Published: March 17, 2011

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

ARTICLE

pubs.acs.org/JPCC

Density Functional Theoretical Study of Perfluoropentacene/NobleMetal Interfaces with van der Waals Corrections: Adsorption Statesand Vacuum Level ShiftsKenji 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, JapanzDepartment 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

’ INTRODUCTION

Organic devices such as organic light emitting diodes,1,2 organicfield effect transistors (OFETs),3,4 and organic photovoltaic cells5,6

have attracted a great deal of interest for their low-cost processingand flexibility.7 Most of the organic molecules in the literature areinsulator or p-type semiconductor, whereas n-type organic semi-conductormaterials, which are required to fabricate complementarycircuits, have been rarely investigated. Perfluoropentacene (PFP,C22F14) is one of a few organic materials examined for n-typeOFETs.8,9 The interaction between PFP andmetal surfaces has thusbeen studied extensively.10�14 Koch et al.10 studied PFP onCu(111)surface to estimate thework-function change (Δ) and the adsorptiongeometry using ultraviolet photoelectron spectroscopy (UPS) andX-ray standing wave (XSW) measurements, respectively. Kochet al.11 also investigatedΔ and electronic structure of PFP onAu(111)using UPS. Recently, Duhm et al.12 studied PFP on Ag(111) toestimate Δ and adsorption geometry. Wong et al.13 also investigatedthe molecular arrangement and the electronic structure of PFP onAg(111) by using low-temperature scanning tunneling microscopyand photoemission spectroscopy. However, theoretical studies on theinterface of PFP with metal surfaces are not available in the literature.

In this study, we have investigated the electronic structuresof PFP on Cu(111), Ag(111), and Au(111) using densityfunctional theoretical calculations within a generalized gradient

approximation (GGA) employing van der Waals (vdW) correc-tions. 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 ads-orbed systems vdW-DF tends to overestimate equilibriumadsorption distances,18�21 which are crucial to predict Δ as willbe discussed in detail; the results of vdW-DF for the PFP/metalsystems are presented in the Supporting Information. We firstcalculate equilibrium adsorption distance and Δ to assess thevdW corrections for the PFP/metal systems. Next, we calculatethe work-function change as a function of distance between PFPand substrate to investigate the nature of the interface dipolelayer, which is an important factor in determining the interfacialelectronic structures.22,23

’THEORETICAL METHODS

DFT Calculations of Adsorption Structures and the Va-cuum Level Shifts. Our calculations were carried out usingSTATE, a first principles molecular dynamics program, which

Received: November 10, 2010Revised: February 20, 2011

ABSTRACT: We have studied the electronic structures ofperfluoropentacene (PFP) on Cu(111), Ag(111), and Au(111)by means of density functional theory with a semiempirical vander Waals method (DFT-D). We show that DFT-D yieldsaccurate equilibrium PFP-metal distances, thereby making anaccurate prediction of the work-function change (Δφ) possible.In order to investigate the nature of the interface dipole layer, wecalculatedΔφ as a function of PFP-substrate distance and foundthat in contrast to pentacene/metal interfaces, the moleculardistortion 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 work-function change does not depend on the substrate work-function at a long distance, while the work-function change varies linearlywith the substrate work-function at a short distance. Our results indicate a transition from Schottky to Bardeen limits as a PFPmolecule approaches the substrate metal surface, as in pentacene/metal interfaces.

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has been successfully applied tometal surfaces and organic/metalinterfaces.18�20,24�29 We employed the Perdew-Burke-Ernzerhof(PBE)30 generalized gradient approximation (GGA) for theexchange-correlation functional. Electron-ion interactions weredescribed by pseudopotentials,31,32 and wave functions and aug-mented charge density were expanded using a plane-wave basisset with the cutoff energies of 25 and 225 Ry, respectively. Thecalculated equilibrium lattice constants of Cu, Ag, and Au are0.365 nm, 0.408 nm, and 0.415 nm, respectively, in reasonableagreement 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 ononly one surface of a slab with its molecular plane parallel to thesurface in a ((43)1/2 � 2(3)1/2) surface unit cell. We assumedthat the surface unit cell of the adsorbed system is commensuratewith the Cu(111) surface unit cell on the basis of the experimentaldata of PFP adsorbed on Cu(111) using scanning tunnelingmicroscopy.10 A 2 � 4 k-point mesh was used to sample thesurface Brillouin zone. We assumed that the adsorption site of PFPis the same as that of pentacene adsorbed on Cu(111);19,20,34 thecenter of the PFP molecules is located at an hcp-hollow site on the(111) surface with the long molecular axis aligned with close-packed 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 denoteby ZC), to calculate adsorption energy and vacuum level shift as afunction of ZC.We also fixed the atoms in the bottom layer of theslab at their respective bulk positions. The remaining degrees offreedom including carbon positions parallel to the substrate werefully optimized, until the maximum force dropped below athreshold value of 0.08 nN.The work-function difference between two surfaces of the slab

was compensated for by using a dipole correction.35 Work-functions were calculated from the difference between EF of thesystem and the average electrostatic potential energy in the vacuumregion, and the vacuum level shifts were calculated from the work-function changes induced by the adsorption of PFP molecules.van der Waals Corrections to the Density Functional

Energy Calculations.The DFT-Dmethod proposed by Grimme15

is based on damped atom-pairwise dispersion corrections of the formC6R

�6 (C6 represents the dispersion coefficient for a given atompair,andR is the distance between the atoms). TheDFT-D total energy isgiven by

EDFT � D ¼ EKS � DFT þ Edisp ð1Þwhere EKS�DFT is the self-consistent Kohn�Sham total energyas obtained from the chosen density functional, and Edisp is thedispersion correction.Themethodwas successfully applied to severalsystems.15,36�39 We used C6 coefficient and vdW radius for golddetermined in our previous study,20 as they are unavailable in theliterature. Those parameters for fluorine, carbon, copper, and silverare adopted from Grimme’s paper.15

’RESULTS AND DISCUSSION

Adsorption States and Electronic Structures. The adsorp-tion energy Ead is defined by

Ead ¼ EðC22F14=metalÞ � EðC22F14Þ � EðmetalÞ ð2Þwhere Ead(C22F14/metal), E(C22F14), and E(metal) are totalenergies of an adsorbed system, an isolated PFP molecule, and aclean metal surface, respectively. A negative value of Ead meansthat the adsorbed system is energetically favorable relative to theisolated state. Figure 2 (a)-(c) shows Ead of PFP on Cu(111),

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

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

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Ag(111), and Au(111) as a function of ZC obtained using GGAand DFT-D.Table 1 summarizes equilibrium distances (ZC

GGA andZCDFT�D) and adsorption energies (Ead

GGA and EadDFT�D) calcu-

lated with GGA and DFT-D, respectively. The potential energycurves calculated with GGA have shallow minima, whereas thosewith DFT-D have deeper minima. Overall, Ead’s obtained usingGGA are significantly small or positive for Cu, meaning that theadsorption of PFP is very weak or even unstable. On the otherhand, inclusion of vdW attraction with DFT-D results in aconsiderably large magnitude of Ead’s, suggesting that vdW forcesare responsible for the adsorption of PFP. Direct comparison ofthe calculated adsorption energy with the experimental one isimpossible, because the adsorption energies for the systemsstudied in the present work have never been measured experi-mentally. It should be noted that in our previous paper onpentacene/metal systems,20 we showed that the vdW-DF meth-od gives reasonable adsorption energy, whereas the DFT-Dmethod slightly overestimates its magnitude. We thus considerthat Ead

DFT�D’s may be overestimated also for PFP on the threemetal surfaces. In the present DFT-D implementation, systemdependency of C6 coefficients

40 as well as the electronic screen-ing at the metal surface42 are not taken into account, leading tothe overestimation of adsorption energies. Application of thelatest version of DFT-D (DFT-D3),41 which takes into accountsystem dependency, may improve adsorption energies. In addi-tion, treating only the topmost layer of metal surface in thedispersion correction may be used to mimic the electronic screeningat the metal surface.42

The equilibrium distances calculated with DFT-D are shorterthan those calculated with GGA. ZC

DFT�D’s for Cu and Ag are inexcellent agreement with the experimental values.10,12 On thecontrary, ZC

GGA’s are significantly overestimated by 0.07�0.10 nm, asseen in other organic/metal interfaces.18�21 For Au, the PFP-metaldistances have never been measured experimentally. However, as willbe discussed later, by comparing the calculated vacuum level shift withthe experimental one, we conclude that DFT-D also predicts theaccurate PFP-Au(111) distance. Therefore our results suggestDFT-Dis able to predict accurate PFP-metal distances as in the cases ofpentacene/metal and benzene/metal systems.20 The distances for thepentacene/metal systems are shorter than those for thebenzene/metalsystems,20 suggesting that the adsorptiondistance is strongly correlated

with the chemical reactivity of the molecule, i.e. pentacene is morechemically reactive thanbenzene.The electron affinity ofPFP is 5.0 eV,whereas that of pentacene is 3.2 eV, indicating that PFP is morechemically reactive thanpentacene.9However, the adsorptiondistancesfor the PFP/metal systems are longer than those for the pentacene/metal systems.20This presumably comes from the repulsion betweenF2p and metal electronic states for the PFP/metal systems.Next, we inspect the electronic structures of the adsorbed

systems at ZCDFT�D by calculating the density of states projected

onto the molecular orbitals of PFP (PDOS). The results areshown in Figure 3. Note that the energy level of LUMOþ1 ismuch higher than that of LUMOby∼1.3 eV, and thus we did notinclude the LUMOþ1 and upper states. The HOMO peaks arelocated at�0.74 eV,�0.82 eV, and�0.46 eV, on Cu, Ag, and Ausurfaces, respectively, which are slightly shallower than theexperimentally determined HOMO derived peaks at �1.35eV,10 �1.82 eV,12 and �0.80 eV,11 respectively. The reasonableagreement comes from the following cancellation; the presentGGA tends to underestimate the HOMO�LUMO gap, whereasit does not describe the energy shift of molecular levels due tosurface polarization of metal substrate,43 and thus the two effectscan cancel the HOMO�LUMO gap calculated with GGA.44

Still, the self-interaction error in GGA causes the shallowerHOMO peaks.45 For Cu and Ag, the LUMO state becomesbroad and is located below EF. However, the hybridization ofLUMO with Cu is weaker compared with those for pentacene/Cu(100)46 and pentacene/Cu(111)19,20 systems. On the otherhand, for Au, the LUMO and HOMO peaks are sharp, and theLUMO state is above EF, suggesting themolecular orbitals do notsignificantly hybridize with the substrate states. The calculatedelectronic structures show that for Au the hybridization of PFPmolecular orbitals with the substrate states is weak, whereas forCu and Ag it is slightly stronger. Because of the longer PFP-metaldistances, the chemical hybridization of PFP with the metal sub-strates is weaker than that of pentacene with the metal substrates.20

VacuumLevel Shift and Slope Parameter.The vacuum levelshift is calculated from the work-function change by the adsorp-tion of PFP. The work function change Δφ

0is defined by

Δφ0 ¼ φðC22F14=metalÞ � φm ð3Þwhere φ(C22F14/metal) and φm are work-functions of an adsorbedsystem and a clean metal surface, respectively. To estimate the

Table 1. Equilibrium Distances (ZCGGA and ZC

DFT�D) andthe Adsorption Energies (Ead

GGA and EadDFT�D) Calculated

Using GGA and DFT-D, Respectively, and the Work-Function Changes (ΔO) Calculated Using GGA at ZC

GGA

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) Ag(111) Au(111)

GGA ZCGGA/nm 0.37 0.42 0.42

EadGGA/eV 0.011 �0.124 �0.082

Δφ/eV �0.16 �0.01 �0.06

DFT-D ZCDFT�D/nm 0.29 0.32 0.32

EadDFT�D/eV �2.17 �2.40 �2.68

Δφ/eV �0.34 �0.21 �0.50

Expt. ZCexp/nm 0.298a 0.316b

Δφexp/eV �0.35a �0.42b,-0.3c �0.50d

aReference 10. bReference 12. cReference 13. dReference 11.

Figure 3. The density of states projected onto the molecular orbitals ofPFP (PDOS) on (a) Cu(111), (b) Ag(111), and (c) Au(111) atZCDFT�D. The energy zero is taken to be the Fermi energy (EF) of the

adsorbed systems. The HOMO and LUMO parts of PDOS near EF aremagnified and displayed in the insets.

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work-function change at the experimental surface molecular den-sity, we used the surfacemolecular density of PFP on ametal (111)surface, nmetal, and correctedΔφ

0by using theHelmholtz equation47

Δφ ¼ Δφ0nmetaln0

ð4Þ

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 calcul-ations. 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 onCu(111)10 andAg(111),13 respectively.We assumed that nAu = nAg,as experimental density of Au is unavailable. In this way, resultingscaling factors (nmetal/n0) are 1.15, 1.3, and 1.3 for Cu, Ag, and Au,respectively.Figure 4 showsΔφ of PFP on Cu(111), Ag(111), and Au(111)

as a function of ZC calculated using GGA. The experimentallydetermined work-function changes on Cu(111),10 Ag(111),12 andAu(111)11 are indicated by horizontal dashed lines, and theequilibrium PFP-metal distances on Cu(111), Ag(111), and Au-(111) [ZC

DFT�D(Cu), ZCDFT�D(Ag), and ZC

DFT�D(Au)] calculatedwithDFT-D are shownby vertical dotted lines. Table 1 summarizesΔφ’s calculated at ZC

GGA and ZCDFT�D for PFP on Cu(111),

Ag(111), and Au(111). The calculated Δφ’s at ZCDFT�D are in

good agreement with the experimental values. In contrast, theabsolute values of Δφ’s at ZC

GGA are significantly underestimated,because ZC

GGA is overestimated, as pointed out by previouscalculations.24 Thus, our results indicate that DFT-D is able topredict the work-function changes for the PFP/metal interfaces aswell as for the pentacene/metal and benzene/metal interfaces.20

Δφ’s for the three metal surfaces have minima at ZC e0.34 nm. The minima are shallower than those for the peta-cene/metal systems studied in ref 20. This is because the effect ofmolecular distortion becomes more significant as the PFPmolecule 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-ringsof PFP (ZF�C) and the work-function changes caused by themolecular distortion (Δφmol) as a function of ZC for PFP(Figure 5). Here, Δφmol’s were estimated from the dipole of anisolated PFP molecule fixed at the adsorbed geometry. On Cuand Ag, ZF�C’s atZC

DFT�D are∼0.008 nm and∼0.004 nm, whichare in good agreement with the experimental values of∼0.01 nm10

and∼0.0 nm,12 respectively. In this way, themolecular distortion ofPFP is different on the three metal surfaces. On the other hand,

previous calculations48 showed that the molecular distortion of anacceptor (F4TCNQ) adsorbed on metals is independent of metalsubstrates, because of the strong chemical hybridization betweenthemolecular orbitals and the substrate states. The distortion of thePFP molecule starts at ZCe 0.30 nm, for example, Δφmol at ZC =0.21 nm reaches ∼0.4 eV, whereas at the equilibrium adsorptiondistanceΔφmol does not significantly affectΔφ. The dependence ofZF�C’s and Δφmol’s on ZC are almost the same for the three metalsurfaces, suggesting that the distortion is governed by the repulsionbetween F 2p and the substrate states. However, Δφmol’s at theequilibrium adsorption distance are different for the three metalsurfaces (0.08 eV, 0.04 eV, and 0.03 eV on Cu, Ag, and Au,respectively). This is attributed to the difference in the equilibriumPFP-metal distance.To single out the electronic factor that contributes to the

formation of the interface dipole at a short distance, we sub-tracted the intramolecular dipole (Δφmol) from the “total” work-function change (Δφ) for PFP on Cu(111), Ag(111), andAu(111). In order to set the same geometric parameters and todiscuss the difference in the electronic factors of the threeadsorbed systems, we rescaled the resulting work functionsΔφ~(�Δφ�Δφmol) according to eq 4 by using nmetal = nCu sothat the surface molecular density is the same on the three metalsubstrates.20 It should be noted that at a long distance, Δφ~ forAu converges to a nonzero value, which comes from the band gaperror, 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 g0.34 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 thehybridization between the PFP molecular orbitals and the metalsubstrate states, as shown in the PDOS calculations and ourprevious calculation.19,20 Tomake clear the relationship betweenΔφ~ and φm, we plottedΔφ~ as a function of φm at several selectedZC’s (0.24 nm, 0.29 nm, and 0.37 nm) in Figure 6 (b). We alsocalculated the slope parameter expressed as20,22,23

S ¼ 1þ k ð5Þwhere k is defined by

k � dðΔ ~φmÞ=dφm ð6ÞAlthough experimentally determined S and k include bothgeometric and electronic contributions, in the present analysiswe can extract electronic contribution by keeping the samegeometric parameters for the three metal surfaces. As seen inFigure 6 (b), k at ZC = 0.24 nm, 0.29 nm, and 0.37 nm areevaluated to be �0.84, �0.79, and �0.056, respectively. The

Figure 4. The work-function change (Δφ) as a function of ZC for PFPon Cu(111), Ag(111), and Au(111). The equilibrium distances onCu(111), Ag(111), and Au(111) calculated using DFT-D(ZC

DFT�D(Cu), ZCDFT�D(Ag), and ZC

DFT�D(Au)) are shown by a verticaldotted line. The experimental values of Δφ on Cu(111), Ag(111), andAu(111) are shown by horizontal dashed lines.

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.

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results indicate that as the PFP molecule approaches the surface,S decreases from∼0.94 to∼0.16, which means a transition fromthe Schottky limit to Bardeen limit. By subtracting the intramo-lecular dipole, we are able to show the transition similar to thatfor the pentacene/metal systems.20 The results infer hybridiza-tion 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 dominatesthe PFP-metal interaction, makes the equilibrium adsorption dista-nces longer. Therefore we suggest the change in the number of Fatoms in fluorinated pentacene canmodify the vacuum level shift.49

In the present study, we assumed that the molecular plane of PFPmolecules is parallel to the surface. Recent experimental results12

pointed out that for a multilayer the plane can be inclined to thesurface, and F�C bonds affect the ionization potential of theorganic film. Although theoretical studies on the inclined structureremain to be future works, we would like to stress that DFT-Dwould be a useful tool to study such systems, because the van derWaals interactions between molecules is dominant and henceinclusion 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 geome-tries, adsorption energies, electronic structures, and the nature ofthe interface dipole. We employed the semiempirical van derWaals (DFT-D)method to include the long-range van derWaalsinteractions. The DFT-D method nicely reproduces the experi-mental adsorption distance between the PFP molecule and themetal substrate. The PFP-metal distances are longer than thoseof the pentacene-metal systems although PFP is more chemicallyreactive than pentacene. This presumably comes from the Pauli

repulsion between F 2p and the substrate states in the PFP/metalsystems.

The work-function change (Δφ) is sensitive to PFP-metaldistance (ZC). Δφ’s as well as the equilibrium ZC’s obtainedusing the DFT-D method are in good agreement with availableexperiments, suggesting that the method is able to predict thevacuum level shift of organic/metal interfaces accurately. We alsofound that the intramolecular dipole (Δφmol) significantly affectsΔφ, when ZC is less than 0.30 nm. To extract the electronic factorfor the interface dipole from geometric factors, we subtractedΔφmol fromΔφ, to find a transition from the Schottky limit to theBardeen limit as the PFPmolecule approaches the substrate. Thetransition infers hybridization between PFP and metal substratesat a short PFP-metal distance.

’ASSOCIATED CONTENT

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

’AUTHOR INFORMATION

Corresponding Author*E-mail: toyoda.kenji@jp.panasonic.com (K.T.); morikawa@prec.eng.osaka-u.ac.jp (Y.M.).

’ACKNOWLEDGMENT

This work is partially supported by a Grant-in-Aid for Scien-tific Research in Priority Areas [Grant No. 19054013] from theMinistry of Education, Culture, Science, Sports and Technology(MEXT), Japan. K.L is supported in part by NSF-DMR-0801343. Numerical calculations were carried out using super-computer facilities at Osaka University, at The Institute for SolidState Physics, The University of Tokyo, at Information Technol-ogy Center, The University of Tokyo, and at Tohoku University.

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