Study On Flexoelectric Effect Of Two-dimensionl Metal Chalcogenide Piezoelectric Semiconductors

In recent years,piezotronics and piezo-phototronics,which are the core mechanisms of the mechanical-optical-electrical interconversion of matter and energy,have become a hotspot of research in microelectronics and solid state electronics.Piezotronics and piezo-phototronics,meanwhile,are one of the important development directions in the areas of information science and technology,energy science and technology,and other national strategic needs.Two-dimensional piezoelectric semiconductors,such as transition metal dichalcogenides(TMDCs)and group IIIA metal chalcogenides,not only possess excellent mechanical flexibility,semiconductor properties,and electromechanical and photoelectric conversion capabilities,but also its planar structure can be highly compatible with existing semiconductor manufacturing technology.Therefore,they are considered as one of the ideal candidates for realizing the next generation of electronic and optoelectronic integrated devices.In addition,the flexoelectric effect,as an enhancement and supplement to the piezoelectric effect,plays a significant role in the low dimensional systems.The flexoelectric effect can greatly improve the piezo-optical-electrical multi-field coupling capability of two-dimensional piezoelectric semiconductors,and enrich the research of piezotronics and piezo-phototronics.Focusing on the above issues,this paper has studyed the flexoelectric effect,flexo-photoelectronic effect,and a deriving light-stimulated artificial synapse based on flexo-photoelectronic effect,with the atomic-scale thickness Mo S₂ and InSe as the main research objects.The specific research results are as follows:(1)Stable and controllable strain gradient in curved atomic-scale thickness Mo S₂ and InSe has been achieved using the suspended structure.The effective out-of-plane piezoelectric response induced by flexoelectric effect is quantified,and the thickness and curvature dependence of flexoelectric effect is further summarized.The strain gradient-induced flexoelectric effect is an important strategy to break through the limitations of 2D piezoelectrics.Whereas,the stable and controllable strain gradient engineering of atomically thick materials and the quantification of flexoelectric effect are still full of challenges,which are also the premise of further research on flexoelectric effect.In this work,the large,stable and controllable strain is introduced into atomically thick Mo S₂ and InSe films through suspended structure.The flexoelectric effect is further quantified as effective out-of-plane piezoelectric coefficient(d ₃₃^eff)by combining suspended structure and piezoresponse force microscopy(PFM)technique.In particular,the d ₃₃^eff as high as 7.5 pm/V is obtained in single-layer Mo S₂,and a promising d ₃₃^eff of 21.9 pm/V are obtained in few-layered InSe.In addition,it is further demonstrated that the flexoelectric coupling strength of a curved 2D semiconductor is directly proportional to the reciprocal of the product of material thickness and radius of curvature.This study contributes to the understanding of the 2D flexoelectric effect and is expected to facilitate the development of 2D electromechanical coupling devices.(2)A light-stimulated Kelvin probe force microscope(KPFM)test system has been built for the characterization of flexo-photoelectronic effect.In situ high-resolution imaging of the couping of flexoelectric polarization with photogenerated carriers has been achieved.The physical mechanism of flexo-photoelectronic coupling on 2D n-type InSe and p-type WSe₂ has been revealed.Piezoelectric/flexoelectric polarization is one of the decisive factors to modulate the performance of electronic and optoelectronic devices.However,there is still a lack of microscopic characterizations and mechanism studies on the kinetic processes of photogenerated carriers in the piezoelectric potential/flexoelectric potential field,including generation,separation,transport and recombination.In this work,a light-stimulated KPFM test platform has been constructed by coupling an external tunable laser to image the flexoelectric potential and photogenerated carrier transport processes in the two-dimensional semiconductor channels of n-type InSe and p-type WSe₂.Energy band analysis shows that the flexoelectric polarization induced by strain-gradient leads the back-to-back built-in electric fields in the curved two-dimensional semiconductor.The built-in electric fields could contribute to the separation of photogenerated electron-hole pairs,and result in the trap of photogenerated holes and electrons in the curved n-type InSe and p-type WSe₂ channels,respectively,thus enabling the regulation of carrier transport behavior in photoelectronic devices.This work provides a new idea for the study of flexo-photoelectronic coupling and is expected to promote the application of 2D flexoelectric effect in the field of photoelectronics.(3)The first light-stimulated artificial synapse based on flexo-photoelectronic effect has been achieved by using a bent InSe channel.The artificial synapse has been further used to achieve light-tunable short-term/long-term synaptic plasticity and learning and memory functions.In the development of future optoelectronic devices,light-stimulated synaptic devices have become one of the most promising candidates.In this work,we propose a new method to achieve the novel light-stimulated synaptic device utilizing the flexo-photoelectronic effect.Specifically,the back-to-back built-in electric fields in the curved InSe channel are used to trap photogenerated holes,which improves the lifetime of photogenerated electrons and greatly improves the photocurrent.Meanwhile,the slow release of trapped holes leads to the memory effect of photocurrent.Due to the photocurrent memory behavior,InSe light-stimulated artificial synapse device enables high-performance simulations of short-term/long-term plasticity,memory and learning behavior of biological synapses.This study has some enlightening significance for the study of flexo-photoelectronic effect and novel light-stimulated synaptic device.