Surface Science for Cathode Development

Wayne HessChemical and Materials Science Division

Pacific Northwest National LaboratoryRichland Washington, USA 99352

Future Light Source Workshop

Electron Sources Working GroupMarch 4-8, 2012, Newport News, VA

Outline

2

*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)

(a) (b)

0

100

Surface Science Capabilities at PNNL / EMSL

EMSL User Facility is well equipped:

*Transmission Electron Microscopy (TEM) 6 aberration corrected instruments (soon)

MgO nanocubes

*Rutherford Backscattering Spectroscopy (RBS)

3

*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

MCP Detector

Time-of-Flight Mass Spectrometer

UV Excitation

Ion Extraction

Resonant Laser Ionization

Pump

Probe

+

Sample

–V

UHV Chamber

Time-of-flight

pump-probe experiment

Laser Induced Reactions of Alkali Halides

4

Br-

ato

m Y

ield

(a

rb.

un

its)

403020100

Delay between Lasers (µs)

Br-atom velocity distributions at7.9 eV excitation energies

Bulk versus Surface Excitation of KBr

Hyperthermal: Surface exciton mechanism

Thermal: Bulk mediated mechanism

Beck, Joly, Hess, Phys. Rev. B 63 (2001) 1254235

6

Bulk and Surface Reactions

K+ Br–

Br–

K+

K+

Br–

(1) Laser excitation of surface(2) Creation of surface exciton(3) Desorption of hyperthermal Br-atom

e-ee

Br

K+ Br–

Br–

K+

K+

Br–

(1) Laser excitation of bulk

ee

(2) Creation of bulk exciton (3) Exciton self trapping

Br-

Br-

(4) Formation of F-H pair

Br 2–

e-

Br 2–

Br 2–

Br 2–

(5) Diffusion of H center along <110>

Br 2–

(6) Desorption of thermal Br-atom

Br“Hyperthermal”

“Thermal”

Model for Surface Exciton Driven Desorption

Hess, Joly, Beck, Henyk, Sushko, Trevisanutto, Shluger, J. Phys. Chem. 109, 19563 (2005)

Surface Exciton Desorption Model

- Results general for alkali halides

Theoretical predictions verified by experiment

- New surface spectroscopy (SESDAD) technique Surf. Sci. 564, L683 (2003)- Velocity control of desorbed atoms (VRAD) Surf. Sci. 564, 62 (2004)

- Experimental exciton energies match calculations CPL, 470, 353 (2009)

7

Vacuum Level

- 2

- 4

- 6

- 8

0

En

erg

y (e

V)

Bulk

6.6 eV

Terrace

6.4 eV

VB VB

Excitonlevels

Br-

atom

Yie

ld (

arb.

uni

ts)

403020100

Delay between Lasers (µs)

Br-atom velocity distributions at7.9 eV excitation energies

Above band gap excitationUncontrolled Br emission

Bulk or Surface Excitation of KBrB

r-at

om Y

ield

(ar

b. u

nits

)

403020100

Delay between Lasers (µs)

Br-atom velocity distributions at 6.4 eV and 7.9 eVexcitation energies

1.2

1.0

0.8

0.6

0.4

Abs

orpt

ion

10987

Energy (eV)

Bulk excito

n bands

Band gapA

bso

rpti

on

Energy (eV)

7 8 9 10

Ab

sorp

tio

n

Photon energy

Surface specific excitationOnly Hyperthermal halogen-atom

emission

8

Laser Induced Reactions of Metal Oxides (MgO)

9

aaa

[100] directions

5-fold / surfaceTerrace Site

3-fold / kinkCorner site

4-fold / stepEdge Site

Corner

Vacuum Level

- 10

- 2

- 4

- 6

- 8

0

En

erg

y (e

V) 4.7 eV

Bulk

7.8 eV

Edge

5.7 eV

Terrace

6.7 eV

1.00

0.90

0.80

0.70

0.60

0.50

0.40

0.30

0.20

0.10

0.00

O-a

tom

Yie

ld (

arb

. un

its)

444036322824201612840

Delay (µs)

1.0

0.8

0.6

0.4

0.2

0.0

No

rma

lize

d Y

ield

(a

rb.

un

its)

0.600.400.200.00

Kinetic Energy (eV)

4.66 eV

7. 9 eV

0.0

28

eV

0.1

2 e

V

Beck, Joly, Diwald, Stankic, Trevisannuto, Sushko Shluger, Hess Surf. Sci. 602, 1968 (2008)

MgO

Mg

O

OO

Mg

Mg

Mg

Mg2+ O2-

The O- Corner Site: A Trapped Hole

Ekin ~0.17 eVO0 – Mg+

DFT Calculations

Sterrer et al. J. Phys. Chem. B 106, 12478 (2002)

EPR

Trevisanutto, Sushko, Beck, Joly, Hess, ShlugerJ. Phys. Chem. C, 13, 1274 (2009).

Mg2+O–

Mg+

O0

10

h

11

Measuring Hybrid Structure Properties

MgO on Ag(100)

Tuning Work functionQuantum yield enhancement – oxides and alkali halidesNanostructures PEEM and TR-PEEMTesting predictions for improved photoemission properties

Schintke et al. Phys. Rev. Lett. 87, 276801 (2001)

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)XPS of 2 ML MgO

On Ag(100) surface

Hybrid Materials: Metal / Metal Oxides

Metal oxidethin film

1. Metal influences oxide filme.g. electron tunnels to hole

2. Oxide film influence on metal surface:Large reduction in work function!

e- e- Metal

substrate

e- e-

e- e-

e- e- e-

e-

e-

e- e- e-

e-

+

e-

Calculated Work Function Reduction

MgO/Ag(100) 2.96 −1.27MgO/Mo(100) 2.15 −1.74MgO/Al(100) 2.86 −1.46

BaO/Ag(100) 2.03 −2.20BaO/Pd(100) 1.99 −3.17BaO/Au(100) 2.33 −2.80

Prada et al. PRB 78, 235423 (2008)Also calculated for Au, Mo, Pd, and Pt

12

Ongoing work: ARPES of clean and 2 ML MgO on Ag(100)

Photoemission from Hybrid Materials

Metal substrate

Multilayer film of CsBr show greatly enhanced quantum efficiency

Enhancement process requiresphotoactivation

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)

CsBr film5 to 25 nm

VB

EF

EVBM

Metal Dielectric

ECBM

E0

CB

F centerband

+

e-

h ~ 3.5 eV

e-

13

JR Maldonado et al. Microelectronic Engineering 86, (2009) 529 & references therein

Metal Nanoparticles & Localized Surface Plasmons

K. A. Willets et al., Annu. Rev. Phys. Chem., 58, 267 (2007)

Silver nanoparticles X.N. Xu

Plasmonic structures absorb light very stronglyHuge optical cross section of localized surface plasmon (LSP) Can tune absorption frequencyHuge optical field enhancementGreatly enhanced photoemission

14

Approach: Photoemission Electron Microscopy

500 nm

mica

50 nm Ag filmSample Sketch

SEM image

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

15

Photoemission Mechanisms

One-Photon Photoemission

hlamp (~ 4.9 eV) > Work Function () of Ag (~ 4.6 eV)

hlamp

15 m15 m

Laser Spot

hlaser

Two-Photon Photoemission (2PPE): fs laser 3.1 eV

EF

Ag

E (eV)

4.6

0

hlamp

hlaser

3.1 LSP

Intensity map calibrated to substrate yield

16

laser

Results: Gold Grating

SEM Image (5 m FoV) PEEM Image (100 m FoV)HIM Image (5 m FoV)

Preliminary results show 106 enhancement of photoemission by gold grating over flat gold film excited with 100 fs pulses at 800nmH. Padmore et al.

Gold gratings are fabricated using nanolithography (LBNL)

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

Ken Beck, Alan Joly, Sam Peppernick, Theva Thevuthasan, Shuttha Shuthanadan, Zihua ZhuPacific Northwest National Lab

Carlos Hernandez-Garcia, Fay Hannon, Marcy StutzmanJefferson Lab

Kathy Harkay, Karoly Nemeth Argonne National Lab

Juan Maldonado Stanford University

Howard PadmoreLBNL

US Department of EnergyEMSL

Acknowledgements

19