Atomic Force Microscopy:
What is AFM?
Atomic Force Microscopy (AFM)--is mainly involved in visual observation and three dimensional analysis including measurement of particle shape, size, and their distribution. AFM also have application in particle surface characterization. The atomic force microscope (AFM) is can be described as special scanning probe microscopes (SPM) which are designed to determine also several important properties like friction, height, magnetism, with using a specific probe. To determine a perfect image, the SPM raster-scanning is conducted over a small area of the sample by using the probe. Overall, Atomic force microscopy (AFM) can be explained as a special technique for determining the surface property of a rigid material into the range of atomic level. AFM uses a special mechanical probe to get a 100,000,000 times surface magnification and it also create 3-D images of the particle surface in nano scale.
History of AFM:
In the year of 1986 by Binning et. al., invented the first Atomic Force Microscope. After that the AFM was continuously development stage in all aspects.
The first image of the biological samples was developed in non-contact mode in the year of 1987.In case of non-contact mode, the cantilever produce an oscillating movement near to its resonant frequency at a very small distance (1-10 nm) above the nanoparticle surface.
The fourth generation of prototype AFM was invented in the middle of 1989. This fourth generation AFM was creating a huge interest in successful commercialization of AFM
The micro-fabricated tips were first invented in 1991. (Prater et al., 1991) Tapping Mode(r) concept was first introduced. In 1993, (See Zhong et al., 1993) where the cantilever perform an oscillation at its resonant frequency and the cantilever gently moves into the surface during the time scanning which particularly reduce the chance of damaging lateral forces.
In 1994 Fluid tapping concept of was a major development for different kind biological samples. Here Modern implementations of oscillating tip and not the sample for finer and easier force control.
Smaller size of the cantilevers were first originated in the year of 1996, which produce a very higher resolution and a very smaller scanning times which significantly reduce the moving mass and increasing the bandwidth detection.
AFM Version 5 was the last prototype AFM with a small cantilever. This prototype used custom designed optics for the optical detection of the cantilever to minimize the spot size on the cantilever. This significantly reduced spot size allows for even smaller cantilevers to be used, resulting in higher speeds and lower forces.
Principle of AFM:
The AFM mainly consists of a special kind of cantilever with a very sharp (probe) at the end of the cantilever which is generally used to scan the entire surface of the specimen. The cantilever is mainly a silicon or silicon nitride based structure with a nanometers tip radius of curvature. When the tip comes into the proximity of a given sample surface, forces acts between the tip and the sample.As a result the cantilever was undergone a deflection according to Hooke's law. The forces AFM include mainly forces, mechanical contact force, capillary forces, chemical bonding, electrostatic forces, magnetic forces , Cantilever forces and solvation forces. Specially, the deflection is calculated with a reflected laser spot originated from the top most cantilever surface into the photo diodes array .In addition the other methods which are mainly used enormously in AFM can be named as optical interferometry, capacitive sensing or piezoresistive AFM cantilevers. These kind of cantilevers are designed with piezoresistive elements that act as a strain gauge.
There is chance of causing damage of the surface if the tip was scanned at a constant height. As a result a risk of collision of tip and surface may exist. For this reason, a constant working force is employed between the tip and the sample to maintain the tip-to-sample distance. The basic function of AFM can be executed in a number of modes based on its application. Although, conventional imaging modes are classified into static modes and dynamic modes where the cantilever is found in different frequency of vibration.
Working of AFM:
AFMs is mainly working by calculating the force between the sample and the probe. Generally, the probe possess a very sharp tip that is around 5 micrometer tall pyramid like structure with 30 nm radius at is end position. Although the resolution lateral view of AFM is around 30nm due to its convolution and the resolution of vertical view can be extended up to 0.1nm.
Figure 1. Schematic diagram of AFM tip and Cantilever
The vertical and lateral deflections of the cantilever are generally calculated to get the resolution image by utilizing the AFM optical lever. The optical lever mainly works on the basis of principle by reflecting a laser beam from the surface of the cantilever. The reflected laser beam imposed on a position-sensitive photo-detector which have four-segment photo-detector. The main differences between all different segments of photo-detector of signals show the zone of the laser spot on the detector and then the angular moment deflections of the cantilever (Figure 2).
Figure 2. AFM is working with an optical lever.
In case of contact mode, AFMs mainly act through the use of feedback mechanism to control the required force on the sample. The AFM not only calculate the force on the sample but also control it. As a result it is allowing acquisition of the proper images at very low energy forces. The feedback loop mainly contains the tube scanner that controls the height of the tip, the cantilever and as well optical lever. A well-constructed feedback loop is very essential to microscope performance.
Other application of AFM:
AFM is mainly used to find problems of material in many cases, including storage of data, biomedicine, chemistry, telecommunications, and aerospace. Incase of data storage, it is assisting researchers to create a disk to create a higher efficiency. In Recent years magnetic storage devices generally have a limited capacity between 20 and 50 gigabits (billions of bits) per square inch of storage medium. Researchers are trying to invent AFM to increase densities to between 40 gigabits and 300 gigabits per square inch. Although no company has yet commercialized AFM technology for this specific purpose, but IBM and others are trying to bring it in the market with lower cost.
Advantages and disadvantages of atomic force microscopy
Just like any other analytical tool, an atomic force microscopy have some limitation and advantages. The appropriateness of a sample whether it should be analyzed by AFM depends on several factors including its advantages and disadvantages.
1. As compare to the scanning electron microscope (SEM) AFM has many advantages.
2. SEM provides a two-dimensional projection image of a given sample but the AFM projected image provides a three-dimensional surface characterization.
3. AFM projected samples view do not need any further special treatments like metal/carbon coatings which may irreversibly degrade the sample.
4. All atomic force microscopy instruments can work perfectly without any costly vacuum environment for proper operation. This makes it feasible to study biological specimens and as well as living organisms.
5. AFM produced images generally deliver higher resolution images than SEM.
The major disadvantages of AFM was discussed here
1. The main disadvantage of AFM over other scanning electron microscope (SEM) is that AFM produce the single scan image size.
2. AFM can only produce an image of a maximum height on the order of 10-20 micrometers and a maximum scanning area of about 150x150 square micrometers.
3. The scanning speed of an AFM is also a big limitation. Conventional AFM cannot perform scanning of surface images as fast as a SEM. It require several minutes for a simple scan. As a result thermal drift in the image make the AFM microscope less suited for measuring true distances between topographical features on different image. As a result image distortion induced by thermal drift is a common phenomenon in case of AFM.
4. AFM images can also be distorted by hysteresis of the piezoelectric material and as well as cross-talk between the x, y, z axes.
5. Due to the limitation of AFM probes, they cannot generally calculate steep walls and overhangs. Specially prepared cantilevers and AFMs can be utilized to actuate the probe sideways as well as up and down to calculate sidewalls, at the cost of more expensive cantilevers.