Graphene quantum dots (GQDs) represent relatively new members of the carbon nanomaterials family. Studies have demonstrated that quantum confinement could appear in graphene with finite size and edge effects-graphene quantum dots (GQDs). GQDs display properties derived from both graphene and quantum dots (QDs), combining the structure of graphene with the edge effects, non-zero band gap, and quantum confinement effects of QDs. They possess unique optical and electrical properties such as:
- strong photoluminescence
- biocompatibility
- exhibit band gap (unlike graphene sheets).
Composition
Graphene quantum dots (GQDs) are defined as graphene sheets with lateral dimensions less than 100 nm in single-, double- and few- (less than 10) layers, that have quantum confinement effect and edge effects. Unlike graphene sheets, they exhibit band gap that is responsible for their unique electrical and optoelectronic properties. An ideal GQD consists of only one atomic layer of carbon atoms. However, most of the synthesized GQDs also contain functional groups like oxygen and hydrogen, and usually have multiple atomic layers with sizes less than 10 nm.
Comparison to quantum dots
GQDs are promising materials as substitutes for Cd, Ir, Ga, S, Se and P quantum dots (QDs) and possess unique structural and photophysical properties. The advantage of GQDs is that demonstrate comparable properties to cadmium-based QDs but without the potentially hazardous health and environmental effects, and at lower cost. In the coming years, they could be utilized as inexpensive and eco-friendly alternatives to QDs for opto-electronic devices such as displays and lighting devices
Compared with QDs, GQDs are potentially superior due to their high photostability against photobleaching and blinking, biocompatibility, and low toxicity.
The shape and reduced size of GQDs offer substantial advantages over graphene in terms of edge sites and biomedical applications. GQDs strong photoluminescence can be tailored for specific applications by controlling their size, shape, defects, and functionality.
Figure 1: Graphene Quantum Dots from DotzNano.
Properties
Theoretical and demonstrated properties include:
- high quantum yield
- high electrical conductivity
- high thermal conductivity
- excellent photostability
- biocompatibility
- superior stability compared to non-carbon QDs.
- highly tunable photoluminescence (PL)
- electrochemiluminescence
- exceptional multi-photon excitation (up-conversion) property
- ease of functionalization
- low-toxicity.
GQDs are interesting in the biomedical field as they are water-soluble, they can be decorated exploiting different chemical approaches and they possess intrinsic characteristics of fluorescence.
Synthesis
There are two main strategies for synthesizing GQDs, the
Top-down method and bottom-up method. In the top-down method, Graphene oxide is manufactured then the graphene sheet is cut through controlled oxidation or the reduction process to the desired size to fabricate GQDs. Specific approaches for preparation include:
- molecular assembly of carbon ring structures.
- chemical exfoliation of graphite nanofibers
- chemical synthesis
- modification of graphite nanoparticles
- electron beam lithography.
- pulsed laser synthesis
- microwave pyrolysis
- organic synthesis using polyphenylene dendritic precursors.
- strong acid assisted cleavage of graphite nanomaterials (nanographite, carbon fibers).
- hydrothermal graphene oxide reduction, resulting in the fracture of GO sheets into ultra-small pieces.
Although the top–down strategy is highly suitable for mass production because of the abundant precursor materials and simple operation, the non-selective chemical cutting leads to poor control over the size and morphology of the ultimate product.
The bottom–up method is based on the gradual growth of small precursor molecules (cyclic molecules, polymers) into nanosized GQDs by carbonization, pyrolysis, chemical vapor deposition, etc., offering high controllability and fewer defects. Poor solubility and aggregation of the resultant product is the main limitation. The bottom-up approach only produces a small quantity of GQDs, limiting its use. The top-down method potentially offers low-cost and high yield and is the main focus of commercial production. There are several technical hurdles that need to be overcome for successful production with low synthesis yields at present due to aggregation that limit industrial scale-up. Also the synthesized GQDs currently display low quantum yield, poor control of the emission wavelength and optical instability. Researchers have recently turned their attentions to developing new methods for synthesis.
Applications
Main markets for GQDs are electronics and photonics, energy storage and conversion, sensors, biomedicine and anti-counterfeiting. Applications include:
Electronics and photonics
- Substitute rare-earth phosphors in white LEDs.
- Broadband photodetectors.
- CMOS-based sensors for ultraviolet, visible and infrared.
- Solar cells.
- Optical gain materials in wide-gamut laser displays and projectors.
Energy storage and conversion
- Composite or coating material for energy storage devices.
- Electrode materials for supercapacitors.
- Flexible supercapacitor devices.
- Lithium-ion batteries.
- A sensitizer material in different types of solar cells to achieve better PCE.
- Catalyst in processes like photocatalytic hydrogen evolution and CO2 reduction, electrocatalytic oxygen reduction, water splitting and CO2 reduction, as well as photoelectron catalysis.
Sensors
- Field-effect transistors.
- Electrochemical sensors.
- Photoluminescence sensors.
- Glucose sensors.
- Gas sensors.
- Humidity sensors.
- PL sensor.
- ECL sensor.
- Metal ion sensing.
Biomedicine and life sciences
- Bioimaging: optical [fluorescence (FL)], two-photon FL, magnetic resonance imaging (MRI), and dual-modal imaging.
- Drug delivery: water solubility, lower cytotoxicity and large specific surface area, which makes them effective drug molecular loading cores.
- Biosensors.
- Magnetic hyperthermia.
- Photothermal therapy.
- Antibacterial coatings, self-sterile textiles, and personal care products.
Anti-counterfeiting
- Fluorophores for ultra-sensitive, multicolour, and multiplexing applications in anti-counterfeiting label and security identification such as passports and ID cards.
- Can be applied on or in products with inks, polymers, resins, solvents and etc. and then applied on the products with various printing techniques. QD-based inks may be applied via conventional screen, flexography, offset, gravure, and ink jet printing processes while the paints are designed to be sprayed onto any surface.
- Liquid tagging: functional fluids that are being used in vehicles. Liquid products such as oil, fuel, adblue or chemicals can be authenticated and tracked in real time.
• Solid tagging: Tags can be incorporated into polymer and plastics parts during the production process.
Pricing
GQDs are currently commercially available, with materials selling for up to $2.5 million/kg, although recent producers have manufactured materials for less. Competing materials such as high quality, non-toxic Cadmium Quantum Dots selling for up to $8 million/kg.
Companies
Dotz Nano Ltd.
Australia
https://www.dotz.tech/
Dotz Nano produces and sells nanomaterial Graphene Quantum Dots, used in imaging and IP protection. The company produces ValiDotz polymer security markers, BioDotz, Fluorensic and InSpec GQD solutions. Dotz’ tracing solution utilises a two-factor authenticator, combining ValiDotz security taggants with geo-specified and encrypted QR codes. “Secured by Dotz” marking units are used for authentication of face masks.
Green Science Alliance Co., Ltd.
Japan
www.gsalliance.co.jp/en/
https://en.green-science-lab.com/research/247
The company is a subsidiary of Fuji Pigment. Fuji Pigment Co., Ltd. was founded in 1944. The Company’s line of business includes the manufacturing of cyclic organic crudes, intermediates, organic dyes, and pigments. The company produces graphene quantum dot inkjet ink. Prepared graphene quantum dot inkjet ink can be printed on various types of substrates including normal paper and films. It is invisible under normal room light although it becomes visible and emission spectra peak will be different depending on the different wavelength of illuminated light so that they can be good anti-counterfeiting inkjet ink because normal fluorescent dye based anti-counterfeiting ink emits only certain spectrum with same peaks no matter what wavelength of light is illuminated.
Quantag Nanotechnologies
Turkey
http://quantagnano.com
The company offers tagging solutions (Quantum Tagging Technology) based on graphene quantum dots for real-time detection with ultra-sensitive sensors. The optical sensor system allows for real-time detection and is compact. The fiber optic based sensors illuminate the digital tags, receive required information, process them instantly and can transfer them to a remote location.
Figure 2: Quantag GQDs and sensor.
