Electronic Dielectric Functions Research Articles

First‐Principle Study of Bandgap Engineering and Optical Properties of Monolayer WSe2 in Second Near‐Infrared Windows

AbstractFluorescence imaging in the second near‐infrared II (NIR‐II) window is opening up new possibilities in bioimaging due to its low scattering rate within the tissue. The integration of 2D materials with NIR‐II fluorescence will enable the development of multifunctional imaging probes. However, there are very few 2D materials that can fluoresce in the NIR‐II range. Monolayer WSe2 is a potential 2D material, but its photoluminescence (PL) around 790 nm is still far from the NIR‐II range due to its bandgap of 1.54 eV. In this study, it is investigated the electronic structures, dielectric functions, and PL spectra for Te, I, and Cr‐doped monolayer WSe2, as well as W and S vacant monolayer WSe2. Most of the defected monolayer WSe2 remain semiconductors, except for a few configurations exhibiting metallic properties after making vacancies. Among the monolayer WSe2 under investigation, the Cr‐doped WSe2 performs the best, exhibiting a strong PL peak in NIR‐II with a decreased bandgap around 1.0 eV. As increasing Cr concentration, the peak shifts further toward the red end of the spectrum due to an enhancement of p–d transition. The results provide a useful guideline for material synthesis applied in NIR‐II bioimaging and other biophysics.

Large bulk photovoltaic effect and second-harmonic generation in few-layer pentagonal semiconductors PdS2 and PdSe2

Recently, atomically thin PdSe2 semiconductors with rare pentagonal Se–Pd–Se monolayers were synthesized and were also found to possess superior properties such as ultrahigh air stability, tunable band gap and high carrier mobility, thus offering a new family of two-dimensional (2D) materials for exploration of 2D semiconductor physics and for applications in advanced opto-electronic and nonlinear photonic devices. In this work, we systematically study the nonlinear optical (NLO) responses [namely, bulk photovoltaic effect (BPVE), second-harmonic generation (SHG) and linear electric-optic (LEO) effect] of noncentrosymmetric bilayer (BL) and four-layer PdS2 and PdSe2 by applying the first-principles density functional theory with the generalized gradient approximation plus scissors-correction. First of all, we find that these few-layer PdX 2 (X = S and Se) exhibit prominent BPVE. In particular, the calculated shift current conductivity is in the order of 130 μA V−2, being very high compared to known BPVE materials. Similarly, their injection current susceptibilities are in the order of 100 × 108 A V−2 s−1, again being large. Secondly, the calculated SHG coefficients (χ (2)) of these materials are also large, being one order higher than that of the best-known few-layer group 6B transition metal dichalcogenides. For example, the maximum magnitude of χ (2) can reach 1.4 × 103 pm V−1 for BL PdSe2 at 1.9 eV and 1.2 × 103 pm V−1 at 3.1 eV for BL PdS2. Thirdly we find significant LEO coefficients for these structures in the low photon energy. All these indicate that 2D PdX 2 semiconductors will find promising NLO applications in light signal modulators, frequency converters, electro-optical switches and photovoltaic solar cells. Fourthly, we find that the large BPVE and SHG of the few-layer PdX 2 structures are due to strong intralayer directional covalent bonding and also 2D quantum confinement. Finally, we also discuss the prominent features of these NLO spectra of these materials in terms of their electronic structure and optical dielectric functions.

Optoelectronic properties of PbSe monolayers from first-principles

In recent years, the two-dimensional (2D) materials have received considerable attention for next-generation technological applications due to their unique physical properties. In this article, we explore the energetic stability, electronic, and optical properties of three different PbSe monolayers (such as α-, β-, and γ- types) for potential optoelectronic and photovoltaic applications. The results presented in this work are obtained from first-principles calculations within density functional theory. The formation and cohesive energies calculated for the PbSe monolayers have been found well-matching to that of stable 2D monochalcogenides which validates their energetic stability. Calculations of the electronic structures show all the three types of monolayers semiconductors in nature with indirect bandgaps of magnitude 0.45, 1.39, and 1.26 eV for α-, β-, and γ-PbSe respectively. The interband optical transitions taking place within these monolayers are identified from the orbital resolved electronic structures and dielectric functions. The β-PbSe exhibited isotropic electronic structures and optical spectra whereas a significant degree of anisotropy is seen in the optical spectra of α- and γ- types of PbSe monolayers. The refraction spectra show the transparent nature of these monolayers ranging from near-infrared to a broad range of the ultraviolet spectrum. Moreover, they show substantially low reflectivity and large optical absorption of the incident light. These features advocate effective applications of these novel PbSe monolayers in cutting-edge optoelectronic and photovoltaic devices.

Electronic Bands and Dielectric Functions of In0.5Tl0.5I Solid State Solution with Structural Defects

We investigate an influence of the crystal structure imperfections on the electronic properties and dielectric functions of the In0.5Tl0.5I semiconductor in the frame of the density functional theory calculations. The tensor of electron effective mass m^{*}_{ij} for the InI, In0.5Tl0.5I and TlI crystals has been calculated for the valence and conduction bands at different K-points of the Brillouin zone. The dielectric functions ε(hν) of the imperfect crystals based on In0.5Tl0.5I solid state solution with iodine vacancy and a thallium interstitial atom were calculated taking into consideration the inter-band and intra-band electron transitions. The studies of the imperfect crystals reveal increased low-frequency and stationary electron conductivity with anisotropy resulted from the anisotropy of the electron effective mass tensor. Our findings explain the origin of crucial changes in the band structure by formation of the donor half-occupied levels close to the unoccupied conduction bands due to the crystal structure defects, i.e., iodine vacancy or a thallium interstitial atom. It has been shown that in the case of real crystals, in particular metal-halides, the proper consideration of defects in quantum-chemical calculations results in a better matching with experimental data and, opposite to the perfect structure calculations, gives opportunities to explain the observed phenomena.

Temperature dependent spectroscopic ellipsometry and Raman scattering of PbTiO_3-based relaxor ferroelectric single crystals around MPB region

To discover intrinsic relationship between optical bandgap and structural transformations in relaxor ferroelectric single crystals, electronic band structures and dielectric functions of xPb(In1/2Nb1/2)O3-(1 − x − y)Pb(Mg1/3Nb2/3)O3-yPbTiO3 single crystals (x∼0.27–0.28, y∼0.29–0.35) around morphotropic phase boundary have been investigated by variable-temperature (200–750 K) spectroscopic ellipsometry. It was found that the discontinuous evolution from the second derivative of dielectric functions corresponds to structural transformation patterns. Using the SCP (standard critical point) model, four typical interband transitions (Ea∼2.8 eV, Eb∼3.6 eV, Ec∼4.6 eV, and Ed∼5.4 eV) can be uniquely assigned. These interband transitions are mainly attributed to the contributions from B-O bonds and multiphase coexistence. Furthermore, a modified phase diagram based on interband transition variations with the temperature and PT composition for PIMN-PT crystals was provided. In order to verify the accuracy of phase transition temperature, temperature-dependent low-wavenumber Raman scattering was used as a support. The present results provide important supports for the theoretical model, which establish a quantitative relationship between the electronic transition and structural transformation for ferroelectric oxides.

Electronic band structure and optical properties of ferroelectric TGS, TGSe and TGFB crystals

Structural and electronic properties of the ferroelectric TGS, TGSe and TGFB crystals were studied by the ab initio method in the framework of the density functional theory. Equation of state (total energy vs. unit cell volume), band structure, density of electronic states and dielectric functions in the range of valence electrons excitations have been calculated using the plane waves and pseudopotentials. The comparable structural and electronic characteristics of TGS, TGSe and TGFB crystals are obtained for the first time and discussed in relation to the reference experimental data. The semiempirical dispersion-correction approach implemented in CASTEP code has been applied to the materials studied and has been shown to produce results more close to the experimental data. Correlation of the total energy difference Ep – Ef for paraelectric and ferroelectric phases and Curie temperatures of the ferroelectric phase transitions has been revealed for TGS, TGSe and TGFB crystals, that was not reported before.

Electronic and magnetic properties of BiFeO3 with intrinsic defects: First-principles prediction

The electronic structure, magnetism, and dielectric functions of BiFeO3 with intrinsic vacancies, including Bi-, Fe-, and O-vacancies (denoted as VFe, VBi, and VO, respectively) are investigated using the first-principles density functional theory plus U calculations. It is revealed that the structural distortions associated with those vacancies impose significant influences on the total density of state and magnetic behaviors. The existence of VBi favors the excitation of the O2p state into the band gap at 0.4 eV, while the O2p and Fe3d orbitals are co-excited into the band gap around 0.45 eV in VFe. Consequently, a giant net magnetic moment of 1.96 μB is generated in VFe, and a relatively small moment of 0.13 μB is induced in VBi, whereas VO seems magnetically inactive. The giant magnetic moment generated in VFe originates from the suppression of the spatially modulated antiferromagnetic spin structure. Furthermore, VFe and VBi have strong influences on dielectric function, and induce some strong peaks to occur in the lower energy level. In contrast, VO has a small effect.

Density-Functional Theory Investigation of Sr2CrOsO6 with Cubic Symmetry Using Modified Becke—Johnson Potential

We investigate the electronic structures and optical dielectric functions of the high temperature phase of Sr2 CrOsO6 with cubic structure by using Tran and Blaha's modified Becke and Johnson exchange potential. In the absence of spin-orbit coupling, the total spin moment is 0μB. When spin-orbit coupling is included, the small total spin moment and an unquenched Os orbital moment appear, and the spin non-conservation gap becomes smaller. The calculated net magnetic moment is smaller than the popular generalized gradient approximation result, and the spin non-conservation gap is larger. The optical dielectric functions with spin-orbit coupling are not very different from the ones without spin-orbit coupling.

Calculation and Analysis of the Dielectric Functions for BaTiO3, PbTiO3, and PbZrO3

The pseudo-potential plane wave method and the generalized gradient approximation (GGA) have been used to calculate the electronic structures and dielectric functions of cubic BaTiO3 (BT), PbTiO3 (PT), and PbZrO3 (PZ). The dielectric constant imaginary parts (e2(ω)) of these three materials have been analyzed with the optical transition theory. Furthermore, the e2(ω) peaks have been fitted by the Lorentz formula, and the fitting results were discussed using the dielectric theory. We hope the results will be helpful for the dielectric spectrum measurements for these three perovskite oxides.

Temperature-dependent dielectric functions and interband critical points of relaxor lead hafnate-modified PbSc1/2Ta1/2O3 ferroelectric ceramics by spectroscopic ellipsometry

The electronic band structures and dielectric functions of (1−x)PbSc1/2 Ta 1/2O3−xPbHfO3 ceramics with different composition have been investigated by variable-temperature spectroscopic ellipsometry. Using the standard critical-point (SCP) model, three typical interband transitions can be observed from the second derivative of dielectric functions. The CP transitions, which are sensitive to B-site order degree, show a redshift trend with the temperature due to the electron-phonon interactions and lattice thermal expansion. The linear temperature coefficients are varied with oxygen vacancy, B-atom (Sc, Ta, Hf) arrangement, and Pb-O bonds owing to addition of PbHfO3.

ELECTRONIC AND ELASTIC PROPERTIES OF ZINC-BLENDE MgSe

In the present study, ground state and elastic properties of semiconductor MgSe in zinc-blende phase are investigated using density functional theory (DFT). Exchange-correlation potentials are approximated with the generalized gradient approximation (GGA). From the calculated bulk modulus, we determine the refractive index, plasmon energy, cohesive energy and micro-hardness of the MgSe semiconductor binary alloy. The density of state (DOS), projected density of state (PDOS), phonon dispersion frequencies, charged density, electronic band structure and dielectric functions are also reported. From the band structure, a direct band gap of 2.50 eV was observed in close agreement with other reported calculations, but lower than the experimental value of 3.60 eV. Along the high symmetries directions, we found a striking resemblance between MgSe and a III–V semiconductor, while the high cohesive energy in MgSe suggests filled bonding orbitals which might result in decrease in atomic volume with corresponding increased compression of the s-orbitals under any transition series.

Electronic band structures, dielectric functions and interband transitions of relaxor ferroelectric (1-x) Pb (Sc1/2Ta1/2) O3-xPbHfO3 ceramics: A spectroscopic reflectance study

Electronic band structures and dielectric functions of relaxor ferroelectric (1-x) Pb (Sc1/2Ta1/2) O3-xPbHfO3 (PSTH) ceramics (0⩽x⩽0.2) have been investigated by far-IR–UV reflectance spectra. Based on generalized four-parameter damped oscillators and Tauc–Lorentz dispersion models, the dielectric functions from 0.006 to 6eV have been successfully extracted at room temperature. The status of structural order can be characterized by combining the strength of the Sc–Ta vibrational mode and the dielectric loss in the frequency range of 280–320cm−1. It was found that the electronic structures (which depend closely on the BO6 (B=Sc, Ta Hf) octahedron) are sensitive to the B-site atom concentration. The interband transitions can be clearly demonstrated by the imaginary part of the dielectric function which corresponds to the transition from the O 2p to B-site (Sc, Ta and Hf) d states and Pb 6p states. The two typical transition energies show the opposite trend with increased PbHfO3 composition. Remarkable variations of the B-site order degree and the lattice expansion due to the introduction of PbHfO3 are responsible for modifications of the dielectric function and electronic structures. These results could be crucial for investigating the potential applications of PSTH-based optoelectronic and pyroelectric devices.

Manganese doping effects on interband electronic transitions, lattice vibrations, and dielectric functions of perovskite-type Ba0.4Sr0.6TiO3 ferroelectric ceramics

The Ba0.4Sr0.6−x Mn x TiO3 (BSMT) ceramics with different Mn composition (from 1% to 10%) have been prepared via the conventional solid-state reaction sintering. The X-ray diffraction analysis shows that the ceramics are polycrystalline with the single perovskite phase. The lattice vibrations and optical properties have been investigated using Raman scattering, spectroscopic ellipsometry (SE), and infrared reflectance spectra. It was found that the optical bandgap for the BSMT ceramics is varied between 3.40 and 3.65 eV. The three first-order Raman-active phonon modes can be observed, and the frequency of the A 1(LO3)/E(LO) mode shows a blue shift of 8 cm−1 with the Mn composition, which can be attributed to the distortion of the TiO6 octahedron. With increasing Mn composition, the frequency of the infrared-active TO4 mode decreases from 532 to 520 cm−1, owing to the local variation of the lattice constant induced by the Mn incorporation. Moreover, the optical functions of the ceramics from the far-infrared to ultraviolet region are obtained based on the SE and reflectance spectra, which is useful for the potential applications in ferroelectric-based optoelectronic devices.

Electronic structures and optical dielectric functions of room temperature phases of SrTiO3 and BaTiO3

We investigate the electronic structures and optical dielectric functions of the room temperature phases of SrTiO3 and BaTiO3 by using Tran and Blaha’s modified Becke and Johnson exchange potential. Calculated energy gaps are substantially better than previous first-principles results with respect to experimental values, further calculated optical dielectric functions as functions of photon energy are in good agreement with experimental curves, and calculated zero frequency refractive indexes are also consistent with experiment. This substantial improvement is achieved because the energy levels of the Ti d states are correctly calculated with this new exchange potential. These make a reasonable, reliable understanding of the electronic structures and optical properties of room temperature phases of SrTiO3 andBaTiO3. This approach should be applicable to other semiconducting materials.

Anhydrous crystals of DNA bases are wide gap semiconductors

We present the structural, electronic, and optical properties of anhydrous crystals of DNA nucleobases (guanine, adenine, cytosine, and thymine) found after DFT (Density Functional Theory) calculations within the local density approximation, as well as experimental measurements of optical absorption for powders of these crystals. Guanine and cytosine (adenine and thymine) anhydrous crystals are predicted from the DFT simulations to be direct (indirect) band gap semiconductors, with values 2.68 eV and 3.30 eV (2.83 eV and 3.22 eV), respectively, while the experimentally estimated band gaps we have measured are 3.83 eV and 3.84 eV (3.89 eV and 4.07 eV), in the same order. The electronic effective masses we have obtained at band extremes show that, at low temperatures, these crystals behave like wide gap semiconductors for electrons moving along the nucleobases stacking direction, while the hole transport are somewhat limited. Lastly, the calculated electronic dielectric functions of DNA nucleobases crystals in the parallel and perpendicular directions to the stacking planes exhibit a high degree of anisotropy (except cytosine), in agreement with published experimental results.

Optical polarized properties of the F 2 center in YAlO 3 crystal

The electronic structures, dielectric functions and absorption coefficient of perfect YAlO 3 (YAP) crystal and YAP crystal containing F 2 center (YAP:F 2) have been studied using the CASTEP code with the lattice structure optimized. The calculated electronic structures indicate that the F 2 center in YAP bring donor energy levels in the forbidden band compared with that of perfect YAP. The optical properties of perfect YAP and YAP:F 2 show anisotropy and their optical symmetry coincides with the lattice structure geometry. The calculated polarized absorption spectra of YAP:F 2 exhibit new absorption bands in the region from 190 to 310 nm, which are in agreement with the experimental results.

Electronic structure and optical properties of CdWO 4 with oxygen vacancy studied from first principles

The electronic structures, dielectric functions and absorption coefficient of both perfect CdWO 4 crystal (CWO) and the CWO crystal containing oxygen vacancy (CWO: V O) have been studied using the CASTEP code with the lattice structure optimized. The calculated total density of states (TDOS) of CWO: V O indicates that the oxygen vacancy would introduce a new electronic state within the band gap compared with that of perfect CWO. The dielectric functions are calculated since the imaginary part of the dielectric function can reduce the optical absorption of a certain crystal, and then the absorption coefficient is calculated. The calculated absorption spectra show that CWO: V O exhibits two absorption bands in the ultraviolet and visible region, peaking at about 3.0 eV (413 nm) and 3.5 eV (354 nm), respectively, which are in agreement with the experimental results showing that the yellow CWO has two optical absorption bands in this region peaking at around 350 nm and 400 nm respectively. It can be concluded that oxygen vacancy causes these two absorption bands. The calculations also indicate that the optical properties of CWO exhibit anisotropy, and can be explained by the anisotropy of the crystal lattice.

Electron low energy-loss functions of Pb(Mg1/3Nb2/3)O3: Theory and experiment

Local low energy-loss functions (ELFs) of chemical ordered nanodomains in relaxor Pb(Mg1/3Nb2/3)O3 (PMN) were obtained by using Kramers–Krönig analysis of experimental electron low energy-loss spectra. Electronic structures and dielectric functions were calculated via ab initio calculation using the full-potential linearized augmented plane-wave method on a 30 atom PMN supercell with B-site cation ordering along the [111] direction. The theoretical ELFs are fundamentally in agreement with the experimental ELFs. Furthermore, we systematically elucidate all the peaks in the low ELF based on the ab initio calculated densities of states. It is found that the Nb 4d–O 2p hybridization is enhanced through ferroelectric transition, which makes the O 2p collective excitation energy upper shift in ELF, while Mg, also as the B-site ion, presents no covalent bonding effect with O.

Electronic and dielectric properties of insulatingZr3N4

The electronic structures and dielectric functions of ${\mathrm{Zr}}_{3}{\mathrm{N}}_{4}$ were studied by a first-principles density-functional method. Three different structure models were used: (1) ZrN in a rocksalt structure with an ordered Zr vacancy, or ${\mathrm{Zr}}_{3}\ensuremath{\square}{\mathrm{N}}_{4},$ (2) $c\ensuremath{-}{\mathrm{Zr}}_{3}{\mathrm{N}}_{4}$ in the cubic spinel structure and (3) $o\ensuremath{-}{\mathrm{Zr}}_{3}{\mathrm{N}}_{4}$ in the orthorhombic structure. All three structures of ${\mathrm{Zr}}_{3}{\mathrm{N}}_{4}$ are found to be insulators with small indirect band gaps. This appears to be consistent with experimental observation that ${\mathrm{Zr}}_{3}{\mathrm{N}}_{4}$ is an insulator. Total-energy (E) calculations show that ${\mathrm{Zr}}_{3}\ensuremath{\square}{\mathrm{N}}_{4}$ has an expanded lattice constant compared to ZrN by about 0.75%, and that $E({\mathrm{Zr}}_{3}\ensuremath{\square}{\mathrm{N}}_{4})lE(o\ensuremath{-}{\mathrm{Zr}}_{3}{\mathrm{N}}_{4})lE(c\ensuremath{-}{\mathrm{Zr}}_{3}{\mathrm{N}}_{4})$ which indicates that the ordered defect model for ${\mathrm{Zr}}_{3}{\mathrm{N}}_{4}$ is valid. It is further demonstrated that past calculations showing ${\mathrm{Zr}}_{3}\ensuremath{\square}{\mathrm{N}}_{4}$ to be a metal were caused by the failure to relax the vacancy structure. The electronic structures and the optical dielectric functions for all three models were calculated and analyzed in detail. The ${\mathrm{Zr}}_{3}\ensuremath{\square}{\mathrm{N}}_{4}$ model again shows the best overall agreement with available experimental data. All three structures have relatively large optical dielectric constants at zero frequency. Possible implications from these calculations are discussed.

Comparison between the electronic dielectric functions of a GaAs/AlAs superlattice and its bulk components by spectroscopic ellipsometry using core levels.

The dielectric functions of a GaAs/AlAs superlattice (SL) as well as it its bulk constituents GaAs and AlAs were determined at T=90 K by means of spectroscopic ellipsometry with rotating analyzer using synchrotron radiation in the photon-energy range between 3\ensuremath{\leqslant}\ensuremath{\Elzxh}\ensuremath{\omega}\ensuremath{\leqslant}25 eV. This broad spectral range gives access to excitations from the Ga 3d core levels lying approximately 20 eV below the conduction-band minimum. These levels are atomiclike with virtually no dispersion throughout the Brillouin zone (BZ). They can thus be used as an energy reference to investigate confinement effects and level splittings on interband transitions in heterostructures compared to their bulk counterparts. The samples were grown by molecular-beam epitaxy and maintained under ultrahigh vacuum conditions from growth to measurement, avoiding the effects of cap or oxide layers, especially for AlAs. The investigation concerns transitions at L- and X-related points in the BZ which is tetragonal in the case of the SL and face-centered-cubic for the bulk samples. For the latter it is found that the L-related ${\mathrm{E}}_{1}$ structure consists of two spin-orbit split groups of three transitions. For a (GaAs${)}_{\mathrm{m}}$(AlAs${)}_{\mathrm{n}}$SL with m=9, n=7 these two groups are blueshifted by about 200 meV compared with GaAs due to confinement effects. Similar shifts are not observed for transitions from the Ga 3d levels to the conduction-band valleys. Therefore, the confinement acts on the highest valence bands. The major part of the SL ${\mathrm{E}}_{1}^{\ensuremath{'}}$ structure is AlAs-like, but the minor one moves to higher energies by 165 and 80 meV compared to AlAs and GaAs, respectively. Since the two parts of the ${\mathrm{E}}_{1}^{\ensuremath{'}}$ structure exhibit a different confinement behavior, they cannot have a common origin in k space. The analysis of X-related ${\mathrm{E}}_{2}$ structures exhibits a blueshift for the SL transitions by about 55 meV compared with AlAs which is consistent with a confinement effect on the lowest conduction band. In correspondence to previous theoretical work a transition above the ${\mathrm{E}}_{2}$ critical point is found which was attributed to a SL specific transition due to the BZ folding. The comparison with the results for the bulk samples suggests a relation to the ${\mathrm{E}}_{2}$(${\mathrm{P}}_{2}$) critical point. Beyond that, an analysis of the interband transition line shapes reveals for all ${\mathrm{E}}_{1}$,${\mathrm{E}}_{1}^{\ensuremath{'}}$ structures, and the AlAs-${\mathrm{E}}_{2}$ one, that a modified Lorentzian profile indicating final-state interaction is more appropriate than two- or three-dimensional critical-point line shapes. This points to deviation from the one-electron band-structure picture even for transitions far above the fundamental absorption edge.