J.A. Ascencio

Structure determination of small particles by HREM imaging: theory and experiment

The characterization of the structure of small particles (with a size of a few nanometers) is very important in order to understand their optical, electronic and magnetic properties. High-resolution electron microscopy is a fundamental tool to obtain a full characterization of nanometric particles. In the present work, we report a systematic theoretical study of the HREM images of metallic particles with nanometric sizes. The images of gold clusters with FCC, decahedral, icosahedral and amorphous structures are reported for a number of orientations with respect to the electron beam. Experimental images of a number of particles are reported and compared with the calculated images and Fourier transforms.

Melting behavior of nanometer sized gold isomers

In the present work, the melting behavior of nanometer sized gold isomers was studied using a tight-binding potential with a second momentum approximation. The cases of cuboctahedra, icosahedra, Bagley decahedra, Marks decahedra and star-like decahedra were considered. We calculated the temperature dependence of the total energy and volume during melting and the melting point for different types and sizes of clusters. In addition, the structural evolutions of the nanosized clusters during the melting transition were monitored and revealed. It is found that the melting process has three characteristic time periods for the intermediate nanosized clusters. The whole process includes surface disordering and reordering, followed by surface melting and a final rapid overall melting. This is a new observation, which it is in contrast with previous reports where surface melting is the dominant step.

Structural analysis and growing mechanisms for long SnO2 nanorods synthesized by spray pyrolysis

Flat-surfaced, rod-like materials were obtained by synthesizing long one-dimensional SnO2 structures using a new spray pyrolysis method. The structure and growing mechanisms were evaluated by using scanning and transmission electron microscopy and atomic force microscopy. Molecular simulation tools and high resolution transmission electron microscopy images allowed the analysis of a dynamic behaviour for energy release which determines how the structures are formed by searching for an axial energy release direction.

Theoretical analysis of hydroxylapatite and its main precursors by quantum mechanics and HREM image simulation

Abstract The possibility to produce hydroxylapatite, for biomaterials applications, from different kind of precursors has motivated many efforts. In this work we have analyzed several common precursors, mainly calcium based systems. Quantum mechanics calculations, by density-functional theory and semi-empirical approximations, were used to study the characteristic crystals and the particular behavior of the PO 4 3− molecules interaction, both by the electronic distributions in order to give a new possibility for the study of these materials and for improving new synthesis methods. High resolution electron microscopy images are also calculated, by a multislice method, to confirm our approaches and also to show a easy way to distinguish this materials experimentally as is proved in this work.

Effects of Minor Element Additions to the Nanocrystalline FeAl Intermetallic Alloy Obtained by Mechanical Alloying

Chemical, microstructural, and mechanical properties of nanocrystalline FeAl intermetallic alloys with Li, Ce, and Ni additions have been assessed. Mechanical alloying and sintering procedures were used to produce and consolidate the alloys. The sintering procedure was based on room temperature uniaxial pressing followed by annealing in air of the pressed specimens. The mechanically alloyed powders have a microstructure consisting of micrometer-size particles that contain FeAl intermetallic nanocrystals. The three minor additional elements form a solid solution with the B2 intermetallic structure of the FeAl alloy. Densification greater than 90% has been obtained. The hardness values are higher than those obtained from specimens produced with conventional casting procedures. High resolution transmission electron microscopy images showed clusters of less than 5 nm with well-defined structure corresponding to Fe 3 Al.

Structural models of inorganic fullerene-like structures

Abstract In the study of fullerene-like structures, some of the more interesting systems are the inorganic cages, made of MoS 2 (usually named inorganic fullerenes), which have many important potential applications as lubricant and catalysts. In the present work, we report calculations for structural models of closed cage of inorganic fullerene-like structures for MoS 2 system. Three cage shapes were found to be the most stable: triangular pyramid, octahedron and dodecahedron. High resolution TEM images of MoS 2 cages structures were calculated to be compared with experimental data. Some examples of triangular pyramid and polyhedron in experimental MoS 2 samples are presented.

Stability and phase transition of sodium nanoclusters

Abstract In this work, structural transformation and some thermodynamic processes of sodium nanoclusters were studied using classic molecular dynamics simulation and density functional theory calculation. These sodium nanoclusters cover cuboctahedra, amorphous, and multi-twinned particles ranging from a few nanometers to tens of nanometers in size. The simulation results indicate different stability regions for different structures and small differences of both melting point and boiling point for all types of clusters. Amorphous transition and Frank–Kasp polyhedron transition from the rounded bcc cluster were observed on the ground state, and also surprising heat capacity curves of amorphous clusters under study were found at regions near the melting.

Synthesis and characterization of quantum dot superlattices.

Gold nanoparticles have been synthesized using n-alkylthiol molecules as a passivating agent. By fixing the length of the thiol chain, it is possible to produce nanocrystal arrays, such as 1D chains, 2D arrays of chains and 3D crystals.