**6.2 2D materials**

Fundamental 2D material research has received a great deal of attention during the past 10 years. The 2D materials maintain a stable mono-layer structure with special chemical and physical properties suitable for simulating synapses. They are recognised for their inter-layer weak van der Waals forces. Haigh et al. demonstrate that the high mobility of 2D synapses allows them to exhibit fast switching speed at low operating voltages [202]. The alteration in electrical, photonic, and electrochemical properties is another distinguishing quality of 2D material synaptic devices [203]. Shi et al. use a CBRAM-based h-BN memristor to exhibit STP and LTP properties [204]. The creation and deformation of the conductive filament caused by ion migration between Cu or Ag electrodes controls weight update. The h-BN exhibits boron vacancies that stimulate resistance changes.

Wang et al. demonstrate how externally introduced oxygen atoms occupy intrinsic defects that can be adjusted as MoS2 sulphur vacancies, causing resistance changes [16]. In [205], authors use bilayer MoS2 vertical memristors to demonstrate STDP features at 0.1–0.2 V voltage. Apart from CBRAM, 2D materials have been integrated as PCM 2-terminal synaptic elements, which have the advantage of better reliability. MoTe2 exhibits an electric field-induced amorphous to crystalline phase transition in TMD materials [206]. Multilevel programming resistance could be facilitated by further device engineering to the device stack. On the other hand, A 3-terminal device uses the gate and the channel as presynaptic and postsynaptic inputs, exhibits superior stability and effective channel conductance control. Chen et al. demonstrate how a graphene and ferroelectric insulator (polyvinylidene fluoride, i.e., PVDF) synapse can imitate the synaptic behaviour as a FeFET device [207]. The ferroelectric material's changing polarisation state affects the carrier concentration in graphene. The polarisation shift is induced when the gate voltage is raised above the threshold voltage. Future research might also look toward Li + ion gated synaptic transistors [208] and various hetero synaptic plasticity implementations [201, 209, 210].
