Density Of The States Research Articles

Enhanced hydrogen storage of a functional material: Hf2CF2 MXene with Li decoration

In this paper, the hydrogen storage properties of the Li-decorated stable Hf2CF2 MXene layer, obtained by the exfoliation of Al from Hf2AlC and F-termination, are considered by using first-principles calculations based on Density Functional Theory. First, the stability characteristics of the host structure (Hf2CF2 layer) are examined by investigating bulk Hf2AlC. To enhance the adsorbed number of H2 molecules, the well-defined initial H2 coordinates are constructed by CLICH (Cap-Like Initial Conditions for Hydrogens) and Monte Carlo-based algorithms. After the geometry optimizations of the designed H2 systems on the Li/Hf2CF2 layer, the adsorption energies of nH2/Li/Hf2CF2 n = 1–10, 15, 20 and 25 systems are calculated, and the suitable values (0.2–0.6 eV/H2) are obtained up to 15H2. For n = 20 and 25 systems, which have adsorption energies of 0.15 eV/H2 and 0.16 eV/H2, respectively. The structural properties and adsorption geometries of these molecules are analyzed. Additionally, the partial density of the states, electron density difference maps, and Mulliken atomic charges are presented to identify the actual binding mechanism of the systems. The results reveal that the Li-decorated Hf2CF2 MXene layer can be preferred for the hydrogen storage applications due to its stable nature and the convenient adsorption characteristics.

Polycomponent doping improved thermoelectric performance of Cu<sub>3</sub>SbSe<sub>4</sub>-based solid solutions

<sec> Cu<sub>3</sub>SbSe<sub>4</sub>, one of the ternary p-type semiconductor materials with chalcopyrite structure, has aroused much interest in thermoelectrics due to its inherent large effective mass and narrow bandgap. Therefore, many researches have been done, which cover the single and/or multi-element doping to manipulate its band structure and introduce the point defects. Although great achievements have been made in recent years, the mechanism in Cu<sub>3</sub>SbSe<sub>4</sub> with respect to the phonon and electronic transport properties needs further investigating. </sec><sec> In this work, first, Sn and S are co-doped into Cu<sub>3</sub>SbSe<sub>4</sub> and then the resulting compound is alloyed with Ga<sub>2</sub>Te<sub>3</sub>, to improve its TE performance and understand the mechanism by calculating the band structure and crystal structure. The calculation of band structure reveals that an impurity band is created within the bandgap after co-doping Sn and S due to their contributions to the density of the states (DOS), which is directly responsible for the significant improvement in carrier concentration (<i>n</i><sub>H</sub>) and electrical property. Therefore, the power factor (PF) is enhanced from 0.52 × 10<sup>–3</sup> to 1.3 × 10<sup>–3</sup> W·m<sup>–1</sup>·K<sup>–2</sup>. </sec><sec> Although the effect associated with the Ga (Te) residing at Sb (Se) sites on the band structure is limited due to the fact that both the single Ga- and single Te-doped band structure remain almost unchanged, the structural parameters (bond lengths and angles) of the polyhedrons [SeCu<sub>3</sub>Sb] and [SbSe<sub>4</sub>] before and after Te and Ga residing at Se and Sb sites respectively change remarkably. This yields the significant distortion of local lattice structure on an atomic scale. Therefore, the phonon scattering is enhanced and the lattice thermal conductivity (<i>κ</i><sub>L</sub>) decreases from 1.23 to 0.81 W·K<sup>–1</sup>·m<sup>–1</sup> at 691 K. The reduction in <i>κ</i><sub>L</sub> prevents the total thermal conductivity (<i>κ</i>) from being enhanced rapidly. As a consequence, the highest ZT value of 0.64 is attained, which is much higher than that of the pristine Cu<sub>3</sub>SbSe<sub>4</sub> (ZT = 0.26). In addition, we not only present a synergistic strategy to separately optimize the phonon and electronic properties, but also fully elaborate its mechanism and better understand that this strategy is an effective way to improve the TE performance of the Cu<sub>3</sub>SbSe<sub>4</sub>-based solid solutions.</sec>

Crystal Structure and Some Thermodynamic Properties of Ca7MgSi4O16-Bredigite

Bredigite with the composition Ca7MgSi4O16 (Ca7MgSi4O16-Bre) has been synthesized by a solid-state reaction method at 1.2 GPa and 1373 K for 7 days, and its structure has been determined by single-crystal X-ray diffraction data. Following a relevant genealogy analysis in the literature, we have refined the structure into two space groups, Pnnm and Pnn2, and found that Ca7MgSi4O16-Bre belongs to the space group Pnnm, which can be essentially derived from the space group Pnn2 via an atomic coordinate transformation (with an average deviation of 0.039 Å only). Furthermore, some thermodynamic properties of the Ca7MgSi4O16-Bre have been obtained in this study. Using first-principles simulations based on density functional theory, the isothermal bulk modulus has been determined as 90.6(4) GPa with a pressure derivative of 5.7(1). Using density functional perturbation technique, the phonon dispersions and vibrational density of the states (VDoS) have been calculated. The VDoS has been combined with a quasi-harmonic approximation to compute the isobaric heat capacity (Cp) and standard vibrational entropy (S2980), yielding Cp = 8.22(2) × 102 − 3.76(6) × 103T−0.5 − 1.384(4) × 107T−2 + 1.61(8) × 109T−3 J mol−1 K−1 for the T range of 298-1000 K and S2980 = 534.1 (22) J mol−1 K−1.

Enhancement of the Seebeck coefficient and power factor in gated silicene superlattices induced by aperiodicity

This paper theoretically investigates the impact of aperiodic sequences in the ballistic transport and thermoelectric effect in silicene gated superlattices. In our analysis, we have implemented the well-known Fibonacci, Thue–Morse, and triadic Cantor type sequences. The transfer matrix technique and the Landauer–Bütikker formalism are used to calculate the transmission probability and the conductance, respectively. The Cutler–Mott formula is employed to estimate the Seebeck coefficient, and the thermoelectric power factor is then obtained. We found that the transmission minibands of aperiodic superlattices exhibit a much more fragmented structure in comparison to that reported in the periodic case. Consequently, the conductance curve presents a more pronounced oscillating shape, which improves the thermoelectric properties. In particular, the Seebeck coefficient has reached values up to 78.2 mV/K for Fibonacci, 233.0 mV/K for Thue–Morse, and 436.3 mV/K for Cantor. In addition, the power factor has been substantially increased, reaching peaks of approximately 8.2, 50.2, and 2.1 nW/K2 for the mentioned sequences, respectively. The best results were obtained for spindown (spinup) charge carriers in the K (K′) valley. Besides, an additional improvement is obtained by considering superior generations of the aperiodic sequences. Finally, our findings are supported through the redistribution of the density of the states, which is induced by the aperiodicity of the nanostructure as well as by the low-dimensionality of the thermoelectric device.

First Principle Study of Adsorption Behavior of PF5 Gas Molecule on S and Mo Vacancy MoS2 Monolayer

The molybdenum disulfide monolayer (MoS2) is gaining more attention due to its attractive electronic property, and it is extensively used in different electronic applications. The presence of vacancies on the MoS2 monolayer leads to an increase in the conductivity of the material. In this work, we have investigated the adsorption behavior and estimated the gas-sensing properties of the S-vacancy (VS) and the Mo-vacancy (VMo) MoS2 monolayers and the two-probe MoS2 devices with phosphorus pentafluoride (PF5) gas molecule. To explore the sensing and electronic properties of VS and VMo MoS2 towards PF5 gas adsorption, the adsorption distance, adsorption energy, charge transfer, band structure, and density of the states have been analyzed using density functional theory in combination with Non-Equilibrium Green's Function. The results show that the PF5 gas molecule is allowed to adsorb on the S- and Mo-vacancy MoS2 monolayers through van der Waals interaction. The PF5 gas molecule shows adsorption distances of 3.3274 A and 2.8673 A, adsorption energies of − 0.1640 eV and − 0.3489 eV, and charge transfers of − 0.025 Q (e) and − 0.053 Q (e) on the VS/VMo MoS2 monolayers. To study the electron transport properties, the device density of the states, the transmission spectra, and the current–voltage characteristics of the VS/VMo MoS2 two-probe devices have been analyzed. The results predicted that the Mo-vacancy MoS2 device shows relatively more adsorption towards the PF5 gas molecule when compared with the VS MoS2 device.

First-principles investigation of the electronic structure, optical and thermodynamic properties on monolayer Sn0.5Ge0.5Se nanosheet

Owing to immense potential applications, in-plane anisotropy has attracted tremendous interest in the mechanical, optical, and thermal properties of two-dimensional (2D) materials for the development of the novel devices. In this work, we used the first-principle calculations to investigate the electronic structure, optical, and thermodynamic properties of the monolayer Sn 0.5 Ge 0.5 Se nanosheet. The nanosheet comprises an armchair and zigzag configurations established towards the X and Y orientation. Band structure and density of the states reveal that the studied nanosheet is a p-type semiconductor with 1.23 eV band-gap. Band structure and density of the states reveal a p-type semiconductor with 1.23 eV band-gap. We estimated the optical properties in different electric polarization. The optical properties confirm the nanosheet is transparent that used for optoelectronic applications. Thermodynamic properties confirm the stability of the nanosheet at a high temperature. • Electronic, optical, and thermodynamical properties of Sn 0.5 Ge 0.5 Se nanosheet were studied. • Band structure and PDOS confirms a p-type semiconductor with a bandgap of 1.23 eV. • The Se-4p electrons and Sn-5s and 5p electrons are dominating near the Fermi level. • Optical properties were studied in different electric polarization. • Thermodynamic studies reveal the nanosheet is stable at high temperatures.

Advanced Study of the Parabolic Trough Collector Using Aluminum (III) Oxide Seal

Solar energy is the main source of all sorts of energy present in nature, i.e. all energy produced from it. So the direct use of solar energy as a useful energy is very important. There are numerous solar thermal projects in which the concentrated type collector heats up to 100 to 400 degrees Celsius. It is used in a wide range of applications, including power generation, industrial steam generation and hot water supply. Parabolic trough collector is chosen for steam production, since high temperatures can be achieved. The temperature of the inlet and outlet water, the mass flow rate, the usable heat gain and also the thermal efficiency of the collector are calculated. Theoretical values for heat loss of vacuum as the annulus gas is measured and compared with other values for heat loss of air as the annulus gas. In order to help preserve the vacuum within the annulus gap, a substitute glass-metal seal was created. The seal helped increased the heat loss and thereby improve the overall efficiency of the collector. It can be deduced from the band structure and density of the states of Al2O3 and is an outstanding insulator content. In addition, the assessment of the absorption coefficient showed a low energy range of 5.34 eV to 11.88 eV, with a median value of 11.50 eV and 11.88 eV respectively for parallel and perpendicular polarizations.

Modifying the Dielectric Properties of the TlGaS2 Single Crystal by Electron Irradiation

The dielectric properties and ac conductivity of an electronically irradiated layered TlGaS2 single crystal are studied in the frequency range of 5 × 104–3.5 × 107 Hz. It is established that electron irradiation of TlGaS2 single crystal samples with doses of 2 × 10l2 to 2.4 × 1013 e/cm2 leads to a decrease in the real component (e') of the complex dielectric permittivity in a high-frequency region (f > 106 Hz) and an increase in its imaginary component (e''), the dielectric loss tangent (tanδ), and ac conductance (σac) across the layers in the entire studied frequency range. At irradiation doses of 2 × 10l2 to 2.4 × 1013 e/cm2, losses in the reach-through conductivity of TlGaS2 occur, and, as the electron irradiation dose accumulates, the dispersion of e'' and tanδ increases significantly. In the frequency range of f = 5 × 104–2 × 107 Hz, in the irradiated TlGaS2 samples, the ac conductivity changes according to the law σac ~ fn (where n = 0.7–0.8), which is typical of the hopping mechanism of charge transfer via localized states near the Fermi level. The parameters of localized states in TlGaS2 (the density of the states near the Fermi level and their energy spread) are estimated versus the electron irradiation dose.

Enhanced photo-absorption of anatase TiO2 with Ni and Eu doping: A first principle study

In this study, structural, electrical and optical properties of doped and un-doped TiO2 were investigated. Calculations for the analysis of all properties were carried in density functional theory implemented in Wien2k code. Comparison of all the properties was carried of un-doped TiO2, and Ni & Eu doped TiO2. The FP-LAPW was used for description of Kohn-Sham (KS) and properties related to ground state were analyzed. Deformation in the structure was observed due to the ionic radii and electronic configuration of dopant atoms. While analyzing electrical properties, change in the density of the states was observed attributed to the fact of replacement of Ti by Ni and Eu. Band gap was reduced in mono-doped and co-doped TiO2 as a result of which co-efficient of absorption was increased in region of visible light thus making it a potential candidate for applications in the field of solar energy.

Cenário da Construção Civil no Brasil durante a pandemia da COVID-19

Esta pesquisa tem por objetivo apresentar o cenário atual da Construção Civil de alguns Estados brasileiros, demonstrando os locais que estabeleceram a parada do setor e os que continuaram a atuação normalmente, sendo feito uma comparação levando em consideração a densidade demográfica dos Estados e a maior e menor quantidade de óbitos até a data atual e também demonstrar as medidas adotas nos canteiros de obras para evitar a propagação da doença. O método de pesquisa foi realizado com base em dados coletados nas Secretarias Estaduais de Saúde e Sindicatos da Indústria da Construção (Sinduscon) dos Estados, de jornais e revistas de 8 Estados brasileiros sendo 4 que possuem menor quantidade de óbitos e 4 que possuem maiores quantidades de óbitos registrados. Foi gerando um valor, chamado índice “I” (Óbitos / Densidade Demográfica), desta forma foi possível entender o impacto que a indústria da Construção Civil pode trazer para a propagação da COVID-19, já que o setor Construção Civil movimenta os outros setores, como lojas de materiais de construção, distribuidoras, restaurantes, transportes públicos, entre outros. Verificou-se quais Estados o setor da Construção Civil está cumprindo com a paralisação do setor e comparar dados dos Estados que estão com os maiores índices desta doença, verificando se há alguma correlação para o aumento de casos confirmados. Os Estados que possuem maiores quantidade de óbitos foram SP > RJ > CE > PE, porém seus índices foram diferentes, relacionando-os com o controle ou descontrole da prevenção do vírus na construção civil e em outros setores, onde se comparados com índice “I” a sequência é SP > CE > PE > RJ, sendo I = 13,57 para São Paulo já Estados que possuem menores quantidades de óbitos foram TO < MS < MT < AC e seus comparativos por índice “I” continuam na mesma sequência, tendo o Tocantins com o menor índice entre todos os Estados, I = 0,53. Foram adotadas algumas medidas essenciais para evitar a propagação do coronavírus como o uso de termômetros na entrada do trabalho, marcação no piso para manter as pessoas distanciadas, distanciamento entre as mesas e retirada dos assentos intermediários, higienização de canteiros, distribuição de cartazes informativos pela obra, redefinição de turnos, entre outras medidas são bastante importantes pra o bem estar e saúde dos operários.

Critical properties and charge transport in ethylene bridged organosilica low-κ dielectrics

Organosilicate-glass-based low-κ films containing both terminal methyl groups and an ethylene bridge between the silicon atoms are spin-on deposited by using 1,2-bis(trimethoxysilyl)ethane and methyltrimethoxysilane, Brij30 template, and thermal curing. The chemical composition, porosity, and internal defects are studied using Fourier-transform infrared spectroscopy, x-ray photoelectron spectroscopy, electron energy loss spectroscopy, UV induced luminescence, and ellipsometric porosimetry. It was found that the studied films contain oxygen-deficient centers (Si—Si bonds). The high defect density of the states near the valence-band edge of the studied low-κ films leads to a relatively small bandgap value of about 6.3 eV. The current–voltage characteristics at different temperatures were analyzed using six theoretical charge transport models where the transport is limited by the traps ionization. It was found that the best qualitative and quantitative agreement between the calculations and experimental data is achieved by using the model of phonon-assisted electron tunneling between the neutral traps and is supplemented by considering the space charge and charge carrier kinetics. Since the thermal and optical energies of the traps in the studied films are 1.6 eV and 3.2 eV, respectively, it is concluded that the traps are responsible for the charge transport in the Si—Si bonds.

A flexible particle Markov chain Monte Carlo method

Particle Markov Chain Monte Carlo methods are used to carry out inference in nonlinear and non-Gaussian state space models, where the posterior density of the states is approximated using particles. Current approaches usually perform Bayesian inference using either a particle marginal Metropolis–Hastings (PMMH) algorithm or a particle Gibbs (PG) sampler. This paper shows how the two ways of generating variables mentioned above can be combined in a flexible manner to give sampling schemes that converge to a desired target distribution. The advantage of our approach is that the sampling scheme can be tailored to obtain good results for different applications. For example, when some parameters and the states are highly correlated, such parameters can be generated using PMMH, while all other parameters are generated using PG because it is easier to obtain good proposals for the parameters within the PG framework. We derive some convergence properties of our sampling scheme and also investigate its performance empirically by applying it to univariate and multivariate stochastic volatility models and comparing it to other PMCMC methods proposed in the literature.

Scalable Processing of Low Voltage Organic Field Effect Transistors With a Facile Soft-Contact Coating Approach

A soft-contact coating (SCC) method, using a rotatable steel sheet as the meniscus guide, is developed for depositing organic semiconductors (OSCs). The method is used to fabricate bottom-gate bottom-contact organic-field effect transistors (OFETs) using blended solution of small molecule OSC and polymer binder. During the coating process, the meniscus guide maintains very weak contact force to the substrate surface, avoiding damages to the pre-fabricated electrodes. Even at a fast coating speed of 20 mm/s, well controlled crystallization can be achieved to form low sub-gap density of the states (DOS) channels for low voltage OFETs of excellent uniformity. The fabricated low voltage OFETs on plastic substrate in full solution processes exhibit high mobility reaching $1.5 ~{\text {cm}}^{{2}}\cdot \,\,\text{V}^{\text {-1}}\cdot \,\,\text{s}^{\text {-1}}$ , and a small threshold voltage dispersion less than 250 mV for measured 159 devices.

Anisotropic Fluorescence Emission and Photobleaching at the Surface of One-Dimensional Photonic Crystals Sustaining Bloch Surface Waves. I. Theory

Photonic crystal (PC) enhanced fluorescence has been proposed as a novel tool for early disease detection in a liquid biopsy format. However, photobleaching of the emitters has never been deeply investigated, although its cross section is expected to increase due to the large field intensity enhancement. Herein, we report on a comprehensive theoretical description of the stationary fluorescence emission of molecular emitters bound to the surface of a one-dimensional photonic crystal (1DPC) biosensor. The model considers coupling of the emission to the large local density of the states provided by the 1DPC, in particular to the Bloch surface waves, which can be characterized by different polarization states. The rotational diffusion equation in the presence of photobleaching was solved analytically by a Laplace spherical harmonics analytical approach. The results show that photobleaching can severely affect the fluorescence emission in terms of total intensity and polarization composition, suggesting that ...

Effects of the Inclusion of Armchair Graphene Nanoribbons on the Electrical Conduction Properties of NN-Heterojunction 4H-6H/SiC Diodes

In recent years, graphene has sparked the interest of researchers due to its promising electrical and physical attributes. These attributes make it highly suitable to develop electronic devices with ultra-high mobility of charge carriers. Meanwhile silicon carbide (SiC), a wide bandgap semiconductor material, is being used for high temperature optoelectronic applications. SiC has more than 250 different crystalline forms, these are called polytypes. Some of these polytypes (such as 4H-SiC, 6H-SiC and 3C-SiC) have exceptional physical and electrical properties. Electronic devices which have SiC and graphene as their constituent materials may combine the outstanding attributes of both materials. This article attempts to simulate electronic devices having SiC and graphene as their constituent materials. For this purpose, simulations of a novel nn-heterojunction 4H-6H/SiC diodes with the inclusion of an armchair nanoribbon layer have been carried out. All of the simulations have been run using QuantumWise Atomistix Toolkit (ATK) software, which is an atomic scale electronic device simulator. The density of the states, charge carrier densities and current-voltage curves of the simulated devices have been computed. The simulation results showed a significant improvement in the electrical conduction properties of nn-heterojunction 4H-6H/SiC diodes after the inclusion of the armchair graphene nanoribbons. These simulations provide the groundwork for our future experiments, which will be targeted on fabricating high mobility diodes and/or field effect transistors.

Impact of surface oxidation on the structural, electronic transport, and optical properties of two-dimensional titanium nitride (Ti3N2) MXene

In this study, the effects of surface oxidation on the structural, electronic transport, and optical properties of two-dimensional titanium nitride (Ti3N2) MXene are examined using density functional theory calculations. Our computations demonstrate that surface-oxidized titanium nitride (Ti3N2O2) MXene is thirty percent more stable than the bare Ti3N2. Adsorption energy calculations show that oxygen terminal groups strongly adhere to the Ti atoms on the surface of the bare Ti3N2. The covalent bonding between outer Ti and N atoms in Ti3N2 MXene is partially transferred to the ionic bonding owing to the surface oxidation and resulting in the formation of covalent bonding between outer Ti and O atoms in the Ti3N2O2 MXene. This surface oxidation has a negative impact on the metallic conductivity of the bare Ti3N2 due to the reduction of density of the states at the Fermi level. The current of Ti3N2 decreases after the termination of the surface atoms with oxygen atoms. Surface oxidation reduces both absorption and the reflectivity of the Ti3N2 in the visible spectrum. However, Ti3N2O2 displays considerably higher absorption and reflectivity compared to the Ti3N2 at higher photon energies. Our results provide a reference for theoretical and experimental studies on the optoelectronics applications of titanium nitride MXenes.

First-Principle Studies on the Ga and As Doping of Germanane Monolayer

The study of energetics, structural, the electronic and optical properties of Ga and As atoms substituted for doped germanane monolayers were studied by first-principles calculations based on density functional theory. Both of the two doping are thermodynamically stable. According to the band structure and partial density of the states, gallium is p-type doping. Impurity bands below the conduction band lead the absorption spectrum moves in the infrared direction. Arsenic doping has impurity level passing through the Fermi level and is n-type doping. The analysis of optical properties confirms the value of bandgap and doping properties.

Energy storage properties of a two-dimensional TiB4 monolayer.

Herein, the energy storage properties of TiB4 monolayers were studied within the density functional theory framework. Both CH4 and H2 were chosen as adsorption molecules, and their interactions with a TiB4 sheet were investigated. TiB4 attracted gas molecules via open Ti sites, and each Ti atom could adsorb a maximum of two molecules. Via the electronic density of the states and atomic charge analysis, we found that the mechanism for gas adsorption was mainly electrostatic. For H2 adsorption cases, orbital interactions also made contributions. As the combustion energy of one CH4 molecule is three times that of one H2, the TiB4-2CH4 compound can achieve the best equivalent gravimetric hydrogen density of 10.14 wt% with the average adsorption energy of 0.38 eV. Ab initio molecular dynamics calculations on this compound showed that there was no kinetic barrier during CH4 desorption. Moreover, the stacking of the TiB4 monolayers could weaken the energy storage capacity. Therefore, it should be avoided in practial usage. Based on the abovementioned results, the TiB4 monolayer was suggested to be a promising candidate for onboard energy storage.

The Gibbs sampler with particle efficient importance sampling for state-space models*

We consider Particle Gibbs (PG) for Bayesian analysis of non-linear non-Gaussian state-space models. As a Monte Carlo (MC) approximation of the Gibbs procedure, PG uses sequential MC (SMC) importance sampling inside the Gibbs to update the latent states. We propose to combine PG with the Particle Efficient Importance Sampling (PEIS). By using SMC sampling densities which are approximately globally fully adapted to the targeted density of the states, PEIS can substantially improve the simulation efficiency of the PG relative to existing PG implementations. The efficiency gains are illustrated in PG applications to a non-linear local-level model and stochastic volatility models.

Size effect on the three state thermal hysteresis of a 2D spin crossover nanoparticles

In this report first we show, in a framework of the Ising-like model, a numerical simulation of a typical two step thermal transition obtained for a square lattice 12x12: a “first-step hysteresis” for a high spin fraction Nhs between 0 - 0,5 and at a higher temperature a “second-step hysteresis” with Nhs between 0.5 and 1. As long as we decrease the number of molecules the temperature range of the “second-step hysteresis” moves to a lower temperature, until is obtained, for a square of 4x4, a clear overlapped case with a three state behaviour. A detailed analysis on the role of the size system (4x4, 5x5, 6x6, 8x8 and 12x12) on the stability of this “Three state behaviour” is presented in this contribution. We study the influence of the surrounding environment for this specific thermal hysteresis. To solve the self-consistent equation related to the average value of the spin-operator <σ>, we use the density of the states calculated using a dynamic programming algorithm that will be presented in this paper.