Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Simultaneous Generation of Direct- and Indirect-Gap Photoluminescence in
  Multilayer MoS2 Bubbles   Transition metal dichalcogenide (TMD) materials have received enormous
attention due to their extraodinary optical and electrical properties, among
which MoS2 is the most typical one. As thickness increases from monolayer to
multilayer, the photoluminescence (PL) of MoS2 is gradually quenched due to the
direct-to-indirect band gap transition. How to enhance PL response and decrease
the layer dependence in multilayer MoS2 is still a challenging task. In this
work, we report, for the first time, simultaneous generation of three PL peaks
at around 1.3, 1.4 and 1.8 eV on multilayer MoS2 bubbles. The temperature
dependent PL measurements indicate that the two peaks at 1.3 and 1.4 eV are
phonon-assisted indirect-gap transitions while the peak at 1.8 eV is the
direct-gap transition. Using first-principles calculations, the band structure
evolution of multilayer MoS2 under strain is studied, from which the origin of
the three PL peaks of MoS2 bubbles is further confirmed. Moreover, PL standing
waves are observed in MoS2 bubbles that creates Newton-Ring-like patterns. This
work demonstrates that the bubble structure may provide new opportunities for
engineering the electronic structure and optical properties of layered
materials.

2
The categories of this paper are: math
30
The categories of this paper are: q-alg
11
The categories of this paper are: hep-lat
35
The categories of this paper are: mtrl-th
12
The categories of this paper are: nucl-th
27
The categories of this paper are: comp-gas
32
The categories of this paper are: atom-ph
26
The categories of this paper are: bayes-an
15
The categories of this paper are: nucl-ex
36
The categories of this paper are: cmp-lg
0
The categories of this paper are: hep-ph
22
The categories of this paper are: chao-dyn
8
The categories of this paper are: hep-ex
18
The categories of this paper are: econ
19
The categories of this paper are: eess
33
The categories of this paper are: plasm-ph