Study On Negative Differential Resistance Effect In Graphene Nanoribbons Heterojunction By First Principle Calculation

Negative differential resistance(NDR)devices are widely used in high-speed logic circuits,fast molecular memories,high-frequency oscillators and molecular neuromorphic devices.Theoretical and experimental studies have shown that edge geometry and passivation can regulate the electrical behaviour of graphene nanoribbons,and certain nanoribbon structures can exhibit NDR.In this paper,first principles calculations are used to study the NDR effect in graphene nanoribbon heterojunctions.According to that the electronic properties of graphene nanoribbons can change between semiconductor and metal with width,edge structure and passivation,we designed three kinds of geometry semiconductor-metal contact,i.e.,"Z","H"and"I",which are all graphene nanoribbons heterojunctions(named as Z-HJ,H-HJ and I-HJ respectively).Through first principle calculation of SIESTA,the NDR characteristics and mechanism of devices with respect to their configurations and dimensions were investigated.The results show that all the heterojunctions have significant NDR effect at low bias voltage,and the location and peak to valley ratio of NDR peak are related to the configuration of the junction,especially in I-HJ,and the negative resistance peak with PVR up to 6.86×10~5 is generated at bias in the range of[0.1V,0.7V].As for the Z-HJ with different widths,its NDR is always found with the peak position and PVR value change with the nanoribbon size.Then,the effects of POM modification on the NDR properties of nanoribbons and their heterojunctions were studied.It is found that almost all the current through the nanoribbons system decreases after POM modification.This inhibition effect of pure nanoribbons is obvious when the bias voltage exceeds 0.6V,while it is significant for the heterojunction when the bias voltage is lower than 0.6V.NDR appears for the nanoribbons which have no NDR effect without the existence of POM,and multiple peaks show up for the I-HJ due to the decoration of POM.Through analysis of electron transmission spectrum,density of states and density of projected states,we found that this is due to the relative shift of the chemical potential of the left and right electrodes after the applied bias voltage,the symmetry is destroyed,and the electron tunneling channel disappears.The NDR devices based on graphene nanoribbons heterojunction have very high PVR,and the NDR can occur at a small bias voltage.The POM modified heterojunction has multiple negative resistance peaks,which can be used in multi valued logic and memory,and is of great significance for low power consumption and miniaturization of logic circuits.