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Quantum dots have emerged over the last few years as a very interesting set of materials. They are materials which have a very small diameter, as small as a few nanometers, and are known to exhibit some interesting florescence properties. The ability to efficiently fluoresce is the main reason why quantum dots have gained so much potential for anti-counterfeiting and packaging applications.
Most quantum dots currently being produced are made of semiconducting inorganic materials, such as cadmium selenide (CdSe), but graphene has emerged as a possible alternative. Graphene quantum dots are not only being trailed in the academic laboratory but also made commercially by a select number of companies.
To date, most of the work is still carried out at the academic level, but graphene quantum dots have the potential to be used in packaging, solar cells, light-emitting diodes (LEDs), electronic displays, fluorescent polymers, antibacterial and disinfection systems, various sensors, batteries, flash memory devices and bioimaging applications.
What are Graphene Quantum Dots?
Like all quantum dots, graphene quantum dots are very small and are realized when graphene flakes have a very small lateral diameter. At this scale, the graphene sheets exhibit different properties to other single sheets with a larger lateral dimension/flake size. These properties include specific optical, electronic (and optoelectronic), spin and photoelectric properties, as well as the ability to fluoresce and emit light in the blue range of the electromagnetic spectrum.
Graphene quantum dots, like all quantum dots, are classed as 0D materials (like Buckminster fullerenes) as their electrons are quantum confined in all three dimensions. This is the phenomenon which causes fluorescence as the quantum states cause the electrons to behave differently than in bulkier materials.
Like many graphene products (and structural forms), graphene quantum dots can be made up of a single layer or a few layers of graphene, with the most common being quantum dots which utilize a few layers of graphene.
Feasible Synthetic Method for Graphene Quantum Dots
Like other graphene products, there are many different methods that can be used to create graphene quantum dots. Early efforts focused on using graphene as the starting material, but because it was expensive at the time this was not developed as the most feasible option. Other methods have also been deemed expensive, such as electron beam lithography (EBL), laser ablation, and various electrochemical synthetic routes, so these are not ideal choices for large-scale production and usage.
While it is not the only commercial option, one method that has arisen in recent years is to use other carbon-based materials, such as coal, as the starting material. The raw material cost of coal and the subsequent synthetic processes are much cheaper than other processes, hence these methods carry more commercial weight than others.
The ability to use coal in a non-combustible way is also an eco-friendlier way of using coal, and because high temperatures are not needed it is generally a relatively eco-friendly route overall. Instead of using high temperatures, acids are used at low processing temperatures before being evaporated and filtered to yield quantum dots. This is just one commercially feasible synthetic method; others include the use of citric acid and urea.
While many nanomaterials, including ultra-thin nanomaterials such as graphene, can be characterized, the incredibly small diameters of quantum dots and their specific properties mean that certain techniques must be used to fully characterize graphene quantum dots. Graphene quantum dots can be analyzed via high-powered microscopy techniques including transmission electron microscopy (TEM) and atomic force microscopy (AFM), as well as different spectroscopy techniques such as photoluminescence spectroscopy, UV-Vis spectroscopy and surface-enhanced Raman spectroscopy (SERS).
As it stands, graphene quantum dots are an emerging technology that is available commercially but not quite at the stage of other graphene materials. However, they show a lot of promise across many areas due to the unique properties that they possess, including the ability to fluoresce. Like many applications graphene, graphene quantum dots have the potential to exponentially grow in the near future and expand into many markets.
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