Theoretical calculation of excitonic binding energies and optical absorption spectra for Armchair graphene nanoribbons

In this paper the excitons of armchair graphene nanoribbons with layers of different width and thickness have been investigated. In this investigation, the band structure and energy gap of armchair graphene nanoribbons have been calculated using a tight-binding model including edge deformation effects (all edge atoms have been passivated with hydrogen atoms). Also, by calculating the conductance in armchair graphene nanoribbons (A-GNRs) optical absorption of armchair graphene nanoribbon in the single-electron approximation has been obtained. Finally, the binding energy of excitons in armchair graphene nanoribbons has been calculated using the Wannier model, Hartree-Fock approximation and the Bethe-Salpeter equation.     

The influence of random edge defects on the electric and optical properties of phosphorene nanoribbons along zigzag direction

Phosphorene nanoribbons are one-dimensional semiconductors with possible edge states falling within its energy bandgap. We build the connection between the possible configurations of edge defects and the corresponding electric and optical properties in practice systems. The influence of the random defects or roughness at the edges of phosphorene nanoribbons cutting along zigzag direction is investigated quantitatively. Theoretical calculations show that the absorption peak due to the transitions involving edge states has an obvious blue shift with the zigzag-type positions at the edges increasing. The absorption thus can be used to estimate the random defects or roughness of the edges of phosphorene nanoribbons.     

Optical studies of carbon nanotubes and nanographites

Resonance Raman and photoluminescence excitation (PLE) spectroscopies are used to study the optical properties of different types of carbon nanostructures such as carbon nanotube, nanoribbons, nanographites and graphite edges. In the resonance Raman experiments of carbon nanotubes, the (n,m) assignment is obtained by comparing the experimental and theoretical diameter and chirality dependence of the optical transitions. The influence of the environment on the optical transitions of the nanotubes is also obtained in the Raman experiments. The PLE measurements in different samples of carbon nanotubes show both direct and phonon-assisted optical transitions, and the results give new evidences that the optical transitions in nanotubes have an excitonic character, which is very strong for the low energy transitions. We also analyze the Raman spectra of nanoribbons and nanographites, showing that this technique is an important tool for defect characterization in graphitic materials, and can be used to distinguish the atomic structure of the graphite edges.     

Characteristics of Li diffusion on silicene and zigzag nanoribbon

We perform a density functional study on the adsorption and diffusion of Li atoms on silicene sheet and zigzag nanoribbons. Our results show that the diffusion energy barrier of Li adatoms on silicene sheet is 0.25 eV, which is much lower than on graphene and Si bulk. The diffusion barriers along the axis of zigzag silicene nanoribbon range from 0.1 to0.25 eV due to an edge effect, while the diffusion energy barrier is about 0.5 eV for a Li adatom to enter into a silicene nanoribbon. Our calculations indicate that using silicene nanoribbons as anodes is favorable for a Li-ion battery.     

单空位缺陷对石墨纳米带电子结构和输运性质的影响

基于第一性原理电子结构和输运性质计算,研究了单空位缺陷对单层石墨纳米带(包括zigzag型和armchair型带)电子性质的影响.研究发现,单空位缺陷使石墨纳米带在费米面上出现一平直的缺陷态能带;单空位缺陷的引入使zigzag型半导体性的石墨纳米带变为金属性,这在能带工程中有重要的应用价值;奇数宽度的armchair型石墨纳米带表现出金属特性,有着很好的导电性能,同时,偶数宽度的armchair型石墨带虽有金属性的能带结构,但却有类似半导体的伏安特性;单空位缺陷使得奇数宽度的armchair石墨纳米带导电 关键词： 石墨纳米带 单空位缺陷 电子结构 输运性质     

扶手椅型石墨纳米带的双空位缺陷效应研究

采用基于密度泛函理论的第一性原理电子结构和输运性质计算,研究了扶手椅型石墨纳米带（具有锯齿边缘）的双空位缺陷效应.研究发现：双空位缺陷的存在并没有改变石墨纳米带的金属特性,但改变了费米面附近的能带结构.同时,双空位缺陷的取向对石墨纳米带的输运性质有很重要的影响.对于奇数宽度的纳米带,斜向双空位缺陷使得石墨带导电性能减弱,而垂直双空位能基本保留原有的线性伏安特性,导电性能降低较少;对于偶数宽度的纳米带,斜向双空位缺陷会使石墨带导电性能明显增强,而垂直双空位缺陷则具有完整石墨带的输运性质. 关键词： 石墨纳米带 585双空位缺陷 电子结构 输运性质     

Optical and dielectric properties of nanocomposites systems based on epoxy resins and reactive polyhedral oligosilsquioxanes

An epoxy network structure made of diglycidylether of bisphenol-A and diamino diphenylsulfone was modified by adding various amounts of an epoxy functionalized polyhedral oligomeric silsesquioxane. The obtained nanocomposites were characterized in terms of optical and dielectric properties. The UV-absorption spectra were collected in the wavelength range of 400–800 nm. The optical data were analyzed in terms of absorption formula for non-crystalline materials. The optical energy gap and other basic constants, such as energy tails, dielectric constants, refractive index and optical conductivity, were determined and showed a clear dependence on the POSS concentration. It was found that the optical energy gap for the neat epoxy resin is less than for nanocomposites, and it decreases with increase in the POSS content. The refractive index of nanocomposites was determined from the calculated values of absorption and reflectance. It was found that the refractive index and the dielectric constants increased with increase in the POSS concentration. The optical conductivity, which is a measure of the optical absorption, increased with the POSS content. Furthermore, it was found that the glass transition temperature and the optical energy gap correlate well with the POSS filler concentration.     

The optical conductivity of bilayer zigzag-edge graphene nanoribbons with external transverse electric fields

The optical absorption properties of bilayer zigzag-edge graphene nanoribbons (BL-ZGNRs) with external transverse electric fields are investigated by taking into account the Coulomb interaction effect in the Hartree-Fock approximation. We study the phase transitions of BL-ZGNRs induced by external electric fields and also the optical selection rules for the incident light polarized along the longitudinal and transverse directions. We find that the excitations from the edge states are crucial for the optical properties of BL-ZGNRs in the antiferromagnetic phase. We show that the low energy part of the optical absorption can be modulated by the external transverse electric field, and there is a broad band low frequency absorption enhancement for the transverse-polarized incident light in the charge-polarized state of BL-ZGNRs.     

Zigzag型石墨纳米带电子结构和输运性质的第一性原理研究

采用第一性原理电子结构和输运性质计算研究了zigzag型单层石墨纳米带(具有armchair 边缘)的电子结构和输运性质及其边缘空位缺陷效应. 研究发现,完整边缘的zigzag型石墨纳米带是具有一定能隙的半导体带,边缘空位缺陷的存在使得纳米带能隙变小,且缺陷浓度越大,能隙越小,并发生了半导体-金属转变. 利用这些研究结果,将有助于在能带工程中实现其电子结构裁剪. 关键词： 石墨纳米带 空位缺陷 电子结构 输运性质     

Effect of particles size on the optical constants of iron/polystyrene composites via UV-radiation

The present study deals with the optical characteristics of polystyrene (PS) composites containing iron particles of different sizes: 5, 40, 110, and 250 μm. The optical absorption spectra were collected in the wavelength range 300–800 nm using a UV-spectrophotometer. The optical results obtained were analyzed in terms of the absorption formula for non-crystalline materials. The optical energy gap and other basic optical constants such as refractive index, dielectric constants, and optical conductivity were investigated and showed a clear dependence on the iron particles size. It was found that the optical energy gap for the iron-filled composites is less than that for the neat PS, and it decreases as the iron particle size decreases. The refractive index of the prepared composites was determined from the collected transmittance and reflectance spectra. It was found that the calculated dielectric constant and refractive index of the composites increase when the iron particles size decreases. The optical dispersion behavior of the composites was described by the single-oscillator model. Enhancement in the optical conductivity was observed with decreasing the iron particles size.     

Zigzag型石墨纳米带电子结构和输运性质的第一性原理研究

采用第一性原理电子结构和输运性质计算研究了zigzag型单层石墨纳米带(具有armchair 边缘)的电子结构和输运性质及其边缘空位缺陷效应. 研究发现，完整边缘的zigzag型石墨纳米带是具有一定能隙的半导体带，边缘空位缺陷的存在使得纳米带能隙变小，且缺陷浓度越大，能隙越小，并发生了半导体-金属转变. 利用这些研究结果，将有助于在能带工程中实现其电子结构裁剪.     

Optical properties of the red bronze K0.33MoO3

The optical reflectivity of the red bronze K_0.33MoO₃ has been measured on single crystals in the spectral energy range between 0.03 and 12 eV at temperatures from 4 K to 300 K using polarized light. The optical constants have been determined by means of a Kramers-Kronig analysis; the data are interpreted that this compound is a 0.5 eV energy gap semiconductor with very strong anisotropy in the infrared and visible energy range.     

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.     

Width dependent structural and electrical properties of zigzag ZnTe nanoribbons

We carry out density functional theory based investigation to understand the structural and electrical properties such as atomic structure, edge energy, band gap, and work function of zigzag ZnTe nanoribbons. It is found that the zigzag nanoribbons may be stabilized by passivating the edge atoms with Hydrogen, Oxygen or Fluorine atoms. Our study reflects that zigzag ZnTe nanoribbons with smaller width behave like semiconductor. However, they exhibit a transition from semiconducting phase to a metallic phase as width increases. A wide variation of band gap is obtained with respect to the choice of edge passivating elements. Work functions of all the nanoribbons are also estimated in order to assess the utility of these nanoribbons in various field emission devices.     

First-principles investigation of armchair stanene nanoribbons

In this study, we systematically investigated the structural, electronic and optical properties of armchair stanene nanoribbons (ASNRs) by using the first-principles calculations. First, we performed full geometry optimization calculations on various finite width ASNRs where all the edge Sn atoms are saturated by hydrogen atoms. The buckled honeycomb structure of two dimensional (2D) stanene is preserved, however the bond length between the edge Sn atoms is shortened to 2.77 Å compared to the remaining bonds with 2.82 Å length. The electronic properties of these nanoribbons strongly depend on their ribbon width. In general, band gap opens and increases with decreasing nanoribbon width indicating the quantum confinement effect. Consequently, the band gap values vary from a few meV exhibiting low-gap semiconductor (quasi-metallic) behavior to ～0.4–0.5 eV showing moderate semiconductor character. Furthermore, the band gap values are categorized into three groups according to modulo 3 of integer ribbon width N which is the number of Sn atoms along the width. In order to investigate the optical properties, we calculated the complex dielectric function and absorption spectra of ASNRs, they are similar to the one of 2D stanene. For light polarized along ASNRs, in general, largest peaks appear around 0.5 eV and 4.0 eV in the imaginary part of dielectric functions, and there are several smaller peaks between them. These major peaks redshifts, slightly to the lower energies of incident light with increasing nanoribbon width. On the other hand, for light polarized perpendicular to the ribbon, there is a small peak around 1.6 eV, then, there is a band formed from several peaks from 5 eV to ～7.5 eV, and the second one from 8 eV to ～9.5 eV. Moreover, the peak positions hardly move with varying nanoribbon width, which indicates that quantum confinement effect is not playing an essential role on the optical properties of armchair stanene nanoribbons. In addition, our calculations of the optical properties indicate the anisotropy with respect to the type of light polarization. This anisotropy is due to the quasi-2D nature of the nanoribbons.     

Even-odd-dependent optical transitions of zigzag monolayer black phosphorus nanoribbons

We analytically study the electronic structure and optical properties of zigzag-edged phosphorene nanoribbons(ZPNRs) using the tight-binding Hamiltonian and Kubo formula. By directly solving the discrete Schrodinger equation, we obtain the energy spectra and wavefunctions for N-ZPNR(where N is the number of transverse zigzag atomic chains) and classify the eigenstates according to the lattice symmetry. Then, we obtain the optical transition selection rule of ZPNRs on the basis of symmetry analysis and analytical expressions of optical transition matrix elements. Under incident light that is linearly polarized along the ribbon, we determine that the optical transition selection rule for N-ZPNR with even-or odd-N is qualitatively different. Specifically, for even-N ZPNRs, the inter-(intra-) band selection rule is ?n =odd(even) because the parity of the wavefunction corresponding to the n-th subband in the conduction(valence) band is(-1)~n[(-1)~((n+1))] owing to the presence of C(2x) symmetry. However, the optical transitions between any subbands are possible owing to the absence of C(2x) symmetry. Our results provide a further understanding on the electronic states and optical properties of ZPNRs, which are useful for explaining the optical experiment data on ZPNR samples.     

The optical responsivity in IR-photodetector based on armchair graphene nanoribbons with p–i–n structure

Armchair graphene nanoribbons (A-GNRs) are an alternative material to use in novel infrared photodetectors, because of their tunable energy gap in the infrared spectrum, and their high quantum efficiency. In this paper, an A-GNR p–i–n structure with all three structural families, different width, and different number of layers to use in IR detectors have been investigated. With calculating the band structure and energy gap using the tight-binding model and by including the edge deformation, the optical absorption in the single electron approximation has been obtained by calculating the optical conductance. Finally, we have calculated the quantum efficiency and the optical responsivity of A-GNR based IR photodetector as a function of incident photon energy, temperature, nanoribbon width and the number of layers. Results show that the responsivity of the A-GNR based IR photodetector increase by increasing the width and number of layers and decrease by increasing the temperature.     

近场光学显微镜研究CdS0.65Se0.35纳米带空间分辨光致荧光谱

通过热蒸发方法成功制备出CdS_0.65Se_0.35纳米带，得到的纳米带表面光滑，宽度厚度均一，表现出很高的结晶质量.使用近场光学显微镜对纳米带室温下的带边荧光波导和光致荧光近场光谱进行研究.发现CdS_0.65Se_0.35纳米带呈现良好的光波导的特性；同时通过近场光学显微镜得到的空间分辨光谱，发现随着传播距离的增大，纳米带的光致荧光光谱有持续的红移现象.这种光谱红移现象是带间跃迁过程中带尾态的吸收效应引起的，并作了光谱带尾态吸收的理论模拟与实验结果进行比较.光波导传输过程中光谱的变化反映了信息在整个传导过程中的情况，体现了信息传递过程中的稳定性和有效性.三元合金材料CdS_0.65Se_0.35纳米带的波导和光谱性质研究，对于其他组分可调的三元合金纳米结构的制备和研究，及发展新型的纳米功能器件有重要意义.     

氧化铟锡薄膜的椭偏光谱研究

用溅射法在Si片上制备了厚度为140nm的氧化铟锡(ITO)薄膜。X射线衍射研究表明所制备的薄膜为多晶结构。在1.5~4.5eV范围内对ITO薄膜进行了椭偏测量。分别用德鲁德-洛伦茨谐振子(Drude Lorenz oscillators)模型、层进模型结合有效介质近似模型对椭偏参量ψ、Δ进行了拟合,得到ITO薄膜的折射指数n的变化范围在1.8~2.6之间,可见光范围内消光系数k接近于零,在350nm波长附近开始明显变化,且随着波长的减小k迅速增加。计算得到直接和间接光学带隙分别是3.8eV和4.2eV。并在1.5~4.5eV段给出一套较为可靠的、具有实用价值的ITO介电常量和光学常量。     

带状碳单层与石墨基底之间相互作用的第一性原理计算

利用基于密度泛函理论的第一性原理计算,研究边缘为Armchair型带状碳单层与石墨基底的相互作用,结果发现,其间的相互作用导致双方发生变形,带状碳单层的禁带宽度较其独立存在时有所减小,但石墨基底的作用并不改变其能带结构的基本特征. 关键词： 带状碳单层 第一性原理计算 能带结构