1：Experimental Design and Method Selection:

First-principles calculations based on density functional theory (DFT) using the projector-augmented wave method (PAW) as implemented in the Vienna Ab initio Simulation Package (VASP). The generalized gradient approximation (GGA) parametrized by Perdew, Burke, and Ernzerhof (PBE) with van der Waals correction proposed by Grimme (DFT-D2) is applied. The HSE06 hybrid functional is used to calculate electronic band structures to overcome bandgap underestimation in GGA-PBE calculations.

2：Sample Selection and Data Sources:

Single-layer Ga2X1X2 (Ga2SeTe, Ga2STe and Ga2SSe) and newly proposed α-Ga2S3/Ga2SSe-A, α-Ga2S3/Ga2SSe-B and α-Ga2S3/Ga2SSe-C van der Waals heterojunctions.

3：List of Experimental Equipment and Materials:

Vienna Ab initio Simulation Package (VASP) software for DFT calculations.

4：Experimental Procedures and Operational Workflow:

Geometric structures are optimized with a plane-wave cutoff energy of 450 eV and a dense k-point grid of 11 × 11 × 1. Equilibrium structures are obtained when the forces acting on all atoms are less than 0.001 eV/?. Optical properties are calculated considering the effective unit cell volume to obtain reliable absorption coefficients.

5：Equilibrium structures are obtained when the forces acting on all atoms are less than 001 eV/?. Optical properties are calculated considering the effective unit cell volume to obtain reliable absorption coefficients. Data Analysis Methods:

5. Data Analysis Methods: The imaginary part of dielectric function due to direct interband transitions is calculated using the Fermi golden rule. The real part of dielectric function is extracted from a Kramers-Kronig transformation. Optical absorption coefficients are obtained based on the complex dielectric function.