Transition metal dichalcogenides (TMDs) exhibit promising catalytic properties for hydrogen generation, and several approaches including defect engineering have been shown to increase the active catalytic sites. Despite preliminary understandings in defect engineering, insights on the role of various types of defects in TMDs for hydrogen evolution catalysis are limited. Screw dislocation-driven (SDD) growth is a line defect and yields fascinating spiral and pyramidal morphologies for TMDs with a large number of edge sites, resulting in very interesting electronic and catalytic properties. The role of dislocation lines and edge sites of these spiral structures on their hydrogen evolution catalytic properties is unexplored. Here we show that the large number of active edge sites connected together by dislocation lines in the vertical direction for a spiral WS 2 domain results in exceptional catalytic properties toward hydrogen evolution reaction. A micro-electrochemical cell fabricated by photo-and electron beam-lithography processes is used to study the electrocatalytic activity of a single spiral WS 2 domain, controllably grown by chemical vapor deposition. Conductive atomic force microscopy studies show improved vertical conduction for the spiral domain, which is compared with monolayer and mechanically exfoliated thick WS 2 flakes. The obtained results are interesting and shed light on the role of SDD line defects, which contribute to large number of edge sites without compromising the vertical electrical conduction, on the electrocatalytic properties of TMDs for hydrogen evolution.

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Tunnelling recombination luminescence in CsI :Na crystals is studied by means of linear polarization, magnetic circular polarization, and optical detection of ESR either under or after X-ray irradiation. Results are well interpreted in terms of a self-trapped exciton (STE) scheme as the final emitting state of tunnelling recombination; here, in the present case with tunnelling, the population for each STE sublevel is derived from the thermal equilibrium spin populations of trapped electrons (NaO) and holes ( v k ) instead of the usual creation rate in UV light excitation case.La luminescence de recombinaison aprhs un transfert par effet tunnel est Btudibe dans un cristal de Cs1:Na & l'aide de la polarisation lineaire, de la polarisation magnetique circulaire et de la detection optique de la RPE soit sous irradiation par RX soit aprhs irradiation.

The emissions around 420 and 370 nm of Cs1:Na excited by the UV absorption associated with the Ka+ ion are studied by measuring the temperature dependence of decay times, the magnetic circular polarization, and ESR by optical detection. The results suggest that the 420 nm band is due to a loralized exciton adjacent to a substitutional E a + ion. The basic properties of the electronic structure and kinetics of the exciton are obtained. These results are discussed in relation with the self-trapped excitons in CsI.Les &missions B 420 et 370 nm, du CsI :Na excitk dans la rBgion d'absorption UV associbe a l'ion XJ+, sont ktudikes en mesurant 1'6volution des temps de ddclin en fonction de la tempCraturc, la polarisation niagnCtiquc circrilaire et la RPE par detection optique. Les r6sultats siiggkrrnt que 1s bande h 420 nm est due B un exciton loc,zlis8 stir un ion S a + en position substitutionnelle.Leu proprieths fondamentales de la structure Clcctroniquc ct la, cindtique de I'exciton sont detcrmin6es. Les rbsultats sont discutds en relation avec I'exciton autopikg6 dans lo CsI. IntrodurtionJn CsI crystals, the 355 and 200 nni eniission bands of the self-trapped escitons (STE) I n the previous paper [7], the absorption, excitation, and emission spectra associated with Na+ ions in Cs1:Na were investigated with both UV and X-ray excitation for 4.2 K < T < 300 K and for different doping rates of Na+. The study of emitting centers was done by measuring the temperature dependence of the decay tiiiies of 420 and 370 bands. Each of the former and the latter emissions was attributed to a single sort of exciton which is composed of singlet + triplet states and triplet state, respect ively .I n the present work, further experiments on those eniissions in CsI : Ka, especially the measurement of the magnetic circular polarization (BICP) and the electron spin resonance (ESR), have been carried out under UV excitation. The analysis of the esperiniental results gives the nature of the electronic structure of the localized excitons and leads to the proposal that the 420 band is due to an exciton localized at a substitutional Nit+ ion.Experimental procedures and results of absorption, excitation, and emission spectra,

The extraordinary mechanical properties of two-dimensional transition-metal dichalcogenides make them ideal candidates for investigating strain-induced control of various physical properties. Here we explore the role of nonuniform strain in modulating optical, electronic, and transport properties of semiconducting, chemical vapor deposited monolayer MoS2, on periodically nanostructured substrates. A combination of spatially resolved spectroscopic and electronic properties explore and quantify the differential strain distribution and carrier density on a monolayer, as it conformally drapes over the periodic nanostructures. The observed accumulation in electron density at the strained regions is supported by theoretical calculations which form the likely basis for the ensuing ×60 increase in field effect mobility in strained samples. Though spatially nonuniform, the pattern-induced strain is shown to be readily controlled by changing the periodicity of the nanostructures thus providing a robust yet useful macroscopic control on strain and mobility in these systems.

The incidence of intra-flake heterogeneity of spectroscopic and electrical properties in chemical vapour deposited (CVD) WS2 flakes is explored in a multi-physics investigation, via spatially resolved spectroscopic maps correlated with electrical, electronic and mechanical properties. The investigation demonstrates that the three-fold symmetric segregation of spectroscopic response, in topographically uniform WS2 flakes are accompanied by commensurate segmentation of electronic properties e.g. local carrier density and the differences in the mechanics of tip-sample interactions, evidenced via scanning probe microscopy phase maps. Overall, the differences are understood to originate from point defects, namely sulphur vacancies within the flake along with a dominant role played by the substrate. While evolution of the multi-physics maps upon sulphur annealing elucidates the role played by S-vacancy, substrate-induced effects are investigated by contrasting data from WS2 flake on Si and Au surfaces. Local charge depletion induced by the nature of the sample-substrate junction in case of WS2 on Au is seen to invert the electrical response with comprehensible effects on their spectroscopic properties. Finally, the role of these optoelectronic properties in preserving valley polarization, affecting valleytronic applications, in WS2 flakes is investigated via circular polarisation discriminated photoluminescence experiments. The study provides a thorough understanding of spatial heterogeneity in optoelectronic properties of WS2 and other two-dimensional transition metal chalcogenides, which are critical for device fabrication and potential applications.

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