2. Nanomaterial
Characterisation
F3370
Probe Force Measurement
Tunneling
• In the microworld, particles may
pass through the energy barrier
under certain conditions tunnelling
may occur
• This is due to their wave-particle
nature
https://www.nanoscience.com/techniques/scanning-tunneling-microscopy/
Scanning Tunneling Microscopy (STM)
• The barrier is the vacuum
between the conductive sample
and the very sharp conductive
tip
• Their distance must be very
small - units of nm
• The sample is on a
piezoelectrically actuated stage
and is scanned under the tip
https://www.nanoscience.com/techniques/scanning-tunneling-microscopy/
STM Modes
• Constant current mode
• The tunneling current is measured when
it increases/decreases, the tip is
raised/lowered by the feedback loop.
• We obtain a 3D topography of the sample
with a lateral resolution of 0.1 nm and a
height resolution of 0.01 nm.
• Constant height mode
• For very smooth samples. The tip height
is stable and from the changes in
tunneling current at the tip position and
voltage we can determine the density of
electron states, defects, molecular
orbitals on the sample surface, etc.
https://afm.oxinst.com/modes/scanning-tunneling-microscopy-stm
Atomic Force Microscopy (AFM)
• Lennard-Jones potential - what forces are "felt" by two objects that
we bring closer to each other - first they attract and then they start to
strongly repel
J. Kámán, Investigationn of the Local ‚Mechanical Properties with Atomic Force Microscopy Techniques, PhD Thesis, Budapest. 2020
AFM
• Measurement of small forces (on
the order of nN) between the tip
on the cantilever and the sample
• Very sharp tip with a radius of
approx. 5 nm
• Also for non-conductive
materials
• No vacuum required
• Lateral resolution 1-5 nm,
vertical resolution 0.1 nm
1https://www.atriainnovation.com/en/atomic-force-microscope-afm-the-key-tool-for-surface-analysis/
2https://nanohub.org/resources/25844/download/App_CantiL_PK14_PG.pdf
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2
AFM Modes
• Contact mode
• The tip is tenths of
nm from the sample
• The tip height is
adjusted to keep the
tip bend constant
• Highest resolution
• May damage the
sample
• Non-contact mode
• Tip is tens of nm
from the sample
• The tip height is
adjusted to keep the
amplitude of its
vibration constant
• Lowest resolution
• Non-destructive
• Tapping mode
• The tip touches the
sample at the point of
deepest deflection
• The height of the tip is
adjusted to keep the
amplitude of its
vibration constant
• Better resolution
• Non-destructive
R. Asmatulu, W.S. Khan, Characterization of electrospun nanofibers, Synthesis and Applications of Electrospun Nanofibers, 2019
AFM Images
https://onlinelibrary.wiley.com/doi/full/10.1002/advs.202201489
AFM – Chemical Identification in Atomic-
resolution
• Non-contact mode - forces from atoms of different elements acting
on the tip are different - individual atoms of the alloy can be
distinguished
H.E. Schaefer, Nanosciience: The Science of the Small in Physics, Engineering, Chemistry, Biology and Medicine, Springer, 2010
Overview of Probe Force Measurements
Methods Shortcut Usage
Atomic force microscopy
Lateral force microscopy
Chemical force microscopy
Magnetic force microscopy
AFM
LFM
CFM
MFM
Topology and surface structure
Surface energy
Chemical analysis of the surface
Magnetic properties
Scanning tunnelling microscopy
Scanning tunnelling spectroscopy
STM
STS
Topology and surface structure
Electron density of states
Atomic probe microscopy
Field ion microscopy
Imaging atomic probe
Atomic probe tomography
APM
FIM
IAP
APT
3D imaging
Chemical composition, atomic distances
Surface imaging by emitted ions
3D position of atoms and their type
Spectroscopic (Photon) Methods
X-Ray Photoelectron Spectroscopy (XPS)
• X-ray photons hit the sample, knock
out photoelectrons and we measure
their energy
h𝜈 = Φ + 𝐸 𝑘
h – Planck constant
ν – frequency of the electromagnetic
wave
Φ – work function
Ek – kinetic energy
• We get the chemical composition and
information about the bonds of the
atoms
http://www.rowbo.info/XPS.html
X-Ray Diffractometry (XRD)
• X-rays are incident on the sample
containing the crystallites and diffract
d – spacing between diffracting planes
λ – wavelength of the beam
ϑ – incident angle
• We obtain information about the
crystal structure (cell type, spacing
between diffracting planes, lattice
parameter, crystallite size, texture,...)
https://wiki.anton-paar.com/cz-cs/rentgenova-difrakce-xrd/
Particle Methods
Mass Spectrometry (MS)
• The sample to be studied is not in
the form of a gas, we ionize it and
use a magnetic field to separate
the charged components based
on its m/q ratio.
1 https://assignmentpoint.com/mass-spectrometry/
2 https://microbenotes.com/mass-spectrometry-ms-principle-working-instrumentation-steps-applications/
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2
Rutherford Backscattering Spectrometry (RBS)
• Light energetic ions are incident
on the measured sample and
reflected from the sample
nucleus. Their energy is studied.
• The chemical composition of the
surface (~ hundreds of nm) layer
of the sample is obtained. The
measurement is very sensitive
for heavy elements and less
sensitive for light elements. The
measurement is non-
destructive.
https://physics.uwo.ca/~lgonchar/research/links/June3_2013/RBS_June3_2013_slides.pdf
Thermodynamic Methods
Thermodynamic Methods
• TGA – thermogravimetric analysis
• Measurement of weight loss/gain as a function of temperature
• DTA – differential thermal analysis
• Thermal differences between measured and reference sample
• State changes, reaction heat, reaction kinetics
• DSC – differential scanning calorimetry
• Comparison of heat flux changes per measured and reference sample
• More modern version of DTA, extra heat
• Thermodynamics is based on the assumption of a large number of
particles. This may not be fulfilled for nanostructures.
Conclusion
• We have introduced different types of methods used to investigate
nanostructures
• Optical
• Electron imaging
• Probe
• Spectroscopic
• Particle
• Thermodynamic