Centre d’Élaboration de Matériaux et d’Etudes Structurales (UPR 8011)


Accueil > Recherche > SINanO : Surfaces, Interfaces et Nano-Objets > Thématiques de Recherche

Nanoparticles : formation mechanisms, size effects, properties, interaction with their environment

SINanO Highlights - The names of the team members are in blue

Au147 nanoparticles : Ordered or amorphous ?

Nathalie Tarrat, Mathias Rapacioli, Fernand Spiegelman

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Structural aspects of the Au147 nanoparticles have been investigated through a density functional based tight binding global optimization involving a parallel tempering molecular dynamics scheme with quenching followed by geometries relaxation at the Density Functional Theory (DFT) level. The focus is put on the competition between relaxed ordered regular geometries and disordered (or amorphous) structures. The present work shows that Au147 amorphous geometries are relevant low energy candidates and are likely to contribute in finite temperature dynamics and thermodynamics. The structure of the amorphous-like isomers is discussed from the anisotropy parameters, the atomic coordinations, the radial and pair distribution functions, the IR spectra, and the vibrational DOS. With respect to the regular structures, the amorphous geometries are shown to be characterized by a larger number of surface atoms, a less dense volume with reduced coordination number per atom, a propensity to increase the dimension of flat facets at the surface, and a stronger anisotropy. Moreover, all amorphous clusters have similar IR spectra, almost continuous with active frequencies over the whole spectral range, while symmetric clusters are characterized by a few lines with large intensities.


Melting of the Au20 Gold Cluster : Does Charge Matter ?

Mathias Rapacioli, Nathalie Tarrat, Fernand Spiegelman

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We investigate the dependence upon the charge of the heat capacities of magic gold cluster Au20 obtained from density functional-based tight binding theory within parallel tempering molecular dynamics and the multiple histogram method. The melting temperatures, determined from the heat capacity curves, are found to be 1102 K for neutral Au20 and only 866 and 826 K for Au20+ and Au20. Both the canonical and the microcanonical aspects of the transition are discussed. A convex intruder, associated with a negative heat capacity, is evidenced in the microcanonical entropy in the case of the neutral cluster but is absent in the melting processes of the ions. The present work shows that a single charge quantitatively affects the thermal properties of the gold 20mer.


Global optimization of neutral and charged 20- and 55-atom silver and gold clusters at the DFTB level

Nathalie Tarrat, Mathias Rapacioli, Jérôme Cuny, Joseph Morillo, Jean-Louis Heully, Fernand Spiegelman

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The global optimization of metallic clusters is an important topic because nanoclusters exhibit structure-dependent properties. In this paper, we present a global optimization study of Ag20, Au20, Ag55 and Au55 in their neutral and charge states (-1, 0, +1) conducted using a Parallel-Tempering Molecular Dynamics algorithm at the DFTB level without pre-screening. For Au20, Ag20 and their ions, the present DFTB low energy structures are in good agreement with previously published calculations and experimental data. In the case of Ag55- and Au55-, the present study is consistent with photo-electron detachment experiments suggesting highly symmetric icosahedral structures for silver and more disordered morphologies for gold. The present results are also compatible with trapped ion electron diffraction experiments and calculations for Ag55+ and Ag55-. We report low-energy isomers of Au55 exhibiting cavities below their external shell. This work quantitatively confirms the relevance of DFTB for structure calculation of noble metal clusters. Furthermore, it also demonstrates the feasibility of global optimization using DFTB, without pre-screening through classical potential, for sizes up to a few tens of atoms and for different charge states.

 


 

Benchmarking DFTB for Silver and Gold Materials : From Small Clusters to Bulk 

Luiz Fernando Lopes Oliveira, Nathalie Tarrat, Jerome Cuny, Joseph Morillo, Didier Lemoine, Fernand Spiegelman, and Mathias Rapacioli

 

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We benchmark existing and improved self-consistent-charge density functional based tight-binding (SCC-DFTB) atomic parameters for silver and gold clusters as well as for bulk materials. In the former case, our benchmarks focus on both the structural and energetic properties of small-size AgN and AuN clusters (N from 2 to 13), medium-size clusters with N = 20 and 55, and finally larger nanoparticles with N = 147, 309 and 561. For bulk materials, structural, energetics and elastic properties are discussed. We show that SCC-DFTB is quite satisfactory in reproducing essential differences between silver and gold aggregates, in particular their 2D-3D structural transitions, and their depen- dency upon cluster charge. SCC-DFTB is also in agreement with DFT and experiments in the medium-size regime regarding the energetic ordering of the different low-energy isomers and allows for an overall satisfactory treatment of bulk properties. A consistent convergence between the cohesive energies of the largest investigated nanoparticles and the bulk’s is obtained. Based on our results for nanoparticles of increasing size, a two- parameter analytical extrapolation of the cohesive energy is proposed. This formula takes into account the reduction of the cohesive energy for undercoordinated surface sites and converges properly to the bulk cohesive energy. Values for the surface sites cohesive energies are also proposed.

 


Prediction of Co nanoparticle morphologies

stabilized by ligands : towards a kinetic model

Van Bac Nguyen, Magali Benoit, Nicolas Combe and Hao Tang

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Cobalt nanoparticles (NPs) synthesized in liquid environments present anisotropic shaped nanocrystals such as disks, plates, rods, wires or cubes. Though the synthesis parameters (precursor, reducing agent, stabilizing ligands, concentration, temperature or rate of precursor injection) controlling the final morphologies are experimentally well controlled, little is known concerning the growth mechanisms at the atomic scale. In this work, we intend to predict the morphology variation of hcp cobalt NPs as a function of the ligand concentration. To this aim, we consider two well-established thermodynamic models and develop a new kinetic one. These models require the knowledge of the adsorption behaviors of stabilizing molecules as a function of surface coverage on preferential facets of NPs. To this end, density functional theory (DFT) calculations were performed on the adsorption of a model carboxylate ligand CH3COO on different Co crystalline surfaces. The shapes of the Co NPs obtained by these models are compared to experimental morphologies and other theoretical results from the literature. While thermodynamic models are in poor agreement with experimental observations, the variety of shapes predicted by the kinetic model is much more promising. Our study confirms that the morphological control of NPs is mostly driven by kinetic effects.


Modeling Iron–Gold Nanoparticles Using a

Dedicated Semi-Empirical Potential :

Application to the Stability of Core–Shell Structures

F. Calvo, N. Combe, J. Morillo, and M. Benoit

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Core–shell nanoparticles made from iron embedded in gold have a strong potential interest in nanomedicine, the Au shell providing an efficient biocompatible coating for the magnetic Fe core. With the aim of determining theoretically the equilibrium morphologies of Fe–Au nanoparticles in a broad size range and with different compositions, a semiempirical many-body Fe–Au potential was designed combining well-established models for the pure metals and introducing dedicated contributions for the interactions involving mixed elements. The potential was parametrized against various energetic properties involving impurities, intermetallics, and finite clusters obtained using density functional calculations in the generalized gradient approximation. The potential was tested to investigate Fe–Au nanoparticles near equiconcentration containing about 1000–2000 atoms at finite temperature using parallel tempering Monte Carlo simulations initiated from the core–shell structure. The core–shell nanoparticles are found to be thermally stable up to about 800 K, at which point the gold outer layer starts to melt, with the iron core remaining stable up to approximately 1200 K. In contrast, the alternative potential developed by Zhou and co-workers (Zhou, X. Z. ; Johnson, R. A. ; Wadley, H. N. G. Phys. Rev. B, 2004, 69, 144113) predicts a strong tendency for mixing, the core–shell structure appearing energetically metastable. The two models also predict significantly different vibrational spectra.


 

Magnetism and morphology in faceted

B2-ordered FeRh nanoparticles

M. Liu, P. Benzo, H. Tang, M. Castiella, B. Warot-Fonrose, N. Tarrat, C. Gatel, M. Respaud, J. Morillo and M. J. Casanove

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Whereas the bulk equiatomic FeRh alloy with B2 structure is antiferromagnetic (AFM) below 370K, we demonstrate that the surface configuration can stabilize the low temperature ferromagnetic (FM) state in FeRh nanoparticles in the 6–10 nm range. The most stable configuration for FM nanoparticles, predicted through first-principles calculations, is obtained in magnetron sputtering synthesized nanoparticles. The structure, morphology and Rh-(100) surface termination are confirmed by aberration-corrected (scanning) transmission electron microscopy. The FM magnetic state is verified by vibrating sample magnetometry experiments. This combined theoretical and experimental study emphasizes the strong interplay between surface configuration, morphology and magnetic state in magnetic nanoparticles. EPL, 116 27006 (2016).


 

Fully Crystalline Faceted Fe–Au Core–Shell Nanoparticles

C. Langlois, P. Benzo, R. Arenal, M. Benoit, J. Nicolai, N. Combe , A. Ponchet, and M. J. Casano

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Iron-Gold core-shell nanocrystals : A question of balance - The nature and the stress state of the exposed facets determine the capacity of nanocrystals to bind to or dissociate a target molecule. They can be mastered by transposing the island growth conditions on a crystalline substrate to core-shell nanocrystals. This has just been demonstrated by a CEMES team and collaborators in Gold-Iron (Fe @ Au) nanoparticles. This work is published in Nanoletters.