Adventures and Opportunities with Ted Madey using Synchrotron Radiation Photoemission

John E. (Jack) Rowe, Physics Department, North Carolina State University

• Introduction

• Brookhaven beam line U4A - Instrumentation for SXPS

• Adventures and Opportunities

• Interface Studies with Soft XPS - using synchrotron radiation

• Bi-metallic Surfaces as Model Catalysts

• Recent Brookhaven NSLS results

• Summary

• Photon energy: 10 - 300 eV

• Surface sensitive: < 30 Å

• High resolution: ~ 0.1 eV

• Non-destructive

• Ideal for Si 2p core level analysis

• Drawback: cannot measure O 1s

U4A @ National Synchrotron Light Source

= 45

Sample

e - Analyzer

Ph

otons

Photoemission spectroscopy at U4A

n =2

1

0

-1

anal 4.7 eV

Si 2p core

VB (Si 3p & 3s)(O 2p & 2s)

h = 150 eV

BE (Si 2p) = 99.3 eV

KEelectron = h - BE - anal 46 eV

BE 4 eVbulk Si

SiO2 film}

Final State

Vacuum Level

Fermi Level

Energy Level Diagram — SXPS

• Binding Energy is measured from the electron kinetic energy (KE) by: h = BE + KE + anal .

(the analyzer workfunction anal is constant.)

• Shifts in the electron binding energy indicate the electronegativity of the environment.

• Core levels such as p, d and f are typically sharp.

• The valence band edge is readily measured from SXPS.

Surface Sensitive Data for Si(100) Surfaces

• Si(100) surface is the one commonly used in “circuit chips”

• Surface atoms are reconstructed into “dimer pairs” to remove half of the dangling bonds

• Work done at Bell Labs with Gunther Wertheim, Phys. Rev. Lett. 67, 120 (1991)

First published Paper of Ted using NSLS - U4AWork was begun in 1993-94

Bi-metallic W(111) with induced {211} facets

Example SXPS data: SiO2/Si(111)

~ 10 Å SiO2

Si

h = 130 eVSecondary electrons

Si 2p core level

Valence band

Photoemission Raw Data: SiO2/Si(111)Si 2p core level

h = 160 eV

Interface suboxide bulk silicon

substrate

SiO2 film

Spin-orbit:• 0.602 eV splitting• 2:1 ratio

~ 10 Å SiO2

Si

Si-SiO2 Interface Model for 5 ÅFilm

Spin-Orbit stripping Si 2p data

• Atomic energy levels are “split” due to the interaction between the spin and orbital angular momentum of the electrons.

• Degeneracy determines branching ratio (2p3/2:2p1/2 = 2:1)

• Raw data can be mathematically split into the 2p3/2 and 2p1/2 components.

Si(111) (majority Si1+)

SiSi

SiSi

SiSi

SiSi Si Si

O O O

More Adventures and a new Opportunity for Ted and me after moving from Bell Labs to North Carolina

Opportunities with Synchrotron Radiation Photoemission

ADVENTURES AND NSLS POSTER PAPER OF THE YEAR (2005) BY ALLY CHAN, RUTGERS POSTDOC

Model Re-oxide catalyst studies

Model Catalyst on Re(1231) Faceted Surface

Figure 1. (Top) Surface structure of atomicallyrough Re (1231). (Bottom) High resolution soft xrayphotoelectron spectra (SXPS) of the planar andfaceted oxygen-covered Re surfaces, and of the thinRe oxide fi lm, formed by different oxygen treatmentsto the Re (1231) surface.

Summary: Ted Madey, Friend, Role Model, and Mentor

A few years after Ted Madey arrived at Rutgers from NIST he contacted me about the possibility of beginning a collaboration using Synchrotron Radiation Photoemission to study surfaces of faceted Tungsten surfaces. He proposed using the Bell Labs beamline, U4A, at the Brookhaven National Synchrotron Light Source, NSLS. Soon thereafter I began an extremely fruitful collaboration with Ted that was both an adventure and an opportunity for me. This collaboration has waned in recent years as my research responsibilities changed, but it was very successful for more than 15 years. I believe this interaction was an opportunity for each of us. This talk will review some of the highlights of our joint work with an emphasis on the science that Ted produced and on the many students and postdocs that he mentored at NSLS and U4A. He was a role model for me as well as an important mentor. His science will continue to be important for many researchers in the field of surface science.