19 research outputs found
Equilibrium structure and shape of Ag and Pt nanoparticles grown on silica surfaces: from experimental investigations to the determination of a metal-silica potential
A combination of experimental and numerical calculations on metallic silver
and platinum nanoparticles deposited on silica substrates is presented, with a
focus on the metal-substrate interactions. Experimentally, the nanoparticles
are elaborated under ultra-high vacuum and characterized by Grazing-Incidence
Small-Angle X-ray Scattering (GISAXS) and High Resolution Transmission
Electronic Microscopy (HRTEM) to determine their structure and morphology, and
in particular their aspect ratio (height/diameter) which quantifies the
metal-substrate interaction. Numerically, the interactions between the metal
and the silica species are modeled with the Lennard-Jones (12, 6) potential,
with two parameters for each metal and silica species. The geometric parameters
were found in the literature, while the energetic parameters were determined
from our experimental measurements of the aspect ratio. The parameters are:
{\sigma}_{Ag-O} = 0.278 nm, {\sigma}_{Ag-Si} = 0.329 nm, {\epsilon}_{Ag-O} = 75
meV, and {\epsilon}_{Ag-Si} = 13 meV for Ag-silica and {\sigma}_{Pt-O} = 0.273
nm, {\sigma}_{Pt-Si} = 0.324 nm, {\epsilon}_{Pt-O} = 110 meV, and
{\epsilon}_{Pt-Si} = 18 meV for Pt-silica. The proposed Ag-silica potential
reproduces quantitatively the unexpected experimental observation of the
variation of the aspect ratio for Ag nanoparticles larger than 5 nm, which has
been interpreted as a consequence of the silica roughness. The nanoparticle
orientation, structure and disorder are also considered. This metal-silica
potential for Ag and Pt should be helpful for further studies on pure metals as
well as their alloys.Comment: 35p, 12 figure
Silver-Platinum nanoparticles and nanodroplets supported on silica surfaces: structure and chemical ordering
Stable and metastable metallic nanoparticles exhibit unique properties
compared to the bulk, with potentially important applications for catalysis.
This is in particular the case for the AgPt alloy that can exhibit the ordered
L1 structure (alternation of pure Ag and Pt (111) planes) in nanometer size
particles. However, for such small systems, the interfaces play an important
role. Therefore, the support used to elaborate the nanoparticles in ultrahigh
vacuum experiments may influence their properties, even in the case of weakly
interacting substrates like amorphous carbon or silica. This work focuses on
the AgPt nanoparticles deposited on silica, and investigates the effect of the
support disorder and roughness on the structure and chemical ordering, in
particular at the interface with the substrate, by Monte Carlo calculations of
the atomic density profiles with semi-empiric potentials
Tuning growth from clusters to continuous ultrathin films: Experiments and molecular dynamics simulations of Pd plasma sputter deposition
Plasma sputter deposition experiments and simple molecular dynamics
calculations are performed for highlighting the effects of plasma ions and
kinetic energy of palladium atoms on the morphology of thin films. A transition
between cluster and continuous film growth is observed. It is attributed to the
kinetic energy of the depositing sputtered palladium atoms and to high binding
energy trapping sites resulting from the effects of ions incident on the
surface during deposition. These high binding energy trapping sites act as
additional nucleation centres that are allowed to be visited by the diffusing
Pd atoms
XMCD studies of Co and Co Pt nanoparticles prepared by vapour deposition
International audienc
Caractérisation structurale et contraintes résiduelles de films d'AIN par diffraction des rayons X
L'état mécanique des couches minces influence considérablement leurs propriétés macroscopiques et notamment leur comportement tribologique. Ainsi, outre les caractéristiques structurales, la détermination des microdéformations et des contraintes résiduelles apparaît comme essentielle. Cette étude porte donc sur la caractérisation par diffraction des rayons X de l'état microstructural et mécanique de couches de nitrure d'aluminium élaborées par deux types de procédé : la nitruration d'aluminium par plasma induit par laser et le dépôt par pulvérisation cathodique. Les résultats montrent l'absence d'orientation préférentielle et de contraintes résiduelles dans les couches d'AIN laser-plasma, alors que les dépôts présentent une direction de croissance privilégiée et de fortes contraintes d'origine thermique.The mechanical state in thin films considerably influences their macroscopic properties, especially their tribological behaviour. It is the reason why microstrains and residual stresses determination is very important. This paper deals with the characterization by X-ray diffraction of the microstructural and mechanical state of aluminum nitride layers elaborated by two kinds of processing : laser induced plasma aluminum nitriding and magnetron sputtering deposition. Results show no preferential orientation and residual stresses in laser-plasma AIN films while deposits offer preferential growth direction and high stresses from thermal ones
Real-time icosahedral to fcc structure transition during CoPt nanoparticle formation.
Nucleation and growth of supported CoPt nanoparticles were studied in situ and in real time by combined grazing incidence small-angle x-ray scattering (GISAXS) and grazing incidence x-ray diffraction (GIXD). GISAXS provides morphological features of nanoparticles as a function of size, shape and correlation distance between particles, while GIXD allows the determination of the atomic structure. We focus on the formation of ultrasmall CoPt nanoparticles, in the 1–4 nm size range at 500○C. The structural analysis method based on the Debye equation is coupled with cluster model calculations performed by Monte Carlo simulations using a semi-empirical tight-binding potential to interpret diffraction spectra and structural transitions. Our results show that the cluster structure evolution during the growth is size-dependent and composition-dependent, yielding an icosahedral to fcc structure transition
Trends in anomalous small-angle X-ray scattering in grazing incidence for supported nanoalloyed and core-shell metallic nanoparticles
As atomic structure and morphology of particles are directly correlated to their functional properties, experimental methods probing local and average features of particles at the nanoscale elicit a growing interest. Anomalous small-angle X-ray scattering (ASAXS) is a very attractive technique to investigate the size, shape and spatial distribution of nanoobjects embedded in a homogeneous matrix or in porous media. The anomalous variation of the scattering factor close to an absorption edge enables element specific investigations. In the case of supported nano-objects, the use of grazing incidence is necessary to limit the probed depth. The combination of grazing incidence with the anomalous technique provides a powerful new method, anomalous grazing incidence small-angle X-ray scattering (AGISAXS), to disentangle complex chemical patterns in supported multi-component nano-structures. Nevertheless, a proper data analysis requires accurate quantitative measurements associated to an adapted theoretical framework. This paper presents anomalous methods applied to nanoalloys phase separation in the 1–10 nm size range, and focuses on the application of AGISAXS in bimetallic systems: nanocomposite films and core-shell supported nanoparticles
Reversed size-dependent stabilization of ordered nanophases
Increasing nanoparticle size usually increases the stability of ordered phases within them. In contrast, the authors show here that the L11 ordered phase only forms in AgPt nanoparticles smaller than 2.5 nm because the segregation of a monolayer Ag shell constrains the nanophase which becomes unstable at larger sizes