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
500 nm
NaCl surface exciton
2
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)
(a)
(b)
100
*Photoemission Electron Microscopy
(PEEM)
Many other techniques:
XRD, EDS, SEM, XPS/UPS,
MBE, FTICR-MS, NMR, STM,
AFM , APT
3
0
Laser Induced Reactions of Alkali Halides
MCP Detector
Time-of-Flight Mass
Spectrometer
Time-of-flight
pump-probe experiment
Ion Extraction
Pump
–V

Resonant Laser
Ionization
+
UV Excitation
Sample
4
UHV Chamber
Probe
Bulk versus Surface Excitation of KBr
Hyperthermal: Surface
exciton mechanism
Br-atom Yield (arb. units)
Br-atom velocity distributions at
7.9 eV excitation energies
Thermal: Bulk mediated
mechanism
0
10
20
30
40
Delay between Lasers (µs)
5
Beck, Joly, Hess, Phys. Rev. B 63 (2001) 125423
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
K+ Br
ee-–
K+ Br–
Br–
Br–
K+
K+ Br–
6
BrBr
K+ Br
ee-–
K+
Model for Surface Exciton Driven Desorption
Surface Exciton Desorption Model
Energy (eV)
0
Vacuum Level
Exciton
levels
-2
-4
6.6 eV
6.4 eV
-6
-8
VB
Bulk
VB
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)
- New surface spectroscopy (SESDAD) technique
- Experimental exciton energies match calculations
- Results general for alkali halides
7
Surf. Sci. 564, 62 (2004)
Surf. Sci. 564, L683 (2003)
CPL, 470, 353 (2009)
Bulk or Surface Excitation of KBr
Absorption
Absorption
Br-atom Yield (arb. units)
units)
Br-atom velocity distributions at
Br-atom6.4
velocity
eV anddistributions
7.9 eVat
7.9 excitation
eV excitation
energies
energies
Above band gap excitation
Uncontrolled Br emission
Surface specific excitation
Only Hyperthermal halogen-atom emission
Bulk
exciton
bands
Band
gap
Photon energy
7
0
10
20
30
Delay between Lasers (µs)
8
40
8
Energy (eV)
9
10
Laser Induced Reactions of Metal Oxides (MgO)
4-fold / step
Edge Site
3-fold / kink
Corner site
Vacuum Level
Energy (eV)
5-fold / surface
Terrace Site
0
7.8 eV
6.7 eV
-2
5.7 eV
4.7 eV
-4
-6
-8
[100] directions
1.00
MgO
4.66 eV
0.90
0.80
0.70
0.60
0.50
0.40
O2-
Mg2+
7. 9 eV
Mg
0.8
O
0.6
O
Mg
0.4
0.2
0.30
Mg
0.0
0.20
0.00
0.20
0.40
0.60
Kinetic Energy (eV)
0.10
0.00
0
4
8
12
16
20
24
28
32
36
40
44
Delay (µs)
9
Bulk Terrace
1.0
Normalized Yield (arb. units)
O-atom Yield (arb. units)
0.12 eV
0.028 eV
- 10
Corner
Edge
Beck, Joly, Diwald, Stankic, Trevisannuto, Sushko
Shluger, Hess Surf. Sci. 602, 1968 (2008)
Mg
O
The O- Corner Site: A Trapped Hole
EPR
Sterrer et al. J. Phys. Chem. B 106, 12478 (2002)
DFT Calculations
Mg2+
10
O–
O0
Mg+
O0 – Mg+
Ekin ~0.17 eV
Trevisanutto, Sushko, Beck, Joly, Hess, Shluger
J. Phys. Chem. C, 13, 1274 (2009).
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- e- e- eeeee
e- eee
eee-
MgO on Ag(100)
Schintke et al. Phys. Rev.
Lett. 87, 276801 (2001)
11
XPS of 2 ML MgO
On Ag(100) surface
*Nemeth et al. Phys. Rev.
Lett. 104, 046801 (2010)
Ag
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
+
Metal oxide
thin film
Metal
substrate
eee-
e-
e-
e-
eee
e- eeee-
MgO/Ag(100)
MgO/Mo(100)
MgO/Al(100)
F
2.96
2.15
2.86
F
−1.27
−1.74
−1.46
BaO/Ag(100)
BaO/Pd(100)
BaO/Au(100)
2.03
1.99
2.33
−2.20
−3.17
−2.80
Prada et al. PRB 78, 235423 (2008)
Also calculated for Au, Mo, Pd, and Pt
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
Enhancement process requires
photoactivation
Quantum Efficiency Enhancement
at 4.8 eV
Metal
Dielectric
E0
eF
Clean
Coated
Factor
Cu F5.0 x10-5
Nb 6.4 x10-7
CsBr film
5 to 25 nm
CB
3.0 x10-3 ECBM
50
-4
5.0 x10
800
hn ~ 3.5 eV
F center
Maldonado et al. J. Appl. Phys. 107, band
013106
EF
e-
+
(2010); Microelectron. Eng. 86, 529 (2009)
Metal
substrate
EVBM
VB
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
14
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
Sample Sketch
50 nm Ag film
mica
SEM image
500 nm
15
Photoemission Mechanisms
Two-Photon
One-Photon Photoemission
Photoemission (2PPE): fs laser 3.1 eV
hnlamp (~ 4.9 eV) > Work Function (F) of Ag (~ 4.6 eV)
Intensity map calibrated to substrate yield
E (eV)
hnlamp
4.6
FAg
3.1
LSP
hnlaser
0
EF
hnlamp
Laser Spot
15 mm
mm
15
16
hnlaser
Results: Gold Grating
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