Quantum size confinement in gallium selenide nanosheets: band gap tunability versus stability limitation

Abstract

Gallium selenide is one of the most promising candidates to extend the window of band gap values provided by existing two-dimensional semiconductors deep into the visible potentially reaching the ultraviolet. However, the tunability of its band gap by means of quantum confinement effects is still unknown, probably due to poor nanosheet stability. Here, we demonstrate that the optical band gap band of GaSe nanosheets can be tuned by ∼120 meV from bulk to 8 nm thick. The luminescent response of very thin nanosheets (<8 nm) is strongly quenched due to early oxidation. Oxidation favors the emergence of sharp material nanospikes at the surface attributable to strain relaxation. Simultaneously, incorporated oxygen progressively replaces selenium giving rise to GaO with a residual presence of GaSe that tends to desorb. These results are relevant for the development and design of visible/ultraviolet electronics and optoelectronics with tunable functionalities based on atomically thin GaSe.

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

“…For the aforementioned properties, GaSe has been proposed for photodetectors with high responsivity (e.g., up to values > 1000 A W −1 at light intensity ≤ 0.1 mW cm −2 , in heterojunction with graphene), nonlinear frequency generation (e.g., second and third harmonic and ultrabroadband radiation generation), spin polarization control (e.g., spintronic logic devices), light‐emitting devices, optical microcavities, and saturable absorbers . Moreover, the number of layers and strain engineering strongly affect the GaSe optoelectronic properties, which can be on‐demand tailored to fulfill the requirements of the final applications . In particular, theoretical calculations demonstrated a c ‐axis confinement‐induced bandgap ( E g ) blueshift, which resembles the behavior of transition metal dichalcogenides .…”

“…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 .…”

“…Gallium selenide (GaSe) is a layered pseudodirect optical bandgap (direct transition positioned just above the indirect one) binary chalcogenide (namely, a group-III properties, [39][40][41][42] which can be on-demand tailored to fulfill the requirements of the final applications. [42,43] In particular, theoretical calculations demonstrated a c-axis confinement-induced bandgap (E g ) blueshift, which resembles the behavior of transition metal dichalcogenides.…”

“…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.…”

Oxidation dynamics of ultrathin GaSe probed through Raman spectroscopy

“…Nonetheless, bulk GaSe is also known to naturally form a native oxide 22 and thermally-and photo-induced oxidation has been reported [23][24][25] . Recently, exposure to intense laser light was found to degrade optical properties and lead to chemical transformations [26][27][28] . Given the interest for few-layer GaSe for electronic and optoelectronic applications, it appears critical to examine the oxidation dynamics of ultrathin GaSe and determine the conditions in which this oxidation can be suppressed or at least minimized.…”