First-Principles Study Of Single Molecular Functional Devices Design

Molecular electronics is a new electronic subject after the microelectronics,nanoelectronics,and now has attracted extensive attention by many researchers.Many molecular devices show the excellent physical properties,such as giant magnetoresistance,spin filtering,molecular rectifying,Kondo effect,etc.Now more and more molecules are designed into molecular devices such as molecular diode,molecular switch,molecular field effect transistor,molecular storage and so on,which become effective substitutes for replacing the traditional silicon-based electronics in the future.In this thesis,basing on density functional theory?DFT?combined with nonequilibrium Green's function?NEGF?method,we have investigated theoretically the electronic transport properties of some low dimensional molecular devices.The system includes dihydroazulene molecule,metallic organic chain(Fe_n+1?C₆H₄?_n),covalent-organic molecule?C₆H₈B₂O₄?and Co-salophen molecule.In these molecular devices,molecular switch,molecular rectifying,spin filtering and negative differential resistance effect,etc have been investigated.The major content of this thesis includes some following aspects:We investigate the molecular switch properties of a dihydroazulene molecule with the open and closed configurations sandwiched between two Au electrodes.This study demonstrates the dihydroazulene molecule shows good switch characteristics from the structure transformation between open and closed configurations by photo-excitation.In the bias region from 0 to 1.4 V,the current through the closed configuration is significantly higher than that of the open configuration within a stable switching ratio at finite bias.The mechanisms of the molecular switch are in detail analyzed by I-V characteristics,transmission spectra,spatial distribution of molecular orbitals and molecular projected self-consistent Hamiltonian?MPSH?states.We investigate the spin filtering transport properties of 1D metallic organic chain(Fe_n+1?C₆H₄?_n)sandwiched between two Au electrodes.Theoretical results reveal that Fe_n+1?C₆H₄?_n molecular chain exhibits robust spin filtering effect,and only the spin-down electrons can transmit through the molecular chain under the bias region from 0 to 1.0 V.The spin-down current also shows negative differential resistance behavior.With the increase of molecular chain length,the spin filtering efficiency is getting higher and higher.The calculated spin filter efficiency is close to 100%in the case of n?3.We find that the effect of spin polarization origin from both Fe_n+1and?C₆H₄?_n.The results can help us understand the spin transport properties of organic molecular chain.We investigate the spin-dependent transport properties of covalent-organic molecule?C₆H₈B₂O₄?embedded between two zigzag-edge graphene nanoribbon?ZGNR?electrodes.The results demonstrates that in the parallel?P?spin configuration of two ZGNR electrodes,the current of spin-up is greater than that of spin-down.The molecular device exhibits a robust spin filtering effect?up to 100%?.In another case of antiparallel?AP?spin configuration,dual spin filtering,spin rectifying as well as negative differential resistance?NDR?effects were discovered.Those features attribute to three factors:spin states match,symmetry of Bloch states,and the symmetry of central molecule's states.The match and mismatch of spin bands between two electrodes decide the spin transport process.These calculated results offer useful ideas for designing high-performance ZGNR-based molecular devices.We investigate the spin-dependent transport properties of a Co-salophen molecule sandwiched between two zigzag-edge graphene nanoribbon?ZGNR?electrodes.The calculation results show the spin-up current is significantly larger than the spin-down one and the spin-down current is nearly zero in the bias range from-1.0 to 1.0 V,which means the device exhibits a near perfect spin filtering effect in the parallel?P?spin configuration of two ZGNR electrodes.Meanwhile,the spin-up current also shows negative differential resistance behavior.For the antiparallel?AP?spin configuration,the device exhibits bipolar spin filtering and spin rectifying effects.The molecular device also showed higher giant magnetoresistance effect.By analyzing the spin-polarization transmission spectrum,the local density of states,the band structure of the electrodes and the projected self-consistent Hamiltonian?MPSH?states of molecular orbitals,the internal mechanism for multiple functional characteristics of the device is explained in detail.These results can be used as good reference significance for the design of the next generation nano devices.