Promising the exceptional properties of quantum dots but without the toxicity, graphene quantum dots could be the next big thing.
Carbon and graphene quantum dots (CDs, GQDs) represent relatively new members of the carbon nanomaterials family. Graphene is a ground-breaking two-dimensional (2D) material that possesses extraordinary electrical and mechanical properties that promise a new generation of innovative devices in flexible displays, transistors, photosensors, RFID tags, solar cells, secondary batteries, fuel cells, supercapacitors, conductive inks, EMI shielding heat insulation, anti-oxidation and LEDs. Studies have demonstrated that quantum confinement could appear in graphene with finite size and edge effects-graphene quantum dots (GQDs).1
Properties
GQDs are promising materials as substitutes for Cd, Ir, Ga, S, Se and P quantum dots (QDs) and possess unique structural and photophysical properties. Theoretical and demonstrated properties include high quantum yield, high electrical conductivity, high thermal conductivity excellent photostability, biocompatibility, highly tunable photoluminescence (PL) property, exceptional multi-photon excitation (up-conversion) property and non-toxicity.
Synthesis
There are two main strategies for synthesizing GQDs:
• 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.
• Bottom-up method: Low molecular weight compounds are polymerized to nm-size to obtain GQDs.
Specific approaches for preparation include:
• molecular assembly of carbon ring structures
• chemical exfoliation of graphite nanofibers2
• chemical synthesis
• modification of graphite nanoparticles
• electron beam lithography3
• pulsed laser synthesis
• microwave pyrolysis 4 5
• hydrothermal graphene oxide reduction, resulting in the fracture of GO sheets into ultra-small pieces. 6 7 8 9
The bottom-up approach only produces a small quantity of GQDs, limiting it’s use. The top-down method potentially offers low-cost and high yield and is the main focus of commercial production.
There are a number of technical hurdles that need to be overcome for successful production with low synthesis yields at present due to aggregation 10 11 that limit industrial scale-up. Also the synthesized GQDs currently display low quantum yield, poor control of the emission wavelength and optical instability. 12 Researchers have recently turned their attentions to developing new methods for synthesis.
Applications
Applications of graphene QDs include catalysis, 13 14
bioimaging (e.g. membrane markers), 15 16 optoelectronics (LEDs), 17 18 19 printing, 20 photodetectors, 21 quantum computing and energy conversion devices. 22 These are also the main market for quantum dots. The advantage of GQDs is that demonstrate similar 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.
Producers
ACS Materials, LLC, USA
www.acsmaterial.com
The company is a graphene producer. They also produce a range of graphene QDs.
KRI, Inc., Japan
http://www.kri-inc.jp
The company produces graphene quantum dots and has developed proprietary synthesis methods.
Shanghai Simbatt Energy Technology Co., Ltd., China
http://www.simbatt.com.cn
The company produces GQD powder and in solution.
References
1. L Li, X Yan, The Journal of Physical Chemistry Letters, 1(17), 2572 (2010).
2. S Schnez, F Molitar, C Stampfer, et al., Applied Physics Letters, 94(1), (2009)
3. H Zhu, X Wang, Y LI, Z Wang, F Yang, X Yang, Chem. Commun, no. 34, 5118 (2009).
4. X Wang, K Qu, B Xu, J Ren, X Qu, Journal of Materials Chemistry, 21(8), 2445 (2011).
5. Libin Tang, Rongbin Ji, et al., ACS Nano, 6(6), 5102 (2012).
6. D Pan, J Zhang, Z Li, M Wu, Advanced Materials, 22(6), 734 (2010).
7. D Pan, L Guo, J Zhang, C Xi, Q Xue, H Huang, et al., Journal of Materials Chemistry
8. J Lu, P S E Yeo, C K Gan, et al., Nature nanotechnology, 6(4), 247 (2011)
9. http://pubs.rsc.org/en/content/articlelanding/2015/nr/c5nr00814j#!divAbstract
10. Zhu, S. J. et al. Strongly Green-Photoluminescent Graphene Quantum Dots for Bioimaging Applications. Chem. Commun. 47, 6858–6860 (2011).
11. Pan, D. Y., Zhang, J. C., Li, Z. & Wu, M. H. Hydrothermal Route for Cutting Graphene Sheets into Blue-Luminescent Graphene Quantum Dots. Adv. Mater. 22, 734–738 (2010).
12. Lin, L. X. & Zhang, S. W. Creating High Yield Water Soluble Luminescent Graphene Quantum Dots Via Exfoliating and Disintegrating Carbon Nanotubes and Graphite Flakes. Chem. Commun. 48, 10177–10179 (2012).
13. Zhuo S, Shao M and Lee S T 2012 ACS Nano 6 1059
14. Li Y, Zhao Y, Cheng H, Hu Y, Shi G, Dai L and Qu L 2012 J. Am. Chem. Soc. 134 15
15. Zhu S, Zhang J, Tang S, Qiao C, Wang L, Wang H, Liu X, Li B, Li Y and Yu W 2012 Adv. Funct. Mater. 22 4732
16. S. Nandi, R. Malishev, K. Parambath Kootery, Y. Mirsky, S. Kolusheva, R. Jelinek, Chem. Commun. 2014, 50, 10299 – 10302.
17. Luk C, Tang L, Zhang W, Yu S, Teng K and Lau S 2012 J. Mater. Chem. 22 22378
18. http://www.nature.com/articles/srep11032
19. Kwon, W. et al. Electroluminescence from graphene quantum dots prepared by amidative cutting of tatterd graphite. Nano Lett.14, 1306–1311 (2014).
20. J. Wang, C.-F. Wang, S. Chen, Angew. Chem. Int. Ed. 2012, 51, 9297 – 9301; Angew. Chem. 2012, 124, 9431 – 9435.
21. http://www.nature.com/articles/srep05603
22. http://www.sciencedaily.com/releases/2015/06/150614225649.htm
