PAES approaches for thin films
surfaces and interfaces studies
Programme / Sub-programme / Module: 5/5.1/ELI-RO
ELI-NP thematic: GDE/II.2 Material science studies with positron beams
Project title / Acronym: PAES approaches for thin films surfaces and interfaces studies/ FILMSURFAPES
Contract no: 17
Project duration : 17.09.2020 - 31.08.2023
Project director: Dr. Ioan-Ovidiu Pana
Project Coordinator: National Institute for Research and Development of Isotopic and Molecular Technologies
Partner 1: National Institute for Research and Development of Materials Physics
Positron annihilation-induced Auger electron spectroscopy, when operated with low energy positron kinetic energies (below ~ 5 eV) is probably the most surface sensitive technique known to date. The origin of this fact stems in the ‚negative positron work-function of most materials or, in other words, to the fact that positrons originating from vacuum should exceed a threshold value of their kinetic energy to penetrate into the bulk of the material. As a consequence, low energy positrons do not penetrate inside most materials and, after forming some metastable positronium-like metastable states near the surface of the material, they annihilate with electrons from the very first atomic layer being a versatile method to study surfaces and interfaces in condensed matter.
The main scientific objective of the project refers to the characterization of surfaces and interfaces by using positron-induced Auger electron spectroscopy (PAES) for thin films and multilayers. It is related to and derives from objectives of Program 5 / Subprogram 5.1 (HG no. 583/2015) namely "Gamma-beam applications at ELI-NP-GDE / II.2" in material science studies with positron beams.
1. preparation by pulsed laser deposition (PLD) and primary physical characterization of thin layers of FePt (L10) with different thicknesses on MgO substrate; - preparation by PLD and characterization of ZnS / FePt (L10) / MgO multilayer systems - obtaining by two different methods (PLD and chemical vapor deposition-CVD) graphene films on MgO substrate;
2. Obtaining the necessary authorizations for the realization and operation of the existing PAES system (with a 22Na positron source) of Partner 1; obtaining and interpreting the PAES spectra as well as their correlation with the other classical measurements.
3. Elaboration of a methodology for characterization of surfaces and interfaces through PAES necessary for the further developments of the low energy positron system within ELI-RO.
4. Establishing partnerships for future collaborations in experiments with low energy positrons within ELI-NP infrastructure.
For the achievement of the above objectives the project includes 4 annual stages 2020-2023. Conceptually, the proposal contains several Tasks that do not necessarily overlap with the mentioned stages. The tasks under consideration are the following:
T1. Preparation as well as theoretical and experimental characterization of thin layers based on FePt (L10). MgO and FePt (L10) / ZnS / MgO.
T2. Complete realization of the PAES spectra measurement system and its authorization by CNCAN;
T3. Obtaining the PAES spectra for thin layers based on FePt (L10). MgO and FePt (L10) / ZnS / MgO as well as their interpretation and correlation with the other experimental data and theoretical calculations. T4. Preparation as well as theoretical and experimental characterization of thin layers based on few graphene layers on MnO substrate.
T5. Obtaining PAES spectra for graphene-based thin films deposited by PLD and CVD and interpretation of these results in correlation with theoretical calculations and other experimental determinations. Elaboration of the methodology necessary for the further development of these types of experiments within ELI-NP.
T6. Dissemination of results by publication in ISI-listed journals and communications at conferences.
Summary of accomplishments during the reporting period:
During the first stage, a series of 5 samples of FePt thin films were prepared by PLD by changing the number of pulses and the heat treatment conditions. The films were prepared on MgO substrates (100). Their characterization was done by Scanning Electrons Microscopy, X-ray diffraction and X-ray reflectometry. The analyzed samples have a granular structure that varies depending on the number of pulses and the heat treatment method. The L10 type crystalline structure was only partially realized for these first prepared samples.
During the second stage, a series of 5 samples of ZnS/FePt/MgO thin films were prepared by PLD by changing the number of pulses for zinc sulfide deposition. The films were prepared on MgO substrates (100). Theoretical investigations regarding layer orientation inside the FePt films were performed. Their characterization was done by X-ray diffraction, X-ray reflectometry, XPS, UPS. Spin resolved XPS was used to investigate the existence of half-metal magnetic ordering in the FePt layer. The magnetic behavior shows that the semiconductor ZnS films have got ferromagnetic order and are coupled to their half-metal substrate (FePtL10). Regarding the PAES installation, the lead shielding was redesigned in order to reduce the initial amount of lead according to the calculations in the technical file. It refers to the CNCAN requirement for the location and construction of the laboratory in order to minimize to zero the irradiation with gamma photons resulting from the de-excitation of the 22Ne* level (hν = 1,275 MeV). The project was executed at the mechanical workshop. In September 2021, the request to obtain the other two authorizations necessary to carry out the research activity with the PAES radiological installation was sent to CNCAN. Also, the procedure for responding to radiological emergencies in the laboratory where the PAES radiological installation is to operate was set-in an updated form according to the notification uet osly received from CNCAN.
In the first part of the third stage, the following steps were done: (i) PLD deposition of graphene layers on MgO substrate and the preparation of graphene layers deposited by PLD on FePt layers previously deposited on MgO substrate; (ii) theoretical calculation (DFT + LDA / GGA and LDA + U) of the band structure for ZnS thin layers. Structural changes on their surface induced by the interface tension with the FePt(L10) thin film.
List of papers:
- M. Stefan , C. Leostean , D. Toloman , A. Popa , S. Macavei , A. Falamas , R. Suciu ,
L. Barbu-Tudoran, O. Marincas, O. Pana, Applied Surface Science 570 (2021) 151172.
- C. M. Teodorescu, M. A. Hușanu, Re-entrant ferromagnetism at ultrahigh temperatures in epsilon–iron as possible origin of the geomagnetic field, Phys. Earth Planet. Inter. 326, 106856 (2022).
List of talks of group members:
-Interplay between ferromagnetism, optical absorption and photocatalytic activity of semiconductors interfaced with a half-metal at nano-scale, O. Pana, M. Stefan, D. Toloman, A. Popa, C. Leostean; ICPAM-13, 13th International Conference on Physics of Advanced Materials, September 24-30, 2021, Sant Feliu de Guixols, Spain
- Surface charge accumulation at ferroelectric thin films and its consequences on long-range ordering and surface chemistry, invited talk at Sabanci University, C. M. Teodorescu, Turkey, April 21, 2021.
- Charge accumulation, conduction band filling and ferroicity, C.M. Teodorescu, International Workshop of Materials Physics, 6th Edition, Măgurele, Romania, September 14–16, 2021 (oral presentation).
- Interplay between surface charge accumulation, conduction band filling and ferroic ordering, C. M. Teodorescu, 13th International Conference on Physics of Physics of Advanced Materials (ICPAM–13), Sant Feliu de Guixols, September 24–30 (plenary lecture).
- Microscopic model for ferroelectricity and its statistical treatment, C. M. Teodorescu IBWAP 2022 – 20th International Balkan Workshop on Applied Physics and Materials Science, Constanța, Romania, July 12–15, 2022 (invited talk).
- Photoelectron spectroscopy: relevant information which can be extracted, C. M. Teodorescu, 4th Autumn School on Physics of Advanced Materials (PAMS–4), Sant Feliu de Guixols, September 24–30 (plenary lecture).
- X-ray absorption and photoelectron spectroscopies, C. M. Teodorescu, RomCat 2022 - The 13th International Symposium of the Romanian Catalysis Society (Summer School), Băile Govora, Romania, June 22–24, 2022 (plenary lecture).
Other deliverables (patents, books etc.).
- C. M. Teodorescu, “Sistem de analiză de timp de zbor în gaz rarefiat, cu determinarea separată a dimensiunilor și sarcinii nanoparticulelor și aerosolilor”, CBI OSIM a2019 00705/04.11.2019. B.O.P.I. Nr. 5/2021, p. H01J (nr. 135011)
16 August 2022