SPM or Scanning probe microscopy describes a range of techniques that utilize a very fine physical probe to scan across surfaces, thereby monitoring the interaction strength between the tip and surface producing images, profiles or maps of the surface.
The year 1982 saw the establishment of SPM as a technique, when the scanning tunneling microscope (STM) was developed by Binnig and Rohrer. In fact, they went on to win the Nobel prize for Physics for their work, in 1986.
STM was invented by Binnig and Rohrer as a method to image surfaces at the atomic level (0.1 nm resolution). However, this technique still requires the surface being examined to have a conductive or partially conductive coating. To produce an image of the sample surface, STM measures the electrical tunneling current between the scanning tip and the sample.
The Principle of STM
The primary principle of STM is that it operates in a vacuum. When a conducting tip is brought near the surface under examination, it initiates a difference in voltage between the probe and surface. This process enables the flow of electrons to tunnel through the vacuum space in-between. Naturally, the tunneling current produced relies upon tip position, the applied voltage, and the local density of the sample.
Thus, the image is created as a result of monitoring the current as the tip scans across the surface. This technique was the fundamental break-through in imaging, since it was the first time surfaces could be visualized at the atomic scale. What’s more, the resolution of STM is not limited by diffraction, rather, it is limited only by the size of the probe-sample interaction volume, which can be in the order of a few picometers.
Scanning Tunneling Microscopy Issue
However, STM suffers from the setback of needing the surface of the sample to be conductive. Thus, this places limits on the type of materials that can be analyzed. To overcome these limitations, further scientific investigation led to the invention of the first atomic force microscope (AFM). The crucial technology across all SPM techniques acted as a feedback loop, thus regulating the gap or distance between the sample and the probe. Today, SPM offers a huge variety of techniques, differing by probe type and interaction measurement required.
The Development of AFM
In contrast, AFM utilizes a very sharp silicon or silicon nitride tip attached to a micro-cantilever arm with a suitable spring constant. This functions as a probe to profile the morphology of the sample surface. However, since AFM does not necessitate that the sample be conductive, no current is measured between the probe tip and the sample surface, in providing an image.
Instead, AFM uses the probe to measure the forces between tip and sample as the tip makes contact with the sample surface. The cantilever then bends due to forces between the tip and the sample surface, as the tip is raster scanned across the sample surface.
Here, beam deflection is used to measure cantilever deflection, most commonly using beam deflection, and the feedback signal measurements generated provide a profile of surface topography. While the principle in AFM remains the same, the type of force measured can vary according to the requirements for imaging.
How Does SPM Work?
Although SPM is an umbrella term that encompasses both the STM and AFM microscopy family, both techniques have developed in slightly different ways. Today, between them, there is a multitude of modes of operation. For instance, AFM can measure forces such as mechanical contact force, van der Waals forces, capillary forces, electrostatic forces, chemical bonding, magnetic forces, and even solvation forces, depending on the situation and its requirements.
While a Magnetic force microscope (MFM) measures the magnetic interactions, Chemical force microscopy (CFM) utilizes chemical interactions existing between a functionalized probe tip and the surface chemistry of a sample. Further, Scanning ion-conductance microscopy (SICM) is when SPM uses an electrode as the probe tip to determine surface topography of nanometer-scale structures in electrolytic aqueous media. In addition, Kelvin probe force microscopy uses a conducting cantilever to scan over a surface at a constant height to map the work function of the surface.
Uses of SPM
SPM techniques provide an excellent method to study surfaces and, if required, even modify them using the techniques of nanolithography. Nanotribology is one of the broader uses of SPM. In this field, nanoscale measurements of friction, wear, adhesion and lubrication are examined. This allows the determination of the effect of atomic interactions and quantum effects on surface friction in the development of materials.
Such techniques are highly significant in industries such as automotive and aerospace. Further, SPM techniques are also extremely vital in electronics – both for the examination of silicon surfaces in component manufacture and the examination of interfaces in semiconductor materials. Further, in biology and medicine, SPM techniques can be used to examine peptide structures and thereby determine their stiffness and structure.
Finally, SPM can also map cell surfaces and determine the difference between healthy and diseased cells by examining the cell membrane’s stiffness.
Advantages of SPM
Now used as a blanket term that includes a multitude of modes and methods, SPM has demonstrated its capability to measure many different types of fundamental force on surfaces and visualize them. While STM provided the first atomic resolution imaging technique that was independent of light’s diffraction limitations, but still needed a conductive surface, AFM was developed as an improvement of STM. In fact, AFM techniques added to the field by allowing a variety of probes and modes to examine surfaces and the forces they display.
Today, SPM has expanded beyond its initial paradigm and provided a method to study surfaces under different conditions. Asylum Research provides a range of accessories to expand SPM’s utility and maximize research into nano-mechanics, nano-electronics, tribology, and biological applications. Using AFM platforms like Cypher and the MFP-3D, Asylum Research can support a full range of biological, mechanical and environmental applications across a wide range of SPM modes – including STM. Further, with innovations such as the blueDrive photothermal excitation, Asylum Research is poised to provide new standards of stability and accuracy to the field of SPM.
- Roland Wiesendanger Scanning Probe Microscopy and Spectroscopy: Methods and Applications, Cambridge University Press, 1994
- Ernst Meyer, Hans Josef Hug, Roland Bennewitz, Scanning Probe Microscopy: The Lab on a Tip, Springer-Verlag, Heidelberg, 2004
- Andrew A. Gewirth, and Brian K. Niece, Electrochemical Applications of in situ Scanning Probe Microscopy, Chem. Rev., 1997, 97 (4), pp 1129–1162
- Leo Gross and Fabian Mohn et al., Organic structure determination using atomic-resolution scanning probe microscopy, Nature Chemistry volume 2, 2010, p 821–825
This information has been sourced, reviewed and adapted from materials provided by Asylum Research - An Oxford Instruments Company.
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