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Surface Science for Cathode Development

Surface Science for Cathode Development. Wayne Hess Chemical and Materials Science Division Pacific Northwest National Laboratory Richland Washington, USA 99352. Future Light Source Workshop Electron Sources Working Group March 4-8, 2012, Newport News, VA. Outline.

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Surface Science for Cathode Development

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  1. Surface Science for Cathode Development Wayne Hess Chemical and Materials Science Division Pacific Northwest National Laboratory Richland Washington, USA 99352 Future Light Source Workshop Electron Sources Working Group March 4-8, 2012, Newport News, VA

  2. Outline *Surface science capabilities at PNNL / EMSL *Excited state reactions of potential cathode coatings: Alkali halides and MgO * Plasmonic excitations of metal nanostructures *Proposed hybrid photocathodes: Cu:CsBr and Ag(100):MgO 500 nm Silver nanoparticle NaCl on silver (100) NaCl surface exciton

  3. Surface Science Capabilities at PNNL / EMSL (a) (b) 100 EMSL User Facility is well equipped: *Transmission Electron Microscopy (TEM) 6 aberration corrected instruments (soon) MgO nanocubes *Rutherford Backscattering Spectroscopy (RBS) 0 *Imaging Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) *Helium Ion Microscopy (HIM) *Photoemission Electron Microscopy (PEEM) Many other techniques: XRD, EDS, SEM, XPS/UPS, MBE, FTICR-MS, NMR, STM, AFM , APT 3

  4. Laser Induced Reactions of Alkali Halides + Pump Sample  Probe MCP Detector Time-of-Flight Mass Spectrometer Time-of-flight pump-probe experiment Ion Extraction –V Resonant Laser Ionization UV Excitation UHV Chamber 4

  5. Bulk versus Surface Excitation of KBr Hyperthermal: Surface exciton mechanism Thermal: Bulk mediated mechanism Beck, Joly, Hess, Phys. Rev. B 63 (2001) 125423 5

  6. Bulk and Surface Reactions K+ K+ Br– Br– Br– Br– K+ K+ e e Br– Br– K+ K+ e- e- Br Br Br- Br- Br2– Br2– Br2– Br2– Br2– (1) Laser excitation of surface (1) Laser excitation of bulk (2) Creation of surface exciton (2) Creation of bulk exciton (3) Desorption of hyperthermal Br-atom (3) Exciton self trapping (4) Formation of F-H pair (5) Diffusion of H center along <110> (6) Desorption of thermal Br-atom “Thermal” “Hyperthermal” 6

  7. Model for Surface Exciton Driven Desorption Surface Exciton Desorption Model Vacuum Level 0 Exciton levels - 2 - 4 6.4 eV Energy (eV) 6.6 eV - 6 - 8 VB Bulk Terrace Hess, Joly, Beck, Henyk, Sushko, Trevisanutto, Shluger, J. Phys. Chem. 109, 19563 (2005) Theoretical predictions verified by experiment - Velocity control of desorbed atoms (VRAD) Surf. Sci. 564, 62 (2004) - New surface spectroscopy (SESDAD) technique Surf. Sci. 564, L683 (2003) - Experimental exciton energies match calculations CPL, 470, 353 (2009) VB - Results general for alkali halides 7

  8. Bulk or Surface Excitation of KBr Bulk exciton bands Absorption Absorption Band gap 7 8 9 10 Energy (eV) Above band gap excitation Uncontrolled Br emission Surface specific excitation Only Hyperthermal halogen-atom emission Photon energy 8

  9. Laser Induced Reactions of Metal Oxides (MgO) Vacuum Level 0 7.8 eV 6.7 eV 5.7 eV - 2 4.7 eV - 4 Energy (eV) - 6 - 8 Corner Edge Terrace Bulk - 10 Mg O O Mg Mg O Mg MgO O2- Mg2+ Beck, Joly, Diwald, Stankic, Trevisannuto, Sushko Shluger, Hess Surf. Sci. 602, 1968 (2008) 9

  10. The O- Corner Site: A Trapped Hole EPR O– O0 Mg2+ Mg+ Sterrer et al. J. Phys. Chem. B 106, 12478 (2002) DFT Calculations O0 – Mg+ Ekin ~0.17 eV hn Trevisanutto, Sushko, Beck, Joly, Hess, Shluger J. Phys. Chem. C, 13, 1274 (2009). 10

  11. Measuring Hybrid Structure Properties Tuning Work function Quantum yield enhancement – oxides and alkali halides Nanostructures PEEM and TR-PEEM Testing predictions for improved photoemission properties e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- - + MgO Ag *Nemeth et al. Phys. Rev. Lett. 104, 046801 (2010) MgO on Ag(100) XPS of 2 ML MgO On Ag(100) surface Schintke et al. Phys. Rev. Lett. 87, 276801 (2001)

  12. Hybrid Materials: Metal / Metal Oxides 2. Oxide film influence on metal surface: Large reduction in work function! 1. Metal influences oxide film e.g. electron tunnels to hole Calculated Work Function Reduction F DF MgO/Ag(100) 2.96 −1.27 MgO/Mo(100) 2.15 −1.74 MgO/Al(100) 2.86 −1.46 BaO/Ag(100) 2.03 −2.20 BaO/Pd(100) 1.99 −3.17 BaO/Au(100) 2.33 −2.80 Prada et al. PRB 78, 235423 (2008) Also calculated for Au, Mo, Pd, and Pt + Metal oxide thin film Metal substrate Ongoing work: ARPES of clean and 2 ML MgO on Ag(100) e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- 12

  13. Photoemission from Hybrid Materials Multilayer film of CsBr show greatly enhanced quantum efficiency Enhancement process requires photoactivation Metal Dielectric E0 DF hn ~ 3.5 eV CB F ECBM F center band CsBr film 5 to 25 nm Quantum Efficiency Enhancement at 4.8 eV CleanCoatedFactor Cu 5.0 x10-5 3.0 x10-3 50 Nb 6.4 x10-7 5.0 x10-4 800 Maldonado et al. J. Appl. Phys. 107, 013106 (2010); Microelectron. Eng. 86, 529 (2009) EF Metal substrate EVBM VB + e- JR Maldonado et al. Microelectronic Engineering 86, (2009) 529 & references therein e- 13

  14. Metal Nanoparticles & Localized Surface Plasmons K. A. Willets et al., Annu. Rev. Phys. Chem., 58, 267 (2007) • Plasmonic structures absorb light very strongly • Huge optical cross section of localized surface plasmon (LSP) • Can tune absorption frequency • Huge optical field enhancement • Greatly enhanced photoemission Silver nanoparticles X.N. Xu

  15. Approach: Photoemission Electron Microscopy Sample Sketch 50 nm Ag film mica SEM image 500 nm 15 Spherical polystyrene nanoparticles vapor deposited on substrate 50 nm silver film over particles and surface LSP field enhancement measured by fs PEEM SEM images of identical region

  16. Photoemission Mechanisms • Two-Photon Photoemission (2PPE): fs laser 3.1 eV Intensity map calibrated to substrate yield hnlamp E (eV) Laser Spot 4.6 FAg hnlamp hnlaser 3.1 LSP 0 EF hnlaser 15 mm 15 mm 16 • One-Photon Photoemission • hnlamp (~ 4.9 eV) > Work Function (F) of Ag (~ 4.6 eV)

  17. Results: Gold Grating Gold gratings are fabricated using nanolithography (LBNL) PEEM Image (100 mm FoV) SEM Image (5 mm FoV) HIM Image (5 mm FoV) laser Preliminary results show 106 enhancement of photoemission by gold grating over flat gold film excited with 100 fs pulses at 800nm H. Padmore et al.

  18. Summary of Hybrid & Plasmonic Materials • Hybrid materials have highly modified optical and electrical properties • - Surface charge and hence chemical potential can be tuned • - Work function can be reduced and QE dramatically increased • - Photoemission can be optimized for photocathode applications • Plasmon excitation allows extreme field enhancement / localization • - Tunable plasmon resonances – UV to IR, broad or narrow by design • - Structures can be both highly absorbing and/or transmissive • - Variety of metals can be used: Al, Mg, Cu, Ag, Au, and alloys 18

  19. Acknowledgements Ken Beck, Alan Joly, Sam Peppernick, Theva Thevuthasan, Shuttha Shuthanadan, Zihua Zhu Pacific Northwest National Lab Carlos Hernandez-Garcia, Fay Hannon, Marcy Stutzman Jefferson Lab Kathy Harkay, Karoly Nemeth Argonne National Lab Juan Maldonado Stanford University Howard Padmore LBNL US Department of Energy EMSL 19

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