Micro-nano Devices Fabrication And Electrical And Optical Properties Of Low-dimensional WTe₂ And GaTe

Recently,the emerging physical phenomena found in low-dimensional material have triggered broad investigations.Among them,nanoelectronics in WTe2 devices have attracted particular attentions due to its exotic band structure and GaTe has been considered as a candidate for radiation detection given its relatively high photoresponsivities,intermediate bandgap and good transport properties.However,both WTe2 and GaTe are obviously environmentally unstable,which will restrict their application in the field of electronics and optoelectronics.Most of the reports on WTe2 are based on bulk form,the thickness dependence of its quantum transport properties in the two-dimensional limit remains under debate.Here,single crystal WTe2 was prepared via the self-flux method,then we show systematic performance of low temperature quantum electronic transport of atomically thin WTe2.It is observed that aging induced localized electronic states explains the low temperature Coulomb gap in transport measurements,leading to the anomalous magnetotransport which appears to be extrinsic.While few-layered WTe2 showed clear metallic tendency in the fresh state,degraded devices exhibited a re-entrant insulating behavior at the begining,and entered a fully insulating state subsequently.Correspondingly,a crossover from parabolic to linear magnetoresistance,and,upon further aging,leads to the appearance of weak anti-localization.The environmental instability of the few-layered WTe2 was investigated by optical microscopy,atomic force microscopy,scanning electron microscopy,and Raman spectroscopy.It was found the lifetime of the four-layered WTe2 in air was 62 minutes.Further systematic Raman study on WTe2 vary thickness,temperature,and laser wavelength was done,demonstrating that there is no strong electron-phonon coupling or phonon-phonon coupling in this system.In addition,we used high-resolution transmission electron microscopy to compare the freshly peeled few-layered WTe2 and the sample after heating in air at 200 ? for 5 minutes.It was found that the fresh sample had obvious lattice characteristics and the thin part exhibiting typical amorphous features after heating was degraded.However,the energy dispersion spectrum before and after heating show that the samples do not contain any oxygen element,excluding the possibility of oxygen or water reacting with the sample.Our study reveals for the first time the correlation between the unusual magnetotransport and disorder in few-layered WTe2.The high-quality monoclinic GaTe single crystal was prepared.For the first time,the Raman spectrum of single layer and double layer GaTe was observed experimentally.Photoluminescence spectroscopy shows that the energy gap of bulk GaTe is 1.655 eV,and it decreases with the increase of incident laser power.The energy gap of a few layered GaTe is 1.675 eV,larger than that of bulk one.The thickness dependent Raman was observed,and the polarized Raman shows that there are obvious in-plane optical anisotropies in GaTe,which is essential for its applications in optoelectronics.Electrical measurement of the devices covered with BN revealed that GaTe had typical p-type semiconductor features and the curve was smoother and the current density was greatly improved compared to the curve without a protective BN.The IV curve exhibits a very good linear relationship,consistent with the description of Ohm's law,indicating a good ohmic contact.