4946 | New J. Chem., 2014, 38, 4946--4951 This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014 Cite this: New J. Chem., 2014, 38, 4946 Plant leaf-derived graphene quantum dots and applications for white LEDs† Prathik Roy, a Arun Prakash Periasamy, a Chiashain Chuang, b Yi-Rou Liou, b Yang-Fang Chen, b Joseph Joly, c Chi-Te Liang* b and Huan-Tsung Chang* a Graphene quantum dots (GQDs) have been prepared for the first time using raw plant leaf extracts of Neem (Azadirachta indica) and Fenugreek (Trigonella foenum-graecum) by a facile, hydrothermal method at 300 1C for 8 hours in water, without the need of any passivizing, reducing agents or organic solvents. High resolution transmission electron microscope studies showed that the average sizes of the GQDs from Neem (N-GQDs) and Fenugreek (F-GQDs) were 5 and 7 nm respectively. N-GQDs and F-GQDs exhibit high quantum yields of 41.2% and 38.9% respectively. Moreover, the GQDs were utilized to prepare a white light converting cap based on the red-green-blue (RGB) color mixing method. Introduction Pristine graphene, prepared by mechanical exfoliation, 1 is the wonder material of the decade. With its exceptional electronic, thermal and mechanical properties, 2–5 graphene continues to attract world-wide interest. However, being a zero-bandgap material, graphene has a few shortcomings. For instance, it is non-luminescent. Furthermore, it has a poor light absorbance of only 2.3%. 2–5 Therefore, for graphene-based optical applications, photoluminescent graphene quantum dots (GQDs) have emerged as an alternative to graphene with many interesting phenomena that might not be possible in its native pristine form. 6 GQDs possess outstanding characteristics such as high surface area and better surface grafting through their p–p conjugated network or surface groups. 6–10 Given the present intense interest in white light- emitting diode (LED) technology, it is very useful if GQD-based materials can find applications in white-light conversion. 11 As a result of their attractive properties, it is highly desirable to synthesize GQDs through various methods such as microwave- assisted hydrothermal methods, 12 electrochemical methods 13 such as cyclic voltammetry (CV), 14 electron beam lithography 15 and ruthenium-catalyzed C60 transformation. 16 GQDs have also been synthesized from a variety of sources such as multi-walled carbon nanotubes (MWCNTs), 17 various forms of graphite, 14,18–20 glucose, 19 carbon fibers, 21 and coal. 22 Ideally, the synthesis of GQDs should not affect the environment as often harsh conditions with strong acids have been used. 23 Moreover, the use of expensive materials and instruments such as MWCNTs and ruthenium may limit its real world applications. Further- more, although electrochemical techniques can be used to synthesize GQDs, the reported quantum yield (QY) is generally low. All these interesting aforementioned results motivated us to investigate whether one can prepare GQDs from an inexpensive, natural, sustainable and renewable source such as green plants, which are the basis of most of the Earth’s ecologies. Natural carbonaceous materials have attracted increasing attention around the world due to their unique morphologies, physical and chemical properties, and excellent applications. Although various carbon species such as carbon fibers, 21 coal, 22 carbon black, 24 candle soot, 25 etc. have been used as a carbon feedstock to prepare GQDs in an elegant manner, these carbon precursors are all related to fossil fuels which are a non-renewable source and may not be sufficiently available in the future. It is therefore imperative to explore more viable carbon sources from natural, sustainable and renewable resources. Carbon nanotubes have recently been synthesized from natural precursors such as camphor, 26 turpentine, 27 eucalyptus, 28 and palm. 29 More recently, Neem oil has been used to prepare aligned CNT bundles. 30 Neem extracts are primarily comprised of hydrocarbons containing low amounts of oxygen, which appears to be an ideal precursor species for synthesizing GQDs since its extracts largely contain hydrocarbons with a low percentage of oxygen (see Fig. S1a, ESI†). 30 Similarly, Fenugreek extracts are also highly carbonaceous with large amounts of hydrocarbons and a Department of Chemistry, National Taiwan University, 1, Section 4, Roosevelt Road, Taipei 106, Taiwan. E-mail: changht@ntu.edu.tw; Fax: +886 2 33661171; Tel: +886 2 33661171 b Department of Physics, National Taiwan University, 1, Section 4, Roosevelt Road, Taipei 106, Taiwan. E-mail: ctliang@phys.ntu.edu.tw; Fax: +886 2 23639984; Tel: +886 2 33665129 c Department of Nanotechnology, Noorul Islam University, Kumaracoil 629180, Tamilnadu, India † Electronic supplementary information (ESI) available: Figures for EDAX, XRD, UV-Vis for Neem and Fenugreek GQDs, photographs for N-GQD/QS/CPY coating, PL spectrum of the uncoated PET cap, photostability of the N-GQD–QS–CPY and table illustrating the lifetimes of GQDs and N-GQD–QS–CPY. See DOI: 10.1039/ c4nj01185f Received (in Montpellier, France) 17th July 2014, Accepted 25th July 2014 DOI: 10.1039/c4nj01185f www.rsc.org/njc NJC PAPER Published on 11 August 2014. Downloaded by National Taiwan University on 16/09/2014 03:05:59. View Article Online View Journal | View Issue