Original Band Gap Research Articles

On the absorption coefficient of GaP1-xNx layers and its potential application for silicon photovoltaics

The absorption coefficient and the energy gap of GaP1-xNx layers has been obtained by spectroscopic ellipsometry for samples grown on Si(001) substrates by chemical beam epitaxy with N mole fractions in the range 0 ≤ x ≤ 0.081. The resulting absorption spectra exhibit a direct band-like behavior near the absorption edge. The absorption coefficient values increase with the N content, reaching values in the range α ∼ 1-2x104 cm−1 in the vicinity of the absorption edge below the original GaP direct bandgap, which are comparable to those obtained for high efficiency solar cell materials. Furthermore, dependence of the absorption coefficient with increasing N content points to a strong GaP Γ-like character of the conduction-band wave function of GaP1-xNx alloys near the Brillouin zone center at k = 0, as predicted by the band anticrossing model. Bandgap energy values obtained by spectroscopic ellipsometry are compared with previous values obtained by photoluminescence measurements on the same samples, observing a shift of about 50–100 meV. Finally, the value of the band anticrossing parameter coupling the N level and the host GaP conduction band has been obtained from the dependence of both, the bandgap and the absorption coefficient, with the N content (2.1 and 3.3 eV respectively).

Dissipative stratified elastic metamaterials for broadband blast-wave impact mitigation

In this paper, impact attenuation and blast-wave mitigation are effectively realized by the dissipative stratified metamaterials with negative mass mechanism due to the coupling of the different resonators. To create a broad bandgap of the metamaterials, the dispersion curve and effective negative mass property are investigated. It is found that the coupling effect of different resonators produced by the relative movement of different main masses can create a broad bandgap and make the original bandgap by starting with a lower frequency, and thus impact input waves could be blocked effectively. Simulation results show that a stratified elastic metamaterial containing 25 units can reduce the amplitude of the load by two thirds under impact loading and almost completely impede its further propagation. Furthermore, the bandgap is broadened by optimization of the structure parameters and dissipative coefficient for blocking transient shock waves propagation. It is shown in numerical experiment that the wider bandgap produced by the new elastic metamaterials can effectively absorbed the energy of the blast and impact waves. Therefore, the results of this study provide a remarkably novel strategy for the design of new blast-resistant materials.

Effects of S substitution on the structural, optoelectronic, and thermomechanical properties of KTaO3 through density functional theory

Driven by a curiosity of how sulfur (S) alters the physical properties of KTaO3, we embarked on a computational journey using the WIEN2k code and the insights of density functional theory (DFT). We aim to unravel the influence of S-substitution on the material's structure, electronic behavior, optical responses, mechanical strength, and thermodynamic characteristics of KTaO3. Our investigation has unveiled a remarkable structural transformation, wherein the cubic phase of KTaO3 undergoes a shift to a tetragonal configuration for KTaO2S and KTaOS2, eventually returning to a cubic phase in KTaS3. Significantly, the band gap values have undergone substantial alterations, presenting energy gaps of 1.808 eV, 0.264 eV, and 0.078 eV for KTaO2S, KTaOS2, and KTaS3, respectively, diverging from the original band gap of 3.572 eV in KTaO3. Importantly, all these compounds have exhibited mechanical and dynamical stability. Furthermore, these compounds have displayed promising optical features characterized by high absorption coefficients (∼105 cm−1), minimal reflectivity (<30%), and robust optical conductivity within the visible spectrum, making them ideal candidates for a range of optoelectronic technologies. Our comprehensive investigation has reinforced the stability of all computed phases, showcasing exceptional electronic, optical, and thermo-mechanical properties, including semiconducting behavior, hardness, ductility, anisotropy, high absorptivity, and low reflectivity. The modified band gap and optical characteristics of KTaO2S indicate considerable potential for its application in solar cells, presenting promising opportunities to improve solar energy conversion efficiency. The thermal conductivity of KTaO3 is fascinating and shows substantial promise for applications as a heat sink material.

Coupled bandgap properties and wave attenuation in the piezoelectric metamaterial beam on periodic elastic foundation

In recent years, the innovative design of metamaterials and the study of bandgap characteristics have become the focus of research. The flexible setting of small added mass in locally resonant metamaterials can effectively regulate bandgap characteristics and wave transmission. In this paper, a novel tunable piezoelectric metamaterial beam on periodic elastic foundation is proposed for realizing the wider bandgap region and the stronger attenuation capability. Based on Timoshenko beam theory, wave equation of the metamaterial beam considering the piezoelectric and the inertial amplification effects is obtained from the transfer matrix method, and the calculational results are compared to that of numerical simulation. The results show that periodic elastic foundation can improve the overall stiffness of the structure, and the bending wave attenuation frequency domain will broaden with the increase of the elastic foundation stiffness. The metamaterial beam exhibits the strong wave attenuation in the bandgap induced by external negative capacitance shunt circuit, and the wave transmission loss is improved about 20 dB than the short circuit case. The coupling effects between the local resonance and the inertial amplification mechanism can cause the coupled bandgaps, which expand the original bandgap range by at least 25%. In addition, the coupled bandgaps boundary can be extended to lower frequencies by adjusting the angle of rigid rod and the additional mass separately, resulting in broadband wave attenuation. This work will provide valuable ideas for the design of advanced smart materials and structures in the field of vibration and noise reduction.

Boron doped mesoporous g-C3N4 nanosheets: Boosting photocatalytic nitrogen fixation performance under visible light

A simple two-step method was used to prepare g-C3N4 nanosheets doped with B atoms. The multi-layer porous morphology prolongs the transmission pathway of visible light inside, thereby improving the ability to capture visible light. The doped photosensitive B atoms can decrease the original bandgap of g-C3N4 and increase the material's sensitivity to visible light. According to the findings, B/g-C3N4 performs superbly in photocatalytic nitrogen fixation, with the best performance being 2 wt% B/g-C3N4 (81.56 mol/L-1.h−1), which has a nitrogen fixation efficiency 2.44 times higher than g-C3N4 (33.52 mol/L-1.h−1). Additionally, 2 wt% B/g-C3N4 exhibits strong photocatalytic stability due to the doping of B atoms.

Carbon dioxide reduction mechanism via single‐atom nickel supported on graphitic carbon nitride

AbstractIn many photocatalytic reaction paths, the breaking of the first CO bond in a CO2 molecule is often the key step that becomes the rate‐controlled reaction step. In this paper, a graphitic carbon nitride (g‐C3N4) supported nickel single‐atom catalyst (Ni@g‐C3N4) was successfully constructed, and the mechanism of CO2 catalytic reduction was systematically studied based on density functional theory (DFT). The introduction of nickel promotes the adsorption of small molecules, especially for the CO2 activation. According to density of states (DOS) and frontier orbital analysis, the photogenerated carriers tend to jump from nitrogen atoms to carbon atoms, forming an electron transfer in real space, after g‐C3N4 is excited by light. With the appearance of nickel‐doped levels, the DOS of Ni@g‐C3N4 is no longer symmetric with respect to the spin up and down, especially around the original band gap of g‐C3N4. Single‐atom nickel has abundant frontier orbitals and high activity and is a favourable place for chemical reactions. The presence of surface hydrogen can promote the recovery of CO2, and the energy barrier of Ni@g‐C3N4 with hydrogen is only 15% of the clean g‐C3N4 surface. This paper provides a new idea for the development of efficient single‐atom catalysts for CO2 reduction.

Low-frequency band gap characteristics of a novel spinning metamaterial pipe with Timoshenko model

As a representative artificial periodic composite, metamaterials have presented broad application prospects in vibration attenuation and noise reduction of engineering devices owing to their extraordinary band gap (BG) properties. In this paper, a novel type of dynamic metamaterial structure is proposed on the basis of a spinning locally resonant (LR) pipe conveying fluid, and the coupled BG characteristics of such meta-structure are investigated. By applying the Timoshenko beam model, the differential equations governing the orthogonally transverse vibrations of spinning pipes conveying fluid are deduced by the Hamilton principle. Based upon the two resulting transverse waves, a dynamic two-dimensional LR pipe structure is developed by periodically encircling the pipe with dual-layer rings. An additional centrifugal force due to spin as well as the conventional inertial force is introduced into the local resonators. It is revealed that the present LR pipe makes a great contribution to the formation of low-frequency BGs below 500 Hz, which is conducive to the vibration suppression of large-size structures in engineering. Moreover, original BGs of non-spin structures will separate to different pseudo BGs towards the precession motions, while the true BGs are actually located in their coincident areas. Numerical results also demonstrate the significant effects of various physical, geometrical and motional parameters on the low-frequency BGs of the pipe.

Disorder‐Driven Collapse of Topological Phases in Photonic Topological Insulator

By embedding the position disorder of rods in 2D gyromagnetic photonic crystal, the influences of increasing random disorder on the photonic topological phases (PTPs) in a photonic topological insulator are numerically investigated. When the disorder is small, the PTPs almost have no change. With the increase in disorder, the PTPs at the edge of the original topological bandgap are first affected, and those at center of the bandgap have the best robustness. As disorder increases to a critical value, the PTPs collapse ultimately. During the collapse of the PTPs, disorders can induce the Anderson localization of electromagnetic (EM) wave, and produce localized hot spots in the system. When the disorder is not too large, the hot spots are excited at the edge area near the excitation source, and have little influence on the one‐way propagation of the topological edge state (TES). As disorder increases continuously, the hot spots would penetrate the system deeply and affect the propagation of EM waves significantly, which leads to the collapse of the PTPs and destruction of one‐way propagation of TES. Even so, there are still remaining long‐range and short‐range orders in the system. This research provides a theoretical model and an experiment platform for further studying the relationship between topological phases and disorders.

Electrostatic Design of Polar Metal-Organic Framework Thin Films.

In recent years, optical and electronic properties of metal–organic frameworks (MOFs) have increasingly shifted into the focus of interest of the scientific community. Here, we discuss a strategy for conveniently tuning these properties through electrostatic design. More specifically, based on quantum-mechanical simulations, we suggest an approach for creating a gradient of the electrostatic potential within a MOF thin film, exploiting collective electrostatic effects. With a suitable orientation of polar apical linkers, the resulting non-centrosymmetric packing results in an energy staircase of the frontier electronic states reminiscent of the situation in a pin-photodiode. The observed one dimensional gradient of the electrostatic potential causes a closure of the global energy gap and also shifts core-level energies by an amount equaling the size of the original band gap. The realization of such assemblies could be based on so-called pillared layer MOFs fabricated in an oriented fashion on a solid substrate employing layer by layer growth techniques. In this context, the simulations provide guidelines regarding the design of the polar apical linker molecules that would allow the realization of MOF thin films with the (vast majority of the) molecular dipole moments pointing in the same direction.

Electronic properties of two-dimensional G/GaN(SiC) van der Waals heterostructures

Based on the first-principles, the van der Waals heterostructures of two-dimensional gallium nitride (GaN) and silicon carbide (SiC) stacked graphene respectively have been researched. The G/GaN(SiC) heterostructure not only maintain the electrical properties of monolayer GaN(SiC), also keep the original electrical properties of graphene. Under the applied external electric field, the two heterostructures have their own Schottky contact critical transition points. When Eext > 0.10V/Å, both heterostructures are converted to ohmic contacts. In addition, in terms of biaxial strain, these two heterostructures maintain a good p-type Schottky barrier. The difference is that GaN in G/GaN turns into a direct bandgap within the biaxial strain range of −7% ~ −9%, while SiC in G/SiC always keeps the original indirect bandgap. The ohmic contact of G/GaN occurs at ε > 7%, while G/SiC occurs at ε > 5%. The charge injection of two heterostructures was further explored by analyzing the electrostatic potential under different biaxial strain. We believe that our work will make a significant contribution to future high-power high-voltage devices and Schottky devices.

Photocatalytic degradation of gaseous VOCs over Tm3+-TiO2: Revealing the activity enhancement mechanism and different reaction paths

In this work, we synthesized thulium (Tm3+) modified TiO2 nanoparticles by a facile sol-gel route. We show that Tm3+-modified TiO2 exhibit superior photocatalytic activity than pure TiO2 towards gas-phase degradation of acetaldehyde, o-xylene, and their mixture. The photocatalytic activity enhances with the increasing Tm content from 0.1 to 0.5 mol%. The 0.5 mol%Tm-TiO2 sample demonstrated the highest photodegradation efficiency (>90.0%) for acetaldehyde and o-xylene. According to the systematic characterization such as photoluminescence (PL), photocurrent, up-conversion spectra, UV–Vis diffuse reflectance spectroscopy and electron spin resonance (ESR) analysis, the enhanced photocatalytic performance can be attributed to introducing the new Tm-4f states between the original band gap of TiO2, which led to the increase of efficiency of light-induced electron-hole separation and the increase of hydrophilicity which resulted in the increased adsorption capacity towards acetaldehyde and o-xylene. Gas Chromatography-Mass Spectrometer (GC–MS) analysis suggested that o-xylene was first oxidized to o-benzaldehyde, which was eventually converted to butanol before being mineralized into CO2. However, new intermediates including phthalide, o-methyl acetophenone, naphthalene and 5-Methyl-4-octanone were detected in the mixture.

Electronic conductance of a lengthy zigzag honeycomb nanotube including some surface-adsorbed molecules

The effect of the adsorption of environmental polyatomic molecules on the electronic transport properties of a lengthy zigzag honeycomb nanotube is investigated using the Green’s function formalism within the tight-binding approach. We follow the theoretical approach in our previous paper, (Nadri et al 2019 Chin. Phys. B 28 017202), which is done for a lengthy armchair honeycomb nanoribbon. Three types of adsorbed molecules are considered: di-, tri- and tetra-atomic ones which are located at a small part in the middle of the nanotube. The results show that in the presence of adsorbed molecules, the original band gap of the nanotube becomes wider and at some molecular concentrations the system converts to a n- or p-type semiconductor. In some molecular concentrations and configurations, a new energy gap is opened that can produce new physical aspects in nanotubes.

Tuning Phase Transitions in Metal Oxides by Hydrogen Doping: A First-Principles Study.

Optimizing physical or chemical properties via electronic phase transitions is important for developing high-performance semiconductor materials. Using first-principles calculations, we established hydrogen doping (H-doping) as an alternative strategy for modulating phase transitions in metal oxide semiconductors. We found that H-doping can induce an insulator-to-metal phase transition in rutile TiO2 or wurtzite ZnO, a metal-to-insulator phase transition in rutile VO2, and sequential insulator-metal-insulator phase transitions in SnO2. H-doping not only creates defect-induced states inside the original bandgap but also induces new occupancy patterns in conduction band edge states. A linear relationship between occupation of the conduction band edge and H-doping concentration was found, offering the possibility of precisely tuning the material's free-carrier concentration, electrical conductance, and photoabsorption ability. We envision that this work will provide a promising way to tune electrical phase transitions in metal oxides.

Alkali metal-substituted bismuth-based perovskite compounds: A DFT study

Abstract In this study, we have performed density functional theory based calculations to investigate the effect of alkali substituents M on the structure and electronic properties of (Bi0.5M0.5)TiO3 compounds with M = Li, Na and K. When the ionic radius of the substituting M cation increases, the corresponding direct band gap of (Bi0.5M0.5)TiO3 also increases. The same trend is found for the O-deficient Bi0.5M0.5TiO3-δ compounds, which were investigated by removing one Oxygen atom from the super cell creating an O-vacancy in it. The presence of the O-vacancy breaks the local symmetry of the crystal structures and generates excess electrons in the O-deficient compounds. This results in the appearance of an indirect bandgap in Bi0.5Na0.5TiO3-δ and Bi0.5K0.5TiO3-δ, except for Bi0.5Li0.5TiO3-δ that still exhibits a direct bandgap. Excess electrons induce the defect states within the original bandgap of the stoichiometric compounds and decrease the bandgap of the O-deficient compounds. In line with the electronic calculations, effective mass calculations have provided a preliminary insight into the photocatalytic performance of (Bi0.5M0.5)TiO3 and reveal a positive impact of K cations on the photocatalytic activity.

The electronic, magnetic and optical properties of single-layer CrS2 with vacancy defects

Abstract Based on the first-principles calculations, we have systematically studied the electronic, magnetic and optical properties of single-layer CrS2 with and without vacancy defects. The S vacancy is the most energetically favorable vacancy among the considered S vacancy, Cr vacancy, S2 double vacancy and CrS double vacancy. The vacancy induced defect states drop into the original band gap region, so the smaller band gaps have been obtained for single-layer CrS2 with vacancies compared with the perfect one. Although the perfect single-layer CrS2 is a nonmagnetic direct band gap semiconductor, the magnetic semiconductor is obtained via either Cr vacancy or CrS double vacancy. The S vacancy and S2 double vacancy are still nonmagnetic semiconductors with reduced band gaps. Furthermore, the static dielectric constants and static refractive index of single-layer CrS2 with vacancies are all bigger than those of perfect CrS2 because the reduction of the band gaps. The first main absorption peak is weakened while the other two main peaks are all significantly enhanced for single-layer CrS2 with vacancies.

From Nonluminescent to Blue-Emitting Cs4 PbBr6 Nanocrystals: Tailoring the Insulator Bandgap of 0D Perovskite through Sn Cation Doping.

All-inorganic cesium lead halide perovskite nanocrystals (NCs) with different dimensionalities have recently fascinated the research community due to their extraordinary optoelectronic performance such as tunable bandgaps over the entire visible spectral region. However, compared to well-developed 3D CsPbX3 perovskites (X = Cl, Br, and I), the bandgap tuning in 0D Cs4 PbX6 perovskite NCs remains an arduous task. Herein, a simple but valid strategy is proposed to tailor the insulator bandgap (≈3.96 eV) of Cs4 PbBr6 NCs to the blue spectral region by changing the local coordination environment of isolated [PbBr6 ]4- octahedra in the Cs4 PbBr6 crystal through Sn cation doping. Benefitting from the unique Pb2+ -poor and Br- -rich reaction environment, the Sn cation is successfully introduced into the Cs4 PbBr6 NCs, forming coexisting point defects comprising substitutional SnPb and interstitial Bri , thereby endowing these theoretically nonluminescent Cs4 PbBr6 NCs with an ultranarrow blue emission at ≈437 nm (full width at half maximum, ≈12 nm). By combining the experimental results with first-principles calculations, an unusual electronic dual-bandgap structure, comprising the newly emerged semiconducting bandgap of ≈2.87 eV and original insulator bandgap of ≈3.96 eV, is found to be the underlying fundamental reason for the ultranarrow blue emission.

On the nature of trapped states in an MoS2 two-dimensional semiconductor with sulfur vacancies

ABSTRACTMolybdenum disulfide (MoS2) is a common two-dimensional semiconductor that has been highly studied as an emerging material for catalysis and electronics. The most common material defects in MoS2 are sulfur vacancies. In order to reveal the nature of the trapped states induced by sulfur vacancies, we perform Density Functional Theory (DFT) combined with quantum dynamics calculations. According to our model, we find that the sulfur vacancies create trap states in the original band gap of monolayer MoS2 that disrupt charge transmission through the monolayer. In addition, we did not find any resonance states among the shallow states in the conduction band continuum.

Ideal half-filled intermediate band position in CuGaS2 generated by Sb-related defect complex: a first-principles study

Cu-based chalcopyrite compounds have attracted much attention for photovoltaic application, while some of them (like CuGaS2) have energy gaps greater than the optimal value. An isolated and half-filled intermediate band located at the lower part of its original band gap exhibits in CuGaS2 with (SbGa + ZnGa) or (SbGa + VCu) defect complex, in line with the intrinsic p-type conductivity of the host, revealed from our first-principles calculations. Subsequently, the absorption coefficients of CuGaS2 can cover the full solar light spectrum efficiently. Based on the defect formation energy calculations, however, these defect complexes are hard to reach a large concentration under equilibrium condition. Nevertheless, non-equilibrium growth methods are suggested to prepare samples inheriting the excellent adsorption coefficients.

Elastic Wave Modulation in Hollow Metamaterial Beam With Acoustic Black Hole

We propose and discuss the elastic wave attenuation of hollow metamaterial beam embedded acoustic black hole. More abundant physical phenomena are given by modal analysis, shows that the band gap of three-dimensional acoustic black hole metamaterial is different from two-dimensional one. Lateral flexural vibrations occurs and make the original first two-dimensional band gap be compressed, and the opening of first three-dimensional band gap are caused by coupling effect between the longitudinal and lateral flexural vibrations. Below 1200Hz, only two band gaps exist, the geometric parameter m1 and angle $\gamma $ could affect the band structure a lot, while the effect of geometric parameter m0 is a little less. Mutual validation of transmission spectra of vibration test and finite element analysis calculation and band gaps, illustrating its validity of the structure design, corresponding results could stimulate the realizations of three-dimensional acoustic black hole structure, particularly paves the way for the bridge from the corresponding theory of low frequency vibration and noise reduction to the practical application.

Preparation, optical, and electrical properties of rubrene∶MoO&lt;sub&gt;3&lt;/sub&gt; films

&lt;sec&gt;In this paper, the mixed films with different rubrene-to-MoO&lt;sub&gt;3&lt;/sub&gt; ratios are deposited on the substrates of Si, indium tin oxide and quartz glass by using the thermal evaporation technique. First, these films are characterized by atomic force microscopy and X-ray diffraction in order to identify their surface morphology and their structure properties. The results show that all the films are amorphous and the film has the best flatness when the rubrene-to-MoO&lt;sub&gt;3&lt;/sub&gt; ratio is 2∶1. Second, the optical properties of the mixed films are investigated by both photoluminescence (PL) spectra and absorption spectra. The optical band gap of rubrene and MoO&lt;sub&gt;3&lt;/sub&gt; are 2.2 eV and 3.49 eV respectively and there is almost no absorption about rubrene and MoO&lt;sub&gt;3&lt;/sub&gt; in the near-infrared (NIR) region. However the PL spectrum shows a peak in NIR region and it indicates that the interface between rubrene and MoO&lt;sub&gt;3&lt;/sub&gt; possesses an abrupt discontinuity at the vacuum level, resulting in electron wave functions overlapping and charge-transfer complex (CTC) forming. The intermediate state within the original band gap of rubrene with energy of 1.25 eV is induced by the CTC, which suggests the possibility of charge transfer exciton generated upon NIR excitation. The absorption spectra of the mixed films show that there is an obvious absorption. All the films have the same absorption peak except the film with a rubrene-to-MoO&lt;sub&gt;3&lt;/sub&gt; ratio of 4∶1 and it indicates that the concentration of MoO&lt;sub&gt;3&lt;/sub&gt; has almost no influence on the absorption of the mixed films. The optical band gaps of the mixed thin films are calculated in a spectral range of 345－1035 nm according to the Tauc equation, and the results show that the optical band gap of the film with a rubrene-to-MoO&lt;sub&gt;3&lt;/sub&gt; ratio of 2∶1 is narrowest (~2.23 eV).&lt;/sec&gt; &lt;sec&gt;In order to study the electrical characteristics of the mixed films, an Al/rubrene:MoO&lt;sub&gt;3&lt;/sub&gt;/ITO device is fabricated. The current density-voltage (&lt;i&gt;J&lt;/i&gt;-&lt;i&gt;V&lt;/i&gt;) characteristic is also investigated. The analysis of the &lt;i&gt;J&lt;/i&gt;-&lt;i&gt;V&lt;/i&gt; measurement for the device indicates that the current conduction in the Al/rubrene:MoO&lt;sub&gt;3&lt;/sub&gt;/ITO device is Ohmic type when the rubrene-to-MoO&lt;sub&gt;3&lt;/sub&gt; ratios are 4∶1 and 2∶1, and it is Schottky type when the ratio is other value. The current for rubrene-to-MoO&lt;sub&gt;3&lt;/sub&gt; ratio of 1∶1 is larger than that for 1∶2, which indicates that the contact is better when the surface is more smooth. These properties of the mixed films can result in the applications in the near-infrared region.&lt;/sec&gt;