Anisotropy of the raman and infrared optical activity on layered metal monochalcogenides: GaSe and GeSe

Abstract

Recent developments in the area of two-dimensional (2D) materials have given new impetus to the studies of nanomaterials. Thence, since the discovery of graphene by Novoselov and Geim in 2004, researches on these materials have increased quickly. Graphene has mechanical, electrical, thermal, and optical unique physical properties with promising potential in technological applications. However, diverse electronic and optoelectronic devices require a band gap, and the quest for new low-dimensional semiconductors with band gap in the infrared to visible electromagnetic range is an active field. Several other two-dimensional materials, such as transition metal dichalcogenides (TMD’s), post-transition metal monochalcogenides, and the group IV monochalcogenides, have been subjects of research aimed at their promising technological applications. This work aims to study the optical properties of two of these materials: the posttransition metal monochalcogenide gallium selenide (GaSe), and the one belonging to the group IV monochalcogenide, germanium selenide (GeSe). The GaSe is a layered semiconductor with lubricating, optoelectronic, photovoltaic and non-linear optics properties, potentially promising in technological applications. However it is still little explored in terms of strain engineering. This work theoretically studies the evolution of active optical modes in Raman and infrared spectroscopies for a GaSe monolayer when subjected to an uniaxial strain. The obtained results clarify some fundamental aspects of strain engineering for the development of nanostructured strain sensors and, through Raman spectroscopy, bring a fast and reliable strategy to identify the structure and crystallographic orientation of the sample. The results for symmetry dependent properties, as mode symmetry assignment and selection rules for Raman and infrared activity, can be extended to all isostructural materials. The GeSe is another layered semiconductor, but with an intrinsic orthorhombic structure. This anisotropic symmetry has attracted particular interest to the materials of its family due to, for example, the expected anisotropic optical response, although the literature still lacks work in this field for the GeSe. This work investigates the anisotropic optical response of the GeSe by polarized Raman scattering in a thin flake. The results, obtained by experimental measurements, show the angular dependence of the Raman active modes, as well as their agreement with results of the theoretical model. This work provides a new tool to obtain the crystallographic orientation of the GeSe and increase the knowledge about its optical response, fundamental prerequisites for both this research field and future technological applications.