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

Latest Graphene Quantum Dots news

A new guide to promising perovskite materials: The Perovskite Handbook

The Graphene-Info team takes pleasure in recommending our new book - The Perovskite Handbook. While not focused on graphene, we believe that any person interested in advanced materials and emerging technologies would find that perovskite materials are an area of focus that should not be ignored.

The Perovskite Handbook

This book gives a comprehensive introduction to perovskite materials, applications and industry. Perovskites offer a myriad of exciting properties and are considered the future of solar cells, displays, sensors, lasers and more. The promising perovskite industry is currently at a tipping point and on the verge of mass adoption and commercialization.

The graphene industry should especially pay attention to perovskites as much work is done on combining these two material technologies to create better solar cells, displays and more.

Chinese team creates new graphene nanomaterials to protect ancient murals

Scientists from the Center for Nano Energy Materials (CNEM) of Northwestern Polytechnical University in China said they have successfully applied graphene‐enhanced nano-materials to protect ancient wall paintings.

The team used a compound of calcium hydroxide and graphene quantum dots in a water solution and applied the material in ancient wall paintings in three tombs of the Tang Dynasty (618-907). "Research shows that the new material is small (an average of 80 nanometers per particle), uniform in size, and very sticky, thus making it good at reinforcing the wall paintings," Wei Bingqing, a CNEM dean, stated.

Graphene Supercapacitors Market Report

NIST team reveals new graphene QDs structure

NIST physicists have spatially and magnetically confined electrons within graphene atoms into cake-like nanostructures, proving theoretical speculations, and promising applications for quantum computing. It was said that the experiment “confirms how electrons interact in a tightly confined space according to long-untested rules of quantum mechanics. The findings could also have practical applications in quantum computing".

Scientists have to confine quantum dots in space to work with them, but the NIST researchers thought of applying a magnetic field to see how electrons orbiting quantum dots would behave. Using a scanning tunneling microscope, the team found that electrons packed together more and arranged themselves into concentric rings that alternate between conducting and insulating energy levels, shaped like a tiered cake.

Dotz Nano reports a successful pilot trial for its graphene-based quantum dots anti-counterfeiting system

Dotz Nano recently reported a successful industrial production pilot to mark special packages with its advanced marker named ValiDotz, to prevent counterfeiting of top brands in China.

Dotz Nano pilot trial image

The production pilot was performed together with Kecai Printing Company (a subsidiary of Brilliant Circle Holding International Limited, the industry leader in China's cigarette packaging industry), at their top-tier Shenzhen facilities, and its results were deemed as a success.

Green Science Alliance launches graphene quantum dots inkjet-suitable ink

Green Science Alliance, part of the Fuji Pigment corporation, has created graphene quantum dot inkjet ink. The prepared graphene quantum dot inkjet ink can be printed on various types of substrates including regular paper and films.

The ink is invisible under normal room light and becomes visible when lit by specific light types. In addition, emission spectra peak will be different depending on the different wavelength of illuminated light, which can be useful for anti-counterfeiting applications.

XFNANO: Graphene and graphene-like materials since 2009 XFNANO: Graphene and graphene-like materials since 2009