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1. Introduction
Two-dimensional transition-metal dichalcogenide (TMDC) materials, such as molybdenum
disulfide (MoS
2
), are promising for the next generation of electronic and optoelectronic
devices [1] because of their unique electrical, optical, and mechanical properties [2–9]. In
contrast to graphene, MoS
2
atomic layers have a band gap which transits from indirect 1.2 eV
to direct 1.9 eV as the thickness of MoS
2
reduces from bulk to monolayer [3,5]. Those unique
properties of MoS
2
make it very suitable for fabrication of electronic and optoelectronic
devices. Single-layer MoS
2
field effect transistors [10–12], heterojunction [13] and
ultrasensitive photodetectors [14], have been reported. Field emission and photoresponse of
MoS
2
thin films are also detailed investigated [15].Even the integrated circuits based on
bilayer MoS
2
transistors have also been demonstrated [16]. It may be predicted that the degree
of integration of MoS
2
transistor-based integrated circuits will be much higher than one of
current Si-based microelectronic integrated circuits because MoS
2
transistors are in nanometer
scale. Heat dissipation or thermal conductivity has been one of the most significant
constraints on design and fabrication of current Si-based integrated electronic circuits.
Consequently, it may be predicted that heat dissipation or thermal conductivity will be critical
in near future in design and fabrication of single layer MoS
2
integrated circuits. For thermal
manipulation, it is very necessary to study electron-phonon interactions, dynamics and
thermodynamics of phonons in MoS
2
thin films.
Raman spectroscopy is a powerful tool to access thermal properties of materials,
especially nanomaterials, and has been extensively used to investigate thermal properties and
microstructures of TMDC films and other materials [17–23], revaling their doping effect [24],
electronic and structural properties [25], and the vibration property of nanoribbons and
nanosheets [26–28]. Several comprehensive studies on the vibration frequency and peak
width evolution of phonon modes of MoS
2
mono- and few-layers with temperature have also
been reported so that thermal conductivity and phonon dynamics of MoS
2
could be revealed
[29–35]. However, some controversial experimental results were reported [29–35]. Both
Taube et al. [29] and Yan et al. [30] investigated the temperature dependence of Raman shifts
of
1
2
and A
1g
modes of monolayer MoS
2
within a low temperature range of 80 – 350 K,
respectively on SiO
2
/Si and sapphire (suspended) substrates. Although the MoS
2
monolayer
samples studied by the two groups were all fabricated from mechanical exfoliation of MoS
2
single crystals, Taube et al. [29] found nonlinear temperature dependence of Raman shifts of
both
1
2
and A
1g
modes, while Yan et al. [30] observed linear temperature dependence for
either a sapphire substrate or the suspended. Inversely, Lanzillo et al. [31] and Najmaei et al.
[32] studied the temperature dependence of Raman shifts of
1
2
and A
1g
modes of monolayer
MoS
2
on SiO
2
/Si substrates within a high temperature range of 300 – 500 K, respectively
prepared by chemical vapor deposition (CVD) and mechanical exfoliation. Both groups
observed linear temperature dependence of Raman shifts of both
1
2
and A
1g
modes. Lanzillo
et al. [31] obtained the first-order temperature coefficient of
1
2
mode as −0.013 cm
−1
/K, but
Najmaei et al. [32] gave out one as −0.0179 cm
−1
/K. An obvious difference on the first-order
Received 9 Mar 2016; revised 17 May 2016; accepted 18 May 2016; published 26 May 2016
30 May 2016 | Vol. 24, No. 11 | DOI:10.1364/OE.24.012281 | OPTICS EXPRESS 12283