The invention of the scanning tunnelling microscope, in 1982 resulted in the emergence of a whole new category of microscopic techniques. All these techniques fall under the family of probe microscopy, which basically involves the interaction of a sample with the sharp tip of a probe at distances of the order of a few μm to a few nm. The atomic force microscope (AFM) is one such scanning probe microscope that builds an image that describes the surface topology of a sample based on the atomic forces of interaction between the tip of the probe and the sample surface. The advantage of using an AFM is that it can be operated in any environment – be it submerged in an aqueous solution or in a gaseous environment. Unlike tunnelling microscopes, there are no focussing optics needed, nor is there a need to evacuate a chamber before measurement. In short, AFM operated on the principle of sensing and not seeing.
The main component of an AFM is the tip attached to one end of a flexible cantilever. A laser beam is applied onto the cantilever and the reflected beam is tracked, amplified and measured to assess the movement of the cantilever. The reflected laser beam is picked up and detected by a photodiode detector that gives a voltage output based on the intensity of the laser beam. The basic AFM setup is shown in fig 1.
The AFM head and base stage comprises of the AFM head, the laser and the photodiode, while the base consists of the sample mounted on a piezoelectric stage. This stage includes optics and positioning mechanisms to focus the laser onto the cantilever, and basic electronics to process the signals falling on the photodiode. The normal and lateral forces, FN and FL respectively, that are deflected by the cantilever and the total intensity of the laser beam Σ are processed by this electronics stage. However, the high voltage electronics stage amplifies that come on to the digital signal processor (DSP) or the low voltage XYZ signals, and drive the piezoelectric scanner at high voltages of about 100V. The signals corresponding to the normal and lateral forces along with Σ are also processed by this high voltage stage and transferred to the DSP which is then processed into the computer. The DSP and the software process these signals received and convert them into images which can later be further processed (Herrero et al 2012).
The main methods used for imaging are contact mode, tapping mode and non-contact mode. In a contact mode, the spring constant of the cantilever is less than the sample surface which causes the cantilever to bend. By maintaining a fixed cantilever deflection, the force between the cantilever and the sample can be kept constant and the image of the surface can be mapped out. This a fast scanning method that is good for rough surfaces. However, this method cannot be used for soft and delicate samples since the contact can damage the sample surface.
In a tapping mode, the cantilever is made to oscillate at its resonant frequency causing the tip to lightly tap over the surface intermittently. By maintaining constant amplitude of oscillation, the tip-sample interactions can be fixed and the image of the sample surface can be drawn out. This is a highly efficient technique that can be used to give high resolution images of soft and delicate surfaces. However, imaging using this technique in liquids is challenging.
In a non-contact mode, the probe does not touch the sample, but oscillated with a certain frequency above the surface. The interactive forces are converted into electrical signals and the sample surface can be imaged. This method improves the probe lifetime, but results in low quality images.
Atomic force microscopy is used in biochemistry to image the structure of biological molecules, cellular constituents and soft tissues. It is also widely and exhaustively used in chemistry, materials science and nanotechnology to study the surface structure of polymers, thin films, nanostructures, etc. It is also used in physics and biophysics to measure atomic forces between the probe tip and the surface.
AFM data gives insights into surface profiles of samples. It gives results on the electrostatic or magnetic interactions between the sample and probe tip, which in turn, gives an estimate as to what the sample composition might be. The AFM tip can also be used to pattern and etch surfaces to modify surface properties. Based on atomic interactions between the tip and the probe, one can derive the surface properties such as possibilities of clustering, agglomeration, uniformity, film thickness, etc.
Works Cited
Moreno-Herrero, Fernando, and Julio Gomez-Herrero. "AFM: Basic Concepts." Atomic
Force Microscopy in Liquid: Biological Applications(2012): 3.
