6 research outputs found
Valley Coherent Hot Carriers and Thermal Relaxation in Monolayer Transition Metal Dichalcogenides
We show room temperature valley coherence with in MoS2, MoSe2, WS2 and WSe2
monolayers using linear polarization resolved hot photoluminescence (PL), at
energies close to the excitation - demonstrating preservation of valley
coherence before sufficient scattering events. The features of the co-polarized
hot luminescence allow us to extract the lower bound of the binding energy of
the A exciton in monolayer MoS2 as 0.42 (+/- 0.02) eV. The broadening of the PL
peak is found to be dominated by Boltzmann-type hot luminescence tail, and
using the slope of the exponential decay, the carrier temperature is extracted
in-situ at different stages of energy relaxation. The temperature of the
emitted optical phonons during the relaxation process are probed by exploiting
the corresponding broadening of the Raman peaks due to temperature induced
anharmonic effects. The findings provide a physical picture of photo-generation
of valley coherent hot carriers, and their subsequent energy relaxation path
ways
Photoresponse of atomically thin MoS2 layers and their planar heterojunctions
MoS2 monolayers exhibit excellent light absorption and large thermoelectric
power, which are, however, accompanied with very strong exciton binding energy
- resulting in complex photoresponse characteristics. We study the electrical
response to scanning photo-excitation on MoS2 monolayer (1L) and bilayer (2L)
devices, and also on monolayer/bilayer (1L/2L) planar heterojunction and
monolayer/few-layer/multi-layer (1L/FL/ML) planar double heterojunction devices
to unveil the intrinsic mechanisms responsible for photocurrent generation in
these materials and junctions. Strong photoresponse modulation is obtained by
scanning the position of the laser spot, as a consequence of controlling the
relative dominance of a number of layer dependent properties, including (i)
photoelectric effect (PE), (ii) photothermoelectric effect (PTE), (iii)
excitonic effect, (iv) hot photo-electron injection from metal, and (v) carrier
recombination. The monolayer and bilayer devices show peak photoresponse when
the laser is focused at the source junction, while the peak position shifts to
the monolayer/multi-layer junction in the heterostructure devices. The
photoresponse is found to be dependent on the incoming light polarization when
the source junction is illuminated, although the polarization sensitivity
drastically reduces at the monolayer/multi-layer heterojunction. Finally, we
investigate laser position dependent transient response of photocurrent to
reveal trapping of carriers in SiO2 at the source junction is the critical
factor to determine the transient response in 2D photodetectors, and also show
that, by systematic device design, such trapping can be avoided in the
heterojunction devices, resulting in fast transient response. The insights
obtained will play an important role in designing fast 2D TMDs based
photodetector and related optoelectronic and thermoelectric devices.Comment: Nanoscale, 201
Visualization of coherent structures in turbulent subsonic jet using planar laser induced fluorescence of acetone
In this paper, we present the molecular density distribution measurement in turbulent nitrogen jet (Re approximate to 3 x 10(3)), using acetone as molecular tracer. The tracer was seeded in the nitrogen jet by purging through the liquid acetone at ambient temperature. Planar laser sheet of 266 nm wavelength from frequency quadrupled, Q-switched, Nd:YAG laser was used as an excitation source. Emitted fluorescence images of jet flow field were recorded on CMOS camera. The dependence of planar laser induced fluorescence (PLIF) intensity on acetone vapor density was used to convert PLIF image of nitrogen jet into the density image on pixel-by-pixel basis. Instantaneous quantitative density image of nitrogen jet, seeded with acetone, was obtained. The arrowhead-shaped coherent turbulent structures were observed in the present work. It was found that coherent structures were non-overlapping with separate boundaries. Breaking of coherent structures into turbulence was clearly observed above four times jet width