Thermal conductivity of crystalline chrysotile asbestos

The thermal conductivity of crystalline chrysotile asbestos made up of hollow tubular Mg₃Si₂O₅(OH)₄ filaments is measured in the range 5–300 K. The paper discusses the possibility of using this material in studies of the thermal conductivity of thin filaments of metals and semiconductors incorporated into the channels of crystalline chrysotile asbestos tubes.     

Thermal and acoustic properties of chrysotile asbestos

The thermal conductivity, specific heat, and sound velocity of crystalline chrysotile asbestos made up of hollow tubular fibrils of composition Mg₃Si₂O₅(OH)₄ have been measured at temperatures of 5–300, 3–65, and 77 K, respectively. An analysis is made of the experimental data obtained.     

Raman spectroscopy of the nickel silicate mineral pecoraite—an analogue of chrysotile (asbestos)

The molecular structure of the mineral pecoraite, the nickel analogue of chrysotile of formula Ni₃Si₂O₅(OH)₄, was analysed by a combination of Raman and infrared spectroscopies. A comparison is made with the spectra of the minerals nepouite and chrysotile and a synthetic pecoraite. Pecoraite is characterised by OH stretching vibrations at 3645 and 3683 cm⁻¹ attributed to the inner and inner surface hydroxyl stretching vibrations. Intense infrared bands at around 3288 and 3425 cm⁻¹ are assigned to the stretching vibrations of water strongly hydrogen‐bonded to the surface of the pecoraite. The asbestos‐like mineral is characterised by SiO stretching vibrations at 979, 1075, 1128 and 1384 cm⁻¹, OSiO chain vibrations at 616 and 761 cm⁻¹ and Ni O(H) vibrations at 397 and 451 cm⁻¹. Copyright © 2008 John Wiley & Sons, Ltd.     

Optical spectroscopy of Nd ions in a nanostructured glass matrix

This paper presents the optical characterization of Nd³⁺ ions in nanostructured SiO₂-Na₂CO₃-Al₂O₃-B₂O₃ (SNAB) CdS glass, synthesized by fusion. Radiative properties of the glass were determined by absorption, luminescence spectroscopy and lifetime measurements. Nd³⁺ emission enhancement and quenching were investigated in the presence of CdS nanocrystals. Nd³⁺-emission quenching was attributed to upconversion mechanisms and nonradiative processes such as multiphonon decay and energy transfer, while the Nd³⁺-emission enhancement was due to energy transfer from the CdS nanocrystals. Changes in the chemical environment around CdS nanocrystals were also confirmed by Judd-Ofelt calculations.     

Investigation of longitudinal vibrations of -O-H groups in chrysotile asbestos by neutron scattering and polarized infrared spectroscopy

The specific features of the vibrational spectra of chrysotile asbestos, which is a natural mineral that represents a system of closely packed tubular fibers with an outer diameter of ∼30 nm, an inside diameter of ∼5 nm, and a length up to a centimeter and more, have been investigated using neutron scattering and polarized infrared spectroscopy. This material can serve as a natural matrix for the preparation of nanostructures by filling channels with various materials.     

Reduction of iron(III) in annealed asbestos/chrysotile

Magnetic and structural properties of Bi substituted YIG with nominal formula of Bi _x Y_3???x Fe₅O₁₂ (x?=?0.0, 0.25, 0.5, 0.75, 1.00) prepared via Mechanochemical Processing (MCP) have been studied with Mossbauer spectroscopy, X-ray diffraction (XRD). The temperature dependence of sublattice magnetic hyperfine field for samples is analyzed. The a–d intersublattice superexchange found to be antiferromagnetic and increases from ??21.97 to ??25.79 k_B as Bi increases from 0.0 to 0.25. The a–a and d–d intrasublattice exchanges for sample x?=?0.0 are 13.18 and 10.55 k_B respectively while for sample x?=?0.25 a–a and d–d intrasublattice exchanges are 7.7 and 8.9 k_B respectively. The correlation of lattice constant and superexchange interaction are discussed.     

Multiple ionic-plasmon resonances in naturally occurring multiwall nanotubes: infrared spectra of chrysotile asbestos

Chrysotile asbestos is formed by densely packed bundles of multiwall hollow nanotubes. Each wall in the nanotubes is a cylindrically wrapped layer of Mg(3)Si(2)O(5)(OH)(4). We show by experiment and theory that the infrared spectra of chrysotile present multiple ionic-plasmon resonances in the Si-O stretching bands. These collective charge excitations are universal features of the nanotubes that are obtained by cylindrically wrapping an anisotropic material. The multiple plasmons can be observed if the width of the resonances is sufficiently small as in chrysotile.     

Light emission from three-dimensional ensembles of CdTe nanocrystal wires templated in nanotubes of chrysotile asbestos

Nanowires have been formed by the infiltration of CdTe nanocrystals (NCs) into nanotubes of chrysotile asbestos (Mg₃Si₂O₅(OH)₄). Photoluminescence of a regular array of these templated nanowires was studied under the excitation of the light from a xenon lamp at different wavelengths. Strong interactions of nanocrystals with structural defects of the template were observed. No dependence of the photoluminescence spectra upon polarisation of the laser beam was observed and no shift of the photoluminescence band was detected in the light polarised along and across nanowires, thus indicating the weakness of the interaction between nanocrystals in the nanotubes.     

Dielectric spectroscopy investigations of nanostructured silicon

Dielectric spectroscopy measurements were performed on planar silicon nanostructures buried within a crystalline Si that form a nanoscale Si-layered system. An insight into the specific behavior of the free-carrier population confined in the surface potential well was then made possible. It was found that the presence or the absence of the SiO₂ passivation modifies considerably relaxation responses of the studied structures. A clear differentiation of two dielectric responses: from the same sample with and without electronic passivation allowed determination of the conduction behavior in the surface c-Si delimited by the nanoscale Si-layered system. The sample with a 100 nm thick SiO₂ layer (and an excellent quality of the SiO₂/c-Si interface) exhibits a fractional power-law dielectric response, corresponding clearly to the generalized Mittag–Leffler pattern. Simultaneously, the dielectric response of a bare sample (after the total RIE of the previously deposited SiO₂ layer, about 10 nm native SiO₂ layer and poor quality of the SiO₂/c-Si interface) is dominated by the conductivity term.     

Defects in the p-n junction and current drift in selenium rectifiers

Drift is considered in relation to a defective p-n junction having localized conduction channels.     

Fabrication of nanostructured silver substrates for surface-enhanced Raman spectroscopy

Four nanostructured Ag substrates have been fabricated with different surface morphologies and tested with surface-enhanced Raman scattering (SERS) experiments by adsorption of adenine. Their SERS efficiency has been compared and related to the surface roughness resulting from atomic force microscopy measurements. Chemical etching of silver by thiourea/Fe(III)nitrate produces homogeneously roughened plates, exhibiting the largest three-dimensional surface and the best SERS enhancement. They mostly exhibit surface protrusions with sizes around 200 nm, thus matching the best condition for obtaining SERS enhancement by laser excitation at 785 nm. This is quite important in the case of biomolecules, whose samples often present strong fluorescence bands, which usually are not observed with red-shifted exciting lines. Moreover, these Ag platforms, owing to their uniform nanostructured surfaces, are suitable for obtaining reproducible results from microRaman investigation. In conclusion, the present nanofabrication of Ag surfaces allows obtaining SERS-active substrates, which combine high reproducibility and sensitivity and can be successfully employed in the molecular recognition of different organic ligands or biomolecules like nucleic acids and proteins.     

Behaviour of polycrystalline selenium layers at high current densities

The paper describes measurements of the current-voltage characteristic of polycrystalline selenium layers at high current density values. At first it is shown that the composition of a counter electrode (Sn, Cd, CdSn) does not influence the character of the dependence of the current density vs. voltage. When measuring the influence of thickness of polycrystalline layers of selenium on the behaviour of the current-voltage characteristic, three regions on the current-voltage characteristic were observed. For the lowest voltage the current density isjU/d and so Ohm's law can be used. For higher voltagejU^1.35/d^1.175. For the highest voltage the relationjU ²/d^1.5 was found. It will probably be possible to explain the results of measurements by means of the existence of space-charge-limited current.I should like to express my sincere gratitude to the specialists from the Institute of the Rectifiers Research at B~chovice for their very careful preparation of the samples, which made possible to carry out really reproducible and comparable measurements.     

Luminescent vacuum ultraviolet spectroscopy of Cr3+ ions in nanostructured aluminum oxide

Impurity Cr³⁺ centers in submicron and nanostructured Al₂O₃ crystals of different phase compositions at temperatures of 300 and 7.5 K were studied by a luminescent vacuum ultraviolet (VUV) spectroscopy method. Photoluminescence (PL) spectra and the energies of ²E, ⁴T₂, and ⁴T₁ excited states of Cr³⁺ ion depend on the type of crystalline samples phase. The PL excitation spectrum of R-line in α-Al₂O₃ nanoscale crystals is formed by intracenter transitions (2.5–5.5 eV region), by charge transfer band (6.9 eV) and by effective formation of impurity-bound excitons (9.0 eV region). Such impurity-bound excitons correspond to O₂p→Al₃s electron transition in surroundings of an impurity Cr³⁺ center. The efficiency of impurity-bound excitons formation decreases with the increase of the grain size above 100 nm. The size dependence is noticeably shown in PL excitation spectra in VUV region. Excitons bound to impurity centers do not appear in nanostructured δ+θ-Al₂O₃ crystals. The effect of the electron excitation multiplication is observed distinctly in nanostrucured α-Al₂O₃ at an excitation energy above 19 eV (more than 2E_g).     

光激电流谱的测试

用光激瞬态电流谱ＯＴＣＳ（ｏｐｉｔａｌｔｒａｎｓｉｔｉｏｎｃｕｎｅｎｔｓｐｅｃｔｒｏｓｃｏｐｙ）方法对ＩｎＰ∶Ｆｅ进行了初步测试。目的是研究半绝缘ＩｎＰ∶Ｆｅ中的深能级，了解其作为光电器件、高速器件衬底的稳定性。在低温下，用强光发现半绝缘ＩｎＰ∶Ｆｅ中存在两深能级，分别是ＥＴ＝０．３４ｅＶ的电子陷阱和ＥＴ＝１．１３ｅＶ的空穴陷阱。     

Experimental and theoretical determination of the low-loss electron energy loss spectroscopy of nanostructured ZnO

The dielectric properties of nanostructured wurtzite-type ZnO are studied by analyzing the low-loss region of the electron energy loss spectroscopy (EELS) in a transmission electron microscope. Characteristic peaks at about 12 and 32 eV in the imaginary part of the dielectric function shift to lower energies as particle size decreases. A comparison of experimental EELS spectra and ab initio density-functional theory calculations (WIEN2k code) within the generalized gradient approximation (GGA), GGA+U and modified Becke-Johnson (mBJ) is presented. The origins of interband transitions are identified in the electronic band structure by calculating the partial imaginary part of the dielectric function and the partial density of states of Zn and O.     

Photo-induced current transient spectroscopy of the ferroelectric-semiconductor TlGaSe2

Systematic studies of crystals from different technological batches have revealed five electrically active defects with a charge exchange thermal activation energy of 0.1–0.7 eV. Specific features of their detection, which are associated with the ferroelectric properties of the material, have been discussed. The pattern revealed in the variation with temperature of the charge exchange enthalpy of the defect detected in the vicinity of the phase transition has been presented.     

X-ray emission and photoluminescence spectroscopy of nanostructured silica with implanted copper ions

Quartz glass samples and compacted SiO₂ nanopowders have been studied by x-ray emission (CuL _{2, 3} transition 3d4s → 2p _{1/2, 3/2}) and photoluminescence spectroscopy following pulsed Cu⁺ ion implantation (energy, 30 keV; pulse current up to 0.5 A; pulse duration, 400 μs; irradiation doses, 10¹⁵, 10¹⁶, and 2 × 10¹⁷ cm^?2). It has been established that ion irradiation gives rise to the formation of glassy and compacted SiO₂ samples of nanosized metallic and oxide phases in the structure. An analysis of CuL x-ray emission spectra has shown that copper nanoparticles are thermodynamically metastable and chemically active because ion beam bombardment transfers them readily to the oxide form. This results from the radiation-stimulated fracture of regular Si-O-Si bonds in amorphous SiO₂ and the formation of defective Si-Si bonds, followed by capture of oxygen by copper atoms. The enhanced degree of oxidation of copper ions in SiO₂ nanostructured pellets can be reduced by coimplantation and thermal annealing. Optical spectroscopy studies suggest that, in glasses and SiO₂ nanostructured pellets, there exist metallic Cu _n ⁰ nanoclusters, which at low temperatures exhibit quantum-confined photoluminescence with a characteristic stepped excitation spectrum.