Theoretical Study Of The Doping Effect On The Conductivity Of Materials

With the continuous development of doping techniques, doping has becomea very important strategy to improve the conductivity of materials. There areseveral commonly used doping techniques, such as chemical doping,electrochemical doping and physical doping. However, the doping mechanism,i.e., the physical and chemical images in the doping process, is still obscure. Howdoes dopant affect the electronic structure of materials? Wether the conductivityof materials could be tuned by changing the electronic structure through dopingtechiques. It is therefore important to investigate the doping effect and the dopingmechanism from a theoretical point of view. The revelation of information aboutthe doping effect and the construction of relationship between the conductivityand the electronic structure have great significance for designing novel materialswith excellent properties. By combining density functional theory and non-equilibrium Green’s function formalism, we calculated the transport properties oftwo different types of materials. The doping effects on their conductivities werediscussed based on the calculated results. This work is composed of two parts:(1) B-, P-doping effects on the conductivity of CNT/oligosilane/CNT hetero-junctions;(2) rare earth element-doping effects on the conductivity of δ-MoN.We calculated the transport properties of thirteen one-dimensional σ-πconjugated CNT/oligosilane/CNT heterojunctions (oligosilane chain(s) grafted tothe mouth of CNTs). The effects of B-doping, P-doping and B-, P-codoping uponthe oligosilane moiety on the conductivity of CNT/oligosilane/CNT hetero-junctions were systematically studied. The mechanism of B-and P-dopings wasexplored. We have found that the B-and P-dopings upon the oligosilane moietycould not only enhance the conductivity but also give rise to multiple NDRbehavior for the CNT/oligosilane/CNT heterojunctions. The effects of dopanttype, dopant concentration and doping position on the conductivity of CNT/oligosilane/CNT heterojunctions were comparatively studied. It is foundthat the B-doped systems show higher conductivity than the P-doped ones owingto their better electron conjugation effect. The increase of dopant concentrationcould significantly reduce the HOMO-LUMO gap and thus enhance NDR effect.The doped system possesses the highest conductivity when the dopant B or P sitsat the center of oligosilane chain due to the strong coupling between theoligosilane and the electrodes. The B-, P-codoping could induce a recifyingeffect and the rectification ratio is up to7.19.We calculated the transport properties of δ-MoN. The rare earth elementdoping effects on the conductivity of δ-MoN were theoretially investigated. Thedoping mechanism was explained from the crystral structures and the electronicstructures. Five special rare earth elements, La, Ce, Pr, Gd and Yb, were seletedas dopants to substitude Mo atoms in δ-MoN. The valence electronconfigurations of La, Ce, Pr, Gd and Yb are4f05d16s2,4f15d16s2,4f35d06s2,4f75d16s2and4f145d06s2, respectively. Two different substitutional sites wereconsidered:2a and6c sites in the Wyckoff positions. Upon the Pr-and Yb-dopings, we found an increase of the conductivity, while a drop under the La-Ce-, and Gd-dopings. For the Pr-doped and Yb-doped systems, the Pr-4f andYb-4f states contribute peaks near the Fermi level, leading to the increase ofcarrier density, which is benefical to improve the conductivity. On the contrary,the La-4f, Ce-4f and Gd-4f states are localized far away from the Fermi level,which can not effectively increase the carrier density. The drop of theconductivity caused by La-Ce-and Gd-dopings should be assigned to thedecrease of carrier mobility well induced by dopants. The similarity ofconductivities between the systems with dopants sitting at2a site and6c site isattributed to their similar bonding characteristics.