Strain‑Engineered GaTe/C₂N Heterostructures: Tunable Band Alignment, Enhanced UV–Visible Absorption, and Photocatalytic Water Splitting

Abstract

GaTe and C₂N monolayers, recently synthesized, exhibit intriguing electronic and optical characteristics. Here we employ first‑principles density functional theory to investigate the structural, electronic, and optical behavior of a GaTe/C₂N van der Waals (vdW) heterostructure. The heterostructure is an indirect‑gap semiconductor with a type‑II band alignment that promotes efficient photocarrier separation. Its visible–UV absorption surpasses that of the constituent monolayers, and vertical strains can tune it into an effective photocatalyst for water splitting at specific pH values. We also examine water adsorption and dissociation on the C₂N surface, revealing energetically favorable pathways for hydrogen evolution. In‑plane biaxial strain further modulates the band gap and transitions between semiconductor, metallic, and different band‑alignment types. These findings position the GaTe/C₂N heterostructure as a versatile platform for next‑generation optoelectronic and photocatalytic devices.

Background

Since the discovery of graphene, the exploration of two‑dimensional (2D) materials has accelerated, driven by their unique electronic, optical, and mechanical properties. Transition‑metal dichalcogenides, group V honeycomb sheets, III‑V binaries, and post‑transition‑metal chalcogenides (PTMCs) have all shown promise in electronics and energy conversion. GaTe, a PTMC, was fabricated via molecular beam epitaxy and is an indirect‑gap semiconductor with a strain‑tunable band gap. C₂N, a recently realized 2D porous crystal, is a direct‑gap semiconductor with uniform pores that facilitate charge separation. However, in monolayer form, both materials suffer from rapid carrier recombination, limiting their photocatalytic and photovoltaic efficiency. Stacking dissimilar 2D layers into vdW heterostructures offers a route to engineer band alignment, reduce recombination, and create new functionalities. Previous GaTe‑based heterostructures (e.g., GaTe/InSe, GaTe/GaSe, GaTe/SnI) have demonstrated type‑II alignment, quantum spin Hall behavior, and Rashba splitting. Likewise, semiconductor/C₂N hybrids (g‑C₃N₄/C₂N, MoS₂/C₂N, CdS/C₂N) have enhanced photocatalytic activity by efficient charge separation.

In this study, we construct a GaTe/C₂N vdW heterostructure and, through hybrid HSE06 calculations, explore its structural, electronic, optical, and photocatalytic properties, including strain‑dependent behavior.

Methods

First‑principles calculations were performed using VASP with a plane‑wave cutoff of 500 eV and the PBE pseudopotential. Hybrid HSE06 corrections were applied to band‑gap predictions. vdW interactions were captured via Grimme’s DFT‑D2. A 25 Å vacuum along z avoided interlayer interactions. Brillouin‑zone sampling used a 21×21×1 k‑mesh for PBE and 11×11×1 for HSE06. Geometry optimizations converged to 10⁻⁵ eV in energy and 0.01 eV/Å in forces.

Results and Discussion

Monolayer Benchmarks

Optimized GaTe and C₂N monolayers (Fig. 1a,b) exhibit lattice constants 4.14 Å (GaTe) and 8.26 Å (C₂N), with Ga–Te and C–N bonds of 2.41 Å and 1.34 Å, respectively. HSE06 predicts indirect band gaps of 2.13 eV for GaTe and 2.44 eV for C₂N, consistent with prior reports. The band dispersions are reliable, justifying their use as building blocks.

Heterostructure Construction

Combining a 2×2 GaTe supercell with a 1×1 C₂N cell yields a minimal 0.48 % lattice mismatch. Three high‑symmetry stackings—α, β, γ—were evaluated; γ is marginally most stable (binding energy −15.80 meV/Å²). Phonon spectra show all positive modes, and ab‑initio MD confirms thermal stability at 300 K. Band structures across stackings are nearly identical; we focus on the γ configuration.

Electronic Structure and Band Alignment

The γ heterostructure has an indirect band gap of 1.38 eV. Projected densities of states reveal the valence‑band maximum (VBM) dominated by Te/Ga p states, while the conduction‑band minimum (CBM) originates from C/N p states. Band‑decomposed charge densities confirm electron–hole separation: electrons localize on C₂N, holes on GaTe, establishing a type‑II alignment with VBO = 1.03 eV and CBO = 0.72 eV. The intrinsic built‑in electric field (E_in) further suppresses recombination, enhancing optoelectronic performance.

Optical Absorption

Compared to the monolayers, the heterostructure displays markedly stronger visible‑UV absorption (Fig. 3), especially between 2.20 and 4.71 eV, due to interlayer charge‑transfer transitions. This broadband response is advantageous for photodetectors and solar energy harvesting.

Strain Engineering

Vertical (normal) strain alters interlayer spacing. Compression reduces the band gap, while tension first increases it modestly before saturating near Δd ≈ 0.8 Å. Type‑II alignment persists across the range, and large tensile strains shift the VBM below the O₂/H₂O oxidation potential, rendering the system suitable for water splitting at acidic pH (e.g., pH = 2).

In‑plane biaxial strain (± 6 %) modulates the band gap and induces transitions: an indirect‑direct‑indirect (Ind‑D‑Ind) sequence near − 3 % and − 8 %, a semiconductor‑to‑metal transition at − 12 %, and band‑alignment shifts from type‑II to type‑I (ε ≥ + 6 %) or type‑III (ε ≤ − 12 %). These transitions arise from differential strain sensitivities of GaTe and C₂N, offering a tunable platform for electronic, optoelectronic, and catalytic applications.

Photocatalytic Water Splitting

Adsorption energies for H, OH, and H₂O on the C₂N surface are −1.03, −0.51, and −0.56 eV, respectively, indicating favorable binding. Water dissociation is endothermic (ΔE ≈ 1.48 eV), but the subsequent formation and release of H₂ require only 0.04 eV, facilitating hydrogen evolution. Combined with the strain‑induced band alignment, the heterostructure offers a viable pathway for photocatalytic hydrogen production.

Conclusions

Hybrid DFT calculations reveal that the GaTe/C₂N vdW heterostructure is an indirect‑gap semiconductor with type‑II alignment, enhanced visible‑UV absorption, and an intrinsic electric field that suppresses carrier recombination. Vertical tensile strain enables photocatalytic water splitting under acidic conditions, while in‑plane strain tunes the band gap and alignment, inducing semiconductor‑metal and band‑type transitions. These strain‑dependent properties position the GaTe/C₂N heterostructure as a promising candidate for multifunctional optoelectronic and photocatalytic devices.