探討配體對有機-無機二維鈣鈦礦發光之影響

Abstract

本研究成功以配體輔助再沉澱法 (ligand-assisted reprecipitation, LARP) 於低溫 (<100 ℃) 合成二維有機-無機鈣鈦礦。引入短碳鏈之乙二胺陽離子 (ethylenediammonium) 作為無機層間之橋接配體，改變合成溫度及反溶劑組合，優化奈米片生長條件，限制奈米片生長。添加與胺類配子形成氫鍵之極性溶劑，提升奈米片之結晶度。使用穿透式電子顯微鏡與高解析粉末X光繞射鑑定二維形貌與晶體結構，此鈣鈦礦半導體之層間距小於1 nm (約0.8 Å)，具有強量子侷限效應，表現出極寬之能隙 (約 3.6 eV) 與紫光範圍之螢光 (約392 nm)。經由紅外光譜與擬合X光光電子能譜發現存在多種氮鍵結與鉛氧化態。此有機-無機鈣鈦礦結構在室溫二維螢光光譜能夠表現出多組螢光，其機制係透過激子-聲子耦誘導自陷激子態 (STE) ，調控電子或能量轉移的途徑。另一方面，質子化胺類配體的存在不僅提供了連接相鄰無機層之強作用力，且能與結構中之鉛元素進行光反應。透過元素分析與熱重分析證實結構中之鉛元素具產氫之光催化能力。原子級厚度之二維有機無機鈣鈦礦不僅有良好的光學性質，組成結構之氫鍵作用力提供研究能量轉移的新方法。未來可以做為摻雜具未成對電子過渡金屬之基質，寬能隙能容納更多自旋量子態，發展量子光學或光催化反應之應用。In this research, two-dimensional organic-inorganic perovskite was successfully synthesized at low temperature (<100 °C) via ligand-assisted reprecipitation (LARP) method. Ethylenediammonium, short carbon chain cation, was introduced as a bridging ligand to connecting two adjacent inorganic layers. Then, optimization as well as confining the growth of nanosheets were through synthetic temperature and anti-solvent combination. Addition of polar solvent forming hydrogen bonds with diammonium ligand enhanced the crystallinity of the nanosheets. Morphology and crystal structure were characterized via transmission electron microscopy (TEM) and high-resolution powder X-ray diffraction (HRPXRD). Thickness of this perovskite semiconductor less than 1 nm (ca. 0.8 Å) induces strong quantum confinement effect, demonstrating an extremely wide bandgap (ca. 3.6 eV) and violet-range fluorescence (ca. 392 nm). Various nitrogen chemical bond and lead oxidation states were figured out through infrared spectroscopy and deconvolved X-ray photoelectron spectroscopy. We proposed a mechanism that exciton-phonon coupling triggers the formation of self-trapped exciton states (STE) , which regulates the pathway of electron or energy transfer. On the other hand, the presence of protonated amine ligands not only provides a strong force to connect adjacent inorganic layers, but also enables to photoreaction with lead elements in the structure. Through elemental analysis and thermogravimetric analysis, it was confirmed that the lead element in the structure has the photocatalytic ability to produce hydrogen. Atomically thin two-dimensional organic-inorganic perovskites not only have excellent optical properties. In addition, the hydrogen bonding as structural framework provides a new insight for studying energy transfer. In the future, it can be used as a matrix for doping transition metals with unpaired electrons, and the wide bandgap can accommodate multiple spin quantum states, and develop applications of quantum-optics or photocatalytic reactions.