Conductive and photoconductive atomic force microscopy are variations of atomic force microscopy (AFM). Conductive AFM simply measures the electrical conductivity of materials while photoconductive AFM measures the conductivity of materials when exposed to light.
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In this article, we discuss what conductive and photoconductive AFM are, how they work, the differences between both, practical uses, and some limitations associated with their use.
What are Conductive and Photoconductive AFM?
Conductive AFM is used to study the electrical properties of materials. These can range from bulk materials and thin films to nanotubes and nanoparticles. Such studies are important to assess the purity of materials, verify the effects of heat treatment, or check for damage in materials. It produces information on both the sample’s topography and conductivity. The major advantages of this method are better lateral resolution and topography-current correlation.
Photoconductive AFM, on the other hand, is a type of atomic force spectroscopy that has been modified to specifically measure the electrical conductivity of a surface due to the presence of light.
This is because some materials can conduct electricity when they are exposed to light with sufficient energy. Materials that do not conduct electricity don’t have free electrons as all have been used for covalent bonds. When exposed to light, this knocks electrons off some of the bonded electrons. These electrons can move freely, and other electrons can move to fill the void left by the knocked-out electrons. The electron movement results in electrical conductivity.
Photoconductive AFM can map out photocurrents, capture film morphology differences, obtain current density voltage plots, help in determining donor-acceptor domains, etc.
How do Conductive and Photoconductive AFM Work?
In conductive AFM, a conductive tip is put in contact with the surface of the sample and scanned. A voltage is then applied between the conductive tip and the sample which helps to generate an image of the current and also generates the topography of the sample. Both make possible the identification of regions on the surface and their corresponding conductivity.
What Differences Exist Between Conductive and Photoconductive AFM?
In photoconductive AFM, a similar instrument setup as in conductive AFM is used. The main difference between both is the inclusion of a light source and an inverted microscope in photoconductive AFM. The microscope concentrates the light source to the area just below the conductive tip to as small as the nanoscale.
In terms of function, photoconductive AFM sets itself apart from conductive AFM in that it can map photocurrents, regions that are photoactive in a material, and to an extent, deduce features below the surface.
In the experimental setup, photoconductive AFM experiments are conducted in the dark with a concentrated light source.
What are the Practical Uses of Conductive and Photoconductive AFM?
Conductive and photoconductive AFM has several applications in nanoelectronics, semiconductor, and solar cell industries where different types of high-resolution measurements are required.
For conductive atomic force microscopy, a recent study in the Advanced Materials Journal showed that it could be used to study the conductive properties of gold-plated metalized DNA nanowires which often have problems with irregularity and electrical connectivity on both ends. Conductive AFM was used because it is suitable for studying conductivities in very long nanowires.
The authors were able to detect the sources of defects which enabled them to produce DNA wires with better conductivities.
Photoconductive AFM is perhaps more well known for its use to study the properties of photovoltaic materials like those used in solar cells and sensors.
A study published in the Surfaces and Interfaces Journal used photoconductive AFM to investigate the underlying microscopic mechanisms behind the photovoltaic degradation behavior of halide perovskite. Halide perovskite is a candidate material for solar cell applications despite its low stability affecting its commercial use.
The authors were able to identify major decomposition mechanisms and newly formed failure regions. This kind of information is valuable for improving the design and performance of perovskite solar cells. The photovoltaics market is one of the most rapidly developing in the renewable energy sectors, and this technique is likely to become more popular as photovoltaic technologies advance.
In another study presented at the Special Issue: 37th International Symposium on Compound Semiconductors (ISCS 2010) photoconductive AFM was used to study photoactive regions of quantum dots by generating photocurrent images which helped to understand charge transport and recombination in their nanostructures. Quantum dots are a central focus in nanotechnology.
Are there Limitations to the Use of Conductive and Photoconductive AFM?
Conductive and photoconductive AFM is one of the most powerful tools to determine at the nanoscale the conductive properties of materials.
However, in practice, both methods have limitations in their uses. For example, in both, the degradation of the conductive tips as well as the use of different types of conductive tips affects the effective measurement of current fluctuations and can lead to erroneous conclusions. This is especially true when comparing results across different experiments.
In most cases, proper monitoring of the equipment and standards can help mitigate this challenge.
Concluding Remarks and Future Directions
Studies using conductive and photoconductive AFM have proven that they are extremely powerful methods for measuring the conductivity of materials at nanoscales. These methods are very valuable for studying and improving the conductivity of materials, especially at nanoscales. As the use of conductive nanomaterials continues to increase, their popularity in product development pipelines is likely to increase as well.
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References and Further Reading
Critchley, L. (2018). AFM Series: An Introduction To Photoconductive AFM (pc-AFM). [Online] AZoOptics.com. Available at: https://www.azooptics.com/Article.aspx?ArticleID=1492
Madl, M., Brezna, W., Strasser, G., Klang, P., Andrews, A. M., Bodnarchuk, M. I., Kovalenko, M. V., Yarema, M., Heiss, W., and Smoliner, J. (2011). AFM-based photocurrent imaging of epitaxial and colloidal QDs. Physica Status Solidi c, 8(2), 426–428. https://doi.org/10.1002/pssc.201000599
Materials science (2022). Conductive Atomic Force Microscopy. [Online] nrel.gov. Available at: https://www.nrel.gov/materials-science/conductive-atomic.html
Stern, A., Eidelshtein, G., Zhuravel, R., Livshits, G. I., Rotem, D., Kotlyar, A., and Porath, D. (2018). Highly Conductive Thin Uniform Gold-Coated DNA Nanowires. Advanced Materials, 30(26), 1800433. https://doi.org/10.1002/adma.201800433
Yang, W., Qin, Q., Wu, S., Gao, J., Tian, G., Hou, Z., Fan, Z., Lu, X., Chen, D., Gao, X., and Liu, J.-M. (2021). Probing the microscopic mechanisms in photovoltaic degradation behaviors of CH3NH3PbI3 perovskite films via photoconductive atomic force microscopy. Surfaces and Interfaces, 27, 101540. https://doi.org/10.1016/j.surfin.2021.101540
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