X-ray Imaging and Spectroscopy of Individual Nanoparticles A. Fraile Rodríguez, F. Nolting
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X-ray Imaging and Spectroscopy of Individual Nanoparticles A. Fraile Rodríguez, F. Nolting Swiss Light Source Paul Scherrer Institut, Switzerland J. Bansmann D ∼8 nm 1 3 Dept of Surface Chemistry and Catalysis Universität Ulm, Germany 1.4 1 2 Intensity [a.u.] 1.3 Institut für Physik, Universität Rostock, Germany 2 1.2 3 1.1 U. Wiedwald 1.0 770 775 780 A. Kleibert 785 790 795 Photon Energy [eV] 800 805 Dept of Solid State Physics Universität Ulm, Germany Magnetism in reduced dimensions Intrinsic properties Interparticle interactions Finite-size effects Nanomagnetism Size, aspect ratio distribution Surface effects Magnetism in reduced dimensions Superparamagnetism Shape-dependent Thermal Switching Superparamagnetic Nanoislands K ani ⋅ Vparticle ≅ k B ⋅ T Superparamagnetic limit: time and thermal stability M. Bode et al. Phys. Rev. Lett. (2004) 92 067201 Magnetism in reduced dimensions Surface and core magnetic orders Surface effects spin glass? dead magnetic layer? bulk-like? • lower coordination number • broken magnetic exchange bonds • frustrated magnetic interactions • surface spin disorder • high-field irreversibilities • high saturation fields • shifted hysteresis loops • reduced M in ferri-, antiferrosystems • enhanced M in metallic ferrosystems Ensembles vs Single-Particle Properties Ensembles: Single Particle experiments: Distributions with respect to nanoparticle size, aspect ratio crystalline structure, defect distribution and chemical composition Correlate the electronic, magnetic and structural properties with the size, aspect ratio, crystalline structure, and chemical composition of each individual particle. The ability to manipulate a single nanoparticle has an increased potential in device manufacturing Courtesy of M. Farle, Uni Duisburg Single Particle Detection: Techniques Available Δx (nm) E-resolution (eV) SP-STM 0.5 < 0.2 EELS 0.5 0.5 Optical Fluorescence <5 0.02 Technique Technique Δx (nm) System (Individual Particles) XPEEM 50 • InAs (D~50 nm), ΔE/E=0.2 eV, Heun et al. • Fe2O3 (D ~ 10 nm), ΔE/E=0.5 eV, Rockenberger et al. Soft x-ray Spectromicroscopy Ni Co chemical bonding electronic properties TEY (a.u.) Intensity (a.u.) Fe Intensity (a.u.) Magnetic Contrast: XMCD Chemical Selectivity S L3 L2 atomic magnetic moments 700 800 Photon Energy (eV) 775 780 785 790 795 800 805 Photon energy (eV) 900 Surface and interface sensitivity hn Magnetic Contrast: XMLD eantiferromagnets Soft x-ray Spectromicroscopy Element specific imaging: PEEM Co Py Substrate Co islands, 778.1 eV Py film, 852.7 eV Magnetization Direction S 5 μm X-ray PhotoEmission Electron Microscopy • probing secondary/Auger/photoemission • spatial resolution: 50 nm Energy Analyzer • electron energy resolution: 0.1 eV • HA~ 30 mT • 100 K < T < 1500 K • ultra high vacuum MCP Phosphor Magnetic Lenses 20kV x-ray s Sample Cobalt particles: Arc ion cluster source R. P. Methling et al., EPJD 16, 173 (2001) • particle size tunable between 4-15nm • size distribution: ΔD/D ~10-15% • in situ deposition Collaboration with J. Bansmann, Uni Ulm, and A. Kleibert, Uni Rostock Particle Size: Scanning Electron Microscopy Co particles, no capping layer Co particles, Al capping layer 100 nm 1 µm D ~10 nm D ~ 8 nm • deposition of Co particles on Si substrates • coverage: 5-10 particles/μm2 • lithographic markers on substrates • low percentage dimers/trimers • crystalline structure Lithographic markers: L. J. Heyderman, PSI Elemental Contrast: X-ray PEEM Co particles D ∼ 13 nm oxidized in air Photon energy 778 eV 2 μm Photon energy 770 eV 2 μm Image (778 eV)÷ Image (770 eV) 2 μm X-ray Imaging of Individual Nanoparticles Co particles D ∼ 8 nm / 8 nm Al capping layer PEEM Elemental Contrast Scanning Electron Microscopy 1 µm 1 µm 1 µm 1 µm The lithographic markers are essential to correlate unambiguously the PEEM observations with the size of the particles imaged by the SEM Lithographic markers: L. J. Heyderman, PSI Individual Particles: X-ray Absorption Spectra Co particles D ∼ 8 nm, no capping layer 1 μm 770 775 780 790 795 800 805 810 Intensity (a.u.) particle/blank particle blank Photon Energy (eV) Movie: 159 images Total acquisition time: 12 hours. 765 770 775 780 785 790 795 800 805 810 Photon Energy (eV) X-ray Absorption: Particle-to-particle variation Co particles D ∼ 8 nm, no capping layer 1.4 Particle 1 Particle 2 Particle 3 BC Intensity (arb.units) 1.3 D A 776 A E 1.2 778 780 Reference CoO thin film BC D E 782 1.1 776 778 780 782 Adapted from Regan et al. PRB 64 (2001) 214422 1.0 770 775 780 785 790 795 Photon Energy [eV] 800 805 X-ray Absorption: Single-Particle vs Ensembles Co particles D ∼ 8 nm, no capping layer 1.4 Particle 1 Particle 2 Particle 3 Ensemble 1.3 Intensity (arb.units) BC D A 776 A E 1.2 778 780 Reference CoO thin film BC D E 782 1.1 776 778 780 782 Adapted from Regan et al. PRB 64 (2001) 214422 1.0 770 775 780 785 790 795 Photon Energy [eV] 800 805 Future: XMCD of individual nanoparticles • in situ Fe clusters (~ 9 nm) supported on ferromagnetic thin films Fe, 708 eV Co, XMCD, 778 eV Fe clusters Co film 1 μm • alloy systems, e.g. FexCo1-x , FexPt1-x • Magnetic transition temperatures on the nanoscale Conclusions • X-ray absorption spectra of individual Co particles as small as 8 nm • Differences in oxide-related features between individual particles were observed • Changes between the spectra of an individual particle and the ensemble were observed D ∼8 nm 1 3 1.4 1 2 Intensity [a.u.] 1.3 2 1.2 3 1.1 1.0 770 775 780 785 790 795 Photon Energy [eV] 800 805 Collaborators F. Nolting, Swiss Light Source Paul Scherrer Institut, Switzerland J. Bansmann, Dept of Surface Chemistry and Catalysis Universität Ulm, Germany A. Kleibert, Institut für Physik, Universität Rostock, Germany U. Wiedwald, Dept. Solid State Physics, Universität Ulm, Germany L. J. Heyderman,Laboratory for Micro- and Nanotechnology Paul Scherrer Institut, Switzerland
