Positrons for Applied Science & Materials Science

42
Positrons for Applied Science & Materials Science K.G. Lynn and M.H. Weber and many others!! Washington State University, Pullman, WA JPOS 09 International Workshop on Positrons at Jefferson Lab Thomas Jefferson National Accelerator Facility Newport News, VA March 25-27, 2009

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Positrons for Applied Science & Materials Science. K.G. Lynn and M.H. Weber and many others!! Washington State University, Pullman, WA. JPOS 09 International Workshop on Positrons at Jefferson Lab Thomas Jefferson National Accelerator Facility Newport News, VA March 25-27, 2009. - PowerPoint PPT Presentation

Transcript of Positrons for Applied Science & Materials Science

Page 1: Positrons for Applied Science & Materials Science

Positrons for Applied Science& Materials Science

K.G. Lynn and M.H. Weberand many others!!

Washington State University, Pullman, WA

JPOS 09International Workshop on Positrons at Jefferson Lab

Thomas Jefferson National Accelerator FacilityNewport News, VAMarch 25-27, 2009

Page 2: Positrons for Applied Science & Materials Science

My Concerns in a low energy positron facility

• Intense positron sources have not fulfilled its promise to DOE/NSF• The intense sources have been small groups trying to move into a larger

facilities based on the researchers interests and lacked the support of the facilitiy and funding agency.

• The beams that have operated have not provided the needed user support and users have gone elsewhere.

• Neither the brightness nor the intensity has been routinely been achieved

• If the Jefferson Lab is planning this facility a real commitment is needed from OS/DOE and local management

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Number 1:

Positrons madethe Newsweekhitlist

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The positron’s death

= g(p)

red shifted

blue shifted

Eventually, in our world of matter, the positron will annihilate with an electron.Two (or rarely three) photons (gamma rays) emerge.

The number of electrons (density) determines how fast this occurs

Basic laws of nature (physics) force certain conditions:2 gammas in opposite direction with small changes in energy (Doppler shifts) and direction.Doppler shifts

Angular deviation from opposite

JPOS 09 Newport News (March 2009)

Page 5: Positrons for Applied Science & Materials Science

Positron characteristics• Unique quantum numbers

– No exchange at the present time

• Annihilation with electrons radiation can be detected– Little interaction with specimen after annihilation

• Electron momentum encoded in -rays– Doppler broadening– Angular correlation

• Lifetime is electron density dependent– Positron lifetimes

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Hits in “Defects”• Vacancy formation enthalpies in metals (90%) (1975-

present)• Voids in neutron irradiation and deformation of metals• Observation of vacancy migration at stage III (1980)

– Major controversy resolved• Vacancies observations in compound semiconductors (1990)• Vacancy character of EL2 in GaAs (1993)• Role of defects in hi-Tc superconductors (1988-92)• Open volume measurements in polymers (ongoing)

– Gas diffusion, Mechanical properties, Aging • Defects at semiconductor interfaces (ongoing)

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Annihilation at high relative momentum• 2D spectrum:

• x: p-parallel <==> Doppler shift• y: Sum energy <==> rest mass + kinetic energy

1022 keV

1092 keV

0 keV340 keV=> 91.3 a.u.-340 keV

JPOS 09 Newport News (March 2009)

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Channeling

Angle

Nor

mal

ized

Yie

ld

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Positron Holography (Never fulfilled)

CdSe

- Electron-electron interaction- Multi-layer contribution

- One positron at a time- Topmost layer only

Now: with electrons Future: with positrons“If positrons were routinely available, all diffraction would be done with them” S.Y. Tong

Page 10: Positrons for Applied Science & Materials Science

Fermi SurfacesYtterbium

Experiment

Theory

Now: 16 dataset Future: Super ACAR 1 shot and depth profiles

Resolution limited by acquisition time

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Quantum dots

0 1 2 3

0.8

1.0

1.2

Rat

io to

bul

k C

dSe

Doppler momentum (a.u.)

1.8 nm

6.0 nm

4.5 nm

“baseline”

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Fe Fe

Cu

Zero point motionenergy

Potential wellin Fe

e+

Cu in Fe

Precipitates-Critical in ReactorSteels

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Atomic scale defects• Missing atoms in crystals are called vacancies• They play a key role in the properties of many metals,

semiconductors and insulators• How to tell the difference between impurities and

dopants– One makes the PC work the other turns it to a pile of junk

• Understanding them drives progress– Electronics, solar cells, sensors, optics, detectors (airports),

lasers– Silicon, silicon carbide, ZnO, GaN, GaAs, YAG,…– Lasers to cut steel, transparent conductors for monitors, sunlight

to electricity, longer lasting cell phones, more gigabytes on DVDs beyond Blue-ray, shorter queues at airport baggage scanners…

JPOS 09 Newport News (March 2009)

Page 14: Positrons for Applied Science & Materials Science

Trapping in negative or missing atoms

<100>-direction (a.u.)

05

10

05

10

-1

0

1

2

3

<010>-direction (a.u.)

05

1015

05

1015

-1

0

1

2

3

<100>-direction (a.u.) <010>-direction (a.u.)

Delocalized Bloch stateLocalized trapped state

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A positron and many electrons

Doppler broadeningConduction electrons: delocalized; low momentum

Bound electrons: localized; highmomentum

Potential of atomic cores

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A positron “likes” vacancy

Doppler broadeningConduction electrons: delocalized; low momentum

Bound electrons: localized; highmomentum

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2400

200016001200800400

Temperature (K)

Ope

n vo

lum

e pa

ram

eter

Tc

Vacancy formation energy

Mo

Now: 1D depth profile Future: 3D map with lifetime

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Depth profiles

0 5 10 15 20 25Positron energy (keV)

0.99

1.00

1.01

1.02

1.03

1.04

1.05

1.06

Nor

mal

ized

S p

aram

eter

0 10 20 30 40 50 60 70 80Depth (nm)

Def

ect c

once

ntra

tion

(cm

)

Mean implantation depth (nm)

d = 150 nmMB E

100 300 500 1000 1500 2000 3000

-3

1020

1021

1019

Now: layer averaged Future: 3D map with nm3 resolution

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SiO2-Si interfacePs trapped in microvoidsat the interface

With broad component

Without broad component

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0 1000 2000 3000 4000 5000

1.00

1.01

1.02

1.03

1.04

1.05

1.06

Ope

n vo

lum

e/da

mag

e

Mean depth (nm)

Sample surface treatment: as cut; polish 1x; polish 2x etch polish after etch Vendor M etched reference

Bulk material level

Colloidal silica (50 nm)

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Defects in matter

The mesh represents electrons “flowing” around atoms in silicon. The atoms are indicated by the red spheres. One atoms is missing and a different atom (green) is replacing a neighboring silicon.

This is hard to “see” but can be detected with positrons.

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Looking for defects

Doppler shift momentum

Total energy

Highly porous material

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Chemical environment

Coincidentpositron annihilationsensitive tocore electrons

Now: 12 hours for 1 sample @ 1 selected depth Future: within hours a full depth profile

0 1 2 3 4 5 6 7 8

1.0

1.5

2.0

2.5

3.0 Si Cu Nb W Pb

Rat

io to

Al

Doppler momentum (a.u.)

x 1/2

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Micro probes

News item in Nature vol. 412, p.764 (2001)W. Triftshauser et al, Phys. Rev. Lett. 87, 067402 (2001)

Combined positron (1-5) and electron (7-6) Microscope (9-10) to probe cracks in metals (11,13). An electrical prism (6) switched between electrons and positrons to combine electron microscope and defect images.Greif et al, Appl. Phys. Lett. vol 71, p. 2115 (1997)

Positron probe thatMeasures the electron density of patterns on silicon with 2 micrometer resolution

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CracksLifetime scale120 170 350 (ps)

Dislocations

Void

Matrix

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The future of Defects 2D lifetime maps

Simulation of the future with e+

Vacancies

Dislocations

Matrix

Precipitate

Small void

TEM

Lifetime scale120 170 350 (ps)

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Stress-Are you feeling some??

stress relieved

under stress

Direct observation of dislocations in metals during elastic deformation

Lifetime

Intensity

Now: stop frame Future: movie

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Lifetime apparatus

Stop: detector

discriminator

Data collectingcomputer

positron beam

Start: e- detector

discriminator

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Positron lifetime

0 40 80 120 160 20010-6

10-5

10-4

10-3

10-2

Are

a no

rm c

ount

s

Time (ns)

Samples: Al(100) 4.1 keV low-k non porous; 2.0 keV low-k 10% porosity; 2.0 keV

Background subtractedNo pores

big space between moleculeslarge pores

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Positron Lifetime

125 150 200175 225

Unit-cell volume (a.u.)

220

260

300

340

Pos

itron

life

time

(ps)

Now: bulk averaged Future: 3D map

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Positronium in Voids & Open Porosity

poro

sity

inte

rcon

nect

ivity

surface

+

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o-Positronium Lifetime

0.1 1 10 1

100

10

Pore radius

o-P

s lif

etim

e

SILICAGEL

ALUMINAGEL

POROUS VYCOR GLASS

SILICAGEL

SODALITE

MS-4A

MS-5A

a-CYCRODEXISTRIN

MS-3A

13X

3A4A

4A13X

13X4A

5A

Now: bulk averaged Future: 3D map

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Separating closed and open porosities (at 2 keV)

0 10 20 30 40 500

5

10

15

20

25

30

35

0.0

0.2

0.4

0.6

0.8

1.0

Por

osity

(%)

(wt%)

closed open total

Ope

n fra

ctio

n

Open/closed porosity differ qualitatively :

25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400

0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

2.25

3/2 r

atio

: Diff

eren

ce to

bul

k S

i

Temperature (K)

3.4 % Porosity - Closed 10.4 % Porosity - Closed

17.3% Porosity - Open Contribution 17.3% Porosity - Closed Contribution

Closed vs open porosity

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Percolation Threshold; Open Porosity

0 10 20 30 40 50

10

20

30

40

PALS, 140 ns lifetime

Inte

nsity

, I4

(%)

porogen load (%)

0 5 10 15 20 25 300

20

40

60

80

100

120

140

porogen load (%)

L Ps

(nm

)

0 5 10 15 20 25 300

10

20

30

40

3 o

-Ps

(%)

porogen load (%)JPOS 09 Newport News (March 2009)

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Two pore diameters

0 200 400 600 80010

20

30

40

50

1.67

2.02

2.43

1.37

1.02

Closed Pore: diameter

Open Pore: Channel Diameter

Pore S

ize (nm)

(ns

)

depth (nm) x density (g/cm3)

Pore 1 Pore 2

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Pores in materials• The size of pores determines

– what size molecules pass– how long a pill can deliver drugs– the function of fuel cells– the mechanical properties of plastics– how fast a computer can calculate– the purity of filtered water

• Filters, membranes, drug-delivery, microelectronics

• How to measure the size? – These are nanometers.

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Ce:YAG Boule

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0 100 200 300 400 500 600 700 8000

1000

2000

3000

4000

5000

Cou

nts

Energy [keV]

After air anneal After Al sputtring and 1st Ar anneal After 3rd Ar anneal

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#2

Zn

A

B C D E

F G

H I J

As rec.: clear

Zn Ti(H) Ti(D)Ti(H dep)

Ti(H dep)Zn

Ti(H dep)O2

Ref [24]#1 #3

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Oxidation of a layer on Si

0 100 200 300 400 500 600 700 800

0.96

0.98

1.00

1.02

1.04 Exposure: 0 min 10 min 120 min

S

depth (nm)

layer Si

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Zero Temperature Limit of 3/2 ratio Extrapolate to 0 K

• Initial Amount of Ps with in T c.f results of Goworek.• Increase in R due to increase in pore lifetimesÞ Less initial Ps but less pick-offÞ “Purification”: Greater relative intensity of self-annihilation

Ps does not die out

3.5% 10.4% 17.3% 21%

0.0

0.2

0.4

0.6

0.8

1.0

3/2 r

atio

: Diff

eren

ce to

Si

Lim

it at

0 K

Porosity

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