Formation Of Subbands Research Articles

Aloe vera Plant Extracted Green Synthesis, Structural and Opto-Magnetic Characterizations of Spinel Co(x)Zn₁₋xAl₂O₄Nano-Catalysts.

Spinel Co(x)Zn₁₋xAl₂O₄ (0 ≤ x ≤ 1) nano-catalysts were synthesized by a simple Aloe vera plant extracted green synthesis route. Powder XRD patterns and Rietveld analysis confirmed the formation of single phase, cubic spinel gahnite structure without other impurities. The lattice parameter increased from 8.089 to 8.125 A with increasing CO²⁺ content. The average crystallite sizes were estimated using Scherrer's method, and it was found to be in the range of 15.72 nm to 26.53 nm. FT-IR spectra showed vibrational stretching frequencies corresponding to the spinel structure. HR-SEM and HR-TEM images showed the features of well particle shaped crystals with nano-sized grains. The elemental compositions of Co, Zn, Al and O were quantitatively obtained from EDX analysis. The band gap energy estimated using Kubelka-Munk method by UV-Visible DRS method, and the values are decreased with increasing the Co²⁺ content (4.12 eV to 3.67 eV), due to the formation of sub bands in between the energy gap. PL spectra showed emission bands in UV as well as in the visible regions for ZnAl₂O₄ and Co-doped ZnAl₂O4, due to the defect centers acting as the trap levels. VSM measurements revealed that pure ZnAl₂O₄ has diamagnetic, while Co doped ZnAl₂O₄ samples (x = 0.2 to 0.8) have superparamagnetism, whereas the sample CoAl₂O₄ has ferromagnetic in nature. Catalytic oxidation of benzyl alcohol to benzaldehyde was found that the sample Co₀.₆Zn₀.₄Al₂O₄ showed 93.25% conversion with 99.56% selectivity, whereas for pure ZnAl₂O₄, the conversion was only 86.31% with 92.85% selectivity.

Photocatalytic reduction of carbon dioxide with water on InVO4 with NiO cocatalysts

InVO4 was synthesized by solid-state reaction method using In2O3 and V3O4 as the starting materials. NiO was added as the cocatalyst. The catalysts were characterized by powder X-ray diffraction, scanning electron microscope, and ultraviolet–visible spectroscopy. The photocatalytic reduction of CO2 with water was carried out in a Pyrex reactor with KHCO3 aqueous solution bubbled with CO2 gas under visible light illumination. Many pinholes were observed on the InVO4 particles after loading with NiO cocatalyst. The band gap of NiO/InVO4 was lower than that of InVO4 due to the formation of subband in valence band by creating more defect sites on the surface of InVO4. Both NiO/InVO4 and InVO4 can reduce CO2 to methanol under visible light irradiation. The reaction rate increased with an increase of the loading of NiO cocatalysts.

Capacitance estimation for InAs Tunnel FETs by means of full-quantum [formula omitted] simulation

We report for the first time a quantum mechanical simulation study of gate capacitance components in aggressively scaled InAs Nanowire Tunnel Field-Effect Transistors. It will be shown that the gate-drain capacitance exhibits the same functional dependence over the whole Vgs range as the total gate capacitance, albeit with smaller values. However, as opposed to the previous capacitance estimations provided by semiclassical TCAD tools, we find that the gate capacitance exhibits a non-monotonic behavior vs. gate voltage, with plateaus and bumps related with energy quantization and subband formation determined by the device cross-sectional size, as well as with the position of channel-conduction subbands relative to the Fermi level in the drain contact. From this point of view, semiclassical TCAD tools seem to be inaccurate for capacitance estimation in aggressively-scaled TFET devices.

Structural, optical and magnetic properties of Zn1−xMnxAl2O4 (0 ⩽ x ⩽ 0.5) spinel nanostructures by one-pot microwave combustion technique

Zn1−xMnxAl2O4 (0⩽x⩽0.5) spinel nanostructures were synthesized by urea assisted microwave combustion method. Structural, vibrational, morphological, optical and magnetic properties were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), high resolution scanning electron microscopy (HR-SEM), UV–visible diffuse reflectance spectroscopy (DRS) and photoluminescence (PL) spectroscopy, and vibrating sample magnetometry (VSM) respectively. The XRD studies showed that the samples have pure cubic spinel phase, which is further validated by Rietveld refinement. The average crystallite size is of the nanoparticles estimated using Debey–Scherrer’s method was found to be in the range of 14–17nm. The lattice parameter is increased from 8.089 to 8.160Å with increasing in Mn2+ content. The William–Hall (W–H) analysis was used to study the effects of lattice strain over the crystallite size. Increase in lattice parameter and increase in porosity were observed on increasing Mn2+ concentration FTIR spectra showed the vibrational stretching frequencies corresponding to the zinc aluminate spinels. The morphology of the samples depicted the formation of well developed nano-sized clusters with homogeneous well crystallized grains without any agglomerations. The optical band gap value of undoped zinc aluminate nanostructure is higher than the reported bulk zinc aluminate. The direct band gap estimated using Kubelka–Munk method decreased with increasing Mn2+ content (5.05–3.49eV), due to the formation of sub bands in the energy gap. The photoluminescence characteristics of undoped and Mn2+ doped zinc aluminates are suggestive of defect controlled process and had an effect on the luminescence. Magnetic measurements revealed that the undoped ZnAl2O4 has diamagnetic behavior while the Mn2+ doped ZnAl2O4 system has superparamagnetic behavior.

Enhanced optical absorption due to E+-related band transition in GaAs:N δ-doped superlattices

The photoabsorption characteristics of GaAs:N δ-doped superlattices (SLs) are investigated. Periodic insertion of N δ-doped layers into GaAs induces the formation of conduction subbands E+ and E−, and each conduction subband forms SL minibands with the GaAs conduction band between the δ-doped layers. In addition to an optical absorption related to the E− band, an abrupt absorption edge originating from the electron transition between the valence band and an E+-related miniband is observed at 1.6 eV in a photoluminescence excitation (PLE) spectrum, indicating that GaAs:N δ-doped SLs are promising candidates for the absorber of intermediate-band solar cells.

Structural, optical and magnetic characterization of Zn1−xNixAl2O4 (0≤x≤5) spinel nanostructures synthesized by microwave combustion technique

Zn1−xNixAl2O4 (0≤x≤0.5) spinel nanostructures were synthesized by microwave combustion technique. Structural, vibrational, optical, morphological and magnetic properties were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), diffuse reflectance (DRS) and photoluminescence (PL) spectroscopy, high resolution scanning electron microscopy (HR-SEM), and vibrating sample magnetometry (VSM), respectively. The XRD pattern confirmed the formation of a single phase cubic spinel structure of ZnAl2O4 (gahnite), except for higher dopant concentrations for which a secondary phase was observed and validated by Rietveld refinement analysis. Different concentration levels of Ni2+ was used as the dopant in the zinc aluminate matrix. Decrease in lattice parameter and increase in porosity were observed on increasing Ni2+ concentration. The average crystallite size of the nanoparticles estimated using Debey–Scherrer׳s method was found to be in the range of 8.89–13.84nm. The William–Hall (W–H) analysis was used to study the effects of lattice strain over the crystallite size. FT-IR spectra showed the vibrational stretching frequencies corresponding to the zinc aluminate spinels. The optical band gap value of undoped zinc aluminate nanostructure is higher than the reported bulk zinc aluminate. The direct band gap estimated using the Kubelka–Munk method decreased with increasing Ni2+ content (5.05–3.98eV), due to the formation of subbands in the energy gap. The HR-SEM images depict the formation of well developed nano-sized clusters with homogeneous well crystallized grains without any agglomerations. The PL characteristics of undoped and Ni2+ doped zinc aluminates are suggestive of defect controlled process. Magnetic measurements revealed that the undoped ZnAl2O4 has diamagnetic behavior while the Ni2+ doped ZnAl2O4 system has superparamagnetic behavior, despite NiO impurities.

Charge transfer processes in CsI:Tl using near-UV light

This paper studies charge transfer processes in CsI:Tl crystals by analyzing the bulk photo-conductivity spectra, the temperature behavior of the bulk photo-conductivity current and the shape and intensity of the activator emission pulse excited by an electron pulse beam and/or laser pulse emission at temperatures between 80 and 400K. The Tl concentration in CsI:Tl crystals varies from 10−3–10−1mass%. It has been determined that near-UV light induces a bulk conductivity in CsI:Tl crystals only when the Tl concentration is greater than 3×10−3mass%. A mechanism is proposed to explain the charge transfer processes with photons whose energy is approximately half the width of the CsI band gap. Near-UV light causes charge transfer from I− to Tl+ ions, forming Tl0 centers in the 6p2P1/2 ground and 6p2P3/2 excited states. The electron, assisted by phonons, leaves the Tl0 center from either the 6p2P1/2 or 6p2P3/2 states and overcomes the 0.13 or 0.30eV energy barrier, respectively, and subsequently populates the activator conduction sub-bands, which are found inside the band gap of CsI:Tl. The formation of activator sub-bands is possible only above the threshold Tl concentration, i.e., above 3×10−3mass%.

Imperceptible and Robust DWT-SVD-Based Digital Audio Watermarking Algorithm

Digital representation of multimedia is more advantageous than the analog one due to potentially improving the portability, efficiency, and accuracy of the information presented. Despite the challenge of having high hiding capacity like other media watermarking, still audios are likely candidates for data hiding due to their possible capabilities of achieving impressive robustness and protection against online music piracy and content identification. In this paper, we propose efficient audio watermarking embedding and extracting techniques, which mainly use Discrete Wavelet Transform (DWT) and Singular Value Decomposition (SVD), in which a new matrix formation of details sub-bands is proposed. Additionally, massive experimental work was conducted to investigate the contributions of operating different watermark intensities and multiple levels of DWT to our proposed techniques. Two performance objectives are employed in this work which involve imperceptibility and robustness. To further boost the imperceptibility, we incorporate the code assignment method to our techniques that do outperform what are closely connected in the literature.

Microwave combustion synthesis, structural, optical and magnetic properties of Zn1−xCoxAl2O4 (0 ⩽ x ⩽ 0.5) spinel nanostructures

Cobalt doped zinc aluminate spinel type nanostructures were synthesized by microwave combustion method. The structural, vibrational, optical, morphological and magnetic properties were studied by using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), diffuse reflectance spectroscopy (DRS), photoluminescence (PL), high resolution scanning electron microscopy (HR-SEM), energy dispersive X-ray diffraction (EDX) and vibrating sample magnetometry (VSM) respectively. The XRD patterns confirmed the formation of single phase ZnAl2O4 cubic spinel without impurities. The lattice parameter increased from 8.081 to 8.116Å with increasing Co2+ content. The average crystallite size of the nanoparticles estimated using Debey–Scherrer’s method was found to be in the range of 11.80–20.21nm. The presence of tensile strain in the zinc aluminates was determined from Williamson–Hall (W–H) analysis. FT-IR spectra showed the vibrational stretching frequencies corresponding to the zinc aluminate spinel structure. The direct band gap estimated using Kubelka–Munk method decreased with increasing Co2+ content (5.01–2.89eV), due to the formation of sub bands in between the energy gap. Optical absorption spectra confirmed the cobalt substitution by the occurrence of a triplet of bands at 542, 584, and 624nm, which is the characteristic of Co2+ ions in the tetrahedral sites. For ZnAl2O4 and Co substituted ZnAl2O4, PL spectra showed the emission bands in UV as well as in the visible regions, due to the defect centers acting as the trap levels. The elemental compositions of Co, Zn and Al were quantitatively obtained from the EDX analysis. HR-SEM images showed the features of well created pore structures and nano sized grains. Magnetic measurements revealed that the undoped ZnAl2O4 has diamagnetic behavior while the Co doped ZnAl2O4 system has superparamagnetic behavior.

Magnetic and thermoelectric properties of the Mn1−X Ni X S solid solutions

The new sulphide Mn1−XNiXS (0 TN. The conductivity type change from the hole to the electronic at X > 0.05 is revealed on the basis of the thermoelectric power measurements. The resistivity and thermopower behaviors are explained in terms of the impurity subband formation into MnS electron excitation gap.

Electrical observation of sub-band formation in SnO2 nanobelts

The electrical observation of energy sub-band formation in the electronic structure, that gives rise to the phenomenon of quantized transport is reported in tin oxide (SnO2) nanobelt back-gate field-effect transistors, at low temperatures. Sub-band formation was observed as current oscillations in the drain current vs. gate voltage characteristics, and was analyzed considering the nanobelt as a "quantum wire" with a rectangular cross-section and hard walls. The lateral quantum confinement in the nanowires created conditions for the successive filling of the first twelve electron energy sub-bands, as the gate voltage increases. When the source-drain voltage is changed, the oscillations are not dislocated with respect to the gate voltage indicating flat-band energies, and that the observations are incompatible with the phenomena of Coulomb blockade and tunnelling oscillations. The separation of the energy sub-bands was found to be in good agreement with the measured cross-section dimensions of the nanobelt and with the threshold temperature, since for T > 60 K the oscillations tend to vanish.

Electric-field-induced destruction of quasi-Landau levels in bilayer graphenenanoribbons

The magneto-electronic properties of bilayer zigzag graphene nanoribbons are investigated by the Peierls tight-binding method. In the presence of magnetic fields, Landau quantization leads to the formation of Landau subbands. For the bilayer nanoribbons, these subbands are partially dispersionless in k-space and are called quasi-Landau levels (QLLs). Perpendicular electric fields, serving as the top gate, push the QLLs to higher state energy and split the flat subbands. From the evidence of band structure and density of states, the QLLs remain dispersionless and the corresponding peaks are still the main structure of density of states, which means that the material properties related to the QLLs are unchanged. However, the wave functions present a totally different evidence that the Landau wave functions are severely mixed, and the corresponding material properties would be strongly affected or destroyed. The wave functions provide an effective way to comprehend the characteristics of the flat subbands and Landau subbands. The energy spectra, density of states, and wave functions are discussed in detail.

Atypical BCS-BEC crossover induced by quantum-size effects

Quantum-size oscillations of the basic physical characteristics of a confined fermionic condensate are a well-known phenomenon. Its conventional understanding is based on the single-particle physics, whereby the oscillations follow the size-dependent changes in the single-particle density of states. Here we present a study of a cigar-shaped ultracold superfluid Fermi gas, which demonstrates an important many-body aspect of the quantum-size effects, overlooked previously. The many-body physics is revealed in the atypical crossover from the Bardeen-Cooper-Schrieffer (BCS) superfluid to the Bose-Einstein condensate (BEC) induced by the size quantization of the particle motion. Quantized perpendicular spectrum results in the formation of single-particle subbands (shells) so that the aggregate fermionic condensate becomes a coherent mixture of subband condensates. Each time when the lower edge of a subband crosses the chemical potential, the BCS-BEC crossover is approached in this subband, and the aggregate condensate contains both the BCS and BEC-like components.

Erratum to: “Formation of Nanoelectrostatic Quantum Dots and Two-Dimensional Subbands by the Random Potential of Mn Donors in p-i-n Resonant Tunneling Heterosystem,”

Erratum to: “Formation of Nanoelectrostatic Quantum Dots and Two-Dimensional Subbands by the Random Potential of Mn Donors in p-i-n Resonant Tunneling Heterosystem,”

Atomistic quantum transport modeling of metal-graphene nanoribbon heterojunctions

We calculate quantum transport for metal-graphene nanoribbon heterojunctions within the atomistic self-consistent Schr\"odinger/Poisson scheme. Attention is paid on both the chemical aspects of the interface bonding as well the one-dimensional electrostatics along the ribbon length. Band-bending and doping effects strongly influence the transport properties, giving rise to conductance asymmetries and a selective suppression of the subband formation. Junction electrostatics and $p$-type characteristics drive the conduction mechanism in the case of high-work-function Au, Pd, and Pt electrodes while contact resistance becomes dominant in the case of Al.

Formation of nanoelectrostatic quantum dots and two-dimensional subbands by the random potential of Mn donors in p-i-n resonant tunneling heterosystems

The influence of the random potential of Mn impurities in p-i-n resonant tunneling structures on the electronic spectrum has been studied. The possibility of the formation of zero-dimensional states in nanoelectrostatic quantum dots, as well as two-dimensional subbands in the region of GaAs quantum well, by the minima of this potential has been shown.

Band gap narrowing and fluorescence properties of nickel doped SnO2 nanoparticles

Nickel-doped tin oxide nanoparticles (sub-5 nm size) with intense fluorescence emission behavior have been synthesized by sol–gel route. The structural and compositional analysis has been carried out by using XRD, TEM, FESEM and EDAX. The optical absorbance spectra indicate a band gap narrowing effect and it was found to increase with the increase in nickel concentration. The band gap narrowing at low dopant concentration (<5%) can be assigned to SnO2−SnO2−x alloying effect and for higher doping it may be due to the formation of defect sub-bands below the conduction band.

Formation of Bandgap and Subbands in Graphene Nanomeshes with Sub-10 nm Ribbon Width Fabricated via Nanoimprint Lithography

We fabricated hexagonal graphene nanomeshes (GNMs) with sub-10 nm ribbon width. The fabrication combines nanoimprint lithography, block-copolymer self-assembly for high-resolution nanoimprint template patterning, and electrostatic printing of graphene. Graphene field-effect transistors (GFETs) made from GNMs exhibit very different electronic characteristics in comparison with unpatterned GFETs even at room temperature. We observed multiplateaus in the drain current-gate voltage dependence as well as an enhancement of ON/OFF current ratio with reduction of the average ribbon width of GNMs. These effects are attributed to the formation of electronic subbands and a bandgap in GNMs. Such mesoscopic graphene structures and the nanofabrication methods could be employed to construct future electronic devices based on graphene superlattices.

Hollow nanocylinder: Multisubband superconductivity induced by quantum confinement

Quantization of the transverse electron motion in high-quality superconducting metallic nanowires and nanofilms results in the formation of well-distinguished single-electron subbands. They shift in energy with changing thickness, which is known to cause quantum-size superconducting oscillations. The formation of multiple subbands results in a multigap structure induced by the interplay between quantum confinement and Andreev mechanism. We investigate multisubband superconductivity in a hollow nanocylinder by numerically solving the Bogoliubov-de Gennes equations. When changing the inner radius and thickness of the hollow nanocylinder, we find a crossover from an irregular pattern of quantum-size superconducting oscillations, typical of nanowires, to an almost regular regime, specific for superconducting nanofilms. At this crossover the multigap structure becomes degenerate. The ratio of the critical temperature to the energy gap increases and approaches its bulk value while being reduced by $20--30\text{ }\mathrm{%}$ due to Andreev-type states driven by quantum confinement in the irregular regime.

Luminescence mechanisms in 6H-SiC nanocrystals

Experimental conditions allowing consequent selection of a dominating photoluminescence mechanism in 6H-SiC nanocrystals at room temperature are reported. Electrostatic screening of surface states involved in radiative transitions can be efficiently achieved by polar ethanol molecules. This leads to a preponderant radiative channel between the electronic levels corresponding to the impurity atoms (N and Al). This radiative channel is deactivated by centrifugation-induced selection of the smallest colloidal 6H-SiC nanocrystals in which the probability to have both donor and acceptor atoms is negligible. Consequently, for these smallest 6H-SiC nanocrystals with switched off transitions between surface states and impurity levels, quantum-confinement effect can be clearly observed. The formation of energy subbands in the 6H-SiC nanocrystals is then evidenced from photoluminescence excitation and absorption measurements performed on the centrifuged colloidal nanosuspension. A most probable mean diameter of 1.9 nm for these particles is deduced from calculation of energy levels in the effective-mass approximation.