ZANG group research

department of chemistry and biochemistry SIU

Atomic Force Microscopy (AFM)

Atomic force microscopy (AFM) was developed when people tried to extend STM technique to investigate the electrically non-conductive materials, like proteins. In 1986, Binning and Quate demonstrated for the first time the ideas of AFM, which used an ultra-small probe tip at the end of a cantilever (Phys. Rev. Letters, 1986, Vol. 56, p 930). Since then, some practical improvement was made on measuring precisely the motion of the cantilever. In 1987, Wickramsinghe et al. developed an AFM setup with a vibrating cantilever technique (J. Appl. Phys. 1987, Vol. 61, p 4723), which used the light-lever mechanism first used by Schamalz in 1929 (Zeitschrift des Vereines deutscher Ingenieure, 1929, Oct. 12, pp1461-1467). Basically, an interferometer was used to measure the amplitude of a cantilever’s vibration. With this optical technique, oscillation amplitudes of the cantilever could be detected as small as  0.3 nm.

The basic components and working principle of an AFM is shown below.

It represents the model of Veeco Explorer^TM, which scans the tip, rather than the sample stage. Most of the AFM setups have the sample stage scanning. The tip scanning mode makes Explorer^TM more flexible to be sit atop different shapes of samples or installed with other optical microscopes for combined investigation of topography and optical properties. Click here for a more detailed scheme of AFM system.

Click here to see the linear and triangle cantilevers.

Click here to see some typical AFM tips.

The central technique of an AFM system is detecting the vertical displacement of the cantilever by the light-lever mechanism. A red laser beam focused on the top surface of the cantilever is used to “amplify” the slight deflection (< 1 nm) of the cantilever. The reflected laser beam is aligned to a position-sensitive photodetector, with which a change of the angle of reflection can be monitored and recorded. The feedback from such a sensor maintains the probe at a constant force, i.e. constant distance from the sample surface. The scanning of the sample stage (or the probe tip in the case of Explorer^TM) is normally driven by piezoelectric ceramics at three dimensional directions (X, Y, Z), which are designed to be moved precisely at angstrom resolution. By raster scanning across the sample, a 3-dimensional topographic map of the sample surface is recorded based on deflections of the cantilever, and thus the height of the surface features can be measured at sub-nanometer scale.

Generally, AFM works in three operation modes based on the interaction between tip and surface. The first one is ‘contact’ mode, in which the tip lightly touches the sample surface and the repulsive force from the surface is used to maintain feedback. The second is ‘non-contact’ mode, in which the tip is quite close to the sample, but not touching it. The interaction between the tip and surface falls into the attractive regime. The force between the tip and sample is quite small, on the order of pN (10 ^-12 N). The stiff cantilever is oscillated at a constant frequency (set point), which is used to maintain the feedback. The detection scheme is based on measuring changes in the resonant frequency of the cantilever. The third operation mode is sort of between the ‘contact’ and ‘non-contact’, commonly referred to as ‘tapping’ mode or ‘intermittent-contact’ mode. In this mode, the stiff cantilever is oscillated closer to the sample than in ‘non-contact’ mode. Part of the oscillation extends into the repulsive regime, so the tip intermittently touches or ‘taps’ the surface. Since tips can get stuck to the soft or wet sample, highly stiff cantilevers are typically used for taping mode AFM. Selection of different operation modes depends on the research needs and sample properties.

(updated on june 24, 2004)