Wide Band Gap Insulators Research Articles

Structural, electronic and optical properties of novel carbonate fluorides ABCO3F (A=K, Rb, Cs; B=Ca, Sr)

In the present study, we performed first principles calculations on the electronic and optical properties of layered alkali–alkaline earth carbonate fluorides which attract attention in the domain of nonlinear optics. The calculated lattice parameters and volumes with and without inclusion of van der Waals (vdW) correction methods to standard density functional methods were compared with experiments. We observed that vdW interactions are predominant in RbCaCO3F and CsCaCO3F as compared with other computed compounds. The calculated bulk modulus from single crystal elastic constants reveals that these materials are all relatively harder than the KH2PO4 (KDP) crystal. We also found that these materials are wide band gap insulators as obtained from Tran–Blaha modified Becke–Johnson potential. The linear optical properties such as dielectric function, refractive indices, birefringence and absorption spectra are presented. Finally, the calculated birefringence values indicate that these crystals could be promising for producing phase matching in the deep ultra-violet region.

First principles calculations of LiNH2BH3, LiNH3BH4, and NaNH2BH3

The structural, bonding and elastic properties of alkali amidoboranes (LiNH2 BH3, LiNH3 BH4, and NaNH2 BH3) have been studied. We employ first principles calculations based on van der Waals (vdW) corrected density functional theory. In the presence of alkali metal the electronic distributions of B–H and N–H bonds are modified and reduce the di-hydrogen bonding. These effects significantly reduce the role of vdW in binding these compounds. Further, the band structure and density of states are calculated to get basic insights on distribution of electronic states. These compounds are found to be wide band gap insulators with the band gap values for LiNH2 BH3, LiNH3 BH4, and NaNH2 BH3 are 4.08, 5.61, and 3.96 eV, respectively. The charge density distribution and bond population analysis are used to understand the nature of bonding. It is noted that these compounds have a strong covalent bonding between B–H and N–H atoms. The calculated elastic constants reveals that these compounds are mechanically stable and LiNH2 BH3 is found to be less plastic compared to the LiNH3 BH4 and NaNH2 BH3.

Novel Electrical Conduction of Insulators under Examination of Defects and Injection and Relationship to Theories of Ferroelectric Domains

ABSTRACTElectrical conductions in insulators such as resistance switching, conduction at interfaces, and conduction at domain boundaries and free surface of ferroelectrics are of interest. These conductions are often attributed to novel mechanism such as ferroelectric polarization. On the other hand, these interpretations appear not fully accepted, because the recent advanced theories of ferroelectric domains disregard screening indicated by these conduction phenomena. That is, these conduction phenomena are quietly regarded as the classical conduction originating from defects. In this paper, we examine these conductions in pure wide bandgap insulators in view of defects, using the direct-accessibility (tangibility) of conduction at free surfaces. Although most of these conductions in ferroelectrics may not be useful in large-scale applications, we show that they have fundamental implications on renovations of ferroelectric basics.

Defect Studies Using Advances with Ion Beam Excited Luminescence

Luminescence techniques offer extremely sensitive probes of imperfections and structural changes in insulating materials. Among the many possible routes of signal excitation are the use of ion beams (IL) and the rarely used combination of ion beams with other excitation sources such as photons (PL), electrons (CL) or X-rays (RL). Ion beams can stimulate intrinsic signals as well as those from defect sites and control of the ion beam energy allows one to vary the depth of material that is being studied. Equally valuable is that variations in the excitation rate change the relative importance of signals from different types of luminescence site. Such flexibility in the excitation methods allows detailed quantification and identification of the structures which have been formed in irradiated material. Additional identification and separation of overlapping features is feasible by lifetime resolved luminescence, and/or by pulsing the ion beam. Ion beam coupled with other excitations can be used to probe defects via the luminescence characteristics of their excited states. Such combinations of IL with PL, CL or RL offer more insights than are possible with each technique independently. For the wide band gap insulators, such as sapphire or silica, the value of full spectral coverage in luminescence detection and photo-excitation will be emphasized, not least because from the optical absorption data it is evident that the simple defect structures become more complex in highly irradiated materials. Possibilities of resolving nanoparticle growth, amorphisation or new phases can all be addressed via the luminescence signals. A comment on systematic errors in data handling and processing is included.

Generation of periodic surface structures on silica fibre surfaces using 405 nm CW diode lasers

Periodic surface structures have been observed on the end surfaces of synthetic silica fibres when they are exposed to long-term irradiation with light from a 405nm CW diode laser. The surface structures are generated when the laser power is at a level which is three magnitudes of order lower than that of the damage threshold. They exhibit multiple bends, break-ups and bifurcations, unlike interference patterns but rather like the effect caused by short-pulsed laser irradiation on wide band-gap insulators. The detailed investigation undertaken in this work has concluded that the key parameters that contribute to the generation of the surface structures are power density, surface roughness, polarisation direction and the presence of ultraviolet defect centres.

Manipulating Mn–Mgk cation complexes to control the charge- and spin-state of Mn in GaN

Owing to the variety of possible charge and spin states and to the different ways of coupling to the environment, paramagnetic centres in wide band-gap semiconductors and insulators exhibit a strikingly rich spectrum of properties and functionalities, exploited in commercial light emitters and proposed for applications in quantum information. Here we demonstrate, by combining synchrotron techniques with magnetic, optical and ab initio studies, that the codoping of GaN:Mn with Mg allows to control the Mnn+ charge and spin state in the range 3≤n≤5 and 2≥S≥1. According to our results, this outstanding degree of tunability arises from the formation of hitherto concealed cation complexes Mn-Mgk, where the number of ligands k is pre-defined by fabrication conditions. The properties of these complexes allow to extend towards the infrared the already remarkable optical capabilities of nitrides, open to solotronics functionalities, and generally represent a fresh perspective for magnetic semiconductors.

Transition of strong, wide band-gap oxide insulators to metallic glasses at 100 GPa (Mbar) pressures, etc

This paper is a review of experimental results observed for shock and statically compressed sapphire (single-crystal Al2O3) and implications of these results. Many of these were unexpected prior to experiments: (i) Hugoniot and 300-K theoretical isotherm are nearly coincident to 340 GPa, (ii) shock-compressed sapphire probably achieves minimum metallic conductivity (MMC) at ~300 GPa, as does fluid H at 140 GPa, (iii) Both crossovers are driven by shock dissipation and both probably occur by a bonds to bands transition (BtBT), (iv) the BtBT is driven by shock-induced damage which breaks bonds at such high densities that wave functions of atoms so formed hybridize into energy bands; (v) disorder caused by sluggish phase transitions in a DAC suggest sapphire also reaches MMC in a DAC at ~300 GPa; (vi) the structural ground states of all solids at very high pressures might be metallic glasses; (vii) Super Earths can have magnetic fields caused only by convecting conducting oxides without the need for Fe; (viii) alumina powders in a DAC might attain MMC at ~300 GPa and perhaps at pressures as low as ~200 GPa. Al2O3 is probably amorphous in a DAC at 200 GPa

Graphane Nanotubes

In this work, one-dimensional graphane nanotubes (GN, stoichiometry CH), built from 2D single-sheet graphanes, are explored theoretically. Zigzag type GN(10,0) and armchair type GN(10,10) structures with varying surface termination were investigated in detail. GN(10,10)-A is found to be the most stable configuration among the GN structures considered. An annealing analysis indicates that graphane-A and GN(10,10)-A are likely to be stable at elevated temperature. A possible reaction path to GN(10,10)-A is suggested by the reaction of single-walled carbon nanotube (10,10) + H(2); the indications are that the GN(10,10)-A can be made at low temperature and high partial pressure of H(2) gas from the corresponding nanotube. The graphane nanotubes are predicted to be wide band gap insulators. A study of the effect of the diameter of GN structures shows, unexpectedly, that the gap increases on reducing the diameter of the graphane nanotubes. We also investigated several partially hydrogenated graphenes and single-walled carbon nanotubes (SWNT); the greater hydrogenation is, the more stable is the resulting structure. The band gap of graphene or SWNT can be tuned via hydrogenation.

Vacancy induced metallicity at the CaHfO3/SrTiO3 interface

Density functional theory is used to study the electronic properties of the oxide heterointerface CaHfO3/SrTiO3. Structural relaxation is carried out with and without O vacancies. As compared to related interfaces, strongly reduced octahedral distortions are found. Stoichiometric interfaces between the wide band gap insulators CaHfO3 and SrTiO3 turn out to exhibit an insulating state. However, interface metallicity is introduced by O vacancies, in agreement with experiment. The reduced octahedral distortions and necessity of O deficiency indicate a less complicated mechanism for the creation of the interfacial electron gas.

Decoherence in fast atom diffraction from surfaces

Diffraction of fast atoms from crystal surfaces at grazing incidence (GIFAD) has now been observed on all types of materials, from wide band gap insulators to metals, including semiconductors. Since mainly the (slow) motion normal to the surface is important diffraction patterns are comparable to those obtained in thermal energies atomic diffraction (TEAS), however, the specific scattering geometry of GIFAD has a strong influence on decoherence phenomena. The contribution of atomic vibrations is much less pronounced than in TEAS but other sources of decoherence such as electronic excitations, clearly observed on metals, can participate due to the comparatively large projectile velocity parallel to the surface. We present here simple models that describe these decoherence effects. The results are in good agreement with the experimental results.

Electronic structure, mechanical properties and thermal conductivity of Ln2Zr2O7 (Ln = La, Pr, Nd, Sm, Eu and Gd) pyrochlore

The electronic structure, chemical bonding and mechanical properties of Ln2Zr2O7 (Ln=La, Pr, Nd, Sm, Eu and Gd) pyrochlore are investigated by local-density approximation of spin polarized scheme+U calculations (U is the Hubbard energy) and further verified by the experimental results. Ln2Zr2O7 compounds are wide band gap insulators, and this is consistent with the experiment results. The calculated spin polarized density of states of them indicates that they are in a ferromagnetic state and the magnetic moment is mainly attributed to the 4f shell of the Ln atoms. For the chemical bonds in the Ln2Zr2O7 crystals, the O–Zr bond is stronger than the O–Ln bond. The hardness, elastic constants, bulk modulus, shear modulus, Young’s modulus and Poisson’s ratio of Ln2Zr2O7 compounds are investigated and the theoretical values are in good agreement with the experiments. The mechanical anisotropic properties are discussed using Zener’s indexes and universal elastic anisotropic index (AU). The sound velocities along [100], [110] and [111] directions are calculated for each Ln2Zr2O7 crystal. The thermal conductivities of Ln2Zr2O7 compounds are evaluated and the obtained thermal conductivity of Ln2Zr2O7 is lower than that of yttria-stabilized zirconia, indicating that they could be good low thermally conductive materials at high temperature.

Anomalous molecular orbital variation upon adsorption on a wide band gap insulator

It is commonly believed that organic molecules are physisorbed on the ideal nonpolar surfaces of wide band gap insulators with limited variation in the electronic properties of the adsorbate molecule. On the basis of first principles calculations within density functional theory (DFT) and GW approximation, we show that this is not generally true. We find that the molecular frontier orbitals undergo significant changes when a hydroxy acid (here we chose gluconic acid) is adsorbed on MgSO(4) x H(2)O(100) surface due to the complex interaction between the molecule and the insulating surface. The predicted trend of the adsorption effect on the energy gap obtained by DFT is reversed when the surface polarization effect is taken into account via the many-body corrections.

Boron-nitride and boron-carbonitride nanotubes: synthesis, characterization and theory

We present in this review a joint experimental and theoretical overview of the synthesis techniques and properties of boron-nitride (BN) and boron-carbonitride (BCN) nanotubes. While their tubular structure is similar to that of their carbon analogues, we show that their electronic properties are significantly different. BN tubes are wide band gap insulators while BCN systems can be semiconductors with a band gap in the visible range.

Accurate Band Gaps of Semiconductors and Insulators with a Semilocal Exchange-Correlation Potential

A modified version of the exchange potential proposed by Becke and Johnson [J. Chem. Phys. 124, 221101 (2006)10.1063/1.2213970] is tested on solids for the calculation of band gaps. The agreement with experiment is very good for all types of solids we considered (e.g., wide band gap insulators, sp semiconductors, and strongly correlated 3d transition-metal oxides) and is of the same order as the agreement obtained with the hybrid functionals or the GW methods. This semilocal exchange potential, which recovers the local-density approximation (LDA) for a constant electron density, mimics very well the behavior of orbital-dependent potentials and leads to calculations which are barely more expensive than LDA calculations. Therefore, it can be applied to very large systems in an efficient way.

High temperature scanning tunneling microscopy of purely ion conducting yttria stabilized zirconia (YSZ)

Scanning tunneling microscopy (STM) on wide band gap insulators is generally not possible. Here we demonstrate that ionic insulators with high ion mobility, such as yttria stabilized zirconia, are an exception to this rule. The ion conductivity ensures that the sample maintains a defined potential relative to the STM tip and thus a stable tunnel junction. Consequently, ion conductors provide an opportunity for characterizing wide band gap insulators, and thin films and nanostructures supported on them by STM.

Thermodynamic and electronic properties of quaternary hydrides Li xNa 1− xMgH 3

The thermodynamic and electronic properties of Li x Na 1− x MgH 3 ( x = 0, 0.25, 0.5 and 0.75) have been investigated using the density functional theory within the generalized-gradient approximation. The obtained cohesive energies indicate that the stability of crystal increases with increasing Li element in Li x Na 1− x MgH 3. The reaction enthalpies for Li x Na 1− x MgH 3 phases have been investigated along four possible dehydrogenation reaction pathways, and the enthalpy change of two pathways is found to be nearly linearly reduced with increasing the Li substitution level from x = 0 to x = 0.75. The electronic densities of states show that with increasing Li substitution at Na sites in NaMgH 3, the band gaps of the wide-band gap insulators Li x Na 1− x MgH 3 become wider, and the contribution of Li s and p states to the total electronic densities of states become larger while Na s and p states become less. The obtained calculation results suggest that Li substitution in NaMgH 3 may result in a favorable modification for onboard hydrogen storage application.

An atomic force pencil and eraser

A method of writing and erasing conducting nanostructures at the interface between the wide-bandgap insulators LaAlO3 and SrTiO3 is presented. New developments for ltrahigh-density information storage look feasible

Electron heating in the conduction band of insulators irradiated by ultrashort laser pulses

We investigate the theoretical problem of electron heating in the conduction band of wide band gap insulators and semiconductors induced by intense femto-second Ti:Sapphire laser pulses. We analyze in detail the heating mechanism due to the sequence of direct interbranch transitions in the conduction band, which has been shown to be of crucial importance in previous work. This analysis is fulfilled by resolving the time dependant Schr\odinger equation (TDSE) in a basis of Bloch functions for the CsI crystal. The field is represented semiclassically and the laser-electron interaction is treated in the dipole approximation. The presented approaches are based on a one-active electron approximation. First the TDSE is solved in a basis of Bloch functions, in one dimension, the influence of laser and crystal parameters on the electron spectra is studied. The electron transfer from the lower conduction band to the higher one is already effective at intensity of $3\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$. Then the problem is solved in three dimension. The electron spectra is consistent with the experimental results, we note in particular the presence of a large plateau at intensities of the order of the terawatt per square centimeter.

Electronically coupled complementary interfaces between perovskite band insulators

Perovskite oxides exhibit a plethora of exceptional properties, providing the basis for novel concepts of oxide-electronic devices. The interest in these materials is even extended by the remarkable characteristics of their interfaces. Studies on single epitaxial connections between the wide-bandgap insulators LaAlO3 and SrTiO3 have revealed them to be either high-mobility electron conductors or insulating, depending on the atomic stacking sequences. For device applications, as well as for a basic understanding of the interface conduction mechanism, it is important to investigate the electronic coupling of closely spaced complementary interfaces. Here we report the successful realization of such coupled interfaces in SrTiO3-LaAlO3 thin-film multilayer structures. We found a critical separation distance of six perovskite unit cell layers, corresponding to approximately 23 A, below which a decrease of the interface conductivity and carrier density occurs. Interestingly, the high carrier mobilities characterizing the separate conducting interfaces are found to be maintained in coupled structures down to subnanometre interface spacing.

Quasiparticle Self-ConsistentGWTheory

In past decades the scientific community has been looking for a reliable first-principles method to predict the electronic structure of solids with high accuracy. Here we present an approach which we call the quasiparticle self-consistent approximation. It is based on a kind of self-consistent perturbation theory, where the self-consistency is constructed to minimize the perturbation. We apply it to selections from different classes of materials, including alkali metals, semiconductors, wide band gap insulators, transition metals, transition metal oxides, magnetic insulators, and rare earth compounds. Apart from some mild exceptions, the properties are very well described, particularly in weakly correlated cases. Self-consistency dramatically improves agreement with experiment, and is sometimes essential. Discrepancies with experiment are systematic, and can be explained in terms of approximations made.