Question

Discuss the use of Atomic Force Microscopy (AFM) and Resonance Raman spectroscopy for determining the sequence of ink- writing on paper. Include an outline of how the AFM signal is generated.

Answer 1

Like all other scanning probe microscopes, the AFM utilises a sharp tip (radius of curvature » 20 nm) moving over the surface of a sample in a raster scan . The tip is mounted on the end of a 100–200 µm long cantilever which bends or deflects in response to the sample topography.  This deformation is detected by reflecting a laser beam off the back of the cantilever onto a two (or sometimes four) segment photodiode. The cantilever deflection gives rise to a modification of the laser beam reflection angle. This change is detected by the photodiodes that are connected to a computer, which reconstructs on the monitor screen the topography of the sample. The nature of the sample has an influence on the instrument resolution: flat and hard samples can be imaged with a higher resolution than rough and soft ones. The resolution can achieve values of less than one a under ideal conditions. A conventional optical microscope is optionally mounted on the top of the AFM so that the tip above the area to be explored can be precisely positioned. The imaging of topography can be achieved in many ways (contact, tapping mode, etc.). Additionally, the AFM offers several different operating modes, such as friction , force volume or phase. These modes give complementary information about the physico-chemical properties of sample surfaces. For this first exploring step of this microscopic technique, only the imaging abilities of the AFM were investigated. This work has been performed by operating the AFM in the tapping mode. In this mode, the cantilever is oscillated near its resonant frequency as it is scanned over the sample surface. The tip is brought closer to the sample surface until it begins to intermittently contact (tap) the surface. This contact with the surface causes the oscillation amplitude to be reduced. This characteristic is used to follow the surface topography. Because the contact with the sample is intermittent, the tip exerts negligible frictional force on the sample, thus, leading to an increase in resolution for the instrument and a high quality image of soft samples, such as ink layers