Tracking the restructuring of oxidized silver-indium nanoparticles under a reducing atmosphere by environmental HRTEM.

Multimetallic nano-alloys display a structure and consequently physicochemical properties evolving in a reactive environment. Following and understanding this evolution is therefore crucial for future applications in gas sensing and heterogeneous catalysis. In view hereof, the structural evolution of oxidized Ag25In75 bimetallic nanoparticles under varying H2 partial pressures (PH2) and substrate temperatures (Ts) has been investigated in real-time through environmental transmission microscopy (E-TEM) while maintaining the atomic resolution. Small Ag25In75 bimetallic nanoparticles, produced by laser vaporization, are found (after air transfer) to contain an indium-oxide shell surrounding a silver-rich alloyed phase. For high PH2 and Ts, the direct reduction of the indium oxide shell, immediately followed by the melting or the diffusion onto the carbon substrate of the reduced indium atoms, is found to be the dominant mechanism. This reduction is concomitant with the growth of the core, indicating a partial diffusion of indium atoms from the shell towards the particle volume. The "surviving" particles therefore consist of a silver-indium alloy, very stable and remarkably resistant against oxidation contrary to native clusters. Interestingly, in the (PH2, Ts) space, the transition from "soft" (core-shell particles for low (PH2, Ts) values) to "strong" reduction conditions (silver-rich alloys for high (PH2, Ts) products) defines an intermediate domain where the preferred formation of Janus structures is detected. These results are discussed in terms of thermodynamic driving forces in relation to alloying and interface energies. This work shows the potential of high-resolution ETEM for unravelling the mechanisms of nanoparticle reorganization in a chemically reactive environment.

Plasmon spectroscopy of small indium-silver clusters: monitoring the indium shell oxidation.

Owing to the very different electrovalences of indium and silver, nanoparticles made of these elements are among the simplest examples of hybrid plasmonic systems retaining a full metallic character. The optical properties of small indium-silver clusters are investigated here for the first time in relation to their structural characterization. They are produced in the gas phase by a laser vaporization source and co-deposited in a silica matrix. The optical absorption of fresh samples is dominated by a strong surface plasmon resonance (SPR) in the near UV, in an intermediate position between those of pure elements. A combination of SPR analysis and electron microscopy imaging provides evidence for the favourable surface segregation of indium. After a prolonged exposure to ambient air and because of the silica matrix porosity, changes in the SPR reflect the spontaneous formation of a dielectric indium oxide shell around a metallic silver core. The metallic character of indium can nevertheless be recovered by annealing under a reducing atmosphere. The reversibility of these processes is directly mirrored in optical measurements through SPR shifts and broadenings as supported by multi-shell Mie theory calculations. By controlling their oxidation level, In-Ag clusters can be considered as new candidates to extend SPR spectroscopy in the UV range and model plasmonic systems consisting of a silver particle of potentially very small size, fully protected by a dielectric oxide shell.

Transition from core-shell to Janus chemical configuration for bimetallic nanoparticles.

In order to determine the possibilities to control the chemical configuration of bimetallic nanoparticles, we have considered CuAg nanoparticles synthesized by a physical route as a model in this study. The synthesis was made by pulsed laser deposition under ultra-high vacuum conditions, via a sequential deposition procedure. We show that the temperature of the substrate and the absolute quantity of Ag in a particle are the main parameters that drive the chemical configuration. To explain the transition from a core-shell configuration to a Janus configuration as a function of Ag quantity, we have conducted density-functional theory calculations and atomistic molecular dynamics simulations to investigate the stability of this system. The results are presented together with the experimental observations.

Flash laser annealing for controlling size and shape of magnetic alloy nanoparticles.

We propose an original route to prepare magnetic alloy nanoparticles with uniform size and shape by using nanosecond annealing under pulsed laser irradiation. As demonstrated here on CoPt nanoparticles, flash laser annealing gives an unprecedented opportunity to control the size and the shape of bimetallic nanoparticles without changing their composition. The mechanisms involved in the complete reshaping of the nanoparticle thin films are discussed and it is also shown that order-disorder phase transformations occur under laser irradiation. This technique is then very interesting for magnetic alloy nanoparticles studies and applications because it opens up a new way to fabricate size-controlled spherical nanoparticles with narrow size dispersion.

Growth and structural properties of CuAg and CoPt bimetallic nanoparticles.

Core/shell CuAg and alloyed CoPt have been synthesized using two vapor phase deposition techniques. For CuAg prepared by Thermal Evaporation (TE), the size and the morphology of the Cu cores are the key parameters to promote the formation of the core/shell arrangement. For CoPt synthesized by Pulsed Laser Deposition (PLD), the growth kinetics of nanoparticles, depending on the deposition rate, the substrate nature and the temperature, controls the nanoparticle morphology. The competition between the growth and the ordering kinetics governs the nanoparticle structure. By reducing the growth kinetics, as-grown L1(0) ordered nanoparticles are obtained according to the bulk phase diagram.

Near-perfect elastoplasticity in pure nanocrystalline copper.

Ductile metals and alloys undergo plastic yielding at room temperature, during which they exhibit work-hardening and the generation of surface instabilities that lead to necking and failure. We show that pure nanocrystalline copper behaves differently, displaying near-perfect elastoplastic behavior characterized by Newtonian flow and the absence of both work-hardening and neck formation. We observed this behavior in tensile tests on fully dense large-scale bulk nanocrystalline samples. The experimental results further our understanding of the unique mechanical properties of nanocrystalline materials and also provide a basis for commercial technologies for the plastic (and superplastic) formation of such materials.