Dielectric Band Gap Research Articles

Interference-based laser-induced micro-plasma ablation of glass

AbstractGlass is one of the most important technical surfaces for numerous applications in automotive, optical, and consumer industries. In addition, by producing textured surfaces with periodic features in the micrometre range, new functions can be created. Although laser-based methods have shown to be capable to produce structured materials in a wide amount of materials, due to its transparency large bandgap dielectrics can be only processed in a controlled manner by employing high-power ultra-short pulsed lasers, thus limiting the employable laser sources. In this article, an interference-based method for the texturing of soda-lime glass using a 15 ns pulsed (1 kHz repetition rate) infrared (1053 nm) laser is proposed, which allows fabricating different periodic patterns with micrometre resolution. This method consists on irradiating a metallic absorber (stainless steel) put in direct contact with the glass sample and inducing locally an etching process on the backside of the glass. Then, the produced plasma at the interference maxima positions leads to the local fabrication of well-defined periodic line-like and dot-like surface patterns. The produced patterns are characterised using white light interferometry and scanning electron microscopy.

The effect of Co-doping on dielectric properties and bandgap of zinc silicate nanowires

In this study, pristine and cobalt (Co) doped zinc silicate (Zn2SiO4) nanowires were synthesized by a simple and cost-effective hydrothermal method. The average length and diameter of Zn2SiO4 nanowires were 1.0–1.5 μm and 40–60 nm, respectively, in electron microscopic studies. In the case of Co-doped Zn2SiO4 nanowires, average length and diameter increase to 4.0–5.0 μm and 70–90 nm, respectively, and a significant increase in particle size was observed. A dielectric constant measurement of pristine and Co-doped Zn2SiO4 nanowires was performed in the frequency range of 20 Hz–2 MHz. The dielectric constant of pristine and Co-doped Zn2SiO4 nanowires was found to be 212 and 1360 at a frequency of 20 Hz, respectively. Such dramatic enhancement in the dielectric constant (approximately six fold) of Co-doped Zn2SiO4 nanowires suggests that these nanowires are promising materials for applications in next generation charge storage devices. UV–Vis spectral analysis shows a remarkable reduction in the optical bandgap of Co-doped Zn2SiO4 nanowires as a result of doping. Such enhancement in the dielectric constant and reduction in the optical bandgap were discussed in light of doping and size enhancement.

BAND STRUCTURE, METALLIZATION AND STRUCTURAL PHASE TRANSITION OF NABR UNDER HIGHPRESSURE

The metallization and the phase transition of the alkali bromide sodium bromide (NaBr) is investigated through its band structure. The equilibrium lattice constant, bulk modulus, pressure derivative of bulk modulus and the phase transition pressure at which the compound undergo structural phase transition from NaCl (B1) to the CsCl (B2) structure is predicted from the total energy calculations. The ground state properties and band gap values are compared with the experimental and other theoretical results. At normal pressure NaBr is a direct band gap insulator. When the pressure is increased there is enhanced overlapping between the wave functions of the neighboring atoms. As a result the widths of the valence and empty conduction bands increase. These changes lead to the narrowing and indirect closing of band gap in NaBr (metallization). It is also confirmed that the metallization and structural phase transition do not occur simultaneously in ionic compounds.

A Monte Carlo calculation of the secondary electron emission in the backward direction from a SiO2 macro-capillary

A Monte Carlo calculation of the secondary electron emission from a SiO2 macro-capillary in the backward direction induced by electron irradiation is presented with the aim to understand transmission guiding of a scanning electron beam through a borosilicate glass macro-capillary. The theoretical modeling of electron transport in SiO2 capillary incorporates the elastic, inelastic and phonon scatterings, resulting, respectively, from the interactions with nucleus, electrons and phonons. The influence on electron inelastic scattering by the insulator bandgap is also considered. In this work a simplified approach has been employed to deal with the charging of the internal wall of the capillary and it is found that at a glancing incident angle, the secondary electrons are mostly originated from the top surface.Graphical abstract

Thermoluminescence, photoluminescence and optically stimulated luminescence characteristics of CaSO4:Eu phosphor: Experimental and density functional theory (DFT) investigations

The CaSO4:Eu phosphor in nanocrystalline form was obtained by chemical method. The sample was annealed at various temperatures and quenched. The structural, electronic and optical properties are studied using various experimental techniques. As synthesized CaSO4:Eu particles have nanorod shapes with diameter of ~15 nm and length of ~250 nm. After annealing (at around 900 °C) a significant increase in their size (~2–4 μm) with phase transformation from hexagonal to orthorhombic was observed. Thermoluminescence (TL) and optically stimulated luminescence (OSL) intensities were found to increase with temperature up to 900 °C and decrease thereafter for 1 Gy of test dose of β-rays from 90Sr-90Yr source. However, the maximum OSL sensitivity was found to be more than that of CaSO4:Eu microcrystalline phosphor (prepared by acid recrystallization method) contrary to the usually found in the literature but much less than that of commercially available α-Al2O3:C phosphor. The activation energy for thermally assisted OSL process was found to be 0.0572 ± 0.0028 eV. The dose ranges of TL and OSL response was found from 0.04 Gy to 100 Gy and 0.02 Gy–100 Gy, respectively. The experimental results are also correlated with computational calculations based on density functional theory (DFT). The crystal structures and electronic structures of both hexagonal and orthorhombic CaSO4 and CaSO4:Eu materials show that they are direct band gap (5.67–5.86 eV) insulators, with Ca2+ substitution by Eu2+ found to introduce donor states in the band gap near Fermi level and the valence band edge of CaSO4 on doping with Eu2+ impurity ions.

Optical absorption in array of Ge/Al-shell nanoparticles in an Alumina matrix

The absorption spectra in array of Ge, Al and Ge/Al-shell nanoparticles immersed in alumina (Al2O3) matrix is calculated in framework of ab initio macroscopic dielectric model. It is demonstrated that absorption is strongly enhanced when germanium nanospheres are encapsulated by Al-shell. Two absorption peaks, appearing in the spectra, correspond to low energy ω+ and high energy ω− plasmons which lie in visible and ultraviolet frequency range, respectively. It is demonstrated that in Ge/Al-shell composite the ω+ plasmon exists only because quantum confinement effect which provides larger Ge band gap (Δ ~ 1.5 eV) and thus prevent decay of ω+ plasmon to continuum of interband electron-hole excitation in semiconducting core. Absorption in visible frequency range enhances additional 3 times when alumina is replaced by large dielectric constant insulator, such as SiC, and additional 6 times when Ge core is replaced by wide band-gap insulator, such as Si3N4. Strong enhancement of optical absorption in visible frequency range make this composites suitable for optoelectronic application, such as solar cells or light emitting devices. The simulated plasmon peaks are brought in connection with peaks appearing in ellipsometry measurements.

Electronic properties and Hirshfeld surface analysis of dihydroxylammonium 5,5′-bistetrazole-1,1′-diolate under pressure

We present ab initio calculations to investigate the pressure induced variations in the electronic structure of dihydroxyl-ammonium 5,5′-bistetrazole-1,1′-diolate (TKX-50) using empirical dispersion correction method proposed by Grimme (DFT-D2). The calculated electronic band structure using Tran-Blaha modified Becke Johnson (TB-mBJ) potential show that TKX-50 is an indirect band gap insulator along C-B direction with a band gap of 4.018 eV, while the band gap obtained with standard generalized gradient approximation is 3.54 eV. The electronic band gap of TKX-50 is found to decrease monotonically as a function of pressure. We also performed the Hirshfeld surface calculations and the corresponding 2-D fingerprint analysis to understand the nature of intermolecular interactions and their quantitative contributions towards the crystal packing. The largest percentage of TKX-50 molecule is due to N...H/H...N and O...H/H...O contacts (∼70%), which represents hydrogen bonding networks as the primary intermolecular interactions and also responsible for the low sensitivity of TKX-50

Two-dimensional electron gas modeling in strained InN/GaN hetero-interface under pressure and impurity effects

The two-dimensional electron gas (2DEG) density modeling in WZ strained InN/GaN hetero-interface with Ga-face arrangement is considered in this paper. We theoretically investigate the role of an externally applied hydrostatic pressure and impurity's position by analyzing their impact in the presence of built electric field due to the dielectric mismatch between the system and its surrounding matrix. To solve the coupling Schrödinger-Poisson equations, as well as the polarization induced charges, finite element method code is used. Pressure-dependence of dielectric constant, band-gap energy, lattices and internal electric field is considered. The 2DEG appears to be strongly-dependent internal and external perturbations exhibiting a non-monotonic behavior. Our results prove that: (1) the InN channel is more populated for on-center impurity compared to off-center one, (2) the electron density in the triangular shape quantum well increases with the pressure; (3) the 2DEG density is enhanced versus the positive impurity's position and dropped versus pressure.

First-principle study of the microstructure and electronic properties for Cr3+ doped yttrium orthoaluminate

Chromium ion (Cr3+) doped YAlO3 host crystal shows strong prospects for the exploitation of new optical materials. However, the microstructure of Cr3+ doped of YAlO3 is still unclear. In this work, we systematically explore the structural evolutions of Cr3+ doped of YAlO3 system by using the CALYPSO method and density functional theory. A novel cage-like structure with P-1 space group is identified for the first time. The impurity Cr3+ ion can naturally substitute the Al3+ ion of the central site in the crystal lattice. The result show that the impurity Cr3+ concentration is determined to be 6.25%, which is almost comparable to the experiments. The perfect agreement between our simulated X-ray diffraction patterns and the measured ones demonstrates the validity of the ground-state structure. The calculation of electronic band structure for Cr3+ doped YAlO3 reveals a closure of the insulator band gap, forming an energy gap of 2.94 eV. The result of electronic density of states shows that it is the impurity Cr3+ ion that leads to a transition from insulator to semiconductor. By analyzing the electron localization function of Cr3+ doped of YAlO3 crystal, we conclude that the Cr–O interaction is dominantly ionic bond. The obtained results could provide an important understanding of the transition-metal doped host materials.

Chemical and structural stability of superconducting In5Bi3 driven by spin–orbit coupling

Relativistic effects play a prominent role in many electronic material properties such as the Rashba and Dresselhaus spin splitting in inversion asymmetric crystals, or the bulk band gap in topological insulators. By contrast, macroscopic material properties are typically not connected to relativistic phenomena. As an exception to this rule, we show that the macroscopic chemical and structural properties of superconducting In5Bi3 are driven by relativistic physics. In the non-relativistic limit In5Bi3 decomposes into elemental indium and bismuth, but the inclusion of relativistic spin–orbit coupling chemically stabilizes the In5Bi3 stoichiometry. Similarly, the structural stability of tetragonal In5Bi3 is driven by the spin–orbit interaction, which eliminates a phonon instability present in the non-relativistic limit. Low-temperature resistivity and heat capacity measurements show that In5Bi3 is a strong coupling superconductor, with a superconducting critical temperature of 4.2 K and a superconducting critical field of 0.3 T. The unconventional interplay between relativity with chemistry and structure, together with the presence of superconductivity, make In5Bi3 a versatile material that provides, for example, a simple model for the study of strong coupling superconductivity in quasiperiodic crystals.

Band Gap of 3D Metal Oxides and Quasi-2D Materials from Hybrid Density Functional Theory: Are Dielectric-Dependent Functionals Superior?

Reproduction of the band gaps of semiconductors and insulators represents a well-known problem for standard DFT approaches based on semilocal functionals. The problem can be partly solved using hybrid functionals, in which a given portion of exact exchange is mixed with the DFT exchange. Recently, a new class of dielectric-dependent functionals has been introduced in which the amount of exact exchange is derived from the static dielectric function of a given compound. In this study we considered in a systematic way on an equal footing a set of 24 nonmagnetic three-dimensional (3D) bulk metal oxides and 24 quasi-two-dimensional (quasi-2D) semiconductors (oxides, hydroxides, chlorides, oxyhalides, nitrides, and transition metal dichalcogenides) and computed the corresponding Kohn-Sham band gaps with three global hybrid functionals and four range-separated hybrid functionals. These in turn were divided into standard (PBE0, B3LYP, HSE06, SC-BLYP) and dielectric-dependent (DD-B3LYP, DD-SC-BLYP, DD-CAM-B3LYP) functionals. We also performed a statistical analysis of the DFT data set along with structural parameters of these 2D and 3D materials. The surprising result is that overall there is no real improvement with the use of dielectric-dependent functionals compared to PBE0, HSE06, and B3LYP. Short-range DD-SC-BLYP gives a minor improvement in the band gaps for bulk metal oxides compared with standard SC-BLYP, but the mean absolute error is still 0.12 eV higher than with B3LYP. The use of dielectric-dependent standard or short-range functionals such as DD-B3LYP or DD-HSE06 worsens the situation. However, the dielectric-dependent version of the long-range-separated functional implemented with the Coulomb attenuating method (CAM), DD-CAM-B3LYP, leads to a clear improvement for band gaps of quasi-2D materials. On the basis of this analysis, the conclusion is that the use of a standard hybrid functional such as B3LYP or HSE06 is recommended for nonmagnetic bulk 3D metal oxides. On the other hand, the treatment of layered materials such as MoO3 or V2O5 benefits from the use of dielectric-dependent range-separated functionals.

Charge Transport in Alkali-Metal Superoxides: A Systematic First-Principles Study

Charge-transport mechanisms within the discharge products of alkali-metal/O2 batteries can strongly influence the performance of these systems. To date, discharge products comprising alkali peroxides (Li2O2 and Na2O2) and superoxides (LiO2, NaO2, and KO2) have been observed. In general, cells that discharge to a superoxide exhibit lower overpotentials than those that form the corresponding peroxide. These lower overpotentials have been hypothesized to originate from more efficient charge transport within the superoxides. While the transport mechanisms in the peroxides have been well studied, consensus regarding the intrinsic conductivity across the alkali-metal superoxides is lacking. Here, we present a systematic first-principles study of charge-transport mechanisms across the alkali-metal (Li, Na, K) superoxides. Our study draws on our prior investigations of the alkali peroxides and of sodium superoxide, while adding new analyses for lithium and potassium superoxides (LiO2 and KO2). In the case of KO2, a nonsymmetrized room-temperature structure is proposed to account for a dynamic Jahn–Teller effect. Band gaps, equilibrium (charged) defect concentrations, mobilities, and intrinsic conductivities are estimated for both LiO2 and KO2. Overall, the alkali superoxides are predicted to be wide band gap insulators, with gaps exceeding 4 eV. These large band gaps imply that negligible transport occurs via band conduction. Compared to the alkali peroxides, ionic conductivities in the superoxides are predicted to be 8–11 orders of magnitude larger, ranging from 4 × 10–9 to 5 × 10–12 S/cm at room temperature. Regarding electronic conductivities, transport in NaO2 and KO2 is predicted to occur via polaron hopping, with low conductivities on the order of 10–19 to 10–20 S/cm. These values are similar to what has been previously reported for the peroxides. Importantly, the present calculations indicate that LiO2 has a much larger electronic conductivity (also arising from polaron hopping) than its Na and K analogues, 9 × 10–12 S/cm at 300 K. These data suggest that the low overpotentials observed for Na- and KO2-based batteries cannot be explained by high intrinsic electronic conductivities. In contrast, the much larger conductivities predicted for LiO2 imply that the superior performance observed in LiO2-based cells may reflect the ability of LiO2 to support higher charge-transport rates.

A Parametric Model for Temperature Dependence of Dielectric Function of AlSb Film.

We present analytic representation of dielectric function (ɛ ═ ɛ1 + iɛ₂) data from 1.7 to 5.0 eV for the temperature from 300 to 803 K of oxide-free AlSb that are the closest representation to date of the intrinsic bulk dielectric response ɛ of the material. Pseudodielectric functions 'ɛ' were measured on a 1.5 µm thick film grown on (001) GaAs by molecular beam epitaxy. Data were obtained with the film in situ to avoid surface oxidation artifacts. The dielectric function parametric model and multilayer calculation were performed to obtain pure dielectric function and fundamental bandgap (E0) of the AlSb film. The ɛ spectrum was successfully reconstructed by seven polynomials and a pole, which can be used to determine ɛ for arbitrary temperatures. Our results should be useful for device design based on AlSb.

Evolution of topological superconductivity by orbital-selective confinement in oxide nanowires

We determine the optimal conditions to achieve topological superconducting phases having spin-singlet pairing for a planar nanowire with finite lateral width in the presence of an in-plane external magnetic field. We employ a microscopic description that is based on a three-band electronic model including both the atomic spin-orbit coupling and the inversion asymmetric potential at the interface between oxide band-gap insulators. We consider amplitudes of the pairing gap, spin-orbit interactions and electronic parameters that are directly applicable to nanowires of LaAlO$_3$-SrTiO$_3$. The lateral confinement introduces a splitting of the $d$-orbitals that alters the orbital energy hierarchy and significantly affects the electron filling dependence of the topological phase diagram. Due to the orbital directionality of the $t_{2g}$-states, we find that in the regime of strong confinement the onset of topological phases is pinned at electron filling where the quasi flat heavy bands start to get populated. The increase of the nanowire thickness leads to a changeover from sparse-to-dense distribution of topologically non-trivial domains which occurs at the cross-over associated to the orbital population inversion. These findings are corroborated by a detailed analysis of the most favorable topological superconducting phases in the electron doping-magnetic field plane highlighting the role of orbital selective confinement.

Semilocal exchange-correlation potentials for solid-state calculations: Current status and future directions

Kohn-Sham (KS) density functional theory (DFT) is a very efficient method for calculating various properties of solids as, for instance, the total energy, the electron density, or the electronic band structure. The KS-DFT method leads to rather fast calculations; however, the accuracy depends crucially on the chosen approximation for the exchange and correlation (xc) functional Exc and/or potential vxc. Here, an overview of xc methods to calculate the electronic band structure is given, with the focus on the so-called semilocal methods that are the fastest in KS-DFT and allow to treat systems containing up to thousands of atoms. Among them, there is the modified Becke-Johnson potential that is widely used to calculate the fundamental bandgap of semiconductors and insulators. The accuracy for other properties like magnetic moment or electron density, that are also determined directly by vxc, is also discussed.

Tuning the superconducting transition of SrTiO3-based 2DEGs with light

The resistivity of the two dimensional electron gas that forms at the interface of strontium titanate with various oxides is sensitive to irradiation from visible light. In this letter, we present data on the interface between the bandgap insulators LaAlO3 (LAO) and SrTiO3 (STO). We operate a light emitting diode at temperatures below 1 K and utilize it to irradiate the LAO/STO interface at ultralow temperatures. On irradiation, the resistance of this system is lowered continuously by a factor of five and the resistance change is persistent at low temperatures as long as the sample is kept in the dark. This makes the characterization of transport properties in different resistive states over extended time periods possible. Our pristine sample gets superconductive below 265 mK. The transition temperature Tc shifts downward on the persistent photo-induced lowering of the resistance. The persistent photoconductance can be completely reverted by heating the structure above 10 K, in which case, Tc takes on its original value. Thus, very similar to field-effect induced changes of the electron density, irradiation at low temperatures offers a versatile tuning knob for the superconducting state of STO-based interfaces which in addition has the advantage to be nonvolatile.

A Broadband TE01–TE11 Mode Converter With Elliptical Section for Gyro-TWTs

The mode converter is a key component in a gyrotron traveling-wave tube (gyro-TWT) application system, which can transform the output circular TE $_{0n}$ modes (in a gyro-TWT loaded with lossy ceramic dielectric or photonic bandgap interaction structure, the output mode is normally a TE $_{0n}$ mode) into a mode conducive to transmission and emission (linearly polarized Gaussian-like HE11 mode). In particular, the bandwidth of the TE01–TE11 mode converter is the most important challenge in the conversion sequence. In this paper, based on the deformation of the transverse cross section and the modified multi-frequency iteration synthesis technique along the longitudinal propagation axis, a novel method for synthesis of a broad bandwidth mode converter is proposed. Then, an application of the method to a wideband serpentine TE01–TE11 mode converter with an elliptical section is given. This novel mode converter including transitional waveguides achieves a 19.18% relative bandwidth (conversion efficiency over 95%) at center frequency 30.5 GHz, which increases nearly 195% compared with the conventional circular design, and its structure length is 1151.65 mm. Overall, this method offers a novel solution to the requirement of broad bandwidth in the high-power microwave transmission system.

A Study on Effect of Sol Aging Time on Optical Properties of ZnO Thin Films: Spectroscopic Ellipsometry Method

Background: Due to wide band gap and large excitonic binding energy, being inexpensive, abundance in nature and easy synthesis ZnO is a promising candidate in many applications such as solar cells. Experimental: In the current work a series of ZnO thin films were deposited on glass substrates using sol-gel method to investigate the change in optical behavior of the film with sol aging time (asprepared, 8, 16, 24 and 32 hours) and the annealing temperature (300ᵒC and 500ᵒC). The optical properties of thin films were explored using spectroscopic ellipsometry method including the real and imaginary part of refractive index, real and imaginary part of dielectric function and band gap energy of the layers in the 300-900 nanometer wavelength range. Results: It can be deduced from the results that sol aging time and annealing temperature, affect the optical properties of the samples. Using single oscillator energy model of Wemple and Di Domenico parameters such as free charge carrier concentration ratio to effective mass, and plasma frequency, were calculated. Conclusion: The films prepared using 24 h aged solution, had the highest transmittance and the largest band gap energy.

Giant Band Gap Reduction and Insulator–Metal Transition in Two-Dimensional InX (X = Cl, Br, I) Layers

Manipulating the electronic structures of semiconductors by an external electric field promises great potential in future electric technology. However, most of the explored low-dimensional systems only show a weak response under external electric field, and the effective factor, which is defined as ΔEg/U (U is the induced electric potential difference and ΔEg is the change of band gap), is very low. Here, we demonstrated that an unexpected giant band gap modulation is realized in indium halides layers, and the effective factor is much higher than the reported results. Through comprehensive first-principles calculations, we found that two-dimensional InX (X = Cl, Br, I) monolayers are semiconductors with band gaps of 2.59, 2.30, and 2.08 eV, respectively. Remarkably, the band gaps of InX (X = Cl, Br, I) monolayers are dramatically reduced by an external electric field, and the band gap reductions under an electric field of 0.5 V/A found for InCl, InBr, and InI monolayers are 1.51, 1.50, and 1.38 eV, respec...

Effect of thermal annealing on the structural, optical and microstructural properties of a-SiC thin films

ABSTRACT)Silicon nanocrystals in a host matrix of silicon carbide SiC are important to high band gap dielectrics for solar cells applications. Forming Si-NC is difficult considering that SiC nano-clusters are easily formed in the host matrix. This analysis focuses on the influence of deposition temperature (Td) and annealing ambience to synthesise SiC film for forming Si-NC in the host matrix. Here Si-NC is obtained by co-sputtering Si and C targets at 350°C and 500°C. Annealing is done at various temperatures using vacuum (VA) or conventional thermal annealing (CTA). Its structural and optical properties are investigated using spectroscopic ellipsometry, Fourier-transform spectroscopy (FTIR) and X-ray spectroscopy (XPS). The findings are its refractive index is between 2.69-4.08, showing Si abundance in the film. FTIR spectra shows change in intensity and position of Si-C bond variations of 620-700 cm−1with change in Td and XPS spectra confirms the presence of Si-C- and Si-Si-related bonds, influenced with increase in Td and VA. CTA samples show increase in Si-O related bonds which indicates increase in oxygen presence. Hence a higher Td=5000Cwith VA appears to be a vital to synthesize Si-NC predominantly for optoelectronic applications.