Query: WS2 Low temperature luminescence

title: Cathodoluminescence in single and multiwall WS2 nanotubes: Evidence for quantum confinement and strain effect
abstract: For nanoparticles with sub 10 nm diameter, the electronic bandgap becomes size dependent due to quantum confinement; this, in turn, affects their electro optical properties. Thereby, MoS2 and WS2 monolayers acquire luminescent capability, due to the confinement induced indirect to direct bandgap transition. Rolling up of individual layers results in single wall inorganic nanotubes (SWINTs) Up to the present study, their luminescence properties were expected to be auspicious but were limited to theoretical investigations only, due to the scarcity of SWINTs and the difficulties in handling them. By optimizing the conditions in the plasma reactor, relatively high yields of WS2 SWINTs 3 7 nm in diameter were obtained in this work, compared to previous reports. A correlative approach, transmission electron microscopy coupled with a scanning electron microscope, was adapted to overcome handling obstacles and for testing individual nanotubes by low temperature cathodoluminescence. Clear cathodoluminescence spectra were obtained from WS2 SWINTs and compared with those of WS2 multiwall nanotubes and the corresponding bulk material. Uniquely, the optical properties of INTs acquired from cathodoluminescence were governed by the opposite impact from quantum size effect and strain in the bent triple S W S layers. The experimental findings were confirmed by the Density Functional and Time Dependent Density Functional theoretical modeling of monolayer and bilayer nanotubes of different chiralities and diameters. This study provides experimental evidence of the quantum confinement effect in WS2 SWINTs akin to WS2 monolayer. The ability to tune the electronic structure with morphology or number of layers may be exploited toward photoelectrochemical water splitting with WS2 catalysts, devising field effect transistors, photodetectors, and so on.

title: Probing of free and localized excitons and trions in atomically thin WSe2, WS2, MoSe2 and MoS2 in photoluminescence and reflectivity experiments.
abstract: We report on detailed temperature dependent (T 7 295 K) optical spectroscopy studies of WSe2, WS2, MoSe2 and MoS2 monolayers exfoliated onto the same SiO2/Si substrate. In the high energy region of absorption type (reflectivity contrast RC) and emission (photo luminescence PL) spectra of all the monolayers resonances related to the neutral and charged excitons (X and T) are detected in the entire measured temperature range. The optical amplitudes of excitons and trions strongly depend on the temperature and two dimensional carrier gas (2DCG) concentration. In the low energy PL spectra of WSe2 and WS2 we detect a group of lines (L) which dominates the spectra at low temperatures but rapidly quenches with the increase in the temperature. Interestingly, in the same energy range of the RC spectra recorded for WS2, we observe an additional line (L 0) which behaves in the same way as the L lines in the PL spectra. The optical amplitude of L 0 and T resonances in the RC spectra strongly increases with the growth of the 2DCG concentration. On the base of these observations we identify the L 0 resonance in the RC spectra as arising from the fine structure of the trion. We also propose that the line interpreted previously in PL spectra of WSe2 and WS2 as related to the biexciton emission is a superposition of the biexciton, trion and localized exciton emission. We find that with the temperature increase from 7 295 K the total PL intensity decreases moderately in WSe2 and WS2, strongly in MoS2 and dramatically in MoSe2.

title: Kinetics of the excitonic radiative recombination in WS2:Br2 and MoS2:Cl2 layered crystals
abstract: Due to their layered structure, 2H layered transition metal dichalcogenide indirect bandgap compounds MoS2, WS2 can be intercalated with foreign atoms and molecules. The 2H MoS2, 2H WS2 and 2H WSe2 crystals intercalated with halogen molecules at low temperatures exhibit a strong luminescence due to the bound excitons recombination. In this work, the results of comparative study of the excitonic luminescence kinetics for 2H WS2:Br2 and 2H MoS2:Cl2 single crystals are presented. A simple kinetic model describing the temperature dependencies of the steady state luminescence intensity and of the radiative decay time is proposed. The lifetimes of the excitonic levels and the activation energy of the excitonic luminescence quenching caused by an extrinsic self trapping process for both studied compounds were determined.

title: Anti Stokes Photoluminescence of Monolayer WS 2
abstract: Anti Stokes photoluminescence excitation of a WS2 monolayer flake between 10 and 300 K is reported herein. Even with continuous wave lasers at low power, the emission of the exciton at excitation 100 meV below its emission energy at room temperature is observed. A mechanism which involves the trions as the intermediate state is proposed, leading to an efficient up conversion process. In addition, it is demonstrated that phonons are the source of the additional energy needed by the system. Overall, the results provide evidence that anti Stokes luminescence in transition metal dichalcogenides is very efficient.

title: Vertical Bridgman growth and low temperature luminescence properties of NaY(MoO4)2 crystal
abstract: NaY(MoO4)2 crystal was investigated as a promising scintillator for neutrinoless double beta decay experiment. In this study, NaY(MoO4)2 crystal was grown by the Vertical Bridgman method using spontaneous crystallization. The as grown NaY(MoO4)2 crystal appeared dark blue in color. The powder X Ray Diffraction, optical transmission spectra at room temperature, luminescence properties and decay time from room temperature to 10 K excited by a 280 nm light emitting diode source were investigated. The transmittance of the annealed NaY(MoO4)2 crystal sample with the thickness of 3 mm was about 80% The NaY(MoO4)2 crystal did not luminescence from room temperature to 200 K and the emission spectra were observed down from 175 K. With the temperature decreasing, the luminescence light yield increased and had maximum peak at 10 K. Compared to Li2MoO4 crystal, the luminescence light yield of NaY(MoO4)2 was much higher at 10 K. The longest average decay time was observed at 10 K (425 ms) and the shortest average decay time was observed at 175 K (88 ms) NaY(MoO4)2 crystal has a higher light yield at cryogenic temperature and no any heavy elements, so it is a promising candidate as a cryogenic detector for investigating the neutrinoless double beta decay (0ubb) mechanism.

title: Edge Site Nanoengineering of WS2 by Low Temperature Plasma Enhanced Atomic Layer Deposition for Electrocatalytic Hydrogen Evolution
abstract: Edge enriched transition metal dichalcogenides, such as WS2, are promising electrocatalysts for sustainable production of H2 through the electrochemical hydrogen evolution reaction (HER) The reliable and controlled growth of such edge enriched electrocatalysts at low temperatures has, however, remained elusive. In this work, we demonstrate how plasma enhanced atomic layer deposition (PEALD) can be used as a new approach to nanoengineer and enhance the HER performance of WS2 by maximizing the density of reactive edge sites at a low temperature of 300 degC. By altering the plasma gas composition from H2S to H2 H2S during PEALD, we could precisely control the morphology and composition and, consequently, the edge site density as well as chemistry in our WS2 films. The precise control over edge site density was verified by evaluating the number of exposed edge sites using electrochemical copper underpotential depositions. Subsequently, we demonstrate the HER performance of the edge enriched WS2 electrocatalyst, and a clear correlation among plasma conditions, edge site density, and the HER performance is obtained. Additionally, using density functional theory calculations we provide insights and explain how the addition of H2 to the H2S plasma impacts the PEALD growth behavior and, consequently, the material properties, when compared to only H2S plasma.

title: WS2 and MoS2 thin film gas sensors with high response to NH3 in air at low temperature.
abstract: Transition metal dichalcogenides (TMDs) have received immense research interest in particular for their outstanding electrochemical and optoelectrical properties. Lately, chemical gas sensor applications of TMDs have been recognized as well owing to the low operating temperatures of devices, which is a great advantage over conventional metal oxide based sensors. In this work, we elaborate on the gas sensing properties of WS<sub>2</sub> and MoS<sub>2</sub> thin films made by simple and straightforward thermal sulfurization of sputter deposited metal films on silicon chips. The sensor response to H<sub>2</sub> H<sub>2</sub>S, CO and NH<sub>3</sub> analytes in air at 30degC has been assessed and both MoS<sub>2</sub> and WS<sub>2</sub> were found to have an excellent selectivity to NH<sub>3</sub> with a particularly high sensitivity of 0.10 0.02 ppm<sup> 1</sup> at sub ppm concentrations in the case of WS<sub>2</sub> The sensing behavior is explained on the bases of gas adsorption energies as well as carrier (hole) localization induced by the surface adsorbed moieties having reductive nature.

title: Photoelectrochemical application of WS2 nanosheets prepared via a low temperature CVD method
abstract: We report the low temperature preparation of WS2 nanosheets by chemical vapor deposition (CVD) method for photoelectrochemical (PEC) water splitting application. The WS2 nanosheets were synthesized onto the FTO, ITO, SiO2, and glass substrates at a relatively low temperature of 500 degC under pressure of 10 2 Torr. Compared to temperatures of conventional WS2 synthesis via CVD method, the deposition was performed at a sufficiently low temperature, which has not been reported for the layer deposition yet. The samples were characterized by X ray diffraction (XRD) scanning electron microscopy (SEM) and ultraviolet visible (UV visible) spectroscopy. The results show that WS2 nanosheets were grown almost vertically and alike a single crystal. Based on the SEM images, the WS2 nanosheets grew homogenously on the FTO substrate and covered the entire surface of the substrate. Linear sweep voltammetry (LSV) applied bias photon to current efficiency (ABPE) photocurrent response, and electrochemical impedance spectroscopy (EIS) of the fabricated nanosheets were measured for the photoelectrochemical response. Because of no transferring to damage layer, the results showed that WS2 nanosheets on FTO substrate had good photoelectrochemical activity. The measured photocurrent of 32 uAcm 2 and ABPE efficiency of 0.84% at a 0.75 V bias (versus RHE) was considerably high compared to the other results reported for WS2 nanosheets.

title: Low temperature luminescence and thermoluminescence from BeO:Zn single crystals
abstract: Low temperature luminescence and thermoluminescence (TL) of BeO:Zn single crystals have been studied in the temperature range of 6 380 K and energy ranges of 1.2 6.5 eV (emission spectra) and 3.7 20 eV (luminescence excitation and reflection spectra) The introduction of zinc impurity ions (0.05 at. into BeO host lattice leads to the creation of both the trapped electron and hole centers: Zn and Zn 2+ O These two new centers are responsible for two TL glow peaks at 307 and 145 K with activation energies of 0.96 and 0.40 eV, and two emission bands at 6.0 and 1.9 2.6 eV. The first emission band is attributed to radiative annihilation of the Zn impurity bound excitons, and the second one is associated with the intracenter electronic transitions in the defect complex comprising zinc impurity ion. The 6.0 eV luminescence center can be excited at 9.6 eV, the low energy tail of the BeO host absorption, but below the first excitonic maximum (10.45 eV) The 1.9 2.6 eV luminescence center can be excited at the BeO optical transparency band. Both emission bands in BeO:Zn appear in the X ray induced luminescence spectra at T 6 K. This indicates that not only these luminescence centers are excited during band to band transitions, but they participate in recombination processes as well. The low temperature T 0 6 K) TL study of BeO:Zn single crystals was made for the first time. Analysis of the low temperature TL glow curves allowed us not only to experimentally determine the energy characteristics of the Zn impurity states in BeO:Zn, but reveal an extremely strong influence of the isovalent zinc impurity on fluctuation rearrangement of BeO host lattice. Note, the fluctuation rearrangement of BeO host lattice, which occurs in the temperature range of self trapped exciton transformation (80 180 K) was previously known only for undoped BeO and BeO crystals with heterovalent impurities [I. N. Ogorodnikov and A. V. Kruzhalov, Proc. SPIE 2967 (1997) 42]

title: Thermoluminescence and low temperature luminescence of beryllium oxide
abstract: Beryllium oxide in the forms of either single crystals (pristine, additively colored) or hot pressed ceramic samples was studied in the energy range of 1.2 6.2 eV using both the thermoluminescence (TL) and steady state X ray induced luminescence (XRL) techniques. The XRL emission spectra were recorded at 6 and 293 K, whereas TL glow curves were studied after X ray exposure at T 0 6 K upon linear heating in the temperature range from 6 to 293 K. A search for TL manifestations of shallow trapping centers was carried out using a sensitive channel for TL registration in the range of more than six decades of change in intensity. The participation of shallow trapping centers in the process of recombination luminescence excitation at 6 293 K; branching electronic excitations between different recombination channels; the dominance of the self trapped exciton and F center emissions in spectra of the low temperature recombination luminescence in BeO at 6 293 K were discussed.