Study On The Molybdenum Oxide-based Synaptic Devices

The data network-based novel technologies,such as artificial intelligence?AI?,big data and the internet of things?IOT?,show significant effect on every aspect of modern life.However,the traditional von Neumann architecture compters,in which memory and processing units are separated physically,are difficult to deal with massive amounts of data and complex scenarios.How to improve the efficiency of storage and computing becomes a hard nut that we need to crack.Different from the traditional computers,human brain can store and process the information simultaneously in the same synaptic operation and it can complete perception,recognition,memory and thinking under a condition of low frequency and energy consumption.Inspired by human brain,researchers are trying hard to implement“neuromorphic” computer.The development of novel functional materials and devices,which can be incorporated into the unique neuromorphic architectures,is a promising way.In this thesis,we fabricated twoterminal and three-terminal resistive switching devices based on layered ?-Mo O₃ and investigated the related physical mechanism,the influential factors and application in synaptic simulation.1.We have investigated a solid state electrochemical cell with a simple Ag/?-Mo O₃/fluorine-doped tin oxide?FTO?sandwich structure,which shows a normal electrochemical metallization?ECM?switching mode after an electroforming process.While in the lower voltage sweep range,the switching behavior changes to?valence change memory?VCM-like mode with the switching polarity opposite to the ECM mode.By measuring current-voltage characteristics under different ambient atmospheres and X-ray photoemission spectra,we have demonstrated electrochemical anodic passivation of the Ag electrode and valence change of molybdenum ions during resistance switching.The crucial role of moisture adsorption in the switching mode transition has been clarified based on the Pourbaix diagram for the Ag-H₂ O system for thefirst time.These results provide a deep insight into the resistance switching mechanism model in solid state electrochemical cells.2.Synaptic functions including potentiation and depression of synaptic weight,transition from short-to long-term plasticity,spike-rate-dependent plasticity,and spike-timing-dependent plasticity behavior were successfully realized in Ag/?-Mo O₃/ FTO cells with VCM-like continual resistance switching.The synaptic plasticity was controlled by tuning the excitatory post-synaptic current?EPSC?decay,which was determined by the number,width,frequency of applied voltage pulse as well as intervals between the pre-and post-spikes.The physical mechanism of the artificial synapse operation is attributed to the interfacial electrochemical reaction processes of the ?-Mo O₃ films with the adsorbed water,in which protons generated by water decomposition under an electric field diffused into the ?-Mo O₃ films and subsequently were intercalated into the lattice,leading to the short-and long-term retention of cell resistance,respectively.These results indicate the possibility of achieving advanced artificial synapses with solid state electrochemical cells and will contribute to the development of smart-terminal network systems.3.We have fabricated for the first time three terminal synaptic transistors based on 2D ?-Mo O₃ nanoflakes,and demonstrated that the ionic liquid?IL?gatingcontrolled electrochemical proton-doping in ambient atmosphere is feasible to modulate the conductance of molybdenum oxide in a non-volatile and analogous way.The dynamic process of protons stemming from IL spray water splitting under the influence of electronical gating is closely resemble the transmission of the chemical signals in biological synapses.The low consumption?9.6 p J per presynaptic pulse?and fast EPSC responding to a short pulse?1 ms in duration time?have been demonstrated.The paired-pulse facilitation,depression and potentiation of synaptic weight,and the transition from short-term to long-term potentiation have also been realized.These resultsare beneficial to the development of synaptic devices based on 2D molybdenum oxide for the application of neuromorphic engineering.