Light-Emitting Copper Nanoclusters: Synthesis, Optical Studies, and Use for Light Emitting Devices and Chemical Sensing

熒光銅納米團簇:合成、光學性質研究以及在發光器件和化學檢測中的應用

Student thesis: Doctoral Thesis

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Award date16 Aug 2017

Abstract

Scientific interest in metal nanoclusters (NCs), such as Au, Ag and Cu, has grown tremendously in the last few decades, as their properties deviate significantly from bulk metals and even nanoparticles, while rather resembling those of molecular compounds. One remarkable research direction focuses on exploring their photoluminescence (PL) in the UV and visible range. With such useful features as being earth abundant, inexpensive, and readily available from commercial sources element, NCs of copper are attractive, and both new synthetic methods and their applications in the fields of chemical sensing, bio-imaging and fabricating of light-emitting devices (LEDs) are exploding nowadays. Especially in the area of LEDs, Cu NCs are promising materials acting as alternatives for commercial phosphors, which are usually rare - earth doped materials facing the problems of the shortage of supply and being destructive to the environment. However, the wider applications of Cu NCs have been limited by their relatively low PL quantum yield (QY) and stability. In this thesis we summarize our recent studies on the synthesis of highly photoluminescent and stable Cu NCs by the stabilization of poly(vinylpyrrolidone) (PVP) and subsequent surface treatment by using electron - rich ligands, by reduction and stabilization of glutathione (GSH) with features of the aggregation-induced emission (AIE) enhancement and through confinement of Cu NCs in the metal-organic frameworks (MOFs). We further fabricated white LEDs using Cu NCs as blue component, all-Cu NC white LEDs (WLEDs) and remote LEDs. These studies are presented in six chapters.
The first chapter introduces general background of Cu NCs, including the synthetic approaches, PL properties and primary applications based on previously reported literature. It covers the strategies to synthesize luminescent Cu NCs using wet chemistry methods, based on the surface ligands used. Further, it reviews the effect of metal core, surface ligands, and environment on the PL properties of Cu NCs (emission color, PL QY and lifetime). To help further understanding of the potential applications of Cu NCs, their use in LEDs and bio-imaging are introduced. Finally, some potential future research directions are discussed.
In the second chapter, PVP supported copper nanoclusters, and their blue emission enhancement by glutathione (GSH) treatment, which are applied in phosphor converted light emitting devices are introduced. Blue emitting Cu NCs are synthesized by the reduction Cu ions into copper atoms and their subsequent clustering on the framework of PVP. The post-preparative surface treatment using GSH helps to enhance the PL QY of Cu NCs up to 27%; they have been employed as blue emitting component for fabricating white WLEDs, in combination with rare-earth phosphors.
The third chapter introduces the concept of fabricating WLEDs based on Cu NCs only, avoiding the use of any rare-earth phosphors. Orange emitting Cu NCs are synthesized by employing GSH as both reducing and stabilizing agent, through injection of precursors into the water/ethanol mixtures to make use of the AIE enhancement effect. Blue Cu NCs are synthesized by reduction of copper ions supported on a PVP backbone. Post-preparative sodium citrate treatment improves both their PL efficiency and stability. Blue and orange emitting Cu NCs are processed into powders and employed as phosphors of both monochrome LEDs and WLEDs.
The fourth chapter of the thesis focuses on fabrication of remote WLEDs using mechanically stable and stretchable composite films comprising aggregated Cu NCs in polyurethane (PU) matrix. These composite films show thermally stable emission with two PL peaks in the blue and orange regions originating from PU and aggregated Cu NCs, with overall absolute PL QY as high as 18%. Applying the luminescent Cu NC/PU films for fabrication of the remote WLEDs avoids the compatibility issues of powdered Cu NC phosphors with silicone resin packaging materials.
The fifth chapter introduces the strategy on how to improve both the stability and the emission intensity of Cu NCs based on their entrapment into MOFs. The latter effect reaches 20 times and is ascribed to a confinement-assisted emission enhancement similar to the AIE of fluorescent species in solution. Towards potential applications of Cu NC/MOF composites as chemical sensors, we demonstrate the selective quenching of their emission by 2,4,6-trinitrotoluene (TNT) attributed to its specific electron-withdrawing interactions with ligands on Cu NCs.
The last chapter provides the summary for this thesis and some perspectives for the future of the field.