C.M. Teodorescu, NIMP Bucharest, Experimental ... - ELI-NP · C.M. Teodorescu, NIMP Bucharest,...

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]



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;



Some main facilities:

Analytical

Cs-corrected

HRTEM, 0.8 Å

Au



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;



Target fabrication

foams/aerogels micro-machining

free standing nm thick foils



0

0

2

00

2

2

exp3

)(R

tv

M

vM

dt

xdtx

0

0

v

R

M

M

3

0

0

0

R

x



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



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)



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





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)



Electron IMFP

substrate

X-ray photoelectron spectroscopy

- charge transfer

- chemical state

- local atomic order



Zr 3d

Pb 4f



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)



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)



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



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



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!



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.)



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



e+ scattering and diffraction

Low energy e+ reflection

source: A. Kawasuso,

Halle Seminar 2000

RHEPD

RHEED

Si(111)H



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



Other advantages of positrons (source: S.W.H. Eijt, 2008, Department of Radiation,

Radionuclides & Reactors, Faculty of Applied Sciences, Delft University of Technology

([email protected])



+ Positrons highly suited for Surface Science

C.M. Teodorescu, NIMP Bucharest, Experimental ... - ELI-NP · C.M. Teodorescu, NIMP Bucharest,...

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