Size dependent electron momentum density distribution in ZnS

Article abstract of DOI:10.1016/j.physb.2011.08.073

In this paper, we present size dependent electron momentum density distribution in ZnS. ZnS nanoparticles of size 3.8 nm and 2.4 nm are synthesized using the chemical route and characterized by X-ray diffraction (XRD). The Compton profile measurements are performed on both the nano-sized as well as bulk ZnS samples employing 59.54 key gamma-rays from ²⁴¹Am source. The results reveal that the valence electron density in momentum space becomes narrower with reduction of particle size. To evaluate the charge transfer on compound formation, the ionic model based calculations for a number of configurations of Zn^+xS ^x (0.0 ≤x ≤ 2) are also performed utilizing free atom Compton profiles. These results suggest different amounts of charge transfer in these materials varying from 1.2 to 2.0 electron from Zn to S atom.

Full text of DOI:10.1016/j.physb.2011.08.073

Physica B 406 (2011) 4307–4311
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Physica B
journal homepage: www.elsevier.com/locate/physb
Size dependent electron momentum density distribution in ZnS
M.C. Mishra a,b, R. Kumar b, G. Sharma c,n, Y.K. Vijay b, B.K. Sharma b
a Department of Physics, Government College Jhunjhunu, Jhunjhunu 303001, India
b Department of Physics, University of Rajasthan, Jaipur 302004, India
c Department of Physics, Banasthali University, Banasthali 304022, India
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper, we present size dependent electron momentum density distribution in ZnS. ZnS nano-
particles of size 3.8 nm and 2.4 nm are synthesized using the chemical route and characterized by X-ray
diffraction (XRD). The Compton profile measurements are performed on both the nano-sized as well as
bulk ZnS samples employing 59.54 keV gamma-rays from 241Am source. The results reveal that the
valence electron density in momentum space becomes narrower with reduction of particle size. To
evaluate the charge transfer on compound formation, the ionic model based calculations for a number
of configurations of ZnþxSꢀx (0.0rxr2) are also performed utilizing free atom Compton profiles.
These results suggest different amounts of charge transfer in these materials varying from 1.2 to
2.0 electron from Zn to S atom.
Received 23 June 2011
Received in revised form
16 August 2011
Accepted 24 August 2011
Available online 26 August 2011
Keywords:
X-ray scattering
Electron momentum density
Ionic model
& 2011 Elsevier B.V. All rights reserved.
Charge transfer
1. Introduction
ZnS nanostructures by employing first-principles calculations based
on generalized gradient approximation (GGA). They observed that
In the last two decades, materials with reduced dimensions
have attracted significant attention and growing interest due to
their unique electronic, optical and magnetic properties, which
are different from their bulk counterparts [1,2]. Among these,
II–VI group semiconductor materials are most attractive because
of their technological applications [3–8]. ZnS is an important
member of this family and hence has been extensively studied. It
is widely used as an important phosphor for photoluminescence
(PL) [9], electroluminescence (EL) [10] devices and is also a very
attractive material in optical applications especially in nanocrys-
talline form [11,12]. It has a wide optical band gap 3.6 eV [13] and
has two different crystal structures (zinc blende and wurtzite),
both of which exhibit direct band gap [14,15].
ZnS nanoclusters have stronger quantum confinement effects than
ZnS nanotubes. Zhang et al. [19] also reported the size dependent
structural and electronic properties of ZnS using first-principles
calculations. Navaneethan et al. [20] reported the temperature
dependence of morphology, structural and optical properties of
ZnS nanostructures. They observed that the absorption edge of the
nanoparticles (295 nm) was shifted towards shorter wavelength
compared to bulk ZnS due to the quantum confinement effect.
Pressure-induced structural transitions of the ZnS nano-particles
with different sizes were investigated by Pan et al. [21] by energy
dispersive X-ray diffraction (EDXD) from ambient pressure to
35.0 GPa. They found that ZnS nano-particles initially in the zinc
blende phase transformed to cubic NaCl structure in the presence of
pressure and the transition was reversible when the pressure was
released. Thus, it is possible to tune the electronic, structural and
optical properties of ZnS nanoparticles by varying their size, making
them promising materials for numerous applications.
It is well known that electron momentum density (EMD) is
also one of the important ground state property. In the last few
decades, measurement of Compton profile has become a powerful
tool for the determination of EMDs. It arises due to the fact that
within impulse approximation, the measured double differential
cross section for inelastically scattered photons is directly related
to Compton profile, J(pz) [22,23],
Regarding nanocrystalline ZnS, a number of investigations have
been performed to explore the electronic, structural and optical
properties [16–21]. Saravanan et al. [16] synthesized the ZnS
nanoparticles using the sol–gel method and characterized them by
XRD and SEM probes. They have also analyzed the bonding feature
of ZnS nanoparticles using the maximum entropy method and found
3
˚
0.433 e/A electron density at the middle of the bond between Zn
and S atom. The variation of admittance with frequency on the CdS
and ZnS quantum dots has been reported by Nath et al. [17], and
these results indicated their potential application in electronics as
nano-tuned and nano-highpass filters. Zhang et al. [18] have studied
the structural character, energetics and electronic properties of
d2
s
¼ Cðo1
,
o2,pz,
fÞJðpzÞ
ð1Þ
dOdo2
n Corresponding author. Tel.: þ91 1438228648; fax: þ91 1438228649.
E-mail address: gsphysics@gmail.com (G. Sharma).
where o1 and o2 are the energies of photons before and after
scattering, respectively. The quantity C(o1 2,pz, ) depends on
,
o
f
0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2011.08.073

4308
M.C. Mishra et al. / Physica B 406 (2011) 4307–4311
the experimental setup while pz is the electron momentum along
the scattering vector (z-axis) and d is the elemental solid angle.
The Compton profile, J(pz), is related to EMD through the relation
2.2. Compton profile measurements
O
The 241Am gamma-ray spectrometer described by Sharma
et al. [26] has been employed for the Compton profile measure-
ments. The incident gamma rays of 59.54 keV from 5 Ci annular
241Am source were scattered at an angle 166173.01 by the bulk
and nanocrystalline samples of ZnS. The details regarding sam-
ples, measurement time and number of counts collected are
given in Table 1. The scattering chamber was evacuated to about
10ꢀ2 Torr with a rotary oil pump to reduce the contribution of air
scattering and the samples were held vertical by affixing on the
back of the lead covered brass slab with a hole of radius 9.05 mm.
The scattered radiation was analyzed using an HPGe detector
(Canberra model, GL0110S), which was cooled with liquid nitro-
gen providing overall momentum resolution 0.6 a.u. The spectra
were recorded with a multichannel analyzer (MCA) with 4096
channels. The channel width was ꢂ20 eV, corresponding to
0.03 a.u. of momentum. To correct for the background, measure-
ment with empty sample holder was performed for 24 h and the
measured background was subtracted from the raw data channel
by channel after scaling it to the actual counting time. Thereafter,
the corrected background spectra were processed for several
corrections like instrumental resolution, sample absorption and
scattering cross section using computer code of the Warwick
group [27,28] to obtain the Compton profiles. After converting the
profiles to the momentum scale, a Monte Carlo simulation was
performed to account for the multiple scattering corrections in
the samples. To account for multiple scattering corrections
history of approximately 107 photons were considered. The effect
of multiple scattering was found to be 4.70%, 2.9% and 2.1% in the
range ꢀ10 a.u. to þ10 a.u. for bulk, samples A and B of nano-ZnS,
respectively. Finally, the corrected profiles were normalized to
20.24 electrons in the range 0 a.u. to þ7 a.u., being the area of
free atom Compton profile [25] in the given range. In the present
experimental setup, the K electrons of Zn contribute up to
2.2 a.u. only.
Z
þ1 Z
JðpzÞ ¼
þ1 rðpx,py,pzÞdpx dpy
ð2Þ
ꢀ1
ꢀ1
where
r(px,py,pz) is the EMD and the pz axis is parallel to the
photon scattering vector [22,23]. Since, valence electron density is
delocalized in r-space, the corresponding quantity in momentum
space i.e. valence electron momentum density is localized. This
produces narrow contribution in the Compton profile, J(pz). For
this reason, the Compton scattering technique is considered as a
promising tool to investigate the behavior of valence electrons.
Although, bulk ZnS has been studied by this technique earlier
[24], to our knowledge there is no reported measurement on
nano-sized ZnS. In this paper, we report the first-ever size
dependent EMD distribution in ZnS. There are no rigorous
theoretical prescriptions for computation of Compton profiles
for nano-ZnS. Hence, the present experimental data is compared
with the corresponding data of bulk sample and also with the free
atom theoretical profile to see the effect of particle size on the
EMD distribution. Accordingly, a new measurement on bulk ZnS
has also been done under identical experimental conditions. In
order to examine the difference in the amount of charge transfer
in nano-samples relative to bulk ZnS, the ionic model based
calculations have also been performed utilizing the free atom
Compton profiles of Zn and S [25]. The paper is organized as
follows: Section 2 gives details of the experiment. In Section 3,
the results and discussions are given. The conclusions are drawn
in Section 4. In this paper, unless stated, all quantities are in
atomic units (a.u.) where e¼_¼m¼1 and c¼137.036, giving unit
momentum¼1.9929 ꢁ 10ꢀ24 kg m sꢀ1
and unit length¼5.2918 ꢁ 10ꢀ11 m.
,
unit energy¼27.212 eV
2. Experiment
2.1. Synthesis of nanoparticles
3. Results and discussion
The aqueous solution of 0.01 M of zinc chloride (ZnCl2) was
prepared in de-ionized water using magnetic stirrer for 3 h. Mercap-
toethanol was added to zinc chloride solution used as a capping
agent at different molar concentrations ranging from 0.005 M to
0.025 M. Hydrogen sulfide gas was produced by following chemical
reaction of iron sulfide and sulfuric acid (H2SO4) acid:
The phase purity and crystal structure of these samples were
analyzed using a Philips X’pert Pro X-Ray diffractometer having
˚
Cu Ka (l¼1.5460 A) source of X-rays. The three main diffraction
peaks indexed at (1 1 1), (2 2 0) and (3 1 1) were observed near 2
y
angles of 28.501, 47.661 and 56.541, respectively, are shown in
Fig. 1. Comparison of recorded XRD pattern with standard JCPDS
80.0020 data file confirms the zinc blende (cubic) structure of
nanocrystalline samples of ZnS (nano-A and nano-B). The average
particle size of ZnS nanoparticles was calculated from the broad-
ening of diffraction peaks using the Debye–Scherrer formula [29].
The particle size obtained from XRD pattern is 3.8 nm and 2.4 nm
for samples A and B, respectively, and it is confirmed that particle
size decreased with increasing capping agent concentration.
The size and morphologies of the synthesized ZnS nanoparti-
cles were examined using scanning electron microscopy (SEM)
H2SO4þFeS-H2SmþFeSO4
By the following reaction,
ZnCl2þH2Sm-ZnSþHCl
this mixture with H2S gas at room temperature turned into milky
color. The precipitate was filtered and washed 3–4 times with
distilled water. Finally the precipitate was dried in vacuum for
24 h to obtain fine ZnS nanomaterial.
Table 1
Some details concerning the samples and measurements.
Sample
ZnS
Sample
thickness
(mm)
Density
(gm/
cm3)a
Counts at Compton
peak ( ꢁ 104)
Multiple scattering
Exposure
time (h)
Normalization
(0 to þ7 a.u.) eꢀ
(ꢀ10 to þ10 a.u.) %
Bulk
0.32
0.32
0.32
1.3606
0.6795
0.4995
2.75
2.60
2.10
4.79
2.9
25
26
25
20.24
20.24
20.24
Nano-A
Nano-B
2.1
a
Effective density of the sample.

M.C. Mishra et al. / Physica B 406 (2011) 4307–4311
4309
A representative TEM image of ZnS (nano-A) is displayed in
Fig. 3. The mean size of these nanoparticles from TEM observation
is about 5–7 nm. In the TEM image, lattice fringes can be clearly
observed, which indicate that the particles are crystalline. How-
ever, the particles were not aggregated into a big structure,
although particles were in contact with each other. Most of the
particles were similar in size and had irregular rounded shapes.
In Fig. 4, the present experimental data on bulk ZnS (obtained
from 59.54 keV gamma-rays) is compared with the earlier pub-
lished data [24]. It is clearly visible that the agreement between
two data is very good and beyond 2.0 a.u. The inset shows the
comparison. The difference curve shown in the inset points out
the subtle differences in low momentum region, which may
perhaps be due to the differences in data correction procedure
in the experiments, specially deconvolution and multiple scatter-
ing corrections.
Nano B
Nano A
20
30
40
50
60
70
2 Theta (degree)
Fig. 1. XRD patterns of ZnS nanoparticles of different sizes.
Fig. 3. Transmission electron microscopy (TEM) image of ZnS nanocrystalline
sample (nano-A).
12
0.6
Ja(pz) - Jb(pz)
0.4
I
Error
10
8
0.2
0.0
Fig. 2. SEM image of ZnS nanocrystalline sample (nano-A).
-0.2
-0.4
and transmission electron microscopy (TEM). Fig. 2 represents
image of ZnS nanoparticles obtained from high resolution SEM at
highest magnification for nano-A sample. The scanning electron
microscopy allows imaging of individual crystallites and the
development of a statistical description of the size and shape of
the particles in a sample. It highlights the presence of spherical,
monodisperse and uniformly distributed nanoparticles. The par-
ticles as seen in the SEM pictures are nanoaggregates of many
nanoparticles and individual crystallite size is expected to be
much less. The actual size of nanoparticles cannot be determined
from SEM. The figure shows that the particle size distribution is
less than 100 nm in diameter for nano-A sample. The discrepancy
with size of ZnS nanoparticles may be because of the fact that
SEM shows the lateral dimension of the particles whereas XRD
gives the regularity in the atomic arrangement.
6
0
1
2
3
4
5
6
7
pz (a.u.)
4
Ja (pz)
Jb (pz)
2
0
0
1
2
3
4
5
6
7
pz (a.u.)
Fig. 4. Comparison of the earlier [Ja(pz)] and present [Jb(pz)] Compton profiles of
bulk ZnS. The difference between the two measurements (
D
J¼[Ja(pz)]ꢀ[Jb(pz)]) is
The TEM is often used to deduce a mean particle size and
size distributions from a limited number of particle images.
also presented in the inset. Experimental errors (7
s) are also shown at few
points.

4310
M.C. Mishra et al. / Physica B 406 (2011) 4307–4311
In Fig. 5, the experimental valence Compton profiles of ZnS
The theoretical Compton profiles of ZnS with varying ionic
arrangements were calculated from the free atom profiles of Zn
and S as taken from Biggs et al. [25]. The valence profiles for
various ZnþxSꢀx (0.0rxr2 in step of 0.5) configurations were
calculated by transferring x electrons from 4s shell of zinc to the
3p shell of sulfur and the valence profiles for ZnþxSꢀx were added
to core contributions to get the total profiles. The difference
profiles (convoluted theory—experiment) for various ZnþxSꢀx
(where x varies from 0 to 2 in a step of 0.5) configurations have
been plotted in Figs. 6–8 for bulk, nano-A and nano-B samples
respectively. From Figs. 6 to 8, it is evident that the effect of
varying charge on Zn and S is visible in the region 0–2.0 a.u. and
all configurations show identical nature for pzZ2.0 a.u. On the
nanomaterials and our new measurement on bulk sample are
presented. The free atom profile given here is convoluted with the
instrumental resolution function (Gaussian of FWHM 0.6 a.u.).
The differences between profiles of nano-A and nano-B with
respect to bulk ZnS, which primarily represents the differences
in valence electron distribution are shown in the inset. From this
figure, it can be seen that the Compton profile for valence
electrons in free atom is the sharpest. The line width increases
with the size of the nanoparticles. Since the Compton profiles of
both the nano-sized samples are sharper than the bulk ZnS, it
indicates the difference in the distribution of valence electron
densities in these samples. The observed narrowness of Compton
profiles can be interpreted due to the differences in density of
valence electrons due to solid state effects, which are smaller in
nanoparticles [30–32]. This also explains that when size of nano-
crystals increases, the broadening of Compton profile also
increases and approaches towards the broad profile due to bulk.
basis of w2 test and also Figs. 6–8, it is obvious that configurations
corresponding to x¼2.0, 1.8 and 1.2 electrons give the best
agreement with the experiment for bulk, nano-A and nano-B
samples, respectively. Hence the ionic model suggests the transfer
of 2.0, 1.8 and 1.2 electrons from the valence 4s state of Zn to the
0.6
8
Zn+1.4S-1.4
Zn+1.6 -1.6
Zn+1.8S-1.8
1.0
0.8
Nano A
Nano A - Bulk
Nano B - Bulk
s
0.4
0.2
0.6
0.4
Zn+2.0S-2.0
6
4
2
0
0.2
Error
0.0
-0.2
-0.4
0.0
0
1
2
3
4
5
6
7
pz (a.u.)
-0.2
-0.4
-0.6
Bulk
Nano A
Nano B
Free Atom
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
pz (a.u.)
pz (a.u.)
Fig. 7. Difference between convoluted ionic Compton profiles and the experi-
mental Compton profile of nano-A. Experimental errors (7 ) are also shown at a
Fig. 5. Experimental valence Compton profiles of bulk and nano-phases of ZnS.
s
The inset shows the differences between nano-A and nano-B to bulk ZnS.
few points. All ionic profiles are convoluted with the Gaussian of 0.6 a.u. FWHM.
1.4
1.0
Zn+1.0S-1.0
Zn+1.2S-1.2
Zn+1.4S-1.4
Zn+1.6S-1.6
Error
Zn+0.5S-0.5
Zn+1.0 -1.0
Zn+1.5S-1.5
Zn+2.0S-2.0
Error
Bulk
Nano B
0.8
0.6
1.2
s
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
-0.4
0.4
I
0.2
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
pz (a.u.)
pz (a.u.)
Fig. 6. Difference between convoluted ionic Compton profiles and the experi-
mental Compton profile of bulk ZnS. Experimental errors (7 ) are also shown at a
few points. All ionic profiles are convoluted with the Gaussian of 0.6 a.u. FWHM.
Fig. 8. Difference between convoluted ionic Compton profiles and the experi-
mental Compton profile of nano-B. Experimental errors (7 ) are also shown at a
few points. All ionic profiles are convoluted with the Gaussian of 0.6 a.u. FWHM.
s
s

M.C. Mishra et al. / Physica B 406 (2011) 4307–4311
4311
3p state of S in bulk, nano-A and nano-B samples, respectively.
Thus, the decreases in the charge transfer values on reduction of
particle size indicate weak ionic bonding in smaller size particle
of ZnS. There are no theoretical calculations reported on nano-
sized ZnS samples, which would have enabled direct comparison
with our data. It is hoped that the present work would stimulate
further work along these lines from which accurate theoretical
Compton profile of ZnS nanomaterials can be computed.
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is significant change in valence electron distribution as we move
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suggests different amounts of charge transfer, which increases with
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This work is financially supported by the University Grant
Commission (UGC) through Emeritus Fellowship and SR/33-37/
2007 to BKS and SR/39-982/2010 to GS. The financial support
provided by DST, New Delhi is also acknowledged.
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