Photoelectron Spectroscopy

Xiaozhe Zhang

10/03/2014

A conception last time remain

• Secondary electrons are electrons generated as ionization

products. They are called 'secondary' because they are generated

by other radiation (the primary radiation). This radiation can be in

the form of ions, electrons, or photons with sufficiently high

energy, i.e. exceeding the ionization potential. Photoelectrons can

be considered an example of secondary electrons where the

primary radiation are photons.

What is photoelectron spectroscopy?

• Photoelectron spectroscopy utilizes photo-ionization

and analysis of the kinetic energy distribution of the

emitted photoelectrons to study the composition

and electronic state of the surface region of a

sample

Auger Electron

Free e-

e- Vacancy

e- of high energy that will occupy the vacancy of the core level

e- released to analyze

1

1, 2, 3 and 4 are the order of steps in which the e-s will move in the atom when hit by the e- gun.

e- gun

2

3

4

Auger electron and photoelectron

Auger electron and photoelectron

Photoelectron spectroscopy

• XPS, also known as ESCA, is the most widely used surface analysis technique because of its relative simplicity in use and data interpretation.

XPS X-ray Photoelectron SpectroscopyESCA Electron Spectroscopy for Chemical AnalysisUPS Ultraviolet Photoelectron SpectroscopyPES Photoemission Spectroscopy

Analytical Methods

Equation

KE=hν-EB-Ø

KE Kinetic Energy (measure in the XPS spectrometer)

Hν photon energy from the X-Ray source (controlled)

Ø spectrometer work function. It is a few eV, it gets morecomplicated because the materials in the instrument will affect it. Foundby calibration.

EB Binding energy(BE), is the unknown variable

Equation

• The equation will calculate the energy needed to get an e-

out from the surface of the solid.

• Knowing KE, hv and Ø the (BE)EB can be calculated.

KE=hv-EB-Ø

KE versus BE(EB)

E E E

KE can be plotted depending on BE

Each peak represents the amountof e-s at a certain energy that ischaracteristic of some element.

1000 eV 0 eV

BE increase from right to left

KE increase from left to rightBinding energy

# o

f e

lect

ron

s

(eV)

Interpreting XPS Spectrum: Background

• The X-Ray will hit the e-s in the bulk (inner

e- layers) of the sample

• e- will collide with other e- from top layers,

decreasing its energy to contribute to the

noise, at lower kinetic energy than the

peak .

• The background noise increases with BE

because the SUM of all noise is taken from

the beginning of the analysis.

Binding energy

# o

f e

lect

ron

s

N1

N2

N3

N4

Ntot= N1 + N2 + N3 + N4

N = noise

Analytical Methods

Orbital splitting

Orbital splitting

XPS Sampling Depth

XPS Sampling Depth

• Sampling Depth is defined as the depth from which 95% of

all photoelectrons are scattered by the time they reach the

surface ( 3λ)

• Most λ‘s are in the range of 1 ~ 3.5 nm for Al Kα radiation

• So the sampling depth (3λ) for XPS under these conditions is

3 ~ 10 nm

XPS spectrum example

• The XPS peaks are sharp.

• In a XPS graph it is possible to see Auger electron peaks.

• The Auger peaks are usually wider peaks in a XPS

spectrum.

• Aluminum foil is used as an example on the next slide.

XPS Spectrum

O 1s

O becauseof Mg source

C

AlAl

O 2s

O Auger

Sample and graphic provided by William Durrer, Ph.D.Department of Physics at the Univertsity of Texas at El Paso

Auger Spectrum

Characteristic of Auger graphsThe graph goes up as KE increases.

Sample and graphic provided by William Durrer, Ph.D.Department of Physics at the Univertsity of Texas at El Paso

Identification of XPS Peaks

• The plot has characteristic peaks for each element

found in the surface of the sample.

• There are tables with the KE and BE already assigned

to each element.

• After the spectrum is plotted you can look for the

designated value of the peak energy from the graph

and find the element present on the surface.

XPS Imaging

Thank you for your time!