Visualizing and imaging nanomaterials is extremely important in characterizing and obtaining critical information about the nanostructures. Optical microscopy provides opportunities to observe, visualize, and capture images of nanoparticles, which help advance nanotechnology's frontiers.
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This article discusses how optical microscopy is used in analyzing, visualizing and imaging nanomaterials as well as its limitations and recent studies related to optical microscopy utilization in nanomaterials research.
What is Optical Microscopy?
Optical microscopy is the type of microscopy that uses different lenses and visible light to magnify a small sample. Optical microscopes are the oldest type of microscopes.
The most fundamental types of optical microscopes might be exceedingly uncomplicated. However, many intricate designs aim to enhance resolution and sample contrast. Historically, these microscopes were simple to create. They continue to be popular today because they utilize visible light and allow materials to be viewed by the naked eye.
How is Optical Microscopy Used in Visualization and Imaging?
Optical visualization is a very effective method for obtaining morphological information about materials. True colored real-time images can be collected with the help of optical visualization, which is advantageous in observing dynamic processes. Compared to scanning probe microscopy and electron microscopy, visualization with optical microscopy is beneficial due to mild operation conditions, simplicity, and low cost.
To produce a micrograph, the image seen via an optical microscope can be taken by a regular light-sensitive camera. Traditionally photographic films were used for imaging in optical microscopy. However, technological developments in complementary metal-oxide-semiconductor and charge-coupled device (CCD) cameras have made digital imaging possible.
Completely digital optical microscopes coupled with CCD cameras are now available. These digital optical microscopes have bypassed the conventional eyepiece, and the images can now be directly seen on computer screens.
How is Optical Microscopy used in Nanomaterial Analysis and Why?
For the characterization of nanomaterials, the ability to directly visualize nanoparticles under ambient conditions is of great importance. Mostly, to observe individual nanoparticles, an electron microscope with high resolution under a high vacuum is required.
Optical microscopy is used in nanomaterial characterization and analysis since it has great convenience due to open space and facile operation. However, there is a significant difference in resolution between optical microscopes and electron microscopes. Hence, effective imaging and visualization are needed for controllable manipulation, accurate location and direct observation of nanoparticles.
What Examples Are There of Optical Microscopy Used to Analyze Nanomaterials?
Over the course of the past several years, a variety of different methodologies for analyzing nanomaterials through optical microscopy have been presented. For instance, researchers have made significant advances in optically visualizing a variety of nanomaterials, such as nanofibers, graphene, and carbon nanotubes, by depositing visible "labels" or by making use of certain optical phenomena.
Such efforts have paved the way for further research to be conducted on the properties and applications of these nanomaterials. Furthermore, optical microscopy has enabled scientists to directly manipulate nanomaterials by designing and modifying setups. This has not only expanded the applications but has provided a better understanding of nanoscale dynamic processes.
Different types of optical microscopes are utilized in characterizing and analyzing nanomaterials. For example, photothermal optical microscopy, Raman microscopy, surface plasmon resonance microscopy, dark-field microscopy, and fluorescence microscopy are applied in nanocatalysis, nanoelectrochemistry and nanosensing to obtain critical information about different nanostructures.
Limitations to Optical Microscopy
One of the limitations of optical microscopy is low resolution, along with the huge beam size of optical microscopes. This limitation is disadvantageous, especially for imaging nanomaterials, since many nanoparticles, nanotubes, or nanostructures are much smaller in size than the optical resolution limit of 200 nm.
For instance, the visualization of carbon nanotubes or carbon nanotubes (CNTs) is not possible through ordinary optical microscopy since the diameters of CNTs are usually much smaller than the illumination beam size. Hence effective imaging and visualization strategies are carefully designed to realize their direct observation, subsequent utilization, performance characterization, controllable manipulation and accurate location.
A study published in 2020 has discussed the Scattering-type scanning near-field optical microscopy (s-SNOM). This study demonstrated that this type of optical microscopy is capable of mapping the complex permittivity of nanomaterials quantitatively as well as their intrinsic optical properties like the refractive index.
The team applied this capability in experiments dealing with three different nanostructured materials, including plasmonic ceramic nanoparticles, optical nanocoatings with controllable color properties, and microcapsules with drug delivery properties. These experiments helped scientists show how complex permittivity mapping with s-SNOM contributes to understanding these materials by giving unique information that is impossible to assess with other techniques.
Another study published in 2021 discusses the advantages of optical microscopy over electron microscopes like SEM and TEM in nanomaterial research. This research considers fast identification, location and manipulation of nanomaterials to be great challenges due to their small sizes.
To obtain certain information about morphology and other properties using the electron microscope, the sample is required to be under a vacuum. This has several disadvantages, like difficulties in carrying out several operations and samples getting damaged after pre-treatment for electron microscopy. On the contrary optical microscopy provides opportunities for manipulation without destroying or damaging samples under normal conditions.
References and Further Reading
Di Gianfrancesco, A. (2017). Technologies for chemical analyses, microstructural and inspection investigations. In Materials for ultra-supercritical and advanced ultra-supercritical power plants (pp. 197-245). Woodhead Publishing. https://doi.org/10.1016/B978-0-08-100552-1.00008-7
Shi, X., Zhao, S., Wang, F., Jiang, Q., Zhan, C., Li, R., & Zhang, R. (2021). Optical visualization and imaging of nanomaterials. Nanoscale Advances, 3(4), 889-903. https://doi.org/10.1039/D0NA00945H
Stanciu, S. G., Tranca, D. E., Pastorino, L., Boi, S., Song, Y. M., Yoo, Y. J., ... & Stanciu, G. A. (2020). Characterization of nanomaterials by locally determining their complex permittivity with scattering-type scanning near-field optical microscopy. ACS Applied Nano Materials, 3(2), 1250-1262. https://pubs.acs.org/doi/abs/10.1021/acsanm.9b02019
Wang, W. (2018). Imaging the chemical activity of single nanoparticles with optical microscopy. Chemical Society Reviews, 47(7), 2485-2508. https://doi.org/10.1039/C7CS00451F
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