Band States Research Articles

Ab-initio atomistic insights into lead-free perovskites for Photovoltaics Technology

The primary objective of this manuscript is to enhance knowledge and comprehension of hexagonal perovskites KNiX3 (X=I, Br). The world is fascinated by halide perovskites because of their advancements in opto-electronic applications, particularly in the photovoltaic field. KNiI3 and KNiBr3 halide perovskites correspond to the two anion I and Br. Here we aim to investigate how opto-electronic applications are affected by the substitution of cations and anion. Within the framework of density functional theory (DFT), the structural, electronic, and optical properties have been computed using the PBE GGA approximation. Br-based perovskites have a shorter bond length than I-based compounds because iodine ions have a larger ionic radius than bromine ions. Thus, the lattice constant increases from I-based potassium halide perovskites to Br-based potassium halide perovskites. The values of the lattice constant match the results of experimental computations. The Density of States (DOS) curves and band structure are used to examine the contribution of the bands. From Br to I, there is a reduction in the band gap, indicating an improvement in photovoltaic applications. Various energy ranges are used to calculate the optical spectra, dielectric function, absorption coefficient, optical reflectivity, and refractive index. KNiX3 (X=I, Br) compound reveals low reflection value which is typically an excellent choice for photovoltaic and opto-electronic applications.

Generation of broadband indistinguishable two-photon state in the telecom band

Generation of broadband indistinguishable two-photon state in the telecom band

Fe-N Co-Doped BiVO4 Photoanode with Record Photocurrent for Water Oxidation.

Bismuth vanadate (BVO) ranks among the most promising photoanodes for photoelectrochemical (PEC) water splitting. Nonetheless, slow charge separation and transport, besides the sluggish water oxidation kinetics, are key barriers to its photoefficiency. Here, we present a co-doping strategy that significantly improves the charge separation performance of BVO photoanodes. We found that, under standard one sun illumination, the Fe-N co-doped BVO photoanode (Fe-N-BVO) by N-coordinated Fe precursor reaches a record photocurrent density of 7.01 mA cm-2 at 1.23 V vs RHE after modified a surface co-catalyst (FeNiOOH), and exhibits an outstanding stability. By contrast, much lower photocurrent density is obtained for the N-doped, Fe-doped and Fe/N-doped BVO photoanode with separated N and Fe precursors. The detailed experimental characterizations show that the high activity of the Fe-N co-doped BVO photoanode is attributed to the enhanced photo-induced bulk charge separation, as well as the accelerated surface water oxidation kinetics. XPS, EXAFS and DFT calculations clearly show that, instead of formation of deep trapping state in the individually doped BVO, the co-doping of Fe-N into BVO generates Fe-based electronic states just below the bottom of conduction band and N-derived states just above the top of valence band. Such modulations in electronic structure enable the efficient trap of the electrons and holes to enhance the separation of photo-induced carriers, but hinder the charge recombination originated from the deep trapping sites.

Bismuth-based ternary chalcogenides Pt3Bi4X9 (X = S, Se) as promising thermoelectric materials

We present a theoretical investigation of thermoelectric transport properties of bismuth-based ternary chalcogenides Pt3Bi4X9 (X = S, Se), which has low thermal conductivity and promising zT as discovered in a recent experiment [Wang et al., J. Am. Chem. Soc. 146, 7352 (2024)], using density functional theory combined with the Boltzmann transport equation within rigid band approximation. We find that the high density of states of valence bands near the Fermi level yields high p-type Seebeck coefficient. The lower effective mass of electron in Pt3Bi4S9 leads to high mobility and long relaxation time, and hence the high n-type electrical conductivity. In contrast, the effective mass of electron is much higher than that of hole in Pt3Bi4Se9 due to the flatted conduction band, which in turn gives rise to higher p-type electrical conductivity. As a result, the p-type zT is much higher than n-type in Pt3Bi4Se9, with an optimal value of 0.5 at 300 K. Considering the experimental carrier concentration for Pt3Bi4S9 (−1.4 × 1019 cm−3) and Pt3Bi4Se9 (−0.898 × 1019 cm−3), calculated n-type zT at 773 K are 0.52 and 0.04, respectively, which are consistent well with the experimental values. Our calculations uncover the upper limit thermoelectric zT of Pt3Bi4X9 and also highlight them as promising thermoelectric materials.

Electronic Structure Of Cobalt Phosphates Co1-xmxpo4 Doped With Iron And Nickel Atoms

In this research, the electronic states, band structures, and bond properties of the framework compounds of CoPO4, Co1-xFexPO4, and Co1–xNixPO4 were investigated by the density functional theory calculations. The potential capabilities of these systems in the photocatalytic water splitting to produce hydrogen were analyzed. The spin-up electron densities of states for the CoPO4, Co1–xFexPO4, and Co1–xNixPO4 systems have band gaps of 2.7, 3.4, and 3.45 eV, respectively. The band of spin-down electron states has several energy gaps above the Fermi level. The density of states of electron with spin up near the Fermi level is obviously greater than that of electrons with spin down. In this case, localized states of electrons appear in the band gap of doped semiconductors due to impurity atoms. The calculated value of the energy at the lower edge of the conduction band for CoPO4 was –0.7 eV, which is more negative than the energy required for water splitting. Meanwhile, the calculated value of the energy at the upper edge of the valence band was 2.01 eV, which is more positive than the oxygen evolution energy of 1.23 eV.

Quantum geometrical properties of topological materials

The momentum space of topological insulators and topological superconductors is equipped with a quantum metric defined from the overlap of neighboring valence band states or quasihole states. We investigate the quantum geometrical properties of these materials within the framework of Dirac models and differential geometry. Their momentum space is found to be always a maximally symmetric space with a constant Ricci scalar, and the vacuum Einstein equation is satisfied in 3D with a finite cosmological constant. For linear Dirac models, several geometrical properties are found to be independent of the band gap, including a peculiar straight line geodesic, constant volume of the curved momentum space, and the exponential decay form of the nonlocal topological marker, indicating the peculiar yet universal quantum geometrical properties of these models.

Generation of squeezed vacuum state in the millihertz frequency band

The detection of gravitational waves has ushered in a new era of observing the universe. Quantum resource advantages offer significant enhancements to the sensitivity of gravitational wave observatories. While squeezed states for ground-based gravitational wave detection have received marked attention, the generation of squeezed states suitable for mid-to-low-frequency detection has remained unexplored. To address the gap in squeezed state optical fields at ultra-low frequencies, we report on the first direct observation of a squeezed vacuum field until Fourier frequency of 4 millihertz with the quantum noise reduction of up to 8.0 dB, by the employment of a multiple noise suppression scheme. Our work provides quantum resources for future gravitational wave observatories, facilitating the development of quantum precision measurement.

Anisotropic electronic structure study of MgB2C2 using soft X-ray emission 0spectroscopy microscopes.

The anisotropic electronic structure of MgB2C2 was studied using soft X-ray emission spectroscopy electron microscopes. MgB2C2 fragments were selected by examining C K-emission profiles. C and B K-emission and Mg L-emission spectra were obtained, revealing common and distinct structures that reflect the mixing of valence orbitals. Since the material is reported to have two-dimensional B-C honeycomb layers, the orientational dependence of these emission spectra was also examined. Experimental data were compared with theoretically calculated partial density of states of the valence bands of the material. The C K-emission profile showed an apparent orientational dependence, while the B K-emission exhibited minimal dependence. This difference originated from the different energy distributions of C-2pz and B-2pz components in the valence bands. The Mg L-emission intensity was very small, likely due to charge transfer from Mg atoms to B-N layers. The Mg L-emission profile showed a peak related to structures in C-K and B-K. An unexpected intensity was observed just above the valence bands, which also showed orientational dependence, possibly due to a small deviation from the ideal composition of Mg:B:C = 1:2:2.

Excited State Band Mapping and Ultrafast Nonequilibrium Dynamics in Topological Dirac Semimetal 1T-ZrTe2.

We performed time- and polarization-resolved extreme ultraviolet momentum microscopy on the topological Dirac semimetal candidate 1T-ZrTe2. Excited state band mapping uncovers the previously inaccessible linear dispersion of the Dirac cone above the Fermi level. We study the orbital texture of bands using linear dichroism in photoelectron angular distributions. These observations provide hints about the topological character of 1T-ZrTe2. Time-, energy-, and momentum-resolved nonequilibrium carrier dynamics reveal that intra- and interband scattering processes play a major role in the relaxation mechanism, leading to multivalley electron-hole accumulation near the Fermi level. We also show that electrons' inverse lifetime has a linear dependence as a function of their excess energy. Our time- and polarization-resolved XUV photoemission results shed light on the excited state electronic structure of 1T-ZrTe2 and provide valuable insights into the relatively unexplored field of quantum-state-resolved ultrafast dynamics in 3D topological Dirac semimetals.

Buckling strain effects on the electron and the phonon spectra, the stability, thermal, and optical characteristics of an MgO nanosheet using PBE/GGA and HSE06

The stability, the electronic, the phonon, the thermal, and the optical properties of a buckled MgO monolayer are investigated using DFT with the PBE/GGA and the HSE06 functionals. Model calculations for a buckled MgO monolayer indicate a more interesting behavior than for its flat phase. The band gap decreases from 4.9 to 2.83 eV within the HSE06 approach due to increased electron density of states in the conduction band in the direction of the buckling. Both phonon dispersion and molecular dynamic calculations confirm the dynamic and the thermal stability of the buckled MgO monolayer. The heat capacity of a buckled monolayer is higher than for the flat one in a high temperature range due to a higher phonon density of states caused by the buckling effects. Furthermore, the buckled MgO monolayer has a higher static dielectric function leading to an increased ability to store electrical energy. The buckling creates wrinkles at the surface of the MgO monolayer that can localize collective electron oscillations, resulting in suppression of the plasmon frequency. The main intensity peak in the optical conductivity is red-shifted from the near-ultraviolet to the visible light region in the presence of buckling which may be beneficial for optoelectronic devices.

Controlling the luminescence of CdSe quantum dots in the fluorinephosphate glass

A series of fluorophosphate glasses with CdSe and CdSe+ZnSe are successfully synthesized. After heat treatment above the glass transition temperature, CdSe quantum dots with sizes 1.5–5.5 nm are formed. The luminescence spectra of CdSe quantum dots are analyzed at various temperatures, excitation energies and chemical composition of glass. The influence of selenium content on the growth features of quantum dots is shown. The size dependence of luminescence quantum yield on the QDs size is found. The 2.0–2.5 nm-sized QDs demonstrate trap emission in the region of 1.7–1.85 eV with a maximum quantum yield of 60 %. It is found that only a broad band due to the transition from the low state of the conduction band and the shallow donor trap states to the deep accepter trap levels is observed. This broad emission band is observed at any selenium cadmium ratio and zinc introduction.

Structural, optoelectronic, and magnetic properties of Q‑carbon studied by hybrid density functional theory ab initio calculations and experiment

This work presents a theoretical study of the structural, optoelectronic, and magnetic properties of Q‑carbon, utilizing hybrid functionals within density functional theory (DFT) ab initio calculations. Moreover, experiments were conducted to measure structural parameters, dielectric functions, and magnetic properties, using various techniques. From the DFT simulations, structural, electronic, and optical properties have been simulated. The band gap energy was calculated using a hybrid approach, which combine HSE functional with different values of exact Hartree-Fock (HF) exchange (α). We propose a theoretical investigation of cubic and quasi-cubic forms of Q‑carbon. Our findings indicate that a non-cubic Q‑carbon cell is energetically more stable and possesses higher cell mass density and bulk modulus compared to a cubic cell. Calculations indicated that Q‑carbon exhibits semiconductor behavior; an indirect band gap of 3.10 eV and a direct band gap of about 3.4 eV was obtained for α = 0.44, in good agreement with experimental values. The density of states analysis allowed us to identify the p orbital of sp2 hybridized atoms as being prevalent on the top of the valence band and in the lowest energy conduction band states. Magnetic moment values of 0.37 μB and 0.41μB was found in two sp2 bonded C atoms, in accordance with the experimental observation. Several optical properties were calculated. The theoretical findings are compatible with the experimental results shown here and available in the literature, with excellent agreement found between the calculated optical band gap and that obtained from absorption measurements.

Annealing influence on stoichiometry and band alignment of 4H-SiC/SiO2 interface evaluated by x-ray photoelectron spectroscopy

Abstract Post oxidation annealing (POA) is a crucial technique for enhancing the performance of SiC Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs). This study investigates the impact of nitrogen-based POA on the 4H-SiC/SiO2 interface, utilizing X-ray photoelectron spectroscopy (XPS) to assess changes in stoichiometry and band alignment. We discovered that high-temperature nitrogen POA significantly refines the interface quality, shifting the SiOxCy binding energy from 101.3 eV (at 400°C) to 102.1 eV (at 1150°C) and reducing the C:Si ratio from 1.120 (at 400°C) to 0.972 (at 1150°C), indicating reoxidation and transition from C-rich interface to Si-rich interface. Despite improvements, the conduction band offset at the interface, decreases from 2.59 eV to 1.62 eV with increasing annealing temperature, suggesting a higher likelihood of electron tunneling. This finding underscores the necessity of evaluating band offsets introduced by POA to ensure the reliability of SiC MOSFETs. Additionally, excessive Ar ion etching introduces residual Ar and surface charges, causing band bending and an increased density of states in the valence band of the 4H-SiC substrate.

Conduction band electronic states of ultrathin furan-phenylene co-oligomer on the surfaces of oxidized silicon and of layer-by-layer grown zinc oxide

The paper reports on results of an investigation of the electronic states of the conduction band of ultrathin films of furan-phenylene co-oligomer 1,4-bis(5-phenylfuran-2-yl)benzene (FP5) and the results of an investigation of the interfacial potential barrier upon the formation of these films on the surfaces of (SiO2)n-Si and of layer-by-layer deposited ZnO. Upon deposition of an 8–10 nm thick FP5 film, the total current spectroscopy (TCS) technique was used for investigation within the energy range from 5 eV to 20 eV above EF. FP5 films on the (SiO2)n-Si surface showed a domain structure with a characteristic domain size of the order of 1 micro.m × 1 micro.m and a surface roughness within the domain under 1 nm. In contrast, FP5 on the ZnO surface showed a granular structure with a grain height of 40–50 nm.

Transport and electronic structure properties of MBE grown Sn doped Ga2O3 homo-epitaxial films

In this work, we report the transport, defect state and electronic structure properties of unintentionally doped (UID) and Sn doped β-Ga2O3 homo-epitaxial thin films grown by molecular beam epitaxy (MBE) with electron density ranging from 2.1 × 1016 to 2.8 × 1019 cm−3. The UID film with an electron density of 2.1 × 1016 cm−3 exhibits a notable RT mobility of 129 cm2/Vs and a peak mobility of 900 cm2/Vs at 80 K, achieving the state-of-the-art level for MBE-grown Ga2O3 films. Temperature dependent Hall measurement reveal that Sn dopants have an activation energy of 56.7 meV. Synchrotron-based photoemission spectroscopy were further used to study insights into the evolution of electronic properties induced by Sn doping. An in-gap defect state was observed at the 1.5 eV above the valence band maximum for the Sn-doped Ga2O3 film. The in-gap state acts as self-compensating centers affecting the overall doping efficiency and mobility. Furthermore, photoemission spectroscopic study also reveals an upward surface band bending existing at the surface region of Sn doped Ga2O3 films. The identification of the in-gap state and surface upward band bending have significant implications for understanding the doping mechanisms in Ga2O3 and its electronic device applications.

Synergistic Effects of Defects and Strain on Photoluminescence in Van der Waals Layered Crystal AgScP2S6${\rm AgScP_{2}S_{6}}$

AbstractMetal thiophosphates (MTPs) are a large family of 2D materials that exhibit large structural and chemical diversity. They also show promise for applications in energy harvesting and photodetection. Strain and defect engineering have previously been demonstrated as useful mechanisms to tune several properties of MTPs such as resistivity, magnetic state, and electronic band gap. However, the effect of these stimuli on engineering tunable light emission in MTPs remains unexplored. Here, it is experimentally demonstrated that sulfur vacancies in metal thiophosphate allow defect‐state‐to‐valence‐band transitions leading to visible light emission at sub‐bandgap energies, while structural defects in the material can enhance and modulate the photoluminescence spectrum. It is also showed that the structural‐defect‐enhanced light emission can be further tuned by temperature‐induced strain gradients.

Deciphering Pauling's Third Rule: Uncovering Strong Anharmonicity and Exceptionally Low Thermal Conductivity in TlAgSe for Thermoelectrics.

The elucidation of chemical bonding, coupled with an exploration of the correlated dynamics of constituent atoms, is essential for unravelling the underlying mechanism responsible for low lattice thermal conductivity (κL) exhibited by a crystalline solid, which is essential for thermoelectrics and thermal barrier coatings. In this regard, Pauling's third empirical rule, which deals with the cationic repulsion due to proximity in the face or edge shared polyhedra in a crystal structure, can bring about the lattice instability required to suppress the κL. Here, we demonstrate the presence of such instability in a ternary selenide, TlAgSe, leading to a ultra-low κL of 0.17 W/m.K at 573 K. Our study reveals the instability arising from Ag-Ag repulsion within edge-shared AgSe4 tetrahedra through investigation of the local structure using synchrotron X-ray pair distribution function (PDF) analysis and supported by first-principles density functional theory calculations. We observe correlation between weakening in the Ag and the Tl-sublattice, providing direct experimental evidence of Pauling's third empirical rule. The correlated rattling of Ag and Tl induces a highly anharmonic lattice and low energy optical phonons, resulting in suppressed sound velocity and ultralow κL in TlAgSe. The electronic origin of soft and anharmonic lattice is the presence of filled antibonding states in the valence band near the Fermi level constructed by Ag(4d)-Se(4p) and Tl(6s)-Se(4p) interactions. This work demonstrates that the evidence of dynamic distortion in a crystal lattice is governed by the third empirical rule given by Pauling, which can act as a potential new strategy for diminishing κL in crystalline solids.

Deciphering Pauling's Third Rule: Uncovering Strong Anharmonicity and Exceptionally Low Thermal Conductivity in TlAgSe for Thermoelectrics

AbstractThe elucidation of chemical bonding, coupled with an exploration of the correlated dynamics of constituent atoms, is essential for unravelling the underlying mechanism responsible for low lattice thermal conductivity (κL) exhibited by a crystalline solid, which is essential for thermoelectrics and thermal barrier coatings. In this regard, Pauling's third empirical rule, which deals with the cationic repulsion due to proximity in the face or edge shared polyhedra in a crystal structure, can bring about the lattice instability required to suppress the κL. Here, we demonstrate the presence of such instability in a ternary selenide, TlAgSe, leading to a ultra‐low κL of 0.17 W/m.K at 573 K. Our study reveals the instability arising from Ag−Ag repulsion within edge‐shared AgSe4 tetrahedra through investigation of the local structure using synchrotron X‐ray pair distribution function (PDF) analysis and supported by first‐principles density functional theory calculations. We observe correlation between weakening in the Ag and the Tl‐sublattice, providing direct experimental evidence of Pauling's third empirical rule. The correlated rattling of Ag and Tl induces a highly anharmonic lattice and low energy optical phonons, resulting in suppressed sound velocity and ultralow κL in TlAgSe. The electronic origin of soft and anharmonic lattice is the presence of filled antibonding states in the valence band near the Fermi level constructed by Ag(4d)−Se(4p) and Tl(6s)−Se(4p) interactions. This work demonstrates that the evidence of dynamic distortion in a crystal lattice is governed by the third empirical rule given by Pauling, which can act as a potential new strategy for diminishing κL in crystalline solids.

Role of local structure in the optical and electronic properties of oxygen vacancies in different crystal phases of Ga2O3

We investigate the structural, electronic, and optical properties of oxygen vacancies (VO) in crystalline α, β, and ε−Ga2O3 using density functional theory (DFT) calculations with the PBE0-TC-LRC functional. Our results reveal that the charge transition levels (CTLs) associated with VO exhibit significant variations depending on the crystal phase and the coordination environment of surrounding atoms. In particular, VOs surrounded by tetrahedral Ga atoms (T-Ga) exhibit deeper CTLs compared to those surrounded by octahedral Ga atoms (O-Ga). We also observe distinct atomic relaxations, with larger displacements of T-Ga atoms compared to O-Ga atoms in the vicinity of VOs. Using linear-response time-dependent DFT, we investigate the optical transitions of VO and identify two distinct types of transitions: defect state to conduction band state and valence band to defect state. These results can be used to better understand the optical properties of VO defects in Ga2O3 films. Published by the American Physical Society 2024

Influence of gadolinium substitution on the crystal structure of NiO and its maximized photocatalytic degradation activity on tetracycline and direct yellow pollutants

Gadolinium-substituted NiO crystal structures with excellent stability were utilized for the first time to degrade antibiotic and organic dye pollutants. A simple hydrothermal technique was used to synthesize different concentrations of Gd3+ substituted NiO, and the catalysts were thoroughly characterized using sophisticated techniques to explore the influence of Gd3+ on the crystal structure and optical characteristics of NiO. The 1.5% Gd3+ substituted NiO showed outstanding photocatalytic activity on tetracycline and direct yellow degradation, with percentages of 90.72 and 95.5%, respectively. The improvement of photocatalytic degradation efficiency is primarily due to the suppression of photoinduced electron and hole recombination via the creation of Gd3+ as the inner energy band state below the conduction band. The recycling experiments revealed that the catalyst is more stable, with no indication of loss after ten cycles. Scavenging investigations also demonstrated that superoxide and holes were responsible for the efficient degradation of pollutants.