Wide Gap Insulators Research Articles

AbInitio Self-Trapped Excitons.

We propose a new formalism and an effective computational framework to study self-trapped excitons (STEs) in insulators and semiconductors from first principles. Using the many-body Bethe-Salpeter equation in combination with perturbation theory, we are able to obtain the mode- and momentum-resolved exciton-phonon coupling matrix element in a perturbative scheme and explicitly solve the real space localization of the electron (hole), as well as the lattice distortion. Further, this method allows us to compute the STE potential energy surface and evaluate the STE formation energy and Stokes shift. We demonstrate our approach using two-dimensional magnetic semiconductor chromium trihalides and a wide-gap insulator BeO, the latter of which features dark excitons, and make predictions of their Stokes shift and coherent phonon generation which we hope will spark future experiments such as photoluminescence and transient absorption studies.

Non-Fermi liquid to charge-transfer Mott insulator in flat bands of copper-doped lead apatite.

Copper-doped lead apatite, called LK-99, was initially claimed to be a room temperature superconductor driven by flat electron bands, but was later found to be a wide gap insulator. Despite the lack of room temperature superconductivity, there is growing evidence that LK-99 and related compounds host various strong electron correlation phenomena arising from their flat electron bands. Depending on the copper doping site and crystal structure, LK-99 can exhibit two distinct flat bands crossing the Fermi level in the non-interacting limit: either a single or two entangled flat bands. We explore potential correlated metallic and insulating phases in the flat bands of LK-99 compounds by constructing their correlation phase diagrams, and find both non-Fermi liquid and Mott insulating states. We demonstrate that LK-99 is a charge-transfer Mott insulator driven by strong electron correlations, regardless of the flat band type. We also find that the non-Fermi liquid state in the multi-flat band system exhibits strange metal behaviour, while the corresponding state in the single flat band system exhibits pseudogap behaviour. Our findings align with available experimental observations and provide crucial insights into the correlation phenomenology of LK-99 and related compounds that could arise independently of superconductivity. Overall, our research highlights that LK-99 and related compounds offer a compelling platform for investigating correlation physics in flat band systems.

Functionalized hexagonal boron nitride bilayers: desirable electro-optical properties for optoelectronic applications.

Structural, electronic, and optical properties of functionalized hexagonal boron nitride (h-BN) bilayer were deeply explored by carrying out the PBE + G0W0 + BSE calculations. Hydrogenation/hydrofluorination/fluorination can cause the planar h-BN bilayer to form a novel diamane-like monolayer by interfacial sp3 atom bonding. These functionalized h-BN bilayers are estimated to be stable dynamically due to their phonon dispersions. The functionalization on h-BN bilayer can induce its electronic nature to be transformed from an indirect wide-gap insulator to direct narrow-gap semiconductor, which is desirable for its application in optoelectronics. In particular, hydrogenated and hydrofluorinated h-BN bilayers have strong absorbance coefficients for the near-infrared and visible part of the incident sunlight (larger than 105 cm-1). More interestingly, the binding energy of the observed first bright exciton can achieve a value beyond 1 eV, which can effectively reduce the recombination of photogenerated electron-hole pairs. These results are potentially important for extending the applications of the h-BN bilayer in optoelectronic devices.

Lizardite–h-BN heterostructures—Application of clay minerals in technology

Graphene has been a subject of great interest not only due to its fascinating properties but also for being the pioneer among 2D van der Waals (vdW) materials. Hexagonal boron nitride, an isomorph of graphene and a wide gap insulator, is commonly referred to as white graphene. The combination of the insulating hexagonal boron nitride (h-BN) with other crystals to form heterostructures provide a path for engineering and manipulating new physics and device properties. In this work, we investigate the vdW heterostructures formed by assembling h-BN and lizardite, a clay-mineral that is abundant in nature and represents the most stable polymorph of the serpentine family. The optoelectronic properties of three distinct heterostructures are presented to discern the characteristics of the systems. We observe that unlike lizardite and h-BN which are insulators, all the three heterostructures exhibit a semiconducting nature. The direct gap of the heterostructure in which two h-BN sheets are simultaneously placed above and below the octahedral and tetrahedral layers also makes it relevant for optoelectronic devices. Additionally, unlike lizardite, the heterostructures demonstrate a polarization-dependent optical properties. The study of the assembled structures combining the clay-mineral with h-BN not only widens the spectrum of vdW heterostructures but also explores their potential within the context of the serpentine family.

Mechanical, optoelectronic and thermoelectric properties of the transition metal oxide perovskites YScO3 and LaScO3: First principle calculation

In this paper, we report a density functional theory (DFT) calculation study based on the full potential linear augmented plane wave (FP-LAPW) to discuss the mechanical stability as well as the optoelectronic and thermoelectric properties of two proposed orthorhombic transition metal oxide perovskites, namely, YScO3 and LaScO3. We found that our proposed perovskites are mechanically stable and elastically anisotropic materials. Also, they present promising thermal barrier coating (TBC) due to their high mechanical parameters. The topology of the band structure is of a wide energy gap insulator character for our compounds. We tested several functionals to recover the observed band gaps; especially the Eg band gap of LaScO3 has been a challenging problem with conventional exchange-correlation (XC) functionals used in the DFT calculations. Thus, the Yukawa-screened hybrid XC YS-PBE0 successfully gives Eg = 5.78 eV which is in good agreement with the experimental value of 5.8 eV. With DF-PT (perturbation theory) taking into account the lattice vibration, we found that our compounds have a high dielectric constant. We also calculated the main optical properties and found that both compounds have a large optical Eg, very small reflectivity, and could absorb light in the UV region and therefore, making them promising host materials for UV applications and ideal for optoelectronic devices.

Ab-initio investigation of the structural stability, electronic and optical properties of the LiBO2 compound by using the G0W0+BSE approach

This article investigates the structural stability, and electronic and optical properties of the LiBO2 compound using density functional theory (DFT) and solving the many-body Bethe-Salpeter equation within the G0W0+BSE approach. The structural properties showed that the monoclinic structure of LiBO2 is thermodynamically and dynamically stable within both LDA and GGA approximations. The band structure results indicated that the direct band gap of LiBO2 at the Γ point is 6.05 eV, which increased to 8.95 eV by considering the quasi-particle effects in G0W0@LDA calculations. The maximum reflectance in the x, y, and z directions is 0.48, 0.86, and 0.54, respectively, with the corresponding static refractive index values of 1.52, 1.61, and 1.54. The direct optical band gap obtained from the imaginary part of the dielectric function is 5.95 eV, 5.95 eV, and 6.10 eV for the various directions x, y, and z respectively. Considering the electron-hole effects in the G0W0+BSE approach, the optical gap showed more accurate results, and the value of 7.54 eV was obtained in the x and z-directions and 8.10 eV in the y-direction. Therefore, excitonic effects within the G0W0+BSE approximation, in comparison to G0W0+RPA, leads to the redshift of the optical absorption spectrum. The calculated exciton binding energy is 1.41 eV for the x and z-directions and 0.85 eV for the y-direction. Finally, it can be stated that LiBO2 is a wide-gap insulator with a gap greater than 7 eV in the monoclinic structure. This indicates the high transparency of the LiBO2 crystal, with the prospect of application in optical instruments.

The suppressing of excitonic effects in Cu-chalcogenides for solar cell applications

Excitonic effect can usually significantly enhance the optical absorption coefficient in wide-gap insulators. Cu-chalcogenides are widely used as thin-film solar cell absorbers due to their high light utilization efficiency. Theoretical investigations on the excitonic absorption in Cu-chalcogenides are rare because of the complicated two-particle nature of an exciton. Here, we theoretically evaluate the continuum-excitonic effect in CuGaS2 and CuInS2 by solving the Bethe-Salpeter equation. Despite the large bandgap of 2.4 eV for CuGaS2, the excitonic effect induces moderate absorption enhancement when compared with the single-particle scenario. Moreover, the experimental results are better reproduced using the latter approach, indicating that the intrinsic excitonic effect is nearly suppressed in the practical experiments. The moderate intrinsic excitonic effect and its further suppressing in the experiments are explained by strong electronic screening effects in these materials. Justification of the simple single-particle approach would be convenient for future theoretical studies, which otherwise require more complex methods with more expensive computational efforts.

18‐Crown‐6 Additive to Enhance Performance and Durability in Solution‐Processed Halide Perovskite Electronics

Recently, an "interlayer" has been often adopted in organic-inorganic hybrid perovskite light-emitting diodes (PeLEDs). The term "interlayer" infers that the layer function is not clear, but it improves electroluminescence (EL) performance. In this respect, it is of interest to determine the exact role of the interlayer and how it works in PeLEDs. In this study, the interlayer is determined to play a crucial role in suppressing the chemical reaction between the metal oxide and hybrid perovskite layers. Nevertheless, the use of an interlayer, a wide gap insulator, does not guarantee the best PeLED performance because it hinders charge injection into the emission layer. Here, a method is proposed that does not apply an "interlayer" but enables simultaneous attainment of high EL performance and outstanding device stability. 18-crown 6-ether (18C6) additive (2.5mg mL-1 ) is found to fully suppress the chemical reaction between the metal oxide and hybrid perovskite layers. With the 18C6 additive, an 82-fold longer device lifetime and very low operating voltage (3.2V at 10 000cd m-2 ) are demonstrated in a PeLED.

Bulk NdNiO2 is thermodynamically unstable with respect to decomposition while hydrogenation reduces the instability and transforms it from metal to insulator

Through CaH2 chemical reduction of a parent R3+Ni3+O3 perovskite form, superconductivity was recently achieved in Sr-doped NdNiO2 on SrTiO3 substrate. Using density functional theory (DFT) calculations, we find that stoichiometric NdNiO2 is significantly unstable with respect to decomposition into 1/2[Nd2O3 + NiO + Ni] with exothermic decomposition energy of +176 meV/atom, a considerably higher instability than that for common ternary oxides. This poses the question if the stoichiometric NdNiO2 nickelate compound used extensively to model the electronic band structure of Ni-based oxide analog to cuprates and found to be metallic is the right model for this purpose. To examine this, we study via DFT the role of the common H impurity expected to be present in the process of chemical reduction needed to obtain NdNiO2. We find that H can be incorporated exothermically, i.e., spontaneously in NdNiO2, even from H2 gas. In the concentrated limit, such impurities can result in the formation of a hydride compound NdNiO2H, which has significantly reduced instability relative to hydrogen-free NdNiO2. Interestingly, the hydrogenated form has a similar lattice constant as the pure form (leading to comparable XRD patterns), but unlike the metallic character of NdNiO2, the hydrogenated form is predicted to be a wide gap insulator thus, requiring doping to create a metallic or superconducting state, just like cuprates, but unlike unhydrogenated nickelates. While it is possible that hydrogen would be eventually desorbed, the calculation suggests that pristine NdNiO2 is hydrogen-stabilized. One must exercise caution with theories predicting new physics in pristine stoichiometric NdNiO2 as it might be an unrealizable compound. Experimental examination of the composition of real NdNiO2 superconductors and the effect of hydrogen on the superconductivity is called for.

Effect of Laser Irradiation on Emissivity of Flame-Generated Nanooxides

The application of pyrometry to retrieve particle temperature in particulate-generating flames strictly requires the knowledge of the spectral behavior of emissivity of light-emitting particles. Normally, this spectral behavior is considered time-independent. The current paper challenges this assumption and explains why the emissivity of oxide nanoparticles formed in flame can change with time. The suggested phenomenon is related to transitions of electrons between the valence and conduction energy bands in oxides that are wide-gap dielectrics. The emissivity change is particularly crucial for the interpretation of fast processes occurring during laser-induced experiments. In the present work, we compare the response of titania particles produced by a flame spray to the laser irradiation at two different excitation wavelengths. The difference in the temporal behavior of the corresponding light emission intensities is attributed to the different mechanisms of electron excitation during the laser pulse. Interband transitions that are possible only in the case of the laser photon energy exceeding the titania energy gap led to the increase of the electron density in the conduction band. Relaxation of those electrons back to the valence band is the origin of the observed emissivity drop after the UV laser irradiation.

Calculation and interpretation of classical turning surfaces in solids

Classical turning surfaces of Kohn–Sham potentials separate classically allowed regions (CARs) from classically forbidden regions (CFRs). They are useful for understanding many chemical properties of molecules but need not exist in solids, where the density never decays to zero. At equilibrium geometries, we find that CFRs are absent in perfect metals, rare in covalent semiconductors at equilibrium, but common in ionic and molecular crystals. In all materials, CFRs appear or grow as the internuclear distances are uniformly expanded. They can also appear at a monovacancy in a metal. Calculations with several approximate density functionals and codes confirm these behaviors. A classical picture of conduction suggests that CARs should be connected in metals, and disconnected in wide-gap insulators, and is confirmed in the limits of extreme compression and expansion. Surprisingly, many semiconductors have no CFR at equilibrium, a key finding for density functional construction. Nonetheless, a strong correlation with insulating behavior can still be inferred. Moreover, equilibrium bond lengths for all cases can be estimated from the bond type and the sum of the classical turning radii of the free atoms or ions.

Probing the Electronic Band Gap of Solid Hydrogen by Inelastic X-Ray Scattering up to 90GPa.

Metallization of hydrogen as a key problem in modern physics is the pressure-induced evolution of the hydrogen electronic band from a wide-gap insulator to a closed gap metal. However, due to its remarkably high energy, the electronic band gap of insulating hydrogen has never before been directly observed under pressure. Using high-brilliance, high-energy synchrotron radiation, we developed an inelastic x-ray probe to yield the hydrogen electronic band information insitu under high pressures in a diamond-anvil cell. The dynamic structure factor of hydrogen was measured over a large energy range of 45eV. The electronic band gap was found to decrease linearly from 10.9 to 6.57eV, with an 8.6 times densification (ρ/ρ_{0}∼8.6) from zero pressure up to 90GPa.

Ultrawide-bandgap Moiré diamanes based on bigraphenes with the twist angles Θ ∼ 30°

Moiré diamanes (Dnθ) based on bigraphenes (BGθ) with a layer rotation angle (θ) of about 30° are considered by ab initio methods. The adsorption of hydrogens or fluorines on the bigraphene surface leads to the formation of interlayer covalent bonds. The resulting structure has only sp3-hybridized atoms, which leads to the appearance of a wide bandgap. Bandgaps of hydrogenated Dn29.4 and Dn27.8 and fluorinated F-Dn29.4 and F-Dn27.8 are 3.6, 3.3, 4.1, and 4.5 eV, respectively, which are larger than the dielectric gaps of ordinary diamanes based on the non-twisted AA- or AB-bigraphenes (≈3 eV). Possible applications of these 2D wide-gap dielectrics were also discussed.

Electronic structure and sodium-ion diffusion in glaserite-type A3−хNa1+х(MoO4)2 (A = Cs, K) studied with first-principles calculations

Here we present an ab initio insight into the electronic structure and sodium diffusion in A3−хNa1+х(MoO4)2 with trigonal (A = Cs) and monoclinic (A = K) glaserite-type structures. Our DFT calculations predict the stability of К3−xNa1+x(МoO4)2 to high sodium content and a significantly smaller homogeneity region for Cs3−xNa1+x(МoO4)2 in accordance with experimental findings. These molybdates are wide-gap insulators with a band gap of 4.2 eV, which is very weakly dependent on the sodium content. The calculated electric field gradients at the Na and A sites were related to the anisotropy of electronic charge distribution and used to predict the quadrupole frequencies of the 133Cs, 39K and 23Na NMR lines in Cs3−хNa1+х(MoO4)2 and K3−хNa1+х(MoO4)2. We examined the possible diffusion paths of Na+ ions and showed that the direct Na−Na migration is unlikely due to a large energy barrier in both molybdates. Instead, the Na+ ions can migrate through the A positions with much lower barriers in A3−xNa1+x(МoO4)2. Therefore, designing stable non-stoichiometric compositions may be a way to reach good sodium-ion diffusion in double molybdates with the glaserite-type structure.

Giant Exciton Mott Density in Anatase TiO2

Elucidating the carrier density at which strongly bound excitons dissociate into a plasma of uncorrelated electron-hole pairs is a central topic in the many-body physics of semiconductors. However, there is a lack of information on the high-density response of excitons absorbing in the near-to-mid ultraviolet, due to the absence of suitable experimental probes in this elusive spectral range. Here, we present a unique combination of many-body perturbation theory and state-of-the-art ultrafast broadband ultraviolet spectroscopy to unveil the interplay between the ultraviolet-absorbing two-dimensional excitons of anatase TiO_{2} and a sea of electron-hole pairs. We discover that the critical density for the exciton Mott transition in this material is the highest ever reported in semiconductors. These results deepen our knowledge of the exciton Mott transition and pave the route toward the investigation of the exciton phase diagram in a variety of wide-gap insulators.

Reexaming the ground state and magnetic properties of curium dioxide

The ground state electronic structure and magnetic behaviors of curium dioxide (CmO$_{2}$) are controversial. In general, the formal valence of Cm ions in CmO$_{2}$ should be tetravalent. It implies a $5f^{6.0}$ electronic configuration and a non-magnetic ground state. However, it is in sharp contrast with the large magnetic moment measured by painstaking experiments. In order to clarify this contradiction, we tried to study the ground state electronic structure of CmO$_{2}$ by means of a combination of density functional theory and dynamical mean-field theory. We find that CmO$_{2}$ is a wide-gap charge transfer insulator with strong 5$f$ valence state fluctuation. It belongs to a mixed-valence compound indeed. The predominant electronic configurations for Cm ions are $5f^{6.0}$ and $5f^{7.0}$. The resulting magnetic moment agrees quite well with the experimental value. Therefore, the magnetic puzzle in CmO$_{2}$ can be appropriately explained by the mixed-valence scenario.

Accurate optical spectra of solids from pure time-dependent density functional theory

We present accurate optical spectra of semiconductors and insulators within a pure Kohn-Sham time-dependent density-functional approach. In particular, we show that the onset of the absorption is well reproduced when comparing to experiment. No empirical information nor a theory beyond Kohn-Sham density-functional theory, such as $GW$, is invoked to correct the Kohn-Sham gap. Our approach relies on the link between the exchange-correlation kernel of time-dependent density functional theory and the derivative discontinuity of ground-state density-functional theory. We show explicitly how to relate these two quantities. We illustrate the accuracy and simplicity of our approach by applying it to various semiconductors (Si, GaP, GaAs) and wide-gap insulators (C, LiF, Ar).

Na2HeO: A possible helium compound from first-principles study

By first-principle study, we propose a possible stable helium-based compound with chemical formula of Na2HeO. It has a structure of space group [Formula: see text]. Energy calculation indicates that it has a local minimum with a lattice constant [Formula: see text] Å. Electronic band structure suggests that it is a wide gap insulator. Analysis on Hirshfeld charges and charge density distribution shows a chemical bond formed between O and He atoms in this structure. Results from both elastic constants and phonon dispersion indicate that this compound in the proposed structure is both mechanically and dynamically stable.

Comment on “Role of the transition state in muon implantation” and “Thermal spike in muon implantation”

Vil\~ao et al. (Phys. Rev. B 96, 195205 (2017) and Phys. Rev. B 99, 195206 (2019)) have reported positive muon spin rotation/relaxation (${\ensuremath{\mu}}^{+}\mathrm{SR}$) experiments on various insulating metal oxides, solar cell materials, and alloyed Ge. They note the presence of a separate early component in the ${\ensuremath{\mu}}^{+}$ polarization time dependence which relaxes more rapidly than the longer lived signal and also exhibits (in ${\mathrm{ZrO}}_{2}$:Mg) a slightly increased frequency at low temperature in transverse field. They interpret this early component as a weakly bound ``transition state'' of the neutral muonium ($\mathrm{Mu}={\ensuremath{\mu}}^{+}{e}^{\ensuremath{-}}$) atom formed epithermally by the incoming 4-MeV muon, in which the hyperfine interaction between the muon and the electron is ``motionally narrowed'' by rapid spin exchange, resulting in a ``diamagneticlike'' Larmor precession signal. This ``diamagneticlike transition state'' supposedly takes microseconds to relax into its final ground state due to the reluctance of the surrounding lattice to deform around the Mu atom. However, numerous earlier experiments (including those in electric fields) on various liquids and solids ranging from wide-gap insulators to narrow-gap semiconductors have convincingly shown that Mu formation proceeds through capture of radiolysis electrons by thermalized muons---a process crucially dependent on the electron mobility rather than epithermal dynamics. Therefore, we question the validity of the interpretation by Vil\~ao et al. and raise some important issues that need clarification.

The Reaction and Microscopic Electron Properties from Dynamic Evolutions of Condensed-Phase RDX Under Shock Loading

Abstract Microscopic electron properties of α-hexahydro-1,3,5-trinitro-1,3,5-triazine (α-RDX) with different shock wave velocities have been investigated based on molecular dynamics together with multi-scale shock technique. The studied shock wave velocities are 8, 9 and 10 km ⋅ s−1. It has been said that the shock sensitivity and reaction initiation of explosives are closely relevant with their microscopic electron properties. The reactions, including the reaction products, which are counted from the trajectory during the simulations are analysed first. The results showed that the number of the products strictly rely on shock wave velocities. The reaction rates and decomposition rates are also studied, which showed the differences between the different shock velocities. The results of electron properties show that α-RDX is a wide-gap insulator in the ground state and the metallisation conditions of shocked RDX are determined, which are lower than under-static high pressure.