AFM EXAM QUESTIONS AND ANSWERS

How does the AFM work? - ANS-The Atomic Force Microscope (AFM) operates by scanning an AFM probe across a sample surface. What does the AFM probe consist of? - ANS-The AFM probe consists of a sharp tip at the end of a flexible cantilever that protrudes from a holder plate also called a holder chip What is the tip made out of? - ANS-The AFM probe is usually made from silicon or silicon nitride, Either the probe or sample is mounted on what? - ANS-a piezoelectric scanner which can move in the x,y, and z directions, and is used to raster scan the probe across the sample surface to acquire an image in 3 dimensions. How does the AFM work? - ANS-In AFM, a sharp microfabricated tip attached to a cantilever is scanned across a sample. The deflection of this cantilever, caused by the forces developed between the tip and the sample, is monitored using a laser and photodiode and is used to generate an image of the surface. A feedback mechanism from the detector system to adjust the piezoelectric scanner is used to maintain either a constant force or a constant deflection (height) between the tip and sample surface, depending on the AFM set-up. How do tip interactions work? - ANS-At very close tip-sample distances (a few angstroms), a strong repulsive force appears between the tip and the sample surface due to the overlap of atomic orbitals. The repulsive force increases as the distance between the tip and sample surface decreases What is contact mode? - ANS-As the probe tip scans the surface at this close distance, a feedback system to the piezoelectric scanner raises and lowers the sample to keep a constant repulsive force between the tip and the sample surface. A plot of this upward and downward motion (z), as a function of the tip x - y position on the sample surface, provides a high-resolution image of the surface topography. This mode of operation is called "contact mode". When the repulsive force is in place, the tip and sample are considered to be in 'contact'. High resolution is possible because very small changes in separation (z) (see Figure 2), lead to large changes in force which is the parameter being monitored. However, for some samples such as biological specimens or soft material such as some polymers, 'contact' of the surface with the tip can damage the sample. What is tapping mode? - ANS-To overcome this problem, an alternative set-up is that the tip vibrates rapidly up and down and only 'taps' or comes very close to the sample surface while at the bottom of its oscillation. This mode, referred to as "tapping mode", "semicontact mode", or "dynamic mode", is the most common mode of operation as it preve