The effects of annealing temperature on photoluminescence of silicon nanoparticles embedded in SiNx matrix

The effects of the annealing temperature on photoluminescence (PL) of non-stoichiometric silicon nitride (SiN_x) thin films deposited by plasma enhanced chemical vapor deposition (PECVD) using ammonia and silane mixtures at 200 °C were investigated. The optical property and the chemical composition of the films annealed at different temperatures were investigated by PL spectroscopy and Fourier transform infrared absorption spectroscopy (FTIR), respectively. Based on the PL results and the analyses of the bonding configurations of the films, the light emission is attributed to the quantum confinement effect of the carriers inside silicon nanoparticles and radiative defect-related states. These results provide a better understanding of optical properties of silicon nanoparticles embedded in silicon nitride films and are useful for the application of nanosize silicon semiconductor material.     

Properties of porous glasses with embedded ferroelectric materials

Methods of introducing two ferroelectric materials, viz. NaNO₂ and (CH₃NH₃)₅Bi₂Cl₁₁ (PMACB), into porous glass matrices and the results of dielectric, pyroelectric, thermal expansion and SEM investigations are presented. The best filling degree was achieved for NaNO₂ introduced from the melt, whereas a significantly worse filling effect was achieved for a water solution as the introducing agent. A splitting was observed in the temperature dependence of dielectric permittivity related to the incommensurate phase of NaNO₂ embedded in porous glass from a water solution, whereas no incommensurate phase was observed in the case of glass filled with NaNO₂ from the melt. The confined materials turned out to possess ferroelectric properties and exhibited phase transitions. The negative size effect was observed for porous glasses filled with NaNO₂ and PMACB. The above conclusions are based on the anomaly of dielectric, pyroelectric and dilatometric properties observed in the phase transition regions.     

Mechanical behaviour of highly porous glasses

The mechanical behaviour of highly porous glassy materials (pore volume higher than 85%) is investigated using Hg porosimetry. Because of the small pore size of these materials, Hg liquid cannot enter their porous network and consequently induces an isostatic pressure. Due to the high compliance of the solid network of these materials, compression results in the sample shrinkage. The experiments described in this paper show that an isostatic pressure applied to highly porous glasses induces an irreversible volume shrinkage which can be associated with an unexpected plastic behaviour and structure strengthening. The magnitude of the plastic shrinkage and the increase of the associated mechanical properties depend on the starting bulk density. The irreversible compaction can be explained by siloxane bond formation between clusters constituting the porous glasses, retaining the strained structure. This densification process could offer a new way to synthesise glasses at room temperature.     

Ferroelectric phase transitions in KNO3 embedded into porous glasses

This paper presents the results of dielectric and dilatometric measurements of porous glasses filled with KNO₃ from water solution and from the melt. It was shown that ferroelectric phase transition is observed during the cooling process only in KNO₃ nanocrystals introduced into the porous matrix. The phase diagram of transition temperatures versus average values of pore dimensions was obtained. The phenomenological model describing the phase transition and size effects in KNO₃ crystals is proposed.     

Preparation and characterization of hcp Co-coated Fe nanoparticles

A relatively simple method was described for coating and modifying iron nanoparticles with a hexagonal close-packed (hcp) cobalt layer. It was realized by depositing cobalt onto the poly(N-vinyl-2-pyrrolidone) (PVP) modified iron particles in ethanol solution. XRD, XPS and HRTEM were used to characterize the product. The Co layers, grown as the exclusive hcp phase, were ascribed to the inducement of the body-centered cubic (bcc) iron cores. Thus-prepared hcp Co-coated Fe particles displayed a much enhanced coercivity and decreased saturation magnetization (M_S) compared with that of the uncoated sample.     

Temperature evolution of copper oxide nanoparticles in porous glasses

The temperature evolution of copper oxide nanoparticles in the temperature range of 1.5–250 K has been investigated by thermal-neutron diffraction. CuO particles were obtained by Cu(NO₃)₂ · 3H₂O decomposition directly in the pores of porous glass with an average pore diameter of 7 nm. The characteristic nanoparticle size and linear thermal expansion coefficients have been determined.     

Development of porous silicon matrix and characteristics of porous silicon/tin oxide structures

Porous silicon (PSi) was formed at different current densities in the range of 5-60 mA/cm² by electrochemical anodized etching in HF for different durations in the range of 10-30 min. Above this PSi structure, SnO₂ films were deposited by the spin coating technique. The PSi has been characterized by X-ray diffraction studies. Peaks pertaining to PSi along with those corresponding to SnO₂ are observed. Atomic force microscopic studies indicate that very fine needle like silicon nanostructures are observed which is the result of the best PSi structure formed at 30 mA/cm². For the SnO₂ covered PSi structures, larger grains are observed with uniform coverage. The PSi samples prepared at current densities above and below 30 mA/cm² show PL spectra with asymmetric and overlapped peaks. The PL profile of thin SnO₂ film coated on PSi shows a peak at 633 nm and a small hump at about 660 nm.     

Structural and optical spectroscopy of LiNbO3:Tm nanocrystals embedded in a SiO2 glass matrix

Transparent SiO₂:Li₂O:Nb₂O₅ glass doped with Tm³⁺ has been prepared by the sol–gel method, and heat-treated in air (HT) at temperatures between 500 and 800 °C. X-ray diffraction (XRD) patterns and Raman spectroscopy show SiO₂ and LiNbO₃ phases in samples HT above 650 °C, and a NbTmO₄ phase for T > 750 °C. The XRD SEM analysis show increasing particle size and number with the increase of HT temperature. Intra-4f¹² transitions due to Tm³⁺ ion dispersed in the matrix are observed in samples with T > 650 °C. The luminescence is dominated by the ¹G₄ → ³F₄ (～650 nm), ¹D₂ → ³F₃ (～780 nm), ³H₄ → ³H₆ (～800 nm), ³H₅ → ³H₆ (～1200 nm) and ³H₄ → ³F₄ (～1500 nm) transitions under resonant excitation to the ion levels.     

Optical phonons and Raman scattering in ternary II–VI spheroidal nanocrystals embedded in a glass matrix

Theoretical and experimental studies of the spatial phonon confinement in ternary CdS_xSe_1−x nanocrystals embedded in a glass matrix formed by the composites (40)SiO₂₋(30)Na₂CO₃–(29)B₂O₃–(1)Al₂O₃ (mol%) + [(2)CdO + (2)S + (2)Se] (wt%) were carried out. From the analysis of the surface phonon modes, the theoretical procedure has allowed the determination of the geometrical characteristics of the nanocrystals. The calculated frequencies were compared with the experimental values obtained from the Raman spectra of CdS_xSe_1−x nanocrystals grown under different thermal treatments. A good correlation between the experimental and calculated CdS-like and CdSe-like surface optical modes was observed. The Raman selection rules and their connection with the nature of the surface optical phonons is discussed in order to use Raman spectroscopy as a probe to determine the composition x and the geometrical shape of the semiconductor nanocrystals.     

FMR and DSC study of maghemite nanoparticles in PMMA polymer matrix

Nanocomposite samples of poly(methyl methacrylate) (PMMA) polymer with a γ-Fe₂O₃ (maghemite) filler have been synthesised and studied by ferromagnetic resonance (FMR) and differential scanning calorimetry (DSC) methods. Two types of samples have been investigated: containing 5 wt%, 10 wt% of a maghemite filler. DSC measurements have revealed an increase in the glass transition temperature T_g and a decrease in the heat capacity c_p with an increase in the concentration of nanoparticles. A FMR study in the 4–300 K temperature range has shown the presence of an asymmetric spectrum that has been analyzed in terms of two Gaussian-shaped components arising from the assumed magnetic anisotropy of the nanoparticles. The FMR investigation has exposed the temperature range of a superparamagnetic regime (60–290 K) and the blocking temperature of T_B ～ 60 K. In that range a shift in the resonance line δH_r and the linewidth ΔH is related by δH_r ～ (ΔH)ⁿ, where n = 2.79 indicates a fair amount of disorder in the maghemite system. An analysis of the FMR spectra in terms of two component lines has shown the importance of the dipole–dipole interaction for higher concentrations of maghemite and for T > 220 K.     

Study of the absorption edge of SnO2 nanoparticles embedded in silica films

Silica thin films with embedded SnO₂ nanoparticles have been grown on transparent substrates by the sol–gel method. Tin dioxide crystals with cassiterite structure are semiconductors with a wide band gap of ~ 3.6 eV. Optical absorption spectroscopy in the near ultraviolet–visible range has been exploited to probe nanostructuring features of such nanocrystals. The results show that the sintering conditions modify crystallite mean size and enable the occurrence of quantum confinement effects. The outcome is in accordance with transmission electron microscopy data conducted on analogous bulk samples.     

Carbon covered magnetic nickel nanoparticles embedded in PBT-PTMO polymer: Preparation and magnetic properties

Fine particles of a face-centered-cubic phase of Ni covered with a graphite layer were prepared and embedded in a PBT-block-PTMO polymer at a concentration of 0.1 wt%. The mean crystalline size of Ni varied from 8 to 30 nm. A magnetic resonance study of the obtained nanocomposites was carried out in the 4–300 K temperature range using an electron paramagnetic resonance spectrometer. An almost symmetrical and very intense magnetic resonance line was recorded for all the investigated samples. The resonance line was centered at g = 2.253(2) (the resonance field H_r = 3003(1) Gs) and had a peak-to-peak linewidth ΔH_pp = 693(2) Gs at room temperature. The amplitude of the resonance line increased with a temperature increase in the low temperature range (T < 40 K) and in the high temperature range (T > 100 K) but was constant at intermediate temperatures. The resonance field H_r decreased and linewidth ΔH_pp increased as the temperature decreased from room temperature what was similar to the changes observed for other systems of nanoparticles. The thermal gradient of the resonance field, ΔH_r/ΔT, strongly depended on the temperature range. The temperature shift of the resonance field and the linewidth were analyzed in terms of the demagnetizing fields of nonspherical agglomerates. A strong change of linewidth and resonance field was registered below 40 K due to the freezing of the spin system’s dynamical magnetic fluctuations. A comparison was made of the results obtained on the Ni/C with the previous measurements on γ-Fe₂O₃ nanoparticles embedded in a copolymer.     

Inclusion of carbon nanotubes in a TiO2 sol-gel matrix

We report here the successful inclusion of carbon nanotubes (CNs) into a TiO₂ matrix prepared by a sol-gel method. The presence of CNs in the sol-gel matrix and the structure of the film were analyzed principally by transmission electron microscopy. Complementary information about the behavior of embedded carbon nanotubes versus heat treatment and ion irradiation were obtained by X-ray photoelectron spectroscopy. The elaboration of an inorganic matrix containing embedded carbon nanotubes leads to a new nanocomposite. The possible applications of this nanocomposite are discussed.     

Study of the oxygen role in the photoluminescence of erbium doped nanocrystalline silicon embedded in a silicon amorphous matrix

We have produced and studied erbium doped nanocrystalline silicon thin films with different oxygen and hydrogen content in order to evaluate the influence of the matrix on the Er³⁺ emission and on the 0.89 eV and 1.17 eV bands. Films were grown by reactive magnetron sputtering on glass substrates under several different conditions (RF power, Er content and gas mixture composition) in order to obtain different microstructures. The structural parameters and the chemical composition of the samples were obtained by X-ray in the grazing incidence geometry, Raman spectroscopy and Rutherford back scattering analysis. Using X-ray technique combined with Raman spectroscopy information on the crystalline fraction and the average crystallite size of Si nanocrystals was obtained. Dependence of the 0.89 eV and 1.17 eV peaks in Si heterogeneous matrixes on the films crystallinity and O/H ratio has been analyzed.     

Raman study of fluorine effects on silica with embedded SnO2 nanoparticles

Silica samples with embedded SnO₂ nanoparticles have been produced by sol–gel method with a synthesis modified by the introduction of fluorinated silicon precursors. The structural features of silica network and SnO₂ nanophase have been mapped inside the samples by means of confocal micro-Raman scattering spectroscopy. The results show that the detection of residual fluorine into the matrix as Si–F groups is accompanied by network dehydration, by intensity depletion of the D₂ peak assigned to three-member rings of coordinated tetrahedra, and by SnO₂ nanoparticles with much larger sizes than in fluorine-free material.     

Phase transitions and macroscopic properties of NaNO3 embedded into porous glasses

The crystal structure and dielectric response of nanocomposite materials on base of porous glasses with average pore diameters of 320, 46 and 7 nm with embedding sodium nitrate have been studied by neutron diffraction and dielectric spectroscopy in low and high temperature phases up to melting. In porous glasses with 46 and 7 nm pores NaNO₃ forms dendrite nanoclusters with "diffraction" sizes of 50(2.5) and 20(2) nm. Decreasing of particle sizes results in decreasing of T_c (temperature of order-disorder orientational transition) and T_melt and in smearing of structure phase transition. The values of critical exponent β for orientational transition are estimated from temperature dependences of intensities of superstructure elastic peaks for these three types of nanocomposite materials.     

Optical constants and band gap expansion of size controlled silicon nanocrystals embedded in SiO2 matrix

Silicon nanocrystals (Si-NCs) with different sizes embedded in SiO₂ matrix were synthesized by phase separation and thermal crystallization of SiO_x/SiO₂ supperlattice approach. The optical constants and band gap expansion of Si-NCs have been investigated by spectroscopic ellipsometry, based on the Maxwell–Garnett effective medium approximation and the Forouhi–Bloomer optical dispersion model. Similar spectra shapes but smaller values of Si-NCs optical constants with respect to bulk crystalline Si is observed. With the size of Si-NCs decreasing from 6 nm to 2 nm, the band gap increases from 1.64 eV to 2.56 eV. The band gap expansion, as compared to bulk crystalline Si, which agrees with the prediction of first-principles calculations based on quantum confinement effect, is presented in this paper.     

Characterisation and mesomorphic behaviour of liquid crystals with dispersed PdCl2 nanoparticles

The synthesis and characterisation are carried on liquid crystalline (LC) p-dodecyloxy benzoic acid (12OBA) with 1 and 2 wt% for PdCl₂ nanoparticles dispersion. Further, characterizations are carried out by different spectroscopic techniques like X-ray diffraction spectrometric studies, scanning electron microscopy, Fourier transform infrared and differential scanning calorimetry (DSC). Textural determinations of the synthesised compounds are recorded by using polarising optical microscope (POM) attached with a hot stage and camera. The results show that the dispersion of PdCl₂ nanoparticles in 12OBA exhibits Nematic phases as same as the pure 12OBA with reduced clearing temperature as expected. Further, the nematic thermal ranges are quenched and the smectic C thermal range has been increased while performing both DSC and POM with the dispersion of 1 wt% PdCl₂ nanoparticles. Size dependence on bonding nature with LC compounds is established.     

Facile and one-pot synthesis of magnetic nanoparticles containing mesoporous carbon

Abstract

Magnetic nanoparticles containing mesoporous carbon materials have been synthesized via an one-pot strategy associated with a direct carbonization process from resol, metal ion sources (Co(NO₃)₂·6H₂O, Ni(NO₃)₂·6H₂O) and triblock copolymer F127. The samples exhibited well-ordered 2-dimensional (2-D) hexagonal mesostructures with p6mm symmetry. The Brunauer-Emmet-Teller (BET) surface area and pore size 1.0wt%Co- and 1.0wt%Ni-FDU-15(700) with 1.0?wt% Co and 1.0?wt% Ni content were 700, 528 m²/g and 17.2, 36.4 Å, respectively, after carbonization at 700?°C. The saturation magnetization values of 1.0wt%Co- and 1.0wt%Ni-FDU-15(700) after carbonization at 700?°C were 1.3 and 1.0?emu/g, respectively.     

Switchable control of hydrophilicity and hydrophobicity in conjugated polymer nanoparticles by carbon dioxide

Abstract

We synthesized yellow-emissive, fluorine-based conjugated polymer to fabricate CO₂-responsive conjugated polymer nanoparticles (CPNPs). The CPNPs were functionalized with tertiary amine to have responsiveness to CO₂. The amine-functionalized CPNPs became hydrophilic by CO₂, bubbling because the bubbling led to formation of cationic ammonium ions at the side chains of the hydrophobic CPNPs. This resulted in high dispersion stability in aqueous phase even after vigorous mixing in the presence of organic phase (1-octanol). Subsequent N₂ bubbling was done to remove CO₂ present in water, leading to deprotonation of the side chains of CPNPs. The CPNPs became hydrophobic and moved to the organic phase. The CO₂-responsive property was based on the amine groups in the side chain of polymer that reversibly interacted with bicarbonate ion (HCO₃^-), formed by dissolving CO₂ in water, generating switchable hydrophilicity and hydrophobicity.