Current status and prospects on chemical structure driven photoluminescence behaviour of carbon dots

https://doi.org/10.1016/j.jphotochemrev.2018.08.001Get rights and content

Highlights

  • Chemical structure controls the luminescence properties of C-dots

  • Graphitic conjugated exhibit size dependent photoluminescence because of quantum confinement effect.

  • Excitation dependent emission of C-dots is due to surface trap states

  • Hetero atom doping tune the emission band of C-dots

  • C-dots are used for optoelectronics, bio-imaging, catalysis, and sensing, depending on the nature of C-dots.

Abstract

Fluorescent carbon dots (C-dots) are new class of nanomaterials with widespread applications in optoelectronics, bio-imaging, catalysis, and sensing. The origin of photoluminescence of carbon dots is a debatable issue which is pretend to depend on the chemical structures such as graphitic conjugated core, molecular fluorophores and the surface defect states found to be dependent on the method of preparation. In this review, we have illustrated the important issues and challenges of the luminescent carbon dots and their potential applications. Graphitic conjugated core containing carbon dots is being synthesized usually from bulk materials like graphite, graphene and graphene oxide which exhibit size dependent photoluminescence behaviour due to quantum confinement. On the other hand, carbon dots synthesized from small molecules exhibit excitation dependent emission due to the presence of surface energy trap states which can be tuned by surface modification. Again, presence of both conjugated core and surface defect generates dual emission property. It is evident that various molecular fluorophores are produced inside carbon dots during low temperature synthesis. Hetero-atom doping is another strategy to tune the photoluminescence properties of carbon dots. Red emitting carbon dots are found to be suitable for bio-imaging applications after surface modification. Again, high quantum yield and solar light absorbing carbon dots are required for light harvesting and optoelectronic applications. Surface modified carbon dots are found to be appropriate for sensing applications. Analysis reveals that carbon dots based hybrid systems provide good applicability towards construction of solar cell devices because of their efficient charge separation.

Introduction

Fluorescent carbon nanomaterials, a new emerging area of research continues to garner scientific attention towards fundamental understanding and development of technologies in several areas of photonics and bio-photonics applications. [[1], [2], [3], [4], [5], [6]] Till date, varieties of fluorescent carbon nanomaterials have been synthesized, namely nanodiamond, fluorescent spherical fullerenes, fluorescent carbon nanotube, fluorescent graphene oxide and carbon dots (carbon nanodots and graphene quantum dots) [2]. Among all these nanomaterials, fluorescent carbon nanoparticles grasp the focus of the researchers due to interesting tunable properties and versatile applicability [[7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]]. Fluorescence properties of carbon nanoparticles were first observed by Scrivens and co-workers during the electrophoretic purification of single walled carbon nanotube by arc-discharge method [19]. The name “carbon dots” was first introduced by Sun et al. [20], which is a general terminology of all the carbon based fluorescent nanoparticles. The origin of photoluminescence of carbon dots is still a debatable issue which is dependent on various parameters such as synthesis procedure, starting precursors, temperature, dopant, surface defects and surface modification [21,22]. Scrivens et al. have demonstrated that the optical properties of carbon dots depend on the particle size whereas Sun et al. have reported that photophysical properties depend on surface nature of carbon dots [19,20]. Studies suggest that the photoluminescence of carbon dots are generally excitation dependent in nature whereas few studies reported the excitation independent emission of the particles. Wei et al. reported size independent tunable photoluminescence of carbon dots whereas Teng’s group showed the size dependent excitation-wavelength-independent photoluminescence [23,24]. Several explanations have been given to explain this complicated optical nature of carbon dots [22,25] which are mainly classified into three category: i) graphitic core driven optical property where quantum confinement effect takes the leading role for photoluminescence, ii) surface mediated optical property depicts the photoluminescence of carbon dots which appears from the surface defect states as well as the synergistic effect of these two factors (i.e. quantum confinement and surface effect) and iii) molecular fluorescence arises due to presence of different π-conjugated islands inside the carbon dots. Apart from the above three approaches, doping is also another strategy for controlling the photoluminescence behaviour. In this review, we discuss all the important issues to unveil the origin of optical properties of carbon dots and how they are carefully controlled. Finally, we highlight the potential applications of carbon dots.

Section snippets

Graphitic core driven optical property

Carbon dot having π-conjugated graphitic core exhibits size dependent photoluminescence property and the photoluminescence of carbon dots arises from π-conjugated graphitic core due to confinement of exciton. Scrivens and co-workers have found that yellow emitting carbon nanoparticles (diameter 18.0 ± 0.4 nm) exhibit red shifting of emission band with increasing the particles size, [19] which is resembled with the photoluminescence nature of quantum dots. Ajayan et al. have synthesized three

Bio-applications

Owing to the tunable optical properties, chemical inertness, biocompatibility, large surface area and easy surface functionality of carbon dots make them potential material for bio-imaging, bio-sensing, bio-medicine and drug delivery [[62], [63], [64]]. Two basic criteria are found to be fulfilled by carbon dots, essential for clinical uses are: they rapidly excreted from the body and exhibit low toxicity while producing reliable optical signal [65]. First, in-vivo and in-vitro bio-imaging with

Conclusions and outlook

The mechanism of photoluminescence depends on the nature precursor, amount of precursors, reaction medium, temperature and time of reaction etc. Cutting down of large conjugated moieties to few nanometre particle exhibits photoluminescence behaviour due to confinement of electron-hole. Surface of the carbon dots contains different functional groups which influences optical behaviour to the carbon dots. Presence of different surface functionalities incorporate defects on the nanoparticles that

Acknowledgements

DAE-SRC Outstanding Investigator Award and TRC (DST) are gratefully acknowledged for financial support. MKB thanks IACS for awarding fellowships and support.

Monoj Kumar Barman obtained his bachelor degree in chemistry from Midnapore College, Vidyasagar University in 2010 and his M. Sc in chemistry IIT Guwahati in 2012. He completed his Ph.D in 2017 under the supervision of Prof. Amitava Patra in the Department of Materials Science at the Indian Association for the Cultivation of Science (Kolkata, India). His research interest includes synthesis and spectroscopic investigation of carbon dots based hybrid systems.

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    Monoj Kumar Barman obtained his bachelor degree in chemistry from Midnapore College, Vidyasagar University in 2010 and his M. Sc in chemistry IIT Guwahati in 2012. He completed his Ph.D in 2017 under the supervision of Prof. Amitava Patra in the Department of Materials Science at the Indian Association for the Cultivation of Science (Kolkata, India). His research interest includes synthesis and spectroscopic investigation of carbon dots based hybrid systems.

    Amitava Patra, senior professor and Head of the Department of Materials Science at Indian Association for the Cultivation of Science (Kolkata, India). He is a fellow of the Indian Academy of Science (FASc), the National Academy of Science (FNASc), India, and the Royal Society of Chemistry.He is a recipient of the DAE-SRC Outstanding Investigator Award, A. V. Rama Rao Foundation Prize in Chemistry, the C. N. R. Rao National Prize for Chemical Research, Asia NANO 2010 Award, CRSI Bronze Medal, MRSI Medal, and Ramanujan Fellowship. He was an advisory board member of Nanoscale, and the Journal of Physical Chemistry. He is an author or co-author of more than 203 scientific papers, five book chapters, and 2 Indian patents. His research interests include excited state dynamics, photo-induced energy transfer, and charge transfer of QD, Au nanoparticles, Carbon dots, polymer- and porphyrin based light harvesting antenna materials, and up and down converted doped nanomaterials.

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