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Gold–copper (Au–Cu) phases were employed already by pre-Columbian civilizations, essentially in decorative arts, whereas nowadays, they emerge in nanotechnology as an important catalyst. The knowledge of the phase diagram is critical to understanding the performance of a material. However, experimental determination of nanophase diagrams is rare because calorimetry remains quite challenging at the nanoscale; theoretical investigations, therefore, are welcomed. Using nanothermodynamics, this paper presents the phase diagrams of various polyhedral nanoparticles (tetrahedron, cube, octahedron, decahedron, dodecahedron, rhombic dodecahedron, truncated octahedron, cuboctahedron, and icosahedron) at sizes 4 and 10 nm. One finds, for all the shapes investigated, that the congruent melting point of these nanoparticles is shifted with respect to both size and composition (copper enrichment). Segregation reveals a gold enrichment at the surface, leading to a kind of core–shell structure, reminiscent of the historical artifacts. Finally, the most stable structures were determined to be the dodecahedron, truncated octahedron, and icosahedron with a Cu-rich core/Au-rich surface. The results of the thermodynamic approach are compared and supported by molecular-dynamics simulations and by electron-microscopy (EDX) observations.

A stress relaxation method for freestanding thin films is developed based on an on-chip internal stress actuated microtensile testing set-up. The on-chip test structures are produced using microfabrication techniques involving cleaning, deposition, lithography, and release. After release from the substrate, the test specimens are subjected to uniaxial tension. The applied load decays with the deformation taking place during relaxation. This technique is adapted to strain rates lower than 10-6/s and permits the determination of the strain rate sensitivity of very thin films. The main advantage of the technique is that the relaxation tests are simultaneously performed on thousands of specimens, pre-deformed up to different strain levels, for very long periods of time without monopolizing any external mechanical loading equipment. Proof of concept results are provided for 205-nm-thick sputtered AlSi0.01 films and for 350-nm-thick evaporated Pd films showing unexpectedly high relaxation at room temperature.

TiO 2 nanotubular structures were prepared by electrochemical oxidation performed in organic electrolytes containing fluorides protracted up to different times. After the annealing treatment, required to obtain crystalline structures, samples were submitted to morphological, electrical and photo-electrochemical analysis. The obtained results suggest that in the presence of porous structures such as nanotube arrays the photo catalytic behaviour of the samples cannot be simply explained considering the different specific areas, but also the electronic properties of the electrodes have to be taken into account. Electrochemical impedance spectroscopy was used to correlate the morphology of these structures with their electronic properties and their working mechanism in the presence of a light radiation. A schematic representation of the nanotube structures as consisting of two layers, compact and porous, proved to be appropriate to clarify the obtained results. © 2012 Elsevier B.V. All rights reserved.

Nanoscience and nanotechnology are among the most widely used terms in the modern scientific and technological literature. The idea of nanotechnology appeared for the first time in the famous talk “There is plenty of room at the bottom” given by the physicist Richard Feynman at the American Physical Society meeting at Caltech on December 29, 1959. Feynman described a process by which the ability to manipulate individual atoms and molecules might be developed, using one set of precise tools to build and operate another proportionally smaller set and so on down to the needed scale. In the course of this, he noted, scaling issues would arise from the changing magnitude of various physical phenomena: gravity would become less important whereas surface effects would become increasingly more significant

A novel stress relaxation technique dedicated to thin freestanding films has been developed as an extension of an on-chip tensile testing method. The technique relies on a structure made of two beams. An actuator beam is deposited with large internal stresses onto a sacrificial layer. The specimen beam is deposited next, partly overlapping with the actuator beam. By etching the sacrificial layer, the two-beam structure is released from the substrate and the test specimen is deformed until force equilibrium is reached. The specimen, being under stress, relaxes at a rate depending on the composition, microstructure, thickness, and temperature. This test is similar to a creep test on a specimen attached to a spring. The strain rate sensitivity exponent and activation volume can be derived from a record of the evolution with time of the displacement undergone by the test specimen. This technique allows measuring the relaxation response of a large amount of test structures, involving different levels of plastic strains, dimensions and loading conditions, without requiring the use of any external device such as a nanoindentor or tensometer. The technique is well suited for very long relaxation measurements. Strain rates varying between 10-6 s-1 down to 10-11 s-1 are accessible and statistically representative data can be generated. The technique has been applied to both 205 nm-thick evaporated Al 1%Si and 355 nm-thick evaporated Pd films showing interesting variations of the activation volume with strain rate and initial level of plastic deformation. Relaxation at room temperature has been studied as well as at higher temperature for the Pd films. In the AlSi films, the initial activation volume amounts to a few b3 presumably due to diffusion mechanisms. The diffusion mechanism requires stress gradients which smooth out during relaxation, leaving room, at lower strain rates, to another dominant relaxation mechanism involving a much larger activation volume. The recovery o

Vacancies play a major role in the electrical and thermal transport as well as the mechanical behavior of materials. To understand the processes occurring in nanomaterials during heat treatment and mechanical deformation, the size effects on the vacancy formation energy and entropy have to be considered. As these material properties are hardly measurable experimentally, particularly in nanoparticles, a theoretical model calculating the size-dependent vacancy formation energy and entropy has been developed in this study. It has been found that size reduction makes the vacancy much easier to form; then the vacancy concentration increases when size reduces and temperature increases. © 2011 American Chemical Society.

In this paper, top-down and bottom-up approaches are used to predict material properties of group III-nitride nanostructures. The. first approach calculates the melting temperature, melting enthalpy, Debye temperature and energy bandgap of InN, GaN and AlN through classical thermodynamics. The second approach calculates the surface energies in the liquid and solid states of the considered nitrides materials through molecular dynamics. Moreover, the liquid and solid surface energies of the zinc-blende and wurtzite III-V materials are compared. Finally, the phase diagram of a ternary III-nitride nanomaterial, AlGaN, is presented and the variation of its energy bandgap with composition is predicted.

Due to the lack of experimental values concerning some material properties at the nanoscale, it is interesting to evaluate this theoretically. Through a "top-down'' approach, a universal equation is developed here which is particularly helpful when experiments are difficult to lead on a specific material property. It only requires the knowledge of the surface area to volume ratio of the nanomaterial, its size as well as the statistic (Fermi-Dirac or Bose-Einstein) followed by the particles involved in the considered material property. Comparison between different existing theoretical models and the proposed equation is done.