Conduction Band States Research Articles

Attosecond state-resolved carrier motion in quantum materials probed by soft x-ray XANES

Recent developments in attosecond technology led to table-top x-ray spectroscopy in the soft x-ray range, thus uniting the element- and state-specificity of core-level x-ray absorption spectroscopy with the time resolution to follow electronic dynamics in real-time. We describe recent work in attosecond technology and investigations into materials such as Si, SiO2, GaN, Al2O3, Ti, and TiO2, enabled by the convergence of these two capabilities. We showcase the state-of-the-art on isolated attosecond soft x-ray pulses for x-ray absorption near-edge spectroscopy to observe the 3d-state dynamics of the semi-metal TiS2 with attosecond resolution at the Ti L-edge (460 eV). We describe how the element- and state-specificity at the transition metal L-edge of the quantum material allows us to unambiguously identify how and where the optical field influences charge carriers. This precision elucidates that the Ti:3d conduction band states are efficiently photo-doped to a density of 1.9 × 1021 cm−3. The light-field induces coherent motion of intra-band carriers across 38% of the first Brillouin zone. Lastly, we describe the prospects with such unambiguous real-time observation of carrier dynamics in specific bonding or anti-bonding states and speculate that such capability will bring unprecedented opportunities toward an engineered approach for designer materials with pre-defined properties and efficiency. Examples are composites of semiconductors and insulators like Si, Ge, SiO2, GaN, BN, and quantum materials like graphene, transition metal dichalcogens, or high-Tc superconductors like NbN or LaBaCuO. Exiting are prospects to scrutinize canonical questions in multi-body physics, such as whether the electrons or lattice trigger phase transitions.

Electronic structure modulation of MoS2 by substitutional Se incorporation and interfacial MoO3 hybridization: Implications of Fermi engineering for electrocatalytic hydrogen evolution and oxygen evolution

The design of earth-abundant electrocatalysts that can facilitate water splitting at low overpotentials, provide high current densities, and enable prolonged operational lifetimes is central to the production of sustainable fuels. The distinctive atomistic and electronic structure characteristics of the edges of MoS2 imbue high reactivity toward the hydrogen evolution reaction. MoS2 is nevertheless characterized by significantly high overpotentials as compared to platinum. Here, we demonstrate that modulation of the electronic structure of MoS2 through interfacial hybridization with MoO3 and alloying of selenium on the anion sublattice allows for systematic lowering of the conduction band edge and raising of the valence band edge, respectively. The former promotes enhanced electrocatalytic activity toward hydrogen evolution, whereas the latter promotes enhanced activity toward the oxygen evolution reaction. Such alloyed heterostructures prepared by sol-gel reactions and hydrothermal selenization expose a high density of edge sites. The alloyed heterostructures exhibit low overpotential, high current density, high turnover frequency, and prolonged operational lifetime. The mechanistic origins of catalytic activity have been established based on electronic structure calculations and x-ray absorption and emission spectroscopy probes of electronic structure, which suggest that interfacial hybridization at the MoO3 interface yields low-lying conduction band states that facilitate hydrogen adsorption. In contrast, shallow Se 4p-derived states give rise to a raised effective valence band maximum, which facilitates adsorption of oxygen intermediates and engenders a low overpotential for the oxygen evolution reaction. The findings illustrate the use of electronic structure modulation through interfacial hybridization and alloying to systematically improve electrocatalytic activity.

The carbon state in dilute germanium carbides

Conduction and valence band states for the highly mismatched alloy (HMA) Ge:C are projected onto Ge crystal states, Ge vacancy states, and Ge/C atomic orbitals, revealing that substitutional carbon not only creates a direct bandgap but also the new conduction band is optically active. Overlap integrals of the new Ge:C conduction band state with states from unperturbed Ge show that the new band cannot be attributed to any single Ge band but is a mixture of multiple Ge states. The Ge Γ conduction band valley state plays the largest single role, but L and X valley states collectively contribute a larger share than Γ due to the multiplicity of degenerate states. C sites structurally resemble uncharged vacancies in the Ge lattice, similar to Hjalmarson's model for other HMAs. C also perturbs the entire Ge band structure even at the deepest crystal core energy levels, particularly if staggered supercells are used to mimic a disordered alloy. Projection onto atomic sites shows a relatively weak localization compared with other HMAs, but it does show a strong anisotropy in probability distribution. L-valley conduction band states in Ge contribute to the conduction band minimum in Ge:C, but the optical transition strength in Ge:C remains within a factor of 2 of the direct gap transition in Ge.

Ultrafast intraband Auger process in self-doped colloidal quantum dots

Summary Investigating the separate dynamics of electrons and holes has been challenging, although it is critical for the fundamental understanding of semiconducting nanomaterials. n-Type self-doped colloidal quantum dots (CQDs) with excess electrons occupying the low-lying state in the conduction band (CB) have attracted a great deal of attention because of not only their potential applications to infrared optoelectronics but also their intrinsic system that offers a platform for investigating electron dynamics without elusive contributions from holes in the valence band. Here, we show an unprecedented ultrafast intraband Auger process, electron relaxation between spin-orbit coupling states, and exciton-to-ligand vibrational energy transfer process that all occur exclusively in the CB of the self-doped β-HgS CQDs. The electron dynamics obtained by femtosecond mid-infrared spectroscopy will pave the way for further understanding of the blinking phenomenon, disproportionate charging in light-emitting diodes, and hot electron dynamics in higher quantum states coupled to surface states of CQDs.

Performance Improvement for Spray-Coated ZnO TFT by F Doping With Spray-Coated Zr–Al–O Gate Insulator

We report the fluorine (F) doping effect on zinc oxide (ZnO) thin-film transistor (TFT) fabricated by spray pyrolysis on spray-coated zirconia alumina oxide (ZAO) gate insulator. F doping was performed by NF3 plasma treatment on ZnO thin film. The ZnO film shows ${C}$ -axis-aligned hexagonal structure, which remains unchanged by F doping. The photoluminescence (PL) spectra and X-ray photoelectron spectroscopy (XPS) show a reduction in oxygen-related defects by F doping. Valance band edge XPS spectra show the shift of Fermi energy level from 2.46 to 3.12 eV which reveals that the more carriers are generated in the conduction band and defect states are reduced. The F-doped ZnO (F:ZnO) TFT exhibits the saturation mobility of 31.59 cm2V−1s−1, the subthreshold swing of 238mV/dec, ON/ OFF current ratio of ~108, and zero hysteresis voltage. The F:ZnO TFT also shows significant improvement of threshold voltage shift ( $\Delta \text{V}_{TH}$ ) under negative/positive gate bias stress compared to pristine ZnO. The enhancement of mobility and stability is attributed to the substitution of oxygen (O) and passivation of oxygen vacancies ( ${V}_{o}$ ) by F in ZnO. Therefore, it is expected that F doping is an effective method to improve the performance and stability of solution-processed oxide TFTs.

Topological surface states in epitaxial (SnBi2Te4)n(Bi2Te3)m natural van der Waals superlattices

Topological insulators are good candidates for charge to spin conversion with high efficiency due to their spin-polarized topological surface states (TSSs). In this work, we provide experimental evidence for two-dimensional (2D) TSSs in ${(\mathrm{Sn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4})}_{n} {({\mathrm{Bi}}_{2}{\mathrm{Te}}_{3})}_{m}$ natural van der Waals superlattices grown by molecular beam epitaxy using angle resolved photoelectron spectroscopy and magnetotransport. While the TSSs overlap with bulk conduction band (BCB) states at the Fermi energy, it is shown that by increasing the Sn composition, the influence of BCB states is reduced and becomes minimum for $\mathrm{Sn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$. The latter compound, found to be in the form of septuplet layers, shows weak antilocalization effect with a prefactor $\ensuremath{\alpha}\ensuremath{\sim}--0.41$, indicating that the TSSs and the bulk behave as one 2D channel in which magnetotransport properties are influenced by large spin-orbit coupling.

The Investigation SnO <sub>2</sub> As the Electron Transport Layer (ETL) on Perovskite Solar Cells

The present study depicts the validation between the simulation and experimental model for representing the improved performance of perovskite solar cell (PSC) by examining defect characteristics, Conduction Band Offset (CBO), the Valence Band Offset (VBO), thickness, Electron Mobility (μ e ), Absorption Coefficient Constant (α), Effective Density of the Conduction Band States (N C ), and Interface Density (N t ). With investigating interface characteristics at the Electron Transport Layer (ETL) / Perovskite and Perovskite / Hole Transport Layer (HTL), Fill Factor (FF) and open-circuit voltage (Voc) can be better determined. The SCAPS-1D software is the mean, which has been utilized to investigate the mentioned parameters and the cell Power Conversion Efficiency (PCE).

Type-II band alignment in single crystalline TiO2 nanowires under twisting

Electronic structures with the type-II band alignment usually exist only in heterostructures. Using the generalized Bloch theorem, we reveal that an effective type-II band alignment can be induced in the single crystalline TiO2 nanowires (NWs) by an axial twisting deformation. With this, we further reveal distinct responses of the the valence band (VB) states and the conduction band (CB) states in the radial dimension of the nanowire. Specifically, in the twisted NW, the VB states migrate from the NW core toward the NW shell while the CB states migrate from the NW shell toward the NW core. Our results pave a new way to realize the type-II band alignment in single crystalline NWs, which is critical for actual applications such as NW-based photovoltaic effect and photocatalytic effect. Further, the significant variation of the quantum states also illustrates the possibility to tune the electronic properties of TiO2 NWs by strain engineering.

Temperature and chemical potential dependence of the parity anomaly in quantum anomalous Hall insulators

The low-energy physics of two-dimensional Quantum Anomalous Hall insulators like (Hg,Mn)Te quantum wells or magnetically doped (Bi,Sb)Te thin films can be effectively described by two Chern insulators, including a Dirac, as well as a momentum-dependent mass term. Each of those Chern insulators is directly related to the parity anomaly of planar quantum electrodynamics. In this work, we analyze the finite temperature Hall conductivity of a single Chern insulator in 2+1 space-time dimensions under the influence of a chemical potential and an out-of-plane magnetic field. At zero magnetic field, this non-dissipative transport coefficient originates from the parity anomaly of planar quantum electrodynamics. We show that the parity anomaly itself is not renormalized by finite temperature effects. However, it induces two terms of different physical origin in the effective action of a Chern insulator, which is proportional to the Hall conductivity. The first term is temperature and chemical potential independent, and solely encodes the intrinsic topological response. The second term specifies the non-topological thermal response of conduction and valence band states. In particular, we show that the relativistic mass of a Chern insulator counteracts finite temperature effects, whereas its non-relativistic mass enhances these corrections. Moreover, we extend our analysis to finite magnetic fields and relate the thermal response of a Chern insulator therein to the spectral asymmetry, which is a measure of the parity anomaly in orbital fields.

Absorption mechanism of dopamine/DOPAC-modified TiO2 nanoparticles by time-dependent density functional theory calculations

Donor-modified TiO2 nanoparticles are interesting hybrid systems shifting the absorption edge of this semiconductor from the ultra-violet to the visible or infrared light spectrum, which is a benefit for several applications ranging from photochemistry, photocatalysis, photovoltaics, or photodynamic therapy. Here, we investigate the absorption properties of two catechol-like molecules, that is, dopamine and DOPAC ligands, when anchored to a spherical anatase TiO2 nanoparticle of realistic size (2.2 nm), by means of time-dependent density functional theory calculations. By the differential absorbance spectra with the bare nanoparticle, we show how it is possible to determine the injection mechanism. Since new low-energy absorption peaks are observed, we infer a direct charge transfer injection, which, unexpectedly, does not involve the lowest energy conduction band states. We also find that the more perpendicular the molecular benzene ring is to the surface, the more intense is the absorption, which suggests aiming at high molecular packing in the synthesis. Through a comparative investigation with a flat TiO2 surface model, we unravel both the curvature and coverage effects.

Lattice strain influence on conduction band nonparabolicity in GaAs and InAs: Application to intraband optical absorption in InGaAs-GaAs asymmetric step quantum wells

We use empirical sps*d5 tight-binding calculations to determine the effects of compressive biaxial lattice strain, perpendicular to the [001] crystal direction, in zinc blende GaAs and InAs. Under that approach, we have been able to compute the behavior of quantities such as the average valence band energy, the energy band gap, the conduction band effective mass, and the spin-orbit split-off energy, as functions of the biaxial strain, within a range from 0 to −7%. Expressions governing these dependencies are reported for both materials. With such information at hand it is possible to calculate the variation of the coefficient of conduction band nonparabolicity due to the presence of strain. Also, the outcome for such quantities allows to evaluate the valence band offset in GaAs/InGaAs heterointerfaces as a consequence of the strain appearing from the difference between the lattice constants of the involved materials. Taking advantage of the above mentioned results, we have performed the calculation of confined conduction band states in step-like asymmetric quantum wells of the GaAs/Inx1Ga1-x1As/Inx2Ga1-x2As/GaAs prototype, using a k→⋅p→ formalism that solves the effective mass equation arising from a bi-cuadratic (nonparabolic) dispersion law. We report the calculation of the optical absorption coefficient related with intraband transitions that involve the ground and first excited energy levels. For that purpose, the study takes into account the variation of the layer widths and compositions.

Heteroleptic Tin-Antimony Sulfoiodide for Stable and Lead-free Solar Cells

Summary The quaternary chalcogenide halides of group IV and V elements have attracted much attention due to their interesting semiconducting properties as well as a suitable band gap for solar cells. Here, for the first time, we report on solar cells using tin-antimony sulfoiodide (Sn2SbS2I3). Sn2SbS2I3 solar cells were fabricated using a chemical single-step deposition process with a solution containing a SbCl3-thiourea complex and SnI2 with the configuration of TiO2 and poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-enzothiadiazole)] as the electron- and hole-transporting layers, respectively. The best-performing cell exhibits a power conversion efficiency of 4.04% under the illumination of standard AM 1.5G conditions (100 mW cm−2). These unencapsulated cells exhibited good stabilities at 80% relative humidity, 85°C in air, and under illumination, respectively. These results provide guidelines for fabrication of lead-free heteroleptic perovskite solar cells by hosting divalent or combinations of monovalent and trivalent metal cations.

Electrical modulation of the LWIR absorption and refractive index in InAsSb-based strained layer superlattice heterostructures

InAsSb-based strained layer superlattices (SLS) have strong fundamental absorption, which can be easily modified in a controlled manner by injecting excess carriers. This makes them attractive for intensity modulation of infrared lasers as well as beam steering and spatial beam shaping with a nanosecond-scale time response. This paper reports the modulation of the fundamental absorption and the refractive index by carrier injection in a 3.45-nm-period InAsSb0.65/InAsSb0.35 SLS with a low temperature energy gap of 85 meV grown by molecular beam epitaxy on a GaSb substrate with a GaInSb metamorphic buffer. The SLS absorber was sandwiched by n- and p-type wider energy gap layers for electrical injection and confinement of excess carriers. The population of conduction band states was obtained by measuring the intensity modulation of a 10.6 μm CO2 laser for temperatures ranging from T = 77 to 200 K. An increase of the electron quasi-Fermi level with electrical injection up to 20 meV was observed. The experimental data imply a decrease in the Auger coefficient with temperature, from 3 × 1024 cm6/s at 77 K to 1 × 1024 cm6/s at T = 200 K attributed to recombination involving two electrons and a heavy hole. The refractive index changes obtained by electrical injection of excess carriers can reach 0.05 at T = 77 K and 0.035 at T = 200 K, which are at least three orders of magnitude greater than those obtained with electro-optical materials.

Effect of Band Symmetry on Photocurrent Production in Quasi-One-Dimensional Transition-Metal Trichalcogenides.

Photocurrent production in quasi-one-dimensional (1D) transition-metal trichalcogenides, TiS3(001) and ZrS3(001), was examined using polarization-dependent scanning photocurrent microscopy. The photocurrent intensity was the strongest when the excitation source was polarized along the 1D chains with dichroic ratios of 4:1 and 1.2:1 for ZrS3 and TiS3, respectively. This behavior is explained by symmetry selection rules applicable to both valence and conduction band states. Symmetry selection rules are seen to be applicable to the experimental band structure, as is observed in polarization-dependent nanospot angle-resolved photoemission spectroscopy. Based on these band symmetry assignments, it is expected that the dichroic ratios for both materials will be maximized using excitation energies within 1 eV of their band gaps, providing versatile polarization sensitive photodetection across the visible spectrum and into the near-infrared.

Anion replacement effect on BaCd2X2 (X = P, As, Sb, Bi) compounds: A first principles study

Cadmium based Zintl phase compounds have been recently studied for the thermoelectric applications. Many interesting experimental results reported on the 1-2-2 cadmium based compounds, however less attention been paid to study them though first principles methods. In the current work, BaCd2X2 (X= P, As, As, Bi) compounds have been studied comprehensively for the basic structural parameters, electronic band profiles, optical characteristics and thermoelectric properties. Poirier–Tarantola equation of state is used to fit the volume and energy data. The calculated results are consistent with the reported experimental results. Electronic band profiles of the compounds reveal that the compounds are semiconductors except BaCd2Bi2 which is metallic. Valence band is composed on mainly X-p states while Ba-d state in conduction band. Compounds show prominent absorption in the visible region with extension to low frequency ultraviolet region. The characteristics optical peaks redshifted by anion replacement from P to Bi. Thermoelectric properties of the compounds are also studied for possible thermoelectric applications.

Study of anion replacement effect on SrCd2X2 (X = P, As, Sb, Bi) compounds by FPLAPW+lo

The structural parameters, optoelectronic and thermoelectric properties of SrCd2X2 (X = P, As, Sb and Bi) compounds have been investigated thoroughly for the first time using first principles methods. The compounds are optimized and obtained structural parameters agree to the experimental data. Electronic band profiles of the compounds reveal that the SrCd2P2, SrCd2As2, SrCd2Sb are direct band gap semiconductors while SrCd2Bi2 is metallic. Valence band is composed on mainly X-p states while Sr-d state in conduction band. Compounds show the strong absorption in the visible region and low frequency ultraviolet region. The characteristics optical peaks shift towards lower energy by replacing the anion from P to Bi. Boltzmann transport theory is implemented through the BoltzTraP code to calculate the electrical and thermal conductivities, Seebeck coefficients and thermo power factor. High absorption peaks and figure of merits of the compounds show that these compounds can be important for optoelectronics and thermo-electric devices.

Multiple optical impacts of anion doping in epitaxial barium titanate films

Multiple strong impacts of anion doping Λ (Λ = oxygen vacancy, nitrogen, or hydrogen) on the optical properties in the near-infrared to vacuum ultraviolet spectral range are experimentally revealed in cube-on-cube-type epitaxial perovskite BaTiO3−δΛδ films (δ ≤ 0.01) grown on (001)SrTiO3 substrates. Compared to the reference undoped films and bulk crystal, the anion-doped films exhibit massive changes of the index of refraction in the transparency range Δn of ∼0.1 to 0.2, a profound bandgap widening ΔEg of ∼0.3 eV to 0.4 eV, a dopant-specific near-edge absorption, and doping-induced interband transitions. Both the conduction-band uplift and the changes in the index of refraction are ascribed to the internal stress and electric field, which result from epitaxially controlled dopant locations in the [Ba–O](001) planes. The dopant-induced in-gap states are responsible for the near-edge absorption, whereas the dopant-induced conduction-band states are suggested to modify the interband transitions and the intrinsic elasto- and electro-optic coefficients.

Simulating the effect of adding BSF layers on Cu2BaSnSSe3 thin film solar cells

In this paper, the device characteristics of Cu2BaSnSSe3 (CBT(S,Se3)) solar cells have been simulated using SCAPS-1D. The simulated results have been validated by comparison with the experimental data reported in the literature. For improving the cell performance and boosting the efficiency, adding different Back Surface Field (BSF) layers is suggested on the absorber layer. Inserting a BSF layer with optimum parameters causes enhancement of open-circuit voltage (VOC), in particular, causes better efficiency on the cell's performance. For this purpose, we optimize the MoSe2 layer, which is formed at the interface between CBT(S,Se3) absorber layer and Mo back contact. Investigation of MoSe2 helps us to represent optimum ranges for bandgap (Eg), electron affinity (χ), charge carrier density, thickness, the mobility of electron and hole (μe and μh) and the effective density of states of conduction band and valence band (NC and NV) of a material to choose as a BSF layer which can be inserted in CBT(S,Se3) solar cells and other types of cells like CZTSSe, perovskite and CIGS. Based on the optimum parameters for an appropriate BSF layer, Cu2BaSnSSe3 solar cell has been simulated by inserting SnS as a BSF layer, and the results indicated improvement in the cell's parameters. We recorded PCE = 7.31%, VOC = 0.867 V, JSC = 16.986 mA/cm2, and FF = 49.63%.

Towards temperature-induced topological phase transition in SnTe: A first-principles study

The temperature renormalization of the bulk band structure of a topological crystalline insulator, SnTe, is calculated using first-principles methods. We explicitly include the effect of thermal-expansion-induced modification of electronic states and their band inversion on electron-phonon interaction. We show that the direct gap decreases with temperature, as both thermal expansion and electron-phonon interaction drive SnTe towards the phase transition to a topologically trivial phase as temperature increases. The band gap renormalization due to electron-phonon interaction exhibits a nonlinear dependence on temperature as the material approaches the phase transition, while the lifetimes of the conduction band states near the band edge show a nonmonotonic behavior with temperature. These effects should have important implications on bulk electronic and thermoelectric transport in SnTe and other topological insulators.

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.