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 not available from their inorganic 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′′ 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 PL intensity on the excitation density over 6 orders of magnitude, which has not been possible even for the very high-quality CdTe and GaAs.
β-ZnTe(en)0.5’s highly-ordered crystallinity enables a systematic investigation of its vibrational property. We apply the orthogonal polarization and the angle-resolved polarization Raman techniques to study β-ZnTe(en)0.5’s vibrational modes. A set of orthogonal polarizations are used to decouple the vibration modes according to their symmetries. A mode-by-mode analysis allows unambiguous assignment for the Raman-active modes. A few exceptions and additional features are discussed. With the assignment, we demonstrated that the Raman tensor could be estimated from both the orthogonal technique and the angle-resolved technique. The two independent measurements yield consistent estimations. In addition, it has been shown that a combination of the two techniques enables unambiguous determination of the crystal orientations.
A distinction 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 activation 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 achieve 1.9x10^8 years, while in the ambient air, its lifetime is on the order of 10^1 years.