Band States Research Articles

Concerted Proton-Coupled Electron Transfer to a Graphite Adsorbed Metalloporphyrin Occurs by Band to Bond Electron Redistribution.

Surface immobilized catalysts are highly promising candidates for a range of energy conversion reactions, and atomistic mechanistic understanding is essential for their rational design. Cobalt tetraphenylporphyrin (CoTPP) nonspecifically adsorbed on a graphitic surface has been shown to undergo concerted proton-coupled electron transfer (PCET) in aqueous solution. Herein, density functional theory calculations on both cluster and periodic models representing π-stacked interactions or axial ligation to a surface oxygenate are performed. As the electrode surface is charged due to applied potential, the adsorbed molecule experiences the electrical polarization of the interface and nearly the same electrostatic potential as the electrode, regardless of the adsorption mode. PCET occurs by electron abstraction from the surface to the CoTPP concerted with protonation to form a cobalt hydride, thereby circumventing Co(II/I) redox. Specifically, the Co(II) d-state localized orbital interacts with a proton from solution and an electron from the delocalized graphitic band states to produce a Co(III)-H bonding orbital below the Fermi level, corresponding to redistribution of electrons from the band states to the bonding states. These insights have broad implications for electrocatalysis by chemically modified electrodes and surface immobilized catalysts.

Impact of resonant state formation and band convergence in In and Sr co-doped SnTe thermoelectric material evaluated via the single parabolic band model

SnTe is a promising thermoelectric material for mid-temperature waste heat recovery. Recently, a high thermoelectric performance (zT) is reported in In and Sr co-doped Sn1−3xInxSr2xTe (x = 0.0 – 0.03). A high zT of 1.075 is achieved when x = 0.02 at 823 K. The In dopant is known to engineer the valence band of the SnTe by forming a resonant state near 300 K. The Sr dopant converges the two valence bands of the SnTe. However, when both In and Sr are doped in SnTe while keeping their atomic ratio constant, their effects on band engineering with increasing are not known. Here, the effects of resonant state formation and band convergence on electronic transport properties of Sn1−3xInxSr2xTe (x = 0.0 – 0.03) are investigated from the estimated temperature-dependent electronic band parameters. Both the resonant state formation and band convergence become most significant at the x = 0.02. According to the single parabolic band model, the zT of the x = 0.02 can be further improved to ∼1.834 (approximately 70 % zT improvement) upon carrier concentration tuning.

ARPES studies of the ground state electronic properties of the van der Waals transition metal trichalcogenide CoPS[formula omitted

The electronic structure of a wide bandgap van der Waals antiferromagnetic material CoPS3 (transition metal phosphorus trichalcogenide) is studied using angle-resolved photoelectron spectroscopy at room temperature which is well above the TN transition temperature for this compound. The observed dispersion of the electronic states in the valence band is correctly described using spin-resolved band structure. This observation points to the importance of the local electron–electron correlations and fluctuations for the description of the electronic and magnetic properties of transition metal phosphorus trichalcogenides and draws further ideas for the studies of electronic structure of CoPS3 and other similar materials.

Electronic Structure and Ultrafast Electron Dynamics in CuO Photocatalysts Probed by Surface Sensitive Femtosecond X-ray Absorption Near-Edge Structure Spectroscopy

CuO is often employed as a photocathode for H2 evolution and CO2 reduction, but observed efficiency is still far below the theoretical limit. To bridge the gap requires understanding the CuO electronic structure; however, computational efforts lack consensus on the orbital character of the photoexcited electron. In this study, we measure the femtosecond XANES spectra of CuO at the Cu M2,3 and O L1 edges to track the element-specific dynamics of electrons and holes. Results show that photoexcitation represents an O 2p to Cu 4s charge transfer state indicating the conduction band electron has primarily Cu 4s character. We also observe ultrafast mixing of Cu 3d and 4s conduction band states mediated by coherent phonons, with Cu 3d character of the photoelectron reaching a maximum of 16%. This is the first observation of the photoexcited redox state in CuO, and results provide a benchmark for theory where electronic structure modeling still relies heavily on model-dependent parametrization.

Non-Hermiticity induced topological edge states with unique transport

We demonstrate that non-Hermiticity solely induced Su-Schrieffer-Heeger (SSH) lattice can result in two unique topological edge state (TES) bands: One is within the band gap, the other is embedded in the bulky bands. The latter are the topological bound states in the continuum (BIC). Different from general TESs, both the TES bands have the unidirectional transport with the direction under two-fold modulation: source spin direction and source position. The TESs with BIC have additional amplifying transport property. All the results have developed the concept of BICs and TESs, and provided a new freedom in the controlling of light transport.

Spectral emission properties of a nitrogen-doped diamond (001) photocathode: Hot electron transport and transverse momentum filtering

The electron emission properties of a single-crystal nitrogen-doped diamond(001) photocathode inserted in a 10kV DC photoelectron gun are determined using a tunable (235-410nm) ultraviolet laser radiation source for photoemission from both the back nitrogen-doped substrate face and the front homo-epitaxially grown and undoped diamond crystal face. The measured spectral trends of the mean transverse energy (MTE) and quantum efficiency (QE) of the emitted electrons are both anomalous and non-monotonic, but are shown to be consistent with (i) the known physics of electron photoexcitation from the nitrogen substitution states into the conduction bands of diamond, (ii) the energy position and dispersion characteristics of the conduction bands of diamond in the (001) emission direction, (iii) the effective electron affinity of the crystal faces, (iv) the strong electron-(optical)phonon coupling in diamond, and (v) the associated hot electron transport dynamics under energy equipartition with the optical phonons. Notably, the observed hot electron emission is shown to be restricted parallel to the photocathode surface by the low transverse effective masses of the emitting band states - a transverse momentum filtering effect.

Structural, Thermoelectric, Electronic, and Magnetic Properties of Pristine Intermetallic Rare-Earth-Based XMn2Si2 (X=Dy, Er) Compounds

Detailed Structural, thermoelectric, electronic and magnetic properties of the ternary rare-Earth based XMn2Si2 (X=Dy, Er) Compounds, are investigated using the full-potential linearized augmented-plane wave (FP-LAPW) method with generalized gradient approximation (GGA+U) in ferromagnetic phase. The basic calculations of optimization are found with the support of (PBE-GGA) to realize theoretical consistency with existing experimental consequences, although for the enhancement of magneto-electronic part the (GGA+U) technique is employed. We have identified theoretically that the ferromagnetic is the most suitable phase among three studied phases for these compounds agree well with previous experimental works. The electronic band structure indicates that these compounds are metallic through both spin channels in the FM phase. A secure hybridization occurs between the elements Dy/Er-f and Mn-d states in the valence band and the Si-p state in the conduction band. The total magnetic moments verify that the rare-Earth based DyMn2Si2 ternary inter-metallic compound showcases stronger ferromagnetic behavior patterns than the ErMn2Si2 compound. We estimated the Seebeck coefficient S, electrical and thermal conductivities, and the ZT in this study over the temperature range of 0 to 800 K. The ErMn2Si2 is a viable contender for high-temperature applications in waste heat management because of its high ZT values in the high-temperature region in thermoelectric devices.

Simulation of Spin Selectivity of Electrical Conductivity of Chiral Platinum Nanotubes

To study the electronic and spin properties of single-walled platinum nanotubes, two rows of chiral nanotubes have been calculated by the relativistic method of symmetrized linearized augmented cylindrical waves: Pt(5, n2) with 1 ≤ n2 ≤ 4 and Pt(10, n2) with 1 ≤ n2 ≤ 9 and radii from 2.24 to 7.78 Å. In all tubes, the intersection of the top of the valence band and the bottom of the conduction band with the Fermi level is observed, which is characteristic of compounds with a semi-metallic band structure. The spin–orbit coupling manifests itself as a splitting of nonrelativistic dispersion curves, which can exceed 0.5 eV for near-Fermi bands and decreases upon transition to the internal states of the valence band and nanotubes of larger diameter. The spin densities of states for electrons with spin up and down at the Fermi level are noticeably different, which can be used to create pure spin currents through nanotubes using alternating electrical voltage. The (5, 3) and (10, 7) nanotubes are the most suitable for this.

Influence of calcination temperature on the microstructure and photocatalysis performance of B/Sm-TiO2 nanomaterials

In this work, we fabricated boron/samarium co-doped TiO2 nanomaterials (B/Sm-TiO2) by a sol-gel method. The effects of calcination temperature on the crystal texture, grain size, and photocatalytic characteristics were investigated by XRD, XPS, SEM, TEM, and UV–vis DRS. Using the Materials Studio software and density functional theory, the band structures and state densities of TiO2 single or codoped B and Sm ions have been determined. The photocatalytic degradation of methyl orange under visible light was used to evaluate the photocatalytic performance of B/Sm-TiO2 nanomaterials. The results demonstrate that the photocatalytic performance of B/Sm-TiO2 nanomaterials is significantly affected by different calcination temperatures. With the increase of calcination temperature, the size of TiO2 particles increases, and the TiO2 phase changes from anatase to rutile with a crystalline transition temperature of 700 °C. The visible light absorption of B/Sm-TiO2 under the forging condition of 600 °C is enhanced accordingly, the degradation efficiency is improved, and the degradation rate of methyl orange reaches 89% after 60 min of light irradiation.

Research on excited states of PPV-COF spectra based on first principles

An in-depth understanding of the excitation mechanism of covalent organic frameworks (COF) materials is very important for the preparation and modification of two-dimensional materials. This paper investigates the photoelectric properties of poly(p-phenylenevinylene) (PPV)-based COFs using first principles and quantum chemistry. Density functional theory (DFT) and time-dependent DFT were employed to analyze the state density and band structure of PPV-COF, confirming that PPV-COF has a direct bandgap of 2.15 eV. We further analyze the electron–hole distribution of all excited states with vibrator strength greater than 0.01, and discuss the mechanism of electron transition after photon absorption in each excited state of PPV-COF. Finally, based on quantitative calculations, UV-visible spectra of PPV-COF are provided. Our results showed that PPV-COF has a good photoelectric performance, which is beneficial to the application of PPV-COF in optoelectronic devices.

On the Donor: Acceptor Features for Poly(3-hexylthiophene): TiO2 Quantum Dots Hybrid Materials Obtained via Water Vapor Flow Assisted Sol-Gel Growth.

Here, we present a novel methodology for the preparation of P3HT:TiO2 quantum dots hybrid materials via water vapor flow-assisted sol-gel growth focusing on the structural, optical and electrical property characterization complemented with first-principles calculations as a promising donor-acceptor system for polymer and hybrid solar cells. X-ray diffraction and UV-Vis spectroscopy analyses suggest that the increasing concentration of TiO2 quantum dots leads to the formation of higher amounts of amorphous regions while the crystalline regions exhibited interesting aspect ratio modifications for the P3HT polymer. Raman spectra evidenced the formation of charge carriers in the P3HT with increasing TiO2 quantum dots content and the P3HT:TiO2 50:50 weight ratio resulted in the best composition for optimizing the bulk electronic conductivity, as evidenced by impedance spectroscopy studies. Our DFT calculations performed for a simplified model of the P3HT:TiO2 interface revealed that there is an important contribution of the thiophene carbon atoms states in the conduction band at the Fermi level. Finally, our DFT calculations also reveal an evident gain of electron density at the TiO2 (101) surface while the thiophene rings showed a loss of the electron density, thus confirming that the P3HT:TiO2 junction acts as a good donor-acceptor system. In our opinion, these results not only present a novel methodology for the preparation of P3HT:TiO2 quantum dots hybrid materials but also reveal some key aspects to guide the more rational design of polymer and hybrid solar cells.

α+Zr92 cluster structure in Mo96

In the evaluation of the half-life of the neutrinoless double-$\ensuremath{\beta}$ decay ($0\ensuremath{\nu}\ensuremath{\beta}\ensuremath{\beta}$) of a doubly closed-subshell nucleus $^{96}\mathrm{Zr}$, the structure of the nucleus $^{96}\mathrm{Mo}$ is essentially important. The $\ensuremath{\alpha}$-clustering aspects of $^{96}\mathrm{Mo}$ are investigated for the first time. By studying the nuclear rainbows in $\ensuremath{\alpha}$ scattering from $^{92}\mathrm{Zr}$ at high energies and the characteristic structure of the excitation functions at the extreme backward angle at the low-energy region, the interaction potential between the $\ensuremath{\alpha}$ particle and the $^{92}\mathrm{Zr}$ nucleus is determined well in the double folding model. The validity of the double folding model was reinforced by studying $\ensuremath{\alpha}$ scattering from neighboring nuclei $^{90}\mathrm{Zr}, ^{91}\mathrm{Zr}$, and $^{94}\mathrm{Zr}$. The double-folding-model calculations reproduced well all the observed angular distributions over a wide range of incident energies and the characteristic excitation functions. By using the obtained potential the $\ensuremath{\alpha}+^{92}\mathrm{Zr}$ cluster structure of $^{96}\mathrm{Mo}$ is investigated in the spirit of a unified description of scattering and structure. The existence of the second-higher nodal band states with the $\ensuremath{\alpha}+^{92}\mathrm{Zr}$ cluster structure, in which two more nodes are excited in the relative motion compared with the ground band, is demonstrated. The calculation reproduces well the ground-band states of $^{96}\mathrm{Mo}$ in agreement with experiment. The experimental $B(E2)$ value of the transition in the ground band is also reproduced well. The effect of $\ensuremath{\alpha}$ clustering in $^{96}\mathrm{Mo}$ on the the half-life of the $0\ensuremath{\nu}\ensuremath{\beta}\ensuremath{\beta}$ double-$\ensuremath{\beta}$ decay of $^{96}\mathrm{Zr}$ is discussed.

Radiative recombination pathways in ordered and disordered CZTSe microcrystals

The photoluminescence (PL) spectra of Cu2ZnSnSe4 microcrystals with two different degrees of disorder were investigated by measuring the dependencies on the temperature and excitation power. Crystals were cooled at different rates after the high-temperature annealing treatment in order to change the degree of disorder. The PL spectra of both samples consist of one asymmetrical band located at 0.88 and 0.92 eV for disordered and ordered crystals, respectively. At low temperatures, both samples have similar origins of PL involving localized electron states. In the case of the ordered sample, we observed deeper localized electron states, thus higher temperatures were needed to empty the electrons from these states to the band states. Changes in the degree of disorder lead to changes in the radiative recombination in CZTSe microcrystals.

Theory of Excitons in Gated Bilayer Graphene Quantum Dots

We present a theory of excitons in gated bilayer graphene (BLG) quantum dots (QDs). Electrical gating of BLG opens an energy gap, turning this material into an electrically tunable semiconductor. Unlike in laterally gated semiconductor QDs, where electrons are attracted and holes repelled, we show here that lateral structuring of metallic gates results in a gated lateral QD confining both electrons and holes. Using an accurate atomistic approach and exact diagonalization tools, we describe strongly interacting electrons and holes forming an electrically tunable exciton. We find these excitons to be different from those found in semiconductor QDs and nanocrystals, with exciton energy tunable by voltage from the terahertz to far infrared (FIR) range. The conservation of spin, valley, and orbital angular momentum results in an exciton fine structure with a band of dark low-energy states, making this system a promising candidate for storage, detection and emission of photons in the terahertz range.

Optimizing Predictive Maintenance Decisions: Use of Non-Arbitrary Multi-Covariate Bands in a Novel Condition Assessment under a Machine Learning Approach

Jointing Condition-Based Maintenance (CBM) with the Proportional Hazards Model (PHM), asset-intensive industries often monitor vital covariates to predict failure rate, the reliability function, and maintenance decisions. This analysis requires defining the transition probabilities of asset conditions evolving among states over time. When only one covariate is assessed, the model’s parameters are commonly obtained from expert opinions to provide state bands directly. However, the challenge lies within multiple covariate problems, where arbitrary judgment can be difficult and debatable, since the composite measurement does not represent any physical magnitude. In addition, selecting covariates lacks procedures to prioritize the most relevant ones. Therefore, the present work aimed to determine multiple covariate bands for the transition probability matrix via supervised classification and unsupervised clustering. We used Machine Learning (ML) to strengthen the PHM model and to complement expert knowledge. This paper allows obtaining the number of covariate bands and the optimal limits of each one when dealing with predictive maintenance decisions. This novel proposal of an ML condition assessment is a robust alternative to the expert criterion to provide accurate results, increasing the expectation of the remaining useful life for critical assets. Finally, this research has built an enriched bridge between the decision areas of predictive maintenance and Data Science.

Insight into Copper Doping Ratio on Optoelectronics Performance of Two-Dimensional Cu-Doped SnO2 Nanosheets: an Experiment and DFT Study

The optoelectronics performances of two-dimensional Cu-doped SnO2 nanosheets were investigated by first-principles calculations on the basis of density functional theory (DFT) and experiments. First, the crystal structures of Cu-doped SnO2 were built and analyzed by using DFT within the generalized gradient approximation (GGA). The total energy of Cu-doped SnO2 was discussed qualitatively and quantitatively from three aspects: charge density, band structure, and state density. The results show that copper doping has little effect on the charge density distribution because of the similar ionic radius of Cu2+, Sn4+, and the close bond lengths of Cu–O and Sn–O. The calculation results of band structure and state density show that the doping of a copper acceptor leads to the appearance of a new energy level at the top of the valence band and decreases the band gap of SnO2. With the guidance of theoretical calculation, the nanosheets of Cu-doped SnO2 are constructed experimentally, and their optoelectronics performances are investigated. The experimental results exhibit that the Cu atom replaces the Sn atom in the SnO2 lattice, while the Cu-doped SnO2 retains the original rutile phase of the intrinsic SnO2. The sample with a doping ratio of 12.5% has more uniform particle morphology and dispersion characteristics. In addition, the 12.5% Cu-doped SnO2 obtained the largest transient photocurrent of 3 μA/cm2 and the lowest resistance to charge transfer, thereby indicating that the material has a remarkably higher optoelectronics performance under this ratio. The theoretical calculation and experimental results are consistent, which indicates that Cu doping effectively enhances the charge transfer in SnO2 and enables it for high-quality application in optoelectronic catalysis.

Bandgap tuning and analysis of the electronic structure of the Cu2NiXS4 (X=Sn, Ge, Si) system: mBJ accuracy with DFT expense

The energy bands and band gaps of Cu2NiXS4 (X ​= ​Sn, Ge, Si) semiconductor materials have been studied and analyzed by using quantum-chemical calculations within the DFT framework. Using different exchange-correlation functionals, the energy gaps of the studied systems were estimated and determined, and their band structure were studied in detail. Based on the results of spin-polarized and spin-orbit mBJ-calculations, bands of t2g states and direct band gaps with values of 1.32, 1.56, and 2.58 ​ eV, were found for Cu2NiSnS4, Cu2NiGeS4, and Cu2NiSiS4, respectively, indicating the suitability of these materials as a suitable light-absorbing layer for a new generation solar cells.

Interaction studies of propylene and butadiene on tricycle graphane nanosheet – A DFT outlook

In this research work, we employed a tricycle graphane nanosheet as a chemical sensor to monitor the toxic hydrocarbon molecules, namely propylene, and 1,3-butadiene, which are emitted from automobile industries. At first, the structural stability and dynamical permanency of tricycle graphane is ascertained based on cohesive energy and phonon-band-spectrum. Sequentially, the electronic properties of tricycle graphane are conferred with the results of the projected density of states spectrum and band structure. The computed band gap of tricycle graphane is 5.53 eV. Chiefly, the adsorption behaviour of target propylene and 1, 3-butadiene on tricycle graphane is explored by determining adsorption energy, relative band gap variation, and Mulliken population analysis. Furthermore, the range of adsorption energy magnitudes (−0.16 eV to −1.03 eV) demonstrates that the target hydrocarbon molecules are physically adsorbed on tricycle graphane material. The overall outcome endorses that the tricycle graphane can be utilised as a prominent sensor to sense the hydrocarbon molecules released from automobiles and monitor air pollutants.

Adsorption and Sensing Performances of MoTe2 Monolayers Doped with Pd, Ni, and Pt for SO2 and NH3: A DFT Investigation

Density functional theory (DFT) calculation was used to study the adsorption and sensing performances of a transition metal atom (TMA) doped MoTe2 monolayer for two industrial toxic and harmful gases, SO2 and NH3, in this study. The adsorption structure, molecular orbital, density of state, charge transfer, and energy band structure were applied to investigate the interaction between the gas and MoTe2 monolayer substrate. The conductivity of the MoTe2 monolayer film doped with TMA (Ni, Pt, Pd) is significantly improved. The original MoTe2 monolayer has poor adsorptive ability for SO2 and NH3, which is physisorption, while for the TMA-doped MoTe2 monolayer, it is significantly enhanced and the adsorption process is chemisorption. All results provide a trustworthy theoretical basis for sensors based on MoTe2 to detect toxic and harmful gases SO2 and NH3. Additionally, it also provides guidance for further research on the transition metal cluster doped MoTe2 monolayer for gas detection.

Physical origins of optical anisotropy in quantum-confined semiconductors: The roles of valence band mixing, transition broadening, and state filling

We investigate experimentally and theoretically the impact of valence band mixing and spectrum of confined states on the polarization of light emitted from or absorbed by GaAs/AlGaAs semiconductor quantum dots and quantum wires with tailored heterostructure potential. In particular, such nanostructures with parabolic-profile confinement potentials, realized by organometallic vapor phase epitaxy inside pyramidal pits, served as model systems for the study. Different degrees of linear polarization (DOLP) of emitted light, depending on the confinement potential profile, the specific excitonic transition, and the level of excitation, are observed. A theoretical model shows that, besides the impact of valence band mixing, the overlap of conduction and valence band wavefunctions as well as state occupation probability and broadening of transitions determine the DOLP. The conclusions are useful for the design of quantum light emitters with controlled polarization properties.