Contrast variation in spin-echo small angle neutron scattering

The use of contrast variation in spin-echo small angle neutron scattering (SESANS) experiments is discussed for the case of colloidal structural investigation. On the basis of calculations for several model systems, we find that the contrast variation SESANS technique, in terms of the measured SESANS correlation function G(z), is not sensitive to the structural characteristics of colloidal suspensions consisting of particles with uniform scattering length density profiles. However, its ability to resolve structural heterogeneity, at both intra-colloidal and inter-colloidal length scales, is clearly demonstrated. The prospect of using this new technique to investigate structural information that is difficult to probe in other ways is also explored.     

Optical properties of inhomogeneous composite materials

Using the condition that the forward scattering amplitude S(0) = 0, we extend the treatment of composite media to optical frequencies and to a model system of metal island spheres suspended in a dielectric. Our results are in good agreement with experimental data from cermets of Au in SiO₂ for volume fractions from 10 to 80% Au in SiO₂.     

Eu(TTFA)3掺杂SiO2杂化胶体球的合成及特性

采用改进的碱催化法和种子法分别制得了稀土配合物Eu(TTFA)₃掺杂的SiO₂杂化胶体球,并用透射电子显微镜和荧光分光光度计对其显微形貌和荧光光谱特性进行了详细地研究.结果表明,两种方法都可以获得单分散性的、稀土配合物掺杂SiO₂杂化胶体球,且都具有Eu³⁺离子典型的荧光光谱特性.Eu(TTFA)₃掺杂入SiO₂胶体球中后,有机配体TTFA在短波长处的吸收明显增强了,最大的吸收峰位也向短波长方向移动大约20～30 nm,Eu³⁺离子5D0→7F2发射跃迁仍然具有良好的窄线发光特征,同时荧光峰值的形态和位置受SiO₂基体的影响发生轻微的变化.     

Synthesis and rheological investigation of a magnetic fluid using olivary silica-coated iron particles as a precursor

A new type of magnetic fluid was prepared by dispersing monodispersed iron–silica (Fe–SiO₂) composite particles in polyethylene glycol (PEG) 400. The composite particles Fe–SiO₂ were synthesized by hydrogen reduction from α-Fe₂O₃–SiO₂ spheres. Their microstructures were observed by a high-resolution transmission electron microscope (HRTEM) and the magnetism was characterized with a superconducting quantum interference device (SQUID) magnetometer. Both steady-state and dynamic rheological properties of the magnetic fluid under different magnetic fields were studied by using a rheometer. Experimental results show that this magnetic fluid has a relatively high magnetoviscous effect at low shear rates. The yield stress of this material shows an increasing trend with a magnetic flux density. Also, viscoealstic properties of such materials are different from conventional ones.     

Optical losses and absorption cross-section of silicon nanocrystals

 Si-rich silicon oxide and SiO₂ (SRSO)/SiO₂ multilayer (ML) samples were grown by reactive magnetron sputtering and then annealed at high temperature to induce the formation of Si-nc with mean size of 3-4 nm and density of about 3.5×10¹⁸ cm⁻³ as deduced from high resolution TEM micrographs. Refractive index and thickness have been determined by m-line measurements, which have shown a birefringence of about 1.5% due to the ML structure. Rib-loaded waveguides have been fabricated to measure propagation losses in the visible-infrared range. The analysis of the different contributions to optical losses such as Mie scattering and scattering due to waveguide roughness has allowed us to isolate the contribution due to the absorption losses and thus to extract the absorption cross-section at different wavelengths. Values of about 3.5×10⁻¹⁸ cm² have been found at 830 nm, increasing with decreasing of the wavelength.     

Leakage current and capacitance characteristics of Si/SiO2/Si single-barrier varactor

We investigate, both experimentally and theoretically, current and capacitance (I–V/C–V) characteristics and the device performance of Si/SiO₂/Si single-barrier varactor diodes (SBVs). Two diodes were fabricated with different SiO₂ layer thicknesses using the state-of-the-art wafer bonding technique. The devices have very low leakage currents (about 5×10^-2 and 1.8×10^-2 mA/mm²) and intrinsic capacitance levels of typically 1.5 and 50 nF/mm² for diodes with 5-nm and 20-nm oxide layers, respectively. With the present device physical parameters (25-mm² device area, 760-μm modulation layer thickness and ≈10¹⁵-cm^-3 doping level), the estimated cut-off frequency is about 5×10⁷ Hz. With the physical parameters of the present existing III–V triplers, the cut-off frequency of our Si-based SBV can be as high as 0.5 THz. Received: 9 February 2001 / Accepted: 9 February 2001 / Published online: 23 March 2001     

Induced SERS activity in Ag@SiO2/Ag core‐shell nanosphere arrays with tunable interior insulator

In this work, we demonstrated a bottom‐up growth of Ag@SiO₂/Ag core‐shell nanosphere arrays with tunable SiO₂ interior insulator and the optimized surface‐enhanced Raman scattering (SERS) substrate based on a nanostructure performed with both high sensitivity and large‐area uniformity. Their morphological, structural, and optical properties were characterized, and the induced SERS activities were investigated theoretically by the FDTD simulation and experimentally using analyte molecules. An ultrathin SiO₂ shell with tunable thickness can be synthesized pinhole‐free by a chemical vapor deposition, working as an interior insulator between the Ag core and Ag out‐layer coating. A detection limit as low as 10⁻¹² M and an enhancement factor up to 3 × 10⁷ were obtained, and the SERS signal was highly reproducible with small standard deviation. The method opened up a way to create a new class of SERS activity sensor with high‐density ‘hot spots’, and it may play an important role in device design and the corresponding biological and food safety monitoring applications. Copyright © 2016 John Wiley & Sons, Ltd.     

Group mean emissivities and opacities of high temperature SiO2

Absorption and extinction coefficients of SiO₂ have been calculated using a mean ion model but with the most important singly and multiply excited states of the dominant ionic species considered explicitly. Free-free, bound-free, bound-bound, and scattering are taken into account. Densities range from 10^-6 to 10³ g cm^-3 and temperatures range from 30 to 10 keV. Emissivities are obtained from detailed balance. Group means are given for some representative cases.     

Enhancement of carrier mobility in pentacene thin-film transistor on SiO2 by controlling the initial film growth modes

Pentacene thin-film transistors (TFTs) were fabricated on thermally grown SiO₂ gate insulator under the conditions of various pre-cleaning treatments. Initial nucleation and growth of the material films on treated substrates were observed by atomic force microscope. The performance of fabricated TFT devices with different surface cleaning approaches was found to be highly related to the initial film morphologies. In contrast to the three-dimensional island-like growth mode on SiO₂ under an organic cleaning process, a layer-by-layer initial growth occurred on the SiO₂ insulator cleaned with ammonia solution, which was believed to be the origination of the excellent electrical properties of the TFT device. Field effect mobility of the TFT device could achieve as high as 1.0 cm²/Vs on the bared SiO₂/Si substrate and the on/off ratio was over 10⁶.     

Spatial modulation of a neutron beam by Larmor precession

We modulated the intensity of a neutron beam using Larmor precession techniques. We simulated triangular coils by using magnetised foils in electromagnets with rectangular pole shoes. Reversing the orientation of two of the magnetised foils in the spin-echo small-angle neutron scattering (SESANS)-instrument in Delft and changing the field settings was sufficient to obtain a modulation with a period in the order of 1 mm. We expect to be able to go to a modulation with a period of .This technique can be used to measure small-angle neutron scattering, analogous to the method used in SESANS, but with the advantage that all the manipulation of the neutron spin occurs before the sample. This means that the technique is insensitive to magnetic perturbations at the sample position. By positioning several of these devices after each other it will be possible to obtain a sharper modulation, or a modulation in two directions.     

Determination of structure parameters and size distribution of colloidal particles by means of small-angle X-ray scattering

A dilute silica suspension was used for examining the possibilities of using the small-angle X-ray scattering in the investigation of objects of colloid dimensions. The average radius of gyration, weight, volume and specific surface of the SiO₂ particles were determined. The particle size distribution curve was determined by integral transformation of the scattering curve and by adjusting the parameters of empirical functions.The authors are indebted to Professor P. W.Schmidt for kindly supplying the SiO₂ suspension.     

Investigation of amorphous states of SiO2 by Raman scattering spectroscopy

The vitreous SiO₂ samples irradiated with fast neutrons at a dose of 5×10¹⁷?2.2×10²⁰ per cm² are investigated by the Raman scattering technique. It is demonstrated that the maximum of the low-frequency Raman spectrum (boson peak) shifts with an increase in the irradiation dose, and the medium-range order size decreases from 25 Å for the initial glass to 19 Å for the sample subjected to irradiation at a maximum dose. It is revealed that the fast relaxation intensity obtained from analysis of the low-frequency Raman spectra linearly correlates with the specific volume of the studied samples.     

Large Scale Synthesis of Highly Stable Fluorescent Carbon Dots Using Silica Spheres as Carriers for Targeted Bioimaging of Cancer Cells

In this paper, fluorescent carbon dots (CDs) loaded on silica (SiO₂) spheres are synthesized by the one‐pot hydrothermal route, and then folic acids (FA) are covalently conjugated on the surface of SiO₂ spheres. The formed SiO₂@CDs‐FA composites can target specific tissues, e.g., cancer. The key of this method is the employment of (3‐aminopropyl)trimethoxysilane as bridge joint, which not only serves as surface passivation agents allowing the large scale synthesis of CDs with high quantum yield, but also enables SiO₂@CDs composites further covalent conjugation of FA. The resultant SiO₂@CDs composites have many advantages such as easy separation and purification, highly stable, well water‐soluble, and biocompatible. Moreover, the SiO₂@CDs‐FA could be used as fluorescent probes for biological imaging in vitro. The uptake of the SiO₂@CDs‐FA into HeLa cells is receptor‐mediated endocytosis, which is confirmed by a comparative study using FR‐negative 293T cells. Findings from this study suggest that the SiO₂@CDs‐FA composites could be used as a platform for cancer diagnosis studies in various biological systems.     

Carbon nano dots scale by focused ion beam system for MIS diode nano devices

Metal-insulator-semiconductor (MIS) structures with a nanocrystal carbon (nc-C) embedded in SiO₂ thin films were fabricated using a focused ion beam (FIB) system with a precursor of low-energy Ga⁺ ion and carbon source. The crystallinity of nc-C was investigated by Raman spectroscopy and atomic force microscopy (AFM). Raman spectra indicate the evidence of crystallization of nc-C after annealed at 600 °C by the sharp peak at 1565 cm⁻¹ in graphite (sp²), while no peak of diamond (sp³) could be seen at 1333 cm⁻¹. The AFM images showed the nc-C dots controlled with diameter of 100 nm, 200 nm and 300 nm, respectively. The above results revealed that the nc-C dots had sufficiently stuck onto SiO₂ films. The hysterisis loop in the capacitance-voltage characteristics appeared in the MIS device with SiO₂/nc-C/SiO₂ structure in which voltage shift is 0.32 V for radical oxidation and 0.14 V for dry oxidation, respectively.     

Retrieval of aerosol physical and chemical properties from mid-infrared extinction spectra

We report several results that validate the accuracy of a retrieval method for the determination of a number of aerosol particle properties from their mid infrared (600-6000 cm⁻¹) extinction spectra. These properties include the number density, chemical composition, phase, size distribution, and to some extent, shape. The approach is based on information obtained in laboratory studies of micron-sized particles using the aerosol flow tube (AFT) technique. We report here experiments in which our method is used to measure a variety of aerosols including SiO₂ micro-spheres as well as solid NaCl, (NH₄)₂SO₄, ice and liquid water particles. The uncertainties in the retrieved aerosol properties associated with the particle shapes (spheres, spheroids, cylinders, hexagonal and rectangular prisms) as well as the effect of variations in the spectral range were evaluated. To assess the accuracy of the retrieved size distributions and particle shapes, the properties calculated from infrared spectra were compared with corresponding properties determined using alternative methods. We used scanning electron microscopy (SEM) for solid (NH₄)₂SO₄ and NaCl aerosols and direct particle imaging with an optical microscope assembly for liquid water aerosols. On the basis of the validation results, we discuss the boundaries of applicability of the most popular spectral model, single scattering by spherical, homogeneous aerosol particles.     

Estimation of various scattering parameters and 2-DEG mobilities from electron mobility calculations in the three conduction bands Γ, L and X of gallium arsenide

The electron drift mobility in Γ conduction band of GaAs has been calculated before, but for the first time, we have made attempts to estimate the electron mobilities in higher energy L and X minima. We have also calculated the value of mobility of two-dimensional electron gas needed to predict hetero-structure device characteristics using GaAs. Best scattering parameters have been derived by close comparison between experimental and theoretical mobilities. Room temperature electron mobilities in Γ, L and X valleys are found to be nearly 9094, 945 and 247 cm²/V-s respectively. For the above valleys, the electron masses, deformation potentials and polar phonon temperatures have been determined to be (0.067, 0.22, 0.39m ₀), (8.5, 9.5, 6.5 eV), and (416, 382, 542 K) as best values, respectively. The 2-DEG electron mobility in Γ minimum increases to 1.54 × 10⁶ from 1.59 × 10⁵ cm²/V-s (for impurity concentration of 10¹⁴ cm⁻³) at 10 K. Similarly, the 2-DEG electron mobility values in L and X minima are estimated to be 2.28 × 10⁵ and 1.44 × 10⁵ cm²/V-s at 10 K, which are about ∼4.5 and ∼3.9 times higher than normal value with impurity scattering present.      

Optoelectrical investigations on MOS-sandwiches at low temperatures

MOS-structures are irradiated with light of energy from 1.5 to 6 eV at different temperatures (300, 77, 12 K) while the resulting photocurrent is measured. At high photon energies (hv>4 eV) the threshold energy and the scattering mean free path for electrons at the Si — SiO₂-interface are determined. They are independent from temperature. At low photon energies (hv<3 eV) electrons are released from traps with energy levels 1.2 and 1.9 eV below the Si-conduction band. The trap concentration is 4.8 10¹³ cm^–3. The capture cross section is measured in a rather direct way. The temperature and electrical field dependence of this cross section is explained by a trapping model.     

Photoluminescence properties of core-shell SiO2/Lu2O3: Eu monodisperse heteronanoparticles

Core-shell monodisperse heteroparticles of the composition SiO₂/Lu_1.86Eu_0.14O₃ have been synthesized using the developed technique for preparing spherical colloidal silicon dioxide particles with the size dispersion in the range 2.0–2.5% and the procedure for producing nanocoatings on the surface of spheres by codeposition. The structure of heteroparticles has been investigated, their excitation and photoluminescence spectra have been analyzed, and the lifetime of the ⁵ D ₀ excited state of Eu³⁺ ions has been examined. It has been revealed that the luminescence decay time for heteroparticles increases by a factor of approximately two compared to that for a powdered luminophor Lu₂O₃: Eu (7 at %) prepared and treated under the same temperature conditions as the SiO₂/Lu₂O₃: Eu (7 at %) heteroparticles. This effect has been attributed to the change in the effective refractive index and the local density of photon states in luminophor nanolayers of heteroparticles.     

Sol-gel growth of Gd2MoO6:Eu nanocrystalline layers on SiO2 spheres (SiO2@Gd2MoO6:Eu) and their luminescent properties

SiO₂@Gd₂MoO₆:Eu³⁺ core-shell phosphors were prepared by the sol-gel process. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectra (EDS), transmission electron microscopy (TEM), photoluminescence (PL) spectra as well as kinetic decays were used to characterize the resulting SiO₂@Gd₂MoO₆:Eu³⁺ core-shell phosphors. The XRD results demonstrate that the Gd₂MoO₆:Eu³⁺ layers on the SiO₂ spheres begin to crystallize after annealing at 600 °C and the crystallinity increases with raising the annealing temperature. The obtained core-shell phosphors have a near perfect spherical shape with narrow size distribution (average size ca. 600 nm), are not agglomerated, and have a smooth surface. The thickness of the Gd₂MoO₆:Eu³⁺ shells on the SiO₂ cores could be easily tailored by varying the number of deposition cycles (50 nm for four deposition cycles). The Eu³⁺ shows a strong PL luminescence (dominated by ⁵D₀-⁷F₂ red emission at 613 nm) under the excitation of 307 nm UV light. The PL intensity of Eu³⁺ increases with increasing the annealing temperature and the number of coating cycles.     

Synthesis and photoluminescence of core–shell structured spherical SiO2@YPO4:Tb3+ phosphors via sol–gel process

Monodispersed SiO₂@YPO₄:Tb³⁺ core–shell submicrospheres were prepared through a simply homogeneous sol–gel method. The resulted SiO₂@YPO₄:Tb³⁺ core–shell particles were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectra (EDS), transmission electron microscopy (TEM), photoluminescence spectra (PL) and kinetic decays. The XRD results demonstrate that the YPO₄:Tb³⁺ layers begin to crystallize on the SiO₂ spheres after annealing at 500 °C and the crystallinity increases with raising the annealing temperature. The FTIR spectra show that the YPO₄:Tb³⁺ shell has linked to the silica surface through forming a Si–O–Y bond. SEM and TEM analysis indicate that SiO₂@YPO₄:Tb³⁺ core–shell submicrospheres have the regular microstructures and uniform size distributions. The emission spectra of the obtained submicrospheres are dominated by ⁵D₄–⁷F₅ transition of Tb³⁺ (545 nm, green), and the emission intensities of Tb³⁺ increase with increasing the annealing temperatures and the number of coating cycles. The optimum concentration for Tb³⁺ was determined to be 5 mol % of Y³⁺ in YPO₄ host.