National Institute of Materials Physics Bucharest-Magurele. Cristian-Mihail Teodorescu, PhD, S.R.1, Surfaces and Interfaces,

Transcription

1 National Institute of Materials Physics Bucharest-Magurele ELI-NP Cristian-Mihail Teodorescu, PhD, S.R.1, Surfaces and Interfaces,

2 National Institute of Materials Physics Bucharest-Magurele NIMP expertise for ELI-NP: 162 R&D personnel 195 ISI papers/year (2-5 outstanding, IF > 7) ~ 10 M budget 5 departments: - multifunctional materials; - magnetism and superconductivity; - nanoscale physics; - optics and spectroscopy; - atomic structure and defects; (a) Characterization of highly irradiated / damaged components and materials: HRTEM; EPR; XPS; XRD; EXAFS; SEM; SPM. (b) Crystal growth; (c) UHV and surface science expertise; (d) Target fabrication laboratory; (e) Positron-related techniques in surface science;

3 Some main facilities: Analytical Cs-corrected HRTEM, 0.8 Å Au

4 NIMP involvment in ELI-NP Target fabrication / characterization: - thin supported foils; - foams, aerogels; - solid rare gases; - micro-engineering; - free standing nm layers; - characterization: XRD, SEM, SPM, XPS, TEM Positron-related activities: - positron diffraction (LEPD, RHEPD); - positron annihilation-induced Auger electron spectroscopy (PAES); - positron annihilation lifetime spectroscopy (PALS); - angular correlation of annihilation radiation (ACAR); - Doppler broadening spectroscopy; - g-induced positron spectroscopy (GIPS); - spin-resolved positron annihilation;

5 Target fabrication free standing nm thick foils foams/aerogels micro-machining

6 exp 3 ) ( R t v M M v dt x d t x 0 0 v R M M R x

7 Positron-related techniques in surface science: Outlook 1. Standard electron-based surface analysis techniques: - Low energy electron diffraction (LEED); - Reflection high energy electron diffraction (RHEED); - Auger electron spectroscopy; - X-ray photoelectron spectroscopy. 2. Positron annihilation-induced Auger electron spectroscopy (PAES) - Principles; - Examples; - Angle-resolved PAES (ARPAES). 3. Positron diffraction: RHEPM. 4. Positron lifetime spectroscopy: - Principles; - Examples; - Spin-resolved positron lifetime spectroscopy. 5. Conclusions

8 LEED Example: Si(001) (2x1)-(1x2) 39.8 ev 66.5 ev 40.4 ev 87.8 ev 50.0 ev ev 57.9 ev ev

9 RHEED no (n = 1) spot, since: E sin d n 2

10 Auger electron spectroscopy Auger electron Element -specific Auger effect core vacancy surface sensitivity ~ a few nm

11 dn/de C.M. Teodorescu, NIMP Bucharest, Positron-related techniques in surface science Elemental analysis O KLL Chemical states C KLL Ti LMM Ti LMV, LVV O KLL TiLMM 2 N.G. Gheorghe et al., OAM-RC 5, 499 (2011)

12 X-ray photoelectron spectroscopy Electron IMFP substrate - charge transfer - chemical state - local atomic order

13 Pb 4f Zr 3d

14 Photoelectron scattering and diffraction X-rays out exp( ikr) r scattering amplitude bcc Co/GaAs(011) f (, k) scattered out X-ray absorber = photoelectron emitter neighboring scatterer interference outgoing - scattered wavefunctions: origins of EXAFS, XANES forward focusing along rows of atoms: photoelectron and Auger electron diffraction M. Izquierdo et al., PRL 94, (2005)

15 Positron annihilation-induced Auger electron spectroscopy Interaction with image charges:, x 0 V ( x) 2 e0 x 0 2x E E n p e 2m 2x 4 0 Quantization: 4me0 1 π 6.12 ev n h 2 4 n 2 x pdx nh stable states near the surface c(2x2) S/Cu(001) Annihilation: with first layer atoms only (1-5 % core e - ) K.O. Jensen and A. Weiss, PRB 1990

16 Au-Cu surface alloy Annihilation probability: K.O. Jensen and A. Weiss, PRB 1990 Au/Cu(001) Cs Ps + Ps + Cs/Cu(001) S Cu(001) K.H. Lee et al., PRL 1994 A.R. Koymen et al., PRL 1992

17 Angle-resolved PAES detailed surface structure Interfaces nicely visible by HRTEM; surfaces not Photoelectron diffraction studies of relaxation / rumpling at BaTiO 3 surfaces Ba 4d Ti 2p Data analysis PED: - multiple scattering; - electrons originate from different shells; - time consuming; - low reliability A. Pancoti et al., PRB, submitted O 1s C 1s

18 e.g. PED-derived adsorption geometry of NH 2 on Si(111) 7 x 7 adsorption on rest atoms N. Franco et al., PRL 1997 S. Bengio et al., PRB 2004 ARPAES could provide a similar information for any system!

19 Experimental setup: 0. Positron source BRIP 10 7 e + /(s mm 2 mrad % BW) [cf. Habs et al., ELI-NP Whitebook, p. 139] 1. UHV chamber (~ 320 k ): - angle-integrated CMA analyzer (basic PAES); - hemispherical analyzer (high resolution & ARPAES); - g detector (Ps identification); 2. Electronics (~ 30 k ): - standard electron spectroscopy; - coincidence measurement. 3. Sample preparation (~ 200 k ): - MBE-based, evaporators, gas adsorption, sputtering, laser deposition, etc. Extreme sample cleaness needed (work in mbar etc.)

20 Positron annihilation lifetime spectroscopy: - concentration of defects with ppb concentration; - pore diameters (precludes difficulties from BET surface area); - metal character; - spin-resolved density of states. 0.5 ns Ps r h dr o-ps vac. = 142 ns p-ps vac. = 125 ps metal-insulator transitions Ps lifetime in Si

21 e + scattering and diffraction source: A. Kawasuso, Halle Seminar 2000 Low energy e + reflection RHEED Si(111)H RHEPD

22 Spin-polarized positrons Surface magnetism Proposed experiment for surface spin-resolved DOS: r e cyclo. eb p e 3.37 μm E c (ev) B(T) DBAR Kawasuso et al., PRB 2011 e.g. 1 st ML of Ni(001) AF coupled 1st ML of Gd(0001) canted, etc.

23 Other advantages of positrons (source: S.W.H. Eijt, 2008, Department of Radiation, Radionuclides & Reactors, Faculty of Applied Sciences, Delft University of Technology

24 + Positrons highly suited for Surface Science