Our current research involves studies of structural determination of amorphous materials in solid and solution states. In all our work, Energy Dispersive X-ray Diffraction (Department of Chemistry, University of Rome "Sapienza") has been used as the principal technique for amorphous characterization of a diverse range of nano-compounds. Ultimately, we would like to exploit the high resolution Transmission Electron Microscopy (recently, installed in the Centro Grandi Apparecchiature, University of Palermo) combined with the structural information of the EDXD facility, which offer the possibilities to achieve a better understanding the real and reciprocal space of the short-range order of amorphous nanomaterials.
Nanomaterials science, the study of the miniaturization of devices into nanometers sizes, is a field characterized by potential applications in area such electronics, optics, catalysis, mechanical, medical, and magnetic data storage. The unique properties and the improved performance of nanomaterials are determined by their sizes, surface morphologies and bulk structures. To fully understand nanomaterials from a structural perspective we need to talk about of the nanophases of nanoparticles distinguishable in two extremes materials types, called as "crystalline and amorphous phases".
Crystalline phases have short- and long-range order of an orderly repeating array of atoms, molecules or ions. Amorphous phases have no crystalline structure. They have short-range order but lack long-range order. A polymer such as polyester is amorphous, showing an atactic configuration, whereas the isotactic polymer readily crystallizes. Ordinary glass is an amorphous solid and it is composed of random meandering chains of silicon and oxygen atoms and other components. While the chains do not exhibit long-range order, short-range order exists.
With this assumption in mind, we digress briefly to discuss on the mainly experimental techniques utilized for determining the structuralaspects of amorphous materials. Usually the techniques that provide qualitative analyses for amorphous identification are the Nuclear Magnetic Resonance, Raman Spectroscopy, Differential Scanning Calorimetry, etc.. A few techniques that are applied almost entirely to the bulk studies of a short-range order are Neutron Scattering, High Energy X-ray Diffraction, High Resolution Transmission Electron Microscopy, and finally the Energy Dispersive X-ray Diffraction. The NS and HDX techniques are only supported by synchrotron sources achieving a high resolution in both direct (nm) and reciprocal space (nm-1) with small corrections of the required data, particularly for absorption. These experimental analyses are desired because of the high resolution for determining the average interatomic distances, bond angles and coordination numbers of disorder materials (short-range order).
The HR-TEM and EDXD techniques belongs to the laboratory facilities easily accessible than at a synchrotron radiation facility, which supply an high resolution investigation but one must bear in mind that to characterize your materials what is the resolution data needed. Other laboratory instrument for structural investigation is the conventional X-ray diffraction, by its nature, provides detailed information about the ordered regions of nanomaterials that are crystalline, but does not bear directly short-range of non-crystalline or glassy materials.
HR-TEM facility has today equipped with widely different capabilities and the highest resolution/highest electron energy TEM, becoming more powerful, specialized, and expensive. Sample preparation in TEM is critical since the specimen sizes accepted are usually less than 500 μm in diameter and 100 nm in thickness, allowing to the probe electron beam to pass through the specimen. This distinguishes TEM from the EDXD technique for which very little preparation of the sample is needed and the probe X-ray beam is a soft radiation avoiding specimen damage, and the instrument is not expensive. A complex variety of operation modes exist in TEM, providing variations or combinations of imaging and diffraction methods. The great advantage is its easy interpretation of imaging in real space for the nonspecialist, giving morphological and structural local informations of nano-particles of different size with a high lateral spatial resolution of 0.2 nm.
The experimental direct informations obtained by EDXD are measured as a structural function SF defined in reciprocal space with an overall range from 2.0 to 170 nm-1 (or from 0.2 to 17.0 Å-1) and the structural information acquired is an average over a large area. Through a Fourier transformation of the experimental SF, the Radial Distribution Function RDF is derived. The RDF allows direct information in real space on the inter- and intramolecular contacts of amorphous nano-object in the range from 0.1 to 10 nm. The acquireddiffraction spectra, as function of the energy of atomic scattering, allows to spatially probing the different chemical species of the nanomaterials investigated. Its main disadvantage is the building processes of the theoretical model to be compared with the experimental data, calculated by Fourier transforming of the theoretical SF of Debye equation.
Recently, we performed experimental and theoretical research on three-dimensional structural determination of a porphyrin hetero-aggregates by RDF analysis has shown the aggregation ability of these amorphous materials to form a nano-object of determinate size in the short-range order. The sample constituted of two porphyrin cationic (CuT4) and anionic (H2TTPS) hetero-assemblies has shown an experimental RDF with shorter-range order distances from 0.1 to 4.7 nm. The porphyrins arranged in a stacking configuration marked by a slipping disorder forming a zig-zag shape, stacked by regular interplanar distance. The same sample was templated with L-Phenylalaline. The structural analysis shows that the chiral template imprints a suprachiral structure to the porphyrin aggregate with a more extended short-range order from 0.1 to 6.0 nm. In the suprachiral aggregate the cationic and anionic porphyrins exhibit a ruffling distortion and form a dimeric arrangement in an α-helix shape (R. Matassa, et al., Adv. Mater. 2007, 19, 3961). Additional research in progress comprise the detection in the short-range order of amorphous organometallic oligomers by RDF of X-ray analysis that cannot be performed in the shorter range by HR-TEM techniques because of the damage radiation problem.
Finally, it is important to underline that structural studies of the amorphous phase of any materials need the knowledge of the crystalphase in helping to understand the phase change materials. As can be seen in a recent studies for commercial DVD-RAM device, which utilizes the phase change crystal-liquid-amorphous (record) and amorphous-crystal (erase) in chalcogenide materials (S. Kohara, et al., Appl. Phys. Lett. 2006, 89, 201910).