Gallium Thiophosphate: An Emerging Bidirectional Auxetic Two-Dimensional Crystal with Wide Direct Band Gap

Abstract

Two-dimensional (2D) materials with negative Poisson’s ratio (NPR) attract considerable attention because of their exotic mechanical properties. We propose a new 2D material, monolayer GaPS4, which shows NPR for both in-plane (−0.033) and out-of-plane (−0.62) directions. Such coexistence of NPR in two distinct directions could be explained by its corner- and edge-shared tetrahedra pucker structure. GaPS4 has an ultralow cleavage energy of 0.23 J m–2 according to our calculation, such that exfoliation of the bulk material is feasible for the preparation of mono- and few-layer GaPS4. Direct wide band gap of 3.55 eV and moderate electron mobility have been revealed in monolayer GaPS4, while the direct gap feature is robust within a strain range of −6% to 6%. These findings render 2D GaPS4 a promising candidate for applications in nanoelectronics and low-dimensional electromechanical devices.

Cross‐Substitution Promoted Ultrawide Bandgap up to 4.5 eV in a 2D Semiconductor: Gallium Thiophosphate

“…The cleavage energy is calculated using the formula E cl = (E − E 0 )/S with the value of 0.15 J m −2 in Figure S1 (Supporting Information), where S is the unit surface area, E and E 0 are the total energy in the cleaved and uncleaved structures, respectively. The value of cleavage energy is close to the previously reported value of 0.23 J m −2 , [60] which is even lower than that of graphene (0.32 J m −2 ) [61] and black phosphorus (0.37 J m −2 ), [62] indicating a weak interlayer interaction, and the exfoliation of 1L GaPS 4 from the bulk structure will be easily achieved.…”

“…All of the peaks are associated with Ga 3+ in various crystals, [ 63 ] demonstrating that Ga atoms are surrounded by S atoms. [ 60 ] In addition, the peaks at around 131.9 and 133.1 eV are assigned to P 2p 3/2 and P 2p 1/2 , respectively. The other two peaks located at 162.0 and 163.2 eV represent the binding energy of S 2p 3/2 and S 2p 1/2 , respectively, which confirms the existence of the S 2− state.…”

“…Whereas, the bulk GaPS 4 exhibits a 3.561 eV direct bandgap with the VBM and CBM both located at B‐point (−0.5, 0, 0), which agrees well with the reported value based on HSE06. [ 60 ] The increasing bandgap with the decreased layer number is the result of the quantum confinement effect. It is worth noting that all these bandgap values, even though based on the hybrid functional DFT calculation, are slightly smaller than the bandgaps extracted from experiments in Figure 3.…”

Giant negative Poisson's ratio in two-dimensional V-shaped materials

“…17 After the first realization of 2D auxetic effects in graphene with thermally-induced ripples, 18 a number of 2D materials have been reported to have NPR. [19][20][21][22][23][24][25][26][27][28][29][30] For example, monolayer black phosphorene with a buckled structure was predicted to have NPR of about -0.027 which has been confirmed in experiments. 11,19 Generally, the auxetic behaviors of 2D materials are correlated to the geometric structures, the deformation mechanisms of the materials dominated by the inter-atomic interaction.…”

“…Some special geometric configurations, such as re-entrant and hinged structures, have inherent auxetic features independent of atomic interactions. [20][21][22][23][24][25][26][27][28] While in other cases, the auxetic behaviors are relevant to the electronic structures of 2D materials which determine the atomic interactions, rather than the geometry. [29][30][31][32] Strong auxetic effect will greatly improve the mechanical performance of materials, such as dentation resistance or hardness.…”