ISSN 1998-0124 CN 11-5974/O4 2019, 12(1): 000–000 https://doi.org/10.1007/s12274-019-2337-4 Research Article Nitrogen-doped graphene quantum dots: Optical properties modification and photovoltaic applications Md Tanvir Hasan 1,§ , Roberto Gonzalez-Rodriguez 1,§ , Conor Ryan 1 , Kristof Pota 2 , Kayla Green 2 , Jeffery L. Coffer 2 , and Anton V. Naumov 1 (  ) 1 Department of Physics and Astronomy, Texas Christian University, TCU Box 298840, Fort Worth, Texas 76129, USA 2 Department of Chemistry and Biochemistry, Texas Christian University, TCU Box 298860, Fort Worth, Texas 76129, USA § Md Tanvir Hasan and Roberto Gonzalez-Rodriguez contributed equally to this work. © Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019 Received: 19 November 2018 / Revised: 11 February 2019 / Accepted: 13 February 2019 ABSTRACT In this work, we utilize a bottom-up approach to synthesize nitrogen self-doped graphene quantum dots (NGQDs) from a single glucosamine precursor via an eco-friendly microwave-assisted hydrothermal method. Structural and optical properties of as-produced NGQDs are further modified using controlled ozone treatment. Ozone-treated NGQDs (Oz-NGQDs) are reduced in size to 5.5 nm with clear changes in the lattice structure and I D /I G Raman ratios due to the introduction/alteration of oxygen-containing functional groups detected by Fourier-transform infrared (FTIR) spectrometer and further verified by energy dispersive X-ray spectroscopy (EDX) showing increased atomic/weight percentage of oxygen atoms. Along with structural modifications, GQDs experience decrease in ultraviolet–visible (UV–vis) absorption coupled with progressive enhancement of visible (up to 16 min treatment) and near-infrared (NIR) (up to 45 min treatment) fluorescence. This allows fine-tuning optical properties of NGQDs for solar cell applications yielding controlled emission increase, while controlled emission quenching was achieved by either blue laser or thermal treatment. Optimized Oz-NGQDs were further used to form a photoactive layer of solar cells with a maximum efficiency of 2.64% providing a 6-fold enhancement over untreated NGQD devices and a 3-fold increase in fill factor/current density. This study suggests simple routes to alter and optimize optical properties of scalably produced NGQDs to boost the photovoltaic performance of solar cells. KEYWORDS nitrogen-doped graphene quantum dots, ozone treatment, optical properties, photovoltaics, solar cells 1 Introduction Graphene quantum dots (GQDs) are zero-dimensional carbon structures well-known for their unique and remarkable properties including high quantum yield (QY) fluorescence emission [1–3], substantial photostability [4, 5], water solubility [5, 6], low cytotoxicity [6, 7], and good biocompatibility [4, 8] which make them promising materials for multiple applications in biotechnology and electronics. A number of such applications have been already explored including light-emitting diodes [5, 9–11], photovoltaic devices [12–15], bio- imaging probes [16, 17], photodynamic therapy [18, 19], and photocatalysis [20, 21]. However, new properties of GQDs explored in this work still leave significant room for further utilization. Graphene quantum dots can be synthesized either by using top-down or bottom-up approaches. The top-down approach involves scission of larger-sized (micrometer) structures into smaller (nanometer-sized) quantum dots. This is accomplished either via strong acidic oxidation using carbon nanotubes, graphite or carbon black [22, 23] as precursors, or electrochemical exfoliation using graphite rods or graphene films [24, 25] as carbon sources. Usually, these methods are time-consuming, complex and provide little control over optical/ structural properties of the end products. Bottom-up methods include step-wise fabrication of carbon-dots (C-dots)/GQDs via solution chemistry [26], or the synthesis of GQDs by pyrolysis/carbonization of organic molecules (carbon precursors) via a hydrothermal method [27, 28]. Although less laborious, these methods still often involve multi-material and multistep synthetic routes that may raise scalability and ecological concerns also producing quantum dots with generally low quantum yields [26]. In our recent work [5], a new bottom-up approach has been utilized to synthesize nitrogen self-doped graphene quantum dots (NGQDs) using a single glucosamine-HCl material as a source of carbon, nitrogen, and oxygen via a microwave-facilitated hydrothermal method yielding fluorescent graphene quantum dots. NGQD intrinsic emission [5] in the visible was previously attributed to the quantum confinement effect of different GQDs sizes [29, 30], whereas near-infrared (NIR) emission may potentially originate from several emissive defect/trap states [31–33]. These QDs exhibited high quantum yields while their synthesis was simplistic, low cost and reproducible. Also, a light emitting device with bright elec- troluminescence and moderate turn-on voltage was fabricated using these NGQDs. In the present work, we focus on further exploring the photovoltaic applications of these NGQDs. We fabricate solar cells utilizing as-prepared NGQDs as a photoactive layer showing power conversion efficiency (PCE) of ~ 0.41% along with moderate current density, open circuit voltage and fill factor (FF). Although it is comparable with previous optimal results for such devices [34–38], we expect that modification of the optical properties of as-prepared NGQDs may significantly improve their photovoltaic performance. A number of methods are used to date to alter the optical pro- perties of GQDs including doping of potassium [39]/hetero-atoms (nitrogen, sulfur, boron, phosphorus), reduction of graphene oxide quantum dots using hydrazine [40], band-gap modulation of GQDs Address correspondence to a.naumov@tcu.edu