Ruggero Frison

Crystal symmetry breaking and vacancies in colloidal lead chalcogenide quantum dots

Size and shape tunability and low-cost solution processability make colloidal lead chalcogenide quantum dots (QDs) an emerging class of building blocks for innovative photovoltaic, thermoelectric and optoelectronic devices. Lead chalcogenide QDs are known to crystallize in the rock-salt structure, although with very different atomic order and stoichiometry in the core and surface regions; however, there exists no convincing prior identification of how extreme downsizing and surface-induced ligand effects influence structural distortion. Using forefront X-ray scattering techniques and density functional theory calculations, here we have identified that, at sizes below 8 nm, PbS and PbSe QDs undergo a lattice distortion with displacement of the Pb sublattice, driven by ligand-induced tensile strain. The resulting permanent electric dipoles may have implications on the oriented attachment of these QDs. Evidence is found for a Pb-deficient core and, in the as-synthesized QDs, for a rhombic dodecahedral shape with nonpolar {110} facets. On varying the nature of the surface ligands, differences in lattice strains are found.

DEBUSSY 2.0: the new release of a Debye user system for nanocrystalline and/or disordered materials

The new release of DEBUSSY is introduced, a free open-source package devoted to the application of the Debye function analysis of powder diffraction data from nanocrystalline, defective and/or nonperiodic materials. The general strategy of the suite remains unchanged, following a two-step approach managed by the CLAUDE and DEBUSSY programs, respectively. The first step essentially consists in generating a database where structural, size and shape information on a nanocrystal population is stored; the second step consists in the calculation, through the Debye scattering equation, of the total diffraction pattern using the previously generated database and a set of model parameters provided by the user and then optimized by the program. The novelties lie in the computational, modelling and graphical levels, and several new programs and features have been added. Among these are a new general comprehensive input file format (.ddb) for the database generation, the automatic management of the space-group symmetry and .cif file, new nanocrystal shapes, size-dependent site occupancy factors and thermal parameters for each atomic species, new lattice expansion functions, and a newly developed algorithm for calculating the standard errors of the optimized parameters. The CLAUDE suite also includes a program for calculation of the pair distribution function. Last but not least, a graphical user interface, which makes it easier to edit input files, execute the programs of the suite in a chain-like way, and plot the results in an automatic or custom manner, is provided.

Lattice parameters and site occupancy factors of magnetite–maghemite core–shell nanoparticles. A critical study

The size-driven expansion and oxidation-driven contraction phenomena of nonstoichiometric magnetite–maghemite core–shell nanoparticles have been investigated by the total scattering Debye function approach. Results from a large set of samples are discussed in terms of significant effects on the sample average lattice parameter and on the possibility of deriving the sample average oxidation level from accurate, diffraction-based, cell values. Controlling subtle experimental effects affecting the measurement of diffraction angles and correcting for extra-sample scattering contributions to the pattern intensity are crucial issues for accurately estimating lattice parameters and cation vacancies. The average nanoparticle stoichiometry appears to be controlled mainly by iron depletion of octahedral sites. A simple law with a single adjustable parameter, well correlating lattice parameter, stoichiometry and size effects of all the nanoparticles present in the whole set of samples used in this study, is proposed.

X-Ray Powder Diffraction Characterization of Nanomaterials

We discuss here what important knowledge can be gained by X-ray diffraction (of course, more specifically, we talk of X-ray powder diffraction) experiments on nanoparticles and nanomaterials in general. Historically, the uses of X-ray diffraction have been to investigate (a) the crystal structure of materials at the atomic scale and (b) their microstructure, a broad term meaning deviations from perfect crystalline order on a scale that is larger than the atomic one but still microscopic. In fact, macroscopic properties of materials tend to depend on both of those. For nanomaterials, the focus is to understand how the small crystal domain extension and related phenomena influence properties that differ – often in a very useful way – from the parent crystalline material. This means that the focus is on the microstructure, due to the fact that reduced domain extension is a (strong) deviation from crystal order – this is by definition something that extends to infinity. Of course there exist crystalline phases that are stable only at the nanoscale – and in such case, the crystal structure determination still is very important. In this contribution, we shall review all modern experimental methods and especially – as this is the core of the method – the data analysis techniques currently used, from the oldest, based on the Bragg formalism to interpret crystal diffraction, to the newest, relying on atomic-scale modeling.

Structural disorder in spinel-like nanoparticles probed by total scattering

Nano-crystalline spinel-like oxides are today widely studied because of their application in many different fields like information technology (ferrofluids, data storage), medicine (drug delivery, medical imaging) and chemistry (catalysis) [1]. Within this class Magnetite (Fe3O4) has a prominent role, however, a detailed understanding of some of its structural and micro-structural aspects – especially at the nanoscale – is still missing and the currently proposed structural models are not yet exhaustive of its magnetic properties. As is well-known the Fe2+ occupying the tetrahedral site is unstable under normal conditions, thus Magnetite is most often observed in a partially oxidised state. We studied the oxidation process of magnetite nano-particles (NP) revealing a partial ordering of the iron vacancies created during the oxidation process, leading to a partial phase transition of the NPs volume. As a second example we studied the cation disorder in the direct spinel Gahnite (ZnAl2O4) at very small NPs sizes, and we observed an important Zn-Al inversion disorder, with an inversion parameter x=0.34, higher than previously reported [3]. Our studies were performed by means of total scattering X-ray powder diffraction data (collected at the MS-Powder X04SA beamline [4] of the SLS synchrotron at PSI, Villigen, CH) and Debye scattering equation analysis [5]. In addition, for the cation disorder in ZnAl2O4 we performed for the first time using Total Scattering Anomalous Modulation Enhanced Diffraction [6] measurements at the Zn K-edge. [1] Laurent S. et al., Chem. Rev., 2008, 108, 2064; Z. Li, S. Zhang, W. E. Lee, J. Eur. Ceram. Soc. 27, 3407, 2007 [2] Morales M. P. et al., Chem. Mat., 1999, 11, 3058. [3] L. Cornu, M. Gaudon, V. Jubera, J. Mat. Chem. C (2013) 1, 5419-5428 [4] P. R. Willmott, D. Meister, S. J. Leake et al., J. Synchrotron Rad. (2013) 20, 667-682. [5] A. Cervellino, R. Frison, F. Bertolotti and A. Guagliardi, J. Appl. Cryst. (2015). 48, 2026-2032. [6] D. Chernyshov, W. van Beek, H. Emerich, M. Milanesio, A. Urakawa, D. Viterbo, L. Palin and R. Caliandro, Acta Cryst. (2011). A67, 327-335

In situ oxidation kinetics of magnetite nanoparticles by total scattering

Iron oxide nanoparticles (NPs) show different structures as a function of oxidation state. In particular, magnetite (Fe3O4) NPs are easily oxidized in air at moderate temperatures, eventually yielding maghemite (Fe2O3). Oxidation proceeds via the creation of iron vacancies. While the vacancies may be created with a random distribution throughout the octahedral Fe sites, they eventually order over a specific subset of these sites, lowering the symmetry from F-centered (magnetite) to P-centered (cubic maghemite). By ex situ X-ray Total Scattering studies of magnetite-maghemite NPs in different oxidation states[1] we have recently studied, by the DFA method[2], the correlation between particle diameter, stoichiometry and lattice parameter in polydisperse NP samples unraveling also the size dependence of lattice parameter and composition. Moreover, we have shown indirect evidence of the formation of a polycrystalline surface layer of maghemite on a magnetite core in the intermediate oxidation states. Motivated by the excellent exsitu results, we have also performed in-situ studies where magnetite NPs were oxidised in air at moderate temperatures (50-200 C). We present here an in-situ study performed at the X04SA-Materials Science beamline of the Swiss Light Source synchrotron[3]. Total Scattering X-ray diffraction patterns were collected every few minutes, while the oxidation was completed within several hours. The mechanism of NPs oxidation whereas a surface oxidised layer is formed by outwards diffusion of Fe, then the vacancies so created order themselves giving rise to the maghemite-magnetite phase transition, will be examined in great detail. We will discuss, on robust statistical basis, the calculation of kinetic and diffusion constants, the temperature effect on the lattice constant and on the thickness of the surface oxidised layer; the different possible structural models for the cubic-maghemite NPs. We thank for support Fondazione Cariplo (2009-0289).