Graphene Quantum Dots: Properties, Synthesis & Applications

Introduction to Carbon and Graphene Quantum Dots

Colloidal semiconductor quantum dots (QDs) have numerous potential applications in solar cells, light emitting diodes, bioimaging, electronic displays, and other optoelectronic devices due to their unique size-dependent electro-optical properties, and have thus been of significant research interest.

However, due to the high market cost of inorganic QDs, on the order of thousands of US dollars per gram, their industrial use has been slow and limited. In addition, application development has been hindered by the high toxicity of inorganic QDs. As a promising cost-effective alternative, carbon quantum dots (CDs, CQDs or C-dots) and graphene quantum dots (GQDs) have recently emerged as a new class of QD materials. CDs and GQDs have advantages of nontoxicity, good solubility, stable photoluminescence, and better surface grafting, thus making them promising candidates for replacing inorganic QDs. Moreover, the recent discovery of a one-step multigram synthesis of GQDs from coal and other carbon sources opens the possibility of their large-scale industrial production.

Synthesis of Graphene Quantum Dots

Previous methods of GQD synthesis involved high-cost raw materials such as graphene¹ or photonic crystals² and fairly low-yield and expensive methods such as laser ablation,³ electron beam lithography,⁴ or electrochemical synthesis.⁵ These factors made GQDs virtually unavailable for commercial applications. More recent research, reports the preparation of GQD from fairly inexpensive organic sources such as citric acid/urea⁶ that offers product cost reduction and availability on a larger scale. However, the synthesis of GQDs from coal⁷ (the least expensive material known) increases the possibility of the use of GQDs in future commercial products. Due to their low production cost, coal-derived GQDs are feasible for large-scale industrial applications and might be successfully used as a cost-effective and eco-friendly alternative to conventional inorganic quantum dots.

In a typical patented process, coal is stirred in concentrated nitric acid and heated at 100^o-120 ^oC for few hours. The solution is cooled, and the nitric acid is evaporated and reused. The GQDs are then filtered using cross-flow ultra-filtration. After purification, the solution is concentrated using rotary evaporation to obtain solid GQDs.

Characterization of GQDs

A variety of high quality GQDs can be produced by controlling the manufacturing process parameters such as raw materials, temperature, and reaction time.

Figure 1 shows representative optical and TEM images of blue luminescent GQDs (Product No. 900708). These images show that the GQDs forms a translucent and stable suspension in water, and typically exhibit disk-shaped structures with a diameter of <5 nm with topographic height of 1–2.0 nm.

Figure 1. Representative optical and TEM images of blue luminescent GQDs. (a) Optical image of 1 Liter of concentrated GQDs suspension. (b) Optical image of diluted GQDs suspension under visible (left) and 365nm UV light (right). (c) Typical TEM image of GQDs. Inset: HR-TEM image of GQD.

Typical photoluminescent (PL) and UV-VIS properties of GQDs (Figure 2) and the PL properties of GQDs offered in our catalog (Table 1).

Figure 2. UV-VIS properties of GQDs. (a) Excitation and emission contour map of GQDs. (b) Photoluminescence emission of GQDs excited at 350nm. (c) Absorption spectra of GQDs.

Table 1. Photoluminescent properties of GQDs

	Violet	Blue (900708, 900726)	Cyan (900707)	Aqua Green (900712, 900713)
Quantum Yield (%)	>70	>70	>30	>35
Maximum excitation (nm)	320	350	420	485
Maximum emission (nm)	404	445	490	525

Applications of GQDs

In contrast to classic QDs, GQDs are biocompatible, photo-stable, with enhanced surface grafting, and inherit superior thermal, electrical, and mechanical properties from graphene. These features can greatly contribute to various state-of-the-art applications including:

• Taggants for security/anti-counterfeiting/brand protection applications⁸
• Bioimaging markers⁹
• Fluorescent polymers¹⁰
• Antibacterial,¹¹ Antibiofouling¹² and Disinfection systems.¹³
  
• Heavy Metals,¹⁴ Humidity and Pressure sensors¹⁵
• Batteries¹⁶
• Flash memory devices¹⁷
• Photovoltaic devices¹⁸
• Light-emitting diodes¹⁹

Summary

Due to the limited availability of GQDs, applications involving them are still being developed and to this end the synthesis of GQDs from coal appears promising as it allows the production of high quality material at a larger scale. The availability of high-quality GQDs in larger quantities to the scientific community will help drive more in-depth studies of the unique properties, as well as accelerate the development of new applications.

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