Microscopy and Surface Analysis 2 pptx

Surface Spectrometric Analysis  Surface spectrometric techniques: – X-ray fluorescence from electron microscopy – Auger electron spectrometry – X-ray photoelectron spectrometry XPS/UPS

Lecture Date: March 17 th , 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, 3rdEd (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, 7 th 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 E g

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

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, 7 th 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 E V

 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

 The workfunction is the ‘barrier” to electron emission –

Material Crystal State Workfunction (eV)

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

Primary photon electron ion

Secondary photon electron ion

photon (X-ray) or electron electron Auger electron spec (AES)

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

K of number

produced photons

K of number



K



K Auger 

  1 

Top Figure from Strobel and Heineman, Chemical Instrumentation, A Systematic Approach, Wiley, 1989.

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 (10-8torr)

– Helps keep surfaces clean and free

from adsorbed gases, etc…

Sample

Energy analyzer

Auger electrons

Electron detector

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

2 1 2 2

2 1

R R

R R k

V ke

 More sophisticated detectors are needed to detect low

numbers of Auger electrons Two types of

electron-multiplier detectors:

Discrete dynode

Continuous dynode

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.)

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

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:

 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.

IP h

Ebinding   

w BE h

Ebinding    

(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):

 Focusing the spot and lateral resolution

-10-m diameter spots are now possible

A Thermo-Electron Dual-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 Ka lines) The Auger peaks

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:

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

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

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:

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

Elemental range Li and higher Z Li and higher Z Na and higher Z All Z

Quantification With calibration With calibration With calibration Correction

necessary Detection limits

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 Analysis

1 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 Spectroscopy

4 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)

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).