Charge Trap Engineering and Synaptic Behavior of Transition Metal Dichalcogenides Transistor, via Molecular Dynamics

The electrical performance of semiconductor devices has a significant dependence on the surface states of the semiconductors because the channel between source and drain regions for charge carrier transports is mostly formed near the surface with applying gate bias. Therefore, it is critical to engineering the surface chemistry of the semiconductor material. Coupling an intrinsically atomically thin body with a finite bandgap, layered transition metal dichalcogenides (TMDCs) have been employed as semiconducting channel platforms with a large ON/OFF ratio and near theoretical subthreshold swing. Here, reconfigurable synapse behaviors of TMDC via molecular adsorption of chemically and the receptive layer are demonstrated with three terminal TMDc synapse devices.<br/>Molecular adsorption on the MoSe₂ surface is induced with dipping in (NH₄)₂S(aq) diluted in H2O with 25%, while the solution temperature is held at 50 degrees celsius. H₂S or HS molecules can be adsorbed on the MoSe₂ surface during this chemical treatment and act as trapping centers for transported carriers. The adsorbed molecules can be desorbed from MoSe₂ surfaces with wash with isopropanol.<br/>To mimic the biological synapse, the electrical synapse response of MoSe₂ transistors is modulated by a pulse generator wherein excitatory modes; thereby synapse plasticity is controlled by spike-like gate bias as a presynaptic input. After applying pulse gate bias with 50 ms pulse duration, for bare MoSe₂ devices, there is no synapse response, including changes in conductance, consistent with the absence of the plasticity. However, after chemical treatment, molecular adsorption increases conductance upon pre-synapse inputs consistent with a significant increase from 5.9 S to 68.6 S (a postsynaptic conductance), consistent with the manifest of plasticity. The observed plasticity can disappear with the desorption of molecules from the surface of synapse devices.<br/>To emulate the visual sensory behavior of humans, the artificial synaptic plasticity of synaptic devices has been explored by applying light pulse. As light stimulation with pulse modulation, the channel’s conductance near-linearly increases as the number of light pulses increases, consistent with optical potentiation. Afterward, electric depression of molecularly functionalized synapse can be observed with the linear decrease of conductance. In addition, although it is crucial to fabricate the receptive layer in which a particular molecule reacts selectively, TMDC materials are absence of dangling bonds and surface state, resulting in degradation of selectivity. Therefore the TiOPc monolayers on the TMDC surface is deposit as the artificial receptive layer to induce the selective electric signal upon molecular absorption.<br/>Consequently, since the present report demonstrates the reconfigurable plasticity of TMDCs synapse devices with the optically sensory response, it can be a milestone to develop a brain-inspired computing system that integrates sensory function upon chemical and optical stimulation