Bulk Electronic Properties Research Articles

Layer-dependent topological surface states in BiSb

The emergence of topological materials has changed the understanding of their electronic and magnetic properties. This work investigated the structural and electronic properties of bulk and a few layers of BiSb material. Interestingly, only the bulk and monolayer systems present semiconductor characteristics. From two monolayers onwards, a semi-metallic character is found. This semi-metallic behavior is due to valance and conduction bands crossing the Fermi level near the Γ point. These band crossings are related to topological surface states, which are achieved without inducing strain into the system, contrary to their bulk and monolayer counterparts. Defects, like deformations, vacancies, or doping, are expected not to affect the surface states. As a probe of concept, the biaxial strain was induced in BiSb surfaces, making no changes in the surface states. These robust surface states are desirable for constructing magnetic-topological junctions for new-generation solid-state storage applications.

Nanoscale Surface and Bulk Electronic Properties of Ti3C2Tx MXene Unraveled by Multimodal X-Ray Spectromicroscopy.

2D layered materials, such as transition metal carbides or nitrides, known as MXenes, offer an ideal platform to investigate charge transfer processes in confined environment, relevant for energy conversion and storage applications. Their rich surface chemistry plays an essential role in the pseudocapacitive behavior of MXenes. However, the local distribution of surface functional groups over single flakes and within few- or multilayered flakes remains unclear. In this work, scanning X-ray microscopy (SXM) is introduced with simultaneous transmission and electron yield detection, enabling multimodal nanoscale chemical imaging with bulk and surface sensitivity, respectively, of individual MXene flakes. The Ti chemical bonding environment is found to significantly vary between few-layered hydrofluoric acid-etched Ti3C2Tx MXenes and multilayered molten salt (MS)-etched Ti3C2Tx MXenes. Postmortem analysis of MS-etched Ti3C2Tx electrodes cycled in a Li-ion battery further illustrates that simultaneous bulk and surface chemical imaging using SXM offers a method well adapted to the characterization of the electrode-electrolyte interactions at the nanoscale.

Anisotropic structural, vibrational, electronic, optical, and elastic properties of single-layer hafnium pentatelluride: an ab initio study.

Motivated by the highly anisotropic nature of bulk hafnium pentatelluride (HfTe5), the structural, vibrational, electronic, optical, and elastic properties of single-layer two-dimensional (2D) HfTe5 were investigated by performing density functional theory (DFT)-based first-principles calculations. Total energy and geometry optimizations reveal that the 2D single-layer form of HfTe5 exhibits in-plane anisotropy. The phonon band structure shows dynamic stability of the free-standing layer and the predicted Raman spectrum displays seven characteristic Raman-active phonon peaks. In addition to its dynamic stability, HfTe5 is shown to exhibit thermal stability at room temperature, as confirmed by quantum molecular dynamics simulations. Moreover, the obtained elastic stiffness tensor elements indicate the mechanical stability of HfTe5 with its orientation-dependent soft nature. The electronic band structure calculations show the indirect-gap semiconducting behavior of HfTe5 with a narrow electronic band gap energy. The optical properties of HfTe5, in terms of its imaginary dielectric function, absorption coefficient, reflectance, and transmittance, are shown to exhibit strong in-plane anisotropy. Furthermore, structural analysis of several point defects and their oxidized structures was performed by means of simulated STM images. Among the considered vacancy defects, namely , , VTeout, VTein, , and VHf, the formation of VTeout is revealed to be the most favorable defect. While and VHf defects lead to local magnetism, only the oxygen-substituted VHf structure possesses magnetism among the oxidized defects. Moreover, it is found that all the bare and oxidized vacant sites can be distinguished from each other through the STM images. Overall, our study indicates not only the fundamental anisotropic features of single-layer HfTe5, but also shows the signatures of feasible point defects and their oxidized structures, which may be useful for future experiments on 2D HfTe5.

Electronic Bulk and Surface Properties of Titanium Dioxide Studied by DFT-1/2.

Transparent conductive oxides, such as TiO2, are important functional materials for optoelectronic and photovoltaic devices. We investigate the electronic bulk properties of the TiO2 phases rutile and anatase with the DFT-1/2 method and obtain a quantitatively good description of their electronic band structures. We then applied this method to the (001) surfaces of rutile and anatase and calculated their ionization potentials (IPs) and work functions (WF). To relate these calculated surface properties with values from experiments, we evaluated the effect of varying the oxygen stoichiometry at the surface on both IP and WF.

A first-principles study of the electronic structure, surface stability, and band alignment of niobium pentoxide

Currently, there is a growing development to find new materials to achieve clean and sustainable energy production, and one of the key processes in this effort is to replace expensive noble element catalysts with less costly transition metal oxides. Niobium pentoxide (Nb2O5) is a wide-gap semiconductor with good catalytic properties which makes it an essential material in this kind of technological applications, however, there are no theoretical studies on the surface physical properties for the different phases of this compound. In this work, we have performed first-principles calculations to study the bulk electronic properties and the low-index non-polar surfaces of the B and R crystallographic phases of Nb2O5. We have used a semi-local exchange–correlation functional (PBEsol) along with the GoWo approximation to calculate their gap and band alignments with respect to the vacuum. It has been found that these phases are indirect wide-gap semiconductors, the calculated gaps for B (R) are EgPBEsol=2.6eV (2.1 eV) and EgGoWo=4.2eV (3.8 eV). On the other hand, it is established that the most stable surfaces are (010) and (001) for B and R, respectively, and their highest conduction band edges are along the (1̄01) and (100) terminations, both being above the CO2/CH2O2 reduction potential at pH = 7. Our results present clues into utilizing these Nb2O5 phases on technological applications such as photocatalytic uses on water splitting and carbon dioxide reduction.

Vacuum Deposited Perovskites with a Controllable Crystal Orientation.

The preferential orientation of the perovskite (PVK) is typically accomplished by manipulation of the mixed cation/halide composition of the solution used for wet processing. However, for PVKs grown by thermal evaporation, this has been rarely addressed. It is unclear how variation in crystal orientation affects the optoelectronic properties of thermally evaporated films, including the charge carrier mobility, lifetime, and trap densities. In this study, we use different intermediate annealing temperatures Tinter between two sequential evaporation cycles to control the Cs0.15FA0.85PbI2.85Br0.15 orientation of the final PVK layer. XRD and 2D-XRD measurements reveal that when using no intermediate annealing primarily the (110) orientation is obtained, while when using Tinter = 100 °C a nearly isotropic orientation is found. Most interestingly for Tinter > 130 °C a highly oriented PVK (100) is formed. We found that although bulk electronic properties like photoconductivity are independent of the preferential orientation, surface related properties differ substantially. The highly oriented PVK (100) exhibits improved photoluminescence in terms of yield and lifetime. In addition, high spatial resolution mappings of the contact potential difference (CPD) as measured by KPFM for the highly oriented PVK show a more homogeneous surface potential distribution than those of the nonoriented PVK. These observations suggest that a highly oriented growth of thermally evaporated PVK is preferred to improve the charge extraction at the device level.

Intrinsic electronic structure and nodeless superconducting gap of YBa2Cu3O7–δ observed by spatially-resolved laser-based angle resolved photoemission spectroscopy

The spatially-resolved laser-based high-resolution angle resolved photoemission spectroscopy (ARPES) measurements have been performed on the optimally-doped YBa2Cu3O7–δ (Y123) superconductor. For the first time, we found the region from the cleaved surface that reveals clear bulk electronic properties. The intrinsic Fermi surface and band structures of Y123 were observed. The Fermi surface-dependent and momentum-dependent superconducting gap was determined which is nodeless and consistent with the d+is gap form.

Effect of Surface Energetics on Phase Stability of CaMnO3

CaMnO3 is a promising starting point for the search of perovskites which are suitable as electrode materials for the hydrogen evolution reaction (HER). In a previous theoretical study [Phys. Status Solidi B 2022, 260, 2200427], it was established that the global hybrid functional PW1PW is well suited to obtain structural, energetic, and electronic bulk properties. Herein, the focus is extended to surface properties of CaMnO3. All symmetry‐inequivalent low‐index surfaces of CaMnO3 are investigated with PW1PW. Based on the experience with polar surfaces, it is decided to employ stoichiometric and symmetric models, in some cases with Schottky defects. From the calculated surface energies, the crystal morphologies are predicted based on the Gibbs–Wulff theorem. The (101), (100), (011), (001), and (010) surfaces (space group no. 62) and the (010), (110), (011), and (101) surfaces (space group no. 20) dominate the surfaces of respective single crystals and should be considered in future theoretical calculations of the HER. Furthermore, it is found that the modification with space group no. 20 is significantly more stable than space group no. 62 for nanoparticles with a diameter below 10 nm.

Optical spectroscopy and band structure calculations of the structural phase transition in the vanadium-based kagome metal ScV6Sn6

In condensed matter physics, materials with kagome lattice display a range of exotic quantum states, including charge density wave (CDW), superconductivity, and magnetism. Recently, the intermetallic kagome metal ${\mathrm{ScV}}_{6}{\mathrm{Sn}}_{6}$ was discovered to undergo a first-order structural phase transition with the formation of a $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\ifmmode\times\else\texttimes\fi{}3$ CDW at around 92 K. The bulk electronic band properties are crucial to understanding the origin of the structural phase transition. Here, we conducted an optical spectroscopy study in combination with band structure calculations across the structural transition. Our findings showed abrupt changes in the optical reflectivity/conductivity spectra as a result of the structural transition, without any observable gap formation behavior. The optical measurements and band calculations actually reveal a sudden change of the band structure after transition. It is important to note that this phase transition is of the first-order type, which distinguishes it from conventional density-wave type condensations. Our results provide an insight into the origin of the structural phase transition in this new and unique kagome lattice intermetallic material.

Structural, magnetic and electronic properties of bulk and monolayers of Ba2TiTe4: An Ab-initio study

We have studied the structural, electronic, magnetic and mechanical properties of bulk (3D) and 2D monolayers (hexagonal and tetragonal) of Ba2TiTe4. The binding energy is calculated to confirm the stability of these studied systems. It is found that bulk (3D) is the most stable as compared to both 2D monolayers i.e. in hexagonal and tetragonal phase. These systems are found to be metallic in nature. Furthermore, magnetic nature of Ba2TiTe4 is discussed and it is found that the bulk Ba2TiTe4 is antiferromagnetic in nature with zero magnetic moment, while both 2D monolayers of Ba2TiTe4 are found to be magnetic. Moreover, the biaxial strain is used to compute mechanical strength, and the behavior of both 2D monolayers changes from metallic to semiconductor. Thus, under the effect of strain 2D monolayers found to be suitable for piezo electronics and spintronics.

Electronic and Optical Properties of Alkaline Earth Metal Fluoride Crystals with the Inclusion of Many-Body Effects: A Comparative Study on Rutile MgF2 and Cubic SrF2

We conducted a systematic investigation using state-of-the-art techniques on the electronic and optical properties of two crystals of alkaline earth metal fluorides, namely rutile MgF2 and cubic SrF2. For these two crystals of different symmetry, we present density functional theory (DFT), many-body perturbation theory (MBPT), and Bethe–Salpeter equation (BSE) calculations. We calculated a variety of properties, namely ground-state energies, band-energy gaps, and optical absorption spectra with the inclusion of excitonic effects. The quantities were obtained with a high degree of convergence regarding all bulk electronic and optical properties. Bulk rutile MgF2 has distinguished ground-state and excited-state properties with respect to the other cubic fluoride SrF2 and the other members of the alkaline earth metal fluoride family. The nature of the fundamental gaps and estimates of the self-energy and excitonic effects for the two compounds are presented and discussed in detail. Our results are in good accordance with the measurements and other theoretical–computational data. A comparison is made between the excitation and optical properties of bulk rutile MgF2, cubic SrF2, and the corresponding clusters, for which calculations have recently been published, confirming strong excitonic effects in finite-sized systems.

Pt2MnGa (001) surface stability and its effect on the magnetic and electronic properties: A DFT study

Pt2MnGa, a tetragonal Heusler alloy with interesting magnetic properties, is gaining interest from the scientific community due to its potential applications in the spintronics industry. We investigate the Pt2MnGa (001) surface atomic arrangements, its surface stability, its magnetic, and electronic properties using density functional theory. Depending on the chemical potential, Pt or Ga atoms could be found at the topmost layer. Three different surface configurations are stable for different growth conditions: a Ga full-monolayer, a Ga half-monolayer, and a Pt full-monolayer. Mn containing surfaces can only exist in out-of-equilibrium conditions. In the stable surface configurations, the magnetic and electronic properties of Pt2MnGa remain almost unaltered. This happens since Mn, responsible of the magnetic properties of Pt2MnGa, is not in the topmost layer of the stable models. • Three stable Pt2MnGa (001) surfaces are found at distinct chemical potential conditions. • Pt2MnGa (001) surface unaltered the bulk magnetic or electronic properties. • The main contribution to the Pt2MnGa’s magnetic properties belongs to the Mn, which is not localized at the surface.

Easily exfoliable monolayer of GdTe[formula omitted]: ab initio study

A recently synthesized material GdTe3 has shown very interesting properties like superconductivity and high carrier mobility in its bulk and flake form. Here, we have shown the formation of a monolayer of GdTe3 from its bulk parent material by means of exfoliation. We have also studied magnetic ground state and electronic properties of bulk and two-dimensional (2D) GdTe3, using ab initio calculations. We find both of them to have the same anti-ferromagnetic spin arrangement in their ground state. Our calculations further reveal that both bulk and 2D-GdTe3 are metallic, mainly because of the electronic contribution arising from the Te-atoms. The magnetic ground state does not change on the inclusion of the Hubbard-U parameter, calculated self-consistently to have a value of 5.7 eV for the Gd-f electrons. The spin–orbit coupling (SOC) calculations in 2D-GdTe3, for both U = 0 and 5.7 eV, show that the system prefers an in-plane anti-ferromagnetic ordering and has an easy magnetization axis along the crystallographic a-direction.

Roles of hydrogen in structural stability and electronic property of bulk hydrogenated amorphous silicon

Hydrogenated amorphous silicon (a-Si:H) has been widely used due to its unique photoelectric properties, but the information about its structure is patchy and the factors that affect its electronic properties are also unclear. In this work, we predicted the bulk a-Si:H structures based on the particle swarm optimization (PSO) technique and explored its structural characteristics under different hydrogen content conditions, then the effect of structural characteristics such as hydrogen content, void size and hydrogen-bonding configuration on structural stability and electronic properties of these structures are revealed. The results indicate that the structures with lower hydrogen content are conductors whereas the structures with higher hydrogen content are semiconductors. The band gaps of bulk a-Si:H structures show a trend with an increase followed a decrease with the increase of hydrogen content, which can be reduced by increasing the void size or adding some Si-H2, Si-H3 or Si-H-Si bonds to structures. The transition-state structure containing Si-H-Si bonds should have better conductivity than the steady-state structures. These results provide comprehensive understanding of the structural characteristics and electronic properties of bulk a-Si:H structures, which provides a theoretical support for the application of a-Si:H in semiconductor devices or as secondarybatteryanodes and have broad implications for further investigating other physical properties of a-Si:H structures.

Transport properties of vertical heterostructures under light irradiation

Electronic and transport properties of bilayer heterostructure under light irradiation are of fundamental interest to improve functionality of optoelectronic devices. We theoretically study the modification of transport properties of bilayer graphene and bilayer heterostructures under a time-periodic external light field. The bulk electronic and transport properties are studied in a Landauer-type configuration by using the nonequilibrium Green's function formalism. To illustrate the behavior of the differential conductance of a bilayer contact under light illumination, we consider tight-binding models of bilayer graphene and graphene/hexagonal boron-nitride heterostructures. The non-adiabatic driving induces sidebands of the original band structure and opening of gaps in the quasienergy spectrum. In transport properties, the gap openings are manifested in a suppression of the differential conductance. In addition to suppression, an external light field induces an enhancement of the differential conductance if photoexcited electrons tunnel into or out of a Van~Hove singularity.

Structural and Electronic Properties of SnO Downscaled to Monolayer.

Two-dimensional (2D) SnO is a p-type semiconductor that has received research and industrial attention for device-grade applications due to its bipolar conductivity and transparent semiconductor nature. The first-principles investigations based on the generalized gradient approximation (GGA) level of theory often failed to accurately model its structure due to interlayer Van der Waals interactions. This study is carried out to calculate structural and electronic properties of bulk and layered structures of SnO using dispersion correction scheme DFT+D3 with GGA-PBE to deal with the interactions which revealed good agreement of the results with reported data. The material in three-dimensional bulk happened to be an indirect gap semiconductor with a band gap of 0.6 eV which is increased to 2.85 eV for a two-dimensional monolayer structure. The detailed analysis of the properties demonstrated that the SnO monolayer is a promising candidate for future optoelectronics and spintronics devices, especially thin film transistors.

Interplay of electrode geometry and bias on charge transport in organic heterojunction gas sensors

Modulation of charge transport in an organic heterojunction sensor is a promising strategy to enhance its gas sensing properties. Herein, a heterostructure consisting of a bilayer assembly of perfluorinated copper phthalocyanine and lutetium bis-phthalocyanine is investigated for ammonia sensor development in two different electrode geometries and in a wide range of applied bias. The microstructure of the heterostructure retains the bulk electronic and structural properties of the individual components and shows granular distribution of phthalocyanine crystallites on the surface. The charge transport of the heterojunction devices depends on the electrode geometry and the applied bias. Notably, interfacial charge transport gets faster, while bulk charge transport remains constant with increasing bias in both devices. But, the small gap between the electrodes fastens the bulk and the interfacial charge transport by 75 and 5 times, respectively, compared to the big gap electrodes. The implication of faster charge transport in the small gap electrodes-based sensor is demonstrated on its ammonia sensing properties, exhibiting higher response and shorter response time. Moreover, the response of the sensor exponentially increases with increasing bias. The high sensitivity and stability of the sensor at different relative humidity make it a suitable NH3 sensor for real environment applications.

Stoichiometry-dependent surface electronic structure of SrTiO3 films grown by hybrid molecular beam epitaxy

We investigate the surface electronic structure of SrTiO3 (STO) films grown by a hybrid molecular beam epitaxy that are both stoichiometric and nonstoichiometric by means of x-ray photoelectron spectroscopy and electron energy loss spectroscopy. Increasing the fraction of the surface that is terminated with an SrO layer is correlated with a decrease in the chemical potential whereby the valence band maximum moves closer to the Fermi level, but without a significant change in the bandgap. Inasmuch as SrO-terminated STO (001) has previously been shown to act as an electron scavenger in which carriers from the bulk are trapped, we argue that the high fraction of SrO in the terminal layer is what lowers the chemical potential in Sr-rich STO. Our experimental results provide important insights into various physical phenomena that can occur on STO (001) surfaces and their effect on bulk electronic properties.

Crucial role of oxygen on the bulk and surface electronic properties of stable \u03b2 phase of tungsten

The A15 β phase of tungsten has recently attracted great interest for spintronic applications due to the finding of giant spin-Hall effect. As β phase is stabilized by oxygen, we have studied the electronic structure of O-doped β-W from first principles calculations. It is found that 20 at.% O-doping makes β phase lower in energy than α-W. These results are in good agreement with energy dispersive X-ray spectroscopy which also shows ~ 16.84 at.% O in 60 nm thick W films. The latter has predominantly β phase as confirmed by grazing incidence X-ray diffraction (XRD). The simulated XRD of bulk β having 15.79 at.% O also agrees with XRD results. Oxygen binds strongly on the surface and affects the Dirac fermion behavior in pure β-W. There is structural disorder, O-inhomogeneity, and higher density-of-states in O-doped β-W at EF compared with pure α. These results are promising to understand the properties of β-W.

Kaolin group minerals under pressure: The study of their structural and electronic properties by DFT methods

Kaolinites have a wide impact in many technological applications, and also are promising for their potential new uses in the future. Because these clay minerals naturally exist as fine particles, there are difficulties in making measurements of their bulk structural and electronic properties. Also, the occurrence of stacking faults and the presence of structural defects at the nanoscale, make these clay minerals quite complex systems to propose accurate models at the crystallographic scale.This work presents a study of six different kaolin group polytypes using calculations based on the Density Functional Theory (DFT), and includes the naturally occurring clay minerals kaolinite, dickite, and nacrite, and also other three pressure-induced polytypes that have been experimentally observed. The idealized structures were optimized using DFT methods, and their responses to hydrostatic pressure were theoretically analyzed from energy vs. volume and enthalpy vs. pressure data. From these, the bulk modulus for each polytype was predicted, and their relative stability and the potential transformations between structures were discussed. Finally, the electronic density of states for each polytype was calculated and compared to each other.The results indicate that although structurally these polytypes present significant differences between each other that lead to different elastic responses, from the perspective of the electronic properties they may be indistinguishable. The analysis performed in this work can be helpful as guidance and for interpretation in future experiments.