The First Principle Study Of The Transport Properties Of C₁₈,C₆₀ And Carbon Nanotube

The smallest functional unit of a modern electronic computer are many tiny Flied Effect Transistors（FET）.The invention of the finned FET has reduced the scale of integrated circuits even further,and the latest manufacturing processes have made it possible to achieve minimum channel lengths of less than 3nm.However,the miniaturisation of devices is being severely tested,as conventional semiconductor devices are facing the thermodynamic limit of physics as a barrier,and Moore’s Law is on the verge of failure.At the microscopic scale,the motion of electrons no longer follows the laws of classical physics,but has quantum effects.How to exploit the microscopic nature of electrons and design new electronic devices capable of reaching smaller scales is an important research direction in spintronics.Carbon,a homologous element of silicon,has similar physicochemical properties to it.In many ways,carbon-based materials exhibit unique advantages not found in silicon-based materials,and a wide variety of semiconductor materials with varying properties can be formed from intrinsic carbon monomers.The excellent electrical and magnetic properties of graphene make it a perfect candidate for use as an electrode in all-carbon devices.Different all-carbon structures in the central region form a variety of different contact patterns with the electrodes,introducing different electrical properties.Therefore,the use of carbon-based materials to create new microelectronic devices,especially all-carbon devices,is a novel research idea.In this paper,we use first-principles approach combining Density Functional Theory（DFT）and Non-equilibrium Greens’Function NEGF to investigate transport properties,of several all-carbon devices based on graphene leads.namely C₁₈ molecules,C₆₀molecular clusters and carbon nanotubes,as follows.1.C₁₈ molecular devices with graphene nanoribbons of different spin polarization directions as electrodes,spin-parallel and anti-parallel states,were investigated.Two types of connections,embedded and suspended,were designed,and their currents,spin polarization rates and magnetoresistance were calculated under different bias pressures,respectively.It is found that both devices have a good spin filtering effect as well as a large TMR value.The embedded structure has a larger current and stronger transmission values due to the stronger interaction of the electrodes with the molecules in the central region compared to the suspended one.The variation of the transmission coefficient and the contrast between the density of states in the central zone and the energy bands of the electrodes are used to explain the reason for this phenomenon,where the symmetry mismatch of the energy bands is the main cause.2.The transport properties of C₆₀ molecular clusters with graphene nanoribbons of different SP directions as electrodes were investigated,and their currents,SP and magnetoresistance were calculated under different bias pressures,respectively.The results show that the devices possess high spin polarizabilities and magnetoresistances,which are explained by the transmission spectra at different voltages and the PDOS of the devices.The results show that there is a significant shift in the transmission spectrum with increasing bias voltage,influenced by the structure of the electrodes and the energy levels in the central region as well.The symmetry of the energy band is the main reason for the effect on the current.3.Single-walled carbon nanotube（SWCNT）devices with a central region were studied,and the transport properties of both structures were calculated for direct contact with a layered graphene electrode,and for contact with a van der Waals heterojunction,and it was found that the current was greatly increased and the contact resistance was significantly reduced for the direct contact structure.The transmission spectra at different voltages and the LDOS in the central region are used to explain the cause of this phenomenon.The results show that both the central region and the electrode in the direct contact structure provide more states,it also shows the metal inducted gap states,making it easier for electrons to be transmitted,while the van der Waals structure has fewer states,making it difficult for electrons to be transported and thus the current is lower.