"Recent Experiments on Materials with Unusual Thermal, Thermoelectric, and Spin-Calortronic Properties", Li Shi, University of Texas at Austin. Host: Sridhar Sadasivam

Materials with high or low thermal conductivity and new thermal energy conversion methods can both help to address various technological challenges and stimulate new fundamental studies. Here, we review results from several recent experiments of materials with unusual thermal, thermoelectric, or spin caloritronic properties. Chemical vapor deposition (CVD), chemical vapor transport (CVT), and travelling solvent floating zone methods are employed to grow one- and two- dimensional materials, continuous graphitic network structures, cubic phase boron arsenide with potentially ultrahigh thermal conductivity, and complex crystals with high magnon thermal conductivity or low lattice thermal conductivity. New four-probe electro-thermal and elastic light scattering measurement methods are established to probe the intrinsic thermal transport property and the relaxation length scales of phonons and magnons in these materials. Some of these materials are integrated with electronic and thermal storage devices to enhance the thermal performance.

"Protein Engineering - From Nature to Nanotechnology", Tijana Grove, Virginia Technical Institute, Grove Lab. Host: Tijana Rajh

Abstract: Proteins and protein assemblies are the biological workhorses that carry out vital functions in all living organisms. The Grove Lab is interested in translating fundamental knowledge and principles of how proteins operate in nature to the growing field of nanotechnology for the design of multifunctional, dynamic materials. Herein, I will present our most recent work on the repeat-protein biomaterials that self-assemble through the combination of head-to-tail stacking and weak dipole-dipole interactions. These ion and proton conducting materials exhibit anisotropic properties at biologically relevant length-scales, nano to micro, and relevant time scales, miliseconds to seconds. I will further highlight the tunable anisotropic nano and meso-scale morphologies, which direct the bending, twisting, or curling of material in response to changes in relative humidity and electric potential.

"Bio-Enigma: High Throughput Screening for Programmable Advanced Materials", Melik Demirel, Pennsylvania State University. Host: Tijana Rajh

Abstract: Recent advances in the nanotechnology of materials combined with parallel improvements in biotechnology and synthetic biology, have demonstrated that more complex biomimetic materials with properties engineered precisely to optimize performance, can be achieved. Specifically, proteins provide unique advantages as advanced materials. For example, proteins can often self-assemble and form network materials with extraordinary properties (PRL’2005) such as extremely high durability or elasticity. More importantly, protein can evolve to new functionalities by gene mutations or duplications, which is unique advantage compared to inorganic materials. Recently, we used a direct correlation between gene duplications and its impact on physical properties to demonstrate that tandem repetition of protein sequences enhances physical properties (PNAS’2016). This method opened the opportunity for the assembly of 2D materials (e.g., graphene, MXene) that are precisely controlled with nm resolution (Carbon’2017) for application in electronic and optical devices. In parallel, we reported the development of a new technique to screen protein evolution based on laser-probing spectroscopy with sub-picosecond resolution (Analyst’2017). Our results demonstrate, for the first time, relative quantification of protein network topology in real time for directed evolution. Hence, combining materials assembly and high-throughput screening, we could answer many fundamental questions in materials research, such as the fundamental long-range order in soft matter as well as development of new tools for advanced materials assembly. Programming physical properties through evolution introduces a new design rule for the understanding of materials design.

"Plasmon-Enhanced Solar Energy Conversion in Semiconductor-Metal Heterojunctions", Nianqiang (Nick) Wu, West Virginia University. Host: Gary Wiederrecht

Solar energy can be converted either to electric energy via photovoltaics or to chemical energy via photocatalysts. Emerging of surface plasmon resonance (SPR) provides a new opportunity to improve the performance of photocatalysts and photoelectrochemical cells. This talk will present our effort on fundamental understanding of the underlying mechanism of plasmon-enhanced solar energy harvesting. In particular, the speech will discuss our newly discovered plasmon-induced resonant energy transfer (PIRET) mechanism in the metal-semiconductor heterojunctions. The PIRET mechanism along with the hot electron transfer process suggests that plasmonic nanostructures can act as photo-sensitizers. The discovery of plasmonic photo-sensitizers has opened a new avenue to develop efficient photocatalysts and solar cells. The theoretical maximum efficiency of solar energy conversion in plasmonic metal-semiconductor heterojunctions is predicted. This talk will demonstrate our effort on development of effective plasmonic metal-semiconductor heterojunctions to enable strong coupling between the plasmonic metal and the semiconductor. Our results show that the performance of solar water splitting by a hematite nanorod array can be greatly improved by plasmon-induced photonic and energy transfer enhancement.

"From Model Systems to Efficient Catalytic Materials: One Nanocrystal Fits All", Matteo Cargnello, Stanford University, Department of Chemical Engineering. Host: Benjamin Diroll

Catalytic processes are ubiquitous in industry and are crucial for the sustainable development and growth of our society. Many important catalyst discoveries in the past and current century allowed the population to grow, and the quality of life to improve considerably. Despite the incredible importance of these discoveries, edisonian approaches have been used in most cases to find efficient catalysts that could be scaled up to the industrial needs and the growing demand. Unfortunately, these approaches can only take us so far, and new solutions to important challenges are needed. A responsible approach in developing catalytic materials is represented by the design of catalytic sites based on the knowledge of reaction mechanisms and structure-property relationships, and in the precise synthesis of these sites at the atomic and molecular level. To achieve this goal, model systems are used to decrease the complexity of realistic catalytic systems, but it is challenging to relate the models with realistic conditions. In order to fill this gap, nanocrystals, in which size, shape and composition can be accurately tuned close to atomic precision, are emerging as ideal tools that share advantages of model systems, yet can be utilized under realistic conditions as efficient catalytic phases. The goal of this talk is to show how this approach can provide not only fundamental understanding of catalytic reactions, but also represent a way to precisely engineer catalytic sites to produce efficient catalysts that are active, stable and selective for several important catalytic transformations. Examples of this approach will be given in the areas of methane activation, photocatalysis, and in the design of active and stable materials for high-temperature reactions.

"Understanding Complex Biological Systems Using Modeling, Experiments, and Analytics", Payel Das, IBM Thomas J. Watson Research Center. Host: Subramanian Sankaranarayanan

Understanding the emerging collective behaviors in complex, dynamic biological systems remains a major challenge. In the first part of my talk, I will discuss use of synergistic experimental-simulation approaches to characterize oligonucleotide dynamics on a biosensor surface and disordered peptide dynamics in solution. In the second part of the talk, I will present application of graph embedding techniques to characterize dynamics of apparently high-dimensional systems, such as peptides and human brain.

"Surface Defected Nanoceria Catalyzing Angiogenesis and its Implications in Wound Care", Sudipta Seal, University of Central Florida: Host: Elena Shevchenko

Angiogenesis is the formation of new blood vessels from existing blood vessels and is critical for many physiological and pathophysiological processes. In this study we have shown the unique property of redox active nanoparticles (Re-NPs) to induce angiogenesis, observed using both in vitro and in vivo model systems. In particular, Re-NPs trigger angiogenesis by modulating the intracellular oxygen environment and stabilizing hypoxia inducing factor 1alpha endogenously. Furthermore, correlations between angiogenesis induction and Re-NPs physicochemical properties including: surface valence state ratio, surface charge, size, and shape were also explored. High surface area and mixed valence states make these nanoparticles more catalytically active towards regulating intracellular oxygen, which in turn led to more robust induction of angiogenesis. Atomistic simulation was also used, in partnership with in vitro and in vivo experimentation, to reveal that the surface reactivity of NPs and facile oxygen transport promotes pro-angiogenesis. The talk will conclude with a nanoceria based optoelectronic sensors for biomarker detection.

"The Emergence of Hybrid Perovskites for High-efficiency Low-cost Optoelectronic Devices", Aditya Mohite, Los Alamos National Laboratory. Host: Xuedan Ma

In this talk, I will describe our recent work on Ruddlesden-Popper halide perovskites as a potential alternative to the bulk hybrid perovskites. I will describe the versatility of this novel system through our efforts on achieving photovoltaic devices, photodetectors and light emitting diodes with technologically relevant stability. At the heart of these high performance devices lies an unusual photo-physical behavior where counterintuitive to classical quantum-confined systems where there exists an internal mechanism for the dissociation of excitons to edges of the perovskite layers. These states provide a direct pathway for dissociating excitons into longer-lived free-carriers, which remain well protected from non-radiative processes..

"Transport and Spectroscopy of Illuminated Molecular Junctions", Abraham Nitzan, University of Pennsylvania. Host: Tal Heilpern

The interaction of light with molecular conduction junction is attracting growing interest as a challenging experimental and theoretical problem on one hand, and because of its potential application as a characterization and control tool on the other. From both its scientific aspect and technological potential it stands at the interface of two important fields: molecular electronics and molecular plasmonics. I shall review the present state of the art of this field and our work on optical response, Raman scattering, temperature measurements, light generation and photovoltaics in such systems.

"Harnessing and Avoiding Loss in Optical Metamaterials", Jason Valentine, Vanderbilt University. Host: Gary Wiederrecht

Optical metamaterials are man-made materials in which structuring is used to control the effective optical properties. Metamaterials have traditionally been made from metals and absorption loss has long been one of the primary impediments to their adoption in practical applications. Over the past several years researchers have come to realize that loss can not only be harnessed for certain applications but also completely avoided when transparency of the material is critical. In this talk, I will start by discussing how absorption loss in plasmonic materials can be utilized in energy conversion devices by harvesting hot electrons within the metal. In this case, the use of metamaterials provides the freedom to engineer the optical absorption while also optimizing the geometry of the metal for efficient capture of the hot electrons. In the second half of the talk I will discuss all-dielectric metasurfaces in which metal, and the accompanying absorption, is completely avoided. As with their plasmonic counterparts, manipulation of the unit cell structure of all-dielectric metasurfaces offers a means to engineer a wide variety of optical properties. This freedom, combined with the reduction in absorption loss, could lead to ultra-thin optical elements and assemblies. Along these lines, I will discuss several implementations of all-dielectric metasurfaces with functionalities that include polarization control, wavefront tailoring, near-unity reflection, and sharp Fano resonances.

"Dimensionally Control of Complex Transition Metal Compounds", Turan Birol, University of Minnesota. Host: Kendra Letchworth-Weaver

The fact that the dimensionality of transition metal compounds’ crystal structures have a strong effect on their macroscopic properties is well known, but the microscopic mechanism by which this happens is not always obvious. First principles methods, such as Density Functional Theory or Dynamical Mean Field Theory, are capable of providing insight that can help uncover the mechanisms of dimensional reduction. In the first half of this talk, I will discuss the microscopic mechanism behind the emergence of novel polar phases in SrTiO3 Ruddlesden-Popper compounds, present results from first principles calculations, and clarify how the interfacial rumpling, only recently observed experimentally in these systems, is responsible of the dimensional reduction. The second half of the talk will be on low connectivity fluoroiridate compounds which, according to our Dynamical Mean Field Theory calculations, host a J=1/2 Mott insulator state because of the interesting interplay between the crystal structure, electronic correlations, and spin-orbit coupling.

"Coherent Quantum Dynamics in Atomically Thin Semiconductors: Excitons, Trions, and Valley Pseudospins", Xiaoqin (Elaine) Li, University of Texas-Austin. Host: Jeff Guest

The near band-edge optical response of atomically thin transitional metal dichalcogenides (TMDs) is dominated by tightly-bound excitons and charged excitons (i.e. trions). A fundamental property of these quasiparticles (excitons and trions) is dephasing time, which reflects irreversible quantum dissipation arising from system (excitons and trions) and bath (vacuum and other quasiparticles) interactions. Using a powerful coherent spectroscopy method known as the two-dimensional Fourier transform spectroscopy, we investigate the ultrafast coherent dynamics of excitons, trions, and valley pseudo-spins in these monolayer semiconductors. These experiments shine new light on the relevant time scales over which these quasiparticles and pseudospins can be coherently manipulated.

"The Hidden Secrets of Nanocrystals, Interfaces, and Surfaces at Atomic Resolution", Christian Kisielowski, Berkeley National Laboratory. Host: Jianguo J.G. Wen

A suitable combination of single digit nanocrystals with their rich variety of tunable surfaces and interfaces allows tailoring materials with novel structure-function relationships. The design of new catalytic materials [1,2] may serve as examples. However, it remains challenging to characterize and understand such materials at the atomic scale with single atom sensitivity in 3D [4] because of their pronounced sensitivity to the probing electron radiation that unintentionally alters their pristine structure, often beyond recognition. We address this challenge by applying low dose-rate (< 10 e/Å2s) in-line holography [4], which combines the best imaging practices developed for biological research with the acquisition of image series. In essence, the new microscopy method exploits reversible object excitations by capturing focus series of entirely noise dominated images that are reconstructed to obtain electron exit wave functions of radiation sensitive matter with unprecedented contrast and resolution. We observe a variety of previously unknown atom configurations that are otherwise hidden behind a barrier of beam-induced object alterations and structures that are greatly affected by an exposure of the material to water vapor or other gases in environmental electron microscopy. Chemical compositions can be determined by contrast measurements alone and functional processes can be triggered and tracked in real time at atomic resolution.
[1] J. Yang, et al., Nature Materials (2016) DOI: 10.1038/NMAT4794
[2] Y.Yu, et al., Nano Letters (2016) DOI: 10.1021/acs.nanolett.6b03331
[3] F.R. Chen et al., Nature Commun. 7:10603 doi: 10.1038/ ncomms10603 (2016)
[4] C. Kisielowski, Advanced Materials 27 (2015) 5838-5844

"Quantum Control Over Diamond Spins with a Mechanical Resonator", Greg Fuchs, Cornell University. Host: Stephen Gray

Creating and studying coherent interactions between disparate solid-state quantum systems is a challenge at the intersection of atomic physics, condensed matter physics, and engineering. In general, different physical realizations of a quantum bit (qubit) operate at different frequencies, on different size scales, and couple to different fields. Nonetheless, efforts to create “hybrid quantum systems” are appealing because they could enable a quantum concert – were parts are played by different physical qubits that each offer the best performance in a particular area. There is a growing consensus that mechanical motion is a “plastic” degree of freedom for solid-state qubits, with the potential to form a coherent interface between them, and with light. This has motivated intense research into the coherent interactions between mechanical resonators and qubits formed from photons, trapped atoms, superconducting circuits, quantum dots, and nitrogen-vacancy (NV) centers in diamond, to name a few. I will describe our experiments to coherently couple NV center spins to gigahertz-frequency mechanical resonators using crystal lattice strain. In high-quality diamond mechanical resonators, we demonstrate coherent Rabi oscillations of NV center spins driven by mechanical motion instead of an oscillating magnetic field. We also use the mechanical resonator to protect the coherence of the NV center spin. Finally, I will describe recent results in which we experimentally examine the spin-strain coupling within the NV center excited-state manifold and explain how it can be used to cool a mechanical mode.