環保奈米材料在固態的光物理性質研究及其在奈米磷光體之應用

Photophysical Properties of Eco-friendly Nanomaterials in the Solid State and Their Applications in Light-conversion Nanophosphors

10.6840/cycu201700496

鄭致和

環保 ； 奈米磷光體 ； 金屬奈米團簇 ； 碳量子點 ； eco-friendly ； nanophosphor ； metal nano-cluster ； carbon dot

中原大學物理學系學位論文

2017年

碩士

院繼祖

繁體中文

含有”鎘”金屬的膠體量子點因為可以調整發光波長、色純度高等特性，目前已經被大量的運用在奈米磷光體中，但是這些量子點都是對環境、生物有害，而且需在有機溶液中合成保存。而一般奈米磷光體的Stokes shift都不大，導致在固態下造成嚴重的再吸收損失和聚集誘導螢光淬滅 (Aggregation-induced quenching, AIQ)。因此發展可以在水溶液中進行合成的環保奈米螢光材料，並且具有較大的Stokes shift，成為未來發展綠色光電 (green photonics) 重要的議題。 本文利用簡易的方法合成出水相的Glutathione-stabilized Au nano-clusters (GSH-AuNCs)、Glutathione-stabilized Cu nano-clusters (GSH-CuNCs) 及綠色碳量子點 (g-CDs)，其中GSH-AuNCs、GSH-CuNCs都具有較大的Stokes shift和聚集誘導螢光增強 (Aggregation-induced emission, AIE) 效應。接著我們利用簡易的表面改質方法製做出固態GSH-AuNCs粉末，與原溶液相比，其增強了固態的量子效率 (photoluminescence quantum yield, PL-QY)、延長了螢光生命期、螢光光譜藍移的特性，意謂著表面改質可減少非輻射複合速率。 GSH-CuNCs溶液的螢光強度相當微弱，因此利用酒精稀出或加入PVA固化之後使GSH-CuNCs螢光強度明顯提升。我們主要利用變溫螢光光譜及螢光生命期光譜來研究GSH-CuNCs的發光機制，根據實驗數據及擬合的分析，我們發現螢光主要包含熱引發的延遲螢光 (Thermally activated delayed fluorescence, TADF) 及磷光，且具有非常短的三重態螢光生命期，原因為重原子效應 (heavy-atom effect) 及微小的單重態/三重態之能量差。 這些環保奈米材料使用較低成本的前驅物 (precursors) 直接在水溶液中合成，且具有特殊光電物理性質，然而目前環保奈米材料在固態時的螢光量子效率較低，是未來必須面對的問題，相信未來這些環保奈米材料在綠色光電科技中會扮演重要的角色。

Nowadays, there usually exists a trade-off between the device efficiency and environmental protection in photonic devices. For example, to harvest all excitons in organic light-emitting materials, expensive heavy metals need to be incorporated into the emitters. Semiconductor nanocrystals with tunable wavelength and pure emission colors have also been commercialized for light-emitting materials in light-conversion nano-phosphors and display backlight. Unfortunately, the most mature semiconductor nanocrystals contain toxic elements (Cd or Pb) and are synthesized in hazardous organic solvent. As a result, it is necessary to develop eco-friendly, non-toxic nanomaterials that can be directly fabricated in an aqueous solution based on cost-effective, element-abundant precursors, while still exhibiting unique photophysical properties. Metal nanoclusters (NCs) with tiny sizes, including AuNCs, AgNCs, and CuNCs can be simply synthesized in an aqueous solution and exhibit some unique photophysical properties that are promising candidates for promising applications in “green photonics”. In the first part, a facile, matrix-free method based on surface modification was used to prepare solid-state non-toxic AuNC nano-phosphors with solid-state enhanced photoluminescence quantum yields (PL-QYs) and lengthened PL lifetime. Those AuNC-nanophosphors also exhibit large Stokes shift due to the emission from intramolecular charge transfer (ICT) state, thus reducing conventional concentration-induced PL quenching and reabsorption losses. In light of our spectroscopic studies and materials characterization, the improved photophysical properties are attributed to surface-modification-induced aggregation, thus restricting molecular surface-ligand motions. Despite aforementioned unique photophysical properties possessed by AuNCs, the issues regarding expensive precursors and long reaction time still need to be addressed. In the second part, we investigated the solid-state photophysical properties of cost-effective, element-abundant CuNCs, which can be simply synthesized in an aqueous solution at room temperature within one hour. The CuNCs exhibit unique solid-state dual-mode emissions of thermally-activated delayed fluorescence and phosphorescence with a short emissive lifetime at room temperature and at ambient environment. Such dual-mode emissions can be attributed to small singlet-triplet energy splitting due to the ICT emission and large spin-orbit coupling arising from heavy-atom effect. To fabricate solid-state nano-phosphors with green PL emission, the carbon nano-dots have also prepared using a simple hydrothermal method. To stabilize the excited states and avoid the formation of solid aggregates, inorganic ionic-crystal matrices were used to protect and disperse the carbon nano-dots. We also investigated the photophysical properties of carbon nano-dots in the solid state. We found some interesting spectral modification behavior when aqueous carbon nano-dots were transferred to solid states and the behind mechanism is still under investigation. Compared with conventional heavy-metal containing semiconductor nanocrystals synthesized in a hazardous solvent, those non-toxic nanomaterials can be directly prepared in an aqueous solution using cost-effective precursors and exhibit some unique photophysical properties, thus would be promising for future applications in “green photonics”. However, the solid-state PL-QY is still poor and need to be further enhanced.

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