10914 Phys. Chem. Chem. Phys., 2010, 12, 10914–10918 This journal is c the Owner Societies 2010

Chemisorption-induced gap state at organic–metal interface: Benzenethiol

on Pt(111)

Shigeru Masuda,*a Toyohiro Kamada,a Keita Sasaki,a Masaru Aokiaa and

Yoshitada Morikawab

Received 15th January 2010, Accepted 1st June 2010

DOI: 10.1039/c001016b

Electron emission spectra obtained by thermal collisions of He*(23S) metastable atoms with

benzenethiol (C6H5SH) on Pt(111) were measured to characterize the chemisorption-induced

gap state (CIGS) formed at the organic–metal interface. First-principles calculations using

density functional theory were also performed for an ordered thiolate (C6H5S) monolayer on

Pt(111). Our data exhibit that the CIGS due to the S 3p–Pt 5d mixings appears just below the

Fermi level (EF) of the substrate, where the local density of states decreases drastically from the

S terminal to the benzene ring. Furthermore, strong benzene p(1e1g)–S 3p couplings are

apparently lifted upon the formation of thiolate. These features indicate that thiolate is

not a good mediator of metal wave functions at EF, which is closely related to tunneling

probability (and eventually electric conductance) in the relevant metal–organic–metal

junctions at zero bias.

1. Introduction

Charge transport across the organic–metal interface has

attracted considerable attention for the basic understanding

and potential use in organic-based electronic devices,1–5

e.g., organic thin-film solar cells, organic light emitting diodes,

functional metal–organic–metal junctions. One of the critical

issues in the field is to control the gap states induced at the

organic–metal interface, as in the case of heterojunctions in

semiconductor devices. When the organic molecule is bound

chemically to a metal substrate, the discrete levels of the

molecule are shifted and broadened to some extent, some of

which may be split to form new electronic states in the

HOMO–LUMO gap (HOMO: highest occupied molecular

orbital and LUMO: lowest unoccupied molecular orbital).

The chemisorption-induced gap state (CIGS) thus formed

mediates the extension of metal wave functions to the chemisorbed

species, so that it plays a crucial role in tunneling probability

and eventually electric conductivity in the relevant molecular

junction. The CIGS is also responsible for the formation of a

dipole layer at the organic–metal interface, which directly

affects the charge injection barrier. Thus, the systematic under-

standing and controlling of the CIGS is a key factor for

fabricating novel organic–metal systems. Here, we report a

new characterization of the CIGS based on the asymptotic

behavior of metal wave functions exposed inside the organic

molecule by taking benzenethiolate (C6H5S) on Pt(111) as a

model system.

Metastable atom electron spectroscopy (MAES) used here is

based on the energy analysis of electrons emitted by thermal

collisions of rare gas metastable atoms such as He*(1s2s, 23S)

with a solid surface.6,7 On a metal surface such as Pt(111), the

He*(23S) atom deexcites to the ground state predominantly via

resonance ionization (RI) followed by Auger neutralization

(AN). Since the AN process produces two holes in the valence

bands, the electron emission spectrum shows a broad structure,

reflecting the self-convolution of the local density of states. On

an insulator surface, such as an ordinary organic film, the RI

process is suppressed and Penning ionization (PI) occurs instead,

where an electron in the valence state of the film fills the He* 1s

hole and simultaneously the 2s electron is emitted. The PI

process yields a single-hole spectrum, as in the case of ultraviolet

photoemission spectroscopy (UPS). The He* atoms do not

penetrate into the bulk, and therefore, the local electronic states

at the outermost layer can be selectively probed. This unique

feature has been applied to identify the CIGS emerged in the

organic–metal systems, e.g., C6H6/Pd(110),8 alkanethiolates

(CnH2n+1S, n = 1–3)/Pt(111),9 C60 on Pt(111).10 Furthermore,

it has been used to clarify the relationship between CIGS and the

charge injection barrier, by taking bathocuproine (BCP) on

polycrystalline Au as a model system.11

The purpose of the present study is two-fold. The first

purpose is to identify the CIGS formed at the C6H5S–Pt(111)

interface by MAES and first-principles calculations using

density functional theory (DFT). Our data show that the

local density of CIGS near the Fermi level (EF) significantly

decreases from the terminal S atom to the benzene ring,

indicating that benzenethiolate is not a good mediator of

metal wave functions. The second purpose is to establish that

the asymptotic feature of CIGS is closely related to the charge

transport across the organic–metal interface. The current–voltage

(I–V) characteristics in benzenedithiolate (BDT, SC6H4S)

bridged by a pair of metal electrodes have been exten-

sively studied both from experimental12–15 and theoretical16–22

points of view. The measured conductances at zero bias in

aDepartment of Basic Science, Graduate School of Arts and Sciences,The University of Tokyo, Komaba, Meguro, Tokyo 153-8902, Japan.E-mail: masuda@piesap.c.u-tokyo.ac.jp

b The Institute of Scientific and Industrial Research, Osaka University,8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan

PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics

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This journal is c the Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 10914–10918 10915

Au–BDT–Au13,14 and Pt–BDT–Pt15 junctions are much lower

than that in a Pt–C60–Pt junction,23 but is the same order as

that in a Pt–hexanedithiolate–Pt junction.24

2. Experimental

The experimental apparatus and related procedure are reported

elsewhere.25,26 The Pt(111) substrate was cleaned by repeated

Ar+ ion sputtering and heating cycles. The clean substrate

showed a well-ordered 1 � 1 pattern, and no impurities were

detected within the limit of Auger electron spectroscopy (AES).

The chemisorbed overlayers and condensed films of C6H5SH

were prepared by exposing the clean Pt(111) substrate held at

200 and 135 K to the gaseous molecules, respectively. The

thickness of the condensed films was estimated by exposure-

dependent UPS and MAES spectra.

3. Computational details

All calculations based on a generalized gradient approxima-

tion (GGA)27 in DFT were performed using a program package

called simulation tool for atom technology (STATE).28 The

pseudopotentials of H 1s, C 2p, and Pt 5d states are con-

structed by Vanderbilt’s ultrasoft scheme,29 whereas those of

other components are constructed by normconserving

scheme.30,31 The cut-off energies for the wave function and

the augmented charge are 25 Ry and 225 Ry, respectively. A

periodic slab model is used, with each slab composed of six

atomic layers, separated by a vacuum region of 15.9 A.

Benzenethiolate is introduced on one side of the slab. For

the structural optimization, benzenethiolate and the top three

layers of the substrate atoms are allowed to relax. For

comparison, a periodic slab with three atomic layers separated

by a vacuum region of 22.8 A was also used.

4. Results and discussion

To clarify the electronic states of C6H5SH without direct

contact with the metal substrate, we measured the He I UPS

and He*(23S) MAES spectra of C6H5SH in the gas and con-

densed phases. Fig. 1 shows the typical data for a condensed

film on Pt(111) with a thickness of about five monolayers. The

binding energy is referred to the Fermi level (EF) of the

substrate. As a reference, the gas-phase MAES spectrum and

some molecular orbitals (MOs) of C6H5SH obtained by

ab initio calculations at the B3LYP level are also shown in

the figure, where the notations are based on the Cs symmetry.

The gas-phase spectrum exhibits several bands due to PI from

the relevant MOs.32,33 The 5a0 0(p4) and 3a0 0(p2) MOs are

composed of antibonding and bonding couplings between

doubly-degenerate benzene (Bz) 1e1g and S 3p orbitals,

whereas the 4a0 0(p3) MO is composed of Bz 1e1g with a nodal

plane on the S atom. The ionization energies of the p4, p3 andp2 bands determined by the UPS spectrum (not shown)

are 8.53, 9.53, and 10.63 eV, respectively. The large energy

splittings correspond well to the calculated values (DE(p4–p3) =1.01 eV and DE(p3–p2) = 1.12 eV), indicating a strong Bz p–S3p conjugation. The DFT calculation for free C6H5S also

shows the strong Bz p–S 3p coupling (DE(p4–p3) = 0.98 eV

and DE(p3–p2) = 1.21 eV) (see below). The 2a0 0(p1) MO is

derived mainly from the lower-lying Bz 1a2u orbital.

The 24a0(sCS) and 22a0(sSH) MOs are distributed along the

molecular skeleton with a large amount of the S 3p components.

As is seen in Fig. 1, the corresponding bands appear in the

condensed-phase spectra. The Pt 5d bands are clearly observed

just below EF in the UPS spectrum, but are missing in the

MAES spectrum. Further, the C6H5SH-derived bands in the

MAES are slightly shifted to the higher binding energy side

relative to those in the UPS spectrum. These features are due

to the fact that electron emission upon He*(23S) impact

selectively takes place at the topmost layer.6,34 The absence

of electron emission near EF in the MAES spectrum clearly

indicates that the topmost molecules (without a chemical bond

to the Pt substrate) are insulating in character, with a wide

HOMO–LUMO gap.

Fig. 2 shows the coverage dependence of the He*(23S)

MAES spectra obtained by exposing the Pt(111) substrate at

200 K to gaseous C6H5SH. At 200 K the molecules are known

to chemisorb on the substrate through the S atom with a

cleavage of the S–H bond, forming thiolate (C6H5S–Pt).35

According to scanning tunneling microscopy (STM) and

AES studies,36 the saturation coverage is estimated to be

0.33. At low exposure (below 1.2 L), the He*(23S) atoms

deexcite predominantly via the two-step process (RI + AN),

as in the case of the clean surface. At high exposure, PI occurs

as a competing process, and its contribution peaks at satura-

tion coverage (2.4 L). Three spectral features can be deduced.

(1) The sCS-derived band is strongly suppressed upon

chemisorption, in spite of a clear appearance of the other

s-derived bands. This suggests that the thiolate species is

bound in a tilted manner through the S–Pt bond. As illustrated

in Fig. 2, in such an orientation, the He*(23S) atoms can access

more readily the outer-located benzene ring than the inner-

located S atom and Pt substrate. The tilted orientation is also

supported by comparison with the MAES spectrum of benzene

chemisorbed on Pt(111), where the RI + AN process is

dominant even at saturation coverage, reflecting their flat

orientation.37

(2) The energy splitting among the p4, p3 and p2 bands is

scarcely seen in thiolate, in contrast to the cases of free

C6H5SH, free C6H5S and condensed C6H5SH as mentioned

above. This indicates that the original p MOs in C6H5SH,

particularly the p4 and p2 MOs including the S 3p components,

are drastically altered by the formation of the S–Pt bond.

(3) A weak emission with an edge structure at EF appears

upon chemisorption, and its intensity grows with increasing

coverage. We assigned it to the CIGS formed at the thiolate–

Pt(111) interface, because the corresponding emission is missing

in the cases of bare Pt(111) and multilayer film. A similar

CIGS with metallic character has been observed in the MAES

spectra of CnH2n+1S (n = 1–3)/Pt(111),9 C6H5S/Au(111),38

C6H5Se/Pt(111),38 C60/Pt(111),

10 etc.

To examine the above-mentioned features, we performed

the first-principles DFT calculations for C6H5S on Pt(111).

The periodic slabs with three and six Pt layers were used, and a

(O3 � O3)R301 overlayer was assumed, based on the STM

studies.36 In both cases, the total energy calculations show that

the three-fold hollow site of the (111) plane is more stable than

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10916 Phys. Chem. Chem. Phys., 2010, 12, 10914–10918 This journal is c the Owner Societies 2010

the terminal and two-fold bridge sites, and the Bz ring of

thiolate is inclined at 91 with respect to the surface normal

(see Fig. 2 and 3). In the case of the bridge configuration, the

Bz ring of thiolate is tilted by 491 from the surface normal, and

consequently the total energy increases due to the steric

repulsion between neighboring molecules. The nearly upright

Fig. 1 He I UPS and He*(23S) MAES spectra of C6H5SH condensed on Pt(111) at 135 K, together with the gas-phase MAES spectrum. The

thickness of the condensed film is estimated to be about five monolayers. The isosurface plots of the 5a0 0(p4), 4a0 0(p3), 3a0 0(p2), and 24a0(sCS) MOs

obtained by ab initio calculations using a 6-31G+(d,p) basis set are also shown.

Fig. 2 (A) He*(23S) MAES spectra of benzenethiolate formed on the Pt(111) substrate at 200 K as a function of exposure. For comparison, the

MAES spectrum of a C6H5SH film with a thickness of five monolayers is also shown. (B) Schematic view of Penning ionization in the

He*(23S)–thiolate collision system, where the adsorption geometry was obtained by the total energy calculations.

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configuration of thiolate is consistent with a previous vibrational

analysis.35 A similar DFT calculation has been performed for

Pt(111)(O3 � O3)R301–CnH2n+1S (n = 1–3),9 which show

that alkanethiolates are bound preferentially on the bridge

sites of the (111) plane.

Fig. 3 shows the local density of states (LDOS) divided

into the Pt, S, C and H atomic orbitals for Pt(111)(O3 �O3)R301–C6H5S. For comparison, the band positions in the

MAES spectra are also shown in the figure. The discrete

molecular levels are broadened due to the orbital mixing with

the substrate bands and the orbital overlap between neighboring

molecules. In particular, the S 3p-derived levels are widely

distributed below and above EF to form a metallic state. The

low-density part just below EF corresponds to the CIGS

observed in the MAES spectra. The LDOS at EF decreases

drastically on going from the S atom to the adjacent C1 atom

and to the outer C2–C4 atoms, indicating that the metallic

wave functions are strongly damped with increasing distance

from the S atom. In other words, benzenethiolate is not a good

mediator of metal wave functions at EF. The prominent

humps around 3 eV below EF are attributed to the C6H5S

p-derived states. The energy splitting among the p-derivedstates is not discernible at the C atoms, as observed in the

MAES spectrum. Furthermore, the corresponding hump is

rarely seen at the S atom. These features clearly indicate that

the strong Bz p–S 3p conjugation is disrupted upon the

formation of the S–Pt bond, causing a marked attenuation

of metal wave functions at the Bz ring. As is seen in Fig. 3, the

higher-lying structures derived from the Bz p1 and s MOs can

be related to the observed bands (although the calculated

LDOS shifts to the lower binding energy). Several limitations

in the GGA or local density approximation (LDA) have been

pointed out when applied to the description of metal–organic

interfaces.39,40 However, the present calculations at the GGA

level are adequate for analysis of the MAES spectra, especially

of the formation of CIGS and weakening of strong Bz p–S 3p

conjugation.

Finally, we briefly address the relationship between the

CIGS and transport properties in the molecular junctions.

The CIGS in organic–metal systems can be classified by the

asymptotic behavior of metal wave functions exposed into the

chemisorbed species. In the damping (or non-resonance) type,

the metal wave function tails a short distance into the organic

molecule and the amplitude attenuates exponentially. In the

propagating (or resonance) type, it penetrates sufficiently inside

the organic molecule and the amplitude does not decay by

coupling with the organic MOs. The CIGSs in alkanethiolate

(CnH2n+1S, n = 1–3)/Pt(111)9 and C60/Pt(111)10 are their

typical examples, respectively. This classification is easily applied

to the metal–organic–metal junction systems. In the damping

type, the metal wave functions strongly attenuate inside the

sandwiched molecule, leading to a substantial gap in the relevant

energy region. In the propagating type, they are distributed

entirely in the system, forming a so-called resonance state. These

features of CIGS (especially near EF) play an essential role in

charge transport across the organic–metal interface.

Fig. 3 Local density of states (LDOS) divided into the Pt, S, C and H atomic orbitals for benzenethiolate on Pt(111). The optimized geometry

based on the total energy calculations is also shown.

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Recently Kiguchi et al. traced the I–V curves for the

Pt–BDT–Pt system using STM and break junction methods,

and obtained the differential conductance of B3 � 10�2 G0

(where G0 is the quantum unit of conductance) at zero bias.15

This indicates that the transmission coefficient through the

junction is only B3% in the Landauer limit.41 Although this

value is higher than those of ordinary Au–BDT–Au junctions,

i.e., 4� 10�4 G0 (1 V bias),12 4� 10�3 G0,14 or 1.1� 10�2 G0,

13

it is much lower than that in a Pt–C60–Pt junction (B0.7 G0)23

and the same order of magnitude as that of a Pt–1,6-hexane-

dithiolate–Pt junction.24 These results may be strange at first

glance, because free BDT and C60 are typical p-conjugatedsystems while alkanethiolate is a non-conjugated system. As

indicated by the present study for C6H5S/Pt(111), the Bz p–S3p conjugation is heavily destroyed upon chemisorption and

the metal wave functions at EF attenuate rapidly from the

terminal S atom to the Bz ring. This would also be the case for

the Pt–BDT–Pt junction system. As a consequence, the charge

transport through BDT is governed mainly by non-resonant

tunneling rather than resonant tunneling, causing low con-

ductivity at zero bias.

5. Conclusion

We have examined the local electronic states of benzenethiolate

formed on Pt(111) using MAES and first-principles DFT

calculations. The chemisorption-induced gap states are derived

from the S 3p–Pt 5d couplings, yielding a sharp Fermi edge.

The local density of states at EF decreases drastically from the

S terminal to the benzene ring. Furthermore, the strong

benzene p–S 3p conjugation is apparently lifted upon the

formation of thiolate. These features indicate that thiolate is

not a good mediator of metal wave functions at EF, which is

closely related to tunneling probability (and eventually electric

conductance) in the relevant metal–organic–metal junctions at

zero bias.

Acknowledgements

This study is financially supported through Special Coordination

Funds of theMinistry of Education, Culture, Sports, and Science

and Technology of the Japanese Government.

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