University of California, Berkeley March 9, 2007

Tony van Buuren

Nanoscale Synthesis and Characterization LaboratoryLawrence Livermore National Laboratory

and

UC MercedSchool of Natural Sciences

Use of NEXAFS in Materials Science

Outline

• Overview of the X-ray absorption process

• How do you measure NEXAFS or XANES– Element specific– Measures partial density of empty states– Sensitive to local bonding– Polarization depended

• Applications of NEXAFS:– Surface Chemistry

• Catalysts– Environmental Chemistry

• Oxy-state -> mobility– Material science experiments using NEXAFS

• Quantum dots• Self assembled monolayer• Chemical mapping (Imaging)• Magnetic structures

Density of states from x-ray absorption

unoccupied, CB

occupied, VB

variable h

core level

emission

VB CB

XES XANES

Intensity

Photon energy

XANES=NEXAFS

XANES = x-ray absorption near-edge structure

NEXAFS = near-edge x-ray absorption fine structure

XAS = x-ray absorption

EELS = electron energy loss spectroscopy, provides very similar information to XANES

XANES: Partial density of unoccupied states

unoccupied, CB

occupied, VB

variable h

core level

Wif ~ fTi2 (Ef-Ei-E)

Il(E) ~ l-1(E)Ml-1(E)2 + l+1(E)Ml+1(E)2

Dipole selection rules apply (l1):s pp s and dd p and ff d and gQuadrupole transitions (l 2 or 0) are typically much (102-103 times) weaker.

Element-specific, angular-momentum resolved density of unoccupied states

XANES edges: 1s – K edge2s, 2p – L edges3s, 3p, 3d – M edges4s, 4p, 4d, 4f – N edges

Zinc K-edge X-ray Absorption Spectroscopy Spectrum

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Pre-Edge

XANES

EXAFS

Pre-Edge Region

•Prior to adsorption of the atom of interest

•Composed of the absorption ‘tails’ of elements with lower binding energies

X-ray Absorption Near Edge Structure(XANES)

•Absorption induces internal electronic transitions•Oxidation state is obtained from the position of the ‘edge’•Edge features are characteristic of the local environment of the atom of interest

Extended X-ray Absorption Fine Structure (EXAFS)

• Oscillations dependant upon type, position and number of neighbouring atoms

•A monatomic gas would not display the fine structure

• Extend for up to 1000eV beyond the edge

• Only elastically scattered electrons contribute to the EXAFS – local order

•Unlike the XANES, interpretation of the EXAFS by inspection is limited

Extended X-ray Absorption Fine Structure (EXAFS)

Irradiate the sample with X-ray photons stepwise over a range encompassing the binding energy of a core electron

Adjacent atoms backscatter the ejected photoelectrons which then interfere with outgoing wave

Constructive interference near the nucleus promotes X-ray photon absorption, destructive interference reduces absorption

XAS is:

•Element specific

•Non-intrusive

Whilst probing:

•Short-range order

Structure Extraction From the EXAFS

Quantitative analysis of the EXAFS was first realised by Sayers et al. working with polycrystalline and amorphous Ge samples

–Fourier transformation of c(k) into real space–Peaks correspond to shells of atoms distributed around the central atom and comprise an entire radial structure function

Data analysis• Subtract pre- and post-edge backgrounds• Create structural models and computer generate

EXAFS and FTs from them to compare with the real data

• Least squares regression gives a fit-parameter, RModel parameters varied include:• Position of backscatterers and their identity• Co-ordination numbers• Thermal disorder (Debye-Waller factors)• Atomic potentials

NEXAFS spectra can be recorded in different ways.

The most common methods are transmission and electron yield measurements. Note that the absorption coefficient µ is obtained either as the logarithm or the direct ratio of the detected intensities It and Ie and incident intensity Io www-ssrl.slac.stanford.edu/stohr/nexafs

NEXAFS measurements are element specific

www-ssrl.slac.stanford.edu/dichroism/xas

X-ray absorption spectra of a wedge sample, revealing the composition at various points along the wedge.

BN Thin Films: NEXAFS Determination of Bonding

•Cubic phase (sp3 bonded) Boron Nitride films and coatings are desirable for their hardness and electronic (wide band gap) properties (GM - G. L. Doll)

•Hexagonal (sp2, graphitic) BN is the energetically favorable phase

•Metastable growth conditions (magnetron sputtering, laser ablation) greatly affect the film’s bonding and morphology

B 1s Photoabsorption,hBN and cBN vs BN film

I. Jimenez, L. J. Terminello, et al. Appl. Phys. Lett. 68, 2816 (1996)

sp2

sp3

p*

Polarization Dependent NEXAFS

Synchrotron radiation sources:→ high flux

→ polarized light

BL8.2BL8.2SSRLSSRL

Bond/functional group orientation:NEXAFS resonance strength E .

Surface Chemistry: Catalysts SnO2 aerogels are attractive gas sensor materials

480 485 490 495 500 505

D4

20 oC

250 oC

400 oC

500 oC

550 oC

bulk

C4B

4

D5

C5B

5

A5

M4

M5

Intensity (arb. units)

Photon energy (eV)

Sn 3d XANES

S. O. Kucheyev, PRB 72 (3): Art. No. 035404 (2005).

Surface Chemistry: Polymers

This clearly illustrates the power of NEXAFS to distinguish chemical bonds and local bonding. In many ways it is superior to XPS, which doe s not provide local

structural information.

Often one can use a spectral "fingerprint" technique to identify the local bonding environment.

Carbon K-edge NEXAFS spectra of different polymers, revealing the sensitivity to molecular functional groups.

www-ssrl.slac.stanford.edu/stohr/nexafs

NEXAFS ImagingChemical mapping of polymer blend

C. Morin J. Electron Spectosc. 137-140 (2004) 785-794

XAS images at 283, 285.1, 288.4 and 290 eV, at the C 1s region of an annealed 28:72 (w/w) PS:PMMA blend thin film spun cast on native oxide Si

(b) Spectra from the indicated spots.

(c and d) Component maps of PS and PMMA derived by singular value decomposition of the C 1s image sequence.

(e) Color coded composite map (red: PS; green: PMMA

Environmental Chemistry: Oxy state

http://wwwssrl.slac.stanford.edu/research/highlights_archive/rocky_flats.pdf

Comparison of plutonium LII XANES spectra for plutonium in oxidation states III, IV, V, and VI with RFETSsoil and concrete samples

Self-Assembled Monolayers (SAMs):molecules which adsorb on a surface,spontaneously order via intramolecular forces.Most common type of SAM: alkanethiols on gold.

Polarization depended NEXAFS to study self assembled monolayers

extremely easy to makedip gold substrate in mM solutionrinse in clean solvent

relatively stableunder ambient conditions ~hrs.Under N2 > 1 year

Mica or 5nm Ti on Si substrateAu(111)

Sulfur bound to gold

Van der WaalsInteractions between chainscause alignment, ordering

Headgroup of molecule changed for chemical functionality

“Hot” Applications

www.sciencemag.orgSCIENCE VOL289 18AUGUST2000

SCIENCE VOL 299 17 JANUARY 2003

60nm

60nm

switchable surfaces switching interlocking molecules -molecular electronics

trapping of proteins, viruses, etc.

Barry Cheung et. Al., LLNL, to be published

C(1s) NEXAFS of Organothiol SAMs

Collect and compare NEXAFS spectra at multiple angles of incidence

• Vary from grazing to normal angles of incidence between X-rays and sample

• Polarization dependent resonances denote well-defined orientation in the orbital of interest

• Peak direction in difference spectra provides a preliminary indication of functional group orientation

C(1s) NEXAFS for MUA SAM on Au(111)

Obtaining Molecular Orientation from C(1s) NEXAFS

Linear regression analysis provides a more quantitative measure of functional group orientation:→ Peak fitting protocols resolve convoluted resonances and provide peak

intensities

→ Linear regression analysis yields bond orientation to within ± 5°

Orientation of MUA on Au(111)?

Carboxyl Group:

Carboxyl group tilted ~ 45° from the Au(111) surface normal

Alkyl Chain:

Hydrocarbon backbone tilted by ~ 42°

T.M. Willey et. al., Langmuir, 2004, 20, 2746

NEXAFS Characterization of MBA SAMs on Au(111)

2-MBA 3-MBA 4-MBA

Quantum Confinement Effects in Semiconductor Nanocrystals (NCs)

Ph

oto

lum

ines

cen

ce/a

rb. u

nit

s

3.02.0Energy (eV)

Ab

sorb

ance/arb

. un

its

37 Å

45 Å

60 Å

85 Å

Semiconductor nanocrystals/‘Quantum dots’

→ Unique, size-dependent, optical and electronic properties

→ Diverse range of potential technological applications

Optoelectronic behavior explained in terms of quantum confinement effects:

Particle in a Box

Silicon nanocrystals are prepared and deposited in situ out of the gas-phase.

19 Å

111220

311

SAD

BF

AFM

C. Bostedt, et al., J. Phys. Condens. Matter 15, 1017 (2003).

TEM

1.0

0.8

0.6

0.4

0.2

0.0

Intensity [arb. units]

4.03.53.02.52.01.51.00.5

Size [nm]

1.1 nm 1.5 nm 2.3 nm 2.8 nm 14 A 15 A 16 A 17 A

Crystalline particles with narrow size distribution.Crystalline particles with narrow size distribution.

First, the size-dependent properties are investigated on sub-monolayer depositions of nanocrystals.

Dilute systems need element specific measurements

Film-morphology:Individual nanocrystals

Substrate:Surface-passivated

germanium

AFM:

X-ray absorption and Emission measurements show shift in valence and conduction band of isolated Si clusters

99 99.5 100 100.5 101 101.5 102 102.5 103

Intensity (Arb. Units)

Absorption Energy (eV)

Bulk Silicon

2 nm Clusters

1.6 nm Clusters

88 90 92 94 96 98 100

Intensity (Arb. Units)

Emission Energy (eV)

Bulk Si

2 nm Clusters

1.6 nm Clusters

Valence BandSoft x-ray emission of Si nanoparticles

Conduction Band Si 2p absorption from nanoparticles

Quantum Confinement in Nanoparticles Measured and Compared to Theory

T. van Buuren, L. Dinh, L. L. Chase, L. J. Terminello, Phys. Rev. Lett. 80, 3803 (1998)

2.4

2.2

2.0

1.8

1.6

1.4

1.2

4.03.02.01.0

Band Gap (eV)

Particle Size (nm)

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0.0

0.2

0.4

CB Shift (eV)

54321

VB Shift (eV)

Particle Size (nm)

CB and VB band edge shift as a function of particle size.

Band gap as a function of particle size

Ratio of CB to VB shift is 1:2

Germanium exhibits much stronger confinement effects than silicon.

3.5

3.0

2.5

2.0

1.5

1.0

Ban

dgap

[eV

]

3.02.52.01.51.0Particle Size [nm]

Ge - extr. band gap Ge - guide to the eye Si - extr. band gap Si - guide to the eye

C. Bostedt, T. van Buuren, APL (2004)

T. van Buuren, Phys. Rev. Lett. 80, 3803 (1998).

The band-gap of the Ge becomes larger than Si at particles sizes below 2.0 nmThe band-gap of the Ge becomes larger than Si at particles sizes below 2.0 nm

CdSe NCs: An Model System of Technological Importance

→ Readily synthesized with narrow size distributions

→ Exhibit size-dependent photoluminescence

→ Extensively studied

Archetypal nanocrystalline binary semiconductor for technological applications

Model system for the study of quantum confinement

Murray, et. al., J. Am. Chem. Soc., 115, 8706 (1993)

5nm

TEM Image of CdSe-TOPO

Synthesis of CdSe NCs

But…

Theories on electronic structure conflict with one another and experimental results

Cd L3-edge XAS: We can probe the bottom of the CB (vacant) DOS directly using this technique

Cd L3-edge XAS

L3-edge formally 2p → s, d empty states

• Bottom of CB comprised of Cd 5s states

• Hybridized pd states located ~ 4-5 eV above CB minimum

We find that only the ‘s’-states move as a function of particle size

J. Lee R. Meulenberg PRL 2007

Magnetic properties of materials can be studied by X-Ray Magnetic Circular Dichroism (XMCD) spectroscopy

Electronic transitions in conventional L-edge x-ray absorption (a), and x-ray magnetic circular x-ray dichroism (b,c), illustrated in a one-electron model.

The transitions occur from the spin-orbit split 2p core shell to empty conduction band states. In conventional x-ray absorption the total transition intensity of the two peaks is proportional to the number of d holes.

By use of circularly polarized x-rays the spin moment (b) and orbital moment (c) can be determined from linear combinations of the dichroic difference intensities A and B,

according to other sum rules. www-ssrl.slac.stanford.edu/dichroism/xas

Circular dichroism at the Iron L-edge

www-ssrl.slac.stanford.edu/dichroism/xas

If the photoelectron originates from the p3/2 level (L3 edge), the angular momentum of the photon can be transferred in part to the spin through the spin-orbit coupling.

Right circular photons (RCP) transfer the opposite momentum to the electron as left circular photons (LCP) photons, and hence photoelectrons with opposite spins are created in the two cases.

Since the p3/2 (L3) and p1/2 (L2) levels have opposite spin-orbit coupling, the spin polarization will be opposite at the two edges. In the absorption process, "spin-up" and "spin-down" are defined relative to the photon helicity or photon

spin.

Backup slides

NEXAFS Quantitative Orientation - Vector

Define polarization: 22

2

||||

||

sEpE

pEP

+=

Intensity is electric field and TDM dot product squared

[ ]2coscossinsin1sincossin αθαφαφθ PPPSI +−+=

Due to 3-fold or higher substrate symmetry,

∫ ∫ ∫ ∫ →→→→2

1)(sin,

2

1)(cos,0)sin(,0)cos( 22 φφφφφφφφ dddd

Squaring the dot product and averaging over azimuthal angle,

( )( ) ( ) ⎥⎦

⎤⎢⎣

⎡−+⎟

⎠

⎞⎜⎝

⎛ −−+= θ 222 sin12

11cos31cos3

2

11

3

1PPSI

For raw intensities, use the ratio method by fitting ( )),(

,

fixedI

I

=θα

θα to experimental data.

(From J. Stohr et. al., Phys. Rev. B, 1987, 36, 7891)

Through this method we can determine quantitatively how molecules in ultrathin organic layers are oriented on surfaces.

NEXAFS Quantitative Orientation - Plane

Define polarization: 22

2

||||

||

sEpE

pEP

+=

Intensity: square of projection of E onto plane or sin(ε)

[ ]22 )coscossinsinsincossin( γθγφγφθ ⋅⋅+⋅⋅+⋅−= EpEsEpESI

Due to 3-fold or higher substrate symmetry,

∫ ∫ ∫ ∫ →→→→2

1)(sin,

2

1)(cos,0)sin(,0)cos( 22 φφφφφφφφ dddd

Squaring the dot product and averaging over azimuthal angle,

( ) ( ) ( )⎥⎦

⎤⎢⎣

⎡−⋅⎟

⎠

⎞⎜⎝

⎛ ⋅++⋅⎟⎠

⎞⎜⎝

⎛ −⋅−⋅−⋅= PPSI 1cos2

1

2

11cos31cos3

4

11

3

2 222 γγθ

For raw intensities, use the ratio method by fitting ( )),(

,

fixedI

I

=θα

θα to experimental data.

(From J. Stohr et. al., Phys. Rev. B, 1987, 36, 7891)

Through this method we can determine quantitatively how molecules in ultrathin organic layers are oriented on surfaces.

NEXAFS Quantitative Orientation - Difference

Use vector or plane intensity:

Take difference spectra between two incident angles

( )( ) ( )⎥⎦

⎤⎢⎣

⎡−⎟

⎠

⎞⎜⎝

⎛ ++⎟⎠

⎞⎜⎝

⎛ −−−= PPSI 1cos2

1

2

11cos31cos3

4

11

3

2 222 γγθ

Through this method we can determine quantitatively how molecules in ultrathin organic layers are oriented on surfaces.

vector:

plane:

( )( ) ( ) ⎥⎦

⎤⎢⎣

⎡−+⎟

⎠

⎞⎜⎝

⎛ −−+= θ 222 sin12

11cos31cos3

2

11

3

1PPSI

),(),(),,( abba IID θθθθ −=

Determine parameter SP from a reference sample with known tilt

Solve for α or γ as all parameters are now known.

In either case, abbaD θθθθ 22 coscos),,( −∝

Run linear regressions of D vs. ab θθ 22 coscos − with multiple spectra