Influence of edge passivation on the transport properties of the zigzag phosphorene nanoribbons

By using first-principles calculation based on density functional theory and non-equilibrium Green's function method, we investigate the transport properties of zigzag phosphorene nanoribbons (ZPNRs). The edges of the ZPNRs can be passivated in three ways named W1, W2, W3. These calculated results show that the electronic transport properties of the ZPNRs can be seriously influenced by the edge passivation ways, and the transport is determined by both the two edges and the interaction between them. Moreover, we find the width of the ZPNR can switch on or switch off the transport channel of the W3-type ZPNR. Furthermore, we present the transmission spectra, the band structures of both left and right electrodes, the molecular energy levels, and transmission eigenstates of the H-S-passivated W3-type ZPNRs to uncover the transport mechanism. This study provides a theoretical support for designing the related nanodevices by changing the passivation ways, which is an effective route for tuning the electronic structures and the transport properties of the phosphorene nanoribbons.     

Band structure and edge states of star-like zigzag graphene nanoribbons

Connecting three zigzag graphene nanoribbons(ZGNRs) together through the sp~3 hybrid bonds forms a star-like ZGNR(S-ZGNR). Its band structure shows that there are four edge states at k = 0.5, in which the three electrons distribute at three outside edge sites, and the last electron is shared equally(50%) by two sites near the central site. The lowest conductance step in the valley is 2, two times higher than that of monolayer ZGNR(M-ZGNR). Furthermore, in one quasithree-dimensional hexagonal lattice built, both of the Dirac points and the zero-energy states appear in the band structure along the z-axis for the fixed zero k-point in the x-y plane. In addition, it is an insulator in the x-y plane due to band gap 4 eV, however, for any k-point in the x-y plane the zero-energy states always exist at k_z = 0.5.     

Electronic and transport properties of zigzag phosphorene nanoribbons doped with ordered Si atoms

Using density functional theory (DFT) and the nonequilibrium Green's function method, we explored the electronic structures and transport properties of zigzag phosphorene nanoribbons (ZPNRs) with ordered doping of Si atoms. Our results show that both pristine and Si-doped ZPNRs exhibit metallic properties and the conductance of the doped ZPNRs nanoribbons can be modulated effectively by changing doping positions and concentrations. As different doping positions, different transmission currents can be obtained even with the same doping concentration. Moreover, current amplification factors vary with the doping concentrations. In addition, compared with the pristine system, negative differential resistance effect can also be observed in doped system (Si3), which occurs in lower bias range.     

Rb吸附石墨烯纳米带电子性质和光学性质的研究

本文基于密度泛函理论的第一性原理方法了计算了Rb、O和H吸附石墨烯纳米带的差分电荷密度、能带结构、分波态密度和介电函数,调制了石墨烯纳米带的电子性质和光学性质,给出了不同杂质影响材料光学特性的规律.结果表明本征石墨烯纳米带为n型直接带隙半导体且带隙值为0.639 eV;Rb原子吸附石墨烯纳米带之后变为n型简并直接带隙半导体,带隙值为0.494eV;Rb和O吸附石墨烯纳米带变为p型简并直接带隙半导体,带隙值增加为0.996eV;增加H吸附石墨烯纳米带后,半导体类型变为n型直接带隙半导体,且带隙变为0.299eV,带隙值相对减小,更有利于半导体发光器件制备.吸附Rb、O和H原子后,石墨烯纳米带中电荷密度发生转移,导致C、Rb、O和H之间成键作用显著.吸附Rb之后,在费米能级附近由C-2p、Rb-5s贡献;增加O原子吸附之后,O-2p在费米能级附近贡献非常活跃,杂化效应使费米能级分裂出一条能带;再增加H原子吸附之后,Rb-4p贡献发生蓝移,O-2p在费米能级附近贡献非常强,费米能级分裂出两条能带.Rb、O和H的吸附后,明显调制了石墨烯纳米带的光学性质.     

电场调控双层石墨烯纳米带的电子结构和光学性质

利用基于密度泛函理论的第一性原理方法,研究了外加电场作用下双层AA堆垛的Armchair边缘石墨烯纳米带(BAGNRs)的电子结构和光学性质. BAGNRs具有半导体特性,其带隙随带宽(宽度为4～12个碳原子)的增加而振荡性减小.当施加电场后,BAGNRs的带隙随着电场强度的增加而逐渐减小,带隙越大对电场值的变化越敏感.当电场值为0.5 V/?时,所有BAGNRs的带隙都为零. BAGNRs具有各向异性的光学性质,其介电函数在垂直极化方向为半导体特性,而在平行极化方向为金属特性.在外加电场的作用下,BAGNRs的介电函数、吸收系数、折射系数、反射系数、电子能量损失系数和光电导率,其峰值向低能量区域移动,即产生红移现象.电场增强了能带间的跃迁几率.纳米带宽度对这些光学性质参数具有不同程度的影响.研究结果解释了电场调控BAGNRs光学性质的规律和微观机理.     

Electronic properties of phosphorene and graphene nanoribbons with edge vacancies in magnetic field

The graphene and phosphorene nanostructures have a big potential application in a large area of today's research in physics. However, their methods of synthesis still don't allow the production of perfect materials with an intact molecular structure. In this paper, the occurrence of atomic vacancies was considered in the edge structure of the zigzag phosphorene and graphene nanoribbons. For different concentrations of these edge vacancies, their influence on the metallic properties was investigated. The calculations were performed for different sizes of the unit cell. Furthermore, for a smaller size, the influence of a uniform magnetic field was added.     

The influence of edge defects on the electrical and thermal transport of graphene nanoribbons

The electrical conductance, thermopower, thermal conductance and figure of merit of graphene nanoribbons (GNRs) are investigated using Green function formalism in the linear response regime. The Hamiltonian of GNR is described by the tight-binding approach and the effect of elastic interactions due to the electron–electron interaction or the thermal environmental fluctuations is considered by dephasing approach within the self-consistent Born approximation. The results show that the dephasing process leads to the reduction of the electrical transport of GNRs. Since the edge configuration of GNRs has the significant role in their electronic properties, it is shown that the electrical and thermal transports of the GNRs are decreased by the edge defects while the reduction of thermal conductance is more efficient, therefore, the thermal efficiency of GNRs is increased.     

Tuning the electronic and magnetic properties of zigzag silicene nanoribbons by 585 defects

Using first-principles calculations, we study the geometric structures, formation energies, electronic and magnetic properties of zigzag silicene nanoribbons (ZSiNRs) with 585 defects. There are two kinds of 585 defects in ZSiNRs referred as 585-I and 585-II, respectively. It is found that no matter which one of the two types, it is the most stable at the edge of the ZSiNR, and at this time it is even more stable than that in an infinite silicene sheet. Utilizing 585 defects, it can transform ZSiNRs from antiferromagnetic semiconductors into metals or half-metals. Especially, defective ZSiNRs may display nearly 100% spin-polarized half-metallic behavior induced by the 585-II defect, which maybe have potential applications in silicon-based spintronic devices. These results present the possibility of modulating the electronic and magnetic properties of ZSiNRs using 585 defects.     

Field-modulated low-energy electronic and optical properties of armchair silicene nanoribbons

The tight-binding model including spin–orbit coupling is used to study electronic and optical properties of armchair silicene nanoribbons (ASiNRs) in electric fields. Perpendicular electric field monotonically increases band-gap, the DOS, and absorption frequency and strength. It does not change spin-degeneracy, edge-states, and optical selection rule. However, parallel electric field strongly modulates energy dispersions resulting in oscillatory band-gaps, shift in edge-states, and destruction of spin-degeneracy. It induces more transition channels and constructs new selection rules that exhibits richer optical spectra. Modulations of electronic and optical properties of ASiNRs have strong dependence on the direction of electric field and nanoribbon's geometry.     

Electronic and magnetic properties of zigzag edge graphene nanoribbons with Stone-Wales defects

The electronic and magnetic properties of zigzag graphene nanoribbons (ZGNRs) with Stone-Wales defects are studied by extensive first-principles calculations. It is shown that the asymmetry distribution of the Stone-Wales defects can induce finite magnetic moment in the defective ZGNRs. As the defect near one of the ribbon edges moving to the centre region, the magnetic moment of the defective ZGNRs gradually decreases to zero, following a transition from metal to semi-half-metal and eventually to semiconductor. In addition, by symmetrically placing an additional defect at the opposite side of the defective ZGNRs, the finite magnetic moment vanishes, and the electronic properties depend on the distance between the defect and the closer ribbon edge. These findings are robust within a wide range of defect concentration.     

铂掺杂扶手椅型石墨烯纳米带的电学特性研究

本文采用基于密度泛函理论(DFT)的第一性原理计算了铂原子填充扶手椅型石墨烯纳米带(AGNR)中双空位结构的电学性能.计算结果表明: 通过控制铂原子的掺杂位置, 可以实现纳米带循环经历小带隙半导体—金属—大带隙半导体的相变过程; 纳米带边缘位置是铂原子掺杂的最稳定位置, 边缘掺杂纳米带的带隙值随宽度的变化与本征AGNR一样可用三簇曲线表示, 但在较大宽度时简并成两条曲线, 一定程度上抑制了带隙值的振荡; 并且铂原子边缘掺杂导致宽度系数N_a = 3p和3p + 1(p是一个整数)的几个较窄纳米带的带隙中出现杂质能级, 有效地降低了其过大的带隙值. 此外, 铂掺杂AGNR的能带结构对掺杂浓度不是很敏感, 从而降低了对实验精度的挑战. 本文的计算有利于推动石墨烯纳米带在纳米电子学方面的应用.     

Field-induced spin-polarized electronic and optical properties of armchair stanene nanoribbons

Electronic and optical properties of armchair stanene nanoribbons are studied within the $s{p}^3$ tight-binding model including spin-orbit coupling in the presence of in-plane electric field. Electric field strongly modulates energy dispersions leading to a zero-gap transition, shift in edge-states, and exhibition of spin-splitting states. Then, the complex dielectric functions in the long wavelength limit is calculated from the gradient approximation. More field-induced transition channels exhibit richer optical spectra which further reveal spin-polarized feature at low frequency. Prominent plasmons in loss spectra come from π–σ mixing orbital. The plasmon peak frequency and height are tuned by field strength. Also, the threshold plasmon frequency linearly decreases as electric field increases and it vanishes at critical field. The reflectance exhibits oscillatory behaviors and shows dip structures with sharp plasmon edge, undergoing a red-shift with increasing field. The calculated results fully show that field-modulations of electronic and optical properties strongly depend on nanoribbon's geometry.     

Electronic structure and optic absorption of phosphorene under strain

We studied the electronic structure and optic absorption of phosphorene (monolayer of black phosphorus) under strain. Strain was found to be a powerful tool for the band structure engineering. The in-plane strain in armchair or zigzag direction changes the effective mass components along both directions, while the vertical strain only has significant effect on the effective mass in the armchair direction. The band gap is narrowed by compressive in-plane strain and tensile vertical strain. Under certain strain configurations, the gap is closed and the energy band evolves to the semi-Dirac type: the dispersion is linear in the armchair direction and is gapless quadratic in the zigzag direction. The band-edge optic absorption is completely polarized along the armchair direction, and the polarization rate is reduced when the photon energy increases. Strain not only changes the absorption edge (the smallest photon energy for electron transition), but also the absorption polarization.     

金掺杂锯齿型石墨烯纳米带的电磁学特性研究

本文采用基于密度泛函理论的第一性原理计算了金原子填充锯齿型石墨烯纳米带 (ZGNRs)中双空位结构的电磁学特性. 计算结果表明: 边缘位置是金原子的最稳定掺杂位置, 杂质原子的引入导致掺杂边缘的磁性被抑制, 不过掺杂率足够大时, 掺杂边缘的磁性反而恢复了. 金掺杂纳米带的能带结构对掺杂率敏感: 随着掺杂率的增大, 掺杂纳米带分别表现半导体特性、半金属特性以及金属特性. 本文的计算表明金原子掺杂可以调制ZGNR的磁性以及能带特性, 为后续实验起指导作用, 有利于推动石墨烯材料在自旋电子学方面的应用.     

Tuning spin-filtering,rectifying, and negative differential resistance by hydrogenation on topological edge defects of zigzag silicene nanoribbons

By first-principles calculations, we propose three heterojunction nanodevices based on zigzag silicene nanoribbons with different edge-hydrogenated topological line defects. The devices all present excellent spin-filtering properties with 100% spin polarization as well as remarkable rectifying effect (with rectification ratio around 10²) and negative differential resistance behaviors. Our findings shed new light on the design of silicon-based nanodevices with intriguing spintronic applications.     

Defect engineering on the electronic and transport properties of one-dimensional armchair phosphorene nanoribbons

We investigate the electronic and transport properties of one-dimensional armchair phosphorene nanoribbons(APNRs) containing atomic vacancies with different distributions and concentrations using ab initio density functional calculations. It is found that the atomic vacancies are easier to form and detain at the edge region rather than a random distribution through analyzing formation energy and diffusion barrier. The highly local defect states are generated at the vicinity of the Fermi level, and emerge a deep-to-shallow transformation as the width increases after introducing vacancies in APNRs.Moreover, the electrical transport of APNRs with vacancies is enhanced compared to that of the perfect counterparts. Our results provide a theoretical guidance for the further research and applications of PNRs through defect engineering.     

层数调控磷烯能带与光学性质的研究

近年来,黑磷作为兼具石墨烯和过渡金属硫化物之长的新型二维材料而倍受关注.本文基于密度泛函理论,研究了不同厚度黑磷的电子结构与光学性质.结果表明,黑磷的性质与其厚度密切相关,可通过厚度调整实现能带与光学性质的可调控性.层间相互作用导致费米能级附近价带和导带的劈裂,是造成黑磷带隙随层数减小的根本原因.黒磷的静态折射率和静态反射率的大小均随层数的增大有增大的趋势,并且各层黑磷的反射峰均位于紫外光波段.黑磷对光的吸收涵盖了可见光到紫外光区域,对光的损失范围小于4eV.本文基于能带图和分波态密度图,从电子跃迁的角度分析了黑磷各项光学性质的变化情况,旨在为黑磷的带隙及光学性质层数可调控性提供理论依据.     

Structural stability and electronic properties of Ni-doped armchair graphene nanoribbons

The size dependent electronic properties of armchair graphene nanoribbons (AGNR) with Ni doped atoms have been investigated using spin-unrestricted density functional theory. We predict antiferromagnetic (AFM) ground states for Ni-termination and one edge Ni-doping. The computed formation energy reveals that one edge Ni-terminated AGNR are energetically more favourable as compared to pristine ribbons. One edge substitutional doping is energetically more favourable as compared to centre doping by ∼1 eV whereas both edge doping is unfavourable. The bond length of substitutional Ni atoms is shorter than that of Ni adsorption in GNR, implying a stronger binding for substitutional Ni atoms. It is evident that binding energy is also affected by the coordination number of the foreign atom. The results show that Ni-interaction perturbs the electronic structure of the ribbons significantly, causing enhanced metallicity for all configurations irrespective of doping site. The band structures reveal the separation of spin up and down electronic states indicating towards the existence of spin polarized current in Ni-terminated and one edge doped ribbons. Our calculation predicts that AGNR containing Ni impurities can play an important role for the fabrication of spin filters and spintronic devices.     

Magnetic and electronic properties of zigzag BN nanoribbons with nonmetallic atom terminations: A first-principles study

The boron nitride (BN) nanosheet is an isostructural analog of graphene and can be viewed as the structure that C atoms in graphene are replaced with alternating B and N. The easily modulated band-gap of BN nanosheet by simply passivating its edge(s) makes it is promising for many potential applications in nanodevices and nanoelectronics. We further systematically theoretically study the magnetic and electronic properties of passivated-ZBNNR by nonmetallic atom(s), here. According to our calculations, all considered structures show magnetic feature, and the ZBNNRs can be metal or half-metal or semiconductor depending on the termination details. The great application-potential of the passivated-ZBNNRs is further confirmed based on our results.