3 research outputs found
Saturation of Two-Photon Absorption in Layered Transition Metal Dichalcogenides: Experiment and Theory
The
saturation of two-photon absorption (TPA) in four types of
layered transition metal dichalcogenides (TMDCs) (MoS<sub>2</sub>,
WS<sub>2</sub>, MoSe<sub>2</sub>, WSe<sub>2</sub>) was systemically
studied both experimentally and theoretically. It was demonstrated
that the TPA coefficient is decreased when either the incident pulse
intensity or the thickness of the TMDC nanofilms increases, while
TPA saturation intensity has the opposite behavior, under the excitation
of 1.2 eV photons with a pulse width of 350 fs. A three-level excitonic
dynamics simulation indicates that the fast relaxation of the excitonic
dark states, the exciton–exciton annihilation, and the depletion
of electrons in the ground state contribute significantly to TPA saturation
in TMDC nanofilms. Large third-order nonlinear optical responses make
these layered 2D semiconductors strong candidate materials for optical
modulation and other photonic applications
A New 2H-2H′/1T Cophase in Polycrystalline MoS<sub>2</sub> and MoSe<sub>2</sub> Thin Films
We
report on 2H-2H′/1T phase conversion of MoS<sub>2</sub> and
MoSe<sub>2</sub> polycrystalline films grown by thermally assisted
conversion. The structural conversion of the transition metal dichalcogenides
was successfully carried out by organolithium treatment on chip. As
a result we obtained a new 2H-2H′/1T cophase system of the
TMDs thin films which was verified by Raman spectroscopy, X-ray diffraction,
and X-ray photoelectron spectroscopy. The conversion was successfully
carried out on selected areas yielding a lateral heterostructure between
the pristine 2H phase and the 2H′/1T cophase regions. Scanning
electron microscopy and atomic force microscopy revealed changes in
the surface morphology and work function of the cophase system in
comparison to the pristine films, with a surprisingly sharp lateral
interface region
Nanopatterning and Electrical Tuning of MoS<sub>2</sub> Layers with a Subnanometer Helium Ion Beam
We report subnanometer modification
enabled by an ultrafine helium ion beam. By adjusting ion dose and
the beam profile, structural defects were controllably introduced
in a few-layer molybdenum disulfide (MoS<sub>2</sub>) sample and its
stoichiometry was modified by preferential sputtering of sulfur at
a few-nanometer scale. Localized tuning of the resistivity of MoS<sub>2</sub> was demonstrated and semiconducting, metallic-like, or insulating
material was obtained by irradiation with different doses of He<sup>+</sup>. Amorphous MoS<sub><i>x</i></sub> with metallic
behavior has been demonstrated for the first time. Fabrication of
MoS<sub>2</sub> nanostructures with 7 nm dimensions and pristine crystal
structure was also achieved. The damage at the edges of these nanostructures
was typically confined to within 1 nm. Nanoribbons with widths as
small as 1 nm were reproducibly fabricated. This nanoscale modification
technique is a generalized approach that can be applied to various
two-dimensional (2D) materials to produce a new range of 2D metamaterials