Opening Band Gap of Graphene by Chemical Doping: a First Principles Study

Single atom chemically doped graphene has been theoretically studied by density functional theory. The largest band gap, 0.62 eV, appears in arsenic atom doped graphene, then 0.60 eV comes by the tin atom, whose deformations can neither be ignored. It is also found that oxygen and iron single atom embedded graphene can open band gap by 0.52 and 0.54 eV, respectively. Moreover, doping O atom shows little distortion and high stability by charge redistribution. The band gap of Fe doped graphene is opened by orbital hybridization. The other heteroatom doped results are a little inferior to them.     

Fluoride-assisted synthesis of anatase TiO2 nanocrystals with tunable shape and band gap via a solvothermal approach

A simple solvothermal approach employing oleic acid has been developed to prepare anatase TiO₂ nanocrystals with different shapes, which were tuned from nanorods to nano-ellipsoids by increasing the amount of NaF from 0 to 0.5 mmol, and the optical band gap decreased from 3.47 eV to 3.29 eV accordingly. However, when the fluoride was changed to NH₄F, the resultant TiO₂ nanocrystals possessed an anatase phase but weremade up of smaller-sized nanocrystals and nanorods, and the band gap was increased to 3.53 eV. The X-ray photoelectron spectroscopy (XPS) results illustrated an increase of fluorine content with an increasing amount of NaF could account for the variation of the shape and optical band gap of TiO₂ nanocrystals. Moreover, the absence of fluorine content brought about less change of shape and increase of optical band gap of the product synthesized in the presence of NH₄F. This result may offer another way to alter the shape and band gap of metal oxide nanocrystals with the assistance of fluoride.     

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.     

聚3-甲基噻吩的光电化学研究

利用光电化学方法研究了聚3-甲基噻吩的光电化学性质.其禁带宽度为1.93 eV.同时确定了它的价带、导带位置.研究还发现聚3-甲基噻吩属于直接跃迁半导体,具有很好的光电流稳定性.得到的最高IPCE值近1.0%.     

DFT+U study of defects in bulk rutile TiO(2)

We present a systematic study of electronic gap states in defected titania using our implementation of the Hubbard-U approximation in the grid-based projector-augmented wave density functional theory code, GPAW. The defects considered are Ti interstitials, O vacancies, and H dopants in the rutile phase of bulk titanium dioxide. We find that by applying a sufficiently large value for the Hubbard-U parameter of the Ti 3d states, the excess electrons localize spatially at the Ti sites and appear as states in the band gap. At U=2.5?eV, the position in energy of these gap states are in fair agreement with the experimental observations. In calculations with several excess electrons and U=2.5?eV, all of these end up in gap states that are spatially localized around specific Ti atoms, thus effectively creating one Ti(3+) ion per excess electron. An important result of this investigation is that regardless of which structural defect is the origin of the gap states, at U=2.5?eV, these states are found to have their mean energies within a few hundredths of an eV from 0.94 eV below the conduction band minimum.     

Low band gap EDOT-benzobis(thiadiazole) hybrid polymer characterized on near-IR transmissive single walled carbon nanotube electrodes

An electron donor/acceptor pi-conjugated polymer composed of a bi-EDOT and benzobis(thiadiazole) repeat unit exhibits two reductions with a band gap ranging from approximately 0.5 to 0.8 eV depending on the method of band gap determination.     

Manipulating the electronic structures of silicon carbide nanotubes by selected hydrogenation

We show that the electronic and atomic structures of silicon carbide nanotubes (SiCNTs) undergo dramatic changes with hydrogenation from first-principles calculations based on density-functional theory. The exo-hydrogenation of a single C atom results in acceptor states close to the highest occupied valence band of pristine SiCNT, whereas donor states close to the lowest unoccupied conduction band appear as a Si atom being hydrogenated. Upon fully hydrogenating Si atoms, (8,0) and (6,6) SiCNTs become metallic with very high density of states at the Fermi level. The full hydrogenation of C atoms, on the other hand, increases the band gap to 2.6 eV for (8,0) SiCNT and decreases the band gap to 1.47 eV for (6,6) SiCNT, respectively. The band gap of SiCNTs can also be greatly increased through the hydrogenation of all the atoms.     

An oligomer study on small band gap polymers

Small band gap polymers may increase the energy conversion efficiency of polymer solar cells by increased absorption of sunlight. Here we present a combined experimental and theoretical study on the optical and electrochemical properties of a series of well-defined, lengthy, small band gap oligo(5,7-bis(thiophen-2-yl)thieno[3,4-b]pyrazine)s ( E g = 1.50 eV) having alternating donor and acceptor units. The optical absorptions of the ground state, triplet excited state, radical cation, and dication are identified and found to shift to lower energy with increasing chain length. The reduction of the band gap in these alternating small band gap oligomers mainly results from an increase of the highest occupied molecular orbital (HOMO) level. The S 1-T 1 singlet-triplet splitting is reduced from approximately 0.9 eV from the trimeric monomer to -0.5 eV for the pentamer. This significant exchange energy is consistent with the fact that both the HOMO and the lowest unoccupied molecular orbital (LUMO) remain distributed over virtually all units, rather than being localized on the D and A units.     

Visible light sensitive photocatalyst, delafossite structured alpha-AgGaO(2)

Delafossite structured alpha-AgGaO(2) powder was successfully synthesized through a cation exchange reaction. alpha-AgGaO(2) has a band gap of 2.4 eV, absorbs visible light up to 520 nm, and effectively decomposes 2-propanol to CO2 via acetone by irradiating with either UV light (300-400 nm) or visible light (420-530 nm). The values of the quantum efficiency are similar (ca. 0.6%) under light irradiations with wavelengths of 365, 390, 430, 470, and 510 +/- 10 nm, but steeply decrease with wavelengths longer than 530 +/- 10 nm, which support a 2.4 eV band gap. In contrast, the other polymorph, alpha-AgGaO(2) powder, which has a band gap of 2.1 eV, shows a negligible activity when irradiating with either UV light or visible light. The higher oxidation activity of alpha-AgGaO(2) is probably due to its larger band gap, which is formed at the top of its valence band in a lower energy region as compared to alpha-AgGaO(2). Moreover, the first-principle calculations of alpha-AgGaO(2) and alpha-AgGaO(2) clearly indicate that alpha-AgGaO(2) has a remarkably larger dispersed valence band as compared to alpha-AgGaO(2), which is advantageous to the photocatalytic activity due to the efficient hole conduction.     

Characterization and photocatalytic activity of Fe- and N-co-deposited TiO2 and first-principles study for electronic structure

Titanium dioxide (TiO₂), co-deposited with Fe and N, is first implanted with Fe by a metal plasma ion implantation (MPII) process and then annealed in N₂ atmosphere at a temperature regime of 400-600 °C. First-principle calculations show that the (Fe, N) co-deposited TiO₂ films produced additional band gap levels at the bottom of the conduction band (CB) and on the top of the valence band (VB). The (Fe, N) co-deposited TiO₂ films were effective in both prohibiting electron-hole recombination and generating additional Fe-O and N-Ti-O impurity levels for the TiO₂ band gap. The (Fe, N) co-deposited TiO₂ has a narrower band gap of 1.97 eV than Fe-implanted TiO₂ (3.14 eV) and N-doped TiO₂ (2.16 eV). A significant reduction of TiO₂ band gap energy from 3.22 to 1.97 eV was achieved, which resulted in the extension of photocatalytic activity of TiO₂ from UV to Vis regime. The photocatalytic activity and removal rate were approximately two-fold higher than that of the Fe-implanted TiO₂ under visible light irradiation.     

The Narrowest Band Gap Ever Observed in Molecular Ferroelectrics: Hexane‐1,6‐diammonium Pentaiodobismuth(III)

Narrow band gaps and excellent ferroelectricity are intrinsically paradoxical in ferroelectrics as the leakage current caused by an increase in the number of thermally excited carriers will lead to a deterioration of ferroelectricity. A new molecular ferroelectric, hexane‐1,6‐diammonium pentaiodobismuth (HDA‐BiI₅), was now developed through band gap engineering of organic–inorganic hybrid materials. It features an intrinsic band gap of 1.89 eV, and thus represents the first molecular ferroelectric with a band gap of less than 2.0 eV. Simultaneously, low‐temperature solution processing was successfully applied to fabricate high‐quality ferroelectric thin films based on HDA‐BiI₅, for which high‐precision controllable domain flips were realized. Owing to its narrow band gap and excellent ferroelectricity, HDA‐BiI₅ can be considered as a milestone in the exploitation of molecular ferroelectrics, with promising applications in high‐density data storage and photovoltaic conversion.     

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.     

Anhydrous crystals of DNA bases are wide gap semiconductors

We present the structural, electronic, and optical properties of anhydrous crystals of DNA nucleobases (guanine, adenine, cytosine, and thymine) found after DFT (Density Functional Theory) calculations within the local density approximation, as well as experimental measurements of optical absorption for powders of these crystals. Guanine and cytosine (adenine and thymine) anhydrous crystals are predicted from the DFT simulations to be direct (indirect) band gap semiconductors, with values 2.68 eV and 3.30 eV (2.83 eV and 3.22 eV), respectively, while the experimentally estimated band gaps we have measured are 3.83 eV and 3.84 eV (3.89 eV and 4.07 eV), in the same order. The electronic effective masses we have obtained at band extremes show that, at low temperatures, these crystals behave like wide gap semiconductors for electrons moving along the nucleobases stacking direction, while the hole transport are somewhat limited. Lastly, the calculated electronic dielectric functions of DNA nucleobases crystals in the parallel and perpendicular directions to the stacking planes exhibit a high degree of anisotropy (except cytosine), in agreement with published experimental results.     

Chemical deposition of semiconducting cadmium selenide quantum dots in thin film form and investigation of their optical and electrical properties

Cadmium selenide quantum dots with cubic crystal structure are chemically deposited in thin film form using selenosulfate as a precursor for selenide ions and ammonia buffer with double role: as a ligand and as a pH value controller. The optical band gap energies of as-deposited and thermally treated cadmium selenide thin films, calculated within the framework of parabolic approximation for the dispersion relation, on the basis of equations which arise from the Fermi's golden rule for electronic transitions from valence to conduction band, are 2.08 and 1.77 eV, correspondingly. The blue shift of band gap energy of 0.34 eV for as-deposited thin films with respect to the bulk value is due to the quantum size effects (i.e., nanocrystals behave as quantum dots) and this finding is in agreement with the theoretical predictions. During the thermal treatment the nanocrystals are sintered, the increase of crystal size being in correlation with the decrease of band gap energy. The annealed thin films are practically non-quantized. From the resistance-temperature measurements, on the basis of the dependence of ln(R/Ω) vs 1/T in the region of intrinsic conduction, the thermal band gap energy (at 0 K) of 1.85 eV was calculated.     

Synthesis, structure, and optical properties of the quaternary seleno-gallates NaLnGa4Se8 (Ln = La, Ce, Nd) and their comparison with the isostructural thio-gallates

Three new quaternary seleno-gallates containing rare-earth metals and sodium cations, have been synthesized by a solid-state route in evacuated quartz ampoules: Na LnGa 4Se 8 ( Ln = La( I), Ce ( II) and Nd ( III)). The synthesis involved the stoichiometric combination of sodium polyselenides, rare-earth metal, Ga 2Se 3, and Se or elemental Ga in place of Ga 2Se 3. Single-crystal structure analysis indicated that the compounds are isostructural to the thio-analogue, NaNdGa 4S 8. The structures of I- III are described in terms of layers of GaSe 4 tetrahedra joined by corner- and edge-sharing; the alkali-metal cations and the trivalent rare-earth metal cations occupy square antiprismatic sites between the layers. The optical properties of the compounds have been investigated and compared with the isostructural thio-gallate. The band gap of I was located around 2.65 eV. The band gaps of II and III were 2.66 and 2.73 eV, respectively, considerably narrower than their thio-analogues ( approximately 3.4 eV). The contraction of the band gap was attributed to the shift of the valence band to higher energy due to the involvement of higher energy (4p) Se orbitals. The 4f --> 5d gap of II is found to be located around 2.32 eV, which is 0.26 eV narrower than the thio-analogue is due to a greater dispersion of the Ln-(5d) band caused by more covalent Ce-Se bonds as well as rising of the f level energy.     

Direct observation of defect levels in InN by soft X-ray absorption spectroscopy

We have used synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to study the electronic structure of nitrogen-related defects in InN(0001). Several defect levels within the band gap or the conduction band of InN were clearly resolved in NEXAFS spectra around the nitrogen K-edge. We attribute the level observed at 0.3 eV below the conduction band minimum (CBM) to interstitial nitrogen, the level at 1.7 eV above the CBM to antisite nitrogen, and a sharp resonance at 3.2 eV above the CBM to molecular nitrogen, in full agreement with theoretical simulations.     

吡唑啉衍生物的电化学性质及其能带结构

有机 /聚合物电致发光是当今世界上的热门研究领域[1 ,2 ] ,因而有机 /聚合物电致发光材料的能带结构成为非常重要的研究课题[3] .目前 ,确定有机 /聚合物电致发光材料的能带结构的方法主要有电化学法[4] ,光谱法[5] ,量子化学计算法[6] ,紫外光电子能谱法[7] ,光电子发射法[8] 等 .这些方法中 ,由于电化学方法操作简单 ,对仪器设备要求不高 ,故被广泛使用 .虽然不同的测试方法测得的能带数据存在一定的系统误差 ,但由于电发光器件中考虑的是材料间的能带匹配 ,因此 ,在不同的条件下测定的能带数据相对来说是可信的 .吡唑啉化合物具有较高的…     

The Electronic and Optical Properties of Au Doped Single-Layer Phosphorene

The electronic properties and optical properties of single and double Au-doped phosphorene have been comparatively investigated using the first-principles plane-wave pseudopotential method based on density functional theory. The decrease from direct band gap 0.78 eV to indirect band gap 0.22 and 0.11 eV are observed in the single and double Au-doped phosphorene, respectively. The red shifts of absorbing edge occur in both doped systems, which consequently enhance the absorbing of infrared light in phosphorene. Band gap engineering can, therefore, be used to directly tune the optical absorption of phosphorene system by substitutional Au doping.     

DFT study on the structural and electronic properties of Pt-doped boron nitride nanotubes

First-principles calculations based on density functional theory were carried out to investigate the structural and electronic properties of Pt substitution-doped boron nitride (BN) nanotubes. The electronic and structural properties were studied for substituted Pt in the boron and the nitrogen sites of the (BN) nanotube. The band gap significantly diminishes to 2.095 eV for Pt doping at the B site while the band gap diminishes to 2.231 eV for Pt doping at the N site. The band density increases in both the valence band and the conduction band after doping. The effects of the hardness and softness group 17 (halogen elements) were calculated by density functional theory (DFT).