Conduction Band States Research Articles

GGA and GGA + U Study of ThMn2Si2 and ThMn2Ge2 Compounds in a Body-Centered Tetragonal Ferromagnetic Phase.

Our study used the full-potential linearized augmented plane waves (FP-LAPW) method to conduct a first-principles evaluation of the structural, electronic, and magnetic properties of ThMn2X2 (X = Si and Ge) compounds. To establish theoretical dependability with the currently available experimental results, computations for the structural findings of ternary intermetallic thorium (Th)-based compounds were achieved using the generalized gradient approximation in the scheme of Perdew–Burke–Ernzerhof (PBE–GGA) potential, while the generalized gradient approximation plus the Hubbard U (GGA + U) approach was employed to improve the electrical and magnetic properties. In contrast with both the paramagnetic (PM) and antiferromagnetic (AFM) phases, the ThMn2X2 compounds were optimized in a stable ferromagnetic (FM) phase, which was more suited for studying and analyzing magnetic properties. The electronic band structures (BS) and the density of state (DOS) were computed using the two PBE–GGA and GGA + U approximations. The thorium (Th)-based ThMn2X2 compound has full metallic character, due to the crossing and overlapping of bands across the Fermi level of energy, as well as the absence of a gap through both spin (up and down) channels. There was a significant hybridization between (Mn-d and (X = Si and Ge)-p states of conduction band with Th-f states in the valence band. The total magnetic moment of ThMn2Si2 in the ferromagnetic phase was 7.94534 μB, while for ThMn2Ge2 it was 8.73824 μB with a major contribution from the Mn atom. In addition, the ThMn2Ge2 compound’s total magnetic moment confirmed that it exhibits higher ferromagnetism than does the ThMn2Si2 compound.

First-Principles Study of the Optical Properties of Zinc Antimonide Using the mBJ Approximation

This computational study focused on the optical properties of zinc antimonide ZnSb. It relates to the complex dielectric function ε (ω), the refractive index n (ω), the extinction function k (ω), the optical conductivity σ (ω), the reflectivity R(ω), the absorption coefficient α (ω) and the energy loss spectrum L(ω). These properties are calculated and discussed for a growing energy of the incident electromagnetic radiation ranging from 0 to 14 eV, comprising infrared, visible and ultraviolet regions. All these properties are obtained using the Full Potential Linearized Augmented Plane Wave (FP-LAPW), by solving Kohn-Sham equations. This method based on Density Functional Theory (DFT), implemented in Wien2k simulation package. This compound is already used in photo-optical applications, it is for this reason that we interested in the calculation of its optical properties according to the energy of the incident photons, in order to open up for it other use ways. Since the zinc antimonide ZnSb is a semiconductor, its optical properties are investigated using Generalized Gradient Approximation plus modified Becke–Johnson as the exchange correlation (GGA-mBJ). Our calculations are performed by considering only the interband transition of electrons between the occupied states in valence band and unoccupied conduction band states along high symmetry points in Brillouin zone. In addition, the relations of the optical properties to these transitions are discussed in detail. We have also verified the Penn’s model by showing the inverse relationship between the static real part of dielectric function ε1(0) and the optical band gap Eg. The results obtained are compared with other results existing in the literature.

A-site cation influence on the conduction band of lead bromide perovskites

Hot carrier solar cells hold promise for exceeding the Shockley-Queisser limit. Slow hot carrier cooling is one of the most intriguing properties of lead halide perovskites and distinguishes this class of materials from competing materials used in solar cells. Here we use the element selectivity of high-resolution X-ray spectroscopy and density functional theory to uncover a previously hidden feature in the conduction band states, the σ-π energy splitting, and find that it is strongly influenced by the strength of electronic coupling between the A-cation and bromide-lead sublattice. Our finding provides an alternative mechanism to the commonly discussed polaronic screening and hot phonon bottleneck carrier cooling mechanisms. Our work emphasizes the optoelectronic role of the A-cation, provides a comprehensive view of A-cation effects in the crystal and electronic structures, and outlines a broadly applicable spectroscopic approach for assessing the impact of chemical alterations of the A-cation on perovskite electronic structure.

Theory of ultrafast magnetization of nonmagnetic semiconductors with localized conduction bands

The magnetization of a nonmagnetic semiconductor by femtosecond light pulses is crucial to achieve an all-optical control of the spin dynamics in materials and to develop faster memory devices. However, the conditions for its detection are largely unknown. In this paper, we identify the criteria for the observation of ultrafast magnetization and critically discuss the difficulties hindering its experimental detection. We show that ultrafast magnetization of a nonmagnetic semiconductor can be observed in compounds with very localized conduction band states and more delocalized valence bands, such as in the case of a p-d charge transfer gap. By using constrained and time-dependent density functional theory simulations, we demonstrate that a transient ferrimagnetic state can be induced in diamagnetic semiconductor ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ via ultrafast pulses at realistic fluences. The ferrimagnetic state has opposite magnetic moments on vanadium (conduction) and oxygen (valence) states. Our methodology outruns the case of ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ as it identifies the key requirements for a computational screening of ultrafast magnetism in nonmagnetic semiconductors.

Improving plasmonic photodetector based on ITO/TiO2/Ag configuration and roughened interface

In this work, plasmonic photodetector based on ITO/TiO2/Ag grating configuration specially with nano-scale roughened interface is proposed to achieve high photoresponsivity, in which hot-electron are effectively excited by taking advantages of the narrow distribution of excitation energy in Ag and the high density of states in TiO2 conduction band. Simultaneously, a roughened structure is introduced at Ag/TiO2 interface to alleviate the momentum mismatch of hot-electron. The influences of the grating structures on incident light absorption and hot-electron excitation are simulated by using finite-difference time-domain method. The thicknesses of both metallic Ag and spacer layers modulate the absorption characteristics, and the introduction of the nano-scale roughened interface does not have effect obviously on the absorptivity and response wavelength. The present work provides a valuable guidance in the fields of interface design and structure optimization of high-efficiency plasmonic photodetectors.

Plasmon assisted high ultraviolet to visible broad band photosensitivity in lateral Ag NPs-TiO2 nanocomposite film

Noble metal-TiO2 composite films attract great attention because of the extension of its spectral absorption in the visible range due to surface plasmon resonance. Among them, Ag-TiO2 composite is considered to be an ideal candidate for the visible light photosensitivity because of narrow energy distribution of plasmonic hot electrons in Ag and the high density of states in the TiO2 conduction band. Here, we report high broad band photosensitivity from ultraviolet (UV) to visible wavelength of sol-gel grown TiO2 film decorated with Ag nanoparticles (NPs) in a simple planar photoconductive geometry. In addition to UV light, Ag NPs-TiO2 composite film (for 1 nm Ag) shows very high photosensitivity values of ∼105 and ∼104 and responsivity values of 0.1 A/W and 7×10−3 A/W at 10 V bias for 680 nm and 550 nm illuminations respectively while pristine TiO2 show photosensitivity values of 6×102 and 3.3×102 and responsivity values of 2.4×10−4 A/W and 6.6×10−5 A/W for both the visible illuminations. A highly enhanced broad band photosensitivity due to transfer of plasmonically generated hot electrons from Ag to TiO2 is quite interesting without having any device fabricated. The photosensitivity values for Ag NPs for 3 nm and 6 nm layer thicknesses on TiO2 decrease because of coalescence of metal NPs on surface which acts as scattering center and/or light shielding.

Perovskite Mediated Vibronic Coupling of Semiconducting SERS for Biosensing

AbstractImproving charge transfer efficiency via vibronic coupling is vital to the performance of semiconducting surface‐enhanced Raman spectroscopy (SERS). Previous attempts have focused on defects‐based metastable‐state assisted band structure matching strategy to enhance vibronic coupling. However, defect‐related charge transfer transitions can be easily deteriorated due to enhanced phonon‐assisted relaxation upon continuous laser irradiation. Herein, perovskite‐based steady‐state assisted band structure matching strategy is proposed to enhance vibronic coupling within perovskite‐molecule charge transfer complex, leading to remarkable Raman enhancement up to 5.9 × 106. Particularly, vibronic coupling can be modulated by tuning valence band position and introducing ultrathin Au coating, which allows selective enhancement of molecules with different band structures, including narrow‐bandgap molecules and wide‐bandgap molecules. Importantly, based on intrinsically stable conduction band and valence band states, this system achieves ultrahigh photostability, preserving 91.3% of the original intensity after 50 000 s of irradiation. This system also provides an outstanding tool for trace molecular detection, allowing sensitive and selective identification of 9 types of gastric cancer related aldehydes, which enables distinguishing the breath of gastric cancer patients from healthy controls with a discriminatory accuracy of 81.09%. This study is anticipated to shed new light into the future strategy design of efficient and stable semiconducting SERS.

Heteroepitaxy of Scandium Delafossite on ZnO

Heteroepitaxy of Scandium Delafossite on ZnO

Photoexcitation and radiation regulation mechanism of Ag anchoring WSe2 heterojunction with plasma coupling effect

The noble metal silver (Ag) combined with tungsten selenide (WSe2) was used to solve the problems of the narrow absorption band range, low luminescence quantum yield, and nonlinear transmittance of WSe2 base materials. Ag/WSe2 films and core–shell Ag@WSe2 heterojunction were designed and fabricated by the two-step magnetron sputtering and hydrothermal/magnetron sputtering method, respectively. X-ray diffraction, scanning electron microscopy and X-ray photoelectric spectroscopy results confirmed that the successful preparation of Ag/WSe2 and Ag@WSe2. Raman results demonstrate the presence of interfacial stress between Ag and WSe2. Based on quantum particle coupling in metal–semiconductor nanocomposites, the adsorbed silver particles provide electrons and through charge transfer, which increases the density of electronic states in conduction band of the WSe2 and thus increases the spontaneous emission rate. Electron-hole pairs generated in the semiconductor through resonant coupling between surface plasmas of the Ag and WSe2 interface will provide strongly enhanced radiative recombination. The absorption and emission of WSe2 nanosheets in the visible light range and linear-nonlinear optical properties depend on the regulation the sputtering power and pressure of Ag particles. Our research results demonstrate that Ag&WSe2 have excellent optical and photoelectric properties, indicating that Ag&WSe2 have a wide application potential in photoelectric functional and optoelectronic devices.

Moiré flat Chern bands and correlated quantum anomalous Hall states generated by spin-orbit couplings in twisted homobilayer MoS2

We predict that in a twisted homobilayer of transition-metal dichalcogenide MoS$_2$, spin-orbit coupling in the conduction band states from $\pm K$ valleys can give rise to moir\'{e} flat bands with nonzero Chern numbers in each valley. The nontrivial band topology originates from a unique combination of angular twist and local mirror symmetry breaking in each individual layer, which results in unusual skyrmionic spin textures in momentum space with skyrmion number $\mathcal{S} = \pm 2$. Our Hartree-Fock analysis further suggests that density-density interactions generically drive the system at $1/2$-filling into a valley-polarized state, which realizes a correlated quantum anomalous Hall state with Chern number $\mathcal{C} = \pm 2$. Effects of displacement fields are discussed with comparison to nontrivial topology from layer-pseudospin magnetic fields.

In-situ STS studies and first principles calculations on bare and Sn adsorbed UHV exfoliated WS2 layers

Abstarct. Two dimensional (2D) derivatives of tin (Sn) have obtained special deliberations recently due to practical realization of planar, as well as, buckled hexagonal lattice of Sn called stanene. However, it has been observed that proper choice of substrate is very important for growth of stanene like films owing to large core size of Sn that prefers sp 3 hybridization over sp 2. Transition metal dichalcogenides (TMDs) like MoS2 or WS2 with honey comb lattice structure seem to be promising substrate candidates for 2D growth of Sn. In the present work, we report mechanical exfoliation of few layers of WS2 under ultra-high vacuum (UHV) conditions and investigations of growth and local electronic structure by in-situ scanning tunneling microscopy (STM) and spectroscopy (STS) studies. Flat WS2 surface with honeycomb lattice structure in the atomic scale with a lattice constant of 0.34 nm is evident in the STM investigations, whereas, STS measurements reveal local density of states (LDOS) of WS2 with a bandgap of approximately 1.34 eV. Density functional theory (DFT) calculations performed by considering bulk WS2 reveal conduction and valence band states comprised of S p and W d at both sides of the Fermi energy (EF) and an indirect bandgap of 1.38 eV. Experimental observations upon Sn adsorption, reveal commensurate growth of Sn atoms on the sulfur `S’ sites with a buckling height of 40 ±10 pm. STS measurements exhibit local electronic structure of the Sn adsorbed surface with clear evidence of in-gap states. DFT calculations quantify the experimental results demonstrating `S’ sites as the most stable sites for the atomic adsorption of Sn with a buckling height of around 80 pm and reveal signature of in-gap hybridized states comprised of Sn p and W d orbitals.

Investigating In2S3 as the buffer layer in CZTSSe solar cells using simulation and experimental approaches

The present study showed the agreement between the simulation model and experimental results in depicting the enhancement in the performance of the kesterite (CZTSSe) solar cells by investigating the electron mobility (μe), conduction band offset (CBO), thickness, the effective density of the conduction band states (NC), valence band offset (VBO), absorption coefficient constant (A), and donor density (ND) using In2S3 instead of the toxic cadmium sulfide (CdS) as the buffer layer. The study of the mentioned parameters leads to a better understanding of the fact that the electro-optical properties of the buffer layer have an important effect on the performance of the device. Our results show that from non-optimal to optimal properties of the buffer layer, the efficiency has increased from 4.4%. to 12.14%.

Strain effect on the intraband absorption coefficient for spherical CdSe/CdS/ZnSe core–shell–shell quantum dots

In this work, we theoretically calculated the intraband absorption coefficient (IAC) between the 1s and 1p conduction band states for a spherical core/shell/shell quantum dot (CSS-QD) as function of inner and outer shells’ sizes with and without the presence of strain effect. The electronic structure for the system is computed in the effective mass approximation framework and the strain effect is considered on the continuum elasticity model. We found out that as we compared the strained and the unstrained system, the former causes that the IAC undergoes a small redshift. A similar behavior occurs as the inner and outer shell size increase. The results show that the strain effect is an important factor that will be considered in these nanostructure type. The incorporation of inner and outer shells allowed us to extend some fractions of eV into the absorption spectrum that could be interesting as a mechanism to control the IAC device design.

Defect tolerance in CsPbI3: reconstruction of the potential energy landscape and band degeneracy in spin–orbit coupling

Different degeneracy between the conduction band and defect state leads to reconstruction of the potential energy surface with spin-orbit coupling and gives rise to defect tolerance in CsPbI3.

Towards complete band structure of microscopic MoS2 flakes

The occupied and unoccupied electronic states of MoS2 monolayer isolated flake were studied using laboratory based photoemission electron microscope (PEEM) nanoESCA equipped with He-I photon source. PEEM real-space imaging allowed selecting the high quality flake. Altogether, the data will allow accurately recovering the band structures of MoS2. The band structures will be used in future pump-probe experiments to explore the dynamics of electrons in the conduction band and photo-induced multitopological states using trefoil polarization.

On the optical anisotropy in 2D metal-halide perovskites.

Two-dimensional metal-halide perovskites (MHPs) are versatile solution-processed organic/inorganic quantum wells where the structural anisotropy creates profound anisotropy in their electronic and excitonic properties and associated optical constants. We here employ a wholistic framework, based on semiempirical modeling (k·p/effective mass theory calculations) informed by hybrid density functional theory (DFT) and multimodal spectroscopic ellipsometry on (C6H5(CH2)2NH3)2PbI4 films and crystals, that allows us to link the observed optical properties and anisotropy precisely to the underlying physical parameters that shape the electronic structure of a layered MHP. We find substantial frequency-dependent anisotropy in the optical constants and close correspondence between experiment and theory, demonstrating a high degree of in-plane alignment of the two-dimensional planes in both spin-coated thin films and cleaved single crystals made in this study. Hybrid DFT results elucidate the degree to which organic and inorganic frontier orbitals contribute to optical transitions polarized along a particular axis. The combined experimental and theoretical approach enables us to estimate the fundamental electronic bandgap of 2.65-2.68 eV in this prototypical 2D perovskite and to determine the spin-orbit coupling (ΔSO = 1.20 eV) and effective crystal field (δ = -1.36 eV) which break the degeneracy of the frontier conduction band states and determine the exciton fine structure. The methods and results described here afford a better understanding of the connection between structure and induced optical anisotropy in quantum-confined MHPs, an important structure-property relationship for optoelectronic applications and devices.

True Fractional Dimensional Nature of Semiconductor Nanostructures: Removing the Anomaly in the Estimation of Quantum Mechanical Properties

Here, we have investigated the fractional dimensional nature of the experimental semiconductor quantum mechanical systems to understand the effect on the accurate prediction of electronic and optoelectronic properties. The formalism presented in this paper, to estimate the actual dimensionality of the system, lifts the anomaly of using the conventional integer density of states (DOS), although the carrier confinement isn't infinite. The effective bound state widths estimated by considering the carrier wavefunction leakage is the basis of this formalism. We have experimentally verified the formalism in AlGaN/GaN high electron mobility transistors (HEMTs). The dimensionality varies over a wide range from 3 to 2.38, as the gate bias sweeps the channel from depletion to strong inversion. However, the manifestation of fractional dimension on Capacitance-Voltage characteristics is insignificant for planar HEMTs. The effect becomes noticeable with the increasing confining dimensions of the heterostructure, which is experimentally verified in AlGaN/GaN nanofins. The effect is manifested as a shift in the threshold voltage and the rate of conduction-band state filling. This efficient and accurate formalism can estimate the dimension of any quantum mechanical system, thereby enhancing the accuracy of the models.

Effect of thermochemical treatments on laser-induced luminescence spectra from strontium titanate: comparison with swift ion-beam irradiation experiments

Results recently reported on the effect of thermochemical treatments on the (He-Cd) laser-excited emission spectra of strontium titanate (STO) are re-analyzed here and compared with results obtained under ion-beam irradiation. Contributing bands centered at 2.4 eV and 2.8 eV, which appear under laser excitation, present intensities dependent upon previous thermal treatments in oxidizing (O2) or reducing atmosphere (H2). As a key result, the emission band centered at 2.8 eV is clearly enhanced in samples exposed to a reducing atmosphere. From a comparison with the ionoluminescence data, it is concluded that the laser-excited experiments can be rationalized within a framework developed from ion-beam excitation studies. In particular, the band at 2.8 eV, sometimes attributed to oxygen vacancies, behaves as expected for optical transitions from conduction-band (CB) states to the ground state level of the self-trapped exciton center. The band at 2.0 eV reported in ion-beam irradiated STO, and attributed to oxygen vacancies, is not observed in laser-excited crystals. As a consequence of our analysis, a consistent scheme of electronic energy levels and optical transitions can now be reliably offered for strontium titanate.Graphical abstract

Oxygen atom adsorption on borophene doped with aluminum, argentum and aurum: A density functional study

In the present study, adsorption of oxygen (O) atoms on aluminum (Al), argentum (Ag) and aurum (Au) doped borophene was investigated using first-principle calculations. The electronic and optical properties of metal-doping borophene were determined using adsorbed O atoms. The obtained results indicate that Dirac cones near fermi-level for pristine borophene tuned to open band-gap and shift valence band due to near fermi-levels of doping and adsorbing atoms. Borophene retained fold strata with the doping of atoms. The electronic properties of borophene changed due to localization and hybridization between B-2s, Al-3p, Ag-4d, Au-5d and O-2p valence electron states in conduction and valence bands. The minimum values of Al, Ag, Au-doped borophene along (1 1 0) were blue-shift predicted by the proposed density function theory. The sharp peaks of electron energy loss spectroscopy were induced by the scattering of valence electrons.

Band well structure with localized states for enhanced charge accumulation on Triboelectrification

We describe an triboelectric nanogenerator embedded with multilayers consisted of PDMS:electron trapping layer (ETL):PDMS (E-TENG) with oxygen vacancies, producing more localized states in the energy gap, that can accumulate much more transferred electrons by triboelectrification at ETL, thereby enhancing surface charge density of E-TENG. We suggest a working mechanism of an E-TENG with band diagram including electron/charge flows by investigating electrical, chemical and physical properties and suggest the optimal characteristics of ETL depending on the degree of oxygen vacancies, rich, medium and poor, for high performance of E-TENG. Furthermore, we propose the main factors such as localized states of ETL and difference conduction band between polymer and ETL with band diagram as a band well structure, presented space for electrons/charges accumulation, for high surface charge density of E-TENG. As a results, maximum output performance of E-TENG with rich oxygen vacancies of ETL (E-TENG-R) is 618.7 V, 40.4 μA and 22.8 nC/cm2, more than three-fold enhancement compare to pristine TENG. Finally, to effectively generate electrical energy, we apply E-TENG-R to kinematic system as a hand handled gear-cam system, 36 times enhancement of working frequency, with ground structure (EHGS) which produces the maximum 200 V and 100 μA. We charged 110 μF until 5.2 V for 25 s using EHGS and achieved operation of wireless humidity sensor system.