Bottom Of Conduction Band Research Articles

Lead-free Dion–Jacobson halide perovskites CsMX2Y2 (M = Sb, Bi and X, Y = Cl, Br, I) used for optoelectronic applications via first principle calculations

In this study, all-inorganic two-dimensional Dion–Jacobson halide perovskites (DJ-HPs) CsMX2Y2 (M = Sb, Bi; and X/Y= I/I, I/Br, Br/Br, I/Cl, Br/Cl, Cl/Cl) have been studied for photovoltaic, photoemission and other potential applications in optoelectronic devices using first principle calculations within the framework of density functional theory (DFT). The ground state crystal symmetry is tetragonal with a P4/mmm (No. 123) space group. In these compounds, Cs+ is a spacer cation, which separates the octahedral layers and balances the charge. The corner sharing M-centered (Sb, Bi) [MX2Y4]3– unit constructs the 2D [MX2Y2]nn– plane, where the X and Y (halogens) atoms occupy the out of plane apical and in-plane bridging sites, respectively. The DJ-HPs with P4/mmm crystal symmetry exhibit pseudo-direct band gap semiconducting nature and the spin orbit (SO) effect reduces the band gaps as well as splits the Sb/Bi-p orbital in the bottom of conduction band (CB) and X/Y-p orbital at the top of valence band (VB). Among these DJ-HPs, the band gap energies of CsSbI2Cl2 (0.87 eV), CsSbBr2Cl2 (1.00 eV), CsSbCl4 (1.40 eV), CsBiI4 (1.10 eV), CsBiI2Br2 (1.40 eV) and CsBiBr4 (1.60 eV) lie in a band gap energy range (0.9–1.6 eV) suitable for photovoltaic applications. The M-s (Sb-5s and Bi-6s) and X/Y-p (Cl-3p, Br-4p and I-5p) orbitals are responsible for the electron transition from the valence to conduction band, while the spacer cation (Cs+) has a limited effect on the charge carrier mobility. The effective masses of the charge carriers and exciton binding energies of these materials are suitable for solar cell and optoelectronic applications. From the optical coefficients it is clear that these DJ-HPs have excellent response to the incident photons in the visible and ultraviolet light region.

Localization Effect in Photoelectron Transport Induced by Alloy Disorder in Nitride Semiconductor Compounds.

Near-band-gap photoemission spectroscopy experiments were performed on p-GaN and p-InGaN/GaN photocathodes activated to negative electron affinity. The photoemission quantum yield of the InGaN samples with more than 5% of indium drops by more than 1 order of magnitude when the temperature is decreased while it remains constant for lower indium content. This drop is attributed to a freezing of photoelectron transport in p-InGaN due to electron localization in the fluctuating potential induced by the alloy disorder. This interpretation is supported by the disappearance at low temperature of the peak in the photoemission spectrum that corresponds to the contribution of the photoelectrons relaxed at the bottom of the InGaN conduction band.

Computer Modelling of Energy Structure of Yb3+ and Lu3+ Doped LaF3 Crystals.

The energy band structure, as well as partial and total densities of states have been calculated for LaF3:Yb and LaF3:Lu crystals within density functional theory using the projector augmented wave method and Hubbard corrections (DFT + U). The influence of geometric optimization on the results of energy band calculations of LaF3:Ln crystals (Ln = Yb, Lu) was analysed and the absence of relaxation procedure is confirmed to negatively influence the energy position of states, and the variability between obtained results of different optimization algorithms are within the calculation accuracy. The top of the valence band of LaF3 is confirmed to be formed by the 2pF--states and the bottom of the conduction band is formed by the 5d-states of La3+. The positions of the 4f-states and 5d-states of activator ions in LaF3 were studied. It is shown that the 4f-states of Yb3+ are slightly above the top of the valence band and the 4f-states of Lu3+ to be 3.5 eV below the top of the valence band. The energy levels of the 5d states of the impurities are energetically close to the bottom of the LaF3 conduction band. The calculated band gap of 9.6 eV for LaF3 is in a good agreement with the experimental result and is not affected by impurity ions.

CMOS-compatible direct laser writing of sulfur-ultrahyperdoped silicon: Breakthrough pre-requisite for UV-THz optoelectronic nano/microintegration

Direct laser writing/ultra-hyperdoping technology was harnessed to fabricate a universal CMOS-compatible Si-based broadband (UV-THz) n-p junction detector, with its spectral response controlled by external liquid helium-nitrogen cooling. Near-far IR (2–40 μm) temperature-dependent photoconductivity (PC) was revealed in a biased n-p junction, made of n-type sulfur-ultrahyperdoped Si (uh-Si, sulfur content ∼1 at. %) sub-micron-thick layer on a surface of a p-doped Si substrate. The observed broadband photoconductivity is provided by a dense quasi-continuum series of sulfur-impurity donor states near the conduction band bottom (so-called “intermediate band”, the bandwidth ≈0.6 eV), corresponding to IR-absorbing neutral and singly-ionized substitutional atomic and cluster centers of sulfur. The “intermediate” donor band supports the gradual “blue” spectral migration of PC maximum due to thermal ionization of deeper states at the temperature increasing in the range of 5–250 K. As a result, in the temperature range of 77–300 K complementary THz (wavenumbers <100 cm−1) spectroscopy indicates the predominating Drude-like conduction-band response of electrons and their gradually raising density ∼1019–1020 cm−3. Illustrating the thermal-ionization depletion of the donor sulfur states from the THz-probed plasma side, this unveils the unprecedently high concentration of electrically-active donor sulfur centers ∼1020 cm−3 and related donor-state density ∼1020 states/eV·cm3 in the uh-Si sample. Overall, these advances enable in situ laser writing of universal liquid helium/nitrogen cooled Si nano/micro/macrodetectors with broad – near-far IR and even THz – spectral response, crucial for photovoltaics, thermal and bio-imaging.

Electronic and photocatalytic properties of ZnO/GaTe heterostructure from first principles calculations

Constructing a novel van der Waals (vdW) heterostructure by integrating diverse two-dimensional (2D) materials is significant to generate the more fascinating performances. In this present contribution, the electronic and photocatalytic properties of ZnO/GaTe heterostructure have been explored using first-principles calculations. The results reveal that the interaction between layers is dominated by the vdW forces. It displays an intrinsic type-II band alignment and a direct band gap of 1.730 eV under hybrid HSE06 functional. The valence band top and conduction band bottom are located at ZnO and GaTe layers, respectively, which guarantees the spatial separation of photogenerated electron-hole pairs and improves the photocatalytic efficiency. Moreover, the band edges all span the energy levels of both the reduction and oxidation potentials of H2O, indicating that its better photocatalytic performance. The modulated photocatalytic properties under varied pH circumstances imply that an acid environment is more favorable to the water splitting process. Furthermore, the band edge alignment exhibits the significant adjustability through varying vertical and in-plane biaxial strains. These theoretical findings indicate that the ZnO/GaTe heterostructure can offer an alternative strategy to design the efficient photocatalysts for water splitting.

The carriers doping effect on electronic and optical behaviors of newly layered Sr1- xHfxFBiS2 alloying materials for light-modulator devices

BiS2-layered superconductors have captivated the colossal attention of the scientific community owing to their structural analogies to other unconventional superconducting materials, termed cuprates and Fe-based superconducting materials. Herein, we inspect how the physical characteristics will be altered in the SrFBiS2 mother compound upon the compositional doping of the hafnium element. The electronic structure characteristics of Sr1−xHfxFBiS2 (x = 0 to 1) are simulated with the virtue of the full-potential linearized augmented plane-wave technique. Accordingly, the evaluated band gap of the parent compound (SrFBiS2) is roughly about 0.88 eV, which vanishes under the substitutional impurity impact of tetravalent (Hf+4). However, a metallic character occurs when Hf is substituted in the SrFBiS2 mother compound, although the bottom conduction band is relocated downward to the Fermi level with the emergence of a viable superconducting state. The band structures and Fermi surface (FS) topology vary obviously with the augmentation of Hf concentration and FS nesting is acquired at the wave vector (π, π, 0). Moreover, the optical characteristics of the explored compounds are probed, while the optical anisotropy in the optical absorption spectra is well marked in the mother material and diminishes through the substitutional Hf impurity effect. The metallic character with Drude-model arises when the incident photon energies span from zero photon energy to infrared spectrum when Hf substitutes Sr in the SrFBiS2 parent compound. The spin orbit coupling (SOC) significantly influences the band structures, Fermi surface topologies, and optical features of these doped systems based on the heavy elements, like Hf or Bi. Hence, the induced nesting FS outcomes may turn Sr1−xHfxFBiS2 into striking materials for newly layered superconductors under the doping effect of hafnium element at low compositional content. Eventually, it is inferred that the SrFBiS2 parent compound would be a potential contender for new superconductivity and may be useful for optical communications and laser devices.

Investigations on structural, electronic and optical properties of ZnO in two-dimensional configurations by first-principles calculations

The electronic structures and optical properties of two-dimensional (2D) ZnO monolayers in a series of configurations were systematically investigated by first-principles calculations with Hubbard U evaluated by the linear response approach. Three types of 2D ZnO monolayers, as planer hexagonal-honeycomb (Plan), double-layer honeycomb (Dlhc), and corrugated tetragonal (Tile) structures, show a mechanical and dynamical stability, while the Dlhc-ZnO is the most energetically stable configuration and Plan-ZnO is the second one. Each 2D ZnO monolayer behaves as a semiconductor with that Plan-, Dlhc-ZnO have a direct band gap of 1.81 eV and 1.85 eV at the Γ point, respectively, while Tile-ZnO has an indirect band gap of 2.03 eV. Interestingly, the 2D ZnO monolayers all show a typical near-free-electron character for the bottom conduction band with a small effective mass, leading to a tremendous optical absorption in the whole visible and ultraviolet window, and this origination was further confirmed by the transition dipole moment. Our investigations suggest a potential candidate in the photoelectric field and provide a theoretical guidance for the exploration of wide-band-gap 2D semiconductors.

Atomic and electronic structures of nitrogen vacancies in silicon nitride: Emergence of floating gap states

We report the density functional calculations with the hybrid approximation to the exchange-correlation energy that clarify the atomic and electronic structures of the N vacancy (${\mathrm{V}}_{\mathrm{N}}$) in ${\mathrm{Si}}_{3}{\mathrm{N}}_{4}$. We find that the gap states induced by the Si dangling bonds around ${\mathrm{V}}_{\mathrm{N}}$ show interesting interplay between the spin splitting and the Jahn-Teller splitting depending on their charges. More surprisingly, we find that a peculiar electron state distributed in the internal space of the host SiN is hidden in the conduction bands and converted to a localized floating state in the fundamental energy gap due to the presence of ${\mathrm{V}}_{\mathrm{N}}$. This theoretical finding indicates ${\mathrm{V}}_{\mathrm{N}}$ being multiply charged from $+1$ to $\ensuremath{-}5$: The charge states from $+1$ to $\ensuremath{-}3$ are stable at respective Fermi-level positions in the gap, whereas the states $\ensuremath{-}4$ and $\ensuremath{-}5$ are metastable for the Fermi level near the conduction band bottom. A possibility of multilevel memory function of ${\mathrm{V}}_{\mathrm{N}}$ in SiN is suggested.

Engineering electronic structures of Janus monolayer group-III monochalcogenides via biaxial strain

We investigate the effects of biaxial strain and spin-orbit coupling on the electronic structures of Janus monolayer group-III monochalcogenides based on first-principle calculations. Due to the spin-orbit coupling and mirror symmetry breaking, Rashba band splitting appears at Γ point of the bottom conduction band and the band gap decreases significantly. The spin-orbit coupling strength and electronic structures can be greatly tuned by biaxial strain, which originate from the change of interaction between pz orbitals. As the lattice constant decreases, the interaction between pz orbitals is enhanced, leading to the decrease (increase) of the energy of corresponding bonding (anti-bonding) state, which is the origin of the indirect-direct-indirect transition of the band gap. In addition, when tensile strain is applied, the pz orbital component of Te atoms increases, leading to the enhancement of the Rashba effect. These results are of great significance for the band engineering and spin-orbitronics of two-dimensional materials.

Bright UV-C Phosphors with Excellent Thermal Stability-Y1-xScxPO4 Solid Solutions.

The structural and luminescence properties of undoped Y1−xScxPO4 solid solutions have been studied. An intense thermally stable emission with fast decay (τ1/e ~ 10−7 s) and a band position varying from 5.21 to 5.94 eV depending on the Sc/Y ratio is detected and ascribed to the 2p O-3d Sc self-trapped excitons. The quantum yield of the UV-C emission, also depending on the Sc/Y ratio, reaches 34% for the solid solution with x = 0.5 at 300 K. It is shown by a combined analysis of theoretical and experimental data that the formation of Sc clusters occurs in the solid solutions studied. The clusters facilitate the creation of energy wells at the conduction band bottom, which enables deep localization of electronic excitations and the creation of luminescence centers characterized by high quantum yield and thermal stability of the UV-C emission.

Lattice strain and band overlap of the thermoelectric composite Mg2Si1−xSnx

${\mathrm{Mg}}_{2}{\mathrm{Si}}_{1\text{\ensuremath{-}}x}{\mathrm{Sn}}_{x}$ solid solutions show enhanced thermoelectric performance when the Sn mole fraction $x$ is approximately $x=0.7$. This has been discussed in terms of complexity of the electronic structure arising from the crossover of two bottom conduction bands, but direct detailed understanding of the origins and the precise nature of this band convergence is limited. Here, we report first-principles calculations of the band edge changes analyzed by band unfolding and crystal orbital Hamilton population techniques. We find that strain is particularly important at this crossing. ${\mathrm{Mg}}_{2}\mathrm{Si}$ and ${\mathrm{Mg}}_{2}\mathrm{Sn}$ show opposite trends in the conduction band edge shifts leading to a band crossover at $x\ensuremath{\sim}0.625$. However, there are also important effects due to disorder. Transport calculations show that enhancement of the figure of merit $ZT$ value owes to the combined effects of lattice strain, band edge overlap, composition change, and disorder.

Photo-induced conversion of type-II CoPc/BiOBr-NSs to S-scheme heterostructure for boosting CO2 photoreduction

With efficient charge separation, S-scheme heterostructures are very attractive for photocatalytic carbon dioxide reduction, but their construction is still facing challenge. Herein, cobalt phthalocyanine (CoPc) was combined with BiOBr nanosheets (NSs) as type-II heterostructure CoPc/BiOBr-NSs. It is more interesting that light-induced oxygen vacancies on BiOBr-NSs can convert type-II CoPc/BiOBr-NSs to S-scheme structures for CO2 reduction. The optimal CoPc/BiOBr-NSs exhibited significantly enhanced CO generation with rate of 59.9 μmol g−1 h−1 (selectivity of 90.4 %), which was 25-fold higher than that of BiOBr-NSs, and much higher than most reported photocatalysts. Photo-induced oxygen vacancies on BiOBr-NSs can generate defect states on bottom of conduction band (CB), which can trap the electrons to have a recombination with holes left on HOMO band of CoPc, boosting charge separation for efficient CO2 reduction. Moreover, the central Co2+ of CoPc can accept the excited electrons from the ligand to act as active site for CO2 reduction. This work provides a new strategy of constructing metal-complex/semiconductor S-scheme photocatalyst for boosting CO2 photoreduction.

Analysis of structural stability and optoelectronic properties of new direct band gap halide double perovskites [formula omitted] : [formula omitted] — a first principle study

A precise and systematic analysis of Cs2XRhCl6: X = N a, K was performed to investigate structural stability, optical and electrical properties using Density Function Theory simulations. Generalized Gradient Approximation(GGA) approximations were used to optimize the lattice constants. The stability of these double perovskites was governed by structural optimization, tolerance factor, and mechanical stability parameters which confirm their cubic structure stability in FM3̄M symmetry. The lattice stability reported through this investigation were in good agreement with previously synthesized, and analyzed double perovskites, like Cs2AgBiX6 (X = Cl, Br). Mechanical stability was obtained from elastic constants and the results of Pugh’s (B/G) and Poisson’s (σ) ratio gives the brittle nature of these Double Perovskites. The positive phonon dispersion considerably support the thermodynamic stability of Cs2NaRhCl6, however Cs2KRhCl6 is shown as thermodynamically unstable, although the elastic moduli values suggesting that both the systems under examination is mechanically stable. The measured electronic bandstructure indicates a direct bandgap along with semiconducting nature, for both GGA and modified Becke Jonhson’s approximations. The electronic transition from the top of valence band [Cl(3p)+Rh(4d)] to the bottom of conduction band [Cl(3p)+Rh(4d)] is fundamentally direct at the X-point of the first Brillouin zone, as suggested by the band structure and density of states. The Calculated value of bandgap using mBJ potential is 1.79 eV for Cs2NaRhCl6 and 1.83 eV for Cs2KRhCl6. Important optical parameters, like dielectric constant, absorption coefficient, energy loss function, reflectivity, and optical conductivity have been computed and discussed in detail. Visible and UV frequency absorption by these materials indicates potential applications in the optoelectronics industry. We were able to show that Cs2NaRhCl6 is plausible candidate material for photoelectric applications due to its significant refractive index and strong absorption coefficient in the range of 2.0-5.0 eV. For potential optoelectronic applications, the theoretical results may help in developing these lead-free Double Perovskites.

Two-dimensional MoTe2/SnSe2 van der Waals heterostructures for tunnel-FET applications

Two-dimensional (2D) van der Waals heterostructures (vdWHs) are attractive candidates for realizing tunnel field-effect transistors (TFETs) for low-power applications. In this work, using first-principles calculations based on density functional theory (DFT), we explore heterostructures composed of 2D $\mathrm{Mo}{\mathrm{Te}}_{2}$ and $\mathrm{Sn}{\mathrm{Se}}_{2}$. Our calculations reveal that upon forming the heterostructures, the valence band top of $\mathrm{Mo}{\mathrm{Te}}_{2}$ and the conduction band bottom of $\mathrm{Sn}{\mathrm{Se}}_{2}$ are almost aligned, forming the nearly broken-gap or type-III band alignment which is highly promising for TFETs. Interestingly, we find that the band alignment can be tuned by applying external electric fields. For positive electric fields, $\mathrm{Mo}{\mathrm{Te}}_{2}$ $(\mathrm{Sn}{\mathrm{Se}}_{2})$ band-edge positions are shifted upward (downward) with respect to the Fermi level, and more electrons are expected to tunnel from $\mathrm{Mo}{\mathrm{Te}}_{2}$ to $\mathrm{Sn}{\mathrm{Se}}_{2}$. Overall, our simulations provide fundamental insights into the electronic properties of $\mathrm{Mo}{\mathrm{Te}}_{2}/\mathrm{Sn}{\mathrm{Se}}_{2}$ stacks, and pave the way for the design and fabrication of future $\mathrm{Mo}{\mathrm{Te}}_{2}/\mathrm{Sn}{\mathrm{Se}}_{2}$-based TFETs.

Transparent conductive SnO2 thin films via resonant Ta doping

Transparent conductive oxide (TCO) thin films are highly desired as electrodes for modern flat-panel displays and solar cells. Alternative indium-free TCO materials are highly needed, because of the scarcity and the high price of indium. Based on the mechanism of resonant doping, Ta has been identified as an effective dopant for SnO2 to achieve highly conductive and transparent TCO. In this work, we fabricated a series of Ta-doped SnO2 thin films (Sn1−xTaxO2, x = 0.001, 0.01, 0.02, 0.03) with high conductivity and high optical transparency via a low-cost sol-gel spin coating method. The Sn0.98Ta0.02O2 film achieves the highest electrical conductivity of 855 S cm−1 with a carrier concentration of 2.3 × 1020 cm−3 and high mobility of 23 cm2 V−1 s−1. The films exhibit a very high optical transparency of 89.5% in the visible light region. High-resolution X-ray photoemission spectroscopy and optical spectroscopy were combined to gain insights into the electronic structure of the Sn1−xTaxO2 films. The optical bandgaps of the films are increased from 3.96 eV for the undoped SnO2 to 4.24 eV for the Sn0.98Ta0.02O2 film due to the occupation of the bottom of conduction band by free electrons, i.e., the Burstein-Moss effect. Interestingly, a bandgap shrinkage is also directly observed due to the bandgap renormalization arising from many-body interactions. The double guarantee of transparency and conductivity in Sn1−xTaxO2 films and the low-cost growth method provide a new platform for optoelectronic and solar cell applications.

High-performance visible-light active Sr-doped porous LaFeO3 semiconductor prepared via sol–gel method

ABSTRACT In this work, we have successfully fabricated Sr-doped porous LaFeO3 samples via sol–gel method. The results reveal that Sr2+ cation is effectively doped into LaFeO3 crystal lattice substituting La3+ cation. The visible light catalytic performance of the materials was evaluated by the degradation of 2,4-dichlorophenol (2,4-DCP) and Rhodamine B (RhB). The amount-optimized Sr-doped porous LaFeO3 sample exhibited outstanding visible-light catalytic performance for the degradation of the model pollutants compared to the porous LaFeO3 alone. The enhanced performance was accredited to the enlarged surface area, absorption extension via the surface states of the introduced Sr2+ below the conduction band bottom of LaFeO3, and promoted charge carrier’s separation as confirmed by various experiments. Experiments of radical trapping reveal that •OH species are dominant intermediate oxidants involved in the oxidation of 2,4-DCP and RhB over the optimized sample. This research will provide new routes for environmental remediation based on the LaFeO3 semiconductor.

Longitudinal magnetoresistance of uniaxially deformed n-type silicon

The study of Galvano-magnetic effects (as well as tensoeffects) in silicon under extreme conditions allows not only to identify the mechanisms of these effects but also to identify the possibility of creating gaussmeters, infrared detectors, sensitive strain gauges, amplifiers and generators of a wide frequency range. The reliability of the mechanism of negative magnetoresistance was verified using uniaxial elastic deformation of the studied crystals. Uniaxial deformation excludes interline transitions of electrons, as a result of which negative magnetoresistance disappears with an increase in uniaxial pressure. When cubic symmetry is violated, anisotropic phenomena occur in such crystals. The multipath of the isoenergetic surface of the bottom of the silicon conduction band causes anisotropies of the effective mass and relaxation time, which are associated with the features of the transfer phenomenon. In particular, magnetoresistance (piezoresistance), which is the most sensitive to the anisotropy of the iso energy surface. The influence of the latter on magnetoresistance is most clearly revealed in the region of strong magnetic fields, where the magnetoresistance is saturated since the longitudinal magnetoresistance is entirely due to the anisotropy of electron mobility.

Longitudinal magnetoresistance of uniaxially deformed n-type silicon

The study of Galvano-magnetic effects (as well as tensoeffects) in silicon under extreme conditions allows not only to identify the mechanisms of these effects but also to identify the possibility of creating gaussmeters, infrared detectors, sensitive strain gauges, amplifiers and generators of a wide frequency range. The reliability of the mechanism of negative magnetoresistance was verified using uniaxial elastic deformation of the studied crystals. Uniaxial deformation excludes interline transitions of electrons, as a result of which negative magnetoresistance disappears with an increase in uniaxial pressure. When cubic symmetry is violated, anisotropic phenomena occur in such crystals. The multipath of the isoenergetic surface of the bottom of the silicon conduction band causes anisotropies of the effective mass and relaxation time, which are associated with the features of the transfer phenomenon. In particular, magnetoresistance (piezoresistance), which is the most sensitive to the anisotropy of the iso energy surface. The influence of the latter on magnetoresistance is most clearly revealed in the region of strong magnetic fields, where the magnetoresistance is saturated since the longitudinal magnetoresistance is entirely due to the anisotropy of electron mobility.

Exploration of the Crystal Structure and Thermal and Spectroscopic Properties of Monoclinic Praseodymium Sulfate Pr2(SO4)3.

Praseodymium sulfate was obtained by the precipitation method and the crystal structure was determined by Rietveld analysis. Pr2(SO4)3 is crystallized in the monoclinic structure, space group C2/c, with cell parameters a = 21.6052 (4), b = 6.7237 (1) and c = 6.9777 (1) Å, β = 107.9148 (7)°, Z = 4, V = 964.48 (3) Å3 (T = 150 °C). The thermal expansion of Pr2(SO4)3 is strongly anisotropic. As was obtained by XRD measurements, all cell parameters are increased on heating. However, due to a strong increase of the monoclinic angle β, there is a direction of negative thermal expansion. In the argon atmosphere, Pr2(SO4)3 is stable in the temperature range of T = 30–870 °C. The kinetics of the thermal decomposition process of praseodymium sulfate octahydrate Pr2(SO4)3·8H2O was studied as well. The vibrational properties of Pr2(SO4)3 were examined by Raman and Fourier-transform infrared absorption spectroscopy methods. The band gap structure of Pr2(SO4)3 was evaluated by ab initio calculations, and it was found that the valence band top is dominated by the p electrons of oxygen ions, while the conduction band bottom is formed by the d electrons of Pr3+ ions. The exact position of ZPL is determined via PL and PLE spectra at 77 K to be at 481 nm, and that enabled a correct assignment of luminescent bands. The maximum luminescent band in Pr2(SO4)3 belongs to the 3P0 → 3F2 transition at 640 nm.

Fe- and Co-based magnetic tunnel junctions with AlN and ZnO spacers

AlN and ZnO, two wide band-gap semiconductors extensively used in the display industry, crystallise in the wurtzite structure, which can favour the formation of epitaxial interfaces to close-packed common ferromagnets. Here we explore these semiconductors as material for insulating barriers in magnetic tunnel junctions. In particular, the {\it ab initio} quantum transport code {\it Smeagol} is used to model the $X$[111]/$Y$[0001]/$X$[111] ($X=$ Co and Fe, $Y=$ AlN and ZnO) family of junctions. Both semiconductors display a valance-band top with $p$-orbital character, while the conduction band bottom exhibits $s$-type symmetry. The smallest complex-band decay coefficient in the forbidden energy-gap along the [0001] direction is associated with the $\Delta_1$ symmetry, and connects across the band gap at the $\Gamma$ point in 2D Brillouin zones. This feature enables spin filtering and may result in a large tunnelling magnetoresistance. In general, we find that Co-based junctions present limited spin filtering and little magnetoresistance at low bias, since both spin sub-bands cross the Fermi level with $\Delta_1$ symmetry. This contrasts the situation of Fe, where only the minority $\Delta_1$ band is available. However, even in the case of Fe the magnitude of the magnetoresistance at low bias remains relatively small, mostly due to conduction away from the $\Gamma$ point and through complex bands with symmetry different than $\Delta_1$. The only exception is for the Fe/AlN/Fe junction, where we predict a magnetoresitance of around 1,000\% at low bias.