Oxide Membranes: More Freedom for Artificial Materials
From superconductivity and ferroic orders to electrical, thermal, and electrochemical properties, oxides show a broad range of phenomena with both scientific and technological implications. The atomic-scale synthesis of oxide heterostructures has successfully demonstrated unique properties emerging at their interfaces. By releasing the oxide thin films from the substrate, we want to grant more freedom in design, manipulation, and characterization of the artificial material.
The key materials development is to synthesize a water-soluble sacrificial layer epitaxially, making virtually all types of complex oxide as a membrane. The comprehensive characterization of the first oxide membranes demonstrated their excellent crystallinity, as well as preserving electromagnetic properties. Such synthetic method provides a new opportunity to integrate complex oxides with other materials systems, such as heterostructures of semiconductors and van der Waals 2D materials.
Extreme Strain Design in 2D Quantum Materials
Strain has been a major parameter to control energy scales and electronic structures in quantum materials. Freestanding 2D materials have unique advantages in strain engineering. First, they can be mechanically coupled to an external platform, allowing a large degree of freedom to access arbitrary strain states by design. Second, the critical fracture strain in very thin materials is often greatly enhanced, suggesting a new opportunity to reach an unprecedented level of strain.
The strain platform based on flexible layers gives an access to unconventional strains of which magnitude and symmetries can be freely chosen. In the example of oxide membranes, we have been able to stretch materials well beyond the conventional limit in crystalline oxides. The mechanical tuning of oxides triggers dramatic resistivity changes in correlated magnetic oxides, across multiple electronic phases. This on-chip strain technique holds a great promise to a generic pathway to control quantum phases of matter.
2D Topological Phase Transition in Crystalline Lattice
Can we make any lattice to be a monolayer? This simple question has been asked repeatedly for decades and has reemerged with the advent of exfoliated atomic crystals. In an example of oxide membranes (SrTiO3), we find that the lattice experiences a dramatic collapse in the crystalline structure in the 2D limit when the layer was lifted off from the substrate. The crossover from algebraic to exponential decay of the crystalline coherence length is analogous to the 2D topological Berezinskii-Kosterlitz-Thouless (BKT) transition, likely driven by chemical energy from the interfacial bond-breaking & passivation. The observation suggests that such abstract concept of topological phase transition may play a critical role in designing realistic materials in the 2D limit.
Topological Insulator Nanostructures
The surface electronic states in topological insulators (TIs) possess unique electronic structures and spin properties, serving as a promising candidate to engineer the quantum computing system. TI nanostructures have large surface-to-volume ratios, enhancing the surface transport compared to the bulk contribution. Moreover, the characteristic 1D (nanowire) & 2D (nanoplate) structures provide intriguing materials platform to design topological electronics across the dimensionality.
My dissertation project was to design (vapor phase growth) and characterize (electronic transport) TI nanomaterials. Based on elemental/electrostatic doping and heterostructure design, I was able to realize surface-dominant, high-mobility transport in nanodevices. Further study of 1D helical states in TI nanowires - 2D surface electrons confined in 1D - revealed the nature of unique spin structures and topological protection via transport.
Atomic Filament Scanning Probe
The resistive switching in oxide junctions is driven by ionic migration in the insulator matrix. We utilized the localized filament to invent a high-resolution electrical scanning probe. By electrostatically forming a metal (Pd) filament in the amorphous alumina coating, we can fabricate an atomic size probe on an AFM tip and map out tiny nanostructures (carbon nanotube) with dramatically improved resolution. The filament tip would provide a nm-resolution scanning probe for wider applications, from a nano-bio interface to electrochemistry.
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Electrochemical Spectroscopy on Nanowire Batteries
Many renewable energy technologies are based on electrochemical systems, involving multiple charge/energy transfers in solids (electrode and catalyis), liquids (electrolyte), and their interfaces. Electrochemical impedance spectroscopy (EIS), analyzing the system in the frequency domain, provides a better understanding of the mechanism by decoupling contributions from each component. We applied EIS on Si nanowire anodes - the first example of nanostructured Li-ion batteries. The evolution of the EIS spectrum during cycling suggested that the formation of solid electrolyte interphase was one of the critical factors to optimize the energy efficiency and lifetime, providing an important guideline to design the next generation battery materials.
Nanotube-Metal Hybrid Electromechanical Systems
Materials can be stronger when they are together. Such a principle of composite materials, like reinforced concrete in modern architecture, would apply to nanoscale structures as well. We designed micromechanical systems made of two distinctive materials - carbon nanotubes embedded in an aluminum matrix. The suspended geometry allowed us to characterize the mechanical properties of the nanocomposite in both dynamic (mechanical resonance) and static (AFM force spectroscopy) methods. Interestingly, we observed that there is a significant improvement in the elastic properties of the metal layer by adding a small fraction of the nanotubes, serving as an attractive strategy to design metal-based nanomechanical structures.