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Importance of Coupling Pattern and Chemical Decoration for Graphene Nanotransistors J. C. Dong,† H. Li,*,† F. W. Sun,‡ and Y. F. Li† †

Key Laboratory for Liquid−Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Jinan 250061, People’s Republic of China ‡ Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands S Supporting Information *

ABSTRACT: The effects of graphene−electrode coupling pattern and chemical decoration of graphene on the electron transport properties of nanoscale graphene field effect transistors (FETs) are systematically investigated. Different from the viewpoint that molecules with thiol ending groups usually connect to Au electrodes through thiolate−gold bonds in molecular devices, the calculated electron transport properties of nanoscale graphene FETs at thiol−gold coupling mode are observed to be well consistent with experimental results. At the thiolate−gold coupling mode nanoscale graphene FETs exhibit pronounced bipolar FET characteristics with on− off ratios up to 320 (n type) and 650 (p type), which are much higher than those of large area graphene FETs. We propose that different coupling patterns between molecules and electrodes are essential factors responsible for the discrepancy between theoretical calculations and experimental studies. Moreover, the performance of nanoscale graphene FETs is observed to be effectively modulated by chemical decoration of graphene. In particular, their on−off ratios can be significantly increased by spacing groups between ending groups and graphene cores. Side substituents of graphene can regulate the performance of these FETs according to their electron-withdrawing ability. Potential implications for the design of high-performance nanoscale graphene FETs can be obtained from these results.

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manufacture n type nanoscale graphene FETs.16 In their study, the scanning tunneling microscope break junction method is used to measure the performance of the nanoscale graphene FETs which are composed of large graphene molecules acting as electron transfer channels and ionic liquids acting as gate electrodes. An on−off ratio as high as 50 is achieved when the graphene molecules are terminated by thiol groups, which is much higher than those of most large area graphene FETs fabricated through top-down approaches. There are several advantages of bottom-up approach over top-down method. It can not only open a sizable band gap for graphene but also offer a precise control of its defect, shape, and size. Moreover, this kind of approach can conveniently reduce the size of electronics to molecular scale, which is considered as the ultimate goal of miniaturization in microelectronics. The study on electron transport properties of graphene devices is an important practical issue in physicochemical and material science. As mentioned above, much research work has been done for regulating the graphene structures and properties in graphene FETs. However, little attention has been paid to the study on coupling patterns between graphene and electrodes, which are critical factors in determining their

INTRODUCTION The successful preparation of graphene and the discovery of its field effect property have led to an explosion of studies on its electronic properties.1−4 In recent years, graphene field effect transistors (FETs) have attracted tremendous interest from both scientists and engineers, and remarkable achievements have been made due to the unique electronic properties of graphene.5−11 All these graphene FETs are observed to exhibit extremely high carrier mobilities that exceed those of conventional Si-based FETs. However, the on−off ratios of these devices are considerably small due to the zero band gap of graphene, making graphene FETs difficult to be switched off. To obtain switching and power gain effects required in logics, methods have been developed to open a band gap for graphene, such as the use of graphene nanoribbons, deforming graphene, and applying electric field to bilayer graphene.12−15 These approaches indeed increase the on−off ratios of graphene FETs to a certain extent, but they also have some intrinsic drawbacks and limitations that cannot be neglected. The unpredictable defects and uncontrollable disordered edges in graphene nanoribbons usually deteriorate their performance. The dimension and shape of these graphenes are difficult to control, resulting in performance instability. Therefore, it is of great importance to develop new techniques to obtain reliable graphene FETs. Worthy of mention is that Zang et al. successfully implement a bottom-up fabrication method to © 2012 American Chemical Society

Received: January 17, 2012 Revised: February 21, 2012 Published: February 27, 2012 6762

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Figure 1. Structure of nanoscale graphene FETs: (a) schematic of a nanoscale graphene FET with thiolate−gold junctions (the dielectric constant and the thickness of dielectric layers are 3.9 ε0 and 3 Å), (b) structures of graphene channels, (c) stable structure of the benzene−graphene FET at the thiol−gold coupling mode, and (d) stable structure of the benzene−graphene FET at the thiolate−gold coupling mode.

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COMPUTATIONAL METHOD All electronic properties of the nanoscale graphene FETs are calculated based on the Nonequilibrium Green Function method in combination with quantum theories, including both the EHT and the DFT. Figure 1a shows a schematic of the nanoscale graphene FETs. A large graphene molecule is chemically bridged between two semi-infinite Au(111) electrodes as the electron transfer channel. Four gate electrodes are employed to four-square surround the channel ever used by Zang et al. The gates are insulated from the channel by dielectric layers. To study the effects of chemical decoration of graphene channel, a series of large graphene molecules are adopted as shown in Figure 1b. They are composed of a graphene core consisting of 13 aromatic rings, two thiol ending groups, and different side substituents. For simplicity, these graphene channels are distinctly labeled by their side substituents and spacing groups. Two different graphene− electrode coupling modes are used to study the effects of coupling patterns on the electron transport properties of these FETs, namely, thiol−gold coupling mode and thiolate−gold coupling mode. As for the thiol−gold coupling mode, these FETs are built based on two different possible coupling geometries. The first one is that the sulfur atoms of thiol groups are located above an Au atom of the electrodes with a vertical distance of 2.39 Å equal to the Au−S bond length. The second one is that the sulfur atoms are situated on the 3-fold hollow sites of the electrodes with a vertical distance of 1.9 Å to ensure the Au−S bond length. In terms of thiolate−gold coupling mode, the FET structure is constructed by situating the sulfur atoms at the 3-fold hollow sites of the electrodes with Au−S bond length of 2.39 Å, in which the passivating hydrogen atoms of the sulfur atoms are removed. To obtain corresponding stable structures, these FETs are fully relaxed. The relaxation is carried out using the DFT method based on local numerical basis orbitals and norm-conserving pseudopotentials using the softpackage Atomistix ToolKit.30 The convergent standard for total energy is 4 × 10−5. The force tolerance on the atoms of graphene and electrodes is 0.05 eV/ Å. The mesh cutoff for the electrostatic potentials is 75 Ha. A 3 × 3 × 10 Monkhorst sampling in the Brillouin zone is utilized for the system. Double-zeta single polarized basis sets of local numerical orbitals and generalized gradient approximations

electron transport properties, especially when the dimensions are diminished to nanoscale.17 Until now, research reports relating to this subject are mainly focused on two terminal molecular devices with couplings between molecules with thiol ending groups and gold electrodes. It is usually acknowledged that molecules with thiol ending groups connect to Au(111) electrodes via a thiolate−gold bond.18−25 However, the calculated conductance of these two terminal devices is in great disagreement with the measured ones.22−25 The reason for this discrepancy still remains ambiguous to date. Some researchers ascribe it to the underestimation of the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) induced by Density Functional Theory (DFT) methods.26,27 However, this kind of explanation is not comprehensive because the calculated conductance of metallic wires at atomic scale based on the DFT method corresponds well to experimental results.28 Others deem that this disagreement results from different electrode configurations between experiments and calculations,21 which may not be a prototypical explanation due to the unstable electrode configuration used in their calculations. Actually, according to the Andreoni's study which shows that the binding energy of a thiol−gold bond is comparable to that of a thiolate−gold bond,29 we would rather believe that this discrepancy is induced by different molecule−electrode coupling patterns in experimental studies and theoretical models. In this paper, nanoscale graphene FETs are constructed using large graphene molecules with thiol ending groups and Au(111) electrodes. The effects of junction patterns between graphene and electrodes on the electron transport properties of these FETs are systematically investigated based on the Nonequilibrium Green Function method combined with the extended Hückel theory (EHT) and the DFT. Moreover, the role of chemical decoration of graphene in modulating the performance of these FETs is also studied. This work can not only help design high-performance nanoscale graphene FETs but also provide a deep insight into the electron transport properties of these FETs. 6763

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(GGA) for exchange correlation potentials are employed. Figure 1c shows the relaxed benzene−graphene FET at the thiol−gold coupling mode. The sulfur atoms locate at the 3-fold hollow site of the electrodes, and the vertical distance between the sulfur atom and the electrode is about 2.3 Å. Besides, the passivating hydrogen atoms are not detached from the sulfur atoms, and there is little change in the configuration of electrodes during relaxation. At the thiolate−gold coupling mode, the benzene−graphene FET structure changes little after relaxation (shown in Figure 1d). The different stable structures of the two coupling modes indicate that nanoscale graphene FETs with different coupling patterns may exhibit different electron transport behaviors. To get a balance between accuracy and efficiency, electron transport properties of the stable nanoscale graphene FETs at both coupling modes are calculated using an EHT model,31 where the electrostatic potential is handled self-consistently. This model divides the FET device into two individual parts: a properly defined channel part and two electrodes.32 A Hamiltonian matrix H and an overlap matrix S are used to describe the channel part. The Hamiltonian matrix H is decomposed into two parts: the core Hamiltonian matrix H0 and the self-consistent potential Usc which is a function of the density matrix ρ. The self-consistent potential Usc is used to deal with the charging and screening effects induced by the external bias Vd, as has been discussed in detail.31 The energy levels of the channel are broadened due to the hybridization with the delocalized wave functions of the electrodes. Thus, self-energy matrices ∑1,2 are used to describe this effect with + 33 the broadening matrices Γ1,2 defined as Γ1,2 = i(∑1,2 − ∑1,2 ). On the basis of the above-mentioned parameters and calculation conditions, the self-consistent calculation of electron transport properties is carried out as follows: first, an initial density matrix is used to calculate the self-consistent potential Usc, and the EHT is used to obtain the H0. The Hamiltonian H is calculated by summing up Usc and H0. Second, the density matrix is calculated through the Nonequilibrium Green Function according to eqs 1 and 2. The electrode electrochemical potentials μ1,2 are defined as μ1,2 = Ef ± 0.5Vd, where Ef is the Fermi level of electrodes. This self-consistent calculation continues until the tolerance of density matrix reaches 10−4. The electron temperature is set to be 298 K. The transmission coefficient T is calculated using eq 3, and the corresponding current Id is manifested by the Landauer− Büttiker equation (eq 4). G(E) = (ES − H − Σ1 − Σ 2)−1 ρ=

1 2π

Figure 2. Transistor characteristics and transmission spectra of the benzene−graphene FET at the thiol−gold coupling mode: (a) and (b) are transfer and output curves, and the red line in (a) is cited from the red line of Figure 3c in ref 16; (c) transmission spectra at different Vg and Vd. The Roman numerals I, II, III, IV, and V denote the resonant peaks. The dashed lines represent the electrochemical potentials (μ1,2) of electrodes.

transistor character. Compared with the transfer curve of a similar graphene FET measured by Zang et al. (red line in Figure 2a),16 the calculated transfer curve shows good consistency. Both of the curves show a two-step increase with the decrease of Vg. In addition, the two FETs have very similar conductance. It should be pointed out that the small discrepancy between the two curves is owing to the different side substituents of graphene channels in the two FETs. To manifest the origin of this n type transistor behavior, transmission spectra of the benzene−graphene FET at different Vd and Vg are illustrated in Figure 2c. Distinct resonance peaks induced by electron transmission through different molecular orbitals of the graphene channel are observed. The equilibrium transmission curve features three neighbor peaks and two separate ones (labeled by Roman numerals) below and above Ef. When an external bias Vd = 0.5 V is applied, the transmission spectrum shifts toward left slightly, and the V resonance peak is split. Because negative gate voltage can decrease the energy levels of the graphene channel, the applied negative gate voltage Vg moves the transmission spectrum toward lower energies. At Vg = −0.5 V, the IV resonance peak is moved at the edge of the effective energy window (μ1−μ2), resulting in a near-resonance tunneling and an obvious increase of the current. When Vg continues to decrease, the molecular energy level of the graphene channel corresponding to the IV resonance peak is shifted under μ2, which leads the graphene channel to be

(1)

∞

∫−∞ dE(f1 GΓ1G+ + f2 GΓ2G+)

(2)

where f1 and f 2 are Fermi functions. T (E , V ) = tr(Γ1G Γ2G+) I=

2e h

(3)

∞

∫−∞ dE(T(E , V )(f1 (E) − f2 (E)))

(4)

where h is the Planck constant and e is the electron charge.

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RESULTS AND DISCUSSION Figure 2 demonstrates the electron transport characteristics of the benzene−graphene FET at the thiol−gold coupling mode. The transfer and output curves shown in Figure 2a and b reveal that the benzene−graphene FET exhibits a pronounced n type 6764

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Figure 3. Eigenstates of the MPSH onto the orbitals of benzene−graphene at the thiol−gold coupling mode. The index of eigenstates counts from 0, and the reference energy is Ef. The isovalue is 0.05.

energies relative to the Ef, the resonances I, II, III, IV, and V shown in Figure 2c are mainly induced by the delocalized orbitals 122, 123, 126, 127, and 130−131, respectively. Interestingly, the transmission resonance V is actually a superposition of two resonances dominated by the orbitals 130 and 131, which can be demonstrated from its splitting shown in Figure 2c and the close energy of the orbitals 130 and 131. Although all these relevant orbitals display π characters, the orbitals 126 and 127 are strongly delocalized throughout the graphene channel, which is not the same as the orbitals 122, 123, 130, and 131 that are mainly distributed at the two ends of the channel. Furthermore, all these orbitals have weight on the sulfur atoms with a p character rather than on the hydrogen atoms of thiol groups, indicating that the graphene channel couples with Au electrodes through S−Au σ bonds. The electron transport properties of the benzene−graphene FET at the thiolate−gold coupling mode are demonstrated in Figure 4. A prominent asymmetric bipolar FET character with small hysteresis can be observed from Figure 4a and b. The on−off ratios for the n and p type FET characters are 320 and 650, respectively, which are at least 1 order of magnitude higher than that at the thiol−gold coupling mode and other results of previous studies.6−15 The subthreshold swings S are −0.2 and 0.14 V/decade, respectively, where S is defined as S = (d log Id/ dVg)−1.36 Compared to the thiol−gold coupling mode, the benzene−graphene FET at the thiolate−gold coupling mode exhibits an outstanding switching performance and stable electron transport character, indicating its potential applications in nanoelectronics. The distinct electron transport behaviors at the two coupling modes suggest that the disagreement between theoretical calculations and experimental studies is due to different interface coupling patterns.

charged. Simultaneously, this charging effect in turn increases the energy levels of the graphene channel and thus makes the position of the IV resonance peak change little. As a result, the transmission resonance IV in the effective energy window does not increase but decreases when Vg changes from −0.5 to −0.6 V, which leads to a decrease of the current. It is worth noting that an undesired negative differential resistance (NDR) appears at Vg = −0.7 V as shown in Figure 2b. Further analysis indicates that when Vd ranges from 0.3 to 0.5 V at Vg = −0.7 V the current is induced by the near-resonance tunneling dominated by the LUMO of the graphene channel. This near-resonance tunneling is suppressed when Vd changes from 0.4 to 0.5 V, resulting in the NDR effect. Thereby, we suggest that nanoscale graphene FETs at the thiol−gold coupling mode should not be ideal, because little change in Vd can lead to arbitrary variations of on−off ratios. To demonstrate the molecular orbitals of the graphene channel that are responsible for the transmission resonances shown in Figure 2c, the equilibrium molecular projected selfconsistent Hamiltonian (MPSH) onto the graphene channel is calculated based on the DFT.30,34 Figure 3 shows the eigenstates and energies of the MPSH. As the isolated graphene channel has 254 valence electrons, its HOMO and LUMO are the orbital 126 and 127, respectively. Because Ef is just between the HOMO and LUMO of the graphene channel, only fractional amounts of charge transfer between the graphene and electrodes at equilibrium,35 implying that the transmission resonances below and above the Ef arise from the occupied and unoccupied molecular orbitals of the graphene channel, respectively. As clearly demonstrated in Figure 3, the MPSH orbitals 124−125 and 128−129 are strongly localized at the graphene core of the channel, implying that these orbitals contribute little to electron transmission. According to the 6765

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benzene−graphene FET at the thiolate−gold coupling mode. The transmission spectrum at Vg = Vd = 0 V characterizes two resonance peaks located below and above the Ef, respectively. Notably, the application of external bias demonstrates that the resonance peaks I and IV are bimodal. The asymmetric bipolar FET character at the thiolate−gold coupling mode is attributed to the fact that positive gate bias elevates the energy levels of the graphene channel and thus moves the resonances I and II into the μ1−μ2 window, while negative gate bias results in the decrease of the molecular energy levels and shifts only one transmission resonance into this window. Hence, it is the asymmetric molecular energy level alignment relative to the Ef and the resonance tunneling effect that lead to the asymmetric bipolar FET character. As shown in Figure 5, the equilibrium eigenstates of the MPSH onto the graphene channel at the thiolate−gold coupling mode are very similar to those at the thiol−gold coupling mode. However, because the two hydrogen atoms of thiol groups at the thiolate−gold coupling mode are removed, the HOMO and LUMO of the graphene channel change to the orbital 125 and 126, respectively. The LUMO is occupied when the graphene channel is connected to electrodes since it is under the Ef. According to the energies relative to the Ef, the delocalized orbitals 122−123 and 130−131 are responsible for the bimodal peaks I and IV, respectively, and the strongly delocalized orbitals 126 and 127 correspond to the resonances II and III. Through comparing the eigenstates of MPSH at both coupling modes, we can find that the coupling strength of thiolate−gold coupling is stronger than that of thiol−gold coupling. A plausible explanation is that the sulfur atoms at the thiol−gold coupling mode are passivated by hydrogen, which can weaken the coupling between sulfur and Au atoms. Moreover, because the hydrogen atoms are located between the sulfur and Au atoms, the local electron density is sharply increased, and hence the sulfur atoms are repulsed from Au electrodes, further abating the coupling strength. This is why the conductance at the thiolate−gold coupling mode is

Figure 4. Transistor characteristics and transmission spectra of the benzene−graphene FET at the thiolate−gold mode: (a) and (b) are transfer and output curves, and (c) transmission spectra at different Vg and Vd.

To shed light on the two different electron transport behaviors, the electron transport mechanism of the benzene− graphene FET at the thiolate−gold coupling mode needs to be clarified. Figure 4c gives the transmission spectra of the

Figure 5. Eigenstates of the MPSH onto the orbitals of the benzene−graphene at the thiolate−gold coupling mode. 6766

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Table 1. As clearly demonstrated in the transfer curves, all FETs exhibit pronounced bipolar characteristics and small hysteresis. The p type gating effect is much stronger than that of n type. The on−off ratios in some FETs are almost as high as 100, which are much larger than those of large area graphene FETs. Compared to the H−graphene FET in on−off ratio at the thiolate−gold coupling mode, the performance of these FETs degrades slightly. However, the performance of these graphene FETs is found to be regulated by these side substituents, suggesting that the electron transport behaviors of nanoscale graphene FETs can be controllably modulated through chemical decoration. It is interesting to note that the change of on−off ratios of these FETs can be reflected by the electronwithdrawing abilities of side substituents. To demonstrate the electron-withdrawing abilities of these side substituents, their average Mulliken populations are calculated and listed in Table 1. From the viewpoint of on−off ratio and subthreshold swing, the performance of graphene FETs with side substituents −NH2, −OH, and −F exhibits a two-step promotion, which is corresponding to the change of the Mulliken populations of these substituents, indicating that strengthening the electronwithdrawing ability of side substituents can improve the on−off ratios of nanoscale graphene FETs. For graphene FETs with side substituents −PH2 and −SH, the on−off ratios decrease first and then increase, which also follows the change of the electron-withdrawing abilities of −SH and −PH2. However, the Cl−graphene FET is an exception.

several orders of magnitude larger than that at the thiol−Au coupling mode. The benzene−graphene contains two aromatic rings between its thiol groups and the graphene core, making it complicated to synthesize. To find out whether it is necessary to reserve these spacing groups, their effect on the performance of the benzene−graphene FET is investigated. Figure 6 shows the

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CONCLUSION

A theoretical study on the effects of the graphene−electrode coupling pattern and chemical decoration of graphene on the electron transport properties of nanoscale graphene FETs is presented. A pronounced n type transistor character dominated by near-resonance tunneling is observed in the electron transport behaviors of the benzene−graphene FET at the thiol−gold coupling mode, which is in good agreement with the measured results of a similar FET. While at thiolate−gold coupling mode, the benzene−graphene FET exhibits an asymmetric bipolar transistor character with on−off ratios as large as 320 (n type) and 650 (p type) dominated by resonance tunneling. The distinct transistor behaviors of the benzene− graphene FET at the two coupling modes indicate that the molecule−electrode coupling pattern is a crucial factor in determining the electron transport properties of molecular devices. These phenomena offer a guideline for future theoretical studies on molecular devices. Moreover, chemical decoration of the graphene channel is found to have a remarkable effect on the performance of nanoscale graphene FETs. Spacing groups between ending groups and the graphene core can significantly increase the on−off ratio of the H− graphene FET through attenuating the off state tunneling effect between source and drain electrodes. Strong electron-withdrawing side substituents are helpful to increase the on−off ratios of nanoscale graphene FETs, indicating that the performance of nanoscale graphene FETs can be controllably modulated. These results provide a crucial simulation input to help guide the design of high-performance graphene FETs with large on− off ratios at nanoscale which are being eagerly pursued. This work is also important for a better understanding of electron transport properties of nanoscale graphene FETs.

Figure 6. Transistor characteristics of the H−graphene FETs: (a) transfer and (c) output curves at the thiol−gold coupling mode (the on−off ratio is 110.5) and (b) transfer and (b) output curves at the thiolate−gold coupling mode (the on−off ratios for n type and p type are 40.05 and 99.10, respectively).

electron transport characteristics of the H−graphene FET, in which the spacing groups are removed. Obviously, the conductance at both coupling modes is larger than those of the benzene−graphene FET. Another remarkable effect is the decrease of on−off ratios, indicating that the spacing groups can improve the switching performance of nanoscale graphene FETs. The transmission spectra shown in the Supporting Information reveal that at the off state electrons need to tunnel from the source electrode to the drain electrode, which means that it is the short channel length of the H−graphene FET that makes the tunneling effect much easier to occur than the benzene−graphene FET. As a result, the off state current of the H-graphene FET is much higher than that of the benzene− graphene FET. Apart from the above changes induced by the removal of spacing groups, the electron transport properties of the H−graphene FET are very similar to those of the benzene− graphene FET. It is well-known that properties of molecules can be modulated by their side substituents. In this study, radicals including −NH2, −PH2, −OH, −SH, −F, and −Cl are employed to substitute the side hydrogen atoms of the H− graphene to explore their effect on the performance of the H− graphene FET. Figure 7 presents the transfer and output curves of these FETs at the thiolate−gold coupling mode. Their on− off ratios and subthreshold swing are calculated and listed in 6767

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Figure 7. Transistor characteristics of graphene FETs decorated by different side substituents at the thiolate−gold coupling mode: (a) transfer and (g) output curves of NH2−graphene FET, (b) transfer and (h) output curves of OH−graphene FET, (c) transfer and (i) output curves of F− graphene FET, (d) transfer and (j) output curves of PH2−graphene FET, (e) transfer and (k) output curves of SH−graphene FET, and (f) transfer and (l) output curves of Cl−graphene FET.

Table 1. On−off Ratios, Subthreshold Swing S of Nanoscale Transistors Based on Different Graphene Channels, and the Corresponding Average Mulliken Population of the Side Substituent of Each Graphene Channela S (V/decade)

on−off ratio graphene channel

n

p

n

p

Mulliken population

NH2−graphene PH2−graphene OH−graphene SH−graphene F−graphene Cl−graphene

4.12 35.24 39.30 35.02 40.74 43.11

16.49 93.03 93.71 55.89 94.90 73.21

−3.30 −0.35 −0.50 −0.60 −0.45 −0.40

1.00 0.30 0.40 0.50 0.35 0.80

6.8415 7.0180 6.9890 7.0150 7.1055 6.9755

a

and 50831003) and the National Basic Research Program of China (2007CB-613901). We also thank the Natural Science Fund for Distinguished Young Scholars of Shandong (JQ200817) for support. This work is also supported by the Natural Science Fund of Shandong Province (ZR2009FM043), by the PhD Dot Programs Foundation of Ministry of Education of China (No. 20090131110025), and by the National Science Fund for Distinguished Young Scholars (No. 2009JQ014) from Shandong University.

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Data in bold account for n type and in italic for p type.

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ASSOCIATED CONTENT

S Supporting Information *

Transmission spectra of the H−graphene FET at the thiol− gold coupling mode and thiolate−gold coupling mode and the eigenstates of the MPSH onto the molecular orbitals of H− graphene at the thiolate−gold coupling mode. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to acknowledge the support from the National Natural Science Foundation of China (Grant Nos. 50971081 6768

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp3005585 | J. Phys. Chem. C 2012, 116, 6762−6769