二維鈣鈦礦晶體之結構與光電特性變化之探討

Abstract

二維Ruddlesden-Popper鈣鈦礦材料（2D RPP，A2A’n-1MnX3n+1），不同於傳統的有機 - 無機混摻的三維（3D）鹵素鈣鈦礦材料，其結構為有機長鏈層和無機鈣鈦礦層交互相疊，而形成類似量子阱的晶體結構。這種獨特的結構使得2D RPP具有比3D鈣鈦礦材料而言更高的水穩定性和更強的激子結合能。此外，2D RPP可以通過改變有機長鏈陽離子，有機陽離子，鹵素陰離子和鹵化金屬八面體的層數來調控螢光波長。由於眾多優越的物理特性，使2D RPP在當前廣泛研究新物理現象以及應用在許多不同光電元件和電子元件中扮演著關鍵的角色。然而由於濕式化學合成過程的複雜性使得合成出高純度同源2D鈣鈦礦仍然是一個巨大的挑戰。其中，有機長鏈陽離子和甲基銨陽離子之間的溶解度差異，導致難以以化學計量法合成出純相同系化合物。除此之外，目前仍然缺乏針對不同元素化合物的2D RPP其結構變化與相變和磁光特性的相關性。因此本論文主要嘗試生長高純度毫米尺寸的單晶2D RPP，以探討2D RPP材料的基礎特性。 在第三章中，我們利用恆溫溶液緩慢蒸發使晶體慢慢析出的方法（SECT）合成了各種不同成分組合且函覆蓋整個可見光範圍帶隙的二維鈣鈦礦材料，如BA2MAn-1PbnI3n+1 (n = 1, 2和3）、BA2MAn-1PbnBr3n+1 (n = 1, 2和3）、BA2FAn-1PbnI3n+1 (n = 2）、PEA2MAn-1PbnI3n+1 (n = 2）、ALA2MAn-1PbnI3n+1 (n = 2）。我們利用X-Ray繞射圖譜和螢光生命週期顯微影像圖譜測量，證實我們合成出的大尺寸2D RPP晶體有良好的結晶度和相純度。我們透過比較含有不同的有機長鏈間隔陽離子，短鍊有機陽離子和鹵素的2D RPP，以討論結構和材料工程以及光學和光物理性質之間的關係。另外，由於2D RPP其具有特殊的金字塔階梯結構和強大的結合能，我們發現這些同源2D大尺寸RPP晶體具有約3.7 uJ / cm2 低閾值無共振腔的雷射行為。 在第四章中，雖然3D鈣鈦礦結構變化所造成的相變已廣被人熟知，然而，2D RPP中不同元素組成與相變的相關性的研究仍然未有一個定論以及系統性的整理。因此透過我們的高純相2D RPP單晶，我們可以降低缺陷態與不純的相所導致的因素，直接探討相變前後的結構變化以及載子動力學。我們比較了不同無機鈣鈦礦層的層數（n值，BA2MAn-1PbnI3n+1 (n = 1, 2和3））、不同的有機間隔陽離子(A，ALA2MAn-1PbnI3n+1，n = 2和PEA2MAn-1PbnI3n+1，n = 2）、不同的有機陽離子分子(A’，BA2MAn-1PbnI3n+1，n = 2)與不同鹵素陰離子（X，BA2MAn-1PbnBr3n+1，n = 2）之2D RPP單晶的的相變所導致的結構和光學性質變化。我們發現，有機長鏈陽離子的排列在相變中扮演著關鍵作用。此外，2D RPP化合物中每個元素部分會影響晶體結構的穩定性並導致相變行為的改變。 在2D RPP材料中，由於特有的有機層-無機層堆疊的異質結構使2D RPP晶體有著結構反稱不對稱性，再加上無機層中含有的重元素，使其存在強自旋軌道耦合並導致電子能帶自旋分裂。然而，針對2D RPP中磁性的相關研究也相當有限。在第五章中，我們透過BA2MAn-1PbnI3n+1 (n = 1, 2和3）和BA2MAn-1PbnI3n+1 (n = 1和2）2D RPP薄膜來探討不同層數的無機鈣鈦礦插層與不同鹵素陰離子的磁光特性。經由不同磁場下的MCD測量，2D RPP晶體顯示出比其3D對應物大得多的有效g因子(-3.34)。此外，量子侷限效應在測量其磁光效應中為重要的變因，2D RPP量子阱的組成會影響較強或較微弱的自旋-軌道耦合。我們同時成功地透過在小磁場下的MCD光譜觀察到由溫度變化引起的自旋-軌道耦合的擾動，其變化趨勢與前一章透過結構和光學測量的結果一致，表明晶體不對稱對於自旋軌道耦合的重要性。我們也發現2D RPP在室溫下也存在著零磁場分裂，這表明2D RPP具有作為自旋極化電子源的巨大潛力。 我們的研究結果表明，由於溶液生長同源有機-無機摻雜二維鈣鈦礦單晶具有可以做為整個可見光範圍的光學增益介質材料的潛力，也因其晶體結構不對稱性而有著強磁光特性，因此二維鈣鈦礦材料可作為尋找新物理性質的材料和有著極具廣闊的應用前景如製作高效能的新型光電子元件和自旋電子元件。

Two dimensional Ruddlesden-Popper perovskite (2D RPP) with chemical formula A2A’n-1MnX3n+1, which is different from traditional halide organic-inorganic 3D perovskite, contains organic spacer layer and inorganic layer, acting as quantum well super-lattices like structure. This unique structure makes 2D RPP have a higher moisture stability, and stronger exciton binding energy than its 3D perovskite counterpart. Besides, the band structure of 2D RPP can be tuned by changing the organic spacer, organic cation, halide anion, and layered perovskite compounds. Because of these superior features, it is expected that 2D RPP plays a key role for the discovery of new physical phenomena and potential novel applications. However, the complexity in the wet-chemical processes make the synthesis of high purity homologous 2D perovskites still a big challenge and the difficulty for the understanding of its fundamental understanding. Among all, the difference in solubility between the organic spacer and methylammonium cation leads to the complication in obtaining pure phase homologous compounds in stoichiometric compositions. In addition, the fundamental mechanisms of the molecular origin for phase transition and magneto-optical properties of the 2D RPP with different element compounds are still lacking. In this regard, this thesis attempts to use high phase purity single crystal 2D RPPs to uncover the intrinsic characteristics of 2D RPP materials, including structure, optical properties, phase transition, and magnetic effects, which can serve as the foundation towards their practical applications. In Chapter three, we synthesized 2D perovskite materials with various compositions of BA2MAn-1PbnI3n+1 (n = 1, 2 and 3), BA2MAn-1PbnBr3n+1 (n = 1, 2 and 3), PEA2MAn-1PbnI3n+1 (n = 1, 2 and 3), ALA2MAPb2I7 (n = 2), and BA2FAPb2I7 (n = 2), which have band gap covering whole visible light range by using slow evaporation at a constant-temperature (SECT) solution growth. Via the XRD and fluorescence lifetime mapping measurements, the large sized 2D RPP crystals show well crystallinity and highly phase purity. We then provided various 2D RPPs with different organic spacers, short organic cations, and halides to discuss the relationship between the structural and materials engineering and optical and photo-physical properties. Furthermore, due to the special pyramid step-liked structure and strong binding energy, we obtained cavity-free lasing behavior with around 3.7 uJ/cm2 low threshold from these homologous 2D large sized RPP crystals. The result demonstrates that solution-growth homologous organic-inorganic hybrid 2D perovskite single crystals open up a new window as a promising candidate for whole visible range of optical gain media for the development of laser devices. 3D perovskites have been known to possess structure related phase transition, however, the detailed study of phase transition for 2D RPPs is still ambiguous and not well-understood. In Chapter four with high pure phase 2D RPP single crystals, we identified the carrier dynamics before and after phase transition directly without defect state induced effect. Besides. we compared the phase transition induced structural and optical properties changes of different dimensionalities (n-value) with BA2MAn-1PbnI3n+1, n = 1, 2 and 3; different organic spacers (A) with (ALA)2MAn-1PbnI3n+1, n = 2 and (PEA)2MAn-1PbnI3n+1, n = 2; different organic cation molecules (A’) with (BA)2FAn-1PbnI3n+1, n = 2 ; and different halide anions (X) with (BA)2MAn-1PbnBr3n+1, n = 2 2D RPP single crystals. We discovered that the correlation of the rearrangement of organic cations with the electron-phonon interactions plays a key role in the phase transition behavior. Moreover, each part of 2D RPP compositions will affect the crystal structure rigidity and lead to the variation of the phase transition behavior. Due to the natural multiple quantum wells like hetero-structure of 2D RPPs and heavy elements of inorganic layers, a strong spin-orbit coupling exists in these materials. Together with the structural inversion asymmetry of 2D RPPs crystal, an intriguing electronic band spin splitting is expected. However, the study of magnetic properties in 2D RPPs is rather limited. In Chapter five, we attempt to figure out the detailed magneto-optical characteristics of different dimensionalities of BA2MAn-1PbnI3n+1, n = 1, 2 and 3 and different halide anions for BA2MAn-1PbnI3n+1, n = 1 and 2 2D RPP thin films. Based on the MCD measurements under different magnetic fields, 2D RPPs crystal shows a much larger effective g factor of -3.34 than that of its 3D counterpart. We also discovered that the quantum confinement effect with a thinner inorganic perovskite layer plays a key role in the measured magnetic field effects. The compositions of the quantum well of 2D RPPs lead to the robust and feeble spin-orbit coupling. The perturbation of the spin-orbit coupling induced by temperature change can be successfully observed via MCD spectroscopy under a small magnetic field. The detected trend is consistent with structure and optical measurements, indicating the importance of crystal asymmetry for the occurrence of spin-orbit coupling. Moreover, we found that the zero field splitting in 2D RPPs exists even at room temperature, indicating that 2D RPPs have a great potential to be a spin polarized electron source. The new physical properties uncovered for organic−inorganic hybrid 2D perovskites in this thesis is very useful for the fundamental understanding of this emerging material system and paving a key step for generating novel optoelectronic and spintronics devices.