Download Moderne Methoden der Materialcharakterisierung

Modern techniques of materials
characterization
Basic concept
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Source – What kind of „probe“ is used?
How does the probe reach the sample?
Interaction between probe and sample
How does the signal of interest reach the analyzer?
Characteristics of the analyzer
Source
Interaction
Sample
Analyzer
What kind of probes are available?
• Each and every analysis technique is based on the interaction
between a probe and a sample. The following probes are generally
available:
• Electrons
• Ions
• Neutrons*
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• Photons
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• Heat*
• A field*
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Hot cathode, field emission
Plasma, liquid metal tips
Nuclear reactions (e.g.
Spallations-sources)
Laser
X-ray
Synchrotron radiation
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electric, magnetic fields
Analysis Techniques (principle)
Signal
Electrons
Electrons
Probe
Ions
Neutrons
Photons
Heat
A field
Ions
Neutrons
Photons
Heat
A field
Energy of a particle → Wavelength
Analysis of the structure
• Usually one starts with the direct physical imaging of a sample
surface
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Optical microscope
SEM/Auger (scanning electron microscopy)
TEM (transmission electron microscopy)
STM/AFM (scanning tunneling microscopy / atomic force microscopy)
LEERM* (low energy electron reflection microscopy)
Indirect analysis of the structure
• Diffraction of electrons, atoms or ions is used to gain insight to the
atomic structure of the sample surface
– XRD (x-ray diffraction) – surface analysis by crazing incidence X-ray
diffraction
– LEED (low electron energy diffraction) - MEED
– ABS (atomic beam scattering)
– LEIS (low energy ion scattering) – MEIS, HEIS
– RBS (Rutherford back scattering)
– RHEED (reflection high energy electron diffraction)
– SEXAFS (surface enhanced X-ray absorption fine structure)
– XANES (X-ray absorption near edge structure)
– SEELFS (surface extended energy loss fine structure)
Chemical analysis of the surface
• Basic determination of elements present at the surface
• Determination of chemical bonding and atomic or molecular states in
the surface region
• Lateral and depth profiling of elemental distribution
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XPS (X-ray photoelectron spectroscopy)
UPS (ultraviolet photoelectron spectroscopy)
AES (Auger electron spectroscopy)
SIMS (secondary ion mass spectrometry)
FTIR (Fourier transform infrared spectroscopy), ATR (attenuated total
reflectance spectroscopy), Raman
Scanning Probe Microscopy
A plethora of possibilities
Basic idea
The SPM family
The scanning part of SPMs
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Based on the piezoelectric
effect:
– Piezo Tri-Pods
– Piezo-Tube-Scanners
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Problems of these
scanners are:
– Hysteresis, creep
– Aging
– Cross-correlations
between the individual
axis
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These are addressed by
extensive calibrationfunctions or closed-loopsystems utilizing laserinterferrometry
Piezo-tube scanner and
sketch of a piezo tripod
AFM - interaction
• Lennard-Jones
potential is often cited
• Consisting of a vander-Waals and a
Pauli-part
• Distance-dependence
of interaction is
changed in case of
nanoscale objects
• Basic behavior,
however, is
comparable
Various AFM modi
Non-contact mode
• Idea here is to
sense the sample
without touching it
→ essential in the
context of most
polymer and
biological
samples
• Cantilever is
operated close to
its resonance
frequency via a
piezo actuator
Electron microscope techniques
Scanning Electron Microscope (SEM)
Transmission Electron Microscope (TEM)
Electron sources
Electron guns:
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Various examples
of gun design
– Thermionic
– Schottky
– Field emission
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Cathode material
– Tungsten
– Lanthanum
hexaboride
(LaB6)
– Others…
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Cathode material
determines
emission current
density
Energy scheme of various gun types
What kind of species are generated?
Probe-sample interaction results in the „generation“ of
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Secondary electrons
Backscattered electrons
X-rays
Auger electrons
Plasmons
Secondary electrons (SE)
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SE (exit energies < 50 eV) are generated if the energy gain of these species
is large enough to overcome the work function
This process needs to be treated quantum mechanically as the scattering of
an electron wave at a potential barrier
SE are only able to escape from a small surface range (probability of
escaping is based on their inelastic mean free path)
Backscattered electrons contribute to the SE yield 
Backscattered electrons (BSE)
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BSE are present in the whole energy range from 50 eV (definition) to the
maximum acceleration energy of the primary electrons (PE)
Their spectrum shows a broad peak overlapped by SE and Auger peaks as
well as plasmon loss
BSE and SE are the most important signals for imaging. Knowledge about
the dependence of the backscattering coefficient and the SE yield on surface
tilt, material and electron energy is essential for any interpretation.
X-ray
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Acceleration of a charged particle (electron) in the screened Coulomb
potential of the nucleus leads – with a low probability – to an emission of a
X-ray quantum (usually elastic scattering is observed)
Electron is decelerated by h (energy of the X-ray quantum) → continuous
X-ray spectrum
This continuous spectrum is superposed on the characteristic X-ray
spectrum generated by filling of inner shell vacancies
X-ray
Diffraction based techniques
X-ray, neutron, and electron based methods
Basic definition of diffraction
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Diffraction is the bending, spreading and
interference of waves when they pass by an
obstruction or through a gap. It occurs with any
type of wave, including sound waves, water
waves, electromagnetic waves such as light and
radio waves, and matter displaying wave-like
properties according to the wave–particle duality.
Thomas Young (1773-1829),
ophthalmologist and physicist
Thomas Young's sketch of two-slit diffraction, which he
presented to the Royal Society in 1803
X-ray sources
Energy regime of Gamma- und X-ray radiation overlap – naming criteria is the
heritage: X-ray is created by electron processes whereas Gamma radiation is
a nuclear reaction product
Typically X-ray radiation is generated by deceleration of electrons
X-ray sources (Synchrotron)
Synchrotron radiation is emitted by
charged relativistic particles deflected by
a magnetic field tangentially to their path
of motion
In order to generate synchrotron radiation
so called storage rings are used that
keep the kinetic energy of the charged
particles constant in order to conserve a
constant energy spectrum of the radiation
Worldwide, about 30 laboratories are able to generate synchrotron radiation. In
Germany there are, among others, BESSY in Berlin, HASYLAB in Hamburg,
DELTA at Universität Dortmund and ANKA in Karlsruhe
A known natural source of synchrotron radiation is for example Jupiter which
bombards its moons with synchrotron radiation
Neutron sources
Nuclear reactor
• Usually fission reactors are used to
generate kinetic neutrons to serve in
diffraction experiments
Spallation source
• Nuclear spallation is one of the processes
by which a particle accelerator may be
used to produce a beam of neutrons. A
mercury, tantalum or other heavy metal
target is used, and 20 to 30 neutrons are
expelled after each impact of a high energy
proton. Although this is a far more
expensive way of producing neutron beams
than by a chain reaction of nuclear fission
in a nuclear reactor, it has the advantage
that the beam can be pulsed with relative
ease.
Bragg relation
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The diffraction equation
postulated by Bragg and
his son in 1914 (Nobel
laureate in 1915)
Waves that satisfy this condition interfere
constructively and result in a reflected wave of
significant intensity
X-ray diffraction – phase analysis
Rietveld method (Hugo Rietveld (1932-) allows a quantitative
phase analysis in the context of X-ray and neutron
diffractogramms
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Analysis of the whole diffractogramm
Refinement of structure- as well as real-structureparameters
– Quantitative phase analysis
– Lattice parameters and temperature effects
– Grain size and micro strain
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Its not a structure analysis!
– Basic lattice parameters,
– phase composition, and
– Space group needs to be known
Hugo Rietveld
Photon-based Techniques
Raman spectroscopy
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The phenomenon behind this technique was first
reported by Sir Chandrasekhara Venkata Raman (18881970) in 1928 – in 1930 he was awarded the Nobel
Prize in physics for his findings
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A small percentage of light scattered at a molecule is
inelastically scattered (1 in 107 photons)
Sir C.V. Raman
Raman spectroscopy - basics
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At room temperature majority of molecules in initial (ground) state  antiStokes signal will be less pronounced: Ratio of anti-Stokes to Stokes can be
used for temperature measurement
The energy of a vibrational mode depends on molecular structure and
environment. Atomic mass, bond order, molecular substituents, molecular
geometry and hydrogen bonding all effect the vibrational force constant
which, in turn dictates the vibrational energy
Vibrational Raman spectroscopy is not limited to intramolecular vibrations.
Crystal lattice vibrations and other motions of extended solids are Ramanactive
Raman scattering occurs when it features a change in polarizability during
the vibration
This rule is analogous to the rule for an infrared-active vibration (that there
must be a net change in permanent dipole moment during the vibration) from group theory it is possible to show that if a molecule has a center of
symmetry, vibrations which are Raman-active will be silent in the infrared,
and vice versa
Raman spectroscopy vs. IR
IR = Change in dipole of molecule
Raman = Polarizability of Molecules
Extended
Equilibrium
Compressed
Raman spectroscopy - examples
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The frequency of the
RBS mode is inversely
proportional to the
diameter of the
nanotube
RBS mode and double
peaked high energy
modes are prove of the
existence of single-wall
nanotubes in a sample
In metallic carbon
nanotubes the lower
high-energy mode is
strongly broadened and
shifted to smaller
energies (1540 cm-1)
http://www-g.eng.cam.ac.uk/edm/research/ramanlab/raman_CNTs.html