Ultrathin 2D GeSe2 Rhombic Flakes with High Anisotropy Realized by Van der Waals Epitaxy

As a layered p-type semiconductor with a wide bandgap of 2.7 eV, GeSe2 can compensate for the rarity of p-type semiconductors, which are desired for the production of high-integration logic circuits with low power consumption. Herein, ultrathin 2D single crystals of beta-GeSe2 are produced using van der Waals epitaxy and halide assistance; each crystalline flake is approximate to 7 nm thick and shaped as a rhombus. The optical and electrical properties of the flakes are studied systematically, and the temperature-dependent Raman spectra of the flakes reveal that the intensity of the Raman peaks decrease with increasing temperature. Low-temperature electrical measurements suggest that the variable-range hopping model is best for describing the electrical transport at 20-180 K; meanwhile, optical-phonon-assisted hopping can account for the transport behavior at 180-460 K. Impressively, the angle-resolved polarized Raman measurements indicate strong in-plane anisotropy of the rhombic GeSe2 flake under a parallel polarization configuration, which may result from the low symmetry of the monoclinic crystal structure of GeSe2. Furthermore, a photodetector based on a rhombic GeSe2 flake is constructed and shown to exhibit a high responsivity of 2.5 A W-1 and a fast response of approximate to 0.2 s.