First-principles Study Of Thermoelectric Properties Of Graphene Nanoribbons And Its Regulation

In recent years, as the rapid progresses in nano-technology and atomic manipula-tion technique, new electrical, thermal and thermoelectric properties, which completely different from bulk materials, have been found in nano-device. These singular proper-ties have attracted more and more attentions from experimental and theoretical studies. In this dissertation, by applying the first-principles calculation in combination with the non-equilibrium Green’s function, we have systematically investigated the thermoelectric properties of GNR.First of all, we investigate the thermal transport properties of PHD in GNRs with different orientation and width. We find that PHD effect on thermal conduction in AAYn is stronger than that of ZZYn. Besides, the effect of phonon dispersion mismatch on thermal conduction is as important as PHD. Interestingly, the ZA-mode will be almost scatted by the PHD at a particular frequency ω0, and a clockwise and counterclockwise circulating thermal current can be observed in AAYn. The thermal conduction ratio η depend on the orientation of the PHD and insensitive to the width of PHD. When N=40, more than97%incident phonons will be scatted by the PHDs. These results will be helpful for designing high-performance thermal insulation or thermoelectric nanodevice which based on graphene.We investigate the effect of magnetic atom on the band structure of ZGNRs. We found that, the band gap in the narrow ZGNRs are opened duo to the fact that part of the P↑/↓(r) will flow from the one side to the other side by the magnetic atom. While for the wide ZGNRs, part of the p↓(r) will flip to P↑(r) to lower its energy in magnetic atom. This behavior will lower the π↑bands and raise the π*↓bands of the magnetic atom applied side, neglect the other side. These results indicate that, the narrow ZGNRs can be semiconducting or metallic by applying or removing a magnetic atom at the edge, and the wide ZGNRs can be spin-up/spin-down half-metallic by magnetic atom and gate voltage. In addition, the Seebeck coefficient of narrow ZGNR will be reversed and enlarged by the magnetic atom. These results will be helpful for designing high-performance elec-tronic/thermoelectric nanodevice.We investigate the electronic transport properties of graphene/carbon tube (GN-R/CNT) intramolecular junctions. Duo to the difference between the delocalization of the highest-occupied-molecular-orbital (HOMO)/lowest-unoccupied-molecular-orbital (LUMO) at positive and negative bias, the ZGNR/(6,6)CNT intramolecular junction-s, which is metal-semiconductor (M-S), display a strong rectifying behavior. But the rectifying behavior is weak in AGNR/(10,0)CNT intramolecular junctions, which is semiconductor-semiconductor (S-S). Both the HOMO and LUMO in M-S/S-S will be located as the gate voltage increasing, resulting in a p-type semiconductor. The results are consistent with the experimental results.Then, we investigate the effect of chemical bonds as well as bridge atom mass on thermoelectric properties of AGNR. We found that the bridge atom mass plays an im-portant role in phonon scattering and the saturated thermal conductance decrease as the mass of bridge atom increasing duo to the fact that the heavier the bridge atom, the harder the atom moving. A simple equation is presented to describe the relationship among the thermal conductance, bridge atom, and temperature. On the other hand, the bridge atom plays a little effect on AGNR electronic properties. The figure of merit ZT will increase as the temperature increasing from100K to500K, and finally reaches0.8。As the tem-perature keep raising (from500K to1000K), the electronic thermal conductance will be lifted, resulting in the max of ZT decrease. The results indicate that the bridge atom can be used to adjust the the thermoelectric properties of AGNR.Base on the studies above, we study the thermoelectric properties of ZGNR with and Au-chains at the two edges (4ZGNRnAu). We found that, duo to the strong phonon scattering by the Au atoms at the edges, the phonon thermal conduction ratio η is lesser than5%at the room temperature. While the electronic transmission near the fermi level will stay duo to the fact that the the electrons in the edge states will transport smoothly by the way of HOMO-1and LUMO channel. As a consequence, the figure of merit ZT of4ZGNRnAu will over1at room temperature. The ZTmax reaches2.3735at500K. The ZTmax appears chain-length and temperature dependent behavior:The longer the chains, or the larger the temperature, the larger is the ZTmax.