Heteroatom-Doping for Carbon Dots: An Efficient Strategy to Improve Their Optoelectronic Properties

Carbon Dots (CDs) are an emerging class of nanomaterials discovered as by-product in 2004 by Scrivens and coworkers. 1 In recent years, they have been shown to be useful for broad applications including fluorescent probes and sensors, biolabeling and medical imaging, LED color display and optoelectronic devices. 2 , 3,4 In contrast to traditional dye molecules and inorganic semiconductor quantum dots (QDs), CDs posses several advantages such as low-cost synthesis process, biocompatibility, high photostability, and excellent fluorescent properties. 5 Despite these fascinating properties, low quantum yields (QYs) and poor solubility severely hinder their widespread applications. 6 Recent studies have been proved that chemical heteroatoms doping is an efficient route to improve the QY, water solubility, fluorescent properties, and other physicochemical properties of CDs and thus expand their application scope. 7 It aims to cause a significant change in electronic structures of the materials which bring to a change in their optical and electrical properties and makes them suitable for potential devices. 8 It is believed that introducing non-metal atomic impurities and metal ions into CDs would affect the interaction between π- and n-states in CDs by the extent of orbital overlap and electron withdrawing/donating abilities of heteroatoms. 9 Among various non-metallic dopants, generally nitrogen (N) is the hetero atom more explored because it is sufficiently electron-rich than carbon, which provides n-type doping characteristics. Sulfur (S), Phosphorous (P) and Boron (B) are also used for the same reason. 10 Compared to most non-metallic heteroatoms, there are more electrons easy to lose and unoccupied orbitals outside of most metal ions (especially transition metal ions) and metal ions provide larger atomic radius than non-metallic ions. 11 Nevertheless, whether the heteroatoms are doped in the rigid core or just in the functional groups on the surface of the CDs is still unclear. Furthermore the exact mechanism of fluorescence emission and its shifts upon different excitations in CDs structures endowing by doping are still under investigation. 5 Therefore, in order to explore and enhance the doped C-Dot photosensitizing capabilities, we analysed a simple route to prepare highly fluorescent CDs by hydrothermal reactions of citric acid (CA) as carbon source and urea as N source. Among the dopants, different precursors both metal and non-metal were selected. Since CDs can be tuned by varying solvents during the synthesis, the experiments were carried out in different reaction mediums by using solvents with high and low polarity. This work would provide a platform to better understand the intricate unclear details of heteroatom-doped CDs and the influence of solvents, precursors, dopants and the surface states on their optical and other properties that is crucial and instructive to improve their performance practical applications in the future. References 1. De Medeiros, T. V. et al. Microwave-assisted synthesis of carbon dots and their applications. Journal of Materials Chemistry C 7 , 7175–7195 (2019). 2. Wang, H. et al. Excitation wavelength independent visible color emission of carbon dots. Nanoscale 9 , 1909–1915 (2017). 3. Zhou, Y. et al. Colloidal carbon dots based highly stable luminescent solar concentrators. Nano Energy 44 , 378–387 (2018). 4. Benetti, D. et al. Hole-extraction and photostability enhancement in highly efficient inverted perovskite solar cells through carbon dot-based hybrid material. Nano Energy 62 , 781–790 (2019). 5. Atabaev, T. S. Doped carbon dots for sensing and bioimaging applications: A minireview. Nanomaterials 8 , (2018). 6. Li, F., Yang, D. & Xu, H. Non-Metal-Heteroatom-Doped Carbon Dots: Synthesis and Properties. Chemistry - A European Journal 25 , 1165–1176 (2019). 7. Jana, J. & Pal, T. An account of doping in carbon dots for varied applications. Natural Resources & Engineering 2 , 5–12 (2017). 8. Barman, M. K., Jana, B., Bhattacharyya, S. & Patra, A. Photophysical properties of doped carbon dots (N, P, and B) and their influence on electron/hole transfer in carbon dots-nickel (II) phthalocyanine conjugates. Journal of Physical Chemistry C 118 , 20034–20041 (2014). 9. Lin, L., Luo, Y., Tsai, P., Wang, J. & Chen, X. Metal ions doped carbon quantum dots: Synthesis, physicochemical properties, and their applications. TrAC - Trends in Analytical Chemistry 103 , 87–101 (2018). 10. Jana, J., Ganguly, M., Chandrakumar, K. R. S., Rao, G. M. & Pal, T. Boron precursor-dependent evolution of differently emitting carbon dots. Langmuir 33 , 573–584 (2017). 11. Sun, H., Wu, L., Wei, W. & Qu, X. Recent advances in graphene quantum dots for sensing. 16 , 433–442 (2013).