Understanding and manipulating quantum superposition are important quests in quantum materials research. One exciting direction emerging in the field is to use the geometry of quantum states – the so-called quantum geometry – as a means of characterizing quantum superposition. This approach has successfully characterized various electric and magnetic properties of materials, which are not easily captured by semiclassical approaches. In this seminar, I will first introduce a general theory of the quantum geometry of resonant optical electric-dipole responses in materials. I will discuss the new insights from this quantum-geometric paradigm, guiding us to discover and investigate new optical phenomena. In the second part, I will talk about an intriguing optical response going beyond electric-dipole transitions: a surprising Kerr effect in certain antiferromagnets that are non-quantized but owe their existence to axion electrodynamics. This response has recently been observed in experiments and used for the optical control of antiferromagnets. Finally, I will give an outlook for future directions.

Weyl semimetals are among the materials proposed to have significant potential in informational technologies and to harbor the necessary elements for quantum computing. They host Weyl nodes at specific points in their Brillouin zone, a pair of relativistic fermions with different chirality, Weyl fermions. The nontrivial momentum-space topology due to the Weyl nodes leads to various fascinating phenomena, such as the chiral anomaly, chiral magnetic effect, anomalous magnetoresistance and Hall effect, large nonsaturating thermoelectric power and ultrafast photocurrents just to name a few. The essential ingredients for the realization of the Weyl semimetal are the absence of inversion symmetry and/or time-reversal symmetry. The RAlX (where R=Rare Earth and X=Ge, Si) family, where both symmetries are broken, has been recently identified as a large class of Weyl semimetal based on systematic first-principles band structure calculations and APRES measurements. In this respect, I will present details and importance of crystal growth of non-centrosymmetric CeAlGe single crystals, their physical properties, anomalous magnetotransport, and discuss the future implications of our findings and the tunability of RAlGe and RAlSi families.  

Cryo–electron microscopy (cryo-EM) has been proven a powerful tool visualizing biological specimen. Method developments in single-particle analysis (SPA) and in situ tomography have enabled more structures to be imaged and determined to attenable resolutions. Sample-to-structure pipeline is essential for an imaging Core in improving the productivity and efficiency in structural determination. As the Director, I am leading the MU Electron Microscopy Core (EMC) in collaboration with MU research laboratories from Physics, Material Science, Computer Science, Biochemistry etc. to develop and implement such a unique pipeline housed in NextGen. Cryo-EM will continue its remarkable growth in technology advancement in the next decades. MU provides great centralized microscopy resource at EMC for scientists from all fields to advance their career and research. We welcome collaborations. 

Abstract:

The conventional electronic system has been developed upon Si-based thin-films because of their cost-effectiveness and mature process. However, there are fundamental limitations in using conventional systems towards future electronics such as wearable devices, biomedical devices, and edge computing devices. One of these most prevalent limitations is that current thin-film electronic systems have been developed on rigid wafers, which causes serious physical constrains because of the thick nature of the materials on the rigid wafers. Thus, an alternative approach has been required to secure mechanically low stiffness of materials and devices.

In this talk, I will discuss about our recent effort to tackle the aforementioned challenge by developing 3D and 2D freestanding nanomembranes. First of all, we have conceived an approach to obtain freestanding single crystalline materials through 2D materials assisted layer-transfer (2DLT). While developing it, we found out that crystallographic information can penetrate through graphene as long as substrates have polarity because of graphene’s information transparency. Also, the slippery nature of graphene enables spontaneous relaxation, which substantially reduces dislocation density in heteroepitaxy. Second, we have developed mechanics to produce 2D materials by playing interfacial toughness contrast. As this approach allows producing various 2D materials at large-scale, various applications can be demonstrated at practical level. Also, we realized that geometrical confinement helps kinetic control of 2D materials, which secures crystallinity, layer-controllability, and heterostructures. As the 3D and 2D freestanding membranes are a new type of building blocks, various opportunities are expected by providing a new platform where physical couplings and practical applications are conceived.

Bio:

Sang-Hoon Bae is an Assistant Professor in Washington University in St. Louis. He was a postdoctoral research associate at Massachusetts Institute of Technology (MIT) working with Professor Jeehwan Kim before joining Washington University. He earned a doctorate in materials science and engineering from University of California, Los Angeles (UCLA) under the supervision of Professor Yang Yang in 2017. He earned bachelor's and master's degrees in materials science and engineering from Sungkyunkwan University (SKKU), Korea, under the supervision of Professor Jong-Hyun Ahn in 2013. He worked at IBM T. J. Watson Research Center (2014) and Samsung Display (2010) as a research intern. He has published more than 70 papers in notable journals including Science, Nature, PNAS, Nature Materials, Nature Nanotechnology, Nature Photonics, and Nature Electronics with over 8000 citations. His h-index is 41 as of March 2023. 

Abstract:

Semiconductor nanocrystals are promising candidates for generating chemical feedstocks through photocatalysis in which photoexcited charge carriers are used to perform charge-transfer reactions. However, the fate of photoexcited charges once they reach the surface, i.e., whether they recombine or are extracted to initiate useful redox reactions, is highly sensitive to the structure of the surface. Our research group is investigating how oxygen vacancies, a common type of surface defect in metal oxide semiconductors, control their photocatalytic activity. Using single-molecule, super-resolution fluorescence microscopy, we image both spatial and temporal variations in the photocatalytic activity of individual semiconductor nanocrystals such as bismuth oxybromide and tungsten oxide. To understand the chemical origins of these variations in activity we apply a combination of ensemble structural characterization, electronic-structure calculations, and the quantitative spatial correlation of multiple fluorogenic probes. Our results show that both photoexcitation and chemical modifications to the surface of semiconductor nanocrystals can be used to tune the concentration and distribution of oxygen vacancies in these materials, which has a significant impact on their resulting catalytic activity. In one system, bismuth oxybromide, a high concentration of oxygen vacancies decreases activity, while in tungsten oxide, it increases activity. Thus, to achieve high performance for a desired catalytic transformation, our results demonstrate it is necessary to tune the concentration and type of defects for the specific photocatalyst. Ultimately, photocatalysts containing a stable, intermediate concentration of oxygen vacancies may prove to be optimal for balancing the activity of both reductive and oxidative transformations in a system that generates chemical fuels from sunlight.

Bio: Bryce Sadtler graduated from Purdue University in 2002 with a B.S. degree in Chemistry. He conducted his graduate studies at the University of California, Berkeley under the guidance of Paul Alivisatos and received a Ph.D. in Physical Chemistry in 2009. He was then a Beckman Institute Postdoctoral Fellow at the California Institute of Technology, where he worked with Nathan Lewis and Harry Atwater. Bryce joined the Department of Chemistry at Washington University in St. Louis in 2014. His research interests include solid-state chemistry and light–matter interactions in nanoscale materials for applications in solar energy conversion and catalysis. He has received an NSF Career award (2018), an ACS PRF Doctoral New Investigator Award (2017), and was named an Emerging Investigator by the Journal of Materials Chemistry (2017). Bryce was promoted to an Associate Professor at Washington University in July 2022.

Abstract:

Surfaces and interfaces, also referred to as phase boundaries, play an important role in material performance for the application in nanodevices. When the surface of a material comes in contact with another phase, interaction occurs at the interface between the phases which initiates the alteration of properties at the phase boundaries. Therefore, the analysis of the surface properties plays a crucial role in understanding the fundamental insights of crystal growth, reaction kinetics and basis for engineering the nanoparticles as well as interpreting the quantum physics and resolving the theoretical prejudice.

Two-dimensional (2D) materials have drawn enormous research interests due to their intriguing physical properties along with potential applications in the next generation nano-devices. The growth of some 2D materials, such as, Xenes, requires supporting substates which have strong impact on the formation of these materials and their properties. Hence, investigating the surface properties of these types of 2D materials has become quite essential. Recently invented 2D boron (B) sheet which is known as borophene, is considered as a promising material; consists of B atoms arranged in triangular lattice with hollow hexagons. The synthesis of 2D B sheet is still challenging and strongly dependent on the supporting substrates and their crystalline faces, and other factors including temperature, deposition rate etc. during preparation. It is evident from surface analysis by Low energy electron diffraction (LEED), Scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) in ultra-high vacuum (UHV) condition that completely different types of B structures are formed on transition metal substrates with different crystalline faces. The dependency of B growth on substrates creates the opportunities to realize the best substrate for the growth of 2D B sheet which can be employed in future applications.

Zoom link available upon request- email sekhrn@missouri.edu

Abstract:

A new instrument is described that employs buffer gas cooling with mm-wave rotational
spectroscopy to probe molecules desorbed from interstellar ice analogues. The unique
combination of these tools has shown, for the first time, direct measurement of products formed
and desorbing from astrochemical ice analogues with both isomer and conformer specificity, and
determined their relative abundances under well-defined, astrochemically-relevant conditions.
Details of the technique, apparatus, and first results will be described in application to the
temperature-programmed desorption of n- and i-propyl cyanide with rotational spectroscopic
detection. This approach represents a new window into the emergence of chemical complexity in
star forming regions.

Abstract

         Steels have important applications in current and advanced nuclear reactors, however, their irradiation tolerance and mechanical properties need to be improved. Bulk ultrafine-grained and nanocrystalline metals possess drastically higher strength than their conventional coarse-grained counterparts due to significant grain boundary strengthening, and are anticipated to have significantly enhanced irradiation tolerance owing to the role of grain boundaries as sinks for irradiation-induced defects. In our 7-year-long and multi-million-dollar DOE project, ultrafine-grained and nanocrystalline austenitic and ferritic steels were manufactured by equal-channel angular pressing (ECAP) and high-pressure torsion (HPT), respectively. The microstructure and mechanical behavior of the steels manufactured by ECAP and HPT were carefully studied. The thermal stability of the ultrafine-grained and nanocrystalline steels was also investigated. For ferritic FeCrAl alloys with different ranges of grain sizes, thermal aging was conducted to study thermally induced α’ Cr precipitation, which typically causes embrittlement. Neutron irradiation was performed to study irradiation behavior of the steels. Ion irradiation was also conducted to compare with the neutron irradiation. Results indicate that the ultrafine-grained and nanocrystalline steels manufactured by ECAP and HPT possess significantly improved hardness/strength compared to their conventionally manufactured coarse-grained counterparts. In FeCrAl alloys, with decreasing grain size, thermally induced α’ Cr precipitation was reduced. In 304 and FeCrAl steels, smaller grains possess reduced irradiation-induced hardening, segregation and precipitation compared to larger grains. Ultrafine-grained and nanocrystalline 304 steels have enhanced phase stability during irradiation compared to the coarse-grained counterpart. These results indicate enhanced irradiation tolerance of ultrafine-grained and nanocrystalline steels.

Bio

          Dr. Wen is an Assistant Professor in Department of Materials Science and Engineering and Department of Nuclear Engineering and Radiation Science at Missouri S&T. He obtained his PhD from University of California – Davis in 2012, and subsequently held postdoctoral appointments at Northwestern University and Idaho National Laboratory. Prior to joining Missouri S&T, he was a Research Assistant Professor at Idaho State University and a staff scientist at Idaho National Laboratory. Dr. Wen has extensive experience in research and development of advanced materials, including those for nuclear applications. He has been leading multiple research projects funded by Department of Energy, National Science Foundation, and Nuclear Regulatory Commission. Dr. Wen has authored or coauthored more than 65 peer-reviewed journal publications, with citations >3,200 and an h-index of 24. He serves on the Editorial Board of the journal Materials Science and Engineering A, and has served as the lead guest-editor of a special issue in AIMS Materials Science. He regularly reviews manuscripts for many journals and research proposals for DOE and NSF.

Switching of control mechanisms during the rapid solidification of alloys

Abstract

The formation of complex solidification patterns is an intrinsic non-equilibrium phenomenon. It is the interplay between capillary and kinetic effects at the solidification front (solid-liquid interface) that produces the complex growth patterns we see in nature. In general, the solidification growth is solely controlled by diffusion. Pure metals are controlled by thermal diffusion, while alloys are controlled by solute diffusion.  However, in the rapid solidification of alloys, the solidification growth may undergo a change from solute diffusion-controlled to thermal diffusion-controlled. The switching of control mechanisms is found to cause the velocity jump and disrupt the microstructure development. In this work, we will investigate two rapid solidification processes, additive manufacturing (AM) and melt spinning (MS), using phase-field modeling. Specifically, the nucleation or the onset of the solidification of AM and MS will be explored. The resulting solidification pathway and the development of inhomogeneous microstructures will be elucidated.

Bio

Dr. Yijia Gu obtained his Ph.D. in Materials Science and Engineering (MSE) with a minor in Computational Science from the Pennsylvania State University in 2014. During his Ph.D., Dr. Gu performed thermodynamic and kinetic modeling work on semiconductor, metal, and ferroelectric materials mostly using the phase-field method. Then, he launched his career at Alcoa Technical Center (ATC, now Arconic Technology Center), where he was first a Senior Engineer and then Staff Engineer. At ATC, he did CALPHAD and kinetic modeling work for alloy design and processing optimization, including both conventional route and additive manufacturing. In 2019, he joined Missouri S&T as an Assistant Professor in the MSE department, where he has been collaborating with colleagues and continues to apply computational materials modeling to the studies of advanced steels, machine learning-assisted alloy design, as well as metal additive manufacturing

Abstract

Radiative cooling is a passive cooling technique featured with zero-energy consumption by emitting terrestrial heat to the universe in form of blackbody thermal radiation. Nocturnal radiative cooling is an ordinary phenomenon of radiative cooling effect happening during a clear and calm night, but day-time sub-ambient radiative cooling material, which can cool an object’s surface temperature below ambient air temperature, has never been found in nature. The first realization day-time radiative cooler has been demonstrated by optimizing spectroscopic property of its surface from ultra-violet to mid-infrared, and indicates the day-time sub-ambient radiative cooling material is a promising cooling solution to save energy from current cooling facilities, such as air conditioners. Therefore, a scalable, low-cost day-time sub-ambient radiative cooling material is demanded to mitigate energy demand in large-scale thermal management systems. In this talk, Dr. Zhai will introduce his research projects related to day-time sub-ambient radiative cooling materials and systems, including scalable-manufactured optical metamaterial, kilo-watt scale radiative cooling collection and storage system, as well as radiative cooling structural materials. He will also discuss potential applications of day-time sub-ambient radiative cooling materials in renewable energy generation, environment sustainability and space cooling in buildings.

Dr. Zhai is an assistant professor in the Department of Mechanical & Aerospace Engineering at the University of Missouri Columbia. Dr. Zhai received his PhD degree from the University of Colorado Boulder and continued his postdoc in the National Institute of Standards and Technology. Dr. Zhai’s research interests focus on investigating novel optical materials with unprecedented properties and developing advanced manufacturing technologies to transform these materials into practical solutions in real-world applications in energy, thermal management, environment sustainability.

Materials and Device Physics for Solid-State Lighting and Renewable Energy Generation

Peifen Zhu

Department of Electrical Engineering and Computer Science, University of Missouri

Abstract:

Efficient use of energy and renewable energy production are of paramount importance to society. Lighting accounts for one-eighth of total U.S. electricity consumption. Light-emitting diodes (LEDs) as a new generation lighting technology have extremely long-life spans and consume much less energy. They are penetrating our daily life and adoption of this technology is expected to reduce energy consumption by 40% in 2030.  Despite rapid advances, LED technology is still in its early stage, and continued innovation and breakthroughs are needed to achieve the full potential of this technology. The relatively higher initial cost of LED over incumbent light sources is hindering the widespread adoption of this technology. Therefore, the utilization of cost-effective approaches to achieve high-efficiency LED is instrumental in the application of this technology in the general illumination market. The periodic nanostructures by low-cost self-assembly process were implemented on both LED and OLED, which resulted in a significant enhancement in power efficiency. The key advantage of the self-assembly process is the ability for implementation of roll-to-roll printing method for large wafer-scale manufacturing processes. Developing efficient, stable, and narrow linewidth down-converter materials as well as engineering the properties of existing materials, which can combine with blue LED chips to generate white light with high color quality, will speed up the adoption of LED in the general illumination market. Both material development and additive manufacturing of white LEDs will be presented. Photocatalysis of CO2 is an environmentally friendly and promising technology to convert CO2 into value-added chemical fuels using solar energy. However, the conversion efficiency is low due to the complex reactions. The efforts to improve the CO2 adsorption capacity, light absorption, and charge separation will also be covered.

Quasiparticles with total angular momentum greater than j=1/2 can emerge in a solid state with strong spin-orbit interaction. While the existence of such high-spin quasiparticles has been known for decades, their implication has been largely overlooked. The possibility of superconductivity beyond spin-triplet in such solid states attracted substantial attention. In this talk, I will talk about unconventional quantum oscillations and superfluid response in half-Heusler YPtBi which is a topological semimetal with j=3/2 quasiparticles. The angle-dependent quantum oscillation exhibits striking anisotropy, and the London penetration depth varies as almost temperature-linear, both of which are not easily expected in a compound with cubic symmetry. These anomalous behaviors can be explained within j=3/2 Fermi surface and high-spin superconductivity.