Article pubs.acs.org/JPCC
Photo-Induced Polymerization and Isomerization on the Surface Observed by Scanning Tunneling Microscopy Xue-mei Zhang,†,⊥ Shan-dong Xu,‡,⊥ Min Li,§ Yong-tao Shen,∥ Zhong-qing Wei,§ Shuai Wang,† Qing-dao Zeng,*,† and Chen Wang*,† †
National Center for Nanoscience and Technology (NCNST), Beijing 100190, P. R. China College of Science, Beijing Forestry University, Beijing 100083, P. R. China § Institute of High Energy Physics, The Chinese Academy of Sciences, CAS Key Lab for Biomedical Effects of Nanomaterials and Nanosafety, Beijing, 100049, P. R. China ∥ Department School of Materials Science and Engineering, Tianjin Key Laboratory of Composite and Functional Materials, Tianjin University, Tianjin 300072, P. R. China ‡
S Supporting Information *
ABSTRACT: In this paper, an azobenzene derivative containing two diacetylene groups is synthesized and its self-assembly at a surface investigated using scanning tunneling microscopy (STM). Both the azo-benzene and diacetylene groups are photoactive, and the results show that surface assemblies of the targeted compound undergo polymerization following irradiation at 254 nm and reversible isomerization following alternating irradiation at 365 nm and with visible light. This is the first report of a STM investigation observing both photopolymerization and photoisomerization simultaneously for the same molecular assembly at an interface. The target molecule allows one to induce sequential and reversible structural changes to surface assemblies via multiple optical treatments, and is thus of both fundamental interest for surface science and engineering. These results provide experimental and theoretical guidance for the fabrication of future molecular optoelectronic devices.
■
Scheme 1. A Schematic Presentation for Reversible cis/trans Photo-Isomerization of Azobenzene Unit
INTRODUCTION Construction of functional nanostructures by means of selfassembly is essential for the future realization of molecule-based electronics and multifunctional optical devices.1,2 In particular, increased control over self-assembly protocols has recently allowed complex and well-ordered nanostructures to be assembled from organic molecules with multiple functional groups at an interface.3,4 Scanning tunneling microscopy (STM) is an ideal technique for observing the formation and final structure of the molecular assemblies due to its ability to resolve spatial information at the atomic scale.5−8 In this paper, the self-assembly of an azobenzene derivative containing two diacetylene groups is investigated at a highly oriented pyrolytic graphite (HOPG) surface. The results show that the formation of various complex nanostructures can be triggered by light irradiation at different wavelengths, due to the photoactive nature of the azobenzene and diacetylene functional groups. Azobenzene derivatives are an important class of photochromic molecules which undergo reversible cis/trans isomerization with suitable light irradiation (Scheme 1).9 They have applications in a wide range of fields including optical switching, holographic storage, light harvesting, long-term © 2012 American Chemical Society
energy storage, and nonlinear optical materials.10−12 Polydiacetylene (PDA), on the other hand, is a well-known quasione-dimensional conjugated polymer formed by polymerization of diacetylene monomers (Scheme 2)13−18 and is one of the most promising topochemical polymerization materials, with potential applications in biosensors, pathogenic agents, and optical, electrical devices.19−21 Up to now, the assemblies of PDA and azobenzene derivatives have separately been studied via STM techniques by researchers, with the results clearly demonstrating the Received: December 2, 2011 Revised: March 25, 2012 Published: April 2, 2012 8950
dx.doi.org/10.1021/jp2115884 | J. Phys. Chem. C 2012, 116, 8950−8955
The Journal of Physical Chemistry C
Article
Scheme 2. A Schematic Presentation for Topochemical Polymerization of Diacetylene
photoinduced polymerization22−30 or isomerization31−44 processes occurred on solid surfaces and resolved at the single molecule level. However, to the best of our knowledge, there have been no STM studies of systems that can undergo both photopolymerization and photoisomerization processes simultaneously. Such systems may allow sequential and reversible changes in a self-assembled nanostructure to be controlled by various photonic inputs. With this aim in mind, a novel diacetylene-substituted azobenzene derivative (Figure 1), di-(10,12-pentacosadiyn-1yl) azobenzene-4,4′-dicarboxylate (Azo-DA), was designed and synthesized (see the Supporting Information). The selfassembly and photochemical response of Azo-DA assemblies on the HOPG surface as investigated by STM are reported below.
Figure 2. (a) STM image of the Azo-DA molecules (before irradiation), Iset = 299 pA, Vbias = 700 mV, scale bar = 5 nm. A schematic model and unit cell are superimposed on it. S is the distance between every fifth alkyl chain. (b) Suggested molecular model of the observed monolayers in part a.
assigned to the azobenzene moiety of the Azo-DA, with the diacetylene units of the molecules presenting lower contrast on both sides. To guide the eye, a four-color model was superimposed on the STM image (Figure 2a). In the middle of each row, the higher contrast with two separated bright spots (indicated by red balls) reflects the characteristics of the trans-azobenzene moiety of a single Azo-DA molecule in its self-assembly.32−37 The long alkyl chains containing the ester and diacetylene groups (indicated by the light blue lines) extend on each side of the azobenzene unit and make an angle (β3) of 129 ± 2° and (β3′) of 125 ± 3° with the azobenzene (Figure 2a), which indicates that the alkyl chains on two sides of a molecule are nearly parallel to each other. In other words, within experimental error, the long chains on both sides of the azoDA molecule adopt a symmetrical adsorption on HOPG. The chain bending of the Azo-DA on the HOPG is defined by the angles β1 or β1′ = 115 ± 2° and β2 or β2′ = 109 ± 2° in Figure 2a. The stacking distance (d) between the diacetylene units on neighboring Azo-DA molecules is measured to be 0.5 ± 0.1 nm, and the distance (S) between every fifth chain is measured to be 2.0 ± 0.1 nm. Combining with the analysis of the unit cell parameters (a = (8.0 ± 0.1) nm, b = (5.0 ± 0.5) Å, and α = (87 ± 1)°), an illustrative model for the Azo-DA arrangement on HOPG over a large area is presented in Figure 2b. The angle (γ) between the alkyl chains and the direction of the lamella axis is determined to be 90 ± 2°, while the angle (θ) between the diacetylene rod and the stacking axis is around 47°. All these structural parameters are in good agreement with previous reports of diacetylene self-assembled monolayers,23,25−28 in which polymerization of the diacetylene upon UV light irradiation is described. Photo-Induced Polymerization of Azo-DA. Following self-assembly of the Azo-DA molecules on the HOPG surface, the sample was irradiated by a UV lamp at 254 nm for 15 min and then recharacterized by STM. Figure 3a shows the monolayer structures of Azo-DA after irradiation. Several bright lines (indicated by red arrows) appeared in the upper part of the image, which were not observed before irradiation.
■
EXPERIMENTAL SECTION STM Investigation. STM measurements were performed under ambient conditions and acquired on a Nano IIIa scanning probe microscope system (Bruker, USA) operating in constant current mode with a mechanically cut Pt/Ir (80/ 20) tip. All solvents for STM experiments were purchased from Acros Company and used without further purification. A droplet of solution containing Azo-DA was drop-cast on a freshly cleaved surface of highly oriented pyrolytic graphite (HOPG, grade ZYB, Advanced Ceramics Inc., Cleveland, USA) for self-assembly studies. Photoirradiation experiments were carried out using a Xenon lamp (50W) with filters (30 nm bandwidth) centered at 365 and 435 nm (Figure S1, Supporting Information), or with the 254 nm light of a low pressure mercury lamp (10 W) without any filter. Samples were placed 25 cm from the light source, and the temperature of the sample was controlled at 20−25 °C throughout the irradiation. All STM experiments were operated in the dark following photoirradiation. The sample preparation for STM investigations was described in the Supporting Information. The graphite substrate was used as a calibration grid. Molecular models were constructed using a HyperChem software package.
■
RESULTS AND DISCUSSION Self-Assembled Structure of Azo-DA. Before any irradiation, the self-assembly of Azo-DA (Figure 1) was investigated at the air/solid interface. A droplet of toluene solution containing Azo-DA was applied to the basal plane of HOPG and then characterized by STM. Figure 2a shows a typical STM image of the self-assembled structure of Azo-DA on HOPG, which shows well-ordered and close-packed lamellae. The very bright features in rows along lamella are
Figure 1. Chemical structure of diacetylene-substituted azobenzene derivatives (Azo-DA). 8951
dx.doi.org/10.1021/jp2115884 | J. Phys. Chem. C 2012, 116, 8950−8955
The Journal of Physical Chemistry C
Article
Figure 3. (a) STM image consisting of polymerized and unpolymerized structure of the Azo-DA molecules upon UV irradiation at 254 nm. Iset = 290 pA, Vbias = 700 mV, scale bar = 10 nm. The polymerized diacentylene units are indicated by red arrows. (b) High resolution image of the outline area (the white frame) in part a; a schematic model and unit cell are superimposed on it. Sp is the distance between every fifth alkyl chain. Iset = 290 pA, Vbias = 699 mV, scale bar = 5 nm. (c) Molecular model for the observed structures in part b.
Figure 4. (a) STM image for the self-assembled structure of nonpolymerized Azo-DA and polymerized Azo-DA ((Azo-poly(DA)) upon UV irradiation at 365 nm for 30 min, Iset = 268 pA, Vbias = 700 mV, scale bar = 10 nm. (b) Submolecular resolved structures of the outlined area (the white frame) in part a, Iset = 275 pA, Vbias = 700 mV, scale bar = 5 nm. A yellow dashed line was drawn to separate the reacted (up) and nonreacted area (down). (c) An illustrative molecular model for the obeserved area in part b. (d) Cross-section analysis for the red dashed line displayed in part b. (e) High resolution image of the outlined area (the red frame) in part a, Iset = 281 pA, Vbias = 700 mV, scale bar = 5 nm. The yellow dotted circles are drawn to hightlight the photoisomerized area for the azobenzene moieties of Azo-poly(DA). (f) Molecular model for the obeserved area in part e. (g) Cross-section analysis for the red dashed line displayed in part e.
individual Azo-poly(DA) molecule can be clearly resolved. The difference in the self-assembled structure of Azo-DA before irradiation with the Azo-poly(DA) structure after illumination is pronounced (Figure 3a and b). First, the polymerized diacetylene units arrange in a characteristic zigzag pattern, as resolved in Figure 3b, and the distance (Sp) for every fifth alkyl chain is determined as 2.2 ± 0.1 nm, which has no significant change after irradiation. Second, the angle (γp = 117 ± 2°) between alkyl chains and the backbone of the Azo-DA becomes larger after polymerization. The unit cell parameters for the observed area are measured to be ap = 7.9 ± 0.1 nm, bp = 5.5 ± 0.5 Å, and αp = 116 ± 1°. Lastly, it is important to note that the azobenzene moiety conformation is unchanged by irradiation in this case, still observed as two well-separated bright spots between the polymerized chains. That is, the azobenzene
With reference to the literature, these bright lines are attributed to the linear arrays of polymerized diacetylene units, effectively cross-linking the Azo-DA molecules as clearly shown in Figure 3b.23−29 The higher contrast of polymerized diacetylene units is attributed to greater π-electron delocalization along the polymer backbone. Analysis of the area of polymerized AzoDA, Azo-poly(DA) (upper part of Figure 3a), shows the angle (γp) between alkyl chains and the backbone of the Azopoly(DA) is 117 ± 2°, much larger than the 90 ± 2° (γ) measured for the nonpolymerized domains (lower part of Figure 3a). After irradiation, the width of a lamella for Azopoly(DA) assemblies (Wp) is measured to be 7.2 ± 0.2 nm, decreasing by 9% compared with that (W = 7.9 ± 0.1 nm) of the nonpolymerized Azo-DA molecule as indicated in Figure 3a. Figure 3b is the high resolution image of the polymerized region outlined in Figure 3a (the white frame), from which the 8952
dx.doi.org/10.1021/jp2115884 | J. Phys. Chem. C 2012, 116, 8950−8955
The Journal of Physical Chemistry C
Article
Figure 5. (a) Asorption conformation structures of Azo-DA and Azo-poly(DA) after irradiation with visible light for 15 min. Iset = 290.1 pA, Vbias = 712.0 mV, scale bar = 5 nm. (b) A suggested molecular model of the observed assemlies in part a.
remained in the thermally stable trans-form during the photopolymerization. Photoisomerization in Azo-DA and Azo-poly(AD) Assemblies. To begin, the solution-phase photoisomerization behavior of the azobenzene in Azo-DA and Azo-poly(DA) were studied in 1-phenyloctane by UV−vis spectroscopy. The results show that about 64% (Azo-DA) and 34% (Azo-poly(DA)) of the molecules in solution (1-phenyloctane) are isomerized from trans- to cis-conformation after illumination by a UV light at 365 nm (Figures S2 and S3, Supporting Information).37 Furthermore, the azobenzene could be switched reversibly between trans- and cis-conformations by alternate light illumination at 365 and 435 nm. These results show that both Azo-DA and Azo-poly(DA) could photoisomerize reversibly at 365 and 435 nm, respectively. The photoisomerization behavior of Azo-DA and Azopoly(DA) was then investigated at the 1-phenyloctane/ HOPG interface by STM. For these experiments, the substrate was covered in the 1-phenyloctane solution of Azo-DA and then irradiated in situ at 365 nm for 30 min. Figure 4a shows its self-assembled structures after 365 nm irradiation. Figure 4b is the high resolution image of the outlined area (the white frame) in Figure 4a, showing the adsorption conformation of the Azo-DA (nonpolymerized) after irradiation at 365 nm. As revealed in this image, two well-separated bright spots observed for the thermally stable trans-Azo-DA isomer are partly replaced by a bright, linear feature after irradiation, attributed to the cisisomer of the Azo-DA molecule. The distance (Li) for the isomerized azobenzene moiety along the black arrows is measured to be 0.9 ± 0.1 nm, shorter than the 1.1 ± 0.1 nm (L) measured in the nonisomerized area. Similar observations have been made in previous STM studies of isomerization of azobenzene derivatives.33,35,36 As deduced from the statistical analysis, the photoisomerized domains of Azo-DA show a unit cell, with parameters ai = 7.8 ± 0.1 nm, bi = 5.0 ± 0.2 Å, αi = 89 ± 2°, and a decrease of 2.5% for the length of one molecule after isomerization from trans to cis. In addition, the crosssection analysis in Figure 4d shows a notable difference in height (1.4 Å) attributed to the different steric conformation between isomerized and nonisomerized azobenzene moieties. Moreover, we think that, in the presence case, the reaction may occur both directly in the solution and on the surface. Figure 4e is the high resolution image of the outlined area (the red frame) in Figure 4a, and importantly, it shows the adsorption conformation of Azo-poly(DA) after illumination by
UV light at 365 nm at the 1-phenyloctane/HOPG interface. Some bright, linear features could also be visualized in this region at the location of the azobenzene groups, which are attributed to the fact that the azobenzene moiety of Azopoly(DA) has undergone photoisomerization from the trans- to cis-form. The image of the Azo-poly(DA) self-assembled structures are fuzzy after isomerization. Also, some defects (indicated by the yellow dotted circles) were observed between the azobenzene group and the polydiacetylene units of cis-Azopoly(DA) molecules. At these defect sites, the phenyl groups may have the possibility of moving out of the HOPG plane,45 resulting in higher constrast in the STM image as observed in Figure 4b and e. Figure 4f depicts a tentative molecular model according to the observed image (Figure 4e). A unit cell is superimposed on the molecular model, as shown in Figure 4f with api = 7.8 ± 0.1 nm, bpi = 5.0 ± 0.5 Å, and αpi = 118 ± 2°. Similar to the photoisomerization of Azo-DA, the cross-section profile along the azobenzene units of Azo-poly(DA) in Figure 4g shows a significant difference in height (1.3 Å). Reversible Photoisomerization in Azo-DA and Azopoly(DA). To investigate the reversibility of photoisomerization in Azo-DA and Azo-poly(DA), the sample which had already been irradiated by UV light at 365 nm was exposed to the visible light for 15 min. Figure 5a shows regions of Azo-DA and Azo-poly(DA) surface assemblies after illumination by visible light. The well-ordered lamella structure, with two rows of bright spots in the middle of a lamella, could be clearly visualized again. A molecular model was proposed in Figure 5b. The unit cell (drawn in yellow) parameters (ar, br, αr) of the unit cell observed in the domain of Azo-DA are measured to be ar = 8.0 ± 0.1 nm, br = 5.0 ± 0.5 Å, and αr = 89 ± 2°, and the unit cell (drawn in pink) parameters (apr, bpr, αpr) of the unit cell observed in the domain of Azo-poly(DA) are measured to be apr = 7.9 ± 0.1 nm, bpr = 5.5 ± 0.5 Å, and αpr = 116 ± 2°. These parameters are in accordance with those observed in the structures of Azo-DA and Azo-poly(DA) before UV irradiation with 365 nm. This proves the reversible nature of the photoisomerization process in nanoassemblies of both AzoDA and Azo-poly(DA). As a summary of all the results and analysis above, on the basis of the observed monolayers, a schematic model of the photopolymerization of Azo-DA, and the reversible photoisomerization of Azo-DA and Azo-poly(DA), between the trans isomer and cis isomer is proposed in Figure 6. 8953
dx.doi.org/10.1021/jp2115884 | J. Phys. Chem. C 2012, 116, 8950−8955
The Journal of Physical Chemistry C
Article
Financial support from National Natural Science Foundation of China (Nos. 21073048, 51173031, 91127043), Key Laboratory of Optoelectronic Materials Chemistry and Physics, Chinese Academy of Sciences (2010KL0010), and the Fundamental Research Funds for the Central Universities (No. HJ2010-18) is also gratefully acknowledged.
■
(1) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418−2421. (2) Barth, J. V.; Costantini, G.; Kern, K. Nature 2005, 437, 671−679. (3) Clever, G. H.; Tashiro, S.; Shionoya, M. J. Am. Chem. Soc. 2010, 132, 9973−9975. (4) Fujita, D.; Takahashi, A.; Sato, S.; Fujita, M. J. Am. Chem. Soc. 2011, 133, 13317−13319. (5) Gimzewski1, J. K.; Joachim, C. Science 1999, 283, 1683−1688. (6) Lehn, J. M. Science 2002, 295, 2400−2403. (7) Li, M.; Deng, K.; Lei, S. B.; Yang, Y. L.; Wang, T. S.; Shen, Y. T.; Wang, C. R.; Zeng, Q. D.; Wang, C. Angew. Chem., Int. Ed. 2008, 47, 6717−6721. (8) Müllen, K.; Rabe, J. P. Acc. Chem. Res. 2008, 41, 511−520. (9) Natansohn, A.; Rochon, P. Chem. Rev. 2002, 102, 4139−4175. (10) Jurt, S.; Aemissegger, A.; Guntert, P.; Zerbe, O.; Hilvert, D. A. Angew. Chem., Int. Ed. 2006, 45, 6297−6300. (11) Perdew, J. P.; Wang, Y. Phys. Rev. B 1992, 45, 13244−13249. (12) Becke, A. D. J. Chem. Phys. 1988, 88, 2547−2553. (13) Bubeck, C.; Tieke, B.; Wenger, G. Mol. Cryst. Liq. Cryst. 1983, 96, 109−120. (14) Wenz, G.; Wegner, G. Mol. Cryst. Liq. Cryst. 1983, 96, 99−108. (15) Enkelmann, V.; Wegner, G. Makromol. Chem. 1977, 178, 635− 638. (16) Enkelmann, V. Adv. Polym. Sci. 1984, 63, 91−136. (17) Wegner, G. Z. Naturforsch., B 1969, 24, 824−832. (18) Baughmann, R. H. J. Polym. Sci., Polym. Phys. Ed. 1974, 12, 1511. (19) Charych, D. H.; Nagy, J. O.; Spevak, W.; Bednarski, M. D. Science 1993, 261, 585−588. (20) Khanarian, G. Thin Solid Films 1987, 152, 265−274. (21) Sugi, M. J. Mol. Electron. 1985, 1, 3−17. (22) Muraoka, T.; Kinbara, K.; Aida, T. Nature 2006, 440, 512−515. (23) Okawa, Y. J.; Aono, M. Nature 2001, 409, 683−684. (24) Dube, H.; Ams, M. R.; Rebek, J. J. J. Am. Chem. Soc. 2010, 132, 9984−9985. (25) Mandal, S. K.; Okawa, Y.; Hasegawa, T.; Aono, M. ACS Nano 2011, 5, 2779−2786. (26) Grim, P. C. M.; De Feyter, S.; Gesquière, A.; Vanoppen, P.; Rücker, M.; Valiyaveettil, S.; Moessner, G.; Müllen, K.; De Schryver, F. C. Angew. Chem., Int. Ed. 1997, 36, 2601−2603. (27) Qiao, Y. H.; Zeng, Q. D.; Tan, Z. Y.; Xu, S. D.; Wang, D.; Wang, C.; Wan, L. J.; Bai, C. L. J. Vac. Sci. Technol., B 2002, 20, 2466−2469. (28) Miura, A.; De Feyter, S.; Abdel-Mottaleb, M. M. S.; Gesquière, A.; Grim, P. C. M.; Moessner, G.; Siefert, M.; Klapper, M.; Müllen, K.; De Schryver., F. C. Langmuir 2003, 19, 6474−6482. (29) De Feyter, S.; De Schryver, F. C. J. Phys. Chem. B 2005, 109, 4290−4302. (30) Okawa, Y.; Mandal, S. K.; Hu, C.; Tateyama, Y.; Goedecker, S.; Tsukamoto, S.; Hasegawa, T.; Gimzewski, J. K.; Aono, M. J. Am. Chem. Soc. 2011, 133, 8227−8233. (31) Mielke, J.; Leyssner, F.; Koch, M.; Meyer, S.; Luo, Y.; Selvanathan, S.; Haag, R.; Tegeder, P.; Grill, L. ACS Nano 2011, 5, 2090−2097. (32) Karapetrov, G.; Fedor, J.; Iavarone, M.; Rosenmann, D.; Kwok, W. K. Phys. Rev. Lett. 2005, 95, 167002−167006. (33) Alemani, M.; Peters, M. V.; Hecht, S.; Rieder, K. H.; Moresco, F.; Grill, L. J. Am. Chem. Soc. 2006, 128, 14446−14447. (34) Comstock, M. J.; Levy, N.; Kirakosian, A.; Cho, J.; Lauterwasser, F.; Harvey, J. H.; Strubbe, D. A.; Fréchet, J. M. J.; Trauner, D.; Louie, S. G.; Crommie, M. F. Phys. Rev. Lett. 2007, 99, 038301−038304. (35) Shen, Y. T.; Guan, L.; Zhu, X. Y.; Zeng, Q. D.; Wang, C. J. Am. Chem. Soc. 2009, 131, 6174−6180.
Figure 6. Schematic model for the photopolymerization and photoisomerization of Azo-DA (Azo-poly(DA)) on HOPG.
■
CONCLUSION In summary, a photosensitive azobenzene derivative containing two diacetylene groups (Azo-DA) was synthesized and its surface self-assembly investigated by STM. Reproducible STM experiments revealed well-ordered 2D monolayer structures of the Azo-DA molecules on HOPG. Azo-DA was observed to photopolymerize via its diacetylene subunits upon irradiation with UV light at 254 nm and to undergo reversible photoisomerization after alternating irradiations with UV light at 365 nm and visible light in both polymerized and nonpolymerized forms. These systems that allow one to optically induce sequential and reversible structural changes to surface assemblies have enormous potential for the development of molecular optoelectronic devices. The studies of AzoDA herein provide novel insight into surface engineering by self-assembly and inspire us to develop further, more complex switchable surface-bound molecular assemblies.
■
ASSOCIATED CONTENT
S Supporting Information *
Synthesis details for Azo-DA, the detailed description of sample preparation for STM images, irradiation experimental information, and UV−vis spectra. This material is available free of charge via the Internet at http://pubs.acs.org.
■
REFERENCES
AUTHOR INFORMATION
Corresponding Author
*Address: National Center for Nanoscience and Technology (NCNST), No. 11 ZhongGuanCun BeiYiTiao, Haidian district, Beijing 100190, P. R. China. Phone: 86-10-82545649. Fax: 8610-62656765. E-mail:
[email protected] (Q.-d.Z.); wangch@ nanoctr.cn (C.W.). Author Contributions ⊥
These authors contributed to this work equally.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (Nos. 2011CB932303, 2011CB933101). 8954
dx.doi.org/10.1021/jp2115884 | J. Phys. Chem. C 2012, 116, 8950−8955
The Journal of Physical Chemistry C
Article
(36) Shen, Y. T.; Deng, K.; Zhang, X. M.; Feng, W.; Zeng, Q. D.; Wang, C.; Gong, J. R. Nano Lett. 2011, 11, 3245−3250. (37) Bleger, D.; Clesielki, A.; Samori, P.; Hecht, S. Chem.Eur. J. 2010, 16, 14256−14260. (38) Gahl, C.; Schmidt, R.; Brete, D.; McNellis, E.; Freyer, W.; Carley, R.; Reuter, K.; Weinelt, M. J. Am. Chem. Soc. 2010, 132, 1831− 1838. (39) Wolf, M.; Tegeder, P. Surf. Sci. 2009, 603, 1506−1517. (40) Piantek, M.; Schulze, G.; Koch, M.; Franke, K. J.; Leyssner, F.; Krüger, A.; Navio, C.; Miguel, J.; Bernien, M.; Wolf, M.; Kuch, W.; Tegeder, P.; Pascual, J. I. J. Am. Chem. Soc. 2009, 131, 12729−12735. (41) Grill1, L.; Dyer, M.; Lafferentz1, L.; Persson, M.; Peters, M. V.; Hecht, S. Nat. Nanotechnol. 2007, 2, 687−691. (42) Leyssner, F.; Hagen, S.; Ovari, L.; Dokic, J.; Saalfrank, P.; Peters, M. V.; Hecht, S.; Klamroth, T.; Tegeder, P. J. Phys. Chem. C 2010, 114, 1231−1239. (43) Ovari, L.; Wolf, M.; Tegeder, P. J. Phys. Chem. C 2007, 111, 15370−15374. (44) Schmidt, R.; Hagen, S.; Brete, D.; Carley, R.; Gahl, C.; Dokic, J.; Saalfrank, P.; Hecht, S.; Tegeder, P.; Weinelt, M. Phys. Chem. Chem. Phys. 2010, 12, 4488−4497. (45) Mali, K. S.; Vanaverbeke, B.; Bhinde, T.; Brewer, A. Y.; Arnold, T.; Lazzaroni, R.; Clarke, S. M.; De Feyter, S. ACS Nano 2011, 11, 9122−9137.
8955
dx.doi.org/10.1021/jp2115884 | J. Phys. Chem. C 2012, 116, 8950−8955
