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 β‐GeSe₂ are produced using van der Waals epitaxy and halide assistance; each crystalline flake is ≈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 ≈0.2 s.