Hierarchical Nanostructure of Natural Biominerals and Man-made Semiconductors

Abstract

Materials with structural hierarchy have become a central focus to inspire new designs of next-generation high-performance materials. Using 3D hierarchical architectures that traverse the atomic, nano-, micro-, to macro-scale with precision, nature and humans exploit exotic physical properties or better performance beyond the inherent properties of the materials, such as diffracting iridescence of nacre, unique quantum effects, and parallel computing. However, visible light is a demarcation point because conventional microscopy such as optical microscope cannot resolve the materials below this length scale. In this thesis, we apply scanning transmission electron microscopy (STEM) to investigate materials down to angstrom length scales using the recent advancement of aberration-corrected electromagnetic lenses. First half of this work provides systematic approach on Nacre to understand the superior toughness, the mesocrystalline order, and the self-correcting growth. The second half of this work provides experimental approach on Group III-Nitrides to understand the structure and chemistry attributable to enhance solar conversion efficiency. The first chapter motivates materials characterization by high-energy electrons for natural biominerals and man-made semiconductors. The exceptional resolving power of STEM with spectroscopic techniques are able to reveal the structural behavior of nacre from macro- to nanoscale and the exotic new phases in group III-nitride at atomic scale. In Chapter II, our investigation of nacre deformation reveals the underlying nanomechanics that govern the structural resilience and absorption of mechanical energy1. Using high-resolution S/TEM combined with in-situ indentation, we observe nanoscale recovery of heavily deformed nacre. The combination of soft nanoscale organic components with inorganic nanograins hierarchically designed by natural organisms results in highly ductile structural materials that can withstand mechanical impact and exhibit high resilience on the macro- and nano-scale. Chapter III presents Nacre’s remarkable medium-range mesocrystal formed through corrective processes that remedy disorder and topological defects2. In layered growth of nanomaterials, external guidelines don’t exist and mesocrystallinity is prohibitive. In rare instances Nature unconsciously assembles mesocrystals—which merits our attention. The entire nanostructure of nacreous pearls is characterized in cross-section to reveal complex stochastic processes that govern ordered nacre growth. Mollusks strike balance between preserving translational symmetry and reducing thickness variation by creating a paracrystal with medium-range order (5.5 µm). This balance allows Pearls to attenuate the initial disorder during early formation and maintain order throughout a changing external environment. In Chapter IV, the thesis extends the InGaN ternary system, that is an optimal photoelectrode for efficient solar hydrogen production3-5. However, it is difficult to grow high crystalline InGaN with uniformly homogeneous indium composition because In-rich crystals are highly strained causing phase segregation and subsequent performance degradation6. Here, aberration-corrected STEM combined with analytic spectroscopy such as EELS and XEDS is used to study crystallinity and compositional uniformity in 1D InGaN heteroepitaxy. Finally, in Chapter V we discuss AlGaN ternary system for high-efficiency deep UV light sources. It is the only alternative technology to replace mercury lamps for water purification and disinfection7-9. At present, however, AlGaN-based mid- and deep UV LEDs exhibit very low efficiency. Here, we investigate the interface phenomenon of 2D AlGaN such as tunnel junction, quantum wall, and nanoclusters in active region to enhance light emitting performance9-12.