"Pickering emulsions: Formation and Applications", Zhaowei Chen, University of North Carolina at Chapel Hill. Host: Chris Fry

Pickering emulsions are emulsions stabilized by colloidal particles. They have attracted attentions from various areas including material science, food industry, mineral process, energy storage and conversion, (bio-)catalysis, drug delivery, cell isolation, etc., as there are so many choices of particles with different physiochemical properties. In this seminar, I will first describe the origin and the development of Pickering emulsions. Specially, the basics of how solid particles can locate at water-oil interfaces to stabilize emulsions will be covered. Secondly, I will show the versatility of Pickering emulsions for biphasic biocatalysis by designing particles that can simultaneously stabilize emulsions and mimic enzyme activities. Lastly, I will discuss our effort in leveraging the inner compartments of Pickering emulsions for cell isolation while maintaining its viability and bioconversion activity.

"Atomically-Precise Engineering of Low-Dimensional Systems", Scott Schmucker, Zyvex Labs, Host: Jeff Guest

As ultra-precise manufacturing technology scales to its atomic limits, we transition into the realm of digital matter and novel composite materials while enabling a burgeoning array of quantum, electronic, and photonic devices. In this talk, we explore several fabrication technologies which enable atomic-precision while elucidating the fundamental science and the engineering applications motivated and enabled thereby.

We first compare several related two-dimensional material systems: graphenic materials and hyperdoped delta layers in silicon, each of which can be chemically and lithographically engineered or combined to form van der Waals and covalent delta-doped heterostructures. We explore the influences of interlayer coupling, interfaces, and defects in layered systems.

We will then expand our discussion beyond layered systems and extend atomic precision into three dimensions. Discussion will focus on scanned probe lithography for the fabrication of donor atom quantum devices in silicon and recent efforts to expand these devices to include acceptor dopants and to enable three-dimensional architectures. We discuss the physics and fabrication of donor atom qubits in silicon, and through a combination of scanning tunneling microscope-driven lithography and Zyvex engineering, we demonstrate the scaling of manufacturing precision to the atomic scale.

November 15, 2016

11:00 am

Bldg. 440, A105-106

Whispering-gallery-mode (WGM) optical microresonators have emerged as excellent platforms for exploring basic science and for fabricating functional devices. They have been used for sensing, cavity-QED, optomechanics, low threshold lasing, and most recently for the realization of parity-time (PT) symmetry in optics. In this talk, after briefly reviewing the physics and the applications of WGM optical microresonators that we have developed in the past few years (e.g., detection and characterization of single nanoparticles and virions), I will discuss the control of optical processes and the flow of light in WGM microresonators. First, I will show how exceptional points (degeneracy at which two or more of the complex eigenvalues and the corresponding eigenstates of a physical system coalesce) in a system of waveguide-coupled microresonators can be utilized to obtain nonreciprocal light transmission (optical diode), to control emission direction of lasers and to achieve loss-induced suppression and revival of lasing. Then, I will report on optomechanics in WGM resonators to generate chaotic light, to transfer chaos between optical fields and to demonstrate stochastic resonance. I will end the talk giving a brief summary of other interesting phenomena we study with WGM resonators (e.g., nonlinear optics, slow- fast light, Brillouin laser, etc.) and discussing some of the opportunities and challenges in the WGM research.

(CANCELED)

November 4, 2016

11:00 am

Bldg. 440, A105-106

"Paper and Circuits, only Atoms Thick", Jiwoong Park, Institute of Molecular Engineering and James Franck Institute, University of Chicago. Host: Nathan Guisinger

2D layered materials are like papers: they can be colored, stitched, stacked, and folded to form integrated devices with atomic thickness. In this talk, I will discuss how different 2D materials can be grown with distinct electrical and optical properties (coloring), how they can be connected laterally to form pattered circuits (stitching), and how their properties can be controlled by the interlayer rotation (twisting). We will then discuss how these atomically thin papers and circuits can be folded to generate active 3D systems.

November 1, 2016

11:00 am

Bldg. 440, A201

Electronic excitations are fundamental physical processes. Spectroscopic information, including absorption and emission spectra, from electron or photon probes is crucial for materials characterization and interrogation. When experimental data are supplemented and interpreted by first principles atomic modeling, a coherent physical picture can be established to provide physical insights into the intriguing structure-property-function relationship of functional materials.

In this talk, the importance of the first principles modeling of electronic excitations is highlighted with three examples. In the first example, we investigated the oxygen 1s core- level binding energy shift of bilayer silica films on Ru(0001) under different surface oxygen coverages in the X-ray photoelectron spectroscopy (XPS) measurement. Our study revealed that the binding energy shift is an electrostatic effect caused by the interplay of the surface and interface dipole moments. In the second example, we raised the question on an inverse problem: how to solve the underlying local structural arrangements from observed spectral features? As a proof of principle, we adopted ab intio X-ray absorption near edge structure (XANES) modeling for structural refinement of local environments around metal impurities of a gold nano cluster. In the third example, we are motivated to develop a local representation of the microscopic dielectric response function of valence electrons, which is a central physical quantity that captures the many-electron correlation effects. Although the response function is non-local by definition, a local representation in real space can provide insightful understanding of its chemical nature and improve the computational efficiency of first principles excited state methods.

October 31, 2016

2:30 pm

Bldg. 440, A105-106

Continuously growing demand for energy storage stimulates research into new electrochemical systems, such as intercalation based beyond lithium-ion (BLI) batteries. Affordability of such batteries, compared to lithium-ion batteries, is related to the higher abundance and therefore lower cost of sodium, magnesium and other BLIs with respect to lithium. However, these ions are heavier and either larger (Na+ or K+ ions) or more highly charged (Mg2+ or Al3+ ions), which leads to slow diffusion and deterioration of battery performance parameters, such as capacity, power density and electrochemical stability over extended cycling. Therefore, development of the methods for improving diffusion of the beyond lithium charge carrying ions is essential for creating next generation energy storage systems with high performance.
In this work, we present a new materials synthesis method that allows insertion of inorganic ions into the crystal structure of battery electrode material using soft chemistry approach, prior to the electrochemical cycling. We use vanadium oxide as high capacity host electrode material. We will discuss the importance of synthesis parameters for the formation of bilayered V2O5, a unique crystallographic form of vanadium oxide with exceptionally large interlayer distance (~10 – 13 Å) that favors diffusion of the charge carrying ions. Using a chemical pre-intercalation technique developed in our laboratory, we have synthesized Li-, Na-, K-, Mg-, and Ca-stabilized bilayered V2O5. We will demonstrate the record high specific capacity of Na-preintercalated V2O5 in Na-ion batteries and discuss strategies for the improvement of capacity retention based on post-synthesis treatment and pillaring of the bilayered V2O5. Our method can be used for the chemical pre-intercalation of a wide variety of alkali and alkaly-earth metal ions, opening an opportunity to create efficient battery electrodes for various intercalation based BLI batteries.

October 19, 2016

11:00 am

Bldg. 440, A105-106

October 7, 2016

11:00 am

Bldg. 440, A105-106

"Molecular Occupancy of Metallic Nanodot Arrays", Haogang Cai, Columbia University

Nanoscale control over the organization of nanomaterials on surfaces enables advances in many areas, from nanoelectronics and plasmonics to biological and medical research. An increasingly effective approach is to immobilize the nanomaterial of choice on patterned arrays of metallic nanodots through site-selective chemistry. In particular, biomolecular nanoarrays have recently been used to probe molecular interaction (improving assay performance of sequencing, biosensing, etc.), and cellular function and behavior (controlling the ligand organization of membrane receptors)., In the latter case, molecular occupancy of each nanodot is crucial, as the nature of the individual interactions being probed is generally sensitive to stoichiometry. In most work to date, it is vaguely assumed that if the nanodot is sufficiently small, it will accommodate only a single molecule. Precise, quantitative testing of this assumption has only been done until now by electron microscopy, an onerous, ex situ technique that is not readily compatible with biological experiments.

In this talk, I will describe the fabrication of molecular-scale (sub-10 nm) metallic nanodot arrays with arbitrary geometry based on electron beam and nanoimprint lithography. I will then present an on-chip technique to measure the molecular occupancy on these nanodots based on fluorescence photobleaching, while accounting for quenching effects by plasmonic absorption. The molecular occupancy can be controlled by adjusting the ratio of mixed self-assembly reagents, which counterbalances the flexibility of dimension control in nanofabrication. Sparsely spaced single molecule arrays were directly observed by conventional epi-fluorescence microscopy, thanks to an improved background passivation scheme to minimize nonspecific adsorption. Finally, this platform with precise single-molecule control (both spatial and stoichiometric) was applied to mimic antigen-presenting surfaces in order to probe the role of ligand geometric organization in receptor-mediated signaling of T cell activation.2,3, I will describe how advanced nanofabrication improves the nanoarray platform, including substrate etching for out-of-plane spatial control, and binary metallic nanodots for heteromolecular arrays. It not only provides insight into fundamental cell biology, but also has translational implications toward adoptive immunotherapies for cancer and other diseases.

H. Cai, D. Depoil, M. Palma, M. P. Sheetz, M. L. Dustin, S. J. Wind, JVST B, 31, 6F902 (2013).
H. Cai, D. Depoil, M. P. Sheetz, M. L. Dustin, S. J. Wind, Biophysical Journal, 108, 631a (2015).
H. Cai, H. Wolfenson, D. Depoil, M. L. Dustin, M. P. Sheetz, S. J. Wind, ACS Nano, 10, 4173 (2016).
H. Cai, S. J. Wind, Langmuir (2016). DOI: 10.1021/acs.langmuir.6b02444
H. Cai, D. Depoil, J. Muller, M. P. Sheetz, M. L. Dustin, S. J. Wind, in The immune synapse: methods and protocols (in press).

September 21, 2016

4:00 pm

Bldg. 440, A105-106

September 15, 2016

11:00 am

Bldg. 440, A105-106

"Control of energy density inside complex nanophotonic structures", Raktim Sarma, Yale University

Interference of the multiply scattered optical waves in a complex medium can lead to many fascinating effects such as localization, non-local speckle correlations and creation of open and closed channels. Besides being of fundamental importance, the ability to control these coherent effects is important as they dictate the spatial distribution of the energy density inside the medium. Since the energy density determines the light-matter interactions inside a scattering system, therefore the ability to manipulate its spatial distribution opens the possibility of tailoring optical excitations as well as linear and nonlinear optical processes such as absorption, emission, amplification, and frequency mixing inside the disordered medium. The potential applications can range from photovoltaics, white LEDs, random lasers, biomedical sensing to optogenetics and radiation treatments.

In this talk, I will demonstrate two different experimental approaches for controlling energy density distribution inside an on-chip scattering medium. The first approach integrates adaptive wavefront shaping technique to selectively couple light to open and closed channels. Using this approach, we are able to vary the total transmission by 470 times and the total energy stored inside the system by 7.4 times which is the highest reported till date [1]. The energy density distribution is changed from an exponential decay to a linear decay and to a profile peaked near the center of the waveguide. In the second approach, we use geometry to control and modify various coherent effects such as localization, non-local speckle correlations, and profiles of transmission eigenchannels to control the energy density distribution [2-4]. Using our approaches we can modify the energy density distribution deterministically and efficiently which highlights the possibility of controlling light-matter interactions inside a disordered medium in an on-chip platform.

[1] Raktim Sarma, Alexey Yamilov, Sasha Petrenko, Yaron Bromberg, Hui Cao, Physical Review Letters 117, 086803 (2016).

[2] Raktim Sarma, Alexey Yamilov, Seng Fatt Liew, Mikhael Guy, Hui Cao, Physical Review B 92, 214206 (2015).

[3] Raktim Sarma, Alexey Yamilov, Pauf Neupane, Hui Cao, Physical Review B (Rapid Comm.) 92, 214206 (2015).

[4] Raktim Sarma, Timofey Golubev, Alexey Yamilov, Hui Cao, Applied Physics Letters 105, 041104 (2014).

September 13, 2016

11:00 am

Bldg. 440, A105-106

"X-ray Diffraction in Nanostructured Devices", Sergio Morelhao, University of Sao Paulo

Nanotechnology, or the ability to control matter in atomic-molecular scales and produce structures with dimensions of a few tens of nanometers, has been providing a constant challenge for techniques of structural analysis. In terms of instrumentation, X-ray probes of high flux combined with zero noise detectors were the first answer to the increasing demand for structural analysis of nanoscale materials by X-rays. However, the dynamical range of X-ray measurements in such modern instruments can be very high, requiring accurate diffraction models that takes into account 2nd-order diffraction phenomena. Here will be described how these phenomena have been either directly exploited or just taken into account in diffraction models for studying a variety of epitaxial systems: semiconductor quantum dots, layered materials of large d-spacing and random stacking sequences, and heteroepitaxy on patterned substrates. Advantages and perspectives in exploiting these diffraction phenomena with highly focused X-ray beams are discussed.

August 24, 2016

4:00 pm

Bldg. 440, A105-106

August 17, 2016

11:00 am

Bldg. 440, A105-106

Electrochemical interfaces are key to the reactivity of heterogeneous catalysts, yet their complexity has hindered a complete understanding of reaction mechanisms on such surfaces. Computational electrochemistry, through tools such as density functional theory (DFT), provides a unique opportunity to explore reaction processes at an atomistic level, but strategies to address the features of real electrochemical interfaces must be developed. For example, DFT alone does not provide a sufficient framework to computationally include an applied electric potential that is referenced to experimentally measured potentials. A number of techniques have been developed to model the electrode-electrolyte interface at an arbitrary, known potential, including coupling full DFT calculations of the electrode surface with a model for the electrolyte that screens the charge of the electrode. However, evaluating the accuracy of these models against experimental data can be challenging because of the complexity of electrochemical data. Selecting well-understood experimental data, I evaluate the failures and successes of a set of continuum electrolyte models, identifying the chemistry that must be captured for accurate results, and defining regimes in which these models can be confidently utilized. I evaluate the surface charge of low index metal surfaces in aqueous electrolyte, and illustrate the significance of correctly describing the metal-water interaction. I then examine the behavior of the electrosorption valency (the change in surface charge upon adsorption) for protons, water, and hydroxide on a set of metal surfaces, and observe the effect of the electrolyte model.

August 10, 2016

4:00 pm

Bldg. 440, A105-106

August 1, 2016

10:30 am

Bldg. 440, A105-106

We model the quantum dynamics of several nanoscale systems, such as coupled quantum dots - metal nanoparticles and nitrogen vacancy centers inside diamond mechanical resonators utilizing the Lindblad form of the density matrix master equation, which allows for inclusion of dephasing and dissipation. In the quantum dot - metal nanoparticle system, we have studied and explained the mechanisms responsible for entanglement generation and the lifetime of that entanglement, while in the nitrogen vacancy system we studied the cooling of the mechanical resonator due to its coupling to the nitrogen vacancy center. Working with the full Hilbert space is computationally difficult; we will discuss our efforts to simulate the full Hilbert space for select systems, as well as the approximations and simplifications we can often make to solve a much easier problem.

July 27, 2016

4:00 pm

Bldg. 440, A105-106

July 15, 2016

3:00 pm

Bldg. 440, A105-106

"Measuring and manipulating light in multimode optical fiber", Prof. Joel Carpenter, The University of Queensland. Host: Chad Husko

In this talk I’ll be discussing how spatial light modulators (SLMs) can be used to characterize the entire linear behaviour of a multimoded device by measuring the mode transfer matrix of the device as a function of wavelength. With this information it is possible to obtain a detailed picture of the couplings between all spatial and polarization modes as well as calculate all parameters of interest such as mode dependent loss (MDL), modal group delays, modal chromatic dispersion and principal modes. Knowledge of the mode transfer matrix also allows us to peer through a multimode fibre as it maps the relationship between an image at one end of the fiber and its corresponding speckle pattern at the other. I will demonstrate this by projecting images to the output of a multimode fiber by coupling in the ‘pre-scrambled’ speckle pattern at the input, which is transformed into an image at the output by the mode-coupling in the fibre. I will also discuss how photonic lanterns can be used to create the multimode analogs of traditionally singlemoded components like wavelength selective switches (WSS) and spectral pulse shapers.

July 13, 2016

4:00 pm

Bldg. 440, A105-106

July 12, 2016

3:00 pm

Bldg. 440, A105-106

Universite Paris-Sud

Diamond is emerging as a new material for making photonic devices thanks to its outstanding properties [1]. Diamond is transparent from the ultraviolet to the far infrared and has a wide variety of optically active color centers that can be used as single photon sources at room temperature. Its high thermal conductivity as well as the absence of two-photon absorption over a wide spectral range make diamond a highly suitable material for nonlinear optics and frequency conversion. Finally, the diamond can be functionalized to produce very stable and selective organic interfaces on its surface. This property is particularly desirable for the development of highly sensitive biosensors.

After presenting a short state of the art of nanophotonics on diamond, I will detail the results we recently achieved at the IEF on photonic crystal (PhC) made in polycrystalline diamond. In particular, I will present the photonic crystal cavities we have fabricated [2,3] and the application of these structures to gas detection [4]. They exhibit wavelength sensitivity reaching 350 nm per unit change of the refractive index of the gaseous environment of the PhC. Moreover, with a simple oxidized surface termination, diamond PhCs can display a high sensitivity to surface adsorption of polar molecules. I will also present some preliminary results on biosensing with these diamond structures.

July 7, 2016

3:00 pm

Bldg. 440, A105-106

Quantum confinement effects in semiconductor nanocrystals allowed newer possibilities in tuning their physical properties by varying the size and shape. Photoexcitation of semiconductor quantum structures generate a bound electron hole pair called exciton. Excitons in type II heterojunction semiconductor nanorods possess novel properties that are fundamentally different from those of individual components. Examples of designing various types of heterojunctions by bringing two dissimilar semiconductor nanorods in contact will be discussed. The charge transfer emission from the heterojunction nanorods was further tuned to near-infrared region by varying the aspect ratio of nanorods. Such systems can assist the spatial separation of excitons upon photoexcitation and their emerging applications in photovoltaics will be discussed. The second part of the presentation will discuss on the optical properties of quantum dots (QDs) with emphasis on the (i) role of crystal structure and surface composition on luminescence (ii) resonance energy transfer and (iii) light induced electron transfer. We have observed that the luminescence properties of CdSe QDs are influenced by its crystal structure. The zincblende CdSe QDs exhibited excellent photostability and high photoluminescence quantum yield compared to hexagonal wurtzite which make the former system ideal for various applications which demand high emission yield. Photoinduced energy and electron transfer from InP QDs to surface bound chromophores were investigated using steady state as well as time resolved absorption and emission spectroscopic techniques. Our results indicate that InP is a versatile material for light harvesting and charge transport applications, meeting various photophysical and biological requirements.

1. K. B. Subila, K. Sandeep, Elizabeth M. Thomas, J. Ghatak^, S. M. Sivaprasad and K. George Thomas (submitted, 2016)

2. A. Thomas, K. Sandeep and K. George Thomas (submitted. 2016)

3. A. Thomas, P. V. Nair and K. George Thomas, J Phys Chem C, 118, 3838–3845 (2014).

4. K. B. Subila, G. Kishore Kumar, S. M. Shivaprasad and K. George Thomas, J Phys Chem Lett, 4, 2774-2779 (2013).

5. P. V. Nair and K. George Thomas J Phys Chem Lett, 1, 2094-2098 (2010).

6. R,Vinayakan, T. Shanmugapriya, P. V. Nair, P. Ramamurthy and K. George Thomas J Phys Chem C, , 111, 10146-10149 (2007).

July 7, 2016

11:00 am

Bldg. 241, D172

"The Productivity Challenge for Wine Grapes", Nick Dokoozlian, E&J Gallo Winery, host: Supratik Guha

Similar to the producers of many other agricultural commodities, wine grape growers will be challenged to increase production in the future while utilizing the same or less land area, water and labor compared to the present. Competitive global markets will also demand continual improvements in fruit quality, production efficiency and sustainability. In contrast to agronomic cropping systems, where genetics, marker-assisted breeding and advanced farming practices have resulted in dramatic yield improvements over the past century, comparatively modest yield improvements have been obtained for wine grapes during this period. Integrated approaches, incorporating clonal selection, germplasm improvement, advanced agronomic practices and remote sensing technologies, are needed to innovate wine grape production systems. A critical component of this work involves the development of robust vineyard productivity and fruit quality measures and metrics, as well as a detailed understanding of the interactions among critical parameters such as environment, genotype and crop load on fruit and wine quality. In addition to funding research, the wine industry must also invest in the infrastructure needed to support the widespread adoption of new technologies. The productivity challenge for wine grapes will be met by advancing highly integrated farming systems – similar to the approach currently used in agronomic cropping systems - in an effort to obtain synergistic improvements in both vineyard yield and fruit quality.

July 6, 2016

2:00 pm

Bldg. 440, A105-106

"Robust Room Temperature Spin Coherence: Towards New Quantum Materials and Technologies", Michael E. Flatte, University of Iowa, hosts: Supratik Guha and Pierre Darancet

Spin coherent quantum dynamics occurs in the electronic, magnetic and optical behavior of many materials at room temperature, including semiconductors (organic and inorganic), ferrites, and complex oxides. Many effects are most dramatic at interfaces or in hybrid structures, and some persist to the single-spin limit. Predicting spin coherent properties requires integrating theoretical techniques to address multiple material classes, and to cope with energy scales ranging from far smaller than the thermal energy to far larger. Quantum-coherent behavior then emerges from an interesting set of excited states and the couplings among them, whose description challenges ab initio approaches. These phenomena imply new categories of quantum materials and technologies.

June 30, 2016

3:30 pm

Bldg. 437, C010

"Engineering Defects to Enable Cost-Effective Solar Cells", David Fenning, University of California, San Diego

To achieve near-term terawatt scale solar power, significant innovation in solar cell technology is required to improve efficiency while simultaneously reducing cost. I will discuss a multi-scale characterization approach integrating synchrotron-based X-ray nanoprobe measurements to understand how nanoscale defect behavior– even at impurity concentrations as low as part per billion and below – dictates the macroscale electronic performance in today’s silicon solar cells. By quantifying the defect kinetics we observe, I will demonstrate how we can use defect simulation to guide the material’s processing from a wafer to a completed cell. Finally, translating these learnings to the emerging class of organic-inorganic lead halide perovskites, I will share insights from our recent synchrotron-based X-ray fluorescence nanoprobe investigations of chemical heterogeneity in perovskite solar cells. By identifying and controlling device-limiting defects in the bulk or at interfaces, we can systematically improve the integration of state-of-the-art materials into high-efficiency, low-cost solar cells.

June 30, 2016

2:00 pm

Bldg. 440, A105-106

"Using the APS X-ray NanoProbe for five dimensional microanalysis of in-situ reactions", Qinang Hu, Oklahoma State University

Progress has been halted in a number of critical fields because it is challenging for most current experimental techniques to make quantitative measurements at the nanoscale. Recent advances of nano scale X-ray imaging make nano-tomography and nano-X-ray fluorescence a reality. The nano-scale X-ray beams in these techniques allow the sample to be imaged nondestructively and provide a high transmission of signal that penetrate through both the sample and a surrounding solution. This can allow in-situ measurements to be made. Moreover, these techniques can be combined to enrich both datasets to become a more powerful technique. This presentation will present a case study of using these techniques to study the reaction of calcium silicates in different solutions. Although these materials have been used as hydraulic cements for over a hundred years, the basic mechanism of the reactions are not well understood. The results give quantitative measurements of 3D structure, chemistry and mass density of the changes that occur over time. Although the methods were developed for one material, they may be adapted to understand changes of a number of materials leading to improve understanding of nanoscale phenomenon.

June 24, 2016

2:00 pm

Bldg. 440, A201

Energy storage system is a critical enabling factor for deploying unstable and intermittent renewable power sources such as solar and wind power sources. Energy storage devices including lithium-oxygen, lithium-sulfur, and redox flow batteries have received extraordinary attention. In this presentation, we will discuss fundamental redox processes and design strategies in these battery systems. We exploit various spectroscopic techniques coupled with single-cell electrochemical characterizations to probe the Li-S and Li-O2 reactions. In addition, we will discuss new flow cathodes that offer high-energy-density redox flow chemistries and alleviate contact issues between insulating active materials and conductive carbon network. The interactions between solid and solution phases and their impacts on fluid viscosity and flow cell performance will be discussed.

June 15, 2016

3:00 pm

Bldg. 440, A105-106

Heterojunction interfaces are common in non-traditional photovoltaic device designs, such as those based on small molecules, polymers, and perovskites. We have examined the effects of the heterojunction interface region on carrier/exciton energetics using mixture of both semi-classical and quantum mechanical methods. Our theoretical analysis has yielded several useful relationships and numerical recipes that should be considered in device design regardless of the particular materials system. On the regularly ordered areas of the heterojunction, dealing with the interface requires a significant set of corrections to the carrier energies, which in turn directly affects device performance. We highlight these formalisms as applied to carriers and charge transfer exicton pairs at C60/subphthalocyanine and pentacene/silicon interfaces. These applications demonstrate the importance and the limits of applicability of our simple classical corrections.

May 18, 2016

4:00 pm

Bldg. 440, A105-106

May 4, 2016

4:00 pm

Bldg. 440, A105-106

May 3, 2016

2:00 pm

Bldg. 440, A105-106

"Negotiating with Batteries", Daniel Steingart, Princeton University

Endeavors in electrochemical energy storage are industrial masochism for the same reason they are academic hedonism: a working, rechargeable battery represents a tight coupling of multiphase phenomena across chemical, electrical, thermal and mechanical domains. Despite these couplings, most treatments of batteries in the academic literature emphasize material challenges and opportunities as opposed to systematic interactions. There are at least three good reasons for this:

To date, tools for examining the structure of “real” cells in operando are largely limited to synchrotron x-ray and neutron methods,

Full cells are products engineered for application demands and not platonic ideals,

Material improvements can have enormous impact on battery performance.

Yet understanding and examining the physical dynamics of cells in a “scaled context” is still a worthwhile academic endeavor. The battery as a system presents problems that are difficult to decouple, but the study of such problems can introduce new opportunities and inform electrochemical reactor and material design strategies.

By studying full “scaled” cell behaviors we have learned how to compensate for certain material disadvantages and to create batteries and components that can meet performance targets while introducing new strategies for materials design. First, I will show that the “dendrite” may not be the universal anathema it is made out to be (at least in a water stable system). Then I will examine a curious battery "trick” unique to the zinc alkaline bobbin that can elucidate phenomena (perhaps) universally applicable to all closed batteries.

April 20, 2016

4:00 pm

Bldg. 440, A105-106

March 31, 2016

10:00 am

Bldg. 440, A105-106

“Exploiting the Post-Graphene Nanomaterials: Synthesis, Stability and Applications”, Joshua D. Wood, Northwestern University

Two-dimensional (2D) nanomaterials like graphene and hexagonal boron nitride (h-BN) have made inroads in exploring novel physics and in applications like touch screens and RF transceivers. Nonetheless, complementary optical, electrical, and topological properties exist in the so-called “post-graphene” nanomaterials like molybdenum disulfide (MoS₂), black phosphorus (BP), germanium monosulfide (GeS), and borophene. Utilizing post-graphene nanomaterials in mainstream applications necessitates advances in how they are synthesized, isolated, and processed. Here, we will detail our progress on these standing questions.

While BP, a stack of phosphorene monolayers, appears to be a viable nanoelectronic and photonic material, its reactivity is poorly understood, limiting massively integrated, reliable devices. We find that exfoliated, unencapsulated BP flakes chemically degrade to oxidized phosphorus compounds from ambient O₂ and H₂O [1], leading to significant losses in BP field-effect transistor performance. However, BP passivation by AlO_x encapsulation preserves device metrics for several months [1]. Instead of encapsulation, we also passivate BP by covalent modification [2], resulting in tunable doping and high device current on/off ratio. By eliminating BP oxidation species, we make stable BP dispersions in organic [3] and aqueous solvents, producing castable solutions that are electronically [3] and optically equivalent to few-layer exfoliated phosphorene. With isolated, passivated BP, we determine BP’s intrinsic thermal properties, including its in situ thermal decomposition to red phosphorus at ~400 °C [4] and its anisotropic, high thermal conductivity (~100 Wm^-1K^-1) [5].

Our previous work shows the importance of growth substrate [6], grain boundary formation [7], and growth pressure [8] on scaling up graphene and h-BN growth. Graphene and h-BN usually grow on a metallic substrate, requiring clean transfer [9] to an insulating surface to examine and use their properties. Similar substrate and transfer issues apply for borophene, an anisotropic, post-graphene nanomaterial constituted of boron atoms stabilized on Ag(111) [10]. We have grown and encapsulated borophene on inexpensive, transferrable Ag(111)/mica, allowing us to manipulate borophene onto arbitrary surfaces and to probe its vibrational properties. Finally, we are modifying our nanomaterial transfer protocols to facilitate the first measurements of borophene nanoelectronic devices.

March 28, 2016

11:00 am

Bldg. 440, A105-106

Epitaxial graphene grown from silicon carbide wafers has been for long time the only route to obtain high quality graphene directly grown at the wafer –level. While encouraging results have been obtained through thermal decomposition of hetero-epitaxial SiC films on silicon wafers, this route has to-date yet to deliver adequate graphene quality.

We demonstrate that the use of hetero-epitaxial silicon carbide films in combination with a catalytic alloy of nickel and copper enables high -quality graphene on silicon. With this approach we obtain 1- 2 layers graphene with uniform coverage over 2” silicon wafers with an average ID/IG ratio as low as 0.2 +/- 0.05 [1], a substantial improvement as compared to the ratio of ~1 and above of graphene through the more conventional thermal decomposition.

This novel approach holds enormous promise for integrated applications also through the capability for straightforward graphene micropatterning through self-aligned synthesis on pre-structured silicon carbide on silicon [2]. We have already indicated the potential for this approach to fabricate a broad range of miniaturised devices such as high throughput molecular recognition for bio -sensing [3], highly –performant electrodes for on-chip supercapacitors [4], as well as nano photonics.

This work is supported by the Australian Research Council through the Future Fellowship FT120100445, and by the AFOSR, Army RDECOM, and ONRG through the joint grant AOARD15IOA053.

March 23, 2016

4:00 pm

Bldg. 440, A105-106

March 9, 2016

4:00 pm

Bldg. 440, A105-106

March 7, 2016

11:00 am

Bldg. 440, A105-106

The mid-infrared (mid-IR) spectral range (wavelengths from ~ 2 μm to ~20 μm) has recently become more important for both applications and fundamental science. However, compared to their counterparts in the visible, optical components in the mid-IR are still significantly under-developed.

We create tunable optical components for the mid-IR by incorporating materials that respond to temperature, current, or applied electric field into photonic structures such as interference coatings and optical metasurfaces. In particular, we exploited the insulator-to-metal phase transition of vanadium dioxide (VO₂) to create temperature- and current-tunable modulators and thermal emitters, and used electrostatic doping in graphene to construct reconfigurable metasurfaces that operate at nanosecond time scales.

February 24, 2016

4:00 pm

Bldg. 440, A105-106

February 24, 2016

11:00 am

Bldg. 440, A105-106

Nanoscale systems provide flexibility to engineer properties by exploiting interfacial phenomena, non-equilibrium dynamics and quantum processes. Computational design of nanoscale electrochemical and light-harvesting systems require new methods for quantum-mechanical calculations of electrons interacting with liquids, phonons and light to take advantage of this added flexibility. Liquids present particular challenges for quantum calculations because of the need for sampling thousands of molecular configurations to calculate equilibrium properties. I present methods that capture equilibrium properties in a single quantum calculation using statistical theories of liquids within the framework of joint density-functional theory (JDFT). I demonstrate applications of these methods to predict energy-level alignment and reaction mechanisms in solid-liquid interfaces relevant for electrochemical energy conversion.

In nanoscale systems, efficient sub-wavelength light capture is possible using plasmonic resonances of metallic nanostructures, which generate energetic carriers that can drive photochemical or photovoltaic energy conversion processes. Combining electromagnetic simulations, electronic structure calculations and Boltzmann transport analyses, I show that both plasmonic hot carrier generation and transport are sensitive to electronic structure and electron-phonon interactions in the material. These calculations reveal strong anisotropies, electron-hole asymmetries and small transport distances for hot carriers in noble metals, elucidating the advances necessary to overcome bottlenecks in plasmonic energy conversion devices. The first-principles methods so developed enable high-throughput computational screening to target desired electronic, optical and chemical properties in nanoscale systems comprising metals, semiconductors and liquids.

February 22, 2016

4:00 pm

Bldg. 440, A105-106

"Multimodal Imaging of Batteries", Doga Gursoy, Argonne National Laboratory, APS Division

Batteries have had a remarkable effect on the adoption of portable electronic devices. Improving batteries requires understanding the sophisticated interaction of many materials at multiple length scales, ranging from micro (structure of various components, transport of ions/charge) to nano (electronic interactions, local charge accumulation). To address this challenge, we are working on developing multimodal imaging systems including electron microscopy and multiple X-ray microscopy techniques to quantify structure at multiple length-scales. Our goal is to unify data from each “modality” with the new numerical methods in an integrated fashion. This will allow us to understand the nano to micro scale evolution of change in the material as a function of electrochemical cycling. This talk will provide a brief overview of the project and the challenges that lie ahead.

February 10, 2016

4:00 pm

Bldg. 440, A105-106

February 5, 2016

11:00 am

Bldg. 440, A105-106

"Heat Transfer Across Heterogeneous Material Interfaces", Shridar Sadasviam, Purdue University

Interfaces between heterogeneous materials play an important role in determining the performance of modern electronic devices. The role of interfaces becomes especially important at small length scales where interfacial properties influence the physics of device operation more significantly than bulk material properties. This talk will focus on the transport of thermal energy across heterogeneous material interfaces. Fundamental understanding of the physics of interfacial heat transfer is important in several applications such as thermal management of computer chips and thermal design of data storage technologies such as phase change memory and heat-assisted magnetic recording. In this talk, I will present our work on two problems in interfacial heat transfer that span different length scales and have applications in electronic thermal management.

The first part of the talk will focus on the modeling of mechanics and heat transfer through carbon nanotube (CNT) arrays used as thermal interface materials (TIMs). An important contribution of this work is the development of a mesoscale simulation approach to bridge the gap between atomistic and continuum methods. We use a coarse-grain bead-spring model to simulate the mechanical compression of CNT TIMs and use the microstructure obtained from mechanics simulations to predict the thermal interface resistance as a function of applied mechanical load on the CNT array. The predictions from mechanics and thermal simulations are compared with experimental data to validate the simulation approach.

The atomistic Green's function (AGF) method has been widely used to study phonon transport across a wide variety of materials such as Si-Ge interfaces, graphene-graphene nanoribbon interfaces and carbon nanotubes with substitutional impurities. However, most prior studies have focused on obtaining the total phonon transmission function, and predictions of polarization-resolved phonon transmission are scarce. Such polarization-specific phonon transmission functions are expected to aid the interpretation of advanced experiments that can probe phonons of a specific polarization and in the development of multi-scale models. The second part of the talk will focus on the development of an eigenspectrum formulation to determine the surface Green's functions in the AGF method. The eigenspectrum formulation leads naturally to the definition of polarization-specific surface Green's functions that are used to obtain polarization-specific phonon transmission functions. We discuss some aspects of numerical implementation of the technique and also demonstrate it by studying phonon transport across a Ge thin film sandwiched between bulk Si contacts and on a C12 graphene sheet doped with C14 atoms.

January 28, 2016

11:00 am

Bldg. 440, A105-106

The lack of an inversion center in a material's crystal structure can result in many technologically useful material properties, such as ferroelectricity, piezoelectricity and non-linear optical behavior. By studying and understanding the fundamental structure-property relationships present in non-centrosymmetric materials, it is possible to purposefully engineer new compounds with desired qualities through crystal engineering. In this work, I show how nanoscale ordering of simple non-polar dielectrics can induce these emergent “acentric” electronic properties and establish a series of design criteria pinpointing the atomic-scale structural features capable of lifting inversion symmetry in ABO3 perovskite heterostructures. Using first principles electronic structure calculations, I investigate a variety of artificial superlattices to show how new forms of these responses can be produced and enhanced. I then extend this structural framework to the family of perovskite-derived brownmillerites (chemical formula ABO2.5), and describe how similar types of cation ordering can break inversion and result in modest spontaneous electric polarizations with coexisting long-range magnetic order. Finally, I demonstrate how the same combination of symmetry principles and density functional theory calculations can lead to fundamental insights in and the design of materials with large second harmonic generation responses.

"Materials Science of Quantum Computing: Epitaxial Aluminum," Christopher J. K. Richardson, University of Maryland

Quantum computing promises to revolutionize information technology as we know it today by changing the physical nature of the fundamental unit of information. However, the physical limitations to creating a quantum computer originate in our ability to manipulate and control the basic materials that comprise quantum computing devices. In this seminar fundamental concepts of quantum computing will be reviewed to provide specific motivation for studying the fabrication of coplanar waveguide resonators with internal quality factors near 106.

Here, high-purity single crystal superconductors are implemented through metamorphic epitaxial aluminum that is grown via molecular beam epitaxy on silicon and sapphire substrates. X-ray diffraction and scanning transmission electron microscopy indicate an abrupt highly ordered interface that results in crystal relaxation within a few monolayers of the substrate interface and no measurable interfacial layers. Quarter-wave coplanar waveguide resonators are fabricated and characterization with charge contrast imaging in a scanning electron microscope, which identifies processing artifacts at the waveguide sidewalls, on the exposed substrate area and on the exposed aluminum surface. Of primary importance are processing induced corrosion defects on aluminum sidewalls, and photoresist residue that is difficult to remove without affecting the superconductor material. Likely correlations between these artifacts and the superconductor resonator quality factor measured at temperatures below 100 mK are discussed in context of device to device variations in resonator performance.

January 13, 2016
4:00 pm
Bldg. 440, A105-106