  Van der Waals (vdW) heterostructures made of two-dimensional materials have
been demonstrated to be versatile architectures for optoelectronic applications
due to strong light-matter interactions. However, most of light-controlled
phenomena and applications in the vdW heterostructures rely on positive
photoconductance (PPC). Negative photoconductance (NPC) has not yet been
reported in vdW heterostructures. Here we report the observation of the NPC in
ReS2/h-BN/MoS2 vdW heterostructures-based floating gate phototransistor. The
fabricated devices exhibit excellent performance of nonvolatile memory without
light illumination. More interestingly, we observe a gate-tunable transition
between the PPC and the NPC under the light illumination. The observed NPC
phenomenon can be attributed to the charge transfer between floating gate and
conduction channel. Furthermore, we show that the control of NPC through light
intensity is promising in realization of light-tunable multi-bit memory
devices. Our results may enable potential applications in multifunctional
memories and optoelectronic devices.
