e-Journal of Surface Science and Nanotechnology

18 July 2015

Conference - ISSS-7 -e-J. Surf. Sci. Nanotech. Vol. 13 (2015) 373-379

Interface Electronic Structures of the L-Cysteine on Noble Metal Surfaces Studied by
Ultraviolet Photoelectron Spectroscopy∗
Kaveenga Rasika Koswattage†
Center for Frontier Science, Chiba University, 1-33 Yayoi-cho, Inageku, Chiba 263-8522, Japan

Hiroumi Kinjo
Graduate School of Advanced Integration Science, Chiba University,
1-33 Yayoi-cho, Inageku, Chiba 263-8522, Japan

Yasuo Nakayama
Department of Pure and Applied Chemistry, Faculty of Science and Technology,
Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan

Hisao Ishii
Center for Frontier Science and Graduate School of Advanced Integration Science,
Chiba University, 1-33 Yayoi-cho, Inageku, Chiba 263-8522, Japan(Received 26 December 2014; Accepted 11 June 2015; Published 18 July 2015)
L-cysteine, one of amino acids, is well known in the bioelectronics ﬁeld as a linker between proteins and metalelectrodes. The interface electronic structures between L-cysteine and metals are therefore very crucial because they
dominate the charge carrier injection characteristics into such biological systems. However the interface electronic
structures of L-cysteine and metals have been not well understood. In this study, the electronic structures at
the interfaces of sequentially deposited L-cysteine layers on the metal surfaces of Au(111), Ag(111), and Cu(111)
were investigated by thickness-dependent ultraviolet photoelectron spectroscopy (UPS). At the contact regions,the electronic structures of L-cysteine revealed modiﬁcation with respect to its bulk phase and signiﬁcant variation
depending on the substrate. The electronic structure at the interfaces including work function, secondary electron
cutoﬀ (SECO), highest occupied molecular orbital (HOMO) onset, position of an interface state, charge injection
barrier, and ionization energy were estimated for each case. [DOI: 10.1380/ejssnt.2015.373]
Keywords: Photoelectron spectroscopy; L-cysteine; Single crystal surfaces; Gold; Silver; Copper; Electronic structure

I.

INTRODUCTION

L-cysteine, as an amino acid on the outer fringes oflarge proteins, can provide a link between those proteins
and inorganic metal substrates. For the application of
biomolecules (e.g., proteins) in electronic devices, it is essential to bond and organize the biomolecules on metalelectrodes. It is expected that the L-cysteine presumably employs multiple functional groups (i.e., SH, NH2 ,and COOH) in its bonding to metal surfaces and anchors
to the metal surface using either chemisorption (similar to thiolate-metal bonding) or physisorption (similar
to the bonding to metal surfaces by the amino and carboxyl functional groups). These bonding strengths of Lcysteine, in particular the involvement of SH with theother functional groups, depend on the metallic partner
properties [1]. Therefore, an understanding of the interaction of L-cysteine with metal surfaces is necessary.
There have been experimental and theoretical researchstudies that examined the behavior of L-cysteine adsorbed
on diﬀerent faces of gold [2–19], silver [20–23], and copper [24–26] metallic single crystals as model systems forunderstanding the interaction of L-cysteine with metal
surfaces and also L-cysteine adsorbed on some other surfaces considering technological interests [27–35]. In the∗ This paper was presented at the 7th International Symposium on
Surface Science, Shimane Prefectural Convention Center (Kunibiki
Messe), Matsue, Japan, November 2-6, 2014.
† Corresponding author: rasika@chiba-u.jp

experimental studies, the L-cysteine sample for investigation was formed using either the self-assembly or evaporation method.
Some studies reported that L-cysteine adsorption on
metallic surfaces involved a strong chemical and electronicinteraction between the cysteine and the surface, and thatthe interactions may strongly inﬂuence the properties of
the L-cysteine–metal interface. For instance, cysteine–
substrate bonding may result in the formation of electronic states in the energy gap of the molecular system,
which are interesting and important to investigate the useof L-cysteine for electronics applications because such astate inﬂuence the charge carrier injection characteristics
across the cysteine/electrode junction [18]. Even thoughsome research eﬀorts have been focused on understanding of the behavior of cysteine on metallic surfaces, itremains diﬃcult to compare the experimental results because of diﬀerences among experimental conditions, sample preparation, etc. Thus, it is necessary to perform a
research study in which a comparison of the behavior ofcysteine on diﬀerent metallic surfaces in the same experimental conditions. Furthermore, detailed understandingsof the interface electronic structure of cysteine on metallic surfaces including charge injection barrier across theinterface, existence of possible interface state and behavior of vacuum level shift have been hardly reported. Suchﬁndings are necessary for designing the function of devices
such as bio-electronic devices and biosensors, because carrier exchange between electrodes and bio-active layer is an
essential process. Consequently, as the initial step, thisstudy focuses on elucidating the electronic structures at

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ISSN 1348-0391 ⃝373

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the interfaces of successive L-cysteine layers and the metal
surfaces Au(111), Ag(111), and Cu(111) by ultraviolet
photoelectron spectroscopy (UPS) using synchrotron radiation, where thickness-dependent UPS is employed fora detailed analysis of the electronic structure at the interfaces.
So far, Beerbom, et al. [36] performed a multistep deposition sequence of cysteine on Au surfaces to investigate
electronic and chemical structure of the interface between
L-cysteine and Au, depositing L-cysteine by repeatedly
dipping the Au substrate into a solution of L-cysteine.
Based on their UPS results, they have suggested the formation of an interface state around 1.5 eV above the
HOMO of L-cysteine. However, this interface state is difﬁcult to be distinguished in their results; they consider
the conclusion to be an assumption limited by weak emission under the experimental conditions. Further, Ogawa,et al. [33, 34] has performed UPS to study the electronic
structures of dental alloys and their interaction with Lcysteine. In the study, they have performed UPS of vacuum evaporated thin ﬁlms of nominal thickness 3 Å andthick ﬁlms of nominal thickness 60 Å on polycrystalline
Au, Ag, Pd, Cu in order to analyze L-cysteine interaction with dental alloys. They have observed the interface electronic structure which was assumed by Beerbom,
et al [36] through their UPS results; however Ogawa, etal. [33, 34] results are not suﬃcient for a systematic elucidation and comparison of the interfaces of L-cysteine
on metallic surfaces because there is no experimental evidence to understand the vacuum level shift at interfaces
and, it is diﬃcult to perform a systematic evaluation of
the interfaces because limited thickness of the ﬁlms are
considered.
In the present study, a vacuum evaporation method
was used to form the L-cysteine ﬁlms on the metallic
substrates due to the following advantages for the investigation. Vacuum deposition can control the L-cysteine
coverage precisely on the substrate, which is importantfor such an interface investigation employing thicknessdependent UPS. In the case of cysteine on metallic substrates, the adsorption conﬁguration involving all thefunctional groups has been determined to be energetically favored at low coverage and the adsorption mechanism changes (chemisorption to physisorption, weakeningof the cysteine–metal bond, etc.) with increase of coverage [21]. Thus, the precise control of the thickness, especially at low coverages, is essential in the present study.
It has been reported that smooth L-cysteine ﬁlms can beformed without decomposition using a vacuum deposition
method [37]. Furthermore, although most amino acids inthe solid states and aqueous solutions tend to exist in
zwitterionic isomers where the amino group is protonatedto ammonium and the carboxyl group is deprotonatedto carboxylate, while amino acids in the gas phase commonly exist in nonzwitterionic neutral forms. In aqueous solution, the zwitterionic form is stabilized by hydrogen bonding with water molecules. However, it has beensuggested that diﬀerent chemical states of L-cysteine adsorbed on metallic substrates using aqueous solution arestrongly correlated to the pH of the solution during adsorption [38–41]. Thus, investigations of cysteine–metalbonding using aqueous solutions have revealed complex
scenarios. In addition, it is essential to perform cysteine374

adsorption processes in an ultraclean condition to enable
essential contamination-free deposition during the experiments, where such a contamination may aﬀect the intermolecular interaction and hence the ordering on thesurface.

II.

EXPERIMENTAL

The clean, atomically ﬂat surfaces of the Au(111),
Ag(111), and Cu(111) single crystals were prepared byrepeated cycles of Ar-ion sputtering and annealing. Lowenergy electron diﬀraction measurements of the metallic substrates were performed to verify the surface quality. The L-cysteine was purchased from Aldrich (Aldrich,
99% L-cysteine), which was inserted into a quartz cylindrical crucible wound with a tungsten wire for heatingand evaporated at a constant rate of 0.3 Å/s (as measured by a quartz microbalance to form cysteine layers on
the substrates). Hence, the thickness values for L-cysteine
coverage are nominal ﬁlm thicknesses. When the desired
rate of evaporation was reached the sample was moved
into position from a preparation chamber, turned to facethe evaporator for the desired time, turned upright again,
and moved into the measurement chamber. Subsequently,
a set of UPS measurements was performed at each stepof the thickness accumulation.
The UPS measurements were performed at the beamline BL8B of the Ultraviolet Synchrotron Orbital Radiation (UVSOR) facility of the Institute for Molecular Science, Japan [42]. The UPS spectra were measured using
an analyzer with a multichannel detector system (VG,
ARUPS10). The angles of incident photons and the detected photoelectrons were 45◦ and 0◦ , respectively, relative to the substrate surface normal. The SECO were
measured with the sample biased at −5 V. All the preparation steps and the measurements were performed atroom temperature. To control for beam damage, adjustment for diﬀerent photon energies of the intense synchrotron beam and sequential short periods of acquisition
on diﬀerent zones of the sample were performed. Further,
for thicker ﬁlms, photoemission-induced sample charging
was carefully monitored by comparing consecutive UPS
measurements at the same experimental conditions.

III.

RESULTS AND DISCUSSIONS

Figure 1 shows the thickness-dependent UPS spectra of
L-cysteine on (a) Au(111), (b) Ag(111), and (c) Cu(111)
for the photon energy of 28 eV. In each ﬁgure, on the left,
the SECO is shown on a magniﬁed scale. On the right,
the region between Fermi level of the metal substrate andthe HOMO of the L-cysteine measured with a relatively
small energy step is also shown. The work functions forclean Au(111), Ag(111) and Cu(111) were estimated tobe 5.2, 4.5, and 4.9 eV, respectively.
Each d band peak of the metallic substrates can be assigned as follows: for Au, the peaks at around 3.6 and
6 eV are Au 5d5/2 and Au 5d3/2 ; for Ag, the peaks ataround 4.8 and 6.1 eV are Ag 4d5/2 and Ag 4d3/2 ; and for
Cu, the peaks at around 2.8 and 3.6 eV are Cu 3d5/2 andhttp://www.sssj.org/ejssnt (J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/)

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FIG. 2: Photon-energy dependent UPS spectra of L-cysteine
thick ﬁlms obtained with photon energies from 30 to 100 eV.

FIG. 1: Thickness-dependent UPS spectra of L-cysteine on (a)Au(111), (b) Ag(111), and (c) Cu(111) at a photon energy of
28 eV. In each ﬁgure, on the left, the secondary energy cutoﬀ(SECO) is shown on a magniﬁed scale. On the right, the region
between the Fermi level of the metal substrate and the HOMOof the L-cysteine measured with relatively small energy step isalso shown.

Cu 3d3/2 [43, 44]. In the case of Au and Cu, the Shockley surface state was observed. For all the substrates, themetal Fermi edges are no longer visible at 64 Å; therefore, at 64 Å, the L-cysteine spectra may represent only
the electronic structure of L-cysteine and the spectra areconsistent with the previous reports of UPS spectra of
thick ﬁlms of L-cysteine [33, 34, 36, 37].
It is clear from Fig. 1 that thin ﬁlm spectra in valance
top region depends on substrate and they are diﬀerent
also from the L-cysteine thick ﬁlms. These features imply
that the electronic structures of L-cysteine on metallic

substrates are strongly dependent on the metallic partner.
In Fig. 1, the feature centered at around 4.8 eV binding
energy for L-cysteine can be attributed to the HOMO
of L-cysteine where the HOMO threshold is estimated tobe 3.2 eV and the HOMO positions of all of the thickest
layers are almost identical for all the substrates. Thisvalue agrees with the HOMO threshold value which wehave roughly estimated from the UPS data of L-cysteine
thick ﬁlm of Kamada, et al. [37]. However, it is diﬀerent
from the value by Beerborm, et al. 3.0 eV [36]. This
discrepancy may be due to the diﬀerence in zwitterionicform.
For the thin ﬁlms on all the substrates, a new structure appears between the Fermi level and the HOMO of
L-cysteine. The structure may be due to the bonding oforbitals of the S atom in the thiol group with the Au 5d,
Ag 4d, and Cu 3d states separately for all the substrates.
However, in the case of Au and Cu, it is diﬃcult to distinguish this new feature from the d-bands contributionof the substrates. In the case of Ag, the new feature appears as one peak. Furthermore, it is also observed inFig. 1 that there is a shift of the peak at ∼5 eV and at∼9 eV (dotted lines). These shifts may be due to the formation of additional chemical bonds between the metal
and cysteine (oxygen or nitrogen metal bonds). We will
discuss brieﬂy this interface state and bonding conﬁguration after discussing the valence electronic structure of
L-cysteine.
It has been veriﬁed that the top of the valence bandof cysteine originates with sulfur using photon-energy
dependent UPS measurements and theoretical calculations [7, 8, 37]. In this experiment, we also performed
photon-energy dependent UPS measurements for the
thickest ﬁlms where the UPS spectra were obtained withphoton energies from 30 to 100 eV, and the results areshown in Fig. 2. The spectra are almost identical tothose of Kamada, et al. [37], and thus the structures A
to E shown in Fig. 2 are labeled in terms of their assignments. In Fig. 2, it is clear that with increasing photon
energy, the intensity of structure A increases, while thoseof structures C to E decrease. This indicates that the top

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Volume 13 (2015)of the valence band has a diﬀerent origin from the otherstructures. The structure around the top of the valenceband can be attributed to sulfur, while structures C to
E can be to carbon, nitrogen, and oxygen, respectively,
considering the photon-energy dependence of the ionization cross-sections of the atoms in the L-cysteine. Hence,
it can be concluded that the sulfur-originated state is located at the top of the valence band and peak at ∼5 eV
and at ∼9 eV are mainly contributed by O, C, and N
atom-originated states of O 2sp, C 2sp, and N 2sp, respectively.
In the following, the new structure between the Fermi
level and the HOMO of the L-cysteine thin ﬁlm will be
brieﬂy discussed considering L-cysteine on Au(111). As
can be found in Fig. 1(a), in particular, an intensity increase is clearly observed in the 1–2 eV binding energy
region, while the Au d bands in the 2–7 eV region show
intensity reduction and shape modiﬁcation. These spectral changes appear to be due to a sulfur-originated stateof L-cysteine HOMO interaction with Au d orbitals. This
interaction can be discussed and interpreted in the framework of an Anderson–Newns model, which describes theinteraction of a localized atomic orbital with an extended
metallic orbital [7, 8, 18]. In this model, for a metal
surface, whose electronic structure is characterized by ahalf-ﬁlled broad s band and completely occupied narrowd bands, the model predicts that the atomic orbitals ofadatoms are modiﬁed by the hybridization of both thesebands. Compared to the broad s band, the narrow d bands
may be viewed as a localized level, and therefore the coupling of the atomic orbital with such bands causes bothbonding and antibonding states, analogous to the bonding and antibonding molecular orbitals, which are formedby the orbitals of two interacting atoms. Thus in the caseof L-cysteine on Au(111), the sulfur 3p orbitals of the Lcysteine molecule (which correspond to its HOMO) couple
to the Au(111) metal d orbitals. That is, molecule–metal
hybridization can occur forming bonding and antibonding
states [18, 45]. Therefore, the spectral change in the UPS
spectra can be attributed to a hybridization of the Au dstate with the S 3p orbitals where the spectral changes
in the 1–2 eV region and above the d band edge can be
attributed to S–Au antibonding and bonding states. In
particular, some theoretical studies have predicted that in
the L-cysteine on Au system, the S–Au antibonding stateis located at around 1.5 eV binding energy and that the
S–Au bonding region is around the lower edge of the Au
d bands [7, 8]. Thus, the increase in the 1–2 eV region
can be attributed to the formation of the S–Au antibonding state, which corresponds to the interface state. This
attribution is in substantial agreement with previous experimental interpretations by Beerborn, et al. though theobserved features in their results are broad and thus precise determination of the location of these states is diﬃcult [36]. Furthermore, the shape modiﬁcation of the dbands region of Au around 2–7 eV is consistent to formation of the S–Au bonding state [7, 8, 18].
Considering the SECO shift that is apparent in thework function change at the interface can be interpreted
as an interface dipole. In the case of L-cysteine on
Au(111), the maximum shift of SECO is 1.1 eV to ahigher binding energy side at 2 Å, and the value is inagreement with the results of Beerbom, et al. [36] for cys376

teine deposition from solution, where the SECO change(as an interface dipole) was attributed to molecule–metal
charge transfer. However, for a system such as L-cysteine
on metallic surfaces, the formation of interface dipoles
can be attributed mainly to the intrinsic dipole of theadsorbed molecule and the charge transfer across the interface. Thus, there can be possible important contributions of the intrinsic molecular dipole moment thatneeds to be considered for the attribution of SECO. However, under the circumstances, it is not so straightforward to assign amount of contributions separately from
intrinsic dipole moments and charge transfer to interface
dipoles because L-cysteine adsorption on gold in diﬀerent adsorption geometries and chemical forms (radical,
anion, and zwitterion) has been theoretically reported
to include an induced intrinsic dipole moment (for theinterface dipole); the interface charge transfer depends
strongly on the molecular conﬁgurations [46]. By considering the SECO value, the electrostatic energy diﬀerence
across the interface (eD) due to the dipole layer is derived to be 1.10 eV, which puts the work function of the
L-cysteine layer to 4.1 eV. Adding the work function of
the cysteine on Au(111) and the charge injection barrier
yields, an ionization energy for the L-cysteine layer canbe estimated as 7.30 eV.
In the case of L-cysteine on Ag(111), the maximum
SECO shift was 0.46 eV to the higher binding energy
side at the nominal thickness of 1 Å. However from thenominal thickness of 2 Å, SECO started to shift to thelower binding energy side, and at 16 Å, the SECO shiftedto a value of around 0.4 eV to the lower binding energy
side where eventually cancel the initial vacuum level shift.
This might be attributed to either of the following reasons concerning over layer formation: 1) weakening theinteraction of L-cysteine with Ag(111) reduces the amountof charge transfer, therefore the strength of the interface
dipole can be decreased, 2) change of the adsorption orientation aﬀects the intrinsic dipole moment of cysteine;parallel to the metal surface reduces the intrinsic dipole
moment contribution to the interface dipole, and 3) a
combination of these two phenomena. Fischer, et al. [20]addressed evaporated L-cysteine on Ag(111) and reported
that L-cysteine forms a dense-packed layer on Ag(111).
In this case, the monolayer coverage is chemosorbed toAg(111) forming silver–sulfur bonding, and L-cysteine exists mainly as a zwitterion for all the layers. Furthermore, when the L-cysteine coverage is increased outside
the monolayer, the next cysteine layers physisorb on topof the chemisorbed monolayer. However, Luque, et al. [21]have reported that there is a weakening of the silver–sulfur
bond with increasing coverage of L-cysteine on silver; thissuggestion is consistent with our UPS results where the
SECO shifts toward the low-BE side at low coverage withincreasing thickness.
In the case of L-cysteine on Ag(111), the feature related
to interface structure appeared as a clear peak centeredat 2.5 eV with a cutoﬀ at 1.9 eV. The reason for the clear
feature is due to a deeply available d band and strong
interaction of thiolate upon the adsorption of L-cysteineon Ag(111). However, it has been reported in the caseof L-cysteine on Ag(111) that the only situation where
the bonding and antibonding states are possible if the adsorbate cysteine is “pushed” into the surface at shorter

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FIG. 3: Schematic of the determined electronic structure at the interface of L-cysteine on (a) Au(111), (b) Ag(111), and (c)Cu(111).

distance than the equilibrium position. The shifting of
the peaks at ∼5 and ∼9 eV for thick ﬁlms toward thehigher binding energy side, as is shown in Fig. 1 (dotted lines), may explain the bonding of amino or carboxylate groups to metal surfaces. As we have described in
the photon-energy dependent UPS results, the features
around ∼5 and ∼9 eV can be assigned as O, C, and N
atom-originated states. Thus, the shifting of peaks at∼5 and ∼9 eV may be due to the interaction of amino
and carboxylate groups (O 2sp and N 2sp orbitals) withsilver surface. This interaction may allow pushing adsorbate L-cysteine in to silver surface at shorter distanceand consequently, satisfying the the possible situation toform bonding and antibonding states due to sulfur-silver
interaction
In the case of L-cysteine on Ag(111) in the Fig. 1 (b),clear spectral changes can be observed comparing 8 Å and
16 Å thickness. Those spectal changes are SECO shiftedto the lower binding energy side by 0.3 eV, the peak centered at ∼9 eV shifting to higher binding energy side, anda disappearance of the peak corosponding to the interface
state. These changes may be due to a formation of second
layer or more on the chemosorbed ﬁrst layer. Because, theformation of over layers can be weakening the interaction
between ﬁrst layer and as a result the SECO shift anddisappearance of the peak of the interface state are possible [21, 22]. Further, the over layers on Ag(111) are
possible to be formed in the zwitterionic form and consequntly a shift of the peak ∼9 eV is possible [21, 22] .
Thus, it may be possible to assign the nominal thicknessof 8 Å layer as around monolayer of L-cysteine.
In contrast to the L-cysteine on Ag(111), the absence ofa clear feature of the L-cysteine on Au(111) from 1 to 2 eV
can be ascribed to the emission of Au(111) and partial
physisorption of L-cysteine on Au(111). There have been
experimentally veriﬁcations at room temperature the Lcysteine molecule is mainly weakly adsorbed on the gold
surface, based on the L-cysteine on Au(111) system [19,23].
Canepa, et al. [19] has performed metastable deexcitation spectroscopy (MDS) investigation of the growthof L-cysteine layer deposited under UHV conditions on
Au(111) and suggested that the early stages of growthcontains of strongly and weakly bound cysteine molecules.
Santos, et al. [23] has reported that L-cysteine is adsorbed
on Ag(111) stronger than on Au(111) through their investigation of electrochemical behavior and nonlinear opticalproperties of L-cysteine on Ag(111).
According to an XPS study of Uvdal, et al., [1] Lcysteine adsorbed on copper is not well organized. When

compared with S 2p XPS results, L-cysteine adsorbed on
copper shows analogous results to S 2p XPS results of
L-cysteine adsorbed on gold. However, the N ls spectra show some new features for the adsorbate, where neither the N 1s nor the O 1s XPS spectra show signiﬁcant changes in peak position during adsorption on gold,when compared with the multilayer ﬁlms. Furthermore,
the angle dependence of the XPS results of O 1s suggests carboxylate groups may be oriented toward the surface. Consequently, Uvdal, et al. [1] suggested that Lcysteine adsorbed on copper employs at least two functional groups where both the thiol and amino groups participate in the reaction with the copper and the carboxylate groups interact weakly. However, according to our
results, the formation of bonding between amino and carboxylate groups with metal surfaces are possible for Cu as
well as Au and Ag. There are research works suggesting
L-cysteine adsorption process involves not only contribution of thiol but also those of the other existent functionalgroup in the cases of Au and Ag by various experimental methods. Tawil, et al. [47] has performed surface
plasmon resonance investigation on L-cysteine on gold
and indicate that the adsorption of L-cysteine involving
both amino and thiol interactions. Brolo, et al. [48] has
performed surface-enhanced Raman spectroscopy (SERS)
and surface-enhanced second-harmonic generation measurements to investigate adsorption behavior of L-cysteine
onto a polycrystalline Ag and found that L-cysteine adsorption on Ag through the sulfure group together a weakbonding through either carboxylate or amino group. Our
UPS results are consistent with above studies as formation of bonding between amino and carboxylate groups
with metallic surfaces are possible in the case of Au andAg which is not consistent with the earlier study of Uvdal,
et al [1]. Furthermore, Honda, et al. [49, 50] has investigated chemical bonding states of Au-cysteine interface
and successfully observed speciﬁc Au–S bond formation
only in mono-layered L-cysteine on gold through S K-edge
NEXAFS and S 1s XPS measurements. They suggested
speciﬁcally strong electron donation from sulfur to gold
which is absent for other metal such as copper due to a
weak bonding. However, our results contradict with theirsuggestion as strong Cu–S bond is possible from our UPS
results.
The charge injection barrier from the L-cysteine HOMO
to the metal Fermi level can be determined as 3.2 eV in
the case of Au, Cu, and Ag. This barrier height is too highto inject carrier, however existence of interface state canbe facilitated carrier injection and the interface sate canbe less than 1 eV from the Fermi level for Cu substrate

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expecting better carrier injection. Considering the work
function of metallic substrates and SECO, the ionizationenergies are identical on all three substrates within theuncertainty of 0.1 eV, namely 7.3 eV on Au(111), 7.2 eV
on each Ag(111) and Cu(111).
Finally, Fig. 3 shows a schematic representation of theelectronic structure at the interface in the cases of Lcysteine on Au(111), Ag(111), and Cu(111), including estimated values of work function, SECO, HOMO cutoﬀ,
position of the interface state, charge injection barrier,and ionization potential for each case separately. As isclear from Fig. 3, the position of the interface state fromthe Fermi level of the metal is diﬀerent on the kind of
metallic partner. It can be suggested that the position of
the interface is dependent on the position of the d bandof the metal.

spectroscopy. In all cases, we have determined the existence of a new state near the Fermi edge and interpreted
it as a metal–cysteine antibonding state. Based on theliterature, the corresponding bonding state is assumed toappear at the lower edge of the d band, which is diﬃcult
to distinguish because of the emission from the d band.
The formation of such interface electronic states, which
are induced by the interaction between the molecule andthe metallic surfaces, can strongly aﬀect the charge transport across the junction. Thus, the position of these newstates relative to the Fermi level is clearly fundamental indetermining the junction properties.

Acknowledgments

We have demonstrated diﬀerent interaction strengthand conﬁgurations of L-cysteine on the metallic surfacesof Au(111), Ag(111), and Cu(111) using photoemission

The UPS experiments were performed under the approval of the Joint Studies Program (25-562) of the Institute for Molecular Science, Japan. This study was also
partially supported by JSPS-KAKENHI (25288114) and
G-COE Program of Chiba University, Japan (Advanced
School for Organic Electronics).

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