Electron Energy-Loss Spectroscopy of transition metal-based materials: from high energy p-d excitations to low energy d-d excitations

Transcription

1 Electron Energy-Loss Spectroscopy of transition metal-based materials: from high energy p-d excitations to low energy d-d excitations A.Gloter 1, L. Bocher 1, F.T. Huang 1, A. Torres-Pardo 1, M. Walls 1, D. Imhoff 1, O. Stephan 1 and C Colliex 1 1 Laboratoire de Physiques des Solides, Université Paris Sud, CNRS UMR 8502, F Orsay, France.

2 Electronic structure of transition metal 3d n materials TM p-d excitations (L 2,3 edges) and the multiplet structures Ion in spherical symmetry Ion in octahedral symmetry Energy ev 730 f> = 2p 5 3d n+1 > TM L 2,3 ( ev) 2p 5 3d 1 has 12 terms 2p 5 3d 6 has 180 terms 2p 5 d terms Fe 2 O 3 Fe L 2,3 i> = 2p 6 3d n > L orbital moment S spin LS term, atomic multiplet 3d 0 has 1 LS term (L=0, S=0) 3d 5 has 37 LS terms Fe 3d 5 (86 terms) GS is 6 S 5/2

3 Oldies but goldies. H. Kurata and C. Colliex PRB (1993); H. Kurata et al. PRB (1993) Atomic multiplet of Mn oxide are really visible.quantification of the valence. C. Colliex et al (1991) EELS Quantification of Fe/O ratio is accurate enough for a differentiation of TM oxide, such as hematite Fe 2 O 3, maghemite Fe 2 O 3 and magnetite Fe 3 O 4

4 We are still doing that but on a nanometer-scale EELS Fine structure Valence quantification Mixed Iron Valences at Bacterial Surfaces From Hydrothermal Vents. Poorly ordered iron hydroxide 500 nm bacteria bacteria The role of bacteria in the precipitation of the iron oxides is not very clear at the hydrothermal vent Fe L3 (a) FeCO3 (b) (c) Fe L2 Synth. 2l-Fh Energy Loss (ev) Gloter et al. EPSL 222 (2004) sum Fe 3+ /SFe = 0.65 with an standard deviation of 0.05 N.B. Here the reference for Fe 3+ is a 2 lines Ferrihydrite whose structure is closed to the investigated phases. If we use hematite reference, the obtained Fe 3+ /SFe ratio increase of 0.03

5 We are still doing that but on a nanometer-scale Iron oxide nanorods obtained by hydrothermal process Fe-L edge surface of particle bulk of particle bulk of wire Energy Loss (ev) SY Chen et al. Phys. Rev. B 79, (2009) Fe 3+ nanorods and mixed valence nano-particles (goethite / maghemite / magnetite / alterated magnetite at particle surface)

6 We are still doing that but on a nanometer-scale Fe/O ratio by EELS Fe 3 O 4 10 nm FeOOH SY Chen et al. Phys. Rev. B 79, (2009) EELS chemical mapping results of the nanoparticle and nanowire. The ratio between Fe and O Fe/O of the particle is about 0.75, which is the same with Fe 3 O 4. For the wire, the ratio ranges from 0.55 to 0.65, which is comparable with a FeOOH compound that had partially transforms to Fe 2 O 3 beam induced dehydration

7 We are still doing (trying to) that but on the angstrœm-scale EELS atomic map of STO-LCMO interfaces as a function of substrate (STO ) orientation Courtesy Mike Walls (LPS), With the University and the Institute of Materials Science of Barcelona STO-LCMO 100 HADF Mn La Ti MnLaTi STO-LCMO 110 HADF Mn La Ti MnLaTi

8 We are still doing (trying to) that but on the angstrœm-scale EELS fine structures with the NION STEM (Convergence semi-angle is 35 mrad, id est, 4 times larger than for non Cs corrected STEM) Third order abberation of the EELS spectrometer are corrected by 3 octupoles. 2a = 70mrad Zero Loss (DE=0.4eV with 0.1 ev/ch) Monochromated EELS Arnold et al. Chem Mat. 21, 635 (2009), a-fe 2 O 3 Fe L 2,3 a-fe 2 O 3 Fe L 2,3 0.1 nm probe and 100 pa of current Nion STEM A. Gloter, L. Bocher (CNRS-Orsay) FeTiO 3 Fe L 3 a-fe 2 O ev Energy loss (ev)

9 f> = a 2p 5 3d n+1 > + b 2p 5 3d n+2 L> 2p 5 3d n+2 L 2p 5 3d n+1 Electronic structure of transition metal 3d n materials Ionic and charge transfer configuration Ion in octahedral symmetry Charge transfer satellite peak Charge transfer satellite peak 3d n+1 L 3d n Cr L 2,3 in Cs[NiCr(CN) 6 ] Cr-C bonding is highly covalent so CT satellite is strong. i> = a 2p 6 3d n > + b 2p 6 3d n+1 L> 3d n+1 L charge transfer configuration Electron transfer from ligand (O 2p) to the TM 3d. Fractional d population / covalency

10 Electronic structure of transition metal 3d n materials Ionic and charge transfer configuration of Ti4+ compounds Ti 4+ > = a 2p 6 3d 0 > + b 2p 6 3d 1 L> Satellite peaks are sensitive to an «average iono-covalency» of the Ti-O bonding. Unfortunalty they have an extremely low x-section. EELS Ti L2,3 Anatase CTM V(e g ) = 2.0 ev Rutile and Titanate tube have similar CT satellite peak, different vs anatase After modelisation, anatase has less electron in the d band (0.07 e-). Rutile CTM V(e g ) = 2.2 ev Nanotube Energy-Loss (ev) 480 Gloter et al. PRB, 80, Covalency measurement on nano-object

11 Electronic structure of transition metal 3d n materials TM dd excitations and the multiplet structures f> = = a 2p 5 3d n+1 > + b 2p 5 3d n+2 L> 2p 5 3d n+1 L 2p 5 3d n+1 L Ion in octahedral symmetry 3d n+1 L 3d n i> = a 2p 6 3d n > + b 2p 6 3d n+1 L> Intra-configuration excitations or d-d excitations (<10 ev)

12 dd excitation of NiO oxide by EELS [02-2] [000] NiO q resolved EELS spectroscopy of NiO (Monochromated Tecnai, NTU, Taiwan) d resolution = 300 nm q resolution = 0.1 A -1 q = 4 A -1 3 T 2 3 T 2 d-d excitations q > 2 A -1 3 T Energy loss (ev) Band gap Cerenkov loss q < 0.1 A -1 A. Gloter, MW Chu, Ultramicroscopy 109 (2009) Low energy dd excitations (optical domain) measurement on nanoobject

13 Electron Energy-Loss Spectroscopy of transition metalbased materials - TM and ligand can be mapped / quantified with atomic resolution. -TM 2p and O 1s excitations give the valence quantifications and the iono-covalence amount. - Decent spectra for TM 2p excitations on single atomic columns are still sparse but will come soon. (0.35 ev energy resolution with 0.12 nm probe, Ti 2p BaTiO 3 -Fe interface) - Optical domain can also be investigated (d-d excitations can have huge intensity). Higher spatial resolution than RIXS or NIXS.