Determination Of Band Structure Research Articles

High-energy photoemission final states beyond the free-electron approximation

Three-dimensional (3D) electronic band structure is fundamental for understanding a vast diversity of physical phenomena in solid-state systems, including topological phases, interlayer interactions in van der Waals materials, dimensionality-driven phase transitions, etc. Interpretation of ARPES data in terms of 3D electron dispersions is commonly based on the free-electron approximation for the photoemission final states. Our soft-X-ray ARPES data on Ag metal reveals, however, that even at high excitation energies the final states can be a way more complex, incorporating several Bloch waves with different out-of-plane momenta. Such multiband final states manifest themselves as a complex structure and added broadening of the spectral peaks from 3D electron states. We analyse the origins of this phenomenon, and trace it to other materials such as Si and GaN. Our findings are essential for accurate determination of the 3D band structure over a wide range of materials and excitation energies in the ARPES experiment.

Determination of electronic band structure of quaternary ferromagnetic Ga0.97-yMn0.03CryAs epitaxial layers

Introducing transition metals to conventional III-V semiconductors anomalously changes their fundamental characteristics, such as electronic, magnetic, and structural properties. In this study, we show that the valence band anti-crossing (VBAC) model can be exploited to calculate the electronic band structure of the quaternary Ga0.97-xMn0.03CrxAs epitaxial layers. In this model, the localized Mn and Cr defect states interact with the valence band states (VB), reconstructing VBs and splitting each VB state. The splitting top of the VB state forms an impurity band (IB) and fundamental VB edge. Photomodulated reflectance (PR) spectroscopy is exploited to determine optical transition energies at room temperature. PR spectra were analyzed with the third derivative functional form (TDFF) signal's line shape. The experimental optical transition energies, including band-to-band and spin split-off band transitions, match the calculated optical transition energies by the VBAC model. In the calculation, the interaction energy between localized Mn/Cr-energy level and valence band edges is experimentally determined as 0.7 eV.

Fabrication of 2D/1D Bi2WO6/C3N5 heterojunctions for efficient antibiotics removal

Photocatalysis is a hopeful way to treat antibiotics-containing wastewater, but its practical application is impeded by the lack of high-performance photocatalysts for now. In the present work, a 2D/1D Bi 2 WO 6 /C 3 N 5 heterojunction photocatalyst was fabricated through a hydrothermal route, and applied to the photocatalytic degradation of tetracycline hydrochloride (TCH). The Bi 2 WO 6 /C 3 N 5 nanocomposites showed superior catalytic activity, compared with a single counterpart. The apparent TCH degradation kinetic constant achieved by the optimized Bi 2 WO 6 /C 3 N 5 nanocomposite was 7.7 times that achieved by Bi 2 WO 6 and 262 times that achieved by C 3 N 5 . Moreover, the influences of the dosage of Bi 2 WO 6 /C 3 N 5 , the initial concentration and pH of TCH solution, and the presence of anions on the photocatalytic TCH degradation were investigated. In addition, the TCH degradation pathway was studied. The collaborative results of X-ray photoelectron spectra, band structure determination, active species capturing experiments, and spin-trapping electron paramagnetic resonance spectra revealed a direct Z -scheme photocatalytic mechanism of Bi 2 WO 6 /C 3 N 5 . • 2D/1D Bi 2 WO 6 /C 3 N 5 heterostructures were successfully fabricated for the first time. • The apparent kinetic constant of the optimized sample was greatly enhanced as compared to a single component. • Factors affecting the tetracycline hydrochloride photodegradation were investigated. • The separation of the photoinduced charge carriers was efficiently promoted. • A possible direct Z-scheme photocatalytic mechanism was proposed.

Eu3+-activated SrY2O4:Ce4+/3+ red-phosphor for WLEDs: Structural and luminescent insights from experimental and theoretical approaches

Herein, single-phase Eu3+-activated SrY2O4:Ce4+/3+ red-emitting phosphor was produced by an adapted Pechini route under softer synthesis conditions (1100 °C in CO reducing atmosphere) compared to those reported so far. A detailed structural study conducted by Rietveld refinement and DFT calculations confirmed the presence of two hexa-coordinated Y3+ sites that are occupied by Eu3+. Cerium is present in a mixed oxidation state, where the proportion of Ce4+/Ce3+ was determined as 70/30% by X-ray photoelectron spectroscopy (XPS), and this influences the SrY2O4 phase stabilization by increasing the point defect concentration. A thorough analysis of the photoluminescent features of Eu3+-activated phosphor revealed its occupancy in at least two non-equivalent local sites belonging to the C1 point group. Furthermore, the excited state dynamics of the phosphors were enlightened by the theoretical band structure determination. Finally, 3%-doped sample exhibited the highest intrinsic quantum yield (81%), which qualifies these materials to be used as red-emitting components for solid-state lighting.

Activation of peroxymonosulfate by S-scheme Bi2S3/doped gCN heterostructure photocatalyst for highly efficient visible light driven tetracycline degradation: Insights into reaction mechanisms

A solid state microwave (MW) assisted hydrothermal method was used to prepare S-scheme heterojunction composite Bi2S3@doped gCN (BS@CN). TC was degraded upto 99.9 % under 20 min visible light, with synergistic action of 0.2 g/L catalyst having 10 wt% Bi2S3 (BS(10)@CN) and 1.5 g/L PMS, for initial TC concentration of 50 mg/L, exhibiting first order rate constant (kapp): 0.215 min−1. The modulation of band-structure due to the generation of heterojunction, coupled with different surface-bound redox cycles, i.e.,Co2 +/Co3 +surf.,Ce3 +/Ce4 +surf. andBi3 +/Bi4 +surf., helped the charge migration, separation of excited e-/h+ and production of reactive species, like,SO4∙-,∙OH,O2∙-,O21, etc., that played important roles towards TC degradation. Band-structure determination, XPS analysis and other characterizations revealed the formation of S-scheme configuration and corresponding surface-bound metastable complexes towards self-regeneration and excellent catalytic activity of the composite, even in the contaminated real-life surface water sources. Extensive intermediate analysis was carried out to develop a detailed TC degradation pathway. The BS@CN photocatalyst can be recycled multiple times, without significant loss of photocatalytic activity (∼88 % TC degradation after 5 cycles). This study demonstrated the design and fabrication of metal oxide doped gCN/metal chalcogenide-based heterojunction photocatalysts that have potential applications towards the removal of refractory contaminants in aqueous medium.

BinPo: An open-source code to compute the band structure of two-dimensional electron systems

We introduce BinPo, an open-source Python code to compute electronic properties of two-dimensional electron systems. A bulk tight binding Hamiltonian is constructed from relativistic density functional theory calculations represented in the basis of maximally localized Wannier functions. BinPo has a Schrödinger-Poisson solver, integrating an electric field-dependent relative permittivity to obtain self-consistently the confining electrostatic potential energy term in the derived tight binding slab system. The band structure, energy slices, and other properties, along with different projections and orientations can be computed. High resolution and publishable figures of the simulations can be generated. In BinPo, priority has been given to ease-of-use, efficiency, readability and modularity, therefore becoming suitable to produce reliable electronic structures simulations at low computational cost. Along with the code itself, we provide files from first-principles calculations for some materials, instructions of use, and detailed examples of its wide range of capabilities. The code was developed with a focus on the ABO3 perovskite structure-based systems, such as SrTiO3 and KTaO3, because of their increasing impact in the materials community. Some features, such as the projection onto orbital states, are restricted to calculations using the relevant t2g orbitals for this family of materials, yet it is possible to include more elements in the basis for the band structure determination of other systems. The use of a relativistic approach allows for the inspection of the role of spin-orbit coupling and the resulting Rashba effect on the systems. We detail the approaches used in the code, so that it can be further exploited and adapted to other problems, such as adding new materials and functionalities which can strength the initial code scopes. Program summaryProgram title: BinPoCPC Library link to program files:https://doi.org/10.17632/zk4xy7vfy4.1Developer's repository link:https://github.com/emanuelm33/BinPoCode Ocean capsule:https://codeocean.com/capsule/0987578Licensing provisions: GPLv3Programming language: PythonExternal routines/libraries: NumPy, SciPy, Matplotlib, ASE.Nature of problem: Compute the band structure and other electronic properties in two-dimensional electron systems.Solution method: Construction of a slab Hamiltonian from one built using a basis of maximally localized Wannier functions. Solve the Schrödinger-Poisson scheme to get the self-consistent potential energy along the slab. Use this potential to compute the band structure and additional properties by solving the Schrödinger equation.Additional comments including restrictions and unusual features: Other features included are the visualization of the self-consistent process and final solutions, band structure and energy slices with different projections. High resolution and customizable figures can be obtained in all cases.

Sodium and potassium ion rich ferroelectric solid electrolytes for traditional and electrode-less structural batteries

The transition to a sustainable society is vital and requires electrification. Sodium and potassium ion-based electrolytes will likely play an important role in energy storage as these elements are very abundant. The latter cations and chloride are especially interesting since life on the planet is somehow based on biological transfers of these ions through cell membranes. K+ is the key charge carrier in plants. Here, we characterize electrochemically, electrostatically, and structurally novel electrolytes, K3ClO and K2.99Ba0.005ClO, and compare their performance with Na3ClO and Na2.99Ba0.005ClO in symmetric and asymmetric structural electrode-less cells, such as K/K2.99Ba0.005ClO in a cellulose membrane/K, Na/Na2.99Ba0.005ClO in a cellulose membrane/Na, Al/K2.99Ba0.005ClO composite/Cu, and Al/Na2.99Ba0.005ClO composite/Cu, at temperatures that range from −45 to 65 °C. An ab initio molecular dynamics structural study followed by band structure determination using density functional theory and hybrid simulations allowed us to compare the amorphous character of the structures, bandgap, and electron localization function for both K3ClO at 25 °C and Na3ClO at 37 °C, temperatures at which preliminary studies indicate that these compounds are already amorphous. As in Na+-based electrolytes, the ferroelectric character of the K+-based electrolytes is well recognizable, especially at −45 °C, where the relative real permittivity achieves 1013 in K/K2.99Ba0.005ClO in cellulose membrane/K symmetric cells for an ionic conductivity of ∼120 mS/cm. As in Na+-based electrodes-less structural battery cells, self-charge and self-cycling phenomena are also demonstrated reinforcing the ferroelectric nature of the A3ClO (A = Li, Na, and K) family of electrolytes. These studies may contribute to understanding the K+ and Na+ transfer behavior in energy harvesting and storage as well as the biologic world.

Determination of Band Structure of Naturally Occurring Goethite with Al Substitution: A Case Study of Zhushan Iron Zone.

The photocatalytic property of Fe oxide minerals has long been considered to play an important role in shaping modern terrestrial environments. However, due to the complexity of natural settings, a precise determination of the band structure of natural goethite has not been achieved. In this work, the mineralogical characteristics of natural goethite samples obtained from Zhushan, China, were systematically studied through X-ray diffraction, transmission electron microscopy, X-ray energy dispersive spectroscopy, and X-ray fluorescence spectroscopy. Afterward, the band structure for both natural and synthetic goethite samples was determined by synchrotron-based X-ray absorption and emission spectra and photoelectron spectroscopy. The band gap of natural goethite (2.25 eV) was narrower than that of its synthetic counterpart (2.55 eV), and the valence band position of natural goethite was slightly lifted (−5.06 eV) compared to that of synthetic goethite (−5.38 eV). Al doping in natural goethite crystal, as revealed by the mineralogical tests, was the main reason that contributed to this difference. The theoretical calculation showed the narrowed band gap was caused by the contribution of Al-2p orbits at the top of the valence band. Therefore, free electrons can be created under light irradiation with a shorter wavelength. The experiments showed that natural goethite can photo-catalytically degrade methyl orange, and the degradation efficiency was better (47.5%) than that of the synthetic goethite group (31.5%). This study, for the first time, revealed the band structure and confirmed the photocatalytic properties of natural goethite, which should play an important role in surface substance evolution and elemental cycling.

Breakdown of the ionization potential theorem of density functional theory in mesoscopic systems.

The ionization potential (IP)-theorem of Kohn-Sham (KS) density functional theory (DFT) states that the energy of the highest occupied molecular orbital (HOMO) ϵHOMO equals the negative of the first IP, thus ascribing a physical meaning to one of the eigenvalues of the KS Hamiltonian. We scrutinize the fact that the validity of the IP-theorem relies critically on the electron density n(r), far from the system, to be determined by HOMO only, behaving as n(r)∼r→∞e-2-2ϵHOMOr. While this behavior always holds for finite systems, it does not hold for mesoscopic ones, such as quasi-two-dimensional (Q2D) electron gas or Q2D crystals. We show that this leads to the violation of the IP-theorem for the latter class of systems. This finding has a strong bearing on the role of the KS valence band with respect to the work-function problem in the mesoscopic case. Based on our results, we introduce a concept of the IP band structure as an observable alternative to its unphysical KS counterpart. A practical method of the determination of the IP band structure in terms of DFT quantities is provided.

The Advantage of Nanowire Configuration in Band Structure Determination (Adv. Funct. Mater. 41/2021)

Band Structure Determination In article number 2105426, Mirjana Dimitrievska, Anna Fontcuberta i Morral, and co-workers demonstrate how nanowires can be effectively used for direct determination of the band structure by means of valence electron energy-loss spectroscopy. This work opens the path for characterizing the band structure of new compounds in a non-destructive and facile manner, strengthening the route of new materials discovery.

A systematic computational study to understand the effect of metals (Mg, Ca, Sr) doping and external isotropic static pressure on phase stability, electronic band structure and optical properties of KNbO3

The phase stability, electronic energy-band structure, and their impact on optical properties of KNbO3in tetragonal phase have been studied, using the ultra-soft pseudopotential method within the generalized gradient approximation for exchange and correlation, with metals (Mg, Ca, Sr) doping and external isotropic pressure (5GPa-50GPa). The band structure shows that the O-2p and Nb-4d orbital electrons play a vital role in the development of upper valence band and band gap energy almost increases linearly with doping, except Mg, and by increasing the pressure. Analysis of DOS describes ionic, K and NbO3and covalent, Nb and O, bonding. In order to understand the influence of doping and external pressure on optical properties of the KNbO3,the dielectric function real and imaginary parts, absorption coefficient, reflectivity, refractive index, loss function and extinction coefficient were computed. Variations in optical spectra profiles were analysed and discussed with electronic band structure determination.

Fabrication of ZnO/Au@Cu2O heterojunction towards deeply oxidative photodegradation of organic dyes

Fabrication of heterojunction is an effective way of improving the photocatalytic performance of semiconductor in the photocatalytic wastewater treatment. ZnO/Au@Cu2O heterojunctions were successfully synthesised using Au nanoparticles loaded onto the surface of ZnO nanorods as heterostructural seeds to induce in situ growth of Cu2O shells. X-ray diffraction, transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, electrochemical workstation and UV–vis diffuse reflection spectrograph were employed to analyze the structures, compositions and photoelectric properties of the synthesised photocatalysts. After 45–90 min of the photocatalysis of ZnO/Au@Cu2O heterojunctions under simulated sunlight, the degradation efficiencies of methyl orange and rhodamine B could reach 93.0% and 99.6%, respectively, which were much higher than those with mono- and bi-component catalysts. The photocatalytic system with ZnO/Au@Cu2O can achieve deep mineralization to decompose organic dye molecules into non-toxic products. The formation of heterostructures not only could expand the spectral absorption range to the visible region but also efficiently facilitate charge separation to transfer more photo-generated holes and electrons to generate oxidative hydroxyl and superoxide radicals. A Z-scheme charge separation mechanism was proposed according to the determination of radical species and energy band structures of ZnO–Cu2O.

Effect of lattice polarisability in optical properties of methylammonium lead iodide by modified Becke-Johnson functionals and Bethe-Salpeter equation

In this work, we used a non-local functional proposed by Tran-Blaha (TB09) with an inclusion of spin-orbit coupling (SOC) in band structure determinations of cubic methylammonium lead iodide (MAPI). We found that the predicted band gap of 1.67 eV is in a good agreement with the experimentally reported values. Moreover, we incorporated TB09 and SOC into the random phase approximation (RPA), and also the Bethe-Salpeter equation (BSE) in order to calculate the dielectric function and the optical response. The BSE calculation significantly features the electron-hole interaction and excitons. However, there are some significant discrepancies between the calculated and the measured dielectric function, which could be explained by the inclusion of the lattice polarisability of MAPI.

Direct comparison of ARPES, STM, and quantum oscillation data for band structure determination in Sr2RhO4

Discrepancies in the low-energy quasiparticle dispersion extracted from angle-resolved photoemission, scanning tunneling spectroscopy, and quantum oscillation data are common and have long haunted the field of quantum matter physics. Here, we directly test the consistency of results from these three techniques by comparing data from the correlated metal Sr2RhO4. Using established schemes for the interpretation of the experimental data, we find good agreement for the Fermi surface topography and carrier effective masses. Hence, the apparent absence of such an agreement in other quantum materials, including the cuprates, suggests that the electronic states in these materials are of different, non-Fermi liquid-like nature. Finally, we discuss the potential and challenges in extracting carrier lifetimes from photoemission and quasiparticle interference data.

Transition Metal Doping of MoS2: A Correlated Experimental and Theoretical Study

Transition metal dichalcogenides (TMDs) are two-dimensional (2D) layered materials covalently bonded within the layers, but with only weak van-der-Waals (vdW) interactions between individual monolayers [1]. Substantial progress has been made in better understanding of these materials, from the nature of their defects [2] to the achievement of large area epitaxially grown films [3-5]. These materials have attracted great attention for applications such as next-generation electronics - including sub-60 mV sub-threshold slope transistors, flexible electronics, and optoelectronics, novel applications in spintronic devices [6] and the use of heterostructures of TMDs for tunnel field effect transistors (TFETs) [7]. However, issues including control of channel and source/drain doping have impeded their implementation into device. Similar to three-dimensional semiconductors, doping of the TMDs is required to modulate carrier concentration, to achieve Ohmic contacts, and to generate n-type and p-type materials which are required for complementary metal-oxide-semiconductor (CMOS) technology and TFET applications. One of the most studied TMDs is MoS2. The transition metals Nb and Re are two candidate dopants for MoS2 with theoretical results showing their suitability as p- and n-type dopants, respectively [8]. Experimental results have confirmed that Nb substitutes at the Mo-site and acts as a p-type dopant in MoS2 [9-11]. While Re has been confirmed as an n-type dopant, with the additional benefit of reducing sulfur vacancies and defect-related gap states [12-13].This study reports on the band structure and electrical characteristics of doped and unintentionally doped chemical vapor transport (CVT) grown MoS2 bulk crystals. We present a direct determination of the valence band structure of the MoS2 and the impact of transition metal doping (Nb and Re) using high-resolution angle-resolved photoemission spectroscopy (ARPES). Structural defects in the form of vacancies are widely known to strongly alter the MoS2 electronic structure. Therefore, we have performed highly-efficient density functional theory (DFT) based simulations to provide insight into the impact of vacancies in addition to the incorporation of transition metal dopants on the MoS2 band structure. Unfolded band structures obtained through our simulations [14], in comparison with the experimentally obtained occupied band structure of doped and un-doped MoS2 have shown excellent agreement and revealed that there has been significant distortion to the band structure due to the presence of vacancies, as well as the introduction of degenerate Nb-doping. Scanning tunneling microscopy (STM) studies revealed high quality crystals with point defects established by our first-principle calculations to be mainly Mo vacancies. We also report our Hall effect analysis to obtain the electrical metrics for the crystals. Secondary ion mass spectrometry (SIMS) shows the impurities present in the crystals, which is then used to explain the difference in transport behaviour and dopant types between similar crystals from different material sources.Figure 1. (a) Unfolded band structure of un-doped MoS2 with Mo vacancy obtained by DFT calculations shown using contour plot of total weight intensity. Black curve: primitive-cell of pristine MoS2 band structure. Mo vacancy induced localized states are located close to the valence band edge - shown by white rectangles. (b) ARPES spectra for a non-intentionally-doped MoS2 crystal acquired along the high symmetry Γ-K direction overlapped with DFT results showing excellent agreement considering the effect of a Mo vacancy. (c) DFT-obtained STM images of a pristine MoS2 (left), and Mo vacancy in MoS2 (right). (d) Charge density difference between pristine MoS2 and MoS2 with Mo vacancy. Red and blue indicate charge accumulation and depletion, respectively, at Mo vacancy sites.Figure 2. In situ STM, XPS and LEED measurements of MoS2. (a) Large-scale STM image with bright and dark defects (300×300 nm). (b) STM image (100×100 nm) with line profiles over the defects 1, 2, and 3. (c) STM image illustrates atomic resolution with interatomic distance 0.32 nm. (d) & (e) corresponds to the binding energies of Mo 3d and S 2s, and S 2p core levels, respectively. (f) LEED showing highly ordered structure.

Mechanically controllable nonlinear dielectrics.

Strain-sensitive Ba x Sr1-x TiO3 perovskite systems are widely used because of their superior nonlinear dielectric behaviors. In this research, new heterostructures including paraelectric Ba0.5Sr0.5TiO3 (BSTO) and ferroelectric BaTiO3 (BTO) materials were epitaxially fabricated on flexible muscovite substrate. Through simple bending, the application of mechanical force can regulate the dielectric constant of BSTO from -77 to 36% and the channel current of BTO-based ferroelectric field effect transistor by two orders. The detailed mechanism was studied through the exploration of phase transition and determination of band structure. In addition, the phase-field simulations were implemented to provide theoretical support. This research opens a new avenue for mechanically controllable components based on high-quality oxide heteroepitaxy.

Determination of electronic band structure by electron holography of etched-and-regrown interfaces in GaN p-i-n diodes

The electrostatic potential variation across etched-and-regrown GaN p-i-n diodes for power electronics has been studied using electron holography in a transmission electron microscope. The potential profiles have been correlated with the composition profiles of Mg, Si, and O obtained by secondary ion mass spectroscopy. Electronic charges obtained from the potential profiles correlate well with the presence of Si and O impurities at regrown interfaces. The overlap of Mg and Si when Mg doped GaN is grown directly over an etched undoped GaN surface results in the formation of a highly doped p-n junction. The introduction of a thin undoped layer over the etched GaN surface prevents the formation of such a junction as the regrowth interface is moved away from the Mg-doped GaN, and results in diodes with improved reverse leakage currents, close to the best values of continuously grown p-i-n diodes. Potential profiles of continuously grown (not etched) p-i-n diodes are compared to those of etched-and-regrown diodes.

The role of strong electron correlations in determination of band structure and charge distribution of transition metal dihalide monolayers

Electronic structure and magnetic properties of the family of first-row transition metal dihalides (TMHal2, TM = V, Cr, Mn, Fe, Co, Ni; H = Br, I) monolayers were studied by means of density functional theory. Strong electron correlations were taken into account by implementing Hubbard U correction in a simplified scheme proposed by Dudarev et al. (Ueff). Ueff correction essentially affects electronic structure of TMHal2 widening the band gap and witnessing their highly spin-polarized nature. Two different ligand orientations namely, H and T configurations of monolayers were considered. Unlike others, FeHal2 monolayers tend to form H structure when Ueff correction is included.

Simple and fast determination of band structure and level repulsion in a mixed-mode metamaterial

Advanced treatment of a reduced order model is shown to agree well with numerical results in a complex mixed-mode resonator array.

Electronic exchange-correlation, many-body effect issues on first-principles calculations of bulk SiC polytypes

The first-principles Projector-Augmented Wave method (PAW) is used to investigate the electronic, phonon band structure and dielectric properties of four bulk silicon carbide (SiC) polytypes. We employ PAW pseudopotential density functional theory with Perdew, Burke and Ernzerhof (PBE) and hybrid HSE06 approximations of the exchange-correlation functional. Many-body effects are incorporated using the GW approximation of the self-interaction to study SiC properties. GW method in its single-shot variant, which is based on the many-body perturbation theory (MBPT), is used to calculate the quasi-particle (QP) energies of the band structure and the dielectric properties for different polytypes. The electronic band structure determination within GW method uses the Wannier procedure where a basis set of maximally localized Wannier function (MLWF) is constructed to interpolate the QP energies of few regular mesh k-points to the high-symmetry lines in Brillouin zone. As a consequence of QP correction to the Kohn–Sham energies, bandgap is increased by upto 3 eV in case of 4H–SiC, as compared to PBE bandgap. GW results are comparable to those of hybrid functionals and are in good agreement with the experimental results. The optical properties are then studied within PBE, HSE06 and include many-body effects. In addition, the phonon band structure has been investigated within HSE06 and compared to previous PBE results. We found good agreement with the previous theoretical results and the experimental available data.