Pasquale Orgiani is responsible of Pulsed Laser Deposition technique within NFFA Laboratory.
His main research activity is the deposition of thin films and heterostructures by Molecular Beam Epitaxy (MBE) and Pulsed Laser Deposition (PLD) techniques and the investigation of the transport properties of both magnetic and superconducting films, with/without external magnetic field.
In the last years he carried out a strong activity on electronic investigation of oxide thin films and heterostructures. His research on superconducting and magnetic oxide is focused on the role of structural properties and transport properties (metal-insulator transition, quantum interference effects at low temperatures, and so on) of such compounds.
The study of ionic materials on nanometer scale is of great relevance for efficient miniaturized devices for energy applications. The epitaxial growth of thin films can be a valid route to tune the properties of the materials and thus obtain new degrees of freedom in materials design. High crystal quality SmxCe1-xO2-δ films are here reported at high doping level up to x=0.4, thanks to the good lat-tice matching with the (110) oriented NdGaO3 substrate. X-ray diffraction and transmission electron microscopy demonstrate the ordered structural quality and absence of Sm segregation at macroscopic and atomic level, respectively. Therefore, in epitaxial thin films the homogeneous doping can be obtained even with high dopant content not always approachable in bulk form, getting even an improvement of the structural properties. In situ spectroscopic measurements by x-ray photoemission and x-ray absorption show the O 2p band shift towards the Fermi level which can favor the oxygen exchange and vacancy formation on the surface when the Sm doping is increased to x=0.4. X-ray absorption spectroscopy also confirms the absence of ordered oxygen vacancy clusters and further reveals that the 5d eg and t2g states are well separated by the crystal field in the undistorted local structure even in the case of high doping level x=0.4.
Nano Lett., 20, 6444–6451, (2020)
Unveiling Oxygen Vacancy Superstructures in Reduced Anatase Thin Films
D. Knez, G. Dražić, S.K. Chaluvadi, P. Orgiani, S. Fabris, G. Panaccione, G. Rossi, and R. Ciancio
Oxygen vacancies are known to play a crucial role in tuning the physical properties and technological applications of titanium dioxide TiO2. Over the last decades, defects in substoichiometric TiO2 have been commonly associated with the formation of TinO2n–x Magnéli phases, which are extended planar defects originating from crystallographic shear planes. By combining advanced transmission electron microscopy techniques, electron energy-loss spectroscopy and atomistic simulations, we reach new understanding of the oxygen vacancy induced structural modulations in anatase, ruling out the earlier shear-plane model. Structural modulations are instead shown to be due to the formation of oxygen vacancy superstructures that extend periodically inside the films, preserving the crystalline order of anatase. Elucidating the structure of oxygen defects in anatase is a crucial step for improving the functionalities of such material system and to engineer devices with targeted properties.
Phys. Rev. Materials, 4, 025801, (2020)
Distinct behavior of localized and delocalized carriers in anatase TiO2 (001) during reaction with O2
C. Bigi, Z. Tang, G.M. Pierantozzi, P. Orgiani, P. K. Das, J. Fujii, I. Vobornik, T. Pincelli, A. Troglia, T.-L. Lee, R. Ciancio, G. Drazic, A. Verdini, A. Regoutz, P.D.C. King, D. Biswas, G. Rossi, G. Panaccione, and A. Selloni
Two-dimensional (2D) metallic states induced by oxygen vacancies (VOs) at oxide surfaces and interfaces provide opportunities for the development of advanced applications, but the ability to control the behavior of these states is still limited. We used angle resolved photoelectron spectroscopy combined with density-functional theory (DFT) to study the reactivity of VO-induced states at the (001) surface of anatase TiO2, where both 2D metallic and deeper lying in-gap states (IGs) are observed. The 2D and IG states exhibit remarkably different evolutions when the surface is exposed to molecular O2: while IGs are almost completely quenched, the metallic states are only weakly affected. DFT calculations indeed show that the IGs originate from surface VOs and remain localized at the surface, where they can promptly react with O2. In contrast, the metallic states originate from subsurface vacancies whose migration to the surface for recombination with O2 is kinetically hindered on anatase TiO2 (001), thus making them much less sensitive to oxygen dosing.
Strada Statale 14 - km 163,5 - 34149 Trieste, ITALY
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NFFA is a Progetto Internazionale financed by MIUR through CNR
(Istituto Officina dei Materiali, Trieste) and Elettra-Sincrotrone Trieste
and managed by the Commissione NFFA chaired by Giorgio Rossi
(Università di Milano and IOM-CNR).