Atomic Force Microscopy (AFM)
The domain of Scanning Tunneling Microscopy (STM) is fairly recent with its beginnings in the early 1980s through the invention of the Scanning Tunneling Microscope in 1981 by Gerd Binning and Heinrich Rohrer which was developed by harnessing the quantum tunneling effect. One of the latest offerings in the repertoire of STM is the Atomic Force Microscope (AFM) which is a state-of-the-art instrument to image, measure and manipulate matter at the nano level. One feature that essentially separates an AFM from other instruments of the same league like a Scanning Electron Microscope (SEM) or a Transmission Electron Microscope (TEM) is that while an AFM 'feels' a surface, the others merely view it from a distance.
The way this 'feeling' of the surface is accomplished is by using a cantilever attached with a sharp probe (tip) that scans the surface by approaching it with a proximity of a few nanometers. This extreme proximity allows the probe to experience a range of forces from the surface ranging from Van der Waal's forces to magnetic forces in the case of certain samples.
AFM is the most basic of scanning probe techniques which provides topographic information and used to study over a range of materials like insulator, conductor & semiconductor. It scans the surface of the sample with a tip which is a couple of micron long & 1-10 nm tip diameter, located at the free end of the cantilever. Force between the tip & sample surface causes the cantilever to bend or deflect. Here a piezoelectric tube scans a micro-machined cantilever across a sample surface. When the sharp tip of this cantilever passes over the surface, the cantilever deflected causing a laser beam reflected off the top of the cantilever to change its angle of reflection. A position sensitive photo detector (PSPD) is used to measure these deflections which are correlated to the surface feature of the sample. A figure to explain the working principle is shown below:
It has two modes of operation:
a. Contact mode
b. Noncontact mode
The contact mode of an AFM uses a stationary probe in a contact range that usually generates repulsive forces between the sample surface and the probe. The deflections in the cantilever due to these forces is mapped by detector. Thus, in effect the probe is in 'contact' with the surface during this mode of operation. A major fallout of this methodology is the damage which may be incurred by the probe while it passes over the topographical variations of the surface. This particular shortcoming is overcome by using a non-contact or a tapping mode in which the cantilever is oscillated at or near its resonant frequency and the changes in the vibrating frequency while the cantilever interacts with the surface are utilized to map the surface features. In other words, the tip 'taps' at the surface and hence the name 'tapping' mode. This method with its limited interaction with the sample surface reduces the chances of damage to the tip.
As shown in the following Fig.1, as the atoms are gradually brought together, they first weakly attract each other. This attraction increases until the atoms are so close together that their electron clouds begin to repel each other electrostatically. This electrostatic repulsion progressively weakens the attractive force as the inter-atomic separation continues to decrease. The force goes to zero when the distance between the atoms reaches a couple of angstroms, about the length of a chemical bond. When the total van der Waals force becomes positive (repulsive), the atoms are in contact.
Fig. Shows the 3D surface image of electrodeposited surface with scan sizes of 40 x 40µm2
The AFM principle can be extended to its variations like Scanning Tunnelling Microscopy (STM), Kelvin Force Microscopy (KFM), Magnetic Force Microscopy (MFM), Current Sensing Atomic Force Microscopy (CSAFM) etc. Of these STM is of foremost importance.
STM works on the principle of quantum electron tunnelling. When a conducting tip is brought very near to the surface to be examined, a bias(voltage difference) applied between the two can allow electrons to tunnel through the vacuum between them. The resulting tunneling currentis a function of tip position, applied voltage, and the local density of states (LDOS) of the sample. The most common conducting tips are made of Pt/Ir alloy or W.