Published: August 18, 2011
r 2011 American Chemical Society 18378 dx.doi.org/10.1021/jp206455a | J. Phys. Chem. C 2011, 115, 18378–18383
ARTICLE
pubs.acs.org/JPCC
Metal�Semiconductor Contacts Induce the Charge-Transfer
Mechanism of Surface-Enhanced Raman Scattering
Zhu Mao,† Wei Song,† Lei Chen,† Wei Ji,† Xiangxin Xue,† Weidong Ruan,† Zhishi Li,† Huijuan Mao,†
Stephen Ma,‡ John R. Lombardi,‡ and Bing Zhao*,†
†State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Qianjin Street 2699, Changchun 130012, P. R. China
‡Department of Chemistry, The City College of New York, New York, New York 10031, United States
bS Supporting Information
’ INTRODUCTION
Control over the electron transport properties of metal�
semiconductor interfaces is important because of the central role
that these junctions play in (opto) electronic devices for Ohmic
and Shottky contacts.1�3 Micro-nanoscale charge transfer (CT)
in such a junction is vital to both the frontier of fundamental
science and applications in molecular electronics. In micro-
nanoscale systems involving CT, some situations are still unclear.
Hence, progress in the area of micro-nanoscale CT requires
interdisciplinary collaboration, a challenging range of experi-
mental techniques for probing CT processes, and a theory for
their interpretation.4
Initial reports of surface-enhanced Raman scattering (SERS)
signals from single molecules adsorbed on colloidal particles5,6
have led to renewed interest in SERS field in the past decade.
Along with its extraordinary specificity with respect to chemical
structure, SERS appears as an ideal process for the realization of
highly sensitive sensing and imaging applications with both metal7
and semiconductor particles.8,9 Two main mechanisms are gen-
erally thought to account for the enhancement in SERS, i.e., the
electromagnetic (EM) and chemical mechanisms.10 Typically,
the chemical enhancement is explained via the CT mecha-
nism.11 In many SERS studies, the difficulty in experimentally
gaining deep insight into the enhancement associated with the
CT mechanism lies in the fact that these chemical enhancements
are normally inextricably linked with EM enhancements.12,13 We
employed a p-aminothiophenol (PATP) molecular layer inter-
connecting the metal and ZnO surface to identify Raman
modes. In this paper, we have fabricated three different kinds
of micro-nanoassemblies (ZnO�PATP�Ag, Au�ZnO�
PATP�Ag, and Cu�ZnO�PATP�Ag) to investigate the SERS
of molecules interconnecting nanoscale Ag and ZnO to study the
CT effect in such connections.
’EXPERIMENTAL SECTION
Silver nitrate (AgNO3) and PATPwere obtained from Aldrich
and Acros Organics Chemical, respectively. Trisodium citrate
(C6H5Na3O7 3 2H2O), zinc nitrate [Zn(NO3)2 3 6H2O], and
sodium hydroxide (NaOH) were purchased from Guangfu
Chemical Co. Pure ethanol (C2H5OH) and ethylenediamine
[C2H4(NH2)2 or EDA] were purchased fromBeijingChemical Co.
All the reagents were used as received without further purification.
The water that was used was purified through a distillation system.
Silver colloid was prepared according to the literature
protocols.14 The particular description is as follows. AgNO3
(36 mg) was dissolved in 200 mL of H2O and the mixture
brought to a boil. A solution of 1% sodium citrate (4 mL) was
added. The solution was kept boiling for ∼1 h. The Ag colloid
was greenish yellow and exhibited an absorption maximum at
420 nm.
In a typical procedure, a gold sheet was washed first with
ethanol and then with water. Subsequently, the gold sheet
together with a mixture solution for the growth of the
ZnO one-dimensional nanorod film15 was transferred into a
Received: July 8, 2011
Revised: August 18, 2011
ABSTRACT: A model metal�semiconductor�molecule�
metal assembly has been designed for probing the charge-
transfer (CT) mechanism of surface-enhanced Raman scatter-
ing (SERS). We measured the SERS of ZnO�PATP�Ag,
Au�ZnO�PATP�Ag, and Cu�ZnO�PATP�Ag assemblies
at excitation wavelengths of 514.5, 785, and 1064 nm. Our
results demonstrate that the metal�semiconductor contact
can alter the charge distribution through p-aminothiophenol
(PATP) molecules. This is attributed to the chemical SERS
enhancement mechanism with additional electrical transport
properties within these assemblies. These inhibit the CT from
the metal to the molecule, resulting in the different degrees to which CT contributes to the overall SERS enhancement of PATP.
18379 dx.doi.org/10.1021/jp206455a |J. Phys. Chem. C 2011, 115, 18378–18383
The Journal of Physical Chemistry C ARTICLE
Teflon-lined autoclave.We prepared themixture solution bymixing
3 mL of an alkali solution of zinc (0.50 M [Zn(NO3)2 3 6H2O]
and 10.00 M NaOH) with 5.0 mL of deionized water and
30.0 mL of pure ethanol (C2H5OH), followed by the addition
of 6.0 mL of ethylenediamine [C2H4(NH2)2 or EDA]. Before
being transferred to a Teflon-lined autoclave, the mixture solu-
tion was pretreated under an ultrasonic water bath for 40 min.
The hydrothermal syntheses were conducted at 180 �C for 20 h
in an electric oven. After cooling to room temperature, the white
nanocrystal ZnO films over a gold sheet (Au�ZnO assembly)
were harvested by successive washings with deionized water and
finally dried with a nitrogen flow. Au�ZnO�PATP assemblies
were obtained by immersing freshly prepared Au�ZnO struc-
tures in a 1 mM PATP solution in ethanol at room temperature
for 30 min. Then the prepared samples were immersed in silver
colloid for 30 min. For the fabrication of Cu�ZnO�PATP�Ag
and ZnO�PATP�Ag assemblies, the same procedure was
conducted except that the ZnO nanorod film was deposited on
the surface of the copper sheet and glass slide, respectively.
The SEM measurements were performed on an EOL JSM-
6700F field emission scanning electron microscope (FE-SEM)
operated at 3.0 kV. Surface-enhanced Raman spectra were
obtained with a FT-Raman spectrometer (Thermo Nicolet
960) equipped with an InGaAs detector and a Nd/VO4 laser
(1064 nm) as an excitation source (laser power of ∼300 mW),
and a Renishaw Raman system model 1000 spectrometer with
the 514.5 nm argon ion laser exciting source (laser power at the
sample position of typically 400 μWwith an average spot size that
was 1 μm in diameter). The spectral resolution was 4 cm�1 at the
excitation wavelength. The 785 nm excitation line was from the
BRM-785 laser and was used as the excitation source.
’RESULTS AND DISCUSSION
The Au�ZnO�PATP�Ag assembly is illustrated in Scheme 1.
The one-dimensional ZnO nanorod film over Au sheets was
prepared by a wet-chemical approach. PATP is considered to be a
good candidate for probing SERSmechanism, because of its well-
established Raman spectral data.16�18 The PATP molecules can
be immobilized to the surface of the assembled ZnO nanorod
through the formation of Zn�S bonds.16 The shape and size of
the ZnO nanorod film over the Au sheet and Au�ZnO�
PATP�Ag assemblies were characterized using scanning elec-
tron microscopy (SEM) as shown in Figure 1. The thin films of
ZnO are composed mainly of needlelike hexagonal rods of ZnO.
The one-dimensional ZnO nanorods (in 100% morphological
yield) over the Au sheet formed a bushlike structure, with sharp
ends projecting out. The mean diameter of the ZnO nanorods
was approximately 400�500 nm (Figure 1A). No apparent
changes were observed after the adsorption of PATP, which
indicates that the PATP molecules that were immobilized to the
surface of the ZnO nanorods did not disturb the distribution of
the nanorods over the Au sheet. Figure 1B shows the deposition
of Ag nanoparticles (NPs) on the surface of the Au�
ZnO�PATP nanoassembly. The ZnO nanorod layers were still
distinguishable after the assembly of Ag (NPs). As one can see,
most of the Ag NPs existed separately on the surface of the ZnO
nanorod film.
We measured the SERS of the Au�ZnO�PATP�Ag assem-
bly with near-infrared (NIR) excitation. This excitation wave-
length is chosen so that it is far from the surface plasmon
resonance of the ZnO nanorods and Ag NPs.16,19 According to
the chemical mechanism of SERS, the enhancement results from
the CT due to the electronic structures of molecules adsorbed on
or interconnecting in the substrate, where Raman peaks are
Scheme 1. Representation of the PATP Interconnecting
One-Dimensional ZnO Nanorods and Ag Nanoparticles over
a Metal Sheet
Figure 1. SEM images of (A) the one-dimensional ZnO nanorod film
over gold sheets and (B) the Au�ZnO�PATP�Ag assembly. The
insets show the corresponding high magnification images.
18380 dx.doi.org/10.1021/jp206455a |J. Phys. Chem. C 2011, 115, 18378–18383
The Journal of Physical Chemistry C ARTICLE
selectively obviously enhanced.11 However, the EM enhance-
ment of the Raman signal is dependent on excitation wavelength
in the visible spectral region.18 Thus, this NIR excitation will
provide an improved understanding of the CT mechanism.
In Figure 2, we show SERS spectra of the Au�ZnO�
PATP�Ag assembly excited at 1064 nm. The Raman peaks
appeared at 1142, 1390, 1436, and 1574 cm�1, assigned to the
non-totally symmetric b2 modes of the PATP molecule, and
1004, 1073, and 1590 cm�1, assigned to the totally symmetric a1
modes.20 Note that the intensity of the b2 modes in the Au�
ZnO�PATP�Ag assembly was weaker than that in the
ZnO�PATP�Ag assembly, while the intensity of a1 modes
showed no change. For comparison, the SERS of the PATP
molecule in these same models at 514.5 nm is shown in Figure 3.
There are no observed differences in these structures because the
introduction of a photon at 514.5 nm (2.40 eV) will be sufficient
to easily transfer an electron to an excited unfilled level of PATP,
which can then easily transfer it to the conduction band of ZnO.
However, the energy of a photon at 1064 nm (1.16 eV) will be
not sufficient to initiate a charge-transfer process in this way.16
Consequently, the Raman spectral features obtained under our
experimental conditions isolate the importance of the CT effect.
Our previous study demonstrated that when the PATP is
attached to the ZnO by the thiol group and the Ag by the amino
group, considerable CT effects were observed in such contacts
(ZnO�PATP�Ag). On the other hand, if the PATP was
assembled inversely (Ag�PATP�ZnO), the CT effect was
inhibited.16
Compared with the two cases described above (ZnO�
PATP�Ag and Au�ZnO�PATP�Ag), the selective enhance-
ment of the b2 modes cannot be explained by the EM mechan-
ism. Thus, we believe that the Herzberg�Teller contribution
also plays an important role in the enhancement of b2 modes that
are attributed to the CT mechanism of SERS.11 That is, the
enhancement of the b2 modes of PATP is associated with the
direction of CT though the assembly system and the extent of
CT. Because the Au�ZnO�PATP�Ag assemblies display a
weaker enhancement of b2 modes than the ZnO�PATP�Ag
assemblies, this should be attributed to the specific Au�ZnO
contacts.
Therefore, we noted that the composite materials exhibit new
advanced functionalities that appear in the hybridized heterostruc-
ture composites because of the proximity of the two functionally
different components.21 Typically, the composites of silver nano-
particles (Ag NPs) and ZnO can exhibit advanced functionalities
in optical and electrical characteristics.22 Because the work func-
tion of ZnO (5.2 eV)23 is higher than the work function of the Ag
NPs (4.26 eV),20 the hybridized heterostructure composites can
cause the transfer of electrons fromAgNPs to ZnO.Therefore, the
strong electronic interaction between plasmonic Ag NPs and ZnO
induces the charge transfer through the interconnecting mol-
ecules. This means that the electrochemical activity of Ag�ZnO
junctions is more efficient than that of single component ZnO.24
Furthermore, PATP molecules interconnecting ZnO and Ag
generated an exactly asymmetric metal�molecule�semiconduc-
tor system. The Au�ZnO contacts in this systemmay depend on
the direction of CT simultaneity. According to semiconductor
theory, between the metal and semiconductor there are
two contact types, Ohmic contact and Schottky contact, that
depend on the work functions of both the metal and the
semiconductor.25 If the work function of the metal (Wm) is
greater than the work function of the semiconductor (Ws), a
positive space charge region will be produced in the me-
tal�semiconductor junction. Accordingly, the region will block
the transfer of electrons between the metal and semiconductor.
This contact type is defined as a Schottky contact. On the other
hand, ifWm is lower thanWs, electrons will flow from themetal to
the semiconductor. The contact type is defined as an Ohmic
contact. In this case, a negative space charge region will appear at
the interface. Then the high electron concentration will create a
high conductivity in the metal�semiconductor junction. In our
models, note that both work function WAg (4.26 eV) and work
function WAu (5.1 eV) are lower than work function WZnO
(5.2 eV), so the ZnO�Ag contacts and the ZnO�Au contacts
are both Ohmic contacts. This means that electrons will flow
from the metal (Au or Ag) to the semiconductor (ZnO) to lower
their energies, until the positions of these Fermi energy levels are
adjusted to the same value. It will also deform the bands (valence
Figure 2. SERS spectra of PATP molecules interconnected in the (a)
ZnO�PATP�Ag, (b) Au�ZnO�PATP�Ag, and (c) Cu�ZnO�
PATP�Ag assemblies. The excitation wavelength was 1064 nm.
Figure 3. SERS spectra of PATP molecules interconnected in the (a)
ZnO�PATP�Ag, (b) Au�ZnO�PATP�Ag, and (c) Cu�ZnO�
PATP�Ag assemblies. The excitation wavelength was 514.5 nm.
18381 dx.doi.org/10.1021/jp206455a |J. Phys. Chem. C 2011, 115, 18378–18383
The Journal of Physical Chemistry C ARTICLE
and conduction) of ZnO to move relatively down to the Fermi
level, i.e., to bend downward. Therefore, either ZnO�Au or
ZnO�Ag contacts will contribute generously to the transfer of
electrons between the metal and semiconductor upon clarifica-
tion of the statements made above.
In addition, when the PATP molecule is in the quinonoid
form,16 it can further interact electrostatically or covalently with
the interconnecting material. Accordingly, the direction of CT
through this system may still match the dipole direction of the
PATP molecule, which is from the amino group toward the thiol
group, but the extent of CT may be lower. In this case, dynamic
CT could still occur through coupling with the vibrations of the
interconnecting molecules, as the energy levels of the system
match the HOMO and LUMO energy levels of the molecules
under excitation by light.26 The selective enhancement of the b2
modes of the PATP molecules may be indicative of such a CT
process. The work function of ZnO (5.2 eV) is larger than those
of silver (4.26 eV) and gold (5.1 eV). As a result, the choice of the
direction of CT (CT fromAg to ZnO and fromAu to ZnO), both
toward ZnO, will occur simultaneously. Therefore, we noted that
less overall CT could be obtained from Ag to ZnO by tunneling
through the interconnecting PATP molecules with the coupling
of gold.27
As further studies of this metal�semiconductor contact effect,
we replaced the gold sheet with a copper sheet. The SEM images
of a ZnO nanorod film over a Cu sheet, a ZnO nanorod film over
a glass slide, the Cu�ZnO�PATP�Ag assembly, and the ZnO�
PATP�Ag assembly are all shown in the Supporting Informa-
tion. The SERS of the PATP molecule in the Cu�ZnO�
PATP�Ag assembly is comparable to that reported previously.
Notably, compared with the ZnO�PATP�Ag assembly
(Figure 2a), the Cu�ZnO�PATP�Ag assembly did not result
in an enhancement of the b2 modes of the interconnecting PATP
molecules (Figure 2c). In other words, CT was inhibited in this
case. The enhancement of the a1 modes at 1590, 1073, and
1004 cm�1 in curve c of Figure 2 is associated with the
contribution of the EM mechanism.10,21
To obtain a quantitative measure of the relative contribution
of the charge-transfer transition to the overall SERS intensity, we
take the SERS of the PATP molecule in these same models at
785 nm (see Figure 4). In Figure 4, we show SERS spectra of
three models (ZnO�PATP�Ag, Au�ZnO�PATP�Ag, and
Cu�ZnO�PATP�Ag assemblies) excited at 785 nm. Note that
the intensities of non-totally symmetric b2 modes appeared at
1142, 1390, and 1436 cm�1 in the Au�ZnO�PATP�Ag and
Cu�ZnO�PATP�Ag assemblies are still different from those of
the ZnO�PATP�Ag assembly. In Figure 5, we show a graph of
the degree of CT as a function of Fermi energy of the contacting
metal at several different wavelengths (514, 785, and 1064 nm).21
These spectra show Fermi energy-dependent variations of both
the totally symmetric (a1) lines and the non-totally symmetric
(b2) lines, and we can use these results to determine the degree of
CT contributions from both the excitation wavelength and the
Fermi energy of the substrate in the SERS spectrum. In Table 1,
we present the measured relative intensities of two selected lines,
the 1073 cm�1 (a1) vibration and the 1390 cm
�1 (b2) vibration,
as a function of applied excitation wavelength. I1390 is the
measured intensity of the line in the region of the spectrum in
which the CT resonance makes an additional contribution to the
SERS intensity. These are chosen because they are relatively
intense, well separated from other nearby lines, and close to each
other. Note that the degree of CT tends to increase with an
increasing Fermi energy of the contacting metal. As the Au sheet
becomes less negative, the degree of CT increases, while the
Au contribution is approximately 2 times that of the Cu con-
tribution.
We may now use this quantitative measure of CT to examine
the effect of other properties on the SERS intensity. In Figure 6,
we plot the degree of CT of the 1390 cm�1 line as a function of
excitation energy. Recently, Lombardi et al.20 gave a detailed
depiction of the CT mechanism of PATP excited at 632.8 nm.
Note that the laser excitations at 514 nm (2.41 eV) and 785 nm
(1.58 eV) are both in the region of a metal-to-molecule charge-
transfer transition from the Ag Fermi level to the π* molecular
level, and the extent of CT tends to increase with an increase in
excitation energy; that at 1064 nm is far from any molecular or
charge-transfer resonance. Presumably, the enhanced 1064 nm
spectrum involves only the CT from Ag to metal�semiconduc-
tor coupling with PATP.
Note that the work function of ZnO (5.2 eV) is also larger than
the work function of copper (4.65 eV).20 We suppose the degree
of CT through the Cu�ZnO�PATP�Ag assembly is less than
Figure 4. SERS spectra of PATP molecules interconnected in the (a)
ZnO�PATP�Ag, (b) Au�ZnO�PATP�Ag, and (c) Cu�ZnO�
PATP�Ag assemblies. The excitation wavelength was 785 nm.
Figure 5. Degree of charge transfer (PCT) as a function of contacting
metal Fermi energy.
18382 dx.doi.org/10.1021/jp206455a |J. Phys. Chem. C 2011, 115, 18378–18383
The Journal of Physical Chemistry C ARTICLE
that of the Au�ZnO�PATP�Ag assembly, because the work
function of copper (4.65 eV) is much lower than that of gold
(5.1 eV). Thus, the degree of CT from Cu to ZnO will be larger
than that from Au to ZnO. In other words, it is more difficult for
electrons to transfer from Ag to ZnO through PATP molecules
with Cu�ZnO contacts in the system. Furthermore, the contact
inhibited the charge distribution through the coupling molecules
in the ZnO�PATP�Ag assembly.
’CONCLUSIONS
In conclusion, we used a PATP molecular layer interconnecting
the metal and ZnO surface to identify Raman modes that are
enhanced through the chemical effects in SERS measurements.
The experimental investigation provides a rather complete picture
of wavelength dependence as well as Fermi energy dependence,
allowing detailed and quantitative analysis of the relative contribu-
tions, both near and far from the charge-transfer resonance. Because
of the typical Au�ZnO�PATP�Ag and Cu�ZnO�PATP�Ag
st
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