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

3. 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

4. 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

5. 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) 5

6. 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

7. 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, (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

8. 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

9. 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

10. The O- Corner Site: A Trapped Hole
Sterrer et al. J. Phys. Chem. B 106, (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

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 - +
MgO e- e- e- Ag
*Nemeth et al. Phys. Rev.
Lett. 104, (2010)
MgO on Ag(100)
XPS of 2 ML MgO
On Ag(100) surface
Schintke et al. Phys. Rev.
Lett. 87, (2001)

12. 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) −1.27
MgO/Mo(100) −1.74
MgO/Al(100) −1.46
BaO/Ag(100) −2.20
BaO/Pd(100) −3.17
BaO/Au(100) −2.80
Prada et al. PRB 78, (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

13. 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 x x
Nb x x
Maldonado et al. J. Appl. Phys. 107,
(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

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
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

16. 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

17. 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.

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