Formation Of Subbands Research Articles

Impact of Cd2+ substituted ZnAl2O4 spinel nanoparticles on structural, optical, morphological, magnetic and nonlinear optical behaviour

Cadmium-substituted ZnAl2O4 (CdxZn(1-x)Al2O4, where x = 0, 0.1, 0.3, and 0.5) nanoparticles were synthesized by the microwave combustion process. The XRD analysis confirmed the cubic system. The crystallite size of Cd2+ doped ZnAl2O4isin the range between 20.82 and 15.12 nm. Fourier transform infrared (FT-IR) spectra are displayed the stretching frequencies. The energy gap values decrease from 4.15 to 2.67 eV with increasing doping concentrations of Cd2+, which is due to the formation of sub-bands and is calculated by the Kubelka-Munk function. HR-SEM pictures showed the features of nanoclusters grain boundaries. The elemental mixing of Zn2+/Cd2+, Al3+andO2− was confirmed by EDX analysis. Magnetic properties reveal that the undoped ZnAl2O4 has diamagnetism and Cd2+ doped ZnAl2O4 system has superparamagnetic behaviour. The nonlinear index of refraction (10−8 cm2/W), nonlinear absorption coefficient (10−4 cm/W) and the 3rd order nonlinear susceptibility (10−6 esu) are found out by Z-scan experiments. The obtained results of optical limiting analysis confirm that these materials are viable candidates for devices like optical power limiters and optical switches.

Broad infrared absorption band through ion beam hyperdoping of silicon with selenium

In this work, we present the formation of silicon layers hyperdoped with selenium through Se implantation followed by pulsed laser annealing. The concentration depth distribution of Se atoms was investigated by Rutherford backscattering and X-Ray photoelectron spectroscopies. Scanning tunneling spectroscopy confirmed the formation of intermediate sub-band within Si band gap. The crystallinity of the doped silicon layer and the fraction of Se atoms in Si lattice sites were determined using backscattering yield analysis. Experimental and theoretical sub-band properties were compared and discussed. Notably, a significant increase in light absorption across a wide spectral region (0.2–23.0 μm) was observed, demonstrating the potential of selenium hyperdoping for enhancing infrared absorption in silicon.

Modeling of 1D confinement in FD-SOI trigate nanowires at deep cryogenic temperatures

Multi-gate FD-SOI qubit MOS devices were electrically characterized. At deep cryogenic temperatures and in linear regime, oscillations in both the current and the transconductance were observed above threshold. These are likely related to the formation of one-dimensional sub-bands in the silicon active region. A compact model taking into account a 1D density of states (DOS) has been developed in order to fathom how the 1D confinement influences both the inversion charge and the transconductance. The impact of temperature was shown to be crucial in order to distinguish the sub-bands contribution to the transconductance oscillations. Furthermore, the impact of drain voltage was also investigated, showing that the oscillations were smoothed out at high drain biases.

Synthesis and characterization of pristine and strontium-doped zinc oxide nanoparticles for methyl green photo-degradation application

Herein we describe an effective route for the degradation of methyl green (MG) dye under visible light illumination by pristine and strontium (Sr)-doped zinc oxide (ZnO) photocatalysts (synthesized by the simple chemical precipitation method). The x-ray diffraction structural analysis has confirmed that both photocatalysts exhibit the hexagonal wurtzite structure; without any additional phase formation in Sr-doped ZnO, in particular. The optical properties of the synthesized photocatalysts have been investigated using UV–vis absorption spectroscopy in the wavelength range of 250–800 nm. Through Tauc’s plot, the slight decrease from 3.3 to 3.2 eV in band gap energy has been elucidated (in the case of Sr-doped ZnO), which has been further confirmed by the quenching in the intensity of Photoluminescence (PL) emission spectrum. This may be due to sub-band level formation between valence and conduction band, caused by the impregnation of Sr2+ ions into ZnO host. The morphological study has also been performed using Field Emission Scanning Electron Microscope, which indicates nanoparticles (NPs) based surface texture for both photocatalysts. During the photocatalytic activity study, after 30 min irradiation of visible light, ∼65.7% and ∼84.8% photocatalytic degradation of MG dye has been achieved for pristine and Sr-doped (2 wt%) ZnO photocatalysts, respectively. The rate of photocatalytic reaction (K) has been observed to be ∼0.06399 min−1 for Sr-doped (2 wt%), whereas nearly half magnitude ∼0.03403 min−1 has been observed for pristine ZnO, respectively. The significantly improved photodegradation activity may be ascribed to the relatively broader optical absorption capability, surface defects and the enhanced charge separation efficiency of the Sr-doped ZnO photocatalyst.

Low-Field Mobility and High-Field Velocity of Charge Carriers in InGaAs/InP High-Electron-Mobility Transistors

Development of transistors for advanced low-noise amplifiers requires better understanding of mechanisms governing the charge carrier transport in correlation with the noise performance. In this article, we report on study of the carrier velocity in InGaAs/InP high-electron-mobility transistors (HEMTs) found via geometrical magnetoresistance in the wide range of the drain fields, up to 2 kV/cm, at a cryogenic temperature of 2 K. We observed, for the first time experimentally, the velocity peaks and found that the peak velocity and corresponding field decrease significantly with the transverse field. The low-field mobility and peak velocity are found to be up to 65 000 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.2\times 10^{{6}}$ </tex-math></inline-formula> cm/s, respectively. Extrapolations to the lower transverse fields show that the peak velocity can be as high as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.7\times 10^{{7}}$ </tex-math></inline-formula> cm/s. The corresponding intrinsic transit frequency can be up to 172 GHz at the gate length of 250 nm. We demonstrated, for the first time, that the low-field mobility and peak velocity reveal opposite dependencies on the transverse field, indicating the difference in carrier transport mechanisms dominating at low and high fields. Therefore, the peak velocity is an appropriate parameter for characterization and development of the low-noise HEMTs, complementary to the low-field mobility. Analysis indicates that the low-field carrier transport is governed by screening of the Coulomb potential of ionized impurities responsible for the carrier scattering. The velocity overshoot is associated with the electron quantization and subband formation caused by the transverse field. The results of the research clarify the ways of the further development of the HEMTs for advanced applications.

Study of sub-band states formation in the optical band gap of CuGaS2 thin films by electronic excitations

The impacts of electronic excitations with three different ion beams, namely Au, Ag, and O ions at 100–120 MeV and fluences of 5 × 1012 ions/cm2, on the structural, optical, morphological, and electrical properties of spray-deposited CuGaS2 (CGS) thin films have been investigated. Powder X-ray diffraction and micro-Raman spectroscopy measurements have revealed a significant modification in crystallinity and an increase in crystallite sizes after irradiation. Atomic force microscopy (AFM) has revealed variations in surface roughness root-mean-square values and grain size following ion irradiation. UV/Vis-NIR spectroscopy has shown that ion-irradiated films exhibit a redshift compared to pristine CGS, attributable to the creation of new sub-band states between the conduction and valence bands. The resistivity of a CGS thin film changed from 5.33 × 10−2 Ωcm to 1.30 × 10−2 Ωcm, 9.6 × 10−2 Ωcm, and 9.63 × 10−2 Ωcm, respectively, after Au-, Ag-, and O-ion-irradiations. The highest photocurrent of 1.196 mA was observed for the Au-ion-irradiated CGS film. By comparing all of the results, it is concluded that an Au-ion-irradiated CGS thin film may act as a suitable photo-absorber for photovoltaic applications.

The Ensemble Machine Learning-Based Classification of Motor Imagery Tasks in Brain-Computer Interface.

The Brain-Computer Interface (BCI) permits persons with impairments to interact with the real world without using the neuromuscular pathways. BCIs are based on artificial intelligence piloted systems. They collect brain activity patterns linked to the mental process and transform them into commands for actuators. The potential application of BCI systems is in the rehabilitation centres. In this context, a novel method is devised for automated identification of the Motor Imagery (MI) tasks. The contribution is an effective hybridization of the Multiscale Principal Component Analysis (MSPCA), Wavelet Packet Decomposition (WPD), statistical features extraction from subbands, and ensemble learning-based classifiers for categorization of the MI tasks. The intended electroencephalogram (EEG) signals are segmented and denoised. The denoising is achieved with a Daubechies algorithm-based wavelet transform (WT) incorporated in the MSPCA. The WT with the 5th level of decomposition is used. Onward, the Wavelet Packet Decomposition (WPD), with the 4th level of decomposition, is used for subbands formation. The statistical features are selected from each subband, namely, mean absolute value, average power, standard deviation, skewness, and kurtosis. Also, ratios of absolute mean values of adjacent subbands are computed and concatenated with other extracted features. Finally, the ensemble machine learning approach is used for the classification of MI tasks. The usefulness is evaluated by using the BCI competition III, MI dataset IVa. Results revealed that the suggested ensemble learning approach yields the highest classification accuracies of 98.69% and 94.83%, respectively, for the cases of subject-dependent and subject-independent problems.

Evolution of the electronic structure of twisted bilayer MoSe2

In this work we examine the evolution of the electronic structure in twisted bilayers of $\mathrm{Mo}{\mathrm{Se}}_{2}$, assuming the moir\'e potential to be a small perturbation to the untwisted limit. Its role in modifying the electronic structure is probed by mapping the calculated band structure for the moir\'e cell onto the primitive cell direction which represents the untwisted limit. At large twist angles such as $19.{03}^{\ensuremath{\circ}}$, we find that the moir\'e cell band structure is identical to the primitive cell one in the low-energy window. There are, however, significant deviations for small twist angles such as $3.{48}^{\ensuremath{\circ}}$ which have large patches of high-symmetry regions of AA and ${\mathrm{AB}}^{\ensuremath{'}}$ stackings. These lead to enhanced interlayer hopping interaction strengths in some regions, and hence stronger perturbations leading to subband formation of the highest occupied band, which has a bandwidth of 19 meV and is found to be localized both in real space as well as in momentum space.

Energetic, structural and electronic features of Sn-, Ga-, O-based defect complexes in cubic In2O3

Defect energy formation, lattice distortions and electronic structure of cubic In2O3 with Sn, Ga and O impurities were theoretically investigated using density functional theory. Different types of point defects, consisting of 1–4 atoms of Sn, Ga and O in both substitutional and interstitial (structural vacancy) positions, were examined. It was demonstrated, that formation of substitutional Ga and Sn defects are spontaneous, while formation of interstitial defects requires an activation energy. The donor-like behavior of interstitial Ga defects with splitting of conduction band into two subbands with light and heavy electrons, respectively, was revealed. Contrarily, interstitial O defects demonstrate acceptor-like behavior with the formation of acceptor levels or subbands inside the band gap. The obtained results are important for an accurate description of transport phenomena in In2O3 with substitutional and interstitial defects.

Resonant Charge-Transfer in Grazing Collisions of H− with Vicinal Nanosurfaces on Cu(111), Au(100) and Pd(111) Substrates: A Comparative Study

We compare the electron dynamics at monocrystalline Cu(111), Au(100) and Pd(111) precursor substrates with vicinal nanosteps. The unoccupied bands of a surface superlattice are populated via the resonant charge transfer (RCT) between the surface and a H − ion that flies by at grazing angles. A quantum mechanical wave packet propagation approach is used to simulate the motion of the active electron, and time-evolved wave packet densities are used to visualize the dynamics through the superlattice. The survived ion fraction in the reflected beam generally exhibits modulations as a function of the vicinal terrace size and shows peaks at those energies that access the image state subband dispersions. Differences in magnitudes of the ion-survival as a function of the particular substrate selection and the ion-surface interaction time, based on the choice of two ion-trajectories, are examined. A square well model, producing standing waves between the steps on the surface, explains the energies of the maxima in the ion survival probability for all the metals considered. This indicates that the primary process of confinement induced subband formation is robust. The work may motivate measurements and applications of shallow-angle ion-scattering spectroscopy to access electronic substructures in periodically nanostructured surfaces.

Perovskite Resonant Tunneling FET with Sequential Negative Differential Resistance Peaks

A vertical resonant tunneling (RT) field effect transistor (VRTFET), fabricated using perovskite (CH3NH3PbI3), has been analyzed for sequential sharp negative differential resistance (NDR) peaks useful in multiple-valued logic devices. NDR peaks are attributed to the sub-bands formation within the parabolic shaped band gap, present at the channel and drain/source interface due to Schottky barriers. Ambipolar CH3NH3PbI3 imparts both p and n mode characteristics with RT NDR peaks. An unprecedentedly high (100 to 1000 V–1) curvature coefficient (ϒ) has been found with two NDR peaks at a short interval, whose positions shift left, with gate bias. Due to the ionic nature of CH3NH3PbI3, hysteresis has also been observed in the transfer characteristics. This structure can overcome the limit of 60 mV/decade as well as a curvature limit of 40 V–1, important parameters for analog and digital applications. So, these devices promise cheaper and easy fabrication at commercial scale operation at ultralow voltage and lo...

Optical and photoelectrochemical studies on photoactive inorganic/organic/organic/interface assemblies of CdS/poly 3-(2-thienyl) aniline/poly 2,2 bithiophene

Assemblies of poly 2,2 bithiophene, poly 3-(2-thienyl) aniline, and CdS were subjected to optical and photoelectrochemical investigation in acetate, citrate, and phosphate aqueous electrolytes. Optical conductivity and dielectric contents reflect the role of CdS on the optical properties of the assemblies. Occlusion of CdS into the organic polymer increased the electron diffusion coefficient and diffusion length by changing both the electron lifetime and electron transport time. O2 enhanced generated photocurrent in presence and in absence of a magnetic field. The magnetic field effects were explained on the basis that external magnetic fields affect the photogeneration of singlet/triplet radical pair processes. The recorded photoactivities indicate formation of hybrid sub-bands due to band alignments between each of the assembly components. The present study also shows that the use of electrolytes can affect the electron life-times at I/O/O/I and possible charge transfer processes.

Structural, magnetic and catalytic properties of La2-xBaxCuO4 (0 ≤ x ≤ 0.5) perovskite nanoparticles

Barium doped La2CuO4 perovskite nanoparticles were synthesized via microwave assisted combustion method. The effects of Ba2+ doping on structural, optical, magnetic and catalytic activity of La2CuO4 have been studied by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) and vibrating sample magnetometer (VSM) techniques. The XRD patterns of pure La2CuO4 and La1.9Ba0.1CuO4 confirmed the formation of perovskite structure without impurities. However, with increasing Ba2+ content in the range 0.2–0.5, phase transformation from orthorhombic to tetragonal structure, occurred. The average crystallite size of orthorhombic and tetragonal phases were in the range 45.2–52.2 nm and 0.5–41 nm, respectively. The appearance of FT-IR bands at around 685 and 517 cm−1 were correlated to the La-O and Cu-O stretching modes of orthorhombic La2CuO4 phase. The direct band gap estimated using Kubelka–Munk (K–M) method decreased with the increase in Ba2+ content (1.88–1.64 eV), due to the formation of sub-bands in the energy band gap. Magnetic measurements of doped La2CuO4 samples showed either para- or ferro-/para- magnetic behaviour at room temperature. The catalytic activity (oxidation) tests carried out in a batch reactor operating under atmospheric conditions indicated that the prepared catalysts, in particular (La1.7Ba0.3CuO4), showed excellent catalytic activity.

Optical, Photoelectrochemical, and Electrochemical Impedance Studies on Photoactive Organic/Inorganic/Interface Assemblies of Poly 2,2 Bithiophene/Poly 3-(2-Thienyl) Aniline (PThA)/TiO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;

Particles of TiO2 modified with poly 3-(2-thienyl) aniline (PThA) and occluded in poly 2,2 bithiophene (PBTh), were subjected to optical, electrochemical impedance spectroscopic (EIS) and photoelectrochemical (PEC) investigation in aqueous, acetate, citrate, and phosphate electrolytes. EIS studies revealed that the assembly film of TiO2/PThA/PBTh possess porous-type structure. They also confirmed the approximate value of Ef obtained from electrochemical studies. Both EIS and optical studies indicated that ac conductivity is much greater than dc conductivity. Guided by the properties of PBTh, no large changes in the energy band structure occurred due to occlusion of TiO2 in PBTh films. Occlusion of TiO2/PThA into the network structure of PBTh inhibits the energy dissipation process and impeded charge polarization process of the material. Photoelectrochemical outcome suggested possible band alignments between the organic film and TiO2 and formation of hybrid sub-bands. Inclusion of TiO2 in the thiophene-based polymers enhanced the charge separation and consequently charge transfer processes and widen the absorption in visible light range.

Creation of Hybrid Photoactive Inorganic/Organic Interface Assemblies of Cadmium Oxide mixtures (CdO2/CdO)/Poly-2,2-Bithiophene; Optical and Photoelectrochemical Investigations

Nanoparticles of cadmium peroxide (CdO2) were immobilized in poly 2,2 bithiophene (PBTh) to build photoactive inorganic/organic interfaces (I/O/I). Studies indicated that the CdO2 initially immobilized in the organic polymer partially decomposed to a low band gap CdO. Therefore we refer to this mixture as CdO2/CdO. The CdO2/CdO/PBTh assemblies were subjected to optical and photoelectrochemical investigations in aqueous electrolytes containing acetate, nitrate, or phosphate. The equilibrium mixture of CdO2/CdO influenced the optical conductivity and dielectric contents of the assemblies. Furthermore, O2 played an important role in the charge separation and transfer processes. The effects of an applied magnetic field were investigated and reported. The results were explained on the basis of formation of hybrid sub-bands due to band alignments between the assembly components. The photo-induced charge generation of PBTh was improved by occlusion of CdO2 in the polymer as was evident by the greater photocurrent generated by CdO2/CdO/PBTh than that generated by PBTh.

Dynamical quantum anomalous Hall effect in strong optical fields

Topological insulators are characterized by the quantum anomalous Hall effect (QAHE) on the topological surface states under time-reversal symmetry breaking. Motivated by recent experiments on the magneto-optical effects induced by the QAHE, we develop a theory for the dynamical Hall conductivity for subgap optical frequency and intense optical fields using the Keldysh-Floquet Green's function formalism. Our theory reveals a nonlinear regime in which the Hall conductivity remains close to ${e}^{2}/2h$ at low frequencies. At higher optical fields, we find that the subsequent collapse of the half quantization is accompanied by coherent oscillations of the dynamical Hall conductivity as a function of field strength, triggered by the formation of Floquet subbands and the concomitant intersubband transitions.

Photoelectrochemical and magnetic studies on photoactive interface thin film assemblies of poly bithiophene/poly 3-(2-thienyl) aniline/ferromagnetic Mg-doped Fe2O3 in aqueous electrolytes

Organic/organic/inorganic interface or (OOI) thin film assemblies were created from poly 2,2 bithiophene and poly 3-(2-thienyl) aniline (PThA) and Mg-doped Fe2O3. Surface modification of Fe2O3 nanoparticles with PThA was achieved using photopolymerization of 3-(2-thienyl) aniline. Particles of PThA/Fe2O3 were occluded during the electropolymerization process of 2,2 bithiophene on a fluorine doped tin oxide glass electrode. The OOI assemblies were subjected to photoelectrochemical investigation in aqueous electrolytes containing acetate, citrate, tartrate, and phosphate. As all components of the assembly are p-type semiconductors, the produced interface are of a p-p junction nature. Our results show that the behavior of the assemblies reflects formation of hybrid sub-bands as a product of band alignments between the organic film and Fe2O3. Furthermore, O2 played an important role in the charge separation and transfer processes. The assembly responses to an applied magnetic field was investigated and reported. The results were explained on the basis that external magnetic fields affect the photogeneration of singlet/triplet radical pair processes. The present studies also show that the longest electron life-time was recorded in citrate buffer.

Fabrication of Al deposited sandwich capacitor structure with CdSe/PVA dielectric thin film by spin coating technique for high power applications: synthesis and characterizations

The capacitor structures were fabricated in the configuration of Al : CdSe/PVA : Al with CdSe/PVA as an insulating dielectric layer for high power applications. The sandwiched layer gave an excellent energy density and a better dielectric strength that was obtained from the amalgamation of CdSe and poly (vinyl alcohol). For the detailed analysis of the interaction between CdSe and PVA, transparent CdSe/PVA composites were synthesized by ultra-sonication technique with micrometer thicknesses at different wt% of CdSe. The UV absorption edge of PVA matrix corresponds to π→π* transition associated with ethylene unsaturation (C=C) was analysed and it was shifted towards higher wavelength with the CdSe incorporation. The sub-band states formation was evaluated, Urbach energy was increased up to ~835 meV, and an increase in structural defect was noticed by widening the tail state within the polymer matrix with the impurity addition. Optical parameters which include extinction coefficient (k) and index of refraction (n) have been determined. Three dielectric relaxations were pronounced as α, β and interfacial polarization and the high relative permittivity and the low values of dissipation factor indicated that the dielectric phenomenon was predominant in all membranes. Inspection of electrical conduction rate to temperatures was also investigated and the temperature coefficient of capacitance and temperature coefficient of permittivity were listed. Thermal stability could be enhanced with CdSe interaction and the variation in thermal parameters was discussed.

Modeling the Effect of Interface Roughness on the Performance of Tunnel FETs

The sub-band formation in the triangular-like potential well in the channel of a tunnel field effect transistor (TFET) results in a delayed onset of the vertical band-to-band tunneling (BTBT) and in a reduction of the ON-current. Furthermore, the roughness of the oxide/semiconductor interface causes density-of-states (DOS) tails, i.e., a smoothing of the otherwise staircase-shaped 2D DOS in the TFET channel. The impact of interface roughness is modeled semi-classically using Ind's perturbation approach. The developed model for the combined effect of the channel quantization and interface roughness on TFET performance has been implemented in a commercial device simulator. Simulations of a TFET with predominantly vertical BTBT show that the sharp onset of tunneling, which ideally originates from the 2D-3D DOS matching, is smoothed by the interface roughness. This leads to a degradation of the sub-threshold swing of the transistor with increasing amplitude and decreasing auto-correlation length of the roughness.

Wurtzite GaAs Quantum Wires: One-Dimensional Subband Formation.

It is of contemporary interest to fabricate nanowires having quantum confinement and one-dimensional subband formation. This is due to a host of applications, for example, in optical devices, and in quantum optics. We have here fabricated and optically investigated narrow, down to 10 nm diameter, wurtzite GaAs nanowires which show strong quantum confinement and the formation of one-dimensional subbands. The fabrication was bottom up and in one step using the vapor-liquid-solid growth mechanism. Combining photoluminescence excitation spectroscopy with transmission electron microscopy on the same individual nanowires, we were able to extract the effective masses of the electrons in the two lowest conduction bands as well as the effective masses of the holes in the two highest valence bands. Our results, combined with earlier demonstrations of thin crystal phase nanodots in GaAs, set the stage for the fabrication of crystal phase quantum dots having full three-dimensional confinement.