In this work, we study the interlayer phonon vibration modes, the layer-numberdependent optical bandgap, and the anisotropic photoluminescence (PL) spectra of atomically thin rhenium diselenide (ReSe₂) for the first time. The ultralow frequency interlayer Raman spectra and the polarization-resolved high frequency Raman spectra in ReSe₂ allow the identification of its layer number and crystal orientation. Furthermore, PL measurements show the anisotropic optical emission intensity of the material with its bandgap increasing from 1.26 eV in the bulk to 1.32 eV in the monolayer. The study of the layer-number dependence of the Raman modes and the PL spectra reveals relatively weak van der Waal's interaction and two-dimensional (2D) quantum confinement in the atomically thin ReSe₂. The experimental observation of the intriguing anisotropic interlayer interaction and tunable optical transition in monolayer and multilayer ReSe₂ establishes the foundation for further exploration of this material in the development of anisotropic optoelectronic devices functioning in the near-infrared spectrum, which is important for many applications in optical communication and infrared sensing.

Highly sensitive phototransistors based on two-dimensional (2D) GaTe nanosheet have been demonstrated. The performance (photoresponsivity, detectivity) of the GaTe nanosheet phototransistor can be efficiently adjusted by using the applied gate voltage. The devices exhibit an ultrahigh photoresponsivity of 274.3 AW^-1. The detectivity of 2D GaTe devices is ~10¹² Jones, which surpasses that of currently-exploited InGaAs photodetectors (10¹¹-10¹² Jones). To reveal the origin of the enhanced photocurrent in GaTe nanosheets, theoretical modeling of the electronic structures was performed to show that GaTe nanosheets also have a direct bandgap structure, which contributes to the promotion of photon absorption and generation of excitons. This work shows that GaTe nanosheets are promising materials for high performance photodetectors.