缺陷对石墨烯纳米带金属度的弱化作用

Weakening effect of defects on the metallicity of graphene nanoribbons

通过在石墨烯纳米带(graphene nanoribbon,GNR)中引入一对跳跃参数相等的零模(即一对零模态的C–C键)构建金属石墨烯纳米带模型,在金属GNR模型的基础上建立单空位缺陷浓度为1.429%和2.857%的石墨烯纳米带模型,并基于密度泛函理论探究缺陷位置对其金属度的影响.研究表明:零模C–C键引入后,GNR表现出比未引入之前宽得多的金属带,其金属带宽由91.49 meV增加为452.92 meV.在金属GNR中,缺陷所处位置对引入单空位缺陷的难易程度影响较大.在GNR超晶胞中引入一个单空位缺陷时,缺陷对距离较近的零模C–C键产生的影响远强于较远的零模,这导致GNR几何结构对称性被破坏,局域的电荷转移加剧.当缺陷位置接近纳米带边缘时,GNR的电子特性容易发生改变,部分GNR带隙被打开,缺陷模型完成从金属向半导体的转变.在缺陷浓度为2.857%的GNR中,由于缺陷位置的对称设置,GNR在几何构型和电荷转移情况方面保留了其原始对称性,最大程度地保证了两零模间的跳跃幅度,使GNR保持金属性不变.石墨烯纳米带缺陷模型的金属带宽均低于仅引入零模C–C键的GNR模型金属带宽,说明缺陷对GNR的金属度有着不可忽视的削弱作用,不仅容易将原本呈现金属特性的GNR转变为半导体,还弱化了其金属度.

Graphene nanoribbon(GNR),one of the most common graphene derivatives,has gradually become a research hotspot with its unique properties of high carrier concentration,high conductivity,atomic-level thickness plane structure,and large carrier mobility.However,in the current manufacturing process,various defects inevitably exist in the synthesis of GNRs.Many studies have shown that defects have a substantial effect on the properties of GNRs.However,the effect of defects on the metallicity of GNRs has never been reported.Recently,Rizzo et al.achieved the regulation of the metallicity of metallic GNRs by inducing the formation of only two new C–C bonds per GNR unit cell and successfully fabricated them using a bottom-up approach.This seemingly insignificant and small change causes a chemical bond rearrangement that causes a substantial change in the electronic structure,which has attracted widespread attention in the scientific community.This kind of GNR with metallic properties has extremely high application value as a connecting device.At the same time,it is also of scientific significance to explore its metallicity and metal bandwidth by exploring the Luttinger liquid of 1-dimensional materials,plasma,charge density wave,and superconductivity.Inspired by this research,this paper hopes to clarify the mechanism and law of the effect of defects on the metallicity of GNRs to design the defect molecular structure with atomically precise control and regulate the metal bandwidth.First,a pair of zero modes with equal jump parameters is introduced into GNRs to construct the metal GNR model.The electronic properties of the model are calculated based on density functional theory.Next,GNR models with single-vacancy defect concentrations of 1.429%and 2.857%were constructed,and the electronic properties of the models were calculated based on density functional theory.Then,the effect of defect location on its metallic character was investigated.Studies have shown that by introducing the zero modes,the GNR exhibits a much wider metal bandwidth that increases from 91.49 to 452.92 meV.This method induces changes in the metal bandwidth because the five-membered ring formed in the GNR after the introduction of the zero modes causes the polarization loss of the sublattice,thereby enhancing the metallicity.Introducing single-vacancy defects into metallic GNRs has little effect on their stability,and the geometry does not distort greatly.When a single-vacancy defect is introduced into the GNR supercell,the effect of the defect on the C–C bonds is much stronger in the close zero mode than in the distant zero mode,destroying the geometrical symmetry of the GNR,and the localized charge transfer is intensified.The electronic properties of GNRs are prone to change when defect sites are near the edge of the nanoribbons.A part of the GNR band gap is opened,and the defect model completes the transition from metal to semiconductor.For GNRs with two symmetric single-vacancy defects in the supercell,the GNRs generally retain their original symmetry in terms of geometry and charge transfer because of the symmetrical arrangement of defect sites.This arrangement ensures that the hopping amplitude between the two zero modes is the same to the greatest extent so that the GNR keeps the metallicity unchanged.All metallic bandwidths of the GNR defect models are lower than those of the GNR model that only introduces zero mode C–C bonds.This comparison shows that defects have a non-negligible weakening effect on the metallicity of GNRs,which not only easily converts the original metallic GNRs into semiconductors but also weakens their metallicity.