by Introduction to AFM
The atomic force microscope (AFM) is one of the foremost tools for imaging, measuring and manipulating matter at the nanoscale. Developed in the 1980s by Gerd Binnig, Calvin Quate and Christoph Gerber at Stanford University and IBM, the AFM works by scanning a probe or cantilever with a very sharp tip over a sample surface. The tiny forces between the tip and sample are measured and used to create high-resolution topographical images of surfaces down to the atomic level.
Atomic Force Microscope Working Principle
At the heart of the Atomic Force Microscope is a microscale cantilever with a sharp tip protruding from its end that is used to scan the specimen surface. The cantilever is typically made of silicon or silicon nitride with a tip radius of curvature on the order of nanometers. When the tip is brought into proximity of a sample surface, forces between the tip and sample lead to a deflection of the cantilever according to Hooke’s law. Piezoelectric elements within the AFM precisely control the position of the sample in the x, y and z directions and a laser spot reflected from the top of the cantilever is used to measure its deflection on photodetectors. By scanning the tip over the sample surface and measuring thousands or millions of points, a three-dimensional topographic image can be generated.
Atomic Force Microscope Imaging Modes
There are three main imaging modes of atomic force microscopy: contact mode, non-contact mode and tapping mode. Contact mode involves dragging the tip across the surface while measuring deflections in all three directions. Non-contact mode oscillates the cantilever close to the sample and detects changes in the oscillation amplitude or frequency. Tapping mode, also called intermittent contact mode, lightly taps the surface and measures van der Waals forces and repulsion from sample features. It allows imaging of a variety of sensitive surfaces without damaging them.
Atomic Resolution Imaging
One of the most amazing capabilities of the AFM is its ability to achieve atomic resolution on a diverse range of samples including semiconductors, thin films and biological molecules. With its sharp silicon or silicon nitride tips, the AFM has imaged individual atoms on surfaces as varied as graphite, silicon, metals and surfaces reconstructed by adsorbed atoms or molecules. Real-time imaging of single atoms moving on surfaces or nanoparticles rearranging has given unprecedented insights into fundamental processes like surface diffusion. Understanding surface structures at the level of individual atoms allows correlating structure to physical properties and driving innovation in nanomaterials.
Biological and Material Applications
Since its invention, the atomic force microscope has found widespread applications both in the biological sciences and material sciences due to its ability to image surfaces with nanoscale resolution under ambient conditions without high vacuum or other special environments. In biology, the AFM has been used to image living cells, proteins, DNA and other biological macromolecules. It provides a less destructive alternative to electron microscopies for 3D imaging of soft, wet biological samples. In materials, the AFM is extensively employed for analyzing the surface topography of semiconductors, metals and ceramics, measuring film thickness and grain size, visualizing defect structures and analyzing nanofabrication processes. Beyond topographic imaging, the AFM is also capable of simultaneously measuring a wide variety of other properties like elasticity, adhesion, charge or magnetic domains with very high sensitivity.
Advancing Nanotechnology
The AFM proved to be a true breakthrough technology not only for nanoscale imaging but also nanomanipulation and nanofabrication. By precisely controlling the position of the sharp tip, individual atoms and molecules can be directly manipulated on surfaces. Building on these capabilities, scientists have constructed nanoscale structures such as quantum corrals and quantum dots by maneuvering single adatoms. Atomic force microscopes have also been developed that can directly pattern materials on the nanoscale by phenomena like dip-pen nanolithography, where the sharp AFM tip is used to “draw” chemical patterns on surfaces through self-assembly processes. New applications continue emerging that are advancing our ability to observe, modify and even fabricate devices and materials at the atomic scale – the realm of nanotechnology.
Emerging Developments
Ongoing developments are further expanding the imaging capabilities of atomic force microscopy. Combining the atomic force microscope with other techniques has enabled correlating nanoscale structural, electrical, magnetic, chemical and other physical property measurements with amazing spatial resolution. New high-speed AFM variants capable of imaging exceptionally fast dynamical processes have also emerged. Developments in ultrahigh vacuum AFMs have attained sub-angstrom resolution for atomic-scale studies of surface structures and dynamics. Advanced AFM modes based on techniques like photocurrent, conductive probe and Kelvin probe are revealing fascinating insights into energy, charge transport and other phenomena at the nanoscale. With its impressive subnanometer resolution, versatility and other advantages, atomic force microscopy will surely continue finding diverse applications across science and technology for many years to come.
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1. Source: Coherent Market Insights, Public Source, Desk Research
2. We have leveraged AI tools to mine information and compile it.
