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Materials Science and Spectroscopy

The research activities of TREETOP - Materials Science, Optical and Spectroscopic Group of the University of Cagliari are focused on the Study, Synthesis, and Characterization of new materials.
Phosphor for lighting applications, Nanosized metal oxides and hybrids materials for photocatalytic applications, the substitution of Critical Raw Materials, Carbon nanodots are just a few of our recent fields of study.
Further, dedicated studies of applied spectroscopy occupy a relevant role in our research: Cultural Heritage, Forensic studies, the detection of viruses, and drugs.

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Optical spectroscopy like Steady time and Time-Resolved Luminescence, Raman Spectroscopy, Absorption, and reflection measurements are commonly applied.
The synthesis and the structural characterization are frequently applied for a deep understanding of the fundamental properties of materials and for the evaluation of new materials for promising applications.

Formation Mechanisms and Phase Stability of Solid-State Grown CsPbI3 Perovskites
Nanomaterials 2021, 11(7), 1823;
CsPbI3 inorganic perovskite is synthesized by a solvent-free, solid-state reaction, and its structural and optical properties can be deeply investigated using a multi-technique approach. X-ray Diffraction (XRD) and Raman measurements, optical absorption, steady-time and time-resolved luminescence, as well as High-Resolution Transmission Electron Microscopy (HRTEM) imaging, were exploited to understand phase evolution as a function of synthesis time length. Nanoparticles with multiple, well-defined crystalline domains of different crystalline phases were observed, usually surrounded by a thin, amorphous/out-of-axis shell. By increasing the synthesis time length, in addition to the pure α phase, which was rapidly converted into the δ phase at room temperature, a secondary phase, Cs4PbI6, was observed, together with the 715 nm-emitting γ phase.
Emission mechanism in single and co-doped Tb:Eu:CaZnOS
Journal of Alloys and Compounds
Volume 868, 5 July 2021, 159007
The study of new phosphors requires in-depth knowledge of the mechanisms and radiative emission paths and, with this aim, the study here reported focuses on the emission properties of CaZnOS crystals single doped with Terbium and co-doped with Terbium and Europium. By studying the optical properties and, in particular, the kinetics of recombination with time-resolved luminescence, the de-excitation mechanisms and the charge transfer processes have been established. A fundamental role is played by the defective centers and their efficient energy transfer process to the excited levels of Terbium, the mechanism being also active among co-doping rare earths (from Tb3+ to Eu3+), allowing further tuning of the emission properties. Beside photoluminescence, the study shows that in case of mechanical stimulus as well, the mechano-luminescence follows the same path, where the defective states of the matrix efficiently excite the levels of dopants, producing a green emission at 545 nm from Tb3+ and a red emission above 600 nm from Eu3+. Therefore, studying the relative ratio of dopants, it is shown how precisely tune across the visible part of the spectrum the mechano-luminescence emission.
Sequential Symmetry-Breaking Events as a Synthetic Pathway for Chiral Gold Nanostructures with Spiral Geometries
Nano Letters.
Symmetry-breaking synthetic controls allow for nanostructure geometries that are counter to the underlying crystal symmetry of a material. If suitably applied, such controls provide the means to drive an isotropic metal along a growth pathway yielding a three-dimensional chiral geometry. Herein, we present a light-driven solution-based synthesis yielding helical gold spirals from substrate-bound seeds. The devised growth mode relies on three separate symmetry-breaking events ushered in by seeds lined with planar defects, a capping agent that severely frustrates early stage growth, and the Coulombic repulsion that occurs when identically charged growth fronts collide. Together they combine to advance a growth pathway in which planar growth emanates from one side of the seed, advances to encircle the seed from both clockwise and counterclockwise directions, and then, upon collision of the two growth fronts, sees one front rise above the other to realize a self-perpetuating three-dimensional spiral structure
Insight into the Molecular Model in Carbon Dots through Experimental and Theoretical Analysis of Citrazinic Acid in Aqueous Solution
J. Phys. Chem. C 2021, 125, 8, 4836–4845
The molecular emission model is the most accredited one to explain the emission properties of carbon dots (CDs) in a low-temperature bottom-up synthesis approach. In the case of citric acid and urea, the formation of a citrazinic acid (CZA) single monomer and oligomers is expected to affect the optical properties of the CDs. It is therefore mandatory to elucidate the possible role of weak bonding interactions in determining the UV absorption spectrum of some molecular aggregates of CZA. Although this carboxylic acid is largely exploited in the synthesis of luminescent CDs, a full understanding of its role in determining the final emission spectra of the produced CDs is still very far to be achieved. To this aim, by relying on purely first-principles density functional theory calculations combined with experimental optical characterization, we built and checked the stability of some molecular aggregates, which could possibly arise from the formation of oligomers of CZA, mainly dimers, trimers, and some selected tetramers. The computed vibrational fingerprint of the formation of aggregates is confirmed by surface-enhanced Raman spectroscopy. The comparison of experimental data with calculated UV absorption spectra showed a clear impact of the final morphology of the aggregates on the position of the main peaks in the UV spectra, with particular regard to the 340 nm peak associated with n-π* transition.
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