On the Modeling of the Diffraction Pattern from Metal Nanocrystals

Abstract

The powder diffraction patterns of spherical nanocrystals made of five different fcc metals were generated using atomistic models within a Molecular Dynamics simulation. Static and dynamic effects are interpreted and discussed within the framework of two different approaches, respectively, based on (1) a Reciprocal Space and (2) a Direct Space representation of diffraction. Chosen elements display a wide range of properties, especially related to material stiffness and elastic anisotropy, so to deeply challenge interpretation paradigms. The effect of the shape on static and dynamic features is also addressed.

A total scattering Debye function analysis study of faulted Pt nanocrystals embedded in a porous matrix

“…For comparison, the values for the unfaulted model are provided: 3.41 (423 K), 3.05 (473 K), 3.06 (523 K). agreement with the values found in our samples and in other recent reports (Gelisio & Scardi, 2014). In our study, the size dependence of the lattice parameters is mostly due to the large number of Pt atoms lying on the surface of the smallest SPH NCs, and was determined through the strain model implemented in DEBUSSY (Cervellino et al, 2015) and reported in equation (7):…”

“…(iii) Surface relaxation effects are reported in very small nanoparticles that, in metals, typically (but not uniquely) lead to average lattice parameters shorter than in the bulk material, giving rise to a (material-dependent) contraction of the metalmetal distances at the surface (Solliard & Flueli, 1985;Qi & Wang, 2005;Qi et al, 2002;Gelisio & Scardi, 2014). A sizedependent effect was modelled for the SPH population by optimizing the two parameters of equation (7) (see below), with a homogeneous shrinkage within each cluster of a defined size.…”

“…However, complex hybrid systems are commonly obtained and, even though the analysis is restricted to the structural and microstructural characterization of the metal nanocrystalline component, conventional X-ray diffraction methods, which inherently disregard the amount of information from the diffuse scattering, may provide a limited outcome. The investigation may be further complicated by the structural defectiveness of metal nanoparticles, which frequently exhibit faulting deformation (Hall, 2000;Zanchet et al, 2000;Ustinov & Budarina, 2002;Cervellino et al, 2003Cervellino et al, , 2004Beyerlein et al, 2010;Beyerlein, Leoni et al, 2011;Scardi et al, 2011;Longo et al, 2014), and by possible surface relaxation effects (Wasserman & Vermaak, 1972;Diao et al, 2003;Qi & Wang, 2005;Gelisio & Scardi, 2014;Scardi & Gelisio, 2016), occurring at their very small sizes. Moreover, even if the geometrical constraints of the host template are expected to induce the formation of (elongated) NCs within the channels, metal nanoparticles may also form on the surface of the silica matrix, the two ensembles being difficult to distinguish by conventional analytical/ imaging methods.…”

Dislocation Effects on the Diffraction Line Profiles from Nanocrystalline Domains

“…[20] As already pointed out, the powder pattern of G35 after energy minimization ( Figure 5) includes effects of an inhomogeneous strain due to grain boundaries and grain-to-grain interactions. [20] Similar effects are also present in the case of an isolated G35 grain, where surface atomic positions are modified by surface tension forces (lower coordination number of surface atoms, causing the so-called surface relaxation effect [39,40] ). We can account for these complex effects, causing a relatively small but measurable strain broadening (or microstrain) component, in a simplified but sufficiently flexible way.…”

“…[20] Modeling results are equally good for the isolated G35 (not shown here for briefness), where the microstrain is caused by the free surface. [40,43] Corresponding rms displacement trends are shown in Figure 8…”