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:

ICFO designs new graphene-based flexible and transparent wearable health trackers

ICFO researchers have recently demonstrated a new class of graphene-based flexible and transparent wearable devices that are conformable to the skin and can provide continuous and accurate measurements of multiple human vital signs.

ICFO's new flexible and transparent graphene health tracker image

These devices can measure heart rate, respiration rate and blood pulse oxygenation, as well as exposure to UV radiation from the sun. While the device measures the different parameters, the read-out is visualized and stored on a mobile phone interface connected to the wearable via Bluetooth. In addition, the device can operate battery-free since it is charged wirelessly through the phone.

German researchers study the biological risk potential of graphene quantum dots

A team of researchers from the Physics, Medicine and Chemistry departments at Heinrich Heine University Düsseldorf (HHU) has examined if graphene nanoparticles are potentially dangerous for the organism and how cells cope with them once they have been incorporated.

Two stem cells containing carbon nanoparticles and cell nuclei imagemicroscopy images of living cells cultured for 36 h without GQDs (left) and with GQDs (right) as observed (top row) and merged with the bright field images (bottom row)

Nanoparticles can be absorbed in body cells, and two aspects to this feature exist. First, it makes nanoparticles good vehicles for transporting a broad range of compounds or substances attached to them into normal diseased cells in a targeted manner. On the other hand, they can also pose health risks.

Dotz Nano shows graphene quantum dots to be effective in treating brain injuries, strokes and heart attacks

Dotz Nano has shared a new research that finds its graphene quantum dots (GQDs) technology effective in treating brain injuries, strokes, multiple sclerosis and heart attacks. According to the company, the study demonstrated that these dots, manufactured from coal, can assist in fighting oxidative stress to assist in treating patients suffering from the serious conditions.

Led by the Company's scientific advisor, Professor James Tour, the study was conducted by five universities and research facilities including Rice University, with the findings covered by multiple medical publications.

Graphene quantum dots could yield an effective antioxidant for various traumatic injuries

Researchers from Rice University, the Texas A&M Health Science Center and the McGovern Medical School at The University of Texas Health Science Center at Houston (UTHealth) have found that graphene quantum dots drawn from common coal may be the basis for an effective antioxidant for people who suffer traumatic brain injuries, strokes or heart attacks.

Graphene quantum dots could yield an effective antioxidant for various traumatic injuries imageCoal-derived graphene quantum dots as seen under an electron microscope

The QDs' ability to quench oxidative stress after such injuries was the subject of a study, which showed that the biocompatible dots, when modified with a common polymer, are effective mimics of the body’s own superoxide dismutase, one of many natural enzymes that keep oxidative stress in check.

Graphene inks help stabilize the stability of perovskite solar cells

Researchers from the Graphene Flagship have developed hybrids of graphene and molybdenum disulphide quantum dots to stabilize perovskite solar cells (PSCs). PSCs are a novel type of solar cells which are efficient, relatively easy to produce, made with cheaper materials and, due to their flexibility, can be used in locations where traditional silicon solar cells cannot be placed.

Graphene inks help stabilize the stability of perovskite solar cells

A collaboration between the Graphene Flagship Partners Istituto Italiano di Technologia, University of Rome Tor Vergata, and BeDimensional resulted in a novel approach based on graphene and related materials to stabilize PSCs, thus addressing the stability issue of PSCs, a major hurdle hindering their commercialization.

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