1. National Institute for Laser, Plasma and Radiation Physics, 409 Atomistilor, Magurele 077125, Romania
2. National Institute of Materials Physics, 105 bis Atomistilor, Magurele 077125, Romania
3. Thermo Fisher Scientific, Materials & Structural Analysis, De Schakel 2, Eindhoven 5651 GE, the Netherlands
4. Faculty of Physics, University of Bucharest, Magurele 077125, Romania
5. CNR-ISM, Istituto di Struttura della Materia,
Area della Ricerca di Roma-Tor Vergata, Via del Fosso del Cavaliere 100, Rome I-00133, Italy