Effects of Annealing on Microstructure and Optical Properties of TiO2 Sculptured Thin Films

The evolution of microstructure and optical properties of TiO2 sculptured thin films under thermal annealing is reported. XRD, field emission SEM, UV-Vis-NIR spectra are employed to characterize the microstructural and optical properties. It is found that the optimum annealing temperature for linear birefringence is 500℃. The maximum of transmission difference for linear birefringence is up to 18%, which is more than twice of that in as-deposited thin films. In addition, the sample annealed at 500℃ has a minimum of column angle about 12℃. The competitive process between the microstructural and optical properties is discussed in detail. Post-annealing is a useful method to improve the linear birefringence in sculptured thin films for practical applications.     

Preparation and characterization of C54 TiSi2 nanoislands on Si (1 1 1) by laser deposition of TiO2

We present the preparation of C54 TiSi₂ nanoislands on Si (1 1 1) with a method of the pulsed laser deposition of titanium oxide thin films. The TiO₂ thin films with nominal thicknesses of 1 nm on Si (1 1 1) were annealed at 850 °C for about 4 h in situ. The X-ray diffraction patterns and the X-ray photoelectron spectra indicate that the nanoislands are in C54 TiSi₂ phase. The characterization using a scanning tunneling microscope shows that the nanoislands with triangular, polygonal and rod-like shapes on Si (1 1 1) exhibit the Volmer-Weber growth mode. The sizes of the polygonal islands distribute in two separated ranges. For the small islands, they have a narrow lateral size distribution centered at 4 nm and a height range in 0.6-3.6 nm, while for the large islands, their lateral sizes are in the range of 12-40 nm and the heights in the range of 4-9 nm. The sizes of the well-shaped triangular islands are intermediate with the lateral sizes in range of 5-20 nm and the heights of 2-3.5 nm. The rod-like islands are about 50-200 nm in length, 5 nm in height and about 15-20 nm in width. The origination of the various shapes of the nanoislands is attributed to the symmetry of Si (1 1 1) substrate and the lattice mismatch between the C54 TiSi₂ and the Si (1 1 1) surface.     

Fabrication of multilayered Ge nanocrystals embedded in SiOxGeNy films

Multilayered Ge nanocrystals embedded in SiO_xGeN_y films have been fabricated on Si substrate by a (Ge + SiO₂)/SiO_xGeN_y superlattice approach, using a rf magnetron sputtering technique with a Ge + SiO₂ composite target and subsequent thermal annealing in N₂ ambient at 750 °C for 30 min. X-ray diffraction (XRD) measurement indicated the formation of Ge nanocrystals with an average size estimated to be 5.4 nm. Raman scattering spectra showed a peak of the Ge-Ge vibrational mode downward shifted to 299.4 cm⁻¹, which was caused by quantum confinement of phonons in the Ge nanocrystals. Transmission electron microscopy (TEM) revealed that Ge nanocrystals were confined in (Ge + SiO₂) layers. This superlattice approach significantly improved both the size uniformity of Ge nanocrystals and their uniformity of spacing on the ‘Z’ growth direction.     

Formation of V2O3 nanocrystals by thermal reduction of V2O5 thin films

We report the formation of homogeneous and stable V₂O₃ nanocrystals, directly from V₂O₅ thin films, at 600 °C, as observed by using in situ electron microscopy experiments. Thermally-induced reduction of V₂O₅ thin films in vacuum is remarkably different when compared to reduction of V₂O₅ single crystals and results in the formation of nanophase V₂O₃. Thermally grown V₂O₃ nanocrystals exhibit hexagon or square shape and are stable at higher temperature as well as room temperature. The formation of stable nanocrystals through the reduction process in a non-chemical environment (vacuum) could provide a basis for understanding the complex processes of vanadium oxide phase transitions and for controlling the chemical processes to produce oxide nanocrystals.     

Microwave plasma synthesis of nano‐crystalline YSZ

Nanocrystalline yttria‐stabilized zirconia (YSZ) powder and compacts have been successfully synthesized by microwave plasma synthesis co‐evaporating Y and Zr precursors from a single source. Both liquid and solid mixtures of chemically homologous precursors were tested. Electron micrographs and X‐ray diffraction studies reveal small crystallites in the range 3–4 nm with a narrow size distribution. Crystallite sizes smaller than 25 nm are preserved even after subsequent annealing at temperatures up to 950 °C. A comparison with other gas‐phase synthesis routes of nanocrystalline YSZ is given. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Characterization of a thin CeO2-ZrO2-Y2O3 films electrochemical deposited on stainless steel

In this paper, we report for the first time formation of a thin CeO₂-ZrO₂-Y₂O₃ films electrodeposited on a stainless steel substrate. The samples have been characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and X-ray diffraction (XRD). The XRD and XPS data indicate formation of a solid solution and additional existence of Ce³⁺ states near the surface. After annealing, SEM examination has shown a microstructure formed by dispersed spherical agglomerates having a size between 20 and 60 nm.     

Deposition and characterization of La2Ti2O7 thin films via spray pyrolysis process

Thin films of La₂Ti₂O₇ have been deposited on fused silica and Si substrates by a spray pyrolysis method using ethylene glycol solution of La(III)-Ti(IV)-citrate complexes as starting material and O₂ as a carrier gas. The composition, crystal structure and morphology of the films are studied.     

Crystallization of amorphous Ge2Sb2Te5 films induced by a single femtosecond laser pulse

Crystallization is achieved in amorphous Ge₂Sb₂Te₅ films upon irradiation with a single femtosecond laser pulse. Transmission electron microscopy images evidence the morphology of the crystallized spot which depends on the fluence of the femtosecond laser pulse. Fine crystalline grains are induced at low fluence, and the coarse crystalline grains are obtained at high fluence. At the damage fluence, ablation of the films occurs.     

Highly selective growth of vertically aligned double‐walled carbon nanotubes by a controlled heating method and their electric double‐layer capacitor properties

Vertically aligned double‐walled carbon nanotubes (DWCNTs) with the highest selectivity of 90% were synthesized by a controlled heating method and their electric double‐layer capacitor characteristics were evaluated. DWCNT arrays had a specific capacitance of 83 F/g, which is one of the highest values among CNT arrays in a nonaqueous solution and is almost equivalent to that for single‐walled CNT (SWCNT) arrays reported previously. At the same specific capacitance, DWCNTs with superior structural properties are more promising for practical capacitors than SWCNTs. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High-Temperature Dielectric Response and Multiscale Mechanism of SiO2/Si3N4 Nanocomposites

The high-temperature dielectric properties of SiO₂/Si₃N₄ nanocomposites are investigated theoretically and experimentally. Its permittivities and loss tangents at the temperature ranging from room temperature to 1300°C at 9.0GHz are measured by the resonant cavity method. The SiO_2/Si₃N₄ nanocomposites show complex dielectric behaviour at elevated temperature, and a multi-scale model is proposed to describe the dependence of the dielectric properties in the SiO₂/Si₃N₄ on its compositional variations. Such a theory is needed so that the available property measurements could be extrapolated to other operating frequencies and temperatures.     

Versatile synthesis of rectangular shaped nanobat-like CuO nanostructures by hydrothermal method; structural properties and growth mechanism

This paper describes a simple and versatile method for growing highly anisotropic rectangular shaped nanobat-like CuO nanostructures by simple, low temperature and cost effective hydrothermal method. Field emission scanning electron microscopy illustrated that these CuO nanostructures have diameter of ∼70 nm, thickness of ∼8 nm and length of ∼174 nm. Structural analysis reveals that the CuO nanostructures have a high crystal quality with monoclinic crystal structure. X-ray photoelectron spectroscopy studies demonstrate that the sample is composed of CuO. The Raman study also indicates the single phase property and high crystallinity of as-grown CuO nanostructures. The plausible growth mechanism for the formation of nanobat-like CuO structure is proposed.     

Preparation of ferromagnetic γ - Fe2O3 nanocrystallites by oxidative co-decomposition of PEG 6000 and ferrocene

Highly crystalline and ferromagnetic γ-Fe₂O₃ nanocrystallites were prepared by controlled oxidative co-decomposition of PEG 6000 and ferrocene at a temperature of 450 ^°C under air atmosphere. The morphology, crystalline structure and preliminary magnetic properties of the as-synthesized nanocrystallites have been characterized by using transmission electron microscope (TEM), X-ray powder diffraction (XRD) and vibrating sample magnetometer (VSM). The highly crystalline γ-Fe₂O₃ nanocrystallites are in quasi-cubic shape with an average size of 30 nm and exhibit room-temperature ferromagnetism. The capping effect of PEG 6000 has also been investigated by thermogravimetry analysis (TGA) and Fourier transform infrared (FTIR) regarding controlling the size of the nanocrystallites and preventing the volatilization of ferrocene and thus raising the yield of the products. This simple method has a high yield of over 80% as well as low cost.     

Characterization of ultra-fast deposited polycrystalline graphite by a CO2 laser-assisted combustion-flame method

High deposition rate, 750 μm/min, crystalline graphite was deposited on WC substrates by a CO₂ laser-assisted combustion-flame method at laser powers between 300 and 800 W. The structures, which were identified as pillars, were characterized by various methods. The pillars were cylindrical in shape and grew to a size of approximately 3 mm in length and in a few minutes. The laser power did not affect the overall length of the pillar, but caused changes in the physical shape. X-ray and electron diffraction results revealed the pillars to be crystalline graphite regardless of the laser power. Investigation of the pillars by scanning electron microscopy showed two distinct microstructural areas: an inner core of dense material surrounded by an outer shell of lamellar-like material. The core/shell microstructure was unaffected by the level of CO₂ laser power.     

Optical properties of nanocrystalline-coated Y2O3:Er, Yb obtained by mechano-chemical and combustion synthesis

Y₂O₃:Er³⁺, Yb³⁺ nanocrystals have been obtained by ball milling and using a combustion synthesis procedure. In both cases the nanocrystals have been successfully coated with SiO₂ following the Stöber method. The average size of the as-synthesized nanoparticles has been estimated from X-ray diffraction patterns and transmission electron microscopy images. The dependence of the optical properties of these samples on synthesis procedure or dopant concentration has been investigated. Emission, excitation and lifetime measurements have been carried out. Upconversion luminescence has been detected in all samples and an enhancement of the red to green emission ratio has been observed in all samples after infrared compared to visible excitation. The mechanisms responsible for the upconversion phenomena have been discussed.     

Thermoelectric effect in laser annealed printed nanocrystalline silicon layers

Nanocrystalline boron and phosphorus doped silicon particles were produced in a microwave reactor, collected, and dispersed in ethanol. Pulsed laser annealing of spin‐coated films of these particles resulted in p‐ and n‐type conductive layers on flexible substrates if a threshold laser energy density of 60 mJ/cm² was exceeded. The thermopower of the laser sintered layers exhibits a distinct maximum at a doping concen‐ tration around 10¹⁹ cm^–3 for both boron and phosphorus doping with an absolute value of the Seebeck coefficient of about 300 µV/K. Since the thermal conductivity of the layers is reduced by nearly the same factor compared to bulk crystalline silicon as the electrical conductivity, these results are promising for the application of such nanocrystalline layers in thin film thermoelectric devices. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of Annealing Temperature on Electrical Properties of Ferroelectric Bi3.25La0.75Ti3O12 Capacitors

Lanthanum-modified bismuth titanate (Bi_3.25La_0.75Ti₃O₁₂, BLT) thin films arefabricated on platinized Si wafers by the sol-gel method, and the effect of annealing temperatures ranging from 650°C to 800°C on the electrical properties of Pt/BLT/Pt capacitors are investigated. It is found that polarization and leakage current of BLT capacitors strongly depend on the annealing temperature although all the capacitors demonstrate very similar characteristics, except the value of polarization, in pulse-width dependence, retention, and fatigue. Remanent polarization increases with the increase of annealing temperature, and annealing temperature of 700°C can yield thelargest remanent polarization, and then polarization decreases with increasing annealing temperature. For the 700°C annealed Pt/BLT/Pt capacitor, the remanent polarization 2P_r and the coercive field 2E_c, at an electric field of 226kV/cm, are 23.8μC/cm² and 130kV/cm, respectively. Dielectric breakdown voltages of BLT films annealed at 750°C and 800°C are much lower than those annealed at 650°C and 700°C. At 100kV/cm, the leakage currents of BLT films prepared at 650°C and 700°C are only1.5×10^-6A/cm² and 8.9×10-7A/cm², espectively. Moreover, all the Pt/BLT/Pt capacitors exhibit excellent retention properties after a cumulative time of 1× m 10⁴s and do not show any significant fatigue up to 1×10¹⁰ switching cyclesat frequency of 1MHz.     

Spatial characterization of the laser-induced plasma plumes generated by IR CO2 pulsed laser on carbon targets

We have used ferrocene and paraffin wax as novel precursor and solvent for the growth of iron oxide nanoparticles. The proposed method of growth has several advantages over existing methods of growth using iron pentacarbonyl a precursor. Highly crystalline and monodispersed particles are obtained which assemble in two- and three-dimensional hexagonal closed packed superlattices. Growth kinetics has been studied by varying concentration of the precursor and time of growth. A phenomenological model has been proposed to explain the growth kinetics.     

On the influence of the surface roughness onto the ultrathin SiO2/Si structure properties

The surface roughness of the semiconductor substrate substantially influences properties of the whole semiconductor/oxide structure. SiO₂/Si structures were prepared by using low temperature nitric acid oxidation of silicon (NAOS) method and then the whole structure was passivated by the cyanidization procedure. The influence of the surface morphology of the silicon substrate onto the electrical properties of ultrathin NAOS SiO₂ layer was investigated. Surface height function properties were studied by the AFM method and electrical properties were studied by the STM method. The complexity of analyzed surface structure was sensitive to the oxidation and passivation steps. For describing changes in the oxide layer structure, several fractal measures in an analysis of the STM images were used. This fractal geometry approach enables quantifying the fine spatial changes in the tunneling current spectra.     

An efficient co-doping of Eu and Er in CaAl2O4 aluminate phosphor

CaAl₂O₄:Eu²⁺ co-doped with varying concentrations of Er³⁺ was prepared by solid-state reaction method. Prepared materials with 1 mol% Eu²⁺ and 2-10 mol% of Er³⁺ were investigated for their photoluminescence properties. Phase, morphology and crystalline structure were investigated by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Broad band UV-excited luminescence was observed for CaAl₂O₄:Eu²⁺, Er³⁺ in the blue region (λ_max=440 nm) due to transitions from 4f⁶5d¹ to the 4f⁷ configuration of the Eu²⁺ ion. The Er³⁺ ion co-doping generates deep traps, which results in longer decay time for phosphorescence.     

Au-assisted electroless etching of silicon in aqueous HF/H2O2 solution

The Au-assisted electroless etching of p-type silicon substrate in HF/H₂O₂ solution at 50 °C was investigated. The dependence of the crystallographic orientation, the concentration of etching solution and the silicon resistivity on morphology of etched layer was studied. The layers formed on silicon were investigated by scanning electron microscopy (SEM). It was demonstrated that although the deposited Au on silicon is a continuous film, it can produce a layer of silicon nanowires or macropores depending on the used solution concentration.