Fluorescence “Giant” Red Edge Effect
3.1 Introduction
The excellent physical and chemical properties of carbogenic nanosystems (fuller- enes, carbon nanotubes, carbon quantum dots, graphene sheets, and nanodiamonds) have inspired researchers to conduct extensive studies with such materials. The expected outcome possesses great potential for a wide variety of applications.
Carbogenic nanosystems have been extensively studied since the discovery of carbon nanotubes (CNTs). Despite the fact that CNTs have enormous potential in electronic applications, large-scale synthesis of such materials has been difficult.
Existing methods are time-consuming, involve high production costs, and produce insufficient yields. CNTs also suffer from dispersion issues. The material is difficult to functionalize and hence shows limited dispersion abilities. Only a few solvents have been reported to be used as dispersion medium but they are mostly high boil- ing solvents. CNTs also suffer from issues related to impurities or the presence of adsorbed solvents. Researchers who use sonication for CNT dispersion in a given medium, report itching of amorphous carbon from the material and hence conduct- ing properties are altered.
3.1.1 Carbon quantum dots
Carbon quantum dots (CQDs) may emerge as a cost-effective alternative to CNTs due to their ease of synthesis and high yield. CQDs have recently gained popularity as a potential substitute to CNTs for feasible applications. CQDs, which are entirely composed of carbon atoms, are abundant on the earth and are not harmful to the environment or human health. CQDs have various properties such as optical fluo- rescence and electrical conductance. These QDs are also used in combination with oxides such as TiO2, SiO2, Cu2O, and ZnO for catalysis via electrochemical reac- tion or energy transfer. CQDs primarily exhibit properties based on relative hybrid- ization volume as a combination of sp2 and sp3 hybridizations. Because of the abundance of trap states, the QDs could be used effectively as charge carrier pock- ets or quantum wells.
Carbon Quantum Dots for Sustainable Energy and Optoelectronics. DOI:https://doi.org/10.1016/B978-0-323-90895-5.00014-X
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Extensive efforts have been made for the development of nontoxic or less toxic and water dispersible fluorescent QDs as alternatives to the semiconductor-based QDs. Carbon-based nanostructures (e.g., CNTs, fullerenes, etc.) including CQDs and graphene nanosheet of less than 100 nm in size, which are known as graphene quantum dots (GQDs) have already been developed. Various methods have been demonstrated in the preparation of fluorescent CQDs. Electrochemical oxidation processes, chemical oxidation methods, hydrothermal methods, and carbonizing organic molecules are some important methods for the preparation of CQDs.
Carbon-based QDs derived from adaptable resources such as graphene materials, graphite, as well as from various carbon resources, such as banana juice, citric acid, carbon soot, glucose, egg material, and so on, exhibit high quantum yield (QY) depending on their synthesis procedure and surface passivation nature. Most of the developed procedures are unsuitable because of high cost of the equipment required, low yield, and the complex procedure. Most of the obtained CQDs have relatively low QY in comparison to conventional semiconductor-based QDs.
Nowadays efforts are made to increase fluorescence QYs of CQDs by passivating them with large organic molecules and polymers. Doping them with various het- eroatoms also has drawn attention due to their tunable properties.
3.1.1.1 Structure of carbon quantum dots
Though there is considerable literature on CQDs, the exact structure of these fasci- nating materials is still unknown.
Until now, graphene and graphene oxide QDs have been reported to be synthe- sized from graphite nanoparticles through conventional Hammer’s method and few of its modifications as shown in Fig. 3.1. Fig. 3.1 shows the synthesis process schematically.
From the figure it is clear that the final structure of the graphene oxide nanoma- terial was easy to predict and it is mainly composed of arrays of sp2carbons with few defects as shown in the figure. The issue with CQDs is that the final structure
Graphite nanoparticles
Graphene oxide quantum dot Graphene quantum dot Exfoliation in
organic solvent
Exfoliation by Hummers method
Figure 3.1 A schematic route showing synthesis of graphene oxide quantum dots and graphene quantum dots from graphite nanoparticles by exfoliation.
of the material is difficult to assign as the source of the materials (mostly organic materials like citric acid, etc.) do not retain any identity. Thus, a hypothetical struc- ture of CQDs has been proposed inFig. 3.2.
It is assumed that CQDs might consist of various allotropic forms of carbon where either one may be prevalent. The structure may have all three different hybridization areas (sp, sp2, and sp3) or a diamond-like crystal structure as shown inFig. 3.2. Some researchers have also predicted that CQDs are composed of dis- crete sp2 islands separated or dispersed over sp3 medium. From the application point of view, it is reasonable to state that the occurrence of more and more sp2 islands would be preferable as it will lower the transition energy and impose better- conducting properties and tunable emission in the near infrared region. Thus the challenge for chemists is to synthesize CQDs with more sp2domains or more con- tinuous sp2domains.
3.1.1.2 Principles of synthesis
To synthesize fluorescent CQDs, a range of different methods have been estab- lished. Some simple but ineffective preparative strategies have also been created, such as:
G Directly oxidizing candle soot into fluorescent CQDs.
G Electrochemical change of CNTs into highly fluorescent nanocrystals of carbon.
G Breaking of nonluminescent graphene sheets into blue fluorescent GQDs hydrothermally.
G Incomplete thermal degradation of presynthesized citrate salts into fluorescent CQDs.
Figure 3.2 Schematic representation of the structure of CQDs.CQDs, Carbon quantum dots.
Courtesy:w.w.w.
Multistep techniques are time-consuming and complicated, but they are very much effective at producing strong green luminescent carbon materials.