Investigation Of Band Structure Research Articles

Constructing Particulate p–n Heterojunction Mo:SrTiO3/NiO@Ni(OH)2 for Enhanced H2 Evolution under Simulated Solar Light

The global environmental issues associated with the use of fossil fuels lead to an urgent need for renewable energy sources, especially those free of CO2 emissions, such as green hydrogen. In this work, we successfully synthesized Mo-doped SrTiO3 by a molten salt method for photocatalytic hydrogen production under simulated solar light (AM 1.5G illumination). As a strategy to enhance the photocatalytic performance of Mo:SrTiO3, nickel-based nanoparticles (NiO@Ni(OH)2) were deposited onto the surface of the particles by modified magnetron sputtering to form a p–n heterojunction (HJ), resulting in the photocatalytic improvement of around 30-fold concerning pristine SrTiO3. Theoretical investigation of the electronic band structure, by DFT, reveals that the addition of Mo as a dopant leads to the formation of midgap states near the conduction band, further attributed to the photoactivity of Mo:SrTiO3 under visible-light illumination (>400 nm). The obtained structure, Mo:SrTiO3/NiO@Ni(OH)2, had its electronic behavior studied by XPS, Mott–Schottky analysis, and UV–vis spectroscopy, leading to the construction of a band diagram that confirms type-II p–n HJ formation. Remarkably, the formation of the HJ was responsible for establishing an internal electric field, which drives the photogenerated holes to the NiO@Ni(OH)2 structure, leading to the suppression of electron–hole recombination, observed by the reduction of the PL signal.

Investigation of Band Structure in Strained Single Crystalline Si1-X Sn X

1. Background and purpose Silicon tin (SiSn) alloys are attractive candidate for the next-generation group-IV semiconductors. It is well known that Si and Ge change from indirect to direct transition types with the addition of Sn. Among them, SiSn alloys are expected to be applied for the near-infrared optical devices because it is predicted to become a direct band-gap appropriate for the optical communication with sufficient Sn content [1]. Although the Sn composition that changes from indirect to direct band-gap has been reported using several calculations, there are few reports of experimental results. In addition, the optical properties of single crystalline Si1-x Sn x have not been sufficiently clarified owing to the difficulty of crystal growth. In this study, we evaluated the optical properties to clarify the band structure of single crystalline Si1-x Sn x . 2. Experimental method Strained 30 nm-thick Si1-x Sn x films were grown on Si substrate by molecular beam epitaxy (MBE), and epitaxial growth was confirmed by X-ray diffraction (XRD) two-dimensional reciprocal space mapping (2DRSM) [2]. The Sn composition was determined by 2DRSM. The Sn fraction x of Si1-x Sn x samples used in this study were 0.005, 0.009, 0.018, 0.022, and 0.06. The spectroscopic ellipsometry measurements were performed with an incident angle of 70°, using a 70 W xenon lamp with a wavelength range of 200 to 1600 nm for the light source. The functions used in the analysis are the Tauc-Lorentz and Lorentz function. 3. Results and Discussion Figure 1(a) and (b) show real part and imaginary part of the complex dielectric functions for Si1-x Sn x (x=0.005 - 0.06) and pure-Si, respectively. From Fig. 1, it can be confirmed that the spectra shift with increasing Sn composition and the peaks appear around 1.8 eV and 2.8 eV for the samples with high Sn composition (2.2% and 6.0%). The peak shift characteristic with the addition of Sn is similar to the results of germanium tin (GeSn) [3].The complex dielectric functions of the semiconductor include much information about the electronic band structure. In Fig. 1, the sharp peaks that are due to direct band transitions show what is known as critical points (CPs). In the case of Si, E'0 CP (Γ) and E 1 CP (L) energy are around 3.2 eV [4]. Therefore, it can be considered that the peak at 2.8 eV indicates the separation of the E'0 and E 1 CP energies due to the change in the band gap at the Γ point with the addition of Sn. The peak at 1.8 eV is unique to Si1-x Sn x , which is not found in Si. We consider that this peak is due to the split-off valence band caused by the Sn addition. The behavior of the valence band is strongly affected by strain. These results suggest a reduction of the band gap at the Γ point and the formation of an optical transition region due to the Sn addition, and reveal important findings for the application of SiSn for the near-infrared devices. Acknowledgment This work was partly supported by the Japan Society for the Promotion of Science (JSPS) (17J08240, 19K21971, and 21H01366).

Investigation of Band Structure in Strained Single Crystalline Si1-X Sn X

We evaluated the optical properties and the band structure of strained single crystalline Si1-x Sn x using spectroscopic ellipsometry. The results suggest a reduction of the band gap at the Γ point and the formation of an optical transition by Van-Hove singularity with higher Sn fraction. In addition, since the reduction of the band gap with increasing Sn fraction at the Γ point is larger than at other points, it is expected the indirect transition type Si1-x Sn x used in this study may eventually change to the direct transition type. Although higher Sn fraction are required to achieve direct transition type Si1-x Sn x , the band structure of strained single crystal Si1-x Sn x was experimentally clarified.

Group IV element allotropes in the Fmmm phase: First-principles calculations

Three new Group IV element allotropes Fmmm C72, Si72, and Ge72 are proposed in this paper, and their physical properties are researched by density functional theory (DFT). Each novel allotrope has mechanical, dynamical, and thermodynamic stability. The study of the elastic modulus shows that Fmmm Si72 has greater elastic anisotropy than Fmmm C72 and Ge72. Fmmm C72 is a superhard material with the hardness of 47.8 GPa. The investigation of the electronic band structures of Fmmm Si72 and Ge72 exhibits they have semiconducting properties, while Fmmm C72 shows metallicity. Fmmm Si72 has a direct band gap (1.41 eV), and it is very suitable for solar cells. In addition, Fmmm Si72 has good absorption capacity in the visible light region, so Si72 has possible applications in photovoltaic applications due to its direct band gap property and good absorption capacity in the visible light region.

Two-Step Process of a Crystal Facet-Modulated BiVO4 Photoanode for Efficiency Improvement in Photoelectrochemical Hydrogen Evolution.

The photoactivity of nanoporous bismuth vanadate (BiVO4, BVO) photoanodes that were fabricated by a two-step process (electrodeposition and then thermal conversion) in photoelectrochemical (PEC) hydrogen (H2) evolution can be enhanced about 1.44-fold by improving the constitutive ratio of (111̅), (061), and (242̅) crystal facets. The PEC characterization was carried out to investigate the factors altering the performance, which revealed that the crystal facet modulation could improve the photoactivity of the BVO photoanodes. In addition, the orientation-controlled BVO thin-film electrodes are introduced as evidence that the present crystal facet modulation is the positive effect for BVO photoanodes in PEC. The investigation of energy band structures and interfacial charge carrier dynamics of the BVO photoanodes reveals that the crystal facet modulation could result in a shorter lifetime of charge carrier recombination and larger band bending at the interface between BVO and electrolytes. This outcome could improve the charge separation and charge transfer efficiencies of BVO photoanodes, promoting the efficiency of PEC H2 evolution. Moreover, this crystal facet modulation can combine with co-catalyst decoration to further improve the solar-to-hydrogen efficiency of BVO photoanodes in PEC. This study presents a potential strategy to promote the PEC activity by crystal facet modulation and important insights into the interfacial charge transfer properties of semiconductor photoelectrodes for the application in solar fuel generation.

Facile preparation of Ag2S/KTa0.5Nb0.5O3 heterojunction for enhanced performance in catalytic nitrogen fixation via photocatalysis and piezo-photocatalysis

In this work, a novel heterojunction composite Ag2S/KTaxNb1-xO3 was designed and synthesized through a combination of hydrothermal and precipitation procedures. The Ta/Nb ratio of the KTaxNb1-xO3 and the Ag2S content were optimized. The best 0.5% Ag2S/KTa0.5Nb0.5O3 (KTN) sample presents an enhanced photocatalytic performance in ammonia synthesis than KTN and Ag2S. Under simulated sunlight, the NH3 generation rate of 0.5% Ag2S/KTN reaches 2.0 times that of pure KTN. Under visible light, the reaction rate ratio of the two catalysts is 6.0. XRD, XPS, and TEM analysis revealed that Ag2S was intimately decorated on the KTN nanocubes surface, which promoted the electron transfer between the two semiconductors. The band structure investigation indicated that the Ag2S/KTN heterojunction established a type-II band alignment with intimate contact, thus realizing the effective transfer and separation of photogenerated carriers. The change in charge separation was considered as the main reason for the enhanced photocatalytic performance. Interestingly, the Ag2S/KTN composite exhibited higher NH3 generation performance under the combined action of ultrasonic vibration and simulated sunlight. The enhanced piezo-photocatalytic performance can be ascribed that the piezoelectric effect of KTN improved the bulk separation of charge carriers in KTN. This study not only provides a potential catalyst for photocatalytic nitrogen fixation but also shows new ideas for the design of highly efficient catalysts via semiconductor modification and external field coupling.

Cylindrical C96 Fullertubes: A Highly Active Metal-Free O2 -Reduction Electrocatalyst.

A new isolation protocol was recently reported for highly purified metallic Fullertubes D5h -C90 , D3d -C96 , and D5d -C100, which exhibit unique electronic features. Here, we report the oxygen reduction electrocatalytic behavior of C60 , C70 (spheroidal fullerenes), and C90 , C96 , and C100 (tubular fullerenes) using a combination of experimental and theoretical approaches. C96 (a metal-free catalyst) displayed remarkable oxygen reduction reaction (ORR) activity, with an onset potential of 0.85 V and a halfway potential of 0.75 V, which are close to the state-of-the-art Pt/C benchmark catalyst values. We achieved an excellent power density of 0.75 W cm-2 using C96 as a modified cathode in a proton-exchange membrane fuel cell, comparable to other recently reported efficient metal-free catalysts. Combined band structure (experimentally calculated) and free-energy (DFT) investigations show that both favorable energy-level alignment active catalytic sites on the carbon cage are responsible for the superior activity of C96 .

Effect of hydrostatic pressure on structural and opto-electronic properties for barium based oxide perovskite

In this study, the effects of hydrostatic pressure ranges from 0 Gpa to 20 Gpa on detailed pressure induced computations of Barium Zirconate BaZrO3, including initial geometry optimization, energy minimization calculations, total energy values, lattice parameters, energy band gaps (Eg), optical and elastic constants are reported. All subjected computational results are collected by applying Local Density Approximation functional proposed by Ceperley-Alder and reformulated by Perdew-Zunger (LDA-CAPZ) using Cambridge Serial Total Energy Package (CASTEP) module under the formulation of Density Function Theory (DFT). The reduced nature of cell volume as well as lattice constants confirm the structural compressibility of BaZrO3 under the application of applied pressure. Increasing bulk modulus values indicate maximum structural stiffness of BaZrO3 under applied pressure. The collected values are compared with available theoretical and experimental parameters. The investigated band structures predict direct bandgap of BaZrO3, that increase under the influence of applied pressure which tend to increase all optoelectronic behavior. So, the current investigation signifies, that these calculations provide a baseline for new researchers to establish further pressure investigations and could be helpful for the uses of BaZrO3 in optoelectronic applications.

Single-particle states spectroscopy in individual carbon nanotubes with an aid of tunneling contacts

Recent studies have demonstrated that the band structure of a carbon nanotube (CNT) depends not only on its geometry but also on various factors such as atmosphere chemical composition and dielectric environment. Systematic studies of these effects require an efficient tool for an in situ investigation of a CNT band structure. In this work, we fabricate tunneling contacts to individual semiconducting carbon nanotubes through a thin layer of alumina and perform tunneling spectroscopy measurements. We use field-effect transistor configuration with four probe contacts (two tunnel and two ohmic) and bottom gates. Bandgap values extracted from tunneling measurements match the values estimated from the diameter value within the zone-folding approximation. We also observe the splitting of Van-Hove singularities of the density of states under an axial magnetic field.

First principles band structure investigation of cubic spinel CuMn2Te4Compound for thermoelectric applications

Spinel CuMn2Te4 material is investigated for its nonmagnetic and ferromagnetic phases by first principles calculation using the FP-LAPW method. Exchange and correlation are treated with GGA. T otal energy calculations reveal that CuMn2T e4 compound tend to stabilise at ferromagnetic phase. Non vanishing bands at the Fermi Energy level illustrates the metallic nature of CuMn2Te4at both of its nonmagnetic and ferromagnetic phases. Transport properties are studied using the BoltzTraP code at 325K. Thermo power of -26.22μv/k is estimated for nonmagnetic CuMn2Te4. At ferromagnetic phase, these values are predicted as 84.85μv/kand 53.39μv/k respectively for spin-up and spin-down states. Computed power factor values are 2.29 x1014Wm-1s-1K-2for NM phase and 2.33×1017Wm−1s−1K−2/1.001×1010Wm−1s-−1K-−2respectivelyfor FM up-spin/down-spin phases. A total magnetic moment of 7.11003 μB/F.U. is obtained for the energetically favourable ferromagnetic phase of CuMn2Te4 compound. High pressure investigations reveal the possibility of electronic topological transition that may lead to changes in the Fermi Surface topology and hence changes in the physical properties of nonmagnetic CuMn2Te4.

Quasiparticle Band Structure and Phonon-Induced Band Gap Renormalization of the Lead-Free Halide Double Perovskite Cs2InAgCl6

The lead-free halide double perovskite Cs2InAgCl6 was recently designed in silico and subsequently synthesized in the lab. This perovskite is a wide-gap semiconductor with a direct band gap and exhibits extraordinary photoluminescence in the visible range upon Na doping. The light emission properties of Cs2InAgCl6 have successfully been exploited to fabricate stable single-emitter-based white-light LEDs with near unity quantum efficiency. An intriguing puzzle in the photophysics of this compound is that the onset of optical absorption is around 3 eV, but the luminescence peak is found around 2 eV. As a first step toward elucidating this mismatch and clarifying the atomic-scale mechanisms underpinning the observed luminescence, here, we report a detailed investigation of the quasiparticle band structure of Cs2InAgCl6 as well as the phonon-induced renormalization of the band structure. We perform calculations of bang gaps and effective masses using the GW method, and we calculate the phonon-induced band structure renormalization using the special displacement method. We find that GW calculations are rather sensitive to the functional used in the density functional theory calculations and that self-consistency on the eigenvalues is necessary to achieve quantitative agreement with experiments. Our most accurate band gap at room temperature is in the range of 3.1–3.2 eV and includes a phonon-induced gap renormalization of 0.2 eV. By computing the phonon-induced mass enhancement, we find that the electron carriers are in the weak polaronic coupling regime, while hole carriers are in the intermediate coupling regime as a result of the localized and directional nature of the Ag eg 4d states at the valence band top.

Construction of direct Z-scheme photocatalyst by the interfacial interaction of WO3 and SiC to enhance the redox activity of electrons and holes

The direct Z-scheme heterojunction structure benefits separation and migration of photoinduced carriers while maintaining original redox ability of each component. Nowadays, most Z-scheme structures are fabricated by g-C3N4 with other narrow band photocatalysts due to its low conduction band (CB). In this paper, SiC, another kind of photoelectric semiconductor with low CB, was employed to prepare direct Z-scheme photocatalyst with 2D WO3 by simple water oxidation precipitation method. The component and interface band structure of Z-scheme heterojunction WO3/SiC (WS) were verified by XPS, KPFM, Mott-Schottky method. The photodegradation efficiency and rate constant values of WS-1 for degrading RhB enhanced 2.5 and 5.3 times respectively compared with pristine WO3. Radical capture experiments and ESR tests affirmed that WS-1 photocatalyst produced •OH and •O2－active species, which further confirmed the photogenerated carriers were transmitted through the Z-scheme mode in principle. Band structure investigation showed that the direct Z-scheme structure assembled by WO3 with high valence band (VB) and SiC with low CB could maintain the high photocatalytic activity of active species. Therefore, this study offers a feasible method for construction of a novel and efficient direct Z-scheme photocatalyst.

Bi spheres decorated g-C3N4/BiOI Z-scheme heterojunction with SPR effect for efficient photocatalytic removal elemental mercury

This work reports the obviously promoted photocatalytic performance and superior cyclic durability of 3D fluffy and hierarchical g-C3N4@Bi/BiOI for heavy metal removal applications. The ternary structure is synthesized via moderate solvothermal method with glycol as chelating agent and reductant, and characterized by structural and morphology analysis. Optical and electrochemical properties are investigated to elucidate the mechanism of enhanced photocatalytic activity, with band structure investigations to unravel the pathway of charge carriers transfer in ternary system. Results indicate that Bi could serve as an electron mediator accepting electrons from g-C3N4 via Schottky barrier and boosting electrons to BiOI irreversibly via hot electron injection, which facilitates separation and migration of electron-hole pairs. This mechanism of charge carriers transfer promotes generation of reactive oxygen species involved in photocatalytic process and in turn enhances photocatalytic activity immensely. The photocatalytic tests demonstrate that 10%g-C3N4@Bi/BiOI exhibits attractive Hg0 removal efficiency. Present work provides perspectives of design on multiple-heterostructure photocatalysts with SPR metal, which would contribute to energy conversion and environmental purification.

Controlling the band structures and electromagnetic density of modes in one-dimensional photonic crystals with Lamb wave

We have modeled and investigated band structures and the electromagnetic density of modes in one-dimensional photonic crystal whose refractive index is modulated by the excitation of the fundamental symmetric mode of Lamb wave using the Wigner phase time approach and transfer matrix method. The Rayleigh–Lamb dispersion curves are plotted to select the proper thickness of each sub-layer of considered photonic crystal structure which is comprised of silica (SiO2) and rutile (TiO2) layers as sub-layers of unit cell. The propagation of Lamb wave into the considered structures produced phononic band gaps in gigahertz frequency for acoustic waves. Also, the propagation of Lamb wave into the considered structures causes compression in SiO2 and dilation in TiO2 or vice versa, which can further increase or decrease the refractive index of both layers. Thus, there can be three combinations of change in refractive indices: case 1 (both layers have an unperturbed refractive index), case 2 (maximum change of the refractive index in SiO2 and minimum change in the refractive layer of TiO2), and case 3 (minimum change of the refractive index in SiO2 and maximum change in the refractive layer of TiO2). The band gap structures of considered structures are plotted for both polarized electromagnetic waves.

Electronic band structure of three-dimensional topological insulators with different stoichiometry composition

We report on a comparative theoretical and experimental investigation of the electronic band structure of a family of three-dimensional topological insulators, ${A}^{IV}{\mathrm{Bi}}_{4}{\mathrm{Te}}_{7\ensuremath{-}x}{\mathrm{Se}}_{x}$ (${A}^{IV}$= Sn, Pb; $x$ = 0, 1). We prove by means of density functional theory calculations and angle-resolved photoemission spectroscopy measurements that partial or total substitution of heavy atoms by lighter isoelectronic ones affects the electronic properties of topological insulators. In particular, we show that the modification of the Dirac cone position relative to the Fermi level and the bulk band gap size can be controlled by varying the stoichiometry of the compound. We also demonstrate that the investigated systems are inert to oxygen exposure.

Hybridization vs decoupling: influence of an h-BN interlayer on the physical properties of a lander-type molecule on Ni(111).

2D materials such as hexagonal boron nitride (h-BN) are widely used to decouple organic molecules from metal substrates. Nevertheless, there are also indications in the literature for a significant hybridization, which results in a perturbation of the intrinsic molecular properties. In this work we study the electronic and optical properties as well as the lateral structure of tetraphenyldibenzoperiflanthene (DBP) on Ni(111) with and without an atomically thin h-BN interlayer to investigate its possible decoupling effect. To this end, we use in situ differential reflectance spectroscopy as an established method to distinguish between hybridized and decoupled molecules. By inserting an h-BN interlayer we fabricate a buried interface and show that the DBP molecules are well decoupled from the Ni(111) surface. Furthermore, a highly ordered DBP monolayer is obtained on h-BN/Ni(111) by depositing the molecules at a substrate temperature of 170 °C. The structural results are obtained by quantitative low-energy electron diffraction and low-temperature scanning tunneling microscopy. Finally, the investigation of the valence band structure by ultraviolet photoelectron spectroscopy shows that the low work function of h-BN/Ni(111) further decreases after the DBP deposition. For this reason, the h-BN-passivated Ni(111) surface may serve as potential n-type contact for future molecular electronic devices.

Effect of exchange and dipolar interlayer interactions on the magnonic band structure of dense Fe/Cu/Py nanowires with symmetric and asymmetric layer widths

We present a systematic experimental and theoretical investigation of the magnonic band structure in dense arrays of both asymmetric and symmetric cross-section trilayered Fe(10 nm)/Cu( $t$)/Py(10 nm) nanowires (NWs). The Cu spacer thickness ($t$) is varied in the range between 0 and 10 nm. The frequency dispersion of collective spin-wave excitations in the above trilayered NW arrays has been studied by the Brillouin light-scattering technique while sweeping the wave vector perpendicularly to the nanowire length over four Brillouin zones of the reciprocal space. The experimental results have been quantitatively reproduced by an original numerical model that includes a two-dimensional Green's function description of the dipole field of the dynamic magnetization and exchange coupling between the layers. We found that, depending on $t$, the Py and Fe magnetic layers within the same nanowire are coupled by either the interlayer exchange or dipolar interactions. This has an impact on both the magnetization reversal and the collective dynamical properties of the artificial crystal. In particular, it is possible to stabilize a magnetization configuration where the layer magnetization vectors point in the same or in the opposite direction over a field range that varies with the Cu thickness. In addition, several modes are detected whose propagation properties (i.e., stationary or dispersive) through the array depend on static magnetization configuration as well as on the relative phase (in-phase or out-of-phase) of dynamic magnetizations between the two layers within the same nanowire.

Structural, magneto-electronic and thermophysical properties of the new d0 quaternary heusler compounds KSrCZ (Z =P, As, Sb)

Investigation of band structure and thermo-physical response of the d0 new quaternary Heusler compounds KSrCZ (Z = P, As, Sb) within the frame work of density functional theory with full potential linearized augmented plane wave method has been analyzed. Results showed that type-Y3 is the most favorable atomic arrangement. All the compounds are found to be half-metallic ferromagnetic materials with an integer magnetic moment of 2.00 μB and a half-metallic gap EHM of 0.292, 0.234, and 0.351 eV, respectively. The half-metallicity of KSrCZ (Z = P, As, Sb) compounds can be kept in a quite large hydrostatic strain. Thermoelectric properties of the KSrCZ (Z = P, As, Sb) materials are additionally computed over an extensive variety of temperature and it is discovered that all compounds demonstrates higher figure of merit. The properties of half-metallicity and higher Seebeck coefficient makes these materials a promising candidates for thermoelectric and spintronic device applications.

Fabrication of nano-CuO-loaded PVA composite films with enhanced optomechanical properties

Herein, flexible, strong and optically tunable nano-CuO-loaded poly(vinyl alcohol) (PVA) composite films were fabricated by means of solution casting approach. The nanocomposite films were characterized using advanced analytical techniques. A small loading with CuO nanofiller resulted in prominent modifications of structural and optomechanical attributes of PVA-based nanocomposites. The comprehensive band structure and mechanical investigations of the prepared samples have been reported. The loading with just 0.5 wt% CuO nanofiller in PVA resulted in significant changes in direct band gap (5.29 to 3.16 eV), indirect band gap (4.91 to 2.88 eV), Urbach energy (0.22 to 1.12 eV), dispersion energy (1.45 to 1.92 eV), oscillator energy (5.74 to 3.25 eV), tensile strength (25.6 to 42.2 MPa), Young’s modulus (144 to 215 MPa), elongation at break (152.4 to 206.1%) and the flexural strength (4.1 to 32.2 MPa). The optical band gap calculated using Tauc’s relation, Wemple and DiDomenico single oscillator model and dielectric loss approach resulted in nearby values. Various theoretical models were employed to validate the experimentally determined Young’s moduli values. A possible application of such nanocomposite films might be in optoelectronic devices.

Band structure and transport property of graphene/h-BN heterostructure under local potentials

We investigate band structure and transport property of lattice-matched graphene/hexagonal boron nitride (h-BN) heterostructure using the tight-binding approach. It shows that local potentials can significantly modify the band structure and the transport property. A method to individually manipulate the edge states by local potentials is proposed, including shifts and other deformations of edge bands. The two-terminal conductance of each layer is quantized but the interlayer conductance is non-quantized due to band mixing. In addition, we explore the Landau level spectrum in graphene/h-BN nanoribbons under both magnetic field and local potentials. The plateaus-like behavior of the interlayer conductance is observed.