Ultra-weak interlayer coupling in two-dimensional gallium selenide

Abstract

Beyond-graphene two-dimensional (2D) materials are envisioned as the future technology for optoelectronics, and the study of group IIIA metal monochalcogenides (GIIIAMMs) in 2D form is an emerging research field. Bulk gallium selenide (GaSe) is a layered material of this family which is widely used in nonlinear optics and is promising as a lubricant. The interlayer coupling in few-layer GaSe is currently unknown, and the stability of different polytypes is unclear. Here we use symmetry arguments and first-principles calculations to investigate the phase stability, interlayer coupling, and the Raman and infrared activity of the low-frequency shear and breathing modes expected in few-layer GaSe. Strategies to distinguish the number of layers and the β and ε polytypes are discussed. These symmetry results are valid for other isostructural few-layer GIIIAMM materials. Most importantly, by using a linear chain model, we show that the shear and breathing force constants reveal an ultra-weak interlayer coupling at the nanoscale in GaSe. These results suggest that β and ε few-layer GaSe show similar lubricant properties to those observed for few-layer graphite. Our analysis opens new perspectives about the study of interlayer interactions and their role in the mechanical and electrical properties of these new 2D materials.

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

“…The crystal structure of the GaSe crystals was evaluated by X‐ray diffraction (XRD) measurements. The XRD pattern (Figure S2, Supporting Information) agrees with the JCPDS 37–931 card, indicating that the as‐synthetized GaSe crystals are in the form of the lowest energy polytype, i.e., the hexagonal ε‐GaSe (space symmetry group: Pm‐), in agreement with previous literature reporting similar GaSe crystal syntheses . The GaSe nanoflakes were produced by LPE of the synthetized crystals in anhydrous IPA followed by sedimentation‐based separation (SBS) to remove un‐exfoliated crystals by ultracentrifugation (see Experimental Section, Supporting Information).…”

“…[78,101] The GaSe nanoflakes were produced by LPE [80,81] of the synthetized crystals in anhydrous IPA followed by sedimentation-based separation (SBS) [104,105] to remove un-exfoliated crystals by ultracentrifugation (see Experimental Section, Supporting Information). Noteworthy, first principle calculations estimated a weak interlayer coupling in GaSe crystals (i.e., cleavage energy ≈0.33 J m −2 ), [103] in agreement with our DFT calculations (cleavage energy of ≈0.29 J m −2 for xL-GaSe with x ≥2), indicating their feasible exfoliation similar to other layered materials, e.g., graphite (cleavage energies of ≈0.37 J m −2 , experimental value) [98] and MoS 2 (theoretical cleavage energy of 0.27 J m −2 ). [97] The use of IPA as solvent has been reported to be effective for exfoliating another Ga-based group-III monochalcogenides (i.e., GaS), [106] as well as other transition metal monochalcogenides (e.g., InSe [107] ) and dichalcogenides (e.g., MoS 2 , [108][109][110] MoSe 2 , [111,112] NbS 2 [113] ).…”

Recent Advances in Quantum Effects of 2D Materials

“…Semimetal chalcogenides, such as InSe and GaSe, are a group of layered materials similar to TMDs . Recently, the widely researched semiconducting InSe is in its γ phase with a rhombohedral structure .…”

“…also mentioned that the PL quenching with decreasing thickness cannot be attributed to the direct‐indirect‐gap crossover, since the energy difference with respect to the Γ‐point is only half of the thermal energy in room temperature . Bulk GaSe is a relatively wide, indirect bandgap semiconductor ( E g ≈2.05 eV at room temperature), which previously attracted some interest because of its nonlinear optical properties . As a 2D material, the QCEs in GaSe are not as significant as in the aforementioned materials, that is, the PL emission peak only shows a slight blue‐shift of ≈30 meV with decreasing thickness from bulk to bilayer …”

“…Semimetal chalcogenides, such as InSe and GaSe, are a group of layered materials similar to TMDs. [14,[32][33][34][35][36][37][38][39] Recently, the widely researched semiconducting InSe is in its γ phase with a rhombohedral structure. [40] As shown in Figure 2c, the primitive unit cell of γ -InSe consists of three layers, each composed of four covalently bonded Se-In-In-Se atomic planes.…”