Langmuir monolayer properties of 4-methylbenzenethiol capped gold nanoparticles

The properties of 4-methylbenzenethiol capped gold nanoparticles have been investigated at the air–water interface. Langmuir isotherms and compression cycles show that a stable monolayer is formed on the water surface. In situ UV-visible spectroscopy confirms that the plasmon absorption band is present which is characteristic of small metallic particles. The monolayer was imaged using real time Brewster Angle Microscopy (BAM). Above surface pressures of 5 mN m^− 1 the monolayer was virtually homogeneous. During decompression, the monolayer fractured into rod-like structures. These rods were initially orientated parallel to the movable barriers containing the monolayer and as the area was increased, their orientation randomized. During subsequent compressions the rods recombined to recreate a homogeneous monolayer.     

Transmission electron microscope characterisation of molar-incisor-hypomineralisation

Molar-incisor-hypomineralisation (MIH), one of the major developmental defects in dental enamel, is presenting challenge to clinicians due, in part, to the limited understanding of microstructural changes in affected teeth. Difficulties in the preparation of site-specific transmission electron microscope (TEM) specimens are partly responsible for this deficit. In this study, a dual-beam field emission scanning electron microscope (FESEM)/focused ion beam (FIB) milling instrument was used to prepare electron transparent specimens of sound and hypomineralised enamel. Microstructural analysis revealed that the hypomineralised areas in enamel were associated with marked changes in microstructure; loosely packed apatite crystals within prisms and wider sheath regions were identified. Microstructural changes appear to occur during enamel maturation and may be responsible for the dramatic reduction in mechanical properties of the affected regions. An enhanced knowledge of the degradation of structural integrity in hypomineralised enamel could shed light on more appropriate management strategies for these developmental defects.     

Synthesis of tadpole-shaped Au nanoparticles using a Langmuir monolayer of fluorocarbon surfactant

Tadpole-shaped nanoplates, linearly arranged nanoparticles and triangular and hexagonal nanoplates were synthesized under a Langmuir monolayer of a cationic fluorocarbon surfactant, FC-4 (C₃F₇O(CF(CF₃)CF₂O)₂CF(CF₃)CONH(CH₂)₃N⁺(C₂H₅)₂CH₃I^?) through interfacial reduction of AuCl₄^? by formaldehyde gas. Reports about such tadpole-shaped nanoparticles are relatively scarce. The predominantly plate-like particles are mainly nearly perfect triangular and hexagonal nanocrystals, of micrometer scale in diameter. The Au nanoparticles are characterized using transmission electron microscopy (TEM) and selected-area electron diffraction (SAED). The atomically flat planar surfaces of the Au nanoplates correspond to {111} planes and the lateral surfaces are {110} planes. The surface pressure strongly influences the formation of different Au nanostructures. A potential mechanism of such diverse morphologies is also discussed.     

Experimental investigation of the reversible collapse of a capped amphiphile Langmuir monolayer

Surface isotherms, Brewster angle microscopy (BAM), surface potential measurements, and ab initio calculations were used to examine Langmuir monolayer films of an amphiphile with a bulky group that caps the terminus of an alkanol chain. BAM images of the collapsed state differ significantly from those of long-chain paraffinic amphiphiles. The collapsed phase BAM images show that aggregates of these amphiphiles extrude to form a three-dimensional phase. These clusters are able to easily reform the pre-collapse monolayer upon release of the applied surface pressure to pre-collapse values. This system may be described over all accessible surface pressures as a two-dimensional liquid-crystal analog at temperatures near and above room temperature. The behavior of this amphiphile is suggested to be related to structural elements of the molecule, particularly the bulky capping group, that may define a class of such systems.     

Thermo chemical stability of cadmium sulfide nanoparticles under intense pulsed light irradiation and high temperatures

Thermo chemical stability of CdS nanoparticles under an Intense Pulsed Light from a xenon flash lamp and high temperature X-ray Diffraction (XRD) were investigated. The CdS nanoparticles were obtained with a chemical bath method. The CdSO₄ (0.16 M) solution was added to an NH₃ (7.5 M) solution under constant stirring. Afterwards, a thiourea (0.6 M) solution was added. The bath temperature and pH were maintained at 65 °C and 10, respectively and the mixture was stirred constantly until a solid precipitate of yellow CdS was produced. Its microstructure was investigated with Scanning Electron Microscopy, and its electronic properties were determined by UV-visible and Photo luminescence Spectroscopy. The microstructure of the sintered CdS nanoparticles, obtained the high temperature XRD, was investigated with EDAX and X-ray micro Tomography. In addition, high temperature XRD and Themogravimetric Analysis tests were conducted over the samples. The CdS nanoparticles’ crystallinity increased with the irradiation exposure and they were thermally stable until 600 °C in argon atmosphere. However new phases start to appear after annealing at 400 °C for 30 min in air atmosphere. The main contribution of this paper was to investigate the stability of CdS nanoparticles under intense light and high temperature conditions. It was found that the number of irradiation shots conducted with the IPL technique increased the crystallinity of the CdS, by increasing the CdS nanopillars formation. A simple mechanism of ultra fast melting and cooling like quenching has been proposed to explain the phenomenon. It is also shown than at temperatures higher than 600 °C, different oxides appeared, while temperatures higher than 900 °C had a complete degradation of the CdS.     

EM208型电镜的维护

本文介绍了EM208型电镜真空系统的特点,而采取相应维护措施的经验。     

Cadmium sulfide and cadmium selenide/cadmium sulfide nanoparticles stabilized in water with poly(cysteine acrylamide)

Cysteine acrylamide (N-acryloyl L-cysteine) stabilizes CdS nanoparticles as the particles form in aqueous dispersions. Cysteine acrylamide also exchanges for citrate on the surfaces of CdSe and core/shell CdSe/CdS nanoparticles to provide greater stability. Heating of the nanoparticle dispersions polymerizes the cysteine acrylamide on the surface to form a more efficient polydentate stabilizer. The polymer-coated nanoparticle dispersions are colloidally stable even after removal of low molecular weight solutes by dialysis. Emission quantum yields of the polymer-coated CdSe and CdSe/CdS samples were 0.9% and 2.6%, respectively, after aging of the samples in light. CdSe/CdS coated with poly(cysteine acrylamide) is colloidally stable for at least two years in the dark at 5 degrees C.     

Transmission electron microscope observations on GaP electroluminescent diode materials

Transmission electron microscope observations were made on GaP material grown by liquid epitaxy, vapour phase epitaxy and Czochralski pulling from the melt. The vapour phase material was found to contain many intrinsic stacking faults. Most were linear but a few were tetrahedral. Both Frank and Shockley partial dislocations were found. The fault density increased with increase in the Te dopant concentration in the vapour phase material. The Czochralski material also contained intrinsic stacking faults, and its defect content was higher than that of layers grown by liquid phase epitaxy. In all specimens with free carrier concentrations greater than 10¹⁸ cm^–3 evidence was found to suggest that precipitation of impurities had taken place.This work was carried out at the Department of Metallurgy, Imperial College, London, SW7, UK.     

An electron microscope investigation of fatigue deformation in copper

The initial fatigue deformation in copper has been shown to be characterised by the presence of deformation bands of high dislocation density, frequently found in conjunction with linear arrays of a dark spot defect. Also noted at this stage was the presence of heavily jogged dislocations. The final fatigue structure consisted of patches of very high dislocation density, occasionally aligned in a specific direction, resolvable as either round or elongated dislocation loops.     

Third-order optical nonlinearity of cadmium sulfide nanoparticles loaded in mesostructured silica materials

Cadmium sulfide nanoparticles have been successfully incorporated in three forms of CTAB-templated mesoporous silica materials: one is the mesoporous silica spheres suspended in ethanol solution, the other is the mesoporous silica spheres spin-coated on glass slide, and the third is the dip-coated mesoporous silica thin film. The mesostructures were characterized by XRD and TEM, respectively. Linear optical properties were investigated using UV-visible spectra, and the diameter of the incorporated CdS nanoparticles was measured to be around 3.1 nm. Z-scan technique manifested that these three composites exhibited distinct third-order optical nonlinearities due to the different preparation techniques. Reverse saturation absorption could be detected in the CdS-loaded mesoporous silica spheres suspended in solution, while those dispersed on glass slide presented saturation absorption. The difference in nonlinear absorption of the two mesoporous silica sphere samples could be attributed to defect-related transitions. On the contrary, the CdS-loaded mesoporous silica thin film showed self-defocusing behavior with no nonlinear absorption signals. Compared to that of the CdS nanoparticles with larger size previously reported, the intrinsic microscopic third-order nonlinear optical susceptibility of those incorporated in CTAB-templated mesoporous thin film was increased as predicted by the quantum theory, and the third-order optical nonlinearity was further determined to arise from intraband transitions induced by quantum confinement.     

Langmuir monolayers of gold nanoparticles

We report an experimental study of Langmuir monolayers of dodecanethiol-ligated gold nanoparticles with in situ optical microscopy and X-ray scattering. The particle spacing increases with thiol concentration but does not depend on surface pressure. Compression beyond the full coverage of the monolayers causes the monolayers to wrinkle. With the presence of excess thiol molecules, the wrinkles unfold back to monolayer upon the expansion of the surface area. A theoretical model based on van der Waals' attraction and tunable steric repulsion is adopted to explain this reversibility.     

Size-controlled one-pot synthesis of fluorescent cadmium sulfide semiconductor nanoparticles in an apoferritin cavity

A simple size-controlled synthesis of cadmium sulfide (CdS) nanoparticle (NP) cores in the cavity of apoferritin from horse spleen (HsAFr) was performed by a slow chemical reaction synthesis and a two-step synthesis protocol. We found that the CdS NP core synthesis was slow and that premature CdS NP cores were formed in the apoferritin cavity when the concentration of ammonia water was low. It was proven that the control of the ammonia water concentration can govern the CdS NP core synthesis and successfully produce size-controlled CdS NP cores with diameters from 4.7 to 7.1?nm with narrow size dispersion. X-ray powder diffraction (XRD), energy dispersive spectroscopy (EDS) analysis and high-resolution transmission electron microscopy (HR-TEM) observation characterized the CdS NP cores obtained as cubic polycrystalline NPs, which showed photoluminescence with red shifts depending on their diameters. From the research of CdS NP core synthesis in the recombinant apoferritins, the zeta potential of apoferritin is important for the biomineralization of CdS NP cores in the apoferritin cavity. These synthesized CdS NPs with different photoluminescence properties will be applicable in a wide variety of nano-applications.     

Towards full-structure determination of bimetallic nanoparticles with an aberration-corrected electron microscope

To fully understand the properties of functional nanostructures such as catalytic nanoclusters, it is necessary to know the positions of all the atoms in the nanostructure. The catalytic properties of metal nanoclusters can often be improved by the addition of a second metal, but little is known about the role of the different metals in these bimetallic catalysts, or about their interactions with each other and the support material. Here we show that aberration-corrected scanning transmission electron microscopy of supported rhodium-iridium clusters, combined with dynamic multislice image simulations, can identify individual atoms, map the full structure, and determine changes in the positions of metal atoms in sequential images. This approach could help in the development of new and improved catalysts and other functional nanostructures.     

Transmission electron microscope study of a directionally solidified Cu-MgCu2 eutectic

The crystallography and the interface structure of a unidirectionally solidified Cu-MgCu₂ eutectic alloy have been examined by transmission electron microscopy. The microstructure of the eutectic was found to be lamellar and regularly interrupted by faults. The preference of the particular orientation relationship could not be explained by relative atomic densities of the planes comprising the interface. Based on the defect contrast observed and extinction distance calculations, it is suggested that the fine array of defects observed at the interface may be characterized as steps with step vectors parallel to or Dislocations were also observed at the interface but they were rarely regular.     

Transmission electron microscope studies of tantalum irradiated by MeV energy α particles

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Transmission electron microscope study of thin Cu-Ni laminates obtained by vapor deposition

Transmission electron microscopy (TEM) was used to study the microstructure of thin Cu-Ni laminates obtained by successive evaporation from two resistively heated sources. Thin (7–500 Å) alternate layers of copper and nickel were deposited up to a total thickness of approximately 4 μm on various substrates (copper, mica and alkali halides). The deposition was accomplished using the sequential opening-closing operation of two mechanical shutters which were electronically controlled with the aid of two quartz thickness monitors.A TEM study of the cross sections of various thickness combinations of Cu-Ni laminates showed that the production of a well-defined lamellar structure, even at the substrate temperature of approximately 300 °C, is possible to a lamellar thickness of atomic dimensions. It is also demonstrated that the fabrication of a synthetic superlattice structure is possible. The origin of the observed image contrast, which defines the structure of alternate layers, is discussed.     

Preparation and characterization of zinc sulfide nanoparticles under high-gravity environment

Nanosized ZnS particles were prepared under high-gravity environment generated by the rotating packed bed reactor (RPBR) using zinc nitrate solution and hydrogen sulfide gas as raw materials. The effects of experimental conditions such as reactant concentration, reaction temperature, rotating speed of the RPBR and aging time, on the preparation of nanosized ZnS particles were investigated. A set of suitable operating parameters (the aging time of 48 h, concentration of zinc nitrate of 0.1 mol/l, reaction temperature of 45 °C and rotating speed of the RPBR of 1500-1800 rotation/min) for the preparation of nanosized ZnS were recommended. Under these optimum conditions, well-dispersed ZnS nanoparticles was obtained. The crystal structure, optical properties, size and morphology of the product were also characterized by XRD, UV-Vis spectrophotometer, and TEM, respectively. Results indicate that the prepared ZnS has a good absorption for light in the wavelength range of 200-330 nm. XRD analysis also shows the prepared ZnS is in a sphalerite crystal phase. The process has great potential of commercialization.     

Thermooptical properties of composites based on cadmium sulfide nanoparticles stabilized in a low-density polyethylene matrix

The thermooptical properties of composites based on a low-density polyethylene matrix containing stabilized cadmium sulfide nanoparticles have been studied. The optical absorption coefficient and refractive index have been measured in the samples with various concentrations of nanoparticles dispersed in the polymer matrix. It is established that, upon reaching a certain temperature, the subsequent heating and cooling of a sample are accompanied by hysteresis in the optical properties.     

Transmission electron microscopy characterisation of metalorganic chemical vapour deposition grown GaN layers

GaN layers grown onto sapphire substrates by metalorganic chemical vapour deposition were characterised by optical microscopy, transmission electron microscopy and atomic force microscopy measurements. Mirror like surfaces were obtained at certain growth conditions despite the hexagonal based pyramids found on the growth surface. The typical pyramids have a base diameter of 20–30 μm and height of about 1.5–3 μm. The GaN layers are of the wurtzite type and epitaxially oriented to the sapphire substrate. Beside the threading dislocations, hexagonal rods of GaN surrounded by inversion domain boundaries are observed. An AlN layer has been formed at the interface region during the nitridation process of sapphire.     

Transmission electron microscopic investigation of the microstructure of Fe-Cr-Al alloys

The microstructure of different Fe-Cr-Al alloys in the composition range of 0 to 18 at % Cr and 7 to 25 at % Al was investigated by transmission electron microscopy (TEM). By systematically varying the compositions, the location of the order-disorder transition was determined. Dark field images confirmed that many samples consisted of a two-phase mixture of small DO₃-ordered particles in a disordered A2 matrix. The mottled contrast exhibited by some samples and sometimes taken as a proof of the existence of ordered particles is shown to be produced by an artefact due to the TEM sample preparation. This revised version was published online in September 2006 with corrections to the Cover Date.