Anomalous Sub-Band Gap Emission at Room Temperature in Mixed-Dimensional WS₂/Cu₂O Heterostructures

Practical implementation of two-dimensional (2D) transition-metal dichalcogenides (TMDCs) in novel optoelectronics requires a fundamental understanding of carrier dynamics and interfacial states at 2D/3D heterostructures. Herein, a mixed-dimensional p-n junction based on monolayer tungsten disulfide ( ${\mathrm{WS}}_2$ ) and cuprous oxide ( ${\mathrm{Cu}}_{2}O$ ) is investigated. An anomalous emission (AE) at 1.66 eV is identified at room temperature, which is in the same spectral range as oxygen-related ${\mathrm{Cu}}_{2}O$ defect luminescence, but is enhanced at the heterointerface under 457 nm excitation. While this sub-bandgap feature resembles the spectral signature of an interlayer exciton (ILX), its combined excitation-wavelength, power, temperature, and bias dependence suggests that the emission is predominantly defect-mediated and modified by the ${\mathrm{WS}}_2$ / ${\mathrm{Cu}}_{2}O$ interface. External modulation of the AE intensity is demonstrated via vertical bias, yielding an approximately 45% maximum-to-minimum modulation of the normalized AE/ ${\mathrm{Cu}}_{2}O$ PL area ratio over the applied bias range from $-2$  V to $+2$  V at 93 K. This modulation is asymmetric, showing a correlation between AE quenching and photocurrent generation under reverse bias. The spectral response peaks near the open-circuit condition, marking a regime of efficient carrier accumulation and interfacial trapping. These findings identify ${\mathrm{WS}}_2$ / ${\mathrm{Cu}}_{2}O$ as a model hybrid interface for probing electrically tunable defect-mediated recombination in mixed-dimensional heterostructures.