Nanometer-sized Metal Particles Research Articles

Tunneling spectroscopy and spectroscopic imaging of granular metallicity of polyaniline

The band-gap electronic structures of two forms of the conducting polymer polyaniline, emeraldine base (EB) and emeraldine salt (ES), have been studied. Tunneling spectroscopy measurements show that ES is metallic in nature with a finite electron density of state at the Fermi energy (EF), while EB behaves as an insulator with a gap at EF in agreement with theoretical calculations. These results allowed us to obtain direct evidence of the granular metallicity of ES by spectroscopic imaging. The spectroscopic images show different spatial distributions of nanometer-sized metallic particles in ES and EB samples, providing important information needed in studying electrical conductivities of conducting polymers.

Spectroscopy and Dynamics of Nanometer-Sized Noble Metal Particles

Electron−phonon coupling in 11 ± 2 nm diameter Au particles and 10 ± 3 nm and 50 ± 10 nm Ag particles has been examined by ultrafast pump−probe spectroscopy. The observed relaxation times are strongly dependent on the pump laser power. At the lowest pump powers used, the time constants for relaxation are 0.8 ± 0.1 ps for the 11 nm Au particles, 1.1 ± 0.1 ps for the 10 nm Ag particles, and 1.0 ± 0.1 ps for the 50 nm Ag particles. The measured relaxation times are similar to those for bulk metals, which implies that there are no size-dependent effects in the dynamics for particles in this size region. The transient absorption/bleach recovery signals for the particles were modeled using the theory developed by Rosei et al. (Surf. Sci. 1973, 37, 689). These calculations yield the transient absorption spectrum as a function of the temperature of the electron distribution. The time dependence of the electronic temperature after pump laser excitation was calculated using the two-temperature model for electron−phonon coupling. The experimental signal versus time traces at selected wavelengths were then simulated by combining the two calculations. The results from the simulations are in semiquantitative agreement with the experimental results. In particular, the low-power relaxation times are correctly predicted by the model calculations. At very high pump laser power (>5 mJ/cm2) the transient bleach signal for Ag shows an unusual 10 ps growth. This growth is attributed to either a change in the dielectric constant of the surrounding medium due to heat transfer from the particles or thermally induced dissociation of adsorbed molecules.

Quantum size colloids: From size-dependent properties of discrete particles to self-organized superstructures

Size quantization effects occurring in nanometer-sized semiconductor and metal particles lead to a variety of novel size-dependent properties. Lately, preparation techniques have been developed which allow a gram scale synthesis of single crystalline nanoparticles with narrow size distributions and unique surface properties. There have also been recent findings concerning changes of structure and dynamics with decreasing particle size along with spectroscopic and transport measurements on individual nanoclusters. The possibility to build up complex ordered superstructures has been demonstrated utilizing self-organization processes of the particles.

Microwear experiments on metal-containing amorphous hydrocarbon hard coatings by AFM: Wear mechanisms and models for the load and time dependence

Metal-containing amorphous hydrocarbon films (MeC:H) consist of nanometer-sized metallic particles embedded in a highly cross-linked hydrocarbon matrix. The coatings have excellent tribological properties and an adjustable electrical conductivity. This is why they are of high interest for industrial applications. Microscopic wear tests have been performed on WC:H and AuC:H surfaces using an atomic force microscope (AFM) with a diamond tip. Periodical breaking-off of material inside the wear trace could directly be observed by AFM imaging during the wear process, indicating material fatigue as one wear mechanism. Furthermore, it was found that columnar growth structure and percolation of the metallic nanoparticles strongly influence the fatigue and wear resistance of the coatings. The load dependence and time dependence of the wear process and especially of the fatigue phenomenon are described by semiempirical microscopic wear models. A comparison with macroscopic tribological tests demonstrates the significance of microwear tests for practical applications.

Growth Characteristics of Au/SiO2 Nanocomposites

AbstractThis paper describes experimental studies of the growth morphology of Au/SiO2 nanocomposites consisting of nanometer-sized metal particles imbedded in a metal oxide, formed by the sequential deposition of metal and metal-oxide layers by RF sputtering. The materials work is aimed at the development of novel resistive-type exhaust gas constituent sensors. Using digitized images from a transmission electron microscope and an automated particle counting routine, we have measured the particle size distributions for films with varying layer thicknesses and deposition conditions. We find strong dependence of the particle size and overall composite structure on the Au and SiO2 layer thicknesses, in agreement with previous work.[1] We also identified four microstructure classes which can occur.

Field Emission TEM and STEM of Ultrafine Metal Particles

Abstract Accurate descriptions of dispersed metals are of central importance in optimizing the activity, selec-tivity and stability of a diverse set of catalysts and chemistries. Improved methods for quantifying metal dispersion, alloying and support interactions help form mechanistic understandings of catalyst function at the atomic scale. In this paper we discuss ideal capabilities of a transmission electron microscope for catalysis, and give an update on the use of a next-generation field-emission TEM/STEM for structural and analytical studies of supported metal catalysts. Characterizing nanometer-sized metal particles requires high performance imaging, high sensitivity analysis, and minimal disruption of the physical state of the particles. No single microscope combines all key performance factors with ease of use. Schottky field-emitter instruments provide high current-density probes with excellent stage stability (nm drifts in > 10 min. observed), < 0.7 eV energy reso-lution for EELS, and a turbo-pumped vacuum system for minimal carbon contamination while using nanometer probes and nanoamps of current. Scanning TEM performance is also key.

Correction of STM tip convolution effects in particle size and distance determination of metal-C:H films

Metal containing amorphous hydrocarbon films consist of nanometer size metallic particles in a hydrogen-carbon matrix. Scanning tunneling microscopy (STM) of Au and Pt containing amorphous C:H-films was performed in order to determine particle size- and distance distributions. To correct tip convolution effects, which lead to apparent particle enlargement and particle hiding (1) a method is presented which allows off-line STM tip radius determination by statistical analysis of apparent particle radii and thereby a correct particle radii determination. (2) A simple Monte Carlo model is proposed to compute fractions of hidden particles and their influence on particle distance distributions. Results of these evaluations are compared with results from small angle X-ray scattering (SAXS), transmission electron microscopy (TEM) and X-ray diffraction (XRD).

Statistical methods for the correction of tip convolution effects in STM imaging of nanometer size particles in metal—C: H films

Metal containing amorphous hydrogenated carbon films are protective coating materials with very low friction coefficients, high hardness and wear resistance, combined with an adjustable electrical conductivity. The structure of these films, consisting of nanometer size metallic particles in a hydrogen—carbon matrix, was investigated by means of scanning tunneling microscopy (STM) in order to determine particle size and particle distance distributions. In doing so, tip convolution effects lead to apparent particle enlargement and particle hiding, falsifying radii and distance determination. In order to correct these errors (1) a statistical method is presented which allows off-line tip radius determination by analysing apparent particle radii, (2) a simple Monte Carlo model is proposed to compute fractions of hidden particles and their influence on particle distance distributions. Both methods are applied to STM measurements of Au-C:H and Pt-C:H samples of different metal content and the result is compared with results of small angle X-ray scattering (SAXS), transmission electron microscopy (TEM) and X-ray diffraction (XRD).

Graphite Electrodes Containing Nanometer-Sized Metal Particles and Their Use in the Synthesis of Single-Walled Carbon Nanotube Composites

Composite graphite/metal(0) electrodes were prepared by the absorbtion of organic solutions of metal carbonyls or aqueous solutions of metal salts into high-purity porous graphite rods. The metal carbonyls were converted to the CO-free metal(0) species by heating the composites under an atmosphere of N2 and then under reduced pressure at 1000 °C. The metal-salt-containing graphite rods were heated under an atmosphere of H2 at 1000 °C to reduce the salts to the corresponding metal(0) species. This procedure permitted dispersion of metal(0) throughout the graphite rod with average metal particle sizes in the range 6.2−11.6 nm by transmission electron microscopy (TEM) analysis. Metals used were Ag, Co, Cu, Fe, La, Ni, and Pt. The composite graphite/metal(0) electrodes were vaporized in a plasma discharge apparatus under a helium atmosphere. The Co, Fe, Ni, and Pt all catalyzed single-walled nanotubule (bucky tube) growth. The soot material from the Co, Fe, and Ni-containing rods had a foam-rubber-like texture. All the metal-containing soots, except for the La-derived material, could be press molded into pellets, without the use of a binder. Data from the trace remaining ligand analyses, powder X-ray diffraction, TEM, conductivity measurements, and surface area analyses are presented. The properties of the arc-derived soots prepared by this method are compared to the soots prepared by standard cored rod/metal(0) methods.

Electron holography of catalysts

The active phase in heterogeneous catalysts consists of nanometer-sized metal or oxide particles dispersed within the tortuous pore structure of a high surface area matrix. Such catalysts are extensively used for controlling emissions from automobile exhausts or in industrial processes such as the refining of crude oil to produce gasoline. The morphology of these nano-particles is of great interest to catalytic chemists since it affects the activity and selectivity for a class of reactions known as structure-sensitive reactions. In this paper, we describe some of the challenges in the study of heterogeneous catalysts, and provide examples of how electron holography can help in extracting details of particle structure and morphology on an atomic scale.Conventional high-resolution TEM imaging methods permit the image intensity to be recorded, but the phase information in the complex image wave is lost. However, it is the phase information which is sensitive at the atomic scale to changes in specimen thickness and composition, and thus analysis of the phase image can yield important information on morphological details at the nanometer level.

Building Heterogeneous Catalysts an Atom at a Time

Nanometer-size metal particles attached to catalyst supports make up the most important class of industrial catalysts, and are vital to our fuel, textile, chemical, and environmental protection industries. How these interesting materials are produced and learning how they work is a fascinating topic, and is well-suited for classroom instruction. One approach is to prepare the nanometer particles by adding one atom at a time at low temperatures, using metal vaporization techniques. These and other recent experiments have shed light on detailed functions of the particles. Especially by studying bimetallic systems, electronic and ensemble (geometric) effects have been clarified. In addition, strong-support-metal-interactions (SMSI) may be explained by geometric decoration effects. Thus, morphology is extremely important. Recent experiments on platinum-tin particles will be briefly reviewed. Studies in this area are leading the way in the new field of nanotechnology.

High-resolution scanning electron microscopy for the characterization of supported metal catalysts

A scanning electron microscope with a short-focal-length “immersion” lens and subnanometer resolution has been used to characterize several oxide-supported metal particle catalysts. Nanometer-sized metal particles in the Pt/TiO2 and Pd/SiO2 systems could be imaged with best clarity at the upper end of the operating voltage range (20–30 kV). However, visibility depended upon an adequate yield of secondary electrons relative to the support: small Pt particles on CeO2 could not be located by secondary electron imaging. Best visibility of the surface topography of the support was obtained at lower accelerating voltages.

Microstructural characteristics and mechanical properties for Al 2O 3/metal nanocomposities

High density Al 2O 3/W nanocomposite has been successfully fabricated by using controlled reduction and hot-pressing of Al 2O 3/WO 3 mixtures. For realizing desirable microstructures, chemical processing was used to prepare the mixtures by using an inorganic salt as a starting material of W03. TEM investigation revealed that the very fine WO 3 particles surrounded Al 2O 3 particles. During the sintering process under the reductive atmosphere, WO 3 was reduced to metal particles. TEM observation indicated that the nanometer-sized W particles were mainly located within the A1 2O 3 matrix grains. The mechanical properties were improved by dispersing small amount of nanometer-sized metal particles and resultant internal stresses.

Synthesis, properties, and characterization of nanometer-size metal particles by homogeneous reduction with alkalides and electrides in aprotic solvents

Synthesis, properties, and characterization of nanometer-size metal particles by homogeneous reduction with alkalides and electrides in aprotic solvents

Silver electrodeposits in ion-exchanged oxide glasses.

Silver electrodeposits have been grown within a wide range of oxide glass sytems. The latter are first of all subjected to an alkali-metal--silver-ion exchange reaction. The experimental configuration ensures two-dimensional growth. The fractal dimension of the electrodeposits has been estimated for each sample. Glasses having lithium ions (in the original composition) show a fractal dimension around 1.85 whereas those containing sodium ions give a fractal dimension of 1.68 for their respective silver deposits. Dendritic growth has been observed in glasses containing nanometer-sized metal particles of either bismuth or aluminum. The fractal dimension of all the electrodeposits appears to increase as the length scale is reduced.

44 O 02 Generation of nanometer-sized metal particles by gas-to-particle conversion

44 O 02 Generation of nanometer-sized metal particles by gas-to-particle conversion

Metal Complexes in Oxidic Matrices

ABSTRACTAlkoxides of the type (RO)nE-X-A in which a functional organic group A is connected to the alkoxide moiety via an inert and hydrolytically stable spacer X, are used to bind metal complex moieties. Sol-gel processing of the resulting metal complexes [(RO)nE-X-A]yMIm under standard conditions results in the formation of metal complex containing gels. If the metal complex moiety MLm is catalytically active, this method results in the heterogenization of homogeneous catalysts. Another application of the metal complex substituted gels is the preparation of composites containing uniform, nanometer-sized metal particles homogeneously dispersed in a ceramic matrix.

Study of surface plasmon excitation of nanometer size metallic particles in the STEM

The study of small particles or microclusters has been very active in recent years. When the particle size becomes smaller, its electronic properties deviate from those of bulk material and become size dependent. This phenomenon is referred to as the quantum size effect. In order to study the electronic states of small particles, the excitation energies of the surface plasmon (SP) have been measured by optical absorption spectroscopy and by broad beam angular resolved electron energy loss spectroscopy (EELS). Changes in the SP excitation energy were observed when the particle diameters were smaller than 40 Å.With the high spatial resolution of the scanning transmission electron microscope (STEM), we hope to overcome the difficulties associated with effects of particle size distribution encountered in previous broad beam measurements. For particles with diameters ranging from 100 Å to 200 Å, the SP energy was found decreasing with decrease of the particle size.

Image processing enhancement of high-resolution TEM micrographs of nanometer-size metal particles

The high-resolution TEM detectability of lattice fringes from metal particles supported on substrates is impeded by the substrate itself. This effect appears in both amorphous and crystalline substrates. In the former case, it is due to substrate-generated phase contrast features; in the latter case it is due to the intermixing of fringes from particle and substrate. Single value decomposition (SVD) and Fourier filtering (FFT) methods were applied to standard high resolution micrographs to enhance lattice resolution from particles as well as from crystalline substrates. SVD produced good results for one direction of fringes, and it can be implemented as a real-time process. Fourier methods are independent of azimuthal directions and allow separation of particle lattice planes from those pertaining to the substrate, which makes it feasible to detect possible substrate distortions produced by the supported particle. This method, on the other hand, is more elaborate, requires more computer time than SVD and is, therefore, less likely to be used in real-time image processing applications.

On the detection and size classification of nanometer-size metal particles on amorphous substrates

The detection and size analysis of small metal aggregates supported on amorphous substrates becomes increasingly difficult when the particle size approaches that of the phase contrast background structures of the support. Standard conventional transmission electron microscopy (CTEM), even with subsequent analog image analysis, becomes inconclusive or fails for particles less than 1 nm in diameter, and clear differentiation of the particles from the background can only be made with considerable effort, typically involving several micrographs taken from the same specimen area under different imaging conditions. The TEM image contrast transfer mechanism is briefly reviewed with emphasis on practical conclusions. In the 1–2 nm particle size range, particles can be distinguished from the amorphous background only when focus, astigmatism, and specimen drift are optimally controlled. Operating at 100 keV and with C s = 2.2 mm, a deviation of 100 nm from the optimum focus condition was, for example, found to increase the “apparent” diameter of 1.3 nm particles by as much as 0.4 nm. Furthermore, the electron exposure required to perform such microscopy may severely complicate the situation by radiation- induced changes in specimen and support. These changes may be intensified by specimen contaminants and an improperly controlled environment. Compared to standard high resolution imaging practice, operation at lower magnifications, lower acceleration voltage, larger illumination aperture, and closer to the Gaussian focus is recommended for routine imaging of very small particles on amorphous substrates.