Surface Analysis with Many Positrons

Alex Weiss

Physics DepartmentThe University of Texas at Arlington

TX 76019, USA

Acknowledgments: Current Students::

1. Saurabh Mukherjee Ph.D. Program in Physics, 2. Karthik Shastry, M.S. Physics

Undergraduate Students:Aale Naqvi

Former Students:

Ph.D.-1. M. Jibally, .2. D.Mehl, 3. K.H. Lee, 4. G. Yang, 5. H.Q. (Amy) Zhou, 6. E. Jung, 7. J.H. Kim 8.Wu-Chi Chen, 9.S. Xie 10. J. Zhu, 11. M. Nadesalingam, 12 R. Sundaramoorthy

MS: 1. C. Lei 2. K.H. Lee 3. L.-W.Tyan 4. J.H. Kim 5. S. Wheeler 6. R.Venkataraman 7. A. Nangia 8.R. Nayak 9. N. Jiang 10. S. Starnes 11. J. Yan 12. S. Kim 13. R. Sundaramoorthy 14. M. Nadesalingam15. S. Mukherjee

Collaborators (UTA): A. R. Koymen, .J.L.Fry, N. Fazleev, W.Chen, M. Tao, K. Rajeshwar, C. Kim,(Japan): Hasegawa, Nagai, (Germany): G. Brauer, (BNL)S. Hulbert, R. Bartynski

Former Post-Docs/Collaborators: J. Kaiser – UTA, K.O. Jensen - U. of East Anglia, U.K., G. A.Mulhollan – SLAC, Rulon Mayer, Arnum Schwab – BNL, Anat Eshed- MIT

Previous and Current Funding: NSF, DOE, Texas-ARP, DOE, TheWelch Foundation

� Lecture I: Positron Surface Spectroscopies

� Review of important positron surface interactions

� Overview of Positron Surface Spectroscopies

� Introduction to PAES spectroscopy

� Lecture II: Applications

� Examples of Applications of Positron Surface Spectroscopies

� Lecture III: High Flux e+ Beams: New Possibilities

� Surface island traps: possible means of increasing e+ densities

� New kinds of surface measurements

Why study surfaces with positrons?

� Corrosion

� Electrical Contacts

Adhesion

Adhesion

Corrosion Catalysis

Ultra Thin layers

Solid or � Adhesion

� Ultrathin Layers

� Heterogeneous Catalysts

� >95% of all products synthesised by Catalysts

� ~20% of world economy depends on Catalysts

Electrical Contacts

Corrosion CatalysisLiquid

Technique Depth Probed in Atomic Layers Comments

Many Techniques to Probe Many Techniques to Probe Surfaces Surfaces

LEED 3-5 Need theory

SIMS 1-5 Destructive of thin films. Hard to qualify

XPS 3-10

e- Auger 3-10

STM ~1 Difficulty with element identification

e+ Auger ~1 Significant advantages over E-AES

� Capable of detecting both structure and elemental

composition

� Highly surface selective due to trapping in surface state

� Capable of probing active sites and chemical mechanisms� Capable of probing active sites and chemical mechanisms

� Capable of probing internal surfaces of porous materials

� Low Background

� Very low damage –low energy dose to surface

e+

beam

Energy Loss:

-plasmons

-electron-hole pairs

-Phonons

Thermalized

After: Schultz and Lynn, Rev. Mod. Phys. 60, 1988

e+

Ps

Thermalized

Positron surface state Diffusion

Annihilation in

the bulk

γ

γ Surface StatePs self-annihilation

Thermally Desorbed

e+

beam

γ

γ

Diffusion

Annihilation with Surface Electrons

Positron may give up all or most of its kinetic energy to an electron at or near the Fermi level and directly drop into the surface state;

Thus transferring the surface state binding energy to the electron in

the process. The maximum kinetic energy for an electron emitted in

this process is given by:

Recently Demonstrated Direct Surface Trapping Mechanism

Φ

Metal Vacuum E=∆E

EEEpE ssK ∆=−+= −ϕ max

eVEeVEeVEeV PKss 2 ,0for so 3,5 Now, max >>==−ϕ

Vacuum level

Φ+

Surfaces potential

Ess

εPs

Positron emission (εe+)

εe+

J. Phys. C: Solid State Phys. 21 (1988)Branching ratios for electron-volt positrons at a Cu(ll0)

surfaceJ A Baker, M Touat and P G Coleman

Positronium emission (εPs)

Surface State Trapping (εS)

εS

positrons in positrons out

LEPD,

positron work function,

positron reemission

spectroscopy,

positron tunneling

spectroscopy

positron backscattering

positrons in

positronium

out

gamma rays out

positrons in

positrons out

positrons in

positrons in electrons out

PAES,

positron-induced secondary electrons

surface

ACAR

positronium

emission

spectroscopyor Ps in Ps diffraction

Principal Advantages of PAES:

1. Higher surface selectivity than electron spectroscopies.

2. Very low background-elimination of primary beam induced secondary electron background.

3. Selective sensitivity to nano structures and impurities.

Low energy (~10ev) positron in.Implantation, thermalization,

diffusion, encountering the surface

Positron trapped in surface state

10-12 -10-11 sec

10-10 sec

Time scales of physical processes underlying PAES

Annihilation of surface state positron with core electron

Emission of Auger electron –

Higher energy (20-800 eV) electron out.

10-14 sec

10-10 sec

Vacuum Level

Valence Band

φEz

Ey

Auger Electron

Ex

γ

γ

E Auger = E x- E z- E y- φφφφ

Elimination of Secondary Electron Background

3 keV e- in

30eV -1keV Auger e-+

0-3 keV secondary e-

EAES 30 eV e+ in

30eV -1keV Auger e-+

0-30 eVsecondarye-

PAES

Why is PAES more surface selective than other electron surface spectroscopies?

EAES samples several layers below the surface

metal vacuumλ- escape depth;

λ~ 4 – 10 Å

1000 Å

Incident electrons ~ keVExcited Atoms

~50% of signal

Electrons:

EAES samples several layers below the surface

PAES probes only top layer

λλλλ ~ 1 Å

Thermalized positron in surface state

~from 2nd layer and

below

Positrons:

~95% of signal from top layer

Construct positron potential:

V+(r) = VH(r) + Vcorr(r).

0 Z- ))(n_(

effZ4

1

04π

2e- = )(

imageV

rr χ

ε

Detailed Calculations: ~95% of positron in surfacestate on Cu annihilate with top atomic layer

Solve for positron-surface-state wave function:

[ ] )(ψEi )(ψ)(V )(V )(ψ2m iii

22

CorrH rrrrr +++ =++∇− h

Calculate annihilation probability ( α positron-electron overlap)

∑∫

+=i

ilndcorln

2)(,

2)(32

, rr r ψψπλ

PAES vs EAES ~1 ML Pd deposited on Cu

Cu MVV Pd NVV Cu LVV

PAES

EAES

Cu MVV Pd NVV Cu LVV

Cu MVV Pd NVV Cu LVV

•TOF method collects all energies simultaneously rather than sequentially stepping an energy window.

UTA Time of Flight (TOF) PAES Spectrometer

•100 times the collection efficiency of previous PAES systems.

•Permits observation of Auger peaks from chemically important light elements (e.g. C, O, N).

22Na Positron Source E X B Plates

Sample Position Sample Preparation Chamber

Retarding Tube

Sample manipulator

Accelerator MCP

1 m

Ion Pump Ion Pump

Positron Annihilation-Induced Auger Spectroscopy (PAES) System with

Time-of-Flight (T-O-F) Energy Analyzer

Sample

MCPEXBPlates D

Permanent magnet

EXBPlates C EXB

Plates BEXB

Plates A

TungstenBarrier C

ModeratorTOF retarding tube

B = 40Gauss

TungstenBarrier B

TungstenBarrier A Accelerator

B = 100Gauss

γe-

Schematic of T-O-F PAES system

Vbia

s

Vrt

d

VEXB

2

V

m

VEXB

1

γ e+

Timing scheme of the Time-of-Flight (TOF) PAES spectrometer

NaI Detector

BaF2 Detector

Magnet

Sample TOF Retarding tubeEXB Plates

e-

e+

Micro-channel Plate (MCP)

Fast pre-amplifier

Time to Amplitude converter(TAC)

stop start

γ

γ

CFD

The energy distribution of outgoing electrons in the TOF technique can be calculated by the equation:

2

startstop

e

2

ekinetic ttL

m21

vm21

E

−−−−========

Counter Fast pre-amplifier Time delayCFD

Multi-channel Analyzer(MCA)

0.007

0.008

0.009

0.010

0.011

0.012

0.013

0.014

0.015200 400 600 800 1000 1200 1400 1600

274.9ns (#1118)

Channel

Cou

nts/

sec

0.003

0.004

0.005200 400 600 800 1000 1200 1400 1600

132.3ns (#1483)

274.9ns (#1118)

Channel

Cou

nts/

sec

TOF-PAES Flight Time Spectrum

Copper Sample

0.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

195.6ns (#1325)

Cou

nts/

sec

Flight time (ns)

650 600 550 500 450 400 350 300 250 200 150 100 50

0.000

0.001

0.002

104.2ns(#1555)

115.1ns(#1527)195.8ns

(#1320)

Cou

nts/

sec

Flight time (ns)

650 600 550 500 450 400 350 300 250 200 150 100 50

Clean Cu Surface Cu Surface with adsorbates

Electron Energy Spectrum

0.3

0.4

0.5

(b)

Experimental data 5 point smoothing

Cu M2,3

VV (59.3 eV)

Inte

nsity

(cou

nt/s

ec)

0.10

0.12

0.14

0.16

0.18

Experimental data 5 point smoothing

Cu M2,3

VV (58.3 eV)

Inte

nsity

(cou

nt/s

ec)

100 200 300 400 500 6000.0

0.1

0.2

Cu M1VV (108.7 eV)

Inte

nsity

(cou

nt/s

ec)

Energy (eV)

100 200 300 400 500 600 700 8000.00

0.02

0.04

0.06

0.08

O KLLN KLL

C KLL(268.0 eV)

Cu M1VV

(108.0 eV)

Inte

nsity

(cou

nt/s

ec)

Energy (eV)

Cu with adsorbates:C (C KLL), N (KLL) and O (KLL)

Clean Cu Surface

Energy spectrum of graphite surface

0.009

0.012

0.015

C KLL (258.4 eV)C

ou

nts

/sec

Auger spectrum of graphite initial surface

100 200 300 400 500 600 700 8000.000

0.003

0.006 O KLL (497.5 eV)Co

un

ts/s

ec

Energy (eV)

Table 2: adsorbate Copper Surface

Core level annihilation probability (%)

Cu 3p Cu 3s C 1s N 1s O 1s

Cu (polycrystalline)Adsorbate surface 0.850 0.191 0.352 0.077 0.070

Core level annihilation probability

*is the theoretical value from reference (Jensen and Weiss), all the adsorbate coverage is 0.5relative to the substrate atomic density for theoretical results.

Cu (110) * with CAdsorbate surface 1.62 0.44 0.14

Cu (110) * with NAdsorbate surface 1.41 0.39 0.21

Cu (110) * with OAdsorbate surface 1.75 0.48 0.10

Core level annihilation probability

Theory Experimental

ps

Comparison of Calculated core-annihilation probabilities (Jensen and Weiss) and experimental results of TOF-PAES as functions of the binding energy of the core levels

Positron removed from surface state

by thermal desorption of Ps

ThermalThermal desorption of Psdesorption of Ps

φ-

Eb

Function of z/γ (desorption rate/ss annihilation rate)

A.P. Mills, Sol. St. Comm., 31 (1979) S. Chu, A.P. Mills, C.A. Murray, PRB, 23 ( 1981)

Rate of Ps desorption:

φ-

Eb

In the positron in a potential well picture:

Where Eb is the binding energy of a positron in the surface state

In the Ps in a potential well picture:Ea is the binding energy of Ps in the well

Energy available when e+ from outside sticks to surfaceEb =Ea-φ- + 1/2Ry

fPs

Ps self-annihilationNo core e-

PAES

fss= 1-fPs(neglecting free e+ channel)

Surface state annihilation~1-10% surface atom core e-followed by Auger emission

1-fPs

RepulsiveRepulsiveRepulsiveRepulsive + van + van + van + van derderderder WaalsWaalsWaalsWaals

Where α(ω)α(ω)α(ω)α(ω) is Ps polarizabilityand ε(ω)ε(ω)ε(ω)ε(ω) is the bulk dielectric function.

Repulsive term approximation: vR(z) = V0e−(z−zo)/l

Positrons can pair up with electrons as Ps Positrons can pair up with electrons as Ps at a surface of quartz and Ps can stick

to the surface.

Saniz, B. Barbiellini, P. M. Platzman, and A. J. Freeman, PRL 99, 096101, (2007);

PRL 100, 019902, (2008).

Michael Schirber, Phys Rev. Focus 20, story 7

Ground state : Ground state : Ground state : Ground state : ----0.112 0.112 0.112 0.112 eVeVeVeV

Excited state: Excited state: Excited state: Excited state: ----0.005 0.005 0.005 0.005 eVeVeVeV

~1/z3

Construct positron potential:

V+(r) = VH(r) + Vcorr(r).

0 Z- ))(n_(

effZ4

1

04π

2e- = )(

imageV

rr χ

ε

Theory used to calculate PAES annihilation probabilities:Positron in potential well at the surface

Jensen and Weiss, PRB 41 (1990) Nieminen and Puska, PRL 50 (1983)

~1/z

Solve for positron-surface-state wave function:

[ ] )(ψEi )(ψ)(V )(V )(ψ2m iii

22

CorrH rrrrr +++ =++∇− h

0 Z- ))(n_(

effZ40 r

Calculate annihilation probability ( α positron-electron overlap)

∑∫

+=i

ilndcorln

2)(,

2)(32

, rr r ψψπλ

Core level annihilation probability

Theory Experimental

ps

Comparison of Calculated core-annihilation probabilities (Jensen and Weiss) and experimental results of TOF-PAES as functions of the binding energy of the core levels

� Properties of positron interactions with surfaces

give positrons significant advantages as probes of

surfaces and surface processes.

� PAES allows measurements of the elemental PAES allows measurements of the elemental

content of the top most atomic layer.

� Temperature dependence of PAES intensities

consistent with surface state origin of PAES signal.

� Theory: Positron in a single particle potential

convenient and accurate calculations of PAES

intensities.