Title Goes Here
Structural, Morphological and Thermoelectric Studies of SnTe-Te
Nanocomposite
Satyendra Singha), Surasree Sadhukhan, Prakash Behera and R.
Venkatesh
Low Temperature Laboratory, UGC-DAE Consortium for Scientific
Research,
University Campus, Khandwa Road, Indore 452 001, M.P, India
a)Corresponding author: [email protected]
Abstract. SnTe-Te nanocomposites have been synthesized by
Microwave assisted solvothermal process to study the structural,
morphological and thermoelectric properties. The X-ray diffraction
studies reveals a cubic FCC nature of the SnTe sample along with
hexagonal structure of Te. Morphological studies from FESEM
measurements indicates that the two-dimensional sheets of SnTe
sample (~370 nm) coexist along with the Te nanorods of length ~1.4
µm and diameter~145 nm. Interestingly, the thermoelectric
properties of nanocomposites with p-type carriers show S=56µV/K at
room temperature which is 36% higher than that of the pure SnTe
nanoparticle as reported in literature. Enhanced thermoelectric
power in SnTe-Te nanocomposites can be attributed to the presence
of Te nanorods which are acting as the phonon scattering centre.
These SnTe-Te nanocomposites synthesized through scalable microwave
synthesis with enhanced thermoelectric power and linear temperature
dependence can open up a new gateway as potential materials for
high temperature thermoelectric applications.
1. INTRODUCTION
Tin telluride is (IV-VI) group compound which is fascinating the
research attention of the researchers due to its promising
thermoelectric properties from several decades. But now a days,
this material is renowned for their novel topological properties.
Tin telluride (SnTe) is a n-type [1] and p-type [2] narrow band gap
semiconductor with direct band gap 0.18e.V. Normally, tin telluride
forms p-type extrinsic semiconductor due to tin vacancies and it is
a low temperature superconductor [3]. Theoretical studies state
that the n-type tin telluride thermoelectric performance may be
particularly good [4]. Tin telluride is a thermoelectric material
at high temperature as well as topological material at low
temperature. SnTe is a novel topological material which is the
counter part of the topological insulator. Topological materials
are those which have conducting surface states but interior part
(bulk) of these materials are insulating due to presence of
internal magnetic field. In topological insulator time reversal
symmetry plays a crucial role but in SnTe topological crystalline
insulator surface states are protected by mirror symmetry instead
of time reversal symmetry [5]. Nanomaterials are responsible for
high thermoelectric performance due to low lattice thermal
conductivity [6]. Topological properties are exhibited in
nanomaterials due to the enhancement of surface to bulk
contribution. Topological Insulators have odd number of Dirac cones
on the surface states while TCI`s have even number of Dirac cones
on the surface states due to which TCI`s have high tunability
properties [7]. Nanocomposite materials are known for their high
thermoelectric performance due to reduction in the lattice thermal
conductivity by scattering of phonons.
2. SAMPLE PREPARATION AND CHARACTERIZATION2.1 Microwave Assisted
Synthesis of SnTe-Te Nano Composites
A microwave-assisted solvothermal synthesis technique was used
for the preparation of SnTe-Te nanocomposites. Microwave synthesis
technique provides fast, scalable, uniform heating, easy handling
and sufficient amount of materials with higher reproducibility.
Temperature can be controlled more precisely in this process as
conditions required. In this synthesize technique, we followed a
procedure similar to R. Metha et al; [8] for the preparation of
sample by this microwave-assisted solvothermal synthesis procedure.
For the synthesis of SnTe-Te nanocomposite materials; SnCl2.2H2O
(451.6 mg), Te powder (382.7 mg), 5 ml TOP, 14 ml Pentane-di-ol and
3 mL TGA have been taken as precursor materials. The dissolved
solution of SnCl2.2H2O, Pentane-di-ol (PD), TGA, Tellurium Powder
and Tri-octyl phosphine (TOP) were mixed, microwave heated for 3
minutes and left up to room temperature for cooling around 1hr. The
obtained black precipitate was centrifuged and washed 3 times with
ethyl alcohol and acetone respectively.
2.2 Characterization
The structural measurements were investigated by XRD (Bruker D8
Advance X-ray diffractometer system with Cu Kα radiation source).
There is presence of tellurium peaks in the xrd measurements
matched with the JCPDS card No: 00-036-1452 as well as SnTe JCPDS
card No: 03-065-7162, followed the microwave synthesis technique.
From the observed pattern of xrd, the composite material shows the
hexagonal Te phase along with SnTe cubic phase. All the diffraction
peaks are indexed to the FCC cubic SnTe (with space group Fm-3m)
and Te (with space group P3121) as shown in Fig 1(a). The Te peaks
and SnTe peaks are shown by blue and red colour respectively in Fig
1(a). The crystallite size of SnTe and Te in prepared nanocomposite
is estimated to be ~54 nm and ~75 nm corresponding to the main peak
as determined by Debye Scherrer formula (τ=Kλ/βcosθ). Where τ is
the crystallite size, K is a constant taken to be 1, λ is the
wavelength of Cu K alpha X-ray (1.54Å), β is the full width at half
maximum (FWHM) obtained from the main peak and θ is the diffraction
angle. The EDS compositional analysis was carried out using “Bruker
made XFlash X130”. The EDS spectrum is taken over the single
Tellurium nanorods present in the SnTe-Te nano composite as shown
in inset of Fig 1(b) which confirms the presence of Te nanorods
along with the SnTe nanoparticles. This is in correspondence with
the structural analysis.
b
a
Figure 1. (a) XRD plot of SnTe nanomaterials along with Te
peaks. Red colour peaks indicate the SnTe peaks while blue colour
indicate Te peaks. (b) shows the eds spectrum of nanocomposites of
SnTe-Te and inset shows the FESEM image of the composite
sample.
2.3 Morphology Studies
Morphological studies of SnTe nanomaterials were studied with
the instrument FEI made “NOVA NANO SEM 450”. The morphology
obtained from FESEM images imply that the SnTe nanoparticle are
coexisting with the Tellurium nanorods as shown in Fig 2a), 2b) and
2c). The cross-sectional edge to edge length of the SnTe is found
to be ~370nm. The length of the Tellurium nanorods were found to be
~1.4 µm with diameter~145 nm with the analysis of Image J
software.
b
1µm
a
3µm
c
500nm
Figure 2. FESEM images of SnTe-Te nanocomposites a), b) and c)
from lower to higher magnification respectively prepared with
microwave assisted solvothermal technique.
2.4 Thermoelectric Properties of the Composite Material of
SnTe-Te
The thermopower measurement has been performed in homemade
Thermoelectric power measurement setup by “DC Differential Sandwich
method”. The thermopower of composite (SnTe-Te) has been shown in
Fig 3 as the function of temperature which indicates the p-type
nature of the composite. Temperature dependence of S is fitted with
linear fit which shows the metallic nature like behavior and the
absence of hump confirms the single band conduction. The linear
increase in S with T shows that this SnTe-Te nanocomposite is
favourable for high temperature thermoelectric applications.
Magnitude of room temperature thermoelectric power of our
nanocomposite sample is compared with the reported SnTe values from
literature is as shown in Fig3.
Figure 3. Graph shows the thermopower of SnTe-Te composite as
the function of temperature with linear fitting. The dots indicate
the room temperature comparison with SnTe.
3.Result and Discussion
Structural and morphological properties confirm the presence of
SnTe-Te nanocomposites. These nano structuring with enhanced
surface to volume ratio by fast, facile and scalable microwave
synthesis technique is beneficial in probing the topological
properties of SnTe system. Nano structuring of SnTe materials are
responsible for the potential applications in thermoelectric
materials which is used in many energy sources. In fact, both SnTe
and Te are good P-type thermoelectric materials which have good
thermoelectric performance. The thermopower of our composite
material is also in line with previous reported literature on this
composite [9]. The Seebeck coefficient for SnTe material addressed
by Dongyang Wang et al; and E. Z. Xu et al; is 21.5µV/K and 41 µV/K
at room temperature respectively. The room temperature thermopower
of composite (SnTe-Te) has been obtained ~56 µV/K which is higher
than the thermoelectric power value reported for SnTe material. The
drop in the lattice thermal conductivity due to the phonon
scattering on the interfaces of (SnTe-Te) is reported to play the
key role for high thermoelectric performance in composites made of
bulk materials [9]. As the thermal conductivity of nanocomposites
would be further lower than the bulk materials. A combined thermal
conductivity measurement, thermopower measurement and electrical
conductivity measurements at high temperature can prove that these
nanocomposites are promising for high temperature thermoelectric
application.
Conclusion
The nanomaterials of TCI`s SnTe have been synthesized
successfully by Microwave-assisted solvothermal synthesis
procedure. A nano composite of SnTe and Te have been obtained
during the synthesis of SnTe nanomaterials in presence of excess
heat. To control the morphology of the SnTe nanomaterials, TGA is
used as the shape directing and reducing agent. The synthesized
SnTe-Te nanocomposites by this fast, facile and scalable microwave
are advantageous for the point of thermoelectric applications and
enhanced surface property of the sample for the topological point
of view. High value of thermopower in composite materials concludes
that composites are useful for thermoelectric performance due to
low lattice thermal conductivity.
Acknowledgments
Authors thank Director and Centre Director of UGC-DAE CSR,
Indore for their support. The authors would like to express their
gratitude to Dr. Mukul Gupta and Dr. D.M.Phase for XRD and FESEM
measurements respectively. LT labmates are acknowledged for their
precious help.
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