Atomic Force Microscope: Applications of AFM, Force Measurement Using AFM and Comparison among AFM, SEM, and TEM

Atomic Force Microscope

Although STM was considered a fundamental advancement for scientific research, it had limited applications, because it worked only on electrically conductive samples. This limitation led the inventors to think about a new instrument that would be able to image insulating samples.

In 1986, Binnig, Quate, and Gerber showed how improvization could be done by replacing the wire of a tunnelling probe from an STM with a lever prepared by carefully gluing a tiny diamond onto the end of a spring made from a thin strip of gold. This was the cantilever of the first atomic force microscope (AFM). The movement of the cantilever was monitored by measuring the tunnelling current between the gold spring and a wire suspended above it. This set-up was highly sensitive to the movement of the probe as it scanned the sample, again moved by piezoelectric elements. This created new excitement in nanometrology.

Atomic force microscopy allows the researcher to see and measure surface structure with unprecedented resolution and accuracy. One can even get images of the arrangement of individual atoms in a sample or see the structure of individual molecules. An AFM is rather different from other microscopes, because it does not form an image by focusing light or electrons onto a surface, like an optical or electron microscope. An AFM physically ‘feels’ the sample’s surface with a sharp probe, building up a map of the height of the sample’s surface. By scanning a probe over the sample surface, it builds a map of the height or topography of the surface as it goes along. This is very different from an imaging microscope, which measures a 2-D projection of a sample’s surface. It has been given the name AFM since it operates by measuring attractive or repulsive forces between the tip and the sample in constant height or constant force mode.

Most practical applications deal with samples of (sub)micrometre dimensions in the X–Y plane and of nanorange in the Z-axis. Since its invention in the 1980s, AFMs have come to be used in all fields of science, such as chemistry, biology, physics, materials science, nanotechnology, astronomy, and medicine.

The basic component of an AFM is the piezoelectric transducer. The piezoelectric transducer moves the tip over the sample surface, a force transducer senses the force between the tip and the surface, and the feedback control feeds the signal from the force transducer back into the piezoelectric, in order to maintain a fixed force between the tip and the sample. Piezoelectric materials are electromechanical transducers that convert electrical potential into mechanical motion. When a potential is applied across two opposite sides of the piezoelectric device, as shown in Fig., it changes geometry. The magnitude of the dimensional change is of the order of 0.1 nm per applied voltage of 1 V. Thus, the ability to control such tiny movement makes piezoelectric materials the key for making measurements in an AFM.

Piezoelectric material
Piezoelectric material

The component of a laser deflectiontype instrument is shown in Fig.

The necessary parts are the X, Y, and Z-piezo that are separately actuated by the X/Y drive and Z-control with extreme precision. The sample is mounted on the XYZ piezo, close to a sharp tip under the inclined cantilever with its mount. The diode laser light is focused at the end of the cantilever, reflected via a mirror to a split diode that provides the feedback signal (topologic information) for maintaining the force by Z-piezo response. The force between an AFM probe and a surface is measured with a force transducer. The force transducer must have a force resolution of 1 nN or less so that the probe is not broken while scanning. The control electronics take the signal from the force transducers and use it to drive the piezoelectrics, so as to maintain the probe–sample distance and thus the interaction force at a set level. Thus, if the probe registers an increase in force (for instance, while scanning, the tip encounters a particle on the surface), the feedback control causes the piezoelectrics to move the probe away from the surface. Conversely, if the force transducer registers a decrease in force, the probe is moved towards the surface. Data sampling is made at discrete steps by means of an ADC. A computer reconstructs the 3-D topological image or projections from the data matrix. Imaging software adds colour, height contrast, and illumination from variable directions.

Laser deflection contact AFM
Laser deflection contact AFM

The electronic control unit is assembled in a separate cabinet in the instrument. 

The primary function of the electronic control unit in an AFM is to

• generate scanning signals for the X–Y piezoelectrics;

• take an input signal from the force sensor and then generate the control signal for the Z piezo;

• control output signals for X–Y–Z stepper motors;

• generate signals for oscillating the probe and measuring phase or amplitude when an oscillating mode is used for scanning; and

• collect signals for display by the computer. One of the major advantages of an AFM is its ability to magnify in the X, Y, and Z axes.

Figure shows a comparison between several types of microscopes. It can be seen that the maximum length of the specimen is limited to slightly more than 100 μm. This is because an AFM requires scanning the probe mechanically over a surface, and scanning such large areas will consume a lot of time. However, the length-scale of an AFM overlaps nicely with a conventional optical microscope. Since AFM is also quite compact in construction, it can be integrated with an optical microscope.

This combination will cover a length-scale from nanometres to millimetres.

Comparison among AFM, SEM, and TEM.

Comparison among AFM, SEM, and TEM.
Comparison among AFM, SEM, and TEM.

Comparison among AFM, SEM, and TEM.
Comparison among AFM, SEM, and TEM.

Applications of AFM

The AFM has been developed into a novel technique for obtaining high-resolution images of both conductors and insulators. As already pointed out, an AFM is compact and hardly requires any specimen preparation. Therefore, it is perceived to be very user-friendly and quite popular among users. One of the most popular applications is in the field of surface science. It can be employed to get an image’s atomic resolution as well as to measure electrical, magnetic, and
mechanical properties of nanomaterials. It is possible to capture images of fine structure of metals and absorbed impurities. An AFM can also be used to study structures of organic and inorganic insulators. It can reveal topographic, tribological, roughness, and adhesion/fouling characterization of a wide variety of nanomaterials.
AFMs can be used to measure an extremely wide range of nanoparticles, including different metal nanoparticles, metal oxide particles, many types of composite metal/organic particles, synthetic polymer particles, nanorods, and quantum dots. Mechanical measurements on 1-D nanostructures such as carbon nanotubes and metal nanowires are carried out using an AFM.

It has many applications in metallurgy. The sintered components in powder metallurgy are subjected to roughness measurement. Even the topology and performance of protective covers or coatings can most reliably be determined by an AFM.

Force Measurement Using AFM

An AFM is not only a microscope. It can be employed to measure tiny forces of the order of tens of piconewtons to tens of nanonewtons. This has been made possible because of the ability
of AFMs to interact with the sample physically. One can give the analogy of a blind person visualizing samples by touching them. With the sense of touch comes the ability to push, pull, deform, and manipulate objects. Thus, the physical contact with the specimen gives the AFM extraordinary sensitivity, which aids in measurement of atomic forces. The atomic forces are created in the process of interaction between molecules in covalent and electrostatic bonds, multiple hydrogen bonding, etc. Even biological processes such as inter-cellular binding and adhesion create tiny forces that can be measured by an AFM.

The AFM tip is engaged with the specimen surface, and a ‘force versus distance’ experiment is performed. The deflection of the lever is recorded over a known distance, and the subsequent electrical signal generated by the photodetector is recorded. The X, Y channels of the AFM scanners are frozen, the feedback loop is suspended, and the tip and the sample are pushed together (or, in some cases, pulled apart) by ramping the Z-piezo channel of the scanner. The resulting photodiode signal is plotted. By calibrating this voltage signal with force, employing any one of several mathematical approaches recommended by research studies, it is possible to determine atomic forces.

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