Understanding of electron transport through nanostructures becomes an important problem with the advancement of the fabrication process to construct atomic-scale devices using single molecules or carbon nanotubes (CNT). Due to the drastic change of transport properties by the contact conditions to electrodes in local electric fields, first-principles calculation approaches with various atomic configurations including even tiny gap structures at contacts are indispensable to understand and characterize the transport properties of nanometer-scale molecular devices.
For these purposes, we have developed an ab initio calculation method based on the density-functional formalism. Using the recursion-transfer-matrix (RTM) method, which is a reliable tool for obtaining accurate scattering waves with plane-wave expansions, combined with non-equilibrium Green's function (NEGF) method, we study the transport properties between metallic electrodes through single molecules. Especially, we investigate how the atomic-scale contacts to electrodes affect quantum transport.
We find that transport behaviors change significantly due to the contacts. When contacts to both electrodes are fairly perfect, transport properties are determined by the HOMO-LUMO states by resonant tunneling processes. However, as the contact to one electrode becomes worse, we find that transport properties are mostly determined by tunneling condition with strong non-linear behaviors of I-V characteristics and molecular states are difficult to observe in the conductance data. Furthermore, we find that negative differential resistance (NDR) appears at some distances between single molecules and one of the electrodes. We will show detailed data of electronic states at contacts in strong electronic fields for various single molecules and will clarify the mechanism for these anomalous transport behaviors in view of the relationship of HOMO-LUMO resonant states and tunneling vs. ballistic transport with various contact conditions to electrodes.
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