Surfactant-free non-aqueous (and/or non-hydrolytic) sol-gel routes constitute one of the most versatile and powerful methodologies for the synthesis of nanocrystalline metal oxides with high compositional homogeneity and purity. A representative example is the solvothermal reaction of zirconium isopropoxide in benzyl alcohol leading to the formation of uniform zirconia nanocrystals, which can also be doped by the addition of various metal complexes to the reaction mixture.
In the present work, the “benzyl alcohol route” was exploited to synthesize various binary (i.e. based on rare earth and group IV metals) and doped metal oxide nanoparticles presenting high crystallinity and uniform particle size (ranging from sub-nm to 5 nm). The main weakness of the surfactant-free non-aqueous route can be overcome, by the in situ formation of organic species, which act as stabilizing and structure-directing agents leading to the self-assembly, in the reaction medium, of the as-synthesized nanoparticles into highly ordered supercrystals with well-defined morphology. In this presentation we will discuss novel structural studies and the 3D assembly mechanism of benzoate capped nanocrystals. Moreover, some peaks in low-frequency Raman spectra can be attribute to THz acoustic vibrations of individual nanocrystals which are only weakly coupled due to the strong acoustic mismatch between the capping ligands and the nanocrystals. By progressively removing the ligands, the acoustic vibrations of the nanocrystals couple resulting in an increase of their frequency, suggesting that these 3D supercrystals can be used for manipulating THz phonons.
 N. Pinna, M. Niederberger, “Surfactant-free non-aqueous synthesis of metal oxide nanostructures”, Angew. Chem. Int. Ed., 2008, 47, 5292
 A. Pucci, G. Clavel, M.-G. Willinger, D. Zitoun, N. Pinna, “Transition metal doped ZrO2 and HfO2 nanocrystals”, J. Phys. Chem. C, 2009, 113, 12048
 A. Pucci, M.-G. Willinger, F. Liu, X. Zeng, V. Rebuttini, G. Clavel, X. Bai, G. Ungar, N. Pinna, “One-step synthesis and self-assembly of metal oxides nanoparticles intp 3D superlattices”, ACS Nano, 2012, 6, 4382
 L. Saviot, D. B. Murray, G. Caputo, M. C. Marco De Lucas, N. Pinna, “THz nanocrystal acoustic vibrations from ZrO2 3D supercrystals”, J. Mater. Chem. C, 2014, DOI: 10.1039/C3TC31833H