Optoelectronic Synapse Behaviors of HfS₂ Grown via Molten Salt Flux Method

Layered two-dimensional materials are promising candidates for next-generation semiconductor platforms owing to their atomically thin bodies, and it is crucial to develop a method for their large-scale synthesis for integrating these materials into the fabrication process. Here, we report the synthesis of a centimeter-scale HfS₂ ingot using the molten salt flux method (MSFM). The structure, crystallinity, and uniformity of the synthesized HfS₂ sample were verified using X-ray diffraction and Raman spectroscopy. The chemical properties were investigated using X-ray photoelectron spectroscopy. A HfS₂ synaptic field effect transistor (FET) was fabricated to confirm its electrical uniformity and semiconducting nature, with an average mobility of 10.6 cm² V^-1 s^-1. The synaptic plasticity of the HfS₂ synaptic FET was investigated by applying light pulses (405 nm) in different modulation configurations. Paired-pulse facilitation was achieved by applying a continuous light pulse with a negative gate bias voltage. The modulation of synaptic weight was demonstrated under different stimulation conditions, which emulates the human brain. Furthermore, the linearity of the HfS₂ synaptic device was optimized based on the frequency of the pulses to enhance learning accuracy. The approach reported here encourages the large-scaled production of transition metal dichalcogenides (TMDs) for use in artificial synaptic transistors.