【作者】 尉靖；

【导师】 曾涛；

【作者基本信息】 哈尔滨理工大学 ， 材料学， 2013， 博士

【摘要】 随着各种掺杂技术的不断完善，掺杂已经成为改善材料导电性的重要手段，目前常用的掺杂方法主要有化学掺杂、电化学掺杂和物理掺杂等。然而，关于导电材料的掺杂机理，即掺杂过程中详细的物理和化学图像，尚不十分明确。比如，掺杂元素如何改变材料的电子结构，能否通过掺杂改变材料的电子结构从而进一步对其导电性进行调控等。因此，从理论角度计算掺杂对材料导电性的影响，并分析其掺杂机理，揭示掺杂对材料电子结构影响的信息，建立电子结构与导电性之间的关系，对于设计出性能优异的导电材料具有重要意义。本文采用密度泛函理论并结合非平衡格林函数方法，从理论角度计算了两种不同类型导电材料的电子输运性质，讨论了掺杂对它们导电性的影响，共进行了两部分研究工作：(1)B、P掺杂对碳纳米管/聚硅烷异质结导电性的影响；(2)稀土掺杂对δ-MoN导电性的影响。研究了碳纳米管口接枝聚硅烷形成的一维σ-π共轭体系，即碳纳米管/聚硅烷异质结的电子输运性质，重点考察了B原子掺杂、P原子掺杂以及B、P共掺杂聚硅烷主链对其导电性的影响，并对掺杂导致其导电性改变的原因和机理进行了深入分析。研究发现，B原子和P原子掺杂不仅显著提高了碳纳米管/聚硅烷异质结的电导率，还引起了多重NDR效应。采用一种比较的方式对影响体系导电性的掺杂因素包括掺杂原子的种类、浓度以及掺杂位置进行了研究。结果发现，B掺杂体系中的电子具有更好的共轭效应，导致B掺杂体系的电导率高于P掺杂体系；掺杂原子浓度的增大能够明显降低HOMO-LUMO能隙，从而显著增强体系的NDR效应；当B原子或P原子取代聚硅烷主链中心位置的Si原子时，中心聚硅烷分子与左、右电极间的耦合程度最好，此时体系的电导率最大；B、P共掺杂导致体系出现了明显的整流效应，其整流比可达7.19。研究了δ-MoN的电子输运性质，讨论了稀土掺杂对其导电性的影响，通过对晶体结构和电子结构的分析探讨了稀土元素的掺杂机理。共选取了五种具有代表性的稀土元素：La、Ce、Pr、Gd和Yb，它们的价电子层结构分别为：4f05d16s2、4f15d16s2、4f35d06s2、4f75d16s2和4f145d06s2，比较全面的考虑了多种稀土元素的4f电子构型对δ-MoN导电性的影响。对于每一种稀土元素，又分别考虑了两种不同的取代位置：Wockoff坐标的2a位置和6c位置。研究发现，除Pr和Yb掺杂提高了δ-MoN的电导率以外，La、Ce和Gd掺杂均使δ-MoN的电导率下降，这主要是由于Pr-4f和Yb-4f轨道电子贡献的态密度峰距离费米能级较近，导致费米能级附近的载流子浓度增大；而La-4f、Ce4f和Gd-4f轨道电子对态密度的贡献均局域在距离费米能级较远的区域，并不能有效增加费米能级附近的载流子浓度，掺杂引起的载流子迁移率的降低是La、Ce和Gd掺杂导致δ-MoN导电性降低的主要原因。当稀土原子取代2a位置和6c位置的Mo原子时对δ-MoN导电性影响的相似性主要归因于两类掺杂体系相似的成键特征。更多还原

【Abstract】 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.更多还原