Precipitation Of Nanocrystals Research Articles

Fourier transform infrared spectroscopy of annealed silicon-rich silicon nitride thin films

A correlation between bonding changes in silicon-rich silicon nitride films, subjected to high temperature annealing under N2 ambient, and the formation of silicon nanocrystals is presented. The postannealing appearance of a shoulder between 1000 and 1100 cm−1 in the Fourier transform infrared (FTIR) spectra of silicon-rich silicon nitride films is attributed to a reordering in the films toward an increased SiN4 bonding configuration resulting from the precipitation of silicon nanocrystals. The FTIR monitoring of bonding changes in these films allows for the indirect verification of silicon nanocrystal formation.

Characterization of amorphous and crystalline silicon nanoclusters in ultra thin silica layers

The nucleation and structure of silicon nanocrystals formed by different preparation conditions and silicon concentrations (28–70 area %) have been studied using transmission electron microscopy (TEM), energy filtered TEM, and secondary ion mass spectroscopy. The nanocrystals were formed after heat treatment at high temperature of a sputtered 10 nm thick silicon rich oxide on 3 nm SiO2 layer made by rapid thermal oxidation (RTO) of silicon. Nanocrystals precipitate when the excess silicon concentration exceeds 50 area %. Below this percentage amorphous silicon nanoclusters were found. In situ heat treatment of the samples in the TEM showed that the crystallization requires a temperature above 800 °C. The nanocrystals precipitate in a 4 nm band, 5 nm from the Si substrate, and 4 nm from the SiO2 sample surface. The silicon nucleates where the excess Si concentration is the highest. The top surface has less excess Si due to reaction with oxygen from the ambient during annealing. The SiO2-RTO layer is more Si rich due to Si diffusion from the SiO2–Si layer into RTO. Twinning and stacking faults were found in nanocrystals with 4–10 nm in diameter. These types of defects may have large effects on the usability of the material in electronic devices. Both single and double twin boundaries have been found in the nanocrystals by high resolution TEM. Image simulations were carried out in order to obtain more information about the defects and nanocrystals. The stacking faults are extrinsic and located in the twin boundaries.

Effect of Iron Substitution on Structure and Optical Properties of Nanocrystalline CaTiO<sub>3</sub>

Nanocrystalline CaTiO3 powders doped with Fe2O3 have been prepared using a soft chemical route. Precipitation of CaTiO3 nanocrystals has been studied by monitoring the exothermic peak in their DSC spectra. The crystal growth temperature of the samples depends on the concentration of iron. Surface morphology, crystal structure, optical and electrical properties of the nanostructures are investigated. X-ray diffraction study shows that the as-prepared powders are amorphous in nature and CaTiO3 phase formation starts at around 500 0C. Rietveld analysis revealed that the particle size of iron substituted CaTiO3 is in nanometer range. Optical bandgap of the nanostructures varies from 4.3 to 3.7 eV for the variation of iron concentration from 0.05 to 0.2 mole %.

Effect of cooling on the optical properties and crystallization of UV-exposed photo-thermo-refractive glass

Photo-thermo-refractive (PTR) glass is a multi-component silicate glass that undergoes a refractive index change after UV-exposure and thermal treatment. This photo-thermo-refractivity is due to the precipitation of sodium fluoride nano-crystals; thus the glass remains highly transparent in the visible and near-IR regions. Up to now, most studies focused on the influence of temperature and duration of thermal treatment on the PTR glass properties, but no attention was given to the cooling step after thermal treatment. In this paper, the influence of cooling on crystallization and resulting optical properties of UV-exposed PTR glass is studied. We show that cooling between the nucleation and growth treatments is a mandatory step to achieve the full benefits of the first heat-treatment, i.e., a large number of small crystals. We also show that the main part of the refractive index change occurs on the cooling path after pre-nucleation. Non-isothermal DSC study associated with in situ pre-nucleation treatment shows that pre-nucleation enhances crystallization only if the temperature is decreased below T g before the second (development) treatment. Using high temperature photometric measurements of the absorption spectra of UV-exposed PTR glasses, we tentatively associate that effect with the presence of liquid drops of a silver containing phase during regular pre-nucleation treatment. This fact explains the necessity to cool such drops below their melting point to obtain nucleation centers for efficient precipitation of NaF nano-crystals.

Silicon nanocluster crystallization in SiOx films studied by Raman scattering

Precipitation and crystallization of Si nanocrystals have been monitored by means of Raman spectroscopy. SiOx films with different compositions have been deposited by low-pressure chemical-vapor deposition technique onto silica substrates and treated to temperatures exceeding 800 °C. The evolution of the Raman signal with the thermal budget reveals that the silicon transition from amorphous to crystalline state shifts to higher temperatures as the Si content in the layers is lowered. A rather complete crystallization of the nanoparticles is achieved after annealing at 1250 °C for a Si excess lower than 20%, while for higher excesses the crystalline fraction reaches only 40%, suggesting the formation of a crystalline core surrounded by an amorphous shell. The Raman spectra have been analyzed by a phonon confinement model that takes into account stress effects. An increasing nanocrystal size, from 2.5 to 3.4 nm, has been estimated when the Si excess varies from 16 to 29 at. %. For small Si nanocrystals a strong hydrostatic stress has been observed, induced by a very abrupt transition with the surrounding SiO2. Its magnitude correlates with the increase in thermal budget required for the crystallization of the amorphous clusters. This study underlines the fundamental role of hydrostatic stress in retarding the crystallization of Si nanoclusters.

Cooperative Quantum Cutting of Nano-Crystalline BaF2:Tb3+, Yb3+ in Oxyfluoride Glass Ceramics

We report on cooperative quantum cutting in Tb3+-Yb3+ codoped glass ceramics. Precipitation of BaF2 nano-crystals is confirmed by XRD and HRTEM analysis. Near-infrared emission due to transition of Yb3+ ions under 485 nm excitation indicates cooperative energy transfer from Tb3+ to Yb3+. The quantum efficiency of this process reaches 145%. The realization of quantum cutting in glass ceramics may have promising applications in solar cells.

Synthesis of Cd-free water-soluble ZnSe 1− xTe x nanocrystals with high luminescence in the blue region

Cd-free core–shell nanocrystals (ZnSe 1− x Te x /ZnS, 0 ⩽ x ⩽ 0.5 ) emitting in the pure blue region were prepared using an aqueous colloidal method followed by post-preparative ultraviolet irradiation. The photoluminescence (PL) efficiency and peak wavelength were maximized (40%, 448 nm) through a heat treatment of the nanocrystals during irradiation in a Zn 2+-containing thiol solution. Because of the small lattice mismatch between ZnSe 1− x Te x and ZnS, post-preparative irradiation at high temperature resulted in the formation of a thick ZnS shell (ca. 1.6 nm) around the core (ca. 2.2 nm in diameter) without deterioration of PL efficiency. Surface substitution of Te by S and size-selective precipitation of nanocrystals before ultraviolet irradiation resulted in intense PL. Quantum mechanical calculations show that the wave function of the electron of the exciton in the Te-containing core extends well into the ZnS shell. The calculations also reveal that a thick shell can confine the electrons inside the particles and thereby improve the PL efficiency and stability against the pH of the solution. Nanocrystals that had been post-preparatively irradiated showed good stability in solution and in a glass matrix even after months of storage in air.

Gain cross-section of 1.06 μm emission in Nd3+-doped SiO2–LaF3 glass–ceramics prepared by sol–gel method

The room temperature near-infrared emission at 1.06μm corresponding to the 4F3/2→4I11/2 laser transition in Nd3+-doped SiO2–LaF3 transparent glass–ceramics prepared by sol–gel method has been characterized in terms of the product of emission cross-section and fluorescence lifetime. Stimulated emission cross-sections have been calculated from experimental emission spectra using the Füchtbauer–Ladenburg equation. The precipitation of LaF3 nanocrystals after heat-treatment, confirmed by X-ray diffraction, induces a significant increment of the 1.06μm near-infrared emission compared with the as made sample. An enhancement of emission cross-section of about 10% has been achieved with increasing temperature of heat-treatment of the nano-ceramming process from 800 to 1000°C, which is related to the growth of LaF3 nanocrystals.

Strain Sensitivity in Ion-implanted Polymers

AbstractIon implantation process was used to fabricate ultra-thin conducting films in inert polymers and to tailor the surface electrical properties for strain gauge applications. To this aim, polycarbonate substrates were irradiated at room temperature with low energy Cu+ ions of 60 keV at 1 μA/cm2 and with doses ranging from 1×1016 to 1×1017 ions/cm2. XRD and TEM measurements on the nanocomposite surfaces demonstrated the spontaneous precipitation of Cu nanocrystals at 1×1016 ions/cm2 fluence. These nanocrystals were located at about 50 nm - 80 nm below the polymer surface in accordance with TRIM calculations. Optical absorption spectra exhibited a surface plasmon resonance (SPR) at 2 eV, in accordance with the formation of Cu nanoparticles. For doses of 5×1016 ions/cm2 the formation of a continuous nanocrystalline Cu subsurface film occurred and a well pronounced SPR peak was observed. Otherwise, for higher doses (1×1017 ions/cm2) a damaged and structurally disordered film was obtained and the SPR peak was smeared out. Electrical conductivity measurements clearly indicated a reduced electrical resistance for the samples implanted with a doses up to 5×1016 ions/cm2, whereas higher doses (1×1017 ions/cm2) resulted detrimental for the electrical properties, probably due to the radiation induced damage. The dependence of electrical resistance from surface load was evaluated during compression tests up to 3 MPa. A significant linear variation of the electrical resistance with the surface load was found and could be related to the changes in the spatial distribution of nanoparticles inside the copper film.

Blue and Yellow Electroluminescence of MOSLED Made on Si-rich SiOx Film with Detuning Buried Si Nanoclusters Size

ABSTRACTWe have demonstrated the blue and yellow electroluminescence of MOSLEDs made on Si-rich SiOx film with buried Si nanoclusters of different sizes. The situation of dehydrogenation of Si nanocrystals within the SiOx film becomes more pronounced then the re-growth of SiO2 matrix along with the prolongation of annealing time period. A linear variation on the O/Si composition ratio of the Si-rich SiOx film related to the deposition recipe is reported, giving rise to the precipitation of Si nanocrystals with different size. With such synthesis conditions, the SiOx films result in relatively strong photoluminescence at blue and yellow colors. From the comparison of the I–V curves we can conclude that there is a linear decrease on the threshold voltage of the SiOx based MOSLEDs by decreasing the thickness of the SiOx layer. According to EL pattern, we could demonstrate that the yellow- and blue-light pattern can be observed at 5.5 and 7.25 MV/cm, respectively.

High performance amorphous steel coating prepared by HVOF thermal spraying

Amorphous steel coating with a composition of Fe(48)Cr(15)Mo(14)C(15)B(6)Y(2) was prepared by means of high velocity oxygen fuel (HVOF) thermal spraying. Microstructural analysis gives the information about porosity, oxidation and nanocrystal precipitation. Properties including the hardness, wear, corrosion and magnetic behaviors of the coating were examined. It was shown that the microhardness and wear resistance of the coating are supenor to those of electroplated Cr and Ni-based amorphous coating. In 1 M HCl aqueous solution, the coating demonstrated a low passive current density, a wide passive region and the same transpassive potential as bulk amorphous sample. Furthermore, the coating remained paramagnetic attribute in room temperature. (C) 2007 Elsevier B.V. All rights reserved.

Enhanced luminescence from transparent Ni 2+-doped MgO–Al 2O 3–SiO 2 glass ceramics by Ga 2O 3 addition

Transparent Ni 2+-doped MgO–Al 2O 3–SiO 2 glass ceramics without and with Ga 2O 3 were synthetized. The precipitation of spinel nanocrystals, which was identified as solid solutions in the glass ceramics, could be favored by Ga 2O 3 addition and their sizes were about 7.6 nm in diameter. The luminescent intensity of the Ni 2+-doped glass ceramics was largely enhanced by Ga 2O 3 addition which could mainly be caused by increasing of Ni 2+ in the octahedral sites and the reduction of the mean frequency of phonon density of states in the spinel nanocrystals of solid solutions. The full width at half maximum (FWHM) of emissions for the glass ceramics with different Ga 2O 3 content was all more than 200 nm. The emission lifetime increased with the Ga 2O 3 content and the longest lifetime is about 250 μs. The Ni 2+-doped transparent glass ceramics with Ga 2O 3 addition have potential application as broadband optical amplifier and laser materials.

Low-plasma and high-temperature PECVD grown silicon-richSiOx film with enhanced carrier tunneling and light emission

Low-plasma and high-temperature chemical vapor deposition of Si-richSiOx for concurrently enhancing the carrier tunneling and light emission efficiency is investigated. TheO/Si compositionratio of the SiOx film significantly decreases from 2 to 1.2 as the substrate temperature increases from 200 to400 °C, corresponding to the enhanced precipitation of Si nanocrystals in the Si-richSiOx. In comparisonwith stoichiometric SiO2, the Si-L2,3 transition induced kinetic energy loss of the primary electron transmitted through the Si-richSiOx samplegrown at 400 °C is red-shifted by 5 eV. The strongest Si nanocrystal related photoluminescence (PL) can be obtained fromthe Si-rich SiOx film prepared at a threshold plasma power of 30 W and substrate temperature of400 °C. In low-plasma and high-temperature deposited samples, the thresholdFowler–Nordheim (F–N) tunneling field and the indium tin oxide(ITO)-SiOx junction potentialbarrier height of ITO/SiOx /p-Si/Al metal-oxide-semiconductor light emitting diodes (MOSLEDs) are concurrentlyreduced due to the increasing density of Si nanocrystals precipitated within theSiOx matrix. A thermal activation energy of 0.8 eV was observed for initiating the F–Ntunneling process in the MOSLEDs. The electroluminescence (EL) intensity andefficiency of the MOSLEDs are improved by at least 10 dB due to the oxygendeficient plasma enhanced chemical vapor deposition (PECVD) of Si-richSiOx at low plasma power and high temperatures.

Evolution of Si (and SiC) nanocrystal precipitation in SiC matrix

Si 1− x C x films with varying ratio of carbon to silicon (C/Si) were fabricated by magnetron co-sputtering from a combined C and Si target. The composition in films was changed by adjusting the ratio of sputtered target's area between C and Si. Analysis of X-ray photoelectron spectroscopy for as-deposited films shows that C/Si atomic ratios of our films have ranges of 0.33–1.02. Thermal annealing of as-deposited films was carried out at various temperatures from 800 to 1100 °C in a conventional furnace. Fourier transform infrared spectra show a shift of Si–C stretching peak towards higher wavenumbers from ∼ 737 cm − 1 to ∼ 800 cm − 1 with increasing annealing temperature. From the results of Raman spectroscopy, X-ray diffraction and transmission electron microscopy, it was found that the dominant type of nanocrystals changes from Si to SiC in the films annealed at 1100 °C when the C/Si atomic ratio increases from 0.33 to 1.02.

Precipitation of nanocrystals in glasses by electron irradiation: An alternative path to form glass ceramics?

This letter demonstrates an alternative method to form gallium silicate glass ceramics using high-energy electron irradiation. Compared with glass ceramics obtained from the conventional thermal treatment method, the distribution and crystal sizes of the precipitated Ga2O3 nanoparticles are the same. An advantage of this method is that the spatial distribution of the precipitated nanoparticles can be easily controlled. However, optically active dopants Ni2+ ions do not participate in the precipitation during electron irradiation.

Calorimetric study of the crystallization kinetics ofCu47Ti33Zr11Ni8Si1metallic glass

The isochronal and isothermal activation energies for the primary crystallization process of ${\mathrm{Cu}}_{47}{\mathrm{Ti}}_{33}{\mathrm{Zr}}_{11}{\mathrm{Ni}}_{8}{\mathrm{Si}}_{1}$ metallic glass powders subjected to varying thermal treatments have been evaluated by differential scanning calorimetry and determined using the Kissinger approach and the Johnson-Mehl-Avrami (JMA) analysis, respectively. The values of the differential Avrami exponent are also determined from the isothermal data. Assuming diffusion-controlled growth, it is shown that thermal treatment of the samples in the supercooled liquid region considerably influences the behavior of the nucleation rate during the crystallization process. Microstructural investigations indicate that the thermal treatment is accompanied by precipitation of fine nanocrystals in an amorphous matrix. The values for the activation energies determined by both the Kissinger approach and the JMA analysis are similar for the as-prepared powder, but a significant difference is found for the thermally treated powders. This discrepancy is explained on the basis of the fundamental assumptions made in the models. It will be shown that the Kissinger method fails if the differential Avrami exponent changes significantly during the transformation process.

Kinetic regularities of the precipitation of PbS nanocrystals in sodium zinc silicate glasses

The kinetics of formation of nanostructured glass-ceramic materials based on silicate glass doped with lead sulfide PbS is investigated using small-angle X-ray scattering and X-ray powder diffraction. The influence of two-stage heat treatment on the properties of the material is studied. It is established that, in the initial stages of the precipitation during secondary heat treatment, the volume of the lead sulfide phase increases proportionally with the cube of the nanocrystal size; i.e., the number of growing particles is constant and does not depend on the temperature of the secondary heat treatment. At high temperatures of the secondary heat treatment, an increase in the heat treatment time leads to a decrease in the volume of the crystalline phase. The inference is made that this effect is possibly caused by the reaction PbS + Na2O = PbO + Na2S, which should bring about a decrease in the lead sulfide concentration in the matrix and the dissolution of precipitated nanocrystals. The observed kinetics of crystal growth has defied description in terms of the known mechanisms. This is explained by the occurrence of a competing process of dissolution of the crystalline phase that is responsible for the absence of the Ostwald ripening stage, which is governed by the universal kinetics of diffusion phase decomposition at the late stages in supersaturated solid solutions.

Localized CO<SUB>2</SUB> Laser Annealing Induced Dehydrogenation/Ablation and Optical Refinement of Silicon-Rich Silicon Dioxide Film with Embedded Si Nanocrystals

CO2 laser annealing induced effects of dehydrogenation, Si nanocrystal precipitation, ablation, and optical refinement in PECVD grown SiO1.25 film are investigated. Dehydrogenation shrinks SiO1.25 thickness by 40 nm after annealing at laser intensity (Plaser) of 4 kW/cm(2) for 1.4 ms. As Plaser increases to 6 kW/cm(2), the photoluminescence (PL) red-shifts to 806 nm due to the size enlargement of Si nanocrystals, while a reduced optical bandgap energy from 3.3 to 2.43 eV and an enlarged refractive index from 1.57 to 1.87 are also observed. Transmission electron microscopy analysis reveals that the randomly oriented Si nanocrystals exhibit an average diameter of 5.3 nm and a volume density of 1.9 x 10(18) cm(-3). CO2 Laser ablation initiates at intensity higher than 7 kW/cm(2), which introduces numerous structural defects with a strong PL at 410 nm. Such an ablation inevitably leads to a blue-shifted optical bandgap energy from 2.43 to 2.76 eV as Plaser enlarges from 6 to 12 kW/cm(2) are concluded.

Luminescence of Er3+-doped nanostructured SiO2–LaF3 glass-ceramics prepared by the sol–gel method

Transparent glass ceramics with composition of 95SiO2–5LaF3 doped with 0.1mol% of Er3+ were synthesized by thermal treatment of precursor sol–gel glasses. Segregated LaF3 nanocrystals in the glass were confirmed from a structural analysis performed by X-ray diffraction. Blue, green and red efficient up-conversion emissions were observed under 980nm excitation at room temperature. Under this excitation near infrared down-conversion at 1.55μm is also observed. These results could be attributed to the precipitation of LaF3 nanocrystals and the incorporation of most Er3+ ions in these nanocrystals. The mechanisms involved in the up-conversion emissions could be ascribed to two and three photon processes.

Microphotoluminescence and Microphotoreflectance Analyses of$hboxCO_2$Laser Rapid-Thermal-Annealed$hboxSiO_x$Surface With Buried Si Nanocrystals

The optical properties of a SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> film rapid-thermal-annealed (RTA) by CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser are primarily investigated. The microphotoluminescence (mu-PL) and high-resolution transmission electron microscopy (HRTEM) analyses indicate that the precipitation of random-oriented Si nanocrystals can be initiated when laser intensity (P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">laser</sub> ) is larger than 4.5 kW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . At P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">laser</sub> of 6kW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , the Si nanocrystal exhibits a largest diameter of 8 nm and a highest density of 4.5times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , which emits strong PL at 790-825 nm. The microphotoreflectance of the CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser RTA SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> film reveals a volume density product dependent refractive index increasing from 1.57 to 1.87 as P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">laser</sub> increases from 1.5 to 7.5 kW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Nonetheless, the laser ablation of the SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> film occurs with a linear ablation slope of 35 nm/kW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at beyond 7.5 kW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , which terminates the enlargement of Si nanocrystals, degrades the near-infrared PL, and slightly reduces the refractive index of the CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser RTA SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> film