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International Journal of Universal Pharmacy and Bio Sciences 3(4): July-August 2014

INTERNATIONAL JOURNAL OF UNIVERSAL

PHARMACY AND BIO SCIENCES IMPACT FACTOR 2.093***

ICV 5.13*** Pharmaceutical Sciences REVIEW ARTICLE……!!!

X- RAY PHOTOELECTRON SPECTROSCOPY: A REVIEW

ARUNA. P *, ASIYA BEGUM

CMR college of pharmacy , Kandlakoya(v) , Medchal, Hyderabad.

KEYWORDS:

X-ray photoelectron

Spectroscopy, Sources,

Detectors and

Applications.

For Correspondence:

ARUNA. P

Address:

CMR college of

pharmacy ,

Kandlakoya(v) ,

Medchal, Hyderabad..

E-mail: pawararuna09@gmail.co

m

ABSTRACT

X ray photoelectron spectroscopy (XPS) is a quantitative

spectroscopic technique that measures the elemental

composition, empirical formula, chemical state and electronic

state of the elements that exist within a material XPS

requires ultra-high vacuum (UHV) conditions. X ray

photoelectron spectroscopy (XPS) is also known as ESCA

(Electron Spectroscopy for Chemical Analysis), an abbreviation

introduced by Kai Siegbahn's research group. XPS is routinely

used to analyze alloys, teeth, ceramics, bones etc.

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INTRODUCTION:

X ray photoelectron spectroscopy (XPS) is a quantitative spectroscopic technique that

measures the elemental composition, empirical formula, chemical state and electronic state of

the elements that exist within a material. XPS spectra are obtained by irradiating a material

with a beam of X-rays while simultaneously measuring the kinetic energy and number

of electrons that escape from the top 1 to 10 nm of the material being analyzed. XPS

requires ultra-high vacuum (UHV) conditions.

X ray photoelectron spectroscopy (XPS) is also known as ESCA (Electron Spectroscopy for

Chemical Analysis), an abbreviation introduced by Kai Siegbahn's research group. XPS is

routinely used to analyze

Inorganic compounds

Metal alloys

catalysts

glasses,

ceramics,

paints,

papers,

inks,

woods,

plant parts,

make-up,

teeth,

bones

medical implants

PRINCIPLE(1)

X-ray photoelectron spectroscopy is a UHV surface technique able to identify the elemental

composition of the outermost atomic layers of a solid (2-10 nm). The fundamental process is

based on the photoelectric effect, the ejection of electrons from the material as a consequence

of the adsorption of very short wave length radiation.

For XPS, these are ‘soft’ X-rays of 200 – 2000 eV photon energy although the X-rays are

usually monochromatic (Al X-rays have an energy of 1486.3 eV). This means that the

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emitted photoelectrons have a particular kinetic energy (Ek) which is measured by a detector.

Ek can be related to the binding energy (EB) of the electron in the solid by the equation.

EB = hν - Ek - Φ

where Φ is the spectrometer work function and hν the energy of the incident radiation.

Therefore, the binding energy is a direct measure of the energy required to remove the

electron from material and is specific to the orbital of the element from which it came. This is

what gives XPS the power to measure not only elemental information but relate peak

intensity to an amount of this element within the sample.

The exact binding energy can be dependent upon both oxidation states and the local chemical

and visible environment. Atoms of higher oxidation state exhibit a higher binding energy due

to the extra coulombic interaction between the photo-emitted electron and the ion core. This

ability to discriminate between different oxidation states and chemical environments is one of

the major strengths of the XPS technique.

Figure 1: x ray photoelectron spectroscopy

INSTRUMENTATION(2)

The Xray photoelectron spectroscopy contains the following components

• Radiation source

• Analysers

• Sample

• Electron collector lens

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• Detectors

• Recorders

Radiation sources

The Source of radiation in XPS originate from either Mg or Al, K, sources giving off 1486

eV and 1258 eV of kinetic energy respectively. For a more versatile light source,

synchrotron radiation sources are also used. Synchrotron radiation is especially useful in

studying valence levels as it provides continuous, polarized radiation with high energies of >

350 eV.

XPS has a choice between a monochromatic beam of a few microns or an unfocused non-

monochromatic beam of a couple centimetres. The main thing to consider when choosing a

radiation source is the kinetic energy involved. The source is what sets the kinetic energy of

the photoelectrons, so there needs to not only be enough energy present to cause the

ionizations, but there must also be an analyzer capable of measuring the kinetic energy of the

released photoelectrons.

In XPS experiments, electron guns can also be used in conjunction with x-rays to eject

photoelectrons. There are a couple of advantages and disadvantages to doing this, however.

With an electron gun, the electron beam is easily focused and the excitation of photoelectrons

can be constantly varied. Unfortunately, the background radiation is increased significantly

due to the scattering of falling electrons. Also, a good portion of substances that are of any

experimental interest are actually decomposed by heavy electron bombardment such as that

coming from an electron gun.

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Analyzers(3)

• There are two main classes of analyzers well-suit for XPS - kinetic energy analyzers

and deflection or electrostatic analyzers. Kinetic energy analyzers have a resolving

power of E/?E, which means the higher the kinetic energy of the photoelectrons, the

lower the resolution of the spectra. Deflection analyzers are able to separate out

photoelectrons through an electric field by forcing electrons to follow different paths

according to their velocities, giving a resolving power, E/?E, that is greater than

1,000.

• Since the resolving power of both types of analyzer is E/?E, the resolution is directly

dependent on the kinetic energy of the photoelectrons. The intensity of the spectra

produced is also dependent on the kinetic energy. The faster the electrons are moving,

the lower the resolution and intensity is. In order to actually get well resolved, useful

data other components must be introduced into the instrument.

Specific Analyzer

• Within the broad picture of two main analyzer classes, there are a variety of specific

analyzers in existence that are used in XPS. The list below goes over several well-

used analyzers, though this list is, by no means, exhaustive. The most common type of

analyzer is a hemispherical analyzer, which will be explained in more depth under the

spherical deflection analyzer topic.

Plane Mirror Analyzer (PMA)

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Figure 2: Schematic of a PMA where the angle between the bottom plate and the

electrons entering is 45 degrees and the angle between the bottom plate and the

electrons exiting is also 45 degrees.

• PMAs, the simplest type of electric analyzer are also known as parallel-plate mirror

analyzers. These analyzers are condensers made from two parallel plates with a

distance, d, across them. Parabolic trajectories of electrons are obtained due to the

constant potential difference, V, between the two plates.

• In order for transmission to occur, the potential must be: V=Eod/eLo. Eo=kinetic

energy of electron in eV and e=charge of the electron.

• To obtain better focus, the electron entrance and exit angle is capable of being shifted

to 30 degrees, but this is not necessarily a good idea as it sacrifices transmission

instead.

Cylindrical Mirror Analyzer (CMA)

Figure 3: Schematic diagram of a CMA where the angle between the centre of the cylinders

and the electrons is 42.3 degrees Rin is the radius of the inner cylinder and Rout is the radius of

the outer cylinder. The electron path should be more parabolic than the overly elliptical

shape shown here.

• CMAs are advantageous over PMAs. They employ geometry to overcome the low

transmission with a PMA. A CMA consists of two cylinders having a potential

difference, V, between them. The entrance and exit slits are all contained on the inner

cylinder.

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• Here: V=1.3E0 ln (Rout/Rin) where L0=6.1(Rin) and E0 is in volts.

• They are good for applications that require a high sensitivity with only a moderate

resolution.

• Cylindrical Deflection Analyzer (CDA)

Figure 4: Schematic of a CDA where the angle the cylinders span is 127 degrees.

• CDAs consist of two cylinders spanning a 127 degree angle. It is this reason that

CDAs are sometimes called "127 degree analyzers."

• The potential difference in a CDA is: 2V=E0(Rin/Rout) where E0 is the energy of

incoming photoelectrons, in eV, that are focused.

• These analyzers have high resolution, however their transmission is low.

Spherical Deflection Analyzer (SDA)

Figure 5: Schematic diagram of an SDA.

• Only photoelectrons of the correct energy are able to pass through the detector with

the right arc and exit instead of colliding with the side walls of the hemispheres and

becoming lost.

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• Since SDAs are the most common, prevalent type of PES analyzer, they will be

discussed in more depth than any of the previous analyzers as a thorough

understanding of how they apply to PES is, theoretically, of greater importance.

• SDAs are similar to CDAs, but they consist of two concentric hemispheres instead. In

an SDA, the transmission of photoelectrons with initial energy, E0, occurs along a

party where R0=(Rin/Rout)/2.

• Here, the potential is different for both the inner and the outer hemisphere: Vin=E0[3-

2(R0/Rin)] and Vout=E0[3-2(R0/Rout)]

• The resolving power of these analyzers is proportional to the radius of the inner and

outer hemispheres. These analyzers are also capable of running in two separate

modes when coupled with an optical system - fixed analyzer transmission mode

(FAT) and fixed retardation energy mode (FRR). In FAT mode, the lens either

retards or accelerates the electrons so that all photoelectrons enter the analyzer with

the same kinetic energy. For this to occur, the analyzer is also arranged so that only

photoelectrons of a specific, fixed kinetic energy will pass through and reach the

detector. In this case, the lens is scanned for different energies. In FRR mode, the

lens only retards the photoelectrons, and it does so in a uniform manner causing all

photoelectrons to be reduced in energy to a fixed value such as 15 eV, 30 eV, or

whatever energy is desired. The hemispheres of the analyzer here have a potential

difference between them that is varied so that photoelectrons of different kinetic

energies can reach the detector. The more common of these two modes is FAT

because it provides a greater signal intensity at low electron kinetic energy and is also

makes quantification of the spectra simpler.

• These analyzers are a particularly good class of deflection analyzer. The slits in an

SDA define the acceptable range of entrance and exit trajectories a photoelectron may

have when entering or leaving the analyzer. The photoelectrons that do make it

through the entrance slits will then only exit if they follow a specific, curved path

down the middle of the two hemispheres. The path they follow has the "correct

energy" for exit to occur, and is determined by the selection of Vin and Vout.

Photoelectrons that are of higher or lower kinetic energy than what is defined by the

hemispheres will be lost through collisions with the walls.

Sample (4)

Samples are usually solid substances. The samples should be stable in a UHV system .

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Polymers with low vapour pressure, wet samples and porous materials need to be handled with

care. Older instruments accept samples: 1×1 to 3×3 cm. Present systems can accept samples

up to 30×30 cm.

Electron Collector Lens(5)

Electron collector lens will collect the emitted photo electrons.

Detectors(6)

• Detection relies on the ability of the instrument to measure energy and photoelectron

output. One type of energy measured is the binding energy, which is calculated

through the following equation

Ke = hν−BE−ϕ

Where,

Ke = Kinetic energy, this is measured

hν = Photon energy from the radiation source, this is controlled by the source

ϕ= Work function of the spectrometer, this is found through calibration

BE= Binding energy, this is the unknown of interest and can be calculated from the

other three variables

• Another part of XPS detection is in the use of electron multipliers.

Photo multiplier tubes

• These devices act as electron amplifiers because they are coated with a material that

produce secondary photoelectrons when they are struck by an electron. Typically,

they are able to produce two to three photoelectrons per every electron they are hit

with. Since the signals in PES are low, the huge amplification, up to 107 and higher

when run in series so the secondary electrons from one multiplier strike the next, they

greatly improve the signal strength from these instrument.

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Recorders(7)

Recorders are used to record the responses obtained from the detector after amplification.

ADVANTAGES(8)

High throughput

Highly automatic

High resolution

Reduced time of analysis

XPS is also able to differentiate between oxidations states of molecules.

XPS has a greater range of potential application since it can probe down to

core electrons.

XPS is good for identifying all but two elements, identifying the chemical

state on surfaces, and is good with quantitative analysis.

XPS is capable of detecting the difference in chemical state between samples.

APPLICATIONS(9)

Direct analysis of most types of solid material/surface as received, including clothing,

soft furnishings, furniture, household wares, paper, glass, plastics and fibres/hair.

Non-destructive analysis i.e. leaving the evidence intact.

Analysis of very small sample areas (less than 0.5 mm with ease and down to 0.05

mm with the very latest instrumentation).

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Analysis of very thin smears/residues (down to ca. 0.000005 mm thick or even less)

which may be invisible optically and are not detectable by other less surface sensitive

techniques.

Physicochemical and structural properties of microbial cell surfaces

Surface analysis of organic and

Inorganic materials

Determining composition and chemical state information from surfaces

chemical state information from surfaces

Depth profiling for thin film composition

Thin film oxide thickness

Analysis of stains and residues on surfaces

Reactive frictional wear of solid-solid reactions

Silicon oxynitried thickness and measurements of dosage

Depth profiling: In depth profiling, a sputter source is used. This removes successive

layers from the surface of a sample and allows for the quanitation of element depth

profiles to be recorded in the near-surface region. This is useful in the composition of

thin films.

Angle dependence measurements: When the angle of measurement is changed, the

depth of the information gathered can be varied by 1-10 nm. The usefulness here is in

determining the concentration of additives in the surface region.

Imaging of surfaces

Utilizing a special imaging mode, the distribution of elements in surface structures

can be determined. This technique is useful in dimensions up to about 3 um.

CONCLUSION

XPS is the most convenient approaches for modern, high-throughput, automatic and ultra-fast

analysis. They provide the most substantial reduction in analysis time.

Also, XPS is the most convenient method of analysis because it can probe down to core

electrons.

It is also convenient to work and gives better resolution.

REFERENCES:

1. A "Handbook of The Elements and Native Oxides". XPS International, Inc. Retrieved

8, December 2012.

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2. Ray, S. and A.G. Shard, Quantitative Analysis of Adsorbed Proteins by X-ray

Photoelectron Spectroscopy. Analytical Chemistry, 2011. 83(22): p. 8659-8666.

3. Ellis, Andrew M.; Feher, Miklos; Wright, Timothy G. Electronic and Photoelectron

Spectroscopy: Fundamentals and Case Studies. Cambridge University Press. 2005.

4. Hufner, Stefan. Photoelectron Spectroscopy: Principles and Applications. Springer

2003.

5. Surface Chemical Analysis — Vocabulary, ISO 18115 : 2001, International

Organisation for Standardisation (ISO), TC/201, Switzerland, [1].

6. G. Hollinger, P. Pertosa, Chapter 3 in ‘Surfaces interfaces et films minces –

Observation et analyse’, Edited by B. Agius et al., Dunod, Paris, 1990.

7. M. Thompson, M.D. Baker, A. Christie, J.F. Tyson, ‘Auger Electron Spectroscopy’,

John Wiley and Sons, New York, 1985.

8. V.I. Nefedov, ‘X-ray Photoelectron Spectroscopy of Solid Surfaces’, English Edition,

VSP.

9. BV, Utrecht, 1988.

10. www.google.com.

11. Wikipedia.org.