Ability to see the surfaces and interfaces at a nanoscale is an important part of nanotechnology research particularly for scientists who study and develop surfaces. There’s a class of nano-characterization tools that are essentially useful in such situations, especially when requirement of rich surface information is in question. This characterization technique is called scanning probe microscopes (SPMs) which uses a physical probe (a cantilever) to scan back and forth over a surface to obtain information. During the process of scanning, computer registers the probe signal with respect to x and y axis coordinate, thus generating an image of the surface. SPMs are quite different compared to optical or electron microscopic techniques as this technique doesn’t use some kind of beam (optical or electron) to “see” the surface directly, instead it “feels” the surface just like a blind man does with his cane.
SPMs are quite popular among scientists because of three main reasons. First, the microscope itself is quite powerful in terms of information it provides. I’ve personally used this tool to image surfaces with a resolution less than one nanometer. Secondly, SPMs can probe in to number of different aspects of the surface which gives the technique much more versatility compared to other microscopic techniques. Although quite frequently probed feature is surface topography (landscape of the surface), one can also probe frictional, phase, magnetic and surface charge profiles using this technique. Thirdly, this technique can generate three dimensional image of the scanned landscape in contrast to the two dimensional images provided by electron microscopy techniques.
SPMs take use of a special cantilever to probe the surface. Cantilever is essentially a flat rod with a tip mounted on one end and other securely fitted to the stage. The tip is quite sharp, sometimes as sharp as a single atom at the end. The cantilever material and the length may change depending on the property it’s designed to probe. The cantilever assembly can be moved back and forth across the sample. The movements can be precisely controlled and high accuracy is maintained during the scanning movements.
Scanning tunneling microscope(STM)
Scanning tunneling microscope is a very powerful SPM technique that works only with the conductive sample. In this technique, a voltage bias is applied between the sample and the cantilever. When the tip is sufficiently close to the sample, a small current starts to flow which is known as the tunnel current. It’s known that the tunnel current decay exponentially with the distance between the sample and the cantilever tip. During the scanning, the STM tip is moved up and down, maintaining a constant tunneling current and respective movements are registered with the coordinates. In the STM, this movement is the probe signal. STM can be used to probe the surfaces with atomic resolution. Some STMs are operated under low pressure conditions to improve the resolution, as the tunnel current decay at a much faster rate in air. STM also can be used to manipulate individual atoms. By increasing the probe current a temporary bond can be made with an absorbed atom on to the surface. The atom can be manipulated using this bond and moved to a different location. The atom can then be dropped at this location by breaking the bond between the STM tip and the atom by lowering the probe current.
Atomic force microscopy (AFM)
Atomic force microscopy uses a different detection principle than that of the STM. A laser is continuously shined on the cantilever and reflection is taken in to a position sensitive photodiode (PSPD) array. When the cantilever is at a straight position, the reflected laser off the cantilever top surface is set to shine in to the origin of the PSPD. When, cantilever tip approaches the sample it can be deflected due to number of different forces such as, mechanical contact, van der Walls forces, electrostatic forces, magnetic forces, etc. This deflection is magnified through the movement of the reflected laser and fed to a computer with corresponding coordinates of the position. This can be used to develop an image of the sample surface. During the scanning, the distance between the sample and the cantilever tip is maintained in such a way that constant force is applied to the surface.
The greatest advantage of AFM over STM is that it can be used to scan samples that are not nonconductive because the method depend on attractive or repulsive forces exerted on the tip of the AFM cantilever by the sample surface.
Usually, the cantilever physically make continuous contact with the sample. This mode of use is referred to as contact mode. However, in certain situations one can use the cantilever in tapping mode or non-contact mode. In tapping mode, cantilever only make contact with the sample intermittently. In non-contact mode, as the name suggests, cantilever doesn’t make mechanical contact with the surface. The probe signal is measured in terms of change of natural frequency rather than the actual deflection of cantilever. It’s observed that the natural frequency of the cantilever is varied depending on the forces applied on to the probe tip.
Based on the sample property that is measured using the AFM, number of other sub categories can be defined such as,
Lateral force microscopy(LFM): In LFM, frictional profile information can be obtained. In this technique, lateral movement of the reflected laser beam is taken as the probe signal. More deflection in lateral direction usually means areas with higher friction
Force modulation microscopy (FMM): In FMM, the tip and sample distance is maintained in such a way that constant cantilever deflection is maintained( in contrast to the constant force in AFM). This method is especially useful in obtaining a profile of sample elasticity across the scanned area.
Phase imaging mode: Phase imaging mode can only be used in either tapping or non-contact mode. In here, probe signal is maintained as the phase different between the signal that oscillate the cantilever and output signal. The phase imaging mode is especially important when information on phase, frictional and adhesive information is important.