Signatures for coexistence of monoclinic and hexagonal phases in GaTe nanoflakes

The burgeoning two-dimensional (2D) layered materials provide a powerful strategy to realize efficient light-emitting devices. Among them, gallium telluride (GaTe) nanoflakes, showing strong photoluminescence (PL) emission from multilayer to bulk crystal, relax the stringent fabrication requirements of nanodevices. However, detailed knowledge on the optical properties of GaTe varies as layer thickness is still missing. Here we perform thickness-dependent PL and Raman spectra, as well as temperature-dependent PL spectra of GaTe nanoflakes. Spectral analysis reveals a spectroscopic signature for the coexistence of both the monoclinic and hexagonal phases in GaTe nanoflakes. To understand the experimental results, we propose a crystal structure where the hexagonal phase is on the top and bottom of nanoflakes while the monoclinic phase is in the middle of the nanoflakes. On the basis of temperature-dependent PL spectra, the optical gap of the hexagonal phase is determined to be 1.849 eV, which can only survive under temperature higher than 200 K with the increasing phonon population. Furthermore, the strength of exciton-phonon interaction of the hexagonal phase is estimated to be 1.24 meV/K. Our results prove the coexistence of dual crystalline phases in multilayer GaTe nanoflakes, which may provoke further exploration of phase transformation in GaTe materials, as well as new applications in 2D light-emitting diodes and heterostructure-based optoelectronics.