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Dr. Nizam M. El-Ashgar Surface Characterization Surface Characterization by Spectroscopy and by Spectroscopy and Microscopy Microscopy
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Surface Chemistry.ppt

Nov 14, 2014

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Faisal Faisal

Definition of Solid Surface:
The boundary layer between a solid and a vacuum, a gas or a liquid.
A part of solid that differs in composition from the average composition of the bulk of the solid.
Top layer of atoms with a non-uniform composition that varies from the bulk.
The surface may be several of atomic layers deep.
surface measurements dose not affect the measurement of average composition of bulk (tiny fraction of the total solid).
Classical Methods:
Provide useful information about the physical nature of surfaces but less about their chemical nature.
Involve obtaining optical and electron microscopic images of surfaces and adsorption isotherms, surface areas, surface roughness, pore sizes and reflectivity.
Spectroscopic Methods: (1950)
Provide information about the chemical nature of surfaces.
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Page 1: Surface Chemistry.ppt

Dr. Nizam M. El-Ashgar

Surface Characterization by Surface Characterization by Spectroscopy and MicroscopySpectroscopy and Microscopy

Page 2: Surface Chemistry.ppt

Importance of surface characterizationImportance of surface characterization

1- Heterogeneous Catalysis.1- Heterogeneous Catalysis.

2- Semiconductor thin film technology.2- Semiconductor thin film technology.

3- Corrosion and adhesion mechanisms3- Corrosion and adhesion mechanisms

4- Activity of metal surfaces.4- Activity of metal surfaces.

5- Studies of the behavior and functions of biological 5- Studies of the behavior and functions of biological membranes. membranes.

Page 3: Surface Chemistry.ppt

IntroductionIntroduction Definition of Solid Surface:Definition of Solid Surface:

• The boundary layer between a solid and a vacuum, a gas or a The boundary layer between a solid and a vacuum, a gas or a liquid.liquid.

• A part of solid that differs in composition from the average A part of solid that differs in composition from the average composition of the bulk of the solid.composition of the bulk of the solid.

- Top layer of atoms with a non-uniform composition that varies Top layer of atoms with a non-uniform composition that varies from the bulk.from the bulk.

- The surface may be several of atomic layers deep.The surface may be several of atomic layers deep.- surface measurements dose not affect the measurement of surface measurements dose not affect the measurement of

average composition of bulk (tiny fraction of the total solid).average composition of bulk (tiny fraction of the total solid).

Classical Methods:Classical Methods:

Provide useful information about the physical nature of surfaces Provide useful information about the physical nature of surfaces but less about their chemical nature.but less about their chemical nature.

Involve obtaining optical and electron microscopic images of Involve obtaining optical and electron microscopic images of surfaces and adsorption isotherms, surface areas, surface surfaces and adsorption isotherms, surface areas, surface roughness, pore sizes and reflectivity.roughness, pore sizes and reflectivity.

Spectroscopic Methods: (1950)Spectroscopic Methods: (1950)

Provide information about the chemical nature of surfaces.Provide information about the chemical nature of surfaces.

Page 4: Surface Chemistry.ppt

Spectroscopic Surface MethodSpectroscopic Surface Method

The chemical composition of the solid surface often differs The chemical composition of the solid surface often differs from the interior or bulk.from the interior or bulk.

Spectroscopic surface methods provide both qualitative and Spectroscopic surface methods provide both qualitative and quantitative chemical information about the composition of quantitative chemical information about the composition of a surface layer of a solid that is a few angstrom units to a a surface layer of a solid that is a few angstrom units to a few tens of angstrom units in thicknessfew tens of angstrom units in thickness

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General Technique in surface spectroscopyGeneral Technique in surface spectroscopy

The solid sample is irradiated with a primary beam made up of The solid sample is irradiated with a primary beam made up of photons, electrons or neutral molecules.photons, electrons or neutral molecules.

This impact on a solid surface and results in formation of a This impact on a solid surface and results in formation of a secondary beam also consisting of photons, electrons, molecules secondary beam also consisting of photons, electrons, molecules or ions form the solid surface.or ions form the solid surface.

The type of particles making up the primary beam is not The type of particles making up the primary beam is not necessarily the same as the particle of secondary beam.necessarily the same as the particle of secondary beam.

The secondary beam which results from scattering, sputtering or The secondary beam which results from scattering, sputtering or emission is studied by a variety of spectroscopic methods.emission is studied by a variety of spectroscopic methods.

Most Effective Surface Methods:Most Effective Surface Methods:

Those in which the primary, the secondary beam or both is made up Those in which the primary, the secondary beam or both is made up of either electrons, ions or molecules and not photons to ensure of either electrons, ions or molecules and not photons to ensure the measurement be restricted to surface not the bulk.the measurement be restricted to surface not the bulk.

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Examples:Examples:

Maximum penetration depth of abeam of 1-keV electrons or ions is 25 Maximum penetration depth of abeam of 1-keV electrons or ions is 25 oo. Whereas the penetration depth of a photon beam of the same . Whereas the penetration depth of a photon beam of the same energy is about 10energy is about 104 4 o.o.

Precautions must be taken in case of using two beams of photons (X-Precautions must be taken in case of using two beams of photons (X-Ray, FL, Raman, IR reflection spectroscopy).Ray, FL, Raman, IR reflection spectroscopy).

Studying surfaces by these methods is possible but bulk inreferences Studying surfaces by these methods is possible but bulk inreferences must be avoided.must be avoided.

Surface Spectroscopic MethodsSurface Spectroscopic Methods

Classified according to the nature of the primary and secondary Classified according to the nature of the primary and secondary beams.beams.

Table:Table:

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Sampling SurfacesSampling Surfaces

Three types of sampling methods are employed regardless of the Three types of sampling methods are employed regardless of the type of spectroscopic surface method being used.type of spectroscopic surface method being used.

First method:First method:

Involves focusing the primary beam on a single small area of the Involves focusing the primary beam on a single small area of the sample and observing the secondary beam.sample and observing the secondary beam.

Second method:Second method:

Mapping the surface in which a region of the surface is scanned by Mapping the surface in which a region of the surface is scanned by moving the primary beam across the surface in a raster pattern of moving the primary beam across the surface in a raster pattern of measured increments (linear or two dimensional mapping).measured increments (linear or two dimensional mapping).

Third method:Third method:

Depth profiling in which a beam of ions from an ion gun is used to Depth profiling in which a beam of ions from an ion gun is used to etch a hole in the surface by sputtering. A finer primary beam is etch a hole in the surface by sputtering. A finer primary beam is used to produce a secondary beam from the center of theused to produce a secondary beam from the center of the

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Surface ContaminationSurface Contamination Contamination of surfaces (by adsorption of some atmospheric Contamination of surfaces (by adsorption of some atmospheric

components: water oxygen and COcomponents: water oxygen and CO22) is a problem in analysis.) is a problem in analysis. Even in vacuum these contaminations occurs in a relatively short time.Even in vacuum these contaminations occurs in a relatively short time.For example:For example:At P of 10At P of 10-6-6 torr, a clean surface will become covered by a monolayer of gas torr, a clean surface will become covered by a monolayer of gas

molecules in just 3 s.molecules in just 3 s.At P of 10At P of 10-8-8 torr coverage occurs in about 1 hr. torr coverage occurs in about 1 hr.At P of 10At P of 10-10-10 torr, 10 hr is required. torr, 10 hr is required.Provision must often be made to clean the sample surface, in the chamber Provision must often be made to clean the sample surface, in the chamber

used for the irradiating the sample.used for the irradiating the sample.Cleaning Methods:Cleaning Methods:Baking the sample at high temperature, sputtering the sample with a beam of Baking the sample at high temperature, sputtering the sample with a beam of

inert gas ions from an electron gun, mechanical scraping or polishing the inert gas ions from an electron gun, mechanical scraping or polishing the surface with an abrasive, ultrasonic washing with various solvents and surface with an abrasive, ultrasonic washing with various solvents and bathing the sample in reducing P to remove oxides. bathing the sample in reducing P to remove oxides.

Other Contaminations:Other Contaminations: The primary beam itself can alter the surface as a measurement progresses, The primary beam itself can alter the surface as a measurement progresses,

damage occurs that depends on the momentum of the primary beam damage occurs that depends on the momentum of the primary beam particles. particles.

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Electron SpectroscopyElectron Spectroscopy

XPS, AES and UPS methods are based on analysis of emitted electrons XPS, AES and UPS methods are based on analysis of emitted electrons produced by various incident beams.produced by various incident beams.

The signal from the analyte consists of a beam consists of abeam of The signal from the analyte consists of a beam consists of abeam of electrons rather than photons.electrons rather than photons.

Determination is of the power of the electrons beam as a function of E (hDetermination is of the power of the electrons beam as a function of E (h ) ) or frequency.or frequency.

Studying needs high resolution spectral measurements of electrons .Studying needs high resolution spectral measurements of electrons . Used to study surfaces.Used to study surfaces.

Types of Electron Spectroscopy:Types of Electron Spectroscopy:

1- Photoelectron Spectroscopy (XPS) or electron spectroscopy for chemical 1- Photoelectron Spectroscopy (XPS) or electron spectroscopy for chemical analysis (ESCA) :analysis (ESCA) :

Sample surface irradiated with monochromatic X-radiation.Sample surface irradiated with monochromatic X-radiation.

2- Auger electron spectroscopy (AES).2- Auger electron spectroscopy (AES).

Excitation occurs by a beam of electrons. Excitation occurs by a beam of electrons.

3- Ultraviolet Photoelectron Spectroscopy (UPS).3- Ultraviolet Photoelectron Spectroscopy (UPS).

A monochromatic beam of UV causes ejection of electrons from the analyte A monochromatic beam of UV causes ejection of electrons from the analyte (not common).(not common).

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Electron spectroscopy is a powerful tool for the identification of all Electron spectroscopy is a powerful tool for the identification of all the elements in the periodic table except H and He.the elements in the periodic table except H and He.

The method permits determination of the oxidation state of an The method permits determination of the oxidation state of an element and the type of species to which it is bonded.element and the type of species to which it is bonded.

The technique provides useful information about the electronic The technique provides useful information about the electronic structure of molecules.structure of molecules.

Applied to gases and solids and to solutions and liquids.Applied to gases and solids and to solutions and liquids. Restricted to surface layer of a few atomic layers thick (20 to 50 Restricted to surface layer of a few atomic layers thick (20 to 50

AAoo) because of poor penetrating power of electrons.) because of poor penetrating power of electrons. Composition of layers is different from average composition of Composition of layers is different from average composition of

entire sample.entire sample.

Valuable Current Applications:Valuable Current Applications:

Qualitative analysis of solid surfaces such as metal alloys, Qualitative analysis of solid surfaces such as metal alloys, semiconductors, and heterogeneous catalysis.semiconductors, and heterogeneous catalysis.

Limited applications of quantitative analysis.Limited applications of quantitative analysis.

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X-Ray Photoelectron SpectroscopyX-Ray Photoelectron Spectroscopy In XPS the kinetic E of emitted electrons is recorded.In XPS the kinetic E of emitted electrons is recorded. The spectrum thus consists of a plot of the number of emitted The spectrum thus consists of a plot of the number of emitted

electrons or the power of the electron beam as a function of the E electrons or the power of the electron beam as a function of the E or or or or of the emitted electrons . of the emitted electrons .

Principles of XPS:Principles of XPS:

XPS provides information:XPS provides information:

1- About the atomic composition of a sample. 1- About the atomic composition of a sample.

2- about the structure and oxidation state of the compounds.2- about the structure and oxidation state of the compounds.

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Physical BasesPhysical Bases

Based upon photon in/electron out process.Based upon photon in/electron out process. Photon E (Einstein ): Photon E (Einstein ): E=hE=hwherewhere : h - Planck constant ( 6.62 x 10 : h - Planck constant ( 6.62 x 10 -34-34 J s ). J s ).    - frequency (Hz) of the radiation.- frequency (Hz) of the radiation. In XPS the photon is absorbed by an atom in a molecule or solid, In XPS the photon is absorbed by an atom in a molecule or solid,

leading to ionization and the emission of a core (inner shell) leading to ionization and the emission of a core (inner shell) electron. electron.

But in case of UPS the photon interacts with valence levels But in case of UPS the photon interacts with valence levels (ionization by lost of one of them)(ionization by lost of one of them)

Process of PhotoionizationProcess of PhotoionizationA + A + hh A A+*+* + + ee--

Where A can be an atom, a molecule or an ion and AWhere A can be an atom, a molecule or an ion and A+* +* is an is an electronicallyelectronically excited cation.excited cation.

E(A) + E(A) + h h = E(A = E(A++ ) + E(e ) + E(e--) ) Since the electron's energy is present solely as kinetic energy (KE)

this can be rearranged to give the following expression for the KE of the photoelectron :

KE = KE = h h – [ E(A+ ) - E(A) ] – [ E(A+ ) - E(A) ]

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The difference in energy between the ionized and neutral atoms, is generally called the binding energy (BE) of the electron

KE = h - BEWhich corrected to:

KE = h - BE –WW is the work function of the spectrometer, which corrects for the

electrostatic environment in which the electron is formed and measured.

BE of an electron is characteristic of the atom and orbital from which the electron was emitted.

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Experimental DetailsExperimental Details

The basic requirements:The basic requirements: a source of fixed-energy radiation (an x-ray a source of fixed-energy radiation (an x-ray

source)  source)  an electron energy analyzer (which can an electron energy analyzer (which can

disperse the emitted electrons according to disperse the emitted electrons according to their kinetic energy, and thereby measure their kinetic energy, and thereby measure the flux of emitted electrons of a particular the flux of emitted electrons of a particular energy)energy)   

a high vacuum environment (to enable the a high vacuum environment (to enable the emitted photoelectrons to be analyzed emitted photoelectrons to be analyzed without interference from gas phase without interference from gas phase collisions)collisions)

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InstrumentationInstrumentation

Expensive: 300,000 – 900,000.Expensive: 300,000 – 900,000. Components:Components:1- Source 1- Source 2- sample holder 2- sample holder 3- Analyzer (same function as 3- Analyzer (same function as

monochromator) .monochromator) . 4- detector4- detector 5- signal processor and read 5- signal processor and read out.out.

Electron spectrometers generally require elaborate vacuum systems to Electron spectrometers generally require elaborate vacuum systems to reduce P in all the components (10reduce P in all the components (10-5 -5 – 10– 10-8-8) torr.) torr.

1- Sources1- SourcesSimplest X-ray sources for XPS are X-ray tubes equipped with Mg or Al Simplest X-ray sources for XPS are X-ray tubes equipped with Mg or Al

targets and suitable filters.targets and suitable filters. Mg KMg K radiation : radiation : hh = 1253.6 eV = 1253.6 eV Al K Al K radiation : radiation : h h = 1486.6 eV = 1486.6 eV The emitted photoelectrons will therefore have kinetic energies in the The emitted photoelectrons will therefore have kinetic energies in the

range ofrange of ca. 0 - 1250 eV or 0 - 1480 eVca. 0 - 1250 eV or 0 - 1480 eVThe KThe K lines for these two elements have considerably narrower band lines for these two elements have considerably narrower band

width (0.8 – 0.9 eV) than that of higher atomic number targets.width (0.8 – 0.9 eV) than that of higher atomic number targets.Narrow bands lead to enhanced resolutions.Narrow bands lead to enhanced resolutions.Relatively sophisticated XPS employ a crystal monochromator to provide Relatively sophisticated XPS employ a crystal monochromator to provide

an X-ray beam having a bandwidth of about 0.3 eV.an X-ray beam having a bandwidth of about 0.3 eV.Monochromators improve S/N ratios.Monochromators improve S/N ratios.

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Page 19: Surface Chemistry.ppt

X-ray Photoelectron SpectrometerX-ray Photoelectron Spectrometer

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2- Sample Holder:2- Sample Holder:

Solid samples: Solid samples: are mounted in a fixed position as close to photon are mounted in a fixed position as close to photon or electron source and entrance slit of the spectrometer as or electron source and entrance slit of the spectrometer as possible.possible.

Sample compartment must be evacuated to a pressure of 10Sample compartment must be evacuated to a pressure of 10-5-5 torr or torr or smaller to avoid attenuation of the electron beam.smaller to avoid attenuation of the electron beam.

Much better vacuums 10Much better vacuums 10-9-9 to 10 to 10-10-10 are required to avoid are required to avoid contamination of the sample surface by substances such as Ocontamination of the sample surface by substances such as O22 or or HH22O that react with or are adsorbed on surface.O that react with or are adsorbed on surface.

Gas Samples:Gas Samples: Are leaked into sample area through a slit of such a Are leaked into sample area through a slit of such a size as to provide P of 10size as to provide P of 10-2-2 torr. Higher P lead to excessive torr. Higher P lead to excessive attenuation of the electron beam due to inelastic collisions. If the attenuation of the electron beam due to inelastic collisions. If the sample P is two low weakened signal are obtained.sample P is two low weakened signal are obtained.

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3- Analyzers:3- Analyzers:Most electron spectrometers are of hemispherical type in which the Most electron spectrometers are of hemispherical type in which the

electron beam is deflected by an electrostatic magnetic field so electron beam is deflected by an electrostatic magnetic field so electron travel in a curved path.electron travel in a curved path.

Radius of curvature depends upon KE of the electron and the Radius of curvature depends upon KE of the electron and the magnitude of the field.magnitude of the field.

By varying the field electrons of various KE can be focused on the By varying the field electrons of various KE can be focused on the detector. P maintained at 10detector. P maintained at 10-5 -5 or lower.or lower.

4- Transducers:4- Transducers:Modern electron spectrometers are based upon solid state, channel Modern electron spectrometers are based upon solid state, channel

electron multipliers.electron multipliers.Consist of tubes of glass that have been doped with lead or vandium.Consist of tubes of glass that have been doped with lead or vandium.When potential of several KV is applied across these materials a When potential of several KV is applied across these materials a

cascade or pulse of 10cascade or pulse of 1066 – 10 – 1088 electrons is produced for each electrons is produced for each incident electron.incident electron.

These pulses are then counted electronically.These pulses are then counted electronically.Two dimensional multichannel electron transducers are offered Two dimensional multichannel electron transducers are offered All resolution elements are stored simultaneously in a computer for All resolution elements are stored simultaneously in a computer for

subsequent display.subsequent display.

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Applications of XPSApplications of XPS

XPS provides:XPS provides:

- Qualitative and quantitative information about the elemental - Qualitative and quantitative information about the elemental composition of matter, particularly solid surface.composition of matter, particularly solid surface.

- Provides useful structural information.- Provides useful structural information.

Qualitative Analysis:Qualitative Analysis:

A low resolution, wide-scan XPS A low resolution, wide-scan XPS (survey spectrum) (survey spectrum) (next page).(next page).

Used to determine the elemental composition of samples.Used to determine the elemental composition of samples.

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XPS SpectraXPS Spectra 1- Characteristic binding energy associated with each core 1- Characteristic binding energy associated with each core

atomic orbital i.e. atomic orbital i.e. each element will give rise to a each element will give rise to a characteristic set of peaks in the photoelectron spectrum characteristic set of peaks in the photoelectron spectrum at kinetic energies determined by the photon energy and at kinetic energies determined by the photon energy and the respective binding energies. the respective binding energies.

2- The presence of peaks at particular energies therefore 2- The presence of peaks at particular energies therefore indicates the presence of a specific element in the sample indicates the presence of a specific element in the sample under study.under study.

3- The intensity of the peaks is related to the 3- The intensity of the peaks is related to the concentration of the element within the sampled region. concentration of the element within the sampled region. (quantitative analysis of the surface composition(quantitative analysis of the surface composition).).

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Page 26: Surface Chemistry.ppt

- With Mg or Al K- With Mg or Al K source, all elements except H and He emit core source, all elements except H and He emit core electrons having characteristic BE.electrons having characteristic BE.

- Survey spectrum has KE range of 250-1500 eV, which correspond Survey spectrum has KE range of 250-1500 eV, which correspond to BE of about 0-1250 eV.to BE of about 0-1250 eV.

- Every element in the P.T. has one or more E levels that will result Every element in the P.T. has one or more E levels that will result in the appearance of peaks in this region.in the appearance of peaks in this region.

- The peaks are well resolved and lead to unambiguous The peaks are well resolved and lead to unambiguous identification provided the element if the element conc. >0.1%.identification provided the element if the element conc. >0.1%.

- Sometimes peak overlap is encountered such as O1s/Sb3d or Sometimes peak overlap is encountered such as O1s/Sb3d or Al2s, 2p/Cu3s, 3p.Al2s, 2p/Cu3s, 3p.

- Overlapping problems can be resolved by investigating other Overlapping problems can be resolved by investigating other spectral regions for additional peaks.spectral regions for additional peaks.

- Peaks from Auger electrons are found in XPS which are readily Peaks from Auger electrons are found in XPS which are readily identified by comparing spectra produced by two X-ray sources identified by comparing spectra produced by two X-ray sources (Mg and Al, K(Mg and Al, K ) )

- Auger peaks remain unchanged while photoelectric peaks are Auger peaks remain unchanged while photoelectric peaks are displaced on the KE scale.displaced on the KE scale.

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Chemical shifts and oxidation statesChemical shifts and oxidation states- One of the peaks of the survey spectrum is examined under One of the peaks of the survey spectrum is examined under

conditions of high E resolution.conditions of high E resolution.- The position of the maximum is found to depend to a small degree The position of the maximum is found to depend to a small degree

upon the chemical environment of the atom responsible for the upon the chemical environment of the atom responsible for the peak.peak.

- So depends on: variations in the number of valence electrons and So depends on: variations in the number of valence electrons and the type of bonds they form so influence the BE of the core the type of bonds they form so influence the BE of the core electrons.electrons.

- BE increases as the oxidation state becomes more positive.BE increases as the oxidation state becomes more positive.- This chemical shift can be explained by assuming that the This chemical shift can be explained by assuming that the

attraction of the nucleus for a core electron is diminished by the attraction of the nucleus for a core electron is diminished by the presence of outer electrons.presence of outer electrons.

- When one of these electrons removed the effective nuclear charge When one of these electrons removed the effective nuclear charge sensed for the core electron is increased so BE increases.sensed for the core electron is increased so BE increases.

- Important application: Important application: Identification of the oxidation states of the Identification of the oxidation states of the elements for different kinds of inorganic compounds.elements for different kinds of inorganic compounds.

-

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Chemical shifts and structureChemical shifts and structure

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- The shift in binding E can be rationalized by taking into account the effect The shift in binding E can be rationalized by taking into account the effect of the various functional groups on the effective nuclear charge of the various functional groups on the effective nuclear charge experienced by the 1s core electrons.experienced by the 1s core electrons.

- As the electron nativity of the attached functional group increases it will As the electron nativity of the attached functional group increases it will withdraw electron density from carbon so the effective nuclear charge withdraw electron density from carbon so the effective nuclear charge increases and the BE increases.increases and the BE increases.

- XPS provides also the relative number of each type of atoms present in a XPS provides also the relative number of each type of atoms present in a compound. compound. Example: Example: The nitrogen 1s spectrum for sodium azide (NaThe nitrogen 1s spectrum for sodium azide (Na++NN33

--) is ) is made up of two peaks having relative areas in the ratio of 2:1 made up of two peaks having relative areas in the ratio of 2:1 corresponding to the two end nitrogens and the center nitrogen corresponding to the two end nitrogens and the center nitrogen respectively.respectively.

Notes:Notes:

The phtoelectrons in XPS are incapable of passing through more than 10-50 AThe phtoelectrons in XPS are incapable of passing through more than 10-50 A00 of a solid. of a solid.

Thus the most important applications of electron spectroscopy are for Thus the most important applications of electron spectroscopy are for surfaces.surfaces.

Examples of uses:Examples of uses:

Identification of active sites and poisons on catalytic surfaces.Identification of active sites and poisons on catalytic surfaces.

Determination of surface contaminants on semiconductors.Determination of surface contaminants on semiconductors.

Analysis of the composition of human skinAnalysis of the composition of human skin

Study of oxide surface layers on metal alloys.Study of oxide surface layers on metal alloys.

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The XPS method has substantial potential in the elucidation of The XPS method has substantial potential in the elucidation of chemical structure.chemical structure.

Its ability to distinguish among oxidation states of an element.Its ability to distinguish among oxidation states of an element.

Quantitative Applications:Quantitative Applications:

For determination of the elemental composition of various inorganic For determination of the elemental composition of various inorganic and organic materials.and organic materials.

Both peak intensities and peak areas have been used as the Both peak intensities and peak areas have been used as the analytical parameter as a function with concentration.analytical parameter as a function with concentration.

Assumption that the surface composition and bulk are the same is Assumption that the surface composition and bulk are the same is Not always correct. Not always correct.

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Chemical ShiftsChemical Shifts

The exact binding energy of an electron depends not The exact binding energy of an electron depends not only upon the level from which photoemission is only upon the level from which photoemission is occurring, but also upon :occurring, but also upon :

1) The formal oxidation state of the atom. 1) The formal oxidation state of the atom. 2) The local chemical and physical environment. 2) The local chemical and physical environment. Changes in either (1) or (2) give rise to small shifts in the Changes in either (1) or (2) give rise to small shifts in the

peak positions in the spectrum - so-called peak positions in the spectrum - so-called chemical shifts chemical shifts

Atoms of a higher positive oxidation state exhibit a Atoms of a higher positive oxidation state exhibit a higher binding energy due to the extra columbic higher binding energy due to the extra columbic interaction between the photo-emitted electron and interaction between the photo-emitted electron and the ion core.the ion core.

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Where do Binding Energy Shifts Come From? Or Where do Binding Energy Shifts Come From? Or element or compound identification.element or compound identification.

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Elemental ShiftsElemental Shifts

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Electronic EffectElectronic Effect

Spin-Orbit Splitting or Spin-Orbit CouplingSpin-Orbit Splitting or Spin-Orbit Coupling Some electronic levels (most obviously 3Some electronic levels (most obviously 3pp and 3 and 3dd ) give rise ) give rise

to a closely spaced doublet, Which appear when spectra to a closely spaced doublet, Which appear when spectra expanded.expanded.

Permitted J values = L Permitted J values = L S S Coupling between L: The Angular Q.N., S: Unpaired SpinCoupling between L: The Angular Q.N., S: Unpaired Spin The lowest energy final state is the one with maximum The lowest energy final state is the one with maximum JJ

(more than half full)(more than half full) The relative intensities of the two peaks reflects the The relative intensities of the two peaks reflects the

degeneracies of the final states (degeneracies of the final states (gJgJ = 2 = 2JJ + 1),  + 1), 22D 5/2: D 5/2: gJgJ = 2x{5/2}+1 = 6 (lower B.E) = 2x{5/2}+1 = 6 (lower B.E) 22D 3/2: D 3/2: gJgJ = 2x{3/2}+1 = 4 (higher B.E) = 2x{3/2}+1 = 4 (higher B.E) These two values determines the probability of transition to These two values determines the probability of transition to

such a state during photoionizationsuch a state during photoionization

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The inner core electronic configuration of the initial state of the Pd is: The inner core electronic configuration of the initial state of the Pd is: (1s)(1s)22(2s)(2s)22(2p)(2p)66(3s)(3s)22(3p)(3p)66(3d)(3d)10 10 ........

with all sub-shells completely full. with all sub-shells completely full. The removal of an electron from the 3d sub-shell by photo-ionization The removal of an electron from the 3d sub-shell by photo-ionization

leads to a leads to a (3d)(3d)99 configuration for the final state - since the d-orbitals configuration for the final state - since the d-orbitals (l (l = 2)= 2) have non-zero orbital angular momentum, there will be coupling have non-zero orbital angular momentum, there will be coupling between the unpaired spin and orbital angular momenta. between the unpaired spin and orbital angular momenta.

Spin-orbit coupling is generally treated using one of two models which Spin-orbit coupling is generally treated using one of two models which correspond to the two limiting ways in which the coupling can occur - correspond to the two limiting ways in which the coupling can occur - these being the LS (or Russell-Saunders) coupling approximation and these being the LS (or Russell-Saunders) coupling approximation and the j-j coupling approximation. the j-j coupling approximation.

If we consider the final ionized state of Pd within the Russell-Saunders If we consider the final ionized state of Pd within the Russell-Saunders coupling approximation, the coupling approximation, the (3d)(3d)99 configuration gives rise to two states configuration gives rise to two states (ignoring any coupling with valence levels) which differ slightly in (ignoring any coupling with valence levels) which differ slightly in energy and in their degeneracy ... energy and in their degeneracy ...

2D 5/2 g J = 2x{5/2}+1 = 6 2D 3/2 g J = 2x{3/2}+1 = 4 2D 5/2 g J = 2x{5/2}+1 = 6 2D 3/2 g J = 2x{3/2}+1 = 4 These two states arise from the coupling of the L=2 and S=1/2These two states arise from the coupling of the L=2 and S=1/2 vectors vectors

to give permitted J values of 3/2 and 5/2. The lowest energy final state to give permitted J values of 3/2 and 5/2. The lowest energy final state is the one with maximum J (since the shell is more than half full), i.e. J is the one with maximum J (since the shell is more than half full), i.e. J = 5/2, hence this gives rise to the "lower binding energy" peak. The = 5/2, hence this gives rise to the "lower binding energy" peak. The relative intensities of the two peaks reflects the degeneracies of the relative intensities of the two peaks reflects the degeneracies of the final states (g J = 2J+1), which in turn determines the probability of final states (g J = 2J+1), which in turn determines the probability of transition to such a state during photoionization.transition to such a state during photoionization.

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s-Orbitals-Orbital

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p-Orbitalp-Orbital

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d-Orbitald-Orbital

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f-Orbitalf-Orbital

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Chemical Shifts-Electronegativity EffectsChemical Shifts-Electronegativity Effects

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Chemical Shifts- Electronegativity EffectsChemical Shifts- Electronegativity Effects

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C1s envelope has been resolved into five components C1s envelope has been resolved into five components of polystyrene surface exposed to an oxygen plasma.of polystyrene surface exposed to an oxygen plasma.

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Applications Applications

of X-ray of X-ray PhotoelectronPhotoelectron

Spectroscopy (XPS)Spectroscopy (XPS)

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XPS Analysis of Pigment from Mummy XPS Analysis of Pigment from Mummy Artwork Artwork

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Analysis Carbon Fiber- Polymer Composite Analysis Carbon Fiber- Polymer Composite Material by XPSMaterial by XPS

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Analysis of Materials for Solar Energy Collection by Analysis of Materials for Solar Energy Collection by XPS Depth Profiling-XPS Depth Profiling-The amorphous SiC/SnO /SnOThe amorphous SiC/SnO /SnO22 Interface Interface

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Polysiloxane Immobilized Ligand Polysiloxane Immobilized Ligand SystemSystem

O

O

O

Si

CH2

CH2 C2H5

C2H5

H2N(CH2)2NH2

O

O

O

Si

CH2

CH2 NH2

NH2

C2H5OH

N

C

O

O

O

O

+

C

O

O

NH

NH

-

C

C

N

Reflux/Toluene

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XPS ResultsXPS Results

SystemSystem ElementElement CC SiSi OO NN

PrecursorPrecursor Core-lineCore-lineC1sC1s Si2pSi2p O1sO1s N1sN1s

285285 102102 532532 399.5399.5

   %Composition%Composition 38.838.8 1717 41.441.4 2.852.85

ProductProduct Core-lineCore-lineC1sC1s Si2pSi2p O1sO1s N1sN1s

285285 102102 532532 399.5399.5

   %Composition%Composition 42.442.4 19.319.3 28.728.7 9.69.6

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ConclusionConclusion

XPSXPS is a powerful technique for is a powerful technique for characterizing solid surfaces.characterizing solid surfaces.

All types of inorganic solids can be All types of inorganic solids can be analyzed.analyzed.

Elemental (except H, He) and chemical Elemental (except H, He) and chemical analyses within a depth of 10 nm.analyses within a depth of 10 nm.

Quantitative technique.Quantitative technique. Extremely useful for surface treatment of Extremely useful for surface treatment of

materialsmaterials..

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Auger Electron Spectroscopy AESAuger Electron Spectroscopy AES

- Auger Electron Spectroscopy (Auger Electron Spectroscopy (Auger spectroscopyAuger spectroscopy or or AES) was developed in the late 1960's.AES) was developed in the late 1960's.

- Deriving its name from the effect first observed by Pierre Deriving its name from the effect first observed by Pierre Auger, a French Physicist, in the mid-1920's.Auger, a French Physicist, in the mid-1920's.

- It is a surface specific technique utilizing the emission of It is a surface specific technique utilizing the emission of low energy electrons in the low energy electrons in the Auger processAuger process

- It is one of the most commonly employed surface It is one of the most commonly employed surface analytical techniques for determining the composition of analytical techniques for determining the composition of the surface layers of a sample. the surface layers of a sample.

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Auger Electron SpectroscopyAuger Electron Spectroscopy

Is based upon a two-step process:Is based upon a two-step process:

First Step: (Ionization)First Step: (Ionization)

Involves formation of an electronically excited ion AInvolves formation of an electronically excited ion A+*+* by exposing the by exposing the analyte to a beam of electrons or sometimes X-rays.analyte to a beam of electrons or sometimes X-rays.

With X-rays:With X-rays:

A + hA + h A A+*+* + e + e--

With an electron beam:With an electron beam:

A + eA + e--ii A A+*+* + e’ + e’--

ii + e + e--AA

Where:Where:

ee--ii represents an incident electron from the source. represents an incident electron from the source.

e’e’--ii represents the same electron after it has interacted with A and represents the same electron after it has interacted with A and has thus lost some of its energy.has thus lost some of its energy.

ee--A A represents an electron that is ejected from one of the inner represents an electron that is ejected from one of the inner

orbitals of A.orbitals of A.

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Energy levels in an isolated, multi-electron atomEnergy levels in an isolated, multi-electron atom

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Alternative nomenclature on the left that is used in Auger Alternative nomenclature on the left that is used in Auger

spectroscopy.spectroscopy.

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Electronic Structure - Solid StateElectronic Structure - Solid State In the solid state the core levels of atoms are little perturbed and In the solid state the core levels of atoms are little perturbed and

essentially remain as discrete, localized (i.e. atomic-like) levels.essentially remain as discrete, localized (i.e. atomic-like) levels. The valence orbitals, however, overlap significantly with those of The valence orbitals, however, overlap significantly with those of

neighboring atoms generating bands of spatially-delocalized energy levels. neighboring atoms generating bands of spatially-delocalized energy levels. The energy level diagram for the solid is therefore closely resemblant of The energy level diagram for the solid is therefore closely resemblant of

that of the corresponding isolated atom, except for the levels closest to the that of the corresponding isolated atom, except for the levels closest to the vacuum level. vacuum level.

The diagram below shows the electronic structure of Na metal:The diagram below shows the electronic structure of Na metal:

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The Auger ProcessThe Auger Process

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- The Auger process is initiated by creation of a core hole - this is The Auger process is initiated by creation of a core hole - this is typically carried out by exposing the sample to typically carried out by exposing the sample to a beam of high a beam of high energy electronsenergy electrons (typically having a primary energy in the range 2 - (typically having a primary energy in the range 2 - 10 keV). 10 keV).

- Such electrons have sufficient energy to ionize all levels of the Such electrons have sufficient energy to ionize all levels of the lighter elements, and higher core levels of the heavier elements.lighter elements, and higher core levels of the heavier elements.

- Ionization is shown to occur by removal of a K-shell Ionization is shown to occur by removal of a K-shell electron, but in practice such a crude method of ionization electron, but in practice such a crude method of ionization will lead to ions with holes in a variety of inner shell levels.will lead to ions with holes in a variety of inner shell levels.

- In some studies, the initial ionization process is instead In some studies, the initial ionization process is instead carried out using soft x-rays ( hcarried out using soft x-rays ( h = 1000 - 2000 eV ). = 1000 - 2000 eV ).

- In this case, the acronym XAES is sometimes used.In this case, the acronym XAES is sometimes used.- However, this change in the method of ionization has no However, this change in the method of ionization has no

significant effect on the final Auger spectrum. significant effect on the final Auger spectrum. -

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Relaxation & Auger EmissionRelaxation & Auger EmissionRelaxation of the excited ion ARelaxation of the excited ion A+* +* can occur in either two ways.can occur in either two ways.

2- X-ray Fluorescence:2- X-ray Fluorescence:

AA+*+* A A++ + h + hff hhf f : Fluorescence photon: Fluorescence photon

The second process recognized as X-ray fluorescence. The energy of The second process recognized as X-ray fluorescence. The energy of the Fl radiation hthe Fl radiation hf f is independent of the excitation E. is independent of the excitation E. So So polychromatic radiation can be used.polychromatic radiation can be used.

2- Auger emission:2- Auger emission:

AA+*+* A A++++ + e + e--A A ee--

AA : Auger electron : Auger electron

The E given up in relaxation results in the ejection of an electron The E given up in relaxation results in the ejection of an electron (auger electron) with KE, (auger electron) with KE, EEkk

The energy of the auger electron is independent of the E of the The energy of the auger electron is independent of the E of the photon or electron that originally created the vacancy in E level Ephoton or electron that originally created the vacancy in E level Eb.b.

So mono-energetic source is not required for excitation.So mono-energetic source is not required for excitation.

So it can be distinguished from XPS by this point.So it can be distinguished from XPS by this point.

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KE of the auger electron is the difference between the E released in KE of the auger electron is the difference between the E released in relaxation of the excited ion (Erelaxation of the excited ion (Ebb- E’- E’bb) and the energy required to ) and the energy required to remove the second electron from its orbit (E’remove the second electron from its orbit (E’bb).).

EEkk = (E = (Ebb – E’ – E’bb) –E’) –E’b b = E= Ebb – 2E’ – 2E’bb

Auger emissions are described in terms of the type of orbitals Auger emissions are described in terms of the type of orbitals involved in the production of the electron.involved in the production of the electron.

Example:Example:

KLL Auger transition involves an initial removal of a K electron KLL Auger transition involves an initial removal of a K electron followed by a transition of an L electron to the K orbital with the followed by a transition of an L electron to the K orbital with the simultaneous ejection of a second L electron.simultaneous ejection of a second L electron.

Other common transitions:Other common transitions:

LMM and MNNLMM and MNN

XPS Spectra:XPS Spectra:

Consist of a few characteristic peaks lying in the region of 20 t0 1000 Consist of a few characteristic peaks lying in the region of 20 t0 1000 eV.eV.

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Auger emissionAuger emission

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In previous example, one electron falls from a higher level to fill an In previous example, one electron falls from a higher level to fill an initial core hole in the K-shell.initial core hole in the K-shell.

The energy liberated in this process is simultaneously transferred The energy liberated in this process is simultaneously transferred to a second electron.to a second electron.

A fraction of this energy is required to overcome the binding energy A fraction of this energy is required to overcome the binding energy of this second electron.of this second electron.

The remainder is retained by this emitted The remainder is retained by this emitted Auger electronAuger electron as kinetic as kinetic energy. energy.

In the Auger process illustrated, the final state is a doubly-ionized In the Auger process illustrated, the final state is a doubly-ionized atom with core holes in the L1 and L2,3 shells. atom with core holes in the L1 and L2,3 shells.

We can make a rough estimate of the KE of the Auger electron from We can make a rough estimate of the KE of the Auger electron from the binding energies of the various levels involved. In this particular the binding energies of the various levels involved. In this particular example, example,

KE = ( EKE = ( EKK - E - EL1L1 ) - E ) - EL23L23 Note : the KE of the Auger electron is independent of the Note : the KE of the Auger electron is independent of the

mechanism of initial core hole formation.mechanism of initial core hole formation. The expression for the energy can also be re-written in the The expression for the energy can also be re-written in the

form : form :

KE = EKE = EKK - ( E - ( EL1L1 + E + EL23L23 ) )

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It should be clear from this expression that the latter two It should be clear from this expression that the latter two energy terms could be interchanged without any effect - energy terms could be interchanged without any effect - i.e. it is actually impossible to say which electron fills the i.e. it is actually impossible to say which electron fills the initial core hole and which is ejected as an Auger electron initial core hole and which is ejected as an Auger electron ; they are indistinguishable.; they are indistinguishable.

An Auger transition is therefore characterized primarily by :-An Auger transition is therefore characterized primarily by :- the location of the initial hole the location of the initial hole the location of the final two holes the location of the final two holes although the existence of different electronic states although the existence of different electronic states

(terms) of the final doubly-ionized atom may lead to fine (terms) of the final doubly-ionized atom may lead to fine structure in high resolution spectra. structure in high resolution spectra.

When describing the transition, the initial hole location is When describing the transition, the initial hole location is given first, followed by the locations of the final two holes given first, followed by the locations of the final two holes in order of decreasing binding energy. in order of decreasing binding energy.

i.e. the transition illustrated is a  KL1L2,3  transition . i.e. the transition illustrated is a  KL1L2,3  transition . If we just consider these three electronic levels there are If we just consider these three electronic levels there are

clearly several possible Auger transitions : specifically, clearly several possible Auger transitions : specifically, K L1 L1, K L1 L2,3 , K L2,3 L2,3K L1 L1, K L1 L2,3 , K L2,3 L2,3

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In general, since the initial ionization is non-selective and the In general, since the initial ionization is non-selective and the initial hole may therefore be in various shells, there will be many initial hole may therefore be in various shells, there will be many possible Auger transitions for a given element - some weak, some possible Auger transitions for a given element - some weak, some strong in intensity. strong in intensity. AUGER SPECTROSCOPYAUGER SPECTROSCOPY is based upon the is based upon the measurement of the kinetic energies of the emitted electrons. measurement of the kinetic energies of the emitted electrons. Each element in a sample being studied will give rise to a Each element in a sample being studied will give rise to a characteristic spectrum of peaks at various kinetic energies. characteristic spectrum of peaks at various kinetic energies.

This is an Auger spectrum of Pd metal - generated using a 2.5 keV This is an Auger spectrum of Pd metal - generated using a 2.5 keV electron beam to produce the initial core vacancies and hence to electron beam to produce the initial core vacancies and hence to stimulate the Auger emission process. The main peaks for stimulate the Auger emission process. The main peaks for palladium occur between 220 & 340 eV. palladium occur between 220 & 340 eV.

The peaks are situated on a high background which arises from The peaks are situated on a high background which arises from the vast number of so-called the vast number of so-called secondary electronssecondary electrons generated by a generated by a multitude of inelastic scattering processes. multitude of inelastic scattering processes.

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Auger spectra are also often shown in a differentiated form : the Auger spectra are also often shown in a differentiated form : the reasons for this are partly historical, partly because it is possible reasons for this are partly historical, partly because it is possible to actually measure spectra directly in this form and by doing so to actually measure spectra directly in this form and by doing so get a better sensitivity for detection. The plot below shows the get a better sensitivity for detection. The plot below shows the same spectrum in such a differentiated form. same spectrum in such a differentiated form.

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Like XPS Auger spectra consist of a few characteristic peaks lying Like XPS Auger spectra consist of a few characteristic peaks lying in the region of 20-1000 eV.in the region of 20-1000 eV.

The derivative of the counting rate as a function of the KE of the The derivative of the counting rate as a function of the KE of the electron dN(E)/dE serves as the ordinate. electron dN(E)/dE serves as the ordinate.

Derivative spectra are standard for Auger spectroscopy in order to Derivative spectra are standard for Auger spectroscopy in order to enhance the small peaks and repress the effect of the large but enhance the small peaks and repress the effect of the large but slowly changing, scattered electron background radiation.slowly changing, scattered electron background radiation.

Well separated peaks obtained (useful for qualitative Well separated peaks obtained (useful for qualitative identification).identification).

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Comparison between XPS and AESComparison between XPS and AES- Auger electron emission and XPS are competitive processes - Auger electron emission and XPS are competitive processes

and their relative rates depend upon atomic number of the and their relative rates depend upon atomic number of the element involved.element involved.

Elements of high Z favor XPS (not sensitive for Z < 10).Elements of high Z favor XPS (not sensitive for Z < 10).

Elements of low Z favor AE.Elements of low Z favor AE.- Both AES and XPS are complementary, the two techniques Both AES and XPS are complementary, the two techniques

involved in a single instrument.involved in a single instrument.- Auger is of particular strength because of:Auger is of particular strength because of:

1- Its sensitivity to atoms of low Z.1- Its sensitivity to atoms of low Z.

2- Its minimal matrix effects.2- Its minimal matrix effects.

3- Its high spatial resolution (detailed exam. Of solid surfaces). 3- Its high spatial resolution (detailed exam. Of solid surfaces). This arises because the primary beam is electrons ( more This arises because the primary beam is electrons ( more tightly focused on surfaces) than X-rays.tightly focused on surfaces) than X-rays.

- Poor information for oxidation state obtained in case of AES.Poor information for oxidation state obtained in case of AES.- Difficulties in quant. analysis in case of AES. Difficulties in quant. analysis in case of AES.

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Instrumentation of AESInstrumentation of AES

Is similar to that for XPS except that the source is usually Is similar to that for XPS except that the source is usually an electron gun rather than an X-ray tube.an electron gun rather than an X-ray tube.

This source consists of a heated tungsten filament (0.1 mm This source consists of a heated tungsten filament (0.1 mm d) and bent into V-shape tip.d) and bent into V-shape tip.

The cathodic filament is maintained at V of 1-50 kV with The cathodic filament is maintained at V of 1-50 kV with respect to the anode contained in the gun.respect to the anode contained in the gun.

Wehnelt cylinder surround the filament (-ve).Wehnelt cylinder surround the filament (-ve). The Electric Field causes electrons to converge on a tiny The Electric Field causes electrons to converge on a tiny

spot called crossover (of diameter dspot called crossover (of diameter d00)) Cathodes contracted from LaBCathodes contracted from LaB66 rods also used in electron rods also used in electron

guns (expensive source and requires better vacuum system guns (expensive source and requires better vacuum system to prevent oxide formation the decrease the efficiency).to prevent oxide formation the decrease the efficiency).

Third type: field emission W or C cathode of very sharp tip Third type: field emission W or C cathode of very sharp tip (100 nm or less). Held at high potential with intense E.F. at (100 nm or less). Held at high potential with intense E.F. at the tip (> 10the tip (> 107 7 v/cm). Provides a beam of electrons of v/cm). Provides a beam of electrons of crossover diameter of only 10 nm (in case of W = 50 crossover diameter of only 10 nm (in case of W = 50 m m and LaBand LaB66 = 10 = 10 m). Disadv: Fragility and requires better m). Disadv: Fragility and requires better vacuum.vacuum.

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Electron guns produce a beam of electrons with e of 1-10 KeV that Electron guns produce a beam of electrons with e of 1-10 KeV that focused on the surface of a sample. focused on the surface of a sample.

AES is of very high spatial-resolution scanning of solid surfaces.AES is of very high spatial-resolution scanning of solid surfaces. Normally electron beams with diameter ranging from 500 to 5 Normally electron beams with diameter ranging from 500 to 5 m m

are used for this purpose.are used for this purpose. Guns producing beams of Guns producing beams of m are called auger microprobes used m are called auger microprobes used

to detect and determine the elemental composition of to detect and determine the elemental composition of inhomogenetiesinhomogeneties

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Applications of AESApplications of AESQualitative Analysis of Solid Surfaces:Qualitative Analysis of Solid Surfaces:

- Auger spectra obtained by bombardment a small area of a surface ( - Auger spectra obtained by bombardment a small area of a surface ( diam 5-500 diam 5-500 m) with a beam of electrons from a gun).m) with a beam of electrons from a gun).

- A derivative electron spectrum is obtained with an analyzer.A derivative electron spectrum is obtained with an analyzer.

Advantages:Advantages:

The low energy auger electrons (20-1000 eV) are able to penetrate The low energy auger electrons (20-1000 eV) are able to penetrate only a few atomic layers (3-20 A) of solid. only a few atomic layers (3-20 A) of solid.

Thus electrons penetrate greater dept below sample surface but only Thus electrons penetrate greater dept below sample surface but only these auger electrons from the first four or five atomic layers these auger electrons from the first four or five atomic layers escape to reach the analyzer. (AES reflects the surface comp. of escape to reach the analyzer. (AES reflects the surface comp. of solids).solids).

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Depth Profiling of Surfaces:Depth Profiling of Surfaces:

- That involves determination of the elemental composition of a - That involves determination of the elemental composition of a surface as it is being etched away (sputtered) by a beam of argon surface as it is being etched away (sputtered) by a beam of argon ions.ions.

- Either XPS or Auger spectroscopy can be used for elemental Either XPS or Auger spectroscopy can be used for elemental detection but AES is the more common.detection but AES is the more common.

- The microprobe (of d 5 The microprobe (of d 5 m) and etching beams are operated m) and etching beams are operated simultaneously.simultaneously.

- Intensity of one or more of auger peaks recorded as a function of Intensity of one or more of auger peaks recorded as a function of time, a depth profile of elemental composition is obtained.time, a depth profile of elemental composition is obtained.

Important for:Important for:

Corrosion chemistry, catalyst behavior and properties of Corrosion chemistry, catalyst behavior and properties of semiconductor junctions.semiconductor junctions.

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Line ScanningLine Scanning

Are used to characterize the surface composition of solids as a Are used to characterize the surface composition of solids as a function of distance along a straight line of 100 function of distance along a straight line of 100 m or more.m or more.

Auger microprobe is used that produces a beam that can be Auger microprobe is used that produces a beam that can be moved across a surface in a reproducible way.moved across a surface in a reproducible way.

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Secondary ion Mass Spectrometry (SIMS)Secondary ion Mass Spectrometry (SIMS) SIMS is the most highly developed of the mass spectrometric SIMS is the most highly developed of the mass spectrometric

surface methods.surface methods. Proven useful for determining both atomic and molecular Proven useful for determining both atomic and molecular

composition of solid surfaces.composition of solid surfaces.

Two types:Two types:

1- Secondary-ion mass analyzers.1- Secondary-ion mass analyzers.

2- Microprobe analyzers.2- Microprobe analyzers.

Both are based upon bombarding the surface of the sample with a Both are based upon bombarding the surface of the sample with a beam of 5-20 keV ions such as Arbeam of 5-20 keV ions such as Ar+, +, CsCs++, N, N22

++, or O, or O++2.2.

The ion beam is formed in an ion gun in which the gaseous atoms or The ion beam is formed in an ion gun in which the gaseous atoms or molecules are ionized by an electron impact source.molecules are ionized by an electron impact source.

The positive ions are then accelerated by applying a high dc The positive ions are then accelerated by applying a high dc potential.potential.

The impact of these primary ion causes the surface layer of atoms of The impact of these primary ion causes the surface layer of atoms of the sample to be stripped (sputtered off) largely as neutral atoms.the sample to be stripped (sputtered off) largely as neutral atoms.

A small fraction however forms positive or negative secondary ions A small fraction however forms positive or negative secondary ions that are drawn into a spectrometer for mass analysis.that are drawn into a spectrometer for mass analysis.

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Secondary mass analyzers:Secondary mass analyzers:

Serves for general surface analysis and depth profiling.Serves for general surface analysis and depth profiling.

The primary ion beam diameter ranges from 0.3-5 mm.The primary ion beam diameter ranges from 0.3-5 mm.

Double-focusing, single focusing, time-of-flight and quadruple Double-focusing, single focusing, time-of-flight and quadruple spectrometers are used for mass determination.spectrometers are used for mass determination.

These spectrometers yield qualitative and quantitative information These spectrometers yield qualitative and quantitative information about all the isotopes (hydrogen through uranium) present on about all the isotopes (hydrogen through uranium) present on surface.surface.

Sensitivities of 10Sensitivities of 10-15-15 g or better are typical. g or better are typical.

Concentration profiles can be obtained with depth resolution of 50 to Concentration profiles can be obtained with depth resolution of 50 to 100 A100 Ao.o.

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Ion Microprobe AnalyzersIon Microprobe Analyzers Are more sophisticated and more expensive instruments that are Are more sophisticated and more expensive instruments that are

based upon a focused beam of primary ions that has a diameter of based upon a focused beam of primary ions that has a diameter of 1 to 2 1 to 2 m.m.

This beam can be moved across a surface for about 300 This beam can be moved across a surface for about 300 m in m in both the x and y directions.both the x and y directions.

A microscope is provided to permit visual adjustment of the beam A microscope is provided to permit visual adjustment of the beam position.position.

Mass analysis is performed with a double-focusing spectrometer.Mass analysis is performed with a double-focusing spectrometer. The ion microprobe version of SIMS permits detailed studies of The ion microprobe version of SIMS permits detailed studies of

solid surfaces. solid surfaces.

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Laser-Microprobe Mass SpectrometryLaser-Microprobe Mass Spectrometry For solid surface studying.For solid surface studying. Ionization and volatilization are accomplished with pulsed neodymium-YAG Ionization and volatilization are accomplished with pulsed neodymium-YAG

laser, which after frequency quadrupling, produces a 0.5 laser, which after frequency quadrupling, produces a 0.5 m spot of 266-m spot of 266-nm radiation.nm radiation.

The power density of the radiation within this spot is 10The power density of the radiation within this spot is 101010 to 10 to 101111 W/cm W/cm2.2.

Attenuation of the power of the beam to 1% by means of a 25-step optical Attenuation of the power of the beam to 1% by means of a 25-step optical filter.filter.

A second lower power He-Ne laser (A second lower power He-Ne laser ( = 633 nm) is collinear with the = 633 nm) is collinear with the ionization beam (illumination beam) so area chosen visually.ionization beam (illumination beam) so area chosen visually.

Advantages:Advantages: Sensitivity is high (< 10Sensitivity is high (< 10-20-20).). Applicable to both inorganic and organic (including biological) samples.Applicable to both inorganic and organic (including biological) samples. Resolution is about 1 Resolution is about 1 m.m. Produces data at rapid rate.Produces data at rapid rate.Applications:Applications:Determination of Na/K conc. ratios in frog nerve fiber.Determination of Na/K conc. ratios in frog nerve fiber.Determination of calcium distribution in retinas.Determination of calcium distribution in retinas.Classification of asbestos and coal mine dusts.Classification of asbestos and coal mine dusts.Determination of Fluorine distributions in dental hard tissue.Determination of Fluorine distributions in dental hard tissue.Analysis of amino acids.Analysis of amino acids.Study of polymer surfaces. Study of polymer surfaces.

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Electron microprobeElectron microprobe

Electron microprobe analysis (EMPA) is a non-destructive method for determining the Electron microprobe analysis (EMPA) is a non-destructive method for determining the chemical composition of tiny amounts of solid materials. chemical composition of tiny amounts of solid materials.

X-ray emission is stimulated on the surface of the sample by a narrow focused X-ray emission is stimulated on the surface of the sample by a narrow focused beam of electrons.beam of electrons.

The resulting X-ray emission is detected and analyzed with either a wavelength The resulting X-ray emission is detected and analyzed with either a wavelength or an energy dispersive spectrometer.or an energy dispersive spectrometer.

InstrumentationInstrumentation

The instrument employs three integrated beams of radiation (electron, light and X-The instrument employs three integrated beams of radiation (electron, light and X-ray) and < 10ray) and < 10 an an system is required and system is required and or E-dispersive X-ray spectrometer. or E-dispersive X-ray spectrometer.

An electron source, commonly a W-filament cathode referred to as a "gun." An electron source, commonly a W-filament cathode referred to as a "gun." A series of electromagnetic lenses located in the column of the instrument, used A series of electromagnetic lenses located in the column of the instrument, used

to condense and focus the electron beam (0.1- 1 to condense and focus the electron beam (0.1- 1 m OD) emanating from the m OD) emanating from the source; this comprises the electron optics and operates in an analogous way to source; this comprises the electron optics and operates in an analogous way to light optics. light optics.

A sample chamber, with movable sample stage (X-Y-Z), that is under a vacuum A sample chamber, with movable sample stage (X-Y-Z), that is under a vacuum to prevent gas and vapor molecules from interfering with the electron beam on to prevent gas and vapor molecules from interfering with the electron beam on its way to the sample; a light microscope allows for direct optical observation of its way to the sample; a light microscope allows for direct optical observation of the sample.the sample.

X-ray produced by the electron beam are collimated. X-ray produced by the electron beam are collimated. A variety of detectors arranged around the sample chamber that are used to A variety of detectors arranged around the sample chamber that are used to

collect x-rays and electrons emitted from the sample.collect x-rays and electrons emitted from the sample.

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Electrons are produced by heating a filament similar to the Electrons are produced by heating a filament similar to the filament in a light bulb (A).filament in a light bulb (A).

These electrons are then formed into a beam by These electrons are then formed into a beam by accelerating them down a column at very high voltages, accelerating them down a column at very high voltages, typically 15 to 20 thousand volts. typically 15 to 20 thousand volts.

The electrons pass through lenses that condense the beam The electrons pass through lenses that condense the beam (B), remove aberrations (C) and focus the beam (D). (B), remove aberrations (C) and focus the beam (D).

When the electrons arrive at the sample (E) the beam is When the electrons arrive at the sample (E) the beam is focused into a spot much smaller than 0.001 millimeter in focused into a spot much smaller than 0.001 millimeter in diameter. diameter.

Upon entering the sample, the electrons interact with the Upon entering the sample, the electrons interact with the atoms in the sample in what is called the atoms in the sample in what is called the interaction volumeinteraction volume, causing X-rays to be produced., causing X-rays to be produced.

Each element produces X-rays with characteristic energiesEach element produces X-rays with characteristic energies.. These X-rays can then be counted by reflecting them These X-rays can then be counted by reflecting them

through a crystal (F) and sending them on to a detector (G).through a crystal (F) and sending them on to a detector (G). By counting the X-rays generated by each element in the By counting the X-rays generated by each element in the

sample and comparing that number to the number of X-ray sample and comparing that number to the number of X-ray generated by a standard of known composition, it is generated by a standard of known composition, it is possible to determine the chemical composition of a spot possible to determine the chemical composition of a spot one one-thousandth of a millimeter in diameter with great one one-thousandth of a millimeter in diameter with great accuracy.accuracy.

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ApplicationsApplicationsProvides a wealth information about the physical and chemical nature Provides a wealth information about the physical and chemical nature

of surfaces (Quantitative and Qualitative analysis).of surfaces (Quantitative and Qualitative analysis). Phase studies in metallurgy and ceramics.Phase studies in metallurgy and ceramics. Investigation of grain boundaries in alloys.Investigation of grain boundaries in alloys. Measurement of diffusion rates of impurities in semiconductors.Measurement of diffusion rates of impurities in semiconductors. Determination of occluded species in crystals.Determination of occluded species in crystals. Study of the active sites of heterogeneous catalysts.Study of the active sites of heterogeneous catalysts.

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Scanning Electron MicroscopyScanning Electron Microscopy Great importance of studying of physical nature of solid surfaces Great importance of studying of physical nature of solid surfaces

in many fields (chemistry, material science, geology and in many fields (chemistry, material science, geology and biology).biology).

Classical methods:Classical methods:

Is the optical microscopy, but of limitted resolution because of Is the optical microscopy, but of limitted resolution because of diffraction effects to about the wavelength of light.diffraction effects to about the wavelength of light.

Higher resolution techniques (Three types)Higher resolution techniques (Three types)

1- Scanning electron microscopy (SEM).1- Scanning electron microscopy (SEM).

2- Scanning tunneling microscopy (STM).2- Scanning tunneling microscopy (STM).

3- Atomic force microscopy (AFM).3- Atomic force microscopy (AFM).

Basics:Basics:

The surface of the solid is swipt in a raster pattern with finely The surface of the solid is swipt in a raster pattern with finely focused beam of electrons or with a suitable probe.focused beam of electrons or with a suitable probe.

The raster is a scanning pattern similar to cathode-ray tube in TV:The raster is a scanning pattern similar to cathode-ray tube in TV:

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Raster (Scanning pattern)Raster (Scanning pattern)

1- Swept across a surface in a straight line (X-direction).1- Swept across a surface in a straight line (X-direction).

2- Returned to its starting position.2- Returned to its starting position.

3- Shifted downward (y direction) by a standard increment.3- Shifted downward (y direction) by a standard increment.

4- Repeating the process until a desired area of the surface has been 4- Repeating the process until a desired area of the surface has been scanned.scanned.

5- During scanning process a signal is received above the surface (Z 5- During scanning process a signal is received above the surface (Z direction) and stored in a computer system which converted to an direction) and stored in a computer system which converted to an image.image.

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Why Study Surfaces?Why Study Surfaces? Surface – the interface between two of Surface – the interface between two of

matter’s common phases:matter’s common phases:• Solid-gas (we will primarily focus on this)Solid-gas (we will primarily focus on this)• Solid-liquidSolid-liquid• Solid-solidSolid-solid• Liquid-gasLiquid-gas• Liquid-liquidLiquid-liquid• The majority of present studies are applied to The majority of present studies are applied to

this type of system, and the techniques this type of system, and the techniques available are extremely powerfulavailable are extremely powerful

The properties of surfaces often control The properties of surfaces often control chemical reactionschemical reactions

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MicroscopyMicroscopy Why is microscopy useful? What can it tell Why is microscopy useful? What can it tell

the analytical chemist?the analytical chemist?• Sample topography Sample topography • Structural stress/strainStructural stress/strain• Electromagnetic propertiesElectromagnetic properties• Chemical compositionChemical composition

Plus - a range of spectroscopic techniques, Plus - a range of spectroscopic techniques, from IR to X-ray wavelengths/energies, from IR to X-ray wavelengths/energies, have been combined with microscopy to have been combined with microscopy to create some of the most powerful create some of the most powerful analytical tools available…analytical tools available…

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Imaging Resolution and MagnificationImaging Resolution and Magnification

Some typical values for microscopic methods:Some typical values for microscopic methods:

MethodMethod ResolutionResolutionMagnificatiMagnificati

onon

(x)(x)

Human EyeHuman Eye 0.1-0.2 mm0.1-0.2 mm --

Optical Optical MicroscopyMicroscopy

0.1-0.2 um0.1-0.2 um ~1200~1200

ElectronElectron

MicroscopyMicroscopy30-50 Å30-50 Å 10-75,00010-75,000

Probe Probe MicroscopyMicroscopy

<1 Å<1 Å > 500,000> 500,000

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Optical Microscopy - HistoryOptical Microscopy - History An ancient technique – the lens has been around for An ancient technique – the lens has been around for

thousands of years. Chinese tapestries dating from 1000 thousands of years. Chinese tapestries dating from 1000 B.C. depict eyeglasses. B.C. depict eyeglasses.

In 1000 A.D., an Arabian mathematician (Al Hasan) made In 1000 A.D., an Arabian mathematician (Al Hasan) made the first theoretical study of the lens.the first theoretical study of the lens.

Copernicus (1542 A.D.) made the first definitive use of a Copernicus (1542 A.D.) made the first definitive use of a telescope.telescope.

As glass polishing skills developed, microscopes became As glass polishing skills developed, microscopes became possible. John and Zaccharias Jannsen (Holland) made the possible. John and Zaccharias Jannsen (Holland) made the first commercial and first compound microscopes.first commercial and first compound microscopes.

Then came lens grinding, Galileo, the biologists, and many Then came lens grinding, Galileo, the biologists, and many great discoveries….great discoveries….

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Modern Optical Microscopy in ChemistryModern Optical Microscopy in Chemistry As optical microscopy As optical microscopy

developed, the compound developed, the compound microscope was applied to the microscope was applied to the study of chemical crystals.study of chemical crystals.

The polarizing microscope The polarizing microscope (1880): can see boundaries (1880): can see boundaries between materials with between materials with different refractive indices, different refractive indices, while also detecting isotropic while also detecting isotropic and anisotropic materials. and anisotropic materials.

http://www.microscopyu.com/articles/polarized/polarizedintro.html

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Optical Microscope DesignOptical Microscope Design

Objective lenses are Objective lenses are characterized NA (numerical characterized NA (numerical apertures)apertures)• The numerical aperture of a The numerical aperture of a

microscope objective is a microscope objective is a measure of its ability to measure of its ability to gather light and resolve fine gather light and resolve fine specimen detail at a fixed specimen detail at a fixed object distanceobject distance

• Large NA = finer detail = Large NA = finer detail = better light gatheringbetter light gathering

http://www.microscopyu.com/articles/polarized/polarizedintro.html

Diagram from Wikipedia (public domain)

Microscope design has not Microscope design has not changed much in 300 changed much in 300 yearsyears• But the lenses are more But the lenses are more

perfect – free of perfect – free of aberrationsaberrations

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Electron Microscopy (EM)Electron Microscopy (EM) Scanning electron microscopy (SEM) – an electron beam is scanned Scanning electron microscopy (SEM) – an electron beam is scanned

in a raster pattern with a beam of energetic electrons and in a raster pattern with a beam of energetic electrons and “reflected” effects are monitored.“reflected” effects are monitored.

Several types of signals are produced from a surface in this process Several types of signals are produced from a surface in this process including:including:

1- Backscattered electrons.1- Backscattered electrons.

2- Secondary electrons.2- Secondary electrons.

(Serve the basis of scanning electron microscopy)(Serve the basis of scanning electron microscopy)

3- Auger electrons.3- Auger electrons.

4- X-ray fluorescence photons 4- X-ray fluorescence photons (used in electron microprobe analysis).(used in electron microprobe analysis).

5- Other photons of various energies.5- Other photons of various energies.

All of these signals have been used for surface studiesAll of these signals have been used for surface studies

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Ice crystalsoptical SEM

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Electron Microscopy: ResolutionElectron Microscopy: Resolution Why can an electron microscope resolve things that are Why can an electron microscope resolve things that are

impossible to discern with optical microscopy?impossible to discern with optical microscopy? Example – calculate the wavelength of electrons accelerated by Example – calculate the wavelength of electrons accelerated by

a 10 kV potential:a 10 kV potential:

nm 0.0123 m1023.1

)V C)(101060.1)(kg102(9.11

s J1063.6

22

2

11

419-31-

34

221

meV

h

eV

m

m

h

m

eVv

eVmv

= 0.123 x 10-3 m. So EM can see >10000x more detail than visible light!

Note: Resolution is limited by lens aberrations!

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Electron Microscopy: ResolutionElectron Microscopy: Resolution

What about What about relativistic corrections?relativistic corrections? The electrons in an EM The electrons in an EM can in some cases be moving pretty close to the speed of can in some cases be moving pretty close to the speed of light.light.

Example – what is the wavelength for a 100 kV potential?Example – what is the wavelength for a 100 kV potential?

nm107.3

)1)(V C)(101060.1)(kg102(9.11

s J1063.6

)1(22

3

)/103(kg)109.11(2

)V C)(101060.1(419-31-

34

2

28-31

419-

2

sm

mceVmeV

h

eV

m

m

h

At high potentials, EM can see atomic dimensions

Using the relativistically corrected form of the previous equation:

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Electron Microscopy: Sample-Beam InteractionsElectron Microscopy: Sample-Beam Interactions

Sample-beam interactions Sample-beam interactions control how both SEM and TEM control how both SEM and TEM (i.e. STEM) operate:(i.e. STEM) operate:• Formation of imagesFormation of images• Spectroscopic/diffractometric Spectroscopic/diffractometric

analysisanalysis There are There are lotslots (actually eight) (actually eight)

types of sample-beam types of sample-beam interactions (which can be interactions (which can be confusing and hard to confusing and hard to remember!)remember!)

It helps to classify these 8 types into two classes of sample-beam It helps to classify these 8 types into two classes of sample-beam interactions:interactions:• bulk specimen interactions (bounce off sample – “reflected”)bulk specimen interactions (bounce off sample – “reflected”)• thin specimen interactions (travel through sample-“transmitted”)thin specimen interactions (travel through sample-“transmitted”)

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SEM: Sample-Beam InteractionsSEM: Sample-Beam Interactions

Backscattered Electrons (~30 keV)Backscattered Electrons (~30 keV)

Caused by an incident electron colliding Caused by an incident electron colliding with an atom in the specimen which is with an atom in the specimen which is almost normal to the incident electron’s almost normal to the incident electron’s path. The electron is then scattered path. The electron is then scattered "backward" 180 degrees. "backward" 180 degrees.

Backscattered electron intensity varies Backscattered electron intensity varies directly with the specimen's atomic directly with the specimen's atomic number. This differing production rates number. This differing production rates causes higher atomic number elements to causes higher atomic number elements to appear “brighter” than lower atomic appear “brighter” than lower atomic number elements. This creates number elements. This creates contrastcontrast in in the image of the specimen based on the image of the specimen based on different average atomic numbers.different average atomic numbers.

Backscattered electrons can come from a Backscattered electrons can come from a wide area around the beam impact point wide area around the beam impact point (see pg. 552 of Skoog) – this also limits the (see pg. 552 of Skoog) – this also limits the resolution of a SEM (along with resolution of a SEM (along with abberations in the EM lenses)abberations in the EM lenses)

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SEM: Sample-Beam Interactions+SEM: Sample-Beam Interactions+

Secondary Electrons (~5 eV)Secondary Electrons (~5 eV)

Caused by an incident electron passing Caused by an incident electron passing "near" an atom in the specimen, close "near" an atom in the specimen, close enough to impart some of its energy to a enough to impart some of its energy to a lower energy electron (usually in the K-lower energy electron (usually in the K-shell). This causes a slight energy loss, a shell). This causes a slight energy loss, a change in the path of the incident electron change in the path of the incident electron and ionization of the electron in the and ionization of the electron in the specimen atom. The ionized electron then specimen atom. The ionized electron then leaves the atom with a very small kinetic leaves the atom with a very small kinetic energy (~5 eV). One incident electron can energy (~5 eV). One incident electron can produce several secondary electrons. produce several secondary electrons.

Production of secondary electrons is closely Production of secondary electrons is closely linked to sample topography. Their low linked to sample topography. Their low energy (~5 eV) means that only electrons energy (~5 eV) means that only electrons very near to the surface (<10 nm) are very near to the surface (<10 nm) are detected. They also don’t suffer from the detected. They also don’t suffer from the backscattered electron lateral resolution backscattered electron lateral resolution problem depicted in Fig. 21-16 of Skoog. problem depicted in Fig. 21-16 of Skoog. Changes in topography in the sample that Changes in topography in the sample that are larger than this sampling depth can are larger than this sampling depth can change the yield of secondary electrons via change the yield of secondary electrons via indirect effects (called collection indirect effects (called collection efficiencies). efficiencies).

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SEM: Sample-Beam InteractionsSEM: Sample-Beam Interactions

Auger Electrons (10 eV – 2 keV)Auger Electrons (10 eV – 2 keV)

Caused by relaxation of an ionized atom Caused by relaxation of an ionized atom after a secondary electron is produced. after a secondary electron is produced. The lower (usually K-shell) electron that The lower (usually K-shell) electron that was emitted from the atom during the was emitted from the atom during the secondary electron process has left a secondary electron process has left a vacancy. A higher energy electron from vacancy. A higher energy electron from the same atom can drop to a lower energy, the same atom can drop to a lower energy, filling the vacancy. This leaves extra filling the vacancy. This leaves extra energy in the atom which can be corrected energy in the atom which can be corrected by emitting a weakly-bound outer electron; by emitting a weakly-bound outer electron; an Auger electron. an Auger electron.

Auger electrons have a characteristic Auger electrons have a characteristic energy, which is unique and depends on energy, which is unique and depends on the emitting element. Auger electrons the emitting element. Auger electrons have relatively low energy and are only have relatively low energy and are only emitted from the bulk specimen from a emitted from the bulk specimen from a depth of several angstromsdepth of several angstroms..

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SEM: Sample-Beam InteractionsSEM: Sample-Beam Interactions

X-ray EmissionX-ray Emission

Caused by relaxation of an ionized atom Caused by relaxation of an ionized atom after a secondary electron is produced. after a secondary electron is produced. Since a lower (usually K-shell) electron Since a lower (usually K-shell) electron was emitted from the atom during the was emitted from the atom during the secondary electron process an inner secondary electron process an inner (lower energy) shell now has a vacancy. A (lower energy) shell now has a vacancy. A higher energy electron can "fall" into the higher energy electron can "fall" into the lower energy shell, filling the vacancy. As lower energy shell, filling the vacancy. As the electron "falls" it emits energy in the the electron "falls" it emits energy in the form of X-rays to balance the total energy form of X-rays to balance the total energy of the atom. of the atom.

X-rays emitted from the atom will have a X-rays emitted from the atom will have a characteristic energy which is unique to characteristic energy which is unique to the element from which it originated. the element from which it originated.

X-ray (elemental) mapping of sample X-ray (elemental) mapping of sample surfaces is a common applications and a surfaces is a common applications and a very powerful analytical approach. very powerful analytical approach.

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SEM: Sample-Beam Interactions – X-raysSEM: Sample-Beam Interactions – X-rays

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SEM: Sample-Beam InteractionsSEM: Sample-Beam Interactions

Cathodoluminescence (CL)Cathodoluminescence (CL) Caused by Caused by electron holeelectron hole pairs, which are pairs, which are

created by the electron beam in certain created by the electron beam in certain kinds of materials. When the pairs kinds of materials. When the pairs recombine, cathodoluminescence (CL) can recombine, cathodoluminescence (CL) can result. CL is the emission of result. CL is the emission of UV-Visible-IRUV-Visible-IR light by the recombination effect. CL is light by the recombination effect. CL is usually usually very weakvery weak and covers a wide range and covers a wide range of wavelengths, and requires high beam of wavelengths, and requires high beam currents, lowering resolution and currents, lowering resolution and challenging detector systems!challenging detector systems!

CL signals typically result from small CL signals typically result from small impurities in an otherwise homogeneous impurities in an otherwise homogeneous material, or lattice defects in a crystal.material, or lattice defects in a crystal.

CL can be used effectively for some CL can be used effectively for some analytical problems. Some “random” analytical problems. Some “random” examples:examples:• Differentiation of anatase and rutileDifferentiation of anatase and rutile• Studying ferroelectric domains in Studying ferroelectric domains in

sodium niobatesodium niobate• Location of subsurface crazing in Location of subsurface crazing in

ceramicsceramics• Forensic analysis of glassesForensic analysis of glasses

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TEM: Sample-Beam Interactions (Thin Sample)TEM: Sample-Beam Interactions (Thin Sample)

Unscattered ElectronsUnscattered Electrons Incident electrons which are Incident electrons which are

transmitted through the thin transmitted through the thin specimen without any interaction specimen without any interaction occurring inside the specimen. occurring inside the specimen.

Used to image - the transmission Used to image - the transmission of unscattered electrons is of unscattered electrons is inversely proportional to the inversely proportional to the specimen thickness. Areas of the specimen thickness. Areas of the specimen that are thicker will have specimen that are thicker will have fewer transmitted unscattered fewer transmitted unscattered electrons and so will appear electrons and so will appear darker, conversely the thinner darker, conversely the thinner areas will have more transmitted areas will have more transmitted and thus will appear lighter. and thus will appear lighter.

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TEM: Sample-Beam Interactions (Thin Sample)TEM: Sample-Beam Interactions (Thin Sample)Elastically-Scattered Electrons Elastically-Scattered Electrons Incident electrons that are scattered Incident electrons that are scattered

(deflected from their original path) by (deflected from their original path) by atoms in the specimen in an elastic atoms in the specimen in an elastic fashion (without loss of energy). These fashion (without loss of energy). These scattered electrons are then transmitted scattered electrons are then transmitted through the remaining portions of the through the remaining portions of the specimen. specimen.

Electrons follow Bragg's Law and are Electrons follow Bragg's Law and are diffracted. All incident electrons have diffracted. All incident electrons have the same energy (and wavelength) and the same energy (and wavelength) and enter the specimen normal to its enter the specimen normal to its surface. So all incident electrons that surface. So all incident electrons that are scattered by the same atomic are scattered by the same atomic spacing will be scattered by the same spacing will be scattered by the same angle. These "similar angle" scattered angle. These "similar angle" scattered electrons can be collated using electrons can be collated using magnetic lenses to form a pattern of magnetic lenses to form a pattern of spots; each spot corresponding to a spots; each spot corresponding to a specific atomic spacing, specific atomic spacing, This pattern This pattern can then yield information about the can then yield information about the orientation, atomic arrangements and orientation, atomic arrangements and phases present in the area beingphases present in the area being examined.examined.

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TEM: Sample-Beam Interactions (Thin Sample)TEM: Sample-Beam Interactions (Thin Sample)

Inelastically-Scattered ElectronsInelastically-Scattered Electrons

Incident electrons that interact with sample Incident electrons that interact with sample atoms inelastically (losing energy during atoms inelastically (losing energy during the interaction). These scattered electrons the interaction). These scattered electrons are then transmitted through the rest of the are then transmitted through the rest of the sample.sample.

Inelastically scattered electrons have two uses:Inelastically scattered electrons have two uses:

1. Electron Energy Loss Spectroscopy (EELS): 1. Electron Energy Loss Spectroscopy (EELS): The The amountamount of inelastic loss of energy by of inelastic loss of energy by the incident electrons can be used to study the incident electrons can be used to study the sample. These energy losses are the sample. These energy losses are unique to the bonding state of each unique to the bonding state of each element and can be used to extract both element and can be used to extract both compositional and bonding (i.e. oxidation compositional and bonding (i.e. oxidation state) information on the sample region state) information on the sample region being examined. being examined.

2. Kakuchi bands: Bands of alternating light 2. Kakuchi bands: Bands of alternating light and dark lines caused by inelastic and dark lines caused by inelastic scattering, which are related to interatomic scattering, which are related to interatomic spacing in the sample. These bands can be spacing in the sample. These bands can be either measured (their width is inversely either measured (their width is inversely proportional to atomic spacing) or used to proportional to atomic spacing) or used to help study the elasticity-scattered electron help study the elasticity-scattered electron patternpattern

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Electron Microscopy: Basic DesignElectron Microscopy: Basic Design

Basic layout of an electron microscope:Basic layout of an electron microscope:

Electrongun

(1-30 keV)

Magneticlenses andscanning

coils

Sample

Detectors

Detectors

electronsphotons

electrons

Computer

Page 108: Surface Chemistry.ppt
Page 109: Surface Chemistry.ppt

Electron Source (Gun)Electron Source (Gun)

Positive electrical potential applied to the Positive electrical potential applied to the anode anode

The filament (cathode) is heated until a The filament (cathode) is heated until a stream of electrons is produced stream of electrons is produced

The electrons are then accelerated by the The electrons are then accelerated by the positive potential down the column (can be up positive potential down the column (can be up to 30 kV)to 30 kV)

A negative electrical potential (~500 V) is A negative electrical potential (~500 V) is applied to the Wehnelt cap applied to the Wehnelt cap

Electrons are forced toward the column axis Electrons are forced toward the column axis by the Wehnelt cap by the Wehnelt cap

Electrons collect in the space between the Electrons collect in the space between the filament tip and Wehnelt cap (a space charge filament tip and Wehnelt cap (a space charge or “pool”)or “pool”)

Those electrons at the bottom of the space Those electrons at the bottom of the space charge (nearest to the anode) can exit the charge (nearest to the anode) can exit the gun area through the small (<1 mm) hole in gun area through the small (<1 mm) hole in the Wehnelt cap the Wehnelt cap

These electrons then move down the column These electrons then move down the column towards the EM lens and scanning systems towards the EM lens and scanning systems

Thermionic electron gun – how it works:

The results:- Electrons are emitted from

a nearly perfect point source (the space charge)

- The electrons all have similar energies (monchromatic)

- The electrons will travel parallel to the column axis

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Electron Optics: Focusing and ScanningElectron Optics: Focusing and Scanning

Apertures – usually made of platinum foil, with circular holes of 2 Apertures – usually made of platinum foil, with circular holes of 2 to 100 to 100 m.m.

The magnetic condenser and objective lens serve to reduce the The magnetic condenser and objective lens serve to reduce the image to final spot size on sample (5-200 image to final spot size on sample (5-200 m).m).

The condenser lens system (one or more lenses) to accelerate The condenser lens system (one or more lenses) to accelerate electrons to reach the objective lens.electrons to reach the objective lens.

The objective lens is responsible for the size of electron beam The objective lens is responsible for the size of electron beam impinging on surface of sample.impinging on surface of sample.

The individual lens is cylindrically symmetrical and of 10-15 cm in The individual lens is cylindrically symmetrical and of 10-15 cm in height.height.

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Magnetic lenses: Circular electro-magnets capable of Magnetic lenses: Circular electro-magnets capable of projecting a precise circular magnetic field in a specified projecting a precise circular magnetic field in a specified region. The field acts like an optical lens, having the same region. The field acts like an optical lens, having the same attributes (focal length, angle of divergence...etc.) and attributes (focal length, angle of divergence...etc.) and errors (spherical aberration, chromatic aberration....etc.). errors (spherical aberration, chromatic aberration....etc.). They are used to focus and steer electrons in an EM (SEM They are used to focus and steer electrons in an EM (SEM and STEM). and STEM).

Goal – a focused, monochromatic (I.e. same Goal – a focused, monochromatic (I.e. same energy/wavelength) electron beam!energy/wavelength) electron beam!

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Scanning with SEMScanning with SEM Accomplished by the two pairs of electromagnetic coils within the objective Accomplished by the two pairs of electromagnetic coils within the objective

lens.lens.

One pair deflects the beam in the X direction across the sample.One pair deflects the beam in the X direction across the sample. The other pair deflects it in the y direction.The other pair deflects it in the y direction. By applying an electrical signal to one pair of the scan coils and varying the By applying an electrical signal to one pair of the scan coils and varying the

electrical signal (x coils) as a function of time, the electron beam is moved electrical signal (x coils) as a function of time, the electron beam is moved in a straight line across the sample and then returned to its original in a straight line across the sample and then returned to its original position.position.

After completion of the line scan, the other set of coils (y coil in this case) After completion of the line scan, the other set of coils (y coil in this case) is used to deflect the beam slightly and the deflection of the beam using is used to deflect the beam slightly and the deflection of the beam using the x coils is repeated.the x coils is repeated.

By rapidly moving the beam the entire sample surface can irradiated with the By rapidly moving the beam the entire sample surface can irradiated with the electron beam.electron beam.

The signals to the scan coils can be either analog or digital (reproducible The signals to the scan coils can be either analog or digital (reproducible movement and location of the electron beam)movement and location of the electron beam)

The output signal from the sample can be encoded and stored in the digital The output signal from the sample can be encoded and stored in the digital form along with digital representations of the x and y positions of the form along with digital representations of the x and y positions of the beam.beam.

The signals are also used to drive the horizontal and vertical scans of a The signals are also used to drive the horizontal and vertical scans of a cathode-ray tube (CRT).cathode-ray tube (CRT).

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The image of the sampleThe image of the sample Produced by using the output of a detector to control the intensity Produced by using the output of a detector to control the intensity

of the spot on CRT.of the spot on CRT.

This method of scanning produces a map of the sample in which This method of scanning produces a map of the sample in which there is one-to-one correlation between the signal produced at a there is one-to-one correlation between the signal produced at a particular location on the sample surface and corresponding point particular location on the sample surface and corresponding point on the CRT on the CRT displaydisplay..

Magnification (M) achievable in the SEM image is given byMagnification (M) achievable in the SEM image is given by

M = W/wM = W/w

Where W is the width of the CRT display and w is the width of the Where W is the width of the CRT display and w is the width of the signal line scan across the sample.signal line scan across the sample.

W is constant so magnification is achieved by decreasing w.W is constant so magnification is achieved by decreasing w.

So focusing the electron beam to infinity small point could provide So focusing the electron beam to infinity small point could provide infinite magnification.infinite magnification.

But variety of factors limit M to a range of 10X to 100,000X.But variety of factors limit M to a range of 10X to 100,000X.

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Sample and sample HolderSample and sample Holder

Sample chambers are designed for rapid changing of samples.Sample chambers are designed for rapid changing of samples. Large capacity vacuum pumps are used to hasten the switch from Large capacity vacuum pumps are used to hasten the switch from

ambient P to 10ambient P to 10-4-4 torr or less. torr or less. Sample holder is capable of holding samples many cm on an edge.Sample holder is capable of holding samples many cm on an edge. It can be moved in the x, y and z directions and can rotated about It can be moved in the x, y and z directions and can rotated about

each axis.each axis.

Samples that conduct electricity are easiest to study, because the Samples that conduct electricity are easiest to study, because the unimpeded flow of electrons to ground minimizes artifacts unimpeded flow of electrons to ground minimizes artifacts associated with buildup of charge.associated with buildup of charge.

It also conduct heat which minimizes the likelihood of their thermal It also conduct heat which minimizes the likelihood of their thermal degradation.degradation.

(biological and most minerals samples do not conduct.(biological and most minerals samples do not conduct.

A variety of techniques have been developed for obtaining SEM A variety of techniques have been developed for obtaining SEM images of non conducting samples.images of non conducting samples.

Most common approaches involve coating the surface of the sample Most common approaches involve coating the surface of the sample with thin metallic film produced by sputtering or by vacuum with thin metallic film produced by sputtering or by vacuum evaporationevaporation

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TransducersTransducers

scintillation devices:scintillation devices: most common type: most common type:The detector consists of a doped glass or plastic target that emits a cascade of The detector consists of a doped glass or plastic target that emits a cascade of

visible photons when struck by an electron.visible photons when struck by an electron.The photons are conducted by a pipe to a photomultiplier tube that is housed The photons are conducted by a pipe to a photomultiplier tube that is housed

outside the high-vacuum region of the instrument.outside the high-vacuum region of the instrument.Gains 10Gains 1055-10-1066..Semiconductor transducers:Semiconductor transducers: (flat wafers of semiconductor material) are also used in electron microscopy.(flat wafers of semiconductor material) are also used in electron microscopy.When high E electron strikes the detector, electron-hole pairs are produced When high E electron strikes the detector, electron-hole pairs are produced

that result in increased conductivity.that result in increased conductivity.Current gains are 10Current gains are 1033-10-104 4

The device is small enough so that it can be placed immediately adjacent to The device is small enough so that it can be placed immediately adjacent to the sample (leads to high collection efficiency).the sample (leads to high collection efficiency).

Easy to use and are less expensive than scintillation transducers.Easy to use and are less expensive than scintillation transducers.X-rays produced:X-rays produced:Usually detected and measured with E-dispersive systems. Usually detected and measured with E-dispersive systems.

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ApplicationsApplications- Provides morphologic and topographic information about the Provides morphologic and topographic information about the

surfaces of solids that usually necessary in understanding the surfaces of solids that usually necessary in understanding the behavior of surfaces.behavior of surfaces.

- First step in the study of the surface properties of a solid.First step in the study of the surface properties of a solid.

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Electron Microscopy: Overall DesignElectron Microscopy: Overall DesignTEM design is similar – however, TEM design is similar – however,

nowadays, TEM systems nowadays, TEM systems usually include a “cryo-stage” usually include a “cryo-stage” for keeping samples extremely for keeping samples extremely cold during analysiscold during analysis

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Transmission Electron Microscopy: ApplicationsTransmission Electron Microscopy: Applications

MorphologyMorphology The size, shape and arrangement of the particles which make The size, shape and arrangement of the particles which make

up the specimen as well as their relationship to each other on up the specimen as well as their relationship to each other on the scale of atomic diameters. the scale of atomic diameters.

Crystallographic InformationCrystallographic Information The arrangement of atoms in the specimen and their degree of The arrangement of atoms in the specimen and their degree of

order, detection of atomic-scale defects in areas a few order, detection of atomic-scale defects in areas a few nanometers in diameter nanometers in diameter

We will discuss this topic further during the crystallography We will discuss this topic further during the crystallography lecturelecture

Compositional InformationCompositional Information The elements and compounds the sample is composed of and The elements and compounds the sample is composed of and

their relative ratios, in areas a few nanometers in diameter their relative ratios, in areas a few nanometers in diameter

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Scanning Probe Microscopy (SPM)Scanning Probe Microscopy (SPM) SPM, also known as profilimetrySPM, also known as profilimetry Are capable of resolving details of surfaces down to the atomic Are capable of resolving details of surfaces down to the atomic

level.level.

Types:Types:

1- The scanning tunnelling microscope: (primary use is 1- The scanning tunnelling microscope: (primary use is measuring surface topography of samples).measuring surface topography of samples).

Unlike Optical and electron micropropes SPM reveal details not Unlike Optical and electron micropropes SPM reveal details not on the lateral x and y of a sample but also on the z axis on the lateral x and y of a sample but also on the z axis which is perpendicular to the surface.which is perpendicular to the surface.

Resolution:Resolution:

SPMs is SPMs is 20 a in the x and y direction bu for ideal samples 20 a in the x and y direction bu for ideal samples best resolution is 1 A. R in z direction is better than 1 A.best resolution is 1 A. R in z direction is better than 1 A.

SEMs is about 50 A.SEMs is about 50 A.

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The scanning tunneling microscopeThe scanning tunneling microscope

Capable of resolving features on an atomic scale on the surface of Capable of resolving features on an atomic scale on the surface of conducting solid surface.conducting solid surface.

Main disadvantages:Main disadvantages:

The requirement that the surface being examined must conduct The requirement that the surface being examined must conduct electricity.electricity.

Principles:Principles:

The surface of the sample is scanned in a raster pattern by a very The surface of the sample is scanned in a raster pattern by a very fine metallic tip. fine metallic tip.

The tip is maintained at a constant distance d above the surface The tip is maintained at a constant distance d above the surface throughout the scan.throughout the scan.

The up and down motion of the tip then reflects the topology of the The up and down motion of the tip then reflects the topology of the surface.surface.

In order to maintain the tip at constant distance from the sample In order to maintain the tip at constant distance from the sample surface, a tunneling current between the tip and the sample is surface, a tunneling current between the tip and the sample is monitored and held at a constant level.monitored and held at a constant level.

The TC is generated by the voltage V that is applied between the tip The TC is generated by the voltage V that is applied between the tip and sample.and sample.

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A TC that passes through a medium that contains no electrons:A TC that passes through a medium that contains no electrons:

Examples of such media:Examples of such media:

Vacuum, nonpolar liquid, or even an aqueous electrolyte solution.Vacuum, nonpolar liquid, or even an aqueous electrolyte solution.

Mechanism of current flow through an insulating medium:Mechanism of current flow through an insulating medium:

Rationalized by quantum mechanics but will not be dealt here.Rationalized by quantum mechanics but will not be dealt here.

Tunneling currents become significant when conductors are within a Tunneling currents become significant when conductors are within a few nanometers of one another and when one of conductors is the few nanometers of one another and when one of conductors is the form of a sharp tip.form of a sharp tip.

Scanning Tunneling current IScanning Tunneling current Itt is given approximately by: is given approximately by:

IIt t = V= Vee-cd-cd

Where V is the bias voltage between the conductors, C is a constant Where V is the bias voltage between the conductors, C is a constant that is characteristic of conductor compositionthat is characteristic of conductor composition

D is the capacity between the lowest atom on the tip and the highest D is the capacity between the lowest atom on the tip and the highest atom on the sample.atom on the sample.

In the tunneling microscope current is held constant by a feedback In the tunneling microscope current is held constant by a feedback mechanism tht moves the tip up and down so d remains constant. mechanism tht moves the tip up and down so d remains constant.

Controlled by a piezoelectric transducer.Controlled by a piezoelectric transducer.

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Scanning Tunnelling Microscopy (STM)Scanning Tunnelling Microscopy (STM)

Besocke-beetle style STM head

Rasteringcontrol

electronicscomputer

DCbias

Piezo actuators

tunnelcurrent

amp

displayX Y

Z

Constant current imaging:A feedback loop adjusts the separation between tip and sample to maintain a constant current. The voltages applied to the piezo are translated into an image.

Image represents a convolution of topography and electronic structure 1/8 in

Slide courtesy of B. Mantooth and the Weiss Group at Penn State

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Scanning Tunnelling MicroscopyScanning Tunnelling Microscopy Tunnelling current is caused by quantum Tunnelling current is caused by quantum

mechanical phenomena (confinement of an mechanical phenomena (confinement of an electron to a “box” with finite walls) electron to a “box” with finite walls)

The tunnelling current The tunnelling current IItt is given by: is given by:

Where: V is the bias voltage C is a constant based on the conducting

materials d is the spacing between the atom at the tip

and the sample atom

Tips are prepared by cutting and Tips are prepared by cutting and electrochemical etching – atomic scale electrochemical etching – atomic scale can be achieved because the tunnelling can be achieved because the tunnelling current falls off exponentially with current falls off exponentially with increasing gap.increasing gap.

R. J. Hamers, “Scanned Probe Microscopies in Chemistry,” J. Phys. Chem., 1996, 100, 13103-13120.

Cdt VeI

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The atomic Force MicroscopeThe atomic Force Microscope

Invented in 1986.Invented in 1986. The Atomic Force Microscope was developed to overcome a basic The Atomic Force Microscope was developed to overcome a basic

drawback with STM - that it can only image conducting or semiconducting drawback with STM - that it can only image conducting or semiconducting surfaces. The AFM, however, has the advantage of imaging almost any surfaces. The AFM, however, has the advantage of imaging almost any type of surface, including polymers, ceramics, composites, glass, and type of surface, including polymers, ceramics, composites, glass, and

biological samples.biological samples. Permits resolution of individual atoms on both conducting and insulating Permits resolution of individual atoms on both conducting and insulating

surfaces.surfaces.

Procedure:Procedure:

A flexible force-sensing cantilever stylus is scanned in rastyer pattern over the A flexible force-sensing cantilever stylus is scanned in rastyer pattern over the surface of the sample.surface of the sample.

The force acting between the cantilever and the sample surface causes The force acting between the cantilever and the sample surface causes minute deflections of the cantilever which are detected by optical means.minute deflections of the cantilever which are detected by optical means.

As in STM the motion of the tip or sometimes the sample is achieved with a As in STM the motion of the tip or sometimes the sample is achieved with a piezoelectric tube.piezoelectric tube.

During a scan the force on the tip is held constant by the up and down motion During a scan the force on the tip is held constant by the up and down motion of the tip, which provides the topographic information.of the tip, which provides the topographic information.

Advantages:Advantages:

It is applicable to nonconducting samples.It is applicable to nonconducting samples.

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An atomically sharp tip is scanned over a surface with feedback mechanisms An atomically sharp tip is scanned over a surface with feedback mechanisms that enable the piezo-electric scanners to maintain the tip at a constant that enable the piezo-electric scanners to maintain the tip at a constant force (to obtain height information), or height (to obtain force information) force (to obtain height information), or height (to obtain force information) above the sample surface.above the sample surface.

The AFM works by scanning a fine ceramic or semiconductor tip over a The AFM works by scanning a fine ceramic or semiconductor tip over a surface much the same way as a phonograph needle scans a record (for surface much the same way as a phonograph needle scans a record (for those of you that remember what a record player was!). those of you that remember what a record player was!).

The tip is positioned at the end of a cantilever beam shaped. The tip is positioned at the end of a cantilever beam shaped. As the tip is repelled by or attracted to the surface, the cantilever beam As the tip is repelled by or attracted to the surface, the cantilever beam

deflects. deflects. A diode laser is focused onto the back of a reflective cantilever.A diode laser is focused onto the back of a reflective cantilever. As the tip scans the surface of the sample, moving up and down with the As the tip scans the surface of the sample, moving up and down with the

contour of the surface, the laser beam is deflected off the attached contour of the surface, the laser beam is deflected off the attached cantilever into a dual element photodiode. cantilever into a dual element photodiode.

The photodetector measures the difference in light intensities between the The photodetector measures the difference in light intensities between the upper and lower photodetectors, and then converts to voltage. upper and lower photodetectors, and then converts to voltage.

Feedback from the photodiode difference signal, through software control Feedback from the photodiode difference signal, through software control from the computer, enables the tip to maintain either a constant force or from the computer, enables the tip to maintain either a constant force or constant height above the sample. In the constant force mode the piezo-constant height above the sample. In the constant force mode the piezo-electric transducer monitors real time height deviation. electric transducer monitors real time height deviation.

A plot of the laser deflection versus tip position on the sample surface A plot of the laser deflection versus tip position on the sample surface provides the resolution of the hills and valleys that constitute the provides the resolution of the hills and valleys that constitute the topography of the surface. topography of the surface.

The AFM can work with the tip touching the sample (contact mode), or the The AFM can work with the tip touching the sample (contact mode), or the tip can tap across the surface (tapping mode) much like the cane of a blind tip can tap across the surface (tapping mode) much like the cane of a blind person. person.

The output from the photodiode then controls the force applied to tip so The output from the photodiode then controls the force applied to tip so that it remains constant.that it remains constant.

The optical control is similar to tunneling-current control system in the The optical control is similar to tunneling-current control system in the STM.STM.

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Measuring forcesMeasuring forces

Because the atomic force microscope relies on the forces between the tip Because the atomic force microscope relies on the forces between the tip and sample, knowing these forces is important for proper imaging. The and sample, knowing these forces is important for proper imaging. The force is not measured directly, but calculated by measuring the force is not measured directly, but calculated by measuring the deflection of the lever, and knowing the stiffness of the cantilever. deflection of the lever, and knowing the stiffness of the cantilever. Hook’s law gives F = -kz, where F is the force, k is the stiffness of the Hook’s law gives F = -kz, where F is the force, k is the stiffness of the lever, and z is the distance the lever is bent.lever, and z is the distance the lever is bent.

AFM Modes of operationAFM Modes of operation

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The tip and cantileverThe tip and cantilever

Performance of AFM depends on the physical charcteristics of the Performance of AFM depends on the physical charcteristics of the cantilever and tip.cantilever and tip.

Early:Early: cantilevers were cut from metal foil and tips were made cantilevers were cut from metal foil and tips were made from crushed diamond particles.from crushed diamond particles.

The tip were glued manually to cantilevers.The tip were glued manually to cantilevers. Currently:Currently: this crude method replaced by semiconductor this crude method replaced by semiconductor

production methods in which integral cantilever/tip assemblies are production methods in which integral cantilever/tip assemblies are produced by etching single chips of silicon, silicon oxides or silicon produced by etching single chips of silicon, silicon oxides or silicon nitride.nitride.

Small and delicate cantilever and tip are suggested.Small and delicate cantilever and tip are suggested. Typically the cantilevers are a few micrometers in width and about Typically the cantilevers are a few micrometers in width and about

one micrometer in thickness.one micrometer in thickness. The pyramid-or cone-shaped tips are a few micrometers in height The pyramid-or cone-shaped tips are a few micrometers in height

and width at the base. and width at the base.

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• Invented in 1986• Cantilever• Tip• Surface• Laser • Multi-segment photodetector

Figure 4. Three common types of AFM tip. (a) normal tip (3 µm tall); (b) supertip; (c) Ultralever (also 3 µm tall).

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Tapping mode operationTapping mode operation

Disadvantages of the contact mode scanning:Disadvantages of the contact mode scanning:

The tip is in The tip is in constant contact (mode)constant contact (mode) with the surfcae of the sample with the surfcae of the sample and the downward force of the tip may not be low enough to and the downward force of the tip may not be low enough to avoided damage to sample surface and distortion of image as avoided damage to sample surface and distortion of image as consequence (specially in soft materials biological samples, consequence (specially in soft materials biological samples, polymers. or semi hard: silicon wafers).polymers. or semi hard: silicon wafers).

Overcome by: Overcome by: Tapping mode operation:Tapping mode operation:

a process in which the tip contacts the surface for only a brief time a process in which the tip contacts the surface for only a brief time periodically and then removed from the surface.periodically and then removed from the surface.

The cantilever is oscilated at frequency of a few hundred kilohertz.The cantilever is oscilated at frequency of a few hundred kilohertz.

The oscillation is driven by a constant driving force and the amplitude The oscillation is driven by a constant driving force and the amplitude is monitored continuously.is monitored continuously.

The cantilever is positioned so that the tip touches the surface only The cantilever is positioned so that the tip touches the surface only at the bottom of each oscillation cycle. (variety materials images at the bottom of each oscillation cycle. (variety materials images that difficult to image by constant-contact mode).that difficult to image by constant-contact mode).

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Contact ModeContact Mode• High resolutionHigh resolution• Damage to sampleDamage to sample• Can measure frictional Can measure frictional

forcesforces Non-Contact ModeNon-Contact Mode

• Lower resolutionLower resolution• No damage to sampleNo damage to sample

Tapping ModeTapping Mode• Better resolution Better resolution • Minimal damage to Minimal damage to

samplesample

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ApplicationsApplications

Seeing surface structures with high resolution.Seeing surface structures with high resolution.

Examples:Examples:

Semiconductor field.Semiconductor field.

Silicon surfaces and their defects.Silicon surfaces and their defects.

Magnetic domains on magnetic materials.Magnetic domains on magnetic materials.

Biotechnology (DNA, chromatin, protein/enzyme interactions, Biotechnology (DNA, chromatin, protein/enzyme interactions, membrane viruses).membrane viruses).

Advantages:Advantages:

Permits imaging of biological samples under water without distortion Permits imaging of biological samples under water without distortion

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DNA image

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2.5 x 2.5 nm simultaneous topographic and friction image of highly oriented pyrolytic graphic (HOPG). The bumps represent the topographic atomic corrugation, while the coloring reflects the lateral forces on the tip. The scan direction was right to left

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Comparison of AFM and other imaging techniquesComparison of AFM and other imaging techniques 1. AFM versus STM:1. AFM versus STM: It's interesting to compare AFM and its precursor -- Scanning Tunneling It's interesting to compare AFM and its precursor -- Scanning Tunneling

Microscope. In some cases, the resolution of STM is better than AFM Microscope. In some cases, the resolution of STM is better than AFM because of the exponential dependence of the tunneling current on because of the exponential dependence of the tunneling current on distance. The force-distance dependence in AFM is much more complex distance. The force-distance dependence in AFM is much more complex when characteristics such as tip shape and contact force are considered. when characteristics such as tip shape and contact force are considered. STM is generally applicable only to conducting samples while AFM is STM is generally applicable only to conducting samples while AFM is applied to both conductors and insulators. In terms of versatility, needless applied to both conductors and insulators. In terms of versatility, needless to say, the AFM wins. Furthermore, the AFM offers the advantage that the to say, the AFM wins. Furthermore, the AFM offers the advantage that the writing voltage and tip-to-substrate spacing can be controlled writing voltage and tip-to-substrate spacing can be controlled independently, whereas with STM the two parameters are integrally independently, whereas with STM the two parameters are integrally linked.linked.

2. AFM versus SEM:2. AFM versus SEM: Compared with Scanning Electron Microscope, AFM provides extraordinary Compared with Scanning Electron Microscope, AFM provides extraordinary

topographic contrast direct height measurements and unobscured views topographic contrast direct height measurements and unobscured views of surface features (no coating is necessary).of surface features (no coating is necessary).

3. AFM versus TEM: 3. AFM versus TEM: Compared with Transmission Electron Microscopes, three dimensional AFM Compared with Transmission Electron Microscopes, three dimensional AFM

images are obtained without expensive sample preparation and yield far images are obtained without expensive sample preparation and yield far more complete information than the two dimensional profiles available more complete information than the two dimensional profiles available from cross-sectioned samples.from cross-sectioned samples.

4. AFM versus Optical Microscope: 4. AFM versus Optical Microscope: Compared with Optical Interferometric Microscope (optical profiles), the AFM Compared with Optical Interferometric Microscope (optical profiles), the AFM

provides unambiguous measurement of step heights, independent of provides unambiguous measurement of step heights, independent of reflectivity differences between materials.reflectivity differences between materials.