Growth mechanism and thermoelectric properties of β-FeSi2 matrix with Si nanowires

A novel composite material consisted of β-FeSi₂ and in situ formed Si nanowires (SiNWs) with the diameter of 50-120 nm, is prepared by mechanical alloying and annealing process. The effects of ball-milling time on the formation of SiNWs are investigated systematically. The growth way of SiNWs is named as autocatalytic growth, in which no catalyst is intentionally added to the system. The possible formation mechanisms of SiNWs are discussed in detail. Due to the increase of boundary scattering, the thermal conductivity of the composite sample with SiNWs decrease significantly. Thus, the composite sample shows excellent thermoelectric performance.     

Photoabsorption properties of β-FeSi2 nanoislands grown on Si(111) and Si(001): Dependence on substrate orientation studied by nano-spectroscopic measurements

Photoabsorption properties of β-FeSi₂ nanoislands epitaxially grown on Si(111) and Si(001) have been discussed using photoabsorption nano-spectroscopy based on scanning tunneling microscope. The obtained spectra exhibit clear features around 0.86-0.91 eV and around 0.71-0.74 eV, which are explained as a direct and an indirect photoabsorption edge of β-FeSi₂, respectively. We also observed a blue shift of spectrum obtained from β-FeSi₂ nanoislands on Si(111) substrates, compared to those on Si(001) substrates. We attributed the dependence on Si-substrate orientation not to a quantum confinement effect but to an effect of elastic strain in the? β-FeSi₂ nanoislands epitaxially grown on the substrate.     

A room temperature solution-phase method to synthesize β-Ni(OH)2 nanoneedles

A simple room temperature solution-phase method has been employed to synthesize β-Ni(OH)₂ nanoneedles in the presence of a suitable cationic surfactant cetyltrimethylammonium bromide for the first time. The chemical composition and morphology of the as-prepared β-Ni(OH)₂ nanoneedles were characterized by X-ray powder diffraction and transmission electron microscopy. The effect of surfactant and NH₄OH on the morphology and size of β-Ni(OH)₂ nanoparticles was discussed in detail. The growth mechanism of the as-synthesized nanoneedles was discussed based on the experimental results. The results indicate that the binding between CTAB and β-Ni(OH)₂ crystal nuclei inhibits crystal growth randomly and favors needle-like single crystal growth with their preferred facets.     

Growth, structure and luminescence properties of multilayer Si/β-FeSi2NCs/Si/…/Si nanoheterostructures

Multilayer structures (up to 15 layers) with β-FeSi₂ nanocrystallites (NCs) buried in silicon crystalline lattice were grown by successive repetition of reactive deposition epitaxy (RDE) or solid phase epitaxy (SPE) of thin iron film on Si(100) or Si(111) substrates and silicon molecular beam epitaxy (MBE) (100-200 nm). Cross-section high resolution transmission electron microscopy (HR TEM) images and ex situ optical and Raman spectroscopy data prove that NCs formed in silicon matrix have the structure and optical properties of β-FeSi₂. The growth conditions provide no dislocations in silicon lattice were found in the course of TEM analysis. Two types of NCs depth distribution were observed: (i) layered that corresponds to iron RDE and (ii) uniform that occurs in the case of iron SPE. The uniform NCs distribution points out the fact that during a growth process NCs moves up to the surface. In spite of small nanocrystallites size (5-50 nm) and their distribution in silicon cap layers the significant photoluminescence (PL) signal at 0.8 eV was observed for all grown samples.     

SiO2/Si(111)表面Ge量子点的生长研究

Si衬底用化学方法清洗后,表面大约残余1.0 nm厚SiO2薄膜.利用原子力显微镜(AFM)和反射高能电子衍射(RHEED)来研究温度和Ge蒸发厚度对在SiO2薄膜表面生长的Ge量子点的影响.实验结果表明,当衬底温度超过500 ℃时,SiO2开始与Ge原子发生化学反应,并形成与Si(111)表面直接外延的Ge量子点.在650 ℃时,只有Ge的厚度达到0.5nm时,Ge量子点才开始形成.     

Effect of the doped nitrogen on the optical properties of β-Ga2O3 nanowires

In this study, optical properties of the nitrogen-doped β-Ga₂O₃ nanowires (N-doped β-Ga₂O₃ NWs) were synthesized by exposing β-Ga₂O₃ NWs under high input power nitrogen plasma (2 kW), using a microwave plasma enhanced chemical vapor deposition (MPECVD) system. The nitrogen contents in the NWs were as-prepared about 7.4, 8.9, 9.7, 13.9, 19.3, and 26.6 at.%, respectively. Low temperature (10 K) cathodoluminescence (CL) spectra exhibit significantly different optical properties for the different nitrogen contents. The CL result of the N-doped β-Ga₂O₃ NWs (210 s N₂ plasma treatment) exhibited four distinct emission peaks at 378, 516, 759, and 970 nm. The possible light emission mechanism including the effect of the nitrogen dopant was discussed.     

Effect of Si/Fe ratio on the boron and phosphorus doping efficiency of β-FeSi2 by magnetron sputtering

Boron-doped or phosphorus-doped β-FeSi₂ thin films have been prepared on silicon substrate by magnetron sputtering. Effects of Si/Fe ratio on the boron and phosphorus doping efficiencies have been studied from the resistivities of doped β-FeSi₂ thin films and current-voltage characteristics of doped β-FeSi₂/Si heterojunctions. The experimental results reveal that the carrier concentration and doping efficiency of boron or phosphorus dopants at the Fe-rich side are higher than that at the Si-rich side. The effect of Si/Fe ratio can be deduced from the comparison of the formation energies under two extreme conditions. At the Fe-rich limit condition, the formation energy of boron or phosphorous doping is lower than that at the Si-rich condition. Therefore, the activation of impurities is more effective at the Fe-rich side. These results demonstrate that the boron-doped and phosphorous-doped β-FeSi₂ thin films should be kept at the Fe-rich side to avoid the unexpected doping sites and low doping efficiency.     

Metalorganic chemical vapor deposition of β-FeSi2 on β-FeSi2 seed crystals formed on Si substrates

We have fabricated a β-FeSi₂ film by metalorganic chemical vapor deposition on a Si(001) substrate with β-FeSi₂ seed crystals grown by molecular beam epitaxy, and investigated the crystallinity, surface morphology and temperature dependence of photoresponse properties of the β-FeSi₂ film. The surface of the grown β-FeSi₂ film was atomically flat, and step-and-terrace structure was clearly observed. Multi-domain structure of β-FeSi₂ whose average size was approximately 200 nm however was revealed. The photoresponse was obtained in an infrared light region (~ 0.95 eV) at temperatures below 200 K. The external quantum efficiency reached a maximum, being as large as 25% at 100 K when a bias voltage was 2.0 V.     

Thermal analysis and mechanism of α-Zn2SiO4:Mn formation from zinc oxalate dihydrate under hydrothermal conditions

Reaction of zinc oxalate dihydrate in high-temperature water at 200–400 °C was investigated to understand the mechanism of α-Zn₂SiO₄:Mn²⁺ formation and specifically why zinc oxalate dihydrate forms α-Zn₂SiO₄ crystalline phase at a higher rate compared with other water soluble and insoluble zinc sources studied in our previous works. From experiments under hydrothermal conditions, it was determined that the dehydration of zinc oxalate dihydrate became suppressed as the temperature and pressure increased. From thermal analyses, zinc oxalate dihydrate, which dehydrates at 160 °C in air, was stable at conditions up to 300 °C in water and this resulted in the lack of formation of the intermediate phase, hemimorphite, which forms at 150–175 °C from other zinc compounds (e.g. ZnO). The stability of the zinc oxalate dihydrate under hydrothermal conditions can possibly be attributed to changes in the chemical equilibrium with temperature and pressure which leads to the compound having less reactivity with silica so that reaction paths to form hemimorphite and β-Zn₂SiO₄ become unfavorable. Zinc oxalate dihydrate as starting material provided α-Zn₂SiO₄ at a higher formation rate than other starting materials.     

Study on crystallization behavior and microstructure of melt-spun Nd2(Fe,Nb)14B/α-Fe alloys

The effects of Nb addition on the crystallization behavior, microstructure, and magnetic properties of melt-spun Nd₂Fe₁₄B/α-Fe nanocomposite magnets have been studied. The addition of Nb can significantly improve the thermal stability of amorphous phase in as-spun alloys, narrow the range between the onset crystallization temperature and the optimal annealing temperature, restrain the initial formation and growth of α-Fe and Nd₂Fe₁₄B. A finer and more homogeneous microstructure can be obtained in the Nb-doped alloy than in the Nb-free alloy. And the Nb addition makes the grains more equiaxed shape. The Nd₁₀Fe₈₃Nb₁B₆ alloy annealed at 715 °C for 10 min exhibits the improved magnetic properties, B _r = 0.90 T, _i H _c = 750 kA/m, (BH)_max = 120 kJ/m³, the intrinsic coercivity and the maximum energy product increase by 25% and 14%, compared with the Nd₁₀Fe₈₄B₆ alloy.     

Growth of uniform lath-like α-(Fe,Al)OOH and disc-like α-(Fe,Al)2O3 nanoparticles in a highly alkaline medium

The effects of aluminium (Al³⁺)-dopant on the precipitation of uniform lath-like α-FeOOH particles, the obtention and growth of α-(Fe,Al)OOH and α-(Fe,Al)₂O₃ solid solutions, particle size and shape were investigated using X-ray powder diffraction, Mössbauer and Fourier transform infrared spectroscopies, field emission scanning electron microscopy and energy dispersive X-ray spectroscopy. Acicular α-FeOOH particles, precipitated in a highly alkaline medium with the addition of tetramethylammonium hydroxide (TMAH), were used as reference material. The influence of Al-dopant was investigated by adding varying amounts of Al³⁺ ions to the initial FeCl₃ solution. In the presence of lower concentrations of aluminium ions (up to 11.11 mol%) α-(Fe,Al)OOH as a single phase was formed, whereas higher concentrations led to an additional obtention and growth of α-(Fe,Al)₂O₃. Al-for-Fe substitution in the α-FeOOH and α-Fe₂O₃ structures was confirmed by a decrease in the unit-cell dimensions, a decrease in the hyperfine magnetic field and an increase in the wave number of the infrared absorption bands. The presence of lower concentrations of aluminium ions (up to 11.11 mol%) in the precipitation system did not affect the size and shape of the α-(Fe,Al)OOH particles, whereas higher concentrations influenced a decrease in the length and aspect ratio. In the presence of 42.86 mol% Al³⁺ ions fairly uniform disc-shaped α-(Fe,Al)₂O₃ were formed.     

Magnetic susceptibility of (Bi2 ? xSbx)Te3 (0 < x < 1) crystals from 2 to 400 K

The magnetic susceptibility (χ) of crystals of (Bi_{2 − x} Sb _x )Te₃ (0 < x < 1) solid solutions has been measured at temperatures from 2 to 400 K. The χ of the crystals containing 10 and 25 mol % Sb₂Te₃ increases with temperature in the range 50 to 220 K, where the Hall coefficient of Bi₂Te₃ increases anomalously. The increase in diamagnetic susceptibility and Hall coefficient with temperature is shown to be caused by a reduction in light-hole concentration, accompanied by a decrease in light-hole effective mass. With increasing Sb₂Te₃ content, the shape of the χ(T) curve changes as a consequence of changes in band structure, which increase the influence of heavy, paramagnetic holes.     

Crystal structure of uranyl μ3-oxoacetate [(UO2)3(μ3-O)2(OOCCH3)2(H2O)2]

The crystal structure of [(UO₂)₃(₃-O)₂(OOCCH₃)₂(H₂O)₂] consists of uranium coordination polyhedra (CPs) combined via common equatorial edges into ribbons, with ₃-O atoms being common for three CPs. The structure was compared with that of known complexes with ₃-oxo atoms.Translated from Radiokhimiya, Vol. 46, No. 5, 2004, pp. 396–400.Original Russian Text Copyright © 2004 by Charushnikova, Krot, Starikova.     

Synthesis and phases evolution of Si–C–Ti from polycarbosilane (PCS) and metal Ti

Si–C–Ti ceramics were synthesized by reactive pyrolysis of polycarbosilane (PCS) precursor filled with metal Ti powder. Pyrolysis of mixture with atomic ratio of Ti:Si through 3:1–3:2 was carried out in argon atmosphere at given temperature up to 1500 °C. The metal–precursor reactions, and phase evolution were studied using X-ray diffraction and scanning electron microscopy with EDX. The Ti₃SiC₂ phase was obtained firstly from reaction of PCS and Ti. Ti₃SiC₂ formation starts at 1300 °C and its amount increases significantly in a narrow temperature range between 1400 °C and 1500 °C. In addition, addition of CaF₂ can promote the formation of Ti₃SiC₂ phase.     

Effects of Gallium on the Structure and Electrical Properties of 0.65 Bi0.94La0.06) (GaxFe1?x)O3-0.35PbTiO3 Ceramics

Crystalline solutions of 0.65 (Bi_0.94La_0.06) (Ga_xFe_1-x)O₃-0.35PbTiO₃ ceramics (BLGF-PT) for x = 0 and 0.05 have been fabricated by the solid-state reaction method. X-ray diffraction (XRD) was utilized to characterize the crystal structure and examine any possible impurities existing in the ceramics. The effects of Ga substitution on dielectric properties of the samples were studied at frequencies from 10² to 10⁶ Hz over a temperature range from 20 to 620degC. The results indicate that Ga modification can reduce the room temperature dielectric loss. The conduction mechanism of the material was investigated using ac conductivity. It is concluded that electrons originating from Fe²⁺ and oxygen ion vacancies are the main charge carriers, and Ga doping could decrease the electronic conduction effectively. The frequency dependence of ac conductivity was found to follow Jonscher's universal power law.     

Synthesis and photoluminescence properties of Eu-doped γ-Ca3(PO4)2

Eu³⁺-doped (1% and 3%) γ-Ca₃(PO₄)₂ was synthesized by high-pressure and high-temperature experimental method and the samples were characterized by X-ray diffraction. The luminescence properties of samples were investigated by emission and excitation spectra. The excitation spectra of Eu³⁺-doped γ-Ca₃(PO₄)₂ showed that samples were mainly attributed to Eu³⁺–O²⁻ charge-transfer band at 270 nm, and some sharp lines were also attributed to Eu³⁺ f–f transitions in near-UV regions with the strongest peaks at 395 nm. Under the 395 nm excitation, the intense red emission peak at 611 nm was observed. The strongest line (395 nm) in excitation spectra of those phosphors matched well with the output wavelength of UV InGaN-based light-emitting diodes (LEDs) chip. The luminescent properties suggested that Eu³⁺-doped γ-Ca₃(PO₄)₂ might be regarded as a potential red phosphor candidate for near-UV LEDs.     

β-Cyclodextrin-assisted preparation of hierarchical walnut-like CeOHCO3 and CeO2 mesocrystals

The hierarchical walnut-like CeOHCO₃ mesocrystals were prepared by a facile hydrothermal method under low temperature with β-cyclodextrin (β-CD) as assistant agent. The hierarchical walnut-like CeO₂ mesocrystals were obtained by thermal decomposition of CeOHCO₃ mesocrystals. The crystal phase, morphology, and structure of CeOHCO₃ and CeO₂ mesocrystals were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and selected area electron diffraction (SAED). The time-dependent experimental results indicated that the morphology transformation from shuttle-like to walnut-like and the crystal phase transformation from orthorhombic to hexagonal simultaneously occurred in the formation processes of CeOHCO₃ mesocrystals. On the basis of the morphological and crystal phase evolution processes, the formation mechanism of hierarchical walnut-like CeOHCO₃ mesocrystals, including dissolution-recrystallization processes, was discussed. β-CD was believed to play an important role in the formation of the hierarchical walnut-like CeOHCO₃ mesocrystals. The effects of reaction temperature, β-CD amount, and concentration of reactants on the morphologies of the products were systematically studied. CeO₂ mesocrystals exhibited the distinct red-shift phenomenon in UV-vis absorption spectra.     

Valence band offset at GaN/β-Si3N4 and β-Si3N4/Si(111) heterojunctions formed by plasma-assisted molecular beam epitaxy

Ultra thin films of pure β-Si₃N₄ (0001) were grown on Si (111) surface by exposing the surface to radio- frequency nitrogen plasma with a high content of nitrogen atoms. Using β-Si₃N₄ layer as a buffer layer, GaN epilayers were grown on Si (111) substrate by plasma-assisted molecular beam epitaxy. The valence band offset (VBO) of GaN/β-Si₃N₄/Si heterojunctions is determined by X-ray photoemission spectroscopy. The VBO at the β-Si3N4 / Si interface was determined by valence-band photoelectron spectra to be 1.84 eV. The valence band of GaN is found to be 0.41 ± 0.05 eV below that of β-Si₃N₄ and a type-II heterojunction. The conduction band offset was deduced to be ~ 2.36 eV, and a change of the interface dipole of 1.29 eV was observed for GaN/β-Si₃N₄ interface formation.     

Novel upconversion phenomenon of Nd sensitized by Yb in Nd-Yb-co-doped β-Na(Y1.5Na0.5)F6

Hexagonal phase Na(Y_1.5Na_0.5)F₆ co-doped with Yb³⁺ and Nd³⁺ was synthesized and its structure was studied by XRD and Raman methods. The Raman scattering spectrum illustrates it is a host with low phonon energy. Emissions in green (⁴G_7/2 → ⁴I_9/2), yellow (⁴G_7/2 → ⁴I_11/2), red (⁴G_7/2 → ⁴I_13/2), and infrared (⁴F_5/2 → ⁴I_9/2, ⁴F_3/2 → ⁴I_9/2) were observed and were the direct result of emissions of the Nd³⁺ ion. The luminescence intensity ratio (I₈₆₃/I₈₀₄) gradually decreased with the increase of excitation power. The long lifetime of 863 nm infrared emission was proved. The upconversion mechanism was deduced to be energy transfer from the Yb³⁺ to Nd³⁺ under 980 nm excitation, by analyzing the energy level structures of Nd³⁺ and Yb³⁺ ions and measuring the power dependence of upconverted emission intensities.