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Atomic Force Microscopy (AFM) has become a well-regarded and well-established technique across a range of industries and scientific fields. AFM is used to map the surface of a material, but the introduction of new imaging modes in recent years has expanded AFM’s capabilities to where it can now be used to probe many different properties of a material. This has also expanded AFM’s application reach. In this article, we look at the many applications where AFM has become a vital imaging and characterization technique.
AFM is an imaging method that uses a piezoelectrically charged probe consisting of a flexible cantilever and an atomically sharp tip to scan the surface of a sample. As the probe scans the surface, the intermolecular interactions between the surface and the tip cause the tip to move towards the surface (and in many cases touch the surface).
This movement also causes a laser beam, which is shone onto the back of the cantilever to deflect. This laser deflection is recorded on a position-sensitive photodiode (PSPD). The whole system is reset using a feedback loop where the probe returns to its original position and the same process occurs for each atom in a sample. This enables the position of each atom to be mapped. The introduction of new imaging modes has enabled the electric, dielectric, mechanical, optical, and thermal properties of a sample to be mapped alongside the topography of the surface.
Soft biological samples may not be the first choice in everyone’s minds, but AFM has an imaging mode known as non-contact mode, which, unlike the other modes, never touches the surface of the sample. The development of this imaging mode has enabled the structure and properties of biomolecules and biological samples to be determined.
The applications of AFM in biology range from examining large tissues down to individual cells and biomolecules such as proteins. AFM has also been used to image other fundamental biological processes and biomolecules, including viruses, membranes, cellular compartments, cellular processes (such as exocytosis), the mechanical properties of cells, the contact forces between cells, how cells and biomolecules interact, as well as DNA and how other biomolecules interact with DNA, to name a few examples of specific analyses.
AFM is also widely used in the applied biological fields such as bio-engineering and can be used to investigate the surface properties of implants and how well they interact with biological matter, especially regarding the nanoscale properties, and how a host’s cells will adhere to an implant so that it doesn’t get rejected by the body.
Given that most nanomaterials have an active surface and, in the case of 2D materials, only possess a surface, a lot of the properties of nanomaterials are governed by the structure of a nanomaterial’s surface, so probing the surface of nanomaterials has become a big application area.
Aside from determining the structural, electrical, mechanical, optical and thermal properties of a nanomaterial, AFM can also be used in conjunction with nanofabrication methods to manipulate the structure (and properties) of nanomaterials. This can take the form of chemically modifying a surface with an AFM tip, or through techniques such as electron beam lithography and dip pen lithography to physically remove atoms from the nanomaterial or for producing a specific pattern on the surface of the nanomaterial.
In addition to being used as a tool for manipulating the nanomaterial, AFM can also be used to measure the subsequent effects of the atomic manipulation post-fabrication.
Semiconducting materials and products that utilize semiconductors are another class of materials where AFM has been a useful tool. AFM is also used as a process control tool in semiconducting manufacturing lines for quality control, failure analysis, defect identification, and the measurement of surface roughness and properties of wafers in a device.
AFM can also be used to investigate the properties of the semiconducting materials used in a device. This can range from measuring the electrical properties of the semiconducting junction, to determining the dopant levels of the semiconducting material, the amount of defects in the material, the charge carrier mobility, and the gate properties of the semiconducting junction (although this last property is usually for metal-oxide semiconductors only).
AFM can also be used on a whole, complete device that uses a semiconducting material, with solar cells (photovoltaics—PVs) being one of the most common examples. This includes measuring the nanoscale properties of a device, to identifying the donor-acceptor regions of a device, the quantum and power efficiencies of a device, and the photoactive and photocurrent regions of a PV device.
Like complete devices that use semiconductors, AFM can also be used to measure the properties of electronic components and the properties of different electronic regions at the atomic level. In many cases, this takes the form of basic electric and dielectric properties of the whole device, or the materials used within the device, but AFM can also be used to determine more exotic electronic properties such as ferroelectric domains and piezoelectric domains.
AFM can be used to determine many different electronic properties, including trapping and resonant electronic structure, as well for determining the safety of the electronic device by measuring the current leakage and whether electrical failure is likely to occur.
Reference and Further Reading