Indirect to direct gap transition in low-dimensional nanostructures of Silicon and Germanium

The electronic band structures of Si and Ge low-dimensional nanostructure such as nanofilms and nanowires have been calculated using first principles based on density functional theory (DFT) with the generalized gradient approximation (GGA). The calculation results show that a direct band gap can be obtained from Si orientation [100] or in Ge orientation [111] confined low dimensional nanostructure. However, an indirect band gap is still kept in the Si orientation [111] or in the Ge orientation [110] confined low dimensional nanostructure. The calculation results are interesting and the transition mechanism from indirect to direct band gap in low dimensional nanostructures is given in the physical structures model.     

Easily attainable new approach to mass yield ferrocenyl Schiff base and different metal complexes of ferrocenyl Schiff base through convenient ultrasonication‐solvothermal method

A new alternative approach with crucial mass yield and high reaction rates is proposed for the synthesis of ferrocenyl Schiff bases using an ultrasonication‐solvothermal method. Equimolar condensation of ferrocenecarboxaldehyde and 2‐aminophenol interact with each other, giving 1‐(1‐[2‐hydroxyphenyl‐2‐imino]methyl)‐ferrocene (FcOH). Furthermore, this ligand forms 1:1 complexes with cobalt(II), nickel(II), copper(II), and palladium(II) ions. From the different spectral data, it is found that metal ions coordinate with ligands through the azomethine group and the deprotonated oxygen of the phenol groups. Moreover, FcOH and their complexes were characterized by elemental analysis, Fourier transform infrared, ¹H nuclear magnetic resonance, and UV‐visible spectrophotometry. The spectral data of FcOH and its metal complexes were discussed in connection with the structural changes due to complexation. Meanwhile, the information about geometric structures can be concluded from the electronic spectra and the magnetic moments. Plainly, electron spin resonance spectra of the Cu(II) complex revealed d_x2?y2 as a ground state, suggesting a square planar geometry around the Cu(II) center. The direct optical band gap energy E_g values of cobalt, nickel, copper, and palladium complexes of FcOH are found to be 3.7, 3.9, 4.6, and 3.65 eV, respectively. 1‐(1‐[2‐Hydroxyphenyl‐2‐imino]methyl)‐ferrocene and its metal complexes were screened for antibacterial activity. The results depict that the metal complexes were found to be more strongly antibacterial than the guardian Schiff base ligand (FcOH) against one or more bacterial species. The minimum inhibitory concentrations of antimicrobial properties of the purified compound were determined using the broth microdilution method.     

Tunable band gap and optical properties of surface functionalized Sc_2C monolayer

Using the density functional theory, we have investigated the electronic and optical properties of two-dimensional Sc_2C monolayer with OH, F, or O chemical groups. The electronic structures reveal that the functionalized Sc_2C monolayers are semiconductors with a band gap of 0.44–1.55 eV. The band gap dependent optical parameters, like dielectric function, absorption coefficients, reflectivity, loss function, and refraction index were also calculated for photon energy up to 20 eV. At the low-energy region, each optical parameter shifts to red, and the peak increases obviously with the increase of the energy gap. Consequently, Sc_2C monolayer with a tunable band gap by changing the type of surface chemical groups is a promising 2D material for optoelectronic devices.     

Dynamically controlled optical nonreciprocity of a double-ladder system with spontaneously generated coherence in moving atomic optical lattice

A four-level double-ladder cold atoms system with spontaneously generated coherence trapped in a moving optical lattice is explored to achieve optical nonreciprocity. When spontaneously generated coherence(SGC) is present, the remarkable contrast optical nonreciprocity of light transmission and reflection can be generated at each induced photonic bandgap in the optical lattice with a velocity of a few m/s. However, when the SGC effect is absent, the optical nonreciprocity becomes weak or even vanishing due to the strong absorption. It is found that the optical nonreciprocity is related to the asymmetric Doppler effect in transmission and reflection, meanwhile the degree and position of optical nonreciprocity can be tuned by the SGC effect and the Rabi frequency of the trigger field.     

Influence of donor and acceptor groups on the S-T energy gap for thermally activated delayed fluorescence emitters

Molecules composed of different donors and acceptors are theoretically designed as potential thermally activated delayed fluorescence emitters, and their singlet–triplet (S-T) energy gap is studied using the optimal Hartree–Fork method. It is found that the S-T energy gap is in reverse proportional to the electron-donating ability. Stronger electron-donating ability of donors will induce smaller highest occupied molecular orbital–lowest unoccupied molecular orbital overlap and also a smaller S-T energy gap. Based on our calculation results, three molecules are proposed to have great potential to be used as thermally activated delayed fluorescence emitters in organic light-emitting diodes.     

理论研究Stone-Wales缺陷和C掺杂对手性BN纳米带的带隙调控

通过第一性原理密度泛函理论的方法,研究了Stone-Wales 缺陷和C掺杂对手性BN纳米带的带隙调控.结果表明,Stone-Wales 缺陷使得BN纳米带的价带顶(VBM)和导带底(CBM)的占据态发生变化,从而引入了缺陷能级降低了带隙,但Stone-Wales 缺陷的个数对带隙的大小影响不明显.电子结构计算表明,带Stone-Wales 缺陷的BN纳米带的缺陷能级主要是由VBM附近形成N-N原子的类π键轨道和CBM附近形成B-B原子的类σ键分布决定.通过在带Stone-Wales 缺陷的BN纳米带中引入C掺杂改变杂质能级的分布,在VBM附近形成了C-C原子的类σ键轨道和CBM附近形成了C-B原子的类σ键,这样可以进一步降低BN纳米带的带隙,拓展了BN纳米带的应用.     

TiO2-based Photocatalysis: Toward Visible Light-Responsive Photocatalysts Through Doping and Fabrication of Carbon-Based Nanocomposites

Elimination of toxic organic compounds from wastewater and provision of safe, clean, and cheap water to communities is currently one of the most important global topics in water-pollution control. TiO₂ has emerged as one of the most fascinating material in the modern era due to its potential catalytic properties. In spite of extensive efforts to apply TiO₂ for environmental remediation, photocatalytic activity in the visible region has remained quite low hence the need to fabricate highly photoactive catalysts through modification of TiO₂ and to apply them for water remediation using the abundantly available solar radiation.

Carbon materials are promising candidates for application as functional materials due to their superior physico-chemical properties. This has prompted investigations into their possible application in environmental clean-up. Nanocomposite assemblies composed of different types and proportions of carbon based materials (i.e., carbon nanotubes, graphene, fullerenes, activated carbon, carbon nanofibers) and TiO₂ have also found widespread applicability in water decontamination studies using UV or visible light. This article surveys the literature and highlights recent progress in the development of TiO₂ and nanocarbon/TiO₂ photocatalysts for water decontamination. The nitty-gritties, on-going challenges, areas of controversy, “hotspots” and possible new directions are outlined in this article.     

沉积压力对氢化非晶硅薄膜特性的影响

采用等离子增强化学气相沉积(PECVD)系统,以乙硅烷和氢气为气源,普通钠钙玻璃为衬底制备了氢化非晶硅(a-Si∶H)薄膜,研究了沉积压力对非晶硅薄膜的沉积速率、光学带隙以及结构因子的影响。采用台阶仪、紫外可见分光光度计、傅里叶变换红外光谱仪和扫描电子显微镜等手段分别表征了a-Si∶H薄膜的沉积速率,光学带隙、结构因子和表面形貌。结果表明： 随着沉积压力的增加,沉积速率呈现先上升后下降的趋势,光学带隙不断下降。当沉积压力小于210 Pa时,以SiH键存在的H原子较多,而以SiH₂或SiH₃等形式存在的H较少; 当沉积压力大于210 Pa时,以SiH₂,(SiH₂)_n或SiH₃等形式存在的H较多。通过结构因子的计算,发现沉积压力在110～210 Pa的范围内沉积的薄膜质量较好。     

环氧树脂胶的太赫兹光谱特性研究

环氧树脂是纤维增强复合材料加工中的一种重要的胶粘剂,太赫兹时域光谱技术已成为纤维增强复合材料无损检测的有力补充手段。固化温度是环氧树脂的重要参数之一,不同的固化温度会影响环氧树脂胶的性能,因此采用太赫兹时域光谱技术分别对室温和高温下固化的环氧树脂胶的太赫兹透射光谱特性进行了系统研究,计算得到了不同温度下固化的环氧树脂胶的折射率和吸收系数,并进行了对比分析。研究表明,由于室温下固化的环氧树脂样本基本没有气泡,而高温下固化的样本存在微量气泡,气泡的存在降低了样品的密度,因此室温下固化的环氧树脂胶样品的折射率和吸收系数均大于高温下固化的样品。在同种固化条件下制备的不同样品间折射率差别较小,同时,室温下固化不同样品间的吸收系数差别亦较小,但高温下固化样品间的吸收系数在0.6～1.5 THz差别逐渐变大,这主要是因为高温下制备的不同样本间的气泡分布不均匀,即密度分布存在差异。室温和高温下固化样品的吸收系数在整体上均随着频率的增加而增加,并且没有明显的吸收峰。此外,由于法布里-珀罗干涉效应的存在,导致有些厚环氧树脂样本的能量透过率在共振峰处要远大于薄样本。该研究对纤维增强复合材料的太赫兹无损检测具有重要的研究意义。