Fig. 1: Design of 2D van der Waals heterojunction (vdWH) spectrometer.

a Schematic drawing of the 2D-vdWH spectrometer. The heterojunction is intercalated by heavy metal Au atoms to construct ReS₂/Au/WSe₂, where the junction would promote the separation of photo-excited electrons and holes. V_ds and V_bg are bias voltage and back gate voltage, respectively. ‘hv’ and the red arrow represent the incident light. b Photo-excited transition path in the heterojunction, including intralayer transition (1) & (2) and interlayer transition (3), where the path (3) corresponds to lower transition energy to near-infrared (IR) band than both band gaps of ReS₂ and WSe₂ in visible (VIS) band. Atomic geometries, band structures, and electron distribution of conduction band minimum in WSe₂ layer (and hole distribution of valance band maximum in ReS₂ layer) for both c pristine ReS₂/WSe₂ and d ReS₂/Au/WSe₂ with Au intercalation heterojunctions, respectively. In the optimized geometry, intervals of S-Au and Se-Au are 2.47 and 2.56 Å, respectively. The lighter dots in d representing the additional few densities of states in k-space by the changed lattices of 2D-vdWH due to the participation of Au atom. The black arrows indicate the transition energy at Г point for the two heterojunctions. The top-view atomic arrangements correspond to the region marked by black dash line in the side-view atomic arrangement. The electron density distribution shows that the electron wavefunctions in the ReS₂ layer are delocalized and partially delivered to the WSe₂ layer through Au atoms. Red and blue represent the relative contribution of the states from electrons in ReS₂ and holes in WSe₂, respectively.