ABSTRACT
The periodic arrangement of atoms in crystalline materials has allowed, in the last hundred years, thousands of chemical species (elements, minerals, functional materials, polymers, drugs, and proteins, to mention a few) to be fully characterized by diffraction methods, mostly relying on the tacit assumption that innite crystals and real samples (with domain size down to a few microns) coincide. However, crystalline materials featuring long-range order of the atomic arrangement at the bulk (micro-/meso-) length scale can undergo relevant structural and microstructural modications when crystal domains are limited to few or few tens of nanometers. In nanosized particles, only shortrange order can take place, and a high fraction of atoms is located at the surface, experiencing a different chemical/ energetic environment compared to atoms in the core. On the sunny side, such a modied atomic arrangement can provide novel and unique chemical-physical properties (optical, electronic, catalytic, magnetic) not found in the same material at the bulk scale, as observed in a number of metals, oxides, semiconductors, ceramics, polymers, and many other molecular systems.1-8 Consequently, the attention of the scientic community to these novel properties and to
the advantages they can provide to our daily life has opened new scenarios in the eld: innovative synthesis of nanoparticles (NPs), nanorods, nanowires or nanosheets, and supramolecular organization or self-assembly of nanoobjects and hybrids are continuously proposed. Nevertheless, the low extension of nanosized materials and their growing complexity with decreasing size have put in evidence the limits of standard crystallographic methods in characterizing this kind of systems. X-ray and neutron powder diffraction, the leading techniques for investigating real complex materials at the atomic length scale, have reached nowadays a high degree of maturity.9 They are currently used to investigate the structure and microstructure of microcrystalline samples through the whole pattern Rietveld modeling of the Bragg peaks.10 However, diffraction patterns collected on nanocrystalline materials show largely smeared, if any, Bragg scattering and a lot of diffuse intensity (between and below the Bragg peaks), which originate from the limited size and shape effects and defects11 (see Figure 58).
