Electron Energy Loss Function Research Articles

Theoretical investigation of some fundamental physical properties of the ternary nitrides Ca4SiN4 and Ca4GeN4 under pressure and temperature effect

We reported for the first time a theoretical study of structural, electronic, optical, elastic and thermodynamic properties of Ca4SiN4 and Ca4GeN4, based on DFT + PP-PW approach. The calculated structural parameters are in good agreement with the experimental data. Both considered alkaline earth metalloid nitrides are more compressible along the c-axis. The band structure calculations showed that the studied compounds are direct bandgap (Γ¬Γ) semiconductors; the energy bandgap given by GGA (LDA) is equal to 1.501 (1.598) eV and 1.462 (1.583) eV for Ca4SiN4 and Ca4GeN4, respectively. The optical parameters including dielectric function, refractive index, extinction coefficient, absorption coefficient, reflectivity and electron energy-loss function were computed for different polarizations of the incident light. Single-crystal elastic constants, polycrystal elastic moduli and their pressure dependence were calculated and discussed. The two nitrides satisfy the mechanical stability criteria over the considered pressure range. We have also investigated their thermodynamic properties by computing Debye temperature, thermal expansion coefficient α and heat capacities CP and CV.

Exploring the electronic and optical anisotropy of quasi-one-dimensional ternary chalcogenide CrSbSe3: a DFT study

The anisotropy of the electronic and optical properties of the quasi-one-dimensional ternary transition metal chalcogenide CrSbSe3 is investigated using density functional theory (DFT) including the on-site Coulomb self-interaction potential (DFT + U) and spin-orbit interaction. The compound is an indirect bandgap semiconductor with bandgap of 0.58 eV. Effective masses and charge recombination rates along different directions show appreciable anisotropy, indicating the possibility of sensor applications. CrSbSe3 exhibits satisfactorily high carrier mobility and charge recombination rates along all directions, making it suitable for solar and photovoltaic applications. The Se-p and Cr-d orbitals contribute to the DOS in the valence band, while DOS contribution to the conduction band bottom comes mainly from Sb-p and Cr-d orbitals. Simulated XPS and HAXPES show that for soft x-rays, the peak in the low binding energy is due to the electron emission from Se-p and Cr-d orbitals while the peak in the high binding energy is due to the electron emission from Cr-d and Se-s orbitals. For hard x-rays, the relative intensity of the low binding energy peak decreases in comparison to the high binding energy peak, indicating enhanced electron emission from Se-s orbitals at higher energy x-rays. CrSbSe3 possesses large anisotropy in optical parameters such as refractive index, optical absorption, reflectivity, optical conductivity and optical extinction. The birefringence and dichroism are large in the IR, visible and near UV regions, with a giant birefringence equal to 0.56 at 650 nm and dichroism equal to 0.57 at 550 nm. The electron energy loss function is anisotropic for high energetic electrons, while for electrons below 4 eV it is isotropic. CrSbSe3 is an ideal material for photovoltaic, sensor and other optoelectronic applications as it possesses a bandgap of 0.58 eV, large optical anisotropy, high values of dielectric constant, refractive index, absorption coefficient and extinction coefficient.

Structural, morphological and optical characterizations of spray pyrolyzed nickel oxide thin films

A simple spray pyrolysis technique was used to prepare thin nickel oxide films on glass substrates at different substrate temperatures (350 °C, 400 °C, and 450 °C). Hydrated Nickel Acetate (Ni(CH3COO)2·4H2O) salt solution was used as a precursor to producing these thin films. The structural properties of the films were characterized by X-ray Diffraction (XRD) spectroscopy. The XRD curve confirms that the films were crystalline, and their preferred growth orientation was along the (111) plane. The values of the lattice parameter, dislocation density, and microstrain were changed with the temperature of the substrate. Scanning Electron Microscope (SEM) micrographs of the surface of the prepared thin films showed uniform morphology with nanorings. A UV-visible Spectrophotometer was used to detect absorption and transmission spectra in the spectral region of 300 nm–900 nm. The films deposited at 400 °C and 450 °C have 80% transparency in the visible wavelength region, while the films deposited at 350 °C have lower transparency. The optical bandgap of the films was varied with substrate temperatures and the values were from 3.55 to 3.65 eV. Other optical metrics, including extinction coefficient, refractive index, optical conductivity, and electron energy loss functions, were also affected by substrate temperature.

Enhanced electronic and optical responses of nitrogen- or boron-doped BeO monolayer: First principle computation

In this work, the electronic and optical properties of a Nitrogen (N) or a Boron (B) doped BeO monolayer are investigated in the framework of density functional theory. It is known that the band gap of a BeO monolayer is large leading to poor material for optoelectronic devices in a wide range of energy. Using substitutional N or B dopant atoms, we find that the band gap can be tuned and the optical properties can be improved. In the N(B)-doped BeO monolayer, the Fermi energy slightly crosses the valence(conduction) band forming a degenerate semiconductor structure. The N or B atoms thus generate new states around the Fermi energy increasing the optical conductivity in the visible light region. Furthermore, the influences of dopant atoms on the electronic structure, the stability, the dispersion energy, the density of states, and optical properties such as the plasmon frequency, the excitation spectra, the dielectric functions, the static dielectric constant, and the electron energy loss function are discussed for different directions of polarizations for the incoming electric field.

First principles investigations of the structural, electronic, mechanical, linear and nonlinear optical properties of RbIO2F2

Rubidium fluoroiodate has been reported to be a potential mid-infrared nonlinear optical material, due to its second harmonic generation (SHG) and Laser damage threshold value of 156 MW/cm2. In this study, the linear and nonlinear optical properties are investigated and analyzed using the density functional theory by full potential linearized augmented plane wave (FP-LAPW) and plane wave pseudo-potential (PP-PW). The FP-LAPW method is used to study the electronic properties and linear optical properties with the recently developed density functional of Tran and Blaha. Furthermore, optical features such as dielectric functions, absorption coefficient, refractive index, reflectivity, electron energy loss function are calculated for photon energies up to 40 eV. The elasticity, SHG response, linear electro-optical effect, and piezoelectricity are studied by the PP-PW approximation. Based on the semi-empirical relations, the Young's modulus, shear and bulk modulus, Poisson's ratio, anisotropic factors, sound velocities, Debye temperature, and hardness have been determined.

Effect of Al doping on morphology and optical properties of spray pyrolized MgO thin films

Thin films of magnesium oxide (MgO) and aluminium (Al) doped MgO were deposited onto glass substrate at 350 °C by spray pyrolysis technique. The microstructure, morphology and optical properties of the MgO films have been investigated and characterized as a function of Al doping concentration (0–2) mol%. X-ray diffraction study confirms the successful fabrication of MgO thin films. The surface morphology of the films reveals that the morphology of the MgO films greatly changes with Al doping concentration. The optical properties like optical transmittance, optical band gap, refractive index, extinction coefficient, real & imaginary part of the dielectric constant, electron energy loss function and optical conductivity have been characterized and discussed. It has been found that the optical properties of MgO films have been changes significantly with Al doping concentration.

Investigation of optoelectronic properties of AgIn1−xGaxY2 (Y = Se, Te) semiconductors

The electronic structure and optical properties of AgIn1−xGaxY2 (Y = Se, Te) are investigated using first-principles calculations based on density functional theory. Both the local density approximation (LDA) and generalized gradient approximation (GGA) are employed in the calculation. The crystal structure of ternary semiconductors is the tetragonal chalcopyrite structure with space group $${\text{I}}\overline{{4}} {\text{2d}}$$ . The lattice parameters a(A) and c(A) are found to vary with the change in Ga composition. The energy gap across the Fermi level in the density of states plots shows that these materials are semiconductors. To compute accurate energy gap values, the LDA + U method is adopted. The calculated elastic constants indicate that these compounds are mechanically stable at normal pressure. The semiconducting nature of these materials may prove their applications in solar cells and photovoltaic absorbers. The optical parameters such as dielectric function, electron energy loss function, refractive index and reflectivity are computed.

Comparative study of the effect of the Hubbard coefficient U on the properties of TiO2 and ZnO

First-principles calculations combined with Hubbard U correction based on Density Functional Theory has been used to investigate the effect of its inclusion on electronic, structural, and optical properties and to reproduce correct band gap for TiO2 (anatase and rutile) and ZnO (wurtzite) structures. The effect of the implementation of U for only metal (Ti or Zn) d state (Ud) or for both 3d (Ud) and Op (Up) states was investigated to exploring changes in valence and conduction bands and their origin. Bader’s charge analysis has been used to revealing the nature of chemical bonding. We can conclude that for both TiO2 polymorphs as the Ud factor increases, both the BG and the lattice parameter increase. On the contrary, for ZnO, as Ud increases the value of BG increases but the cell parameter decreases. The optimal values correspond to Ud =8 eV (TiO2) and Ud =13 eV (ZnO). The corresponding optical properties using Ud, such as real and imaginary parts of dielectric function, reflectivity R(ω), refractive index n(ω), extinction coefficient k(ω), absorption coefficient α(ω) and electron energy loss function L(ω) has been calculated. It is important to highlight that the curves obtained considering the inclusion of the Ud show an excellent agreement with those reported experimentally.

Ab-initio study about the electronic, optical and thermoelectric nature of α-, β-, and γ-phases of CdS semiconductor: using the accurate m-BJ approach

In this work the first principles-based calculations with the FP-LAPW (full potential linear augmented plane wave method) are employed to investigate structural, thermal and optoelectronic properties of the three different phases of CdS binary compound. Our electronic band structure calculations display a direct type band transition with a gap value (Γv − Γc) equal to 0.9 eV, and 1.2 eV for β and γ phases respectively. Due to inexistence of VBM (valence band maxima) and the CBM (conduction band minima) located at the same Γ-point an indirect band transition was predicted in case of α-phase. Our calculated T-DOS and P-DOS visualizes to be shifted successively towards higher values along the energy axis. The σ s−p bonding character in the V/B is responsible for the instigation of relatively localized states S-p, Cd-d and Cd-p states close to Fermi energy. We also computed and discussed the important optical constants like the complex dielectric constant components, electron energy loss functions, reflectivity spectra, the absorption coefficients, refractive indices, real component of optical conductivity and the extinction coefficient. The inter band contribution resulting due to the corresponding optical nature was also studied and discussed in detail for these three phases. The temperature dependent thermoelectric parameters were studied to explore the thermoelectric behaviour. The thermoelectric parameters like thermal conductivity, Seebeck coefficient, specific heat capacity, Power factor, Electrical conductivity, Susceptibility and figure of merit of the three phases are investigated for their possible thermoelectric applications. The present work could be concluded as a theoretical qualitative type calculation related to optoelectronic and thermoelectric nature of the three studied phases and their efficient device application.

Structural, electronic, bonding and optical properties of LiBaPO4: A comparative study of monoclinic and trigonal phases

Crystal structure, electronic, bonding and optical properties of monoclinic and trigonal phases of LiBaPO4 compound have been explored using the density functional theory based orthogonalized linear combination of atomic orbitals method. The outcomes of band structure and density of states reveal the insulating direct band gap (E g = 4.62 eV) and semiconducting indirect band gap (E g = 1.12 eV) natures of monoclinic and trigonal phases respectively. The highest bond order and shortest bond length of P–O bond disclose its highest strength over Ba–O and Li–O bonds in both phases of LiBaPO4. with %BO values of 53.20 and 59.34 in M-LiBaPO4 and T-LiBaPO4 respectively. Optical properties in the form of dielectric function, refractive index, optical conductivity and electron energy loss function have been comprehensively described. Dielectric studies demonstrate that the trigonal phase may find its use as a photovoltaic material for solar cell applications.

First principle investigation of the structural, electronic and optical properties of methylammonium lead iodide: implications for photovoltaic applications

In the present work structural, electronic and optical properties of methylammonium lead iodide have been investigated by full potential linearised augmented plane wave (FP-LAPW) method with the local density approximation. The structural properties; lattice constants, bulk modulus and derivative of bulk modulus are calculated. The direct energy band gap of is found to be 1.52 eV at an R-R point which makes the material suitable for photovoltaic applications. The optical properties; dielectric function, extinction coefficient, refractive index, absorption coefficient, reflectivity, electron energy loss function and photoconductivity of CH3NH3PbI3 are discussed and the analysis confirms reliability of CH3NH3PbI3 material for the solar cell absorption layer.

Synthesis and optical absorption properties of nanocrystalline rare earth hexaborides Nd<sub>1–</sub><i><sub>x</sub></i>Eu<i><sub>x</sub></i>B<sub>6</sub> powders

Nanocrystalline rare earth hexaborides Nd<sub>1–</sub><i><sub>x</sub></i>Eu<i><sub>x</sub></i>B<sub>6</sub> powders are successfully synthesized by the simple solid-state reaction in vacuum condition for the first time. The effect of Eu doping on the crystal structure, grain morphology, microstructure and optical absorption properties of nanocrystalline NdB<sub>6</sub> are investigated by X-ray diffraction, scanning electron microscope (SEM), high resolution transmission electron microscopy (HRTEM) and optical absorption measurements. The results show that all the synthesized samples have a single-phase CsCl-type cubic structure with space group of <i>Pm-</i>3<i>m</i>. The SEM results show that the average grain size of the synthesized Nd<sub>1–</sub><i><sub>x</sub></i>Eu<i><sub>x</sub></i>B<sub>6</sub> powders is 50 nm. The HRTEM results show that nanocrystalline Nd<sub>1–</sub><i><sub>x</sub></i>Eu<i><sub>x</sub></i>B<sub>6</sub> has good crystallinity. The results of optical absorption show that the absorption valley of nanocrystalline Nd<sub>1–</sub><i><sub>x</sub></i>Eu<i><sub>x</sub></i>B<sub>6</sub> is redshifted from 629 nm to higher than 1000 nm with the increase of Eu doping, indicating that the transparency of NdB<sub>6</sub> is tunable. Additionally, the X-ray absorption near-edge structure spectra <i>μ</i>(<i>E</i>) around the Nd and Eu <i>L</i><sub>3</sub> edges for nanocrystalline NdB<sub>6</sub> and EuB<sub>6</sub> show that total valence of Nd ion is estimated at +3 in nanocrystalline NdB<sub>6</sub> and total valence of Eu ion in nanocrystalline EuB<sub>6</sub> is +2. Therefore, the Eu-doping into NdB<sub>6</sub> effectively reduces the electron conduction number and it leads the plasma resonance frequency energy to decrease. In order to further qualitatively explain the influence of Eu doping on the optical absorption mechanism, the first principle calculations are used to calculate the band structure, density of states, dielectric function and plasma resonance frequency energy. The calculation results show that the electron band of NdB<sub>6</sub> and EuB<sub>6</sub> cross the Fermi energy, indicating that they are typical conductors. In addition, the plasmon resonance frequency can be described in the electron energy loss function. The plasmon resonance frequency energy of NdB<sub>6</sub> and EuB<sub>6</sub> are 1.98 and 1.04 eV, which are corresponding to the absorption valley of 626.26 and 1192.31 nm, respectively. This confirms that the first principle calculation results are in good consistence with the experimental optical absorption valley. Therefore, as an efficient optical absorption material, nanocrystalline Nd<sub>1–</sub><i><sub>x</sub></i>Eu<i><sub>x</sub></i>B<sub>6</sub> powders can expand the optical application scope of rare earth hexaborides.

First-principle calculation of electronic and optical properties of VO2 by GGA-1/2 quasiparticle approximation

Correctly elucidating the strong electron correlations of vanadium dioxide (VO2) is of great significance to understand the physics of the metal–insulator transition (MIT) and develop potential applications of VO2. Standard density functional theory is believed to be inappropriate to describe the MIT of VO2. Herein, the recently developed GGA-1/2 quasiparticle approximation is employed to perform first-calculations on VO2. The electronic structures of the metallic and insulating phases of VO2 are well described. The GGA-1/2 calculations indicate that the preferential occupancy of the d// orbitals leads to strong Mott–Hubbard correlation, which induces the splitting of the d// orbitals and the MIT of VO2. The calculations on electron energy-loss function reveal that the satellite electronic energy-loss spectroscopy peak of metallic phase VO2 is resulting from the plasma resonance. This work demonstrates that the GGA-1/2 approach facilitates the electron correlation calculations of VO2 and suggests that the strong Coulomb correlation is necessary to trigger the MIT.

Optoelectronic properties of confined water in angstrom-scale slits

The optoelectronic properties of confined water form one of the most active research areas in the past few years. Here we present the multiscale methodology to discern the out-of-plane electronic and dipolar dielectric constants (${\ensuremath{\varepsilon}}_{\ensuremath{\perp}}^{\text{el}}$ and ${\ensuremath{\varepsilon}}_{\ensuremath{\perp}}^{\text{dip}}$) of strongly confined water. We reveal that ${\ensuremath{\varepsilon}}_{\ensuremath{\perp}}^{\text{el}}$ and ${\ensuremath{\varepsilon}}_{\ensuremath{\perp}}^{\text{dip}}$ become comparable for water confined in angstrom-scale channels (with a height of less than $15\phantom{\rule{0.28em}{0ex}}\AA{}$) within graphene (GE) and hexagonal boron nitride (hBN) bilayers. Channel height ($h$) associated with a minimum in both ${\ensuremath{\varepsilon}}_{\ensuremath{\perp}}^{\text{el}}$ and ${\ensuremath{\varepsilon}}_{\ensuremath{\perp}}^{\text{dip}}$ is linked to the formation of the ordered structure of ice for $h\ensuremath{\approx}(7\text{--}7.5)\phantom{\rule{0.28em}{0ex}}\AA{}$. The recently measured total dielectric constant ${\ensuremath{\varepsilon}}_{\ensuremath{\perp}}^{T}$ of nanoconfined water [L. Fumagalli et al., Science 360, 1339 (2018)] is corroborated by our results. Furthermore, we evaluate the contribution from the encapsulating membranes to the dielectric properties, as a function of the interlayer spacing, i.e., the height of the confining channel for water. Finally, we conduct analysis of the optical properties of both confined water and GE membranes, and show that the electron energy loss function of confined water strongly differs from that of bulk water.

First principles probes of electronic and optical behaviours of zinc doped cuprous oxide for catalysis applications

In order to examine the Zn doping impact on the electronic and optical behaviours of the host Cu2O material at various concentrations (x = 3.125%, 12.5%, and 25%) of Zn, we have performed first principles calculations. The computed electronic density of states of the pure bulk material Cu2O is established which corroborates the available previous experimental and theoretical reports. The Zn doped host material, made Cu2O to act as an n-type semiconductor, although the donor levels are formed nearby the lower conduction band. The bonding character showed that the orbital ordering disappeared on Cu atoms when cuprous oxide is doped with Zn. The interesting optical aspects, like the dielectric function, absorption coefficient, refractive index, optical conductivity, reflectivity, and electron energy loss function, are examined. We anticipate that this theoretical work will serve for further experimental realizations to engineer the electronic and optical behaviors of Cu2O doped with Zn for photocatalysis, sensors, and electronic device applications.

First-principles structural, electronic, optical and bonding properties of scandium-based ternary indide system Sc5T2In4 (T = Ni, Pd, Pt)

The structural, electronic and optical properties of rare-earth, scandium-based compounds within the family of RE5-T2-In4 (T = Ni, Pd, Pt) intermetallics are calculated by using density functional theory-based orthogonalized linear combination of atomic orbitals method. The studied compounds crystallize in orthorhombic structure with space group Pbam (No. 55). The electronic properties exhibit conducting features of all three compounds. Charge transfer and crystal strength analysis were carried out by computing bond order and effective charge. Regarding optical properties the complex dielectric function, optical conductivity and electron energy loss function have been assessed. All the three compounds demonstrate optically anisotropic behaviour for energy values up to 7.0 eV and turned towards isotropic nature at higher energy (>7.0 eV). Optical conductivity spectra designate that the compounds are optically active for visible to UV light. Two major peaks are observed in energy loss spectra, at 13.2 and 32.6 eV, which correspond to plasmonic resonance points. The interatomic bonding characteristics between distinct pair of atoms within each compound have also been elaborated in comprehensive manner.

Optical and electronic properties of zigzag boron nitride nanotube (6,0): DFT study

The optical and electronic properties of boron nitride nanotubes have been studied using the full potential linear augmented plane wave method in the framework of density functional theory. The electronic properties such as band structure and density of states have been investigated using the generalized gradient approximation. In addition, the optical parameters of boron nitride nanotube (6,0) have been investigated such as real and imaginary parts of the dielectric function, electron energy loss function, refractive index, extinction coefficient, and optical conductivity. The obtained values are in better agreement with the available experimental data.

Sputtering of LiF and other halide crystals in the electronic energy loss regime

Sputtering experiments were performed by irradiating LiF, NaCl, and RbCl crystals with various swift heavy ions like S, Ni, I, Au with energies between 60 and 210 MeV, C60 clusters between 12 and 30 MeV or Pb ions between 730 and 6040 MeV. Sputtered species are collected on arc-shaped catchers and subsequently analyzed by elastic recoil detection analysis or Rutherford backscattering analysis. The study focuses on angular distributions and total yields for LiF and covers a broad range of experimental parameters including cleaved or rough sample surfaces, ion fluence, beam incident angles, and different ion velocities leading to electronic energy loss (Se) values from 5 to 45 keV/nm. In most cases, the angular distribution has two components, a jet-like peak perpendicular to the surface sample superimposed on a broad isotropic cosine distribution whatever is the beam incident angle. The observation of the jet depends mainly on the surface flatness and angle of ion incidence. However, the jet does not appear clearly when irradiated with C60 cluster. The sputtering yield is stoichiometric and characterized by huge total yields of up to a few 105 atoms per incident ion. The yield follows a power law as function of electronic energy loss, Y follows an exponential law with Sen with n ~ 4. While the azimuthal symmetry for sputtering is observed at low ion velocity (~1 MeV/u), it seems to be lost at high velocity (>4 MeV/u). The data provide a comprehensive overview how the angular distribution and the total sputtering yield scale with the energy loss, beam incidence angle and ion velocity. Complementary experiments have been done with NaCl and RbCl targets confirming the observation made for LiF.

Insight DFT studies about the optoelectronic properties of Fe and Ga doped Mg-based hydrides: Efficient materials for optical devices

We reported and discussed results for the first time for the electronic and optical properties of Fe doped MgH2 and Ga doped MgH2Fe hydride materials by first-principles calculations using generalized gradient approximation (GGA) within the density functional theory (DFT). The effective contributions of various electronic bands and density of states calculations confirms a quite weak bonding nature between Fe, Mg and H atoms in Fe doped MgH2 system. Also, the addition of Ga, the interaction between Mg and H atoms show weak bonding along with extensive reduction in the PDOS of both Mg and H atoms closer to Fermi level. The optical properties such as dielectric function, refractive index, extinction coefficient, absorption, reflectivity spectra, electron energy loss function and the real part of optical conductivity were calculated and discussed in detail. The observed sharp energy loss peak values for both the studied systems match up with the edge of plasma energy and corresponds to plasma frequency which indicates transition between semiconductor and dielectric characters. The base for the absorption was discussed in detail in addition to observed spectral peaks on the basis of their electronic band structures and crystal field nature.

Electronic, thermal, and optical properties of graphene like SiCx structures: Significant effects of Si atom configurations

We investigate the electronic, thermal, and optical characteristics of graphene like SiCx structure using model calculations based on density functional theory. The change in the energy bandgap can be tuned by the Si atomic configuration, rather than the dopants ratio. The effects of the concentration of the Si atoms and the shape of supercell are kept constant, and only the interaction effects of two Si atoms are studied by varying their positions. If the Si atoms are at the same sublattice positions, a maximum bandgap is obtained leading to an increased Seebeck coefficient and figure of merit. A deviation in the Wiedemann-Franz ratio is also found, and a maximum value of the Lorenz number is thus discovered. Furthermore, a significant red shift of the first peak of the imaginary part of the dielectric function towards the visible range of the electromagnetic radiation is observed. On the other hand, if the Si atoms are located at different sublattice positions, a small bandgap is seen because the symmetry of sublattice remains almost unchanged. Consequently, the Seebeck coefficient and the dielectric function are only slightly changed compared to pristine graphene. In addition, the electron energy loss function is suppressed in Si-doped graphene. These unique variations of the thermal and the optical properties of Si-doped graphene are of importance to understand experiments relevant to optoelectronic applications.