AFM Flashcards
AFM probe - mounting
materials
tip mounted on cantilever mounted onto carrier chip
generally made out of Si/ SiN (can use other materials depending on application)
Backside of cantilever can be coated with a refelective metal coating such as Au or Al (BUT can cause cantilever bending if Temp changes (drift in deflection)
Typical vibrations that AFM has to be insensitive to
Building 1-100Hz
Sound waves 1-10 kHz
samples often placed on vibration table
what’s k, how is it dependent
spring coefficient - the lower the k the more sensitive the cantilever
needs to be calibrated
Methods for calibrating k
Dimensional relationship
Static experiments - apply known force and measure deflection
Dynamic methods - combine resonant frequency such as thermal
sample requirements
flat (less than 10 microns high)
clean
very small samples e.g. particles must be adhered to surface
no need for coating, grounding staining etc
Key forces
1 - low distances - strong repulsion Hard sphere repulsion Pauli exclusion electron electron coulombic interaction 2 - larger distances - attractive forces van der waals electrostatic chemical force
Other forces at play - capillary + friction
capillary forces - will damage fragile samples/single molecules
do not occur when imaging in liquid but solvation forces have an effect
Force curves - how, what do you investigate?
Young’s modulus, adhesion - push into sample
pulling away - bond breaking, protein folding
Calibration required -
cantilever deflection, cantilever spring constant
tip shape/size - use probe of known size, calibration samples available
Modes (3)
contact mode- working in repulsive regime:
1. constant force
deflection set to specific value (Set point) feedback moves the z piezo to maintain constant set point
movement of Z piezo recorded as topography image
2. constant height
tip deflection recorded
can be calibrated to determine sample heights
risk of cantilever colliding with large features on surface\
can have applications when scanning faster than feedback loop can handle
Tapping mode
(dynamic mode, AC mode, AM-AFM)
resonate tip close to resonant frequency
typical amplitude 20-100 nm
Oscillation is dampened when the tip comes into contact with surface to detect short range forces
feedback loop maintains constant amplitude - set point - typically 50-60% below that it’s too hard and can cause sample damage and above that it’s ‘soft tapping’
Non-contact mode
cantilever oscillates but never makes contact with sample
usually a bit above resonant frequency
relies on long range interactions (van der waals) - net positive
need UHV conditions and adsorbed layer has to be thick
- either tip gets stuck in liquid or it cannot detect the forces
capable of atomic resolution
tuning a cantilever?
A bit below its resonant frequency to ensure repulsive interaction between tip and surface leading to better imaging
tuning makes cantilever less susceptible to external vibrations
What is phase imaging
Monitor movement of cantilever for any feedback compared to the shake piezo
Measure of energy dissipation, sensitive to:
viscoelasticity, adhesion, contact area (impacted by slope of sample - sample topography will impact phase image)
Used to measure composition, mechanical properties but should be used with caution
what is lateral force imaging
measures friction forces across surface
measures torsional deflection (twist) of tip as it moves along surface
Imaging in liquid - what, how, issues
possible
allows for in situ imaging of bio samples
good for fragile samples as it removes capillary forces
can work in open or close cells
issues:
air bubbles on tip,
might be difficult to find resonant frequency in liquid
Scanning artefact
tip - blunt tip (triangles) or double tip
Bow/tilt - very common can correct
Gains setttings - too high/too low - a bit shifted, not sharp
Drift:
1. piezo drift - no easy fix, occurs when moving over large surfaces , use data from later scans
2. thermal drift - expansion/contraction can cause features to shift - maintain constant temperature in lab/scanner
nanomechanical imaging modes
AM-FM FM
Amplitude modulated frequency modulated
both cantilever at resonant frequency
AM - measures topography and loss tangent
FM - Stiffness and Elasticity and dissipation
2nd frequency v small doesnt have effect on overall
nanoelectrical imaging modes
e.g Charge transfer in nanoparticles and substrates (catalysts) and between metal nanostructures and biological molecules (biological sensors)
C-AFM
developed from contact mode
uses conductive tip Si or SiN coated with Pt-Ir
Bias applied to sample and conductivity collected simultaneously with sample
Kelvin probe microscopy (KPFM)
requires conductive tip
measures contact potential difference - can therefore calculate work function or surface potential
AC biased applied to tip and the potential difference causes the tip to oscillate
bias recorded as surface potential
Each line scanned twice - first topography collected as in standard topography experiment
Probe then lifted and longer distance interactions measured
PFM
probes piezoelectric response of sample
conductive tip scanned across sample whilst AC bias is applied
movement usually quite small - need to amplify signal
use large voltages or contact resonance
