Download Surface Characterization by Spectroscopy and Microscopy

Chapter 21
Surface Characterization
by Spectroscopy and
Microscopy
Introduction

The most inclusive way to define a
surface is to state that a surface of
interface exists in any case where there
is an abrupt change in the system
properties with distance, with many
degrees of abruptness.
A crystalline solid in contact with its own
vapor at low temperature, effectively
has an interface that is one atomic
distance in width. A more diffuse
interface is present in the extreme case,
where we may consider a system near
its critical point, such as a liquid in
contact and hence at equilibrium with its
own vapor at high temperature and
pressure. Typical properties exhibiting
abrupt change at an interface are
density, crystal structure, crystal
orientation, chemical composition and
ferromagnetic or paramagnetic ordering.
Surface of a Solid


The surface of a solid in contact with a liquid or
gaseous phase usually has very different chemical
composition and physical properties from the interior
of the solid
Characterization of these surface properties is often
important in many fields, including heterogeneous
catalysis, semiconductor thin-film technology,
corrosion and adhesion mechanisms, activity of metal
surfaces, embrittlement properties, and studies of the
behavior and functions of biological membranes
Surface Measurements
 Classical methods
 useful information about the physical nature
of surfaces but less about their chemical
nature
 They involve obtaining optical and electron
microscopic images, as well as
measurements of adsorption isotherms,
surface areas, surface roughness, pore sizes
and reflectivity
Measurements (cont.)
 Spectroscopic
methods
provided information about the chemical
nature of surfaces, as well as determine
their concentration
 began in the 1950s

Measurements (cont.)
 Microscopic

methods
imaging surfaces and determining their
morphology, or physical features
The figure below illustrates the general principle
by which a spectroscopic examination of surface is
performed.
Three types of sampling methods are used regardless
of the spectroscopic surface method.

Primary beam focused on a single small area
of the sample and observing the secondary
beam.
 The surface can be mapped, by moving the
primary beam across the surface in a raster
patter of measured increments and observing
changes in the secondary beam.
 A beam of ions from an ion gun is used to
etch a hole in the surface by sputtering.
During this process a finer primary beam is
used to produce a secondary beam from the
center of the hole. This provides analytical
data on the surface composition as a function
of depth. This method is known as depth
profiling.
Spectroscopic Surface Methods
The chemical composition of a surface
of a solid is often different from the
interior of the solid
 One should not focus solely on this
interior bulk composition because the
chemical composition of the surface
layer of a solid is sometimes much more
important

Electron spectroscopy

The first three methods listed in the above
table are based upon the analysis of emitted
electrons produced by various incident
beams. In electron spectroscopy, the
spectroscopic measurement consists of the
determination of the power of the electron
beam as a function of the energy (or
frequency hv) of the electrons.

The most common type is based upon
the irradiation of the sample surface
with monochromatic X-radiation. This is
called X-ray photoelectron spectroscopy
(XPS). This method is also known as
Electron spectroscopy for chemical
analysis (ESCA).

The second type of electron
spectroscopy is called Auger electron
spectroscopy (AES). Auger spectra are
most commonly excited by a beam of
electrons, although X-rays are also
used.

The third type of electron spectroscopy
is ultraviolet photoelectron spectroscopy
(UPS).
In
this
method,
a
monochromatic beam of ultraviolet
radiation causes the ejection of
electrons form the analyte. This method
is not as common as the first two
methods.

Electron spectroscopy can be used for
the identification of all of the elements in
the periodic table except for helium and
hydrogen. The method also permits the
determination of the oxidation state of
an element and the type of species to
which it is bonded. This technique also
provides useful information about the
electronic structure of molecules.
Understanding how the SEM
works
Understanding how the SEM
works
SEM Setup

X-Ray Photoelectron Spectroscopy
(XPS), not only provided information
about the atomic composition of the a
sample, but also information about the
structure and oxidation state of the
compounds being examined

The kinetic energy of the emitted
electron Ek is measured in an electron
spectrometer. The binding energy of
the electron Eb can be calculated
Eb = hv – Ek - w
where, w is the work function of the spectrometer, a
factor that corrects for the electrostatic environment
in which the electron is formed and measure.

Secondary-ion mass spectrometry
(SIMS) is the most highly developed of
the mass spectrometric surface
methods, with several manufacturers
offering instruments for this technique.
SIMS is useful from determining both
atomic and the molecular composition
of solid surfaces.

In secondary-ion mass analyzers that
serve for general surface analysis and
for depth profiling, the primary ion beam
diameter from 0.3 to 0.5mm. Doublefocusing, single-focusing, time-of-flight
and quadrapole spectrometers are used
for mass determination.

Ion microprobe analyzers are more
sophisticated (and thus more
expensive) instruments that are based
upon a focused beam of primary ions
that has a diameter of 1 to 2 m. This
beam can be moved across a surface
for about 300 m in both x and y
directions.
Scanning Electron Microscopy
In many fields of chemistry, material
science, geology and biology, detailed
knowledge of the physical nature of the
surface of solids is of great importance.
The classical method of obtaining this
information was optical microscopy. the
resolution of optical microscopy is
limited by diffraction effects to about the
wavelength of light.
Current surface information at considerably
higher resolution is obtained by three
techniques:



Scanning electron microscopy (SEM)
Scanning tunneling microscopy (STM)
Atomic force microscopy (AFM)
A raster is a scanning pattern similar to that
used in a cathode-ray tube or in a
television set in which the electron beam
is:
swept across a surface in a straight line
(x direction)
 returned to its starting position
 shifted downward (y direction) by a
standard increment

This process is repeated until a desired
area for the sample has been scanned
Below is a schematic of a combined instrument that
is both a scanning electron microscope and a
scanning electron microprobe.
Scanning Probe Microscopes

Scanning probe microscopes (SPMs)
are capable of resolving details or
surfaces down to the atomic level.
Unlike optical and electron
microscopes, scanning probe
microscopes reveal details not only on
the lateral x and y axis of a sample but
also the z axis.
The figure below is a contour map
The figure below shows the most common method
of detecting the deflection of the cantilever holding
the tip.
The diagram below shows a
common design for an AFM.
Below is a micrograph of an SiO2 cantilever

One disadvantage of the contact mode
scanning is that the downward force of
the tip may not be low enough to avoid
damage to the sample surface, causing
image distortion. This effect can be
overcome by using a tapping mode of
operation in which the cantilever is
oscillated at a frequency of a few
hundred kilohertz.
CFM studies provide specific qualitative analytical
information as well as information on the spatial
arrangement of analytes on the surface.
References...
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http://www.mcs.com/~wbstine/spm/spm.html
http://www.chem.wisc.edu/~hamers/gallery/in
dex.html
http://www.molec.com/gallery/
http://www.csc.fi/lul/chem/graphics.html
http://www.kri.physik.unimuenchen.de/crystal/stm/
http://www.imb-jena.de/IMAGE.html
References (cont.)
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http://www.people.vcu.edu/~srutan/chem4
09/pp222_231/sld002.htm
http://www.ipfdd.de/research/res15/res15.htm
l
http://www.uksaf.org/tech/sem.html
http://www.mse.iastate.edu/microscopy/home
.html
http://www.llnl.gov/str/Scan.html
http://stm2.nrl.navy.mil/how-afm/howafm.html