Shape effects on the electronic structure and the optical gain of InAsN/GaAs nanostructures: From a quantum lens to a quantum ring

The electronic structures of self-assembled InAs_1−xN_x/GaAs nanostructures from quantum lens to quantum rings (QRs) are calculated using the 10-band k.p method and the valence force field (VFF) method. With the variation of shape of the nanostructure and nitrogen (N) content, it shows that the N and the strains can significantly affect the energy levels especially the conduction band because the N resonant state has repulsion interaction with the conduction band due to the band anticrossing (BAC). The structures with N and greater height have smaller transition energy, and the structures with N have greater optical gain due to its overwhelming greater value of factor _fc+_fv−1$f_{\text{c}}+f_{\text{v}}-1$. After analyzing the shape effect, we suggested that the nanostructures with volcano shape are preferred because the maximum optical gain occurs for quantum volcano. With our simulation result, researchers could select quantum dots (QDs) structures to design laser with better performance.     

Mechanism of fluorescence energy transfer in aqueous solution cadmium sulfide quantum dots

Cadmium sulfide (CdS) quantum dots (QDs) prepared by a convenient chemical method have been characterized using absorption, fluorescence, and photoluminescence excitation techniques. The photoluminescence excitation studies show that there is an electron transfer from the surface adsorbate (thiourea) to CdS QDs in aqueous solution. The excitation band with peak maximum at 5.8 eV is assigned to the electronic transitions in the chemisorbed thiourea, whereas the excitation band between 3.45 and 3.7 eV corresponds to the band-to-band transition within the nanocrystalline CdS host. The absorption spectroscopy of the CdS QD solutions shows a strong absorption peak which is generated from thiourea. The band-edge fluorescence of the CdS QDs has also been investigated. It is shown that the fluorescence property of the CdS QDs can be enhanced by adding cadmium chloride (CdCl₂) solution.     

Spectroscopical splitting of Cu ion energy levels in magnesium lead fluoro silicate glasses

Homogeneous 40.0 MgO-(10-x) PbF₂-50.0 SiO₂: x CuO glasses were prepared using melt-quenching technique under controlled conditions. Spectroscopic studies (UV-vis absorption, ESR, FT-IR) are carried out for these glasses. One broad characteristic visible absorption band is observed around 700-850 nm in these glasses, the optical band gap decreases as the content of the CuO increases in the glass network up to 0.7 mol % then reversal trend is observed. ESR spectra of all these glasses show resonance peaks characteristic of Cu²⁺ ions and hyperfine splitting is resolved with increasing the CuO content in the glass network. From the observed ESR spectra, the spin-Hamiltonian parameters have been evaluated and indicate that Cu²⁺ ions have octahedral coordination with a strong tetragonal distortion in these glasses. By correlating ESR and optical absorption data, the molecular orbital coefficients have been evaluated. FT-IR spectra give important information about the nature of bonds in the glass matrix. The density of the glasses is also measured and is found to decrease with the increase CuO contents in the glass matrix. The physical parameters along with spectroscopic parameters are measured.     

An extreme change in structural and optical properties of indium oxynitride deposited by reactive gas-timing RF magnetron sputtering

The indium oxynitride (InON) films were achieved by reactive RF magnetron sputtering indium target which has the purity of 99.999% with a novel reactive gas-timing technique. The structural, optical and electrical properties in a series of polycrystalline InON films affected by gas-timing of reactive N₂ and O₂ gases introduced to the chamber were observed. The X-ray photoelectron spectroscopy revealed that the oxygen content in thin films that compounded to indium and nitrogen, which increased from 10% in indium nitride (InN) to 66% in indium oxide (In₂O₃) films. The X-ray diffraction peaks show that the phase of deposited films changes from InN to InON and to In₂O₃ with an increasing oxygen timing. The hexagonal structure of InN films with predominant (0 0 2) and (0 0 4) orientation was observed when pure nitrogen is only used as sputtering gas, while InON and In₂O₃ seem to demonstrate body-center cubic polycrystalline structures depending on gas-timing. The surface morphologies investigated from atomic force microscope of deposited films with varying gas-timing of O₂:N₂ show indifferent. The numerical algorithm method was used to define the optical bandgap of films from transmittance results. The increasing oxygen gas-timing affects extremely to the change of crystallinity phase from InN to InON and to In₂O₃, the increase of optical bandgap from 1.4 to 3.4 eV and the rise of sheet resistance from 15 Ω/□ to insulator.     

Influence of Mg Composition on Optical Properties of Exciton Confinement in Strained Wurtzite ZnO/MgxZn1-xO Cylindrical Quantum Dots

Based on the effective-mass approximation and variational approach, excitonic optical properties are investigated theoretically in strained wurtzite (WZ) ZnO/Mg_xZn_1-xO cylindrical quantum dots (QDs) for four different Mg compositions: x=0.08, 0.14, 0.25, and 0.33, with considering a three-dimensional carrier confinement in QDs and a strong built-in electric field effect due to the piezoelectricity and spontaneous polarization. The ground-state exciton binding energy, the interband emission wavelength, and the radiative lifetime as functions of the QD structural parameters (height and radius) are calculated in detail. The computations are performed in the case of finite band offset. Numerical results elucidate that Mg composition has a significant influence on the exciton states and optical properties of ZnO/Mg_xZn_1-xO QDs. The ground-state exciton binding energy increases with increasing Mg composition and the increment tendency is more prominent for small height QDs. As Mg composition increases, the interband emission wavelength has a blue-shift if the dot height L<3.5 nm, but the interband emission wavelength has a red-shift when L>3.5 nm. Furthermore, the radiative lifetime increases rapidly with increasing Mg composition if the dot height L>3 nm and the increment tendency is more prominent for large height QDs. The physical reason has been analyzed in depth.     

Influence of Mg Composition on Optical Properties of Exciton Confinement in Strained Wurtzite ZnO/Mg_x Zn_(1-x) O Cylindrical Quantum Dots

Based on the effective-mass approximation and variational approach, excitonic optical properties are investigated theoretically in strained wurtzite (WZ) ZnO/Mg x Zn 1-x O cylindrical quantum dots (QDs) for four different Mg compositions: x = 0.08, 0.14, 0.25, and 0.33, with considering a three-dimensional carrier confinement in QDs and a strong built-in electric field effect due to the piezoelectricity and spontaneous polarization. The ground-state exciton binding energy, the interband emission wavelength, and the radiative lifetime as functions of the QD structural parameters (height and radius) are calculated in detail. The computations are performed in the case of finite band offset. Numerical results elucidate that Mg composition has a significant influence on the exciton states and optical properties of ZnO/Mg x Zn 1 x O QDs. The ground-state exciton binding energy increases with increasing Mg composition and the increment tendency is more prominent for small height QDs. As Mg composition increases, the interband emission wavelength has a blue-shift if the dot height L 3.5 nm, but the interband emission wavelength has a red-shift when L 3.5 nm. Furthermore, the radiative lifetime increases rapidly with increasing Mg composition if the dot height L 3 nm and the increment tendency is more prominent for large height QDs. The physical reason has been analyzed in depth.     

Stranski–Krastanov growth of InGaN quantum dots emitting in green spectra

Self-assembled InGaN quantum dots (QDs) were grown on GaN templates by metalorganic chemical vapor deposition. 2D–3D growth mode transition through Stranski–Krastanov mode was observed via atomic force microscopy. The critical thickness for In_0.67Ga_0.33N QDs was determined to be four monolayers. The effects of growth temperature, deposition thickness, and V/III ratio on QD formation were examined. The capping of InGaN QDs with GaN was analyzed. Optimized InGaN quantum dots emitted in green spectra at room temperature.     

Impurity band in SnBi<Subscript>4</Subscript>Se<Subscript>7</Subscript>: thermoelectric power and electrical resistivity measurements

Thermoelectric power and electrical resistivity measurements on polycrystalline samples of Bi₂Se₃ and stoichiometric ternary compound in the quasi-binary system SnSe–Bi₂Se₃ in the temperature range of 90–420 K are presented and explained assuming the existence of an impurity band. The variation of the electron concentration with temperature above 300 K is explained in terms of the thermal activation of a shallow donor, by using a single conduction band model. The density of states effective mass m ^*=0.15m ₀ of the electrons, the activation energy of the donors, their concentration, and the compensation ratio are estimated. The temperature dependence of the electron mobility in conduction band is analyzed by taking into account the scattering of the charge carriers by acoustic phonon, optical phonon, and polar optical phonon as well as by alloy and ionized impurity modes. On the other hand, by considering the two-band model with electrons in both the conduction and impurity bands, the change in the electrical resistivity with temperature between 420 and 90 K is explained.     

Cooperative downconversion and near-infrared luminescence of Tb<Superscript>3+</Superscript>–Yb<Superscript>3+</Superscript> codoped lanthanum borogermanate glasses

In the present paper, we investigate the near-infrared (NIR) luminescence of Tb³⁺–Yb³⁺ codoped lanthanum borogermanate (LBG) glasses under visible and ultraviolet light excitation. The results indicate that NIR quantum cutting occurs through cooperative energy transfer from Tb³⁺ to Yb³⁺ ions when only 4f ⁸ levels of Tb³⁺ ions are excited in the wavelength region of 300–490 nm. The highest quantum efficiency under the excitation ⁵ D ₄ level of Tb³⁺ at 484 nm is 146%. Ultraviolet excitation that populates the charge transfer band (CTB) of Yb³⁺ near 270 nm does not result in quantum cutting as the fast nonradiative decay from CTB to ² F _5/2 level dominates. These materials are expected to be used as a converting layer for silicon solar cells to enhance their efficiency by splitting each high-energy photon into two NIR photons.     

Exciton in wurtzite GaN/AlxGa1−xN coupled quantum dots

Based on effective-mass approximation, we present a three-dimensional study of the exciton in GaN/Al_xGa_1−xN vertically coupled quantum dots (QDs) by a variational approach. The strong built-in electric field due to the piezoelectricity and spontaneous polarization is considered. The relationship between exciton states and structural parameters of wurtzite GaN/Al_xGa_1−xN coupled QDs is studied in detail. Our numerical results show that the strong built-in electric field in the GaN/Al_xGa_1−xN strained coupled QDs leads to a marked reduction of the effective band gap of GaN QDs. The exciton binding energy, the QD transition energy and the electron-hole recombination rate are reduced if barrier thickness L^AlGaN is increased. The sizes of QDs have a significant influence on the exciton state and interband optical transitions in coupled QDs.     

Magnetic and electronic properties of Cr- and Mn-doped SnO2: ab initio calculations

The ab initio calculations, based on the Korringa–Kohn–Rostoker (KKR) approximation method combined with the coherent potential approximation (CPA), indicated as KKR–CPA, have been used to study the stability of ferromagnetic and ferrimagnetic states, for systems that are SnO₂ doped and co-doped with two transition metals, that is, chromium and manganese. Our results indicate that the ferromagnetic state is more stable than the spin-glass state for the (Sn_1−xCr_xO₂; x = 0.07, 0.09, 0.12 and 0.15)-doped system, while the spin-glass state is more stable than the ferromagnetic state for the (Sn_1−xMn_xO₂; x = 0.02 and 0.05)-doped system. However, the ferromagnetic and/or the ferrimagnetic states are stable for the (Sn_0.98−xMn_0.02Cr_xO₂; x = 0.05, 0.09 and 0.13)-doped system depending on the Cr concentration. Moreover, we estimated the Curie temperature (T_c) for the Cr-doped tin dioxide (SnO₂), and we explained the origin of magnetic behaviour through the total density of states for different doped and co-doped SnO₂ systems.     

Analysis of the magnetic losses in iron-based soft magnetic composites with MgO insulation produced by sol-gel method

This work investigated the magnetic losses of heat treated iron-based soft magnetic composites with a thin MgO insulating layer produced by sol-gel method. The samples were characterized by energy dispersive X-ray spectroscopy, X-ray analysis and Fourier transform infrared spectroscopy. The results show that the surface of the powders contains a thin layer of MgO insulation. The loss results indicate that the hysteresis part for both the core loss and total loss factor was approximately the same for the MgO-insulated compacts and conventional SOMALOY^TM samples with phosphate insulation after annealing at 600 °C. But the MgO-insulated compacts exhibited significantly lower eddy current contribution of both core loss and total loss factor with respect to SOMALOY^TM samples after annealing. Also the contribution of eddy current in the iron particles for MgO insulated compacts (k_p=0.91) was noticeably higher than this contribution for SOMALOY^TM samples (k_p=0.18) after annealing due to the higher electrical resistivity of the MgO-insulated compacts.     

The Dynamic Resistance of CdS/CdSe/ZnS Co-Sensitized TiO2 Solar Cells

Quantum dots' sensitized solar cells (QDSSCs) can create the high-performance and low-cost photovoltaic in the future. In this study, we synthesized the film of TiO₂/CdS/CdSe/ZnS photoanodes by successive ionic layer adsorption reaction (SILAR) method. The absorption spectra, photoluminescent spectra and electrochemical impedance spectra (EIS) of the film TiO₂/CdS/CdSe/ZnS photoanodes show that the structure of energy levels in the conduction band (CB) of photoanode materials CdS, CdSe, and ZnS quantum dots (QDs) can absorb a great number of photons in each region and inject stimulated electrons quickly into the conduction band (CB) of TiO₂. Furthermore, we also studied the influence of the SILAR cycles on the dynamic resistance, the lifetime of electrons in QDSSCs through Nyquist and Bode.     

Electronic properties of orthorhombic LiGaS<Subscript>2</Subscript> and LiGaSe<Subscript>2</Subscript>

We report theoretical calculations of the band structure and density of states for orthorhombic LiGaS₂ (LGS) and LiGaSe₂ (LGSe). These calculations are based on the full potential linear augmented plane wave (FP-LAPW) method within a framework of density functional theory. Our calculations show that these crystals have similar band structures. The valence band maximum (VBM) and the conduction band minimum (CBM) are located at Γ, resulting in a direct energy band gap. The VBM is dominated by S/Se-p and Li-p states, while the CBM is dominated by Ga-s, S/Se-p and small contributions of Li-p and Ga-p. From the partial density of states we find that Li-p hybridizes with Li-s below the Fermi energy (E _F), while Li-s/p hybridizes with Ga-p below and above E _F. Also, we note that S/Se-p hybridizes with Ga-s below and above E _F.     

Preparation and purification of l-cysteine capped CdTe quantum dots and its self-recovery of degenerate fluorescence

l-cysteine capped CdTe quantum dots (QDs) were prepared in aqueous solution by a simple and efficient method, showing many advantages such as short synthesis period, the broaden range of starting pH value and the wide fluorescence emission wavelength range. A novel purification process was designed to remove excess Cd²⁺ which has potential cytotoxicity for bio-analysis. Three-dimensional fluorescence charts of pre- and post-purification showed that the purified QDs were of better luminescent performance. The prepared QDs were of cubic crystal structure with an average size of 2-6 nm, which were characterized by XRD and HRTEM. It is confirmed by IR spectra that the l-cysteine ligands were conjugated with CdTe cores via covalent bond. The degenerate fluorescence of QDs can be self-recovered in the presence of l-cysteine without other processing steps.     

Emission features of Ho ion in Nb2O5, Ta2O5 and La2O3 mixed Li2O-ZrO2-SiO2 glasses

Li₂O-ZrO₂-SiO₂: Ho³⁺ glasses mixed with three interesting d-block elemental oxides, viz., Nb₂O₅, Ta₂O₅ and La₂O₃, were prepared. Optical absorption and photoluminescence spectra of these glasses have been recorded at room temperature. The luminescence spectra of Nb₂O₅ and Ta₂O₅ mixed Li₂O-ZrO₂-SiO₂ glasses (free of Ho³⁺ ions) have also exhibited broad emission band in the blue region. This band is attributed to radiative recombination of self-trapped excitons (STEs) localized on substitutionally positioned octahedral Ta⁵⁺ and Nb⁵⁺ ions in the glass network. The Judd-Ofelt theory was successfully applied to characterize Ho³⁺ spectra of all the three glasses. From this theory various radiative properties, like transition probability A, branching ratio β_r and the radiative lifetime τ_r, for ⁵S₂ emission levels in the spectra of these glasses have been evaluated. The radiative lifetime for ⁵S₂ level of Ho³⁺ ions has also been measured and quantum efficiencies were estimated. Among the three glasses studied the La₂O₃ mixed glass exhibited the highest quantum efficiency. The reasons for such higher value have been discussed based on the relationship between the structural modifications taking place around the Ho³⁺ ions.     

第一原理研究Sn(O1-xNx)2材料的光电磁性质

基于第一原理的密度泛函理论, 以量子化学从头计算软件 为平台研究了Sn(O_1-xN_x)₂材料的光电磁性能, 分析了体系的态密度、 能带结构、 磁性、 介电虚部及折射率. 计算结果表明, N替代O后, 随着掺杂浓度的增加, 体系的带隙先减小后增大, 掺杂量为12.50%时带隙最窄. 由于N 2p轨道电子的贡献, 在0.55-1.05 eV范围内产生了浅受主能级, 价带和导带处的能级均出现了劈裂及轨道的重叠现象, Sn-O键的键强大于N-O键的键强. 从磁性来看, N原子决定了磁矩的大小. 从介电虚部可知, 掺杂后体系的光学吸收边增宽, 主跃迁峰发生红移, 反射率和介电谱相对应, 各峰值与电子的跃迁吸收有关.     

Size-dependent radiative emission of PbS quantum dots embedded in Nafion membrane

PbS quantum dots (QDs) have been incorporated in a Nafion membrane, where the QD sizes were adjusted by changing the reaction time due to the steady growing process. The radiative emissions of the samples were investigated by optical absorption, photoluminescence (PL), and time-resolved PL spectroscopy. Size-tunable emissions are shown by the PL spectrum in a range of 1.84–1.65 eV, and the emission mechanism was investigated based on a four-band envelope-function model. Possible energy transitions for the radiative emission are listed. The PL lifetime depending on the particle size is about one microsecond, and PL decay curves exhibit a trend of decreasing decay time with an increase of the PbS QD size.