Soft x-ray spectroscopies: Photoemission and x-ray absorption

Bryan Doyle

S@S 2009, Pretoria, 10th February 2009

Physics at the “surface”

Plan of talk

o Soft x-rays

o X-ray absorption

o Case 1: carbon nanotubes

o Photoemission

o Case 2: organic molecules on surfaces

o Case 3: YbGaGe and zero thermal expansion

o ARPES – electronic structure in the valence band

Soft x-rays

Soft x-rays

• from the ultraviolet (UV) to hard x-rays 20 – 1600 eV

• correspond to many atomic energy levels – K shell of light elements (organics), L shells of transition metals, M shells of rare-earths

• high cross sections for absorption and electronic excitations

• limited penetration depth: 20 – 500 nm

• experiments carried out in high vacuum < 10-6 mbar

occupied valence band

unoccupied valence band

core level

EF

h

X-ray absorption

if the photon energy is just higher than the binding energy of the core level

bound state

X-ray absorption (NEXAFS/XANES): a brief introduction

2p3/2-3d (LIII)

2p1/2-3d (LII)

O

NH

NH

O

O

O

NH

O

NH2

NH2

O

O

OOH

OH

OH

OOH

O

O

OOH

OH

OH

OOH

HDA

hOK-edge(530 eV)

hCK-edge(280 eV)

NH2

hNK-edge(400 eV)

Case 1: Functionalization and doping of carbon nanotubes

Nanotube functionalization: grafting of an amine

525 530 535 540 545 550

inte

nsity

(A. U

.)

Energy (eV)

Pristine

525 530 535 540 545 550

HDA

Oxidized

inte

nsity

(A. U

.)

Energy (eV)

Pristine

525 530 535 540 545 550

Oxidized

inte

nsity

(A. U

.)

Energy (eV)

Pristine

525 530 535 540 545 550

inte

nsity

(A. U

.)

Energy (eV)

Pristine

C--C

=OHD

AC-

-C=O

OH

Oxygen K-edge

525 530 535 540 545 550

Energy (eV)

CH3COOH

*C=O

*C-OH

CH3COCH3

(Acetic acid)

(Acetone)

chemical environment of the oxygen

M.-R. Babaa, J.-L. Bantignies, L. Alvarez, P. Parent, F. Le Normand, M. Gulas, J. Mane Mane, P. Poncharal and B.P. Doyle, J. Nanosci. Nanotechnol. 7 (2007) 3463-3467

: carbon hybridized sp2

Multiwalled tubes30 % nitrogen

Single walled tubes1 % nitrogen

Substitutional doping of nanotubes: CNx

h

macroscopic probe

high energy resolution

nanoprobe

spatial resolution

Electron Energy Loss Spectroscopy (EELS)

*he-

*

1s

N K-edge 1s *1s *

NEXAFS e-

Use complementary spectroscopies: EELS and NEXAFS

High density of nitrogen within the tube cavity

(a)

(b)

(a)

(b)

(c) (d)Carbon

Nitrogen

(c) (d)Carbon

Nitrogen

(d)Carbon

Nitrogen

Energy Loss (eV)

Inte

nsity

(a.u)

Energy Loss (eV)

Inte

nsity

(a.u)

EELS on multiwalled nanotubes

395 400 405 410 415 420

inte

nsity

(A. U

.)

Energy (eV)

1s*

Multiwalled tubes: NEXAFS spectroscopy

Nitrogen K-edge

?

395 400 405 410 415 420

inte

nsity

(A. U

.)

Energy (eV)

*

molecular N2

400,0 400,4 400,8 401,2 401,6 402,01,30E-011

1,40E-011

1,50E-011

1,60E-011

1,70E-011

1,80E-011

1,90E-011

Inte

nsity

(A. U

.)

Energy (ev)

MWNT

S. Enouz, J. L. Bantignies, M. R. Babaa, L. Alvarez, P. Parent, F. Le Normand, O. Stéphan, P. Poncharal, A. Loiseau and B.P. Doyle, J. Nanosci. Nanotechnol. 7 (2007) 3524-3527

Molecular N2 mostly adsorbed on the internal tube walls

N2

The state of molecular N2 in MWNTs

405 410 415 420 425

Abso

rptio

n in

tens

ity (A

. U.)

Energy (eV)

405 410 415 420 425

Abso

rptio

n in

tens

ity (A

. U.)

Energy (eV)

N 1s Rydberg series

Double excitation

Shape resonance

N2

MWNT CNx

405 410 415 420 425

Abso

rptio

n in

tens

ity (A

. U.)

Energy (eV)

405 410 415 420 425

Abso

rptio

n in

tens

ity (A

. U.)

Energy (eV)

N2

MWNT CNx

405 410 415 420 425

Abso

rptio

n in

tens

ity (A

. U.)

Energy (eV)

N2

MWNT CNx

395 400 405 410 415 420

inte

nsity

(A. U

.)

Energy (eV)

chemical sites are the same but the relative concentrations are very different

Presence of substitutional N verified

Single walled tubes

Multiwalled tubes

Comparison of the N chemical environments in single walled and multiwalled tubes

Photoemission: a brief introduction

• based on photoelectric effect: photon in – electron out

• other names are photoelectron spectroscopy (PES), XPS, UPS

ANALYSER

h

SAMPLE

MICRO-CHANNEL PLATES

PHOSPHOR SCREEN

CCD CAMERA

EXPERIMENTAL CHAMBER

ELECTROSTATIC LENSES

occupied valence band

unoccupied valence band

core level

Photoemission: x-rays in electrons out

h

valence band photoemission

EF

core level photoemission

Information obtained is: The binding energy of the electron emitted: • electronic structure (VB)• chemical state (core levels)• different bonding configurations

But electrons have a much smaller escape depth than photons!

100

10

1

0,1 10

01000

101h (eV)

Mea

n fr

ee p

ath

(nm

)Electron inelastic mean free path

Experiments usually carried out in ultra high vacuum < 10-9 mbar

O

S

O

nn

1. OFET2. OLED3. Solar Cells4. Antistatic coatings5. Anticorrosion coatings6. “smart windows”7. Sensors8. ...

Interface with metal of great importance in any device

Case 2: 3,4-ethylenedioxythiophene (EDOT) on noble metal surfaces

thiol or poly-Sthiophene

C(1s) h = 385 eV

S(2p) h = 260 eV

C3C2C1

10 mM aqueous solution

Pt polycrystalline surface20

15

10

5

0

Inte

nsity

(arb

.uni

ts)

294 292 290 288 286 284 282 280 278 276Binding energy (eV)

14

13

12

11

10Inte

nsity

(arb

.uni

ts)

540 536 532 528Binding Energy (eV)

O(1s) h = 630 eV

6543210in

tens

ity (a

rb.u

nits

)

540 536 532 528binding energy (eV)

10

8

6

4

2

0

inte

nsity

(arb

.uni

ts)

175 170 165 160binding energy (eV)

40

30

20

10

0

inte

nsity

(arb

.uni

ts)

292 290 288 286 284 282 280binding energy (eV)

Au polycrystalline surfaceC 1s

h = 385 eV

S 2p h = 260 eV

thiophene

thiolor

poly-S

atomicS

SOx

10 mM aqueous solution

O1s h = 630 eV

3.0

2.5

2.0

1.5

1.0

0.5

0.0

inte

nsity

(arb

.uni

ts)

290 288 286 284 282 280binding energy (eV)

4

3

2

1

0

inte

nsity

(arb

.uni

ts)

175 170 165 160binding energy (eV)

Au(111) single crystalC 1s

h = 385 eV

S 2p h = 260 eV

thiophene

thiolor

poly-S

atomicS

NO oxygen!

exposure to EDOT vapours

X-ray photoemission – conclusions

Most solids expand upon heating.

This is due to the anharmonic potential formed from the sum of the internuclear forces

However some systems exhibit negative (or zero) thermal expansion: Invar (Fe65Ni35) and some oxides (ZrW2O8, Y2W3O12, CuScO2).

Case 3: Rare earth valence transitions as a mechanism for zero or negative thermal expansion: the case of YbGaGe

Zero thermal expansion (ZTE) reported in YbGaGe

J.R. Salvador et al., Nature 425 (2003) 702

= -3×10-7 K-1

2 calculated Yb valences:Yb(1) = 2.6 ; Yb(2) = 2.0

Yb3+

Yb2+

Magnetic susceptibility

YbGaGe

Yb2+ (r = 1.16 Å), significantly larger than

Yb3+ (r = 1.008 Å)

They proposed temperature dependent electron transfer: between Yb (4f) and Ga (4p)

Density of States

Yb2+ (4f14)

Yb 5d

Yb 4f

Yb2+ (4f13 5d1)

Yb2+ (4f14)

Yb 5d

Yb 4f

Ga 4p

Ener

gyEn

ergy

Yb3+ (4f13)

Density of States

Yb 5d

Yb 4f

Ga 4p

Yb 5d

Yb 4f

increasing temperature

Yb2+ ion

S. Schmidt et al., PRB 71 (2005) 195110

J.L. Sarrao et al., PRB 54 (1996) 12207

YbInCu4: Resistivity

h = 1486.6 eV

S. Schmidt et al., PRB 71 (2005) 195110

4f electrons in the rare earths reflect the valence

h = 5.95 keV

H. Sato et al., PRL 93 (2004) 246404

Photoemission

L.I. Johansson et al., PRB 21 (1980) 1408

Yb3+

Yb2+

Shows a transition at T = 42 K

YbGaGeProblem! Other groups cannot replicate the data

Direct spectroscopic probe of valence needed

Normal thermal expansion found:S. Bobev et al., Solid State Commun. 131 (2004) 431

No magnetic transition:Y. Janssen et al., J. Alloys Compd. 389 (2005) 10

Normal thermal expansion found:Y. Muro et al., J. Phys. Soc. Jpn. 73 (2004) 1450

from original ref.

h = 182 eVon resonance

Yb 4f12 and 4f13 as a function of temperature

no clear change with temperature

Need to determine parameters for fit

Yb2+: h = 72 eV to increase surface sensitivityYb3+: h = 182 eV to increase intensity

1

4

4

12

13

14131

f

fh I

In 2 hYb nv

Use high energy photons to be more bulk-sensitive

h = 1421 eV

Electron mean free paths:

35 Å at 1421 eV10 Å at 182 eV

again no clear change with temperature

YbGaGe conclusions

B.P. Doyle, E. Carleschi, E. Magnano, M. Malvestuto, A. Dee, A.S. Wills, Y. Janssen and P.C. Canfield, Phys. Rev. B 75 (2007) 235109

No valence change detected over temperature range of interest (T: 115 K 316 K: v = 2.29)

Other groups fail to replicate ZTE results (although doping does lower thermal expansion coefficient)

Photoemission is a sensitive tool to measure valence changes (our limit <0.03)

Now are looking at other candidates: Yb2.75C60,Yb8Ge3Sb5

Angles resolved photoemission (ARPES)

Information obtained:

Which electrons are free to move and how?

€

Ar k ,E,σ( )

Courtesy: A. Damascelli

ARPES: Widespread impact in complex materials

Diamond

Nature 2005

Nature 2003

Nanotubes

Nature 2000

Quasicrystals

CMR's

Nature 2005Science 2006

Graphene

C60

Science 2003

as well as high TC superconductors

In conclusion …

• soft x-ray techniques can also be applied to ex-situ samples

• greater bulk sensitivity now available with (1) hard x-ray photoemission < 15 keV (2) very low energies < 15 eV (ARPES only)

• most techniques also available as microscopies

• There are many available soft x-ray beamlines in the world today

Thanks to …

• L. Pasquali and F. Terzi, University of Modena, Italy • E. Carleschi and E. Magnano, TASC Laboratory, Italy

• M. Malvestuto, Elettra, Italy

• J.-L.Bantignies and L. Alvarez, Université Montpellier II, France

• A.S. Wills and A. Dee, University College London, UK

• Y. Janssen, Brookhaven National Laboratory, USA

• P.C. Canfield, Ames Laboratory and Iowa State University, USA

and to all of you for listening!