Both AFM and STM are surface microscopy techniques that can be used to determine the topology of a surface. They are both widely used throughout the chemical and nanoscience fields in both academia and industry. In this article, we look at what both of these techniques are, as well as showing which applications suit each method.
What is AFM?
Atomic force microscopy, otherwise known as AFM, is a technique that physically touches the sample to measure the features of a surface. In their most basic sense, they measure the force between a probe and a surface.
The critical element of an AFM machine lies in its cantilever beam, i.e., the probe. The cantilever beam physically taps the surface of a substrate and can be done so using a variety of modes and tips. The tips range from sharp tips for hard surfaces to colloidal probes for softer matter.
As the tip approaches the surface of the material, the intermolecular attraction between the surface and the tip causes the cantilever to move towards the surface and ‘tap’ it. A laser beam is employed within AFM machines to detect any movement from the cantilever.
When a cantilever moves towards the surface, the laser beam deflects off it, and the positional change is registered by the laser hitting a position-sensitive photo-diode (PSPD). AFM instruments then utilize a feedback loop system to generate a high-resolution topographic map once the vertical and lateral deflections of the cantilever have been measured.
While the tapping mode is most common, AFM does have a non-contact method – which never touches the surface of the material. In this mode, the tip vibrates above the resonance frequency surface of the sample but can quickly become ineffective due to the long-range interaction forces of the sample.
What is STM?
Scanning tunneling microscopy (STM) is different to AFM, in that it uses tunneling electrons and the piezoelectric effect to generate an image of a surface. STM uses a conducting (quartz) tip to scan the surface. When the tip approaches the surface, a voltage difference (also known as a bias) is generated through the piezoelectric effect. The voltage difference allows the electrons to tunnel between the tip and the surface. A feedback loop is then utilized to maintain the current so that an image of the surface can be generated using a combination of the tip position, applied voltage and the local density of states (LDOS).
There are two modes of STM-constant voltage and constant height. In constant height mode, the tip remains at a constant height, and the difference in the applied voltage is key to determining the position (especially the height) of atoms at the surface. In constant voltage mode, the voltage bias remains constant throughout. This means that the tip always stays the same distance away from the surface, regardless of its height. In this mode, the change in the height of the tip plays a crucial role in determining the topography and the relative positions of surface atoms.
Applications of AFM and STM
AFM is used across a wide range of industries. It’s multiple operating modes, and various cantilever tips lends itself as a flexible microscopy instrument. While it is used throughout chemistry and nanotechnology applications, it has also found itself in biologically related fields, such as in biomaterials, biomolecules, cells, and tissues.
Some of the most common areas of use for AFM instruments include catalytic surface planes, energy storage, food science, graphene and other 2D materials, magnetics and data storage, nanomaterial characterization, piezo and ferroelectrics, polymers, semiconductors, microelectronics, photovoltaics and solar cells, thin films and coatings.
STM is a bit more niche in its applications, although it is readily used across various aspects of chemistry, physics, and nanotechnology. Some applications include measuring the friction, surface roughness, materialistic defects and surface reactions in materials. They are also widely employed in the semiconductor and microelectronic industries
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