arXiv subjects

David Amans

Publications and source records attributed to David Amans.

6 recordsLinked to original sources

arXiv2019

Comparison of aluminum oxide empirical potentials from cluster to nanoparticle

Aluminum oxide nanoparticles are increasingly sought in numerous technological applications. However, as the nanoparticles grow during the synthesis, two phase transitions occur. At the nanoscale, numerical simulation of the stability of the alumina phases requires the use of empirical potentials that are reliable over a large range of system sizes going from a few atoms to several hundred thousand atoms. In this work, we confronted four different empirical potentials that are currently employed for bulk alumina. We found that only two of them are correct at the molecular level when compared to DFT calculations. Furthermore, the two potentials remain the best at the nanoscale as they reproduce one or two phase transitions that were observed experimentally: from amorphous solid to cubic crystal ({\gamma}) and from cubic to hexagonal ({\alpha}, i.e. corundum) crystal.

cond-mat.mes-hall↗

arXiv2017

Atomistic Mechanisms for the Nucleation of Aluminium Oxide Nanoparticles

A predictive model for nanoparticle nucleation has not yet been successfully achieved. Classical nucleation theory fails because the atomistic nature of the seed has to be considered since geometrical structure as well as stoichiometry do not always match the bulk values. We present a fully microscopic approach based on a first-principle study of aluminium oxide clusters. We have calculated stable structures of AlxOy} and their associated thermodynamic properties. From these data, the chemical composition of a gas composed of aluminium and oxygen atoms can be calculated as a function of temperature, pressure, and aluminium to oxygen ratio. We demonstrate the accuracy of this approach in reproducing experimental results obtained with time resolved spectroscopy of a laser induced plasma from an Al2O3} target. We thus extended the calculation to lower temperatures, i.e. longer time scales, to propose a scenario of composition gas evolution leading to the first alumina seeds.

physics.chem-ph↗

arXiv2013

Shells of crystal field symmetries evidenced in oxide nano-crystals

By the use of a point charge model based on the Judd-Ofelt transition theory, the luminescence from Eu3+ ions embedded in Gd2O3 clusters is calculated and compared to the experimental data. The main result of the numerical study is that without invoking any other mechanisms such as crystal disorder, the pure geometrical argument of the symmetry breaking induced by the particle surface has influence on the energy level splitting. The modifications are also predicted to be observable in realistic conditions where unavoidable size dispersion has to be taken into account. The emission spectrum results from the contribution of three distinct regions, a cluster core, a cluster shell and a very surface, the latter being almost completely quenched in realistic conditions. Eventually, by detailing the spectra of the ions embedded at different positions in the cluster we get an estimate of about 0.5 nm for the extent of the crystal field induced Stark effect. Due to the similarity between Y2O3 and Gd2O3, these results apply also to Eu3+ doped Y2O3 nanoparticles.

cond-mat.mes-hall↗

arXiv2009

Facile and rapid synthesis of highly luminescent nanoparticles via Pulsed Laser Ablation in Liquid

This paper demonstrates the usefulness of pulsed laser ablation in liquids as a fast screening synthesis method able to prepare even complex compositions at the nanoscale. Nanoparticles of Y2O3:Eu3+, Lu2O2S: Eu3+, Gd2SiO5:Ce3+ and Lu3TaO7:Gd3+,Tb3+ are successfully synthesized by pulsed laser ablation in liquids. The phase and stoichiometries of the original materials are preserved while the sizes are reduced down to 5-10 nm. The optical properties of the materials are also preserved but show some small variations and some additional structures which are attributed to the specificities of the nanoscale (internal pressure, inhomogeneous broadening, surface states...)

cond-mat.mtrl-sci↗

arXiv2005

Higher Order Harmonics of Modulational Instability Induced by Vacuum Fluctuations

We study the higher order harmonics of scalar modulational instability in the regime where it arises spontaneously through amplification of vacuum fluctuations. We obtain detailed predictions concerning the detunings, intensities, growth rates and spectral widths of the harmonics. These predictions are well verified by experimental results obtained by propagating high intensity light pulses through optical fibers.

physics.optics↗

arXiv2004

Provably Secure Experimental Quantum Bit-String Generation

Coin tossing is a cryptographic task in which two parties who do not trust each other aim to generate a common random bit. Using classical communication this is impossible, but non trivial coin tossing is possible using quantum communication. Here we consider the case when the parties do not want to toss a single coin, but many. This is called bit string generation. We report the experimental generation of strings of coins which are provably more random than achievable using classical communication. The experiment is based on the ``plug and play'' scheme developed for quantum cryptography, and therefore well suited for long distance quantum communication.

quant-ph↗