Wide Band Gap Insulator Research Articles

A long-cycle-life reversible Li-CO2 battery enabled by engineering the active sites of graphene with Pd nanoparticles.

Li-CO2 batteries have been recognized as an emerging technology for energy storage systems owing to their high theoretical specific energy and environmentally friendly CO2 fixation ability. However, their development for applications requires a high energy efficiency and long cycle-life, this is currently limited to the formation of wide-bandgap insulator Li2CO3 during discharge. Here, nanoparticle Pd supported on reduced graphene oxide (rGO) is utilized as cathodes for Li-CO2 batteries, Pd nanoparticles as active centers significantly enhance CO2RR/CO2ER reaction activity, which can support the fast formation and decomposition of Li2CO3 in organic electrolytes and achieve a high discharge capacity of 7500 mAh g-1. It also performs remarkably high cycling stability of over 500 cycles with a long cycle-life of 5000 hours. The observed super electrochemical performance is attributable to the thick electrode design and uniform distribution of ultrafine catalyst nanoparticle Pd. When Li2CO3 is adsorbed on Pd particle, the Li-O bond in Li2CO3 will be elongated due to the interactions of two nucleophilic O atoms with Pd, resulting in a weakening of the Li-O bond and activation of Li2CO3. Our work suggests a way to design catalysts with high activity that can be used to activate the performance of Li-CO2 batteries.

First principles study of the electronic structure, mechanical and optical properties of cubic perovskite NaCaCl3 under pressure

Based on the first-principles methods, the electronic, mechanical and optical properties of NaCaCl3 crystal were investigated under various pressures for the first time. At 0 GPa, the lattice constant and bulk modulus were calculated to be 5.278 Å and 27.011 GPa, respectively. Analysis of Poisson's ratio and Pugh's ratio indicated that NaCaCl3 exhibits ductile characteristics, which increases with the increasing pressure. Based on the crystal mechanical stability condition, the phase transition pressure of NaCaCl3 was calculated to be 5.12 GPa. The electronic property demonstrated that NaCaCl3 exhibits an indirect and wide band gap insulator (3.84 eV) at 0 GPa, and the band gap increases with the increasing pressure. The results of optical properties show that all spectrums of NaCaCl3 exhibit a blue shift when the pressure increases. The excellent ultraviolet absorption suggests that NaCaCl3 has a potential application in vacuum ultraviolet devices. And it is also suitable as transparent coating and window materials in the infrared and visible regions due to small reflectivity and absorption.

Imogolite Nanotubes and Their Permanently Polarized Bifunctional Surfaces for Photocatalytic Hydrogen Production.

To date, imogolite nanotubes (INTs) have been primarily used for environmental applications such as dye and pollutant degradation. However, imogolite's well-defined porous structure and distinctive electro-optical properties have prompted interest in the system's potential for energy-relevant chemical reactions. The imogolite structure leads to a permanent intrawall polarization arising from the presence of bifunctional surfaces at the inner and outer tube walls. Density functional theory simulations suggest such bifunctionality to encompass also spatially separated band edges. Altogether, these elements make INTs appealing candidates for facilitating chemical conversion reactions. Despite their potential, the exploitation of imogolite's features for photocatalysis is at its infancy, thence relatively unexplored. This perspective overviews the basic physical-chemical and optoelectronical properties of imogolite nanotubes, emphasizing their role as wide bandgap insulator. Imogolite nanotubes have multifaceted properties that could lead to beneficial outcomes in energy-related applications. This work illustrates two case studies demonstrating a step-forward on photocatalytic hydrogen production achieved through atomic doping or metal co-catalyst. INTs exhibit potential in energy conversion and storage, due to their ability to accommodate functions such as enhancing charge separation and influencing the chemical potentials of interacting species. Yet, tapping into potential for energy-relevant application needs further experimental research, computational, and theoretical analysis.

Phonon-Mediated Quasiparticle Lifetime Renormalizations in Few-Layer Hexagonal Boron Nitride.

Understanding the collective behavior of the quasiparticles in solid-state systems underpins the field of nonvolatile electronics, including the opportunity to control many-body effects for well-desired physical phenomena and their applications. Hexagonal boron nitride (hBN) is a wide-energy-bandgap semiconductor, showing immense potential as a platform for low-dimensional device heterostructures. It is an inert dielectric used for gated devices, having a negligible orbital hybridization when placed in contact with other systems. Despite its inertness, we discover a large electron mass enhancement in few-layer hBN affecting the lifetime of the π-band states. We show that the renormalization is phonon-mediated and consistent with both single- and multiple-phonon scattering events. Our findings thus unveil a so-far unknown many-body state in a wide-bandgap insulator, having important implications for devices using hBN as one of their building blocks.

Exploring bonding nature, electronic, optical and thermoelectric properties of ternary compound Na2ZnCl4 via first-principles calculations

• Structural studies of the orthorhombic phase Na 2 ZnCl 4 compound have been studied • The electronic properties of the compound, such as band structure and density of states signify its insulating nature with a wide - bandgap of 4.51 eV. • The analysis of the charge density distribution for this compound along the ( 2 1 ¯ 0 ) plane indicates partially ionic and partially covalent bond between atoms. • Various optical properties, including the absorption coefficient, dielectric function, energy loss function, reflectivity and refractive index were studied. • The results from the Figure of merit provide material applications in thermo-electric devices. First-principles calculations for orthorhombic Na 2 ZnCl 4 performed using Density Functional Theory (DFT) to analyze the structural, electronic, optical and thermoelectric properties using WIEN2k code. The exchange correlation of PBE (GGA) is used. Geometrical optimization is performed with c/a variation to find the optimized lattice constant which is in good agreement with the experimental results. Electronic property calculations, including the band structure and density of states (DOS) indicates that the compound is a wide-bandgap insulator (WBGI) with a bandgap of 4.51 eV. The bonding nature is determined from charge density plot. Thermoelectric studies provide the figure of merit for this compound is around 0.52 at 900 K. The calculated optical and thermoelectric properties demonstrate the application of these compound in optoelectronic and thermo-electric devices.

Efficient GW calculations in two dimensional materials through a stochastic integration of the screened potential

Many-body perturbation theory methods, such as the G0W0 approximation, are able to accurately predict quasiparticle (QP) properties of several classes of materials. However, the calculation of the QP band structure of two-dimensional (2D) semiconductors is known to require a very dense BZ sampling, due to the sharp q-dependence of the dielectric matrix in the long-wavelength limit (q → 0). In this work, we show how the convergence of the QP corrections of 2D semiconductors with respect to the BZ sampling can be drastically improved, by combining a Monte Carlo integration with an interpolation scheme able to represent the screened potential between the calculated grid points. The method has been validated by computing the band gap of three different prototype monolayer materials: a transition metal dichalcogenide (MoS2), a wide band gap insulator (hBN) and an anisotropic semiconductor (phosphorene). The proposed scheme shows that the convergence of the gap for these three materials up to 50meV is achieved by using k-point grids comparable to those needed by DFT calculations, while keeping the grid uniform.

Concurrent weak localization and double Schottky barrier across a grain boundary in bicrystal SrTiO3

Stoichiometric ${\mathrm{SrTiO}}_{3}$ (STO) is a wide band-gap insulator in which oxygen deficiency generates a low-temperature metallic ferromagnetic state. Here, we report weak ferromagnetic-like and metallic transport in oxygen-deficient bicrystal of $\mathrm{STO}$ with a low-temperature electron mobility and surface carrier density of $2800\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ and $3.500\ifmmode\times\else\texttimes\fi{}{10}^{16}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$, respectively. Moreover, we show that magnetotransport behavior in the bulk and across the grain boundary (GB) in $\mathrm{STO}$ single crystals are significantly different. In the bulk, our magnetotransport results agree with those reported in the literature. Importantly, low-temperature magnetotransport across the GB shows a disorder-induced metal-insulator transition, and it is modeled as a quasi--two-dimensional system below 10 K that accounts for weak localization (WL), supported by electron-electron, electron-phonon, and Hikami-Larkin-Nagaoka (HLN) quantum interference models. The HLN model accounts for realistic length scales: the dephasing length ${L}_{\ensuremath{\phi}}$, and the spin-orbit (SO) scattering length ${L}_{\mathrm{SO}}$, which are comparable to the electron mean-free path, resembling the surface-conducting nature across the GB. Our experimental work shows the importance of structural defects in the interpretation of magnetotransport in ${\mathrm{SrTiO}}_{3}$ and shows that WL is concurrent with a double Schottky barrier at the GB interface.

A Noble-Metal-Free Spintronic System with Proximity-Enhanced Ferromagnetic Topological Surface State of FeSi above Room Temperature.

Strongly spin-orbit coupled states at metal interfaces, topological insulators, and 2D materials enable efficient electric control of spin states, offering great potential for spintronics. However, there are still materials challenges to overcome, including the integration into advanced silicon electronics and the scarce resources of constituent heavy elements of those materials. Through magneto-transport measurements and first-principles calculations, here robust spin-orbit coupling (SOC)-induced properties of a ferromagnetic topological surface state in FeSi and their controllability via hybridization with adjacent materials are demonstrated. In comparison to the case of its naturally oxidized surface, the ferromagnetic transition temperature is greatly increased beyond room temperature and the effective SOC strength is almost doubled at the surface in proximity to a wide-bandgap fluoride insulator. Those enhanced magnetic properties enable room-temperature magnetization switching, being applicable to spin-orbit torque based spintronic devices. Realization of strong SOC in the noble-metal-free silicon-based compound will accelerate spintronic applications.

Large bandgap insulating superior clay nanosheets

One can find conductive, semiconducting, and insulating single nanosheets with unique electronic properties that are tied to their two-dimensional (2D) structure. Here, we report on wide-bandgap 2D insulator nanosheets obtained by delamination of a synthetic 2D fluorohectorite clay mineral showing one of the largest bandgap insulators in the world. The bandgap was determined experimentally to be up to 7.1–8.2 eV measured by electron energy-loss spectroscopy in a high-resolution transmission electron microscope. The experimental data were supported by DFT calculations giving a bandgap of 5.5 eV. The single fluorohectorite clay crystalline nanosheets are 0.95-nm, and they can be synthetized with high-aspect ratios and lateral dimensions up to dozens of microns. These properties render these nanosheets promising candidates for practical applications in manually assembled or self-assembled electronic heterostructures, potentially serving as insulating nanosheets in graphene or various (semi)conductive 2D material-based devices.Impact statementProperties of the synthetic fluorohectorite clay presented in this article render these 0.95-nm-thin nanosheets promising candidates for practical applications in manually assembled or self-assembled electronic heterostructures, potentially serving as insulating nanosheets in graphene or various (semi)conductive 2D material-based devices.The information provided in this work can be essential for the growing community focused on the study of 2D materials and their wide range of applications.Graphical abstract

Structural, electronic, optical, and thermoelectric properties of Li2ZnCl4 based on density functional theory computations

Theoretical predictions of ground-state properties can be validated for any material based on first-principles calculations. In the present study, we analyzed the compound Li2ZnCl4 to assess its feasibility for application in optical, electrical, and thermoelectric devices. We performed geometry optimization for spinel (cubic) and olivine (orthorhombic) structures because this compound undergoes a phase transition between these two structures at a given temperature. The theoretical results suggested that this compound is stable in an orthorhombic structure. Calculations were performed to determine the electronic, optical, and transport properties of the cubic and orthorhombic structures. According to the calculated electronic properties, we conclude that Li2ZnCl4 is a wide bandgap insulator. The effective masses of the electrons and holes were also calculated. These optical and thermoelectric studies indicate that Li2ZnCl4 is a potential candidate for use in optoelectronic and thermoelectric devices.

Uncovering the morphological effects of high-energy Ga+ focused ion beam milling on hBN single-photon emitter fabrication.

Many techniques to fabricate complex nanostructures and quantum emitting defects in low dimensional materials for quantum information technologies rely on the patterning capabilities of focused ion beam (FIB) systems. In particular, the ability to pattern arrays of bright and stable room temperature single-photon emitters (SPEs) in 2D wide-bandgap insulator hexagonal boron nitride (hBN) via high-energy heavy-ion FIB allows for direct placement of SPEs without structured substrates or polymer-reliant lithography steps. However, the process parameters needed to create hBN SPEs with this technique are dependent on the growth method of the material chosen. Moreover, morphological damage induced by high-energy heavy-ion exposure may further influence the successful creation of SPEs. In this work, we perform atomic force microscopy to characterize the surface morphology of hBN regions patterned by Ga+ FIB to create SPEs at a range of ion doses and find that material swelling, and not milling as expected, is most strongly and positively correlated with the onset of non-zero SPE yields. Furthermore, we simulate vacancy concentration profiles at each of the tested doses and propose a qualitative model to elucidate how Ga+ FIB patterning creates isolated SPEs that is consistent with observed optical and morphological characteristics and is dependent on the consideration of void nucleation and growth from vacancy clusters. Our results provide novel insight into the formation of hBN SPEs created by high-energy heavy-ion milling that can be leveraged for monolithic hBN photonic devices and could be applied to a wide range of low-dimensional solid-state SPE hosts.

Mechanism of In-Plane and Out-of-Plane Tribovoltaic Direct-Current Transport with a Metal/Oxide/Metal Dynamic Heterojunction.

Interfacial layer engineering has been demonstrated as an effective strategy for boosting power output in semiconductor-based dynamic direct-current (DC) generators, although the underlying mechanism of power enhancement remains obscure. Here, such ambiguity has been elucidated by comparing fundamental tribovoltaic DC output characteristics of prototypical metal-oxide-metal heterojunctions prepared by atomic-layer deposition (ALD) with a vertical (out-of-plane carrier transport through the interfacial layer) and a horizontal (in-plane carrier transport along the interfacial layer) configuration such that the influences from nonequilibrium electronic excitation and interfacial capacitive amplification can be individually tuned and investigated. It is found in the case of Al/TiO2/Ti vertical configurations that the open-circuit voltage (VOC) increases linearly from -0.03 to -0.52 V as the thickness of titanium oxide (tTiO2) increases from 0 to 200 nm with a linear amplification coefficient of -2.31 mV nm-1, which is validated by a parallel-capacitor theoretical model with tribovoltaic electronic excitation. In contrast, the VOC output with the horizontal configuration is ∼55 mV, where the potential difference is merely associated with the accumulation of surface charges and the subsequent charge rearrangement in the depletion region. Meanwhile, it is measured that the short-circuit current density (JSC) shows an initial increasing trend when tTiO2 increases, reaches its peak value at 0.21 A m-2 at tTiO2 = 20 nm, and then decreases as tTiO2 increases further. From current-voltage (I-V) characterization, it is proposed that such DC output variation with an optimal interfacial layer thickness stems from the competition of amplified voltage and increased resistance with increasing interfacial layer thickness, with the main charge transport mechanism switching from quantum tunneling to thermionic emission/trap-assisted transport. In contrast, tribovoltaic excitation is proven to be significantly weaker when a wide band-gap insulator (Al2O3) is involved. The elucidation of the fundamental mechanism of power enhancement by the interfacial layer in this work is of great significance in providing instructional direction for the development and optimization of high-performance DC nanogenerators.

The electronic properties of SrTiO3-\u03b4 with oxygen vacancies or substitutions

The electronic properties, including bandgap and conductivity, are critical for nearly all applications of multifunctional perovskite oxide ferroelectrics. Here we analysed possibility to induce semiconductor behaviour in these materials, which are basically insulators, by replacement of several percent of oxygen atoms with nitrogen, hydrogen, or vacancies. We explored this approach for one of the best studied members of the large family of ABO3 perovskite ferroelectrics — strontium titanate (SrTiO3). The atomic and electronic structure of defects were theoretically investigated using the large-scale first-principles calculations for both bulk crystal and thin films. The results of calculations were experimentally verified by studies of the optical properties at photon energies from 25 meV to 8.8 eV for in-situ prepared thin films. It was demonstrated that substitutions and vacancies prefer locations at surfaces or phase boundaries over those inside crystallites. At the same time, local states in the bandgap can be produced by vacancies located both inside the crystals and at the surface, but by nitrogen substitution only inside crystals. Wide-bandgap insulator phases were evidenced for all defects. Compared to pure SrTiO3 films, bandgap widening due to defects was theoretically predicted and experimentally detected.

New Two-Dimensional Wide Band Gap Hydrocarbon Insulator by Hydrogenation of a Biphenylene Sheet.

Based on first-principles calculations, the ground state configuration (Cmma-CH) of a hydrogenated biphenylene sheet ( Science 2021, 372, 852) is carefully identified from hundreds of possible candidates generated by RG2 code ( Phys. Rev. B. 2018, 97, 014104). Cmma-CH contains four inequivalent benzene molecules in its crystalline cell due to its Cmma symmetry. Hydrogen atoms bond to carbon atoms in each benzene with a boat-like (DDUDDU) up/down sequence and reversed boat-1 (UUDUUD) sequence in adjacent benzene rings. Cmma-CH is energetically less stable than the proposed allotropes of hydrogenated graphene, but the formation energy for hydrogenating a biphenylene sheet is remarkably lower than that for hydrogenating graphene to graphane. Our results of mechanical and dynamical stability also confirm that Cmma-CH is a stable 2D hydrocarbon, which is expected to be realized experimentally. Especially, biphenylene undergoes a transition from normal metal to a wide band gap insulator (4.645 eV) by hydrogenation to Cmma-CH, which has potential applications in nanodevices at elevated temperatures and high voltages.

Hybrid assemblies of octagonal C and BN monolayers and their electronic properties

Two-dimensional materials and their assemblies have attracted considerable attention due to their versatile properties for various applications. Among them, recently proposed octagonal monolayers (o-MLs) of C and BN are investigated for thermal, dynamical, and energetic stability. These robust o-MLs are then probed for patterned hybrid assemblies due to inherent low lattice mismatch and metallic and wide bandgap insulator combination to study their electronic structure for applications. Carbon substitution in boron nitride in the form of eight membered rings is found to be stable, and the variation of the substituted rings in the patterned hybrid o-MLs changes the structure from an insulating phase to a metallic phase. Such predicted structures may provide impetus for their practical realization.

Nanoparticles of ZnO and Mg-doped ZnO: Synthesis, characterization and efficient removal of methyl orange (MO) from aqueous solution

Nanoparticles of zinc oxide and of ZnO doped with MgO in different concentrations (1, 2 and 4 mol%) were synthesized in a controlled and reproducible way, using the Pechini polymer precursor method. To determine the physicochemical and structural characteristics of the synthesized nanoparticles, Fourier transform IR (FTIR), X-ray diffraction (XRD), UV–Vis spectroscopy and transmission and scanning electron microscopy (TEM and SEM) were used. Characterization revealed the particles obtained to be nanometric in size (<50 nm) and with a deformed hexagonal morphology. Taking into account the doping percentage, the energy gap value varied between 3.3 eV for pure ZnO and 3.45 eV for ZnO with 4 mol% of Mg, which indicates that the optical properties of these nanoparticles were affected by dopant concentration. The effect of doping with Mg2+ on the capacity for removal of pollutant molecules by ZnO, for different working conditions, was evaluated by studying the removal of methyl orange (MO) in aqueous solution. Irradiation of the compounds led to a greater removal of MO from the solution such that all ZnO samples doped with MgO showed higher photoactivity than ZnO. The ZnO nanoparticles doped with 2% Mg were the most efficient in removing MO, achieving a removal percentage of ~73% after 2 h of testing and a totally transparent solution after 3 h of treatment. The kinetics of removal of MO promoted by this sample was best represented by pseudo-first-order kinetics. The results of this work showed that on combining a photosensitive semiconductor, ZnO, with a wide band gap insulator, MgO, Zn–Mg solid solutions are obtained that showed adequate capacity to remove contaminating organic molecules, specifically MO.

Comparison of coherent phonon generation by electronic and ionic Raman scattering in LaAlO3

In ionic Raman scattering, infrared-active phonons mediate a scattering process that results in the creation or destruction of a Raman-active phonon. This mechanism relies on nonlinear interactions between phonons and has in recent years been associated with a variety of emergent lattice-driven phenomena in complex transition-metal oxides, but the underlying mechanism is often obscured by the presence of multiple coupled order parameters in play. Here, we use time-resolved spectroscopy to compare coherent phonons generated by ionic Raman scattering with those created by more conventional electronic Raman scattering on the nonmagnetic and non-strongly-correlated wide band-gap insulator LaAlO$_3$. We find that the oscillatory amplitude of the low-frequency Raman-active $E_g$ mode exhibits a sharp peak when we tune our pump frequency into resonance with the high-frequency infrared-active $E_u$ mode, consistent with first-principles calculations. Our results suggest that ionic Raman scattering can strongly dominate electronic Raman scattering in wide band-gap insulating materials. We also see evidence of competing scattering channels at fluences above 28~mJ/cm$^2$ that alter the measured amplitude of the coherent phonon response.

Nano‐Sized Au Particle‐Modified Carbon Nanotubes as an Effective and Stable Cathode for Li−CO2 Batteries

AbstractLithium‐carbon dioxide (Li−CO2) batteries show a great potential in the fields of CO2 fixation and energy storage, and moreover the research of Li−CO2 batteries is of great significance to the practical application of Lithium‐air (Li‐air) batteries. However, the sluggish kinetics and high charging potential causing by the wide band gap insulator Li2CO3 make it extremely important to research cathode catalysts that can promote the decomposition of Li2CO3 and reduce the charge potential. Here, we prepared a composite of Au nanoparticles uniformly distribute on carbon nanotubes (CNTs) network (AuNPs/CNTs) as an efficient cathode catalyst. Benefitted by the small sized Li2CO3 which is easier to decompose during charge process, the AuNPs/CNTs‐based batteries exhibit a high specific capacity of 6399 mAh g−1, a low overpotential of 1.53 V and a stable cycling performance of 46 cycles, demonstrating the conspicuous catalytic performance of AuNPs/CNTs.

Boron Nitride Nanotube (BNNT) Membranes for Energy and Environmental Applications.

Owing to their extraordinary thermal, mechanical, optical, and electrical properties, boron nitride nanotubes (BNNTs) have been attracting considerable attention in various scientific fields, making it more promising as a nanomaterial compared to other nanotubes. Recent studies reported that BNNTs exhibit better properties than carbon nanotubes, which have been extensively investigated for most environment-energy applications. Irrespective of its chirality, BNNT is a constant wide-bandgap insulator, exhibiting thermal oxidation resistance, piezoelectric properties, high hydrogen adsorption, ultraviolet luminescence, cytocompatibility, and stability. These unique properties of BNNT render it an exceptional material for separation applications, e.g., membranes. Recent studies reported that water filtration, gas separation, sensing, and battery separator membranes can considerably benefit from these properties. That is, flux, rejection, anti-fouling, sensing, structural, thermal, electrical, and optical properties of membranes can be enhanced by the contribution of BNNTs. Thus far, a majority of studies have focused on molecular simulation. Hence, the requirement of an extensive review has emerged. In this perspective article, advanced properties of BNNTs are analyzed, followed by a discussion on the advantages of these properties for membrane science with an overview of the current literature. We hope to provide insights into BNNT materials and accelerate research for environment-energy applications.

Orientation-dependent electric transport and band filling in hole co-doped epitaxial diamond films

Diamond, a well-known wide-bandgap insulator, becomes a low-temperature superconductor upon substitutional doping of carbon with boron. However, limited boron solubility and significant lattice disorder introduced by boron doping prevent attaining the theoretically-predicted high-temperature superconductivity. Here we present an alternative co-doping approach, based on the combination of ionic gating and boron substitution, in hydrogenated thin films epitaxially grown on (111)- and (110)-oriented single crystals. Gate-dependent electric transport measurements show that the effect of boron doping strongly depends on the crystal orientation. In the (111) surface, it strongly suppresses the charge-carrier mobility and moderately increases the gate-induced doping, while in the (110) surface it strongly increases the gate-induced doping with a moderate reduction in mobility. In both cases the maximum total carrier density remains below 2·1014 cm−2, three times lower than the value theoretically required for high-temperature superconductivity. Density-functional theory calculations show that this strongly orientation-dependent effect is due to the specific energy-dependence of the density of states in the two surfaces. Our results allow to determine the band filling and doping-dependence of the hole scattering lifetime in the two surfaces, showing the occurrence of a frustrated insulator-to-metal transition in the (110) surface and of a re-entrant insulator-to-metal transition in the (111) surface.