Size-dependence of relaxation in random-field Ising model

Monte Carlo simulations were made of two- and three-dimensional Ising models containing up to 150³ and 1395² sites in a randomly oriented field ±H. A spontaneous magnetization was observed to build up at low enough temperatures, even if initially the spins were disordered. The time required for this build-up was observed to increase with some power of the system size for short times, and possibly exponentially with the square root of the length for larger times.Dedicated to B. Mühlschlegel on the occasion of his 60th birthday     

Intervalley-scattering-induced electron-phonon energy relaxation in many-valley semiconductors at low temperatures

We report on the effect of elastic intervalley scattering on the energy transport between electrons and phonons in many-valley semiconductors. We derive a general expression for the electron-phonon energy flow rate at the limit where elastic intervalley scattering dominates over diffusion. Electron heating experiments on doped n-type Si samples with electron concentrations (3.5-16.0) x 10(25) m(-3) are performed at sub-Kelvin temperatures. We find a good agreement between the theory and the experiment.     

Controlled synthesis of cuprous oxide nanospheres and copper sulfide hollow nanospheres

Uniform Cu₂O nanospheres have been successfully synthesized by reducing CuSO₄ with ascorbic acid in sucrose solution at room temperature. The diameter of the Cu₂O nanospheres can be tuned from 90 to 280 nm by adding different amounts of sucrose in the solution. Furthermore, CuS hollow nanospheres with different diameters have been obtained based on the Kirkendall effect using the as-prepared Cu₂O nanospheres as sacrificial templates. Cu₂O/Cu_7.2S₄ core/shell nanospheres and Cu_7.2S₄ hollow nanospheres are obtained as the intermediate products at different stages of the conversion process. Through the post-treatment of sodium citrate solution, Cu_7.2S₄ hollow nanospheres can be changed into CuS hollow nanospheres. The products are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and field-emission scanning electron microscopy (FESEM). Optical properties of the products have also been studied.     

脉冲激光烧蚀中电声弛豫时间的确定

为了提高脉冲激光制备薄膜的质量,准确掌握电声弛豫时间是关键,它对脉冲激光脉宽和能量密度的选取起着决定性的作用. 文中以铝靶材为例,利用经典的双温方程通过时域有限差分法(FDTD)得到电子、离子亚系统的温度随时间和位置演化的图像,进而得到电声弛豫时间的准确值. 这样便能准确划分热烧蚀和非平衡烧蚀,从而更好地控制激光的烧蚀过程. 同时找出了电声弛豫时间随激光脉宽以及能量密度变化的规律. 关键词： 飞秒激光 电声弛豫时间 双温方程 激光能量密度     

Synthesis of hollow ferrite nanospheres for biomedical applications

Hollow ferrite spheres of 220-340 nm diameter were synthesized at 60 °C as multi-functionalized magnetic carriers which are potentially applicable both as drug delivery systems (DDS) and hyperthermia treatment. We found that SH and OH groups on the silica template spheres enabled the fabrication of continuous ferrite shells of 20-30 nm in thickness. Transmission electron microscopy and energy-dispersive spectroscopy revealed that the templates were dissolved by a NaOH solution, yielding hollow particles exhibiting saturation magnetization of 78 emu/g. The results suggested that the ferrite shells are porous and the pores work as pathway for releasing drugs from the hollow particle inside.     

The tilt effect in the electron-phonon interaction with finite relaxation times of the electrons

A theoretical study of the tilt effect is presented namely the dependence of the longitudinal sound absorption and velocity on the angle between the magnetic field and the propagation vector. As has been shown, the tilt effect is a strong nonadiabatic effect which does not contain the small parameter s/v. The influence of the finite electron relaxation time on the singularities of the absorption and of the sound velocity is investigated. The tilt effect proves to be well-distinguishable if the condition ωτ > 1 is satisfied.     

SERS enhancement of gold nanospheres of defined size

Monodisperse, citrate‐stabilized gold nanoparticles of sizes ranging from 15 to 40 nm were synthesized and characterized by small angle X‐ray scattering and UV‐vis experiments. Identical surface properties of nanoparticles of different sizes to avoid variation in the chemical surface‐enhanced Raman scattering (SERS) enhancement, as well as selection of experimental conditions so that no aggregation took place, enabled the investigation of enhancement of individual nanospheres. Enhancement factors (EFs) for SERS were determined using the dye crystal violet (CV). EFs for individual gold nanospheres ranged from 10² to 10³, in agreement with theoretical predictions. An increase of the EFs of individual spheres with size can be correlated to changes in the extinction spectra of nanoparticle solutions. This confirms that the increase in enhancement with increasing size results from an increase in electromagnetic enhancement. Beyond this dependence of EFs of isolated gold spheres on their size, EFs were shown to vary with analyte concentration as a result of analyte‐induced aggregation. This has implications for the application of nanoparticle solutions as SERS substrates in quantitative analytical tasks. Copyright © 2011 John Wiley & Sons, Ltd.     

Synthesis and characterizations of hollow spheres and nanospheres of Au

Hollow spheres and nanospheres of Au have been prepared by a simple reaction of HAuCl₄·4H₂O, NaOH and (NH₂OH)₂·H₂SO₄ in the presence of gelatin. The role of gelatin and the effect of the temperature of the reaction in producing the spherical particles of Au are discussed. The products were characterized by powder X-ray diffraction (XRD), transmission electron microscopy and UV–Vis absorption spectroscopy. The sizes of the nanospheres of Au were estimated by the Debye–Scherrer formula according to the XRD spectrum. PACS  81.05.Bx; 81.05.Rm; 81.07.-b; 81.16.Be; 81.20.Fw     

Synthesis and formaldehyde sensing performance of LaFeO3 hollow nanospheres

Metal oxide semiconductors with hollow structure and morphology have attracted considerable attentions because of their promising application on gas sensors. In this paper, LaFeO₃ hollow nanospheres have been prepared by using carbon spheres as templates in combination with calcination. Based on the observation of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), and transmission electron microscope (TEM), the structure and morphology of the products were characterized. It has been revealed that as-prepared LaFeO₃ samples have a uniform diameter of around 300 nm and hollow structures with thin shells of about 30 nm consisting of numerous nanocrystals and nanopores. Owing to the hollow and porous structure, large surface area and more surface active sites, the sensor based on LaFeO₃ hollow nanospheres exhibited high response, good selectivity and stability to formaldehyde gas (HCHO). It suggests that the as-prepared LaFeO₃ hollow nanospheres are promising candidates for good performance formaldehyde sensor.     

The synthesis of hollow CdS nanospheres packed with square subunits

Hollow CdS nanospheres packed with square subunits were fabricated via microwave irradiation and pentaerythriol. The products were characterized with XRD, TEM, HRTEM, EDX and UV–vis spectrum. The results indicate that pentaerythriol is necessary in the formation of square subunits with high degree of crystallinity. A possible growth mechanism was proposed.     

Synthesis of hollow silica-sulfur composite nanospheres towards stable lithium-sulfur battery

Herein, porous hollow silica nanospheres were prepared via a facile sol-gel process in an inverse microemulsion, using self-assemblies of chiral amphiphile as a soft template and fine water droplets as a hard template. The shells of the hollow silica nanospheres are composed of flake-like nanoparticles with dense big holes on the surface. After covering a layer of sulfur on the silica nanospheres, followed by hydrothermal treatment in a D-glucose aqueous solution, silica-sulfur and silica-sulfur-carbon nanospheres were successfully fabricated. The silica-sulfur composites exhibit a stable capacity of 454 mAh g^?1 at current density of 335 mA g^?1 after 100 cycles with capacity retention of 85%, demonstrating a promising cathode material for rechargeable lithium-sulfur batteries. We believe that the approach for synthesis of porous hollow silica nanospheres and its carbon spheroidal shell can also be applicable for designing other electrode materials for energy storage.     

Fano resonances in heterogeneous dimers of silicon and gold nanospheres

We theoretically investigate the optical properties of dimers consisting of a gold nanosphere and a silicon nanosphere. The absorption spectrum of the gold sphere in the dimer can be significantly altered and exhibits a pronounced Fano profile. Analytical Mie theory and numerical simulations show that the Fano profile is induced by constructive and destructive interference between the incident electric field and the electric field of the magnetic dipole mode of the silicon sphere in a narrow wavelength range. The effects of the silicon sphere size, distance between the two spheres, and excitation configuration on the optical responses of the dimers are studied. Our study reveals the coherent feature of the electric fields of magnetic dipole modes in dielectric nanostructures and the strong interactions of the coherent fields with other nanophotonic structures.     

Controlled synthesis of gold nanospheres and single crystals in hydrogel

Narrow-dispersed gold nanospheres, regular single-crystal nanoplates and nanobulks were prepared, respectively, by reducing HAuCl₄ within a hydrogel system under UV irradiation. The formation of gold products with different geometric shape and size was found to depend on both the microenvironment of the gel matrix and the initial concentration of HAuCl₄. The resultant gold particles were investigated by UV–vis spectroscopy, X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and energy-dispersion X-ray spectroscopy. Electronic Supplementary Material The online version of this article at (doi: ) contains supplementary material, which is available to authorised users.     

The effect of phonon focusing on the electron-phonon relaxation and electron transport in potassium crystals

The influence of anisotropy of elastic energy on electron-phonon relaxation and the role of shear waves in the electrical resistance of potassium crystals are investigated. It is shown that, at temperatures much lower than the Debye temperature (T<< θ_D), the contribution of slow quasi-transverse phonons to the electrical resistance of potassium crystals exceeds that of longitudinal phonons by an order of magnitude. Earlier, the Bloch-Grüneisen theory left aside this component under the above conditions. At the same time, at high temperatures(T>>θ_D), the contribution of longitudinal phonons to the electrical resistance turns out to be 4 times greater than the total contribution of electron relaxation by fast and slow transverse modes. The role of shear waves in the electrical resistance of potassium crystals is analyzed. It is shown that, at low temperatures, this mechanism provides 32% of the total electrical resistance. It is 4 times higher than the contribution of longitudinal phonons to the electrical resistance and should be taken into account when analyzing the electrical resistance of alkali metals. The distribution function of the most effective phonons for electrical resistance is defined, and the inelasticity of electron-phonon scattering is analyzed. It is shown that the calculated results of the electrical resistance of potassium in the temperature range from 40 to 400 K, taking into account the anisotropy of elastic energy, are in good agreement with the experimental data without the use of fitting parameters.     

Synthesis,formation mechanism and electric property of hollow InP nanospheres

The present work reports on novel four-layer thermally driven piezoresistive cantilevers implemented in one- and two-dimensional arrays for parallel proximity scanning. There, the heater (metallic meander), the piezoresistive deflection sensor, and the metal actuation film with significantly higher thermal expansion coefficient make up separate layers. Actuation efficiency and cross-talk of the novel cantilever design are studied and compared with two recent designs: thin metallic film and ion-implanted heater. The novel actuator, integrated on a 240 μm long and 3 μm thick silicon cantilever and supplied by V _dc=1 V enables deflections up to 5 μm of the AFM-tip with an actuation efficiency of about 170 nm/mW and suppressed cross-talk between actuator and sensor.     

A self-generated template route to hollow carbon nanospheres in a short time

A self-generated template route was reported to prepare hollow carbon nanospheres. The hollow spheres were obtained through the direct pyrolysis of ferrocene for 1 h. The external diameter of the hollow carbon nanospheres was 50-150 nm and the thickness of the wall was about 15 nm. A possible formation mechanism of the hollow carbon nanospheres was discussed.