Formation of net-like patterns of gold nanoparticles in liquid crystal matrix at the air-water interface

Controlled patterning and formation of nanostructures on surfaces based on self-assembly is a promising area in the field of "bottom-up" nanomaterial engineering. We report formation of net-like structures of gold nanoparticles (Au NPs) in a matrix of liquid crystalline amphiphile 4'-n-octyl-4-cyanobiphenyl at the air-water interface. After initial compression to at least 18?mN?m(-1), decompression of a Langmuir film of a mixture containing both components results in formation of net-like structures. The average size of a unit cell of the net is easily adjustable by changing the surface pressure during the decompression of the film. The net-like patterns of different, desired average unit cell areas were transferred onto solid substrates (Langmuir-Blodgett method) and investigated with scanning electron microscopy and X-ray reflectivity (XRR). Uniform coverage over large areas was proved. XRR data revealed lifting of the Au NPs from the surface during the formation of the film. A molecular mechanism of formation of the net-like structures is discussed. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s11051-012-0826-4) contains supplementary material, which is available to authorized users.     

Laser-assisted generation of gold nanoparticles and nanostructures in liquid and their plasmonic luminescence

Nanoparticles (NPs) and surface nanostructures (NS) are produced via laser ablation of a bulk gold target in liquid using second harmonics of 10 ps Nd:YAG laser (532 nm) with repetition rate of 50 kHz. The morphology and plasmon photoluminescence (PL) properties of obtained nanoscale objects are described. Transmission electron microscopy and field emission scanning electron microscopy are used for morphology characterization of NPs and NS, respectively. Plasmon PL of both gold NPs and NS is experimentally studied using the third harmonics of the Nd:YAG picosecond laser (355 nm) as a pump. The wavelength of intensity maximum of PL of Au NPs colloidal solution virtually coincides with the position of Au NPs plasmon absorption peak. Real-time excitation of both plasmon PL and Raman scattering of surrounding liquid by picosecond laser pulses in aqueous colloidal solution is also investigated. The efficient cross section of plasmon PL of Au NPs colloid is evaluated using Raman scattering of water as a comparative parameter. The results are in good agreement with values obtained in previous works. Plasmon PL from self-organized NS on the Au surface produced via laser ablation is observed for the first time. Its spectrum is compared to PL spectra of both aqueous colloidal solutions of NPs and of NPs deposited on a Si wafer. The obtained experimental data are discussed with reference to the band structure of bulk Au.     

磁控溅射法制备金团簇纳米颗粒及性能表征

 采用磁控溅射法制备金团簇纳米颗粒,用透射电镜(TEM)、X射线衍射(XRD)、紫外可见光分光光度计(UV-Vis)和X射线光电子能谱(XPS)等分析手段对其表征,研究了金团簇纳米颗粒的形貌、颗粒度、结构、光吸收性质及物质成份。研究结果表明：制备的金团簇纳米颗粒呈球形,平均粒径在10 nm左右,粒径分布均匀,无团聚、氧化现象,颗粒的结构为面心立方。在519 nm处出现团簇颗粒的表面等离子共振吸收峰,测试得到Au(4f_7/2)和Au(4f_5/2)电子的结合能分别为83.3 eV和86.9 eV,并且没有出现金的氧化产物。     

磁控溅射法制备金团簇纳米颗粒及性能表征

采用磁控溅射法制备金团簇纳米颗粒,用透射电镜(TEM)、X射线衍射(XRD)、紫外可见光分光光度计(UV-Vis)和X射线光电子能谱(XPS)等分析手段对其表征,研究了金团簇纳米颗粒的形貌、颗粒度、结构、光吸收性质及物质成份。研究结果表明：制备的金团簇纳米颗粒呈球形,平均粒径在10 nm左右,粒径分布均匀,无团聚、氧化现象,颗粒的结构为面心立方。在519 nm处出现团簇颗粒的表面等离子共振吸收峰,测试得到Au(4f7/2)和Au(4f5/2)电子的结合能分别为83.3 eV和86.9 eV,并且没有出现金的氧化产物。     

Formation of GaAs nanowhisker arrays by magnetron sputtering deposition

The possibility is demonstrated of fabricating arrays of cone-shaped GaAs nanowhiskers with a surface number density of up to 10⁹ cm^-2, a characteristic height ranging from 300 to 10000 nm, and a transverse size of approximately 200 nm at the base and from 200 to 10 nm or smaller at the top. The characteristic height of GaAs nanowhiskers varies in direct proportion to the effective thickness of the deposited material layer and in inverse proportion to the transverse nanowhisker size at the top. The growth of GaAs nanowhiskers is studied as a function of the deposition rate, the temperature, and the crystallographic orientation of the substrate. From an analysis of the obtained dependences of the nanowhisker size on these parameters, it is concluded that GaAs nanowhiskers are formed through the diffusion mechanism.     

Sonication-assisted layer-by-layer deposition of gold nanoparticles for highly conductive gold patterns

Sonication-assisted layer-by-layer (LBL) deposition of gold nanoparticles (GNPs) was carried out in an attempt to prepare highly conductive gold patterns on polyimide substrates. First, sonication time was optimized with GNPs (12.8 nm) whose size was large enough to be analyzed by FE-SEM in order to evaluate the surface coverage. Next, multilayer formation (4, 8 and 12 layer) was confirmed using ethanedithiol (EDT) as linker molecules under optimized conditions by measuring their UV absorption, near-IR (NIR) transmittance, thickness, and electrical conductivity. Finally, 20-layer films using small GNPs (2.5 nm) were prepared with or without patterning, followed by sintering at 150 °C for 1 h, which provided clean gold patterns with high electrical conductivity (2.5 × 10⁵ Ω⁻¹ cm⁻¹).     

Magnetron and pulsed laser deposition of silver and gold nanoparticles and discontinuous films and their optical properties

Ag and Au nanoparticles are obtained by magnetron sputtering and pulsed laser deposition under different conditions, and the features of their absorption spectra associated with plasmon resonances are investigated. Optimal deposition conditions for obtaining small (5?C10 nm) silver nanoparticles with a high density of surface distribution include an increased argon pressure (2.5 × 10^?2 Torr) and a low discharge voltage (100 V). Gold nanoparticle arrays obtained by pulsed laser deposition at a temperature of 200°C in vacuum are more uniformly distributed on the substrates than those deposited at room temperature in argon. It is shown that the maximum of the plasmon absorption shifts toward shorter wavelengths with a decrease in the equivalent thickness of metal films and depends not only on this thickness but also on the type of substrate, which is responsible for the morphology of nanoparticle arrays.     

Formation of nanoparticles in liquid phase processes

The thermodynamic and kinetic regularities of the formation of inorganic nanoparticles in solutions containing additives of highly adsorbable organic compounds were examined. The dependences of the surface tension on the nanoparticle size and surface coverage and of the size dependences of the rate constants of all reactions were taken into account. A kinetic equation describing the formation of nanoparticles was derived, important limiting cases considered, and simplified analytical solutions obtained. It was demonstrated that, at the initial stage, the nanoparticle grows linearly with time, whereas at later stages (large nanoparticle radii) and high intensity of organic compound adsorption, it grows in the formation regime. The properties of size distribution function of nanoparticles under steady-state conditions are analyzed. It was shown that the presence of a highly adsorbable organic compound in the solution substantially diminishes the variation of nanoparticles in size and makes them smaller.     

Formation of silicon nanoparticles and web-like aggregates by femtosecond laser ablation in a background gas

We show that the mechanism of nanoparticle formation during femtosecond laser ablation of silicon is affected by the presence of a background gas. Femtosecond laser ablation of silicon in a H₂ or H₂S background gas yields a mixture of crystalline and amorphous nanoparticles. The crystalline nanoparticles form via a thermal mechanism of nucleation and growth. The amorphous material has smaller features and forms at a higher cooling rate than the crystalline nanoparticles. The background gas also results in the suspension of plume material in the gas for extended periods, resulting in the formation (on a thin film carbon substrate) of unusual aggregated structures including nanoscale webs that span tears in the film. The presence of a background gas provides additional control of the structure and composition of the nanoparticles during short pulse laser ablation. PACS 81.16.-c     

Formation and encapsulation of gold nanoparticles using a polymeric amine reducing agent

We report on the use of poly(allylamine) hydrochloride (PAH) as a reducing agent for the controlled formation of gold nanoparticles (AuNPs) in the size range of 5–50 nm. The formation of AuNPs using this polymer matrix allows for the AuNPs to be imbedded in the polymer matrix, once formed. The kinetics of AuNP formation are shown to be pseudo first-order in [HAuCl₄] at room temperature. The kinetics of AuNP formation are controlled by the ratio of reducing agent to HAuCl₄ as well as the overall concentration of the PAH and HAuCl₄. Additionally, at low PAH:HAuCl₄ mole ratios, the plasmon resonance wavelength can be controlled through the ratio of the reactants. This plamson resonance shift is shown to be related to AuNP size by means of TEM imaging data on the AuNPs.     

Formation of fractal aggregates during green synthesis of silver nanoparticles

The aggregation behavior of silver nanoparticles (AgNPs) prepared by a green synthesis procedure using starch as the stabilizer was studied by the small angle X-ray scattering (SAXS) technique. The protecting ability of starch was affected by the presence of NaOH leading to different aggregation behaviors. In all the samples, mass as well as surface fractal regimes were observed. Assuming spherical form, the radii of nanoparticles were in the range of 11–17 nm.     

Formation of gold nanoparticles in gold ruby glass: The influence of tin

A ¹¹⁹Sn Mössbauer study was carried out of tin(IV) complexes with 2-benzoylpyridine thiosemicarbazone (H2Bz4DH) and its N(4)-methyl (H2Bz4M) and N(4)-phenyl (H2Bz4Ph) derivatives: [Sn(2Bz4DH)Cl₃] (1), [Sn(2Bz4DH)PhCl₂] (2), [Sn(2Bz4M)Cl₃] (3), [H₂2Bz4M]₂[Ph₂SnCl₄] (4), [Sn(2Bz4Ph)PhCl₂] (5), [Sn(2Bz4Ph)Ph₂Cl] (6), in which H2Bz4R stands for the neutral ligand and 2Bz4R stands for the anionic thiosemicarbazone. In addition, ¹¹⁹Sn Mössbauer studies of the tin(IV) complexes [Sn(H4Bz4DH)₂Cl₄H₂O] (7), [Sn(H4BzPS)₂Cl₄H₂O] (8) with 4-benzoylpyridine thiosemicarbazone (H4Bz4DH) and the correspondent semicarbazone (H4BzPS) were performed. The isomer shifts decrease upon coordination due to the variation in the percentage of s character as tin changes from approximately sp³ hybridization in the tin salts to sp³d² in the octahedral or sp³d³ in the heptahedral complexes. The Mössbauer parameters of compound (4) showed the existence of two tin(IV) sites, which have been attributed to the presence of the cis and trans isomers.     

Novel bidecahedral morphology in gold nanoparticles frozen from liquid

A bidecahedral morphology in which two truncated decahedral structures share two tetrahedral units, involving two types of symmetric-tilt grain boundaries, is observed as a novel and rare morphology of gold nanoparticles frozen from liquid in free space. This low-symmetry polyhedral morphology with eight multiply twinned domains is intermediate between the icosahedral and decahedral motifs.     

Surface modification of gold nanoparticles and their monolayer behavior at the air/water interface

Gold nanoparticles were prepared by two different methods. The first method was chemically grafting the particles with different lengths of alkylthiol (C₆SH, C₁₂SH and C₁₈SH). For the second method, the Au particles were surface modified first by mercaptosuccinic acid (MSA) to render a surface with carboxylic acid groups which play a role to physically adsorb cationic surfactant in chloroform. This method was termed physical/chemical method. In the first method, the effects of alkyl chain length and dispersion solvent on the monolayer behavior of surface modified gold nanoparticles was evaluated. The gold nanoparticles prepared by 1-hexanthiol demonstrated the narrowest size distribution. Most of them showed narrower particle size distributions in chloroform than in hexane. For the physical/chemical method, the particles can spread more uniformly on the water surface which is attributed to the amphiphilic character of the particles at the air/water interface. However, the particles cannot pack closely due to the relatively weak particle-particle interaction. The effect of alkyl chain length was also assessed for the second method.     

Video-frequency scanning transmission electron microscopy of moving gold nanoparticles in liquid

Immobilized gold nanoparticles were imaged in a liquid containing water and 50% glycerol with scanning transmission electron microscopy (STEM). The specimen was enclosed in a liquid compartment formed by two silicon microchips with electron transparent windows. A series of images was recorded at video frequency with a spatial resolution of 1.5nm. The nanoparticles detached from their support after imaging them for several seconds at a magnification of 250,000. Their movement was found to be much different than the movement of nanoparticles moving freely in liquid as described by Brownian Motion. The direction of motion was not random-the nanoparticles moved either in a preferred direction, or radially outwards from the center of the image. The displacement of the gold nanoparticles over time was three orders of magnitude smaller than expected on the basis of Brownian Motion. This finding implies that nanoscale objects of flexible structure or freely floating, including nanoparticles and biological objects, can be imaged with nanoscale resolution, as long as they are in close proximity to a solid support structure.     

Optical frequency mixing at coupled gold nanoparticles

We present nonlinear-optical four-wave mixing (4WM) at coupled gold nanoparticles. By decreasing the interparticle distance from large separation to touching contact, the 4WM yield increases by 4 orders of magnitude. The reason for this dramatic enhancement lies in the shift of the localized plasmon resonance to infrared wavelengths as the dimer is formed, making one of the input wavelengths doubly resonant. At the touching point, the 4WM signal changes discontinuously because of a sudden charge redistribution imposed by the formation of a conductive bridge. The 4-wave mixing signal provides an ultrasensitive measure for the contact point between a pair of particles and it can be employed as a spatially and temporally controllable photon source.     

Formation of silver nanoparticles during deposition onto viscous-fluid epoxy resin

A new method is proposed for synthesizing metallic nanoparticles in a polymer matrix. These nanoparticles are synthesized during thermal vacuum evaporation of a metal (4.8 × 10⁻⁶ g/cm²) onto the surface of viscousfluid epoxy resin (at a viscosity of 20–120 Pa s) having room temperature, which is well below the glass transition temperature of the polymer. As a result, epoxy resin layers containing silver nanoparticles in their volume form; these nanoparticles are studied by transmission electron microscopy and optical absorption spectroscopy. Various types of disperse structures formed by metallic nanoparticles in the polymer are detected. The morphology of the composite material is found to be controlled by the polymer viscosity and the metal deposition time.     

Light-induced changes of the refractive indices in a colloid of gold nanoparticles in a nematic liquid crystal

It was shown that irradiation of a nematic liquid crystal doped with metal nanoparticles in the visible near the plasmon resonance band led to strong thermal changes of the refractive indices. The effect was studied by recording of dynamic optical gratings in the colloid. Nanoparticles "worked" as effective nano-heaters in a matrix causing the order parameter decrease around the particles. A large nonlinearity parameter (n (2) ≈ 10(-2) cm(2)/kW and fast response (≈ 0.7 ms), with no detectable particles' aggregation and excellent photo- thermo-stability make these colloids potentially attractive nonlinear optical media. Application of a dynamic holography technique allowed measuring the coefficients of thermal conductivity of the liquid crystal along the director k (||) = (0.4 ± 0.02) W m(-1)K(-1) and perpendicular to the director k (⊥) = (0.2 ± 0.01) W m(-1)K(-1).