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AFM has become a widely used technique for many reasons. It is most frequently used to determine the topography of a surface. While this is the principle of conventional AFM techniques, there is also a range of AFM variations. In this article, we look at one of the AFM variations known as Kelvin Probe Force Microscopy (KPFM), which is a technique that measures the potential difference between a sample and the AFM tip.
cross all variations of AFM, there is a common theme. The position location mechanism is generally the same. The areas in which they differ is often in the tip, how the tip interacts with a material and the information that can be generated. In a lot of cases, the many AFM variants are a combination of AFM operating principles with the interaction principles of other techniques.
What is Kelvin Probe Force Microscopy (KPFM)?
Kelvin Probe Force Microscopy (KPFM) is a variation of AFM that only operates in non-contact mode. KPFM is seen as a technique that bridges the areas of conventional AFM and surface potential microscopy (SPM). Unlike conventional AFM which deduces the positioning of atom on a surface, KPFM is used to measure the work function (as a function of potential difference) between the tip and the surface. It is a method more associated with analyzing the electronic properties and has found a lot of use within the nanotechnology and semiconductor fields in recent years.
How KPFM Works
KPFM uses basic AFM principles, such as using a tip. Although the tip used in KPFM is a conductive tip so that a potential difference between the surface and the tip can be generated. KPFM utilizes the same position locating principles as AFM, in which the cantilever moving towards the surface determines the relative position of a localized work function. Its location is mapped by a laser beam deflecting off the cantilever onto a position-sensitive photodiode (PSPD).
However, rather than mapping the position of atoms, it is more concerned with measuring the contact potential difference between a point of the surface and the tip. The cantilever beam acts as a reference electrode and forms a capacitor and it is then scanned across the surface. In normal AFM, the cantilever and tip are piezoelectrically driven, however, in KPFM, an alternating current (AC) is applied to the tip. When the tip encounters a direct current (DC) from the surface, the current is offset, and this causes the cantilever to move towards the sample.
The electrostatic interactions can also be used to deduct the work function of a material (i.e., the amount of energy required to remove an electron from a solid to a point where it is out of the solid). The work function can be deduced because it is the difference in the work functions between the tip and the materials that generate the contact potential difference.
However, there are a couple of different ways in which the surface can be scanned. These are known as intermittent mode and non-contact mode, which are also referred to as amplitude modulation (AM) and frequency modulation (FM) operations, respectively. In both modes, the tip-sample interactions can become perturbed by the attractive and repulsive forces, and this can cause changes in the oscillation of the cantilever.
AM Mode
In the AM mode, changes in the oscillation of the amplitude provides the feedback signal required to image a large area of the surface. As the distance between the tip and the sample increases, so does the oscillation amplitude. This is attributed to the loss of electrostatic interactions between the tip and the sample. The amplitude can be monitored and regulated using a feedback loop, and this keeps the distance between the tip and the sample constant. It also enables the cantilever to return to its original setpoint. The AM mode therefore represents the direct force between the tip and the sample.
FM Mode
FM mode operates using a slightly different principle. In FM mode, the change in oscillation frequency can provide information about the interactions between the tip and the sample by measuring the variation in distance between them. FM mode uses a feedback loop to regulate the frequency change and for continually keeping the cantilever at a defined setpoint, which enables the topography of the surface to be generated.
The change in oscillation frequency is dependent upon the force gradient between the tip and the sample and for restoring the force to its original setpoint. So, FM mode detects the force gradient rather than the actual force, but this enables FM mode to have a higher spatial resolution than AM mode.
Applications
Because KPFM is concerned with potential differences, it has found a lot of use in measuring the electrical/electronic properties of various materials. Some examples of this include deducing the electronic structure of metallic nanostructures, including catalysts, biological sensors and the charge transfer mechanisms between nanostructures. Other applications include mapping the deposition of metal substrates onto a surface plane, characterizing the electrical properties of semiconductors, and for identifying the electrical properties of commercial devices, such as transistors and solar cells.
Because KPFM also measures the work function, it can also be used to deduce localized phenomenon on the surface of a material, such as the extent of catalytic activity, surface reconstruction mechanisms, doping, adsorbates and band-bending in semiconductors, surface defects, charge trapping, and surface corrosion.
Sources and Further Reading
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