Charge separation in heterostructures of InP nanocrystals with metal particles

Article abstract of DOI:10.1021/jp051201y

The optical and electron paramagnetic resonance (EPR) properties of InP nanocrystals, in which metallic gold or indium is present as an incorporated part of the nanocrystals, have been studied. A study of Au/InP quantum rods supports different carrier localization regimes compared to metal-free quantum rods, including the charge-separated state for which the electron and hole are located in different parts of the heterostructure. They also show that elongated semiconductors that grow on metallic catalysts have electronic properties that are different from those of pure semiconductor nanocrystals of the same shape. We have also developed a simple method for growing melted indium particles on the surface of colloidal spherical InP nanocrystals, and in these In/InP nanocrystals the emission is completely quenched while the absorption spectrum moves to red due to the strong mixing of the semiconductor and metal electronic states. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp051201y

J. Phys. Chem. B 2005, 109, 18243-18249
18243
Charge Separation in Heterostructures of InP Nanocrystals with Metal Particles
Nada M. Dimitrijevic´ and Tijana Rajh
Chemistry DiVision, Argonne National Laboratory, 9700 South Cass AVenue, Argonne, Illinois 60439
S. Phillip Ahrenkiel, Jovan M. Nedeljkovic´, Olga I. Mic´ic´,* and Arthur J. Nozik
National Renewable Energy Laboratory, 1617 Cole BouleVard, Golden, Colorado 80401
ReceiVed: March 8, 2005; In Final Form: July 27, 2005
The optical and electron paramagnetic resonance (EPR) properties of InP nanocrystals, in which metallic
gold or indium is present as an incorporated part of the nanocrystals, have been studied. A study of Au/InP
quantum rods supports different carrier localization regimes compared to metal-free quantum rods, including
the charge-separated state for which the electron and hole are located in different parts of the heterostructure.
They also show that elongated semiconductors that grow on metallic catalysts have electronic properties that
are different from those of pure semiconductor nanocrystals of the same shape. We have also developed a
simple method for growing melted indium particles on the surface of colloidal spherical InP nanocrystals,
and in these In/InP nanocrystals the emission is completely quenched while the absorption spectrum moves
to red due to the strong mixing of the semiconductor and metal electronic states.
Introduction
tip. Liquid metallic droplets (Au or In) have been widely used
in the SLS method as the catalyst to synthesize wirelike
Photogenerated electrons confined to nanometer-size regions
of a semiconductor have been the subject of intensive studies.
Various properties of semiconductor quantum dots (QDs) and
quantum rods (QRs) can be altered by bringing them in contact
with different materials. An insulator shell provides a well-
defined potential barrier that confines the electron and hole
inside nanocrystals with a large excitonic binding energy that,
for example, for 4.2-nm InP QDs is 120 meV.1 The exciton
binding energy in QRs is even larger due to dielectric enhance-
ment caused by the difference between the dielectric constant
of the semiconductor nanocrystal and the surrounding medium.2,3
The electron and hole are at a distance larger than the QR radius
and interact predominantly through the surrounding medium,
which has a smaller dielectric constant. This leads to the
formation of one-dimensional excitons with a large binding
energy.2 For InP quantum wires with a 3.8-nm diameter, the
exciton binding energy is about 190 meV with a dielectric
constant 2 for the surrounding medium.2 If the difference of
the dielectric constant of the constituent parts of the QRs is
varied, it is possible to change the binding energy of the
excitons. The presence of a metal on the nanocrystals can
produce this effect and also increase delocalization of electrons
inside the nanocrystals; this lowers the confinement energy
compared to core QDs with no metal. With recent advances in
techniques for synthesizing different semiconductor and metallic
nanocrystals, it is now possible to synthesize quantum structures
with a metal nanoparticle incorporated onto or into the
semiconductor nanocrystal.
crystallites of zinc blende cubic-lattice semiconductors (InAs,
InP, and GaAs) with controllable diameters and excellent
crystallinity.6-8 However, the presence of metal as an incorpo-
rated part of the semiconductor nanostructure can be expected
to change the optical and electronic properties of the nano-
crystals. Their electronic states are a mixture of the semiconduc-
tor and metal electronic states.
For semiconductor nanocrystals with large band gap (ZnO,
TiO2),9,10 the presence of metal on their surface promotes
interfacial charge-transfer kinetics between the semiconductor
and metal and improves the charge separation. For semiconduc-
tor nanocrystals that are in the strong confinement regime, there
are only a few published studies of the effect of the metal on
the electronic properties of the semiconductor. It has been found
that luminescence enhancement of CdSe QDs near a nanostruc-
tured gold surface occurs when the spacing between the QD
and gold is in the range of a few nanometers due to the locally
enhanced electromagnetic field produced by the gold nano-
particles.11,12 However, when the distance between the gold
particle and the QD is further decreased and results in intimate
contact, quenching of the emission occurs. Very recently, Banin
et al.13 showed that the growth of gold tips on CdSe QRs with
different gold sizes was caused by a strong coupling between
the gold and the CdSe parts. In that case, the emission is
quenched by the metal tips via nonradiative pathways created
by the proximity of the metal. The different electronic properties
of semiconductor-metal nanocrystals compared to those of pure
semiconductor nanocrystals create the advantage of having better
charge separation inside the nanocrystals and formation of metal
anchor points, which can serve as a contact for wiring
nanocrystals onto an electrical circuitry. However, there are no
available data in the literature that show whether charge
separation occurs and where the photoinduced electron and hole
are located in the semiconductor quantum structure when metal
nanoparticles are incorporated.
Most synthetic methods for growing elongated nanocrystals
in solution use a colloidal metallic catalyst to initiate the growth
process. The growth process proceeds via a solution-liquid-
solid (SLS)4 or a vapor-liquid-solid (VLS) mechanism,5 and
the final product contains metallic catalyst particles on the wire
* Author to whom correspondence should be addressed. E-mail:
Olga_Micic@nrel.gov.
10.1021/jp051201y CCC: $30.25 © 2005 American Chemical Society
Published on Web 09/08/2005

18244 J. Phys. Chem. B, Vol. 109, No. 39, 2005
Dimitrijevic´ et al.
We report here on the studies of the optical and electron
paramagnetic resonance (EPR) properties of InP nanocrystals
in which metallic gold or indium is present as an incorporated
component. The combination of metal and semiconductor
provides new functionalities to the nanostructures and the
possibility of charge separation. InP QRs are so far produced
using gold as the growth catalyst, and hence the understanding
of the contribution of the metal component in charge separation
processes is investigated in InP-Au QRs. EPR investigation
of charge separation in InP-In QRs was not feasible due to
the extremely large hyperfine coupling of In that would lead to
weak EPR signals. Our experiments studying paramagnetic
species with the EPR techniques show that charge separation
occurs in Au/InP quantum rods and that the electron is located
in the metal particle while the hole remains in the interface
between the InP rod and Au particle. EPR measurements also
show that elongated semiconductors that grow on metallic
catalysts have different electronic properties compared to pure
nanocrystals of the same shape. For spherical semiconductor
QDs, the presence of metal drastically interferes with the optical
properties of the QDs. Growth of Au on InP nanoparticles was
proved unsuccessful, and we developed a simple method for
growing melted indium particles on the surface of colloidal
spherical InP nanocrystals. We have shown that in In/InP
nanocrystals the absorption spectrum moves to lower energy
due to the strong mixing of the semiconductor and metal
electronic states.
We have prepared InP QDs capped with TOPO/TOP with a
4.8-nm diameter and a standard deviation of about 8%. Details
of the nanocrystal synthesis may be found in ref 16. For the
synthesis of the 3.5-nm InP quantum dots, the preparation
described by Peng et al.17 was followed with some changes.
The difference is that we used oleic acid instead of the palmatic
or myristic acid that was used by Peng et al. With oleic acid,
we prepared highly fluorescent InP QDs with slightly broader
size distribution (10%).
The metallic indium was deposited on the 3.5-nm InP QDs
capped with oleate or 4.8-nm InP capped with TOPO/TOP by
using spontaneous decomposition of C5H5In. Typically, 1 mg
of the 3.5-nm InP QDs was dissolved in 3 mL of hexane that
contains 10 mg of HDA, and 3 mg of C5H5In dissolved in
3 mL of hexane was added very slowly drop by drop over a
period of 2 h by vigorously stirring at 40-50 °C in the glovebox.
Then, the reaction mixture was cooled to room temperature.
Optical changes of the In/InP QD composites were followed
for two weeks. We found that pure InP QDs are formed when
In0/InP particles are mixed with P(SiMe3)3 in TOPO/TOP
solution and heated at 260 °C for 2 h.
2. Nanocrystals Characterization. The EPR measurements
were made at Argonne National Laboratory. X-band EPR
experiments were collected on a Bruker ESP300E spectrometer
equipped with a Varian rectangular cavity TE102 and a variable
temperature cryostat (Air Products) cooled to helium temper-
atures. Samples were excited either by a 300-W ILC xenon lamp
with a 320-nm cutoff filter or, for higher radiant power (1-10
photons per particle), with a YAG-OPO laser (550-nm, and
10-20 mJ/pulse). The samples were kept under an argon
atmosphere. Samples were excited directly in the EPR cavity
at 4 K. Preliminary scans were obtained on the samples before
irradiation to check for spurious signals. Temperature control
for sample annealing was achieved with a Lake Shore 320
autotuning temperature controller. The sample was equilibrated
for 20 min at the temperature before the spectrum was recorded
at the temperature indicated on the figure. The g-factors were
calibrated by comparison to Mn2+ standard in SrO matrix (g )
2.0012 ( 0.0002)18 and with a 1,1-diphenyl-2-picrylhydrazyl
(DPPH) standard (g ) 2.0036 ( 0.0003). A high concentration
of particles (concentration of particles 10-5 to 10-4 M) was
used in order to improve the quality of detection, as described
previously.19
Experimental Procedure
1. Synthesis of Nanocrystals. All compounds used in this
work are extremely sensitive to oxygen and moisture, and they
were manipulated in a Schlenk line or glovebox under rigorous
air- and water-free conditions.
For preparation of In0 nanoparticles, C5H5In is used; it
spontaneously decomposes and yields indium particles. Indium
particles of 8-nm diameter were synthesized by decomposition
of 5-10 mg of C5H5In in 10 mL of toluene containing 0.1 mL
of TOA in the absence of light at 50 °C.14 The 160-Å In0
particles were synthesized by decomposition of 100 mg of
(CH3)5C5In in 56 mL of octadecene in the presence of 6.25 g
of poly(1-hexadecene-co-vinylpyrrolidinone) (PHVP). The 2-nm
thiol-derivatized gold nanoparticles were prepared using a two-
phase liquid-liquid system described elsewhere.15
InP QRs without residual metallic catalysts were synthesized
by reacting P(SiMe3)3 with In0 nanoparticles dispersed in organic
solvent.14 Short nanorods (6-nm diameter and 15-nm length)
were prepared when a solution of 8-nm indium particles that
contained 1 mg of In0 were mixed with 30 mg of P(SiMe3)3 in
2.5 mL of TOP, 2.5 mL of toluene, and 30 mg of hexadecy-
lamine (HAD) and heated for 2 h at 110 °C in a closed system
saturated with nitrogen. Larger rods (9-nm diameter and 66-
nm length) were prepared by reaction of 1-3 mg of 16-nm
indium particles with 20-30 mg of P(SiMe3)3 in 2-4 mL of
1-octadecene overnight at 220 °C.
For the preparation of InP QRs in the presence of 2.0-nm
Au catalyst, we used the procedure developed by Banin et al.6
Briefly, a stock solution containing 85 mg of (SiMe3)3P, 1 mL
of InCl3/TOP solution (0.15 g/mL), 0.5 mL of gold/toluene
solution (6.7 mg/mL), and 0.5 mL of TOP was injected into
the growth solution (2 g of TOPO) at 360 °C under vigorous
stirring. Less than 3 s later, 2 g of cold TOP was injected to
additionally quench the reaction temperature to 220 °C, and then
the reaction mixture was cooled to room temperature. The
precipitation with methyl alcohol followed with centrifugation
was used to obtain fractions of QRs with different sizes.
For microstructural analysis, colloidal samples were deposited
on C-coated Cu grids. Characterization was carried out with a
Philips CM200 STEM operated at 200 kV or a Philips CM30
TEM operated at 200 kV. For high-angle annular dark-field
imaging, a JEOL JEM-2010F STEM operated at 200 kV was
used. Bright-field and lattice images were acquired with an
objective aperture that admitted contributions from low-index
reflections. Selected-area patterns were acquired with an aperture
having a projected diameter of approximately 7 µm in the image
plane. EDX spectra were acquired with a Kevex Li-drifted Si
detector using the Emispec ES Vision software. X-ray diffraction
patterns were acquired on a Scintag ×1 diffractometer using
Cu KR radiation.
Optical absorption spectra were collected at room temperature
using a Cary 5E UV-vis-NIR spectrophotometer. Steady-state
photoluminescence spectra were collected at room temperature
using a SPEX model 1691 fluorolog (excitation with a Xe lamp
and cooled photomultiplier tube detector).
Results and Discussion
1. Paramagnetic Electron and Hole Species. We have
studied the paramagnetic species formed in illuminated hetero-

Charge Separation in Heterostructures of InP
J. Phys. Chem. B, Vol. 109, No. 39, 2005 18245
Figure 1. TEM images showing (a) In0 nanoparticles, and (b-e) InP
QRs. (b) QRs (6-nm diameter) with aspect ratios of 1.4-2.5 prepared
from a solution of 8-nm In0 particles at 110 °C. (c) QRs (9-nm diameter)
with aspect ratio of 7 (tapered shape) prepared from a solution of
16-nm In0 particles heated at 260 °C. (d, e) High-resolution images of
the individual QRs.
structure Au/InP QRs that are synthesized by the SLS mecha-
nism with gold nanoparticles as the catalyst. These results have
been compared with those obtained for InP QRs that are metal-
free and also for pure Au nanoparticles. With this analysis, we
want to clarify the nature of the electron and the hole located
in a QR in order to understand if charge separation occurs in
Au/InP QRs. The nature of the paramagnetic species formed in
Au nanoparticles is well-known,20 and there are data in the
literature showing their behavior in different matrixes.21
We first studied InP nanorods that do not contain gold
particles and are catalyst-free. In this procedure, quantum rods
were produced via the reactions of monodispersed colloidal
indium droplets with P(SiMe3)3 (the source of P3- ions) in
organic solvent.14 We prepared indium particles by thermal
decomposition of C5H5In (Figure 1a). The dimension of the QRs
depends on the dimension of the In0 particles, the stabilizer used,
and the temperature. Parts b and c of Figure 1 show InP
nanorods prepared with trioctylphosphine/hexadecylamine (TOP/
HDA) or hexadecene-co-vinylpyrrolidinone polymer (PHVP)
stabilizers. Colloidal InP QRs having ∼6-nm diameter and
15 nm-length with a TOP/HDA stabilizer were prepared from
8-nm In0 droplets (Figure 1b), while QRs that have a 9-nm
diameter and 60-nm length were prepared with PHVP from
16-nm In0 droplets (Figure 1c). The PHVP polymer causes QRs
to grow in one direction and to be straight, although the
diameters changed during the growth process and the QRs have
a tapered shape (Figure 1c). Both QRs have diameters that are
significantly smaller than the corresponding bulk InP exciton
Bohr diameter (20 nm). Rods with 9-nm diameter and 66-nm
length have strong confinement only in the radial direction,
while along the long axis they behave as bulk InP material.
In our previous work, we reported the observation of
nonradiative and radiative paramagnetic species of spherical InP
QDs by EPR22 and by optically detected magnetic resonance
(ODMR)23 spectroscopy, respectively. Results obtained by EPR
techniques for QDs and QRs are presented in Figure 2. When
spherical nanocrystals were illuminated with a Xe lamp (Figure
2a) with a light intensity producing about one exciton per
Figure 2. EPR spectra of (a) InP QDs (diameter 4.7 ( 0.5 nm), and
(b) QRs (6 × 13 nm) obtained upon excitation with white light (λ )
450 nm) at 4.2 K. The EPR frequency is 9.345 GHz, MA is 6.3 G,
time constant is 1.28 ms, and microwave power is 2 mW. The bars
indicate hyperfine quintet that would be obtained due to the interaction
of unpaired electron with four neighboring phosphorus atoms 31P (I )
1/2, 100% natural abundance) with hyperfine coupling of A ) 32 G in
zinc blende crystalline structure either at an In vacancy site or at an
interstitial site (g ) 2.001).
particle, two paramagnetic species were observed that exhibited
EPR signals at g ) 2.055 and g ) 2.001. When the light was
turned off, the signal at g ) 2.001 immediately disappeared at
8 K, while the signal at g ) 2.055 disappeared only slowly and
could still be detected, even upon heating to room temperature.
Our previous analysis22 showed that the signal at g ) 2.055
was caused by trapped electrons located on the surface, while
the signal at g ) 2.001 arose from delocalized holes in the QD
core.
In rod-shaped nanocrystals (Figure 2b), slightly different
paramagnetic species are formed during illumination compared
to spherical InP nanocrystals. The difference arises due to
different defect chemistry arising from the different synthetic
approaches used for the preparation of the QDs and QRs. Dots
are prepared in excess of indium (In/P ) 1.5-2), while rods
are prepared in a large excess of phosphorus (P/In ) 20-30).
The same signal at g ) 2001 (delocalized hole) arises for rods
and dots, while the signal at g ) 2.055 (electron adduct on the
surface) is not formed in the rod sample. Instead, a new quintet
spectrum indicative of hyperfine interaction with nuclear spin
appears in the rod samples. Due to the absence of higher-order
spin effects,24 the splitting is believed to be caused by nuclear
spin 1/2, with an isotropic g value of 2.004. Because phosphorus
31P has an I ) 1/2 nucleus with 100% abundance, it is apparent

18246 J. Phys. Chem. B, Vol. 109, No. 39, 2005
Dimitrijevic´ et al.
that the spectrum arises from a radical interacting with four
phosphorus atoms. Indium atoms, which are coordinated with
four phosphorus atoms in the zinc blende structure, have a
nuclear spin of I ) 9/2 and a large magnetic moment. Therefore,
if a radical were centered at In atoms, a ten-line spectrum with
large hyperfine coupling would be observed, as unpaired
electrons would reside in the 5s orbitals of In. We have looked
for the paramagnetic signature of this radical in a broad magnetic
field range (500-12000 G) and did not observe additional lines
indicative of an In-centered radical, suggesting that In is not
participating in radical interactions. Therefore, we propose that
acceptor states arising from In vacancies are formed in the QRs;
these defect states are positively charged in the dark (empty of
electrons) to maintain charge neutrality and do not become
ionized because the ionization energy for creating holes in the
valence band from this acceptor state is relatively large (about
200 meV for QDs22). However, under illumination, the In
vacancy acceptor states become populated with electrons trapped
from the conduction band and free holes are formed in the
valence band. These holes produce the EPR signal at g ) 2.001,
while the electrons trapped in the In vacancy state produce the
quintet spectrum.
Different EPR spectra were observed for Au/InP QR com-
posites. To prepare Au/InP QR composites, we employ gold
nanoparticles to catalyze and direct one-dimensional rod growth
in solution. The procedure used is the same as that used by
Banin et al.6 for the synthesis of InAs and InP QRs with
∼2-nm Au nanoparticles. Figure 3 shows TEM images and the
absorption spectrum of Au/InP QRs with a short length obtained
after size-selective precipitation. The quantum rods have a mean
diameter of 4 nm and varying length of 12-20 nm. Z-analysis
of TEM images (bright images, Figure 3b) shows that the Au
particles are not attached at the rod tip but are located inside
the InP nanorods, close to the rod tip.
Photoinduced EPR signals of electron and hole adducts
obtained for Au/InP QRs are presented in Figure 4 and compared
to the EPR signals observed in pure 2-nm Au particles. EPR
spectra in Figure 4 are obtained under the same conditions as
those for pure InP QRs and QDs in Figure 3. Comparison of
the EPR signals for Au and Au/InP QRs shows that both spectra
are composed of two components: one a multiline spectrum
with different hyperfine coupling and a narrow singlet center
line. The multiline spectrum is reminiscent of the oxidized gold
Figure 3. TEM images (a-c) and absorption spectrum (d) of the first
fraction of InP QRs synthesized by SLS mechanism with 2-nm Au
nanoparticles. (a) An ensemble of rods where many rods are kinked.
(b, c) High-resolution images where gold particles can be identified
on the rods as a bright image located inside QRs. (d) Absorption spectra
of Au/InP rods.
2+
cluster Au2 in which gold ions are associated with metallic
Au atoms.21 In this species, unpaired electrons of gold ions
interact with two gold nuclei, which have nuclear spin of 3/2
and 100% natural abundance, resulting in a seven-line spectrum
(g ) 2.007). The hyperfine splitting of 15 G measured in isolated
Au nanoparticles is three times smaller than the one of free
The signal for electron trapping sites at g ) 2.006 is not the
same as the signals present in either the metal-free InP dots or
rods. This signal is connected with the new location of electrons
in the presence of Au particles in InP QRs. During illumination,
only the InP part of Au/InP QRs is photoexcited because the
absorption coefficient of InP nanocrystals is larger than that of
the surface plasmon resonance band in the absorption spectrum
of 2-nm Au particles.25 The contribution of the surface plasmon
band at 520 nm cannot be seen in the absorption spectrum of
Au/InP rods (Figure 3d).
Photogenerated electrons can be transferred from InP to Au
nanoparticles in a Au/InP nanojunction. This transfer is ther-
modynamically favorable because in aqueous solution the Fermi
level of Au is 0.45 V10 versus NHE at pH 7, while bulk InP
has a conduction band potential of -0.6 V versus NHE at pH
7. For quantum rods, the conduction electron level will shift to
even more negative values due to size quantization effects. The
transfer of electrons from InP to Au continues until the two
2+
Au2 clusters21 and is in agreement with shared spin density
within Au22+ clusters. An even smaller hyperfine splitting was
measured in the samples in which gold particles are associated
with InP QRs, indicating the electron density of these clusters
is perturbed by the presence of InP; that in turn suggests that
the interface between Au particles and InP rods is the place of
localization of photogenerated holes. The central singlet line
has the same width and the same g ) 2.006 for both Au/InP
QRs and Au particles. While the shape of the signal is indicative
of CESR (conduction electron spin resonance), the small g shift
with respect to the value of the free electron resonance suggests
trapped electrons exist at the Au nanoparticle surface site.

Charge Separation in Heterostructures of InP
J. Phys. Chem. B, Vol. 109, No. 39, 2005 18247
QR sample that corresponds to the InP part. The electron transfer
from the InP part to the Au particles competes with recombina-
tion of the electron-hole pair in the InP part of the rod, and a
weak band edge can be seen. The EPR signal for excitons that
form in the InP part during the absorption of light and
recombined to yield emission cannot be seen because EPR can
detect only paramagnetic species that reach a steady-state
concentration of ∼4 × 1012 spins per cm3 at 10 K for a 20-G-
wide signal.
2. Absorption and PL Spectra of In0/InP QD Composite.
We also studied the heterostructure of spherical InP QDs with
indium particles. In0 particles are grown on InP QD surfaces
that were initially passivated with oleic acid (3.5 nm in diameter)
or with TOPO/TOP stabilizer (4.8 nm in diameter). The
3.5-nm InP QDs stabilized with oleic acid are shown in Figure
5a. A metallic indium island is grown on the QD surface by
spontaneous decomposition of C5H5In in a toluene solution at
50 °C. A C5H5In solution was added very slowly over a period
of 2 h to allow In0 nucleation on the InP surface. Indium grown
on the surface of QDs is clearly identified by TEM (Figure 5b)
as a point with enhanced contrast due to the higher electron-
scattering cross section of indium compared to InP QDs. TEM
images (Figure 5) show that metallic indium forms islands of
irregular shape, which do not completely cover all the InP
nanocrystals. We used the same molar concentration of In0 and
InP QDs, which means that the volume of In0 particles that are
formed should be smaller than that of InP QDs because of the
difference in density between indium (7.3 g cm-3) and InP (4.8
g cm-3).
In addition to In0/InP QD characterization with TEM, optical
absorption measurements have been used to characterize the
coupling of In0 and InP QDs. Figure 5c shows the evolution of
the absorption spectra of 3.5-nm InP QDs after growth of
metallic indium on the QD surface. The band edge peak of InP
QDs, which is at 590 nm (2.1 eV), is red-shifted after growth
of metallic indium (Figure 5c, curve 1). A red-shift of 50 and
70 meV in the In0/InP QD composite is observed immediately
upon the measurement (Figure 5c, curve 2) and after aging of
the sample for two weeks (Figure 5c, curve 3), respectively.
Absorption of light by In0 above 400-nm wavelength does not
interfere with the absorption spectrum of InP QRs because the
plasmon surface band is at 240 nm.26 The changes of the position
of the first exciton by addition of indium on the surface can be
attributed to the delocalization of the electron over the whole
In0/InP composite and also to the change of polarization energy
of the exciton due to the increase of the dielectric constant of
the shell around InP in the presence of the In0 island. The
Coulomb attraction energy will decrease only slightly due to
the asymmetry of In0 growth on InP QDs. Figure 5c, curve 4,
shows that annealing at 180 °C for a half hour increased the
width at the peak of the first exciton and also affected its position
by a red-shift of about 160 mV. Melted indium (mp 157 °C) at
180 °C is spread better on the QD surface after annealing, and
that improves the contact between the In0 droplets and InP QDs.
At 180 °C, the particles do not aggregate; they are soluble in
nonpolar solution, and TEM analysis shows that most In0/InP
particles are individual dots of about 3.5-5.0-nm diameter
(Figure 5c). When strong contact between InP and In is reached,
then strong mixing of the semiconductor and metal electronic
states can occur that exhibit broadened levels and reduced band
gap (Figure 5c, curve 4). Similar results were observed by
loading In0 on 4.8-nm InP QDs that were capped with TOPO/
TOP.
Figure 4. (a) EPR spectra of 2-nm Au nanoparticles, and (b) of
Au/InP QR composite (4 × 15 nm) obtained upon laser excitation at
450 nm (15 mJ/pulse) and at 4.2 K. The EPR frequency is 9.345 GHz,
MA is 6.3 G, time constant is 1.28 ms, and microwave power is
2 mW. The spectra were deconvoluted into a sum of two radical species
using the Simfonia (Bruker) simulation program. The 0.5 Lorentzian/
Gaussian line width of 17 G (a) and 8 G (b) and g tensors of 2.023
+
(a) and 2.020 (b) were used for simulation of (Au)n radical species,
while 17 G (a) and 10G (b) and g tensor of 2.0025 were used for an
electron adduct radical species of (Au)n-. The bars indicate hyperfine
+
septet obtained for (Au)n radical species due to the interaction of
unpaired electrons with two gold atoms 197Au (I ) 3/2, 100% natural
abundance) with (a) hyperfine coupling of A ) 16 G and (b) A )
7.5 G, indicating different electron density of bare and InP coupled
gold nanoparticles. The inset shows comparison between EPR spectra
obtained by illumination of InP (Figure 2b) and Au/InP QRs. The
difference is produced by the presence of metal and not by size or
shape differences.
systems attain equilibration and the Fermi levels of the two
phases become equal. As a result of charge equilibration between
semiconductor and the metal nanoparticles, excess electrons
accumulate in the Au particles and we can expect to see the
EPR signal as shown in Figure 4b.
Our EPR results undoubtedly show that the electron in
Au/InP QRs does not feel the change of electron density of
InP, and it means that the electron is far from the InP part. The
electron and the hole are located in the vicinity of the Au
particles but in different places. Although the EPR study shows
that the carrier charges are in different places in the Au/InP
QRs, we can see very weak band edge emission in our Au/InP

18248 J. Phys. Chem. B, Vol. 109, No. 39, 2005
Dimitrijevic´ et al.
tip via a new nonradiative pathway. The emission of In0/InP
QD composites annealed at 180 °C can be recovered when
metallic indium is converted into InP by reaction with P(SiMe3)3.
The absorption spectrum is slightly shifted to the red with a
long tail (Figure 5d, curve 2), indicating the broad size
distribution of InP QRs as the emission spectrum is recovered
(Figure 5d, curve 4). This experiment also confirms that strong
PL quenching in the In0/InP composite is a consequence of the
presence of In0 on the nanocrystal surface because when In0 is
removed by reaction with P(SiMe3)3, emission again appears.
In conclusion, we have demonstrated that heterostructures of
semiconductor and metal particles improve the photoinduced
charge-transfer efficiency. The interaction of metal with semi-
conductor nanocrystals provides new functionalities to the
nanostructures and the possibility of charge separation. EPR
studies show that charge separation occurs in elongated semi-
conductor quantum rods that grow on metallic catalysts and that
the electron and the hole are located in the vicinity of metal
particles but at completely different places. The results elucidate
the difference in the electronic properties of elongated semi-
conductors that grow on metallic catalysts compared to pure
semiconductor nanocrystals of the same shape. When melted
indium particles are grown on the surface of colloidal spherical
InP nanocrystals (In0/InP QDs), the optical properties change
and the band gap moves to lower energy due to the strong
mixing of the semiconductor and metal electronic states while
emission is completely quenched.
Our study shows that the presence of metal as an incorporated
part of the semiconductor quantum structures such as dots and
rods produces large changes in their electronic properties. Soon,
we plan to study the conductivity of this heterostructure using
time-resolved terahertz spectroscopy and ultrafast electron
dynamics to distinguish differences in electronic structure
between metal-semiconductor and metal-free semiconductor
nanocrystals. This study will help in understanding this class
of materials and their possible use in optoelectronic applications
such as solar cells.
Acknowledgment. The work at ANL (Contract No. W-31-
109-ENG-38) and NREL was supported by the U.S. Department
of Energy, Office of Science, Office of Basic Energy Sciences,
Division of Chemical Sciences, Geosciences and Biosciences.
Use of John Cowley Center for High Resolution Microscopy is
gratefully acknowledged, and we thank Renu Sharma for
assistance with TEM HRTEM investigations.
Figure 5. (a) TEM images showing a high-resolution TEM micrograph
of individual, spherical 3.5-nm InP QDs. (b) In/InP QDs after annealing
at 180 °C. (c) Absorption spectra of 3.5-nm InP QDs (curve 1) and
after deposition of In0: immediately (curve 2), after two weeks (curve
3), and after heating at 180 °C (curve 4). (d) Absorption and emission
spectra of pure InP QDs (curves 1, 3), and after addition of In0 droplet
(the sample is the same as that in Figure 5a, curve 4) and exposure to
P(SiMe3)3 and heated at 260 °C for two hours (curves 3, 4, respectively).
References and Notes
(1) Ellingson, R. J.; Blackburn, J. L.; Nedeljkovic´, J. M.; Rumbles,
G.; Jones, M.; Fu, H.; Nozik, A. J. Phys. ReV. B 2003, 67, 075308.
(2) Shabaev, A.; Efros, A. L. Nano Lett. 2004, 4, 1821.
(3) Muljarov, E. A.; Zhukov, E. A.; Dneprovskii, V. S.; Masumoto,
Y. Phys. ReV. B 2000, 62, 7420.
(4) Trentler, T. J.; Goel, S. C.; Hickman, K. M.; Viano, A. M.; Chiang,
M. Y.; Beatty, A. M.; Gibbons, P. C.; Buhro, W. E. J. Am. Chem. Soc.
1997, 119, 2172.
(5) Lieber, C. M. Solid State Commun. 1998, 107, 607.
(6) Kan, S.; Mokari, T.; Rothenberg, E.; Banin, U. Nat. Mater. 2003,
2, 155.
(7) Yu, H.; Li, J.; Loomis, R. A.; Wang, L.-W.; Buhro, W. E. Nat.
Mater. 2003, 2, 517.
(8) Ahrenkiel, S. P.; Mic´ic´, O. I.; Miedaner, A.; Curtis, C. J.;
Nedeljkovic´, J. M.; Nozik, A. J. Nano Lett. 2003, 3, 833.
(9) Wood, A.; Giersig, M.; Mulvaney, P. J. Phys. Chem. B 2001, 105,
8810.
(10) Jakob, M.; Levanon, H.; Kamat, P. V. Nano Lett. 2003, 3, 353.
(11) Shimizu, K. T.; Woo, W. K.; Fisher, B. R.; Eisler, H. J.; Bawendi,
M. G. Phys. ReV. Lett. 2002, 89, 117401.
(12) Kulakovich, O.; Strekal, N.; Yaroshevich, A.; Maskevich, S.;
Gaponenko, S.; Nabiev, I.; Woggon, U.; Artemyev, M. Nano Lett. 2002, 2,
1449.
The emission is partially quenched after addition of metallic
indium on the surface and completely quenched after annealing
at 180 °C. The mechanism responsible for quenching of PL
can be explained by electron tunneling from electron-hole pairs
in the InP, followed by electron capture in indium particles and
nonradiative recombination with holes in the InP core. Energy
transfer from photoexcited InP particles to In0 particles does
not occur because the surface plasmon band for In0 particles is
at higher energy (240 nm) than that of the first exciton energy
of InP QDs. Our results are in agreement with recently published
results of Banin et al.13 that showed when gold tips are grown
on CdSe nanorods, a change of optical properties of the nanorods
occurs that is caused by strong coupling between the gold and
semiconductor parts, and that emission is quenched by the gold

Charge Separation in Heterostructures of InP
J. Phys. Chem. B, Vol. 109, No. 39, 2005 18249
(13) Mokari, T.; Rothenberg, E.; Popov, I.; Costi, R.; Banin, U. Science
2004, 304, 1787.
(21) Claus, P.; Bru¨ckner, A.; Mohr, C.; Hofmeister, H. J. Am. Chem.
Soc. 2000, 122, 11430.
(14) Nedeljkovic´, J. M.; Mic´ic´, O. I.; Ahrenkiel, S. P.; Miedaner, A.;
Nozik, A. J. J. Am. Chem. Soc. 2004, 126, 2632.
(15) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R.
J. Chem. Soc., Chem. Commun. 1994, 801.
(16) Mic´ic´, O. I.; Jones, K. M.; Cahill, A.; Nozik, A. J. J. Phys. Chem.
B 1998, 102, 9791.
(17) Battaglia, D.; Peng, X. Nano Lett. 2002, 2, 1027.
(18) Rosenthal, J.; Yarmus, L. ReV. Sci. Instrum. 1966, 37, 381.
(19) Rajh, T.; Ostafin, A. E.; Mic´ic´, O. I.; Tiede, D. M.; Thurnauer, M.
C. J. Phys. Chem. 1996, 100, 4538.
(22) Mic´ic´, O. I.; Nozik, A. J.; Lifshitz, E.; Rajh, T.; Poluektov, O. G.;
Thurnauer, M. C. J. Phys. Chem. B 2002, 106, 4390.
(23) Langof, L.; Ehrenfreund, E.; Lifshitz, E.; Mic´ic´, O. I.; Nozik, A. J.
J. Phys. Chem. B 2002, 106, 1606.
(24) Kennedy, T. A.; Wilsey, N. D.; Krebs, J. J.; Stauss, G. H. Phys.
ReV. Lett. 1983, 50, 1281.
(25) Palpant, B.; Pre´vel, B.; Lerme´, J.; Cottancin, E.; Pellarin, M.;
Treilleux, M.; Perez, A.; Vialle, J. L.; Broyer, M. Phys. ReV. B 1998, 57,
1963.
(20) Halperin, W. P. ReV. Mod. Phys. 1986, 58, 533.
(26) Zhao, Y.; Zhang, Z.; Dang, H. J. Phys. Chem. 2003, 107, 7574.