The conductivity and TEMF of MoS2 with Mo2S3 additive

Full text

The conductivity and TEMF of MoS2 with Mo2S3
additive
G.E. Yakovleva1, A. S. Berdinsky 1, A. I. Romanenko2, S. P. Khabarov1 and V.E. Fedorov2
1 Novosibirsk State Technical University/Semiconductor Devices & Microelectronics, Novosibirsk, Russia
2 Nikolaev Institute of Inorganic Chemistry, Russian Academy of Sciences, Novosibirsk, Russia
e-mail address: berdinsky.alexander@gmail.com
Abstract - Transition-metal chalcogenides are prospective
thermoelectric materials. One of them is a molybdenum
disulfide MoS2, which has a layered structure. MoS2 has a
good potential to have a high value of thermoelectric quality
factor due to a high value of thermo-EMF and low value of
thermal conductivity. But a low value of its electrical
conductivity suppresses the thermoelectric quality factor ZT
on the level of 0.1 at high temperatures. Present work shows
the influence of metal addition to MoS2 on electrical
conductivity and Seebeck coefficient (SC) of final mixture.
Mo2S3 was chosen as a metal addition. Mo2S3 has an
electrical conductivity of 330 S/m and thermo-EMF of 10
μV/K at 300K. Bulk powder samples of MoS2 with addition
of 3, 6, 10, 30 and 60 wt% Mo2S3 were studied. An electrical
conductivity of samples was measured in temperature
range: 77 K – 423 K. All samples have shown semiconductor
hopping conductivity with variable hopping length. The SC
was measured in the temperature range of 300K - 500K.
The addition of Mo2S3 decrease SC from 300μV/K to
75μV/K.
I. INTRODUCTION
Thermoelectric materials have a great interest for
study. Environmental problems associated with non-
renewable natural resources and environmental pollution
are the cause of such interest [1].
The effectiveness of a thermoelectric material is
characterized by dimensionless thermoelectric quality
factor ZT, which is calculated by the formula “(1)”.
ZT = S2σT/λ, (1)
where S – Seebeck coefficient, σ – electrical
conductivity, λ – thermal conductivity, T – temperature
[2].
The search of new materials with high ZT consists of
research of completely new composite materials.
Thermoelectric materials have a different temperature
range where the quality factor reaches its maximum.
Therefore thermoelectric materials are classified as low,
middle or high temperature materials according to a
temperature range with a highest value of quality factor.
Such materials as transition-metal chalcogenides are
of great interest in recent time. One of the brightest
representatives of such materials is a molybdenum
disulfide. Molybdenum disulfide has its own distinctive
electronic, optical and catalytic properties [3].
Molybdenum disulfide has a layered structure. Single-
layer in disulfide molybdenum is a two-dimensional
quasi-crystal, which consists of close-packed layer of
molybdenum in between of two close-packed layers of
sulfur S-Mo-S. Layers are held together due to weak Van
der Waals forces, whereas atomic structures of layers are
tied together by strong covalent forces. This material is
structurally similar to graphene and hexagonal boron
nitride (h-BN) [4].
Recent reports demonstrated thermopower of MoS2 in
a range of 600 to 700 µV/K at a high temperature.
Electrical and thermal conductivity of MoS2 have different
values in a various sources. The value of the electrical
conductivity is in a range of 0.01 to 10 S/m at room
temperature. The value of thermal conductivity is in a
range of 0.1 to 1 W/(m*K) [5-7].
Molybdenum disulfide has a potential to have a high
value of thermoelectric quality factor due to high value of
thermo-EMF and low value of thermal conductivity. But
the low value of electrical conductivity suppresses its
thermoelectric quality factor on the level of 0.1 at high
temperatures [8].
Present work shows the influence of metallic addition
to molybdenum disulfide on its electrical conductivity
and Seebeck coefficient. Molybdenum(III) sulfide Mo2S3
was chosen as a metallic addition. Mo2S3 has an electrical
conductivity of 330 S/m and thermopower of 10 μV/K at
room temperature.
II. EXPERIMENTAL
A. Preparation of sample
MoS2 and Mo2S3 were synthesized by a standard high
temperature ampule method using high purity elements in
stoichiometric ratio. The composite samples are prepared
by means of intimately mixing molybdenum disulfide
with 3, 6, 10, 30, 60 wt% Mo2S3.
The powdered composite materials were pressed to
obtain 12 mm in diameter pellets-shape samples. The
samples shape was further trimmed to rectangular form
for study of thermoelectric and electronic properties. Such
shape of the samples was obtained by means of diamond
cutter. The ohmic contact to the samples was obtained
using silver paste.
MIPRO 2015, 25-29 May 2015, Opatija, Croatia
12

B. Electrical properties
The samples under study have n-type conductivity.
Seebeck coefficient of the samples was measured at the
temperature range of 300 to 500 K. The results on
Seebeck coefficient are presented in Fig.1.
Seebeck coefficient of pure molybdenum disulfide has
value of 100 to 300 µV/K over a temperature range of 300
to 500 K. It is observed that Seebeck coefficient is
decreased with introduction of Mo2S3 to the MoS2.
Introduction of 60wt% of Mo2S3 has the highest effect on
the Seebeck coefficient of the composite. At 60wt% of
Mo2S3 Seebeck coefficient became negative which means
than composite changed its conductivity to p-type.
Electrical conductivity was measured at the
temperature in the range of 77 to 500 K. The measured
results are displayed in Fig.2.
The results of electrical conductivity measurement
showed that the samples with addition of 60wt% of Mo2S3
have a higher electrical conductivity. The results are
proved by the fact that Mo2S3 is a metal with high
electrical conductivity. The results in Fig.2 clearly shown,
that MoS2 sample itself as well as all sample with Mo2S3
additive demonstrated 3D hopping conductivity with
variable range hopping .
Thus all the samples belong to family of materials,
where hopping conductivity with variable range hopping
(VRH) is realized. Such type of conductivity was
discussed by Mott in 1968 [9]. According to Mott’s law,
the resistivity varies with temperature by the formula
“(2)”.
ρ(T)=ρ0·exp[(T0/T)1/4] (2)
where ρ0 is the resistivity at infinite temperature and T0 is
the characteristic temperature
Thermoelectric power factor of the samples was
calculated by the formula “(3)”.
P = S2σ (3)
where S – the Seebeck coefficient, σ – electrical
conductivity.
The results of the thermoelectric power factor
calculation are presented in Fig. 3. One can see that
addition of 60wt% Mo2S3 dramatically changes the
thermoelectric power factor in comparison with MoS2,
e.g. thermoelectric power factor increased in 3.5 times in
the average range of the temperatures. The smaller
additions of Mo2S3 show the trend in decreasing of
thermoelectric power factor.
III. CONLUSION
We have measured MoS2 pellet-shape pressed samples
with different amount of Mo2S3 additive for analysis of
TEMF and conductivity. It was shown that addition of
Mo2S3 decreases Seebeck coefficient from 300μV/K to
75μV/K. The addition of 60wt% Mo2S3 gives the
advantage in thermoelectric power factor.
It was shown that all samples have disordered
structure with 3D hopping type of conductivity with
variable hopping length. It was also shown that the
temperature dependence of the samples omit Mott’s law.
By the variation in MoS2/Mo2S3 composites is possible to
change thermoelectric and conductive properties by
controlling of Mo2S3 contents.
Figure 1. The Seebeck coefficient as function of temperature.
Figure 3. The power factor as function of temperature.
Figure 2. The resistivity as function of temperature.
13

ACKNOWLEDGMENT
This study was supported by Russian Scientific
Foundation, Project 14-13-00674. The authors are
gratefully acknowledges to Dr. Kozeeva L.P. for the help
in samples preparing.
REFERENCES
[1] Jong-Young Kim, Soon Mok Choi, Won-Seon Seo, and Woo-
Seok Cho, “ Thermal and electronic properties of exfoliated metal
chalcogenides,” Korean Chem. Soc. Korea, vol. 31, pp. 3225 –
3227, September 2010.
[2] V. L. Kalikhman and Ya. S. Umanskii, “ Transition – metal
chalcogenides with layer structures and features of the filling of
their brillouin zones,” Soviet physics uspekhi. Russia, vol. 15, pp.
728 -741, June 1973.
[3] Wen Huang, Haixia Da, and Gengchiau Liang, “ Thermoelectric
performance of MX2 ( M=Mo,W; X = S, Se) monolayaes,” Journal
of applied physics. Singapore, vol. 113, pp. 1 -7, March 2013.
[4] G. H. Lee, Y. J. Cui, N. Petrone, C. H. Lee, M. S. Choi, D. Y. Lee,
C. Lee, W. J. Yoo, K. Watanabe, T. Taniguchi, C. Nuckolls, P.
Kim and J. Hone, “ Flexible and transparent MoS2 field – effect
transistors on hexagonal boron nitride – graphene
heterostructures,” ASCNANO, vol. 7, pp. 7931 -7936, August
2013.
[5] S. K. Srivastava and B. N. Avasthi, “ Indium intercalation
compounds of molybdenum disulfide; InxMoS2 (0≤ x≤ 1),”
Synthetic Metals. India, vol. 11, pp. 193-205, February 1985.
[6] S. K. Srivastava and B. N. Avasthi, “ Preparation and
characterization of molybbdenum disulphide catalysts,” Journal of
materials science. India, vol. 28, pp. 5032-5035.
[7] X. Liu, G. Zhang, Q. X. Pei, and Y. W. Zhang, “ Phonon thermal
conductivity of monolayer MoS2 sheet and nanoribbons,” Applied
physics letters. Singapore, vol. 103, pp. 1 – 5, September 2013.
[8] Guo Huai-Hong, Yang Teng, Tao Peng, and Zhang Zhi-Dong, “
Theoretical study of thermoelectic properties of MoS2,” Chin.
Phys. B. China, vol. 23, pp. 1 – 7, November 2013.
[9] N. F. Mott and E. A. Davis, “ Electronic processes in non-
crystalline materials,” Clarendon, Oxford,1979.
14