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pH-Dependent Synthesis of Novel Structure-Controllable PolymerCarbon NanoDots with high acidophilic Luminescence and Super Carbon Dots Assembly for White-Light-Emitting Diodes Siyu Lu, Ridong Cong, Shou-Jun Zhu, Xiaohuan Zhao, Junjun Liu, John S Tse, Sheng Meng, and Bai Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11579 • Publication Date (Web): 21 Jan 2016 Downloaded from http://pubs.acs.org on January 21, 2016
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pH-Dependent Synthesis of Novel Structure-Controllable Polymer-Carbon NanoDots with high acidophilic Luminescence and Super Carbon Dots Assembly for White-Light-Emitting Diodes Siyu Lu†§, Ridong Cong$, Shoujun Zhu†,£, Xiaohuan Zhao†, Junjun Liu†, John S.Tseж§, Sheng Meng‡ and Bai Yang†* † State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University Changchun, 130012, P. R. China. ‡ Beijing National Laboratory for Condensed Matter Physics, and Institute of Physics, Chinese Academy of Sciences, 100190, Beijing, China. § Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada. £ Department of chemistry, Stanford University, Stanford, California 94305, United States. $ Hebei Key Laboratory of Optic Electronic Information Materials, College of Physics Science and Technology, Hebei University, Baoding, 071002, People's Republic of China. Ж State Key Laboratory of Suprhard Materials, Jilin University Changchun, 130012, P. R. China.
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Abstract: We use a pH-dependent solubility equilibrium to develop a one-pot aqueous synthesis of polymer carbon nanodots with novel structures. The chemical structure and photoluminescence (PL) were heavily influenced by the synthesis pH, with crosslinked polymer–carbon film (pH > 7), polymer carbon nano-sheets (3 < pH < 7), and amorphous carbon structures (1 < pH < 3) achieved by altering the initial pH. Although pH-dependent structures frequently occur in typical semiconductors and supramolecular architectures involving metal, this is the first experimental work describing it in carbon nanodots. Super-small carbon nanodots (SCNDs, ~0.5 nm) were obtained at pH < 1; their direct white emission can be easily applied as an inexpensive color-changing layer in white LEDs. Investigation of the PL mechanism of the SCNDs revealed an uncommon multi-level highly emissive recombination channel, which could be possibly derived from the wide distributions of surface-state PL centers. Theoretical calculation of the single-layer of the carbon dots further explored their band gap changes.
KEYWORDS: carbon dots, structure controllable, molecular state, fluorophore, carbon core state
INTRODUCTION Throughout the history of fluorescent materials, considerable effort has been devoted to the development of heavy-metal-free and low-toxicity photoluminescence (PL) nanoprobes. Carbon nanodots (CNDs) are promising in this regard, and have thus drawn much attention during the past decade.1–4 They show high photostability without blinking,5,6 and have tunable emissions,7 chemical inertness,8 good biocompatibility,9 and low toxicity.10 They can be conveniently surface-modified, and are cheaply produced.11,12 We have previously reported that their quantum yield can be as high as 80%, which is comparable to even organic dyes.13 2 ACS Paragon Plus Environment
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Their strong properties make CNDs promising for use in applications such as biomedicine, optoelectronic devices, sensors, and assembly composites. Since the discovery of CNDs composed of a batch of single-wall nanotubes in 2004, thousands of starting materials and hundreds of approaches have been reported for their synthesis.14–17 Reactants can vary from multi-component chemical mixtures to natural products, while syntheses have been reported using laser ablation, pyrolysis, electrochemical oxidation, hydrothermal reactions and microwave treatment, among others.18,19 Although syntheses can vary greatly, the resulting CNDs are generally spherical, smaller than 10 nm, and present only blue PL that is quenched at low pH, thereby restricting their applicability.20–25 Their chemical structure is generally closely related to their PL properties, and CNDs with novel structures sometimes show unique optical properties. Therefore, fabricating CNDs with novel structures is important to understand the PL mechanism and broaden their applicability. Herein, we report a new, and facile route to synthesize four novel structures of polymer-CNDs (p-CNDs) by the hydrothermal process of two amino acids at different temperatures and pH values. The p-CNDs showed polymer chains coexisting with CNDs throughout the whole reaction, which represent a key factor for the formation of the novel structures. Changing the pH and temperature controlled the product structures, with polymer–carbon film (p-CNDs-1), polymer–carbon nano-sheets (p-CNDs-2), amorphous CNDs (p-CNDs-3), and super-small CNDs (SCNDs) all possible. Although pH-dependent structures are frequently observed in typical semiconductors and supramolecular architectures involving metal, this is the first experimental report of them in CNDs.26–28 The p-CNDs-1, p-CNDs-2, and p-CNDs-3 all emitted strong blue luminescence at high acidophilic condition, with the fluorescence intensity in acid at least doubled that seen in a neutral solution. Therefore, these materials are able to detect H+ sensitively and selectively. Additionally, the fluorescence of the three p-CNDs could also be quenched by Fe3+. Unlike the CNDs, the SCNDs exhibited a strong white fluorescence, which can be used for direct white-light generation in white LEDs. The 3 ACS Paragon Plus Environment
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PL mechanism of the SCNDs was also investigated through the theoretical calculation of their designed single layer. Results and discussion Synthesis and characterization of film carbon dots, carbon nanosheets and amorphous nanoparticles. Our previous work used the one-step hydrothermal treatment of L-serine and L-tryptophan (molar ratio 3:1) in alkaline conditions (pH ~8.1) at 200 °C to obtain p-CNDs-1.29 The resulting carbon material comprised a large smooth film with embedded carbon dots, which were detectable by transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) (Figure 2a). The material displayed extremely acidophilic high luminescence (61.12%). This promising novel structure led to the current work, an exploration of the relationship between the structure and PL emission of the p-CNDs. Aqueous L-serine and L-tryptophan were at different pH values (Figure 1), with all other conditions kept constant. Synthesis at 3 < pH < 7 generated regular layered structures (p-CND-2), with an average diameter of 50 nm (Figure 1c, d). At 1 < pH
