10. Atomic Force Microscopy Flashcards
Define atomic force microscopy
A method of imaging the surface of a sample.
What are the two types of atomic force microscopy?
- Contact mode
- Non-contact mode (tapping)
Describe contact mode AFM
The tip stays in contact with the surface, tracing the sample surface due to repulsive force.
What is contact mode AFM used for?
Hard materials
Describe non-contact mode AFM
Height information is found by recording oscillation frequency.
What is non-contact mode AFM used for?
Soft/biological materials
Give the equation for Hooke’s law
F = force
k = spring constant
z = extension/compression
Define Hooke’s law
The strain in a solid is proportional to the applied stress within the elastic limit of that solid.
What is Hooke’s law used for?
It is used to measure a force from the stretch of a spring of known stiffness.
How can the stiffness of the AFM cantilever be determined?
By using Hooke’s law to find the spring constant (stiffness).
Give the equation for stiffness based on the dimensions of an AFM cantilever
k = stiffness
E = Young’s Modulus
I = geometric moment of inertia
L = length of the cantilever
How can the deflection of the AFM cantilever be detected?
By reflecting a laser beam off the back of the cantilever and collecting the reflected light on a split photodiode detector.
How can the spring constant of a cantilever be calibrated?
By measuring the natural response of the cantilever to thermal fluctuations, specifically the mean square displacement.
Give the equation for cantilever equilibration
k = spring constant
<z²> = mean square displacement
kB = Boltzmann constant
T = temperature
What is the typical cantilever spring constant?
0.01 - 50 N/m
