原子吸附与扩散引起类石墨烯材料电子性质改变的理论研究
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摘要
自2004年以来,石墨烯成为凝聚态物理、材料、化学、光电子、生物与能源等领域的研究热点。近几年,类石墨烯二维纳米材料也渐成研究前沿。尤其是新型的类石墨烯材料氮化硼和硅烯的制备成功,更大大鼓舞了科研工作者的研究热情。低维纳米材料具有与其化学结构相对应的、独特的光电子性质以及其它性能,在纳米光电子、洁净能源、生物医学等方面具有广泛的应用前景。
计算机模拟是沟通理论和实验的桥梁,具有研究周期短,工作效率高等优点,它已经成为理论分析和实验研究之外的第三大工具。在材料设计和性能预测方面,基于密度泛函理论的第一性原理计算方法往往被认为是最行之有效的方法,适于研究纳米材料的稳定结构、电子结构、磁学性质、光学吸收与激发以及化学反应过程等。本论文采用了第一性原理计算方法,着重研究了金原子和氢原子在二维和一维纳米体系表面上的吸附与扩散过程以及由此对体系的电子性质造成的影响。研究工作中涉及到了石墨烯、二维氮化硼、硅烯、硅烷以及它们相对应的纳米带结构。
论文第一章讲述该论文的研究背景和选题意义；第二章主要介绍了本论文所采用的计算方法的理论基础和主要程序包；第三章到第五章,详细介绍并总结了作者在攻读博士学位期间所做的主要工作和取得的研究成果。主要内容和结论如下：
1.我们从理论上系统的研究了金原子和金二聚体在完整的和有缺陷的石墨烯及氮化硼上的稳定吸附结构、能量、电子转移情况以及衬底自身能带的变化情况。为了得到最稳定的吸附构型,我们考虑几十种初始状态。优化后发现,金原子不论在完整的还是有缺陷的石墨烯表面上都喜欢稳定在碳原子上方,而金二聚体则不同,始终稳定在桥位上。然而,当金原子及其二聚体在氮化硼衬底表面上时均喜欢呆在原子上方这个位置。把完整衬底和带有缺陷的衬底加以对比,就会发现,石墨烯的硼、氮替代缺陷和氮化硼中的碳替代缺陷都能有效固定住金纳米颗粒。从能量角度来说,六个被研究的衬底中碳替代氮的氮化硼衬底最好。原子吸附以后,衬底的能带结构发生一些变化。既有能带的平移也有新的局域能带出现在费米能级附近,这种变化在氮化硼体系中表现的最明显。接下来我们对碳替代氮的氮化硼体系做了态密度和投影态密度的分析,发现衬底和金原子之间的相互租用之所以会加强,根本在于他们之间发生了电荷转移。投影态密度显示费米能级附近的能带主要来自金的5s、6d轨道和C、B、N的2p轨道交叠。这样,我们既得到了两种体系的异同之处,还发现氮化硼对金的催化过程有促进作用,而不是惰性的。
2.虽然BN单层具有很好的热稳定性和化学稳定性,但是BN纳米结构的宽带隙阻碍了它们在电子器件领域的应用。对其进行掺杂或对边界进行修饰是改变其电子性质的一种有效途径。本文将通过密度泛函理论计算来研究Au原子在BN和A/Z-BNNRs两种纳米带上的吸附和扩散情况。当Au原子吸附在BN单层上时,经充分优化后,Au原子只能够稳定在BT(B原子正上方)和NT(N原子正上方)两个位置。经对比,BT比NT吸附能大0.206eV,更有优势。同时,BN衬底的B原子向上凸起0.182A,但BN整体结构不会改变。另外Au的扩散结果显示,Au沿B-N-B锯齿形路径势垒最低,只有0.007eV。这使得Au原子可以在BN片上沿锯齿形路径自由扩散。金在纳米带上面的扩散研究表明,在两种纳米带上Au原子都倾向于稳定吸附在边缘的B位置上。另外,Au原子可以很容易的从内部区域偏析到A-BNNRs的两边以及Z-BNNRs的B边界,并且纵向扩散时,次边界路径是很有利的。Au原子沿A-BNNRs次边界的势垒为0.111eV,而在Z-BNNRs的次边界扩散几乎没有势垒。如果沿边界纵向扩散,则需要克服大一些的势垒,范围在0.345-0.507eV之间。能带结构显示低浓度的Au原子吸附后BNNRs仍然是宽带隙半导体；提高Au浓度以后,A-BNNRS带隙变小,而Z-BNNRs带隙消失,对外呈现金属性质。对BNNRs电子性质的成功调节有望在将来的试验中得到证实。这样,通过Au的修饰达到了调节Z-BNNRs体系的电子性质的目的。进一步分析发现,相邻Au原子之间的电荷沿着边缘的锯齿形状分布是导致带隙闭合的原因所在。
3.我们采用第一性原理,针对硅烯、硅烷及其纳米带的稳定结构和电子结构的调控规律进行了研究。已经报道的硅烷构型有四种,其中chair和boat构型的能量比较相近。首先我们优化了两种硅烷结构,发现chair比boat的能量低38meV/Si,这与已有文献Lew et al和Wang et al的结论一致。即使这样,我们仍然给出boat构型的能带结构,结果显示两种硅烷结构都是自旋兼并的宽带隙半导体。其中,chair构型是间接带隙的,VBM和CBM分别对应G点和K点；boat构型是直接带隙的,VBM、CBM都在G点。对比GNRs,根据硅烷纳米带边界类型也可以将其分为A/ZSilicane-NRs两类,对应的带宽用NA和Nz表示。经优化得到稳定结构,伴随带宽变大,Z-和A-纳米带的带隙都逐渐减小,且在相同宽度时,前者的带隙比后者的带隙小。相应的boat构型纳米带也给出了相似的结果,且都无自旋劈裂。硅烯纳米带A/Z-Silicene-NRs的性质和边界形状、H的饱和情况以及外界条件有关。在无外加电场、边界饱和一个H时,ZSilicene-NRs基态是AFM,亚稳态是FM,电子性质丰富。从应用角度出发,我们希望获得铁磁半导体性质的ZSilicene-NRs。所以,我们以Silicane-NRs和Silicene-NRs做为结构单元,把它们沿带宽方向自左向右拼接。并把对应宽度定义为N、M,得到杂化体系A/Z-N/M-NRs。先看A-N/M-NRs(N+M=9),对N、M两侧褶皱程度都是从左到右逐渐减小,带隙随M呈现震荡减小趋势。震荡规律可总结为3n+2<3n<3n+l,其中n为任意正整数。再看Z-N/M-NRs (N+M=8),它的界面处值得特别关注,因为是zigzag链,加H时有半饱和与全饱和之分。经优化,形变部分主要集中在界面处。(1)界面半饱和情况：硅烯一侧的轨道分布呈铁磁序分布,主要来自Pz轨道的贡献,且从中心区域向边沿区域递减。能带发生明显劈裂,产生1单位磁矩,且劈裂的缺陷态对应的带隙随M可调节。(2)界面全饱和情况：硅烯一侧的轨道分布呈反铁磁序分布,因为界面处和边沿处的Si原子静电势不完全相等,能带有微小劈裂。VBM和VBM之间的带隙也是随M可以调节的。同时,也测试了boat结构的杂化体系,结果相似。上述结果表明,基于硅烯我们能够得到铁磁半导体性质的结构,为制备新型纳米电子器件提供了理论依据。
Since2004, graphene has become a hot topic in condensed matter physics, chemistry, photonics, biology and energy and other fields. Meanwhile, two-dimensional graphene-like materials also become the research frontier. In particular, the successful preparation of boron nitride and silicene has greatly inspired the enthusiasm of scientific research workers. Low-dimensional materials with its unique chemical structure and corresponding optoelectronic and other properties, will have broad application prospects in nano-photonics, clean energy, bio-medicine.
Computer simulation is the communication bridge between theory and experiment, the high efficiency and shortly research cycle making it become the third largest tools beyond theoretical analysis and experimental research. In terms of material-designing and properties prediction, first-principles calculations based on density functional theory is often considered as the most effective way to study nanomaterials for structural stability, electronic structure, magnetic properties, optical absorption and excitation and chemical reaction processes. First-principles calculation was performed to explore the atomic adsorption and diffusion processes on the two-dimensional and one-dimensional system, which will impact the electronic properties of the systems in turn. The materials involved in our work include two-dimensional (2D) graphene, boron nitride, silicene, silicane and their corresponding one-dimensional (1D) nanoribbons.
The thesis is organized as follows:
Chapter I gives a brief introduction of research background and motivation. Chapter II introduces the theoretical fundamentals used in our research work. Chapters III to VI describe in detail and summarize the work done during my Ph.D degree studies. The main content and results in this dissertation are listed as follows:
1. Energetic and structural properties of gold atom (Au) and gold dimer (Au dimer) adsorbed on pristine and defective graphene (Gra) and boron nitride monolayer (BN) are investigated using density functional theory. The electron transfering as well as band structure changing are also given in the results. We considered dozens of initial configurations to get the most stable adsorption structure. After optimization, Au atom prefers stabling at atop site to the other site when it adsorbed on graphen, whereas Au dimer like bridge site. Interestingly, Au/Au-dimer prefers a top sites over other hollow or bridge sites on pristine and alternatively defective BN. Through comparing the perfect and defective substrate, it is easy to find that the doped defects can trap Au/Au-dimer effectively. From the point of view of energy, BN-NC is the best surface among the six tested. There are some changes took place for band structure after atomic adsorption, including both band translation and new bands appearing at Fermi energy, which is very obvious in BN system. In the following, we obtained DOS and PDOS of Au/Au-dimer on BN-NC, founding the reason of the strengthen interaction between Au and substrate is that charge transfer occurring between them. The contribution mainly comes from the hybridization of Au-5d,6s, C-2p, N-2p and B-2p. In this way, we got the similarities and differences between the two systems and obtained that pristine or defective BN can no longer be treated as inert substrates.
2. Although BN monolayer has good thermal and chemical stability, but the band gap hinders their application in the electronic devices. Doping or modification of edges is an effective way of tailoring the electronic properties of nanoribbons. In this contribution, we performed first-principles calculations within density functional theory (DFT)to investigate the adsorption and diffusion of Au adatoms on BNNRs with zigzag or armchair edges. When the Au atoms adsorbed on BN monolayer, they can only stable at BT (atop at B atom) and NT (atop at N atom) two positions after full optimization. The B site configuration is marginally favored over the N site configuration on the BN sheet by about0.206eV, and both of them are significantly more favorable than the hollow site configuration. Meanwhile, the boron atoms move slightly upward by0.182A°but the original structure of BN is preserved. In addition, we found that Au diffusion from the B site to the neighboring N site encounters the lowest barrier of0.007eV when it on the BN sheet. This implies that Au adatom prefers to diffuse along the path B-N-B when it sites on BN sheet. It is clear that the energetically preferable site for the Au adatom on the BNNRs is not right above a B atom but has a small deviation. Au adatom can easily segregate from inner region toward both edges of A-BNNRs and to the B edge of Z-BNNRs. The longitudinal diffusion along subedges is energetically favorable. For A-BNNRs, the energy barrier is only0.111eV but negligible for Z-BNNRs. In contrast, the longitudinal diffusion of Au atom along edge has to overcome higher energy barriers,0.345-0.507eV. The electronic structure calculations indicate the wide-band-gap features are preserved in the Au/BNNRs as the Au adatoms concentration is low. When increasing the Au concentration, the band gap for Z-BNNRs is closed, yet, for A-BNNRs, narrowing the band gap of the system with more localized bands emerging within the band gap. These results are wished to be validated by further experiments. Thus, by Au doping can realize the purpose of tailoring the electronic properties of nanoribbons. Further analysis revealed that the band gap closure in Au-doped Z-BNNRs is related to the interaction between the Au atomic chain and the edge B atoms.
3. First-principles calculations were performed to explore the intriguing electronic and magnetic properties of silicane and hybrid silicane-silicene nanoribbons. There have been reported four kinds of silicane configurations, in which the chair structure is the most energetic favorable one and followed by the boat one. At the beginning, we optimized the two configurations of silicane and found that the chair-silicane is more stable than the boat one by about38meV/atom. This is agreement with Lew et al. and Ding et al. very well. Yet, we still display the band structures of boat-silicane and the results tell us that both of them are wide gap semiconductor with spin degenerate. The corresponding valance band maximum (VBM) and conduction band minimum (CBM) are at the G point and K point for chair one, whereas, both VBM and CBM are at G point for boat one. Like the carbon counterparts, all the Silicane-NRs can be classified into armchair and zigzag ones according to the edge-shape and the width across the ribbon can be classified by the number of dimer lines (NA) or zigzag chains (Nz). We can see that the band gaps decrease with width increasing and gas of ASilicane-NRs are larger than that of ZSilicane-NRs with the same width conditions. Corresponding boat-NRs resembles the above results with no spin-spilt band structures. In the absence of an electric-field, the ZSilicene-NRs terminated with mono-hydrogen display a semiconducting feature at the ground AFM state, while the metstable ferromagnetic state is metallic. From the application point of view, the ferromagnetic semiconducting characteristic is highly desired in the ZSilicene-NRs. So we consider hybrid Silicane-NRs with Silicene-NRs side by side and the former side abbreviated as N while the latter one abbreviated as M, thus the hybrid system denoted as A/Z-N/M-NRs. For A-N/M-NRs with N+M=9, the band gaps present oscillatory behavior and can be classified into three families with M=3n,3n+1and3n+2(where n is a positive integer), respectively. Yet in Z-N/M-NRs, the zigzag chain at the interface can be either fully hydrogenated or half hydrogenated, which will lead to a very different results.(1) The interface half hydrogenated:The ground state consist of ferromagnetic ordering along the zigzag chains of the ZSilicene-NRs part and descending from central region to edge area which mainly come from Pz oribtals. The imbalance between the spin up and spin down states results in a net magnetic moment of1.000μB per unit along the ribbon and the gaps can be tuned by M.(2) interface fully hydrogenated:the states between the ZSilicene-NRs edge and the Silicane-Silicene interface have opposite spins and approximately zero magnetic moment in the unit cell. However, because of the chemical potential difference between the ZSilicene-NRs edge and the interface, the bands nearest the Fermi level are not completely degenerate. Gaps between VBM and CBM also varying with M and the boat configurations give the similar results as chair ones. Thus, the hybrid systems have rich electronic properties for gap engineering and for applications as a spin filter.
计算机模拟是沟通理论和实验的桥梁,具有研究周期短,工作效率高等优点,它已经成为理论分析和实验研究之外的第三大工具。在材料设计和性能预测方面,基于密度泛函理论的第一性原理计算方法往往被认为是最行之有效的方法,适于研究纳米材料的稳定结构、电子结构、磁学性质、光学吸收与激发以及化学反应过程等。本论文采用了第一性原理计算方法,着重研究了金原子和氢原子在二维和一维纳米体系表面上的吸附与扩散过程以及由此对体系的电子性质造成的影响。研究工作中涉及到了石墨烯、二维氮化硼、硅烯、硅烷以及它们相对应的纳米带结构。
论文第一章讲述该论文的研究背景和选题意义；第二章主要介绍了本论文所采用的计算方法的理论基础和主要程序包；第三章到第五章,详细介绍并总结了作者在攻读博士学位期间所做的主要工作和取得的研究成果。主要内容和结论如下：
1.我们从理论上系统的研究了金原子和金二聚体在完整的和有缺陷的石墨烯及氮化硼上的稳定吸附结构、能量、电子转移情况以及衬底自身能带的变化情况。为了得到最稳定的吸附构型,我们考虑几十种初始状态。优化后发现,金原子不论在完整的还是有缺陷的石墨烯表面上都喜欢稳定在碳原子上方,而金二聚体则不同,始终稳定在桥位上。然而,当金原子及其二聚体在氮化硼衬底表面上时均喜欢呆在原子上方这个位置。把完整衬底和带有缺陷的衬底加以对比,就会发现,石墨烯的硼、氮替代缺陷和氮化硼中的碳替代缺陷都能有效固定住金纳米颗粒。从能量角度来说,六个被研究的衬底中碳替代氮的氮化硼衬底最好。原子吸附以后,衬底的能带结构发生一些变化。既有能带的平移也有新的局域能带出现在费米能级附近,这种变化在氮化硼体系中表现的最明显。接下来我们对碳替代氮的氮化硼体系做了态密度和投影态密度的分析,发现衬底和金原子之间的相互租用之所以会加强,根本在于他们之间发生了电荷转移。投影态密度显示费米能级附近的能带主要来自金的5s、6d轨道和C、B、N的2p轨道交叠。这样,我们既得到了两种体系的异同之处,还发现氮化硼对金的催化过程有促进作用,而不是惰性的。
2.虽然BN单层具有很好的热稳定性和化学稳定性,但是BN纳米结构的宽带隙阻碍了它们在电子器件领域的应用。对其进行掺杂或对边界进行修饰是改变其电子性质的一种有效途径。本文将通过密度泛函理论计算来研究Au原子在BN和A/Z-BNNRs两种纳米带上的吸附和扩散情况。当Au原子吸附在BN单层上时,经充分优化后,Au原子只能够稳定在BT(B原子正上方)和NT(N原子正上方)两个位置。经对比,BT比NT吸附能大0.206eV,更有优势。同时,BN衬底的B原子向上凸起0.182A,但BN整体结构不会改变。另外Au的扩散结果显示,Au沿B-N-B锯齿形路径势垒最低,只有0.007eV。这使得Au原子可以在BN片上沿锯齿形路径自由扩散。金在纳米带上面的扩散研究表明,在两种纳米带上Au原子都倾向于稳定吸附在边缘的B位置上。另外,Au原子可以很容易的从内部区域偏析到A-BNNRs的两边以及Z-BNNRs的B边界,并且纵向扩散时,次边界路径是很有利的。Au原子沿A-BNNRs次边界的势垒为0.111eV,而在Z-BNNRs的次边界扩散几乎没有势垒。如果沿边界纵向扩散,则需要克服大一些的势垒,范围在0.345-0.507eV之间。能带结构显示低浓度的Au原子吸附后BNNRs仍然是宽带隙半导体；提高Au浓度以后,A-BNNRS带隙变小,而Z-BNNRs带隙消失,对外呈现金属性质。对BNNRs电子性质的成功调节有望在将来的试验中得到证实。这样,通过Au的修饰达到了调节Z-BNNRs体系的电子性质的目的。进一步分析发现,相邻Au原子之间的电荷沿着边缘的锯齿形状分布是导致带隙闭合的原因所在。
3.我们采用第一性原理,针对硅烯、硅烷及其纳米带的稳定结构和电子结构的调控规律进行了研究。已经报道的硅烷构型有四种,其中chair和boat构型的能量比较相近。首先我们优化了两种硅烷结构,发现chair比boat的能量低38meV/Si,这与已有文献Lew et al和Wang et al的结论一致。即使这样,我们仍然给出boat构型的能带结构,结果显示两种硅烷结构都是自旋兼并的宽带隙半导体。其中,chair构型是间接带隙的,VBM和CBM分别对应G点和K点；boat构型是直接带隙的,VBM、CBM都在G点。对比GNRs,根据硅烷纳米带边界类型也可以将其分为A/ZSilicane-NRs两类,对应的带宽用NA和Nz表示。经优化得到稳定结构,伴随带宽变大,Z-和A-纳米带的带隙都逐渐减小,且在相同宽度时,前者的带隙比后者的带隙小。相应的boat构型纳米带也给出了相似的结果,且都无自旋劈裂。硅烯纳米带A/Z-Silicene-NRs的性质和边界形状、H的饱和情况以及外界条件有关。在无外加电场、边界饱和一个H时,ZSilicene-NRs基态是AFM,亚稳态是FM,电子性质丰富。从应用角度出发,我们希望获得铁磁半导体性质的ZSilicene-NRs。所以,我们以Silicane-NRs和Silicene-NRs做为结构单元,把它们沿带宽方向自左向右拼接。并把对应宽度定义为N、M,得到杂化体系A/Z-N/M-NRs。先看A-N/M-NRs(N+M=9),对N、M两侧褶皱程度都是从左到右逐渐减小,带隙随M呈现震荡减小趋势。震荡规律可总结为3n+2<3n<3n+l,其中n为任意正整数。再看Z-N/M-NRs (N+M=8),它的界面处值得特别关注,因为是zigzag链,加H时有半饱和与全饱和之分。经优化,形变部分主要集中在界面处。(1)界面半饱和情况：硅烯一侧的轨道分布呈铁磁序分布,主要来自Pz轨道的贡献,且从中心区域向边沿区域递减。能带发生明显劈裂,产生1单位磁矩,且劈裂的缺陷态对应的带隙随M可调节。(2)界面全饱和情况：硅烯一侧的轨道分布呈反铁磁序分布,因为界面处和边沿处的Si原子静电势不完全相等,能带有微小劈裂。VBM和VBM之间的带隙也是随M可以调节的。同时,也测试了boat结构的杂化体系,结果相似。上述结果表明,基于硅烯我们能够得到铁磁半导体性质的结构,为制备新型纳米电子器件提供了理论依据。
Since2004, graphene has become a hot topic in condensed matter physics, chemistry, photonics, biology and energy and other fields. Meanwhile, two-dimensional graphene-like materials also become the research frontier. In particular, the successful preparation of boron nitride and silicene has greatly inspired the enthusiasm of scientific research workers. Low-dimensional materials with its unique chemical structure and corresponding optoelectronic and other properties, will have broad application prospects in nano-photonics, clean energy, bio-medicine.
Computer simulation is the communication bridge between theory and experiment, the high efficiency and shortly research cycle making it become the third largest tools beyond theoretical analysis and experimental research. In terms of material-designing and properties prediction, first-principles calculations based on density functional theory is often considered as the most effective way to study nanomaterials for structural stability, electronic structure, magnetic properties, optical absorption and excitation and chemical reaction processes. First-principles calculation was performed to explore the atomic adsorption and diffusion processes on the two-dimensional and one-dimensional system, which will impact the electronic properties of the systems in turn. The materials involved in our work include two-dimensional (2D) graphene, boron nitride, silicene, silicane and their corresponding one-dimensional (1D) nanoribbons.
The thesis is organized as follows:
Chapter I gives a brief introduction of research background and motivation. Chapter II introduces the theoretical fundamentals used in our research work. Chapters III to VI describe in detail and summarize the work done during my Ph.D degree studies. The main content and results in this dissertation are listed as follows:
1. Energetic and structural properties of gold atom (Au) and gold dimer (Au dimer) adsorbed on pristine and defective graphene (Gra) and boron nitride monolayer (BN) are investigated using density functional theory. The electron transfering as well as band structure changing are also given in the results. We considered dozens of initial configurations to get the most stable adsorption structure. After optimization, Au atom prefers stabling at atop site to the other site when it adsorbed on graphen, whereas Au dimer like bridge site. Interestingly, Au/Au-dimer prefers a top sites over other hollow or bridge sites on pristine and alternatively defective BN. Through comparing the perfect and defective substrate, it is easy to find that the doped defects can trap Au/Au-dimer effectively. From the point of view of energy, BN-NC is the best surface among the six tested. There are some changes took place for band structure after atomic adsorption, including both band translation and new bands appearing at Fermi energy, which is very obvious in BN system. In the following, we obtained DOS and PDOS of Au/Au-dimer on BN-NC, founding the reason of the strengthen interaction between Au and substrate is that charge transfer occurring between them. The contribution mainly comes from the hybridization of Au-5d,6s, C-2p, N-2p and B-2p. In this way, we got the similarities and differences between the two systems and obtained that pristine or defective BN can no longer be treated as inert substrates.
2. Although BN monolayer has good thermal and chemical stability, but the band gap hinders their application in the electronic devices. Doping or modification of edges is an effective way of tailoring the electronic properties of nanoribbons. In this contribution, we performed first-principles calculations within density functional theory (DFT)to investigate the adsorption and diffusion of Au adatoms on BNNRs with zigzag or armchair edges. When the Au atoms adsorbed on BN monolayer, they can only stable at BT (atop at B atom) and NT (atop at N atom) two positions after full optimization. The B site configuration is marginally favored over the N site configuration on the BN sheet by about0.206eV, and both of them are significantly more favorable than the hollow site configuration. Meanwhile, the boron atoms move slightly upward by0.182A°but the original structure of BN is preserved. In addition, we found that Au diffusion from the B site to the neighboring N site encounters the lowest barrier of0.007eV when it on the BN sheet. This implies that Au adatom prefers to diffuse along the path B-N-B when it sites on BN sheet. It is clear that the energetically preferable site for the Au adatom on the BNNRs is not right above a B atom but has a small deviation. Au adatom can easily segregate from inner region toward both edges of A-BNNRs and to the B edge of Z-BNNRs. The longitudinal diffusion along subedges is energetically favorable. For A-BNNRs, the energy barrier is only0.111eV but negligible for Z-BNNRs. In contrast, the longitudinal diffusion of Au atom along edge has to overcome higher energy barriers,0.345-0.507eV. The electronic structure calculations indicate the wide-band-gap features are preserved in the Au/BNNRs as the Au adatoms concentration is low. When increasing the Au concentration, the band gap for Z-BNNRs is closed, yet, for A-BNNRs, narrowing the band gap of the system with more localized bands emerging within the band gap. These results are wished to be validated by further experiments. Thus, by Au doping can realize the purpose of tailoring the electronic properties of nanoribbons. Further analysis revealed that the band gap closure in Au-doped Z-BNNRs is related to the interaction between the Au atomic chain and the edge B atoms.
3. First-principles calculations were performed to explore the intriguing electronic and magnetic properties of silicane and hybrid silicane-silicene nanoribbons. There have been reported four kinds of silicane configurations, in which the chair structure is the most energetic favorable one and followed by the boat one. At the beginning, we optimized the two configurations of silicane and found that the chair-silicane is more stable than the boat one by about38meV/atom. This is agreement with Lew et al. and Ding et al. very well. Yet, we still display the band structures of boat-silicane and the results tell us that both of them are wide gap semiconductor with spin degenerate. The corresponding valance band maximum (VBM) and conduction band minimum (CBM) are at the G point and K point for chair one, whereas, both VBM and CBM are at G point for boat one. Like the carbon counterparts, all the Silicane-NRs can be classified into armchair and zigzag ones according to the edge-shape and the width across the ribbon can be classified by the number of dimer lines (NA) or zigzag chains (Nz). We can see that the band gaps decrease with width increasing and gas of ASilicane-NRs are larger than that of ZSilicane-NRs with the same width conditions. Corresponding boat-NRs resembles the above results with no spin-spilt band structures. In the absence of an electric-field, the ZSilicene-NRs terminated with mono-hydrogen display a semiconducting feature at the ground AFM state, while the metstable ferromagnetic state is metallic. From the application point of view, the ferromagnetic semiconducting characteristic is highly desired in the ZSilicene-NRs. So we consider hybrid Silicane-NRs with Silicene-NRs side by side and the former side abbreviated as N while the latter one abbreviated as M, thus the hybrid system denoted as A/Z-N/M-NRs. For A-N/M-NRs with N+M=9, the band gaps present oscillatory behavior and can be classified into three families with M=3n,3n+1and3n+2(where n is a positive integer), respectively. Yet in Z-N/M-NRs, the zigzag chain at the interface can be either fully hydrogenated or half hydrogenated, which will lead to a very different results.(1) The interface half hydrogenated:The ground state consist of ferromagnetic ordering along the zigzag chains of the ZSilicene-NRs part and descending from central region to edge area which mainly come from Pz oribtals. The imbalance between the spin up and spin down states results in a net magnetic moment of1.000μB per unit along the ribbon and the gaps can be tuned by M.(2) interface fully hydrogenated:the states between the ZSilicene-NRs edge and the Silicane-Silicene interface have opposite spins and approximately zero magnetic moment in the unit cell. However, because of the chemical potential difference between the ZSilicene-NRs edge and the interface, the bands nearest the Fermi level are not completely degenerate. Gaps between VBM and CBM also varying with M and the boat configurations give the similar results as chair ones. Thus, the hybrid systems have rich electronic properties for gap engineering and for applications as a spin filter.
引文
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