Size-dependent ultrafast magnetization dynamics in iron oxide (Fe3O4) nanocrystals

Optically induced ultrafast demagnetization and its recovery in superparamagnetic colloidal iron oxide (Fe3O4) nanocrystals have been investigated via time-resolved Faraday rotation measurements. Optical excitation with near-infrared laser pulse resulted in ultrafast demagnetization in approximately 100 fs via the destruction of ferrimagnetic ordering. The degree of demagnetization increased with the excitation density, and the complete demagnetization reached at approximately 10% excitation density. The magnetization recovered on two time scales, several picoseconds and hundreds of picoseconds, which can be associated with the initial reestablishment of the ferrimagnetic ordering and the electronic relaxation back to the ground state, respectively. The amplitude of the slower recovery component increased with the size of the nanocrystals, suggesting the size-dependent ferrimagnetic ordering throughout the volume of the nanocrystal.     

氨化Si基Ga2O3/In2O3制备GaN薄膜

研究了Ga2O3/In2O3 膜反应自组装制备GaN薄膜,再将Ga2O3/In2O3膜在高纯氨气气氛中氨化反应得到GaN薄膜,用X射线衍射 (XRD),傅里叶红外吸收(FTIR),扫描电镜(SEM)和透射电镜(TEM)对样品进行结构、形貌的分析.测试结果表明,用此方法得到了六方纤锌矿结构的GaN多晶膜,且900℃时成膜的质量最好.     

Magnetic properties of gamma-Fe2O3/poly(ether-ester) nanocomposites

The magnetic properties of gamma-Fe2O3 nanoparticles embedded in a thermoplastic elastomer poly(ether-ester) copolymer by the in situ polycondensation reaction process have been investigated by means of magnetization and ferromagnetic resonance (FMR) measurements at low filler concentrations of 0.1 and 0.3 wt% with the magnetic additive introduced in the polymer matrix in powder and solution form. The magnetic behavior of the magnetopolymeric nanocomposites indicates significant interparticle interaction effects that depend mainly on the dispersion state of the magnetic nanoparticles as well as their concentration, consistent with the variation of the particle microstructure characterized by magnetic aggregates in the nanometer and micron scale for the solution and powder dispersions, respectively. The magnetization and FMR results at different filler concentrations and dispersions show a close correspondence to the relaxation processes of the copolymer, implying the coupling of polymeric and magnetic properties.     

Differences in electrophysical and gas sensing properties of flame spray synthesized Fe2O3(gamma-Fe2O3 and alpha-Fe2O3)

Nanoscaled Fe2O3 powders as candidates for gas sensing material for hydrogen detection were synthesized by the high temperature flame spray assisted combustion of ferrocene dissolved in benzene. X-ray diffraction (XRD) and selected area electron diffraction (SAED) show that the as prepared nanopowder consists of maghemite (gamma-Fe2O3) with low crystallinity. Thermal post-treatment causes a phase transformation towards hematite (alpha-Fe2O3) accompanied by an increase in the crystallinity. Upon exposure to air and hydrogen at elevated temperatures, both phases show a significant variation of conductivity and activation energy-as evidenced by impedance spectra-and thus a favorable sensor response, surpassing even that of flame-synthesized nanocrystalline tin dioxide. The conductivity has been identified as of electronic origin, affected by trap states located in the region adjacent to grain boundaries. Quantitative analysis of the impedance spectra with equivalent circuits shows that the conductivity is thermally activated and affected by the interaction of hydrogen with the sensor material. The calculated Debye screening length of gamma-Fe2O3 and alpha-Fe2O3 is about 27 nm and 16 nm, respectively, what contributes significantly to the sensitivity of the material. Gamma-Fe2O3 and alpha-Fe2O3 exhibit high sensor response towards hydrogen in a wide concentration range. Gamma-Fe2O3 shows n-type semiconducting behavior up to 573 K. Alpha-Fe2O3 shows p-type semiconducting behavior, as reflected in the dynamic changes of the resistivity. For both sensor materials, 523 K was the optimal operating temperature.     

Growth mechanism and characterization of chemical oxide films produced in peroxide mixtures on Si(100) surfaces

The growth kinetics of silicon chemical oxides in H₂O₂-containing solutions at various pH values and temperatures was studied by electrochemical impedance spectroscopy (EIS), ellipsometry and X-ray photoelectron spectroscopy. Infrared (IR) spectroscopy was also used to investigate the evolution of the surface chemistry from the initial hydrogen coverage as Si-H bonds to the subsequent oxidation states by analysing the Si-O-Si stretching vibration modes. Successive EIS diagrams obtained as a function of time constituted a series of semicircles indicating that the semiconductor/oxide/electrolyte junction can be modelled as a resistance-capacity (RC) circuit. It was then observed that the resistance term increased with time to almost 1 MΩ cm² after 3 h in SC1 solution. It is generally known that, in alkaline solutions such as SC1, the oxidation rate reaches rapidly a steady regime controlled by the interfacial charge transfer reaction and the subsequent dissolution of the generated oxide. Accordingly, a mechanism involving a diffusion process of reactants through the oxide barrier followed by a simultaneous dissolution of the layer is proposed. Likewise, in acidic media such as SC2, even though the solubility is extremely low, we have extended our model based on the competition between the oxidation rate at the surface and the dissolution of the built-up oxide, and thus it was possible to evidence oxide solubility at the nanoscopic scale. In addition, thickness measurements by ellipsometry together with the observed gradual change in the IR spectrum of the Si-O-Si vibration mode were interesting parameters revealing that the oxide growth proceeds simultaneously with an evolution of the structure leading to a more compact and insulating dielectric layer. The whole of the results lead to a model for the oxidation process accounting for the observed structure and complex impedance properties under various conditions of chemical treatment.     

Photodegradation of 2-mercaptobenzothiazole in the gamma-Fe(2)O(3)/oxalate suspension under UVA light irradiation

The aim of this study is to investigate the effect of various factors on the photodegradation of organic pollutants in natural environment with co-existence of iron oxides and oxalic acid. 2-Mercaptobenzothiazole (MBT) was selected as a model pollutant, while gamma-Fe(2)O(3) was selected as iron oxide. The crystal structure and morphology of the prepared gamma-Fe(2)O(3) was determined by X-ray diffractograms (XRD) and scanning electron microscopy (SEM), respectively. The specific surface area was 14.36 m(2)/g by Brunauer-Emmett-Teller (BET) method. The adsorption behavior of gamma-Fe(2)O(3) was evaluated by Langmuir model. The effect of the dosage of iron oxide, initial concentration of oxalic acid (C(ox)(0)), initial pH value, the light intensity and additional transition metal cations on MBT photodegradation was investigated in the gamma-Fe(2)O(3)/oxalate suspension under UVA light irradiation. The optimal gamma-Fe(2)O(3) dosage was 0.4 g/L and the optimal C(ox)(0) was 0.8 mM with the UVA light intensity of 1800 mW/cm(2). And the optimal dosage of gamma-Fe(2)O(3) and C(ox)(0) for MBT degradation also depended strongly on the light intensity. The optimal gamma-Fe(2)O(3) dosage was 0.1, 0.25 and 0.4 g/L, and the optimal C(ox)(0) was 1.0, 0.8, and 0.8 mM with the light intensity of 600, 1200 and 1800 mW/cm(2), respectively. The optimal initial pH value was at 3.0. The additional transition metal cations including Cu(2+), Ni(2+) or Mn(2+) could significantly accelerate MBT degradation. This investigation will give a new insight to understanding the MBT photodegradation in natural environment.     

Boron ions B interaction with Si(100) and Ge(100) surfaces

The goal of this work is simulation of possible structures, formed by boron ions (B⁺) during adsorption on Si(100) and Ge(100) surfaces. Calculations were carried out using a semi-empirical method, known as the Modified Neglect of Differential Overlap method (MNDO). The surface was simulated using of Si₄₉(Ge₄₉) and Si₆₃(Ge₆₃) clusters. Results of quantum-chemical calculations the boron ion (B⁺) interaction with Si(100) − 2 × 1 and Ge(100) − 2 × 1 surfaces are presented and show adsorption barriers for boron ions and migration barriers of adsorbed boron ion (during migration along surface dimer row and along surface dimer).     

Crystalline structures and misfit strain inside Er silicide nanocrystals self-assembled on Si(001) substrates

The morphology and crystalline structure of Er silicide nanocrystals self-assembled on the Si(001) substrate were investigated using scanning tunneling microscopy (STM) and transmission electron microscopy (TEM). It was found that the nanowires and nanorods formed at 630?°C has dominant hexagonal AlB(2)-type structure, while inside the nanoislands self-organized at 800?°C the tetragonal ThSi(2)-type structure is prevalent. The lattice analysis via cross-sectional high-resolution TEM demonstrated that internal misfit strain plays an important role in controlling the growth of nanocrystals. With the relaxation of strain, the nanoislands could evolve from a pyramid-like shape into a truncated-hut-like shape.     

Template-free synthesis of self-assembled Co3O4 micro/nanocrystals

In this article, we have reported the fabrication of various morphological porous Co3O4 by thermal decomposition of cobalt oxalate at open atmospheric conditions. Uniform cobalt oxalate microrods and microneedles were synthesized without using any surfactants or templates in large scale. The cobalt oxalate preparation method was played a crucial role on the crystal structure and its morphology. The as prepared cobalt oxalates and its corresponding cobalt oxides were characterized by using the thermogravimetric analysis, X-ray diffraction (XRD), field-emission scanning electron microscope (FE-SEM), transmission electron microscopy (TEM) and nitrogen adsorption analysis. The XRD results indicated that the orthorhombic and monoclinic cobalt oxalates were formed in different experimental conditions. The influence of preparation method of cobalt oxalates and cobalt precursors on the final morphology has been investigated. The M-H loop of the Co3O4 porous microrods and microneedles showed the presence of paramagnetic properties at room temperatures. A plausible mechanism of both cobalt oxalates and Co3O4 formation was proposed based on the experimental results.     

Growth kinetics and thermodynamic stability of octadecyltrichlorosilane self-assembled monolayer on Si (100) substrate

We have studied the growth kinetics and thermodynamic stability of octadecyltrichlorosilane (OTS) self-assembled monolayers on Si (100) substrate in order to understand its role in controlling the adhesion and surface hydrophobicity. Time-dependent contact angle measurements, using water as a function of OTS concentration, show rapid monolayer formation in the initial stage followed by a slow attainment of full coverage and the overall kinetics approximately follows the Langmuir adsorption isotherm. The adsorption rate constant (k_a = 150 M^− 1 s^− 1) is found to be significantly greater than the desorption rate constant (k_d = 0.156 s^− 1) while the Gibbs free energy (ΔG_ads) change amounts to − 4.2 kcal/mol suggesting thermodynamic stability of OTS monolayer on a silicon surface. Partial monolayer formation by a ‘uniform’ growth mechanism, even at low coverage, is revealed by atomic force microscopy (AFM) in conjunction with grazing angle FTIR spectroscopy. Analysis of the interfacial adhesion properties using Zisman plot suggests a critical surface tension (γ_c) of 20.7 dyn/cm for OTS monolayer on Si (100) surface.     

Crystal shape of GaAs nanocrystals deposited on Si(100) by molecular beam epitaxy

The shape changes in GaAs nanocrystals deposited on Si substrate have been studied as a function of the coverage by transmission electron microscopic observations in order to see the growth mechanism from the viewpoint of the surface energy and the lattice strain energy between the nanocrystal and the substrate. When GaAs was deposited on the Si(100) surface, the shape of the GaAs nanocrystals changes from stepped mound, hut cluster to dome structure with increasing the coverage. The shape changes are responsible for decreasing the total free energy caused by the lattice strain energy with the substrate and surface energy depending on the crystal size.     

GISAXS study of Si nanocrystals formation in SiO2 thin films

We present a study on amorphous SiO/SiO₂ superlattice using grazing incidence small-angle X-ray scattering (GISAXS). Amorphous SiO/SiO₂ superlattices were prepared by high vacuum evaporation of 3 nm thin films of SiO and SiO₂ (10 layers each) on Si(100) substrate. After the evaporation, samples were annealed at 1100 °C for 1 h in vacuum, yielding Si nanocrystals formation. Using a Guinier approximation, the shape and the size of the crystals were obtained.     

Growth and characterisation of SiC films deposited on a-SiO2/Si (100) substrates

Abstract

The growth of polycrystalline SiC films has been carried out by low pressure chemical vapour deposition in a horizontal quartz reaction chamber using tetramethylsilane and H2 as the precursor gas mixture. Silicon (100) wafers were used as substrates. A thin Si O₂ amorphous layer of ~6 nm was formed before SiC deposition to reduce the strain induced by the 8% difference in thermal expansion coefficients between SiC and Si. Samples were. analysed by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and infrared reflectivity. The structure of films grown at temperatures between 950 and 1150°C varies from amorphous to polycrystalline SiC. Preferential [111] orientation and columnar growth of polycrystalline films develops with increasing temperature.

MST/3317     

Preparation of epitaxial Y1-xHoxBa2Cu3O7-δ high Tc films on (100)Zr(Y)O2 substrates

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

镍酸镧导电薄膜的热处理制度与电性能研究

研究了镍酸镧(分子式LaNiO3,简称LNO)导电薄膜的制备,及其与性能之间的关系。实验结果表明热处理制度和热处理过程中通氧是LNO薄膜获得良好电性能的关键。实验发现LNO薄膜在退火温度5500C时就已经晶化,并且随着退火温度的升高,薄膜电性能也随之提高,但是当温度高于860℃后,LNO薄膜发生了结构相变并伴有NiO的析出,因此导致了薄膜电阻率的升高。较短的预处理时间和较长的退火处理时间有助于薄膜晶粒的长大,从而提高薄膜的电性能。同时也讨论了LNO薄膜电阻率和工作温度的关系。     

Dissociation of dislocations in kyanite: (Al2 O3  Si O2)

A study of the crystal structure of kyanite allows us to predict that there is one possible burgers vector $\text{c}\text{?}\text{= 5,57}\text{A}\text{?}$ and only one easy glide plane (100), because only in that plane the gliding dislocations do not cut neither the strong Si-O bonds nor the chains of Al O₆ octaedra.TEM observations confirm these hypotheses. Moreover the dislocations are widely dissociated in two colinear partials. The following reaction $\text{c}\text{?}\text{→}\text{1}\text{2}\text{c}\text{?}\text{+}\text{1}\text{2}\text{c}\text{?}$ preserves the anionic lattice as well as the chain structure.     

Growth and properties of thin Ag films on Pt(100) surfaces

Adsorption of Ag on Pt(100) surfaces including the multilayer regime has been studied using the low energy electron diffraction, X-ray photoelectron spectroscopy, ultraviolet electron spectroscopy, and thermal desorption spectroscopy methods. Deposition of Ag sub-monolayers on Pt(100) below 300 K leads to the formation of three-dimensional crystallites, which decompose into a pseudomorphic layer upon annealing above ≈ 350 K. Deposition of Ag multilayers proceeds along the same growth mode. Adsorbed Ag lifts the hex reconstruction of the Pt(100) surface. Up to an Ag coverage of ≈ 0.8, Ag desorbs according to a zero-order desorption rate law, E_des = 322 kJ mol⁻¹. This desorption order is interpreted by the presence of a mixed Pt/Ag layer on top of the square Pt(100) substrate into which all excess Pt atoms from the hex surface are expelled upon lifting of the reconstruction. Therefore, Ag desorption reflects the decomposition of a monolayer-confined Ag-Pt alloy. Between Ag coverages of Θ ≈ 0.7 and Θ = 1, first-order desorption is observed, while above Θ = 1 zero-order desorption is again observed (E_des = 288 kJ mol⁻¹) A 760 K annealed Ag monolayer suffices to suppress CO adsorption. XP spectra reveal a binding energy shift of the Ag 3d_5/2 level from 367.5 eV (monolayer) to 368.1 eV (thick multilayer). In UP spectra an Ag/Pt(100) interface state at 1.5 eV below the Fermi level is observed, which is interpreted to stem from Pt 5d electrons. Low-temperature deposited Ag sub-monolayers exhibit in UP spectra a broad 4d band structure, which collapses upon annealing into two extremely narrow Ag 4d emission peaks, ΔE ≈ 0.2 eV, at 4.6 eV and 4.8 eV. They are interpreted as emission from chain-like structures of Ag atoms in the mixed Pt/Ag layer structures which are formed in the annealing process.