PHOTOELECTRON SPECTROSCOPY CIRCE: PHOTOELECTRON SPECTROSCOPY Lucia Aballe Michael Foerster Virginia Perez Carlos Escudero Jordi Prat

overview basic principles some instrumentation x-ray photoelectron spectroscopy - chemical analysis photoelectron microscopy - chemical mapping Near ambient pressure photoemission

basic principles PHOTOELECTRIC EFFECT: photon in – electron out electrons absorb photon energy and “escape” K L M e- e- e- Ekin= hn – Ebinding- F hn monochromatic discovery photoelectric effect: Hertz 1886, Einstein 1905 development of XPS: 1960’s K. Siegbahn (1981 Physics Nobel Prize) Auger decay: EKLM= EK-EL-EM

basic principles vacuum analyzer sample hn - fs fs fa measured spectrum Ekin = hn - Eb - fa valence band EF hn sample analyzer vacuum core level fs fa surface hn - fs

theory of photoemission Fermi´s Golden Rule for N-particle states: N-electron ground state of energy EN, 0 N-electron excited state of energy EN, s (N-1 electrons in the solid and one free photoelectron of momentum k and energy  ) main term dipole transition operator normally simplified one-electron approximation dipole transition Δ l = ±1, Δs = 0 (allowed transitions: 1s → 2p, 2p → 3d, 3d → 4f,…)

relevant energies binding energies 0 eV (Fermi level) to several keV (deep shells) non interacting chemical bonding

cross sections cross sections are energy and Z dependent h o t n e r g y ( V ) C u Z = 2 9 R a l i m - f c 1 3 4 5 6 barn/atom atomic dipole calculation ( 1 barn = 10 -24 cm2 ) cross sections are energy and Z dependent absorption more likely than scattering in soft x-ray region photo-ionization cross section is core level dependent

to remember known photons energy, polarization, incidence angle + analysis photo-emitted electrons energy, angle, spin conservation rules energy, momentum ” sample properties composition, chemical state, depth profile, electronic and magnetic properties,…

x-ray photoelectron spectroscopy XPS wide scan XPS wide scan also Electron Spectroscopy for Chemical Analysis identification of species each core atomic orbital has a characteristic binding energy each element/compound has a characteristic spectrum Different elements can have similar energies. Some tricks Check for different core levels Check for Auger lines Change photon energy to check cross section - Spin orbital splitting and peak area ratios

(Ba,Sr)TiO3 thin film magnetron sputtering

(photon attenuation negligible within photoelectron escape depth l) xps: surface sensitivity surface sensitivity short mean free electron path in solids limited escape depth information comes from surface layer (~3l) I(z)=I0exp-z/l.cosq ~constant (photon attenuation negligible within photoelectron escape depth l) surface sensitivity can be varied changing hn and thus Ekin changing the geometry

quantification: Ii depends on Ii = ni σi λi K quantity of i atoms differential cross-section of relevant level of species I escape probability of electrons instrumental (photon flux, detection efficiency, acceptance solid angle) hn choice Different types of backgrounds , the last one with a smooth step is the Shirley Background

other spectral contributions electrons loose energy on the way to the surface ’ background ’ plasmon loss peaks electrons are emitted via Auger decay (fixed Ekin!) deeper = more collisions Ekin = EK - EL1 - EL3 ion left in excited state ’ “shake-up” satellite peaks

xps: chemical shifts binding energy depends on: oxidation state 3 20 315 310 305 reduced RhCl /Al 2 O impregnated XPS Rh 3d region 3d 5/2 3/2 315 binding energy depends on: oxidation state local chem-phys environment qualitatively: higher oxidation state less core shielding extra coulomb interaction between photo-electron & ion core ’ higher binding energy more subtle final state effects also cause shifts binding energy (eV)

xps: spin-orbit splitting initial state binding energy (eV) DESO = 2.18 eV I ratio= 4:3 4f7/2 4f5/2 W 4f 5p3/2 25 45 35 or final state without spin-orbit interaction spin-orbit interaction lifts degeneracy ΔESO “spin-orbit doublet” splitting and intensity ratio largely independent of compound degeneracy = 2j+1 determines probability of each final state

xps: peak shape deconvolution each core level peak might contain different components non-equivalent atomic environments fit each component typically convolution of : lorentzian intrinsic width (final state lifetime, temperature) gaussian experimental width (incident radiation, analyzer resolution) asymmetry electron-electron interaction – metal-like compounds C 1s Nylon 6 3 1 2 4 —(CH2(CH2)3CH2—C—NH)n— II O 1 4 3 2 Al2p www.casaxps.com

Surface core levels shifts: high resolution core level spectroscopy Atoms at surfaces have lower atomic coordination than bulk atoms. This leads to different core level binding energies T B hν=70 eV res. 80 meV 4f7/2 Lower atomic coordination: lower binding energy

Edge atoms in stepped surface may be identified

Surface core level shifts allow to identify chemisorption sites: S. Lizzit et al. (Elettra SuperEsca) Ru 0001 3d5/2 Binding energy eV

basic instrumentation analyzer energy-filter electrons detector multiply & count electrons channeltron pre-lenses decelerate & focus electrons - resolution - transmission 2D micro-channel plate sample environment typically UHV - electron path - sample purity delayline high resolution: narrow slit, large R, low E0 (shielding, power supplies) sometimes flood gun to avoid charging other analyzers: cylindrical mirror analyzer, time of flight

CIRCE beamline T G M3a and M3b KB M1 M2 monochromator grating illuminated by a V collimating beam: no E slit M1 : deflecting mirror and collimating (sagittal cylinder) M2: plane deflecting mirror in mono G: Gratings : LE (700 l/mm) : 100-600 eV, ME(900 l/mm): 500-1500 eV, HE(1200 l/mm) 900-2000eV Refocusing and branching mirrors Exit slit Refocusing to sample : KB and toroidal Mono scan: rotate M2 and grating Gratings water cooled

CIRCE optics performance

Stability of the microscope and optics mounted on a monolithic structure Vibrations at the sample manipulator of the microscope

CIRCE summary performances PEEM NAPP

BL24, CIRCE: Variable polarization soft X-ray beamline for Photoemission Electron Microscopy and Near Ambient Pressure Photoemission spectroscopy PEEM NAPP

Photoemission Microscope and Low Energy Electron Microscope LEEM: e- backscatered from sample form a magnified image. Structural info. Res : 10 nm XPEEM: several laterally resolved operation modes. ΔE= 0.15 eV; Sample at 15-20 keV and 2 mm : discharges. Microchannel plate intensifier and phosphorus screen and CCD

PEEM sample environment Four axes manipulator , sample hating (e- bombard), cooling (~), p~ 10 -10 mbar Sample beam geometry: Azimuthal rotation possible ; incidence angle : 16 deg (grazing) up to 74 deg. Vertical pol. Horiz. Pol Circular pol.

In the last years, several developments of non-standard sample environments: Magnets for XMCD M. Foerster, L. Aballe

(1/2, 0) diffracted beam from Si (100) 2x1 reconstruction. Examples of LEEM: (1/2, 0) diffracted beam from Si (100) 2x1 reconstruction. Dark field image showing monoatomic steps. In the red profile hat shows the steps, two lines indicate 10 nm separation. Electron energy 4.6 eV Anneal SiC to form graphene layers Reflectivity curves at selected positions show characteristic features of graphene single layer, two and three layers

PEEM examples Silicon wafer covered with Au nanoparticles after a special thermal treatment. Only below the Au Si is oxidized

PEEM applications LEM “Moons” : SrO termination

XMCD laterally resolved . Permalloy nanostructures on Si. Saturate the sample and measure in remanence Fe L3 elemental image Fe XMCD contrast Xrays m

Near Ambient Pressure Photoemission Electrons need vacuum to travel far away (UHV photoemission and storage rings). In air or gases , the mean free path is quite small 200 400 600 800 1000 2 4 6 Elastic Mean Free Path in Oxygen Gas Electron Kinetic Energy [eV] Mean Free Path [Torr-mm] For 500 eV electrons: P ~ 4 Torr for a 1 mm travel P ~ 45 Torr for a 0.1 mm travel In spite of this in 1985 someone already overcome this difficulty

Solution: Capture electrons before they collide with gas molecules by means of differential pumping h e- gas previous designs: conventional X-ray source new design: X-rays from synchrotron Use electrostatic focusing !

“ambient” pressure photoemission

The usage of NAPP is increasing a lot in the last years : Gas-solid interfaces extensively Investigated H. Bluhm et al.

near ambient pressure PES Dissolution of KBr salts Ghosal et al. Science (2005) ALS, BL 9.3.2 relative humidity (RH) increased from 5% to deliquescence point an abrupt raise of the Br/K XPS peaks areas takes place

αPtO2 has also been suggested to exist after oxygen exposure on Pt110 based on ambient photoemission and calculations : O chemisorbed bulk Pt Oxide (αPtO2) 0.5 Torr 473K (DFT calculated shift) 10 -6 Torr CO on O precovered Pt110 (0.5 Torr O2 exposure) Bhlum et al ALS Exposure at room T of a clean Pt110 surface to O2 reveals that at pressures of 50 torr the photoemission 4f peak has an extra component attributed to chemisorbed oxygen. Additional Exposure to 0.5 Torr causes a.nother peak (red) assigned to alpha PtO2 on the basis of calculations. The oxide peak was also bserved on Pt nanoparticles synthetized from solution chemistry . Particles of 1.5 nm shoed an oxide peak as major component of the Pt 4f. The pre-covered oxygen surface was prepared with X ray beam assisted. CO exposure at T= 270-275 K causes decrease of the chemisorbed and oxide concentration and increase of the on top CO. top CO

CH3CH2OH + 3H2O 6H2 + 2CO2 Pd Rh CeO2 RhPd ? RhPd CeO2

Model catalyst RhPd/CeO2: Δ

Oxidized metal atoms in supported metal particles have been seen many cases. Example: CH3CH2OH + 3H2O 6H2 + 2CO2 RhPd particles, supported on CeO2 and unsupported Photoemission. T: 550 ⁰C P: 0.05 mbar The reactivity is higher for the supported catalyst. Pd is oxidized under reaction conditions Here are Pd, Rh and O spectra taken at 550C under reaction conditions (ethanol hydrogenation). The first group at the top part are results for the unsupprted RhPd particles (unsupported means that they are lying on a W foil) , at the lower part are the results for particles supported on CeO2. Different photon energies were used to explore the composition of the particles. It was found that the particles have an excess of Pd atoms at their surface. The spectra of Pd show that the supported particles are partially oxidized (blue peaks). This is not the case for the unsupported particles. The Rh spectra also show an oxide peak For the supported particles the oxygen spectra shows the presence of OH groups in the supported particles. The oxidation of Pd and Rh shows that the CeO2 is an active part of the catalysis providing oxygen to the metals. The oxidized metals are likely the active particles for the reaction

reaction 550ºC red ox O-Ce O-H O-M Pd 3d Rh 3d O 1s CeO2 RhPd 875 eV

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