Lecture Date: March 25 th , 2013 Microscopy and Surface Analysis 2
Jan 03, 2016
Lecture Date: March 25th, 2013
Microscopy and Surface Analysis 2
Basic Considerations for Surface Spectroscopy
Common sampling “modes”– Spot sampling
– Raster scanning
– Depth profiling
Surface contamination: – The obvious contamination/alteration of surfaces that can be the
result of less-than careful sample preparation
– Solid surfaces can adsorb gases: At 10-6 torr, a complete monolayer of a gas (e.g. CO) takes just 3
seconds to form. At 10-8 torr, monolayer formation takes 1 hour.
– Most studies are conducted under vaccuum – although there are newer methods that don’t require this.
D. M. Hercules and S. H. Hercules, J. Chem. Educ., 1984, 61, 403.
Surface Spectrometric Analysis
Surface spectrometric techniques:– X-ray fluorescence (from electron microscopy)
– Auger electron spectrometry
– X-ray photoelectron spectrometry (XPS/UPS)
– Secondary-ion mass spectrometry (SIMS)
Depth profiling – if you are going to study surfaces with high lateral resolution (e.g. using microscopy), then wouldn’t it be nice to obtain information from various depths within the sample?
The Basic Idea Behind Surface Spectrometry
Surface
Primaryphotonelectron
ion
Secondaryphotonelectron
ion
Photons, electrons, ions: they can go in and/or out!!!
Leads to lots of techniques, and lots of acronyms!
Primary Secondary Name of Technique
photon (X-ray/UV) electron XPS (ESCA) and UPS
photon (X-ray) or electron electron Auger electron spec. (AES)
ion ion SIMS (secondary ion MS)
photon ion LMMS (laser microprobe MS)
electron Photon (X-ray) SEM “electron microprobe”
Electron Microprobes and X-ray Emission
Electron microscopy (usually SEM) can also be used to perform X-ray emission analysis in a manner similar to X-ray fluorescence analysis
– see the X-ray spectrometry lecture for details on the spectra
The electron microprobe (EM) is the commonly used name for this type of X-ray spectrometry
Both WDS and EDS detectors are used (as in XRF), elemental mapping
This technique is not particularly surface sensitive!
Electron Microprobes: X-ray Emission
Electron Spectroscopy
Spectroscopic methods that measure electron energies (electron spectroscopy) are often surface sensitive, because electrons can’t escape from deep inside a material
Major forms of electron spectroscopy:– Auger electron spectroscopy
– X-ray/UV photoelectron spectroscopy
– Electron energy loss spectroscopy (EELS)
Electron Spectroscopy: Surface Sensitivity
Electrons can only escape from shallow depths in the surface of a sample, because they will undergo collisions and lose energy.
XPS/AES region, electrons that have
not been inelastically scattered from shallow regions
(mostly excitation of conduction-band
electrons)
Deep electrons that undergo inelastic collisions but lose
energy (exciting e.g. phonons)
Auger Electron Spectrometry (AES)
The Auger process can also be a source of spectral information. Auger electrons are expelled from atomic/molecular orbitals and their kinetic energy is characteristic of atoms/molecules
However, since it is an electron process, analysis of electron energy is necessary!
– This is unlike the other techniques we have discussed, most of which measure photon wavelengths or energy
Auger electron emission is a three-step (three electron) process, that leaves an atom doubly-ionized
AES: Basic Mechanism
See Figure 21-7 in Skoog, et al. for a related figure.
AES: Naming Auger Electrons
Auger electrons are created from outer energy levels (i.e. less-tightly bound electrons, possibly valence levels).
Their name indicates their origin:
This example would be called a
LMM Auger electron. Other Common types
are denoted KLL and MNN.
AES: Efficiency of Auger Electron Production
Two competing processes:
– X-ray fluorescence
– Auger electron emission
Auger electrons predominate at lower atomic number (Z)
Photoelectron emission does not compete!
created vacanciesshellK ofnumber
produced photonsK ofnumber K
KAuger 1
Top Figure from Strobel and Heineman, Chemical Instrumentation, A Systematic Approach, Wiley, 1989.
Ion
Gun
AES: Spectrometer Design
AES instruments are designed like an SEM – often they are integrated with an SEM/EDXA system
Unlike an SEM, AES instruments are designed to reach higher vacuum (10-8 torr)
– Helps keep surfaces clean and free from adsorbed gases, etc…
Basic components:– Electron source/gun
– Electron energy analyzer
– Electron detector
– Control system/computer
– Ion gun (for depth profiling)
ElectronGun
Sample
Energyanalyzer
Augerelectrons
Electrondetector
AES (and XPS): Electron Energy Analyzers Two types of electron energy analyzers (also used in XPS):
Cylindrical mirror analyzer(higher efficiency)
More common for AES
(Right) Diagram from http://www.cea.com/cai/auginst/caiainst.htm(Left) Diagram from Strobel and Heineman, Chemical Instrumentation, A Systematic Approach, Wiley, 1989.
Concentric hemispherical analyzer(higher resolution) – better resolution, mostly
for XPS/UPS
21
22
21
RR
RRk
VkeKEelectron
Electrons only pass if their KE is:
AES: Detectors
More sophisticated detectors are needed to detect low numbers of Auger electrons. Two types of electron-multiplier detectors (adapted from MS detectors):
Discrete dynode
Continuous dynode
Both types of detector are also used in XPS/UPS!!!
AES: Surface Analysis
AES is very surface sensitive (10-50 Ǻ) and its reliance on an electron beam results in excellent lateral resolution
Diagram from http://www.cea.com/cai/auginst/caiainst.htm
The electron beam does not have to be monochromatic– Note: an X-ray beam can also
be used for AES, but is less desirable b/c it cannot currently be focused as tightly (as is the case in XPS)
Auger electrons typically have energies of < 1000 eV, so they are only emitted from surface layers.
AES: Spectral Interpretation AES Electron Kinetic Energies* versus Atomic Number
(Most intense peaks only. Valid for CMA-type analyzers.)
*Data is from J.C. Vickerman (Ed.), "Surface Analysis: The Principal Techniques“, John Wiley and Sons, Chichester, UK, 1997 . Image from http://www.cem.msu.edu/~cem924sg/KineticEnergyGraph.html (accessed 12-Nov-2004)
AES: Typical Spectra
AES: Elemental Surface Analysis
Very common application of AES - elemental surface analysis
For true surface analysis, AES is better than SEM/X-ray emission (electron microprobe) because it is much more surface sensitive
AES can be easily made quantitative using standards.
Image from http://www.cem.msu.edu/~cem924sg/ (accessed 12-Nov-2004)
AES: Chemical Shifts
Chemical information (i.e. on bonding, oxidation states) should be found in Auger spectra because the electron energy levels are sensitive to the chemical environment.
In practice, it is usually not found because too many electron energy levels are involved – it is difficult to calculate and simulate Auger spectra.
X-ray Photoelectron Spectrometry (XPS)
Photoelectron spectroscopy is used for solids, liquids and gases, but has achieved prominence as an analytical technique for solid surfaces
XPS: “soft” x-ray photon energies of 200-2000 eV for analysis of core levels
UPS: vacuum UV energies of 10-45 eV for analysis of valence and bonding electrons
Photoelectric effect: Proposed by A. Einstein (1905), harnessed by K. Siegbahn (1950-1970) to develop XPS
XPS: Basic Concepts
Like Auger electrons, photoelectrons can not escape from depths greater than 10-50 A inside a material
Schematically, the photoelectron process is:
eAhA *
cationatom or molecule
Like in AES, the kinetic energy of the emitted electron is measured in a spectrometer
XPS: Review of X-ray Processes
XPS: Photoelectron Emission and Binding Energy
The kinetic energy of the emitted electron can be related to the “binding energy”, or the energy required to remove an electron from its orbital.
– Higher binding energies mean tighter binding – e.g. as atomic number goes up, binding energies get tighter because of increasing number of protons.
IPhEbinding wBEhEbinding
(gas)
(solid)
http://www.chem.qmw.ac.uk/surfaces/scc/scat5_3.htm
Quick Review of Electron Energy levels in Solids
In solids, atomic and molecular energy levels broaden into bands that in principle contain as many states as there are atoms/molecules in the solid.
P.A. Cox, "The Electronic Structure and Chemistry of Solids" Oxford University Press, 1987.C. Kittel, Solid-state Physics, 7th Ed, Wiley, 1999.
W. A. Harrison, Electronic Structure and the Properties of Solids, Dover, 1989.
Bands may be separated by a band gap with energy Eg
Energy Bands in the Solid State Bands are continuous and delocalized over the material
Band “widths” are determined by size of orbital overlap
P.A. Cox, "The Electronic Structure and Chemistry of Solids" Oxford University Press, 1987.C. Kittel, Solid-state Physics, 7th Ed, Wiley, 1999.
W. A. Harrison, Electronic Structure and the Properties of Solids, Dover, 1989.
The highest-energy filled band (which may be only partially filled) is called the valence band
The lowest-energy empty band is called the conduction band
The Workfunction: A Barrier to Electron Emission How does the electronic arrangement in solids affect
surfaces? In particular, how can an electron be removed?
P.A. Cox, "The Electronic Structure and Chemistry of Solids" Oxford University Press, 1987.C. Kittel, Solid-state Physics, 7th Ed, Wiley, 1999.
W. A. Harrison, Electronic Structure and the Properties of Solids, Dover, 1989.
For some electron being removed, its energy just as it gets free is EV
The energy required to remove the electron is the workfunction (typically several eV)
Free electron!
The Workfunction: A Barrier to Electron Emission
Workfunctions vary from <2 eV for alkali metals to >5 eV for transition metals.
Data from CEM 924 Lectures presented at MSU (2001).
The workfunction is the ‘barrier” to electron emission – like the wall in the particle-in-a-box concept.
Material Crystal State Workfunction (eV)
Na polycrystalline 2.4
Cu polycrystalline 4.4
Ag polycrystalline 4.3
Au polycrystalline 4.3
Pt polycrystalline 5.3
W polycrystalline 4.5
W(111) single crystal 4.39
W(100) single crystal 4.56
W(110) single crystal 4.68
W(112) single crystal 4.69
XPS: Binding Energy
The workfunction w is usually linked to the spectrometer (if the sample is electrically connected)
In gases, the BE is directly related to IP– Ionization potential – the energy required to take an electron out
of its orbital all the way to the “vacuum” (i.e. far away!)
– PE spectroscopy on gases is used to check the accuracy of modern quantum chemical calculations
In conducting solids the workfunction is involved
Koopman’s Theorem: binding energy = -(orbital energy)– Orbital energies can be calculated from Hartree-Fock
Another definition for XPS binding energy: the minimum energy required to move an inner electron from its orbital to a region away from the nuclear charge. Absorption edges result from this same effect
XPS: Sources Monochromatic sources using electrons
fired at elemental targets that emit x-rays. – Can be coupled with separate post-source
monochromators containing crystals, for high resolution (x-ray bandwidth of <0.3 Å)
XPS Sources (hit core electrons):– Mg K radiation: h = 1253.6 eV
– Al K radiation: h = 1486 eV
– Ag Lradiation: h = 2984.3 eV
UPS Sources (hit valence electrons):– He(I) radiation: h = 21.2 eV (~58.4 nm)
h = 23 eV (~53.7 nm)
– He(II) radiation: h = 41 eV (~30.4 nm)
Focusing the spot and lateral resolution - 10-m diameter spots are now possible
A Thermo-ElectronDual-anode (Al/Mg)
XPS source
XPS: Spectral Interpretation
Orbital binding energies can be interpreted based on correlation tables, empirical trends and theoretical analysis.
Peaks appear in XPS spectra for distinguishable atomic and molecular orbitals.
Auger peaks also appear in XPS spectra – they are easily distinguished by comparing the XPS spectra from two sources (e.g. Mg and Al K lines). The Auger peaks remain unchanged with respect to kinetic energy, while the XPS peaks shift.
XPS: Binding Energy Ranges
XPS Photoelectron Binding Energies versus Atomic Number (Z)
*Data from C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder and G.E. Muilenberg, Eds., "Handbook of X-ray Photoelectron Spectroscopy,"Perkin-Elmer Corp., Flying Cloud, MN, 1979.
Image from http://www.cem.msu.edu/~cem924sg/BindingEnergyGraph.html (accessed 12-Nov-2004)
XPS: Typical Spectra
An XPS survey spectrum of stainless steel:
Spectrum image from http://www.mee-inc.com/esca.html
XPS: Typical Spectra
An expanded XPS spectrum of the C1s region of PET:
Spectrum image from http://www.mee-inc.com/esca.html
XPS: Chemical Shifts
Peaks appear in XPS spectra for distinguishable atomic and molecular orbitals.
Effects that cause chemical shifts in XPS spectra:
– Oxidation states
– Covalent structure
– Neighboring electron withdrawing groups
– Anything else that can affect ionization/orbital energies
XPS: Depth Profiling
Option 1: Sputtering techniques– Disadvantage – can damage the surface
– Advantage – wide range of depths can be sampled (just keep sputtering), e.g. 100 A
Option 2: Angle-resolved XPS (AR-XPS)– Reducing the photoelectron take-off angle
(measured from the sample surface) reduces the depth from which the XPS information is obtained. XPS is more surface sensitive for grazing take-off angles than for angles close to the surface normal (longer PE paths).
– The most important application of angle resolved XPS (AR-XPS) is in the estimation of the thickness of thin films e.g. contamination, implantation, sputtering-altered and segregation layers.
For more on AR-XPS, see Briggs and Seah, Practical Surface Analysis, 2nd Ed., Vol. 1. “Auger and X-ray Photoelectron Spectroscopy,” Wiley, 1990, pp. 183-186, 244-250
normal
grazing
Sample
electron
Depth Profiling with Angle-Resolved XPS
AR-XPS data is often acquired by tilting the specimen
Example: gallium arsenside with a thin oxide layer on its surface:
AR-XPS figure from C. R. Brundle, J. F. Watts and J. Wolstenholme, in Ewing’s Analytical Instrumentation Handbook 3rd Ed., Dekker 2005.
bulk
surface(grazing)
Sample
electron
Grazing angle(X-ray takeoff angle)
XPS: Applications A modern application of XPS – study the nature of PEG as a surface
coating to prevent biofouling in biosensors– Biofouling: the tendency of proteins to adsorb to silicon-based surfaces
XPS can be used, with AFM, to observe the coating of PEG onto silicon surfaces (PEG-silane coupling) - Increased C 1s C-O signal indicates greater grafting density
S. Sharma, et al., “XPS and AFM analysis of antifouling PEG interfaces for microfabricated silicon biosensors”, Biosensors and Bioelectronics, 20 227–239 (2004).
XPS: Quantitative Applications
Quantitative XPS is not as widely used as the qualitative version of the technique.
Variations in instrument parameters and set-up have traditionally caused problems with reproducibility
Using internal standards, XPS can achieve quantitative accuracies of 3-10% in most cases (and getting better every year, as more effort is put into this type of analysis)
AES and XPS: Combined Systems
Dual Auger/XPS systems are very common, also combined with a basic SEM
– Note - SAM = scanning Auger microprobe
Auger is seen as complementary to XPS with generally better lateral resolution
Both are extreme surface sensitive techniques:– AES better elemental quantitative analysis
– XPS contains more chemical information
Also, remember that Auger peaks are often seen in XPS spectra (and are hence useful analytically) – they can be identified by changing source, so that the X-ray peaks shift (the Auger peaks do not).
Comparison of XPS, AES and Other Techniques
* = yes, with compensation for the effects of sample chargingSIMS = secondary ion mass spectrometry, discussed in the “Ion and Particle Spectrometry” Lectures.
Characteristic AES XPS SEM/X-ray EM SIMS
Elemental range Li and higher Z Li and higher Z Na and higher Z All Z
Specificity Good Good Good Good
Quantification With calibration With calibration With calibration Correction necessary
Detection limits
(atomic fraction)
10-2 to 10-3 10-2 to 10-3 10-3 to 10-8 10-3 to 10-8
Lateral resolution (um)
0.05 ~1000 0.05 1
Depth resolution (nm)
0.3-2.5 1-3 1000-50000 0.3-2
Organic samples No Yes Yes* Yes
Insulator samples Yes* Yes Yes* Yes*
Structural information
Elemental Elemental and Chemical
Elemental Elemental and Chemical
Destructiveness Low Very Low Medium Medium
See Strobel and Heineman, Chemical Instrumentation, A Systematic Approach, Wiley, 1989, pg. 832.
XPS: New Applications
A recent report in Chem. Commun. (2005) by Peter Licence and co-workers describes the use of XPS to study ionic liquids
Normal liquids evaporate under ultrahigh vacuum (UHV), ionic liquids do not (they have a vapor pressure of nearly zero!)
Why? Ionic liquids have become important for electrochemistry, catalysis, etc…
See C&E News Oct. 31, 2005, pg 10.
Further Reading
Hand-out Review Article: C. R. Brundle, J. F. Watts, and J. Wolstenholme, “X-ray Photoelectron and Auger Electron Spectroscopy”, in Ewing’s Analytical Instrumentation Handbook, 3rd Ed. (J. Cazes, Ed.), Marcel-Dekker 2005.
Electron Microscopy and Electron Microprobe/X-ray Emission Analysis1. J. I. Goldstein et al., Scanning Electron Microscopy and X-ray Microanalysis, 3rd Ed., Kluwer Academic, 2003.2. J. J. Bozzola et al., Electron Microscopy: Principles and Techniques for Biologists, 2nd Ed., Jones and
Bartlett, 1998.3. J. W. Edington, N. V Philips, Practical Electron Microscopy in Materials Science, Eindhoven, 1976.
Electron Microscopy and Electron Diffraction/Electron Energy Loss Spectroscopy4. A. Engel and C. Colliex, “Application of scanning transmission electron microscopy to the study of biological
structure”, Current Biology 4, 403-411 (1993). (STEM and EELS)5. W. Chiu and M. F. Schmid, “Electron crystallography of macromolecules”, Current Biology 4, 397-402 (1993).
(ED and LEED)6. W. Chiu, “What does electron cryomicroscopy provide that X-ray crystallography and NMR cannot?”, Annu.
Rev. Biophys. Biomol. Struct., 22, 233-255 (1993). (Electron Cryomicroscopy/Imaging)7. L. Tang and J. E. Johnson, “Structural biology of viruses by the combination of electron cryomicroscopy and
X-ray crystallography”, 41, 11517-11524 (2002). (Electron Cryomicroscopy/Imaging)
Optical Microscopy8. R. H. Webb, "Confocal optical microscopy“, Rep. Prog. Phys. 59, 427-471 (1996).
Force Microscopy:9. R. J. Hamers, “Scanned probe microscopies in chemistry,” J. Phys. Chem., 100, 13103-13120 (1996).
Further Reading
Surface Spectrometric Methods (XPS and AES)10. T. L. Barr, Modern XPS, Boca Raton: CRC Press (1994).11. M. Thompson, M. D. Baker, A. Christie, and J. F. Tyson, Auger Electron Spectroscopy, New York:
Wiley (1985). 12. N. H. Turner, “X-ray Photoelectron and Auger Electron Spectroscopy”, Applied Spectroscopy
Reviews, 35 (3), 203-254 (2000).