Strong quantum confinement effects in SnS nanocrystals produced by
ultrasound-assisted method
Yashar Azizian-Kalandaragh
1*
, Ali Khodayari
2
, Zaiping Zeng
3
, Christos S. Garoufalis
3,4
Sotirios Baskoutas
3
, Lionel Cervera Gontard
5
1
Department of Physics, University of Mohaghegh Ardabili, P.O. Box 179, Ardabil, Iran
2
Department of Chemistry, University of Mohaghegh Ardabili, P.O. Box 179, Ardabil, Iran
3
Materials Science Department, University of Patras, 26504 Patras, Greece
4
Department of Environment Technology and Ecology, Technological Institute of Ionian Islands, 2
Kalvou Sq, 29100, Zakynthos, Greece
5
Instituto de Ciencia de Materiales de Sevilla (CSIC), 41092, Sevilla, Spain
Abstract
Nanocrystalline SnS powder has been prepared using tin chloride (SnCl
2
) as a tin ion source and
sodium sulfide (Na
2
S) as a sulfur ion source with the help of ultrasound irradiation at room
temperature. The as-synthesized SnS nanoparticles were quantitatively analyzed and
characterized in terms of their morphological, structural and optical properties. The detailed
structural and optical properties confirmed the orthorhombic SnS structure and a strongly blue
shifted direct band gap (1.74 eV), for synthesized nanoparticles. The measured band gap energy
of SnS nanoparticles is in a fairly good agreement with the results of theoretical calculations of
exciton energy based on the potential morphing method (PMM) in the Hartree Fock
approximation.
Keywords: Quantum Confinement effect; SnS; Semiconductor nanoparticles; X-Ray Diffraction;
Potential Morphing Method; Ultrasound irradiation.
*Corresponding author: Y. Azizian-Kalandaragh, E mail: yashar.a.k@gmail.com
Fax: (+98) 451 551 4701, Tel: (+98) 9148805131
1. Introduction
Quantum confinement effect in semiconductor nanomaterials has been of special interest during
the last decades. Quantum confined semiconductor nanocrystals, which exhibit properties
different from bulk materials, are a new class of materials that hold considerable attention for
numerous applications in the field of optoelectronics. Modification of molecular design and
morphology of such nanostructures provides a powerful approach to control their electronic and
optical properties. Reduction in the size of particles to nanometer ranges, changes the degree of
confinement of charge carriers, which affect the electronic and optical properties of
semiconductor materials (Alivisatos, 1996; Henglein, 1989; Liu et al., 2006; Ögüt et al., 1997;
Rama Krishna & Friesner, 1991; Trindade et al., 2001). These unique characteristics of
semiconductor nanostructured materials originate from the quantum confinement effects. From
theoretical point of view, as the radius of particle approaches the exciton Bohr radius of a given
material, quantization of the energy bands become apparent and a blue shift in the exciton
transition energy can be observed (Baskoutas & Terzis, 2006; Wang & Herron, 1990). Among
the extensively studied IV-VI semiconductor materials, tin sulfide is very important narrow gap
material because of its low toxicity and wide applications as an absorber layer in solar cells, near
infrared materials, holographic recording media and solar control devices (Liu et al., 2010;
Rudel, 2003; Winship, 1998). It is important and necessary to study the band gap changes in
semiconductor nanostructures in order to gain a better understanding for their relevant properties.
Also band gap engineering of the semiconductor nanostructures by the control of nanostructure
sizes is important. Experimental studies showed that semiconductor SnS exhibit p and n type
conduction and has both a direct optical gap located at 1.3 eV and indirect optical band gap
located at 1.1 eV (Bashkirov et al., 2011; Ning et al., 2010; Yue et al., 2009). In order to obtain
nanostructured SnS, the following methods are used: spray pyrolysis of the water solution
(Reddy et al., 1999; Thangaraju & Kaliannan, 2000), vacuum evaporation (Johson et al., 1999),
chemical vapor deposition (Ortiz et al., 1996; Price et al., 2000), chemical bath deposition
(Engelken et al., 1987; Tanusevski, 2003), electro deposition and electrochemical deposition
(Chazali et al., 1998; Takeuchi et al., 2003), chemical synthesis (Gou et al., 2005), microwave
assisted synthesis (Chen et al., 2004), mild solution route (Li et al., 2002), modified solution
dispersion method (Zhao et al., 2004), two gas process (Reddy & Reddy, 2002), solvothermal
process (Panda et al., 2006; Paul & Agarwal, 2007; Paul et al., 2008; Qian et al., 1999),
successive ionic layer adsorption and reaction (SILAR) method (Ghosh et al., 2008),
hydrothermal synthesis (Biswas et al., 2007), and molecular beam epitaxy (Nozaki et al., 2005).
Generally, most of the above mentioned methods require high temperature as well as the use of
highly sensitive toxic solvents. Our attempt is to obtain high quality materials under normal
laboratory conditions, using safer precursors by applying ultrasonic waves. Previously ultrasonic
waves have been used for the preparation of nanomaterials (Azizian-Kalandaragh et al., 2009;
Azizian-Kalandaragh & Khodayari, 2010a,b; Bhattacharyya & Gedanken, 2008; Goharshadi et
al., 2009; Suslick, 1990; Suslick et al., 1990; Wang et al., 2002; Zhu et al., 2006).
Ultrasonic waves have been shown to cause physical and chemical effects such as fragmentation
to small particles and acceleration of reactions, which may be used for the preparation of new
materials with desirable properties.
During sonication, ultrasonic longitudinal waves are radiated through the solution causing
alternating high and low pressure regions in the liquid medium. Millions of microscopic bubbles
form and grow in the low-pressure stage, and subsequently collapse in the high-pressure stage.
Hot spots that are localized regions of extremely high temperatures as high as 5000 K, and
pressures of up to ~ 1800 atm can occur from the collapsing bubbles, and cooling rates can often
exceed ~10
10
K s
-1
. The energy released from this process, known as cavitation, would lead to
enhanced chemical reactivity and accelerated reaction rates (Suslick, 1998).
In this paper we report the preparation of SnS nanocrystals with the help of ultrasonic
irradiation. We have chosen this method because of its many advantages, such as easier
composition control, low toxicity, better homogeneity, low processing temperature, easier
fabrication of large numbers of nanoparticles, lower cost, and possibility of using high purity
starting materials. In this paper we also report the morphological, optical and structural
properties of SnS nanocrystals. As our results indicate, the absorption edge is shifted towards the
lower wavelength side (i.e. blue shift) and direct energy gap of SnS nanocrystals is estimated to
1.74eV.The results are compared to theoretical calculations based on the potential morphing
method (Rieth et al., 2002) (PMM) in the Hartree Fock approximation (Baskoutas, 2005a,b;
Baskoutas et al., 2006a,b; Baskoutas & Terzis, 2006; Poulopoulos et al., 2011). This method, based on
the adiabatic theorem of quantum mechanics which states that if the Hamiltonian of the system
varies slowly with time then the n
th
eigenstate of the initial Hamiltonian will be carried into the
n
th
eigenstate of the final Hamiltonian, solves the Schrödinger equation for any arbitrary
interaction potential. In the present case, the PMM based results exhibit a fairly good agreement
with the experimental data. This combined experimental and theoretical work provides a better
insight on the quantum confinement effects in SnS nanoscaled systems.
2. Experimental details
2.1 Materials and Instruments
Sodium sulfide hydrate was obtained from Sigma-Aldrich, Triethanolamine (TEA) was obtained
from Rankem ,Tin (II) chloride dihydrate, polyvinyl alcohol (PVA) and absolute ethanol were
obtained from Merck. All the reagents used as-received without purification.
X-ray diffraction (XRD) analysis of drop-coated films on an ordinary glass substrate from the
SnS nanocrystals was carried out on a Philips X’ Pert Pro with CuKα radiation. The optical
properties of sample were monitored on a Carry 5 UV-Visible spectrophotometer (model
Varian). Scanning electron microscopy (SEM) measurements were performed on a LEO 1430VP
instrument operated at an accelerating voltage of 15 kV. The elemental analyses of the products
were obtained by energy disperse X-ray analysis (EDAX) on the same LEO 1430VP instrument
with accelerating voltage of 15 kV. Samples for SEM and EDAX studies were prepared by
placing drops of the SnS nanostructured solutions on gold and palladium-coated SEM stage.
Transmission electron microscopy (TEM) images of the sample were taken on a Philips CN10,
TEM performing at an accelerating voltage of 100 kV. Samples for the electron microscope and
EDAX analyses were prepared by ultrasonically dispersing the prepared SnS nanoparticles into
absolute ethanol and distilled water, placing a drop of this suspension onto a special stage coated
with gold and palladium alloys.
2.2 Preparation of SnS nanocrystals
In a typical procedure, for preparation of 0.2M solution of tin chloride, 0.90g of tin chloride
powder is dissolved in 20 ml TEA, then 0.31g of sodium sulfide was dissolved in 20ml distilled
water (0.2M). These two solutions were mixed and were put in a 100 ml round bottom flask. The
pH value of the mixture was 12. The mixture solutions were kept under high intensity ultrasonic
transductor at room temperature for 2 hours. During irradiation 5ml of aqueous solutions of PVA
(1%) were added to the mixture. At the end of the reaction, a great amount of black precipitates
were obtained. After cooled to room temperature, the precipitates were centrifuged, washed by
distilled water and absolute ethanol in sequence and dried in vacuum. Plenty of SnS
nanoparticles have been prepared using this method and the yield of this preparation is high in
comparison with most of chemical preparation methods. The final products were collected for
characterizations. The products were characterized by XRD, SEM, TEM, EDAX and UV-Visible
spectroscopy.
The formation mechanism of SnS nanocrystals with the reaction equation can be expressed as
follows:
SnCl
2
.H
2
O+Na
2
S. xH
2
O SnS + 2NaCl + xH
2
O
The role of PVA is to stabilize the nanostructures preventing them from coagulation.
3. Theory
In the effective mass approximation the Hamiltonian for the electron hole system can be
written as (Baskoutas, 2005a,b; Baskoutas et al., 2006a,b; Baskoutas & Terzis, 2006;
Poulopoulos et al., 2011)
eh
h
h
e
e
h
h
e
e
r
e
rVrV
mm
H
1
22
2
00
2
*
2
2
*
2
(1)
where
*
e
m
*
h
m
is the effective electron (hole) band mass,
is the effective dielectric constant,
r
eh
is the electron – hole distance in three dimensions and
he
V
0
is the finite depth well
confinement potential of electron (hole). As in our previous work (Poulopoulos et al., 2011;
Baskoutas et al., 2006) we will also use here a reliable expression for the dielectric constant
developed by Hanken (1956) and used by several authors for example Nanda et al., (2004);
Pellegrini et al., (2005) and which has the following form