3 research outputs found
Effect of strain and sulfur vacancies on the luminescence and valley polarization properties of CVD grown monolayer MoS films
Using temperature dependent photoluminescence (PL), polarization resolved PL
and Raman spectroscopy, we investigate the effect of in situ vacuum annealing
as well as the relaxation of strain on the luminescence and the valley
polarization properties of large area strictly monolayer (1L)-MoS, grown on
sapphire and SiO/Si substrates by a microcavity based chemical vapor
deposition (CVD) technique. The study shows that the strain as well as the
physisorption of air molecules at the sulfur vacancy () sites play key
roles in governing the optical quality of CVD grown 1L-MoS. Removal of air
molecules from the sites enhances the relative strength of the
A-exciton/trion transition as compared to the broad luminescence (BL) band
arising from those defects at low temperatures. It has also been found that
such removal helps in improving the valley polarization property of the film.
Relaxation of biaxial tensile strain, which has been achieved by post growth
transferring of 1L-MoS film from the sapphire to a SiO/Si substrate by
a polystyrene assisted transfer process, is also found to be helpful to get
back the high polarization character (80%) of the valleys. The study
further shows that the transfer process not only facilitates the removal of
physisorbed air molecules from the sites but also puts in place a long
lasting capping layer on MoS that shields the film from reacting with air
and hence enhances the relative yield of A-exciton/trion transition by
suppressing the BL transition. The study thus creates an opportunity to use CVD
grown large area 1L-MoS for the development of optoelectronic as well as
valleytronic devices for practical applications for the future
Influence of Defects on the Valley Polarization Properties of Monolayer MoS Grown by Chemical Vapor Deposition
Here, the underlying mechanisms behind valley de-polarization is investigated
in chemical vapor deposited 1L-MoS. Temperature dependent polarization
resolved photoluminescence spectroscopy was carried out on as-grown,
transferred and capped samples. It has been found that the momentum scattering
of the excitons due to the sulfur-vacancies attached with air-molecule defects
has a strong influence on the valley de-polarization process. Our study reveals
that at sufficiently low densities of such defects and temperatures, long range
electron-hole exchange mediated intervalley transfer due to momentum scattering
via Maialle-Silva-Sham (MSS) mechanism of excitons is indeed the most dominant
spin-flip process as suggested by T. Yu et al. The rate of momentum scattering
of the excitons due to these defects is found to be proportional to the cube
root of the density of the defects. Intervalley transfer process of excitons
involving -valley also has significance in the valley de-polarization
process specially when the layer has tensile strain or high density of
defects as these perturbations reduce to -energy separation.
Band-structural calculations carried out within the density functional theory
framework validate this finding. Experimental results further suggest that
exchange interactions with the physisorbed air molecules can also result in the
intervalley spin-flip scattering of the excitons, and this process gives an
important contribution to valley depolarization, specially at the strong
scattering regime
An electroplating-based plasmonic platform for giant emission enhancement in monolayer semiconductors
Two dimensional semiconductors have attracted considerable attention owing to
their exceptional electronic and optical characteristics. However, their
practical application has been hindered by the limited light absorption
resulting from their atomically thin thickness and low quantum yield. A highly
effective approach to manipulate optical properties and address these
limitations is integrating subwavelength plasmonic nanostructures with these
monolayers. In this study, we employed electron beam lithography and
electroplating technique to fabricate a gold nanodisc (AuND) array capable of
enhancing the photoluminescence (PL) of monolayer MoS giantly. Monolayer
MoS placed on the top of the AuND array yields up to 150-fold PL
enhancement compared to that on a gold film. We explain our experimental
findings based on electromagnetic simulations