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Investigation on structural aspects of ZnO nano-crystal using
radio-active ion beam and PAC
Bichitra Nandi Ganguly
a,
⇑
, Sreetama Dutta
a
, Soma Roy
a
, Jens Röder
b,c
, Karl Johnston
b,d
,
Manfred Martin
c
, ISOLDE-Collaboration
b
a
Saha Institute of Nuclear Physics, Kolkata 700064, India
b
Physics Department, ISOLDE/CERN, Geneva, Switzerland
c
Physical Chemistry, RWTH-Aachen, Aachen, Germany
d
Experimental Physics, University of the Saarland, Saarbrücken, Germany
article info
Article history:
Received 7 January 2015
Received in revised form 6 August 2015
Accepted 31 August 2015
Keywords:
ZnO nano-crystal
Perturbed angular correlation (PAC)
Radio-active ion beam
Hyperfine interaction
abstract
Nano-crystalline ZnO has been studied with perturbed angular correlation using
111m
Cd, implanted at
ISOLDE/CERN and X-ray diffraction using Rietveld analysis. The data show a gradual increase in the crys-
tal size and stress for a sample annealed at 600 °C, and reaching nearly properties of standard ZnO with
tempering at 1000 °C. The perturbed angular correlation data show a broad frequency distribution at low
annealing temperatures and small particle sizes, whereas at high annealing temperature and larger crys-
tal sizes, results similar to bulk ZnO have been obtained. The ZnO nano-crystalline samples were initially
prepared through a wet chemical route, have been examined by Fourier Transform Infrared Spectroscopy
(FT-IR) and chemical purity has been confirmed with Energy Dispersive X-ray (EDAX) analysis as well as
Transmission Electron Microscopy (TEM).
Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction
ZnO is one of the most versatile wide band gap (3.37 eV) semi-
conductor materials known for its multifarious applications in
technology [1,2]. The aqueous precursor-derived ZnO material is
a promising alternative to organic semiconductors and amorphous
silicon materials in applications such as transparent thin film tran-
sistors at low temperatures [3]. Also, the properties of ZnO transis-
tors depend on its chemical preparation (such as extraction of the
precipitate from acidic or basic solutions), drying and sintering
processes and also on the traces of impurities imparted to the sys-
tem [4–6]. Chemical structure analysis suggests that the hydrated
zinc cation, the pH of the medium and ligand type play a critical
role in aqueous precursor based ZnO-nano-crystalline material.
ZnO is a bio-friendly oxide semiconductor and an inexpensive
luminescent material. It is expected to have a wide range of appli-
cations in room temperature ultraviolet (UV) lasing [7], biosensors
[8], bio-imaging [9], drug delivery [10], piezoelectric transducers
[11] and other usages as doped-ceramic compounds [12]. For all
such purposes, growth of ZnO nano-size material through a chem-
ical route is a necessity. Most existing preparation techniques rely
strongly on ZnO grain manipulation processes while drying and
sintering the hydrated ZnO precipitate. Also, the defect structure
in the evolution process of nano material is vital as there lies strong
evidence that defects have a role to play in ferromagnetic order in
such materials and the ferromagnetism coupling may be mediated
by carriers [13]. Radiation induced doping [14] and direct radiation
effect on the crystal structure can induce static charge localization
effects [15], these subtle details in structural analysis play a vital
role.
Thus to begin with, a systematic study has been carried out to
examine various physical aspects of chemically grown nano-
crystalline ZnO. Firstly, the chemical speciation and purity of the
as-grown material along with grain growth and crystallinity are
checked using several analytical methods like FT-IR spectroscopy,
X-ray diffraction method with Rietveld analysis, etc., size and mor-
phology effects through TEM and EDAX have been examined for
purity check.
In order to gain detailed information of the growth of nano-
crystals of ZnO, the perturbed angular correlation method (PAC)
has been found suitable [5,6,16]. This method is employed in order
to gain information on doping and its microenvironment in atomic
scale [20–22]. But in such cases for the ZnO host,
111
In has been
used as the probe [5,16–19] which undergoes electron capture
(EC) decay, and after-effects of the
111
In(EC) ?
111
Cd process
http://dx.doi.org/10.1016/j.nimb.2015.08.098
0168-583X/Ó 2015 Elsevier B.V. All rights reserved.
⇑
Corresponding author. Fax: +91 33 23374637.
E-mail address: bichitra.ganguly@saha.ac.in (B.N. Ganguly).
Nuclear Instruments and Methods in Physics Research B 362 (2015) 103–109
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B
journal homepage: www.elsevier.com/locate/nimb
renders the probe into a highly unstable charged state as found in
other substrates [23,24]. Also, it is envisaged that some effects
could be associated with the valance state of radioactive parent
probe nuclei, for example: when the parent nucleus
111
In
3+
replaces Zn
2+
, it may probably induce more defects than
111m
Cd
2+
.
The situation is clearly different than having
111m
Cd
2+
as a tracer
ion located at isovalent cationic site at the semiconductor oxide
material [18,22]. The benefit is the use of a meta-state for PAC,
as during the decay, no charge change appears, reducing the effects
usually observed by transmutation.
It is thus important to mention that a suitable radioactive
dopant ion serving as PAC probe (for example:
111m
Cd
2+
, by expo-
sure of the ZnO samples to radioactive ion beam at ISOLDE, CERN)
becomes a necessity. Further, facilitating the diffusion of the
dopant ion into ZnO grains is followed by gradual and controlled
annealing steps under vacuum. An improvement of grain size
consistency by controlling the formation of ultrafine grains and
prevention of abnormal grain growth could yield a better result
for fabricating nano-material for a device related purpose.
Although we find ZnO nano-material has been studied in various
ways, but a systematic monitoring of the structural distribution
in the presence of a dopant like Cd
2+
through PAC [22] at trace level
in the host medium along with sintering processes are sparse. In
this work, we have purportedly used
111m
Cd
2+
such that its direct
route to stable state will not alter any other characteristics of the
medium except the electric field gradient (EFG) tensor, which is
the observable parameter from PAC measurement. Such a parame-
ter is strongly dependent on the distribution of the electronic
charge density around the site where the radioactive probe atom
has been hosted in the lattice.
In this article, we develop a meaningful comparison of the
changes that have been observed for pure ZnO nano-crystal, start-
ing from the defect structure caused due to radioactive ion beam
irradiation by using the technique: PAC measurements, after
subjecting the material through a few sequential and controlled
vacuum annealing steps. The same is compared with X-ray diffrac-
tions studies and Rietveld analysis, for the structural aspects of
ZnO nano-crystalline material.
2. Experimental
2.1. Chemical synthesis of pure zinc oxide
ZnO nano particles were prepared by the wet-chemical route
from zinc acetate, Zn(CH
3
COO)
2
2H
2
O (extra pure AR grade mate-
rial, from SRL, India). The desired weight of zinc acetate was dis-
solved in triple distilled water (TDW) and a (1:1/vol) ammonia
solution (Merck India) was added to this solution drop by drop,
maintaining a pH 7.5. Initially zinc is precipitated as zinc
hydroxide. After centrifugation, the precipitate was collected and
re-dispersed into TDW to remove excess ions. Finally, the precipi-
tate was recollected and dried at 100 °C for 12 h in vacuum oven
and sintered at different temperatures (200, 600 and 1000 °C for
30 min) under vacuum to evolve ZnO nano-crystallites. These
ZnO nano-grains were characterized by Fourier Transmission
Infrared (FT-IR) Spectroscopy [25] (as pellets in KBr, without mois-
ture) using a Perkin Elmer FTIR system, Spectrum 100) in the range
of 400–4500 cm
1
, with a resolution of 0.4 cm
1
. The results are
shown in Fig. 1 and Table 1.
2.2. Electron microscopic studies
For the elemental purity check of the ZnO samples, Energy
Dispersive X-ray analysis (EDAX/SEM) and scanning microscopic
analysis of the grain surface was performed using Quanta
FEG-200, FEi Company USA. As prepared ZnO sample sintered at
200 °C, has been dispersed in pure water then TEM examination
was performed after placing a drop of the same on the carbon
coated grid and dried under vacuum. Measurements were done
with (Tecnai S-twin, FEI) using an accelerating voltage of 200 kV,
having a resolution of 1 Å. The results of such analysis are shown
in Fig. 2.
2.3. Characterization with X-ray powder diffraction measurements
The phase structures of the samples were identified by X-ray
diffraction, Rigaku TTRAX3 diffractometer with CuK
a
1,2
radiation
(k = 1.541 Å and 1.5444 Å) has been used. The data have been col-
lected in the range (2h) 10–100° with a step size of 0.02°. Si has
been used as an external standard to deconvolute the contribution
of instrumental broadening [26]. The measured XRD pattern is
shown in Fig. 3a.
Additionally, Rietveld refinement analysis was performed with
GSAS [27,28], calibrating the instrumental parameters with a stan-
dard Si measurement. The results are shown in Fig. 3b. In all the
fits, instrumental parameters were kept constant.
From Thompson–Cox–Hastings pseudo-Voigt function (TCH) of
the Lorentzian and Gaussian term, size and strain parameters can
be retrieved directly from GSAS profile parameters [29], using
Lorentzian TCH term with parameters GX and GY:
C
L
¼
GX
cos h
þ GY tan h ð1Þ
For the Williamson-Hall analysis, the integral breath (b) holds
with D
v
as the volume weighted average,
e
str
the strain and k as
the wavelength:
0
1000 2000 3000 4000 5000
20
40
20
40
1020
660
448
ZnO annealed at 100
0
C
Wave number (cm
-1
)
1569
1430
3600
1020
A
ZnO annealed at 200
0
C
% Transmission
448
660
1020
0
Fig. 1. FT-IR spectrum representing the characteristic frequencies of ZnO material
synthesized from aqueous route and dried in vacuum oven at 100 °C for 12 h and
then dried to 200 °C.
Table 1
IR spectroscopic group frequencies [25] for the prepared pure ZnO nano-crystalline
sample, dried at 100 °C and compared to that after drying to 200 °C (refer to Fig. 1).
Absorption wave number cm
1
Functional group frequencies
500, 525–560 ZnAO stretching
1020 HAOAH bending
1430, 1570 AC@O stretching
3500 and greater AOH stretching
104 B.N. Ganguly et al. / Nuclear Instruments and Methods in Physics Research B 362 (2015) 103–109
fb
obs
b
inst
g¼
k
ðD
v
cos hÞ
þ 4
e
str
ðtan hÞð2Þ
A correction of the FWHM to integral breath and correcting
GSAS centi-degrees, size and strain can be calculated by:
D
v
¼ 36000
k
p
2
GX
ð3Þ
e
str
¼
p
2
ðGY GY
inst
Þ
144000
ð4Þ
Gaussian TCH term with parameters GU and GP (GV and GP can
be considered as instrumental constants):
C
G
¼ GU tan
2
h þ GV tan h þ GW þ
GP
cos h
ð5Þ
Williamson-Hall analysis:
fb
2
obs
b
2
inst
g¼
k
2
fðD
v
Þ
2
cos
2
hg
þ 16ð
e
str
Þ
2
tan
2
h ð6Þ
Applying the same corrections as for the Lorentzian part results
in:
D
v
¼ 18000
k
sqrtð2
p
3
GPÞ
ð7Þ
e
str
¼
sqrtf2
p
3
ðGU GU
inst
Þg
72000
ð8Þ
The volume weighted average D
v
can be converted into the diam-
eter of spheres with identical size: d = 4/3D
v
(see Tables 2 and 3).
2.4. Local structure investigation with perturbed angular correlation
using radioactive ion beam of
111m
Cd
The time differential perturbed angular correlation technique is
based on the modulation of angular correlation of the successive
radiations emitted during a nuclear decay cascade due to hyperfine
interactions between the electromagnetic moments of the
intermediate nuclear level with its immediate neighboring elec-
tronic environment. Suitable isotopes for standard PAC have an
intermediate level (sensitive level) which has a half-life between
10 and 1000 ns (see Fig. 4), while the half-life of the parent isotope
is sufficiently long lived to provide a measurement.
111
In with a
2.8 days half-life and simple cascade is one of the most and
widely-used PAC probes. However, the so-called after effects,
resulting from the change of electronic charge state from
111
In
3+
to
111
Cd
2+
, can be problematic especially in semiconductors [30].
In order to minimize these effects, the
111m
Cd
2+
radioactive ion is
more suitable as it does not undergo any such transmutation.
However, its short half life of 48 min restricts its availability to
on-site measurements at a facility such as ISOLDE at CERN.
A PAC machine consists of a setup of usually four or six detectors
which measure the time difference between the
c
1
and
c
2
-ray as
shown in Fig. 4, from which the radioactive decay curve of the inter-
mediate level of
111m
Cd with a life time of 84.5 ns has been
obtained. Due to the anisotropy of the emitted radiation, ripples
on the decay curve become visible when electric quadrupole
or/and magnetic dipole interaction, e.g. electric field gradient
(EFG) or local magnetic fields in magnetic materials, are non-zero
as shown in Fig. 4 (see in the middle). The counting rate ratio, R(t),
is obtained from the coincidence data by:
Rðt Þ¼2
N
180
ðtÞN
90
ðtÞ
N
180
ðtÞþ2N
90
ðtÞ
ð9Þ
The counting rate ratio can be described in practical use by [31]:
Rðt ÞA
eff
22
X
i
f
i
G
i
22
ðtÞð10Þ
where A
22
eff
is the effective anisotropy, f the fraction per site i and G
22
the perturbation factor. For quadrupole interaction, including the
finite time resolution here in the last term, the following expression
was used for fitting the data:
G
i
22
ðtÞ¼s
i
20
ð
g
i
Þþ
X
3
j¼1
s
i
2j
ð
g
i
Þ cosð
x
i
j
tÞe
x
i
j
d
i
t
m
Q
i
e
ð
x
i
j
s
r
Þ
2
16 ln 2
ð11Þ
Fig. 2. EDAX data: a purity check of the ZnO sample as synthesized through sol–gel chemical route, inset shows the ZnO nano particles with fringed structure of the
crystalline sample from TEM picture.
B.N. Ganguly et al. / Nuclear Instruments and Methods in Physics Research B 362 (2015) 103–109
105
In this expression,
m
Q
is the quadrupole interaction constant
with d its distribution,
x
is the quadrupole circular frequency
and
s
is the instrumental time resolution. The term:
d
n
ð
x
i
;
s
r
Þ¼e
ð
x
i
j
s
r
Þ
2
16 ln 2
ð12Þ
takes into account the Gaussian and Lorentzian time resolution
broadening [32,33]. More detailed description of the PAC method
can be found at [31,34–36].
Perturbed Angular Correlation (PAC) spectroscopy was per-
formed in order to study the local structure in polycrystalline
nano-ZnO.
111m
Cd as PAC probe was implanted into cold pressed
pellets of the previously prepared material. Room temperature
implantations were performed at ISOLDE/CERN [37] at 30 keV.
111m
Cd
+
was produced following irradiation of a molten Sn target
by 1.4 GeV protons, which were subsequently plasma ionized
and mass separated. A typical irradiation dose was about 3 10
13
ions per sample.
PAC measurements were performed at room temperature (RT)
using two Digital Time Differential PAC machines (DTDPAC)
[38,39] with four BaF
2
detectors resulting in 12 single spectra of
90° and 180° per measurement. All the detected
c
-rays were saved
as time and energy values on the DTDPAC machine’s hard disks.
Due to the short half-life of the probe, energy windows shifted
slightly with the rather fast decrease of the radioactivity. Therefore
the recorded data were reprocessed by separating the data into
several parts, the optimal energy windows were determined for
each part and the results of all parts were finally merged together.
The data were analyzed with XFIT program, using the XPAC for the
same program [40].
3. Result and discussion
3.1. FT-IR and EDAX study
ZnO samples prepared by aqueous sol–gel technique were dried
under vacuum at 100 °C over night, and the characteristic FT-IR
frequencies were checked as shown in Fig. 1 and Table 1 for any
residual organic groups. These data represent the as-prepared
ZnO material. Some residual acetate groups (C@O) could be
detected. Presence of moisture is also detected; this could be
dependent on the ambient condition. It was considered as the pre-
cursor material for ZnO, which was sintered later to proceed for
evolution of pure and dried material of ZnO. The elemental purity
of ZnO was checked with the help of EDAX spectrum as shown in
Fig. 2. From TEM results of pure ZnO samples (dried at 200 °C
was used), the fringe structure of ZnO sample showed crystallinity
of the material (as shown by the arrow). The mean size as esti-
mated from the TEM image has been about 10 nm (average size
as measured with the size bar shown in the figure) and clearly indi-
cates that the initially annealed ZnO nano-particles are crystalline
with a wurtzite structure [41]. No other impurities were observed.
3.2. X-ray diffraction (XRD) study
The as-dried precipitate of ZnO material (at 100 °C) can be con-
sidered as a precursor of the ZnO material without the typical ZnO
phase, but mainly zinc acetate phase and an unknown
phase. Further annealing of this sample stage by stage from
(200–1000 °C) or precisely at 200, 600 and 1000 °C, pure ZnO
40 60 80 100
35.5 36.0 36.5 37.0
Intensity (arb. unit)
2θ (degree)
ZnO annealed at 200
O
C
ZnO annealed at 600
O
C
ZnO annealed at 1000
O
C
Intensity (arb. unit)
2θ (degree)
(a)
(b)
Fig. 3. (a) X-ray diffraction pattern (raw data as obtained) ZnO-nano crystalline
grains at different sintering temperatures. (b) Rietveld fit with GSAS. Graph is scaled
twice.
Table 2
Lattice parameters at different sintering temperatures as per the Rietveld GSAS refinement analysis, Scherrer constant K =1.
Temperature (°C) a (Å) c (Å) Texture Index (GSAS) Strain (%) Size (nm) (diameter of spherical particles)
200 3.2500 5.2070 1.078 Negligible 26
600 3.2496 5.2057 1.147 0.0856 46
1000 3.2499 5.2056 1.021 0.0188 76
Table 3
The unit cell parameters after sintering at 1000 °C as per Rietveld refinement analysis.
Spacegroup a (Å) c (Å)
a
, b (°)
c
(°)
P 63 m c 3.2499 5.2056 90 120
Atom/charge xyz100 * Uiso
Zn + 2 0.3333 0.6666 0 2.917
O 2 0.3333 0.6666 0.3759 3.517
106 B.N. Ganguly et al. / Nuclear Instruments and Methods in Physics Research B 362 (2015) 103–109
structure was produced, no characteristic peaks from intermedi-
ates such as Zn(OH)
2
could be detected in the samples. XRD results
shown in Fig. 3a give us the characteristic diffraction pattern of the
crystallites under the particular configuration, through the Bragg
angles. The appearance of characteristic diffraction peaks for a pure
ZnO sample corresponding to (100), (002), (101), (102), (110),
(103) and (112) planes is in good agreement with the standard
XRD peaks of crystalline bulk ZnO with hexagonal wurtzite struc-
ture [JCPDS card No. 36-1451, a = 3.2501 Å, c = 5.2071 Å, space
group: P63mc (186)]. The gradual changes of the FWHM of charac-
teristic peak (002) is shown as an inset to show the refinement of
evolved structure, at 1000 °C, the best result is obvious from the
spectrum.
The Rietveld refinement analysis was performed using GSAS.
For performing the Rietveld fitting, the instrumental parameters
were determined by using a standard Si sample. (Fitted data
according to GSAS labeling were LX, LY, shft, ptec, lattice parame-
ters, fractional coordinates and Uiso as well as spherical harmonics,
preferred orientation with 6th order.) Lattice parameters are
shown at different sintering temperatures in Table 2 and the unit
cell parameters after sintering at 1000 °CinTable 3.
While fitting these nano-crystallite parameters, an unusual
peak shape was observed which could not be fully adjusted by
keeping the instrumental parameters fixed. This may indicate an
irregular structure or larger variation in crystallite size. The fit
could be improved by adding a Gaussian component by parameters
GU and GP, see Fig. 3b for the sample sintered at 1000 °C. (In
General, the Lorentzian term of the Thompson–Cox–Hastings
pseudo-Voigt function mainly fits sufficiently well crystallized
material with Bragg–Brentano X-ray powder diffractometers, so
that Gaussian terms (GU, GP) are rarely used.)
From Thompson–Cox–Hastings pseudo-Voigt function (TCH) of
the Lorentzian and Gaussian term, size and strain parameters can
be retrieved directly from GSAS profile parameters. The analysis
of the powder, which was sintered at different temperatures, has
been summarized in Table 2. The crystalline size is assumed by a
model for spherical particles of the same size. It increases steadily
with increasing sintering temperature, while the lattice parame-
ters stay almost unchanged. As the sintering temperature gradually
increases and the crystal size increases, the lattice strain is found to
be at maximum at 600 °C. Lattice strain arises generally due to
vacancies, crystal imperfection, dislocations sinter stress, stacking
faults etc., which in case of nano-materials can also be induced
by surface tension effects. Also, the texture is found to be strongest
at 600 °C sintered sample, where also stain is the highest as well.
3.3. Perturbed
c
–
c
angular correlation measurements
In order to understand the changes in the local structure and
the material properties induced due to subsequent annealing of
ZnO, perturbed angular correlation spectroscopy (PAC) was per-
formed with the probe
111m
Cd
2+
and results are shown in
Figs. 5a, b, c.
The PAC experimental runs have been performed at room tem-
perature, at first, without annealing ZnO sample (precursor mate-
rial) and then after subsequent annealing of the ZnO samples at
temperatures 600 °C, and 1000 °C respectively after implantation
of
111m
Cd
2+
for 30 min to anneal implantation defects. However,
the high annealing temperatures for the same time duration are
sufficient to bring in change in nano materials in terms of increase
in crystal size and their properties as well. At room temperature,
the
111m
Cd
2+
doped precursor ZnO sample shows no frequencies
corresponding to standard ZnO, but a wide distribution of frequen-
cies can be seen in Fig. 5a. The sample sintered at 600 °C shows a
known frequency from
m
Q
30 Hz, (can be compared with earlier
results [5]), with a wide distribution of about 4 ± 12 MHz and
g
0.5 ± 0.5, while the material sintered at 1000 °C shows
m
Q
31 Hz and
g
0.1. Due to low statistics at the 600 °C
Fig. 4. Decay scheme of
111m
Cd
48
, IT, T
1/2
= 48.54 m, isomeric level with 5/2
+
spin where EFG tensor interacts with nuclear quadrupole moment Q, an illustration of the typical
PAC measurement.
B.N. Ganguly et al. / Nuclear Instruments and Methods in Physics Research B 362 (2015) 103–109
107
tempered sample, errors are considerably high, reducing the signif-
icance of the data. However it is clearly visible that the amplitude
at 210 ns is nearly absent in this sample as compared to the
1000 °C sample. As the A
22
is a nucleus property and in a measure-
ment is decreased by the detector size and distance, it is machine
related observed A
eff
is at around 0.1. As reference on this, the
observed fraction of the sample at 600 °C is significantly lower
with only 50% than compared to the sample at 1000 °C with 83%,
which indicates that many probes of the 600 °C tempered sample
are on non contributing locations. This may indicate a highly dis-
torted environment and can be probably caused by the higher shell
quantity considering the core–shell model of small particles [5,18].
Comparing the three measurements, the development indicates a
steady growth of crystallinity, in agreement with the X-ray diffrac-
tion data in case of the growth of crystal size, and can be compared
with results of reference [22]. It is possible to assume also that
there could be an incomplete radiation/implantation damage
recovery of the material structure. However, the re-
crystallization of nano-ZnO is probably more significant here. The
asymmetry parameter (
g
) is also larger at 600 °C, but the error is
in the same order, so that its significance with the current statistics
of the measurement is not high enough. However, it would indi-
cate a highly distorted environment and the same could be due
to random distribution of static crystal defects. The observed dif-
ferences overlap well with the observations of the X-ray data and
their Rietveld analysis. The deviation from normal peak profile
and as well the increased stress at the 600 °C indicate that the
nano-ZnO sample undergoes changes during the re-
crystallization (through vacuum annealing stages), which the data
of both the methods (namely X-ray diffraction analysis as well as
PAC) indicate. Barbosa et al. [22] observed a second frequency in
thin film ZnO at 800 °C annealing temperature. It is interesting that
our data at 600 °C near this temperature also show alterations
from the normal bulk properties. But it needs to be mentioned that
our data were measured at room temperature and that nano mate-
rials are mainly from low crystalline quality where this frequency
might be difficult to observe. Nevertheless, both data indicate that
there might be an effect worth to be further investigated. The PAC
results are shown in Table 4.
As the density functional theory calculations [5] of the lattice
sites, the EFG have been performed with good agreement of Cd
2+
impurities into ZnO host nano-structure, through the measure-
ments. This confirms well with the expectations for incorporation
of Cd
2+
in ZnO as both the metals are from the same IIB group of
elements, having similar crystal radii (see preference of crystal
radii over ionic radii in crystals [42]) with Cd
2+
in tetragonal coor-
dination with r
Cd+2
= 0.97 Å and for Zn
2+
in equivalent environment
Fig. 5a. PAC measurements after implantation of
111m
Cd
2+
at RT in ISOLDE
experiment, without annealing, showing the experimental R(t) spectrum and the
hyperfine interactions.
Fig. 5b. PAC measurements after doping
111m
Cd
2+
in ISOLDE experiment and after
annealing at 600 °C, showing the experimental R(t) spectrum and the hyperfine
interactions. The data shown in the legend are from the fit program. Error corrected
data are in Table 4.
Fig. 5c. ZnO nano-crystalline material was doped with
111m
Cd
2+
in ISOLDE
experiment and annealed at 1000 °C, measured in PAC set up. The experimental R
(t) spectrum shows well defined hyperfine interaction and the Fourier transforms of
the hyperfine parameters are also represented (top). The data shown in the legend
are from the fit program. Error corrected data are in Table 4.
Table 4
Hyperfine interaction parameters of ZnO derived from the PAC in the ZnO samples
showing profound effect on annealing temperature (refer to Fig. 5a, b, c).
T(°C) (annealing
temperature)
Fraction
(%)
m
Q
(MHz) d (MHz)
g
RT (precursor) 78.8 ± 78 59.3 ± 200 59.2 ± 300 0 ± 0.2
600 50.4 ± 50 30.4 ± 12 4.00 ± 12 0.47 ± 0.5
1000 82.9 ± 22 31.2 ± 2 0.59 ± 0.5 0.12 ± 0.3
108 B.N. Ganguly et al. / Nuclear Instruments and Methods in Physics Research B 362 (2015) 103–109
with r
Zn+2
= 0.74 Å. As Cd
2+
is larger than Zn
2+
, the local structure
may be slightly distorted, which may cause a larger effect in highly
distorted material, where restoring forces are smaller. The final
sintering at high temperatures of 1000 °C transforms the material
in normal bulk ZnO.
The performed measurements suggest that the preparation of
nano-ZnO by the wet chemical method provides different materi-
als properties (surface defects are common features in nano-size
particles, see Tables 2 and 3) which are possibly scalable with sin-
tering conditions. But due to radioactive ion beam implantation
method, there are damages caused which are gradually recovered
through sequential sintering processes.
4. Conclusion
ZnO material prepared through wet chemical route, sintered at
200 °C already forms ZnO with small crystallite size 626 nm (TEM
examination shows smaller size), but has been continuously grow-
ing with further tempering at higher temperature.
Perturbed angular correlation using the
111m
Cd probe confirmed
the results with previous works and shows that the material at
lower sintering temperatures possesses a higher order distorted
local structure, due to stress in the nano-material as shown from
the X-ray diffraction studies and due to higher ratio of shells in
the core–shell model, which also results in a disordered material.
The ZnO structure is partially recovered after sintering at
1000 °C, but still shows effects of nano-material properties with
crystal sizes below the 100 nm regime. PAC data of the sample at
1000 °C shows analyzed results that are comparable to bulk mate-
rial. The result of the sample tempered at 600 °C, is interesting
since it indicates increased stress from Rietveld analysis and a
defect rich environment from PAC data.
Structurally different, evolving nano-ZnO crystalline grains
could be further useful for future study of semi-conductor applica-
tion in requirements like thin film transistors, optoelectronic
devices and development of flexible electronics etc. with new
properties.
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