Current Research
Introduction to AFM
Atomic force microscopy (AFM) has become a useful tool for measuring forces at the nanoscale.
Atomic force microscopy (AFM) has become a useful tool for measuring forces at the nanoscale. The instrument uses a tip attached to the end of a cantilever to interact with a sample surface. A laser is bounced off of the back of the cantilever into a photodetector (Fig 1A). When the cantilever bends, the laser changes position on the photodetector (Fig 1B). In this manner, the instrument measures how the cantilever deflects as it is subjected to forces.
Piezoelectric materials are a special kind of ceramic which change shape in response to an applied voltage (Fig. 2A). A piezoelectric sample stage is used in a closed feedback loop to control the distance between the tip and the sample surface (Fig 2B).
The deflection signal of the cantilever is used in conjunction with the spring constant of the cantilever in order to calculate forces of interaction. By assuming that the cantilever behaves like a spring, attractive and repulsive forces of interaction can be calculated using the deflection of the cantilever (x) and the cantilever’s spring constant (k) according to Hooke’s law:
Note that the value is negative because this is a restoring force. In other words, if a spring is stretched or compressed, the resulting force acts in the opposite direction of the displacement in order to restore the spring back to its equilibrium position. An AFM cantilever is no different – it starts at an equilibrium position and as it is displaced due to interactions with its environment, it exerts a force in the opposite direction in order to regain its equilibrium position. By detecting the deflection of the cantilever and modeling it as a spring, forces of interaction between the tip and the sample can be measured with piconewton accuracy.