Conduction Band States Research Articles

Pressure-induced enhancement in the thermoelectric properties of monolayer and bilayerSnSe2.

The electronic structures of monolayer and bilayer SnSe2 under pressure were investigated by using first-principles calculations including van der Waals interactions. For monolayer SnSe2, the variation of electronic structure under pressure is controlled by pressure-dependent lattice parameters. For bilayer SnSe2, the changes in electronic structure under pressure are dominated by intralayer and interlayer atomic interactions. The n-type thermoelectric properties of monolayer and bilayer SnSe2 under pressure were calculated on the basis of the semi-classical Boltzmann transport theory. It was found that the electrical conductivity of monolayer and bilayer SnSe2 can be enhanced under pressure, and such dependence can be attributed to the pressure-induced changes of the Se–Sn antibonding states in conduction band. Finally, the doping dependence of power factors of n-type monolayer and bilayer SnSe2 at three different pressures were estimated, and the results unveiled that thermoelectric performance of n-type monolayer and bilayer SnSe2 can be improved by applying external pressure. This study benefits to understand the nature of the transport properties for monolayer and bilayer SnSe2 under pressure, and it offers valuable insight for designing high-performance thermoelectric few-layered SnSe2 through strain engineering induced by external pressure.

Infrared/terahertz spectra of the photogalvanic effect in (Bi,Sb)Te based three-dimensional topological insulators

We report on the systematic study of infrared/terahertz spectra of photocurrents in (Bi,Sb)Te based three dimensional topological insulators. We demonstrate that in a wide range of frequencies, ranging from fractions up to tens of terahertz, the photocurrent is caused by the linear photogalvanic effect (LPGE) excited in the surface states. The photocurrent spectra reveal that at low frequencies the LPGE emerges due to free carrier Drude-like absorption. The spectra allow to determine the room temperature carrier mobilities in the surface states despite the presents of thermally activate residual impurities in the material bulk. In a number of samples we observed an enhancement of the linear photogalvanic effect at frequencies between 30{\div}60 THz, which is attributed to the excitation of electrons from helical surface to bulk conduction band states. Under this condition and applying oblique incidence we also observed the circular photogalvanic effect driven by the radiation helicity.

Highly Localized Charge Transfer Excitons in Metal Oxide Semiconductors.

The ability to observe charge localization in photocatalytic materials on the ultrafast time scale promises to reveal important correlations between excited state electronic structure and photochemical energy conversion. Of particular interest is the ability to determine hole localization in the hybridized valence band of transition metal oxide semiconductors. Using femtosecond extreme ultraviolet reflection absorption (XUV-RA) spectroscopy we directly observe the formation of photoexcited electrons and holes in Fe2O3, Co3O4, and NiO occurring within the 100 fs instrument response. In each material, holes localize to the O 2p valence band states as probed at the O L1-edge, while electrons localize to metal 3d conduction band states on this same time scale as probed at the metal M2,3-edge. Chemical shifts at the O L1-edge enable unambiguous comparison of metal-oxygen (M-O) bond covalency. Pump flux dependent measurements show that the exciton radius is on the order of a single M-O bond length, revealing a highly localized nature of exciton in each metal oxide studied.

Shell Thickness-Dependent Tunable Threshold Voltage Single Quantum Dot Rectification Diode

Ambient atmosphere single colloidal quantum dot (QD) rectifying diode with tunable threshold voltage has been fabricated by using a type-II heterojunction core/shell structure with a device geometry ITO/ZnO/QDs. Specifically, in our work we have used ZnTe/CdS core/shell QDs in which hole wave function strongly confined to the core, whereas the lowest-lying conduction band state resides in the shell. Current–voltage (I–V) characterization of this device has been done using an ambient atmosphere scanning tunneling microscope. The scanning tunneling spectra (STS) shows high rectification with a ratio of 103. The rectification is found to arise because of the bias-dependent band alignment of ZnO/QDs heterojunction and the effect of shell of each QD that presents a barrier for hole tunneling into the substrate. This barrier is overcome by the externally applied bias. This mechanism is distinct from the rectification observed in conventional p–n junction diodes. In particular, we find that even for QDs with opt...

Simultaneous Conduction and Valence Band Quantization in Ultrashallow High-Density Doping Profiles in Semiconductors

We demonstrate simultaneous quantization of conduction band (CB) and valence band (VB) states in silicon using ultrashallow, high-density, phosphorus doping profiles (so-called Si:P δ layers). We show that, in addition to the well-known quantization of CB states within the dopant plane, the confinement of VB-derived states between the subsurface P dopant layer and the Si surface gives rise to a simultaneous quantization of VB states in this narrow region. We also show that the VB quantization can be explained using a simple particle-in-a-box model, and that the number and energy separation of the quantized VB states depend on the depth of the P dopant layer beneath the Si surface. Since the quantized CB states do not show a strong dependence on the dopant depth (but rather on the dopant density), it is straightforward to exhibit control over the properties of the quantized CB and VB states independently of each other by choosing the dopant density and depth accordingly, thus offering new possibilities for engineering quantum matter.

Parallel and antiparallel angular momentum transfer of circularly polarized light to photoelectrons and Auger electrons at the Ni L3 absorption threshold

We report the investigation of the angular momentum transfer of light to photoelectrons and Auger electrons at the Ni ${L}_{3}$ absorption threshold. Upon core-level excitation by circularly polarized light, the angular momentum of light, or helicity, is transferred to the emitted photoelectron. The angular momentum of the emitted photoelectron (${m}_{\mathrm{f}}$) is the sum of the helicity ($\ensuremath{\sigma}$) and the orbital magnetic quantum number of the initial state (${m}_{\mathrm{i}}$). Here the quantization axis was defined as the direction along the incident light. This can be measured by the parallax shift of the forward focusing peak (FFP) direction in the photoelectron intensity angular distribution. At the absorption threshold, the excited core-level electron is promoted to a conduction-band state and the angular momentum of the light is partially transferred to Auger electrons. We measured photoelectron and Auger electron intensity angular distributions from the Ni(111) surface at the ${L}_{3}$ absorption threshold. We observed a significant angular circular dichroism of the $[101]$ FFP for the ${L}_{3}{M}_{4,5}{M}_{4,5}$ Auger electrons. Furthermore, we discovered non-negligible reversal angular circular dichroism contrasts for the triplet components in the case of the ${L}_{3}{M}_{2,3}{M}_{4,5}$ Auger electrons, suggesting that the angular momentum of light was transferred to the emitted electron in the antiparallel way.

Electrical tuning of spin splitting in Bi-doped ZnO nanowires

The effect of applying an external electric field on doping-induced spin-orbit splitting of the lowest conduction-band states in a bismuth-doped zinc oxide nanowire is studied by performing electronic structure calculations within the framework of density functional theory. It is demonstrated that spin splitting in Bi-doped ZnO nanowires could be tuned and enhanced electrically via control of the strength and direction of the applied electric field, thanks to the nonuniform and anisotropic response of the ZnO:Bi nanowire to external electric fields. The results reported here indicate that a single ZnO nanowire doped with a low concentration of Bi could function as a spintronic device, operation of which is controlled by applied lateral electric fields.

First principle study of electron and band structure of BixBa1-xTiO3

Some perovskite structured catalysts have narrower forbidden band widths than pure TiO2, and they have been widely used in a number of photo-catalytic reactions. The ions in the perovskite may be replaced by other ions while maintaining the structure unchanged for its tailorable character. BiTiO can form into the typical perovskite composite oxide BiTiO3 under specific preparation conditions. The regulation of the energy gap of the perovskite BaTiO3 can be realized by substituting Bi for Ba to form the BixBa1-xTiO3 perovskite structure to improve its photo-catalytic activity. But the improvement mechanism and the electron and band structures of BixBa1-xTiO3 are still not very clear. In this study, we exhibit a detailed theoretical investigation to predict the electronic structure, band gap and optical absorption properties of BixBa1-xTiO3 structures based on the first-principles plane-wave ultrasoft pseudopotential method. The exchange and correlation interactions are modeled using the generalized gradient approximation and the Perdew-Burke-Ernzerhof exchange-correlation functional. The cutoff kinetic energy of the electron wave function is 340 eV, and the k-point sampling sets 333 division of the reciprocal unit cell based on the Monkhorst-Pack scheme. In the geometrical optimization, all forces on atoms are converged into less than 110-5 eV/atom, the maximum ionic displacement is within 0.001 and the total stress tensor decreases to the order of 0.05 GPa. The DFT calculation results reveal that the symmetry and binding energy decline in the BixBa1-xTiO3 structure, and the bond lengths of BaO and TiO decrease a little after Ba has been substituted by Bi atom, except for the structure of Bi0.5Ba0.5TiO3. The photo-catalysts of BixBa1-xTiO3 are direct band gap semiconductors, and the substitution Bi can regulate the band gaps of BixBa1-xTiO3. The band gaps become wider from x=0.125 to x=0.750 with the carrier concentration decreasing, and then decreases with the higher carrier concentration increasing when x=0.875. It is predicted that the band width of Bi-based perovskite will be much lower than that of Ba-based perovskite. In the case of the density of states we reveal that the top of the valence band is hybrided by O-2p and Bi-6s and the bottom of the conduction band state is mainly constituted by the Ti-3d state. The electron transport properties and carrier types are mainly determined by Ti-3d, O-2p state and Ba-5p electronic states in BaTiO3 and Ti-3d, O-2p, Bi-6s and Bi-6p electronic states in BixBa1-xTiO3 respectively. The absorption spectra indicate that the ultraviolet absorption performance can be improved in BixBa1-xTiO3 system, which may effectively improve the photo-catalytic activity of BaTiO3.

A Unifying Perspective on Oxygen Vacancies in Wide Band Gap Oxides.

Wide band gap oxides are versatile materials with numerous applications in research and technology. Many properties of these materials are intimately related to defects, with the most important defect being the oxygen vacancy. Here, using electronic structure calculations, we show that the charge transition level (CTL) and eigenstates associated with oxygen vacancies, which to a large extent determine their electronic properties, are confined to a rather narrow energy range, even while band gap and the electronic structure of the conduction band vary substantially. Vacancies are classified according to their character (deep versus shallow), which shows that the alignment of electronic eigenenergies and CTL can be understood in terms of the transition between cavity-like localized levels in the large band gap limit and strong coupling between conduction band and vacancy states for small to medium band gaps. We consider both conventional and hybrid functionals and demonstrate that the former yields results in very good agreement with the latter provided that band edge alignment is taken into account.

The effects of strain and vacancy defects on the electronic structure of Cr2O3

In this study, density functional theory calculations have been carried out to investigate the electronic structure of Cr2O3 after applying strain and doping vacancy defects. Results indicate that the tensile strain will decrease the gap, while the compressive strain will increase the gap and the planar biaxial strain has more remarkable effects on electronic structure modulation than that of out-of-plane strain. With respect to the vacancy defects, we find both the Cr vacancy and O vacancy will introduce extra energy bands into the band gap, and the Cr vacancy could reduce the band gap remarkably. Cr vacancy leads to conduction band states closing to Fermi level and accounting for a pronounced narrowing band gap. Tensile strain draw conduction band minimum with t2g (dxy, dxz, dyz) and dx2−y2 orbitals nearer to EF, while compressive strain pushes dxy, dxz, dyz and dx2−y2 orbitals away EF, which are responsible for the pronounced narrowing band gap under tensile strain and increasing under compressive strain. In addition, both the strain and vacancy could also influence the magnetic moments of Cr2O3. Interestingly the O vacancy migration appears in Cr2O3 under the planar biaxial tensile strain ranging from 5% to 8%.

Real- and reciprocal-space attributes of band tail states

Band tail states are localized electronic states existing near conduction and valence band edges. Band tail states are invariably found to exhibit an exponential distribution defined by a characteristic (Urbach) energy. To a large extent, the band tail state density of states determines the electronic performance of an amorphous semiconductor (or insulator) in terms of its mobility. Real-space assessment of a suitable density of states model for extended (delocalized) conduction or valence band states and nearby localized band tail states leads to an expression for the peak density of band tail states at the mobility edge and for the total band tail state density. Assuming a continuous density of states and its derivative with respect to energy across the mobility edge, these densities are found to depend on only two parameters – the Urbach energy and an effective mass characterizing the extended state density above the mobility edge. Reciprocal-space assessment is then employed to deduce a probability density function associated with band tail states. The full width at half maximum of the resulting Gaussian probability density function is found to be equal to the average real-space distance of separation between band tail states, as estimated from the total band tail state density. This real- and reciprocal-space insight may be useful for developing new high-performance amorphous semiconductors and for modeling their electronic properties.

First-Principles Study of Electron Injection and Defects at the TiO2/CH3NH3PbI3 Interface of Perovskite Solar Cells.

We investigated electron injection rates and vacancy defect properties by performing first-principles calculations on the interface of an anatase-TiO2(001) and a tetragonal CH3NH3PbI3(110) (MAPbI3(110)). We found that the coupling matrix element between the lowest unoccupied molecular orbital of MAPbI3 and the TiO2 conduction band (CB) minimum is negligibly small, the indication being that electron-injection times for low-energy excited states are quite long (more than several tens of picoseconds). We also found that higher-lying CB states coupled more strongly; injection was expected to take place on a femtosecond time scale. Furthermore, we found that vacancy defects in the TiO2 layer produced undesired defect levels that caused hole traps and recombination centers. Whereas most of the vacancy defects in the MAPbI3 layer produced no additional states in the MAPbI3 gap, a Pb vacancy (VPb) at the interface created an energy level below the MAPbI3 CB edge and had a lower energy of formation than the VPb defect in bulk because of the interaction with the TiO2 surface.

Gigantic 2D laser-induced photovoltaic effect in magnetically doped topological insulators for surface zero-bias spin-polarized current generation

A new kind of 2D photovoltaic effect (PVE) with the generation of anomalously large surface photovoltage up to 210 meV in magnetically doped topological insulators (TIs) has been studied by the laser time-resolved pump-probe angle-resolved photoelectron spectroscopy. The PVE has maximal efficiency for TIs with high occupation of the upper Dirac cone (DC) states and the Dirac point located inside the fundamental energy gap. For TIs with low occupation of the upper DC states and the Dirac point located inside the valence band the generated surface photovoltage is significantly reduced. We have shown that the observed giant PVE is related to the laser-generated electron–hole asymmetry followed by accumulation of the photoexcited electrons at the surface. It is accompanied by the 2D relaxation process with the generation of zero-bias spin-polarized currents flowing along the topological surface states (TSSs) outside the laser beam spot. As a result, the spin-polarized current generates an effective in-plane magnetic field that is experimentally confirmed by the kII-shift of the DC relative to the bottom non-spin-polarized conduction band states. The realized 2D PVE can be considered as a source for the generation of zero-bias surface spin-polarized currents and the laser-induced local surface magnetization developed in such kind 2D TSS materials.

X-ray spectroscopic study of amorphous and polycrystalline PbO films, \u03b1-PbO, and \u03b2-PbO for direct conversion imaging

We investigated the electronic structure of Lead Oxide (PbO) – one of the most promising photoconductor materials for direct conversion x-ray imaging detectors, using soft x-ray emission and absorption spectroscopy. Two structural configurations of thin PbO layers, namely the polycrystalline and the amorphous phase, were studied, and compared to the properties of powdered α-PbO and β-PbO samples. In addition, we performed calculations within the framework of density functional theory and found an excellent agreement between the calculated and the measured absorption and emission spectra, which indicates high accuracy of our structural models. Our work provides strong evidence that the electronic structure of PbO layers, specifically the width of the band gap and the presence of additional interband and intraband states in both conduction and valence band, depend on the deposition conditions. We tested several model structures using DFT simulations to understand what the origin of these states is. The presence of O vacancies is the most plausible explanation for these additional electronic states. Several other plausible models were ruled out including interstitial O, dislocated O and the presence of significant lattice stress in PbO.

The effect of density-of-state tails on band-to-band tunneling: Theory and application to tunnel field effect transistors

It is demonstrated how band tail states in the semiconductor influence the performance of a Tunnel Field Effect Transistor (TFET). As a consequence of the smoothened density of states (DOS) around the band edges, the energetic overlap of conduction and valence band states occurs gradually at the onset of band-to-band tunneling (BTBT), thus degrading the sub-threshold swing (SS) of the TFET. The effect of the band tail states on the current-voltage characteristics is modelled quantum-mechanically based on the idea of zero-phonon trap-assisted tunneling between band and tail states. The latter are assumed to arise from a 3-dimensional pseudo-delta potential proposed by Vinogradov [1]. This model potential allows the derivation of analytical expressions for the generation rate covering the whole range from very strong to very weak localization of the tail states. Comparison with direct BTBT in the one-band effective mass approximation reveals the essential features of tail-to-band tunneling. Furthermore, an analytical solution for the problem of tunneling from continuum states of the disturbed DOS to states in the opposite band is found, and the differences to direct BTBT are worked out. Based on the analytical expressions, a semi-classical model is implemented in a commercial device simulator which involves numerical integration along the tunnel paths. The impact of the tail states on the device performance is analyzed for a nanowire Gate-All-Around TFET. The simulations show that tail states notably impact the transfer characteristics of a TFET. It is found that exponentially decaying band tails result in a stronger degradation of the SS than tail states with a Gaussian decay of their density. The developed model allows more realistic simulations of TFETs including their non-idealities.

Behavior of temperature-dependent dc-photoconductivity in hot-wall deposited CaAl2Se4 layers

The dc-photoconductive characteristic on the hot-wall grown CaAl2Se4 (CAS) layers was explored as a function of temperature. From the photocurrent (PC) measurement, three PC peaks A, B, and C corresponded to the intrinsic transitions, which represent the band-to-band transitions from the valence-band states of Γ2(A), Γ3 + Γ4(B), and Γ3 + Γ4(C) to the conduction-band state of Γ1, respectively. Based on these PC results, the optical band-gap energy was well matched by E g (T) = E g (0) − 4.94 × 10−3 T 2/(T + 552), where E g (0) is found to be 3.8239, 3.8716, and 3.8801 eV for three peaks A, B, and C, respectively. Thus, the effect of the crystal field and spin-orbit splitting (These values were extracted out to be 47.7 and 8.5 meV, respectively.) was observed and calculated by means of the PC spectroscopy. However, PC intensity gradually decreased with decreasing temperature unlike an ordinary behavior. In the log J ph vs 1/T plot, two dominant traplevels were observed to be 20.81 meV at temperatures of 300 - 70 K and 1.18 meV at temperatures below 70 K. Consequently, we extract out that these trapping centers caused by native defects in CAS confine the PC intensity as temperature decreases.

Analysis of crystalline phases and integration modelling of charge quenching yields in hybrid lead halide perovskite solar cell materials

Organic inorganic metal halide perovskites (OIHPs) has emerged as promising photovoltaic materials the latest years. Many OIHPs, however, have complex material compositions with mixed cation and halide compositions, phase mixtures, as well as beneficial remains of PbI2 in the final solar cell materials where the complex material composition with dual conduction and valence band states and its effects on the performance remain unclear. Here, we report an approach to analyze the phase mixture, order-disorder phases and the emissive electronic states via a 4-state model of the photoluminescence yield. The approach is applied to scaffold layer perovskite materials with different mixed halide composition. The optical transitions and the full emission spectra are de-convoluted to quantify the band gaps and charge quenching yields in the OIHPs. An approach to extract the excited state coupling parameters within the 4-state model is also briefly given. The integration model is finally utilized in charge quenching yield analysis for the different materials and correlated with solar cell performance from MAPbI3 and MAPbI3−xClx in mesoporous TiO2 layers where inclusion of Cl improves crystal formation and is compared to alternative approaches using optimized solvents and anti-solvent methods. A band gap grading effect was found to be present for the scaffold MAPbI3 and increased for MAPbI3−xClx, beneficial for decreased hole concentration at the back contact and thus reducing back contact recombination.

Selectivity of photoelectrochemical CO2 reduction modulated with electron transfer from size-tunable quantized energy states of CdSe nanocrystals

We investigate the product selectivity of CO2 reduction using NiO photocathodes decorated with CdSe quantum dots (QDs) of varying size in a photoelectrochemical (PEC) cell. Size-tunable and quantized energy states of conduction band in CdSe QDs enable systematic control of electron transfer kinetics from CdSe QDs to NiO. It turns out that different size of CdSe QDs results in variation in product selectivity for CO2 reduction. The energy gap between conduction band edge and redox potential of each reduction product (e.g., CO and CH4) correlates with their production rate. The size dependence of the electron transfer rate estimated from the energy gap is in agreement with the selectivity of CO2 reduction products for all reduction products but CO. The deviation in the case of CO is attributed to sequential conversion of CO into CH4 with CO adsorbed on electrode surface. Based on a premise that the CdSe QDs would exhibit similar surface configuration regardless of QD size, it is concluded that the electron transfer kinetics proves to alter the selectivity of CO2 reduction.

Insight into Electronic and Structural Reorganizations for Defect-Induced VO2 Metal-Insulator Transition.

An oxygen vacancy defect in monoclinic VO2 has been shown to modulate the metal-insulator transition (MIT) at room temperature. However, as the electronic and structural reorganizations occur simultaneously, the origin of MIT is still unclear. Here we performed first-principles calculations to examine electronic variations separately from structural reorganizations during MIT. It was found that the oxygen defect induces electronic reorganization by creating polarized 3d orbitial electrons, while structure reorganization makes the conduction band edge states available for occupation. The conduction band states thus hold polarized charges that delocalize over space, bestowing metallic property on the originally insulated VO2. A linear relationship for the number of polarized electrons and the defect concentration is revealed, which would lead to cost-effective control of VO2 MIT behavior by defect engineering.

Ferroelastic modulation and the Bloch formalism.

The key to the development of advanced materials is to understand their electronic structure-property relationship. Utilization of this understanding to design new electronic materials with desired properties led to modern epitaxial growth approaches for synthesizing artificial lattices, which for almost half a century have become the mainstay of electronic and photonic technologies. In contrast to previous scalar modulation approaches, we now study synthetic crystal lattices that have a tensor artificial modulation and develop a theory for photons and conduction band states in these lattices in a regime with an unusual departure from the familiar consequences of translational symmetry and Bloch's theorem. This study reveals that a nonmagnetic crystal lattice modulated by a purely geometrical orientational superlattice potential can lead to localized states or to spiral states for electrons and photons, as well as weakly or strongly localized states that could be used to markedly slow down the propagation of light and for optical energy storage applications.