Optical spectra and excited state relaxation dynamics of Sm<Superscript>3+</Superscript> in Gd<Subscript>2</Subscript>SiO<Subscript>5</Subscript> single crystal

Single crystals of gadolinium orthosilicate Gd₂SiO₅ containing 0.5 at% and 5 at% of Sm³⁺ were grown by the Czochralski method. Optical absorption spectra, luminescence spectra and luminescence decay curves were recorded for these systems at 10 K and at room temperature. Comparison of optical spectra recorded in polarized light revealed that the anisotropy of this optically biaxial host affects the intensity distribution within absorption and emission bands related to transitions between multiplets rather than the overall band intensity. It has been found that among four bands of luminescence related to the ⁴G_5/2→⁶H_J (J=5/2–11/2) transitions of Sm³⁺ in the visible and near infrared region the ⁴G_5/2 →⁶H_7/2 one has the highest intensity with a peak emission cross section of 3.54×10⁻²¹ cm² at 601 nm for light polarized parallel to the crystallographic axis c of the crystal. The luminescence decay curve recorded for Gd₂SiO₅:0.5 at% Sm³⁺ follows a single exponential time dependence with a lifetime 1.74 ms, in good agreement with the ⁴G_5/2 radiative lifetime τ _rad=1.78 ms calculated in the framework of Judd-Ofelt theory. Considerably faster and non-exponential luminescence decay recorded for Gd₂SiO₅:5 at% Sm³⁺ sample was fitted to that predicted by the Inokuti-Hirayama theory yielding the microparameter of Sm³⁺–Sm³⁺ energy transfer C _da=1.264×10⁻⁵² cm⁶×s⁻¹.     

Casimir friction force between a SiO<Subscript>2</Subscript> probe and a graphene-coated SiO<Subscript>2</Subscript> substrate

The possibility of mechanical detection of Casimir friction with the use of a noncontact atomic force microscope is discussed. A SiO₂ probe tip located above a graphene-coated SiO₂ substrate is subjected to the frictional force caused by a fluctuating electromagnetic field produced by a current in graphene. This frictional force will create the bend of a cantilever, which can be measured by a modern noncontact atomic force microscope. Both the quantum and thermal contributions to the Casimir frictional force can be measured using this experimental setup. This result can also be used to mechanically detect Casimir friction in micro- and nanoelectromechanical systems.     

Friction and wear properties of ZrO<Subscript>2</Subscript>/SiO<Subscript>2</Subscript> composite nanoparticles

In this article, the lubrication properties of ZrO₂/SiO₂ composite nanoparticles modified with aluminum zirconium coupling agent as additives in lubricating oil under variable applied load and concentration fraction were reported. It was demonstrated that the modified nanoparticles as additives in lubrication can effectively improve the lubricating properties. Under an optimized concentration of 0.1 wt%, the average friction coefficient was reduced by 16.24%. This was because the nanoparticles go into the friction zone with the flow of lubricant, and then the sliding friction changed to rolling friction with a result of the reduction of the friction coefficient.     

Nonradiative relaxation of photoexcited O<Stack><Subscript>1</Subscript><Superscript>0</Superscript></Stack> centers in glassy SiO<Subscript>2</Subscript>

The processes involved in the excited-state relaxation of hole O ₁ ⁰ centers at nonbridging oxygen atoms in glassy SiO₂ were studied using luminescence, optical absorption, and photoelectron emission spectroscopy. An additional nonradiative relaxation channel, in addition to the intracenter quenching of the 1.9-eV luminescence band, was established to become operative at temperatures above 370 K. This effect manifests itself in experiments as a negative deviation of the temperature-dependent luminescence intensity from the well-known Mott law and is identified as thermally activated external quenching with an energy barrier of 0.46 eV. Nonradiative transitions initiate, within the external quenching temperature interval, the migration of excitation energy, followed by the creation of free electrons. In the final stages, this relaxation process becomes manifest in the form of spectral sensitization of electron photoemission, which is excited in the hole O ₁ ⁰ -center absorption band.     

RETRACTED ARTICLE: Refractive-index tailoring and morphological evolutions in Gd<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> and ZrO<Subscript>2</Subscript>–SiO<Subscript>2</Subscript> composite thin films

Refractive-index tailoring and morphological evolutions in two different thin film composite systems of gadolinia–silica (Gd₂O₃:SiO₂) and zirconia–silica (ZrO₂:SiO₂) deposited through reactive electron-beam codeposition processes are discussed in this research paper. For Gd₂O₃:SiO₂ the refractive-index tuning has been achieved from 1.45 to 2.18, whereas in the case of ZrO₂:SiO₂ the achieved tunable range is from 1.45 to 2.45 in the ultraviolet region. Under certain compositional mixings with lower silica fractions both the systems demonstrated relative microstructural and morphological densifications. Such evolutions were very successfully derived through phase-modulated ellipsometry and atomic force microscopy. The composition-dependent refractive-index tailoring and microstructural densifications have been investigated by adopting Tauc–Lorentz and single-effective-oscillator models. The morphological correlation functions have also very aptly supported such evolutions in these composite films. These experimental results indicate their favourable properties and applicability down to the extreme ultraviolet wavelength region of the electromagnetic spectrum. PACS 42.79.Wc; 78.66.-w; 78.20.Ci; 61.16.Ch; 42.70.-a;68.55.-a; 68.35.Bs; 81.15.Ef     

Preparation and investigation of composite SiO<Subscript>2</Subscript>-TiO<Subscript>2</Subscript> films

SiO₂-TiO₂ films [Si:Ti = 1:(0.06–2.3)] are obtained by the sol-gel method. The structural and photoluminescent properties of the films and powders heat-treated at different temperatures are studied. It is shown that after 700°C the composite consists of TiO₂ crystallites that are structurally similar to anatase and distributed in an amorphous SiO₂ matrix. The photoluminescence spectra have maxima at 450–500 nm. The photoluminescence intensity depends on the treatment temperature and TiO₂ content. __________ Translated from Zhurnal Prikladnoi Spektroskopii Vol. 74, No. 3, pp. 357–361, May–June, 2007.     

Mössbauer studies of magnetic Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>/SiO<Subscript>2</Subscript> nanocomposites

A large variety of glass and glass ceramics may be obtained by sol-gel process from hydrolysis of tetraethoxysilane. The transformation involves hydrolysis and polycondensation reactions leading to the growth of clusters that eventually collide together to form a gel. The structure and properties of the final product have been found to be strongly dependent on the initial conditions of preparation. Silica nanocomposites based on Fe₂O₃/SiO₂ were prepared with the help of ultrasonic activation and subsequent annealing in nitrogen atmosphere or air with concentrations of iron oxide of about 20 to 30wt.%.     

Fabrication and structural characterization of Mg<Subscript>2</Subscript>SiO<Subscript>4</Subscript> nanowires

This article demonstrates the first reported successful synthesis of Mg₂SiO₄ nanowires. We have thermally heated Au-coated Si substrates, using a quartz tube with its inner surface pre-coated with MgO nanostructures. We have characterized the sample morphologies by using scanning electron microscopy and transmission electron microscopy (TEM). X-ray diffraction analysis and high-resolution TEM observation coincidentally revealed that the nanowires were crystalline with an orthorhombic Mg₂SiO₄ structure. We have discussed the possible growth mechanism of Mg₂SiO₄ nanowires. PACS 81.07.-b; 81.05.Zx; 61.10.Nz; 68.37.Hk; 68.37.Lp     

Facile growth of monolayer MoS<Subscript>2</Subscript> film areas on SiO<Subscript>2</Subscript>

Areas of single-layer MoS₂ film can be prepared in a tube furnace without the need for temperature control. The films were characterized by means of Raman spectroscopy, photoluminescence, low-energy electron diffraction and microscopy, and X-ray photoelectron spectroscopy and mapping. Transport measurements show n-doped material with a mobility of 0.26 cm² V^-1 s^-1.     

Synthesis and nanostructural investigation of TiO<Subscript>2</Subscript> nanorods doped by SiO<Subscript>2</Subscript>

TiO₂ Nano rods can be used as dye-sensitized solar cells, various sensors and photocatalysts. These nanorods are synthesized by a hydrothermal corrosion process in NaOH solution at 200°C using TiO₂ powder as the source material. In the present work, the synthesis of TiO₂ nanorods in anatase, rutile and Ti₇O₁₃ phases and synthesis of TiO₂ nanorods by incorporating SiO₂ dopant, using the sol–gel method and alkaline corrosion are reported. The morphologies and crystal structures of the TiO₂ nanorods are characterized using field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD) study. The obtained results show not only an aggregation structure at high calcination temperatures with spherical particles but also Ti–O–Si bonds having four-fold coordination with oxygen in SiO^4 −.     

A computer study of the Raman spectra of the (GaN)<Subscript>129</Subscript>, (SiO<Subscript>2</Subscript>)<Subscript>86</Subscript>, and (GaN)<Subscript>54</Subscript>(SiO<Subscript>2</Subscript>)<Subscript>50</Subscript> nanoparticles

The Raman spectra of the (GaN)₁₂₉, (SiO₂)₈₆, and (GaN)₅₄(SiO₂)₅₀ nanoparticles were calculated using the molecular dynamics method. The spectrum of (SiO₂)₈₆ had three broad bands only, whereas the Raman spectrum of (GaN)₁₂₉ contained a large number of overlapping bands. The form of the Raman spectrum of (GaN)₅₄(SiO₂)₅₀ was determined by the arrangement of the GaN and SiO₂ components in it. The nanoparticle with a GaN nucleus had a continuous fairly smooth spectrum over the frequency range 0 ≤ ω ≤ 600 cm⁻¹, whereas the spectrum of the nanoparticle with a SiO₂ nucleus contained well-defined bands caused by vibrations of groups of atoms of different kinds and atoms of the same kind.     

Influence of two-particle excited states on the interatomic exchange interaction in La<Subscript>2</Subscript>CuO<Subscript>4</Subscript>

The effective spin Hamiltonian for undoped cuprates is constructed in the framework of the realistic multiband p-d model with the parameters calculated from first principles. The exchange interaction parameter is defined as the sum of the antiferromagnetic and ferromagnetic contributions, which are determined by the two-hole triplet terms. The ferromagnetic and antiferromagnetic contributions of the excited terms compensate each other to a large extent. It is shown that the antiferromagnetic contribution of the two-hole ground singlet ¹ A _1g to the exchange interaction is dominant. Original Russian Text. V.A. Gavrichkov, S.G. Ovchinnikov, 2008, published in Fizika Tverdogo Tela, 2008, Vol. 50, No. 6, pp. 1037–1041.     

Ferroelectrical properties of SiO<Subscript>2</Subscript>-TGS nanocomposite

Dielectric properties of the composites based on nanodisperse silica hydrosol and ferroelectric triglycinesulfate (TGS) are investigated. The studies allow us to expect the presence of the ferroelectric state in the composites.     

The enhanced catalytic degradation of SiO<Subscript>2</Subscript>/Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>/C@TiO<Subscript>2</Subscript> photo-Fenton system on p-nitrophenol

Heterogeneous photo-Fenton SiO₂/Fe₃O₄/C@TiO₂ (SFCT) catalyst with a core-multishell structure and a diameter of about 550 nm was successfully prepared and was characterized by scanning electron microscopy (SEM), TEM, XRD, Raman, and Fourier transform infrared (FT-IR). The results illustrated that anatase TiO₂ coexisted with rutile TiO₂, in which the anatase phase was the main crystal phase. In addition, the catalytic activity of SFCT catalyst had been evaluated in the catalytic degradation on p-nitrophenol (PNP). The influence factors on the PNP degradation, including SFCT component ratio (m _SFC/ m _TiO2), H₂O₂ dosage, solution pH, and PNP concentration, had been investigated. And the contrast experiments about the photo-Fenton catalytic mechanism revealed that the SFCT-2 catalyst possessed a superior activity in the neutral environment due to the optimal activity matching between Fe₃O₄ and TiO₂, and it exhibited the stable catalytic performance after five successive recycles. Therefore, the SFCT-2 catalyst had a promising application for the photo-Fenton degradation of organic contaminant.     

Electroluminescence in SiO<Subscript>2</Subscript> layers in various structures

Electroluminescence (EL) in an electrolyte-insulator-semiconductor system is used to study oxide layers in Si-SiO₂-Si₃N₄ and Si-SiO₂ structures prepared using various techniques. The EL spectra of all the structures contained the 2.7-eV emission band characteristic of the radiative relaxation of excited sililene centers. A comparative study of the conditions conducive to the formation of such luminescence centers in various structures containing SiO₂ layers was made, and their nature was refined.     

Reluctance of the YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>x</Subscript> oxygen-deficient ceramics

Influence of an external magnetic field on the reluctance of the YBa ₂ Cu ₃ O _x ceramics is investigated. A significant reluctance of the oxygen-deficient ceramics (with critical temperature T_c < 77 K) is established for a sample in the normal state at T < 160 K. It is demonstrated that after ceramics annealing that restores the oxygen content to a nearly optimum value, the magnetic field has essentially no effect on the sample reluctance at temperatures exceeding T_c. To explain the revealed mechanisms, a model involving ferromagnetic clusters effectively decreasing the free carrier density is used. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 68–71, April, 2007.     

Comparison between Si/SiO<Subscript>2</Subscript> and InP/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> based MOSFETs

The discrete breathers in graphane in thermodynamic equilibrium in the temperature range 50–600 K are studied by molecular dynamics simulation. A discrete breather is a hydrogen atom vibrating along the normal to a sheet of graphane at a high amplitude. As was found earlier, the lifetime of a discrete breather at zero temperature corresponds to several tens of thousands of vibrations. The effect of temperature on the decay time of discrete breathers and the probability of their detachment from a sheet of graphane are studied in this work. It is shown that closely spaced breathers can exchange energy with each other at zero temperature. The data obtained suggest that thermally activated discrete breathers can be involved in the dehydrogenation of graphane, which is important for hydrogen energetics.