Joint research project

X-ray meso-crystallographic studies of self-assembled 3D nanocrystal superlattices

Project leaders
Cinzia Giannini, Daniela Zanchet
Agreement
BRASILE - CNPq - Conselho Nacional Desenvolvimento Cientifico y Tecnologico
Call
CNR/CNPq 2012-2013
Department
Molecular Design
Thematic area
Chemical sciences and materials technology
Status of the project
New

Research proposal

Nanocrystal superlattices (SLs) are attracting significant interest due to novel and peculiar collective properties arising from the interactions of the nanocrystals (NCs) forming the SL. A large variety of SL structures can be obtained, involving one or more types of NCs, with different sizes and concentrations. Engineering of the SL properties relies on accurate structural and morphological characterization, which is able to provide not only a fundamental feedback for synthesis procedures, but also relevant insights on their structural properties for possible applications.
The new physical or chemical properties of these nanocrystals SLs intimately depend on i) their morphological properties, i.e. the NCs shapes, sizes and spatial organization, and ii) their structural/crystallographic properties; i.e. type, positions and symmetry relation of the NCs in the unit cell, unit cell size, space group.
The structural complexity of these materials can be inspected at different length scales, studying the atomic/molecular ordering by Wide Angle X-ray Scattering (WAXS) or X-ray Powder Diffractometry (XRPD) and the morphological conformation by Small Angle X-ray Scattering (SAXS). Specific measurement geometries (GISAXS, GIWAXS) have to be used for studying nanomaterials laying on top of surfaces with respect to materials produced in powders or similar states. These techniques, especially if used in a synergic combination, allow to pinpoint the crystallographic position of all nanocrystals in the superlattice unit cell. XAFS analysis will complement WAXS-GIWAXS-XRPD inspecting the atomic environment and electronic properties of the nanocrystals, with chemical sensitivity.
In detail:
SAXS-GISAXS will be used to study the nanocrystal shapes and sizes but with precise constraints: in case of samples without a size distribution it is possible to precisely measure the nanocrystals shape; in case of known shape it is possible to find the size distribution. Nanoparticles can be either on a surface, or embedded in a matrix. Besides the morphological analysis (shape&size) of the nanocrystals, these techniques can provide precise structural information about their relative positions in case of periodicity on a nanometric scale.
WAXS-GIWAXS-XRPD data contain information on the atomic crystal structure, the size and size distribution, surface and size related unit cell deformation (strain) and defects. These effects will be extracted from higher order peak shifts (strain), peak width anisotropy (size and shape) or peak positions and relative intensities (crystal structure). Being several effects all convoluted in the same profile, it is quite helpful to determine size&shape separately from SAXS-GISAXS data, especially if measured simultaneously on the same sample.
XAFS analysis will give information about the atomic environment and electronic properties of the nanocrystals, with chemical sensitivity. Modifications of the coordination numbers, interatomic distances and structural disorder will be complementary to the information obtained by WAXS-GIWAXS-XRPD and due to the intrinsic complexity of these systems the combination of these techniques is in fact a requirement to fully characterize them. Particle size information is indirectly obtained by the reduction of the coordination numbers.

References
Quantum Dots: Synthesis and Characterization, D. Dorfs , R. Krahne , C. Giannini , A. Falqui, D. Zanchet and L. Manna, Book Title: Comprehensive Nanoscience and Technology (vol. 2), Editors: Guozhong Cao, Duncan Gregory, Thomas Nann, Edited by: Elsevier, 2011

Research goals

The unconventional assembly of nanocrystals towards functional materials is the area where this proposal aims at providing a key contribution. This will be achieved via ground-breaking advances in the fabrication of shape controlled nanocrystals, via solution approaches, in their organization following radically new concepts and in the study of their assembly related properties.
The aim of the research activity is to gain access to structural (atomic models), micro-structural (domain size and lattice strain) and morphological (domain shape) information from spatial regions of different extension (from millimetric to micrometric) of new nanostructured materials and assembled superlattices, whose functionality depends on the aforesaid structural properties. The combined use of the SAXS/GISAXS/WAXS/GIWAXS/XAFS imaging techniques will allow to obtain a map of micro/nano structural and chemical characteristics of these spatially heterogeneous systems in a non-destructive manner.
Indeed, X-ray Scattering related techniques allow to probe matter at different length scales (from Angstrom to nanometer ), in non invasive way and without charge build-up, eventually with a depth sensitivity, allowing at the need in situ monitoring of deposition, growth, annealing, gas adsorption processes and permitting various sample environments: ultra-high vacuum, liquid, gas.

Last update: 27/11/2021