Hard X-Ray Photoelectron Spectroscopy (HAXPES)

and Ambient Pressure XPS (APXPS)

Jürg Osterwalder,

Physics Department, University of Zürich, Switzerland

School on Synchrotron and Free-Electron-Laser Based Methods: Multidisciplinary Applications and Perspectives ICTP Trieste and ELETTRA, April 4-15, 2016

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Literature:

Special Issue “Recent advances in Hard X-ray Photoelectron Spectroscopy (HAXPES)”, J. Electron Spectrosc. Relat. Phenom. 190, Part B, Pages 125-314 (2013)

“Some future perspectives in soft- and hard- x-ray photoemission”, C. S. Fadley and S. Nemšák, J. Electron Spectrosc. Relat. Phenom. 195, 409–422 (2014).

… and literature therein.

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Contents:

• Hard x-ray photoelectron spectroscopy (HAXPES) • Case studies using HAXPES • Photoelectron diffraction with hard x-rays (HXPD) • X-ray standing wave photoelectron spectroscopy (XSW-PS) • Ambient pressure XPS (APXPS, APPES, NAPP) • XSW and APXPS combined (SWAPPS)

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The primary reason for higher energies

Data from Tanuma, Powell, Penn, Surf. Interf. Anal. 43, 689 (2011).

C. S. Fadley and S. Nemšák, J. Electron Spectrosc. Relat. Phenom. 195, 409–422 (2014).

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•  More bulk sensitive spectra: a versatile tool for any new material, multilayer nanostructure, or nano-/meso- scale device

•  Easier quantitative analysis: IMFPs & analyzer trans. ∼ constant •  Less inelastic background/Auger interferences •  Less elastic scattering/surface scattering in ARXPS depth profiling •  HXPD Kikuchi bands: atomic structure, dopants, lattice distortions •  Valence-level DOS-like spectra (RT) or HARPES •  Higher ambient pressures at sample •  Standing wave, total reflection effects

The many reasons for higher energies

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Price to pay: low photoionization cross sections

Relativistic subshell cross sections for Mn and O As a function of photon energy over the region 1-10 keV.

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Example: valence band of Ag

Ag 4d

Ag 5s,p

HAXPES: • no cleaning necessary (<5% surface signal) • s and p relative contribution increased • DOS information Data from G. Panaccione,

M. Grioni / VOLPE, ESRF

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Photoemission cross sections: beyond the dipole approximation

R. Guillemin et al., Rad. Phys. Chem. 75, 2258 (2006).

Photoemission matrix element

Dipole approximation

Cross section

becomes zero for

Include next higher term:

photon polarization

electron momentum

photon propagation

n,l

n,l

n,l

n,l

Dipolar angular distribution parameter

hν = 4 keV – wave length 3.1 Å

9 C. S. Fadley and S. Nemšák, J. Electron Spectrosc. Relat. Phenom. 195, 409–422 (2014).

These differential cross sections can be calculated

Mn 3d orbitals (hν = 833 eV)

code available (Fadley group)

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Contents:

• Hard x-ray photoelectron spectroscopy (HAXPES) • Case studies using HAXPES • Photoelectron diffraction with hard x-rays (HXPD) • X-ray standing wave photoelectron spectroscopy (XSW-PS) • Ambient pressure XPS (APXPS, APPES, NAPP) • XSW and APXPS combined (SWAPPS)

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HAXPES study of CrO2

M. Sperlich et al., PRB 87, 235138 (2013).

Rutile structure Co O

Valence spectra: SXPS sees Fermi edge HAXPES no Fermi edge

CrO2 is expected to be a half-metallic ferromagnet.

XPS gives controversial results, depending on the photon energy

SXPS: Fermi edge as expected

HAXPES: no Fermi edge ???? Should not be affected by surface effects!

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HAXPES study of CrO2

M. Sperlich et al., PRB 87, 235138 (2013).

O 1s

O 1s spectra: •  SXPS shows contamination from surface phases (Co2O3, adsorbed O) • HAXPES shows clean spectrum, with a strong metallic asymmetry and weak Co 3d – O 2p charge transfer satellite

Cr 2p spectra: •  SXPS shows weak shoulder on the low binding energy side. • HAXPES shows pronounced peak at this position: well-screened final state is representative for bulk

Cr 2p

=> Bulk CrO2 is metallic ! Then why do we see no Fermi edge ?

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Momentum conservation in HAXPES Doppler effect on Auger emission from a free atom:

Ne KLL: 1s-1Ne+2 2p-2 (1D2) + Auger electron

M. Simon et al., Nat. Commun. 5, 4069 (2014).

Photoelectron is emitted predominantly towards or away from spectrometer (polarization) => Momentum of Ne Auger emitter either adds or is subtracted to Auger electron energy.

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HAXPES study of CrO2: beware of recoil effects !

Y. Takata et al., PRB 75 233404 (2007). Y. Takata et al., PRL 101, 137601 (2008).

The recoil energy of the emitted photoelectron is transferred to a single ion in the solid, even in the case of valence emission. (momentum conservation, phonon bath)

⇒  The photoelectron kinetic energy is accordingly reduced, the Fermi edge apparently shifted to ‘higher binding energy’.

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Contents:

• Hard x-ray photoelectron spectroscopy (HAXPES) • Case studies using HAXPES • Photoelectron diffraction with hard x-rays (HXPD) • X-ray standing wave photoelectron spectroscopy (XSW-PS) • Ambient pressure XPS (APXPS, APPES, NAPP) • XSW and APXPS combined (SWAPPS)

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Hard X-ray photoelectron diffraction (HXPD)

A. Winkelmann et al., New J. Phys. 10, 113002 (2008).

Evolution of XPD patterns as a function of kinetic energy: diamond(111)

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Hard X-ray photoelectron diffraction (HXPD)

Multiple scattering cluster simulation

4.1 nm SiO2/Si(001) 7.0 nmSiO2/Si(001)

Native

SiO2 / Si(001)

XPD through amorphous overlayers

Monochromatized Cr Kα1 lab source (hν = 5414 eV)

M. Kobata et al., Anal. Sci. 26, 227 (2010).

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Lab Instrument for Hard X-ray photoelectron diffraction (HXPD)

K. Kobayashi Laboratory

M. Kobata et al., Anal. Sci. 26, 227 (2010).

Cr Kα1 x-ray source

Objective lens

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Contents:

• Hard x-ray photoelectron spectroscopy (HAXPES) • Case studies using HAXPES • Photoelectron diffraction with hard x-rays (HXPD) • X-ray standing wave photoelectron spectroscopy (XSW-PS) • Ambient pressure XPS (APXPS, APPES, NAPP) • XSW and APXPS combined (SWAPPS)

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X-ray optical effect: standing waves near Bragg condition

rocking curve or energy scan

on-Bragg

off-Bragg

0.0017°

Bragg’s law:

X-ray standing wave (XSW)

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Amplitude of x-ray field modulates photoemission intensity

rocking curve or energy scan

reflectivity

photoemission signal

Example: A XSW study of the adsorption Of methylthiolate on Au(111)

M. G. Roper et al., Chem. Phys. Lett. 389, 87 (2004).

-H energy scan

S 1s

Au MNN

Triangulation: • S in atop site, • Au-S distance: 2.5 Å

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The standing wave wedge (SWEDGE) method

C. S. Fadley, J. Electron Spectrosc. 178-179, 2 (2010)

Bragg’s law:

Larger lattice spacing d ⇒  Longer wavelength of the standing wave

SWEDGE: • artificially grown multilayers as superlattices • grow wedge structure on top of this structure • grow sample on top of wedge

⇒  Laterally probing the sample along the wedge allows to shift the SW maximum through the interface

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The standing wave wedge (SWEDGE) method

C. S. Fadley, J. Electron Spectrosc. 178-179, 2 (2010)

Cr 3p / Fe 3p ratio for two types of scans:

Photoelectrons predominantly from the Cr or the Fe side of the interface !

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Application to a buried TMR interface (soft x-rays) Tunneling magneto- resistance

hν = 1000 eV

B 1s

C. S. Fadley, J. Electron Spectrosc. 178-179, 2 (2010)

CoFe – Al2O3 - CoFe

(FM – I – FM)

TMR enhanced with amorphous CoFeB buffer layer => Segregated boron layer at interface

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Application to a buried TMR interface (hard x-rays)

C. S. Fadley, J. Electron Spectrosc. 178-179, 2 (2010)

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Contents:

• Hard x-ray photoelectron spectroscopy (HAXPES) • Case studies using HAXPES • Photoelectron diffraction with hard x-rays (HXPD) • X-ray standing wave photoelectron spectroscopy (XSW-PS) • Ambient pressure XPS (APXPS, APPES, NAPP) • XSW and APXPS combined (SWAPPS)

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Ambient pressure XPS

X-ray source

Electrostatic transfer lens • focus at original sample position • massive differential pumping

D. F. Ogletree et al., Rev. Sci. Instrum. 73, 3872 (2002)

Goal: sample environment: pressures up to several tens of mbar Why: • study solid-gas interfaces at higher pressures • study solid-liquid interfaces (vapor pressure) Problems: • inelastic attenuation of photoelectrons in ambient pressure • electron energy analyzer / detector requires UHV

28 H. Bluhm et al., J. Phys. Chem. B 108, 14340 (2004)

Methanol oxidation on a copper catalyst Surface: Cu foil at 400°C

Total pressure: 0.6 mbar (O2 and methanol)

methanol:O2 ratio

O 1s region

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Methanol oxidation on a copper catalyst

The Cu surface at a methanol:O2 ratio = 3:1

H. Bluhm et al., J. Phys. Chem. B 108, 14340 (2004)

Valence region

C 1s region

O 1s region

CHx methanol fragments on the surface Only at ratio 6:1

Unique to this method: Gain information on the catalyst surface

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Oxidation of Au with ozone

A. Y. Klyushin et al., PCCP. 16, 7881 (2014).

Au foil at 100°C ozone pressure of 0.3 mbar

Oxide layer present only during ozone exposure ! And only at temperatures below 300°C.

… and heating during ozone exposure

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Thickness of the Au oxide layer

Vary the photon energy Oxide vs. metal ratio

=> Thickness 2.9 Å

A. Y. Klyushin et al., PCCP. 16, 7881 (2014).

This system can only be studied under ozone ambient pressure !

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APXPS at solid liquid interfaces

S. Axnanda et al., Sci. Rep. 5, 09788 (2015).

Phenomena at the solid-liquid interface: solvation, Helmholtz layer, catalysis, … Buried interface, difficult to study with XPS, but …

Optimizing the photon energy: Simulation of a system with 1 nm Fe on Si + amorphous carbon layers of various thicknesses …

Representing 1 nm Helmholtz layer underneath a typical water layer

Place a beaker of electrolyte into the vacuum chamber

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operando electrochemical XPS

In situ electrochemical cell Water vapor pressure ~ 25 mbar

working- reference- counter -

electrode

Prior to immersion: O 1s shows signals from water vapor and from a condensed water film.

During immersion: Measure cyclic voltammograms, Sample can be cleaned.

After immersion: Liquid water film pulled out of the liquid, Potential control of liquid water and vapor.

S. Axnanda et al., Sci. Rep. 5, 09788 (2015).

hν = 4 keV

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Surface analysis under potential control

S. Axnanda et al., Sci. Rep. 5, 09788 (2015).

pote

ntia

l sw

eep

reducing

oxidizing

neutral

• oxidation of Pt • no shift (grounded)

Species from the liquid:

• Shifts follow the potential of the reference electrode • O 1s from water vapor follows the potential of the liquid

Needs a continuous water film that is pulled out of the liquid

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Contents:

• Hard x-ray photoelectron spectroscopy (HAXPES) • Case studies using HAXPES • Photoelectron diffraction with hard x-rays (HXPD) • X-ray standing wave photoelectron spectroscopy (XSW-PS) • Ambient pressure XPS (APXPS, APPES, NAPP) • XSW and APXPS combined (SWAPPS)

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Combination of XSW and APXPS methods: SWAPPS

For studies of the electrochemical double layer at the surface

S. Nemsak et al., Nat. Commun. 5, 5441 (2014).

Oxygen species:

And their rocking curves:

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Vertical distribution of cations (Cs and Na)

S. Nemsak et al., Nat. Commun. 5, 5441 (2014).

Rocking curves show a shift of 0.04° between the two cations

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Sensitivity of rocking curves to vertical distribution

S. Nemsak et al., Nat. Commun. 5, 5441 (2014).

X-ray optical calculations using YXRO code

Water delta layer at various positions above the oxide surface.

https://sites.google.com/a/lbl.gov/yxro/home

Cs+ about 0.4 Å further away than Na+ ions

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Quantitative analysis of SWAPPS data

S. Nemsak et al., Nat. Commun. 5, 5441 (2014).

Fitting the rocking curves with Individual concentration profiles

Resulting best fit profiles

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Sensitivity to the local potential

Calculated standing wave profile of electric field intensity |E2|

XPS spectra show also shifts of binding energies (0.2 – 0.4 eV)

S. Nemsak et al., Nat. Commun. 5, 5441 (2014).

⇒  Likely one sees the profile of the local electrostatic potentia within the Helmholtz layer.

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Solid-liquid interfaces: wide open field of studies

Proposal for a solid-liquid APXPS chamber at the Swiss Light Source

J. Osterwalder, J. van Bokhoven, M. Ammann, April 2016

If funded, there will be positions for 1 senior postdoc and 1 PhD student