The Control Of Physical Properties And Device Applications Of Novel Van Der Waals Materials

Since the discovery of graphene in 2004,van der Waals materials are widely used in many fields of scientific research.There are a wide variety of van der Waals materials,including insulators,semiconductors,metals,superconductors,topological insulators,and recently discovered magnetic materials,which have many excellent properties and potential applications.Also,the van der Waals heterostructures formed by stacking van der Waals materials show great potential in basic physics research and promising device applications.In this work,the control of properties of van der Waals materials,such as Bi₄I₄?Fe₃Ge Te₂?Re S₂ and their heterostructures are investigated and we design promising devices,such as strain-assisted magnetization reversal,floating gate non-volatile memory and light-tunable multibit memory devices based on these van der Waals materials.First of all,we introduced several kinds of van der Waals materials,which have been widely focused on.Then,we introduced the device applications based on van der Waals materials.In Chapter 2,we introduced the methods for device fabrication andmeasurement platforms in our lab.In Chapter 3,we studied the transport properties of Bi₄I₄ which is a weak topological insulators and observed the phase transition from?-Bi₄I₄ to?-Bi₄I₄ near room temperature.Also,by analyzing Sd H oscillation in the low temperature,we found that?-Bi₄I₄ is a trivial topological phase.Also,we observed that temperature of transistion from?-Bi₄I₄ to?-Bi₄I₄ decreases with decreasing thickness,which indicates that the topological states of Bi₄I₄ are controlled by the sample thickness.In Chapter 4,we apply uniaxial tensile strain to the vd W magnet Fe₃Ge Te₂(FGT)thin flakes,and observe a dramatic increase of the coercive field(H_c)by more than 150%with an applied strain of 0.32%.Moreover,we investigate the change of the transition temperatures between different magnetic phases under the strain,and obtained the phase diagram of FGT in the strain-temperature plane.Comparing the phase diagram with the theoretical results,we attribute the strain-tunable magnetism to the sensitive change of magnetic anisotropy energy.Remarkably,strain engineering allows us to achieve an ultra-sensitive magnetization reversal,which may promote the development of novel straintronic device applications.In Chapter 5,we studied the control of the electronic transport properties of Re S₂and van der Waals heterostructure based on Re S₂.First,we studied the electronic transport properties of Re S₂ via the control of ionic liquid.We found that with the increase of the ionic liquid gate voltage,Re S₂ showed a transition from the metal state to the insulation state,and the carrier concentration of Re S₂ increased at first and then descends.The results may be related to the disordered introduction of ionic liquid.Next we studies the control of the electronic transport properties of Re S₂/Graphene/h BN and h BN/Re S₂/h BN van der Waals heterostructure.In Re S₂/Graphene/h BN,we observed that the Landau level of Graphene splits,which may be the spin or valley splitting in Graphene induced by Re S_2.In h BN/Re S₂/h BN,we observed Sd H oscillation which may result from the complex band structure of Re S₂,interface interaction,and electrodes made by Graphene.In Chapter 6,we observed negative photoconductance(NPC)in van der Waals heterostructures for the first time.First,the Re S₂/h BN/Mo S₂ heterostructures exhibited excellent performance of nonvolatile memory.More interestingly,we observed a gate-tunable transition between the positive photoconductance(PPC)and the NPC under the light illumination.The observed NPC phenomenon can be attributed to charge transfer between the floating gate and the conduction channel.Furthermore,we show that control of NPC through light intensity is promising in realization of light-tunable multibit memory devices.In Chapter 7,we summarizes our researches and makes a further prospect for the future research directions.