Directing Assembly of Organic Electronics Inspired by Living Systems
University of Illinois at Urbana-Champaign
Molecular assembly, crystallization and controlled phase transition have played a central role in the development of modern electronics and energy materials. Recent years, printed electronics based on semiconducting molecular systems have emerged as a new technology platform that promise to revolutionize the electronics and clean energy industry. In contrast to traditional electronic manufacturing that requires high temperature and high vacuum, these new electronic materials can be solution printed at near ambient conditions to produce flexible, light-weight, biointegrated forms at low-cost and high-throughput. However, it remains a central challenge to control the morphology of semiconducting molecular systems across length scales. The significance of this challenge lies in the order of magnitude modulations in device performance by morphology parameters across all length scales. This challenge arises from the fact that directed assembly approaches designed for conventional hard materials are far less effective for soft matters that exhibit high conformational complexity and weak, non-specific intermolecular interactions. On the other hand, biological systems have evolved to assemble complex molecular structures highly efficiently. We are eager to transfer the wisdom of living systems to developing printed electronics technologies as to enable next generation electronics for clean energy and healthcare. In this talk, we present new insights and strategies we recently developed for controlling multi-scale assembly and transformation of semiconducting molecules. We learned from living systems and designed bioinspired assembly processes, allowing molecules to put themselves together cooperatively into highly ordered structures otherwise not possible with significantly improved electronic properties. We discovered molecular design rules that impart dynamic and switchable electronic properties through the mechanism of molecular cooperativity – a mechanism ubiquitous in nature. These new solid-state properties could potentially enable new sensing and actuation mechanisms not possible before. We further developed 2D and 3D printing process to realize on-the-fly morphology control down to the molecular and nanoscale.
Electronic Transport in Strain-Engineered Graphene
University of Illinois at Urbana-Champaign
There is wide interest in using strain-engineering to modify the physical properties of 2D materials, for both basic science and applications. Deformations of graphene, for example, can lead to the opening of band gaps, as well as the generation of pseudo-magnetic fields and novel quantum Hall states. We demonstrate how controllable, device-compatible strain patterns in graphene can be engineered by depositing graphene on corrugated substrates. We discuss several techniques for creating corrugated substrates, focusing on periodic spherical curvature patterns in the form of closely packed nanospheres. We show how the smaller nanospheres induce larger tensile strain in graphene, and explain the microscopic mechanism of this. We also present experimental results demonstrating how a nearly periodic array of underlying nanospheres creates a strain superlattice in graphene, exhibiting mini-band conductance dips and magnetic field effects that depend on the magnitude of induced strain.
Elucidating the Mechanisms for Atomic Layer Growth through In Situ Studies
Argonne National Laboratory
Atomic Layer Deposition (ALD) provides exquisite control over film thickness and composition and yields excellent conformality over large areas and within nanostructures. These desirable attributes derive from self-limiting surface chemistry, and can disappear if the self-limitation is removed. Understanding the surface chemical reactions, i.e. the ALD mechanism, can provide insight into the limits of self-limitation allowing better control, successful scale up, and the invention of new processes. In situ measurements are very effective for elucidating ALD growth mechanisms. In this presentation, I will describe investigations into the growth mechanisms of ALD nanocomposite films comprised of conducting (e.g. W, Mo and Re) and insulating (e.g. Al2O3, ZrO2 and TiO2) components using in situ measurements. These ALD nanocomposites have applications in particle detection, energy storage, and solar power. We have performed extensive in situ studies using quartz crystal microbalance (QCM), quadrupole mass spectrometry (QMS), Fourier transform infrared (FTIR) absorption spectroscopy, and current-voltage measurements. These measurements reveal unusual ALD chemistry occurring upon transitioning between the ALD processes for the two components. This results in unique reaction products that affect the properties of the films in beneficial ways. The knowledge gained from our in situ studies of the ALD nanocomposite films has helped us to overcome problems encountered when we scaled up the ALD processes to large area substrates. Beyond fundamental understanding, in situ measurements are extremely effective in ALD process development and process monitoring. I will end my talk by describing our recent work combining in situ measurements and machine learning to accelerate ALD process development.
Integrated Micro- and Nanofluidic Devices for Single-Particle Tracking of Biological Processes
We are developing integrated micro- and nanofluidic devices to study virus assembly and bacterial development at the single-particle level. Analysis of single particles provides unprecedented insight into biological processes that are often missed when a population is studied as an ensemble. For fabrication of these integrated devices, we combine focused-ion beam milling or electron-beam lithography with photolithography, wet-chemical etching, and cover plate bonding. To characterize capsid assembly of hepatitis B virus, we are using resistive-pulse sensing as a label-free, nondestructive technique. This single-particle method permits real-time detection and has sufficient sensitivity to monitor assembly at biologically relevant concentrations and over a range of reaction conditions. Assembly in the presence of potential antivirals and chaotropes produces a variety of particle morphologies, including normal capsids, kinetically trapped intermediates, and aberrant structures. To study development of bacteria, we have integrated nanochannel arrays into a microfluidic platform that physically trap bacteria. The nanochannels confine growth of bacteria in one dimension, and when coupled with fluorescence microscopy, these devices measure bacterial processes with improved temporal and spatial resolution. Growth and division rates, subcellular functions, epigenetic effects, and antibiotic response are easily tracked for extended periods of time and across multiple generations.