低维石墨烯纳米体系的电子结构及输运机理研究

发布时间：2018-03-27 12:07

本文选题：密度泛函理论　切入点：非平衡格林函数　出处：《湖南大学》2016年博士论文

【摘要】：本论文在阐述了分子电子学产生背景和分子器件应用前景的基础上,采用了基于密度泛函理论和非平衡格林函数输运理论相结合的第一性原理计算方法,系统地研究了分子体系的电子结构和输运性质。着重研究了配位原子掺杂、分子链的长度、电极-分子的接触类型、分子结的构型、化学修饰以及门压等对体系电子输运性质的影响。观察到明显的负微分电阻行为、电学整流效应,深入探讨了分子器件中的开关比、自旋过滤以及自旋极化等,并给出了相应的输运量子调控的理论机理。本论文主要研究以下几部分内容:运用基于密度泛函数理论和非平衡态格林函数电子输运计算相结合的第一性原理方法,我们研究了用双N原子取代掺杂两个准相邻晶格C原子的扶手型石墨烯纳米条带的电子能带结构及电荷输运特性。研究结果表明N2A A掺杂扶手性纳米条带的电学性能根据N2A A的位置不同可以呈现半导体和金属特性,因此我们可以通过调控N2A A位置实现扶手型石墨烯纳米条带的开关效应。我们所构建的体系在半导体和金属性质之间的转变主要归根于N原子和C原子的Pz轨道之间的耦合作用。此外在这种双N掺杂的扶手型纳米条带器件中,我们可以观察到明显的负微分电阻效应。因此通过调控N2A A位置可以使分子体系呈现完全不同的电荷输运性质。接着,我们研究了在不同宽度的N2A A掺杂的扶手型石墨烯纳米条带器件的输运性质。我们用N2A A取代分子器件中一个电极的中心或者边缘的双C原子,而体系的另一个电极则不掺杂。结果表明,分子器件的电子输运性质不仅依赖双N原子的掺杂位置,而且也受纳米条带宽度的影响。通过调节扶手型石墨烯纳米带的宽度以及N2A A的位置,可以有效的调控器件的整流率。进一步发现,宽度为10的边界N2A A掺杂的扶手型石墨烯纳米分子器件的整流比最大,而且我们证明了体系左右电极能带的匹配以及其对应的分子投影自洽哈密顿量间的耦合作用是产生明显整流效应的根本原因。运用密度泛函理论与非平衡格林函数相结合的方法,我们研究了边缘钝化的锯齿形石墨烯纳米带异质结的输运特性。计算结果表明sp2和sp3共存可以使锯齿形石墨烯纳米带变成具有稳定带隙的半导体材料。我们发现边界为sp2和sp3钝化的锯齿形石墨烯纳米条带之间轨道对称不匹配抑制了电导边界态,而且带隙的大小主要是由sp2边界锯齿形石墨烯纳米带的第二最高价带与第二最低导带之间的能量差来决定。研究发现sp2和sp3边界钝化的锯齿形石墨烯纳米条带的半导体特性也受条带宽度的影响,在宽度为奇数时,分子器件的能隙都较小,且带隙随其宽度的增加递减地很快。而当宽度为偶数时,体系的带隙都相对比较大,其随宽度递减比较缓慢。我们研究了单碳链通过不同的连接方式夹在两个锯齿形石墨烯纳米条带电极中间组成的碳链分子结的自旋输运性质。通过改变碳链与锯齿形石墨烯之间的连接方式调制器件自旋输运特性。结果表明,当体系左右电极处于反铁磁态时,整体电导能力较弱,表现为半导体性质。而体系处于铁磁态时,自旋向下的电子被抑制,仍然保持绝缘体性质,而自旋向上的电子电导能力较好,表现为金属性,因而在费米能附近出现完美的自旋分离效应。在碳链长度确定的情况下,53连接的构型的自旋向上的电子比56/36分子结的导电性能较好。其次,这些不对称分子器件的自旋输运在一定的能量范围内都表现100%的自旋电流极化效应。进一步研究表明,这些完美自旋过滤效应主要与不对称体系的分子轨道与电极耦合强度有关。研究了一种由铬卟啉连接两条单原子碳链夹在半无线长的锯齿形石墨烯纳米条带中间所组成的分子器件的自旋输运性质。研究结果表明,当体系处于P态时,费米能处自旋向下的电子被抑制,其自旋向上的电子态密度分布却是离散的,说明自旋向上的电子在体系中电导较好,表现为金属性;而当体系处于AP态时,体系中的电子的传输能力被强烈抑制,使体系呈半导体性。此外,体系在P态与AP态都能具有100%的自旋电流极化效应,而且在AP中有整流比高达104的整流效应。因此,我们构建的器件中能表现有整流效应、负微分电阻效应以及自旋极化为100%的自旋分离效应等多种分子开关行为共存性质。进一步的研究表明,这些自旋效应主要归根于锯齿形石墨烯纳米带的π和π*子带所固有的自旋输运选择以及分子轨道与电极之间耦合强度。
[Abstract]:In this paper the background and application prospect of molecular devices described in molecular electronics, the calculation method of density functional theory and non equilibrium Green function combined transport theory based on the first principle, systematic study of the electronic structure and transport properties of the molecular system. Focusing on the coordination of atoms and molecules the length of the chain, the contact type electrode molecule, molecular configuration, chemical modification and portal pressure transport properties of electrons. System was observed in the negative differential resistance behavior, electrical rectification effect, in-depth study of the molecular switch devices in the ratio of spin filtering and spin polarization, and gives the the transport mechanism of quantum control theory. This thesis mainly studies the following contents: Based on density functional theory and nonequilibrium Green function electron transport calculation combining The first principle method, we study the two atoms of armchair graphene nano doped two quasi lattice adjacent C atoms with electronic band structure and charge transport properties. The results show that the N2A doped A nanoribbons help chiral electrical properties according to the N2A A in different locations can present semiconductor and metal the characteristics, so we can achieve the armchair graphene nanoribbon switch effect by regulating the N2A A position. The coupling effect between the change in our constructed system between the semiconductor and metal properties mainly due to N and C atoms in the Pz orbital. In addition in the armchair with the double doped nano devices, we can observe the negative differential resistance effect. Therefore by regulating the N2A A position can make the system presents a completely different molecular charge transport properties. Then, we study in With the width of N2A A doped armchair graphene nanoribbon devices. We replace the transport properties of a molecular electrode devices in the center or edge using N2A A double C atom, and another electrode system is not doped. The results showed that the doping position of molecule device transport properties I only rely on atoms, but also influenced by nano strip width. By adjusting the width of the armchair graphene nano and the position of N2A A, can control the device effectively rectification rate. Further found that the width of armchair graphene nano molecular device N2A doped A 10 boundary of the rectification ratio but, we prove that the system can bring about electrode matching and their corresponding molecular projected self consistent Hamiltonian between coupling is the fundamental reason of the obvious rectifying effect. By using the density functional theory and non equilibrium Green function Combined method, we study the transport properties of zigzag graphene nano edge passivation with heterojunction. The calculation results show that SP2 and SP3 can make the coexistence of zigzag graphene nanoribbons into semiconductor materials with stable band gap. We found that the inhibitory conductance boundary boundary state between SP2 and SP3 passivation of sawtooth the shape of graphene nanoribbons orbital symmetry does not match, but the size of the band gap is mainly composed of SP2 boundary zigzag graphene nanoribbons with the highest price between second and second of the lowest conduction band energy difference to decide. The study found that the zigzag graphene nano SP2 and SP3 boundary passivation strips by semiconductor properties the effect of strip width, width in odd, molecular devices, energy gap is smaller, and the gap with the width decreasing quickly. When the width is even, the band gap of the system are relatively large, with the The width decreases slowly. We study the single carbon chain claw spin carbon chain molecule graphene nanoribbons composed of intermediate electrode junction in two saw the transport properties through different connections. By changing the carbon chain and zigzag graphene connection modulation devices spin transport properties. The results showed that when the system around the electrode in the antiferromagnetic state, the overall conductance capacity is weak, performance for the semiconductor properties. And the system is in a ferromagnetic state, spin down electrons are suppressed, and still maintain the insulator properties, electronic conductivity ability to spin up good performance for metal, so the Fermi energy in near perfect spin separation effect. The carbon chain length is determined, the conductive properties of 53 spin configuration and electronic upward than 56/36 molecular junctions is better. Secondly, spin these asymmetric molecular devices in a transport The energy range are spin polarized current effect 100%. Further study showed that the perfect spin filtering effect of molecular orbital and asymmetric system mainly related to electrode coupling strength. A study by chromium porphyrin linking the two atom carbon chain clip in spin transport properties of zigzag graphene nano wireless half long the band is composed of middle molecular devices. The results show that, when the system is in the P state, the Fermi energy at the electron spin down is suppressed, the spin up of the electron density distribution is discrete, electron spin up in system performance for metallic conductance is better; and when the system is in a AP state when the electron transfer capability in the system is strongly inhibited, so that the system is semiconductor. In addition, the current spin polarization effect system can have 100% in the P and AP States, and in AP the whole flow ratio Rectification effect up to 104. Therefore, we can construct the device performance rectification effect, negative differential resistance and spin polarization of 100% spin separation effect and other molecular switch coexistence properties. Further studies show that the spin effect is mainly attributed to zigzag graphene nanoribbons and pi pi * spin with inherent transmission coupling strength between the transport and molecular orbitals and electrodes.

【学位授予单位】：湖南大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O469

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