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 *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 NaCl surface exciton 500 nm Silver nanoparticle NaCl on silver (100)

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

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

Bulk versus Surface Excitation of KBr Hyperthermal: Surface exciton mechanism Thermal: Bulk mediated mechanism Animation … Selective excitation of surface atom at terraces Beck, Joly, Hess, Phys. Rev. B 63 (2001) 125423 5

Bulk and Surface Reactions (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 “Hyperthermal” “Thermal” Br Br e Br2– K+ Br– e- K+ Br– Br2– Br2– Br– Br– Br2– Br2– K+ Br- K+ e K+ Br– K+ Br– e- 6

Model for Surface Exciton Driven Desorption Surface Exciton Desorption Model Vacuum Level Exciton levels - 2 - 4 6.4 eV Energy (eV) 6.6 eV - 6 - 8 VB 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) - Results general for alkali halides 7

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

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

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

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 - + MgO e- e- e- 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)

Hybrid Materials: Metal / Metal Oxides 1. Metal influences oxide film e.g. electron tunnels to hole 2. Oxide film influence on metal surface: Large reduction in work function! 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 e- e- e- e- Ongoing work: ARPES of clean and 2 ML MgO on Ag(100) 12

Photoemission from Hybrid Materials Multilayer film of CsBr show greatly enhanced quantum efficiency VB EF EVBM Metal Dielectric ECBM E0 DF CB F Quantum Efficiency Enhancement at 4.8 eV Clean Coated Factor 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) Enhancement process requires photoactivation e- hn ~ 3.5 eV CsBr film 5 to 25 nm F center band + e- Metal substrate JR Maldonado et al. Microelectronic Engineering 86, (2009) 529 & references therein 13

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

Approach: Photoemission Electron Microscopy 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 mica 50 nm Ag film Sample Sketch 500 nm SEM image 15

Photoemission Mechanisms 4/21/2017 Photoemission Mechanisms Two-Photon Photoemission (2PPE): fs laser 3.1 eV One-Photon Photoemission hnlamp (~ 4.9 eV) > Work Function (F) of Ag (~ 4.6 eV) Intensity map calibrated to substrate yield EF FAg E (eV) 4.6 hnlamp hnlamp 15 mm 15 mm hnlaser 3.1 LSP Laser Spot hnlaser Tremendous Increase in the photoemission from the NPs due to excitation of the LSP! 16

Results: Gold Grating laser Gold gratings are fabricated using nanolithography (LBNL) SEM Image (5 mm FoV) HIM Image (5 mm FoV) PEEM Image (100 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.

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

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