nanomaterials Review Applications of Carbon Dots in Optoelectronics Evgeniia A. Stepanidenko 1 , Elena V. Ushakova 1,2 , Anatoly V. Fedorov 1 and Andrey L. Rogach 2,3, *   Citation: Stepanidenko, E.A.; Ushakova, E.V.; Fedorov, A.V.; Rogach, A.L. Applications of Carbon Dots in Optoelectronics. Nanomaterials 2021, 11, 364. https:// doi.org/10.3390/nano11020364 Academic Editor: Antonio Bartolomeo Received: 30 December 2020 Accepted: 24 January 2021 Published: 1 February 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Center of Information Optical Technology, ITMO University, 197101 Saint Petersburg, Russia; stepanidenko.e@mail.ru (E.A.S.); elena.ushakova@itmo.ru (E.V.U.); a_v_fedorov@inbox.ru (A.V.F.) 2 Centre for Functional Photonics (CFP), Department of Materials Science and Engineering, City University of Hong Kong, Kowloon, Hong Kong, China 3 Shenzhen Research Institute, City University of Hong Kong, Shenzhen 518057, China * Correspondence: andrey.rogach@cityu.edu.hk Abstract: Carbon dots (CDs) are an attractive class of nanomaterials due to the ease of their synthesis, biocompatibility, and superior optical properties. The electronic structure of CDs and hence their optical transitions can be controlled and tuned over a wide spectral range via the choice of precursors, adjustment of the synthetic conditions, and post-synthetic treatment. We summarize recent progress in the synthesis of CDs emitting in different colors in terms of morphology and optical properties of the resulting nanoparticles, with a focus on the synthetic approaches allowing to shift their emission to longer wavelengths. We further consider formation of CD-based composite materials, and review approaches used to prevent aggregation and self-quenching of their emission. We then provide examples of applications of CDs in optoelectronic devices, such as solar cells and light-emitting diodes (LEDs) with a focus on white LEDs. Keywords: carbon dots; emission; composite materials; solar cells; light emitting diodes 1. Introduction Alongside well-known light-emitting materials such as organic dyes, rare-earth ele- ments, and semiconductor quantum dots (QDs), carbon-based luminescent nanomaterials such as carbon dots (CDs) are gaining increasing popularity currently [1,2]. CDs are tiny nanoparticles composed of sp 2 - or sp 3 -carbon domains and rich in oxygen- and nitrogen- containing functional groups. According to Refs. [3,4], luminescent carbon nanoparticles can be classified as graphene quantum dots (GQDs), carbon dots (or nanodots), and poly- mer dots. GQDs are composed of one to several layers of graphene often functionalized with molecular groups at the edges, and possess a quantum-sized effect. CDs are mostly spherical or quasispherical nanoparticles, where the degree of crystallinity can vary over; they often demonstrate excitation-dependent photoluminescence (PL). Polymer dots can be considered as aggregated or cross-linked oligomers/polymers and contain emissive groups within the polymer network. This review will focus on spherical or quasispherical luminescent carbon nanoparticles, which are designated hereafter as CDs. A considerable research interest in these nanoobjects is fueled by the possibility of tuning the CD emission over a wide spectral range, which can be accomplished at the synthetic stage by varying precursors and reaction conditions [5–7]. Importantly, CDs do not contain any toxic ele- ments and are considered as non-toxic materials, in a stark contrast to most of the “classical” Cd- or Pb-based semiconductor QDs [8,9]. Moreover, the PL quantum yield (QY) of CDs can easily reach very high values, especially in the blue and green spectral range [10,11]. Being a biocompatible and non-toxic alternative to QDs, CDs have been widely used in bioimaging, sensing, and other biomedical applications [12–19]. Composite materials based on CDs have been often reported to have phosphorescence observed at room temperature, which is a useful property for data encryption [20–22]. CDs even found some applications in agriculture, where they were used to improve plant growth and production [23]. Due to their strong absorption [24], CDs were used as sensitizers in solar cells [25,26]. Considering Nanomaterials 2021, 11, 364. https://doi.org/10.3390/nano11020364 https://www.mdpi.com/journal/nanomaterials