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Photoelectron spectroscopy Electromagnetic radiation has the
properties of both a wave and a particle .The radiation is usually
described in terms of its energy, E, or its wavelength, . The
relation between these units is expressed in following equation:
E=hv=hc/ where h is Plancks constant, v is the frequency of the
wave, c is the speed of light in vacuum and is the wavelength of
the light . Photons are particles of light and with the right
energy they can interact with matter .If an atom or molecule
absorbs a photon, the electronic structure of the atom or molecule
will adjust itself to the added quantum of energy .This process can
be used to explore the different energy levels of atoms or
molecules, and one can obtain useful chemical information about
different species. X-ray photoelectron spectroscopy (XPS) is the
most widely used surface
analysis technique to provide both quantitative atomic
concentration and
chemical state information of the detected elements .X-ray
irradiation
of surfaces results in the emission of photoelectrons whose
energies are
characteristic of the elements .The information depth is
approximately
57 nm. Angle-resolved XPS offers non-destructive resolution
of
structures within the XPS sampling depth, e .g .layer ordering,
composition and thickness can be determined.
Moreover, XPS can be utilized for sputter depth profiling to
characterize
thin films and multi-layer systems by quantifying matrix-level
elements as a function of depth.
Principles of X-RAY Photoelectron Spectroscopy
Briefly, XPS is based on the principle that, when a surface is
irradiated with X-rays, i will be ejected. If X-ray lines of
sufficiently narrow widths are used, the photoelectrons have
characteristic energies related directly to the atomic levels from
which they came; XPS commonly uses either the AlK(1486.6 eV) or the
MgK(1253.6 eV ) lines .With such low -
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energy excitation, the photoelectrons must originate in the
outer few monolayers only if they are to escape without energy loss
.The resultant energetic electrons are then collected and counted,
after dispersion, by an electrostatic analyzer .A photoelectron
energy spectrum then consists of a plot of counts as a function of
kinetic energy .Such spectra may be used
for analytical purposes or to gain insight into the chemical
bonding of the elements present .The technique is fully
quantitative insofar as the area under a characteristic peak can be
related directly to the concentration of the corresponding atomic
species in the surface layer .The sensitivity of XPS is of the
order of 0.1 at% for most elements. Chemical information can be
extracted from the spectrum by detailed considerations of the
position (with a typical resolution of 0.1 eV) and shape of peak
envelopes .This information can be interpreted readily by
comparison with spectra of standard compounds, recorded in the same
experiment,
or by consulting the voluminous and mature literature on XPS
investigations of various compound.
The basic requirements for a photoemission experiment (XPS)
are:
1. a source of fixed-energy radiation
2. an electron energy analyzer (which can disperse the emitted
electrons according to their kinetic energy, and thereby measure
the flux of emitted electrons of a particular energy)
3. a high vacuum environment (to enable the emitted
photoelectrons to be analyzed without interference from gas phase
collisions)
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XPS can provide typical information such as:
determining the composition of a surface )elemental and chemical
composition quantification.
mapping the spatial distribution of the surface
constituents.
in-depth profiling these constituents into the bulk of the
material.
determining an over-layer or thin film thickness
identifying particles
quantifying light element impurities
Limitations
All elements are detectable except for H and He
Sample has to be a solid at RT and stable under vacuum
conditions, powders are possible
Depending on the chemical composition samples might be
sensitive
to X-ray irradiation
Basic Instrument
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n For each and every element, there will be a characteristic
binding energy associated with each core atomic orbital i.e. each
element will give rise to a characteristic set of peaks in the
photoelectron spectrum at kinetic energies determined by the photon
energy and the respective binding energies.
n The presence of peaks at particular energies therefore
indicates the presence of a specific element in the sample under
study - furthermore, the intensity of the peaks is related to the
concentration of the element within the sampled region. Thus, the
technique provides a quantitative analysis of the surface
composition and is sometimes known by the alternative acronym ,
ESCA (Electron Spectroscopy for Chemical Analysis).
n The most commonly employed x-ray sources are those giving rise
to :
Mg K radiation : hv = 1253.6 eV
Al K radiation : hv = 1486.6 eV
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References
1- L .J .Sathre, T .D .Thomas, S .Svensson, J .Chem .Soc.,
Perkin Trans .2, 749, 1997
2- 15 C .D .Wagner, J .Electron Spectrosc .Relat .Phenom .47,
283 1988
3- J .C .Vickerman, Surface analysis the principal techniques,
John Wiley & Sons 1997
4- D .Briggs and M .Seah, Practical surface analysis, John Wiley
& Sons 1997 O .Svren, Electron Spectroscopy and Chemical
Reactivity, University of Bergen, June. 1997.
5- T .Fujikawa, R .Suzuki, H .Arai, H .Shinotsuka, L .Kvr, J
.Electron Spectrosc .Relat .Phenom., in press; T .Fujikawa, R
.Suzuki, L .Kvr, ibid, 151,2006,170
6- L .Kvr, M .Novk, S .Egri, I .Cserny, Z .Bernyi, J .Tth, D
.Varga, W .Drube, F .Yubero, S .Tougaard, W .S .M .Werner, Surf
.Interface Anal .38,2006,569.