|December 18, 2019
"Quantum Transduction in Magnonic Systems", Alec Dinerstein, NST, Argonne. Host Pierre Darancet
Hybrid quantum systems combine multiple degrees of freedom (e.g., photonic, mechanical, magnetic, electronic) to optimize quantum functionality. A non-trivial aspect of building such a system is efficiently transferring quantum states from one degree of freedom to another, a process known as quantum transduction. Recent demonstrations of strong coupling between superconducting microwave circuits and magnonic modes in solid-state magnetic systems, some of which have occurred at Argonne, have indicated that these systems could be good candidates for creating efficient quantum transducers. In this talk, I'll explore possible schemes for quantum transduction using these current generation magnonic systems.
|October 15, 2019
"Polymer Informatics: Current Status & Critical Next Steps", Rampi Ramprasad, Materials Science and Engineering, Georgia Institute of Technology, Host: Subramanian Sankaranarayanan
The Materials Genome Initiative (MGI) has heralded a sea change in the philosophy of materials design. In an increasing number of applications, the successful deployment of novel materials has benefited from the use of computational, experimental and informatics methodologies. Here, we describe the role played by computational and experimental data generation and capture, polymer fingerprinting, machine-learning based property prediction models, and algorithms for designing polymers meeting target property requirements. These efforts have culminated in the creation of an online Polymer Informatics platform to guide ongoing and future polymer discovery and design. Challenges that remain will be examined, and systematic steps that may be taken to extend the applicability of such informatics efforts to a wide range of technological domains will be discussed. These include strategies to deal with the data bottleneck, new methods to represent polymer morphology and processing conditions, and the applicability of emerging algorithms for design.
|October 7, 2019
Hybrid van der Waals Heterosctuctures Composed of Conventional Semiconductors and Two-Dimensional Materials, Jinkyoung Yoo, Center for Intergrated Nanotechnologies, Los Alamos National Laboratory. Host: Xuedan Ma
Emerging nanomaterials have attracted much attention due to their novel functionalities, but have also been hindered by lack of scalable synthesis and of ways of controlling characteristics. Atomically thin two-dimensional (2D) materials are good examples of novel materials set promising exotic properties and requiring established manufacturing approaches for practical applications. Heterostructuring is a powerful and general strategy to control physical properties of materials. Moreover, heterostructuring can offer novel characteristics differentiating the heterostructure from individual component in a structure. Recently, 2D/2D heterostructures prepared by stacking are being explored to observe quantum phenomena. However, fabrication of 2D/2D heterostructures has been limited by difficulty in preparation of individual 2D layers in controlled manner. Heterostructuring with 2D and conventional materials in other dimensions (e.g. bulk-like structure for 3D and nanowires for 1D) has shown great potential for multi-dimensional heterostructures.
In this presentation I'll discuss how to prepare multi-dimensional heterostructures composed of 2D and conventional materials. The experimental approach is epitaxial growth Si, Ge, and ZnO on various 2D materials including graphene, hexagonal boron nitride, transition metal dichalcogenides. Absence of surface dangling bonds on a 2D material provides a unique opportunity to overcome materials compatibility issues. Nucleation strategy and novel characteristics of multi-dimensional heterostructures will be discussed in detail.
|September 27, 2019
Synchrotron X-ray STM Investigation of Nanoscale Materials, Hao Chang, Plasma Science & Fusion Center, Massachusetts Institute of Technology, Host: Saw Wai Hla and Volker Rose
To date many emerging techniques have been combined with STM to discover and explore new phenomena. On top of the overview of the development of the Synchrotron X-ray STM (SXSTM), this talk explicitly demonstrates novel application of SXSTM which combines synchrotron X-ray and scanning tunneling microscopy to study magnetic, electronic, and structural properties of materials interfaces. A nanofabricated tip is used to capture X-ray magnetic circular dichroism and near edge X-ray absorption fine structure signals, which explain charge transfer and magnetic properties of oxide materials interfaces with chemical and elemental sensitivities. Using X-ray absorption spectroscopy and spectroscopic imaging, the effect of charge transfer at the interfaces formed by transition metals of cobalt and nickel in nanoscale clusters and islands on a Cu (111) surface has been explored. Finally, X-ray standing wave formed by the interference of the incident and diffracted X-ray beams is used to characterize the structural properties of a cobalt thin film grown on a Au (111) surface. These results open novel research directions where material characterizations will be able to perform simultaneous chemical, structural and magnetic contrast potentially down to atomic scale.
|September 26, 2019
Illuminating the Role of Nanoscale Defects on Diminished Semiconductor Performance by Scanning Probe Microscopy. Sarah Wieghold, Department of Chemistry and Biochemistry, Florida State University. Host: Saw Wai Hla and Volder Rose
Nanomaterials attract great attention in the development of tunable energy conversion and storage systems for renewable energy vectors. Their great potential is related to a high material efficiency and optical/electrical properties that can be tuned by size, shape and interaction with the solid support or solvents. However, while their macroscopic ensemble properties have been extensively studied by e.g. time-resolved optical spectroscopy, X-ray diffraction or efficiency measurements of entire device architectures, the properties on the scale of individual atoms or molecules are still vastly unexplored. One reason is the fundamental mismatch between the spatial extension of optical fields and the electron wave functions in atoms or molecules, which hinders access to the photophysical processes in close proximity to where they occur. As a result, it often remains difficult to trace the origin of the macroscopically measurable properties and identify the root cause of e.g. PCE drop in solar cells, non-emissive losses in LEDs or material degradation and pinpoint whether these mechanisms are determined by the bulk, surface, interlayers or defect properties of the material.
Here I will present an approach to investigate charge carrier mechanisms at the interface of bulk perovskite thin films and create a link to their elemental composition by using synchrotron-based X-ray fluorescence and atomic force microscopy. To further investigate light-matter interactions at the nanoscale for various types of nanomaterials, I utilize single molecule absorption detected by scanning tunneling microscopy. This technique is based on a change in the local density of states upon photon absorption, and thus visualizes the localized excitation. Taking advantage of Stark shifts caused by the applied electric field in the STM, different energy levels can be tuned into resonance with the excitation wavelength.
|September 25, 2019
Time-resolved X-ray Absorption Spectroscopy (TR-XAS): Watching Intermediates Dance, Cunming Liu, XSD/APS, Argonne National Laboratory. Host: Saw Wai Hla and Volker Rose
The introduction of pump-probe techniques to the field of X-ray absorption spectroscopy (XAS) has allowed the monitoring of both electronic and structural evolution between intermediates of photoreaction systems with unprecedented accuracy, both in time and in space. In this talk, I will at first present the TR-XAS technique at APS sector 11-ID-D, which has been developed in the ultrafast laser pump-synchrotron X-ray probe configuration. Then, I will present the application of our TR-XAS technique to measure two representative photoreaction sample systems. In the solution case of spin-crossover (low spin (LS) ↔ high spin (HS)) Fe monomer, we have precisely determined the light-induced Fe-N bond length change from LS to HS states in two different solvents. In the suspension example of lead-free perovskite nanocrystals, we have found that upon photoexcitation, the electron is delocalized but the hole is localized at Br atom via likely forming Br2 dimer as Vk center. At the same time, the hole localization results in a local structural distorted state, which is long-lived (~ 60 µs) compared to the recombination of charge carriers (~ 20 ns). Additionally, I will present our recent preliminary TR-XAS results of 2D and 3D lead perovskite thin films from our new home-built sample platform.
|September 11, 2019
Metasurfaces - Achromatic Polarization Conversion and Efficient Optical Modulation, Hoo-Tong Chen, Center for Integrated Nanotechnologies, Los Alamos National Laboratory. Host: Xuedan Ma and Haidan Wen.
Two-dimensional plasmonic metamaterials – metasurfaces – offer tremendous opportunities in realizing exotic optical properties and functionalities. Through tailoring the resonant response of basic building blocks as well as their mutual interactions, they enable effective control of the amplitude, phase, and polarization state of optical reflection, transmission, and scattering. In this talk, I will present plasmonic metasurfaces consisting of a few planar layers of subwavelength metallic structures, demonstrating optical functionalities such as antireflection and perfect absorption. By judicious design of anisotropic resonances, the off-resonance phase dispersion can be tailored to achieve achromatic phase retardation, through which we demonstrate ultrabroadband linear polarization rotation and linear-to-circular polarization conversion (i.e., achromatic half and quarter waveplates). In the second topic, I will present hybrid metasurfaces by integrating functional materials such as semiconductors and graphene at critical regions of the resonators, allowing enhanced light-matter interactions and accomplishing dynamic switching, active tuning, and enhanced nonlinearity. In particular, I will show hybrid graphene metasurfaces for efficient optical modulation at mid-infrared wavelengths for imaging applications. The augmented metasurface functionalities through both structural design and materials integration will provide promising opportunities of metasurfaces for real-world applications.
|September 6, 2019
Electrocatalyst for Oxygen Reduction and Evolution Reactions, Hong Yang, Dept. of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign, Hosts: Xiao-Min Lin and Chengjun Sun
Chemical transformation processes reply heavily on the catalysts made of carefully designed nanoparticles on supports. Many of these transformations involve oxygen species, which are important in chemical, petrochemical and energy industries, ranging from chemical production, such as epoxidation and automobile exhaust treatment, to the electrocatalysts for hydrogen-powered fuel cells and metal-air batteries. In this presentation, I will present our recent efforts in the following areas related to the catalyst and nanomaterial developments for sustainable technologies: 1) low- and non-platinum group metal (PGM) oxygen reduction catalysts and 2) ternary metal oxide electrocatalysts, such as pyrochlores (A2B2O7), for oxygen evolution reaction. I will focus on illustrating the design principles of high performance catalysts through examining the structure-catalytic property relationship under dynamic, reactive conditions and by developing quantitative analytical models for the synthesis of nanostructured catalyst materials.
|August 9, 2019
Fabrication and Functionality of Molecular Nanosystems at Interfaces, Prof. Dr. Johannes Barth, Molecular Nanoscience & Chemical Physics of Interfaces, Physics Dept., Technical University of Munich. Host: Saw Wai Hla
The utilization and organization of molecular species is an important issue for advancing nanoscale science and underpins the development of novel functional materials. To this end we explore molecular bonding and assembly at well-defined homogenous surfaces, textured templates, nanoelectrodes and 2D-sheet layers. The developed bottom-up fabrication protocols employ tailored building blocks and exploit both supramolecular engineering and on-surface covalent synthesis. Structure formation, chemical conversions, electronic and other characteristics are addressed by a multitechnique experimental approach, whereby scanning probe microscopy provides molecular-level insights that are frequently rationalized with the help of computational modeling and x-ray spectroscopy investigations. We work toward a rationale for the control of single molecular units and the design of nanoarchitectures with distinct functional properties.
|July 30, 2019
Nanotube Electro-Mechanical Resonators, Adrian Bachtold, ICFO, The Barcelona Institute of Science and Technology. Host: Daniel Lopez
Mechanical resonators based on carbon nanotubes feature a series of truly exceptional properties. Carbon nanotubes are so small that they make the lightest resonators fabricated thus far. The mechanical vibrations are enormously sensitive to the dynamics of the electrons through the nanotube, and vice versa. Taking advantage of this coupling, we developed a novel detection method that allows us to measure the mechanical vibrations of nanotube resonators with an unprecedented sensitivity. In this talk, I will discuss our efforts to cool the amplitude of the thermal vibrations to a few quanta . Cooling is achieved using a simple yet powerful method, which consists in applying a constant (DC) current of electrons through the suspended nanotube in a dilution fridge. I will also present results where we increase the effect of the electron-phonon interaction to an unprecedented level, enabling the demonstration of polaron physics in an electro-mechanical resonator.
|July 12, 2019
Angstom Scale Chemical Analysis via Scanning Tunneling Microscopy and Tip-Enhanced Raman spectroscopy, Nan Jiang, Department of Chemistry, University of Illinois at Chicago. Host: Saw Wai Hla
My research group is broadly interested in spectroscopically determining how local chemical environments affect single molecule behaviors. We focus on highly heterogeneous systems such as molecular self-assembly and bimetallic catalysis, developing and using new imaging and spectroscopic approaches to probe structure and function on nanometer length scales. This talk will focus on Tip-Enhanced Raman Spectroscopy (TERS), which affords the spatial resolution of traditional Scanning Tunneling Microscopy (STM) while collecting the chemical information provided by Raman spectroscopy. By using a plasmonically-active material for our scanning probe, the Raman signal at the tip-sample junction is incredibly enhanced, allowing for single-molecule probing. This method, further aided by the benefits of ultrahigh vacuum, is uniquely capable of obtaining (1) single molecules chemical identification; (2) the molecular mechanism of chemical bond formation under near-surface conditions using self-assembly concepts; (3) adsorbate-substrate interactions in the ordering of molecular building blocks in supramolecular nanostructures. By investigating substrate structures, superstructures, and the adsorption orientations obtained from vibrational modes, we extract novel surface-chemistry information at an unprecedented spatial (<1nm) and energy (<10 wavenumber) resolution. We are able to interrogate the impact of changes in the chemical environment on the properties of supramolecular nanostructures, and thereby lay the foundation for controlling their size, shape and composition.
|June 18, 2019
Thermal-emission Engineering: Challenges and Opportunities, Mikhail A. Kats, Department of Electrical and Computer Engineering, University of Wisconsin - Madison. Host: Daniel Lopez
Thermal emission (thermal radiation) is the phenomenon responsible for most of the light in the universe. Though understanding of thermal emission dates back over a century, recent advances have encouraged the re-examination of this phenomenon and its applications. This talk will describe our group's advances and outline future work in the measurement and manipulation of thermal emission. First, I will discuss our efforts to improve thermal-emission metrology, especially for low-temperature thermal emitters and/or those with temperature-dependent emissivity. Then, I will describe our use of phase-transition materials including vanadium dioxide and the rare-earth nickelates to demonstrate new phenomena, including negative- and zero-differential thermal emittance, radiative thermal runaway, and thermo-dichroism. I will also discuss our recent demonstration of nanosecond-scale emissivity modulation. The talk will include discussion of exciting opportunities of thermal-emission engineering for infrared camouflage and thermoregulation.
|June 17, 2019
Scalable Nanosystems for Neuromorphic Computing, Vinod K. Sangwan, Department of Materials Science and Engineering, Northwestern University. Host: Daniel Lopez
Advances in silicon-based digital electronics and improved understanding of the human brain have spurred tremendous interest in artificial intelligence and neuromorphic computing. Last few decades saw a rapid rise in both computation power and theoretical framework that resulted in a more efficient solution for specialized cognitive tasks. Since logic processing and memory are intimately connected, the neural computation is not burdened by von Neumann bottleneck. However, progress in artificial intelligence has fallen short of initial predictions and inspired the new approach of hardware implementation using a nano-ionic device that mimics biological neuron at a fundamental level. I will discuss an artificial synapse realized in a device called memtransistor that is based on a single layer semiconducting MoS2. The internal resistance of the device is tuned not only by the biasing history but also by a third gate terminal. Furthermore, open architecture channel allows multiple electrodes resulting in elusive heterosynaptic plasticity -a necessary ingredient for hyper-connectivity. In addition, an artificial neuron is needed to integrate signals received via synapses and fires a charge wave along axon causing subsequent synaptic switching. Thus, I will discuss a practical route to design artificial neurons based on films from solution-processed two-dimensional materials that embody two coupled state variables (temperature and charge) needed for action-potential based computation. An alternative artificial neuron based on heterojunctions will also be presented to circumvent the issues of stochasticity. Finally, a new fabrication method and a new memtransistor crossbar architecture will be discussed to achieve the desired scaling. I will conclude with future goals in fundamental research and applications.
|June 11, 2019
High Frequency Piezo-optomechanics for Superconducting Microwave and Optical Interface, Xu Han, Yale University, Host: Daniel Lopez
Coherent microwave-optical photon conversion is pivotal in the development of scalable quantum networks, which incorporate powerful local microwave quantum processors, such as superconducting qubits, with long-distance quantum channels through optical fibers. However, due to the dramatic mismatch in frequency and wavelength, establishing effective coupling between microwave and optical photons remains a challenging task. In this talk, I will demonstrate an efficient superconducting piezo-optomechanical interface where the microwave-optical interaction is mediated and significantly enhanced by high frequency phonons. Moreover, the implementation of high frequency mechanics can greatly suppress added thermal noise, paving the way for quantum operation. I will first introduce high frequency piezo-optomechanical resonators above 10 GHz, next demonstrate multimode strong coupling in superconducting electromechanics, and then integrate the two systems together and present our most recent progress on bidirectional coherent microwave-optical photon conversion. At last, two promising future directions will be discussed. One is entanglement-based microwave-optical quantum transduction, which will be able to bypass the stringent requirements for direct conversion. The other one is the exploration of nonlinear micro-electromechanical system (MEMS) in the quantum regime for novel applications such as quantum memory.
|May 28, 2019
Solid State Quantum Emitters for Quantum Communication and Metrology Applications, Kristiaan De Greve, Physics Department, Harvard University, Host: Daniel Lopez
In this seminar, I will briefly touch upon three different quantum platforms and materials systems that, each in their own way, exemplify different aspects of the overarching theme of my research work thus far: quantum control of solid-state quantum emitters.
In the first half of my talk, I will discuss a series of cryogenic experiments performed on single spins at high magnetic fields in III-V quantum dots. At the single-qubit level, using ultrafast optical pulses, we demonstrated full optical control of both single electron and hole spin qubits, and we identified the dominant dephasing mechanisms in each case . We then moved on to two-qubit operation, where we demonstrated, through a novel, ultrafast non-linear measurement technique, the highest to-date solid-state spin-photon entanglement fidelity, as confirmed by full state tomography. I will then briefly analyze the fundamental limitations of this materials system in terms of scalability, and potential solutions thereof.
One such solution we recently explored, involves the creation of a fully novel type of quantum device, combining the advantages of an electrostatically controlled quantum dot with the versatile optical access provided by optical quantum dots. For this, we harness the strong optical response and tight excitonic binding energy of monolayer transition metal dichalcogenides (TMDs). I will show new results demonstrating both the observation of Coulomb blockade in quantum dot transport devices, as well as electrostatic control of the optical emission of rudimentary nanowire-quantum dots.
Time permitting, I will also briefly discuss molecular defects in wide-bandgap semiconductors, particularly the nitrogen-vacancy (NV) center in diamond. In a first series of experiments, we used a scanning-NV magnetometer with a scanning magnetic gradient to map out, with sub-nanometer and single-spin resolution, the magnetic and spin environment of shallow NV centers. Such shallow NVs are commonly used for high-resolution magnetometry experiments. We observed a pronounced and dominant spin noise contribution which could be attributed to spin-1/2 defects at the diamond surface. We then developed and studied in detail a novel oxygenation procedure that reduced the surface spin noise by over an order of magnitude – allowing for the observation of the NMR signal of a single, denatured ubiquitin protein.
|May 23, 2019
Electronic States in Electron-Doped Rare-Earth Nickelates From First Principles, Michele Kotiuga, Department of Physics and Astronomy, Rutgers University. Host: Pierre Darancet
Correlation effects in transition metal based materials give rise to many interesting and exotic properties. The rare-earth nickelates, with a rich composition-phase diagram, are no exception. Doping rare-earth nickelates can lead to electron localization, introducing defect states that are unlike typical shallow or deep donor states familiar in conventional semiconductors. We present first-principles density-functional-theory-based calculations of rare-earth nickelates, with a focus on lanthanum nickelate and samarium nickelate, in which we add electrons to the material. Here, we investigate doping concentrations on the order of one electron per formula unit with the goal of changing the orbital occupation and triggering a phase transition, akin to the phase control seen with strain modulation. We carry out calculations where a uniform compensating background charge ('jellium") has been added to maintain charge neutrality when electrons are added, as well as supercell configurations with defects that electron dope the system, and superlattices where an electron is transferred at interfacial layers. In particular, we explore the effects of intercalated hydrogen and lithium as well as oxygen vacancies in samarium nickelate as well as lanthanum nickelate/strontium iridate superlattices. In comparing these calculations, we find the jellium-background calculations capture the changes to the electronic structure seen with the explicit inclusion of defects and interfaces. The resulting changes to the electronic structure, intimately linked to structural changes, cannot be understood with a rigid shift of the states: the bands are reorganized and the character of the gap is fundamentally altered. This class of doping effects introduces a new knob to turn in the field of materials design.
|May 22, 2019
An Inside View of the Physical Review Family of Journals, Yan Li, American Physical Society, Host: Maria Chan
The Physical Review journals of the American Physical Society (APS) have a long tradition of publishing important physics papers and serving as the bedrock of physics research. In the past decade, the APS has launched several new journals to broaden this collection, including Physical Review X, a highly selective open-access journal; Physical Review Applied, dedicated to publishing high-quality papers that bridge the gap between engineering and physics; and Physical Review Materials, a new broad-scope journal serving the multidisciplinary community working on materials research. Physical Review Research, the latest fully open access title in the Physical Review family of journals, will be open for submission soon.
In this talk, I will give a brief overview of our journal family and the peer review process, and offer some guidelines on how to communicate effectively with editors and referees to facilitate the review. I will also discuss the current scope and standards for papers on condensed matter and materials physics published in Physical Review B.
|May 20, 2019
Light-matter interactions for optical communications and energy-related applications, Ankun Yang, Department of Materials Science and Engineering, Stanford University. Host: Daniel Lopez
Light-matter interactions are closely related to everyday life and are the fundamental basis for optical communications and light microscopy and spectroscopy. In the first part of my talk, I will present my study of small-size lasers toward on-chip optical communications. Plasmon lasers represent a type of small lasers that support ultrasmall mode confinement and ultrafast dynamics with device feature sizes below the diffraction limit. However, plasmon-based lasers show emission with limited far-field directionality. In addition, most plasmon-based lasers rely on solid gain materials, e.g., inorganic semiconducting nanowire or organic dye in a solid matrix, that preclude the possibility of dynamic tuning. I will show that arrays of gold nanoparticles surrounded by liquid dye molecules exhibit directional lasing emission that can be modulated by the dielectric environment. By integrating gold nanoparticle arrays within microfluidic channels and flowing in liquid gain materials with different refractive indices, dynamic tuning of the lasing wavelength has been achieved.
In the second part of the talk, I will present more recent research using in situ light microscopy and spectroscopy approaches to study electrochemical devices. Lithium sulfur (Li-S) batteries are attractive candidates for energy storage with high energy density. Sulfur, the charge product in Li-S batteries, was believed to be solid, while we discovered that sulfur can stay in super-cooled state as liquid sulfur. To reveal the implications of this finding, I use a typical 2D material molybdenum disulfide (MoS2) as a platform to show distinct growth behaviors of sulfur on the basal plane (liquid) and edges (solid). Through correlating the sulfur states (liquid or solid) with the electrochemical performances, liquid sulfur is demonstrated to have much faster kinetics compared to solid sulfur. Using a similar in situ optical set-up, I will also show ion intercalation of 2D materials through electrochemical approach as a promising low-temperature modification strategy to manipulate the material properties for nanoelectronics devices. I will conclude by presenting future prospects for exploiting light-matter interactions in nanostructured materials and systems for applications in both optical communications and energy-related applications.
|May 3, 2019
Mechanism and New Applications of Large and Persistent Photoconductivity, Rafael Jaramillo, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Host: Gary Wiederrecht
Large and persistent photoconductivity (LPPC) in semiconductors is due to the trapping of photo-generated minority carriers at crystal defects. Theory suggests that anion vacancies in II-VI semiconductors are responsible for LPPC due to negative-U behavior, whereby two minority carriers become kinetically trapped by lattice relaxation following photoexcitation. By performing a detailed analysis of photoconductivity in CdS, we provide experimental support for this negative-U model of LPPC. We also show that LPPC is correlated with sulfur deficiency. We use this understanding to vary the photoconductivity of CdS films over nine orders of magnitude, and vary the LPPC characteristic decay time from seconds to 10,000 seconds, by controlling the activities of Cd2+ and S2- ions during chemical bath deposition. We suggest a screening method to identify other materials with long-lived, non-equilibrium, photo-excited states based on the results of ground-state calculations of atomic rearrangements following defect redox reactions, with a conceptual connection to polarons and organic dyes.
We apply our knowledge of defect physics in CdS to propose and design a new type of semiconductor device – the donor level switch (DLS), which operates by switching individual defects between deep-donor and shallow-donor states. We study DLS behavior by making two-terminal devices using hole injection layers to control the charge state of sulfur vacancies. We also apply our knowledge to study the influence of LPPC on the performance of CIGS thin-film solar cells. If time allows we will also cover recent results from our group on infrared optical properties and phase-change functionality in transition metal di-chalcogenides (TMDs), and early results on growth and the opto-electronic performance of sulfide perovskite semiconductors.
|April 3, 2019
Band engineering design and demonstration for high performance avalanche photodetector, Jiyuan Zheng, Department of Electrical and Computer Engineering, University of Virginia, Host: Supratik Guha
If you need to measure incredibly low light signals, you have to make a compromise. If you want the best performance, you must select a photomultiplier tube or superconductor. However, photomultiplier tube is fragile and bulky. Superconductor needs to work under low temperature. The alternative, addressing these weaknesses, is the avalanche photodiode (APD), but it is let down by its poor controllability.
If you need to have a high speed low noise photodetector in long haul telecommunications, APD is the first choice. However, the randomness process in APD limited the noise performance, which restricts the traffic capability.
In order to improve the controllability and noise performance of APD, Band engineering design is deployed in the material lattice structure. Firstly, a high gain, low noise APD has been demonstrated on AlN/GaN periodically stacked material. A record stable high linear gain of over 104 has been realized. Secondly, an extremely low noise APD has been demonstrated on AlInAsSb and InAlAsdigital alloy. A record low excess noise factor of 0.02 has been realized based on these lattice structure in 1550 nm fiber optic window. Both of the two types of devices were verified by experiments and first principle study.
|March 28, 2019
Accelerating the Discovery of Energy Materials, Linda Hung, Toyota Research Institute, Host: Maria Chan
The Accelerated Materials Design and Discovery (AMDD) program at the Toyota Research Institute (TRI) is working toward a next-generation platform for materials research. Our team is composed of research scientists and software engineers, together with collaborators at national labs and universities. Our projects focus on automated or high-throughput experiments and computations, as well as the development of software and machine learning tools that enable us to fully leverage the diverse datasets being generated. In this talk, I will give an overview of TRI-AMDD and highlight a few projects: MatScholar, synthesizability networks, and chemical convolutional neural networks.
|March 14, 2019
Transition Metal Dichalcogenides Based Vertical Devices for Memory Application, Feng Zhang, Purdue University, Host: Daniel Lopez
Transition metal dichalcogenides (TMDs) have attracted attention as potential building blocks for various electronic applications due to their atomically thin nature. Innovation in modern storage applications is intimately connected with the development of the entire field of modern information technology. Denser, faster and less power consuming memories are highly sought after by the industry. Resistive random access memory (RRAM) occurs promising as an emerging technology due to its potential scalability, high operation speed, high endurance and ease of process flow. However, reliable and repeatable operation is a potential challenge in future applications since switching involves the uncontrollable motion of individual atoms. In this talk, I will discuss the first experimental demonstration of a vertical MoTe2 based bipolar RRAM device that exhibits a new switching mechanism, i.e. a structural transition from the stable semiconducting 2H phase to a more conductive 2Hd state, which is induced by an electric field. Set and reset voltages are tuned by the MoTe2 flake thickness. 5 ns switching speeds are achieved. This new switch mechanism holds the promise for faster and more reproducible switching if compared with conventional RRAM devices that are based on the migration of ions or the amorphous-to-crystalline transition in phase change memories (PCMs). Moreover, vertical TMD based memory selectors will also be discussed, showing promising characteristics for future application memory applications.
|February 22, 2019
Manipulating Exciton Dynamics for Energy Conversion Applications, Mathew Sfeir, City University of New York, Host: Pierre Darancet
The promise of nanotechnology lies in the emergence of novel electronic and photonic phenomena with the potential for new device technologies. For example, optical transitions at the nanoscale frequently result from strongly absorbing excitons, whose electronic structure and dynamics are shape, size, and morphology dependent. However, since excitons are bound states, device concepts that exploit the unique photophysics of excitons, for example, organic photovoltaics or multiple exciton generation solar cells, depend critically on the ability to direct specific favorable conversion processes and suppress unfavorable ones. Here I will discuss how ultrafast optical spectroscopy is an important tool to understand the success (and failure) of nanoscale device concepts. I will demonstrate how the unique optical signature of excitons allow for dynamical tracking of energy conversion processes on ultrafast time scales. These optical signatures can be used to understand the harvesting of excitons in nanoscale lasers, photocatalytic water splitting devices, and photovoltaics. I will show how the first few picoseconds after light absorption are crucial to understanding the overall performance of a nanomaterial-based energy device.
|January 15, 2019
Switchable geometric frustration in an artificial-spin-ice/superconductor hetero-system, Jing Xu, Department of Physics, Northern Illinois University. Host: Xufeng Zhang
Superconducting (SC) and ferromagnetic (FM) hybrid systems provide an intriguing combination of two contrasting phenomena and their mutual interaction has been extensively studied to tailor their electromagnetic behavior. Here, a novel FM/SC hybrid structure consisting of a unique artificial spin structure deposited onto a superconducting MoGe film will be presented. The spin structure can produce magnetic charge ice structures mimicking those of a square artificial spin ice with the added value that the long-range ordering can be easily realized and controlled with the application of an in-plane magnetic field. The magnetic charge ice can affect the behavior of superconducting vortices present in the underlying MoGe film. The novelty of the magnetic spin ice structure and its impact on vortex matching effect and dynamics will be presented including a reconfigurable matching effect and rectification effect in an artificial-spin-ice/superconductor hybrid sample. By controlling the magnetic charge symmetries with an in-plane external magnetic field, those effects can be achieved with in situ tunable amplitude and polarization.