Synaptic devices based on two-dimensional layered single-crystal chromium thiophosphate (CrPS4)

Two-dimensional (2D) van der Waals (vdW) materials have recently attracted considerable attention due to their excellent electrical and mechanical properties. TmPSx (where Tm = a transition metal), which is a new class of 2D vdW materials, is expected to show various physical phenomena depending on the Tm used. In this paper, the unprecedented synaptic behavior of a vertical Ag/CrPS4/Au capacitor structure, where CrPS4 is a single-crystalline 2D vdW layer, is reported. Multi-stable resistive states were obtained using an external voltage of less than 0.3 V. Both short-term plasticity and long-term potentiation were observed by controlling the interval of the external voltage pulse. Simple mechanical exfoliation was used to develop a synaptic device based on a very thin CrPS4 layer with a thickness of ~17 nm. Therefore, it was demonstrated that vertical Ag/CrPS4/Au capacitors could be promising inorganic synaptic devices compatible with next-generation, flexible neuromorphic technologies. A capacitor made of a chromium thiophosphate (CrPS4) layer sandwiched between a silver electrode and a gold electrode behaves like a synapse. A Korea-based team led by Bae Ho Park from Konkuk University, Seoul, peeled very thin, crystalline CrPS4 layers of different thicknesses from the bulk material and fabricated top and bottom electrodes to prepare a series of Ag/CrPS4/Au capacitors. Under electrical pulses of different amplitudes, durations and intervals, these structures mimicked short-term and long-term variations in synaptic strength—neural behaviors involved in learning and memory. The devices’ behavior is attributed to the electrochemically induced migration of silver ions across the CrPS4 layer, leading to the formation and rupture of a conducting filament between the electrodes. The Ag/CrPS4/Au structure is promising for computing technologies that imitate neural pathways in the nervous system. We suggest synaptic devices using cation migration along thickness direction in a new class of 2D layered materials. An electrochemically active metal, such as Ag and Cu, is used for the operation of the synaptic device and chromium thiophosphate (CrPS4) single crystal is used as an electrolyte material. Multi-stable resistive states, short-term plasticity, and long-term potentiation are observed by controlling external voltage pulse with height smaller than 0.3 V. Given that simple mechanical exfoliation can generate very thin CrPS4 layers, the vertical Ag/CrPS4/Au capacitor offers a promising inorganic synaptic device compatible with next-generation flexible neuromorphic technology.