C.M. Teodorescu, NIMP Bucharest, Experimental ... - ELI-NP · C.M. Teodorescu, NIMP Bucharest,...
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C.M. Teodorescu, NIMP Bucharest, Positron-related techniques in surface science
Experimental Programme at ELI-NP, 3-5 Oct. 2012
National Institute of Materials Physics Bucharest-Magurele
ELI-NP
Cristian-Mihail Teodorescu, PhD, S.R.1, Surfaces and Interfaces, [email protected]
C.M. Teodorescu, NIMP Bucharest, Positron-related techniques in surface science
Experimental Programme at ELI-NP, 3-5 Oct. 2012
National Institute of Materials Physics Bucharest-Magurele
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;
NIMP expertise for ELI-NP:
(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;
C.M. Teodorescu, NIMP Bucharest, Positron-related techniques in surface science
Experimental Programme at ELI-NP, 3-5 Oct. 2012
Some main facilities:
Analytical
Cs-corrected
HRTEM, 0.8 Å
Au
C.M. Teodorescu, NIMP Bucharest, Positron-related techniques in surface science
Experimental Programme at ELI-NP, 3-5 Oct. 2012
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;
C.M. Teodorescu, NIMP Bucharest, Positron-related techniques in surface science
Experimental Programme at ELI-NP, 3-5 Oct. 2012
Target fabrication
foams/aerogels micro-machining
free standing nm thick foils
C.M. Teodorescu, NIMP Bucharest, Positron-related techniques in surface science
Experimental Programme at ELI-NP, 3-5 Oct. 2012
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C.M. Teodorescu, NIMP Bucharest, Positron-related techniques in surface science
Experimental Programme at ELI-NP, 3-5 Oct. 2012
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
C.M. Teodorescu, NIMP Bucharest, Positron-related techniques in surface science
Experimental Programme at ELI-NP, 3-5 Oct. 2012
LEED
40.4 eV
50.0 eV
57.9 eV
66.5 eV
87.8 eV
104.4 eV
122.8 eV
39.8 eV
Example: Si(001) (2x1)-(1x2)
C.M. Teodorescu, NIMP Bucharest, Positron-related techniques in surface science
Experimental Programme at ELI-NP, 3-5 Oct. 2012
RHEED
no (n = 1)
spot, since: 2
22 5.37
sind
nE
Auger electron spectroscopy
Auger
effect
Auger
electron
Element
-specific
core vacancy surface sensitivity ~ a few nm
C.M. Teodorescu, NIMP Bucharest, Positron-related techniques in surface science
Experimental Programme at ELI-NP, 3-5 Oct. 2012
C.M. Teodorescu, NIMP Bucharest, Positron-related techniques in surface science
Experimental Programme at ELI-NP, 3-5 Oct. 2012
Elemental analysis
dN
/dE
O KLL
Ti LMM
Ti
LMV,
LVV
C KLL
2LMM Ti
KLL O
Chemical
states
N.G. Gheorghe
et al., OAM-RC
5, 499 (2011)
C.M. Teodorescu, NIMP Bucharest, Positron-related techniques in surface science
Experimental Programme at ELI-NP, 3-5 Oct. 2012
Electron IMFP
substrate
X-ray photoelectron spectroscopy
- charge transfer
- chemical state
- local atomic order
C.M. Teodorescu, NIMP Bucharest, Positron-related techniques in surface science
Experimental Programme at ELI-NP, 3-5 Oct. 2012
Zr 3d
Pb 4f
C.M. Teodorescu, NIMP Bucharest, Positron-related techniques in surface science
Experimental Programme at ELI-NP, 3-5 Oct. 2012
Photoelectron scattering and diffraction
X-ray absorber
= photoelectron
emitter
neighboring
scatterer
r
ikrout
)exp(
outscattered kf ),(
interference
outgoing - scattered
wavefunctions:
origins of EXAFS, XANES
forward focusing along
rows of atoms:
photoelectron and
Auger electron
diffraction
scattering amplitude
M. Izquierdo et al., PRL 94, 187601 (2005)
X-rays bcc Co/GaAs(011)
C.M. Teodorescu, NIMP Bucharest, Positron-related techniques in surface science
Experimental Programme at ELI-NP, 3-5 Oct. 2012
Positron annihilation-induced Auger electron spectroscopy
0 x 2
0 x,
)(2
0
x
e
xV
Interaction with image charges:
Quantization: nhpdx
x
0
0
2022
2
0
2
x
e
m
pE
2
2
22
4
0 eV 12.6
4
π
2
14
nhn
meEn
stable states
near the surface
Annihilation:
with first layer
atoms only
(1-5 % core e-)
K.O. Jensen and
A. Weiss, PRB 1990
c(2x2) S/Cu(001)
C.M. Teodorescu, NIMP Bucharest, Positron-related techniques in surface science
Experimental Programme at ELI-NP, 3-5 Oct. 2012
Au-Cu surface alloy Annihilation probability:
K.O. Jensen and A. Weiss, PRB 1990
A.R. Koymen et al., PRL 1992
Cs/Cu(001)
Cu(001)
S
Cs
Ps+
Ps+
Au/Cu(001)
K.H. Lee et al., PRL 1994
C.M. Teodorescu, NIMP Bucharest, Positron-related techniques in surface science
Experimental Programme at ELI-NP, 3-5 Oct. 2012
Angle-resolved PAES → detailed surface structure
Interfaces nicely visible by HRTEM; surfaces not Photoelectron diffraction
studies of relaxation / rumpling
at BaTiO3 surfaces
Ba 4d Ti 2p
O 1s C 1s
Data analysis PED:
- multiple scattering;
- electrons originate from
different shells;
- time consuming;
- low reliability
A. Pancoti et al., PRB, submitted
C.M. Teodorescu, NIMP Bucharest, Positron-related techniques in surface science
Experimental Programme at ELI-NP, 3-5 Oct. 2012
e.g. PED-derived adsorption geometry of NH2 on Si(111) 7 x 7
N. Franco et al., PRL 1997
S. Bengio et al., PRB 2004
→ adsorption on rest atoms
ARPAES could provide a similar
information for any system!
C.M. Teodorescu, NIMP Bucharest, Positron-related techniques in surface science
Experimental Programme at ELI-NP, 3-5 Oct. 2012
Experimental setup:
0. Positron source BRIP
107 e+/(s mm2 mrad2 0.1 % 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 10-11 mbar etc.)
C.M. Teodorescu, NIMP Bucharest, Positron-related techniques in surface science
Experimental Programme at ELI-NP, 3-5 Oct. 2012
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.
↑↑ o-Ps vac. = 142 ns
↑↓ p-PS vac. = 125 ps
metal-insulator transitions
rh dr
Ps
Ps lifetime in Si
0.5 ns
C.M. Teodorescu, NIMP Bucharest, Positron-related techniques in surface science
Experimental Programme at ELI-NP, 3-5 Oct. 2012
e+ scattering and diffraction
Low energy e+ reflection
source: A. Kawasuso,
Halle Seminar 2000
RHEPD
RHEED
Si(111)H
C.M. Teodorescu, NIMP Bucharest, Positron-related techniques in surface science
Experimental Programme at ELI-NP, 3-5 Oct. 2012
Spin-polarized positrons
DBAR
Kawasuso et al.,
PRB 2011
Proposed experiment for surface spin-resolved DOS:
e.g. 1st ML of Ni(001) AF coupled
1st ML of Gd(0001) canted, etc.
Surface magnetism
(T)
(eV) μm 37.3
.
B
E
p
eBr
c
e
e
cyclo
C.M. Teodorescu, NIMP Bucharest, Positron-related techniques in surface science
Experimental Programme at ELI-NP, 3-5 Oct. 2012
Other advantages of positrons (source: S.W.H. Eijt, 2008, Department of Radiation,
Radionuclides & Reactors, Faculty of Applied Sciences, Delft University of Technology