Photoluminescence enhancement in nanocomposite thin films of CdS–ZnO
Pushan Ayyub, Parinda Vasa, Praveen Taneja, Rajarshi Banerjee, and B. P. Singh
Citation: J. Appl. Phys. 97, 104310 (2005); doi: 10.1063/1.1899248
View online: http://dx.doi.org/10.1063/1.1899248
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JOURNAL OF APPLIED PHYSICS 97, 104310 共2005兲
Photoluminescence enhancement in nanocomposite thin films of CdS–ZnO
Pushan Ayyub,a兲 Parinda Vasa, and Praveen Tanejab兲
Department of Condensed Matter Physics, Tata Institute of Fundamental Research, Homi Bhabha Road,
Mumbai 400005, India
Rajarshi Banerjee
Department of Materials Science & Engineering, University of North Texas, Denton, Texas 76203
B. P. Singh
Department of Physics, Indian Institute of Technology, Mumbai 400076, India
共Received 10 January 2005; accepted 7 March 2005; published online 3 May 2005兲
We show that the photoluminescence emitted from a dense, two-component quantum dot ensemble
on a thin film is significantly higher and decays much faster than that from quantum dots of either
of the two pure systems 共CdS and ZnO兲. The semiconductor nanocomposite, in which the
characteristic grain size of each species was 2 – 3 nm, was deposited directly on Si wafers by
high-pressure magnetron sputtering, and exhibits a single, relatively sharp optical absorption
edge. © 2005 American Institute of Physics. 关DOI: 10.1063/1.1899248兴
I. INTRODUCTION
The electronic and optical properties of quasizero dimensional semiconductors or quantum dots are fairly well
understood. Yet there is an overwhelming interest in these
systems, largely due to the size-controlled tunability of their
band gap1 and the high quantum efficiency for photoluminescence 共PL兲2 arising from quantum confinement. Exciting applications are envisaged for such systems in optoelectronic
devices, lasers, optical communications, and quantum computing. Of particular interest is the lasing action arising from
photon localization in an ensemble of light-emitting particles. Repeated scattering of light from grain boundaries in
such systems could result in random lasing.3 Since lasing
originates from interference effects, it is essential for the
emitted light to have some degree of coherence. Coherence
is also important for applications related to quantuminformation processing.4 Such applications demand a high
luminescence output with appreciable coherence from a
nanocrystalline semiconductor in a device-compatible, thin
film geometry. In this article we show that the PL emitted by
an optically flat, quantum dot solid 共QDS兲 thin film deposited on Si wafers shows a significant enhancement due to
quantum confinement. Further, in a separate article we show
that the light emitted from such a nanocrystalline ensemble
has an appreciable degree of spatial coherence.5 It is important to note that while the optoelectronic properties of lowdimensional semiconductors have been investigated
widely6,7 this is one of the few studies on dense ensembles of
QDs or QDSs.8,9
A major problem with isolated semiconductor nanoparticles is that they often have surface electronic states within
the highest occupied molecular orbital/lowest unoccupied
molecular orbital 共HOMO–LUMO兲 gap that provide nonradiative decay channels and lead to a severe degradation in
a兲
Author to whom correspondence should be addressed; electronic mail:
[email protected]
b兲
Current address: Brandeis University, MA 02454.
0021-8979/2005/97共10兲/104310/4/$22.50
optoelectronic properties. This problem has been addressed
by “capping” the semiconductor nanoparticle with a thin
layer of a higher band gap material.10 Techniques involving
semiconductor-doped glasses,11 zeolite encapsulation,12 sol–
gel synthesis,13 and colloidal precipitation14 can produce
capped, nearly monodisperse nanoparticles suitable for basic
studies, but are not very useful for fabricating large area thin
films that can be integrated into devices. Here, we demonstrate a single-step process 共without the necessity of forming
a core–shell structure兲 for the deposition of nanocomposite
thin films directly on Si wafers. The nanocomposite film is
constituted of a random assembly of nanoparticles of two
distinct semiconductors 共CdS and ZnO兲, and we show that
the larger band gap component of the nanocomposite effectively passivates the component with the smaller band gap.
II. SYNTHESIS AND MICROSTRUCTURE
Bulk CdS is a direct band gap semiconductor 共Eg
= 2.42 eV at 300 K兲 with a hexagonal wurtzite structure. It
undergoes a size-induced structural transformation to a cubic
zinc blende structure below 2 – 3 nm.15 The larger band gap
component in our nanocomposite was ZnO 共Eg = 3.44 eV at
300 K兲, a semiconductor with a crystal structure similar to
CdS. Nanostructured films 共average grain size ⬇2 – 20 nm兲
of most metals, semiconductors, and oxides can be sputtered
onto desired substrates by dc/rf sputtering16 at relatively high
gas pressures 共⬇10– 200 mTorr兲 and low substrate temperatures 共77– 300 K兲. In particular, uniform, nanocrystalline
films of pure CdS can be synthesized by this technique,17 but
need to be chemically passivated by a prolonged in situ exposure to H2 + N2 gas mixture. The present study reports the
deposition of CdS:ZnO nanocomposite thin films on Si wafers or quartz plates by rf magnetron sputtering, typically
carried out in flowing Ar 共99.999%兲 at 170 mTorr, the substrate being held at 0 ° C. The sputtering targets were sintered 2 in. pellets composed of a mixture of CdS and ZnO in
the molar ratios 45:55, 40:60, and 33:67.
97, 104310-1
© 2005 American Institute of Physics
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104310-2
J. Appl. Phys. 97, 104310 共2005兲
Ayyub et al.
FIG. 2. Optical absorption spectrum from: 共A兲 bulk CdS film 共t
= 1.3 ␮m , dav ⬎ 5 nm兲, 共B兲 nanocrystalline CdS film 共t = 0.2 ␮m , dav
= 4 nm兲, and 共C兲 CdS:ZnO nanocomposite film 共40/ 60, t = 1.3 ␮m , dav
= 3 nm兲. Here t = film thickness and dav = mean particle size obtained from
the quantum shift of the optical band gap.
FIG. 1. 共Top兲 Bright-field TEM micrograph of nanocomposite CdS:ZnO
thin film sputter deposited on Si. The micrographs show two nanodispersed
phases with different contrasts, each phase consisting of crystallites of average size 2 – 3 nm. 共Center兲 Dark-field TEM micrograph from the same
area, highlighting only a fraction of the particles that contribute to a segment
of a single diffraction ring. 共Bottom兲 Selected area electron diffraction pattern from the same region, indexed on the basis of the hexagonal 共h兲 and
cubic 共c兲 polymorphs of CdS and ZnO.
The x-ray diffraction 共XRD兲 spectra of the sputterdeposited nanocomposite films show a single broad hump,
which could arise from a superposition of the stronger diffraction lines of CdS and ZnO, substantially broadened due
to small particle size. The microstructure and chemical composition of the ion-milled nanocomposite CdS:ZnO film
were obtained using transmission electron microscopy
共TEM兲 in the imaging and diffraction modes, respectively. A
FEI CM-200 TEM 共200 keV兲 with a LaB6 emitter was used.
Bright-field TEM micrographs of a typical nanocrystalline
CdS:ZnO film 共Fig. 1, top兲 indicate that the average in-plain
grain size is ⬇3 nm and the size distribution is quite narrow.
Note that it is not possible to obtain images at higher resolution since this results in electron beam induced annealing
of the densely packed nanoparticles. However, the almost
monodispersed, individual nanocrystals can be more clearly
identified as white dots in dark field TEM micrographs 共Fig.
1, center兲 formed by intensity contributions from a single
diffraction ring. All the distinct rings in the selected area
diffraction pattern from the sample 共Fig. 1, bottom兲 can be
indexed consistently to the hexagonal and cubic polymorphs
of CdS and ZnO. We point out that the TEM diffraction
patterns were recorded using a selected area aperture of diameter ⬃500 nm. Hence the compositional data refer to an
average from a statistically significant number of nanopar-
ticles. Energy dispersive x-ray spectrometry indicated that
the CdS and ZnO phases were approximately stoichiometric,
with Cd: S = 1 : 1.20 and Zn: O = 1 : 1.18.
The nanocomposite CdS:ZnO thin films therefore consist
of densely packed, randomly intermixed, nearly uniformsized arrays of individual CdS and ZnO nanoparticles, the
characteristic grain size for each phase being about 3 nm. An
approximate estimate of the particle size distribution obtained from an analysis of the XRD line shape and the optical absorption spectra is 3共±1兲 nm. In the case of nanocrystalline CdS thin films deposited under similar conditions, the
size distribution was measurably broader: 4共±2兲 nm. Competing growth of the two crystallographic phases in the nanocomposite probably leads to a narrower size distribution. A
core–shell structure is not expected to originate from the
sputtering process; neither is there any evidence for such a
structure or for the formation of a solid solution between
CdS and ZnO. Since the ZnO phase 关whose HOMO–LUMO
gap is quantum shifted deep into the ultraviolet 共UV兲兴 essentially provides a passivating matrix 共see below兲, the system
can be modeled as a random, three-dimensional ensemble of
nearly identical CdS quantum dots. Such a system has been
termed a disordered quantum dot solid8 and is described by
coexisting discrete 共localized兲 and band-like 共delocalized兲
electronic states.
III. OPTICAL PROPERTIES
The optical absorption spectrum of a typical nanocomposite CdS:ZnO 共40/ 60兲 thin film deposited on quartz shows
a single, sharp absorption edge at 405 nm 共Fig. 2兲, which
corresponds to the quantum shifted excitonic energy in nanoCdS. In particular, the absorption edge is much sharper than
that from the pure nano-CdS sample, indicating a narrower
size distribution in the former. The absorption edge from a
large particle 共bulk兲 CdS film is also shown for comparison.
Using standard expressions,18 we find that the observed blueshift in the gap energy 共from 514 to 405 nm兲 in the nanocomposite film corresponds to a particle diameter of 3 nm
for CdS nanoparticles, which is in excellent accordance with
the size obtained from TEM. Clearly, the observed absorp-
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104310-3
J. Appl. Phys. 97, 104310 共2005兲
Ayyub et al.
FIG. 3. Normalized photoluminescence emission spectra recorded with an
excitation wavelength of 391 nm 共second harmonic of Ti:sapphire laser兲
from 共A兲 bulk CdS film 共t = 1.3 ␮m , dav ⬎ 5 nm兲, 共B兲 nanocrystalline CdS
film 共t = 0.5 ␮m , dav = 4 nm兲, 共C兲 CdS:ZnO nanocomposite film 共r
= 45/ 55, t = 0.6 ␮m , dav = 3 nm兲, 共D兲 CdS:ZnO nanocomposite film 共r
= 40/ 60, t = 1.3 ␮m , dav = 3 nm兲, and 共E兲 CdS:ZnO nanocomposite film 共r
= 33/ 67, t = 1.5 ␮m , dav = 3 nm兲. Here r = the molar ratio of two semiconductors. Curve 共A兲 has been multiplied by 5.
tion edge is not related to the HOMO–LUMO gap in ZnO
nanoparticles, which is expected at ⬇320 nm for ⬇3 nm
particles, its “bulk” band gap being 360 nm.
The photoluminescence emission spectra were obtained
using the second harmonic of a Ti:sapphire laser
共80 fs, 100 MHz兲 with a fundamental of 782 nm. The power
incident on the sample was ⬃40 mW. The PL spectra were
recorded with a 0.32 m Jobin Yvon monochromator, using
UV blocking filters to cut down the scattered light from the
sample. Note that the spectra 共in Fig. 3兲 were normalized for
the film thickness and absorption coefficient at the pump
wavelength. The pure nanocrystalline CdS film 共B兲 expectedly shows a higher PL than the bulk CdS film 共A兲 due to
quantum confinement effects. However, the normalized PL
emission in all the CdS:ZnO nanocomposite films is substantially higher, indicating that the presence of ZnO nanoparticles intermixed with the CdS nanoparticles gives rise to an
appreciable degree of surface passivation. The PL intensity is
a rather sensitive function of the CdS / ZnO ratio and peaks at
a value of 2:3 共molar ratio兲. This particular nanocomposite
film shows significant enhancement in the spectrally integrated PL emission: by 2 orders of magnitude over the bulk
CdS film, and more than six times over nano-CdS films.
Note that the peak of the PL emission profile occurs in the
480– 540 nm range in nanocrystalline CdS as well as CdS–
ZnO nanocomposite, indicating a similar physical origin.
The maximum in the PL emission is “Stokes shifted” with
respect to the absorption edge in CdS and CdS:ZnO due to
trapping and capture in defect states, and is a favorable feature in many applications.19
We ascribe the PL enhancement to the presence of ZnO
nanoparticles in the nanocomposite that effectively quenches
nonradiative decay processes. Also, unlike the pure nanoCdS films which need prolonged postdeposition surface
treatment with a H2 + N2 mixture to prevent chemical
degradation,17 the as-deposited CdS:ZnO films are inherently
inert to atmospheric degradation, and do not require such
treatment. We also point out that in pure CdS, there is a
growth in particle size in the sputtered films with increasing
film thickness. However, the presence of two components in
the nanocomposite film effectively retards the particle
growth for either phase.
The external PL quantum efficiency 共QE兲 for the nanocomposite sample was measured using the second harmonic
of a Ti:sapphire laser 共400 nm, 80 fs兲. The spectrally integrated PL produced by an incident power of 10 mW was
detected with a calibrated photodiode, the scattered laser
light being blocked by a suitable long pass filter. Appropriate
corrections were made for the absorption cross section at the
excitation wavelength and the filter transmission. The angular variation in the PL signal was also measured and used in
the calculation of the QE. The external QE at room temperature for the nanocomposite 共40/ 60兲 sample was found to be
⬇0.1%, which is at least five times more than that measured
for nano-CdS under similar conditions. The internal quantum
efficiency of all the samples is expected to be higher due to
self absorption within the samples. This compares favorably
with the value of external QE 共=0.22% 兲 measured from light
emitting diodes made with CdSe共CdS兲 core–shell
structures.20
We also carried out time-resolved fluorescence measurements using the second harmonic from a Ti–sapphire laser
共440 nm, 1 ps兲 as excitation source. The majority species in
the CdS:ZnO 共40/ 60兲 nanocomposite was found to have a
lifetime ⬍50 ps 共as obtained from an exponential fit兲, while
both nano-CdS 共166 ps兲 and nano-ZnO 共329 ps兲 had substantially longer lifetimes. The short decay time in the nanocomposite appears to be a direct consequence of the enhanced oscillator strength in this system. The enhanced PL
and fast relaxation are both desirable for device applications.
An earlier measurement of time-resolved PL in nanocrystalline CdS and CdSe also yields a decay time in the picosecond range.21 In that case too, the authors invoked the participation of “intrinsic nanocrystalline states or intrinsically
unpassivated interface states.”
In a separate article,5 using the Young’s double slit experiment, we have shown that the PL emission from the
nanocomposite thin films has significant spatio-temporal coherence, a fact that would be crucial for their use in quantum
information processing as well as for lasing action from
light-emitting particles.
IV. CONCLUSIONS
The optoelectronic properties of a nanoscale dispersion
of two different semiconductors 共in which we expect an appreciable overlap of wave functions between particles兲 cannot be expressed simply as a linear superposition of the properties of the individual components. This therefore opens up
a region of phase space with both the crystallite size as well
as the component fraction as coordinates. The CdS:ZnO
nanocomposite exhibits much higher photoluminescence
emission and shorter decay times than individual CdS or
ZnO nanocrystals of similar dimensions. The increase in the
PL due to decreasing size can be empirically related to the
faster relaxation dynamics, while a further increase takes
place in the nanocomposite due to efficient surface passiva-
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104310-4
tion, which reduces the effect of the nonradiative surfacemediated decay channels. The self-passivated nanocomposite
thin film structure can also be formed more efficiently and in
a device-compatible manner than the core–shell structure.
ACKNOWLEDGMENTS
It is a pleasure to thank G. Krishnamoorthy for the fluorescence lifetime measurements, A. S. Vengurlekar and K. L.
Narsimhan for stimulating discussions, and S. Bhattacharya
for his advice and a critical reading of the manuscript.
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