Article last updated on: Jan 25, 2019

What are quantum dots?

Quantum dots, or QDs, are semiconductor nanoparticles or nanocrystals, usually in the range of 2-10 nanometers (10-50 atoms) in size. Their small size and high surface-to-volume ratio affects their optical and electronic properties and makes them different from larger particles made of the same materials. Quantum dots confine the motion of conduction band electrons, valence band holes, or excitons (bound pairs of conduction band electrons and valence band holes) in all three spatial directions. Quantum dots are also sometimes referred to as ‘artificial atoms’, a term that emphasizes that they are a single object with bound, discrete electronic states, similarly to naturally occurring atoms or molecules.

Quantum Dots - fluorescent image

Many types of quantum dot are fluorescent - they emit light of specific frequencies if electricity or light is applied to them. These frequencies can be tuned by changing the dots' size, shape and material, opening the door to diverse applications. Generally speaking, smaller dots appear blue while larger ones tend to be more red. Specific colors also vary depending on the exact composition of the QD.

Applications

Thanks to their highly tunable properties, QDs are attracting interest from various application developers and researchers. Among these potential applications are displays, transistors, solar cells, diode lasers, quantum computing, and medical imaging. Additionally, their small size enables QDs to be suspended in solution, which leads to possible uses in inkjet printing and spin-coating. These processing techniques may result in less-expensive and less time consuming methods of semiconductor fabrication.

Quantum dots are considered especially suitable for optical applications, thanks to their ability to emit diverse colors, coupled with their high efficiencies, longer lifetimes and high extinction coefficient. Their small size also means that electrons do not have to travel as far as with larger particles, thus electronic devices can operate faster. Examples of applications that take advantage of these electronic properties include transistors, solar cells, quantum computing, and more. QDs can greatly improve LED screens, offering them higher peak brightness, better colour accuracy, higher color saturation and more.



QDs are also very interesting for use in biomedical applications, since their small size allows them to travel in the body, thus making them suitable for applications like medical imaging, biosensors, etc.

What is graphene?

Graphene is a material made of carbon atoms that are bonded together in a repeating pattern of hexagons. Graphene is so thin that it is considered two dimensional. Graphene's flat honeycomb pattern gives it many extraordinary characteristics, such as being the strongest material in the world, as well as one of the lightest, most conductive and transparent. Graphene has endless potential applications, in almost every industry (like electronics, medicine, aviation and much more).

Graphene structure photo

The single layers of carbon atoms provide the basis for many other materials. Graphite, like the substance found in pencil lead, is formed by stacked graphene. Carbon nanotubes are made of rolled graphene and are used in many emerging applications from sports gear to biomedicine.

Graphene quantum dots

The term graphene quantum dots (GQDs) is usually used to describe miniscule fragments, limited in size, or domains, of single-layer to tens of layers of graphene. GQDs often possess properties like low toxicity, stable photoluminescence, chemical stability and pronounced quantum confinement effect, which make them attractive for biological, opto-electronics, energy and environmental applications.

The synthesis of graphene quantum structures, such as graphene quantum dots, has become a popular topic in recent years. While graphene usually does not have a bandgap - which is a problem for many applications - graphene quantum dots do contain a bandgap due to quantum confinement and edge effects, and that bandgap modifies graphene's carrier behaviors and can lead to versatile applications in optoelectronics. GQDs were also found to have four quantum states at a given energy level, unlike semiconductor quantum dots, which have only two. These additional quantum states, according to researchers, could make GQDs beneficial for quantum computing.

Additional properties of GQDs such as high transparency and high surface area have been proposed for energy and display applications. Because of the large surface area, electrodes using GQDs are applied for capacitors and batteries.

Various techniques have been developed to produce GQDs. Top-down methods include solution chemical, microwave, and ultrasonic methods. Bottom-up methods include hydrothermal and electrochemical methods.

Further reading

The latest graphene quantum dots news:

Researchers achieve direct visualization of of quantum dots in bilayer graphene

Researchers at UC Santa Cruz have reported the first direct visualization of quantum dots in bilayer graphene, revealing the shape of the quantum wave function of the trapped electrons. The finding of this research could provide important fundamental knowledge, required for developing quantum information technologies based on bilayer graphene quantum dots.

Direct visualization of quantum dots reveals shape of quantum wave function imageImage from Nano Letters

"There has been a lot of work to develop this system for quantum information science, but we've been missing an understanding of what the electrons look like in these quantum dots," said corresponding author Jairo Velasco Jr., assistant professor of physics at UC Santa Cruz.

Graphene quantum dots help check water content in soil

A recent study by researchers at Gauhati University, Indian Institute of Technology Bombay and DAIICT in India has demonstrated a soil moisture sensor made from graphene quantum dots, which are nanometer-sized fragments of graphene.

Water sensors are vital for various agriculture applications, like keeping track of the watering schedule for a large number of plants, such as for a field of crops. Soil moisture sensors measure the water content in the soil to avoiid crop destruction by under or over watering the field.

Researchers create a mechanically-tunable graphene quantum dot

Researchers at Delft University of Technology (TU Delft) recently presented what they say is the first mechanically-tunable monolayer graphene QD whose electronic properties can be modified by in-plane nanometer displacements.

TU Delft team creates novel GQD image

The ability to precisely manipulate individual charge carriers can be considered as a cornerstone for single-electron transistors and for electronic devices of the future, including solid-state quantum bits (qubits). Quantum dots (QDs) are at the heart of these devices.

Graphene quantum dots could be an efficient anti-inflammatory therapy for colitis

Researchers from Seoul National University have examined graphene quantum dots' (GQDs') efficiency as anti-inflammatory therapy for colitis. The team speculated that graphene quantum dots may be suitable for treating intestinal bowel diseases (IBDs) because of their low toxicity in vivo and ease of clearance.

In their study, GQDs were intraperitoneally injected to dextran sulfate sodium (DSS)–induced chronic and acute colitis model, and its efficacy has been confirmed. In particular, GQDs effectively prevented tissue degeneration and ameliorated intestinal inflammation.

US energy department funds project for coal-derived graphene production process

The US Department of Energy’s (DOE) Office of Fossil Energy (FE) has selected three projects to receive approximately USD$3 million in federal funding for cost-shared research and development projects.

Among these projects is a laboratory-scale coal-derived graphene process – the University of North Dakota will demonstrate a laboratory-scale coal-derived graphene process to produce graphene oxide, reduced graphene oxide, and graphene quantum dots starting from domestic US coal.