Effects of electric and magnetic fields on the electronic properties of zigzag carbon and boron nitride nanotubes

We have investigated the electronic properties of zigzag CNTs and BNNTs under the external transverse electric field and axial magnetic field, using tight binding approximation. It was found that after switching on the electric and magnetic fields, the band modification such as distortion of the degeneracy, change in energy dispersion, subband spacing and band gap size reduction occurs. The band gap of zigzag BNNTs decreases linearly with increasing the electric field strength but the band gap variation for CNTs increases first and later decreases (Metallic) or first hold constant and then decreases (semiconductor). For type (II) CNTs, at a weak magnetic field, by increasing the electric field strength, the band gap remains constant first and then decreases and in a stronger magnetic field the band gap reduction becomes parabolic. For type (III) CNTs, in any magnetic field, the band gap increases slowly until reaches a maximum value and then decreases linearly. Unlike to CNTs, the magnetic field has less effects on the BNNTs band gap variation.     

Tight binding description on the band gap opening of pyrene-dispersed graphene

Opening up a band gap in graphene holds a crucial significance in the realization of graphene-based electronics. Doping with organic molecules to alter the electronic properties of graphene is perceived as an effective band gap engineering approach. Using the tight binding model, we examined the band gap opening of monolayer graphene due to the adsorption of pyrene molecules on both of its sides. It was found that the breakdown of the sublattice symmetry in pyrene-dispersed graphene leads to a band gap of ～10 meV.     

Lead‐Free Halide Double Perovskite Cs2AgBiBr6 with Decreased Band Gap

Environmentally friendly halide double perovskites with improved stability are regarded as a promising alternative to lead halide perovskites. The benchmark double perovskite, Cs₂AgBiBr₆, shows attractive optical and electronic features, making it promising for high‐efficiency optoelectronic devices. However, the large band gap limits its further applications, especially for photovoltaics. Herein, we develop a novel crystal‐engineering strategy to significantly decrease the band gap by approximately 0.26 eV, reaching the smallest reported band gap of 1.72 eV for Cs₂AgBiBr₆ under ambient conditions. The band‐gap narrowing is confirmed by both absorption and photoluminescence measurements. Our first‐principles calculations indicate that enhanced Ag–Bi disorder has a large impact on the band structure and decreases the band gap, providing a possible explanation of the observed band‐gap narrowing effect. This work provides new insights for achieving lead‐free double perovskites with suitable band gaps for optoelectronic applications.     

Engineering the Band Gap States of the Rutile TiO2(110) Surface by Modulating the Active Heteroatom

Introducing band gap states to TiO₂ photocatalysts is an efficient strategy for expanding the range of accessible energy available in the solar spectrum. However, few approaches are able to introduce band gap states and improve photocatalytic performance simultaneously. Introducing band gap states by creating surface disorder can incapacitate reactivity where unambiguous adsorption sites are a prerequisite. An alternative method for introduction of band gap states is demonstrated in which selected heteroatoms are implanted at preferred surface sites. Theoretical prediction and experimental verification reveal that the implanted heteroatoms not only introduce band gap states without creating surface disorder, but also function as active sites for the Cr^VI reduction reaction. This promising approach may be applicable to the surfaces of other solar harvesting materials where engineered band gap states could be used to tune photophysical and ‐catalytic properties.     

Electronic Properties of Vinylene-Linked Heterocyclic Conducting Polymers: Predictive Design and Rational Guidance from DFT Calculations

The band structure and electronic properties in a series of vinylene-linked heterocyclic conducting polymers are investigated using density functional theory (DFT). In order to accurately calculate electronic band gaps, we utilize hybrid functionals with fully periodic boundary conditions to understand the effect of chemical functionalization on the electronic structure of these materials. The use of predictive first-principles calculations coupled with simple chemical arguments highlights the critical role that aromaticity plays in obtaining a low band gap polymer. Contrary to some approaches which erroneously attempt to lower the band gap by increasing the aromaticity of the polymer backbone, we show that being aromatic (or quinoidal) in itself does not ensure a low band gap. Rather, an iterative approach which destabilizes the ground state of the parent polymer toward the aromatic ? quinoidal level crossing on the potential energy surface is a more effective way of lowering the band gap in these conjugated systems. Our results highlight the use of predictive calculations guided by rational chemical intuition for designing low band gap polymers in photovoltaic materials.     

First-principles band gap criterion for impact sensitivity of energetic crystals: a review

In this article, we review some recent studies in predicting impact sensitivity for different classes of energetic crystals based on first-principles band gap. Based on these investigations on metal azides, a first-principles band gap criterion is founded to measure impact sensitivity for a series of energetic crystals. For energetic crystals with similar structure or with similar thermal decomposition mechanism, the smaller the band gap is, the easier the electron transfers from the valence band to the conduction band, and the more they becomes decomposed and exploded. Applications of this criterion on other series of energetic crystals show that the first-principles band gap criterion is applicable to different series of energetic crystals with similar structure or with similar thermal decomposition mechanism. This criterion may be useful for molecular design of high-energy density materials.     

Fluorine substituted conjugated polymer of medium band gap yields 7% efficiency in polymer-fullerene solar cells

Recent research advances on conjugated polymers for photovoltaic devices have focused on creating low band gap materials, but a suitable band gap is only one of many performance criteria required for a successful conjugated polymer. This work focuses on the design of two medium band gap (~2.0 eV) copolymers for use in photovoltaic cells which are designed to possess a high hole mobility and low highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels. The resulting fluorinated polymer PBnDT-FTAZ exhibits efficiencies above 7% when blended with [6,6]-phenyl C(61)-butyric acid methyl ester in a typical bulk heterojunction, and efficiencies above 6% are still maintained at an active layer thicknesses of 1 μm. PBnDT-FTAZ outperforms poly(3-hexylthiophene), the current medium band gap polymer of choice, and thus is a viable candidate for use in highly efficient tandem cells. PBnDT-FTAZ also highlights other performance criteria which contribute to high photovoltaic efficiency, besides a low band gap.     

A Semiempirical Study of Carbon Nanotubes with Finite Tubular Length and Various Tubular Diameters

A semiempirical PM3 quantum computational method has been used to generate the electronic and optimized geometrical structure of SWNT of zigzag and armchair types. We shed light on the electronic structures of SWNT with various diameters and lengths of the tube. Particularly, the calculated HOMO, LUMO and band‐gap of SWNT are not monotonic but exhibit a well‐defined oscillation, which depends on the tubular diameter and the tubular length. Calculated HOMO, LUMO and band‐gap of the zigzag SWNTs have oscillated with tubular diameter as they contain an odd or even number of benzenoids in the circular plane of the carbon nanotube. The zigzag SWNTs with an odd number of benzenoids have a higher band‐gap than those of SWNTs with an even number of benzenoids in the circular plane of the carbon nanotube. Calculated results also reveal that the tubular length in the zigzag SWNTs influences the band‐gaps very little. For the armchair SWNT, calculated HOMO, LUMO and band‐gap contained the oscillate depending on the number of carbon sections in the tubular length axis. Their repeat sections are 3n‐1, 3n and 3n+1. The armchair SWNT with 3n+1 sections has a high band‐gap while the SWNTs with 3n‐1 sections have a low band‐gap. The tubular diameters of armchair SWNT influence the HOMO, LUMO and band gap very little.     

Effect of Pressure on Cesium Iodide Band Gap

The evolution of cesium iodide band gap as a function of pressure is studied in the range from 0 to 60 GPa. Within this range, two structural phase transitions occurred, and the band gap was affected by the compression pressure and structural rearrangement. The band gap estimation under pressure, as obtained by the density functional theory methods, successfully reproduced the experimental trend of the optical gap and electrical resistivity, namely, a general decreasing tendency, an early maximum, and a discontinuous peak around 40 GPa.     

The electronic structure of polydiacetylenes: cyclic versus linear models

The electronic structure of polydiacetylenes is investigated with SCF calculations as a function of backbone structure and chain length. The first dipole-allowed excitation (band gap) is red-shifted for the transformation from polydiacetylene (PDA) to polybutatriene (PBT) for cyclic chains The band gap for linear chains is blue-shifted for short chains, for linear chains containing 36 (or more) carbon atoms the band gap is red-shifted.     

Achieving indirect-to-direct band gap transition and enhanced photocatalytic performance in blue phosphorene through doping and strain

Recently, blue phosphorene (BP) has demonstrated great potential in the field of photocatalytic water splitting due to the ultrahigh carrier mobility. However, the practical application of BP as an efficient photocatalyst is greatly limited by its indirect band gap. In this work, we investigate the synergistic effect of substitutional doping and biaxial strain on the electronic and photocatalytic properties of BP using hybrid density functional calculations. The results show that As/Sb doping not only reduces the band gap of BP without introducing any midgap states but also turns it into direct band gap semiconductor, which can be ascribed to the p states of the dopants appearing around the band edges. For these As/Sb-doped BP systems, the band gaps, band edge positions, and optical absorption abilities can be further tuned by applying a biaxial strain. In particular, we predict that compressive strains are more propitious for the doped systems than the tensile strains since the requirements for water splitting are satisfied, meanwhile preserving the direct band gap characteristics. Besides, our calculations also show that the band gap and the reducing and oxidizing power of multilayer BP are highly dependent on the layer thickness. These results suggest feasible modulation strategies for enabling BP to be a visible-light-driven photocatalyst for water splitting.     

光子晶体的制备及应用

光子晶体（PC）是具有光子带隙的周期性电介质结构，频率落在光子带隙中的光将被禁止传播。本文对光子晶体的能带结构、带隙的产生、制备方法及应用作了简要的介绍，重点介绍了光子晶体的制备技术及其在光电领域中的应用前景。     

A constrained variational approach to the designing of low transport band gap materials: A multiobjective random mutation hill climbing method

Neutral polythiophene (PT) and polyselenophene (PSe) are semiconductors with band gaps of about 2 eV. We have proposed and implemented a constrained variational method in which total energy of neutral PT or PSe oligomers is minimized under the constraint that the band gap measured by HOMO–LUMO energy difference is also a minimum in each case. The constrained (bimodal) minimization has been carried out by an adaptive random mutation hill climbing method within the basic framework of Su‐Schrieffer‐Heeger type of model. We show that the “band‐gap constrained minimization” automatically creates electron deficient quinoid regions (QR) in the PT or PSe chains, embedded in aromatic regions (ARs), on both sides. We have investigated how the number and distribution of such QRs can reduce the band gap. Band gap constrained electronic structure calculations thus provide designing clues for low transport band gap materials based on molecular chromophores. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Lead-Free Halide Double Perovskite Cs2AgBiBr6 with Decreased Band Gap

Environmentally friendly halide double perovskites with improved stability are regarded as a promising alternative to lead halide perovskites. The benchmark double perovskite, Cs₂AgBiBr₆, shows attractive optical and electronic features, making it promising for high-efficiency optoelectronic devices. However, the large band gap limits its further applications, especially for photovoltaics. Herein, we develop a novel crystal-engineering strategy to significantly decrease the band gap by approximately 0.26 eV, reaching the smallest reported band gap of 1.72 eV for Cs₂AgBiBr₆ under ambient conditions. The band-gap narrowing is confirmed by both absorption and photoluminescence measurements. Our first-principles calculations indicate that enhanced Ag–Bi disorder has a large impact on the band structure and decreases the band gap, providing a possible explanation of the observed band-gap narrowing effect. This work provides new insights for achieving lead-free double perovskites with suitable band gaps for optoelectronic applications.     

Density functionals from many-body perturbation theory: the band gap for semiconductors and insulators

Theoretically the Kohn-Sham band gap differs from the exact quasiparticle energy gap by the derivative discontinuity of the exchange-correlation functional. In practice for semiconductors and insulators the band gap calculated within any local or semilocal density approximations underestimates severely the experimental energy gap. On the other hand, calculations with an "exact" exchange potential derived from many-body perturbation theory via the optimized effective potential suggest that improving the exchange-correlation potential approximation can yield a reasonable agreement between the Kohn-Sham band gap and the experimental gap. The results in this work show that this is not the case. In fact, we add to the exact exchange the correlation that corresponds to the dynamical (random phase approximation) screening in the GW approximation. This accurate exchange-correlation potential provides band structures similar to the local density approximation with the corresponding derivative discontinuity that contributes 30%-50% to the energy gap. Our self-consistent results confirm substantially the results for Si and other semiconductors obtained perturbatively [R. W. Godby et al., Phys. Rev. B 36, 6497 (1987)] and extend the conclusion to LiF and Ar, a wide-gap insulator and a noble-gas solid.     

DFT description on electronic structure and optical absorption properties of anionic S-doped anatase TiO2

Plane-wave-based pseudopotential density functional theory (DFT) calculations are used to characterize the doping effect of S substituting for O in anatase TiO(2). Through band structure calculation, a direct band gap is predicted in TiO(2)(-)(x)S(x). Electronic structure analysis shows that the doping S could substantially lower the band gap of TiO(2) by the presence of an impurity state of S 3p on the upper edge of the valence band. Excitations from the impurity state of S 3p to the conduction band may be responsible for the red shift of the absorption edge observed in the S-doped TiO(2). The band gap lowering and red shift of the absorption edge are found to increase as the sulfur concentration increases.     

N／F掺杂和N-F双掺杂锐钛矿相TiO2（101）表面电子结构的第一性原理计算

采用密度泛函理论(DFT)平面波赝势方法计算了N/F掺杂和N-F双掺杂锐钛矿相TiO2(101)表面的电子结构.由于DFT方法存在对过渡金属氧化物带隙能的计算结果总是与实际值严重偏离的缺陷,本文也采用DFT+U(Hubbard系数)方法对模型的电子结构进行了计算.DFT的计算结果表明N掺杂后,N2p轨道与O 2p和Ti 3d价带轨道的混合会导致TiO2带隙能的降低,而F掺杂以及氧空位的引入对材料的电子结构没有明显的影响.DFT+U的计算却给出截然不间的结果,N掺杂并没有导致带隙能的降低,而只是在带隙中引入一个孤立的杂质能级,反而F掺杂以及氧空位的引入带来明显的带隙能降低.DFT+U的计算结果与一些实验测量结果能够较好地符合.     

Longitudinal polarizability of carbon nanotubes

The longitudinal polarizabilities of carbon nanotubes are determined using first principles density functional theory. These results demonstrate that the polarizability per atom of a nanotube in the axial direction is primarily determined by the band gap. In fact, polarizability per atom versus inverse band gap yields a linear trend for all nanotubes and methods utilized in this study, creating a universal relationship for longitudinal polarizability. This can be explained by examining the terms in the sum over states equation used to determine polarizability and noting that the vast majority of the polarizability arises from a few elements near the band gap. This universal trend is then used with experimentally determined band gaps to predict the experimental polarizability of carbon nanotubes.     

含噻吩窄带隙共轭聚合物类太阳能电池材料的研究进展

含噻吩的窄带隙共轭聚合物类太阳能电池材料因其良好的稳定性和可加工性,已成为新型太阳能电池的研究热点。本论文主要介绍了用于太阳能电池的窄带隙共轭聚合物研究进展,按其结构特征分为烷基/烷氧基取代聚噻吩、含苯基聚噻吩、基于噻吩并吡嗪的共聚物、基于噻吩并噻唑的共聚物、基于噻吩并吩噻嗪的共聚物、基于烷基芴的共聚物以及其它种类的窄带隙的共轭聚合物,并对它们的结构特点、光学带隙、合成方法进行了归纳与总结。本文最后简要介绍了该研究领域目前所面临的一些问题,同时讨论了该类材料在此领域今后的发展趋势。     

N/F掺杂和N-F双掺杂锐钛矿相TiO2(101)表面电子结构的第一性原理计算

采用密度泛函理论(DFT)平面波赝势方法计算了N/F掺杂和N-F双掺杂锐钛矿相TiO2(101)表面的电子结构. 由于DFT方法存在对过渡金属氧化物带隙能的计算结果总是与实际值严重偏离的缺陷, 本文也采用DFT+U(Hubbard 系数)方法对模型的电子结构进行了计算. DFT的计算结果表明N掺杂后, N 2p轨道与O 2p和Ti 3d价带轨道的混合会导致TiO2带隙能的降低, 而F掺杂以及氧空位的引入对材料的电子结构没有明显的影响. DFT+U的计算却给出截然不同的结果, N掺杂并没有导致带隙能的降低, 而只是在带隙中引入一个孤立的杂质能级, 反而F掺杂以及氧空位的引入带来明显的带隙能降低. DFT+U的计算结果与一些实验测量结果能够较好地符合.