restructuring of pentacene adlayer on the reconstructed Au(111) at 0.5 V in 0.1 M HClO4. The time differences two images are √ between √ 10 s. Two rotational domains of the ( 31 31)R8.9 structure were found in (a). The less important domain II comprising ca. 100 pentacene molecules appeared to shift to the right and rotated by 19 to join the major domain I in 10 s. All images are 4040 nm. Figure 7. Potential-dependent STM images showing the restructuring of pentacene adlayer as the potential was decreased from 0.5 (a) to 0.35 V (b), then to 0.25 V (c) and finally to 0.15 V (d-f) in 0.1 M HClO4. The whole sequence was recorded in a time span of 40 min. √ All√images are 6666 nm. Initially, domains I and II hosted the ( 72 7)R19.1 and (310)-pentacene structures, respectively. √ They were gradually replaced by a new structure, ( 31  10), marked by the dotted rectangles.

Figure 6. In situ STM images showing pentacene adlayer on Au (111) at 0.5 V (a and d), and 0.8 V (b and c) in 0.1 M HClO4. Most ordered pentacene adlattice and the reconstructed Au(111) structure seen at 0.5 V disappeared at 0.8 V. Shifting the potential back to 0.5 V could not restore the reconstructed Au(111) and the ordered pentacene structure, except those areas with surviving herringbone structures. All images are 100100 nm.

10 and 20 s later are shown Figure 5b and c, where the more important domain I expanded at the expense √ of√the less important domain II. The ordered structure of ( 31  31)R8.9 seen in domain I was stable against prolong STM imaging, without transforming to other structure. These STM results indicate that pentacene admolecules, even shown to bind strongly with Au (111) at a binding energy of ∼100 kJ/mol,14 could at least rotate freely on the substrate at room temperature. Close inspection of this sequence of STM images show that those pentacene molecules located at the boundary line were the last √ ones√to settle down and realigned themselves with the major ( 31  31)R8.9 adlattice. Thus, pentacene molecules in the number of one hundred moved laterally and rotated horizontally by 19 in a concerted manner to merge into the more important structure. This structural transformation could be driven by the lattice strain existing at the domain boundary, which eventually eliminated defects in the adlayer and produced a uniform molecular adlayer. However, since the tip of the STM could exert forces on the sample, the realignments of pentacene molecules seen in Figure 5 could result from the tip-and-substrate interaction. Indeed, imaging with different parameters could affect the structure of pentacene adlayer in some cases.14 For example, a bias voltage larger than (300 mV destabilized the pentacene adlayer, as also noted in vacuum.9,14 It appears that the tip could interact Langmuir 2009, 25(17), 9887–9893

Figure 8. In situ STM images (a) and (b) and ball model (c), √ showing the Au(111)-( 31  10)-pentacene structure, θ = 0.0666. This structure derived from (3  10) was observed after the potential was switched from 0.5 to 0.25 V in 0.1 M HClO4.

with the Au(111) sample to an extent strongly enough to destabilize the adlayer.37-39 Also, STM imaging of pentacene on Au(111) was attempted in air, but in vain. This difficulty could result from a strong, moisture-mediated interaction between the tip and the pentacene-coated Au(111). Potential Induced Phase Transition of Pentacene Adlayer. We also gathered STM results to elucidate the effect of potential on the arrangement of pentacene admolecules on Au(111). Shown in Figure 6 are time-dependent STM images recorded in a time span of 8 min after the potential√was shifted √ from 0.5 to 0.8 V. At 0.5 V a long-range ordered ( 31  31)R8.9 structure was observed on a mainly reconstructed Au(111) (Figure 6a), as expected. The depressed steaks were not always observed and are thought to stem from adsorption of contaminations. The next two images (Figure 6b and c) were acquired 4 and 8 min after the potential was increased from 0.5 to 0.8 V, which show that the ordered pentacene structures along with the herringbone features disappeared. The pentacene adlayer became mostly disordered. Protruded islands monatomic (Δz = 0.25 nm) in height are aggregates of gold atoms ejected from the uppermost layer of the Au(111) surface as its reconstructed surface was removed . (37) Meepagala, S. C.; Real, F. Phys. Rev. B 1994, 49, 10761. (38) D€urig, U.; Gimzewski, J. K.; Pohl, D. W. Phys. Rev. Lett. 1986, 57, 2403. (39) Sun, Y.; Mortensen, H.; Sch€ar, S.; Lucier, A.-S.; Miyahara, Y.; Gr€utter, P.; Hofer, W. Phys. Rev. B 2005, 71, 193407.

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Pong et al. Table 1. Summary of Pentacene Adlattices Identified on Au(111) Electrode in 0.1 M HClO4a

structure coverage (pentacene/gold atom) dosing [pentacene], μM √ √ 0.2-0.7 31  31 2/31 = 0.0645 16 0.2-0.7 1/15 = 0.0666 80 √ 3  10 0.2-0.35 31  10 1/15 = 0.0666 80 √ √ 0.2-0.5 72 7 1/14 = 0.0714 80-400 a Pentacene molecules were adsorbed from benzene dosing solution containing pentacene whose concentrations are listed in the last column. potential, V

The potential back to 0.5 V to check if this potential initiated process was reversible. This led to a STM image shown in Figure 6d, where patches of ordered pentacene arrays were discerned on some reconstructed Au(111) structures. Most of the pentacene adlayer still appeared disorder, residing on the unreconstructed (11) structure. Neither the ordered pentacene domains nor the reconstructed Au(111) structure grew with prolong potential holding at 0.5 V. These findings then suggest that pentacene admolecules preferred to have ordered adlattices on the reconstructed Au(111) structure, as also observed in vacuum.12,14 The STM results shown in Figure 7a-f were recorded in a time span of 40 min as the potential was decreased progressively from 0.5 to 0.15 V in 0.1 M HClO4. Despite a slight drift in the course of imaging, these STM results are able to show changes occurring at roughly the same areas on Au(111). Figure 7a acquired at 0.5 √V shows √ two highly ordered arrays identified respectively as ( 7 2 7)R19.1 and (310) in domain I and II on the reconstructed Au(111). Decreasing the potential from 0.5 to 0.35 V triggered restructuring of the pentacene adlayer preferentially at the domain boundaries between the ordered structures. These changes are highlighted by dotted rectangles marked in the STM images shown in Figure 7b-e. The areas of these new structures increased with time until the potential was made so negative that other electrode processes occurred to displace the ordered pentacene structures. The newly produced structure prevailed between 0.15 and 0.25 V. The STM results shown in Figure 8 highlight the new structures seen at E < 0.35. This adlattice resembles that of (310), except the shorter vector is rotated from that of Æ110æ azimuth by 9 while the long axis remained unchanged. In other √ words, the shorter vector √ of the unit cell is now aligned in the 31 direction, yielding a ( 3110) structure. The pentacene coverage is calculated as 4/60= 0.0666, which equals √ to that of (3  10). Thus, the restructuring from (3  10) to ( 31  10) was not caused by desorption of pentacene admolecules. Typically, √ √ ordered pentacene adlattice, such as the most compact ( 72 7)R19.1 structure seen in the domain I, simply disappeared at negative potentials without transforming first to another loosely packed structure, as seen in the series of STM images shown in Figure 7d-f. The STM images shown in Figure 7d-f were recorded 1, 2, and 8 min after the potential was made to 0.15 V. The pentacene adlayer started to desorb, leading to the fuzziness in some local areas (Figure 7d). More pentacene admolecules disappeared with prolong STM imaging, as seen in Figure 7e and f. It took about 20 min to remove all the pentacene structures at 0.15 V, yielding typical herringbone features (not shown here). This potential induced phase transition was reversible, as ordered pentacene adlattices were restored by shifting the potential positively to 0.5 V. Because of the high concentration of proton in 0.1 M HClO4, the changes observed in Figure 7 are thought to result from the adsorption of protons at E < 0.2 V. Because pentacene molecule was not soluble in the aqueous solution, they probably stayed near the electrified interface and were readsorbed once the potential was made positive again. Furthermore, the restored pentacene adlayer was highly ordered with a single ordered 9892 DOI: 10.1021/la900978v

domain spanning 50 nm or more. It appears that pentacene admolecules were not reduced at 0.15 V, so that ordered pentacene structures could be restored by changing potential to E > 0.3 V. These potential changes in electrified interface of Au(111) are mostly consistent with those observed with coronene, fullerene molecules, etc.18,21 In fact, it is shown that one can use a similar method to prepare ordered C60 and C70 molecular adlayers.20 Table 1 summarizes the pentacene structures observed in this study. The coverages of these pentacene adlattices, defined as ratio of the number of pentacene molecule/one gold atom, vary from 0.0645 to 0.0714 with the dosing concentration of 16 to 400 μM. To compare our results with those observed in vacuum, we convert the coverage of those pentacene structures found in vacuum by assuming an atomic density of 1.51015 atoms/cm2 for 0.0256 the Au(111) surface.12,14 The resultant values lie between √ and the ordered pentacene √ 0.0417 √ for √ √ √ √adlattices of (6  127), (3 3 19), (2 7 37), and √ ( 21 43); √ whereas those more compact structures of (22 7) and (2 31) have coverages of 0.0833. Thus, none of these structures has coverage comparable to those found in this study. This may well be the reason why different pentacene structures are observed in these independent studies. One can apply the ideal gas law to evaluate the dosage level used in solution and compare with those used in vacuum. Using the relationship of P=CRT, where P, C, R, T represent pressure, concentration, ideal gas constant, and temperature, respectively, we estimate an equivalent pressure of 1910-3 torr for a 10-6 M pentacene dosing concentration. This could be a thousand times higher than the normal dosing pressure (10-6 torr) used in ultrahigh vacuum.12 Even with this highest dosage level (400 μM) used in this study, we could not obtain any ordered structure with a coverage higher than 0.0714. It was also difficult to deposit multilayer pentacene thin film on Au(111) by simply dipping the electrode in a pentacene solution. It appears that two layers of pentacene molecule could have only π-π interaction, a van der Waals interaction too weak to render crystallization from the dosing solution at room temperature. The most √ densely √ packed pentacene structure observed in this study is ( 72 7)R19.1, which has a√dimension of 0.781.56 nm. This structure resembles the (22 7) structure observed in vacuum by France et al.,12,14 except the spacing between two pentacene molecules is 32% larger. Although we could increase the surface coverage of pentacene by raising [pentacene], we would observed disordered pentacene adlayer residing on a Au (111)-(1  1) structure. This finding suggests that the lack of ordered structure arises from pentacene molecules were adsorbed in various configurations ranging from tilt to vertical, which made it difficult for pentacene admolecules to pack orderly. Overall, this study shows the importance of dosing conditions in guiding the adsorption of pentacene molecules on Au(111), which could be the reason for the different pentacene structures observed by Yang et al.19

Conclusions High-quality molecular STM images reveal ordered √ resolution √ √ pentacene structures of ( 31 31)R8.9, (310), ( 3110), and Langmuir 2009, 25(17), 9887–9893

Pong et al.

√ √ ( 7  2 7)R19.1 deposited irreversibly from benzene dosing solutions containing 16 to 400 μM. The most compact structure, √ √ ( 7  2 7)R19.1, seen in this study comprises pentacene admolecules lying parallel to the Au(111) substrate. The saturated coverage is 0.0714. Most ordered pentacene adlattices are found on the reconstructed Au(111), and they are stable between 0.2 and 0.8 V in 0.1 M HClO4. At E < 0.2 V the adsorption of proton gains importance to displace pentacene admolecules from the Au (111) electrode. However, increasing the potential from 0.2 back to 0.5 V restore ordered pentacene adlattices with a degree of ordering comparable to that at the beginning of experiment. Increasing the potential to E > 0.8 V lifts the reconstructed Au (111) and disrupts ordered pentacene structures. These processes are reversible if the potential is made negative enough (∼0.2 V) to

Langmuir 2009, 25(17), 9887–9893

Article

restore the reconstructed Au(111) structure. At room temperature (25 C) pentacene admolecules on Au(111) can be forced to move and realign with those in a neighboring domain. These restructuring events can be driven by lattice strains existing at domain boundaries and by the tip-and-sample interactions. Acknowledgment. We thank technical help from Prof. C. C. Su (Institute of Organic and Polymeric Materials, National Taipei University of Technology). This research is supported by the National Science Council of Taiwan (NSC 98-2113-M-008-001 and NSC97-2628-M-008-019). Financial assistance for this research was partially provided by Industrial Technology Research Institute of Taiwan under contract number ITRI 98-B-08 8351AA5140.

DOI: 10.1021/la900978v

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