Oxidation dynamics of ultrathin GaSe probed through Raman spectroscopy

Abstract

Gallium selenide (GaSe) is a 2D material with a thickness-dependent gap, strong non-linear optical coefficients and uncommon interband optical selection rules, making it interesting for optoelectronic and spintronic applications. In this work, we monitor the oxidation dynamics of GaSe with thicknesses ranging from 10 to 200 nm using Raman spectroscopy. In ambient temperature and humidity conditions, the intensity of all Raman modes and the luminescence decrease rapidly with moderate exposure to above-gap illumination. Concurrently, several oxidation products appear in the Raman spectra: Ga 2 Se 3 , Ga 2 O 3 and amorphous and crystalline selenium. We find that no safe measurement power exists for optical measurements on ultrathin GaSe in ambient conditions. We demonstrate that the simultaneous presence of oxygen, humidity, and abovegap illumination is required to activate this photo-oxidation process, which is attributed to the transfer of photo-generated charge carriers towards aqueous oxygen at the sample surface, generating highly reactive superoxide anions that rapidly degrade the sample and quench the optical response of the material.

Solution‐Processed GaSe Nanoflake‐Based Films for Photoelectrochemical Water Splitting and Photoelectrochemical‐Type Photodetectors

“…Additionally, the Raman spectra of GaSe crystals and exfoliated flakes do not present signatures attributed to Ga 2 Se 3 , Ga 2 O 3 , and amorphous/crystalline Se (a‐/c‐Se) modes, which are observed at ≈155, ≈200, and between 135–160 cm −1 , respectively. This indicates that the LPE of GaSe crystals in anhydrous IPA does not cause significant additional surface oxidation of the native material . This conclusion is further supported by X‐ray photoelectron spectroscopy (XPS) analysis (Figures S4 and S5 and Tables S2–S7, Supporting Information).…”

“…Despite these encouraging driving factors, the PEC properties of GaSe are still experimentally uncharted. A certain reluctance to study the (photo)electrochemical properties of GaSe and other group‐III monochalcogenides undoubtedly originated from their tendency to undergo surface oxidation . The latter can occur either in a two‐step reaction: 1) GaSe + 1/4O 2 = 1/3Ga 2 Se 3 + 1/6Ga 2 O 3 followed by Ga 2 Se 3 + 3/2O 2 = Ga 2 O 3 + 3Se; or in a single‐step reaction: 2) GaSe + 3/4O 2 = 1/2Ga 2 O 3 + Se .…”

“…The latter can occur either in a two‐step reaction: 1) GaSe + 1/4O 2 = 1/3Ga 2 Se 3 + 1/6Ga 2 O 3 followed by Ga 2 Se 3 + 3/2O 2 = Ga 2 O 3 + 3Se; or in a single‐step reaction: 2) GaSe + 3/4O 2 = 1/2Ga 2 O 3 + Se . The presence of humidity and light has been reported to accelerate further the surface oxidation of GaSe . However, it is worth noticing that the material oxidation process can be controlled by properly adjusting (photo)electrochemical conditions such as potential, pH and the dissolved O 2 concentration.…”

“…A certain reluctance to study the (photo) electrochemical properties of GaSe and other group-III monochalcogenides undoubtedly originated from their tendency to undergo surface oxidation. [41,[70][71][72][73][74][75] The latter can occur either in a two-step reaction: 1) GaSe + 1/4O 2 = 1/3Ga 2 Se 3 + 1/6Ga 2 O 3 followed by Ga 2 Se 3 + 3/2O 2 = Ga 2 O 3 + 3Se; [76] or in a singlestep reaction: 2) GaSe + 3/4O 2 = 1/2Ga 2 O 3 + Se. [77] The presence of humidity and light has been reported to accelerate further the surface oxidation of GaSe.…”

Synthesis, Structure and Application of Intramolecularly‐Coordinated Gallium Chalcogenides: Suitable Single‐Source precursors for GaxSey Materials

“…The bands at ≈250 and ≈290 cm −1 represent longitudinal and transverse mode of the crystalline Ga 2 Se 3 and correspond to a group of at least three vibrational modes with various symmetry representations (A 1 , B 1 , B 2 ) that have been assigned to localized vibrational modes of vacancy‐disordered Ga 2 Se 3. Due to the lack of the signal at 115 cm −1 attributed to the Ga−O bond, we can deduce a low degree of oxidation, if any . The histogram of the hydrodynamic diameter D H of the re‐dispersed powder has bimodal character with the higher population at the 500–600 nm (60 % of the cumulative intensity) (Figure S9).…”