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1Formation and Structure of Glass Feb 7, 2007. Jain/Lehigh slide
Lecture 7Chemical/Electronic Structure of Glass
Syllabus Topic 6. Electronic spectroscopy studies of glass structure
Fundamentals and Applications of
X-ray Photoelectron Spectroscopy (XPS)a.k.a. Electron Spectroscopy for Chemical Analysis (ESCA)
2Formation and Structure of Glass Feb 7, 2007. Jain/Lehigh slide
Bibliography
1. D. Briggs and M.P. Seah, Practical surface analysis, vol 1. Auger
and XPS. Wiley, 1990.
2. M.A. Sherwood in Surface imaging and visualization, A.T. Hubbard,
ed. Ch 63. CRC Press.
3. D.A. Skoog, F.J. Holler, T.A. Nieman, Principles of Instrumental
Analysis, Brooks/Cole. Ch. 21.
4. NIST X-ray Photoelectron Spectroscopy Database:
http://srdata.nist.gov/xps/
5. http://www.emsl.pnl.gov/new/emsl2002/tutorials/engelhard_xps.pdf
6. www.courses.vcu.edu/PHYS661/pdf/08TechSpectroscopy041.ppt
3Formation and Structure of Glass Feb 7, 2007. Jain/Lehigh slide
Outline
Introduction – chemical structure, electron spectroscopy
XPS as a tool for composition analysis
Surface limitation of XPS
Chemical structure
Spectrometer
Video tour of Scienta instrumenthttp://rm1.cc.lehigh.edu:8080/asxgen/dept/IMI/VirtualGl
assCourse/Video/XPS_miller150.wmv
Charging issues - special to glass
4Formation and Structure of Glass Feb 7, 2007. Jain/Lehigh slide
Different Aspects of the Static Structure
Physical structurePhysical arrangement of
atoms with respect to
each other (RM-O, M-O2,
CN, etc..)
•Short range order.
•Medium range structure.
Chemical structure•Nature of bonding
(covalency, ionicity,
basicity, etc...) between
different kinds of atoms.
•Charge distribution.
Vibrational structure•Bond strength
•Local vibrations of the
mobile atom.
•Vibrations of network
structural units of small and
medium size.
N a S i B O n B O
R a n d o m n e tw o r k st r u c tu r e o f a s o d iu m s il ic a te g la s s in tw o -d im e n s io n (a f te r W a r r e n a n d B is c o e )
5Formation and Structure of Glass Feb 7, 2007. Jain/Lehigh slide
There are different manifestations of the same
overall glass structure
The various aspects are strongly interdependent, yet we
examine one aspect at a time due to the limitations of the
techniques and our own vision!
6Formation and Structure of Glass Feb 7, 2007. Jain/Lehigh slide
Electron Spectroscopy Techniques
• Photon in, Electron out
• Photoemission Spectroscopy (PES)
X-ray Photoemission Spectroscopy (XPS - 200 to 2,000 eV source)
Ultraviolet Photoemission Spectroscopy (UPS - 10 to 50 eV source)
• Auger Electron Spectroscopy (AES)
•Electron in, Electron out (inelastic)
•Auger Electron Spectroscopy (AES)
•Electron Energy Loss Spectroscopy (EELS)
7Formation and Structure of Glass Feb 7, 2007. Jain/Lehigh slide
XPS: basic process
•Developed in the mid 1960s by K. Siegbahn et al.
•Awarded the Nobel Prize for Physics in 1981
8Formation and Structure of Glass Feb 7, 2007. Jain/Lehigh slide
Auger spectroscopy
ESCA => XPS + Auger
Auger (vs. XPS): Independent of input energy,
Sensitivity for low Z atoms,
High spatial resolution if e are used as excitation beam,
Minimal matrix effects,
Difficult quantitative analysis.
Auger electron:
KL1L2,3
9Formation and Structure of Glass Feb 7, 2007. Jain/Lehigh slide
XPS: What do we detect?
e: discrete peak, e’: background that decreases with increasing k.e. Also a step at each peak.XPS info is from 10-100 A, depending on k.e.
K.E. = h – BE; if BE is w.r.t. vacuum level
Often for solids BE is defined w.r.t. Fermi
level. The measured KE should be corrected
for contact potential (i.e. work function of
sample and spectrometer), and any surface
charging of an insulating sample. Then
K.E. = h - BE - (sample) - (spectro.) - S
10Formation and Structure of Glass Feb 7, 2007. Jain/Lehigh slide
Relative intensity of XPS signal from different shells
Calculated cross sections (1.5 keV photon energy) give probabilities for observing electrons from various energy levels.
The intensity of XPS signal decreases with increasing shell, the valence band being the weakest.
11Formation and Structure of Glass Feb 7, 2007. Jain/Lehigh slide
Binding energy of electrons in various elements
BE for a particular electron shell is characteristic of the element
=>
Identify the element from its BE. However, watch out for interference as
the BE of different shell electrons of different elements may overlap!
12Formation and Structure of Glass Feb 7, 2007. Jain/Lehigh slide
Survey spectrum Identification of almost all elements in one experiments
Survey spectrum of an in situ fractured sodium diborate (Na2O-2B2O3) glass
Core levels and Auger
Valence band
13Formation and Structure of Glass Feb 7, 2007. Jain/Lehigh slide
Core level peak attributes:
•Height
•Area
•Asymmetry
•Position
•Spin-orbit split (for higher
than s-levels)
W 4f core level spectrum of W metal
exposed to air. Proctor and Sherwood, Anal. Chem. (1982).
14Formation and Structure of Glass Feb 7, 2007. Jain/Lehigh slide
What can we learn from core level spectrum?•Compared to valence electrons, the core levels are
little affected by the bonding
•Core level BE is characteristic of the element, but it
is chemically shifted (0 to 10 eV) by interaction with
valence electrons and surrounding (mostly nearest
neighbor) atoms.
•Symmetric for insulator but asymmetric for metals
from excitation of conduction electrons.
•(p3/2, p1/2), (d5/2, d3/2), (f7/2, f5/2).. levels show two
peaks from spin-orbit coupling. The ratio of
respective peaks are: 2/1; 3/2; 4/3
=>
Core level XPS gives qualitative and quantitative
chemical structure around particular element
(excluding H and He).
•The area under a core level peak is concentration of the element =>
Get composition using reference sensitivity factors.
15Formation and Structure of Glass Feb 7, 2007. Jain/Lehigh slide
Change in composition along the glassware height
0
0.1
0.2
0.3
0.4
0.5
0 1 2 3 4 5 6Going down from the Rim
Na/Si
Al/Si
Si/O
Variation of surface composition
5
4
3
2
1
16Formation and Structure of Glass Feb 7, 2007. Jain/Lehigh slide
After washing
• Na
• Ca
• Al
• Si/O
Rim region
0
0.1
0.2
0.3
0.4
0.5
0.6
Na/Si Al/Si Ca/Si Si/O
Unwashed
washed
Milky region
0
0.1
0.2
0.3
0.4
0.5
Na/Si Al/Si Ca/Si Si/O
Unwashed
Washed
Effect of corrosion on surface composition: leaching vs. uniform dissolution
•Rim is richer in alumina.
17Formation and Structure of Glass Feb 7, 2007. Jain/Lehigh slide
XPS sees only the surface region
(XPS info is from 10-100 A, depending on k.e.)
18Formation and Structure of Glass Feb 7, 2007. Jain/Lehigh slide
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
Na/Si Ca/Si Si/O
Bulk
Surface
Glass composition: surface vs. bulkHigh resolution X-ray photoelectron spectroscopy
Preferential loss of alkali and alkaline earth oxide from the surface.
19Formation and Structure of Glass Feb 7, 2007. Jain/Lehigh slide
x-ray
beam
Electron
detectorLaser light
beam
Take-off angle
= 90
specimen
x-ray
beam
Electron
detector
Laser light
beam
Take-off angle
= 5
specimen
Surface sensitivity => Angle Resolved XPS
(XPS info is from 10-100 A, depending on k.e.)
Obtain depth profile of the chemical info!
20Formation and Structure of Glass Feb 7, 2007. Jain/Lehigh slide
Composition as f(depth, ambient)
As50Se50 film on Si, irrad.
in air for ~12 h
0
10
20
30
40
50
60
70
80
0 5 10 15 20 25 30 35
Depth (Angstrom)
Co
ncen
trati
on
(%
)
O (Irradiated in air)
As (Irradiated in air)
Se (Irradiated in air)
As (as-prepared film)
Se (as-prepared film)
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