Narrow Conduction Band Research Articles

Obtaining giant Rashba-Dresselhaus spin splitting in two-dimensional chiral metal-organic frameworks.

Two-dimensional (2D) nonmagnetic semiconductors with large Rashba-Dresselhaus (R-D) spin splitting at valence or conduction bands are attractive for magnetic-field-free spintronic applications. However, so far, the number of 2D R-D inorganic semiconductors has been quite limited, and the factors that determine R-D spin splitting as well as rational design of giant spin splitting, remain unclear. For this purpose, by exploiting 2D chiral metal-organic frameworks (CMOFs) as a platform, we theoretically develop a three-step screening method to obtain a series of candidate 2D R-D semiconductors with valence band spin splitting up to 97.2 meV and corresponding R-D coupling constants up to 1.37 eV Å. Interestingly, the valence band spin texture is reversible by flipping the chirality of CMOFs. Furthermore, five keys for obtaining giant R-D spin splitting in 2D CMOFs are successfully identified: (i) chirality, (ii) large spin-orbit coupling, (iii) narrow band gap, (iv) valence and conduction bands having the same symmetry at the Γ point, and (v) strong ligand field.

Structural, Optical, Photoelectrochemical, and Electronic Properties of the Photocathode CuS and the Efficient CuS/CdS Heterojunction.

We report a facile synthesis of CuS and CdS nanoparticles using a cheap solution-processed chemical bath method forming heterogeneous nucleation. The optical, structural, photoelectrochemical (PEC), and electronic properties are studied by implementing relevant experimental techniques. The estimated optical band gap of ∼2.10 eV of CuS designates potential application in inexpensive photocatalysis and solar cells. Further, the valence band and conduction band positions of CuS and CdS are evaluated using cyclic voltammetry curves. Narrow conduction and valence band offset potentials measured at the CuS/CdS heterojunction are encouraging factors for the PEC application. The electronic properties are supported by the current density vs potential plots (J–V) with an improved short-circuit current density of 0.71 mA cm–2 for the heterojunction compared to bare CuS showing 0.15 μA cm–2. The determined PCE of the heterojunction CuS/CdS is 0.65%.

One-dimensional narrow-band conductors

We review one-dimensional lattice models and the corresponding results that describe the low-temperature properties of quasi-one-dimensional lattice systems with long-range interaction. A widely known example is narrow-band low-dimensional conductors with long-range interelectron repulsion. The models deal with particles that live on the one-dimensional host lattice (chain), translation invariant or disordered, and interact via the long-range repulsive potential. The results are presented concerning the translation invariant host chain, in particular: the low-temperature thermodynamics incommensurable ground states and related devil-staircase form of various characteristics as functions of pertinent parameters, the self-localization of particles, a new branch of elementary excitations, etc. In the disordered case, where the sites of the host chain fluctuate randomly around the sites of the periodic chain, the low-temperature thermodynamics and the structure of the ground state are discussed in the framework of a certain model, which we call the cluster model and which seems to be a fairly reasonable approximation for low temperatures and small concentration of particles. Using analytical and numerical tools we analyze the thermodynamics and the ground state of the model. The latter proves to be a sequence of random domains and we study in detail their distribution.

Spin-polarized current in wide bandgap hexagonal boron nitrides containing 4|8 line defects

Line defects (LDs) which are commonly present in two-dimensional materials play an important role in spintronic applications for generating and transporting spin current. We theoretically predict of the electronic and magnetic properties of n-type doped monolayers of hexagonal boron nitride (h-BN) with 4|8 LDs. The LDs can create two deep narrow bands in the band gap of the h-BN sheet with an energy separation about 2.75 eV at Γ point. Interestingly, at a certain concentration of electrons introduced by n-type doping, a tantalizing spontaneous one-dimensional ferromagnetic ordering emerges due to the partially-occupied narrow conduction band. Moreover, it is found that this electron doping induced spin polarization (of charge current) is up to 90% in the bias voltage range from 0.1 V to 0.4 V. Our findings indicate that the h-BN monolayer can functionalize a semiconductor for spintronic devices applications, which is beyond a common sense of h-BN tunneling barrier.

Temperature Dependence of the Resistance of Thin Poly(Diphenylene Phthalide) Films

The temperature dependence of the resistivity of thin poly(diphenylene phthalide) films in a highly conductive state is studied. A metallic type of conductivity in the range of low temperatures is observed, suggesting the possibility of charge transfer in the films along a narrow conduction band in the band gap.

Electronic, structural, and optical properties of Y2WO6, a host material for inorganic phosphors

Optimization by first principles DFT-based electronic structure methods of the crystal structures for the five polymorphs of Y2WO6 reported in the literature yields results in good agreement with those determined experimentally by X-ray diffraction. The monoclinic P2/c phase appears to be the most stable one at ambient conditions, although high temperature orthorhombic phases with larger molar volumes could be favoured upon replacement of Y3+ cations by larger Ln3+ ones, and hence, provide plausible structures for Y2WO6:Ln3+ phosphors at ambient conditions. For all polymorphs the top of the valence band is dominated by O2p orbitals with a relatively narrow WO6-centred conduction band appearing just below a broad Y4d-centred band. Insertion energies for Eu3+ replacing Y3+ are estimated to be in the range of 3–4 eV per cation, with the smaller values corresponding to substitutions into the larger octacoordinated Y3+ sites.

Persistent antiferromagnetic order in heavily overdoped Ca1−xLaxFeAs2

In the Ca1−xLaxFeAs2 (1 1 2) family of pnictide superconductors, we have investigated a highly overdoped composition (x = 0.56), prepared by a high-pressure, high-temperature synthesis. Magnetic measurements show an antiferromagnetic transition at TN = 120 K, well above the one at lower doping (0.15 < x < 0.27).Below the onset of long-range magnetic order at TN, the electrical resistivity is strongly reduced and is dominated by electron–electron interactions, as evident from its temperature dependence. The Seebeck coefficient shows a clear metallic behavior as in narrow band conductors. The temperature dependence of the Hall coefficient and the violation of Kohler’s rule agree with the multiband character of the material. No superconductivity was observed down to 1.8 K. The success of the high-pressure synthesis encourages further investigations of the so far only partially explored phase diagram in this family of Iron-based high temperature superconductors.

Synthesis, crystal structure, absorption properties, photoelectric behavior of organic–inorganic hybrid (CH3NH3)2CoCl4

Lead‐free and organic–inorganic hybrid (CH3NH3)2CoCl4 [(MA)2CoCl4] single crystal and thin film are prepared. The single crystal diffraction data of (MA)2CoCl4 are assigned to monoclinic, P2(1)/C space group (7.6590 × 12.6908 × 10.89350 Å, 90.0 × 96.5320 × 90.000). The absorption edge of (MA)2CoCl4 reaches 730 nm. The band gap for (MA)2CoCl4 is determined to be approximately 1.63 eV. To the best of our knowledge, this is the first study on (MA)2CoCl4 for optoelectronic applications. A low‐cost photodetector based on (MA)2CoCl4 thin film is efficient under different monochromatic light from 330 nm to 400 nm with different chopping frequencies (1.33–60 Hz). The strongest photoresponse (Ion − Ioff) is under 330 nm monochromatic light with 1.33 Hz according to our optimal condition. The calculational results by density functional theory show that the narrow valence bands and conduction bands are derived from the 3p orbitals of Cl and 3d orbitals of Co.

Physical and photo-electrochemical properties of MgFe2O4 prepared by sol gel route: application to the photodegradation of methylene blue

The spinel MgFe2O4 synthesized by sol gel method, was characterized by physical and electrochemical methods. The thermal analysis (TG) gives an optimal synthesis temperature of 650 °C where the X-ray diffraction pattern exhibits a single phase crystallizing in a cubic symmetry (Space Group: Fd\(\stackrel{-}{3}\)m) with spherical crystallites (~ 0.45 µm). The morphology observed through the SEM image shows elliptical and spherical grains of ~ 0.8 µm. The spinel possesses a direct optical transition (2.16 eV) due to electrons transfer Fe3+: d−d transition, coming from the crystal field splitting in agreement with the red color. The low electrons mobility is assigned to a narrow conduction band of Fe3+: 3d parentage with activation energy (0.14 eV) in conformity with a conduction mechanism by small polaron hopping. The semi-logarithmic plot confirms the chemical stability of oxide. Unlike other spinels, the capacitance plot exhibits n type conduction confirmed by chrono-amperometry with a flat band potential (Efb) of 0.27 V. The electrochemical impedance spectroscopy displays a semi-circle attributed to the bulk behavior. As application, the photodegradation of methylene blue (10 mg L− 1) was successfully on the spinel suspension; 87% are eliminated after 2 h of exposure to sunlight irradiation.

Synthesis, Crystal Structure, UV-Vis Adsorption Properties, Photoelectric Behavior, and DFT Computational Study of All-Inorganic and Lead-Free Copper Halide Salt K2Cu2Cl6.

In this study, all-inorganic copper halide salt K2Cu2Cl6 single-crystal and thin films were prepared. The single-crystal diffraction data belonged to the monoclinic K2Cu2Cl6 (space group = P2(1)/C, unit cell parameters of a = 4.0340 Å, b = 13.7987 Å, c = 8.7445 Å, α = 90.000, β = 97.123, and γ = 90.000). As far as we know, this is the first study of the copper halide salt K2Cu2Cl6 for optoelectronic applications. The band gap of K2Cu2Cl6 is calculated to be approximately 1.85 eV. A low-cost photodetector based on the K2Cu2Cl6 thin film was efficient under different monochromatic light from 330 to 390 nm with different chopping frequencies (1.33–30 Hz). Density functional theory (DFT) computational results indicate that the valence bands (VBs) and conduction bands (CBs) are shifted up in energy using the orbital-dependent correction to the DFT energy. Partial density of states reveals that the VBs and narrow CBs are derived from the hybrid orbitals of Cu2+ 3d and Cl– p, respectively.

Rattling of Oxygen Ions in a Sub-Nanometer-Sized Cage Converts Terahertz Radiation to Visible Light.

A simple and robust approach to visualization of continuous wave terahertz (CW-THz) light would open up opportunities to couple physical phenomena that occur at fundamentally different energy scales. Here we demonstrate how nanoscale cages of Ca12Al14O33 crystal enable conversion of CW-THz radiation to visible light. These crystallographic cages are partially occupied with weakly bonded oxygen ions and give rise to a narrow conduction band that can be populated with localized, yet mobile electrons. CW-THz light excites a nearly stand-alone rattling motion of the encaged oxygen species, which promotes electron transfer from them to the neighboring vacant cages. When the power of CW-THz light reaches tens of watts, the coupling between forced rattling in the confined space, electronic excitation and ionization of oxygen species, and corresponding recombination processes result in emission of bright visible light.

Disorder engineering and conductivity dome in ReS2 with electrolyte gating.

Atomically thin rhenium disulphide (ReS2) is a member of the transition metal dichalcogenide family of materials. This two-dimensional semiconductor is characterized by weak interlayer coupling and a distorted 1T structure, which leads to anisotropy in electrical and optical properties. Here we report on the electrical transport study of mono- and multilayer ReS2 with polymer electrolyte gating. We find that the conductivity of monolayer ReS2 is completely suppressed at high carrier densities, an unusual feature unique to monolayers, making ReS2 the first example of such a material. Using dual-gated devices, we can distinguish the gate-induced doping from the electrostatic disorder induced by the polymer electrolyte itself. Theoretical calculations and a transport model indicate that the observed conductivity suppression can be explained by a combination of a narrow conduction band and Anderson localization due to electrolyte-induced disorder.

Electronic and magnetic properties of electron-dopedV2O5andNaV2O5

Because of its narrow split-off conduction band, doping of ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ leads to interesting strongly correlated electrons. We study the effects of doping on ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$'s electronic and magnetic properties, either by adding electrons compensated by an artificial homogeneous background, or a virtual crystal approximation (VCA), by changing the atomic number ${Z}_{\mathrm{V}}$, so as to keep charge neutrality, or by explicitly introducing Na as a dopant. The former two are considered as a way to simulate injected charge by gating, the latter occurs in the vanadium bronze ${\mathrm{NaV}}_{2}{\mathrm{O}}_{5}$. We also simulate ${\mathrm{Na}}_{1\ensuremath{-}x}{\mathrm{V}}_{2}{\mathrm{O}}_{5}$ using a virtual crystal approximation by changing the atomic number $10\ensuremath{\le}{Z}_{\mathrm{Na}}\ensuremath{\le}11$. The differences in the band structure, which result from how the electrons added to the band are compensated by positive charge in the three models, are compared. The electronic band structures are calculated using the quasiparticle self-consistent $QSGW$ method including a lattice polarization correction and the local spin density functional method with Hubbard-$U$ corrections (LSDA+$U$). For ${\mathrm{NaV}}_{2}{\mathrm{O}}_{5}$, the half-filling leads to a splitting of the up- and down-spin lowest ${d}_{xy}$ band. The spins are found to prefer an antiferromagnetic ordering along the chain direction. Other spin configurations are shown to have higher energy and the exchange interactions are extracted and compared with literature. The optical conductivities are calculated and compared with experiment. Similar results are found for simply doping the band compensated by a background or virtual crystal approximation. However, the position of the occupied bands depends on the method chosen for compensating the charge. The most realistic way to simulate gating in which the compensating charge is kept away from the ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ layer is the VCA with varying ${Z}_{\mathrm{Na}}$. The splitting between the up- and down-spin bands depends on the filling. We find that below a certain concentration of about 0.88 electrons per V, the FM arrangement becomes preferable over the antiferromagnetic one. The magnetic moments then gradually decrease as we lower the filling of the split-off band.

Transformation of the band spectrum of Hg-based HTSC and features of the temperature dependences of the thermoelectric power coefficient

The temperature variations of the thermoelectric power coefficients of the Hg-based high-temperature superconductors HgBa2Can−1CunO2n+2+δ (n = 1, 2, 3) with oxygen doping and cation substitutions are analyzed in terms of a narrow conduction band model. The parameters of the band spectrum in the vicinity of the Fermi level are determined and the manner in which they transform is examined. A correlation is found between the effective band width and the superconducting transition temperature Tc.

Preparation-method-dependent morphological, band structural, microstructural, and photocatalytic properties of noble metal–GaNbO4 nanocomposites

We report the distinct physicochemical and photophysical properties of gallium niobate photocatalysts (bandgap: ∼3.1 eV), prepared by a solid-state (SS) reaction and sol–gel (SG) method and dispersed with a noble metal (∼0.5% of Pt, Au, or RuOx) cocatalyst. SG–GaNbO4 comprised smaller size particles (∼20–50 nm) and a larger surface area (∼160 m2 g−1) compared to SS–GaNbO4 (particle size ∼30–150 nm, surface area ∼27 m2 g−1). XRD patterns revealed a preparation-dependent variation in the relative intensity of prominent reflections. In TEM examination, SG samples exhibited small-range grain boundaries and heterogeneous metal/substrate interfacial contacts, while SS–GaNbO4 had long-range ordering. Laser-Raman and thermoluminescence investigations revealed that lattice distortion, defect-induced inter-bandgap charge trapping states, and the local environment around the metal/semiconductor interfaces may also depend on the preparation method. Metal–GaNbO4 nanocomposites showed no activity for the dissociation of pure water under UV (>250 nm) irradiation, despite the favourable conduction and valence band potentials. This was attributed to the sharp Ga and Nb d-levels in the narrow conduction band of GaNbO4, as confirmed by ab initio electronic structure calculation. These photocatalysts, however, showed good activity for semiconductor-mediated photo-dissociation of aqueous methanol to produce H2; a cocatalyst-dependent activity trend, Pt > RuOx > Au, was observed. Doping of S at ∼5% of the oxygen sites led to decreased photoactivity, ascribed to the presence of localized S 3p states just above the O 2p valence level. In conclusion, besides band characteristics, certain morphological and microstructural properties play a crucial role in the photoactivity of the metal/oxide nanocomposites.

Ab-initio investigation of spin states of sodium cobaltate Na2/3CoO2

Resent experiments in the lamellar system NaxCoO2 detected a transition of Co planes into a puzzling metallic state at x ≥ 2/3, which co-exists with a robust arrangement of the 3d cobalt electrons: The triangular Co lattices are disproportionated in the spinless Co3+ sites (Co1), and Co3.44+ sites (Co2) with enhanced magnetism forming conducting sublattices. This textures concur with a tightening of the ferromagnetic (FM) interaction in planes, and emerge when the sodium ions become arranged in layers in between the CoO2 slabs. In the present research we have investigated ab-initio the appearance of such state in Na2/3CoO2. Towards this end in view we studied an interplay between the electronic coupling to the superstructure of the Na+ ions and local correlations of the itinerant d electrons treated within the GGA+U approximation. Employing the exact crystallographic supercell, the electronic organization has been analyzed upon increasing the energy U of the Coulomb repulsion within the 3d shells at T = 0. The metallic ground state, being a spin density wave with the inplane FM and antiferromagnetic interplane correlations, has been obtained and established to posses two regimes. When U > 2 eV, a crossover develops from a uniform state of the d-lattice to the regular phase with the spin/charge disproportionation between the sites. In particular at the representative value U = 5 eV, the Co13+ sites with suppressed magnetism appears, while the spin-active Co4+ holes are accumulated by the Co2 sites. A related formation of an isolated, narrow conduction band at the Fermi level implies a considerable enhancement of the electron correlations in the crystal field imposed by the Na+ patterns.

Lone conduction band in Cu2ZnSnSe4

We present experimental proof for a narrow first conduction band in Cu2ZnSnSe4 semiconductor films as it has been predicted by theoretical calculations. The optical absorption characteristics of Cu2ZnSnSe4 thin films are analyzed by optical transmission spectroscopy. The experimental data show strong evidence for three absorption edges, as expected from theory, involving the valence band and two conduction bands, which are separated by a second band gap.

Spectroscopic characterization of a multiband complex oxide: Insulating and conducting cement 12CaO·7Al2O3

Natural 12CaO$\cdot$7Al$_2$O$_3$ (C12A7) is a wide bandgap insulator, but conductivity can be realized by introducing oxygen deficiency. Currently, there are two competing models explaining conductivity in oxygen-deficient C12A7, one involving the electron transfer via a "cage conduction band" inside the nominal band gap, the other involving electron hopping along framework lattice sites. To help resolve this debate, we probe insulating and conducting C12A7 with X-ray emission, X-ray absorption, and X-ray photoemission spectroscopy, which provide a full picture of both the valence and conduction band edges in these materials. These measurements suggest the existence of a narrow conduction band between the main conduction and valence bands common in both conducting and insulating C12A7 and support the theory that free electrons in oxygen-deficient C12A7 occupy the low-energy states of this narrow band. Our measurements are corroborated with density functional theory calculations.

A Theoretical Study Showing K2picene as a Parent Semiconductor for Organic Superconductivity

Based on the density functional theory (DFT), a theoretical determination method is applied to find structural and electronic properties of potassium-doped picene. In a unit cell of K 2 picene observed experimentally, the atomic positions and the electronic state are determined within a generalized-gradient approximation of DFT using the plane-wave expansion method. Some metastable structures of K 2 picene are found in the optimization simulations. Notable properties of a structure with AB stacking, which is the lowest in the energy than others, are 1) appearance of electron doped molecular bands with the band width of 0.1–0.4 eV, 2) gap formation with a gap of 0.04–0.1 eV at the Fermi energy, and 3) narrow conduction bands with the band width of ∼0.1 eV. In all meta-stable structures determined in this study, these characteristics are commonly found. Assuming further electron doping in the conduction bands of K 2 picene, electron–correlation effects may appear as the pair-hopping processes between doped ...

Optimal Bandwidth for High Efficiency Thermoelectrics

The thermoelectric figure of merit (ZT) in narrow conduction bands of different material dimensionalities is investigated for different carrier scattering models. When the bandwidth is zero, the transport distribution function (TDF) is finite, not infinite as previously speculated by Mahan and Sofo [Proc. Natl. Acad. Sci. U.S.A. 93, 7436 (1996)], even though the carrier density of states goes to infinity. Such a finite TDF results in a zero electrical conductivity and thus a zero ZT. We point out that the optimal ZT cannot be found in an extremely narrow conduction band. The existence of an optimal bandwidth for a maximal ZT depends strongly on the scattering models and the dimensionality of the material. A nonzero optimal bandwidth for maximizing ZT also depends on the lattice thermal conductivity. A larger maximum ZT can be obtained for materials with a smaller lattice thermal conductivity.