OIST-UC Santa Barbara Mini Symposium "Materials of Tomorrow: Harnessing Responsiveness, Intelligence, and Sustainability"

Polymer-based mixed ionic/electronic conductors (MIECs) are gaining prominence for their broad application spectrum, from energy storage in batteries and ultracapacitors to biosensors and actuators. Initially, MIECs struggled with ionic mobility due to hydrophobic backbones, but advancements have been made by incorporating polar oligoethylene glycol (oEG) side chains, enhancing ionic conductivity and overall performance. Further research into polythiophenes with varied LiTFSI doping levels has revealed that side chain architecture significantly influences material morphology and conductivity. These findings underscore the critical role of oEG side chain design in optimizing MIECs' ionic and electronic properties, and will be discussed in detail. 

We will present recent work on the plasma-enhanced chemical vapor deposition of nanocrystalline diamond films, which are commonly grown on substrates seeded with nanodiamonds dispersed in a liquid such as water. Our research aims to unravel the processes governing film growth by adjusting initial seed density and modulating the concentrations of methane, oxygen, and hydrogen in the gas feed. This approach enables precise control over film morphology. In parallel, we are developing diamond processing techniques to forge innovative paths for developing practical devices with applications in thermal management, quantum sensing, nanofluidics, and membrane science.

Photoinduced carrier generation is the initial process in solar cells and persistent luminescent materials. Because charge generation in organic materials is more difficult than inorganic materials, the electron donor-acceptor interface is generally used for photoinduced charge separation. However, the generated charge is quickly deactivated by recombination in the organic semiconductors. We have reduced charge recombination probability and achieved stable charge-separated states by controlling the donor/acceptor concentration and energy levels. These long-lasting organic charges enable persistent luminescence and stimulated luminescence.

The primary focus of our research team is to deepen understanding of emergent phenomena driven by many electrons in low-dimensional quantum materials and their interfaces. It is widely recognized that the development of emergent phenomena at the nano-scale holds immense significance for advancing future nano-electronics, nano-spintronics, nano-optics, and the broader field of hybrid quantum device applications, surpassing conventional silicon technology. To support this endeavor, we have recently established an advanced and state-of-the-art laboratory facility that synergistically combines synthesis and spectroscopic techniques, empowering our team to search various unexplored areas. I will introduce our recent findings on the rich electronic emergence within 2D material and strongly correlated oxide categories in my talk. Through these examples, the main aim of this talk is to explore potential synergetic collaborations.

Excitons – the few-particle state composed of an electron and hole bound by Coulomb interaction – are one of the most fundamental excited states in semiconductors and insulators. The recent emergence of 2D semiconductors such as transition metal dichalcogenides has offered a unique playground with a robust and rich variety of bright, dark and valley-polarized excitonic states. We have developed a unique instrumentation - Time-Resolved-XUV -µ-angle-Resolved Photoemission Spectroscopy (TR-XUV-µ-ARPES), that gives us direct access to the momentum-resolved dynamics of photoexcited states. I will present our work on the direct visualization of dark excitons and the measurement of the intrinsic wavefunction of the Wannier exciton. Finally, I will show our latest results on valleytronics providing a complete picture of the excitonic landscape after a valley-selective photoexcitation.

With the exception of bones and teeth, the human body is composed of soft and hydrated tissues with freeform geometric construction. Living cells and tissues can endure substantial loads and strains, execute intricate processes of heat and mass transfer, and are in a constant state of remodeling, rebuilding, and adapting to external stimuli. Nature’s elegant solutions to mechanical and transport challenges across sliding interfaces (e.g., cartilaginous joints, eyeblinks) includes fragile biopolymer gels, which dissipate energy under shear yet rapidly recover to provide load support. Understanding the fundamental mechanisms by which biological gels sustain large strains and plastic deformations is often frustrated by the inherent complexity of biological samples: denaturation/degradation, small volumes, and high sample-to-sample variability. High water content hydrogels have emerged as synthetic model materials to enable investigations of interfacial phenomena in biological gels. This presentation is focused on our recent efforts integrating custom-built tribological instrumentation, advanced microscopy, and bio-inspired materials design to shed light on Nature’s design rules for low friction across biological sliding interfaces.

Sustainability is an important goal in contemporary materials science, but many commercial applications still use conventional polymers that lack viable end-of-use recycling or degradation pathways. This talk will discuss new design strategies rooted in novel chemistry to render common pressure-sensitive adhesives recyclable and degradable without sacrificing compelling performance. Insights spanning monomer selection, copolymerization kinetics, and material properties provide a route to improve the sustainability of adhesives found in everyday consumer products and advanced technology alike.

Polymeric battery binders are a ubiquitous component in composite lithium-ion cathodes, providing critical structural functionality. However, industry standard binders, such as polyvinylidene fluoride (PVDF) are insulating to both electrons and ions, detrimentally adding resistance to the overall system. Mixed ion-electron conducting polymers are promising
materials for next generation battery binders, as they can provide the adhesive properties of
traditional binders while also facilitating charge transport. However, simultaneously optimizing electronic, ionic, and lithium transport within a single system has proved a challenge, particularly given the need to maintain the mechanical function required of a binder. Further the practical requirements of a battery binder are extreme including: high solids-loading processability, electrochemical stability, and resistance to solvation in the battery electrolyte.   In this talk, I will show that complexation between two oppositely charged polyelectrolytes offers unique opportunities for solvent-lean processing of semiconducting polymers, facilitated by the formation of a polymer-dense fluid phase known as the coacervate. These electrostatically stabilized complexes, comprising of a blend of a charged conjugated polymer with an oppositely charged polyelectrolyte, reduce kinetic limitations in LiFePO4 cathodes. Further, complexation overcomes inherent dissolution issues associated with the single component conjugated polyelectrolytes, while also enhancing electronic conductivity compared to the single component polymers. Across a wide variety of specific polymer chemistries, these conducting binders dramatically improve both rate capability and cycle stability, compared to the industry standard, insulating PVDF binder. Further, via manipulation of electrostatic parameters, including polymer charge fraction and counterion concentration, we can adjust the morphology of these polymer complexes from a homogeneously mixed state to a weakly structured state in which the local ordering arises from backbone-immiscibility-induced segregation. Our findings demonstrate that structural disorder along the CPE backbone is alleviated in strongly mixed complexes due to the nanoconfinement induced via the charge complexation.  As a result, charge transport is improved in charge complexed materials in comparison to the analogous conjugated polymers.  

Advancing the goal of materials-by-design requires the ability to screen materials for function. This is the first step en route to a paradigm of dialing up the optimal material structure and composition to serve a particular function. Several issues that make even this task of screening somewhat complex. The first is that many properties of interest are not tractably calculated in a reliable way, because the underlying science is as-yet not established. The second is that materials optimization is frequently based on much more than a single performance criterion. In this talk, I will describe computational proxies that have allowed us to establish guidelines to find better phosphor materials for solid-state white lighting, better magnetocaloric materials, and some recent work on low-k  dielectrics. Separately, I will describe the computational screening of all inorganic photovoltaic materials.

Geometric frustration and hopping interference effects manifest in hexagonal lattice types are predicted to lead to a variety of unusual electronic states, ranging from quantum spin liquid states to orbital-only magnetism to unconventional superconductivity.  In this talk, I will present results from the experimental search for some of these unusual states in triangular lattice insulators and kagome metals being explored in our group.  In particular, I will focus on layered triangular lattice insulators with spin-orbit entangled magnetic moments and on kagome lattice metals whose electron fillings populate saddle points at their Fermi energies.  I will try and motivate why these compounds are interesting from the perspective of new electronic states stabilized by the frustration effects manifest in each of their lattice types.

Conjugated polyelectrolytes (CPEs) are materials that comprise a conjugated polymer backbone with pendant ionic groups and counter ions. Their pendant ionic side chains make CPEs soluble in polar solvents including water, methanol, dimethylformamide, and dimethylsulfoxide, which presents the opportunity to move away from environmentally unfriendly halogenated solvents toward more green, sustainable solvents. CPEs have been used in sensing of small ions as well as biomolecules, proteins, and DNA, as interfacial interlayers in OLEDs, organic photovoltaics (OPVs), and perovskite solar cells, and to assist in charge injection from the electrodes into the active layer in OFETs and as active layers in OLEDs, polymer light-emitting electrochemical cells (PLECs), OPVs, dye sensitized solar cells (DSSCs), thermoelectrics, and organic electrochemical transistors (OECTs). Among reported CPEs, some can be doped in the presence of a proton source, and thus, have a neutral pH ~ 7. This doping process has been referred to as “self-dope” in the literature because a proton source comes from a solvent used to dissolve the CPE as opposed to the traditional doping method by adding a dopant molecule. These self-doped CPEs can be used as a conductive buffer layer in OLEDs, OPVs, and organic photodetectors (OPDs) to replace poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). In this talk, I will discuss the development of CPEs and how chemical structures such as counterion size, type of charged groups, alkyl chain length, charge density, and conjugated backbone can be used to tune the optical, electronic, and ionic charge transport, and therefore device performance such as OECTs and OPDs.