Multibit Nonvolatile Optoelectronic Memory Based On Graphdiyne/hexagonal Boron Nitride/graphene Heterostructures

Memory devices have been an indispensable component in the modern electronic industry.To increase the data storage capabilities of memories,great efforts have been made for the scaling down of device size to integrate more memory cells in a unit area.At present,this approach is approaching its theoretical limit,and further scaling down will suffer from the dramatically increased fabrication complexity and reduced storage reliability.Alternatively,multibit memories,which can store more data in one single device,provide a promising approach to improve the data storage capability.In the nonvolatile memories with floating-gate structure,the storage states（channel currents）are associated with the number of charges accumulated in the floating gate or charge trapping layer,and therefore the charge trapping capability of the charge trapping layer is crucial for the multilevel storage of a memory.Moreover,light controlling in recently developed optoelectronic memories has become a low-power consumption stimulus with nondestructive programming/erasing processes compared with electrical tuning.These processes can be achieved via transforming light signal to electrical response to achieve multibit storage.Graphdiyne,a new two-dimensional（2D）carbon allotrope containing only sp and sp² carbon atoms with high degrees ofπ-conjugation,has attracted tremendous attention since its first synthesis in 2010.Graphdiyne is predicted to exist superior mechanical,optical,thermal,and electrical properties,and has been widely applied in photo/electro-catalysis,energy storage and conversion,water purification,and humidity sensors.Especially,the abundant charge trapping sites and strong absorption of graphdiyne make it a promising candidate for a photoresponsive charge trapping layer in optoelectronic memories.However,limited by the small area and poor quality of the synthesized graphdiyne film,the applications of graphdiyne in optoelectronic devices,especially in optoelectronic memory,are still in its infancy.Recently,a“solution-phase van der Waals epitaxy”method was developed to synthesize highly crystalline and ultrathin graphdiyne film on graphene and other 2D materials,paving the way to fabricate and investigate optoelectronic devices based on graphdiyne and its heterostructures.In this thesis,high-quality graphene and hexagonal boron nitride（hBN）films were grown via chemical vapor deposition（CVD）approaches on Cu foils,and hBN/graphene vertical heterostructures were fabricated on Si O₂/Si substrate serving as the epitaxial surface for the synthesis of graphdiyne.Large-area high-quality pyrenyl graphdiyne（Pyr-GDY）film was synthesized on top of the hBN/graphene heterostructure via the van der Waals epitaxial growth method to fabricate Pyr-GDY/hBN/graphene vertical heterostructure.And then,a top-floating-gated multibit nonvolatile optoelectronic memory based on this Pyr-GDY/hBN/graphene heterostructure was constructed.In this device,Pyr-GDY was used as a photoresponsive charge trapping layer,while graphene and hBN serve as the channel and dielectric layer,respectively.Benefitting from the excellent charge trapping capability of Pyr-GDY,an ultrathin hBN film（3 nm）can be used to separate the graphene channel and Pyr-GDY,significantly improving the storage performances of the memory device,such as a large memory window(137 V at V_g-max=±90 V),small programming/erasing voltages（±30 V）,and a small light pulse energy（3 n J）.The reliability and stability of the device is demonstrated by its long retention time（>10⁵ s）and 100 cyclic programing/erasing（P/E）endurance.Moreover,the huge difference of currents between the programmed and erased states(>130μA at V_ds=0.1 V)and prolonged retention time（>10⁵ s）provide a way to achieve multibit storage,for which8 and 9 distinct storage states（3-bit）are obtained by applying periodic electrical/optical pulses,respectively.This thesis opens the way for the applications of graphdiyne in optoelectronic memory,which is an important step toward versatile graphdiyne-based optoelectronic devices in the future.