Embedded Metal Nanoparticles Research Articles

Thermal stability of Fe, Co, Ni metal nanoparticles

AbstractThe thermal stability of metal nanoparticles is discussed in both melting thermodynamics and interface thermodynamics. Emphases are put on the size and shape dependence of melting temperature, critical size and vacancy‐formation energy of both freestanding and embedded metal nanoparticles. The melting temperature depression and superheating phenomena for reported nanoparticle systems are explained. Critical sizes of Fe, Co, Ni metal nanocrystals that the crystals keep their crystallinity are calculated and the corresponding minimum melting temperatures predicted. The vacancy‐formation energies of Fe, Co, Ni small particles are also calculated as a reference. Theoretical predictions are consistent with experimental results. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A study on percolation threshold of Ag-MgF2 nanoparticle cermet films

Several methods are introduced to calculate the percolation threshold of nanoparticle cermet films made of ceramic matrix with embedded metal nanoparticles. For Ag-MgF2 nanoparticle cermet films, the percolation threshold of 0.14, which reasonably agrees with the experimental results, is calculated by using Priou's percolation threshold theory. The factors that affect cermet film percolation threshold are also discussed.

Catalytic and Optical Behavior of Polymer Embedded Metal Nanoparticles

AbstractVery small metal aggregates (nanoaggregates) embedded in an inert polymer matrix, which simply stabilizes the nanodispersion, show unique catalytic and optical behaviour.Indeed Ru‐nanoparticles in vinyl‐aromatic polymers or polyphosphazenes display an enhanced reactivity for hydrogenation of different groups, such as aromatic rings, keto and nitro groups, oximes and nitriles which are converted into the corresponding saturated derivatives under mild conditions.The optical properties of terthiophenethiol‐coated gold nanoparticles in UHMWPE (concentration lower than 4 wt.%) are characterized by poor dichroism due to their centrosymmetric structure. High dichroism (R = 30 at 400 nm, DR = 30) is on the contrary recorded for the terthiophene band showing the chromophores to be sensitive to mechanical orientation also when complexed with gold. On the other hand, nanocomposites based on polymers containing vinyl alcohol units and dispersed gold nanoparticles with average diameter ranging from 3 to 20 nm have been efficiently prepared by an UV photo‐reduction process. Uniaxial drawing of the irradiated Au/polymer nanocomposites favours the anisotropic distribution of packed assemblies of gold particles, providing oriented films with polarization‐dependent tunable optical properties.

Metal nanoparticle-doped coloured films on glass and polycarbonate substrates

In a program on the development of metal (e.g. Au, Ag, Cu and their alloy) nanoparticles in sol-gel derived films, attempts were made to synthesize different coloured coatings on glasses and plastics. The absorption position of surface plasmon resonance (SPR) band arising from the embedded metal nanoparticles was tailored by controlling the refractive index of the matrix for the development of different colours. Thus different coloured (pink to blue) coatings on ordinary sheet glasses were prepared by generating Au nanoparticles in mixed SiO2-TiO2 matrices having refractive index values ranging from about 1.41 to 1.93. In another development,in situ generation of Ag nanoparticles in the inorganic-organic hybrid host leads to the formation of different abrasion resistant coloured coatings (yellow to pink) on polycarbonate substrates after curing. As expected, the SPR peak of Ag or Au is gradually red-shifted due to the increase of refractive index of the coating matrices causing a systematic change of colour

Evolution of poling-assisted bleaching of metal-doped nanocomposite glass with poling conditions

Poling-assisted bleaching was studied with respect to poling time, voltage, and temperature in glass samples having a gradient of silver nanoparticles across the depth. The optical extinction band due to the particles’ surface plasmon resonance (around 415 nm) was measured and the Maxwell–Garnett effective medium theory was used to fit extinction spectra. Fitting allowed us to determine the evolution of the thickness of the near-surface layer where the particles’ volume filling factor has dropped to zero as a result of the bleaching process. At 280 °C, bleaching started with a voltage as low as 200 V and saturated with time after about 1 h. Tight glass-electrode contact, voltage of at least 1 kV, and temperature higher than 200 °C were required in order to obtain significant and uniform bleaching in the poled area. The results were discussed in terms of the underlying electric-field-assisted dissolution of embedded metal nanoparticles.

Light‐Absorption Phenomena in Novel Low‐Reflectance Cathodes for Organic Light‐Emitting Devices Utilizing Metal–Organic Mixtures

Low‐optical‐reflectance cathodes (< 2 %, ∼ 45× lower than standard metal cathodes) utilizing a metal–organic mixed layer (MOML) have recently been developed and shown to dramatically enhance display contrast for organic light‐emitting devices. MOML characterization reveals an organic matrix with embedded metal nanoparticles (see Figure), whose surface plasmon resonance is responsible for the layer's light‐absorption properties.

Zero permittivity materials: Band gaps at the visible

We theoretically discuss the possibility of having materials with zero effective permittivity that would create band gaps in a wide range of frequencies up to the visible. The physical realization of these materials is also discussed in terms of embedding metallic nanoparticles and nanowires in a dielectric medium. In the limit of long wavelengths, these composites will behave like a homogeneous medium with zero permittivity that will completely reflect electromagnetic waves. We present transmittivity calculations by using finite-difference time domain for periodic structures that proves the concept and shows the validity of the long wavelength approximation. The striking result is that the cutoff frequency ωc is determined by the lattice parameter of the composite. By properly choosing the lattice constant of the composite and permittivity of metal and dielectric constituents, we can have full band gaps at any frequency range but especially in the visible.

Determination of thermally induced microstructural changes of embedded metal nanoparticles by lateral and cross-sectional transmission electron microscopy

The size and shape of silver and indium nanoparticles embedded in a plasma polymer matrix formed from benzene or styrene monomer were determined by transmission electron microscopy (TEM) in lateral direction and by cross-sectional transmission electron microscopy (XTEM) in vertical direction. In multilayer assemblies consisting of plasma polymer/plasma polymer metal composite/plasma polymer [ABA], silver as well as indium nanoparticles were embedded only in one plane. XTEM micrographs on samples before and after appropriate thermal annealing (450 K) showed, that the multilayer [ABA] structure was not destroyed. At a higher annealing temperature and time (570 K, 3 h) only parts of the plasma polymer matrix were left. With all thermal annealed samples, reshapening and coalescence of the embedded silver particles was found. For multilayers with embedded indium particles, the second plasma polymer layer was formed mainly as a shell around the large particles. During thermal annealing in the electron microscope, the formation of indium oxide particles inside these shells was observed.