Essay About Surface Characterization And X-Ray Photoelectron Spectroscopy

X-Ray Photoelectron Spectroscopy
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X-ray Photoelectron Spectroscopy
– A Report
I. Overview
This report is a detailed study on X-ray Photoelectron Spectroscopy (XPS), a surface
characterization technique. The first question that probably comes in your mind is,
what exactly is surface characterization? Well, surface characterization is a method
which enables us to know everything about the surface in focus. Through this
method, we know the type of atoms present on the surface, the concentrations of the
atoms, the exact locations of the atoms on the surface, the bond lengths and angles of
the molecules on the surface, the strength of the bonding to the surface, and finally
the effect the surface bond will have on surface reactivity. Basically, upon
characterization, you will know the surface like the back of your hand.
The next question that might be asked would be, why do we need all this information
about a surface? The answer to this question is a lot more universal and general than
you would expect. All this information about a surface comes in handy when you are
working with or analyzing a specific surface. This method can analyze a surface as it
is received and we can use the information, or we can use it to compare after the
surface has undergone certain treatment. Some examples of this treatment are
cutting, scraping, fracturing, Ultra High Vacuum (UHV), ion beam etching, exposure
to heat, exposure to reactive gases or solutions, exposure to ion beam implant, and
exposure to UV light amongst many others. The treatments serve purposes like
exposure to bulk chemistry (UHV), cleaning surface contamination (ion beam
etching), and study of changes (due to exposure to heat, reactive gases, solutions, ion
beam implant and UV light) and surface characterization can help in analysis.
An overview on surface characterization has been provided in this section. In the
coming sections, you will learn everything there is to know about X-ray
Photoelectron Spectroscopy (one of the surface characterization techniques) ranging
from the technical side with physical and chemical principles to the practical side
with applications of this technique.
II. Introduction
X-ray Photoelectron Spectroscopy (XPS) is one of the surface characterization
techniques and is used to measure the empirical formula for pure materials, elements
that contaminate a surface, elemental composition of the surface (1-10 nm), chemical
or electronic state of the elements that exist in the surface, uniformity of elemental
composition across the top of the surface (line profiling), and uniformity of elemental
composition as a function of ion beam etching (depth profiling).
Image 1: Shows the basic parts of the XPS system.
X-ray Photoelectron Spectroscopy
– A Report
Looking at Image 1, you can figure out how the XPS functions. First, a focused beam
of X-rays is shot towards the surface. As this is happening, the electron detector and
electron energy analyzer will measure the number of electrons and the kinetic
energy of the electrons that escape from the surface. When you irradiate a material
with X-ray beams, it is highly likely that the electrons that escape as a result are only
from the top 1-10 nm of the material being analyzed. The reason for this can be
understood in a much simpler way if you compare this with an analogy of sitting on
the beach. When you are on the beach and under the sun for too long, although your
entire body is there, only the surface gets sun-burned and thus substantially affected.
The XPS technique works in a similar fashion only affecting the molecules close to
the surface. Once the electrons are detected by the electron energy analyzer and the
electron detector, a signal is transmitted and we are able to see and analyze the
collected data.
Now that you know how the XPS system works, we will discuss some essential and
some interesting facts about this technique. They are as follows:
* The detection of the XPS technique is limited. The technique detects elements
with an atomic number between that of lithium (Z=3) and lawrencium
(Z=103). As obvious from this limitation, the XPS technique cannot detect
hydrogen (Z=1) and helium (Z=2). Also, the detection limits of this technique
for most of the elements are in the PPTh or parts per thousand range.
* The XPS technique can be performed using two different types of systems.
The first type is a commercially built XPS system which uses either a highly
focused 20 to 200 micrometer beam consisting of monochromatic aluminum
K-alpha X-rays or a broad 10-30 mm beam