Evidence for fast decay dynamics of the photoluminescence from Ge nanocrystals embedded in SiO2

We have investigated the origin of room temperature photoluminescence from ion-beam synthesized Ge nanocrystals (NCs) embedded in SiO₂ using steady state and time-resolved photoluminescence (PL) measurements. Ge NCs of diameter 4-13 nm were grown embedded in a thermally grown SiO₂ layer by Ge⁺ ion implantation and subsequent annealing. Steady state PL spectra show a peak at ∼2.1 eV originating from Ge NCs and another peak at ∼2.3 eV arising from ion-beam induced defects in the SiO₂ matrix. Time-resolved PL studies reveal double exponential decay dynamics on the nanoseconds time scale. The faster component of the decay with a time constant τ₁∼3.1 ns is attributed to the nonradiative lifetime, since the time constant reduces with increasing defect density. The slower component with time constant τ₂∼10 ns is attributed to radiative recombination at the Ge NCs. Our results are in close agreement with the theoretically predicted radiative lifetime for small Ge NCs.     

Surface acoustic phonon modes of Ge nanocrystals embedded in SiO2

Ge nanocrystals (NCs) embedded in SiO₂ are synthesized by ion implantation, and the surface vibrational modes of the Ge NCs are investigated using the low-frequency Raman scattering (LFRS) technique. LFRS studies show distinct low-frequency Raman modes in the range 6.5-21.2 cm⁻¹ for the Ge NCs depending on the implant dose and annealing temperature. These low-frequency Raman modes are attributed to the confined surface acoustic phonon modes of Ge NCs with (0,0) spheroidal mode and (0,3) torsional modes. Our results are in excellent agreement with the recent theoretical predictions of surface vibrational modes in Ge NCs.     

Double layer oxidation resistant coating for carbon fiber reinforced silicon carbide matrix composites

Double layer coatings, with celsian-Y₂SiO₅ as inner layer and Y₂Si₂O₇ as outer layer, were prepared by microwave sintering on the surface of carbon fiber reinforced silicon carbide matrix composite. Both celsian, Y₂SiO₅ and Y₂Si₂O₇ were synthesized by in situ method using BAS glass, Y₂O₃ and SiO₂ as staring materials. The sintering temperature was 1500 °C, and little damage was induced to the composite. The composition and micrograph of the fired coating were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The oxidation and thermal shock resistance of samples with doubled-layered coating were characterized at 1400 °C in air. After 150 min oxidation and thermal cycling between 1400 °C and room temperature for 15 times, the weight loss of double layer-coated sample was 1.22% and there were no cracks in the coating.     

Correlation between optical properties and Si nanocrystal formation of Si-rich Si oxide films prepared by plasma-enhanced chemical vapor deposition

We have investigated the phase separation and silicon nanocrystal (Si NC) formation in correlation with the optical properties of Si suboxide (SiO_x, 0 < x < 2) films by thermal annealing in high vacuum. The SiO_x films were deposited by plasma-enhanced chemical vapor deposition at different nitrous oxide/silane (N₂O/SiH₄) flow ratios. The as-deposited films show increased Si concentration with decreasing N₂O/SiH₄ flow ratio, while the deposition rate and surface roughness have strong correlations with the flow ratio in the N₂O/SiH₄ reaction. After thermal annealing at temperatures above 1000 °C, Fourier transform infrared spectroscopy, Raman spectroscopy, and transmission electron microscopy manifest the progressive phase separation and continuous growth of crystalline-Si (c-Si) NCs in the SiO_x films with increasing annealing temperature. We observe a transition from multiple-peak to single peak of the strong red-range photoluminescence (PL) with increasing Si concentration and annealing temperature. The appearance of the single peak in the PL is closely related to the c-Si NC formation. The PL also redshifts from ∼1.9 to 1.4 eV with increasing Si concentration and annealing temperature (i.e., increasing NC size). The good agreements of the PL evolution with NC formation and the PL peak energy with NC size distribution support the quantum confinement model.     

Thermal oxidation temperature dependence of 4H-SiC MOS interface

The thermal oxidation temperature dependence of 4H-silicon carbide (SiC) is systematically investigated using X-ray photoelectron spectroscopy (XPS) and capacitance-voltage (C-V) measurements. When SiC is thermally oxidized, silicon oxycarbides (SiC_xO_y) are first grown and then silicon dioxide (SiO₂) is grown. It is identified by XPS that the SiO₂ films fall into two categories, called SiC-oxidized SiO₂ and Si-oxidized SiO₂ in this paper. The products depend on thermal oxidation temperature. The critical temperature is between 1200 and 1300 °C. The interface trap density (D_it) of the sample possessing Si-oxidized SiO₂, at thermal oxidation temperature of 1300 °C, is lower than SiC-oxidized SiO₂ at and below 1200 °C, suggesting that a decrease of the C component in SiO₂ film and SiO₂/SiC interface by higher oxidation temperature improves the metal-oxide-semiconductor (MOS) characteristics.     

Mechanochemical Route for the Synthesis of Cobalt Ferrite–Silica and Iron–Cobalt Alloy–Silica Nanocomposites

CoFe₂O₄–SiO₂ and Fe–Co alloy–SiO₂ nanocomposites were prepared by high energy ball milling of Si, -Fe₂O₃, Co₃O₄ and SiO₂ mixtures, followed by thermal treatment under appropriate oxidative/reductive conditions. XRD and TEM measurements carried out after 50 h of milling show highly dispersed unidentified reaction products. Further thermal treatment at 900°C in air leads to spherical CoFe₂O₄ nanocrystals (NCs) with a narrow size distribution centered around 2.5 nm, uniformly dispersed in the amorphous SiO₂ matrix. Reduction of this sample in a H₂ flow at 800°C produces a mixture of dispersed metallic cobalt and iron silicate with traces of Fe–Co alloy NCs. Reduction of the as-milled sample, on the contrary, leads almost completely to Fe–Co alloy NCs uniformly dispersed in the SiO₂ matrix and with an average particles size around 11.2 nm.     

Radiative quantum efficiency of CdSe/ZnS core-shell colloidal solutions: Size-dependence

Thermo-optical parameters of CdSe/ZnS core-shell nanoparticles suspended in toluene were measured using a thermal lens (TL) technique. TL transient measurements were performed using the mode-mismatched dual-beam (excitation and probe) configuration. A He-Ne laser at λ_p = 632.8 nm was used as the probe beam and an Ar⁺ laser (at λ_e = 514.5 nm) was used as the excitation beam for studies as a function of both core size and concentration of CdSe/ZnS nanocrystals. The fraction thermal load (φ) and radiative quantum efficiencies (η) of the CdSe/ZnS were determined. Dependence on core size (∼2-5 nm) and concentration (∼0.01-0.62 mg/ml) was observed for both φ and η parameters.     

Dependence of photoluminescent properties of cubic Y2O3:Tb nanocrystals on particle size and temperature

Temperature-dependent spectral properties in the cubic Y₂O₃:Tb³⁺ nanocrystals (NCs, 10-70 nm) under 488 nm excitation were studied and compared to that in the bulk. In NCs, emission lines assigned to the ⁵D₄-⁷F_J (J=1-6) transitions of Tb³⁺ ions and a broad band originated from oxygen defects were observed. As a function of temperature, two intensity maximums of the ⁵D₄-Σ⁷F_J transitions appeared in the NCs, at ∼250 and ∼500 K, while in the bulk only one maximum appeared at ∼250 K. The relative intensity of the maximum at ∼500 K to that at ∼250 K increased with decreasing particle size. The intensity maximum of the band emissions that came from the oxygen defects appeared in the range of 500-600 K. The appearance of intensity maximum as a function of temperature was attributed to the rivalry between thermal quenching process and phonon-assisted excitation. The appearance of two maxima in the NCs was attributed to the luminescence contributed by different Tb³⁺ centers, the internal and the surface. The emission for the surface Eu³⁺ centers has higher quenching temperature in contrast to that for the internal centers.     

Low cost fabrication of SiO2 optical waveguides by laser direct writing on Ti-doped Sol-Gel films

A commercial direct laser writing (DLW) system operating at 1070 nm was used to fabricate SiO₂ optical waveguides on silicon wafers. A Ti-doped SiO₂ Sol-Gel film was deposited on the SiO₂/Si substrate by the dip-coating technique, based on which SiO₂ optical waveguides were patterned by DLW using a Ytterbium fiber laser and followed by chemical etching. The effects of laser parameters and the preheated temperature of Sol-Gel films on the dimensions of optical waveguides were studied systematically. The differences of etching rate between laser irradiated and non-irradiated areas in Sol-Gel films preheated at various temperatures are characterized by measuring the thickness of the films. Results demonstrate that the available laser power density range for laser densification and the width of the patterned optical waveguides are influenced strongly by the preheated temperature of the Sol-Gel films. The width of the optimized optical waveguide in this work is 25 μm. The minimum propagation loss of the fabricated optical waveguides is 1.7 dB cm⁻¹ at the wavelength of 1550 nm.     

Studies on fabrication of Ag/Tl2Ba2Ca2Cu3O10/CdSe heterostructures with electrochemical technique

The Ag/Tl₂Ba₂Ca₂Cu₃O₁₀/CdSe heterostructure was fabricated at room temperature by soft electrochemical processing technique for the first time. The formation of the heterostructure with non-diffusive interfaces was confirmed by X-ray diffraction. The crystallite sizes determined for Tl-2223 and CdSe films were 33 nm and 25 nm, respectively. The Tl₂Ba₂Ca₂Cu₃O₁₀ film electrodeposited onto Ag-substrate has shown the superconducting transition temperature T_c at 116.5 K and J_c = 2.1 × 10³ A/cm². These values were found to improve after the deposition of CdSe onto Ag/Tl-2223 films. The effect of red He-Ne laser irradiation on the superconducting properties of heterostructure are studied and discussed at length in this paper.     

Preferred orientations in nano-gold/silica/silicon interfaces

Gold in contact with silicon substrates Si(1 0 0), Si(1 1 1), and SiO₂ is studied by thermal evaporation and annealing in N₂ using the modified sphere-plate technique. The final orientation distribution of crystalline Au films grown on Si substrate systems that incorporate a native amorphous oxide layer of silica and Au on amorphous silica (SiO₂ glass) substrates is influenced by preferred orientations and twinning. Experimental evidence suggests that the orientation of Au{1 1 1} close packed planes (multiply twinned) was found to be of low-energy as the annealing temperature was increased to 530 °C and 920 °C. Additional orientations were observed for Au{1 0 0} on Si(1 0 0) substrates and Au{1 0 0}, {1 1 0}, and {3 1 1} on SiO₂ substrates. After annealing at 920 °C the size distribution of the gold particles was determined to be within the range of 20-800 nm while the morphology of gold surface appears spherical to faceted in character. These results show similarities to recent findings for smaller nano-size 1D particles, islands and thin Au films on silicon annealed over lower temperature ranges.     

Nitric acid oxidation of Si (NAOS) method for low temperature fabrication of SiO2/Si and SiO2/SiC structures

We have developed low temperature formation methods of SiO₂/Si and SiO₂/SiC structures by use of nitric acid, i.e., nitric acid oxidation of Si (or SiC) (NAOS) methods. By use of the azeotropic NAOS method (i.e., immersion in 68 wt% HNO₃ aqueous solutions at 120 °C), an ultrathin (i.e., 1.3-1.4 nm) SiO₂ layer with a low leakage current density can be formed on Si. The leakage current density can be further decreased by post-metallization anneal (PMA) at 200 °C in hydrogen atmosphere, and consequently the leakage current density at the gate bias voltage of 1 V becomes 1/4-1/20 of that of an ultrathin (i.e., 1.5 nm) thermal oxide layer usually formed at temperatures between 800 and 900 °C. The low leakage current density is attributable to (i) low interface state density, (ii) low SiO₂ gap-state density, and (iii) high band discontinuity energy at the SiO₂/Si interface arising from the high atomic density of the NAOS SiO₂ layer.For the formation of a relatively thick (i.e., ≥10 nm) SiO₂ layer, we have developed the two-step NAOS method in which the initial and subsequent oxidation is performed by immersion in ∼40 wt% HNO₃ and azeotropic HNO₃ aqueous solutions, respectively. In this case, the SiO₂ formation rate does not depend on the Si surface orientation. Using the two-step NAOS method, a uniform thickness SiO₂ layer can be formed even on the rough surface of poly-crystalline Si thin films. The atomic density of the two-step NAOS SiO₂ layer is slightly higher than that for thermal oxide. When PMA at 250 °C in hydrogen is performed on the two-step NAOS SiO₂ layer, the current-voltage and capacitance-voltage characteristics become as good as those for thermal oxide formed at 900 °C.A relatively thick (i.e., ≥10 nm) SiO₂ layer can also be formed on SiC at 120 °C by use of the two-step NAOS method. With no treatment before the NAOS method, the leakage current density is very high, but by heat treatment at 400 °C in pure hydrogen, the leakage current density is decreased by approximately seven orders of magnitude. The hydrogen treatment greatly smoothens the SiC surface, and the subsequent NAOS method results in the formation of an atomically smooth SiO₂/SiC interface and a uniform thickness SiO₂.     

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 atomically smooth SiO2/SiC interfaces at ∼120 °C by use of nitric acid oxidation method

Conventional thermal oxidation of SiC requires heating at ∼1100 °C. In the present study, we have developed a method of oxidizing SiC at low temperatures (i.e., ∼120 °C) to form relatively thick silicon dioxide (SiO₂) layers by use of nitric acid. When 4H-SiC(0 0 0 1) wafers are immersed in 40 wt% HNO₃ at the boiling temperature of 108 °C and the boiling is kept for 5 h after reaching the azeotropic point (i.e., 68 wt% HNO₃ at 121 °C), 8.1 nm thick SiO₂ layers are formed on the SiC substrates. High resolution transmission electron microscopy measurements show that the SiO₂/SiC interface is atomically flat and the SiO₂ layer is uniform without bunching. When SiC is immersed in an azeotropic mixture of HNO₃ with water from the first, the SiO₂ thickness is less than 0.3 nm. The metal-oxide-semiconductor (MOS) diodes with the SiO₂ layer formed by the nitric acid oxidation method possess a considerably low leakage current density.     

Microstructure and magnetic properties of FePt:Ag nanocomposite films on SiO2/Si(1 0 0)

FePt:Ag nanocomposite films were prepared by pulsed filtered vacuum arc deposition system and subsequent rapid thermal annealing on SiO₂/Si(1 0 0) substrates. The microstructure and magnetic properties were investigated. A strong dependence of coercivity and ordering of the face-central tetragonal structure on both Ag concentration and annealing temperature was observed. With Ag concentration of 22% in atomic ratio, the coercivity got to 6.0 kOe with a grain size of 6.7 nm when annealing temperature was 400 °C.     

Investigations on structural and magnetic properties of cobalt ferrite/silica nanocomposites prepared by the coprecipitation method

Magnetic nanocomposites consisting of cobalt ferrite nanoparticles embedded in silica matrix were prepared by the coprecipitation method using metallic chlorides as precursors for ferrite. Subsequently composites were annealed at 100, 200 and 300 °C for 2 h. The samples were structurally characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). The magnetic properties were measured in the temperature range of 10-300 K using vibrating sample magnetometer (VSM). The effects of thermal treatment on structural and magnetic properties of nanocomposites were investigated. When the samples were annealed, CoFe₂O₄ nanocrystallites were observed in the SiO₂ matrix, whose size increases with increase in annealing temperature. The coercivity and saturation magnetization of nanocomposite (annealed at 300 °C for 2 h) are much higher than that of bulk cobalt ferrite. The realization of adjustable particle sizes and controllable magnetic properties makes the applicability of the CoFe₂O₄ nanocomposite more versatile.     

Ultrasonic-assisted fabrication and characterization of PVC-SiO2 nanocomposites having bovine serum albumin as a bio coupling agent

In this work, SiO₂ nanoparticles (NPs) were modified with bovine serum albumin (BSA) under ultrasound irradiations as a green and fast route to achieve their good dispersion. Subsequently, different weight percentages of the modified NPs (3, 6, and 9 wt%) were incorporated in poly(vinyl chloride) (PVC) as the matrix. Thermogravimetric analysis of the SiO₂-BSA NPs indicated that 12 wt% of the modifier was loaded on the surface of SiO₂ NPs. Encapsulation of the SiO₂-BSA resulted in a meaningful improvement in the optical, mechanical and thermal characteristics of the prepared PVC nanocomposites (NCs). X-ray diffraction (XRD) patterns for the PVC/SiO₂-BSA NCs showed a crystalline behavior for the NC with 6 wt% of the SiO₂-BSA originated from the phosphate buffer on the NPs. Water contact angle of the PVC/SiO₂-BSA NCs showed that the hydrophilicity enhanced with increasing of the NPs contents.     

Controllable fabrication and characterization of biocompatible core-shell particles and hollow capsules as drug carrier

SiO₂@CdSe core-shell particles were fabricated by controllable deposition CdSe nanoparticles on silica colloidal spheres. Step-wise coating process was tracked by the TEM and XRD measurements. In addition, SiO₂@CdSe/polypyrrole(PPy) multi-composite particles were synthesized based on the as-prepared SiO₂@CdSe particles by cationic polymerization. The direct electrochemistry of myoglobin (Mb) could be performed by immobilizing Mb on the surface of SiO₂@CdSe particles. Immobilized with Mb, SiO₂@CdSe/PPy-Mb also displayed good bioelectrochemical activity. It confirmed the good biocompatible property of the materials with protein. CdSe hollow capsules were further obtained as the removal of the cores of SiO₂@CdSe spheres. Hollow and porous character of CdSe sub-meter size capsules made them becoming hopeful candidates as drug carriers. Doxorubicin, a typical an antineoplastic drug, was introduced into the capsules. A good sustained drug release behavior of the loading capsules was discovered via performing a release test in the PBS buffer (pH 7.4) solution at 310 k. Furthermore, SiO₂@CdSe/PPy could be converted to various smart hollow capsules via selectively removal of their relevant components.     

Electric-field-controlled photoluminescence of CdSe nanocrystal-doped SiO2 on Si

Dense-packed CdSe nanoclusters synthesized by sequential ion implantation of Cd⁺ and Se⁺ in thermally grown SiO₂ are subjected to high electric field strengths in a metal oxide semiconductor (MOS) structure. The nanocrystal-containing device shows efficient CdSe band-edge photoluminescence (PL) when excited by a cw-HeCd laser operating at a wavelength of 442 nm at room temperature. An effective PL quenching and enhancement has been observed. Depth-resolved μ-PL measurements reveal an exponential decrease, which is depth-correlated with a layer of nanoparticles near the surface, whereas the optical non-linearity of the PL increases in parallel. The PL spectra and particle size distribution suggest an energy transfer from the nanoscopic to adjacent large particles. It can be concluded from these results that charge injection into the near-surface region of the nanocluster/SiO₂ system might be the reason for the asymmetric and hysteretic electro-optic response.     

Migration of CrSi2 nanocrystals through nanopipes in the silicon cap

CrSi₂ nanocrystals (NC¹) were grown by reactive deposition epitaxy of Cr at 550 °C. After deposition the Cr is localized in about 20-30 nm dots on the Si surface. The NCs were covered by silicon cap grown by molecular beam epitaxy at 700 °C. The redistribution of NCs in the silicon cap was investigated by transmission electron microscopy and atomic force microscopy. The NCs are partly localized at the deposition depth, and partly migrate near the surface. A new migration mechanism of the CrSi₂ NCs is observed, they are transferred from the bulk toward the surface through nanopipes formed in the silicon cap. The redistribution of CrSi₂ NCs strongly depends on Cr deposition rate and on the cap growth temperature.