Characterization of phonon modes by XPD on a (2×2) s -V 2 O 3 layer on Pd(111)

Frascati, December 13, 2012.

Characterization of phonon modes by XPDCharacterization of phonon modes by XPDon a (2×2) on a (2×2) ss-V-V22OO33 layer on Pd(111) layer on Pd(111)

Mauro SambiMauro Sambi

Dipartimento di Scienze ChimicheDipartimento di Scienze ChimicheUniversità di PadovaUniversità di Padova

Via Marzolo 1Via Marzolo 135131 Padova35131 Padova

[email protected]@unipd.it


“despised and rejected by men” (Isaiah 53, 3)


DFT modelDFT modelSTMSTM

(2×2) surface-V(2×2) surface-V22OO33/Pd(111) - structural determination/Pd(111) - structural determination

M. Sambi, M. Petukhov, B. Domenichini, G. A. Rizzi, S. Surnev, G. Kresse, F. P. Netzer and G. GranozziSurf. Sci. 534 (2003) L234.

XPD FSXPD FS

V 2p, SSC-SW based on DFTV 2p, SSC-SW based on DFT V 2p, expt. KE=972 eVV 2p, expt. KE=972 eV

zzV-OV-O(expt.)=0.72±0.07 Å(expt.)=0.72±0.07 Å

zzV-OV-O(DFT)=0.723 Å(DFT)=0.723 Å


Prehistory: Formate adsorption on Cu (100)Prehistory: Formate adsorption on Cu (100)

a)a) Cross Bridge (CB)Cross Bridge (CB)b)b) Diagonal Atop (DA)Diagonal Atop (DA)c)c) Short Bridge (SB)Short Bridge (SB)

M. Sambi, G. Granozzi, M. Casarin, G. A. Rizzi, A. Vittadini, L. S. Caputi and G. Chiarello: Surf. Sci., 315 (1994) 309.

O-C FSO-C FS


Quantitative determination: SSC-SW simulationsQuantitative determination: SSC-SW simulations

dd(Cu-O)(Cu-O) = 1.95±0.05 Å = 1.95±0.05 Å

<<(OCO)(OCO) = 129°±5° = 129°±5°

ω ≈ 70 cmω ≈ 70 cm-1-1


Back to (2×2) Back to (2×2) ss-V-V22OO33/Pd(111) – FS peaks azimuthal broadening/Pd(111) – FS peaks azimuthal broadening

M. Sambi, M. Petukhov, B. Domenichini, G. A. Rizzi, S. Surnev, G. Kresse, F. P. Netzer and G. GranozziSurf. Sci. 534 (2003) L234.

V 2p, SSC-SW based on DFT,V 2p, SSC-SW based on DFT,simple DW attenuationsimple DW attenuation

V 2p, expt. KE=972 eVV 2p, expt. KE=972 eV


Soft phonon mode involving in-plane displacementsSoft phonon mode involving in-plane displacementsof O scatterers with respect to V emitters?of O scatterers with respect to V emitters?

DFT prediction:DFT prediction:

ωω=14.7 cm=14.7 cm-1-1!!

M. Sambi, S. Surnev, G. Kresse, F. P. Netzer and G. Granozzi, Phys. Rev. B68 (2003) 155417

(2×2) (2×2) ss-V-V22OO33/Pd(111) – a vibrational study/Pd(111) – a vibrational study

θ = 68°


HARMONIC OSCILLATOR MODELHARMONIC OSCILLATOR MODEL

γγ - angular displ. from equilibrium - angular displ. from equilibriumωω - frequency - frequencyNNνν - normalisation constant - normalisation constant

HHνν - ν - νthth order Hermite polynomial order Hermite polynomial

Boltzm

ann-weighted sum

γ


γ

The convolution procedure


The smallest cluster: 4 atoms

ωω=50±10 cm=50±10 cm-1-1


The effect of correlated atomic displacements - the largest cluster: 73 atoms

ωω=45±30 cm=45±30 cm-1-1


ωω=40±25 cm=40±25 cm-1-1

The optimal cluster: 34 atoms


The linear oscillator model


Conclusions

- Anisotropic deformation of FS features in XPD scans are indicative of the presence of soft phonon modes involving the emitters and/or the scatterers of the photoelectron wave;

- Such modes cannot be accounted for by isotropic DW attenuation;

- A simple model based on a harmonic torsional/linear oscillator has been adopted to account for the anisotropic deformation (azimuthal broadening vs polar narrowing) of V – O FS features in a s-V2O3 monolayer film;

- The frequency of oscillation obtained by fitting the XPD curves with the model is of the same order of magnitude of the DFT computed one;

- In a periodically ordered overlayer, the extent of anisotropic deformation of FS peaks depends on the coherence length of the vibrational mode as seen by the photoelectron emitter, which is limited to 2 – 4 coordination circles around the emitter.


THANK YOU FOR YOUR ATTENTION!


XPD – simulations: checking non-structural parametersXPD – simulations: checking non-structural parameters

- SS C-SW simulations (Fadley code). MS was checked to be negligible (Van Hove MSCD code) at the outgoing photoelectron KE (952 eV);

- Simulations are substrate-insensitive and barely sensitive to the presence of V scatterers coplanar with the V emitters: FS dominates; first order features arise from V -> NNN O atoms.

- Simulations are largely insensitive to the choice of the inelastic attenuation length in a broad range (5 – 100 Å), due to the essentially non-attenuating nature of the oxide monolayer.

- The same for the choice of the inner potential, due to the high photoelectron KE.

- Scattering phase shifts were calculated by MUFPOT. MT radii were varied in a reasonable interval to check their effect of FS peaks FWHM – negligible.

Characterization of phonon modes by XPD on a (2×2) s -V 2 O 3 layer on Pd(111)

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