Electron Microscopy : (1) Auger electron Microscope - Principle -Diagram - Theory -Application (2) ESCA (Electron spectroscopy for chemical analysis)/XPS - Principle - Diagram -Theory -Application

1. Surface analytical
Techniques: (3.1)
(3.1.4)-Electron spectroscopy (ESCA and Auger)
By,
SACHIN ROHIDAS KALE
Class: M.Sc
Part-I
Sem-ll
Roll no. 804428/4/2018 Electron microscopy. 1

2. Electron Microscopy :
(1) Auger electron Microscope
- Principle
-Diagram
- Theory
-Application
(2) ESCA (Electron spectroscopy for chemical analysis)/XPS
- Principle
- Diagram
-Theory
-Application
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3. Electron Spectroscopy (ES):
Principle: “the signal produce by excitation of analyte consists of a
beam of e and measurements are made of the power of this beam
as a function of e energy. exitation of the analyte is possible by
irradiation with e-.”
• The 2nd method involes Auger electron spectroscopy (AES) in
which a beam of e is used.
• The 1st method involves irradiation with monochromatic X-
radiation for ESCA/XPS (X-ray photoelectron spectroscopy).
ESCA/AES both permit to find Oxidation state of element & the kind
of bonding and electronic structure .
NOTE: This technique can identify any metal or element except He
and H2.
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5. • Ionisation process inlvolves: S + hv (X-ray) S+* + e-
• The intershell e are ejected to give S+* in the excited state,
creating the vacancy in the inner shell. After excitation,e- in the
outer shell fall in to vacancies in the inner shell with
simultanious emmision of X ray photons as :
S+* S+ + hv (X ray fluorescence).
• It is also possible to lose energy when the outer shell e is
absorbed by a second e from a vacant orbital,such a second e
can be ejected from the sample giving a doubly charged ion
causing the “AUGER EFFECT”
S+* S2+ + e- ( Auger electron)
• In ESCA the entire energy of the incident photon is absorebed
and the emmited e- possess kinetic energy. the process of
ionisation proceeds as : S + e- S+* + 2e- ( Continuum)
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7. • Since the kinetic energy of each e- emmited during ESCA and AES
is related to the energy of the orbital from which the e- is ejected
and since orbital energies are pecuilar to atom or molecules. ES
can be used for qualitative analysis.
• Further as the no. of emitted e- is propotional to the
concentration of the emitter. it can also be used for quantitative
analysis.
The electron
transformation is
shown in the
figure:
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8. Auger Emmision spectroscopy:
• After initial ejection of inner shell e- , an outer shell e- falls in to
the vacancy i.e. from the K-level to the L level and from there
once again to the L level giving the transition KILL.
• The energy lost when the e- falls into the vacancy is used to eject
a second outer shell (i.e L) e- from the atom. Since all two e- are
ejected. a doubly charged + ve ion is created.
• Thus the KILL auger process is one involving initial ejection from
the 1s level followed by relaxation of 2s e in to the vacancy 1s
and subsequent emission of a 2p e- . If V is used to indicate a
valancy orbital similar to a KILL transition. we may have MMV
transition also.
• Wherein the e- initially ejected from the M shell, falls to a
vacancy with simultaneous emission of an e- from a valance
orbital. The product has a vacancy in the M shell and in the
vacancy orbital.
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9. Following transformation
is shown in figure:
• The kinetic energy of the ejected
auger e matches the difference
between the e lost by the e (b) as it
falls into the innershell and the
energy requred to remove the
auger e (c) from the atom.
• The e that is lost when the e (b)
falls to a lower shell is absorbed by
a auger e. a portion of the
absorbed energy is used to remove
the auger e from the atom and the
remainer appars as KE of the
ejected e.the energy of the auger
e is given by equation :
A = ( Ea – Eb ) - Ec = Ea – Eb - Ec
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10. • In the above process it is presumed that Ea > (Eb + Ec). The
simultaneous emission of the two auger e- is a double auger
process.
• 2° effect is a vacancy cascade effect due to injection of an auger
e- from the inner shell.
• Such a vacancy filled if e- from higher energy level fall into the
vacancy with the simultaneous emission of a second auger e with
the formation of triply positive ion.
• Auger emission can be observed with X-rays or positive ions but
one prefers e- bombardment due to a better focusing effect in
AES.
• the only limitation is the large background signal caused by
scattered incident electron ,hence the 1st derivative of emitted e
intensity as a function of kinetic energy is recorded.
• e- bombardment some times causes damage to the sample.
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11. Instrumentation of ES:
• Most instrument permits both ESCA and AES measurements and
are made up of a source, sample holder or container analyzer
analogue to monochromator ,detector and signal processing
unit. and they also need a high vacuum of 10-8 to 10-10 torr.
(1) Source and sample holder :
• The e- are directed through a slit to the e- analyzer. The source
consisting of an X-ray tube (in ESCA) or an e- gun or discharge
tube. e- guns produce a beam of e- with the energy 1 to 10 kev
for producing auger e-. A beam of 500 to 5 µm is used with
microprobes.
• While in ESCA we use X-ray source the Kex line for two elements
have a narrow band width of 0.8 to 0.9 ev to give a better
resolution.
• Solid samples are mounted in a fix positions close to the e-
source. The vacuum is used to avoid attenuation of the e- beam.
The sample is freed from moisture and O2 and is cleaned by
Argon sputtering .
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12. (2) Analyser :e- analyzers are of two type as follows :
(a) Retarding field.
(b) Dispersion type.
(1) Retarding field type –
• e- pass from the sample to a cylindrical collector through
metallic grids , giving 70% of transmission.
• The potential across the grid is gradually raised to retard the
flow of e-. the signal so collected are amplified, which in turn are
successively decreasing.
• Such instrument lack the resolution of dispersion instruments.
(2) Dispersion type model –
• the e beam is deflected by an electrostatic field when such beam
travels in a circular path. In such a case the radius of the circle
depends upon the intensity of field.
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13. (3) Detectors and Magnetic sheild :
• Multichannel photo detectors are available.
• Resolution elements of electron spectra are monitored
simultaneously .
• The path of e- in the analyzer is affected by the earths magnetic
fields.
• For better resolution , external fields are reduced to 0.1 mG by
ferromagnetic shielding.
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14. Application of Auger Electron
Spectroscopy (AES) :
• Chemical peaks can be seen and studied by using auger spectral
peaks , except H2 and He .
• To find oxidation state of the element/atom.
• Quantitative analysis by AES is restricted to elemental analysis.
• auger spectral peak in which the area under the curve/peak is
measured to give a Quantitative picture.
• Also used for depth profile analysis, wherein a change in the peak
shape of differentiated spectra affects the usual peak height
estimate of concentration.
• AES is very efficient technique for elements with low atomic
no.(N<10) and high spatial resolution.
• AES is highly used for qualitative analysis of solid surface, which
involve determination of elemental composition of surface as it is
being etched or sputtered by argon ion.
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15. Electron spectroscopy for chemical
analysis (ESCA) :
• This was dicoverd in 1981 by Siegbalin ,who was later awarded by
nobel prize for his discovery figure 31.1 depicts ESCA transition.
• The e from the inner K shell (lower lines ) and e from outer pr valancy
shells (upper lines) undergo transitions . the reaction can be
represented as : A + hv A+* + e-
• If A represents a atom/molecule/ion and A+* is the excited ion with a
higher positive charge i.e. A*> A as far as charge is concerned.
• The Ek kinetic energy of emitted e- is measured. The binding energy of
the electron, Eh is obtained which is termed as the core energy of
binding. Eh = hr - Ek - W , W is a work function.
• More then one peak can be observed due to 1s e can be seen due to
2s,2p e. ESCA is more useful for charecterisation rather than for
determination as it is concerned with core e binding energy.
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16. • X –ray are the only source of excitation of e commonly used.
usually the e whose binding energy is less then X ray energy is
ejected what we measure is Ek i.e. kinetic energy by an analyzer .
ESCA furnishes information on the outer 2nm of a surface layer.
Chemical shift in ESCA :
• if W kept constant in the above eq. ,we can calculate the chemical
shifts of say ,
A metal oxide by the equation : Eoxide = Ek(metal) - Ek(oxide)
• The chemical shift in figure (31.3) indicate transition of e in ESCA
measurements . This is usually used for surface of solid . The
chemical group which are bound to an atom can change the e
density and caused the chemical shift in ESCA peak position.as in
the NMR spectroscopy, chemical shift can be used for qualitative
analysis.
• chemical shifts for various compounds have been investigated,
specially the chemical shift corresponding to the ejection of an 1s
e from C has been studied in depth. however C compounds easily
get contaminated.
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17. • Chemical shifts can be measured relative to a reference
compound eg. N2 for nitrogenous compound . Chemical shifts
can be measured for all elements which have an inner shell e in a
chemical compound which can be ejected (except H2 and He).
Application of ESCA:
This technique is extremely used for chemical analysis .
• It can be use for elemental composition of a sample if it is
present in amounts more then 10:1 % .
• The identification of a oxidation state of inorganic compounds
can easily be carried out
• ESCA can be used for the Quantitative analysis. only in rare case
the peak overlaps encountered eg. O(1S), Sb (3d) ,Al (2s),Cu (3s
3p) etc.
• This can be eliminated by measuring spectra at an additional
peak such the auger peak which as a rule remains unchanged on
the kinetic energy scale.
• while photoelectric peaks are displaced. Table 31.1 shows
various e spectroscopic method while 31.2 shows chemical shifts
with respect to the oxidation states.
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18. 28/4/2018 Electron microscopy. 18
(1) Instrumental Analysis by Douglas A. SKOOG - F
JAMES HOLLER Crouch.
(2) Physical Principles of Electron microscopy and
Introduction to AEM.
(3) Authers Ray F. Egerton ( INTERNET) .
Reference :

19. 12/4/2018 TEM and Electron microscopy. 19