Lecture Date: March 17 th, 2008 Microscopy and Surface Analysis 2.

Lecture Date: March 17th, 2008

Microscopy and Surface Analysis 2

Reading Assignments for Microscopy and Surface Analysis

Skoog, Holler and Nieman, Chapter 21, “Surface Characterization by Spectroscopy and Microscopy”

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.

Introduction to the Solid State

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



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?



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




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

Not particularly surface sensitive!

Electron Microprobes: X-ray Emission

Electron Spectroscopy

Electron spectroscopy – measuring the energy of electrons.

Major forms:– 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: Basic Mechanism

Auger electrons are created from outer energy levels (i.e. less-tightly bound electrons, possibly valence levels).

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:

http://www.cea.com/cai/auginst/caiainst.htm

AES: Detectors

More sophisticated detectors are needed to detect low numbers of Auger electrons. Two types of electron-multiplier 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

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)

http://www.cem.msu.edu/~cem924sg/KineticEnergyGraph.html

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)

http://www.cem.msu.edu/~cem924sg/KineticEnergyGraph.html

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 not (usually) there 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 in AES, 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

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 w.r.t. 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)

http://www.cem.msu.edu/~cem924sg/BindingEnergyGraph.html

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.

Optional Homework Problems (for Study!)

Skoog, Holler and Nieman, Chapter 21.

Problems: 21-1, 21-2, 21-4, and 21-8.

Further Reading

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).

Lecture Date: March 17 th, 2008 Microscopy and Surface Analysis 2.

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Transcript of Lecture Date: March 17 th, 2008 Microscopy and Surface Analysis 2.