Two‐Dimensional MXene Synapse for Brain‐Inspired Neuromorphic Computing

Abstract

formula M n+1 X n T x (n = 1-3) has been studied all over the world, since the discovery of Ti 3 C 2 T x in 2011. [10][11][12][13] These 2D MXenes exhibit many fascinating properties, such as metallic conductivity, [14] high transparency, [15] water dispersibility, [16] thermal stability, [17] good mechanical properties, [18] and antibacterial activity, [19] which are required for functional electronic and optoelectronic devices. Recently, resistive switching properties required to mimic the dynamics of biological synapses, including short-term and long-term plasticity, [8,20] have been demonstrated in MXenes. This indicates the applicability of these 2D materials to artificial synapses required for brain-inspired neuromorphic computing. [8,[21][22][23] To date, only a few studies on MXene synapse electronics have been reported. In 2019, Yan et al. used Ti 3 C 2 T x MXene as an active layer of artificial synapses. [24] Here, Ti and oxygen vacancies acted as trap sites for hopping carriers, which enabled fundamental synaptic operations such as "update" and "memorize" of an internal conductivity in response to external electrical stimuli. [8] In 2020, Wei et al. coupled Ti 3 C 2 MXene with a solid lithium-polymer-electrolyte layer to implement the synaptic functionalities. [25] The inherently large interlayer spacing, good conductivity, and fast ion diffusion facilitated the migration of lithium ions in the MXene layer, which enabled multistate conductivity control. Although resistive switching phenomenon in the MXene layer has been reported in a few studies, desirable synaptic characteristics, including linear long-term potentiation (LTP)/long-term depression (LTD) characteristics, a large number of conductance states, large conductance on/off ratio (dynamic range), and so on, have not been demonstrated yet. Furthermore, the applicability of 2D MXene synapses for neural networks (NNs) have not been demonstrated (see also Table S1, Supporting Information).Here, we report an artificial 2D MXene synapse fabricated with Ti 3 C 2 T x MXene nanosheets, which successfully mimics the dynamics of biological synapses, such as excitatory/inhibitory postsynaptic currents (EPSC/IPSC), paired-pulse facilitation (PPF), and LTP/LTD characteristics. Through in-depth analyses such as X-ray photoelectron spectroscopy (XPS), transmission electron microscope (TEM), and energy dispersive X-ray spectroscopy (EDS), we reveal that such synaptic functionalities originated from the gradual formation and annihilation of the conductive metallic filaments on the MXene surface with distributed functional groups. We also discuss the synaptic MXenes, an emerging class of two-dimensional (2D) transition metal carbides and nitrides, have attracted wide attention because of their fascinating properties required in functional electronics. Here, an atomic-switch-type artificial synapse fabricated on Ti 3 C 2 T x MXene nanosheets with lots of surface functional groups, which successfully mimics the dynamics of biological synapses, is reported. Through ...

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