Atomic Force Microscopy(AFM)

 

Introduction

Atomic force microscopy or AFM is similar to STM relies on a very sharp tip, but in this case, the tip is brought close enough to the surface that the intermolecular forces between tip and  the surface can be measured. It is a method to see a surface in its full, three-dimensional picture, down to the nanometer scale. This method applies to hard and soft synthetic materials as well as biological structures (tissues, cells, biomolecules), irrespective of opaqueness or conductivity.

 Working of AFM

AFM provides a 3D profile of the surface on a nanoscale, by measuring forces between a sharp probe (<10 nm) and surface at very short distance (0.2-10 nm probe-sample separation). The probe is supported on a flexible cantilever. The AFM tip gently touches the surface and records the small force between the probe and the surface. Most commonly, the sharp tip is attached to a flexible microcantilever – essentially a microscopic diving board – which bends under the influence of force. The bending produces a change of angle of inclination, measured by bouncing a laser beam off of the cantilever and into a position-sensitive detector (split photodiode), the output of which gauges small movements of the laser spot. The vertical tip movement in turn is quantified from this cantilever bending.

Fig: Schematic diagram of AFM instrument

Dynamiic AFM

Here cantilever driven near resonance. The cantilever's resonant frequency, phase and

amplitude are affected by short-scale force gradients. Non-contact AFM, Tapping mode AFM, Amplitude, Modulated AFM, Frequency Modulated AFM are all dynamic AFM.

Conclusion

AFM is a versatile tool to investigate

·         topography of surfaces

·         properties of surfaces

·         properties of single molecules

·         forces within molecules

But it always consider experimental conditions and artefacts on measurements