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.     

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.     

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.     

拓扑相MoS2电子结构及光学性质的第一性原理研究

基于密度泛函理论的第一性原理对二维拓扑相1T′-MoS₂和2M-MoS₂的电子结构、有效质量和光学性质进行研究,并将其与二维H-MoS₂进行对比分析.研究表明,电子有效质量大小关系为：2M-MoS₂22,空穴有效质量大小关系为：T′-MoS₂<2M-MoS₂2,但2M-MoS₂的空穴有效质量和T′-MoS₂相差不大,二者均适用于高性能电子器件.由于拓扑相1T′-MoS₂和2M-MoS₂均存在能带反转,导致带间相关性以及导带和价带的波函数重叠增强,进而光电流响应增强,二者的光学性质均优于H-MoS₂. 2M-MoS₂具有较大的吸收系数和光电导率,2M-MoS₂对红外光和紫外光有着优...     

锯齿型石墨烯纳米窄带中量子霍尔体系的电场调控

应用含自洽格点在位库仑作用的Kane-Mele模型,研究锯齿型石墨烯纳米窄带平面内横向电场对边界带能带结构和量子自旋霍尔(QSH)体系的影响.研究结果显示,当电场强度较弱时,外加电场的方向可以调控自旋向下的两个边界带一起朝不同方向移动,导致波矢q=0.5处自旋向下的两个纯边界态的能量简并劈裂方向可由电场调控;当电场强度进一步增强到超过0.69 V/nm,自旋向下的两个边界带出现较大带隙,能带反转,而自旋向上的电子结构无能隙,系统呈现半金属性,同时QSH体系不再是B类.特别当电场强度为1.17 V/nm时,在自旋向下能带的能隙中,q=0.5处存在自旋向上的纯边界态,意味着在8格点边界处可以产生自旋向上的纯边界电流.当电场强度持续增加时,QSH系统从B类到C类经历3个阶段的变化.当电场强度超过1.42 V/nm后,自旋向上的两个边界带也出现能带反转,分别成为导带和价带,系统成为C类的普通量子霍尔体系.     

Ab-initio investigation of Ni(Fe)/ZrO2(0 0 1) and Ni-Fe/ZrO2(0 0 1) interfaces

The atomic and electronic structures of Me/ZrO₂(0 0 1) interfaces, where Me is Ni, Fe or a Ni-Fe alloy, are investigated by the plane wave pseudopotential method within density-functional theory. The work of separation of metal films from oxide substrate for the O- and Zr-terminated Me/ZrO₂(0 0 1) interfaces is calculated. High adhesion at both Me/(ZrO₂)_O and Me/(ZrO₂)_Zr interfaces is found. The effect of oxygen vacancies on the adhesion at the metal-ceramic interfaces is also investigated. It is shown that Ni(Fe)-O interaction at the O-terminated interface weakens in the presence of interfacial oxygen vacancies. At interfaces with Ni-Fe alloys the adhesion depends strongly on the composition of the interfacial layers and their magnetic properties.     

石墨烯纳米带的制备与电学特性调控

石墨烯由于其独特的晶体结构展现出了特殊的电学特性,其导带与价带相交于第一布里渊区的六个顶点处,形成带隙为零的半金属材料,具有优异的电子传输特性的同时也限制了其在电子学器件中的使用.因而科研人员尝试各种方法来打开其带隙并调控其能带特性,主要有利用缺陷、应力、掺杂、表面吸附、结构调控等手段.其中石墨烯纳米带由于量子边界效应和限制效应,存在带隙.本综述主要介绍了制备各类石墨烯纳米带的方法,并通过精确调控其细微结构,从而对其进行精确的能带调控,改变其电学特性,为其在电子学器件中的应用提供一些可行的方向.     

石墨烯纳米带能带结构及透射特性的扭曲效应

利用基于密度泛函理论的第一性原理方法, 系统研究了石墨烯纳米带(GNRs)电学性质的扭曲效应. 结果表明: 锯齿型石墨纳米带(ZGNRs)的带隙对扭曲形变最不敏感, 在扭曲过程中几乎保持金属性不变, 其次是W=3p-1型扶手椅型石墨烯纳米带(AGNRs), 扭曲时带隙也只有较小的变化. W=3p+1型AGNRs的带隙对扭曲最为敏感, 扭曲发生时, 呈现宽带隙半导体、中等带隙半导体、准金属、金属的变化, 其次是W=3p型AGNRs, 扭曲时带隙变化也较为明显. 换言之, GNRs在无扭曲时带隙越大, 扭曲发生后带隙变化(变小)越明显. 对于整个电子结构及透射系数来说, 扭曲对AGNRs影响较大, 而对ZGNRs的影响相对小些. 研究表明: 由于石墨烯容易变形, 其相关电子器件的设计必须适当考虑扭曲对电学性质的影响.     

Latent track creation in fused silica by 200 MeV silver beam

Studies of formation of latent tracks in swift heavy ion irradiated SiO₂ are presented. Fused silica (SiO₂) were irradiated with 200 MeV silver (Ag) ion beam at varying fluences. Radiation-induced effects were studied by ultraviolet(UV)/Visible optical absorption spectroscopy and transmission electron microscopy (TEM). UV/Visible absorption study indicated E′ centers and oxygen deficiency centers having characteristic absorption occurred at 5 eV. The density of these color centers calculated from the absorption peak intensity showed Poisson-type variation with irradiation fluence. The defects are thus entirely confined to the latent tracks created by swift heavy ions in SiO₂. The track radius estimated from optical absorption study was found to be 5.1 nm. Similar results were obtained from TEM studies of the irradiated samples.     

Electrical and optical properties of pure and KClO3-doped P(MMA-CO-4VPNO) polymer electrolyte films

Ion-conducting polymer electrolyte films based on a copolymer poly(methyl-methacrylate-co-4-vinyl pyridine N-oxide) [P(MMA-CO-4VPNO)] complexed with potassium chlorate (KClO₃) were prepared by solution cast technique. The complexation of KClO₃ salt with the polymer was confirmed by X-ray diffraction and infrared studies. The electrical conductivity and optical absorption of pure and KClO₃-doped P(MMA-CO-4VPNO) polymer electrolyte films have been studied. The electrical conductivity increased with increasing dopant concentration, which is attributed to the formation of charge transfer complexes. The variation of electrical conductivity with temperature shows two regions with two activation energies. Optical properties like direct band gap, indirect band gap, and optical absorption edge were investigated for pure and doped polymer films in the wavelength range 300–550 nm. It was found that the energy gaps and band edge values shifted to lower energies on doping. The behavior is in an agreement with the activation energies obtained from the conductivity data.     

Band gap opening in zigzag graphene nanoribbon modulated with magnetic atoms

The effects of magnetic atom on the band structure of zigzag-edged graphene nanoribbons are investigated by the density functional theory. The results show that for narrow zigzag-edged graphene nanoribbons, the band gap can be opened duo to the spin-up/spin-down charges being re-enriched on the edge sites. However, for the wide zigzag-edged graphene nanoribbons, a spin-up/spin-down half-metallic property can be observed. Moreover, it is found that the Seebeck coefficients in the narrow zigzag-edged graphene nanoribbons are reversed and enlarged, which provides a way to design novel thermoelectric device.     

（110）应变对立方相Ca2Si能带结构及光学性质的影响

采用第一性原理贋势平面波方法对(110)应变下立方相Ca2P0.25Si0.75的能带结构及光学性质进行模拟计算,全面分析了应变对Ca2P0.25Si0.75能带结构、光学性质的影响。计算结果表明：在92%~100%压应变范围内随着应变的逐渐增大导带向低能方向移动,价带向高能方向移动,带隙呈线性逐渐减小,但始终为直接带隙;在100%~102%张应变范围内随着应变的增加,带隙呈逐渐增大,应变达到102%直接带隙最大Eg=0.54378eV;在102%~104%应变范围内随着应变的增加,带隙逐渐减小;当应变大于104%带隙变为间接带隙且带隙随着应变增大而减小。施加应变Ca2P0.25Si0.75的介电常数、折射率均增大;施加压应变吸收系数增加,反射率减小;施加张应变吸收系数减小,反射率增加。综上所述,应变可以改变Ca2P0.25Si0.75的电子结构和光学常数,是调节Ca2P0.25Si0.75光电传输性能的有效手段。     

（110）应变对立方相Ca2Si能带结构及光学性质的影响

采用第一性原理贋势平面波方法对(110)应变下立方相Ca2P0.25Si0.75的能带结构及光学性质进行模拟计算,全面分析了应变对Ca2P0.25Si0.75能带结构、光学性质的影响。计算结果表明：在92%~100%压应变范围内随着应变的逐渐增大导带向低能方向移动,价带向高能方向移动,带隙呈线性逐渐减小,但始终为直接带隙;在100%~102%张应变范围内随着应变的增加,带隙呈逐渐增大,应变达到102%直接带隙最大Eg=0.54378eV;在102%~104%应变范围内随着应变的增加,带隙逐渐减小;当应变大于104%带隙变为间接带隙且带隙随着应变增大而减小。施加应变Ca2P0.25Si0.75的介电常数、折射率均增大;施加压应变吸收系数增加,反射率减小;施加张应变吸收系数减小,反射率增加。综上所述,应变可以改变Ca2P0.25Si0.75的电子结构和光学常数,是调节Ca2P0.25Si0.75光电传输性能的有效手段。     

GaAs半导体中的三光子吸收

本文从直接能隙半导体中三光子吸收跃迁速率的全量子理论表达式出发,在二能带和四能带理论模型下,分别就抛物线和非抛物线型能带,计算了GaAs半导体的三光子吸收系数。其结果与实验值作了比较。同时还给出了GaAs中三光子吸收系数的色散曲线。     

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.     

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.     

Synthesis and characterization of magnesium oxide nanocrystallites and probing the vacancy-type defects through positron annihilation studies

Magnesium oxide nanocrystallites exhibit certain abnormal characteristics when compared to those of other wide band gap oxide semiconductors in the sense they are most prone to water absorption and formation of a hydroxide layer on the surface. The problem can be rectified by heating and pure nanocrystallites can be synthesized with controllable sizes. Inevitably the defect properties are distinctly divided between two stages, the one with the hydroxide layer (region I) and the other after the removal of the layer by annealing (region II). The lattice parameters, the optical band gap and even the positron annihilation characteristics are conspicuous by their distinct behavior in the two stages of the surface configurations of nanoparticles. While region I was specific with the formation of positronium-hydrogen complexes that drastically altered the defect-specific positron lifetimes, pick-off annihilation of orthopositronium atoms marked region II. The vacancy clusters within the nanocrystallites also trapped positrons. They agglomerated due to the effect of the higher temperatures and resulted in the growth of the nanocrystallites. The coincidence Doppler broadening spectroscopic measurements supported these findings and all the more indicated the trapping of positrons additionally into the neutral divacancies and negatively charged trivacancies. This is apart from the Mg²⁺ monovacancies which acted as the dominant trapping centers for positrons.