Decrease Of Effective Mass Research Articles

Study on Pressure-Induced Band Gap Modulation and Physical Properties of Direct Band Gap Ca3NX3 (X = Cl, Br) for Optoelectronic and Thermoelectric Applications

Utilizing density functional theory, we have comprehensively analyzed the structural, electronic, mechanical, transport, and thermodynamic properties of Ca3NCl3 and Ca3NBr3 photovoltaic materials. Furthermore, we investigated the impact of pressures ranging from 0 to 80 GPa and observed that the applied pressure decreases the atomic distances, resulting in significant changes in the physical properties of both materials. We employed GGA-PBE and TB-mBJ functionals to calculate the band structures and density of states, confirming their semiconducting nature. At 0 GPa, both compounds initially display a direct bandgap (Γ-Γ) of 1.5 eV for Ca3NCl3 and 1.26 eV for Ca3NBr3 based on GGA-PBE calculations. In contrast, the TB-mBJ calculations show an increase in the bandgap to 2.25 eV for Ca3NCl3 and 1.84 eV for Ca3NBr3, respectively. When a pressure of 80 GPa was applied, the direct bandgap of Ca3NCl3 and Ca3NBr3 decreased to 0.23 eV and 0.12 eV with GGA-PBE, and to 0.92 eV and 0.74 eV with TB-mBJ, respectively. It was also observed that the effective mass decreases under applied pressure, making these materials promising candidates for solar cell applications. The mechanical stability of both perovskites was maintained up to 80 GPa. Poisson's ratio and Pugh's ratio indicate that both compounds are brittle at 0 GPa but tend to become more ductile under applied stress. The optical properties reveal that pressure alters the dielectric function, absorption spectra, and optical conductivity, causing a red shift from higher to lower photon energies. Several thermodynamic properties, including heat capacity, Debye temperature, thermal expansion coefficient, and entropy, have been calculated, with their dependencies on temperature and pressure illustrated. Finally, the thermoelectric response of these materials is evaluated by examining how various transport parameters - such as the Seebeck coefficient, electrical and thermal conductivity, and power factor - depend on the temperature.

Tunable electronic and optical properties of h-BP/MoS2 van der Waals heterostructures toward optoelectronic applications

The integration of two-dimensional materials (2D) into van der Waals heterostructures (vdWHs) is widely recognized as an effective strategy for the development of multifunctional devices. This study aims to construct stable ultrathin 2D vdWHs using h-BP and MoS2 monolayers, and analyze the impact of strain on the electronic properties of h-BP/MoS2 vdWHs, with a specific focus on the mechanism of biaxial strain regulation. The h-BP/MoS2 vdWHs demonstrate the type II band alignment and possess direct bandgap, which can be effectively controlled through appropriate application of biaxial strain. The power conversion efficiency (PCE) can reach 21.85 % achieved suggests that the material under consideration has potential as a suitable candidate for 2D excitonic solar cells. The observed decrease in effective mass for both electrons and holes implies an enhanced carrier mobility, thereby facilitating the utilization of h-BP/MoS2 vdWHs in the development of high-efficiency excitonic solar cells. These findings indicate that the h-BP/MoS2 vdWHs possess the capability to serve as a reconfigurable system in the construction of highly efficient solar cells.

Structural, electronic and mechanical properties of Mo2GeC under strain engineering

A new type of nitride and carbide with excellent properties of metals and ceramics, known as MAX phase materials, is a new guide for structural modification in various engineering applications and novel technologies. The Mo 2 GeC MAX phase is regarded as a promising material for applications such as new coatings, high pressure resistant materials, and fission and fusion program reactors. In this work, the structural, electronic, and mechanical properties of Mo 2 GeC under strain engineering are predicted by first-principles calculations. When Mo 2 GeC is tensioned or compressed by various strain conditions, effective mass decreases and increases in electrical conductivity are observed. Under uniform biaxial strain, the degeneracy at the top of the valence band is significantly increased. Mo 2 GeC possesses small Young’s modulus, shear modulus, and large bulk modulus, indicating that it can be desired for large strain engineering applications. However, the layered structure of Mo 2 GeC leads to low shear strength and hardness, which makes it less resistant to shear deformation. We also investigate the structural deformation and mechanical properties of Mo 2 GeC in different directions of uniaxial tensile strain up to the fracture limit. For tension along the a -direction, the strain ε value is 45% when Mo 2 GeC reaches the ideal tensile strength. The different bond strengths of Mo 2 GeC explain its anisotropy and show more significant differences under different strain conditions. Our results provide a strategy for tuning the various properties of Mo 2 GeC. Mo 2 GeC is subjected to uniaxial strain in different directions

Additive effect of lanthanide compounds into perovskite layer on photovoltaic properties and electronic structures

Fabrication and characterization of lanthanide compound doped CH3NH3PbI3 (MAPbI3) perovskite solar cells have been performed for improving the photovoltaic properties with long-term stabilities of conversion efficiency. The purpose of this research is to investigate additive effect of formamidinium iodine (FAI) and lanthanide compound into the perovskite layer for improving carrier diffusion and mobility in the perovskite layer. Incorporation of FAI and europium chloride (EuCl2) into the perovskite crystals maintained the stability of conversion efficiency for 30 days. The photovoltaic properties were based on the crystal growth and orientation with suppression of decomposition, thermodynamic stabilities, and the narrow band dispersion with decrease of effective mass. In contrast case, addition of samarium or terbium compound reduced the stabilities, and suppressed the carrier mobility owing to the flat band dispersion of localized d orbital of samarium atom or 4 f orbital of terbium atom near valence band state. The annealing treatment improved the crystal orientation related to the carrier diffusion with inhibiting recombination, improving short circuit current density and external quantum efficiency. The perovskite crystal doped with EuCl2 and FAI have high advantage to apply for practical use of the photovoltaic devices with long-term stability.

Laser-driven detonation wave in a dielectric film prior to ablation

Illumination of hafnium dioxide (HfO2) thin films with a nanosecond pulsed laser reaching peak intensities of 180 GW / cm2 has been shown to trigger a laterally expanding plasma wave prior to ablation that reaches speeds of up to 100 km / s. The phenomenon has recently been described as a defect-initiated laser-driven detonation (LDD) wave propagating in the electron–hole subspace, but the electron/hole effective masses that would be required for quantitative agreement were found to be at least 100 times heavier than band structure calculations predicted for HfO2. The results are re-examined in the context of a more general description of LDD that accounts for a change in the electron effective mass between the pre- and post-detonation states. Reasonable agreement between theory and experiment is found under the assumption that the electron’s effective mass decreases by a factor of 20 and approaches the free electron mass in the final state.

The mechanism of the effect of V doping on the thermoelectric properties of ZnO ceramics

In this paper, the mechanism of the effect of V doping on the thermoelectric properties of ZnO is studied by experimental test and first principles calculation. The results show that the carrier concentration and conductivity increase significantly after V doping, which is mainly due to the donor effect caused by V substituting for Zn site, and the band gap becomes narrower. Due to the influence of 3d orbit of V ion, the density of states near Fermi level increases. The effective mass decreases and the Seebeck coefficient decreases with V doping. The thermal conductivity decreases with the decrease of lattice thermal conductivity, which is mainly due to the decrease of Young's modulus due to the change of strain and stress field caused by V doping. While the power factor increases, the thermal conductivity decreases, and the ZT value increases significantly in the whole temperature range, with the highest value reaching 0.302 ​at 873 ​K for Zn0.985V0.015O, which is 3.02 times of undoped ZnO.

Effect of band structure adjustment based of Cu site on the thermoelectric properties of BiCuSeO

The effect of Ti doped at Cu site on the thermoelectric properties of BiCuSeO was studied by experimental method and first principles calculation. The samples were prepared by high-temperature solid-state reaction, mechanical alloying and spark plasma sintering. The results show that Ti doping can cause the lattice contraction and decrease the lattice constant. Ti doping can increase the bandgap and lengthen the Cu/Ti–Se bond, resulting in the decrease of carrier concentration. Ti doping can reduce the effective mass and the Bi–Se bond length, correspondingly improve the carrier mobility. Ti doping can decrease the density of states of Cu-3d and Se-4p orbitals at the top of valence band, but Ti-4p orbitals can obviously increase the density of states at the top of valence band and finally increase the electrical conductivity in the whole temperature range with the highest value of 192 Scm−1 at 873 K. With the decrease of effective mass, Ti doping would reduce the Seebeck coefficient, but the gain effect caused by the increase of electrical conductivity is more than the benefit reduction effect caused by the decrease of Seebeck coefficient, and the power factor shows an upward trend. Ti doping can reduce Young's modulus, lead to the increase of defect scattering and strain field for the heat-carrying phonons, correspondingly reduce the lattice thermal conductivity and total thermal conductivity. It is greatly increased for the ZT values in the middle- and high-temperature range, with the highest value of 1.04 at 873 K obtained by 4% Ti-doped sample.

Effect of Hydrostatic Pressure and Temperature on Quantum Confinement of AlGaN/GaN HEMTs

In this paper, an analytical model for quantum confinement electron density in two-dimensional quantum well, has been investigated. In order to obtain the exact AlGaN/GaN HEMTs parameters such as electron density, the wave function, band gap, polarization charge, effective mass and dielectric constant, the hydrostatic pressure and temperature effects are taken into account. It has been found that the electron density decreases with increasing temperature and increases with increasing hydrostatic pressure. With increasing hydrostatic pressure, the effective mass decreases and the quantum confinement electrons are increased in the quantum well. Also with increasing hydrostatic pressure, the height of wave functions increase and decreases electron wave functions to penetrate the quantum barrier but increasing the temperature behaves the opposite of increasing the pressure. However, with increasing temperature, the effective mass is increased and the quantum confinement electrons are reduced. The calculated results for electron density are in good agreement with existing experimental data.

Thermoelectric Properties and Low-Energy Carrier Filtering by Mo Microparticle Dispersion in an n-Type (CuI)0.003Bi2(Te,Se)3 Bulk Matrix.

We investigate the thermoelectric properties of (CuI)0.003Bi2Te2.7Se0.3/Mo (Mo: 0.0, 0.9, 1.3, 1.8, 3.1, and 4.3 vol %) composites, which were synthesized by extrinsic phase mixing with hot press sintering. From X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX) measurements, we confirm that micro-sized Mo particles are dispersed homogeneously in the (CuI)0.003Bi2Te2.7Se0.3 matrix without doping. While the electrical resistivity of Mo-dispersed (CuI)0.003Bi2Te2.7Se0.3 composites is not changed significantly, the Seebeck coefficient is significantly increased. Because the work function (5.3 eV) of the (CuI)0.003Bi2Te2.7Se0.3 compounds, measured by ultraviolet photoelectron spectroscopy (UPS), is larger than that of Mo particles (4.95 eV), we expect the potential barrier near the interfaces between the BTS matrix and Mo particles. The band bending effect and potential barrier can give rise to the low-energy carrier filtering. For a low concentration dispersion of Mo particles (<2 vol %), a decrease of Hall carrier concentration, an increase of Hall mobility, a decrease of effective mass, and an increase of Seebeck coefficient also support the formation of low-energy carrier filtering. The Mo dispersion does not affect the decrease in the lattice thermal conductivity but enhances the power factor significantly, leading to the high ZT value above 1.0 at room temperature, which is a high level in n-type thermoelectric room-temperature applications.

Enhanced thermoelectric performance of nanostructured manganese telluride via antimony doping

Abstract Manganese telluride is a good thermoelectric material for medium temperature applications. Pristine MnTe is a p-type semiconductor with an average hole concentration of 10 19 c m − 3 . Many dopants like Sodium, Copper and Sulphur were incorporated in the MnTe and their thermoelectric properties were studied. Antimony is an ideal dopant to alter the thermoelectric properties. Compounds belonging to the series M n 1 − x S b x T e with (x = 0, 0.005, 0.01, 0.015, 0.02) were prepared using a combination of High Energy Ball Milling (HEBM), melt quenching and hot press techniques. Structural, electrical and thermo-electric properties of these compounds were determined. Carrier concentration and mobility were measured by Hall measurement techniques. Charge carrier concentration increases with Sb doping while mobility and effective mass decreases. Seebeck coefficient and electrical conductivity were measured in the temperature range 100 K–730 K, and thermal conductivity was evaluated in the temperature range 100 K–300 K. The power factor and figure of merit (ZT) were also estimated. The highest power factor achieved is at 730 K and is 1768 μ W m − 1 K − 2 for M n 0.98 S b 0.02 T e . The highlight of the present study is the rise in electrical conductivity and reduction in the thermal conductivity with Sb doping.

Strain-induced majority carrier inversion in ferromagnetic epitaxial LaCoO3−δ thin films

Tensile-strained $\mathrm{LaCo}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ thin films are ferromagnetic, in sharp contrast to the zero-spin bulk, although no clear consensus has emerged as to the origin of this phenomenon. While magnetism has been heavily studied, relatively little attention has been paid to electronic transport, due to the insulating nature of the strain-stabilized ferromagnetic state. Here, structure, magnetism, and transport are studied in epitaxial $\mathrm{LaCo}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ films (10--22-nm thick) on various substrates (from 1.4% compressive to 2.5% tensile strain), using synchrotron x-ray diffraction, scanning probe and transmission electron microscopy, magnetometry, polarized neutron reflectometry, resistivity, and Hall effect. High quality, smooth films are obtained, exhibiting superstructures associated with both oxygen vacancy ordering and periodic in-plane ferroelastic domains. Consistent with prior work, ferromagnetism with an approximately 80--85 K Curie temperature is observed under tension; polarized neutron reflectometry confirms a relatively uniform magnetization depth profile, albeit with interfacial dead layer formation. Electrical transport is found to have similar semiconducting nature to bulk, but with reduced resistivity and activation energy. Hall effect measurements, however, reveal a striking inversion of the majority carrier type, from $p$-type in the bulk and under compression to n-type under tension. While thus far overlooked, ferromagnetism in epitaxial $\mathrm{LaCo}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ films is thus directly correlated with $n$-type behavior, providing important insight into the ferromagnetic state in this system. Aided by density functional theory calculations, these results are interpreted in terms of tensile-strain-induced orbital occupation and band structure changes, including a rapid decrease in effective mass at the ${e}_{g}$-derived conduction band minimum, and corresponding increase at the valence band maximum.

Analysis of the influence of Ti doping on the electrical properties of SnO2

The electronic structure, energy band and density of states (DOS) were calculated from the density function theory (DFT) for pure SnO2 and Ti doped SnO2 of various concentrations. The results reveal that: the 3d electronic state of Ti atom has a great contribution to the conduction band at the Fermi level, as a result, the conduction band shifts to the lower energy level, the forbidden bandwidth is smaller and the electrical conductivity is improved than that of pure SnO2. With the decreases of Ti doping concentration, the forbidden bandwidth decreases gradually, in other words, the electrical performance is gradually improved, and when the doping concentration of Ti is 6.25%, the forbidden bandwidth is at the lowest level, that is 0.148 eV. According to the semiconductor theory, the relative electron hole number and the electron hole effective mass of Ti doped system were calculated. The results show that with the decreases of Ti doping concentration, the relative electron hole number increases and the electron hole effective mass decreases. When the doping concentration of Ti is 6.25%, the hole effective mass is 0.242 m0, the relative quantity of hole is 0.5355. At last, the experiment verification reveals that when the doping concentration of Ti is 6.25%, the experimental value of electric conductivity is the largest, that is 58.09%IACS. In summary, when the doping concentration of Ti is 6.25%, the SnO2 material has better electrical properties. This research provides theory and data support for further study on AgSnO2 contact materials.

Phase transition for the system of finite volume in the ϕ4 theory in the Tsallis nonextensive statistics

We studied the effects of nonextensivity on the phase transition for the system of finite volume [Formula: see text] in the [Formula: see text] theory in the Tsallis nonextensive statistics of entropic parameter [Formula: see text] and temperature [Formula: see text], when the deviation from the Boltzmann–Gibbs (BG) statistics, [Formula: see text], is small. We calculated the condensate and the effective mass to the order [Formula: see text] with the normalized [Formula: see text]-expectation value under the free particle approximation with zero bare mass. The following facts were found. The condensate [Formula: see text] divided by [Formula: see text], [Formula: see text], at [Formula: see text] ([Formula: see text] is the value of the condensate at [Formula: see text]) is smaller than that at [Formula: see text] for [Formula: see text] as a function of [Formula: see text] which is the physical temperature [Formula: see text] divided by [Formula: see text]. The physical temperature [Formula: see text] is related to the variation of the Tsallis entropy and the variation of the internal energies, and [Formula: see text] at [Formula: see text] coincides with [Formula: see text]. The effective mass decreases, reaches minimum, and increases after that, as [Formula: see text] increases. The effective mass at [Formula: see text] is lighter than the effective mass at [Formula: see text] at low physical temperature and heavier than the effective mass at [Formula: see text] at high physical temperature. The effects of the nonextensivity on the physical quantity as a function of [Formula: see text] become strong as [Formula: see text] increases. The results indicate the significance of the definition of the expectation value, the definition of the physical temperature, and the constraints for the density operator, when the terms including the volume of the system are not negligible.

Annealing effect on effective mass of two-dimensional electrons in InGaAsN/GaAsSb type II quantum well

The InP-based InGaAs/GaAsSb type II multiple quantum well is the system for developing optical devices for 2 – 3 μm wavelength regions. By doping nitrogen into InGaAs layers, the system becomes effective to fabricate the optical devices with longer wavelength. The epitaxial layers of InGaAsN/GaAsSb on InP substrates are grown by the molecular beam epitaxy. The electrical resistance has been measured as a function of the magnetic field up to 9 Tesla at several temperatures between 2 and 8 K. The effective mass is obtained from the temperature dependence of the amplitude of the Shubnikov-de Haas oscillations. We have reported the nitrogen concentration dependence of the effective mass on the InGaAsN/GaAsSb type II system. The effective mass increases as the nitrogen concentration increases from 0.0 to 1.5 %. In this report, the annealing effect on the effective mass is investigated. The effective mass decreases by the annealing. This result suggests that some amount of nitrogen atoms of the InGaAsN layers are considered to diffuse to the GaAsSb layers by the annealing.

Electronic properties of MoS2 nanoribbon with strain using tight‐binding method

AbstractThe tight binding method was used to calculate the band structures of and its nanoribbon structures. We studied the influences of the quantum confinement effect and the strain effect to the band structure. The tensile strains were applied on both the confined and the transport directions of the nanoribbon. We found that the bandgap and the effective mass decrease with an increasing strain. In addition, the tensile strain along the transport direction has a better effect on reducing the hole effective mass. Although external strains can reduce the carrier effective mass, the valence band edge actually changes from the K valley to the valley with a significantly larger effective mass.Sructure profile (real space and k‐space) and valence band maximum under different tensile strains.

Drastic difference between hole and electron injection through the gradient shell of CdxSeyZn1-xS1-y quantum dots.

Ultrafast fluorescence spectroscopy was used to investigate the hole injection in CdxSeyZn1-xS1-y gradient core-shell quantum dot (CSQD) sensitized p-type NiO photocathodes. A series of CSQDs with a wide range of shell thicknesses was studied. Complementary photoelectrochemical cell measurements were carried out to confirm that the hole injection from the active core through the gradient shell to NiO takes place. The hole injection from the valence band of the QDs to NiO depends much less on the shell thickness when compared to the corresponding electron injection to n-type semiconductor (ZnO). We simulate the charge carrier tunneling through the potential barrier due to the gradient shell by numerically solving the Schrödinger equation. The details of the band alignment determining the potential barrier are obtained from X-ray spectroscopy measurements. The observed drastic differences between the hole and electron injection are consistent with a model where the hole effective mass decreases, while the gradient shell thickness increases.

The investigation of pressure effect on the optical properties, spontaneous polarization and effective mass of BaHfO3: ab initio study

The optical properties, spontaneous polarization (Ps) and the effective mass of the cubic perovskite BaHfO3 (BHO) under pressure effect have been investigated based on the full potential linearized augmented plane wave method and the generalized gradient approximation implemented in the WIEN2K code. The effect of the pressure is imposed on the electronic and optical properties. The results found that the optical absorption of the BHO increases with pressure in the visible range. The band gap energy and effective mass decreases. The gap varies from the direct to indirect one. The Ps was calculated also with pressure, it increases in a quasi-linear behavior as the pressure increases. The cubic BHO show promising properties, it can be used as a piezoelectric material; and can be investigated also in optoelectronic applications.

Making a soft relativistic mean-field equation of state stiffer at high density

We study relativistic mean-field (RMF) models including nucleons interacting with scalar, vector and iso-vector mean fields and self- and cross- mean-field interaction terms. Usually, in such a models the magnitude of the scalar field increases monotonically with the nucleon density, and the nucleon effective mass decreases. We demonstrate that the latter quantity stops to decrease and the equation of state stiffens, provided the mean-field self-interaction potential rises sharply in a narrow vicinity of the values of mean fields corresponding to nucleon densities $n> n_{*}>n_0$, where $n_0$ is the nuclear saturation density. As the result the limiting neutron star mass increases. This procedure offers a simple way to stiffen the equation of state at densities above $n_{*}$ without altering it at densities $n\le n_{0}$. The developed scheme allows an application to neutron stars of the RMF models, which are well fitted to finite nuclei but do not fulfill the experimental constraint on the limiting neutron star mass. The exemplary application of the method to the well-known FSUGold model allows us to increase the limiting neutron star mass from $1.72~M_\odot$ to $M \geq 2.01~M_\odot$.

Strain induced change of band structure and electron effective mass in wurtzite ZnO: A first-principles study

The first-principles hybrid density functional theory is applied to the contrastive investigation on the structural and electronic properties of wurtzite ZnO under hydrostatic strain and uniaxial strain along c axial, and the effect of strain on electron effective mass. It is observed that the structural transformation of the wurtzite ZnO occurs under tensile hydrostatic strain and compressive uniaxial strain. The transformation of ZnO from wurtzite phase to graphitelike phase is accompanied by the transition from a direct to indirect band gap. The deformation potentials that quantify the changes of band gap with strain are also reported. When the hydrostatic strain changes from compression to tension, the electron effective masses at Γ point in Σ, T and Δ directions, and average electron effective mass decrease linearly. For the uniaxial strain, the changes of electron effective mass are dissimilar to the hydrostatic results.

A first-principle study of the effect of W-doping on physical properties of anatase TiO2

The experimental studies of the effect of W-doping on conductivity of anatase TiO2 have opposite conclusions when the W-doping concentration is in a range from 0.02083 to 0.04167. To solve the conflict, two supercell models for Ti0.97917W0.02083O2 and Ti0.95833W0.04167O2 are set up for optimizing their geometries and calculating their band structures and the densities of states based on the first-principles plane-wave norm-conserving pseudopotential of the density functional theory. The electron concentration, electron effective mass, electronic mobility, and electronic conductivity are calculated as well. The calculated results show that both electronic conductivity and conductive property of the doped system increase while the electron effective mass decreases, with the increase of W-doping concentration in the presence or absence of electron spin. The conductive property of Ti0.95833W0.04167O2 system is better than that of Ti0.97917W0.02083O2 system, which is further proved by the analyses of ionization energy and Bohr radius. To analyze the stability and formation energy of W-doped anatase TiO2, two more supercell models for Ti0.96875W0.03125O2 and Ti0.9375W0.0625O2 are set up combined with the geometry optimization. The calculated results show that the total energy and the formation energy increase while the stability of the doped system decreases, with the increase of W-doping concentration in a range from 0.02083 to 0.04167 in the presence or absence of electron spin. Meanwhile the W-doping becomes more difficult. A comparison of the doped system with the pure anatase TiO2 shows that the lattice constant along the a-axis of the W-doped anatase TiO2 increases, and its lattice constant along the c-axis and volume increase as well. The calculated results agree with the experimental results. The doped system becomes a half-metal diluted magnetic semiconductor with a room temperature ferromagnetism in the presence of electron spin.