D. Romeu

Strong white and blue photoluminescence from silicon nanocrystals in SiNx grown by remote PECVD using SiCl4/NH3

Strong white and blue photoluminescence (PL) from as-grown silicon nanocrystals (nc-Si) in SiNx films prepared by remote plasma enhanced chemical vapour deposition using SiCl4/NH3 mixtures is reported. The colour and intensity of the PL could be controlled by adjusting the NH3 flow rate. Samples with white emission were annealed at 1000 °C, obtaining a strong improvement of the PL intensity with a blue colour. The PL can be attributed to quantum confinement effect in nc-Si embedded in SiNx matrix, which is improved when a better passivation of nc-Si surface with chlorine and nitrogen atoms is obtained. The size, density and structure of the nc-Si in the as-grown and annealed films were confirmed and measured by high-resolution transmission electron microscopy.

The relation between dispersion and particle size on supported catalysts

Abstract The relation between catalyst dispersion and particle size is determined using computer calculations taking into account the particle roughness observed in weak beam thickness fringes. It is shown that a complex relationship exists between these parameters. Therefore the use of simple formulae to correlate chemisorption measurements with electron microscopy is questioned.

Distribution of surface sites on small metallic particles

Abstract The surface structure of small metallic particles on supported catalysts is determined using weak-beam techniques. It is found that surfaces of the particles have a rough structure corresponding to incomplete layers and steps. This structure can be reproduced with a theoretical calculation of the particle structure assuming that the gross crystal structure of the particle is retained but it has one or more incomplete shells of atoms. The calculation enables us to determine the probability of various sites as a function of particle size. Sites of coordination from B2 to B6 are calculated. The results indicate that single-atom and B2 sites, which do not oscillate in population with increasing particle size, are most likely responsible for catalytic activity in hydrogenolysis. On the other hand, sites of higher coordination (B3–B6), which have an oscillatory behavior with size, are most likely responsible for isomerisation and particle size effects.

Helical Growth of Ultrathin Gold-Copper Nanowires

In this work, we report the synthesis and detailed structural characterization of novel helical gold-copper nanowires. The nanowires possess the Boerdijk-Coxeter-Bernal structure, based on the pile up of octahedral, icosahedral, and/or decahedral seeds. They are self-assembled into a coiled manner as individual wires or into a parallel-ordering way as groups of wires. The helical nanowires are ultrathin with a diameter of less than 10 nm and variable length of several micrometers, presenting a high density of twin boundaries and stacking faults. To the best of our knowledge, such gold-copper nanowires have never been reported previously.

On the structure of small palladium particles

The study of small noble metal particles is becoming increasingly important in many fields in physics (1). The advent of high-resolution electron microscopy (HREM) has allowed a deeper understanding of structural aspects of small particles. This work reports the study of particles of palladium with a diameter less than 3 nm. Specimens were prepared by in-situ deposition of Pd onto thin carbon films under near-UHV conditions in the specimen preparation chamber. Faulted decahedral MTP was grown using a recursive (R) growth model which generates infinite, space-filling structures reproducing the structure of crystals, twinned particles and quasicrystals. R growth consists of the formation of a cluster by iterative addition of points (atoms) from a given star vector. The method presented sheds some light on a point that has been controversial in the past about the nature of MTPs. Some authors have claimed that these structures can be considered as FCC twins with a disclination to close the resulting gap. The fact that they can be obtained quite simply from stable smaller units appears to make the disclination unnecessary.

Gold-copper nanostars as photo-thermal agents: Synthesis and advanced electron microscopy characterization

Nanoalloys have emerged as multi-functional nanoparticles with applications in biomedicine and catalysis. This work reports the efficient production and the advanced transmission electron microscopy characterization of gold-copper pentagonal nanostars. The morphology of the branches is controlled by the adequate choice of the capping agent. When oleylamine is used rounded nanostars are produced, while pointed nanostars are obtained by using hexadecylamine. Both types of nanostars were proved to be thermally stable and could therefore be used as therapeutic agents in photo-thermal therapies as confirmed by the near-infrared absorption spectra.

MODELING GRAIN BOUNDARIES IN SOLIDS USING A COMBINED NONLINEAR AND GEOMETRICAL METHOD

The complex arrangements of atoms near grain boundaries are difficult to understand theoretically. We propose a phenomenological (Ginzburg–Landau-like) description of crystalline phases based on symmetries and some fairly general stability arguments. This method allows a very detailed description of defects at the lattice scale with virtually no tunning parameters, unlike the usual phase-field methods. The model equations are directly inspired from those used in a very different physical context, namely, the formation of periodic patterns in systems out-of-equilibrium (e.g. Rayleigh–Benard convection, Turing patterns). We apply the formalism to the study of symmetric tilt boundaries. Our results are in quantitative agreement with those predicted by a recent crystallographic theory of grain boundaries based on a geometrical quasicrystal-like construction. These results suggest that frustration and competition effects near a defect in crystalline arrangements have some universal features, of interest in solids or other periodic phases.

Three-dimensional decagonal structure for AlCoCu alloys as random coincidence networks with Fibonacci spaced lines

Abstract The decahedral recursive model is used to explain at an atomic level the microcrystalline O1, O2, O1′ and O2′ and decagonal states of AlCuCrFe and AlCuCo(Si) systems. Electron microscopy results, including the presence of Fibonacci sequenced lines in DAl65Cu20Co15Si2 are also explained by random accretion of clusters. In spite on an apparent conflict, experimental results are seen to be compatible with the random tiling hypotheses. A quasiperiodic state is obtained when the disorder is maximized.

Recurrence properties of O‐lattices and the classification of grain boundaries

A recurrence relation is shown to exist between O-lattices of rotation-related grain boundaries (GBs) when a suitable parametrization of the rotation angle is introduced. This relation allows the basis vectors of any O-lattice to be calculated by a simple vector addition if the basis vectors of any two orientations are known. Its main usefulness, however, lies in the fact that it induces a partition of the angular space into disjoint sets, which groups grain boundaries into a finite number of equivalence classes, each represented by a special singular boundary (normal form). This shows that the O-lattice theory contains within it a much sought after general classification scheme for interfaces independent of the crystal system and therefore completely general.

A pseudoinverse for Frank's formula

In a previous communication, it has been argued that in the Frank-Bilby equation and in the o-lattice equations a pseudoinverse must be used when the reduced displacement-field matrices are not invertible. Here an explicit expression for the pseudoinverse of Franks formula is derived using a direct vector approach.