Thermal control of oxygen-induced emission states in carbon dots for indoor lighting applications

Highlights

• Development of a novel synthetic method for thermally controlling the oxygen content of carbon dots.

• Examining the effect of oxygen on the optical properties of carbon dots.

• Using pump-probe spectroscopy to investigate the photoexcited charge-carrier dynamics of carbon dots.

• Demonstration of carbon dot-based panchromatic indoor lighting devices simulating sunlight.

Abstract

Understanding the role of heteroatoms in carbon dots (CDs) has been recognized as a critical factor for the design and engineering of their optical properties. Oxygen is one of the most prevalent heteroatom defects in CDs; however, its effects on the optical properties are still unclear because it has not been possible to precisely control the degree of oxidation of CDs. Here, we report the synthesis of CDs in a controlled process that allows thermal control of the oxygen content using benzyl alcohol as an oxidizing solvent. The results suggest that oxygen-induced defects reduce the optical band gap of CDs, and their emission is red-shifted from blue (428 nm) to red (628 nm). The photoexcited charge-carrier dynamics of the CDs were thoroughly studied using transient absorption spectroscopy and fluorescence decay measurements. Furthermore, the bright multicolored emission of our CDs renders them suitable for sun-like panchromatic indoor lighting applications. We believe that these new insights into oxygen-induced defects in CDs will result in significant progress in their practical use.

Introduction

Nanoparticles have unique mechanical, optical, magnetic, and catalytic properties depending on their size and chemical composition. Many studies have investigated the effects of size, heteroatom incorporation (i.e., chemical doping), and surface modification [[1], [2], [3]]. It has been demonstrated that the optical properties of semiconductor nanocrystals (quantum dots) can be widely tuned by varying their composition and size-controlled quantum confinement.

Carbon dots (CDs) are a class of luminescent carbon nanomaterials with quasi-spherical morphology and a characteristic size of less than 10 nm [4,5]. CDs have attracted increasing attention for both fundamental and practical research owing to their attractive properties, particularly their fluorescence (FL), bioavailability, and thermochemical stability [6,7]. Consequently, they have been applied in various areas including biomedicine [8], light-emitting devices [9,10], photochemical sensors [11,12], data security [13,14], photovoltaics [15,16], and photocatalysis [17,18]. Furthermore, improvements in the functionality and diversity of CDs and products containing them have recently been accelerated by the development of simple and efficient methods for their preparation and functionalization [19,20].

The chemical structure of CDs is complex and still not completely known; however, the core is typically described as partially graphitic or polycrystalline with a mixture of graphitic and polymeric structures, and the surface is thought to be covered with heteroatom-containing functional groups and chemical defects [21,22]. Although they have various structures and compositions, CDs share several optical properties: (i) a large Stokes shift [23], (ii) excitation-dependent [24] or even multichromatic fluorescence [25], and (iii) a wavelength-dependent FL lifetime [26]. The contributions of certain physical/chemical structures in CDs to these optical properties are unclear [27,28]; however, incorporating heteroatoms into CDs by doping has been found to considerably enhance the color purity and emission intensity of their FL [29,30].

An understanding of the roles of heteroatoms in CDs is considered critical for designing a robust synthetic scheme and ultimately tailoring the optical properties for specific applications. Heteroatoms with lone electron pairs, such as nitrogen and oxygen, are particularly interesting because they can easily form strong covalent bonds with carbon and generate defect states (or intragap states) [31]. Previous studies have demonstrated that the effect of heteroatoms on the optical band gap of CDs depends strongly on the chemical structure of the precursors and the chemical states of the heteroatoms in CDs [[32], [33], [34]]. Ding et al. synthesized CDs with tunable FL from blue to red, which was attributed to surface oxidation [35]. Wang et al. reported a facile method for the preparation of CDs with full-spectrum emission, which was attributed to the combined effects of particle size and sp2-hybridized carbon [36]. Ding et al. designed a solvothermal method for CDs with tunable FL ranging from 443 nm (blue) to 745 nm (red), which was ascribed to the synergistic effects of the p-electron system, graphitic nitrogen, and surface states [37]. These studies offer valuable insights into the FL mechanism and highlight the potential of CDs for practical applications. However, most previous research has focused on nitrogen because it can significantly change the optical properties of CDs. In contrast to nitrogen, oxygen has generally been regarded as a negative defect that degrades the optical properties, especially the FL color purity. Moreover, the effects of oxygen on CDs have rarely been investigated because the precise control of the oxygen content or the degree of oxidation in CDs is still limited (Table S1, Supplementary Material) [29].

In this paper, we present a new approach to tailoring the optical band gap of CDs by controlling the degree of oxidation using citric acid (CA) as a carbonization source and benzyl alcohol (BzOH) as an oxidizing solvent. The FL color (wavelength) of the CDs can be varied from blue (428 nm) to red (628 nm) by varying the reaction temperature, which affects the oxidizing power of BzOH. The chemical origin of the FL of the CDs was systemically investigated using Stern–Volmer quenching analysis. The correlation between the oxygen-induced structural/chemical changes and carrier dynamics of the CDs was thoroughly studied using pump-probe spectroscopy. Finally, we demonstrated the practical utility of the CDs by fabricating sun-like white light-emitting devices (LEDs) with freestanding and flexible phosphor films based on CDs, which may replace toxic or costly rare-earth phosphors such as yttrium, lanthanide, europium, and terbium.