Zero-Bias Photodetection and Opto-Synaptic Plasticity in BP/MoS₂ and WS₂/PdSe₂ van der Waals Heterostructures

van der Waals (vdW) heterostructures assemble atomically thin crystals into defect-free interfaces, where band alignment, interlayer coupling, and built-in electric fields can be engineered for multifunctional, low-power optoelectronics. This Spotlight provides a concise, critical overview of two complementary platforms - BP/MoS₂ and WS₂/PdSe₂ - highlighting how their type-II or type-I band alignment enables optoelectronic operations. In back-gate transistors, both BP/MoS₂ and WS₂/PdSe₂ heterojunctions exhibit transfer curves with high ON/OFF ratios that can reach 10⁷-10⁸ and hysteresis, which is reduced in a vacuum. In BP/MoS₂, deterministic stacking and contact/work function engineering stabilize a type-II band offset that supports zero (V _ds = 0 V) or low (|V _ds| ≤ 5 V) bias operation with fast, dual-time scale photoresponse (from hundreds of ms to ∼1 s) and linear behavior up to 50 μW optical power. Pressure tunes transport mechanisms (from thermionic to band-to-band tunnelling and ohmic), and four-probe data provide a band offset of 68 meV and zero-bias operation at 600 nm, consistent with MoS₂-driven visible selectivity. Similarly, WS₂/PdSe₂ stacks enable visible-band photodetection at low bias, with pressure-tunable transport and photoresponse; the pressure-dependent photocurrent makes them promising for optoelectronic pressure sensing. In a gate-assisted regime (negative V _gs in high vacuum), persistent photoconductivity enables opto-synaptic plasticity in WS₂/PdSe₂ heterojunctions with paired-pulse facilitation (PPF) of 137% and post-tetanic potentiation (PTP) of 300% under 1 s pulse trains. Overall, this Spotlight examines synthesis pathways, interfacial structure, and device-level behavior, and discusses stability, uniformity, and readout compatibility as practical levers for translating single-device demonstrations into reproducible arrays.