Formation and Healing of Defects in Atomically Thin GaSe and InSe

Abstract

GaSe and InSe are important members of a class of 2D materials, the III-VI metal monochalcogenides, which are attracting considerable attention due to their promising electronic and optoelectronic properties. Here an investigation of point and extended atomic defects formed in mono-, bi-, and few-layer GaSe and InSe crystals is presented. Using state-of-the-art scanning transmission electron microscopy (STEM), it is observed that these materials can form both metal and selenium vacancies under the action of the electron beam. Selenium vacancies are observed to be healable; recovering the perfect lattice structure in the presence of selenium or enabling incorporation of dopant atoms in the presence of impurities. Under prolonged imaging, multiple point defects are observed to coalesce to form extended defect structures, with GaSe generally developing trigonal defects and InSe primarily forming line defects. These insights into atomic behavior could be harnessed to synthesize and tune the properties of 2D post transition metal monochalcogenide materials for optoelectronic applications.

The role of traps in the photocurrent generation mechanism in thin InSe photodetectors

“…20,33,51,54,56 Recently, both theoretical calculations and experimental scanning transmission electron microscopy (STEM) reported that the selenium vacancies V Se in InSe crystals can be passivated by chemical dissociation of O atoms at these sites. 20,22,51 Similarly, Po-Hsun et al demonstrated that the change of Raman and X-ray photoelectron spectra (XPS) of pristine InSe flakes exposed to ambient air is comparable to that observed when exposing InSe to dry oxygen atmosphere. The authors attribute these observations to the formation of a surficial InSe 1Àx O x layer that encapsulates the InSe beneath and promotes a long-term stability of thin InSe devices.…”

“…[43][44][45][46][47][48][49] Similarly, in the case of InSe, the defects related to In adatoms and Se vacancies, which can be related to the In-rich atmosphere in which high-quality InSe crytals are grown, 50,51 may play important roles during the photocurrent generation process in thin InSe photodetectors. 20,[50][51][52] Interestingly, the presence of defects in InSe crystals, especially selenium vacancies (V Se ), 22 is predicted to promote the physical adsorption and chemical dissociation of O 2 /H 2 O molecules and V Se can act as preferential sites for the adsorption of these molecules. 20,[52][53][54][55] This phenomenon can help in explaining the performance degradation reported in various studies on thin InSe devices.…”

Harnessing the Electron Beam to Study Reactions in Graphene Liquid Cells and Degradation in Sensitive 2D Materials

“…These materials have shown relatively high electron mobilities, strong second harmonic generation in monolayers and a direct bandgap in bulk form which transitions to a quasi-direct bandgap when reduced to the monolayer limit. [4] These properties have been successfully applied in few-layer photodetection and field effect devices and when used together in GaSe/InSe heterostructures have shown tunable photoemission from interlayer excitons. The performance of devices based on GaSe and InSe can be affected by the presence of impurities or by exposure to air, light, and/or moisture, yet, little has been done to characterise the structure of these defects.…”

“…The performance of devices based on GaSe and InSe can be affected by the presence of impurities or by exposure to air, light, and/or moisture, yet, little has been done to characterise the structure of these defects. We have used atomic resolution STEM together with electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy to characterise individual point and extended defects in these materials [4]. By analysing annular dark field (ADF) STEM images of mono-, bi-, and few-layer GaSe and InSe crystals we observe that these materials can form both metal and selenium vacancies under the action of the electron beam.…”

“…All scale bars: 1 nm. Adapted with permission from the American Chemical Society, reference [4], copyright 2019.…”