Noble

ARTICLE pubs.acs.org/JPCC

Density Functional Theoretical Study of Perfluoropentacene/Noble Metal Interfaces with van der Waals Corrections: Adsorption States and Vacuum Level Shifts Kenji Toyoda,*,† Ikutaro Hamada,‡ Kyuho Lee,z Susumu Yanagisawa,§ and Yoshitada Morikawa*,§ †

Advanced Technology Research Laboratories, Panasonic Corporation, Japan WPI-Advanced Institute for Materials Research, Tohoku University, Japan z Department of Physics and Astronomy, Rutgers, The State University of New Jersey, United States § Department of Precision Science and Technology, Graduate School of Engineering, Osaka University, Japan ‡

bS Supporting Information ABSTRACT: We have studied the electronic structures of perfluoropentacene (PFP) on Cu(111), Ag(111), and Au(111) by means of density functional theory with a semiempirical van der Waals method (DFT-D). We show that DFT-D yields accurate equilibrium PFP-metal distances, thereby making an accurate prediction of the work-function change (Δφ) possible. In order to investigate the nature of the interface dipole layer, we calculated Δφ as a function of PFP-substrate distance and found that in contrast to pentacene/metal interfaces, the molecular distortion has a significant influence on Δφ at a short distance. However, by subtracting the contribution of the molecular distortion from the total work-function change, we show that the workfunction change does not depend on the substrate work-function at a long distance, while the work-function change varies linearly with the substrate work-function at a short distance. Our results indicate a transition from Schottky to Bardeen limits as a PFP molecule approaches the substrate metal surface, as in pentacene/metal interfaces.

’ INTRODUCTION Organic devices such as organic light emitting diodes,1,2 organic field effect transistors (OFETs),3,4 and organic photovoltaic cells5,6 have attracted a great deal of interest for their low-cost processing and flexibility.7 Most of the organic molecules in the literature are insulator or p-type semiconductor, whereas n-type organic semiconductor materials, which are required to fabricate complementary circuits, have been rarely investigated. Perfluoropentacene (PFP, C22F14) is one of a few organic materials examined for n-type OFETs.8,9 The interaction between PFP and metal surfaces has thus been studied extensively.10
14 Koch et al.10 studied PFP on Cu(111) surface to estimate the work-function change (Δ) and the adsorption geometry using ultraviolet photoelectron spectroscopy (UPS) and X-ray standing wave (XSW) measurements, respectively. Koch et al.11 also investigated Δ and electronic structure of PFP on Au(111) using UPS. Recently, Duhm et al.12 studied PFP on Ag(111) to estimate Δ and adsorption geometry. Wong et al.13 also investigated the molecular arrangement and the electronic structure of PFP on Ag(111) by using low-temperature scanning tunneling microscopy and photoemission spectroscopy. However, theoretical studies on the interface of PFP with metal surfaces are not available in the literature. In this study, we have investigated the electronic structures of PFP on Cu(111), Ag(111), and Au(111) using density functional theoretical calculations within a generalized gradient r 2011 American Chemical Society

approximation (GGA) employing van der Waals (vdW) corrections. The vdW corrections include semiempirical vdW method (DFT-D)15 and van der Waals functional (vdW-DF).16,17 Here, we focused on DFT-D as the vdW corrections, because for adsorbed systems vdW-DF tends to overestimate equilibrium adsorption distances,18
21 which are crucial to predict Δ as will be discussed in detail; the results of vdW-DF for the PFP/metal systems are presented in the Supporting Information. We first calculate equilibrium adsorption distance and Δ to assess the vdW corrections for the PFP/metal systems. Next, we calculate the work-function change as a function of distance between PFP and substrate to investigate the nature of the interface dipole layer, which is an important factor in determining the interfacial electronic structures.22,23

’ THEORETICAL METHODS DFT Calculations of Adsorption Structures and the Vacuum Level Shifts. Our calculations were carried out using

STATE, a first principles molecular dynamics program, which Received: November 10, 2010 Revised: February 20, 2011 Published: March 17, 2011 5767

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Figure 1. (a) Top view and (b) cross-sectional view of PFP on a (111) surface.

has been successfully applied to metal surfaces and organic/metal interfaces.18
20,24
29 We employed the Perdew-Burke-Ernzerhof (PBE)30 generalized gradient approximation (GGA) for the exchange-correlation functional. Electron-ion interactions were described by pseudopotentials,31,32 and wave functions and augmented charge density were expanded using a plane-wave basis set with the cutoff energies of 25 and 225 Ry, respectively. The calculated equilibrium lattice constants of Cu, Ag, and Au are 0.365 nm, 0.408 nm, and 0.415 nm, respectively, in reasonable agreement with the experimental values33 of 0.361 nm, 0.409 nm, and 0.408 nm, respectively. We used a repeated slab model to represent the surface, in which one slab consists of four atomic layers. A vacuum region of ∼2 nm was inserted in between the slabs. PFP is adsorbed on only one surface of a slab with its molecular plane parallel to the surface in a ((43)1/2  2(3)1/2) surface unit cell. We assumed that the surface unit cell of the adsorbed system is commensurate with the Cu(111) surface unit cell on the basis of the experimental data of PFP adsorbed on Cu(111) using scanning tunneling microscopy.10 A 2  4 k-point mesh was used to sample the surface Brillouin zone. We assumed that the adsorption site of PFP is the same as that of pentacene adsorbed on Cu(111);19,20,34 the center of the PFP molecules is located at an hcp-hollow site on the (111) surface with the long molecular axis aligned with closepacked metal atom rows as shown in Figure 1. In geometry optimization, we fixed the height of carbon atoms from the first-layer of the clean (111) surface (hereinafter denote by ZC), to calculate adsorption energy and vacuum level shift as a function of ZC. We also fixed the atoms in the bottom layer of the slab at their respective bulk positions. The remaining degrees of freedom including carbon positions parallel to the substrate were fully optimized, until the maximum force dropped below a threshold value of 0.08 nN. The work-function difference between two surfaces of the slab was compensated for by using a dipole correction.35 Workfunctions were calculated from the difference between EF of the system and the average electrostatic potential energy in the vacuum region, and the vacuum level shifts were calculated from the workfunction changes induced by the adsorption of PFP molecules. van der Waals Corrections to the Density Functional Energy Calculations. The DFT-D method proposed by Grimme15

Figure 2. The adsorption energy (Ead) as a function of PFP-metal distance (ZC) calculated using GGA and DFT-D for PFP on (a) Cu(111), (b) Ag(111), and (c) Au(111).

is based on damped atom-pairwise dispersion corrections of the form C6R
6 (C6 represents the dispersion coefficient for a given atom pair, and R is the distance between the atoms). The DFT-D total energy is given by EDFT
D ¼ EKS
DFT þ Edisp

ð1Þ

where EKS
DFT is the self-consistent Kohn
Sham total energy as obtained from the chosen density functional, and Edisp is the dispersion correction. The method was successfully applied to several systems.15,36
39 We used C6 coefficient and vdW radius for gold determined in our previous study,20 as they are unavailable in the literature. Those parameters for fluorine, carbon, copper, and silver are adopted from Grimme’s paper.15

’ RESULTS AND DISCUSSION Adsorption States and Electronic Structures. The adsorption energy Ead is defined by

Ead ¼ EðC22 F14 =metalÞ
EðC22 F14 Þ
EðmetalÞ

ð2Þ

where Ead(C22F14/metal), E(C22F14), and E(metal) are total energies of an adsorbed system, an isolated PFP molecule, and a clean metal surface, respectively. A negative value of Ead means that the adsorbed system is energetically favorable relative to the isolated state. Figure 2 (a)-(c) shows Ead of PFP on Cu(111), 5768

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Table 1. Equilibrium Distances (ZCGGA and ZCDFT
D) and the Adsorption Energies (EadGGA and EadDFT
D) Calculated Using GGA and DFT-D, Respectively, and the WorkFunction Changes (ΔO) Calculated Using GGA at ZCGGA and ZCDFT
D, along with the Experimentally-Determined Adsorption Distances (ZCexp) and Work-Function Changes (ΔOexp) for PFP on Cu(111), Ag(111), and Au(111) Cu(111) GGA

ZGGA C /nm EGGA ad /eV

Expt. a

Au(111)

0.37

0.42

0.42

0.011


0.124


0.082


0.16


0.01


0.06

0.29

0.32

0.32

/eV EDFT
D ad


2.17


2.40


2.68

Δφ/eV


0.34


0.21


0.50

Zexp C /nm Δφexp/eV

0.298a
0.35a

0.316b
0.42b,-0.3c


0.50d

Δφ/eV DFT-D

Ag(111)

ZCDFT
D/nm

b

c

d

Reference 10. Reference 12. Reference 13. Reference 11.

Ag(111), and Au(111) as a function of ZC obtained using GGA and DFT-D. and Table 1 summarizes equilibrium distances (ZGGA C DFT
D DFT
D ) and adsorption energies (EGGA and E ) calcuZC ad ad lated with GGA and DFT-D, respectively. The potential energy curves calculated with GGA have shallow minima, whereas those with DFT-D have deeper minima. Overall, Ead’s obtained using GGA are significantly small or positive for Cu, meaning that the adsorption of PFP is very weak or even unstable. On the other hand, inclusion of vdW attraction with DFT-D results in a considerably large magnitude of Ead’s, suggesting that vdW forces are responsible for the adsorption of PFP. Direct comparison of the calculated adsorption energy with the experimental one is impossible, because the adsorption energies for the systems studied in the present work have never been measured experimentally. It should be noted that in our previous paper on pentacene/metal systems,20 we showed that the vdW-DF method gives reasonable adsorption energy, whereas the DFT-D method slightly overestimates its magnitude. We thus consider ’s may be overestimated also for PFP on the three that EDFT
D ad metal surfaces. In the present DFT-D implementation, system dependency of C6 coefficients40 as well as the electronic screening at the metal surface42 are not taken into account, leading to the overestimation of adsorption energies. Application of the latest version of DFT-D (DFT-D3),41 which takes into account system dependency, may improve adsorption energies. In addition, treating only the topmost layer of metal surface in the dispersion correction may be used to mimic the electronic screening at the metal surface.42 The equilibrium distances calculated with DFT-D are shorter ’s for Cu and Ag are in than those calculated with GGA. ZDFT
D C excellent agreement with the experimental values.10,12 On the contrary, ZGGA C ’s are significantly overestimated by 0.07
0.10 nm, as seen in other organic/metal interfaces.18
21 For Au, the PFP-metal distances have never been measured experimentally. However, as will be discussed later, by comparing the calculated vacuum level shift with the experimental one, we conclude that DFT-D also predicts the accurate PFP-Au(111) distance. Therefore our results suggest DFT-D is able to predict accurate PFP-metal distances as in the cases of pentacene/metal and benzene/metal systems.20 The distances for the pentacene/metal systems are shorter than those for the benzene/metal systems,20 suggesting that the adsorption distance is strongly correlated

Figure 3. The density of states projected onto the molecular orbitals of PFP (PDOS) on (a) Cu(111), (b) Ag(111), and (c) Au(111) at ZDFT
D . The energy zero is taken to be the Fermi energy (EF) of the C adsorbed systems. The HOMO and LUMO parts of PDOS near EF are magnified and displayed in the insets.

with the chemical reactivity of the molecule, i.e. pentacene is more chemically reactive than benzene. The electron affinity of PFP is 5.0 eV, whereas that of pentacene is 3.2 eV, indicating that PFP is more chemically reactive than pentacene.9 However, the adsorption distances for the PFP/metal systems are longer than those for the pentacene/ metal systems.20 This presumably comes from the repulsion between F 2p and metal electronic states for the PFP/metal systems. Next, we inspect the electronic structures of the adsorbed by calculating the density of states projected systems at ZDFT
D C onto the molecular orbitals of PFP (PDOS). The results are shown in Figure 3. Note that the energy level of LUMOþ1 is much higher than that of LUMO by ∼1.3 eV, and thus we did not include the LUMOþ1 and upper states. The HOMO peaks are located at
0.74 eV,
0.82 eV, and
0.46 eV, on Cu, Ag, and Au surfaces, respectively, which are slightly shallower than the experimentally determined HOMO derived peaks at
1.35 eV,10
1.82 eV,12 and
0.80 eV,11 respectively. The reasonable agreement comes from the following cancellation; the present GGA tends to underestimate the HOMO
LUMO gap, whereas it does not describe the energy shift of molecular levels due to surface polarization of metal substrate,43 and thus the two effects can cancel the HOMO
LUMO gap calculated with GGA.44 Still, the self-interaction error in GGA causes the shallower HOMO peaks.45 For Cu and Ag, the LUMO state becomes broad and is located below EF. However, the hybridization of LUMO with Cu is weaker compared with those for pentacene/ Cu(100)46 and pentacene/Cu(111)19,20 systems. On the other hand, for Au, the LUMO and HOMO peaks are sharp, and the LUMO state is above EF, suggesting the molecular orbitals do not significantly hybridize with the substrate states. The calculated electronic structures show that for Au the hybridization of PFP molecular orbitals with the substrate states is weak, whereas for Cu and Ag it is slightly stronger. Because of the longer PFP-metal distances, the chemical hybridization of PFP with the metal substrates is weaker than that of pentacene with the metal substrates.20 Vacuum Level Shift and Slope Parameter. The vacuum level shift is calculated from the work-function change by the adsorp0 tion of PFP. The work function change Δφ is defined by Δφ0 ¼ φðC22 F14 =metalÞ
φm

ð3Þ

where φ(C22F14/metal) and φm are work-functions of an adsorbed system and a clean metal surface, respectively. To estimate the 5769

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Figure 4. The work-function change (Δφ) as a function of ZC for PFP on Cu(111), Ag(111), and Au(111). The equilibrium distances on Cu(111), Ag(111), and Au(111) calculated using DFT-D (ZDFT
D (Cu), ZDFT
D (Ag), and ZDFT
D (Au)) are shown by a vertical C C C dotted line. The experimental values of Δφ on Cu(111), Ag(111), and Au(111) are shown by horizontal dashed lines.

work-function change at the experimental surface molecular density, we used the surface molecular density of PFP on a metal (111) 0 surface, nmetal, and corrected Δφ by using the Helmholtz equation47 nmetal Δφ ¼ Δφ0 ð4Þ n0 where Δφ is the corrected work-function change, and n0  A0
1 with A0 being the area of the surface unit cell used in our calculations. For Cu and Ag, we used nCu = 1.27  1014 cm
2 and nAg = 1.45  1014 cm
2 based on the experimental data of PFP on Cu(111)10 and Ag(111),13 respectively. We assumed that nAu = nAg, as experimental density of Au is unavailable. In this way, resulting scaling factors (nmetal/n0) are 1.15, 1.3, and 1.3 for Cu, Ag, and Au, respectively. Figure 4 shows Δφ of PFP on Cu(111), Ag(111), and Au(111) as a function of ZC calculated using GGA. The experimentally determined work-function changes on Cu(111),10 Ag(111),12 and Au(111)11 are indicated by horizontal dashed lines, and the equilibrium PFP-metal distances on Cu(111), Ag(111), and Au(Cu), ZDFT
D (Ag), and ZDFT
D (Au)] calculated (111) [ZDFT
D C C C with DFT-D are shown by vertical dotted lines. Table 1 summarizes and ZDFT
D for PFP on Cu(111), Δφ’s calculated at ZGGA C C are in Ag(111), and Au(111). The calculated Δφ’s at ZDFT
D C good agreement with the experimental values. In contrast, the are significantly underestimated, absolute values of Δφ’s at ZGGA C is overestimated, as pointed out by previous because ZGGA C calculations.24 Thus, our results indicate that DFT-D is able to predict the work-function changes for the PFP/metal interfaces as well as for the pentacene/metal and benzene/metal interfaces.20 Δφ’s for the three metal surfaces have minima at ZC e 0.34 nm. The minima are shallower than those for the petacene/metal systems studied in ref 20. This is because the effect of molecular distortion becomes more significant as the PFP molecule moves close to the surface, unlike for the pentacene/ metal systems. To evaluate the effect of the molecular distortion, we calculated the average height of F atoms relative to the C-rings of PFP (ZF
C) and the work-function changes caused by the molecular distortion (Δφmol) as a function of ZC for PFP (Figure 5). Here, Δφmol’s were estimated from the dipole of an isolated PFP molecule fixed at the adsorbed geometry. On Cu are ∼0.008 nm and ∼0.004 nm, which and Ag, ZF
C’s at ZDFT
D C are in good agreement with the experimental values of ∼0.01 nm10 and ∼0.0 nm,12 respectively. In this way, the molecular distortion of PFP is different on the three metal surfaces. On the other hand,

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Figure 5. The average height of F atoms relative to the C-rings of PFP (ZF
C) as a function of ZC for PFP on Cu(111), Ag(111), and Au(111). The work-function changes caused by the molecular distortion (Δφmol) for PFP on Cu(111), Ag(111), and Au(111) are displayed in the inset.

previous calculations48 showed that the molecular distortion of an acceptor (F4TCNQ) adsorbed on metals is independent of metal substrates, because of the strong chemical hybridization between the molecular orbitals and the substrate states. The distortion of the PFP molecule starts at ZC e 0.30 nm, for example, Δφmol at ZC = 0.21 nm reaches ∼0.4 eV, whereas at the equilibrium adsorption distance Δφmol does not significantly affect Δφ. The dependence of ZF
C’s and Δφmol’s on ZC are almost the same for the three metal surfaces, suggesting that the distortion is governed by the repulsion between F 2p and the substrate states. However, Δφmol’s at the equilibrium adsorption distance are different for the three metal surfaces (0.08 eV, 0.04 eV, and 0.03 eV on Cu, Ag, and Au, respectively). This is attributed to the difference in the equilibrium PFP-metal distance. To single out the electronic factor that contributes to the formation of the interface dipole at a short distance, we subtracted the intramolecular dipole (Δφmol) from the “total” workfunction change (Δφ) for PFP on Cu(111), Ag(111), and Au(111). In order to set the same geometric parameters and to discuss the difference in the electronic factors of the three adsorbed systems, we rescaled the resulting work functions Δφ~(Δφ
Δφmol) according to eq 4 by using nmetal = nCu so that the surface molecular density is the same on the three metal substrates.20 It should be noted that at a long distance, Δφ~ for Au converges to a nonzero value, which comes from the band gap error, as in the case of the pentacene/Au interface.20 In Figure 6 (a), we plotted Δφ~’s for the three metal surfaces. At ZC g 0.34 nm, Δφ~’s are almost independent of φm. On the other hand, at ZC e 0.34 nm, Δφ~’s vary with φm, which comes from the hybridization between the PFP molecular orbitals and the metal substrate states, as shown in the PDOS calculations and our previous calculation.19,20 To make clear the relationship between Δφ~ and φm, we plotted Δφ~ as a function of φm at several selected ZC’s (0.24 nm, 0.29 nm, and 0.37 nm) in Figure 6 (b). We also calculated the slope parameter expressed as20,22,23 S ¼ 1þk

ð5Þ

k  dðΔ φ~m Þ=dφm

ð6Þ

where k is defined by

Although experimentally determined S and k include both geometric and electronic contributions, in the present analysis we can extract electronic contribution by keeping the same geometric parameters for the three metal surfaces. As seen in Figure 6 (b), k at ZC = 0.24 nm, 0.29 nm, and 0.37 nm are evaluated to be
0.84,
0.79, and
0.056, respectively. The 5770

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repulsion between F 2p and the substrate states in the PFP/metal systems. The work-function change (Δφ) is sensitive to PFP-metal distance (ZC). Δφ’s as well as the equilibrium ZC’s obtained using the DFT-D method are in good agreement with available experiments, suggesting that the method is able to predict the vacuum level shift of organic/metal interfaces accurately. We also found that the intramolecular dipole (Δφmol) significantly affects Δφ, when ZC is less than 0.30 nm. To extract the electronic factor for the interface dipole from geometric factors, we subtracted Δφmol from Δφ, to find a transition from the Schottky limit to the Bardeen limit as the PFP molecule approaches the substrate. The transition infers hybridization between PFP and metal substrates at a short PFP-metal distance.

’ ASSOCIATED CONTENT

bS

Supporting Information. The results of vdW-DF for the PFP/metal systems are presented. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 6. (a) Δφ~ as a function of ZC for PFP on Cu(111), Ag(111), and Au(111). Δφ~ is calculated by subtracting Δφmol from Δφ. (b) Δφ~ as a function of the metal work-function (φm) at ZC= 0.24 nm, 0.29 nm, and 0.37 nm. k indicates the slope of the line of Δφ~ as a function of φm at each ZC.

results indicate that as the PFP molecule approaches the surface, S decreases from ∼0.94 to ∼0.16, which means a transition from the Schottky limit to Bardeen limit. By subtracting the intramolecular dipole, we are able to show the transition similar to that for the pentacene/metal systems.20 The results infer hybridization between PFP and metal substrate at ZC e 0.34 nm . Chemical hybridization for the PFP/metal interfaces at the equilibrium is, however, not stronger than that for the pentacene/ metal interfaces,20 because the Pauli repulsion, which dominates the PFP-metal interaction, makes the equilibrium adsorption distances longer. Therefore we suggest the change in the number of F atoms in fluorinated pentacene can modify the vacuum level shift.49 In the present study, we assumed that the molecular plane of PFP molecules is parallel to the surface. Recent experimental results12 pointed out that for a multilayer the plane can be inclined to the surface, and F
C bonds affect the ionization potential of the organic film. Although theoretical studies on the inclined structure remain to be future works, we would like to stress that DFT-D would be a useful tool to study such systems, because the van der Waals interactions between molecules is dominant and hence inclusion of van der Waals forces is crucial.

’ CONCLUSIONS We have presented a first-principles study of PFP on Cu(111), Ag(111), and Au(111) to investigate their adsorption geometries, adsorption energies, electronic structures, and the nature of the interface dipole. We employed the semiempirical van der Waals (DFT-D) method to include the long-range van der Waals interactions. The DFT-D method nicely reproduces the experimental adsorption distance between the PFP molecule and the metal substrate. The PFP-metal distances are longer than those of the pentacene-metal systems although PFP is more chemically reactive than pentacene. This presumably comes from the Pauli

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (K.T.); morikawa@ prec.eng.osaka-u.ac.jp (Y.M.).

’ ACKNOWLEDGMENT This work is partially supported by a Grant-in-Aid for Scientific Research in Priority Areas [Grant No. 19054013] from the Ministry of Education, Culture, Science, Sports and Technology (MEXT), Japan. K.L is supported in part by NSF-DMR0801343. Numerical calculations were carried out using supercomputer facilities at Osaka University, at The Institute for Solid State Physics, The University of Tokyo, at Information Technology Center, The University of Tokyo, and at Tohoku University. ’ REFERENCES (1) Hung, L. S.; Chen, C. H. Mater. Sci. Eng., R. 2002, 39, 143–222. (2) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913–915. (3) Dimitrakopoulos, C. D.; Malenfant, P. Adv. Mater. 2002, 14, 99–117. (4) Katz, H. E.; Bao, Z. J. Phys. Chem. B 2000, 104, 671–678. (5) Tang, C. W. Appl. Phys. Lett. 1986, 48, 183–185. (6) Hoppe, H.; Sariciftci, N. S. J. Mater. Res. 2004, 19, 1924–1945. (7) Drury, C. J.; Mutsaers, C. M. J.; Hart, C. M.; Matters, M.; de Leeuw, D. M. Appl. Phys. Lett. 1998, 73, 108–110. (8) Inoue, Y.; Sakamoto, Y.; Suzuki, T.; Kobayashi, M.; Gao, Y.; Tokito, S. Jpn. J. Appl. Phys. 2005, 44, 3663–3668. (9) Yokoyama, T.; Park, C. B.; Nishimura, T.; Kita, K.; Toriumi, A. Jpn. J. Appl. Phys. 2008, 47, 3643–3646. (10) Koch, N.; Gerlach, A.; Duhm, S.; Glowatzki, H.; Heimel, G.; Vollmer, A.; Sakamoto, Y.; Suzuki, T.; Zegenhagen, J.; Rabe, J. P.; Schreiber, F. J. Am. Chem. Soc. 2008, 130, 7300–7304. (11) Koch, N.; Vollmer, A.; Duhm, S.; Sakamoto, Y.; Suzuki, T. Adv. Mater. 2007, 19, 112–116. (12) Duhm, S.; S. Hosoumi, I. S.; Gerlach, A.; Oehzelt, M.; Wedl, B.; Lee, T.-L.; Schreiber, F.; Koch, N.; Ueno, N.; Kera, S. Phys. Rev. B 2010, 81, 045418. (13) Wong, S. L.; Huang, H.; Huang, Y. L.; Wang, Y. Z.; Gao, X. Y.; Suzuki, T.; Chen, W.; Wee, A. T. S. J. Phys. Chem. C 2010, 114, 9356–9361. 5771

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp1107262 |J. Phys. Chem. C 2011, 115, 5767–5772