J. Phys. Chem. C 2007, 111, 9235-9239
9235
Effect of Thermal Annealing on Hydrogen Bond Configurations of Host Lattice Revealed in VOPc/TCDB Host-Guest Architectures Xiang-Hua Kong,†,§ Ke Deng,‡ Yan-Lian Yang,‡ Qing-Dao Zeng,† and Chen Wang*,‡ Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100080, China, and National Center for Nanoscience and Technology, Beijing 100080, China ReceiVed: January 15, 2007; In Final Form: April 23, 2007
The two-dimensional (2D) self-assembled 1,3,5-tris(10-carboxydecyloxy) benzene (TCDB) networks connected by hydrogen bonds are shown to accommodate molecules such as vanadyl phthalocyanine (VOPc) on highly oriented pyrolytic graphite (HOPG) surfaces when examined by scanning tunneling microscopy (STM) under ambient conditions. Thermally annealed adlayers were investigated with STM, which demonstrated that thermal annealing could induce a distinctive phase transition from the host-guest architecture VOPc/TCDB (II) (consisting of VOPc dimers or two VOPcs entrapped in one TCDB cavity) to the architecture VOPc/TCDB (I) (consisting of VOPc monomers or one VOPc entrapped in one TCDB cavity). On the basis of density functional theory (DFT) calculations, we suggest that the transition is associated with the transformation of the hydrogen bond configurations in the TCDB dimers. The experimental results combined with the theoretical calculations revealed that monomer-entrapped VOPc/TCDB (I) is thermodynamically stable for the VOPc/ TCDB complex architectures.
Introduction Two-dimensional (2D) self-assembly or self-organization of molecules on solid surfaces plays a significant role in the “bottom-up” strategy for construction of molecular architectures. Common to the various studies in this area, precise control of the structures of 2D molecular assemblies is a crucial issue and has been subjected to focused attentions.1-10 Molecular ordering could be tuned by modifying chemical structures,11 adjusting film-growth conditions,12 or exerting external stimuli such as light irradiation,13 electric field,14,15 or electrochemical potential change.16,17 Thermal-annealing effect on self-assembled monolayers (SAMs) is another possible venue to affect molecular ordering.18-24 For example, it has been reported that irreversible transformation of monolayers of stilbenoid dendrimer (SD12, carbon side chains (n ) 12)) could be induced by thermal annealing from hexagonal to parallelogram assemblies, accompanied by a molecular conformation change.20 Rohde and co-workers recently reported that the thermal-annealing process could convert the structure of the adlayer of bis(4,4′-(m,m′-di(dodecyloxy)phenyl)-2,2′-difluoro-1,3,2-dioxaborine) on highly oriented pyrolytic graphite (HOPG) from lamellar to hexagonal arrangements and also induce trans-to-cis isomerization of adsorbed molecules.25 These reported sensitivities of SAMs to temperatures indicated that the thermal induced structural transitions are indispensable for potential application in molecular-based nanodevices. Although thermal-annealing effects on single-component systems have been intensively studied, there have been few reports on thermal-annealing induced phase transition of multicomponent complex architectures. Ruben et al. recently mentioned the phase transition of 4,4′,4′′-benzene-1,3,5-triyl* To whom correspondence should be addressed. Tel/ Fax: +86-1062562871. E-mail:
[email protected]. † Institute of Chemistry, Chinese Academy of Sciences. ‡ National Center for Nanoscience and Technology. § Also in CAS Graduate School, Beijing, China.
tribenzoic acid (BTA)/macrocycle (mt-33) complex architectures by thermal annealing, while phase separation occurred between the host (BTA) and the guest (mt-33) molecules due to reorganization of BTA.24 Recently, multicomponent host-guest architectures are gaining interests from both scientific and technological points of view. The host networks can serve as templates to accommodate and arrange the functional building blocks into well-defined supramolecular architectures or functional devices. By utilizing various geometries and functionalities of the building units, a broad range of host structures provide us with a rich ground for explorations.26-35 For example, the modular assembly of polytopic organic carboxylate linker molecules and iron atoms on a Cu(100) surface provided versatile templates for manipulation and organization of functional species at the nanometer scale. Only single C60 guest molecules could be entrapped in the pores formed by Fe-TPA (1,4-dicarboxylic benzoic acid) and Fe-TMLA (trimellitic acid), while the Fe-TDA (4,1′,4′,1′′-terphenyl-1,4′′-dicarboxylic acid) could host C60 monomers, dimers, or trimers.26 We have previously reported that copper phthalocyanine (CuPc) molecules and coronene molecules could be entrapped in the cavities formed by 1,3,5-tris(10-carboxydecyloxy) benzene (TCDB) to obtain host-guest architectures.36 It was found that when the hydrogen bond configurations in TCDB dimers were kept intact, a TCDB cavity could accommodate either one or two coronene molecules, while only one CuPc molecule could be accommodated in all stoichiometries. Further investigations on the thermal-annealing effect could be beneficial for general understanding of the complex architectures utilizing the TCDB network as the template. In the present work, vanadyl phthalocyanine (VOPc) molecules are employed as an example and the thermal-annealing effect on the host-guest architectures of VOPc/TCDB on HOPG surfaces is investigated by scanning tunneling microscope (STM). By thermal annealing of the as-prepared adlayers, phase transition from dimer-entrapped to monomer-entrapped VOPc/
10.1021/jp070328f CCC: $37.00 © 2007 American Chemical Society Published on Web 06/05/2007
9236 J. Phys. Chem. C, Vol. 111, No. 26, 2007 TCDB architectures has been identified. The phase transition of the VOPc/TCDB complex architectures was examined by STM under ambient conditions, and the transition mechanism is proposed based on density functional theory (DFT) calculations.
Kong et al. SCHEME 1: The “Ball-and-Stick” Models of VOPc (a) (top view), (b) VOPc (side view), and TCDB (c)
Experimental Section TCDB was synthesized by a method similar to that of 1,3,5tris(carboxymethoxy) benzene.37 Ethyl 11-bromo undecanoate and phloroglucinol were in turn added to the stirred mixture of anhydrous potassium carbonate in acetone and formed 1,3,5tris(10-ethoxycarbonyldecyloxy) benzene with the catalysis of sodium iodide anhydrous and tetra-n-butyl-ammonium bromide. Subsequently, TCDB was separated when 1,3,5-tris(10-ethoxycarbonyldecyloxy) benzene was added to a mixture of ethanol and water at pH ) 7. VOPc was purchased from Alfa and used as received. The guest species were introduced through a coadsorption approach and were observed using STM under ambient conditions. The solvent used in the STM experiments was toluene (HPLC grade, Acros). The concentrations of all the solutions used were less than 1 mM. The molar ratio of TCDB to VOPc was approximately 2:1. The samples were prepared by depositing a droplet of the solution containing the host and the guest compounds on the freshly cleaved HOPG surfaces. After the solvent was completely evaporated, the STM measurements were performed with a Nanoscope IIIA system (Veeco Metrology, USA). The tips were mechanically formed Pt/Ir wires (80/ 20). The prepared adlayers were heated at predetermined temperatures step-by-step and subsequently were examined by STM at room temperature. All the STM images were recorded using constant current mode. The specific tunneling conditions are given in the figure captions. We performed theoretical calculations using density functional theory (DFT) provided by the DMol3 code.38 The Perdew and Wang parametrization39 of the local exchange-correlation energy is applied in the local spin density approximation (LSDA) to describe the exchange and correlation. We expand the allelectron spin-unrestricted Kohn-Sham wave functions in a local atomic orbital basis. In such, a double-numerical basis set polarization is described. All calculations are all-electron ones and are performed with the extra-fine mesh. A self-consistent field procedure is done with a convergence criterion of 10-5 a.u. on the energy and electron density. Results and Discussion VOPc, as a member of the phthalocyanine family, has potential applications relating to nonlinear optical and electronic characteristics.40-42 Scheme 1 shows the “ball-and-stick” models of the guest molecule of VOPc (parts a and b of Scheme 1) and the host molecule of TCDB (Scheme 1c) herein investigated. Our previous investigations demonstrated that copper phthalocyanine molecules could be immobilized in the TCDB cavities and form monomer-entrapped CuPc/TCDB (I) host-guest architectures.36 In this study, not only monomer-entrapped but also dimer-entrapped architectures could be observed in the TCDB/VOPc binary assembling systems. When VOPc and TCDB molecules were co-deposited on HOPG surfaces at room temperature (about 26 °C), architecture VOPc/TCDB (II) with entrapped VOPc dimers was the predominant characteristic, as shown in Figure 1a. Large-area and high-quality STM images could be readily obtained. The molecules formed a lamellar structure with aligned bright spots, marked as region II. Except for the regions occupied with
entrapped VOPc dimers, some TCDB molecules were selfassembled into regions with only individual VOPcs entrapped sporadically, shown as region A in the inserted image in Figure 1a. Further investigations demonstrated that thermal annealing of the as-prepared adlayers could induce phase transitions of the complex architectures from VOPc/TCDB (II) to VOPc/ TCDB (I). Parts b-f of Figure 1 display the annealing effect observed with STM. The packing arrangements had no observable changes when the annealing temperature was below 40 °C. The compact lamellae with VOPc dimers in one cavity were the predominant characteristics, as shown in Figure 1a. Annealing at temperatures of 40, 60, 70, and 80 °C revealed gradual disappearance of the architecture with VOPc dimers and the simultaneously increase of the assemblies of sporadically entrapped individual VOPcs, as shown in parts b-e of Figure 1. More and more lamellae of VOPc/TCDB (II) were gradually destroyed, and the entrapped VOPc molecules were eventually lost upon thermal annealing, as shown by the black arrows in parts b-d of Figure 1. During the process of transition from occupancy of VOPc dimers to individual VOPc molecules, the VOPc molecules ejected from the cavities under thermal activation became unobservable due to their high mobility at the molecular interface, as shown by region B in Figure 1e. Simultaneously, more VOPc molecules were sporadically entrapped in the pure TCDB regions than the original sample, as shown in region A in Figure 1e. After annealing for 15 min at 90 °C, large scale regions with VOPc monomers were formed stably while no observable lamellae were seen with VOPc dimers, marked as I in Figure 1f. A nearly quadrate lattice was formed with bright spots representing VOPcs. The structural transformation from type II to I was irreversible by cooling down the annealed adlayers at high temperatures, which indicated that type I architectures is the thermodynamically stable state. Parts a and b of Figure 2 display the typical high-resolution STM images of the two types of the binary complex architectures. On the basis of the adlayer symmetry and intermolecular distances, a unit cell is superimposed in parts a and b of Figure 2, respectively. The white squares and the white circles drawn in parts a and b of Figure 2 represent VOPc and the benzene cores of TCDB, respectively. For architecture VOPc/TCDB (II), the unit cell parameters are a ) 5.0 ( 0.1 nm and b ) 2.0 ( 0.1 nm, respectively. The value of a is much larger than that of the original unfilled TCDB unit cell (3.9 ( 0.2 nm) (see Table 1). That is to say, the host cavities of TCDB expand significantly when the guest molecules VOPc are enclosed within them; while
Effect of Thermal Annealing on H-Bond Configurations
J. Phys. Chem. C, Vol. 111, No. 26, 2007 9237
Figure 1. (a) Typical large-scale STM image of VOPc/TCDB SAMs on HOPG at room temperature (26 °C). Tunneling conditions are 685 mV and 419 pA. The scan size of the inset is 30 nm × 30 nm. (b-f) The typical images revealing the phase transition process by thermal annealing on the SAMs as shown in (a). The black arrows in parts b-d point to the lamellae where VOPc/TCDB (II) architectures were destroyed: (b) after heating for 1h at 40 °C, (c) after heating for 15 min at 60 °C, (d) after heating for 15 min at 70 °C, (e) after heating for 15 min at 80 °C, and (f) after annealing for 15 min at 90 °C. Tunneling conditions are as follows: (b) 671 mV, 411 pA; (c) 658 mV, 403 pA; (d) 764 mV, 352 pA; (e) 674 mV, 417 pA; (f) 736 mV, 459 pA. (Parts a-f represent the typical images of the adlayers at specified temperatures, while they do not correspond to the same area.)
Figure 2. Typical high-resolution STM images of the two types of complex architectures. Tunneling conditions are as follows: (a) 634 mV, 384 pA; (b) 662 mV, 518 pA. (a) Dimer-entrapped architecture VOPc/TCDB (II). Unit cell parameters: a ) 5.0 ( 0.1 nm, b ) 2.0 ( 0.1 nm, and R ) 80 ( 1.0°. (b) Monomer-entrapped architecture VOPc/ TCDB (I). Unit cell parameters: a ) 3.0 ( 0.1 nm, b ) 2.3 ( 0.1 nm, and R ) 82 ( 1.0°. (c) and (d) Schematics depicting the suggested molecular models of VOPc/TCDB (II) and VOPc/ TCDB (I), respectively. Unit cells are superimposed in Figure 2 with black quadrangles. The white squares and the white circles drawn in parts a and b represent VOPc and the benzene cores of TCDB molecules, respectively. The black ellipses drawn in parts c and d point out the hydrogen bonds in TCDB dimers.
for monomer-entrapped type VOPc/TCDB (I), the unit cell parameters are a ) 3.0 ( 0.1 nm and b ) 2.3 ( 0.1 nm, respectively. Herein, the host cavity dimensions of TCDB are reduced greatly when individual VOPc guest molecules are entrapped in them. The dimer-entrapped architecture VOPc/ TCDB (II) is formed by VOPc and TCDB molecules with a ratio of 2:2, while the architecture VOPc/TCDB (I) is composed
of VOPc monomers and TCDB host molecules with a ratio of 1:2. The difference in ratios is due to the phase transition upon thermal annealing but not the compositional change before and after annealing. The overall molar ratio of VOPc and TCDB was still approximately 1:2 on the surfaces. We have performed DFT calculations to analyze the observed different complex structures. The proposed models are shown in parts c and d of Figure 2 for the two types of architectures. The computational parameters of the unit cell agree well with the experimental values, which are listed in Table 1. Theoretical results indicate that every two TCDB molecules interact with each other through H-O‚‚‚H hydrogen bonds to form a cavity and allow VOPc monomers or dimers to be entrapped in the cavity. The distinct difference is that for the monomer-entrapped structure the TCDB dimer could form double pairs of hydrogen bonds in a unit cell (as pointed out by the two black ellipses in Figure 2d), resulting in strong interactions for the host lattice (about 48.4 kcal/mol, much higher than that of an unfilled TCDB dimer) and relatively small size. While for the VOPc dimerentrapped structure, the TCDB dimer could form only a single pair of hydrogen bonds in a unit cell (as pointed out by the black ellipse in Figure 2c) and results in relatively weak interaction (about 18.2 kcal/mol) with expanded size. Furthermore, our theoretical results show that the monomer-entrapped structure is more stable (with binding energy 130.4 kcal/mol for a unit cell) than the dimer-entrapped structure (with binding energy 96.5 kcal/mol), which reveals that monomer-entrapped type VOPc/TCDB (I) is the thermodynamically stable state. This conclusion agrees well with the experimental results. On the basis of the theoretical results, it could be proposed that the thermal stability of the hydrogen-bonded TCDB dimers is the key factor in the formation of different complex architectures. In the binary assemblies, it is presumably easier for the TCDB molecules to form single-pair hydrogen-bonded dimers than to form double-pair hydrogen-bonded dimers because of the collision probability of host molecules. The above calculations clearly suggest that TCDB dimers with double pairs of hydrogen bonds should be more stable than those with a single pair of hydrogen bonds at elevated temperatures. Such a
9238 J. Phys. Chem. C, Vol. 111, No. 26, 2007
Kong et al.
TABLE 1: Measured and Calculated Unit Cell Parameters, the Calculated Hydrogen Bonding Interaction and Binding Energies of TCDB Networks, VOPc/ TCDB (II) Architecture, and VOPc/TCDB (I) Architecture hydrogen-bonding per TCDB dimer
unit cell parameters
TCDB VOPc/TCDB (II) VOPc/TCDB (I)
a/nm
measured b/nm
R/deg
a/nm
3.9 ( 0.2 5.0 ( 0.1 3.0 ( 0.1
2.2 ( 0.1 2.0 ( 0.1 2.3 ( 0.1
73 ( 2.0 80 ( 1.0 82 ( 1.0
3.80 5.00 3.15
SCHEME 2: Schematic Illustration of the Phase Transition Process from Dimer-entrapped VOPc/ TCDB (II) to Monomer-entrapped VOPc/TCDB (I) Host-Guest Architectures by Thermal Annealinga
calculated b/nm R/deg 2.25 2.10 2.38
73.0 81.0 83.0
number
interaction (kcal/mol)
binding energy per unit cell (kcal/mol)
double-pair single-pair double-pair
25.3 18.2 48.4
71.1 96.5 130.4
understanding of the thermodynamics of the 2D assembled molecular nanostructures and further controllable construction of functional thin films. Conclusion
a Annealing temperatures: (1) 26 °C, (2) 40-70 °C, (3) 80 °C, and (4) 90-98 °C. The green squares represent VOPc molecules, and the pink ellipses represent TCDB cavities. The alkyl chains of TCDB are omitted for simplification.
difference in thermal stability could be further enhanced by guest molecule inclusions. Once single-pair hydrogen-bonded TCDB dimers are formed, they could be disrupted or even broken by the thermal motions of entrapped molecules. When preparation temperature is low (for example 26 °C), the thermal motions of entrapped molecules are relatively weak and the single-pair hydrogen-bonded TCDB dimers can be sustained. Thus, it could be observed that the architecture with entrapped VOPc dimers was formed by these single-pair hydrogen-bonded TCDB dimers and VOPc dimers. When the temperature increases, the strength of thermal motions of the entrapped molecules also increases. As a result, single-pair hydrogen-bonded TCDB dimers could become less stable until all were broken by the stronger thermal motions of molecules. On the other hand, the double-pair hydrogen-bonded TCDB dimers could sustain and only monomerentrapped architecture could be observed. Scheme 2 schematically illustrates the phase transition process from dimer-entrapped to monomer-entrapped VOPc/TCDB architectures. The thermal motions of the entrapped guest molecules (green) become pronounced and eventually break the single-pair hydrogen-bonded TCDB dimers when thermally annealed from 40 to 80 °C, resulting in the diffusing of the entrapped guests from the original host cavities (pink) to the nearby hollow cavities or escaping from the host cavities and becoming unobservable due to their high mobility. At sufficiently high annealing temperature, the formation of monomerentrapped complex architecture is mainly due to the formation of double-pair hydrogen-bonded TCDB dimers. The intermolecular interactions, especially van der Waals forces between VOPc and TCDB molecules, modified the complex architecture and led to different molecular packing arrangements with high stability of the complex architectures. This observation manifests that the architecture is the result of a delicate balance mainly between the hydrogen bonds of TCDB dimers and the intermolecular interactions especially van der Waals forces. The above analyses reveal that the involved interactions leading to host lattice formation are sensitive to thermal conditions and the resulted host lattice could be self-adjusted under the influence of thermal annealing and guest molecule inclusion. These investigations would be beneficial for better
Dimer-entrapped architectures were apt to form when the VOPc/TCDB host-guest architectures were prepared at room temperatures. The thermal-annealing process rendered the packing arrangements to be tuned from a dimer-entrapped architecture to a monomer-entrapped one. DFT calculations show that the interaction of hydrogen bonding in the monomerentrapped architecture is much stronger than that in the dimerentrapped architecture. On the basis of the computational results, we suggest that during the temperature range of annealing (from 30 to 90 °C), the observed phase transition from dimer-entrapped structure to monomer-entrapped structure is associated with the competition between the thermal motions of the molecules and the stability of hydrogen bonding in TCDB dimers. The experimental results combined with the theoretical calculations revealed that monomer-entrapped VOPc/TCDB (I) is thermodynamically stable for the VOPc/TCDB architectures. It sheds light on the investigation of interfacial thermal stability of molecular architectures with guest inclusion properties, which might be constructive for studying functional molecular devices in general. Acknowledgment. Financial support from the National Natural Science Foundation of China (Grant Nos. 90406019, 20473097, 20573116, and 90406024) and Chinese Academy of Sciences are gratefully acknowledged. The authors also wish to thank the reviewers for constructive comments and providing updated references on thermal effects on molecular architectures. References and Notes (1) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424. (2) Fang, H. B.; Giancarlo, L. C.; Flynn, G. W. J. Phys. Chem. B 1999, 103, 5712. (3) Gleiche, M.; Chi, L. F.; Fuchs, H. Nature 2000, 403, 173. (4) Barth, J. V.; Weckesser, J.; Cai, C. Z.; Gu¨nter, P.; Bu¨rgi, L.; Jeandupeux, O.; Kern, K. Angew. Chem., Int. Ed. 2000, 39, 1230. (5) He, Y. F.; Ye, T.; Borguet, E. J. Am. Chem. Soc. 2002, 124, 11964. (6) De Feyter, S.; De Schryver, F. C. Chem. Soc. ReV. 2003, 32, 139. (7) Pan, G. B.; Liu, J. M.; Zhang, H. M.; Wan, L. J.; Zheng, Q. Y.; Bai, C. L. Angew. Chem., Int. Ed. 2003, 42, 2747. (8) Yoshimoto, S.; Tsutsumi, E.; Honda, Y.; Murata, Y.; Murata, M.; Komatsu, K.; Ito, O.; Itaya, K. Angew. Chem., Int. Ed. 2004, 43, 3044. (9) Lei, S. B.; Wang, C.; Wan, L. J.; Bai, C. L. J. Phys. Chem. B 2004, 108, 1173. (10) Yoshimoto, S.; Higa, N.; Itaya, K. J. Am. Chem. Soc. 2004, 126, 8540. (11) Qiu, X. H.; Wang, C.; Zeng, Q. D.; Xu, B.; Yin, S. X.; Wang, H. N.; Xu, S. D.; Bai, C. L. J. Am. Chem. Soc. 2000, 122, 5550. (12) Gong, J. R.; Wan, L. J.; Yuan, Q. H.; Bai, C. L.; Jude, H.; Stang, P. J. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 971. (13) Xu, L. P.; Yan, C. J.; Wan, L. J.; Jiang, S. G.; Liu, M. H. J. Phys. Chem. B 2005, 109, 14773. (14) Yang, Y. L.; Chan, Q. L.; Ma, X. J.; Deng, K.; Shen, Y. T.; Feng, X. Z.; Wang, C. Angew. Chem., Int. Ed. 2006, 45, 6889. (15) Alemani, M.; Peters, M. V.; Hecht, S.; Rieder, K.-H.; Moresco, F.; Grill, L. J. Am. Chem. Soc. 2006, 128, 14446.
Effect of Thermal Annealing on H-Bond Configurations (16) Ye, T.; He, Y. F.; Borguet, E. J. Phys. Chem. B 2006, 110, 6141. (17) Yoshimoto, S.; Yokoo, N.; Fukuda, T.; Kobayashi, N.; Itaya, K. Chem. Commun. 2006, 500. (18) Yoshida, S.; Itoh, M.; Yamamoto, N.; Nagamura, T.; Oyama, M.; Okazaki, S. Langmuir 1999, 15, 6813. (19) Bondos, J. C.; Drummer, N. E.; Gewirth, A. A.; Nuzzo, R. G. J. Am. Chem. Soc. 1999, 121, 2498. (20) Li, C. J.; Zeng, Q. D.; Liu, Y. H.; Wan, L. J.; Wang, C.; Wang, C. R.; Bai, C. L. ChemPhysChem 2003, 4, 857. (21) Konishi, Y.; Yoshida, S.; Sainoo, Y.; Takeuchi, O.; Shigekawa, H. Phys. ReV. B 2004, 70, 165302. (22) Perdiga˜o, L. M. A.; Perkins, E. W.; Ma, J.; Staniec, P. A.; Rogers, B. L.; Champness, N. R.; Beton, P. H. J. Phys. Chem. B 2006, 110, 12539. (23) Lee, J.; Dougherty, D. B.; Yates, J. T. J. Am. Chem. Soc. 2006, 128, 6008. (24) Ruben, M.; Payer, D.; Landa, A.; Comisso, A.; Gattinoni, C.; Lin, N.; Collin, J.-P.; Sauvage, J.-P.; Vita, A. D.; Kern, K. J. Am. Chem. Soc. 2006, 128, 15644. (25) Rohde, D.; Yan, C. J.; Yan, H. J.; Wan, L. J. Angew. Chem., Int. Ed. 2006, 45, 3996. (26) Stepanow, S.; Lingenfelder, M.; Dmitriev, A.; Spillmann, H.; Delvigne, E.; Lin, N.; Deng, X. B.; Cai, C. Z.; Barth, J. V.; Kern, K. Nat. Mater. 2004, 3, 229. (27) Barth, J. V.; Costantini, G.; Kern, K. Nature 2005, 437, 671. (28) Gong, J. R.; Yan, H. J.; Yuan, Q. H.; Xu, L. P.; Bo, Z. S.; Wan, L. J. J. Am. Chem. Soc. 2006, 128, 12384.
J. Phys. Chem. C, Vol. 111, No. 26, 2007 9239 (29) Wu, D. X.; Deng, K.; Zeng, Q. D.; Wang, C. J. Phys. Chem. B 2005, 109, 22296. (30) Spillmann, H.; Kiebele, A.; Sto¨hr, M.; Jung, T. A.; Bonifazi, D.; Cheng, F. Y.; Diederich, F. AdV. Mater. 2006, 18, 275. (31) Pan, G. B.; Cheng, X. H.; Ho¨ger, S.; Freyland, W. J. Am. Chem. Soc. 2006, 128, 4218. (32) Theobald, J. A.; Oxtoby, N. S.; Phillips, M. A.; Champness, N. R.; Beton, P. H. Nature 2003, 424, 1029. (33) Yoshimoto, S.; Suto, K.; Tada, A.; Kobayashi, N.; Itaya, K. J. Am. Chem. Soc. 2004, 126, 8020. (34) Mena-Osteritz, E.; Ba¨uerle, P. AdV. Mater. 2006, 18, 447. (35) Griessl, S. J. H.; Lackinger, M.; Jamitzky, F.; Markert, T.; Hietschold, M.; Heckl, W. M. Langmuir 2004, 20, 9403. (36) Lu, J.; Lei, S B.; Zeng, Q. D.; Kang, S. Z.; Wang, C.; Wan, L. J.; Bai, C. L. J. Phys. Chem. B 2004, 108, 5161. (37) Lu, J.; Zeng, Q. D.; Wang, C.; Zheng, Q. Y.; Wan, L. J.; Bai, C. L. J. Mater. Chem. 2002, 12, 2856. (38) Becke, A. D. J. Chem. Phys. 1988, 88, 2547. (39) Perdew, J. P.; Wang, Y. Phys. ReV. B 1992, 45, 13244. (40) Fang, S. L.; Hoshi, H.; Kohama, K.; Maruyama, Y. J. Phys. Chem. 1996, 100, 4104. (41) Blochwitz, J.; Pfeiffer, M.; Fritz, T.; Leo, K. Appl. Phys. Lett. 1998, 73, 729. (42) Hipps, K. W.; Barlow, D. E.; Mazur, U. J. Phys. Chem. B 2000, 104, 2444.
