Conduction Band States Research Articles

Coloration of tungsten oxide electrochromic thin films at high bias potentials and low intercalation levels

The optical absorption of amorphous tungsten oxide thin films was studied at low intercalation levels. The electronic density of states was obtained by an electrochemical method and bias potential regions were assigned to the conduction band and gap states according to the experimentally estimated conduction band edge. Differences between the coloration in the conduction band and gap states were observed when comparing the experimental results to a theoretical site-saturation model that considers electronic transitions between localized tungsten sites. The model could reproduce the optical response due to conduction band states. However, it underestimated the rate of change of the optical absorption coefficient with intercalation level for gap states. This discrepancy is discussed in the context of small-polaron optical absorption.

Pyramidal core-shell quantum dot under applied electric and magnetic fields

We have theoretically investigated the electronic states in a core/shell pyramidal quantum dot with GaAs core embedded in AlGaAs matrix. This system has a quite similar recent experimental realization through a cone/shell structure [Phys. Status Solidi-RRL 13, 1800245 (2018)]. The research has been performed within the effective mass approximation taking into account position-dependent effective masses and the presence of external electric and magnetic fields. For the numerical solution of the resulting three-dimensional partial differential equation we have used a finite element method. A detailed study of the conduction band states wave functions and their associated energy levels is presented, with the analysis of the effect of the geometry and the external probes. The calculation of the non-permanent electric polarization via the off-diagonal intraband dipole moment matrix elements allows to consider the related optical response by evaluating the coefficients of light absorption and relative refractive index changes, under different applied magnetic field configurations.

Atomic reconstruction in twisted bilayers of transition metal dichalcogenides.

Van der Waals heterostructures form a unique class of layered artificial solids in which physical properties can be manipulated through controlled composition, order and relative rotation of adjacent atomic planes. Here we use atomic-resolution transmission electron microscopy to reveal the lattice reconstruction in twisted bilayers of the transition metal dichalcogenides, MoS2 and WS2. For twisted 3R bilayers, a tessellated pattern of mirror-reflected triangular 3R domains emerges, separated by a network of partial dislocations for twist angles θ < 2°. The electronic properties of these 3R domains, featuring layer-polarized conduction-band states caused by lack of both inversion and mirror symmetry, appear to be qualitatively different from those of 2H transition metal dichalcogenides. For twisted 2H bilayers, stable 2H domains dominate, with nuclei of a second metastable phase. This appears as a kagome-like pattern at θ ≈ 2°, transitioning at θ → 0 to a hexagonal array of screw dislocations separating large-area 2H domains. Tunnelling measurements show that such reconstruction creates strong piezoelectric textures, opening a new avenue for engineering of 2D material properties.

Oxygen vacancy in ZnO-w phase: pseudohybrid Hubbard density functional study

The study of zinc oxide, within the homogeneous electron gas approximation, results in overhybridization of zinc 3d shell with oxygen 2p shell, a problem shown for most transition metal chalcogenides. This problem can be partially overcome by using LDA + U (or, GGA + U) methodology. However, in contrast to the zinc 3d orbital, Hubbard type correction is typically excluded for the oxygen 2p orbital. In this work, we provide results of electronic structure calculations of an oxygen vacancy in ZnO supercell from ab initio perspective, with two Hubbard type corrections, UZn-3d and UO-2p. The results of our numerical simulations clearly reveal that the account of UO-2p has a significant impact on the properties of bulk ZnO, in particular the relaxed lattice constants, effective mass of charge carriers as well as the bandgap. For a set of validated values of UZn-3d and UO-2p we demonstrate the appearance of a localized state associated with the oxygen vacancy positioned in the bandgap of the ZnO supercell. Our numerical findings suggest that the defect state is characterized by the highest overlap with the conduction band states as obtained in the calculations with no Hubbard-type correction included. We argue that the electronic density of the defect state is primarily determined by Zn atoms closest to the vacancy.

Local photodoping in monolayer MoS2

Inducing electrostatic doping in 2D materials by laser exposure (photodoping effect) is an exciting route to tune optoelectronic phenomena. However, there is a lack of investigation concerning in what respect the action of photodoping in optoelectronic devices is local. Here, we employ scanning photocurrent microscopy (SPCM) techniques to investigate how a permanent photodoping modulates the photocurrent generation in MoS2 transistors locally. We claim that the photodoping fills the electronic states in MoS2 conduction band, preventing the photon-absorption and the photocurrent generation by the MoS2 sheet. Moreover, by comparing the persistent photocurrent (PPC) generation of MoS2 on top of different substrates, we elucidate that the interface between the material used for the gate and the insulator (gate-insulator interface) is essential for the photodoping generation. Our work gives a step forward to the understanding of the photodoping effect in MoS2 transistors and the implementation of such an effect in integrated devices.

What Does the Transient Absorption Spectrum of CdSe Quantum Dots Measure?

The roles of conduction and valence band state filling and stimulated emission in the room temperature transient absorption spectroscopy of CdSe and CdSe/CdS quantum dots are discussed. Photolumine...

Electrical detection of excitonic states by time-resolved conductance measurements

We present time-resolved conductance measurements and charge spectra for the conduction-band states of InAs quantum dots after creating metastable holes by illumination. We demonstrate an electrical way of measuring the conduction-band energy offset and inverse tunnel rates of electrons from a two-dimensional electron gas into neutral (${X}^{0}$), single positively (${X}^{1+}$), and double positively (${X}^{2+}$) charged exciton states. The experiment also gives information about the metastable hole storage time and discharge dynamics.

Target power influence on optical and electrical properties of amorphous titanium oxide deposited by reactive grid-assisted magnetron sputtering

Titanium oxide (TiOx) thin films were deposited by reactive grid-assisted magnetron sputtering on glass substrates as function of the target power. Films were characterized by mechanical profilometry, optical spectrophotometry, four-probe method, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) in order to investigate the effect of the target power on electrical, structural and electronic properties. The electrical properties were measured after deposition and one year later to evaluate the stability of the sheet resistance. Data for suboxides show the decreasing of the optical gap, sheet resistance and density of electronic states in conduction and/or valence bands, and increasing of the optical gap entropy, optical absorption and film thickness as the target power increases. The absorption coefficient indicates the presence of mid-gap states in suboxides, inducing the redshift of the absorption onset. The TiO2 reference sample presents only a single transition in the ultraviolet part. XRD show amorphous structure for all samples. Ti2p and O1s spectra from XPS indicate an oxidized surface with a small contribution of Ti3+ states in either titanium suboxide or dioxide, where the surfaces with the highest concentrations of hydroxyl radicals presented the best stability of the sheet resistance after one year.

PbS and PbS/CdS quantum dots: Synthesized by photochemical approach, structural, linear and nonlinear response properties, and optical limiting

Abstract

Modulation of band alignment with water redox potentials by biaxial strain on orthorhombic NaTaO3 thin films.

Photocatalysis-assisted water splitting using semiconductor materials greatly depends on the bandgap size and the alignment of band edges relative to the reaction potentials. We used ab initio computational methods to show that the biaxial strain on [100]-oriented orthorhombic NaTaO3 thin films grants the modulation of surface states, favoring either the hydrogen evolution reaction (HER) or the oxygen evolution reaction (OER), which basically rules the perovskite photocatalytic performance. Under compression, the outermost TaO6 and TaO4 polyhedra become more distorted, and electrostatic repulsion increases the energy of Ta 5d surface states. As they overcome the O2/H2O potential, they cease to contribute to the OER. At the same time, the H+/H2 remains below the conduction band, leveraging the HER over the OER. The tensile strain lowers the outermost polyhedra distortions, stabilizing both Ta 5d surface and conduction band states, and increasing the charge centered around surface Ta atoms. Consequently, the bands are better aligned with O2/H2O and H+/H2 potentials, which benefits the overall water splitting photocatalysis. Our results evidence that combining facet and strain engineering is an effective way of altering the photocatalytic activity of orthorhombic [100] NaTaO3 thin films.

Conduction band modifications by d states in vanadium doped CdO

3d transition metal ions have been shown to act as donors and to introduce highly localized d-levels when incorporated in CdO. In this work, we synthesized CdO thin films doped with a 3d metal V (CVO) with a V mole fraction x up to 0.1 by radio-frequency magnetron sputtering. CVO thin films exhibit a monotonic increase in the electron concentration n with x up to a saturation value of ∼1.0 × 1021 cm−3 at a dopant concentration x > 0.045. In contrast to CdO doped with shallow donors such as In where the electron mobility μ is > 100 cm2/V-s even at a high In doping level of >5%, in CVO the μ decreases continuously from ∼100 to <10 cm2/V-s with increasing x. Spectroscopic ellipsometry measurements show a rapid increase in the effective mass in CVO, consistent with the continuous reduction of μ. The electrical and optical properties of CVO are analyzed in terms of the anticrossing interaction of the localized V d-levels with the extended conduction band (CB) states of the host CdO. Such anticrossing interaction splits the CdO CB into two non-parabolic subbands, E+ and E-. The optical absorption edge of CVO with x > 0.045 is consistent with transitions from the valence band to the empty E+ subband while the much reduced μ can be explained by the increase in effective mass due to flattening of the E- subband. Such band anticrossing interaction is a common phenomenon for most transition metal dopants with d electrons in metal oxides and can explain the optoelectronic properties of these materials.

Electronic structure of ZnO thin films doped with group III elements

First-principles calculations based on density functional theory are performed to understand the electronic properties of III group elements doped ZnO as functions of the dopant atoms content. The results reveal that the lattice constants of Ga, In, Tl, Y, Sc and La doped ZnO increase linearly with the doping increasing, while for B and Al doped ZnO observed decreases lattice parameters with increases dopant content in ZnO. According to the results, the optical band gap of all ZnO doped systems is broadened due to the increase of doping concentrations. Substitutional In, Al, Ga atoms contribute significantly delocalized s orbitals in the conduction band states, which are expected to increase the mobility of the material. The covalent bonding of Ga, In, Tl, Y, Sc and La doped ZnO is found to weaken with the increase of dopant content, whereas we observed more covalent bonds in B and Al doped ZnO. The simulation and calculation results are in good agreement with the existing experimental data and the study can provide a theoretical basis for future applications of group III elements doped ZnO.

Electronic structure and optical characteristics of AA stacked bilayer graphene: A first principles calculations

First-principle calculations based on full potential linearized augmented plane-wave method have been performed to investigate the electronic and optical properties of 1 × 1 and 2 × 2 supercells of non-Bernal AA stacked bilayer graphene. The exchange-correlation effects in the present work have been treated using the local density approximation which show good agreement of the calculated structural and electronic properties with previous reports. From the calculated electronic properties, semimetallic nature is found for 1 × 1 supercell of bilayer graphene, while a pseudogap is observed for the 2 × 2 supercell. Our results indicate that the pseudogap in 2 × 2 supercell of bilayer graphene originates from the increased number of valence and conduction band states contributing in the electronic structure of this configuration and suppressing the states in the vicinity of Fermi level. In order to understand the effects of non-Bernal AA stacking of two graphene layers on the optical response, we have computed the optical properties in terms of complex dielectric function for electromagnetic radiations having polarization vectors parallel and perpendicular to the xy-plane. Our calculated optical properties show a significant enhancement of optical absorption of electromagnetic radiations having polarization vector parallel to xy-plane, which is ascribed to the augmentation of 2D honeycomb lattice of carbon atoms in form of bilayer graphene. These results highlight tunable electronic and optical characteristics of graphene that might prove advantageous for its employment in electronic devices.

Thermoelectric properties improvement in quasi-one-dimensional organic crystals

The charge and energy transport in highly conducting quasi-one-dimensional organic crystals of p-type tetrathiotetracene-iodide, TTT2I3, and of n-type tetrathiotetracene-tetracyanoquinodimethane, TTT(TCNQ)2, is studied. Two electron-phonon interactions are considered simultaneously. One interaction is of the acoustic deformation potential type and the other one is of polaronic character. Charge transport along the conducting molecular chains is bandlike, whereas in the transversal directions, it is of the hopping type. It is shown that due to a partial compensation of these interactions for a narrow interval of states in the one-dimensional conduction band, the relaxation time is of Lorentzian shape and shows a distinct dependence on carrier energy with a pronounced maximum. The scattering of charge carriers on adjacent molecular chains and by impurities and structural defects limits the height of this maximum. However, rather high relaxation times might be anticipated in the case of perfect single crystals. As the carriers in these states show an enhanced mobility, this will lead to a simultaneous increase of electrical conductivity and Seebeck coefficient. It is proposed that, if the above-mentioned crystals could be accomplished by means of sufficient purification and by optimization of the carrier concentration, so that the Fermi level is close to energetic states for which the relaxation time has a maximum, one might achieve values for the thermoelectric figure of merit ZT∼5 in crystals of tetrathiotetracene-iodide and ZT∼1.5 in those of tetrathiotetracene-tetracyanoquinodimethane.

Theoretical estimation of tunnel currents in hetero-junctions: The special case of nitride tunnel junctions

In tunnel junctions, an electron current is transformed into a hole current via a quantum tunnel effect through the semiconductor bandgap. We derive a complete theory for the current through tunnel junctions based on Kane's approach and extended to the general case of a nonconstant electric field and arbitrary potentials in heterostructures. The theory mixes an analytical approach based on Fermi's golden rule and the numeric calculation of wave functions in the heterostructure. The parallel component of the transport is included in the calculation and the symmetry of the conduction and valence band states are taken into account in the transition rates. The calculation is limited to the elastic case and leads to a simple and fast estimation of the tunnel current in any semiconductor junction. We applied our calculation to III-nitrides due to the importance of tunnel junctions in these materials, since they allow circumventing the problem of insufficient p-type doping in GaN and AlGaN. Our approach is also particularly relevant in III-nitride heterojunctions owing to the large band offsets and varying piezoelectric fields present in these materials. The resulting dependence of the inverse current-voltage characteristics on several parameters is studied, making it possible to optimize thickness, doping, and composition of a smaller gap semiconductor layer inserted in the junction. Among all parameters, we show the importance of the doping levels in the n and p regions, while a thin undoped interlayer with a smaller bandgap energy critically enhances the tunnel transparency.

Non-conventional ferromagnetism and high bias magnetoresistance in [formula omitted]: A simple phenomenological approach

A simple phenomenological approach is used to describe magnetization and magnetoresistance in thin films of oxygen-deficient rutile titanium dioxide (TiO2). Magnetization curves were successfully simulated by considering the ferromagnetic (FM) contribution of magnetically interacting oxygen vacancies (VO’s) forming a single-domain using the Stoner-Wohlfarth (SW) model combined with paramagnetic (PM) contribution of isolated VO’s described by a Langevin function. Whereas approximately half of the VO’s behave as ferromagnetically coupled, rotating coherently with the applied magnetic field, the remaining half of the VO’s tend to align individually with the local magnetic field overcoming the thermal fluctuation deviations. The conduction mechanism is described by variable range hopping (VRH) regime with electron hopping between VO’s centers in the TiO2-x matrix. The magnetoresistance (MR) curves are consistently simulated through the scaling with square of PM contribution of magnetization, indicating that resistance depends on the relative orientation of the magnetic moments associated with non-interacting VO’s dispersed in TiO2 matrix at low temperatures. Density functional theory (DFT) calculations were performed to validate the parametrization of the phenomenological approach used to described the ferromagnetic groundstate of the VO network and spin polarization in the VRH regime in the TiO2-x matrix. Spin-resolved density of states (DOS) calculations consistently reveal that magnetoresistance can be ascribed to electron hopping from defect-states located immediately below Fermi level to the bottom of conduction band states with an opposed spin-polarization.

Density Functional Theory Study of ZnIn2S4 and CdIn2S4 Polymorphs Using Full‐Potential Linearized Augmented Plane Wave Method and Modified Becke–Johnson Potential

ZnIn2S4 and CdIn2S4 are ternary sulfide semiconductors with attracting photoelectric and catalytic properties, which are closely related with their band structure. The full‐potential linearized augmented plane wave (FP‐LAPW) method is used to calculate the band structures of cubic and hexagonal polymorphs of ZnIn2S4 and CdIn2S4. Among the four systems studied, the hexagonal CdIn2S4 structure with a space group P63mc (No. 186) is newly predicted, and the phonon calculation shows that its structure is dynamically stable. Owing to the well‐known bandgap underestimation problem of local density approximation (LDA) and generalized gradient approximation (GGA), the effects of Hubbard model and modified Becke–Johnson (mBJ) potential to band structure are investigated. The calculated outcomes of cubic ZnIn2S4 and cubic CdIn2S4 with mBJ‐GGA + U are in good agreement with the experimental data. For hexagonal ZnIn2S4, it is shown that electronic states in high valence bands and low conduction bands have different components. The origin of discrepancy between the theoretical bandgap of hexagonal ZnIn2S4 and experimental value obtained from optical absorption measurement is discussed based on the spatial separation of electron and hole states and optical transition probability.

Photoinduced Charge Separation in Retinoic Acid on TiO2: Comparison of Three Anchoring Modes

A complete understanding of the interaction between the dye and the semiconductor is essential for the design of a dye-sensitized solar cell (DSC) because the chemical interactions and the electron coupling between semiconductor and dye directly impact electron transfer and the performance of the dye-sensitized photoanode. Three anchoring modes of retinoic acid (RA), which contains a carboxylic acid group adsorbed on TiO2, were studied by DFT calculations. Of the three anchoring modes, the mode involving surface Ti–OH groups has the highest driving force for photoinduced electron transfer. The mixing of the RA’s lowest unoccupied molecular orbital (LUMO) with the conduction band states of TiO2 is also the highest. The calculated results are supported by EPR measurements, which show enhanced light-driven charge separation from increasing the density of the Ti–OH groups on the surface of TiO2. The anchoring modes affect the bond length, degree of conjugation, energies of the highest occupied molecular orbit...

Absence of Interlayer Tunnel Coupling of K-Valley Electrons in Bilayer MoS_{2}.

In Bernal stacked bilayer graphene interlayer coupling significantly affects the electronic band structure compared to monolayer graphene. Here we present magnetotransport experiments on high-quality n-doped bilayer MoS_{2}. By measuring the evolution of the Landau levels as a function of electron density and applied magnetic field we are able to investigate the occupation of conduction band states, the interlayer coupling in pristine bilayer MoS_{2}, and how these effects are governed by electron-electron interactions. We find that the two layers of the bilayer MoS_{2} behave as two independent electronic systems where a twofold Landau level's degeneracy is observed for each MoS_{2} layer. At the onset of the population of the bottom MoS_{2} layer we observe a large negative compressibility caused by the exchange interaction. These observations, enabled by the high electronic quality of our samples, demonstrate weak interlayer tunnel coupling but strong interlayer electrostatic coupling in pristine bilayer MoS_{2}. The conclusions from the experiments may be relevant also to other transition metal dichalcogenide materials.

Faceting-Controlled Zeeman Splitting in Plasmonic TiO2 Nanocrystals.

Dynamic manipulation of discrete states in nanostructured materials is critical for emerging quantum technologies. However, this process often requires a correlation of mutually competing degrees of freedom. Here we report the control of magnetic-field-induced excitonic splitting in colloidal TiO2 nanocrystals by control of their faceting. By changing nanocrystal morphology via reaction conditions, we control the concentration and location of oxygen vacancies, which can generate localized surface plasmon resonance and foster the reduction of lattice cations leading to the emergence of individual or exchange-coupled Ti(III) centers with high net-spin states. These species can all couple with the nanocrystal lattice under different conditions resulting in distinctly patterned excitonic Zeeman splitting and selective control of conduction band states in an external magnetic field. This work demonstrates the concept of using nanocrystal morphology to control carrier polarization in individual nanocrystals using both intrinsic and collective electronic properties, representing a unique approach to multifunctionality in reduced dimensions.