The sol-gel glass transformation of silica

Abstract

Large monolithic xerogel silica glasses were successfully made from tetramethyl-orthosilicate and distilled water using the combination of an acidic drying control chemical additive (DCCA) and a specially designed drying chamber. The acidic DCCA increases the gel strength by formation of a fibrillar ultrastructure, and the drying chamber reduces the catastrophic capillary forces inside the wet gel body.

Partially densified monolithic gels up to 850°C were routinely made for physical property tests and compared to commercial fused silicas. Although the mechanical properties of the porous gel-silica monoliths such as microhardness, Young's modulus, toughness, flexural strength, density are relatively lower than fused silica, the optically transparent porous gel silica has a UV cut-off ranging from 250–300 nm. Such a porous gel with excellent optical transmission and a highly uniform pore radius of 10–50 Å offers a unique, chemically stable matrix for impregnation with a second phase of optically active organic or inorganic compounds.

The processing and properties of Types I and II fused quartz optics and Types III and IV synthetic fused silica optics are compared with the new organometallic sol-gel derived gel-silica optics. Fully dehydrated and densified gel-silica has excellent transmission from 165 nm to 4400 nm with no OH absorption peaks. This optical transmission is equivalent to the best Type IV fused silica. The other physical properties and structural characteristics of the dehydrated dense gel-silica are similar to fused quartz and fused silica. However, the dense gel-silica has a lower coefficient of thermal expansion of 2.0 × 10^?7 cm/cm compared with 5.5 × 10^?7 cm/cm for standard vitreous silicas. The CTE value is temperature independent from 80 K to 500 K. Sol-gel silica optics can be made as complex shapes by casting of the sol into inexpensive plastic molds.     

Linear optical absorption and photoluminescence emission properties of gold nanoparticles prepared by laser ablation technique

Gold nanoparticles of average size varying between 1.1 and 3.3?nm are prepared by 1064?nm Nd:YAG laser ablation of solid gold target kept in ethylene glycol medium. The measured UV-Visible absorption spectra showed the presence of sharp absorption peaks in the UV and in the visible regions due to the interband transition and surface plasmon resonance (SPR) oscillations in Au nanoparticles, respectively. The increase in linewidth of the SPR peaks with the reduction in particle sizes is observed due to intrinsic size effects. The prepared samples exhibit photoluminescence (PL) emissions in the UV-Visible region peaked at ??354?nm due to the recombination of electrons with holes from sp conduction band to d-band of Au. The peak PL intensity in the sample prepared with 60 minutes of laser ablation time is enhanced by a factor of ??2.5 compared to that obtained in the sample prepared with a laser-ablation time duration of 15 minutes.     

Spatially selective modification of optical and magneto-optical properties in Fe- and Au-doped glasses irradiated with femtosecond-laser

The optical property and the magneto-optical response were space-selectively modified in transparent Fe³⁺- and Au³⁺-doped glasses by using infrared femtosecond- (fs-) laser irradiation and subsequent annealing. This irradiation process induces the precipitation of not only magnetic spinel-type Fe-oxide nanoparticles but also Au nanoparticles inside the glasses, which shows localized surface plasmon resonance absorption at the wavelengths larger than 500 nm. As the annealing time and the temperature increases, the position of the LSPR peaks exhibits red shifts, which is due to the growth of Au nanoparticles. Faraday rotation angles as a function of wavelength were measured, and the difference spectra exhibit distinct positive peaks, indicating that the coupling between the LSPR due to the Au nanoparticles and the diamagnetism of the matrix glass is effective. To decrease the coupling with the diamagnetic glass, a two-step annealing process (at 450 °C for 90 min and at 550 °C for 30 min) was carried out after irradiation with fs-laser. The preliminary annealing at the lower temperature contributes to the precipitation of ferrimagnetic magnetite nanoparticles. Au nanoparticles were subsequently grown by annealing at 550 °C. In this case, effective coupling between the LSPR and ferrimagnetic nanoparticles has significantly suppressed the intensity of the positive peak in the Faraday spectra compared with the single annealing process.     

SHINERS and plasmonic properties of Au Core SiO2 shell nanoparticles with optimal core size and shell thickness

Shell‐isolated nanoparticles (NPs)‐enhanced Raman spectroscopy (SHINERS) can be potentially applied to virtually any substrate type and morphology. How to take a step forward to prepare SHINERS NPs (SHINs) with superior performance is critical for the practical applications of surface‐enhanced Raman scattering (SERS) in the breadth and depth. Here, we present a method to obtain 120 nm diameter gold NPs coated with ultrathin silica shells (1–4 nm). The silica shell can be controlled growth through carefully tuning a series of parameters, such as amount of 3‐aminopropyl triethoxysilane used, pH, reaction time, and reaction temperature. We compare the enhancement factor of the obtained 120 nm Au with a 4 nm silica shell NPs to the 55 nm Au with a 4 nm silica shell NPs, and the activity of a 120 nm SHINs is nearly 24 times that the 55 nm SHIN from a single particle view. We also compare the enhancement factor of 1 nm silica shell Au@SiO₂ NPs with the bare Au NPs. The enhancement factor of 1 nm silica shell Au@SiO₂ NPs was found to be about twice that of the bare particles. For a deeper understanding of the source of the giant enhanced electrical field of the 1 nm silica shell Au@SiO₂ NPs, we study the plasmonic property of single 1 nm silica shell Au@SiO₂ NP on a gold film substrate through correlation of the structure of single NP using SEM with its SPR spectroscopy. We find that the multipolar interaction between the single Au@SiO₂ NP and gold film substrate is important for the SERS. Our studies on the performance of 120 nm SHINs and the plasmonic property of these particles can significantly expand the applications of SHINERS technique and improve the understanding of physical nature of SHINs. Copyright © 2013 John Wiley & Sons, Ltd.     

Synthesis and thermal stability of gold nanowires within monolithic mesoporous silica

This paper describes the preparation, by a novel and simple method, and the thermal stability of gold nanowires within monolithic mesoporous silica, involving soaking monolithic mesoporous silica in HAuCl₄ aqueous solution, followed by drying and subsequent step-annealing. It has been shown that reduction of Au³⁺ within silica pores can occur during the drying process at 80 °C without any special reduction treatment. After initial annealing at 300 °C, Au nanowires are formed within the pores and are stable at temperatures up to 500 °C. Increasing the annealing temperature leads to a wire-to-rod-to-sphere morphological transformation of the Au nanowires. The surface-mediated reducing groups (-OH) on the silica pore are responsible for the low-temperature reduction of Au³⁺ ions, and the formation of Au nanowires is attributed to the uni-directional diffusion of Au atoms and the confinement of the pore channels. Spheroidization and breaking at some defects in the Au nanowires during annealing at elevated temperature result in the wire-to-rod-to-sphere transformation, accompanied by a blue-shift of the surface plasmon resonance over a very wide region in the optical spectrum. PACS 81.07.-b; 81.40.-z; 81.05.Rm     

Structure Characterization of HSQ Films for Low Dielectrics Using D5 as Sacrificial Porous Materials

Low-density materials, commercially available hydrogensilsesquioxane （HSQ） offer a low dielectric constant. HSQ films can be obtained by spin on deposition （SOD）. In this work, low-dielectric-constant HSQ films are prepared by using D5 （decamethylcyclopentasiloxane） as sacrificiaJ porous materials. The dielectric constant of silica films significantly changes from 3.0 to 2.4. We report the structural aspects of the films in relation to their composition after annealed at 300℃, 400℃, and 500℃ for 1.5h in nitrogen ambient and annealed at 400℃ for 1.5h in vacuum. Si-OH appears after annealed at 400℃ for 1.5h in vacuum. The results indicate that the proper condition is in nitrogen ambient. Intensity of the Sill peak increases with the increasing temperature. Fourier transform infrared spectroscopy is used to identify the network structure and cage structure of Si-O-Si bonds and other possible bonds. Dielectric constant k is significantly lowered by annealing at 350℃ for 1.5h in nitrogen ambient. The I-V and C-V measurements are used to determine the dielectric constant, the electric resistivity and the breakdown electric field.     

Experimental characterization of bi-directional terahertz emission from gold-coated nanogratings

The THz radiation emission of Au-coated nanogratings (fused silica substrate, 30?nm Au layer thickness, 500?nm grating constant) upon fs laser irradiation (785?nm, 150?fs, 1?kHz,???1?mJ/pulse) is observed in both directions along the laser beam axis (forward and backward) and for both, illumination of the Au/air or the Au/silica interface. THz radiation along the laser beam propagation is emitted in a narrow solid angle of about 15°?fwhm independent on the laser pulse fluence, the angle of incidence and the nanograting profile. The bar width and groove depth of the nanograting as well as the angle of laser beam incidence strongly affect the THz radiation yield. The energy of single THz light pulses is measured absolutely (2?fJ in the 0.3?C0.38?THz range) using a highly sensitive and fast superconducting transition edge sensor. The bi-directional emission of THz radiation is in agreement with the model assumption of surface plasmon polaritons propagating simultaneously on both Au layer interfaces (Au/air and Au/silica).     

基于金银合金薄膜的近红外表面等离子体共振传感器研究

利用淀积在玻璃衬底上的金银合金薄膜作为表面等离子体共振(SPR)芯片, 构建了Kretschmann结构的近红外波长检测型SPR传感器. 采用不同浓度的葡萄糖水溶液测试了金银合金薄膜SPR传感器的折射率灵敏度. 实验结果表明随着入射角从7.5°增大到 9.5°, SPR吸收峰的半高峰宽从292.8 nm 减小到 131.4 nm, 共振波长从 1215 nm蓝移到 767.7 nm, 折射率灵敏度从35648.3 nm/RIU 减小到 9363.6 nm/RIU.在相同的初始共振波长(λ_R)下获得的金银合金薄膜SPR折射率灵敏度高于纯金膜(纯金膜在λ_R=1215 nm下的折射率灵敏度为29793.9 nm/RIU). 利用1 μmol/L的牛血清蛋白(BSA)水溶液测试了传感器对蛋白质吸附的响应.结果表明, BSA分子吸附使得金银合金薄膜SPR吸收峰红移了12.1 nm而纯金膜SPR吸收峰仅红移了9.5 nm. 实验结果还表明, 在相同λ_R下, 金银合金薄膜SPR吸收峰的半高峰宽大于纯金膜的半高峰宽, 因此其光谱分辨率比纯金膜SPR传感器低. 关键词： 金银合金薄膜 表面等离子体共振 波长检测型 高灵敏度     

Ion beam shaping of embedded metal nanoparticles by Si+ ion irradiation

Fine Co and Pt nanoparticles are nucleated when a silica sample is implanted with 400 keV Co⁺ and 1370 keV Pt⁺ ions. At the implanted range, Co and Pt react to form small Co _x Pt_(1?x) nanoparticles during Si⁺ ion irradiation at 300 °C. Thermal annealing of the pre-implanted silica substrate at 1000 °C results in the formation of spherical nanoparticles of various sizes. When irradiated with Si⁺ ions at 300 °C, particles in the size range of 5–17 nm undergo rod-like shape transformation with an elongation in the direction of the incident ion beam, while those particles in the size range of 17–26 nm turn into elliptical shape. Moreover, it is suspected that very big nanoparticles (size >26 nm) decrease in size, while small nanoparticles (size <5 nm) do not undergo any transformation. During Si⁺ ion irradiation, the crystalline nature of the nanoparticles is preserved. The results are discussed in the light of the thermal spike model.     

Reply to ‘On substructure in titanium alloy martensite’

Thermal stability of nanocrystalline Cu prepared by compacting nanoparticles (mean grain size about 50?nm) under high pressure has been studied by means of positron annihilation lifetime spectroscopy and X-ray diffraction. A gradual increase of mean grain size in the sample is observed with an increase in ageing time at 180°C, indicating an increase of volume fraction of the ordered regions. Furthermore, during the ageing, the increase in average size of the vacancy clusters in grain boundaries is confirmed by the positron lifetime results. The recrystallization is observed at the temperature of about 180°C, and becomes significant above 650°C. Three annealing stages, which are at the intervals 180–400°C, 400–650°C and 650–900°C have been characterized by positron average lifetime. The average volume of the defects almost remains constant in the interval 400–650°C but becomes considerably smaller in the interval 650–900°C.     

Preferred orientation of ZnO nanoparticles formed by post-thermal annealing zinc implanted silica

High quality zinc oxide nanoparticles with (002) preferred orientation were prepared by post-thermal annealing zinc implanted silica at 700 °C using two methods. One method was annealing zinc implanted silica at 700 °C for 2 h in oxygen ambient; the other method was sequentially annealing zinc implanted silica at 700 °C in nitrogen and oxygen ambient for 1 h, respectively. X-ray diffraction (XRD), absorption and microphotoluminescence (micro-PL) results indicated that the latter method could create high quality ZnO nanoparticles with (002) preferred orientation and narrow size-distribution. X-ray photoelectron spectra (XPS) showed the formation of ZnO nanoparticles on a silica surface, where the ZnO nanoparticle content increased with increasing oxidation time in an oxygen environment. The processes of the transformation from Zn to ZnO are discussed.     

胶体金纳米颗粒的表面等离子体发射特性

利用电化学方法制备出粒径为20-80 nm的胶体金纳米颗粒。研究其荧光发射光谱特性,在485nm处观察到表面自由电子集体激发导致的表面等离子体共振发射峰,其位置不随激励光波长的变化而移动。当激励光波长为485 nm时,观察到最强的发射峰。在240和640 nm处,还观察到倍频发射峰和3/4分频发射峰。增加金纳米颗粒粒径,观察到发射谱的峰值增大而发射峰的位置只有很小的红移。     

Microstructure and optical absorption of Au-MgF2 nanoparticle cermet films

The microstructure and optical absorption of Au-MgF2 nanoparticle cermet films with different Au contents are studied. The microstructural analysis shows that the films are mainly composed of the amorphous MgF2 matrix with embedded fcc Au nanoparticles with a mean size of 9.8-21.4nm. Spectral analysis suggests that the surface plasma resonance （SPR） absorption peak of Au particles appears at λ=492-537nm. With increasing Au content, absorption peak intensity increases, profile narrows and location redshifts. Theoretical absorption spectra are calculated based on Maxwell-Garnett theory and compared with experimental spectra.     

Optical property of nanocomposite of mesoporous silica thin films incorporated with gold nanoparticles

Amino-functionalized mesoporous silica thin films (MTFs) are produced using surface active agent F127,and then gold nanoparticles are introduced into the pore channels to prepare the Au/SiO 2 nanocomposite.After assembling the gold,the amino-functionalized MTF undergoes some shrinkage but remains a periodic structure as demonstrated by X-ray diffraction (XRD) patterns.The nanocomposite shows an acute characteristic diffraction peak assigned to (111) plane of the face-centered-cubic structure of gold,indicating that gold nanoparticles crystallize well and grow in a preferred orientation in the pore channels.The surface plasma resonance (SPR) absorption peak near 570 nm undergoes a red-shift accompanied by a strengthening of intensity when HAuCl 4 is used to react with the amino groups on the internal pore surfaces for 4,6,and 8 h.The simulative results are consistent with the experimental ones shows that the absorption property of the Au/SiO 2 nanocomposite is influenced by the dipping time,which affects the size and volume fraction of embedded gold nanoparticles.     

Irradiation effect of low-energy ion on polyurethane nanocoating containing metal oxide nanoparticles

Irradiation effect of low-energy ion beam has been investigated on nanocoating developed with silica, titania and silica–titania core–shell nanoparticles embedded in an organic binder for nanopaint application. In this work, we have taken polyurethane as a model organic binder. Silica nanoparticles have been prepared through sol–gel synthesis with a particle size of 85?nm. Titania and core–shell nanoparticles have been prepared through both sol–gel and peptization process. Particle sizes obtained were 107?nm for titania and 240?nm for core–shell nanoparticles prepared through sol–gel process and 75?nm for TiO₂ and 144?nm for core–shell nanoparticles prepared through peptization process. The coating formulations were developed with the above nanoparticles individually and nanoparticle concentration was varied from 1 to 6?wt% and the best performance in terms of hydrophobicity was obtained with 4?wt % of the nanoparticles in polyurethane coating formulation. All the coating formulations prepared were applied on a glass substrate and dried at 100°C. The dry film thickness obtained was around 100?µm in each case. These films dried on glass substrate were irradiated by nitrogen and argon ion beam with energy of 26?keV at fluences of 10¹⁴ to 10¹⁶?ions/cm². The anti-algal property of the irradiated samples was improved and hydrophobicity was reduced.     

Effects of temperature gradient induced nanoparticle motion on conduction and convection of fluid

The continuous synthesis of nickel nanoparticles (NiNPs) in a static microchannel T-mixer by the reduction of NiCl₂·6H₂O in the presence of ethylene glycol without a stabilizing/capping agent was investigated. The nanoparticles were formed in accordance with the modified polyol process with hydrazine used as a reducing agent and NaOH as a catalyst for nanoparticle formation. The reaction mechanism for NiNP formation was investigated in batch with the help of Fourier transform infrared spectroscopy and X-ray diffraction (XRD) techniques. Parameters were found for reducing reaction times from 60 to 1?min. The effects of temperature (60?C120?°C) and NaOH concentration (0.1 and 0.5?M) on batch-processed particle characteristics were also studied using XRD, transmission electron microscope and electron microprobe analysis. Average particle size was reduced from 9.2?±?2.9 to 5.4?±?0.9?nm at higher temperature and NaOH concentration. Adaptation of this chemistry to a static microchannel T-mixer for continuous synthesis resulted in smooth, spherical particles. Increases in the reaction temperature from 120 to 130?°C resulted in a narrow size distribution of 5.3?±?1?nm and also resulted in magnetic properties of 5.1?emu/g (saturation magnetization), 1.1?emu/g (remanent magnetization), and 62?Oe (coercivity).     

Photoconductivity studies on amorphous and crystalline TiO<Subscript>2</Subscript> films doped with gold nanoparticles

In this work, amorphous and crystalline TiO₂ films were synthesized by the sol–gel process at room temperature. The TiO₂ films were doped with gold nanoparticles. The films were spin-coated on glass wafers. The crystalline samples were annealed at 100°C for 30 minutes and sintered at 520°C for 2 h. All films were characterized using X-ray diffraction, transmission electronic microscopy and UV-Vis absorption spectroscopy. Two crystalline phases, anatase and rutile, were formed in the matrix TiO₂ and TiO₂/Au. An absorption peak was located at 570 nm (amorphous) and 645 nm (anatase). Photoconductivity studies were performed on these films. The experimental data were fitted with straight lines at darkness and under illumination at 515 nm and 645 nm. This indicates an ohmic behavior. Crystalline TiO₂/Au films are more photoconductive than the amorphous ones.     

Silver nanoparticles produced by PLD in vacuum: role of the laser wavelength used

In this work we report the results of investigation of silver (Ag) nanoparticles prepared on a silica substrate by laser ablation. Our attention was focused on the mean diameter, size distribution and optical absorption properties of nanoparticles prepared in vacuum by using different laser wavelengths. The fundamental wavelength and the second, third, and fourth harmonics of a nanosecond Nd:YAG laser were used for nanoparticles fabrication. The corresponding values of the laser fluence for each wavelength were: 0.6 J/cm² at 266 nm, 0.8 J/cm² at 355 nm, 2.8 J/cm² at 532 nm, and 2 J/cm² at 1064 nm. The Ag nanoparticles produced have mean diameters in the range from 2 nm to 12 nm as the nanoparticles’ size decreases with the decrease of the wavelength used. The presence of the Ag nanoparticles was also evidenced by the appearance of a strong optical absorption band in the measured UV-VIS spectra associated with surface plasmon resonance (SPR). A redshift and widening of the absorption peak were observed as the laser wavelength was increased. Some additional investigations were performed in order to clarify the structure of the Ag nanoparticles.     

Charge states of the activator and size effects in BaF2: Eu nanophosphors

ABSTRACT

According to the spectra of stationary X-ray excited luminescence (XEL) of BaF₂: Eu nanophosphors at 80 and 294 K, it was revealed that the thermal annealing of fine-grained nanoparticles (d?=?35?nm) in the range of 400–1000°C, which is accompanied by an increase of their sizes in the range of 58–120?nm, does not result in effective changes of the charge state of Eu^{3 +} → Eu^{2 +} activator, in contrast to CaF₂: Eu nanoparticles. The maximum light output of X-ray excited luminescence of BaF₂: Eu nanophosphors in the 590?nm emission band of Eu³⁺ ion was observed at an annealing temperature of 600°C with the average size of nanoparticles 67?nm. The subsequent growth of annealing temperatures, especially in the range of 800–1000°C, causes decrease in the light output of X-ray excited luminescence due to the increase of defect concentration in the lattice as a result of sharp increase of nanoparticle sizes and their agglomeration. In BaF₂: Eu nanoparticles of 58?nm size, according to the thermostimulated luminescence (TSL) spectrum, transformation of Eu³⁺ → Eu²⁺ under the influence of long-time X-ray irradiation was revealed for the peak of 151?K. Thus, X-ray excited luminescence spectra of BaF₂: Eu nanophosphors are formed predominantly due to the emission of Eu³⁺ ions, while emission of Eu²⁺ ions is observed in the TSL spectra.     

The characteristics of lysine-mediated self-assembly of gold nanoparticles on the ITO glass

In this study, lysine was investigated as a cross-linker to induce the self-assembly of gold nanoparticles (GNPs) with the variation of solution pH, dosage amounts of lysine, and GNP size. Lysine molecules at acidic pH ranges induced the aggregation of Au colloids via α, ?-amine mediated self-assembly of GNPs, consequently leading to the generation of secondary peak at longer wavelength for aggregated GNPs. At intermediate and basic pH ranges, however, the ionization of carboxylic acid groups in lysine hindered the cross-linking between Au colloids with the consequent disappearance of secondary peak. For the array of small Au colloids (ca. 43 nm), lysine induced heavily-aggregated GNPs on the ITO glass at strongly acidic condition (pH2～3) through its molecular bridging effect. For the array of large Au colloids (ca.70 nm), lysine produced one-dimensional assembly of GNPs on the ITO glass at slightly acidic condition (at pH4.7) through zwitterions-mediated interactions.