XAS analysis on nanosystems

Transcription

1 XAS analysis on nanosystems Félix Jiménez-Villacorta Spanish CRG Beamline (SpLine) ESRF XLIV Zakopane School of Physics Breaking Frontiers: Submicron Structures in Physics and Biology May

2 XAS analysis on nanosystems Outlook XAS as a tool - X-ray Absorption Spectroscopy and Materials Science - XAS in nanosystems (nanoparticles, nanolayers, ) XAS analysis in nanostructured systems - EXAFS vs XANES - EXAFS analysis: particle dimensions - XANES analysis: electronic structure 2

3 XAS analysis on nanosystems Outlook XAS as a tool - X-ray Absorption Spectroscopy and Materials Science - XAS in nanosystems (nanoparticles, nanolayers, ) XAS analysis in nanostructured systems - EXAFS vs XANES - EXAFS analysis: particle dimensions - XANES analysis: electronic structure 3

4 XAS and Materials Science X-ray absorption spectroscopy I = I 0 exp[-μt] μt = ln [ I 0 / I ] 2.5 V K-edge ln(i0/i1) XANES EXAFS Energy (ev) 4

5 XAS and Materials Science EXAFS: Extended X-ray Absorption Fine Structure Interference between photoelectron ejected and backscattered (by nearest neighbours). Structural information Mainly single scattering. Local order geometry around an absorbing atom. 5

6 XAS and Materials Science XANES: X-ray Absorption Near-Edge Spectroscopy Transition core level unoccupied states Electronic configuration. Local-projected density of states. Intrinsic information of short-range structure Multiple scattering. Pertubated electronic structure f E F Absorption cross section g Core hole 6

7 XAS and Materials Science Detection modes: Transition core level unoccupied states Electrons ħω F, I F Fluorescence ħω, I 0 ħω, I Transmission ħω, I Elastic or inelastic scattering Resonant scattering f Electrons TEY E F Transmission TM g Fluorescence FY 7

8 XAS and Materials Science XAS classical detection modes: Transmission mode (TM): μt= ln (I 0 /I) Fluorescence yield mode (FY): μ~i f /I 0 Total electron yield (TEY): μ~i/i 0 I-V 8

9 XAS and Materials Science What is it XAS useful? Materials Solid state physics: hole-doped systems (perovskites), hybridizations (intermetallic alloys) Amorphous materials Diluted systems: aqueous (ionic, metalloproteins, ) Diluted systems: solid state (DMS, doped-sc, ) Nanoparticles & nanolayers: finite size and surface effects (catalysis, nanomagnetism, ) 9

10 XAS analysis on nanosystems Outlook XAS as a tool - X-ray Absorption Spectroscopy and Materials Science - XAS in nanosystems (nanoparticles, nanolayers, ) XAS analysis in nanostructured systems - EXAFS vs XANES - EXAFS analysis: particle dimensions - XANES analysis: electronic structure 10

11 EXAFS in nanosystems Materials science: control (tunability) in preparation and characterization of materials at the nanometric scale Total number of atoms 13 Surface atoms (%) 92 Nanoscience!! XAS as local probe - short-range structure - electronic configuration S.C. Tjong and H. Chen, Mat. Sci. Eng. R 45, 1 (2004) 35 Applied to nanosystems -Finite size effects - Surface effects 11

12 EXAFS in nanosystems Nanoparticles & thin films: loss of coordination χ( k) = S 2 0 j N kr j 2 j e 2 2 j ( 2k σ ) e ( 2R j / λ ( k )) f j ( k) sin(2kr j + Φ j ( k)) k-weighted EXAFS signal (Å -1 ) in-situ oxidized non-oxidized Fe foil T S =300K T S =250K T S =200K T S =170K Fe foil T S =300K T S =250K Fourier Transform Magnitude non-oxidized in-situ oxidized Ts=300 K Fe foil T S =300K T S =250K T S =200K T S =170K Ts=170 K Fe foil) T S =300K T S =250K T S =200K T S =170K Granular Fe thin films Grain size vs substrate temperature T S =200K T S =170K Wavenumber (Å -1 ) Distance (Å) Reduction Fe-Fe 1 st & 2 nd neighbours: decreasing grain size F. Jiménez-Villacorta, A. Muñoz-Martín and C. Prieto, J. Appl. Phys. 96, 6224 (2004). 12

13 EXAFS in nanosystems Nanoparticles & thin films: loss of average coordination number Spherical particles -R.B. Greegor, F.W. Lytle, J. Catal., 63, 476 (1980). -M. Borowski, J. Phys. IV (France), 7, C2-259 (1997). - I. Arcon, A. Tuel, A. Kodre, G. Martín, A. Barbier, J. Synch. Rad., 8, 575 (2001). Hemispherical (droplets) -A.I. Frenkel, C.W. Hills, R.G. Nuzzo,, J Phys. Chem., 105 (51),12689 (2001). -J.C. Cezar, H.C.N. Tolentino, M. Knobel, Phys. Rev. B, 68 (5), 4404 (2003). 13

14 EXAFS in nanosystems Spherical nanoparticles I. Arčon, A. Tuel, A. Kodre, G. Martin, A. Barbier, J. Synchrotron Rad., 8, (2001), p Columnar growth (nanocrystalline thin films) d d R R-d < Ñ 10 d 3 d 3 d d >= R R α R R 2 2 Normalized average CN Sphere h=2r h=5r h=9r Average Radius ((α/2) 1/3 R) (Å) -F. Jiménez-Villacorta, A. Muñoz-Martín and C. Prieto, J. Appl. Phys. 96, 6224 (2004). 14

15 XAS in nanosystems EXAFS: difficult to see, easy to analyze quantitatively: Data treatment to obtain the EXAFS signal. Analytical equation (approach) for single scattering processes. χ( k) = S 2 0 j N kr j 2 j e 2 2 j ( 2k σ ) e ( 2R j / λ ( k )) f j ( k) sin(2kr j + Φ j ( k)) XANES: easy to see, difficult to analyze quantitatively: Little data treatment (larger signal). No simple equation for XANES. Many-body processes. 15

16 XANES in nanosystems Qualitative XANES Information obtained by XANES: 3 regions Pre-edge Bond half-filled states Hybridization Local geometry Edge Determination of E 0 : Oxidation state White line: Charge transfer d-band splitting (sometimes: L-edge) XANES region Intermixing electronic structure & multiple scattering 16

17 Quantitative XANES analysis Quantitative XANES XANES: a probe of electronic structure above the Fermi level. Comparison with other band structure calculation methods (based on LCAO and full periodic potentials): tight-binding (TB), augmented plane wave (APW), OPW, XANES calculations within the core-hole final-state approximation (screened core hole) and reproducibility of experimental measurements advantages respect theoretical calculations (ground-state approximation). 17

18 Quantitative XANES analysis Quantitative XANES: usual strategies Atomic Potentials: Spherical atomic potentials: Muffin-Tin (MT) approximation, Mattheis and Norman prescription rules. Parameters: overlapping (interstitial constant 0) Electronic exchange-correlation potentials: energy dependent self energy Metals Hedin-Lundqvist (HL) self energy Insulators Hedin-Lundqvist (HL) or Dirac- Hara (DH) self energy Molecules Dirac-Hara (DH) or Ground State (Xα) self energy XANES Cu K-edge (fcc) Xα Energy dependent HL overlap 18

19 XANES in nanosystems Quantitative XANES: added strategies for XANES in nanosystems. Modification of electronic structure due to finite size effects. Small cluster calculations Contribution to experimental XANES from interface and surface atoms with other local environment. Two-potential calculations (at least). C. T. Meneses, W. H. Flores, and J. M. Sasaki Chem. Mater. 2007, 19,

20 XANES in nanosystems Quantitative XANES: Strategies Examples: catalysis and magnetic nanoparticles 1st example: Pt nanoparticles (Finite size effects). Modification at TiO 2 /Co 3 O 4 and phase formations in Mn/ZnO (Interface effects). Two-phases (interface): Granular magnetic systems: Fe/Si 3 N 4 multilayers (Finite size + interface effects). FEFF8 code A.L. Ankudinov, B. Ravel, J.J. Rehr, and S.D. Conradson, Phys. Rev. B 58, 7565 (1998). 20

21 XANES in nanosystems Finite size effects 1st example: Bazin (Pt nanoparticles) FEFF7 Numerical calculations of Pt clusters (Pt13, Pt19, Pt43, ) Several configurations considering contributions. Same potential Different potentials, inner or surface atoms D. Bazin, D. Sayers, J. J. Rehr, and C. Mottet J. Phys. Chem. B 1997, 101,

22 XANES in nanosystems Finite size effects 1st example: Bazin (Pt nanoparticles) l-dos Pt cluster (3871 ats.) edge atoms Average DOS of Pt13 cluster D. Bazin, D. Sayers, J. J. Rehr, and C. Mottet J. Phys. Chem. B 1997, 101,

23 XANES in nanosystems Finite size effects 2nd example: Ankudinov & Rehr (Pt nanoparticles) FEFF8 Pt clusters (Pt13, Pt19, Pt43, ) Self-consistency. Same potential Different potentials: inner or surface atoms A. Ankudinov, J. J. Rehr, J.J. Low and S.R. Bare, J. Chem. Phys. 116, 1911 (2002). A. Ankudinov, J. J. Rehr, J.J. Low and S.R. Bare, Topics in Catalysis 18, 3 (2002). 23

24 XANES in nanosystems Contribution of phases Multilayers t Mn >3 nm FM at RT Multilayers t Mn <2 nm SPM 0.03 M (μ B /Mn) (ZnO 30Å /Mn 60Å ) 5 (ZnO 30Å /Mn 30Å ) 10 (ZnO 30Å /Mn 15Å ) 20 (ZnO 30Å /Mn 7Å ) 43 (ZnO 30Å /Mn 1Å ) 75 1/M (μ Β /Mn) H = 1kOe T (K) Temperature (K) E. Céspedes, G.R. Castro, F Jiménez-Villacorta, A de Andrés and C Prieto, J. Phys.: Condens. Matter 20, (2008) 24

25 XANES in nanosystems Contribution of phases Fingerprints multilayers Mn/ZnO (with t Mn >3 nm) Rocksalt (RS) Wurtzite (W) Normalized absorption Thickest Mn layers Energy (ev) Mn in RS (ZnO 30Å /Mn 30Å ) 10 (ZnO 30Å /Mn 60Å ) 5 Mn in W [RS] = 70%; [W] = 30% t Mn = 3 nm t Mn = 6 nm [RS] = 25%; [W] = 75% E. Céspedes, G.R. Castro, F. Jiménez-Villacorta, A. de Andrés and C. Prieto, J. Phys.: Condens. Matter 20, (2008) [W] t=6nm /[W] t=3nm 2.5 [M s ] t=6nm /[M s ] t=3nm 2.5 Only Mn substituting Zn in W phase contributes to the observed FM 25

26 XANES in nanosystems Contribution of phases M (μ B /Mn) Oxidation state E 0 multilayers Mn/ZnO (with t Mn <3nm) (ZnO 30Å /Mn 60Å ) 5 (ZnO 30Å /Mn 30Å ) 10 (ZnO 30Å /Mn 15Å ) 20 (ZnO 30Å /Mn 7Å ) 43 (ZnO 30Å /Mn 1Å ) 75 1/M (μ Β /Mn) H = 1kOe T (K) Temperature (K) E 0 shifted to higher energies with reducing t Mn. Mixture of Mn oxides evolving from MnO (thickest Mn layers) MnO 2 (for the thinnest Mn layers) Normalized Absorption Thinnest Mn layers E 0 = ev Mn valence Samples MnO 2 MnO MnOOH 1 0 Metallic Mn Energy (ev) Energy (ev) Mn metal foil (ZnO 30Å /Mn 60Å ) 5 (ZnO 30Å /Mn 7Å ) 43 (ZnO 30Å /Mn 1Å ) 75 E. Céspedes, G.R. Castro, F Jiménez-Villacorta, A de Andrés and C Prieto, J. Phys.: Condens. Matter 20, (2008) 26

27 XANES in nanosystems Interface effects Examples: magnetic systems: TiO 2 /Co 3 O 4 (1-5 %) powders Magnetic properties of Co 3 O 4 associated to TiO 2 crystallization. 500 nm TiO 2 Anatase (little FM signal) vs TiO 2 Rutile (non-magnetic) T=500 ºC TiO 2 Anatase T=700 ºC TiO 2 Rutile A. Serrano et al. Phys. Rev. B 79, (2009) M (emu/g) M(emu/g Co3O4 ) Milled 500ºC 600ºC 700ºC Milled 500ºC 600ºC 700ºC H (KOe) H (Oe) (a) (c) 27

28 XANES in nanosystems Interface effects Examples: magnetic nanoparticles XANES signal: contribution of 2 kind of absorbers in Co 3 O 4 normal spinel: Co-th in A sites (8 sites). Co-oh in B sites (16 sites). XANES shows enhanced contribution of Co 2+ absorbers in the octahedral sites when A-TiO 2. TiO 2 Anatase (little FM signal) vs TiO 2 Rutile (non-magnetic) A. Serrano et al. Phys. Rev. B 79, (2009) 28

29 XANES in nanosystems M(emu/g Co3O4 ) Milled 500ºC 600ºC 700ºC H (Oe) (c) Co 2+ oh Co 2+ oh Ti4+ Ti 4+ Rutile Anatase 29

30 XANES in nanosystems A. Serrano et al. Phys. Rev. B 79, (2009) Interface effects TiO 2 anatase induces an enhanced contribution of Co-oh (Co 2+ ) in A sites at the interfaces TiO 2 /Co 3 O 4 interfaces cause FM. XANES calculations (FEFF8): -Co-th (Co 2+ in A) in Co 3 O 4 -Co-oh (Co 3+ in B) in Co 3 O 4 -Co-oh (Co 2+ ) at the interface CoO6/TiO 2 (ana) Absorption (Norm.) Absorption (norm.) Energy (ev) Energy (ev) Co-th (A sites) Co-oh (B sites) Calculated XANES Absorption (Norm.) Co-th (A sites) Co-oh (B sites) Calc. XANES Co 3 O 4 Calc. XANES Co-oct/TiO 2 _ana (interface) Energy (ev) Co-th (A sites) Co-oh (B sites) Calculated XANES 30

31 XANES in nanosystems Finite size + interface effects Magnetic nanoparticles & multilayers Granular magnetic systems: Fe/Si 3 N 4 multilayers (Fe layer thickness: 10 1 nm). Important reduction of M and SPM when reducing the Fe layer thickness. F. Jiménez-Villacorta, E. Céspedes, M. Vila, A. Muñoz-Martín, G. R. Castro and C.Prieto, J. Phys. D.: Appl. Phys (2008). Magnetization (emu/cm 3 ) (a) t Fe =5 nm 0 t Fe =5 nm Magnetic Field (Oe) (b) Magnetization (emu/cm 3 ) t Fe =2.5 nm T=5 K T=10 K T=20 K T=50 K T=100 K T=300 K t Fe =2.5 nm Magnetic Field (Oe) T=5K T=10K T=20K T=35K T=300K 31

32 XANES in nanosystems Finite size + interface effects Granular magnetic systems: Fe/Si 3 N 4 multilayers Evolution of the XAS spectrum (both XANES and EXAFS) with reduction of nominal Fe layer thickness. kχ(k) (A -1 ) Fe Foil t Fe =5 nm t Fe =2.5 nm t Fe =1.4 nm Wavenumber (A -1 ) As Prepared t Fe =10 nm Fourier transform magnitude Fe-Fe Fe-N Re Re Re Re Re As Prepared Fe foil Fe Foil t Fe =10 nm t Fe =10 nm t Fe =5 nm t Fe =5 nm t Fe =2.5 t Fe =2.5 nm t Fe =1.4 t Fe =1.4 nm F. Jiménez-Villacorta, E. Céspedes, M. Vila, A. Muñoz-Martín, G. R. Castro and C.Prieto, J. Phys. D.: Appl. Phys (2008) Distance (A) 32

33 XANES in nanosystems Finite size + interface effects Contribution of two Fe environments Normalized Absorption FeSN7 [Fe(10nm)/Si 3 N 4 (3nm)] FeSN8 [Fe(5nm)/Si 3 N 4 (3nm)] FeSN9 [Fe(2.5nm)/Si 3 N 4 (3nm)] FeSN10 [Fe(1.4nm)/Si 3 N 4 (3nm)] Fe Foil Energy (kev) - Small Fe cluster, R~4 Ǻ (to simulate nano-fe) - Small FeN-th, R~4 Ǻ (to simulate interface region) Normalized Absorption (b) A B C D 2 nd derivative Energy (ev) E [Fe(1.4nm)/Si 3 N 4 (3nm)] 75 FeN Fe (3 shells) 0.6(FeN)+0.4(Fe) F. Jiménez-Villacorta, E. Céspedes, M. Vila, A. Muñoz-Martín, G. R. Castro and C.Prieto, J. Phys. D.: Appl. Phys (2008). 33

34 XANES in nanosystems Finite size + interface effects Fe/Si 3 N 4 multilayers l-dos shows 3d broadening due to pd hybridization in the FeN zinc-blende phase at the interface. Moreover, this results in a reduction of the l-dos at the Fermi level Normalized Absorption FeSN7 [Fe(10nm)/Si 3 N 4 (3nm)] FeSN8 [Fe(5nm)/Si 3 N 4 (3nm)] FeSN9 [Fe(2.5nm)/Si 3 N 4 (3nm)] FeSN10 [Fe(1.4nm)/Si 3 N 4 (3nm)] Fe Foil Energy (kev) Stoner criterion for FM: IxJ>1 Non-magnetic FeN (tetrahedral) phase. F. Jiménez-Villacorta, E. Céspedes, M. Vila, A. Muñoz-Martín, G. R. Castro and C.Prieto, J. Phys. D.: Appl. Phys (2008). local-dos local-dos (a) (b) Fe N ε ε F (ev) XANES Fe 4 shells XANES Fe 3 shells Fe Fe (4 shells)(3 shells) sdos pdos ddos XANES FeN 34 sdos pdos ddos

35 Summary XAS is a powerful tool to determine local order (short-range structure). Hence, it is an ideal technique in nanosystems (i.e., particles, nanocrystalline materials, thin films, ). These finite size and surface effects are the responsible of the peculiar physical properties of nanosystems respect to their bulk counterparts. XANES is quite sensitive to modifications of the electronic structure of nanosystems, due to such important contribution of finite size and surface effects. Strategies for the qualitative and quantitative XANES analysis of the electronic features of materials at the nanoscale are needed. They are based in approximations to the problem based in small size. 35

36 Acknowledgements Germán R. Castro (SpLine-ESRF; ICMM-CSIC) Juan Rubio-Zuazo (SpLine-ESRF; ICMM-CSIC) Miguel Ángel García (UCM) Eva Céspedes (ICMM-CSIC) Carlos Prieto (ICMM-CSIC) Spanish Ministry of Science XLIV Zakopane School of Physics Breaking Frontiers: submicron Structures in Physics and Biology May

37 Thank you very much for your attention! 37