J. Phys. Chem. B 2000, 104, 11853-11858
11853
Poly(N-vinylcarbazole) (PVK) Photoconductivity Enhancement Induced by Doping with
CdS Nanocrystals through Chemical Hybridization
Suhua Wang,† Shihe Yang,*,† Chunlei Yang,‡ Zongquan Li,‡ Jiannong Wang,‡ and Weikun Ge‡
Department of Chemistry, The Hong Kong UniVersity of Science and Technology, Clear Water Bay,
Kowloon, Hong Kong, and Department of Physics, The Hong Kong UniVersity of Science and Technology,
Clear Water Bay, Kowloon, Hong Kong
ReceiVed: February 9, 2000; In Final Form: June 7, 2000
We have functionalized poly(N-vinyl carbazole) (PVK) by controlled sulfonation. CdS nanocystals of 3-20
nm across were synthesized in the sulfonated PVK matrix with the CdS molar fraction of ∼1-18%. The
CdS nanoparticle size increased with the molar fraction of CdS. At high CdS molar fractions, the CdS
nanocrystals exist in both cubic and hexagonal phases. Photoluminescence efficiency of PVK decreases when
the molar fraction of CdS increases due to quenching through interfacial charge transfer. Photoluminescence
attributable to the CdS nanocrystals can be observed only at low molar fractions of CdS. Significant
enhancement in photoconductivity induced by the chemical doping of CdS in PVK has also been demonstrated.
Introduction
Nanocomposites consisting of inorganic nanoparticles and
organic polymers often exhibit a host of mechanical, electrical,
optical and magnetic properties, which are far superior compared
with those of the individual components.1-4 These desirable
properties are derived from a complex interplay between the
building blocks and the interfaces separating the building
blocks.
Poly(N-vinyl carbazole) (PVK) is a hole transport organic
semiconducting polymer. It has been widely used as an
electronic and optical material.4 CdS is a well-known inorganic
semiconductor. Hybrids of CdS nanoclusters and PVK promise
both the excellent carrier generation efficiency and mobility of
the inorganic semiconductor and the processibility of the organic
polymer. Indeed, a photoconductivity enhancement of PVK has
been observed when CdS nanoclusters were finely dispersed in
the PVK matrix. In this case, the CdS nanoclusters act as a
sensitizer for the photogeneration of charges and the PVK
polymer serves as the carrier transporting medium.5
It should be pointed out that the nanocomposite CdS/PVK
reported previously was prepared simply by mixing PVK and
CdS nanoclusters or their precursors.4 This procedure introduced
inevitably capping molecules or precursor molecules aside from
PVK and CdS. The effect of these molecules on the electrical
and optical properties of the nanocomposite remains to be
investigated. We have recently taken a new approach for the
preparation of a truly two-component nanocomposite of CdSPVK by direct chemical hybridization. The thrust for pursuing
this synthetic approach is to better control surface properties of
the CdS nanoparticles in the nanocomposite for the enhancement
of photoconductivity. The electronic, structural, and compositional properties of the nanoparticle surface are the key to the
engineering of interfacial charge-separation characteristics in
the nanocomposite. Our method consists of (1) sulfonation of
PVK,6 (2) preparation of the precursor PVK(SO3)2Cd, and (3)
* Corresponding author (E-mail: [email protected]).
† Department of Chemistry.
‡ Department of Physics.
formation of the CdS-PVK nanocomposite. The maximum
molar fraction of CdS in the nanocomposite was ∼18% (∼5%
v/v) based on the sulfonation degree of PVK, as determined by
both X-ray photoelectron spectroscopy (XPS) and secondary
ion mass spectrometry (SIMS).7 The average size of the
particles, as determined using transmission electron microscopy
(TEM), was 3-20 nm depending on the molar ratio CdS:PVK.
The absorption edge is blue-shifted by 15-70 nm from that of
bulk CdS. Decreased photoluminescence and enhanced photoconductivity due to the dispersion of CdS nanoclusters in PVK
matrix have been observed.
Experimental Section
1. Synthesis of Sulfonated PVK6. A 0.76-mL (8.1 mmol)
aliquot of acetic anhydride (99+%, Aldrich) was dissolved in
4.0 mL of 1,2-dichloroethane in a 50-mL flask. Then, 0.28 mL
(5.0 mmol) of 95% sulfuric acid was added in a dropwise
manner into the aforementioned solution at 10 °C, and a
transparent colorless solution was obtained. The concentration
of acetyl sulfate in this solution was 1.0 M. The solution was
stored for later use in the sulfonation of PVK.
PVK (secondary standard, Aldrich) was dried in a vacuum
oven at 50 °C for 10 h before use. PVK (250 mg) was dissolved
in 5.0 mL of tetrahydrofuran (THF) at room temp. Seven
identical solutions were prepared in this way. A different amount
of the sulfonating agent was added in a dropwise manner into
each of the seven PVK solutions under magnetic stirring,
resulting in seven solutions labeled as PVK-00, PVK-10, PVK15, PVK-30, PVK-40, PVK-50, and PVK-60 (Table 1). The
two-digit numbers in the sample labels indicate rough molar
ratios (x100) of the sulfonating agent to PVK. These solutions
containing the sulfonating agent were heated to ∼75 °C and
refluxed for 5 h. Then, 1.0 mL of ethanol was added to terminate
the sulfonation reaction.6 At this stage, ∼20 mL of cyclohexane
was added to the solution to precipitate the sulfonation products.
The precipitate was vacuum filtrated, washed with ethanol (for
samples with a ratio of sulfonation agent to carbazole of PVK
<40%) or cyclohexane (for samples with a ratio of sulfonation
agent to carbazole of PVK >40%), and dried in a vacuum oven
10.1021/jp0005064 CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/28/2000
11854 J. Phys. Chem. B, Vol. 104, No. 50, 2000
Wang et al.
TABLE 1: Characteristics of PVK Solutions
product
fraction of
sulfonation, %
molar fraction
of CdS in
PVK-CdS
volume fraction
of CdS in
PVK-CdS (%)
PVK-00
PVK-10
PVK-15
PVK-30
PVK-40
PVK-50
PVK-60
0
2.51
5.96
11.7
16.7
18.2
25.0
0
1:80
1:34
1:17
1:12
1:11
1:8
0
0.55
1.32
2.57
3.67
4.22
5.54
at 50 °C overnight. Finally, a light gray powder was obtained.
Sulfonation of PVK was also carried out at different temperatures, but the optimum temperature for the sulfonation of PVK
was 75 °C. The sulfonated PVKs reported in this work were
all synthesized at this temperature.
2. Preparation of Cd-Exchanged Sulfonated PVK. First,
100 mg of sulfonated PVK was dissolved in 40 mL of THF.
Then, 20 mL of 1 M CdCl2 aqueous solution (for PVK-10 and
PVK-15, 1.0 g of CdCO3 powder was used) was slowly added
to the THF solution. After the mixture was stirred for 24 h, the
solvent was removed by rotary evaporation at 75 °C for 30 min
to precipitate the Cd-exchanged sulfonated PVK from the
solution (for PVK-10 and PVK-15, the mixture was filtered to
remove remnant CdCO3). The solid was washed six times with
double deionized water, and then was dried in a vacuum oven.
In XPS spectra of the Cd-exchanged sulfonated PVK, peaks
corresponding to the Cd 3d5/2 core levels were clearly observed
at 406.3 eV. The atomic ratio of cadmium to sulfur shows that
roughly all the sulfonic acid groups are exchanged by the
cadmium ion.
3. Formation of the Chemically Derivatized Nanocomposite CdS-PVK. Cd-exchanged sulfonated PVK (40 mg) was
dissolved in 20 mL of THF, and 10 mL of hydrogen sulfide
gas was then injected into the solution. Immediately, the
originally colorless solution turned yellow. Argon gas was
bubbled into the yellow solution at a rate of 3 mL/min to remove
the excessive hydrogen sulfide dissolved in the solution. The
appearance of the yellow color indicated the formation of
the chemically derivatized nanocomposite CdS-PVK in the
solution.
4. Sample Characterization. The size, shape, and crystal
structures of the CdS nanoparticles were determined using a
JEM100 CXII and JEM2010 high-resolution transmission
electron microscopes (HRTEM). The microscopes, operated at
200 keV, have a spatial resolution of 0.17 nm. TEM samples
were prepared by placing a drop of the colloidal suspension on
holey carbon-coated copper grids, the excess solvent was
evaporated, and the sample was dried in a vacuum. X-ray
diffraction measurements were carried out on a PHILIP8338
model diffractometer with Cu KR incident radiation. The
samples for X-ray diffraction measurements were prepared by
rotary evaporation of the CdS nanoparticle suspension to a fine
powder.
Ultraviolet-visible (UV-vis) absorption spectra were obtained using a Milton Roy Spectronic 300 spectrometer using
THF solution samples. Photoluminescence spectra were recorded
using a He-Cd laser as the excitation light source at room
temperature for both solutions and films. A Spex500 spectrometer, a photomultiplier tube (PMT), and a photon counter were
used as the detection system. For photocurrent spectroscopy, a
150 W Xe lamp served as a source of white light, which passed
through a Spex1681 monochromator and was chopped at a
frequency of 400 Hz before being focused onto the sample
Figure 1. XRD pattern of the CdS nanoparticles in the chemically
derivatized nanocomposites (CdS:PVK ) 1:12). The vertical lines
indicate the standard XRD stick patterns for (b) hexagonal and (2)
cubic phases of CdS. The peak intensities of the cubic phase are
normalized to that of the (311) diffraction and the hexagonal phase to
that of the (103) diffraction.
surface. The signal from the two electrodes of the sample was
detected by a SR830 lock-in amplifier. All the measurements
were taken for short circuits without external bias. The currentvoltage measurement was carried out using a Hewlett-Packard
4115A semiconductor analyzer.
The pristine PVK and doped PVK films were prepared by
spin coating on an indium-tin-oxide (ITO) glass substrate. For
good film quality, a solvent mixture of THF and chloroform
(volume fraction of chloroform: 1-10%) was used. After drying
the fresh film in vaccuo for 1 h, Al was evaporated on the
surface of the nanocomposite film.
Results and Discussion
1. Nanocomposite Structures. A typical XRD pattern of the
yellow CdS-PVK nanocomposite powder (PVK-30) is presented in Figure 1. Careful analysis of the XRD pattern shows
the coexistence of cubic and hexagonal phases in the nanocomposite. Nearly all the diffraction peaks can be reasonably
assigned to the reflections from the lattice planes of the cubic
and hexagonal phases of CdS. For the cubic phase, the (200)
diffraction peak at the angle of 31° is not obvious probably
because it is masked by the strong (101) reflection of the
hexagonal phase. The (100) and (101) reflections of the
hexagonal phase are both masked by the strong (111) reflection
of the cubic phase at the angle of 26.5°, but they are still
recognizable as two shoulders of the strong peak at the angle
of 26.5°. Furthermore, as can be seen from the XRD pattern,
the cubic phase is predominant over the hexagonal phase in
the CdS-PVK nanocomposite.
It is noteworthy that the XRD pattern of our CdS nanoparticles synthesized at room temperature is similar to that of the
CdS nanoparticles that were prepared by annealing at 300 °C.8
At 300 °C, transition occurs from cubic (β-CdS or Hwaleyite)
to hexagonal (R-CdS or Greenockite) phase; thus, it is reasonable
for the coexistence of the two phases in the CdS nanoparticles
annealed at this temperature.9 However, our synthesis was
carried out at room temperature and the coexistence of cubic
and hexagonal phases may be related to the method we used to
produce the CdS-PVK nanocomposite; that is, rapid nucleation
and growth from a H2S supersaturated solution. Moreover, the
cubic CdS is a metastable phase at room temperature.10 At the
initial instant of nucleation, the β-CdS phase may form
CdS-Induced PVK Photoconductivity Enhancement
J. Phys. Chem. B, Vol. 104, No. 50, 2000 11855
Figure 2. TEM images of the CdS nanoparticles in the chemically
derivatized nanocomposites (CdS:PVK ) 1:12) at different magnification scales.
preferably to the R-CdS because its isotropy is more compatible
with a spherical nucleus than that of the anisotropic hexagonal
structure.8 However, the small free energy difference between
the cubic and hexagonal phases is likely to result in the
formation of hexagonal phase nanocrystals.8 The fabrication of
the two-phase CdS nanoparticles in this way could otherwise
be attained at a much higher temperature (∼300 °C) by such
methods as chemical bath deposition and aqueous solution
precipitation at room temperature.8,11
The mean particle size is estimated to be ∼4 nm from the
XRD line width using the Scherer equation.12 This value is
smaller than that estimated from the TEM image (inset of Figure
2b), which shows CdS particles with sizes in the range 4-20
nm. This result is understandable because the CdS nanoparticles
prepared at this large molar ratio of CdS:PVK has a broad size
distribution, and the XRD peak width is primarily determined
by the small particles in the ensemble.
TEM images reveal CdS nanoparticles in both spherical and
cubic shapes (Figure 2). The two CdS nanoparticles shown in
Figure 2 are both single nanocrystallites. The CdS nanocube
has definitely a cubic phase based on the two perpendicular
sets of fringes at d spacings of 2.94 Å (200) and 2.84 Å (200),
and one diagonal set of fringes at a d spacing of 1.96 Å (220;
see arrow in Figure 2). This result is also supported by the
selected electron diffraction pattern from the nanoparticle. On
the other hand, only one fringe spacing (3.41 Å) was observed
for the spherical nanoparticle. This spacing can be indexed either
to (111) of cubic CdS (3.36 Å) or to (002) of hexagonal CdS
(3.36 Å). It is therefore unclear whether this spherical particle
has a cubic or hexagonal structure.
We found that the CdS nanoparticle size is very much affected
by the molar ratio CdS:PVK. The XRD and TEM patterns
already presented were obtained for a sample with a CdS:PVK
molar ratio of 1:12. For samples with CdS:PVK molar ratios <
1:34, no X-ray diffraction patterns could be observed, presumably due to the small size of the CdS nanoparticles. This result
was confirmed by TEM measurements presented next.
Figure 3. High-resolution TEM image of the CdS nanoparticles with
the CdS:PVK molar fraction of 1:80.
Figure 3 shows a HRTEM image of CdS nanoparticles
prepared with a much lower Cd:PVK molar ratio (1:80). Most
CdS nanoparticles are ∼3 nm in size, as can be seen in Figure
3a, in which four nanoparticles are apparent with fringe spacing
of 3.4 Å. Again, this spacing can be assigned either to (111) of
cubic CdS (3.36 Å) or to (002) of hexagonal CdS (3.36 Å).
Occasionally, particles as large as 12 nm also can be found
(Figure 3b). The large CdS nanoparticle in Figure 3b has a
triangular shape, and its hexagonal structure can be appreciated
from the observed fringe spacings of (100) and (010) (3.60 Å).
2. Optical Properties. Figure 4 shows the UV-vis absorption
spectra of the PVK (curve a), sulfonated PVK (curve b), and
the chemically derived nanocomposite CdS-PVK (PVK-30 and
PVK-15; curves c and d, respectively). It is clear that there is
not much difference between the UV-vis spectra of the
sulfonated PVK and the chemically derivatized nanocomposite
CdS-PVK, except that a new broad absorption tail appears in
the latter above 370 nm. For the CdS-PVK nanocomposite with
a molar ratio of 1:12, the absorption starts at ∼520 nm (curve
d) and increases with decreasing wavelength. The absorption
onset is determined to be 491 nm (2.53 eV) by plotting (Rhν)2
against hν and extrapolating (Rhν)2 to zero (R is the absorption
coefficient and hν is the photon energy), where the photon
energy stands for the absorption onset (Figure 4, inset 1). This
11856 J. Phys. Chem. B, Vol. 104, No. 50, 2000
Wang et al.
Figure 4. UV-vis spectra of the (a) PVK, (b) sulfonated PVK, (c) chemically derivatized nanocomposite CdS-PVK (CdS:PVK ) 1:34), and (d)
chemically derivatized nanocomposite CdS-PVK (CdS:PVK ) 1:12). Inset 1: plot of (Rhν)2 versus hν, obtained from the spectra labeled as c and
d. The absorption edges are fitted to a direct transition.19 Inset 2: an enlargement of the spectra.
tail is attributed to the band gap absorption of the CdS
nanoparticles and thus the band gap is calculated to be 2.53
eV. For these CdS nanoparticles in the chemically derivatized
nanocomposites CdS-PVK, a particle size of 6.1 nm is
estimated from the band gap absorption onset using the Brus
equation based on the effective mass approximation for semiconductor nanoparticles.13 For the CdS-PVK nanocomposite
with a molar ratio of 1:34, a band gap of 2.79 eV is obtained
(curve c), and the average particle size is estimated to be
3.8 nm.
When the as-prepared chemically derivatized nanocomposite
(CdS/PVK > 1:34) was excited with laser at a wavelength of
325 nm, the photoluminescence spectrum did not change, but
the intensity of the photoluminescence was reduced compared
with that of the pure PVK solution with the same concentration
(Figure 5, curves 1 and 2). As can be seen from Figure 5, the
higher the CdS:PVK molar ratio, the lower the intensity of the
photoluminescence. The PL appears to be from the transition
in the carbazole moieties of PVK, and the band gap emission
from CdS was not observed. Both the reduced PL of PVK and
the absence of band-gap luminescence from CdS can be
attributed to the quenching by interfacial change transfer.
Because amines (e.g., carbazole moiety in this system) are
effective complexing agents with the surfaces of CdS clusters,
the quenching of the fluorescence of PVK in the chemically
derivatized nanocomposite CdS-PVK probably results from the
close contact between the polymer and the CdS nanoparticles.
This close proximity makes possible the charge carrier transport
through the interface between PVK and CdS nanoparticles.
When photons are absorbed, electrons were excited into the
lowest unoccupied molecular orbital (LUMO) of the carbazole
groups in PVK and holes were left in the highest occupied
molecular orbital (HOMO). The excited electrons, in the pure
PVK solution, will return from the conduction band back to
the valance band through a radiative process. In the system of
the as prepared PVK-CdS nanocomposites, however, the
excited electrons can also choose to migrate from PVK to the
CdS nanoparticles. This possibility is illustrated by the relative
energy levels of PVK and CdS,11,14 as shown in Figure 6a. Such
an interfacial charge-transfer brings down the transition probability from LUMO to HOMO and thus reduces the PVK
photoluminescence.
Electron-hole pairs in the CdS nanocrystals also form
through a direct absorption, as shown in Figure 6b. In this case,
electrons from PVK migrate favorably to CdS to fill the holes,
creating holes in PVK, or equivalently, the holes have been
transferred to PVK. As a consequence, for the as-prepared large
CdS nanoparticles capped by PVK (Cd:PVK > 1:34), virtually
no band gap luminescence could be observed.15
However, when the surface of the CdS nanoparticles was
modified with dodecanthiol, which separates the nanoparticle
and PVK, the photoluminescence was significantly enhanced,
and in fact, the photoluminescence intensity is nearly comparable
to that of the pure PVK solution (Figure 5, curve 3). Because
the -SH group of dodecanthiol has a stronger interaction with
the surface of CdS nanoparticles than the carbazole moiety of
PVK, it replaces PVK as a capping molecule on the nanoparticle
surface. Furthermore, the formation of the complex between
CdS-Induced PVK Photoconductivity Enhancement
J. Phys. Chem. B, Vol. 104, No. 50, 2000 11857
Figure 7. Photoluminescence spectra of (a) PVK and (b) CdS-PVK
in THF with a CdS:PVK molar ratio of 1:80.
Figure 5. Photoluminescence spectra of pure PVK (curve 1), asprepared CdS-PVK (curve 2), and thiol-activated CdS-PVK (curve
3) in THF: (a) CdS:PVK ) 1:34; and (b) CdS:PVK ) 1:12.
Figure 6. A schematic illustration of the energy band profiles of the
chemically derivatized CdS-PVK nanocomposites and the routes for
excitation and charge transfer. The band positions of CdS nanoparticles
were estimated based on the electron binding energies as measured by
valence band photoemission.18,20 The curved arrows indicate the
direction of carrier transfer.
Cd2+ and -SH also removed the trapping sites on the CdS
surfaces. Because of the space layer of the dodecanthiol
Figure 8. Photoconductivity spectra of PVK and the CdS-PVK
nanocomposite with CdS:PVK ) 1:80. Inset: CdS-PVK nanocomposite film configuration for photoconductivity measurements.
separating PVK and the nanoparticle, interfacial carrier transport
was significantly hindered. As a result, the PVK in the
nanocomposite is similar to that in the pure PVK solution, and
thus the emission from the thiol-modified nanocomposite is
enhanced compared with that of the as-prepared CdS-PVK
nanocomposite.
In general, the intensity of the photoluminescence from PVK
(375-400 nm) decreases when the filling factor of CdS becomes
larger in the CdS-PVK nanocomposite. Moreover, although
the band gap photoluminescence was not observed from large
CdS nanoparticles (corresponding to high filling factors of CdS),
it was detected when the filling factor of CdS is <1:34. For
example, with a CdS:PVK molar ratio of 1:80, photoluminescence was detected from the sample in THF solution peaking
at ∼430-440 nm (Figure 7). The band gap is estimated to be
2.86 eV, which is apparently due to the presence of CdS.
3. Photoconductivity of the Nanocomposite. For electrical
measurements, the CdS-PVK nanocomposite (CdS:PVK )
1:34) was casted on an ITO glass by spin-coating to form a
transparent film. The film thickness was ∼150 nm, as determined with an Alpha-step 200 surface profiler. To form a
cathode, Al was evaporated on the surface of nanocomposite
film, as illustrated in the inset of Figure 8. The size of the device
is ∼0.5 cm2.
To investigate the effect of the CdS nanoparticles doped into
the PVK film, we examined the spectral response of the
photoconductive device with pure PVK and CdS-doped PVK.
11858 J. Phys. Chem. B, Vol. 104, No. 50, 2000
Figure 8 shows the wavelength dependence of the short-circuit
photocurrent for a pure PVK film and a CdS-PVK nanocomposite film. For the pure PVK film, the photocurrent action
spectrum appears to be very similar to the absorption spectrum
(symbatic response) due to the small film thickness of our
sample.16,17 Because only the electrons created by light close
to the aluminum electrode will be effective in generating the
photocurrent, the polymer layer between the ITO and aluminum
electrodes should be sufficiently thin to observe the symbatic
response. When the CdS nanoparticles are doped into the PVK
film, the peak shifts to a longer wavelength by 6-7 nm with a
long tail. This photocurrent spectrum, again, closely resembles
the corresponding absorption spectrum. The red-shift of the peak
and the appearance of the long tail indicates the photoabsorption
of the CdS nanoparticles. In addition, the doping of CdS
nanoparticles into the PVK matrix aggrandizes the photocurrent
efficiency of PVK by a factor of 1.7 at the wavelength indicated
by the vertical dashed line in Figure 8. Considering the
integration of the short-circuit current over the wavelength, the
enhancement factor of the photocurrent efficiency of the hybrid
nanocomposite is estimated to be nearly 3. These results show
that the CdS nanoparticles have broadened the spectral range
of the photocurrent response and improved the photoconductivity
of PVK film.
Photoconductivity is a convolution of photoinduced charge
generation and charge transport. Because a low density of
nanoparticles has a small effect on the transport property of the
polymer matrix, their main function is the enhancement of the
charge generation efficiency.17 The PVK polymer matrix is
mainly responsible for the charge transport. CdS can serve as
electron traps in PVK matrix judging from their band structures
(Figure 6).18 After electron-hole pairs (or excitons) are generated by photoabsorption in PVK molecules, which can only
absorb photons with energy >3.6 eV, the excited electrons can
move to the conduction band of CdS in the CdS-PVK
nanocomposites (Figure 6a). Such an interfacial charge transfer
brings down the recombination probability of electrons and
holes, and thus increases the lifetime of the holes in the HOMO
of PVK (Figure 6c). As a result, the holes in the HOMO of
PVK have more chances to migrate and the photoconductivity
of the nanocomposite increases. As illustrated in Figure 6b, the
CdS nanoparticles can absorb photons with energy 2.5 eV < E
< 3.6 eV, creating electron-hole pairs in the conduction and
valence bands of CdS, respectively. The holes in the valence
band of CdS have higher energy than that in the HOMO of
PVK, hence will intend to move to PVK, being accumulated in
the HOMO of PVK (Figure 6c). It is interesting that the
excitation of either PVK or CdS has the same effect of creating
a hole in PVK, and this results explains why the CdS
nanoparticles can enhance the photoconductivity and broaden
the photocurrent response of PVK (Figure 8).
Perhaps more convincing about the good performance of our
hybrid nanocomposite is to make a direct comparison with that
of a blend nanocomposite with the same filling factor. Although
there are a number of reports on the photoconductivity of blend
nanocomposites of CdS-PVK, direct comparison of the data
with our results is difficult because different experimental
conditions involved (e.g., different filling factors, substrates,
measurement techniques among other things). Nevertheless, we
examined the photoconductivity of a hybrid nanocomposite
Wang et al.
(PVK-10-CdS) and a blend nanocomposite (containing alkylthiol-capped CdS nanoparticles) under the same conditions. It
is encouraging that the photoconductivity enhancement factor
of the hybrid nanocomposite (2-3) is always larger than that
of the blend nanocomposite (1-2). Because this nanocomposite
sample contains only ∼1 wt % CdS, further photoconductivity
enhancement is expected by increasing the CdS filling factor
of the hybrid nanocomposite.
Conclusion
In this work, we synthesized CdS nanocystals of a few
nanometers across in sulfonated PVK matrixes with the CdS
molar fraction of ∼1-18%. The sulfonated PVKs were prepared
by controlled sulfonation of the PVK polymer. The CdS
nanoparticles size increased with the molar fraction of CdS. At
high CdS molar fractions, the CdS nanoparticles exist predominantly in the cubic phase with a minor contribution from the
hexagonal phase. Significant enhancement in photoconducty due
to the chemical doping of CdS in PVK has been demonstrated.
Photoluminescence results indicate that interfacial electron
transfer occurs between the CdS nanoparticles and the PVK
molecules. This transfer reduces the photoluminescence efficiency, but enhances the photoconductivity of the CdS-PVK
nanocomposite.
Acknowledgment. This work was supported by an RGC
grant administered by the UGC of Hong Kong.
Supporting Information Available: Figure of conductivity
data. This material is available free of charge via the Internet
at http://pubs.acs.org.
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