(6ex) Understanding and Designing Non-Equilibrium States at Soft/Hard Material Interfaces

My career objective is to interface soft and hard materials with controlled non-equilibrium interfacial states to create material platforms that address societal challenges in energy, electronics, health and water. Interfaces between soft and hard matter ubiquitously exist in natural and man-made systems such as cartilage/bone interface in body, fluid/electrode interface in catalyst, and solid/electrolyte interphase in battery. These interfaces often carry deterministic impacts on material behavior, property and functionality by governing transport and transformation of mass and charge. Such exchanges fundamentally link to the free energy information encoded in the interfacial material states. Soft and hard materials differ profoundly in the strength and density of chemical bonds, giving rise to largely mismatched energy density at interface. How to control this energy discontinuity stand as a core consideration that dictates the scope of soft/hard hybrid systems.

The nonstoichiometric coordination environments at the interface create spaces for materials away from equilibrium (e.g., dangling molecules, glassy materials, grain boundaries, and metastable phases). Conventional soft/hard composites have overlooked these domains due to the shortage of generalizable methodologies that can afford predictable control. Nevertheless, non-equilibrium materials encompass free energy levels common to neither soft nor hard material bulks, and thus can be exploited to design the interfacial energy landscape. Their time-dependent and stimuli-responsive features further promise otherwise unreachable hierarchy and dynamics. Therefore, understanding and designing the compositional, spatial, structural, and temporal characters of interfacial non-equilibrium states will offer unique opportunities to build unprecedented soft/hard material platforms.

My academic training in hard (PhD) and soft matter (postdoc) will allow me to establish a compelling interdisciplinary research program on interface science. I have accumulated expertise in a broad range of synthesis and characterization methods relevant to oxides, halides, fluidic and polymeric systems, particularly in confined environments. Based on these foundations, I propose the following future research themes:

1. Soft/hard heterostructures with programmable hierarchy and phases: The increasing demands of sophisticated functionalities force us to go beyond monolithic materials and pursue hybrid matters. I propose to develop soft/hard 3D heterostructures that have controllable hierarchy and phases using liquid crystal elastomers (LCE)/halide perovskites as an exemplary soft/hard system. To unleash the potential that non-equilibrium interfaces could bring, I plan to locally nucleate and grow perovskites inside the open modules defined by LCE networks. Such a unique synthesis route may lead to the following outcomes: (i) promoted or retarded crystallization kinetics of perovskites due to the nanoscale confinement of LCE networks; (ii) programmable surface reconstruction of perovskites by controlling LC isotropy; (iii) configurable size and geometry distributions of perovskites via design of the LCE network. This platform contains rich diffusion and crystallization questions, for instance, whether the nucleation and growth of inorganic in confined and/or disordered polymeric surfaces follows concepts of classical nucleation theory or falls within the broader context of “nonclassical” pathways involving formation, aggregation, and transformation of transient material states (i.e., Ostwald-Lussac law). It will also bring technological implications to optoelectronics as the received hybrid is virtually a flexible optoelectronic unit that features stimuli-responsive light emission and long-term stability.

2. Soft/hard grain-scale epitaxy: Epitaxy means the growth of a crystalline film on top of a crystalline substrate undergoing a small lattice mismatch. It is a vital concept in solid-state semiconductor field, conventionally irrelevant to soft materials. However, the nature of crystallinity and lattice refers to structural order, which do exist in crosslinked polymer network. The crosslinking mesh can be treated as the “lattice” for polymer though it has larger degree of freedom to motion and assembly compared with the solid lattice. The "unit cell" of crosslinked polymer networks is at the scale of grain sizes, super lattice, or artificially created patterns of a hard material. Based on that fact, I propose to selectively bind the crosslinking unit cell with the grain boundaries by controlling the assembly of bridging molecules, which I name as, grain-scale epitaxy. This epitaxial soft/hard interface could provide on-demand control of interfacial ion/charge transfers. For example, it could enable directional lithium ion transport from electrolyte to electrode in solid-state batteries. Combining with the soft/hard heterostructure proposed above, this design would yield a previously unseen hard/hard heterostructure that has commensurate grain arrangements.

3. Multifunctional encapsulations for soft bioelectronics: Exquisite control of soft/hard interfaces will have broad technological impacts in energy, electronics and bioengineering. I propose to develop a multifunctional encapsulation platform for soft bioelectronics. The encapsulation will present the following characteristics: (i) flexible and transparent; (ii) low water permeability under deformation; (iii) anti-fatigue; (iv) interfacial adhesion to skin or organs. Its design will entail a crosslinked polymer matrix with infusion of dense hydrophobic oil, inclusion of phase-separated droplets, and interfacial bonding molecules. The fluidic oil will serve as a dynamic and replenishable barrier to seal the free volume created during deformation. The droplets will contain precursors that undergo solidification upon mechanical perturbation or air exposure at crack tip. The stimuli-responsive solidification will locally bridge the crack and prevent its further propagation, while maintaining the bulk intact and elastic. This theme will provide transformative insights for the protection and electrode design of bioelectronics, and other emergent technologies, such as perovskite optoelectronics and soft robotics.

Teaching Interests:

After going through bachelor and PhD courses, as well as 11 courses audited during postdoc, I gained a clear vision about what expectations are for a great teacher. I have met several remarkable professors who not only taught me knowledge but also ignited my passion to subjects of thermodynamics and kinetics, intermolecular interactions, and fracture mechanics. Inspired by different aspects from each of them, I develop my teaching philosophy with emphasis on three core principles: (i) fostering the students’ interest to the subject and science in general; (ii) inspiring the students to find the core of scientific questions; (iii) cultivating students’ self-learning ability and collaborative and independent problem-solving skills. As engineers, we have plenty of practical implementations in real life to draw from to intrigue students. The history of science and scientists (e.g., rubber and Goodyear) is always a fantastic aspect to introduce. Fundamental questions will help the students to see the heart of a phenomenon. During the lecture, I will ask questions like: what is the difference between the gas diffusion (or charge transfer) in a single crystal and that in a glassy material? Where does the difference come from? I will also encourage students to ask questions and nurture an active classroom. Team projects and in-class discussions will serve as versatile media to connect students.

I am interested in teaching engineering courses such as “Statistical Thermodynamics” and “Kinetics and Phase Change” at introductory or advanced level. In these courses, I plan to teach students topics including Brownian motion, Boltzmann distribution, thermodynamic laws, thermodynamic functions, specific heat, statistical ensembles, chemical equilibrium and kinetics, transition rate theory, atom movements and point defects, kinetics of phase change, nucleation and growth, and etc., covering a whole spectrum of solid-state and soft-matter, atomic and molecular interactions. I would like to develop two more graduate-level classes including “Surface Chemistry” and “Material Interfaces”. For example, in the course of “Surface Chemistry”, I would like to deliver students historical motivations of surface chemistry; the fundamental knowledge of surface energy, bonding, diffusion, electronic orbitals, interfacial band structures; and the vastly exciting advancements of practical applications such as catalyst, battery, optoelectronics, photoelectrochemical fuel production, physical vapor deposition, chemical vapor deposition, atomic layer deposition, etc.

Selected publications (^* represents equal contribution):

1. Yanhao Yu^*, C. Sun^*, X. Yin, J. Li, S. Cao, C. Zhang, P. M. Voyles, X. Wang. “Metastable Intermediates in Amorphous Titanium Oxide: A Hidden Role Leading to Ultra-Stable Photoanode Protection” Nano Lett., 18, 5335-5342 (2018).

2. Yanhao Yu, X. Wang. "Piezotronics in Photo-Electrochemistry", Adv. Mater., 1800154, (2018).

3. Yanhao Yu^*, Z. Zhang^*, Q. Liao, Z. Kang, X. Yan, Y. Zhang, X. Wang. “Enhanced Photoelectrochemical Efficiency and Stability Using a Conformal TiO₂ Film on a Black Silicon Photoanode” Nature Energy, 2, 17045 (2017).

4. F. Wu^*, Yanhao Yu^*, H. Yang, L. N. German, Z. Li, W. Yang, L. Huang, W. Shi, L. Wang, and X. Wang. “Simultaneous Enhancement of Charge Separation and Hole Transportation in TiO₂-SrTiO₃ Core-Shell Nanowire Photoelectrochemical System” Adv. Mater., 29, 1701432 (2017).

5. Yanhao Yu, Z. Li, Y. Wang, S. Gong, and X. Wang. “Sequential Infiltration Synthesis of Doped Polymer Films with Tunable Electrical Properties for Efficient Triboelectric Nanogenerator Development” Adv. Mater., 27, 4938-4944 (2015).

6. W. Yang^*, Yanhao Yu^*, M.B. Starr, X. Yin, Z. Li, A. Kvit, S. Wang, P. Zhao, and X. Wang. “Ferroelectric Polarization-Enhanced Photoelectrochemical Water Splitting in TiO₂-BaTiO₃ Core-Shell Nanowire Photoanodes” Nano Lett., 15, 7574-7580 (2015).

7. Yanhao Yu, J. Li, D. Geng, J. Wang, L. Zhang, T. Andrew, M. Arnold, and X. Wang. “Development of Lead Iodide Perovskite Solar Cells Using Three-Dimensional Titanium Dioxide Nanowire Architectures” ACS Nano, 9, 564-572 (2015).

8. Yanhao Yu, X. Yin, A. Kvit, and X. Wang. “Evolution of Hollow TiO₂ Nanostructures via the Kirkendall Effect Driven by Cation Exchange with Enhanced Photoelectrochemical Performance” Nano Lett., 14, 2528-2535 (2014).