Design Of Low-Dimensional Nanostructures Based On The ?A Elements And Their Electronic Transport Properties

With the continuous development of modern integrated circuit technology,electronic components become smaller and smaller.When the device size is smaller than the critical size,it is hard to maintain the Moore's law due to quantum effect.At the same time,the growth in big data and the Internet of Things has enhanced the demand for low-power data storage and processing.Thus,there is a demand for alternative,beyond traditional silicon technologies.Low-dimensional nanomaterials containing IVA elements show many novel physical properties due to their unique structures.With the increasing demand for research and development of nano-electronic devices,it is urgent to explore new materials and design new structures,so as to construct a new generation of electronic devices.It is particularly important to study its internal mechanism such as electronic transport properties.In this thesis,a series of structures are designed by using density functional theory combined with the non-equilibrium Green's function's first-principles calculation,and the electronic transport properties of low-dimensional nanomaterials containing IVA elements,such as carbon materials,SiC nanoribbons,Pb nanowires,2D layered Fe3GeTe2,are systematically studied.The relationship among the structure of low-dimensional nanomaterials containing IVA elements,their electronic structure and electronic transport properties is revealed.The main contents are as follows:(1)Based on some recently successfully synthesized materials,carbon-based molecular devices such as ring-,calabash-,taper-,fusiform-and hourglass-shaped structures are constructed,and their electronic transport properties are studied.It is found that polyynic and cumulenic C18 have similar transmission performance,but different electrodes(1D carbon chain,2D graphene and 3D Ag bulk)have different transmission characteristics,including ohmic,quasi-Schottky and current limiting characteristics.The electronic transport properties of calabash-shaped two-node hollow fullerene structure are studied,and it is found that the device is highly sensitive and selective to greenhouse gas CF4 molecule,and can be used as a sensor.And the performance of this CF4 sensor is not interfered by the change of device placement direction.Electronic transport properties of taper-shaped,fusiform-shaped and hourglass-shaped carbon nano-devices are predicted theoretically.Fusiform-shaped devices exhibit negative differential resistance effect,while taper-and hourglass-shaped devices follow the Ohm's law.There are significant differences in electronic transport between spin-up and spin-down states of these three devices.Interestingly,the taper-shaped device shows almost perfect spin polarization(-100%)under positive bias,which indicates that it may be able to filter the current in a specific spin direction.In addition,high rectification ratio(?7.5×105)and spin diode characteristics also appear on taper-shaped devices.(2)"Y"-shaped PbS/PbSe nanostructures based on SiC nanoribbons are designed,and the electronic transport properties of devices constructed by these structures are studied.It is found that the ?-? curve of tri-SiC-PbS device follows the Ohm's law,and the ohmic characteristic remains unchanged but the slope increases obviously after the gate voltage is applied.And it does not change with the gate voltage changing again,so it can be used as a stable electronic device that is not affected by the gate voltage.It is found that the ?-? curve of the tri-SiC-PbSe device shows a zero value under low bias voltage,which turns into an "on" state with the increase of bias voltage.This distinct switching state as well as obvious forward and reverse rectification behaviors can be applied to circuit switches.The tri-SiC devices composed only of SiC nanoribbons are also studied and compared.It is found that the current is insensitive to the change of gate voltage,and the ?-? curves under different gate voltages follow the symmetrical and "W"-shaped conductance characteristics at zero point.The effects of the length,width and combination angle of the nanoribbons with "Y"-shaped structure on the electronic transport properties are also studied.It is found that the change of width makes the cross-sectional area in the transport direction significantly narrower,and the electronsstates are more localized,which has a significant impact on ?-? curves.(3)The electronic transport properties of Pb nanowires and Pb nanowires doped with Si are studied.Pb nanowires with different diameters are spiral structures with different helicity.On the ?-? curves,there are nonlinear curves that break the Ohm's law.With the increase of diameter,the delocalization of electronic structure becomes stronger and stronger,and the Pb nanowire with larger diameter exhibits stronger transport properties.PbSi nanowires are segregated in structure.Doping has a greater influence than the diameter on the electronic transport properties of PbSi nanowires,and its conductance has a larger amplitude with the increase of doped Si atoms.The?-? curves of these PbSi nanowires are also nonlinear and have more fluctuation than that of Pb nanowires,showing obvious negative differential resistance effect.The conductance of PbSi nanowires encapsulated with the gate electrode is symmetrical,and the ?-? curves maintain the characteristics of negative differential resistance effect,which can be explained by Schottky contact.(4)A germanium-based magnetic heterojunction is designed and its electronic transport properties are studied.It is found that the designed magnetic heterojunction has a magnetoresistance ratio up to 700%,which is several orders of magnitude higher than that reported by the traditional magnetic heterojunction devices.This is due to the strong transmission coefficient of the spin-up channel in the P state and almost zero in the AP state.Through the calculation of complex energy band,it is found that the energy band of electrode material and barrier is matched ?1 symmetry,and these ?1 states have the lowest barrier height,which indicates that electron tunneling plays a leading role in this magnetic heterojunction.In a word,the research content of this thesis is of great significance to deeply understand the electronic transport properties of low-dimensional nanostructures based on IVA elements,and provides theoretical guidance for developing electronic devices,logic devices and information storage devices used as new generation computers.