Atomic Force Microscopy Flashcards
Describe the piezo element.
generates a voltage when force is applied.
How does the contact mode of the AFM work?
constant force applied by adjusting tip-sample separation.
Describe how the intermittent contact mode for an AFM works.
vibrated at resonance. relative phase between driving and detected signal is recorded.
Why does phase signal vary over a region with no topographic variation?
There is a viscoelastic interaction between the tip and sample.
Which size of cantilever experiences larger thermal fluctuations?
larger.
Which size of cantilever has a wider frequency spectrum?
smaller.
Explain what causes the measurement of thermal fluctuations for larger cantilevers to be higher.
When measurements are limited to a certain bandwidth, the area under the curve caused by thermal noise is less for smaller cantilevers.
What is the effect of thermal noise on the AFM?
limits the minimum detectable :
change in force.
shift in frequency
What is the effect of an elastic interaction?
a shift in the resonant peak.
What is the effect of a viscous interaction?
changes the quality factor.
Describe the jump-to-contact condition.
If the gradient of the force field > k, the cantilever will jump-to-contact without reaching an intermediate equilibrium position. Will deflect more returning to equilibrium.
What is the purpose of tethering a molecule between the AFM and a surface?
It can be stretched and its elastic response can be measured.
Describe contour length vs persistence length.
Complete length of polymer and 2*length of segment length (Kuhn length)
When a polymer stretches, what is the effect on the energy?
Free energy increases (H = U -TS)
entropy decreases, bending energy doesn’t change.
In the low force regime, the polymer acts as…
spring with spring constant proportional to the temperature.
Force to stretch an entropic spring and its spring constant depend on…
absolute temperature.
