Room temperature CdSe Semiconductor nano-crystals
Nayan Mishraa V. D. Abrahama
a
Laxmi Institute of Technology epartment of Physics, Oriental University, Indore , M. P. , India
Abstract
CdSe quantum dots of tunable size have been assembled on glass/ ITO substrates using a wet synthesis route in the
presence of the cadmium and selenium precursors and surfactant. The physical properties of the quantum dots have been
investigated using X-ray diffraction, electron microscopy and optical spectroscopy techniques. It is shown that strong
near band edge luminescence can be obtained from these quantum dots. The luminescence behavior has been interpreted
in the light of the quantum confinement effects.
1:3:1.37 molar ratios in triple distilled
water. The pH of the bath was adjusted to 11
with the help of ammonia solution. The size of
the CdSe nano-crystal was controlled by
adding an appropriate concentration of
mercapto-ethanol as surfactant in the bath. The
value of surfactant to selenium atom in the
synthesis bath was varied from 0 to 2.42.
1.0. Introduction:
Quantum dot structures
based on II-VI semiconductors are presently
of significant interest due to their
advantageous linear and non linear optical
properties. The electronic and optical
properties of these quantum dots are size
dependent [1-3]. These semiconductor
quantum
dots
reveals
considerable
applications in tunable displays [4-5], light
emitting diodes [6], optoelectronic devices [78], biological labels [9] etc.
CdSe is an important semiconductor of
the group II-VI family. The scientific interest
emerged are due to the tenability of its
absorption edge over the entire visible
spectrum by controlling the size. Synthesis of
nano-crystalline CdSe performance in vacuum
[10-11] or arrested precipitation in solution
[12-14]. But the study on the deposition of
CdSe quantum dots as thin films found to be
not substantial. This paper reports the results
on synthesis of quantum dot CdSe thin film
(Q-CdSe) in an aqueous medium and their
characterization.
2.2 Characterisation Techniques
X-ray diffraction patterns were recorded in
the thin film grazing angle mode of XRD6000
(SHIMADZU,
JAPAN)
powder
diffractometer, using CuKα line. Beam
divergence was restricted with the help of 0.15
mm slit on the source side. Drive axis was 2θ
for scan range between 200-650 covered in
10/minute with step size of 0.020 .The
instrument propagation error in the d-value
was ± 0.003 Å.
TEM characterization was carried out
using Techni 20G2 Transmission Electron
Microscope operated at 200 kV. The bright
and dark field images were acquired by
applying axial tilt and selecting a small portion
of the brightest diffraction ring.
The HOMO-LUMO gap of the Q-CdSe
films was determined from optical absorption
studies with the help of UV-Vis
spectrophotometer
(model
UVPC1601,
SHIMADZU Corporation JAPAN) in the
spectral range between 300-800 nm using a
spectral bandwidth of 2 nm at room
temperature.
2 Experimental Details:
2.1 Deposition of CdSe films
The Glass\Indium tin oxide coated
substrates used for deposition of Q-CdSe film
were first thoroughly cleaned in a detergent
(Extron MA 01 MERCK) and then ultrasonificated. The CdSe quantum dots were
synthesized at room temperature in a reaction
matrix containing sodium seleno sulphite,
cadmium chloride and tri-ethanolamine in
1
Photoluminescence (PL) emission spectra for
Q-CdSe films were recorded from a computer
controlled
rationing
luminescence
spectrophotometer
LS55
(Perkin-Elmer
Instruments, UK) with λ accuracy = ±1.0 nm and
λ reproducibility = ± 0.5 nm. A tunable 20 kW
pulse <10 s from a Xenon discharge lamp
was used as the excitation source for recording
photoluminescence emission spectra. A gated
photo multiplier tube was used as a detector.
Prior to the PL experiments signal-to-noise
ratio was adjusted to 500:1, using the Raman
band of water with excitation at 350 nm.
manifested as over lapping of the higher angle
reflexes at 2  =42.25 o and 51o.Our XRD
results thus clearly suggest that particle size of
CdSe can be effectively controlled with the
use of surfactant. The average particle sizes
experimentally determined from the observed
line widths using Scherer’s formula was found
to be 13.2nm and 1.9nm for CKF and CK-VIII
respectively. In fact the particle size was
found to systematically decrease with
increasing use of surfactant to selenium ratio
in the growth matrix. Fig 2 represents the
3 Results and Discussion:
3.1 Structural Analysis
The result of XRD characterization of two
Q-CdSe films sam1 and sam2 grown using
surfactant to selenium ratio of 0 and 2.42 have
been shown in fig 1.The as prepared CdSe
film sam1 is dominated by reflections arising
from (111), (220) and (311) planes at 2  =
25.8 o, 42.25 o and 51o.
(111)
250
200
Fig-2. Observed dependence of particle size with
surfactant to selenium ratio during synthesis.
100
sam2
50
(311)
150
(220)
intensity(a.u)
sam1
0
20
30
40
50
60

Fig-1- Grazing angle XRD spectra of two QCdSe films sam1 and sam2
All the observed peaks were carefully
analyzed using a peak fit algorithm assuming
Gaussian line shape. It was found that all the
observed peaks matched with the zinc –
blended structure of CdSe. The increase in the
concentration of the surfactant during the
synthesis of CdSe quantum dots had two fold
effect on the observed XRD spectra .The line
widths of XRD reflexes broadened and the
higher angle reflexes also weakened in
intensity .The cumulative effect of peak
broadening and intensity weakening was
Fig-3(a) Dark field image and (b) the SAD pattern of
Q-CdSe sample2 grown using surfactant to selenium
ratio of 2.42.
2
dependent of particle size with the variation in
the surfactant to selenium ratio. It is therefore
concluded that the surfactant employed in the
studies can effectively control the size of CdSe
quantum dots. The smallest synthesized size of
Q-CdSe was 1.9 nm.
Figure 3(a & b) show the dark field TEM
image and the corresponding diffraction
pattern of Q-CdSe sam2 sample. The selective
area diffraction pattern (fig.2b) revealed the
presence of zinc blended phase of CdSe. The
average particle size determined from the
TEM data was 1.6 nm which is in close
agreement with XRD result
Sam2
Sam1
3.2. Optical Characterization:
3.2.1. Optical absorption studies:
Fig-4 Optical absorption spectra for two Q-CdSe
samples1 and sample2 deposited on ITO/glass
substrate.
The optical absorption spectra for CdSe
samples sam1 (13nm), sam2 (2.1 nm) have
been shown in fig4. The sharp rise in the
absorption onsets correspond to the
fundamental absorption edges at 580nm and
426nm for sam1 and sam2 respectively. The
absorption edges in CdSe correspond to the
transition 1S3/2 -1Se, which obviously shifted
to blue side following reduction in the size of
Q-CdSe.The observed blue shifted absorbance
is in agreement with the quantum confinement
effect in nano particles.
In addition to fundamental absorption onset,
each of the CdSe sample exhibited a second
absorption onset on the blue side. The second
absorption onset in CdSe may be attributed to
the spin orbit interaction giving rise to the
transition labeled as 2S3/2-1Se.
Sam
1
Sam2
3.2.2. Photoluminescence Studies
The Photoluminescence emission spectra
of CdSe samples sam1 along with sam2 have
been shown in figure 5 respectively. The
excitation wavelengths were 640nm (sam1)
and
415nm
(sam2).The
observed
luminescence in sam1, peak at 770nm was
rather weak and red shifted vis-à-vis the
corresponding absorption edge 680nm.
However, the PL emission increased
dramatically for sam2. The observed PL line
shape was deconvoluted assuming Gaussian
shape and the results have also been shown in.
Fig-5- Photoluminescence emission spectra of two QCdSe films sam1and sam2 CdSe
Fig-5 PL spectra for two Q-CdSe samples1 and
sample2 deposited on ITO/glass substrate.
3
Fig 5. The observed luminescence spectrum
was found to consist of four band E1, E2, E3
and E4.The peak E1 positioned at 457 nm was
slightly
stoke
shifted
vis-a-vis
the
corresponding absorption edge by 31 nm. The
peak E1 can therefore be associated with
excitonic luminescence. The other side bands
E2, E3 and E4 may originate due to the
presence of surface defects or traps
References
1 Alivisatos A P, Science 271(1996) 933.
2 Nirmal M, Brus L, Accounts Chem. Res.32
(1999) 407.
3 Norris D J and Bawendi M G, Phys. Rev.B
53 (1996) 16338.
4 Goldman E R, Anderson G P, Tran P T,
Mattoussi H, Charles P T, Mauro J M,
Anal. Chem. 74(2002) 841.
5 Mattoussi H, Mauro J M, Goldman E R,
Anderson G P, Sander V C, Mikulec F V,
Bawendi M G, J. Am. Chem. Soc., 122,
(2000) 12142.
6 Matsumura N, Endo H, Saraie J,
Phys.Status Solidi B, 229(2002)
1039.
7 Hermandez C F, Shuh D J, Kippelen
B, Marder S R, Appl. Phys. Lett., 85,
(2002) 534.
8 Coe S, Woo W K, Bawendi M,
Bulovic V, Nature 420 (2002) 800.
9 Urbieta A, Fernandez P, Piqueras J,
J.Appl.Phys., 96,(2004) 2210.
10 Shan CX, Liu Z, Ng C M, Harl S K,
Appl. Phys. Lett., 86 (2005) 213106.
11 Schallenberg T, Borzenko T,
Schmidt G, Obert M, Bacher G,
Schumacher C, Karczewski G,
Molenkamp L W, Rodt S, Heitz R,
Bimberg D, Appl. Phys. Lett.,82
(2003) 4349.
12 Murray C B, Norris D J, Bawendi M
G, J. Am. Chem. Soc. 115 (1993)
8706.
13 Trojnec F, Cingolani R, Cannoletta
D, Mikes D, Nemec P, Uhlirova E,
Rohovec J, Maly P, J. Crystal
Growth, 209 (2000) 695.
Conclusions
CdSe quantum dots of tunable size can be
grown as thin film using an aqueous root in
the presence of the surfactant. The role of the
surfactant in controlling the particle size has
been demonstrated using XRD, TEM, optical
absorbance and luminescence spectroscopy.
The as synthesized Q-CdSe samples have been
shown to grow with a preferred orientation of
the zinc blended phase. The quantum
confinement effect in Q-CdSe samples have
been demonstrated by the observation of blue
shifted absorption edge and PL emission. It is
also shown that the quantum dots give intense
excitonic luminescence
Acknowledgments
I gratefully acknowledge Dr N.P Lalla
Dr. Mukul Gupta UGC-DAE CSR, Indore for
providing
facilities
for
TEM
&
XRD
experiments.
14 Chaure S, Chaure N B, Pandey R K,
Physica E, 28 (2005) 439
4