Changes In Electronic Band Structure Research Articles

Electronic band structure change with structural transition of buckled Au2X monolayers induced by strain.

This study investigates the strain-induced structural transitions of η ↔ θ and the changes in electronic band structures of Au2X (X = S, Se, Te, Si, Ge) and Au4SSe. We focus on Au2S monolayers, which can form multiple meta-stable monolayers theoretically, including η-Au2S, a buckled penta-monolayer composed of a square Au lattice and S adatoms. The θ-Au2S is regarded as a distorted structure of η-Au2S. Based on density functional theory (DFT) calculations using a generalized gradient approximation, the conduction and the valence bands of θ-Au2S intersect at the Γ point, leading to linear dispersion, whereas η-Au2S has a band gap of 1.02 eV. The conduction band minimum depends on the specific Au-Au bond distance, while the valence band maximum depends on both Au-S and Au-Au interactions. The band gap undergoes significant changes during the η ↔ θ phase transition of Au2S induced by applying tensile or compressive in-plane biaxial strain to the lattice. Moreover, substituting S atoms with other elements alters the electronic band structures, resulting in a variety of physical properties without disrupting the fundamental Au lattice network. Therefore, the family of Au2X monolayers holds potential as materials for atomic scale network devices.

Electronic Band Structure Changes across the Antiferromagnetic Phase Transition of Exfoliated MnPS3 Flakes Probed by μ-ARPES.

Exfoliated magnetic 2D materials enable versatile tuning of magnetization, e.g., by gating or providing proximity-induced exchange interaction. However, their electronic band structure after exfoliation has not been probed, presumably due to their photochemical sensitivity. Here, we provide micrometer-scale angle-resolved photoelectron spectroscopy of the exfoliated intralayer antiferromagnet MnPS3 above and below the Néel temperature down to one monolayer. Favorable comparison with density functional theory calculations enables identifying the orbital character of the observed bands. Consistently, we find pronounced changes across the Néel temperature for bands consisting of Mn 3d and 3p levels of adjacent S atoms. The deduced orbital mixture indicates that the superexchange is relevant for the magnetic interaction. There are only minor changes between monolayer and thicker films, demonstrating the predominant 2D character of MnPS3. The novel access is transferable to other MPX3 materials (M: transition metal, P: phosphorus, X: chalcogenide), providing several antiferromagnetic arrangements.

Reversible Thermal Conductivity Modulation of Non-equilibrium (Sn1–xPbx)S by 2D–3D Structural Phase Transition above Room Temperature

We demonstrated a reversible two-dimensional (2D) to three-dimensional (3D) crystal-structure transition and associated thermal conductivity switching (κ) well above room temperature (RT) in (Sn1–xPbx)S bulk polycrystals, solid solutions of 2D-layered SnS and 3D-cubic PbS. The direct phase boundary between the 2D and 3D structures does not exist in (Sn1–xPbx)S under the thermal equilibrium condition due to the solubility limit x = 0.5 in the 2D phase and x = 0.9 in the 3D phase. While, by applying a non-equilibrium synthesis process that combined a high-temperature solid-state reaction at 973 K and subsequent rapid thermal quenching to RT, the Sn solubility limit in the 3D (Sn1–xPbx)S was not changed but the Pb solubility limit was considerably expanded up to x = 0.9 in 2D (Sn1–xPbx)S. As a result, the direct 2D–3D phase boundary was formed in the Pb-rich (Sn0.2Pb0.8)S, which showed the reversible 2D–3D (3D–2D) structural phase transition at ∼573 K (473 K). This transition temperature is much higher than that previously reported for (Sn0.5Pb0.5)Se, which requires temperatures lower than RT for the reversible structure transition. The electronic band structure change from a 2D semiconducting state to a 3D metallic state caused a 20-times increase in electron κ (κele) during the structural phase transition in the (Sn0.2Pb0.8)S. Still, κele negligibly contributes to the total κ. In contrast, the lattice κ (κlat) was largely decreased by the 3D–2D structure transition due to the formation of a 2D-layered structure with strong phonon scattering, resulting in a 1.8-times modulation of the total κ (κ3D phase/κ2D phase) at 463 K. The realization of 2D–3D structural phase transition above RT in (Sn1–xPbx)S would accelerate the development of thermal management materials through a crystal-structure dimensionality switch using non-equilibrium phase boundaries.

Unconventional magnetoresistance and electronic transition in Mn3Ge Weyl semimetal

Weyl semimetals are well known for their anomalous transport effects caused by a large fictitious magnetic field generated by the non-zero Berry curvature. We performed the analysis of the electrical transport measurements of the magnetic Weyl semimetal Mn$_{3}$Ge in the a-b and a-c plane. We have observed negative longitudinal magneto-resistance (LMR) at a low magnetic field ($B<1.5$ T) along all the axes. The high field LMR shows different behavior along x and z axes. A similar trend has been observed in the case of planar Hall effect (PHE) measurements as well. The nature of high field LMR along the x axis changes near 200 K. Dominant carrier concentration type, and metallic to semimetallic transition also occur near 200 K. These observations suggest two main conclusions: (i) The high field LMR in Mn$_3$Ge is driven by the metallic - semimetallic nature of the compound. (ii) Mn$_3$Ge compound goes through an electronic band topological transition near 200 K. Single crystal neutron diffraction does not show any change in the magnetic structure below 300 K. However, the in-plane lattice parameter (a) shows a maximum near 230 K. Therefore, it is possible that the change in electronic band structure near 200 K is driven by the a lattice parameter of the compound.

The effects of defects on the defect formation energy, electronic band structure, and electron mobility in 4H–SiC

With the first-principle method, we studied the effects of the type and position of defects on the defect formation energy, electronic band structure, and electron mobility of the 4-layer hexagonal system silicon carbon (4H–SiC). The vacancy defect formation energy is smaller than the interstitial defect formation energy. The C vacancy defect formation energy is the smallest, while the Si interstitial defect formation energy is the largest. The defect formation energy is little affected by the defect position. The electronic band structure shows semi-metallic property due to the vacancy defect and the interstitial defect, and it shows a smaller bandgap due to the antisite defect. The electronic band structure changes little while the defect position changes. The electron mobility is reduced in varying degrees according to different defect types. The electron mobility changes little while the defect position changes.

Metallic vs. semiconducting properties of quasi-one-dimensional tantalum selenide van der Waals nanoribbons.

We conducted a tip-enhanced Raman scattering spectroscopy (TERS) and photoluminescence (PL) study of quasi-1D TaSe3-δ nanoribbons exfoliated onto gold substrates. At a selenium deficiency of δ ∼ 0.25 (Se/Ta = 2.75), the nanoribbons exhibit a strong, broad PL peak centered around ∼920 nm (1.35 eV), suggesting their semiconducting behavior. Such nanoribbons revealed a strong TERS response under 785 nm (1.58 eV) laser excitation, allowing for their nanoscale spectroscopic imaging. Nanoribbons with a smaller selenium deficiency (Se/Ta = 2.85, δ ∼ 0.15) did not show any PL or TERS response. The confocal Raman spectra of these samples agree with the previously-reported spectra of metallic TaSe3. The differences in the optical response of the nanoribbons examined in this study suggest that even small variations in Se content can induce changes in electronic band structure, causing samples to exhibit either metallic or semiconducting character. The temperature-dependent electrical measurements of devices fabricated with both types of materials corroborate these observations. The density-functional-theory calculations revealed that substitution of an oxygen atom in a Se vacancy can result in band gap opening and thus enable the transition from a metal to a semiconductor. However, the predicted band gap is substantially smaller than that derived from the PL data. These results indicate that the properties of van der Waals materials can vary significantly depending on stoichiometry, defect types and concentration, and possibly environmental and substrate effects. In view of this finding, local probing of nanoribbon properties with TERS becomes essential to understanding such low-dimensional systems.

Structural and Physical Properties of Ultrathin Bismuth Films

Bismuth films are interesting objects for research because of the many effects occurring when the film thickness is less than 70 nm. The electronic band structure changes significantly depending on the film thickness. Consequently, by changing the film thickness, it is possible to control the physical properties of the material. The purpose of this paper is to give a brief description of the basic structural and physical properties of bismuth films. The structural properties, namely, morphology, roughness, nanoparticle size, and texture, are discussed first, followed by a description of the transport properties and the band structure. The transport properties are described using the semi-metal–semiconductor transition, which is associated with the quantum size effect. In addition, an important characteristic is a two-channel model, which allows describing the change in resistivity with temperature. The band structure of bismuth films is the most interesting part due to the anomalous effects for which there is still no unambiguous explanation. These effects include anomalous spin polarization, nontrivial topology, and zone changes near the edge of the film.

Spectroscopic and color chromaticity analysis of rhodamine 6G dye-doped PVA polymer composites for color tuning applications

Herein we report the successful fabrication of highly flexible, reversibly stretchable and transparent rhodamine 6G/poly(vinyl alcohol) (Rh6G/PVA) polymer composite (PC) films using a user-friendly solution casting method by adding different amounts, viz. 0, 0.5, 1, 1.5 and 2 wt%, of Rh6G dye. The FTIR studies reveal that an interaction between PVA matrix and added Rh6G dye and room-temperature photoluminescence (RTPL) emission intensity is decreased with the increase in the Tg is observed from DSC thermograms. The developed films show appreciable optical and RTPL properties as established by a UV–visible and fluorescence techniques such that the electronic spectral studies substantiate the changes in electronic band structure of PC films, leading to appreciable changes in the optoelectronic properties. These prepared films with excellent optoelectronic properties alongside appreciable flexibilities and stretchabilities aid their applications to organic light emitting diodes (OLED) with enhanced RTPL studies.

Tensile strain for band engineering of SrTiO3 for increasing photocatalytic activity to water splitting

SrTiO3 is a well-known highly active photocatalyst with high energy conversion efficiency. In this study, we investigated the formation of oxygen vacancy by using the chemo-mechanical effect that was introduced by the dispersion of metal particles into grain and photocatalytic activity to water splitting. Au dispersion on SrTiO3 followed by sintering treatment was studied for introduction of chemo-mechanical strain because of a different thermal expansion coefficient; the introduced chemo-mechanical strain generated oxygen vacancy in SrTiO3. Thus, induced chemo-mechanical strain shows change in electronic band structure resulting in increasing lowest unoccupied molecular orbital (LUMO) level with increasing Au content. Since photoluminescence was significantly decreased by sintering after Au dispersion, the introduced strain effects may work for increasing a charge separation efficiency and adsorption site in water splitting. Therefore, the photocatalytic activity was much increased by sintering treatment after Au dispersion on SrTiO3.

Properties of monolayer black phosphorus affected by uniaxial strain

Abstract First-principles methods including density functional theory and hybrid functional is used to survey the properties of monolayer black phosphorus affected by uniaxial strain . Based on structural properties, band structure, and the density of states, the strained monolayer black phosphorus is analyzed detailedly. We discuss the uniaxial strain from −10% compressive strain to 10% tensile strain and find it has a significant impact on black phosphorus properties. The changes of structural properties indicate the close dependence of bond angle and bond length. The band structure of monolayer black phosphorus could be modulated by the applied strain. Especially, direct-indirect-direct-indirect transition is found while the compressive strain applied on armchair direction. Furthermore, the density of states of strained monolayer BP has been analyzed. The reason that the electronic band structure change induced by strain is explained.

Double Resonance Raman Scattering in Single-Layer MoSe2 under Moderate Pressure**Supported by the Strategic Priority Research Program (B) of the Chinese Academy of Sciences under Grant Nos XDB30000000, XDB28000000 and XDB07030100, the National Natural Science Foundation of China under Grant Nos 11774395, 11727902 and 91753136, and the Beijing Natural Science Foundation under Grant No 4181003.

Pressure-dependent properties in layered transition-dichalcogenides are important for our understanding of their basic structures and applications. We investigate the electronic structure in MoSe2 monolayer under external pressure up to 5.73 GPa by Raman spectroscopy and photoluminescence (PL) spectroscopy. The double resonance out-of-plane acoustic mode (2ZA) phonon is observed in Raman spectroscopy near 250 cm−1, which presents pronounced intensity and pressure dependence. Significant variation in 2ZA peak intensity under different pressures reflects the change in electronic band structure as pressure varies, which is consistent with the blue shift in PL spectroscopy. The high sensitivity in both Raman and PL spectroscopy under moderate pressure in such a two-dimensional material may have many advantages for optoelectronic applications.

Plasmons in Li under compression

We report the high-pressure behavior of plasmon in polycrystalline Li up to 15 GPa at room temperature studied by inelastic x-ray scattering and ab initio calculation. The plasmon energy () increases with decreasing atomic volume (), and the slope exhibits a discontinuity at bcc → fcc structural phase boundary reflecting the electronic band structure change. The plasmon peak width () versus momentum transfer (q) curve of bcc-Li below 6.5 GPa keeps similar parabola-like shape. Above 8.4 GPa, where Li is in fcc, it changes from that of bcc-Li and has a convex shape.

Layered ferrielectric crystals CuInP2S(Se)6: a study from the first principles

ABSTRACTLayered CuInPS(Se) crystals in ferrielectric and paraelectric phases were investigated for the first time via the plane-wave pseudo-potential methodology within density functional theory (DFT). Was proved that structural and electronic properties of the van der Waals layered crystals should be calculated applying dispersion correction to the DFT formalism. The partial and projected density of states were used to evaluate the electronic band structure changes under phase transition. Instability of the CuInPS(Se) crystals was successfully explained by the second-order Jahn-Teller effect.

On topology-tuned thermoelectric properties of noble metal atomic wires

We have carried out a systematic first principles study of thermal and thermoelectric transport properties of different noble metal (viz. Ag, Cu, Au, and Pt) atomic wires modelled in different topologies by using the Landauer-Büttiker approach in the linear response regime. Both electronic and phononic contributions are considered, with the respective transmission functions determined using the non-equilibrium Green's function approach, based upon the density functional theory and the general utility lattice program, respectively. To explore the role of topology, we have considered the linear, ladder, zigzag (ZZ), and double zigzag (DZZ) topologies for each of the four wires. Numerical results are presented for the electronic κel and phononic κph thermal conductance, thermopower S, and total figure of merit ZT. In addition, the validity of the Wiedemann-Franz law in these atomic wires has been studied. Quite generally, we find a significant dependence of these properties on the wire topology. Under ambient conditions, the valence electrons are found to be the dominant carriers of heat energy in all the wires, except for the Ag, Cu, and Au wires in the DZZ topology due to their semi-metallic nature. In relative terms, the phononic contribution is minimum (∼ 4%) for the Pt wire in linear topology, while maximum (∼ 15%) for the Cu wire in ZZ topology. However, the phonon contribution increases steadily with decreasing temperature to become as large as about 30% at T = 100 K (for Cu wire in ZZ topology). In overall, the Pt wire is found to have the maximum total thermal conductance in all topologies. S and ZT, apart from depending on topology and type of metal wire, exhibit an oscillatory behaviour as a function of chemical potential μ of electrodes. S has been found to change even its sign at characteristic μ, with a maximum value of ∼±1.5 mV/K for the Au wire in DZZ topology at μ = ∓0.15 eV. Also, ZT is found to be maximum (∼ 3) for this wire, with the presence of two pronounced peaks at μ ∼±0.6 eV, apparently located at somewhat higher μ as compared to S. An analysis of our results reveals that the change in thermoelectric properties with topology stems primarily from a change in electronic band structure and phonon spectra.

Yellow–green luminescent poly(vinyl alcohol) nanocomposite thick films as effective spectral manipulators

ABSTRACTHighly flexible and visibly transparent yellow–green fluorescent poly(vinyl alcohol) (PVA) based nanocomposite (NC) films with rubidium zincate (Rb2ZnO2) nano impregnates were developed through aqueous solution casting. The introduced rubidium zincate nanofillers impart UV shielding properties to PVA host, through UV to visible (fluorescent yellow–green) photon cutting. The rubidium zincate nanofiller impregnated PVA systems were also characterized for their structural, morphological, optical, and electrical behaviors by various analytical tools. The Fourier Transform Infrared (FTIR) and X‐ray diffraction (XRD) studies shed light on the changes in gross structural properties of PVA. WhileScanning Electron Microscopic (SEM) profiles support uniform filler dispersions, which aid high visible transparencies. Further, the UV–visible spectral measurements depict the changes in electronic band structure that validates novel fluorescent emission properties, established through fluorescent emission studies. The NC films show excellent visible transparency (72–84%), near hydrophobic surface wettabilities (ϴ = 82°–93°) and enhanced charge conduction, in addition to high flexibilities and UV photons induced fluorescent emission ( λEmi=585 nm). The yellow–green luminescent, Rb2ZnO2 nanofillers reinforced PVA NC films are effective spectral manipulators, which may be used as competent packaging materials for photonic display devices and to extend the short wavelength limit of solar cells. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46854.

Specific thermoelectric features of novel CaPd3B4O12 (B = Ti, V) perovskites following DFT calculations

Perovskite materials demonstrate excellent elastic and thermoelectric properties. We report for the first time theoretical investigation of CaPd3B4O12 (B = Ti, V) perovskite and perform the electronic calculations using full potential linear augmented plane wave (FP-LAPW) method within a framework of DFT approach. The transport properties were calculated using semi-local Boltzmann transport theory. As the Pd2+ occupied at A′-sites in perovskite their orbitals are very close to Fermi level and cause drastic changes in electronic band structure and transport properties of CaPd3B4O12 (B = Ti, V) perovskite. Both materials exhibit good elastic properties. The thermoelectric figure of merit for CaPd3Ti4O12 is (ZT = 0.8) so this material is good for cooling devices and thermoelectric applications. Our investigated results are in good agreement with experimental reported results.

First principles investigation on the electronic structure and magnetism of dilute Fe impurity in [formula omitted][formula omitted] alloys

Employing the all electron augmented plane wave + local orbital (APW + lo) method based on the density function theory (DFT), we have performed ab initio calculations of electronic structure and magnetic properties of dilute Fe impurity in PdHx alloys (0⩽x⩽1.0). Here we present our results for the H induced changes in the density of state (DOS), local magnetic moment and the macroscopic magnetic properties. The calculations performed for PdHx without the Fe impurity reveal H concentration dependent lattice expansion and Pd-H bonding which results in large change in electronic band structure and the DOS at the Fermi energy. The nonmagnetic DOS calculated for Fe doped PdHx is analyzed within Stoner model and the condition for local moment formation is found to hold for all concentrations of H. With an increase of the H content in the Pd matrix, the local moment of Fe declines from 3.43μB at x = 0 to 2.72μB at x = 1.0. More significantly, the giant magnetic moment, associated with the Fe impurity in Pd, progressively decreases with H concentration as the positive spin polarization of Pd-4d band electrons diminish and crossover to negative sign for x > 0.75. We show that H induced changes in s-d and d-d hybridization plays a crucial role on the magnetic properties of Fe doped PdHx alloys. In particular, we suggest that the rapid decline of the giant moment in PdHx alloys results from the weakening of ferromagnetic d-d exchange interaction between Fe-3d and Pd-d band electrons caused by changes in density of states arising from H induced filling up of the Pd-4d band.

Pressure-Induced Structural Evolution and Optical Properties of Metal-Halide Perovskite CsPbCl3

Metal-halide perovskites have emerged as the most promising semiconductor materials for advanced photovoltaic and optoelectronic applications. Herein, we comprehensively investigate the optical response and structural evolution of metal-halide perovskite CsPbCl3 (ABX3) upon compression. Band gap realized a pronounced narrowing under mild pressure followed by a sharp increase, which could be ascribed to Pb–Cl bond contraction and inorganic framework distortion, respectively. The transformation of the crystal structure is confirmed and analyzed through in situ high-pressure X-ray diffraction and Raman experiments, consistent with the evolution of optical properties. Combining with the first-principles calculations, we understand the electronic band structure changes and phase transition mechanism, which are ascribed to severe PbCl6 octahedral titling and twisting. Our results demonstrate that the high-pressure technique can be used as a practical tool to modify the optical properties of metal-halide perovsk...

Substitute ftalosiyanin yüzeyinde amonyak adsorpsiyon kinetiği

The sensing performance of 2(3),9(10),16(17),23(24)- tetrakis-[4-nitro-2-(octyloxy)phenoxy]phthalocyaninato zinc(II) thin film towards ammonia and the influence of relative humidity on it were studied. The results show that the exposure to ammonia gas leads to an increase in sensor current, which is unexpected because of the strong electron donating character of ammonia and p-type semiconductivity of phthalocyanine. This unexpected behavior is related to the change in electronic band structure of the Pc molecule. It was observed that relative humidity enhances the ammonia adsorption. Several adsorption kinetics model were employed to represent the ammonia adsorption on the sensor surface. Analysis of the experimental data indicated that a simple adsorption model can be used to represent our adsorption system.

Effect of Ce0.5Zr0.5O2 nano fillers on structural and optical behaviors of poly(vinyl alcohol)

The effect of Cerium doped zirconium oxide (Ce4+ doped zirconia) nano fillers on electronic band structure and hence the optical absorption and emission properties of poly(vinyl alcohol) (PVA) have been established. The solution casted PVA/Ce0.5Zr0.5O2 nanocomposite films were subjected to electronic (UV–Visible), fourier transform infrared (FTIR), scanning electron microscopic (SEM) and fluorescent spectral studies. The FTIR studies reveal a positive interaction between the Ce0.5Zr0.5O2 nano fillers and PVA matrix through hydroxyl groups. The electronic spectral studies reveal the changes in electronic band structure of composite with nano filler doping. The SEM images reveal homogeneous filler dispersions, while fluorescent spectral studies establish the dopant dependent photonic emission. The refractive index dispersion of developed films was established by single oscillator model developed by Wemple–Didomenico that shows a monotonic dependence on Ce0.5Zr0.5O2 loadings.