Long-term stability, vibrational and optical properties of β-ZnTe(en)0.5 and related organic-inorganic hybrid superlattices
Organic-inorganic hybrids may offer material properties enhanced compared to or not available from their inorganic and organic components. However, they are typically less stable and disordered. A group of highly ordered II-VI based hybrid structures has been shown to possess various unusual properties and potential applications. As a prototype, β-ZnTe(en)0.5 can be viewed as a superlattice with alternating layers of two-monolayer thick (110) ZnTe and single-molecule length ethylenediamine. In contrast to all the known inorganic superlattices where interfacial diffusion is inevitable, we demonstrate in this thesis that β-ZnTe(en)0.5 exhibits an unusually high degree of crystallinity, as is evidenced by < 25′′ x-ray diffraction (XRD) rocking curve linewidth and < 1 cm-1 Raman linewidth, which are comparable to many high-quality binaries. Besides manifesting in the macroscopic scale crystallinity characterization, it also shows an exceptionally low level of microscopic scale defects, as suggested by the observed linear dependence of photoluminescence (PL) intensity on the excitation density over 6 orders of magnitude, which has not been possible even for the very high-quality CdTe, GaAs, and hybrid perovskite MAPbI3.β-ZnTe(en)0.5’s exceptionally high crystallinity enables a systematic investigation of its vibrational property, which makes it possible to compare the experimental results with theoretical predictions without the extrinsic interferences that inevitably exist in other superlattices. We perform on-axis and angle-resolved polarization selective Raman measurements to study β-ZnTe(en)0.5’s vibrational modes. A set of on-axis polarization configurations are used to identify and analyze the vibration modes according to their symmetries. A mode-by-mode analysis allows for unambiguous assignments of the Raman-active modes. With the assignments, the Raman tensors can be estimated from both the on-axis and the angle-resolved measurement. The two independent measurements yield consistent estimations. In addition, it has been shown that the angle-resolved polarization measurement enables unambiguous determination of the crystal orientations. A few exceptions and additional features are discussed. A distinction of β-ZnTe(en)0.5 among the hybrid materials is its unprecedented ambient long-term stability over 15 years, which is still limited by extrinsic mechanisms but is already the longest documented hybrid semiconductor. In this work, we used Raman spectroscopy to investigate its degradation in air and a protected condition and framed the factors contributing to its long-term stability into (1) intrinsic effect such as large formation energy and large kinetic barrier in excess of the formation energy; (2) extrinsic factors, including surface or edge effect, where degradation can initiate through processes such as oxidation, and the structural defects, which may provide more accessible paths for degradation. Based on this approach, we estimate the room-temperature lifetime of β-ZnTe(en)0.5 in a protected environment can be as long as 1.9x108 years, while in the ambient air, its lifetime is on the order of 10 years.