First-Principles Investigation on Transport Properties of Graphene-Based Systems

Abstract

The studies on the electronic and transport properties of graphene-based materials are necessary for understanding the experimental results and predicting possible applications. In our works, first-principles calculations, in which nonequilibrium Green?s function (NEGF) is combined with Density Functional Theory (DFT), are used to study the electronic and transport properties of graphene-based materials We first study the electronic and transport properties of carbon chains sandwiched between graphene electrodes. Carbon chain can be regarded as the extreme of graphene nanoribbon and may be the smallest unit for interconnection. Our results show that a long enough carbon chain possesses an entirely open transport channel, which is robust against hydrogen impurities and structural imperfections in carbon chains. However, the oxygen impurities, as the epoxy group, in this system dramatically decrease the conductance, indicating that the low conductance of carbon chain measured in experiments may be contributed to oxygen impurities. Besides that, negative differential resistance effect is found in double carbon chains. Moreover, we study the spin transport and find that perfect spin filter and spin valve effect simultaneously exist in the same system.

Next, spin transport properties of zigzag GNRs (ZGNRs) are investigated. The results show that ZGNR can play the role of a bipolar spin diode, in which spin polarized currents can be selected by controlling the bias and magnetic configuration. We attribute these interesting properties to the symmetry matching of wave functions of the two different spin subbands of ZGNRs. The controllable spin polarized currents enable us to theoretically design spin transistors and logic gates. Our results demonstrate that ZGNR can be a potential candidature for integrating logic operations and digital storage for carbon-based spintronics.

Besides, graphene-based spin caloritronics, a new research field which explores the possibility to directly generate spin currents and operate spintronics devices using temperature difference, is investigated. We predict that magnetized ZGNRs (M-ZGNRs) possess several intriguing properties. A strongly spin polarized current can be generated in M-ZGNRs using temperature difference instead of external electric bias. Moreover, this thermally induced spin polarized current in M-ZGNRs can be controlled by thermal (i.e. temperature), electrical (gate voltage) or magnetic means, thereby providing a rich set of thermal spin components, including spin filter, spin diode, spin field effect transistor (FET) and magnetoresistance (MR) device.

In addition, we study the transport properties of ZGNRs-based heterostructure, which consists of hydrogen terminated ZGNR (ZGNR-H) and oxygen terminated ZGNR (ZGNR-O). We find that ZGNR-H/ZGNR-O heterostructure integrates several useful functions on controlling both charge and spin currents. We find a large transmission gap near the Fermi energy and the transmission spectrum is highly asymmetric, which is very favourable to creating currents by temperature difference. Moreover, we find spin filter and MR effects with either electric or temperature bias.

Finally, we study the effect of different edge functional groups on the electronic and transport properties of ZGNRs. we find the metallic behaviour of ZGNRs with various edge functional groups under finite bias. The existence of edge states is robust against these chemical functional groups except for the case of edge oxidization, which changes dramatically the band structure of ZGNRs and gives rise to three complete open conductance channels. The good conductance of edge oxidization shows little width dependence and removes the requirement for symmetry compared to hydrogen terminated ones. On the other hand, Oxygen-containing absorbents and other defects can deteriorate the conductivity.