Study On Photodetectors And Neuromorphic Devices Based On Transition Metal Dichalcogenides

Transition metal dichalcogenides(TMDs),a representative member of the two-dimensional material family,are attractive in the field of optoelectronic devices due to their adjustable band gap,high light absorption coefficient,high carrier mobility and surface without dangling bonds.So far,chemical vapor deposition(CVD)can realize the preparation of large-area,high-quality TMDs individual materials and their heterostructures.However,the CVD-grown TMDs will generally introduce defects and impurities.The defect-induced photogating effect will produce a higher optical gain under illumination,but will also sacrifice the response rate of the device,showing the persistent photoconductivity(PPC)effect.This effect is similar to biological synaptic function:in the former,the stimuli affect the carrier concentration in the channel,while in the latter,the stimuli influence the ion concentration in the postsynaptic end.Moreover,the influence after stimuli will not disappear immediately with the disappearance of stimuli.In recent years,TMDs optoelectronic devices have been extended to biomimetic research based on the PPC effect,i.e.,neuromorphic photonics,for example,photonic synapses,optoelectronic synapses,even bionic eyes and bionic brains.The realization of bionic function by optical simulation has become a hot issue at home and abroad.Therefore,the research of TMDs multifunctional optoelectronic devices is of great significance to neuromorphic photonics and artificial intelligence,especially in decreasing power consumption and expanding applications.The main research contents in this dissertation are as follows:(1)Various molybdenum-based and tungsten-based TMDs materials are prepared by CVD,including large-size single crystal and large-area thin film,as well as lateral or vertical heterostructures.Meanwhile,the effect of the main parameters of CVD,such as growth temperature,growth time,precursor,substrate position,carrier gas and its flow rate,on as-grown molybdenum-based and tungsten-based TMDs samples has been revealed.In addition,the growth and mechanism of spiral,dendrite or quasi-one-dimensional molybdenum-based and tungsten-based TMDs was further studied.(2)A large-area and high-quality monolayer molybdenum disulfide/tungsten disulfide(Mo S₂/WS₂)lateral heterojunction was prepared by one-step liquid phase CVD.In order to confirm the interfacial quality,the lateral structure was characterized by a Raman-AFM confocal spectrometer.The photodetector based on this lateral heterojunction as channel layer possesses the photoresponse of 570 AW^-1 and photodetectivity of 7.17×10¹¹ Jones under 405 nm laser irradiation.Due to the defect-induced photogating effect,the response time of the device gets longer,i.e.,PPC effect.By testing the noise current,the existence of 1/f noise in the device has an adverse effect on the device performance including photodetectivity and response rate.(3)Large-area uniform multilayer MoS₂ films were prepared by space-confined low pressure CVD.The film is used as a channel layer to fabricate the back-gate photonic synapse.This transistor can simulate a variety of biological synaptic functions and can respond to ultra-short light pulses as low as 5?s with ultra-low power consumption(40a J),which is far less than the power required for a biological synapse to respond to a synaptic event.The photonic synapse can simulate vision perception,memory storage,associative learning,emotion regulation,information preprocessing,etc.In addition,in the aspect of visual perception,the device can realize the detection of visible light to near infrared light.This photonic synapse integrates sensing-memory-preprocessing capabilities,which can effectively alleviate the high power consumption and low efficiency caused by the von Neumann bottleneck.(4)A device based on the mesh-like MoS₂ channel is designed and fabricated,with which the electric field distortion effect induced by carrier trapping was explored.This effect was used to simulate the full memory trace,including encoding,storage and retrieval,rather than the simple simulation of partial memory storage process as commonly reported.In addition,this Mo S₂ device further simulates the effect of pressure-induced stress hormones on memory function.The encoding of time information is very important for biomimetic neural activity.The study of this device performance can provide a reference for exploring dynamic neural networks.(5)The above devices also achieve the encoding of more complex time information,which is different from the traditional works about the illumination wavelength,intensity,polarization or the interstimulus interval coding.In addition,the transmitted data is encrypted and decrypted by introducing dual laser stimulation to realize more secure data transmission and storage.In this dissertation,taking molybdenum-based and tungsten-based TMDs prepared by CVD as the research object and based on defect-induced photogating effect,PPC effect and local electric field distortion effect,the performance and application of TMDs optoelectronic devices in the following three aspects are studied,including photodetectors with high responsitivity and detectivity,photonic synapses with ultrahigh sensitivity and ultralow power consumption,as well as multifunction and safer illumination duration encoders.Multi-functional,low-power,integratable and safe TMDs optoelectronic devices have great application prospects in the fields of neuromorphic photonics and artificial intelligence(AI),and have guiding significance for the further research and development of intelligent devices.