Atomic Origami: A Technology Platform for Nanoscale Machines, Sensors, and Robots, Itai Cohens, Cornell University, Host: Xiao-Min Lin

What would we be able to do if we could build cell-scale machines that sense, interact, and control their micro environment? Can we develop a Moore's law for machines and robots? In Richard Feynman's classic talk ​"There's Plenty of Room at the Bottom" he foretold of the coming revolution in the miniaturization of electronics components. This vision is largely being achieved and pushed to its ultimate limit as Moore's Law comes to an end. In this same lecture, Feynman also points to the possibilities that would be opened by the miniaturization of machines. This vision, while far from being realized, is equally as tantalizing. For example, even achieving miniaturization to micron length scales would open the door to machines that can interface with biological organisms through biochemical interactions, as well as machines that self-organize into superstructures with mechanical, optical, and wetting properties that can be altered in real time. If these machines can be interfaced with electronics, then at the 10's of micron scale alone, semiconductor devices are small enough that we could put the computational power of the spaceship Voyager onto a machine that could be injected into the body. Such robots could have on board detectors, power sources, and processors that enable them to make decisions based on their local environment allowing them to be completely untethered from the outside world​.In this talk I will describe the work our collaboration is doing to develop a new platform for the construction of micron sized origami machines that change shape in fractions of a second in response to environmental stimuli. The enabling technologies behind our machines are graphene-glass and graphene-platinum bimorphs. These ultra-thin bimorphs bend to micron radii of curvature in response to small strain differentials. By patterning thick rigid panels on top of bimorphs, we localize bending to the unpatterned regions to produce folds. Using panels and bimorphs, we can scale down existing origami patterns to produce a wide range of machines. These machines can sense their environments, respond, and perform useful functions on time and length scales comparable to microscale biological organisms. With the incorporation of electronic, photonic, and chemical payloads, these basic elements will become a powerful platform for robotics at the micron scale. As such, I will close by offering a few forward looking proposals to use these machines as basic programmable elements for the assembly of multifunctional materials and surfaces with tunable mechanical, optical, hydrophilic properties.

Epitaxial Nanocomposite: A Pathway for Tunable Functionalities, Quanxi Jia, State University of New York (SUNY), Host: Liliana Stan

Epitaxial nanocomposites provide a pathway to produce tunable and improved properties that are often not accessible from the individual constituents. Over the years, new discoveries and major advances have been made to synthesize epitaxial nanocomposite films and to gain fundamental understanding of their physical properties such as ferromagnetism ferroelectricity, and multiferroicity. In this talk, I will overview our effort to understand, exploit, and control competing interactions of a range of epitaxial nanocomposite metal oxide films. Using both ferroelectric and ferromagnetic oxides as model systems, we have illustrated that certain physical properties of the materials could be systematically tuned by controlling the strain state of the epitaxial nanocomposite films. Our phase field simulations have suggested that the ultimate strain in the interested phase is related to the vertical interfacial area and interfacial dislocation density of the epitaxial nanocomposite films.

Extending The Scale and Enhancing the Yield of Self-Assembled Structures, James Alexander Liddle, National Institute of Standards and Technology (NIST), Host: Ralu Divan

Self-assembly is ubiquitous in biological systems, but remains challenging for synthetic structures. These typically form under diffusion-limited, near-equilibrium conditions. DNA-mediated self-assembly is a powerful method with which to build multi-functional, molecularly-addressable nanostructures of arbitrary shape. While there have been many recent developments in DNA nanostructure fabrication that have expanded the design space, fabrication based on DNA alone can suffer from low yields and is hampered by the need to strike a balance between size and mechanical rigidity. Despite recent efforts, typical assembly protocols, employing large numbers of discrete components, offer little control over the assembly pathway, limiting structure size, complexity, and yield.

We have been working to both understand the factors that limit the yield of self-assembled structures, and to devise approaches to overcome them. In this talk, I will discuss our attempts to build a simple, but predictive model, that describes the process of forming a single fold in a DNA origami structure. Using this model, we show that yield decreases exponentially as a function of the number of discrete components used to assemble a structure. To circumvent this limit, we have developed a two-stage, hierarchical self-assembly process, to create large structures with high yield.4 Our process employs a limited number of discrete, sequence-specific element to shape the structure at the nanoscale and control the large-scale geometry. A generic building block – a DNA binding protein, RecA – rigidifies the structure without requiring any unnecessary information to be added to the system.

Expanding the self-assembly toolbox by blending sequence-specific and structure-specific elements, enables us to make micrometer-scale, rigid, molecularly-addressable structures. More generally, our results indicate that the scale of finite-size self-assembling systems can be increased by minimizing the number of unique components and instead relying on generic components to construct a framework that supports the functional units.

Overcoming Materials Science Roadblocks to Reach the Next Frontiers of Carbon Nanoelectronics, Water Separation, and Beyond, Mike Arnold, Dept. of Materials Science and Engineering, University of Wisconsin-Madison, Host: Nathan Guisinger

My research addresses fundamental challenges in controlling the growth, processing, ordering, and heterogeneity of nanomaterials and in understanding phenomena beyond the scale of single nanostructures – that must be overcome to exploit nanomaterials in technology.

In this seminar, I will present on 3 recent advances from my laboratory: (1) We have pioneered a scalable approach for assembling parallel arrays of ultrahigh purity semiconducting nanotubes. This approach has allowed us to create carbon nanotube field effect transistors (FETs) with current density that exceeds Si and GaAs, for the first time, which has been a goal of the nanoelectronics field for 20+ years. (2) We have discovered how to drive graphene crystal growth on Ge(001) surfaces with a giant anisotropy. This giant anisotropy is remarkable because it enables the rational synthesis of narrow, long, smooth, and oriented nanoribbons of graphene that are semiconducting whereas unconfined graphene is typically a semimetal. This result opens up the possibility of realizing hybrid carbon nanoelectronics directly on conventional group IV or III-V semiconductor wafer substrates. (3) Laminates of graphene oxide nanosheets have been shown to exhibit high water permeance and salt rejection. We have used experiments and modeling to show that the water transport pathways through such laminates are not as expected.

This work has implications in extending Moore's Law, creating ultra-low energy logic circuits, developing higher bandwidth RF communication devices, and realizing next-generation water separation membranes.

"Quantum-Sized" Metal Nanoparticles for Photochemical Energy Conversion, Yugang Sun, Temple University. Host: Gary Wiederrecht

Generation of hot carriers in transition metal catalysts through photoexcitation has been demonstrated to be a promising approach capable of significantly lowering activation temperature of the catalysts, which could have a widespread impact on substantially reducing the current energy demands and improving the selectivity of heterogeneous catalysis. Plasmonic nanoparticles made of Au, Ag, Cu, and Al are recently focused because they can actively absorb light at the corresponding surface plasmon resonance (SPR) frequencies, which are usually in the visible spectral region. The high optical absorptions lead to the generation of hot carriers in plasmonic nanoparticles, on which the hot carriers can directly drive chemical transformations. Despite the promise, plasmonic metal nanoparticles are not useful catalysts for a wide range of important reactions. In contrast, platinum-group metals (PGMs) such as Pt, Pd, Ru or Rh are excellent catalytic materials but exhibit SPR in the ultraviolet (UV) spectral region, which represents a significant disadvantage for photocatalysis due to the poor overlap with the solar spectrum. Although increasing size of PGM nanoparticles shifts SPR absorption to the red, it increases cost and reduces surface area and thus catalytic activity. Moreover, increasing the size of metal nanocrystals significantly reduces the yield of hot electron generation, lowering the efficiency of photochemical energy conversion. In this presentation, a new light absorption model will be discussed to demonstrate a transformative way to enhance optical absorption in small PGM nanoparticles in the visible spectral region by adjusting their dielectric environment instead of changing their size. In this model, the ​"quantum-sized" metal nanocrystals are attached to surfaces of transparent silica spheres, which can support a variety of dielectric scattering resonances (e.g., Fabry-Perot or Whispering Gallery modes depending on the size of silica spheres) capable of creating strong electric fields near the silica surface. The intensified nearfields can dramatically enhance the absorption cross-section of the metal nanocrystals, which are on the silica surface, thus improving the yield of ​"hot electrons" in the metal nanocrystals. This new model provides a unique opportunity to efficiently generate hot carriers in the PMG metal nanoparticles upon excitation of solar energy.

First Glimpse of a New Type of Two-Dimensional Crystals, Gong Gu, University of Tennessee, Host: Lifen Wang

Traditional and new mainstream semiconductors are all sp3-coordinated crystals, including III-V, IV-IV, and, to a lesser extent, II-VI compounds, which are collectively referred to as octet compounds. Among octet compounds that exist in sp3-coordinated polymorphs, only boron nitride is known to exist also in sp2-coordinated forms, the most common of which is hexagonal BN (h-BN). Given the tremendous interest in two-dimensional (2D) crystals, a natural question is whether h-BN-like polymorphs can exist for at least some octet compounds other than BN. A theoretical study, based on an energetic consideration, predicted that each cation-anion bilayer in a wurtzite {0001} film would collapse into a planar, h-BN-like structure if and only if the film thickness is below a certain threshold. This transformation to the nonpolar structure is deemed a new stabilization mechanism for the otherwise polar crystals to avert the polar field in the ultra-thin limit; a multitude of known mechanisms counter the would-be catastrophic divergence of potential due to the polar field for bulk crystals. While the h-BN-like ultra-thin films were hailed as ​"precursors to wurtzite films," experimental evidences have been illusive despite efforts to grow ultra-thin films. This talk presents the discovery of h-BeO, the h-BN-like form of BeO, made in a serendipitous experiment at CNM. Nanocrystals of BeO formed in graphene-sealed liquid cells were identified by HRTEM and EELS. Since h-BeO and the usual wurtzite BeO (w-BeO) have nearly identical basal plane lattice constants, we resorted to the ​"fine structure" of EELS, or energy loss near edge structure (ELNES), to show the sp2 electron configuration. Furthermore, we measured h-BeO thicknesses significantly larger than the thermodynamic threshold above which w-BeO is more stable. I will explain why this can be achieved, as well as why previous attempts did not lead to h-BN-like films. Our theoretical work further shows that the h-BN-like thin films of the octet compounds with wurtzite bulks are not so much like h-BN. They constitute a new type of 2D materials, of which we just had a first glimpse.

"Multimodal Microscopy Applied to Emerging Energy materials: Perovskite Solar Cells", David S. Ginger, University of Washington, Host: Pierre Darancet

From halide perovskite solar cells, to new polymer electrolytes for batteries, many emerging materials being explored for solar energy harvesting and storage show performance that depends sensitively on nanoscale structure. Rapid advances in the capability and accessibility of scanning probe microscopy methods have made it possible to study processing/structure/function relationships ranging from photocurrent collection, to ion uptake, to photocarrier lifetimes with resolutions on the scale of tens of nanometers or better in these materials. Importantly, such scanning probe methods offer the potential to combine measurements of local structure with local function, and they can be implemented to study materials in situ or devices in operando to better understand how materials evolve in time in response to an external stimulus or environmental perturbation. This talk highlights recent advances in the development and application of both scanning probe and optical microscopy methods to help address such questions while filling key gaps between the capabilities of conventional electron microscopy and newer super-resolution optical methods, with a specific focus on perovskite semiconductors. This talk will emphasize the application of multimodal microscopy to characterize perovskite solar cells, and discuss how these insights led us to surface passivation schemes that can achieve 96% of the Shockley-Queisser quasi-Fermi level splitting in these materials.

Apr. 18, 2018

Theory and Practice of Nanoparticle Self Assembly, Nicholas A. Kotov, University of Michigan. Host: Gleiciani de Queiros Silveira

Inorganic nanoparticles (NPs) have the ability to self-organize into variety of structures with sophisticated and dynamic geometries. Analysis of experimental data for different types of NPs indicates a general trend of self-assembly under a wider range of conditions and having broader structural variability than self-assembling units from organic matter. Remarkably, the internal organization of self-assembled NP systems rival in complexity to those found in biology which reflects the biomimetic behavior of nanoscale inorganic matter. In this talk, the following questions will be addressed:

• What are the differences and similarities of NP self-organization compared with similar phenomena involving organic and biological building blocks?
• What are the forces and related theoretical assumptions essential for NP interactions?
• What is the significance of NP self-assembly for understanding emergence of life?
• What are the technological opportunities of NP self-organization?

Self-organization of chiral nanostructures will illustrate the importance of subtle anisotropic effects stemming from collective behavior of NPs and non-additivity of their interactions. The fundamental significance of studies in this area from this and other groups will be discussed in relation to the origin of homochirality on Earth and spontaneous compartmentalization (protocells). The practicality of self-organization of nanoparticles will be discussed in relation to charge storage technologies, DNA/protein biosensing, chiral catalysis, and combating antibiotic resistant bacteria and other infections including rapidly mutating viruses.

"Emerging Materials for Nanophotonics and Plasmonics", Alexandra Boltasseva, Purdue University, Host: Gary Wiederrecht

The fields of nanophotonics and plasmonics have taught us unprecedented ways to control the flow light at the nanometer scale, unfolding new optical phenomena and redefining centuries-old optical elements. As we continue to transfer the recent advances into applications, the development of new material platforms has become one of the centerpieces in the field of nanophotonics. In this presentation, I will discuss emerging material platforms including transparent conducting oxides, transition metal nitrides, oxides and carbides as well as two- and quasi-two-dimensional materials for future practical optical components across the fields of on-chip optics and optoelectronics, sensing, spectroscopy and energy conversion.

"Quantum Dynamics of Confined Molecules", Pierre-Nicholas Roy, University of Waterloo, Host: Stephen Gray

Molecular assemblies are often described using classical concepts and simulated using Newtonian dynamics or Classical Monte Carlo methods. At low temperatures, this classical description fails to capture the nature of the dynamics of molecules, and a quantum description is required in order to explain and predict the outcome of experiments. In this context, the Feynman path integral formulation of quantum mechanics is a very powerful tool that is amenable to large-scale simulations. We will show how path integral simulations can be used to predict the properties of molecular rotors trapped in superfluid helium and hydrogen clusters. We will show that microscopic Andronikashvili experiments can be viewed as a measurement of superfluidity in a quantum mechanical frame of reference. We will also show that path integral ground state simulations can be used to predict the Raman spectra of parahydrogen clusters and solids. We will present ongoing work on the simulation of molecular rotors confined in endohedral fullerene materials such as H2O@C60. The questions we will address include symmetry breaking, spin conversion, the nature of dipole correlations and dielectric response, and entanglement measures.

"AWE-somes: All Water-Emulsion Bodies formed by Polelectrolytes at Interfaces", Kathleen J. Stebe, University of Pennsylvania, Host: Xiao-Min Lin

Interfaces between fluids are rich environments to trap materials and build films. Particles and molecules adsorb at interfaces to lower the interfacial energy, and so can be collected from bulk fluid phases to form interfaces covered with monolayer or multilayer structures. This system is an excellent platform for capsule formation. By placing droplets in an external phase, materials from either the dispersed or continuous phases can be incorporated into films. Judicious selection of these components can lead to highly versatile, tailored structures. We are developing encapsulation methods via interfacial complexation of polyelectrolytes and other charged species in all aqueous two phase systems to make multi-functional all water emulsion bodiesAWE-somes. Such capsules might be particularly interesting for sequestration of delicate components, including proteins and microbes, which should not be placed in contact with oils or hydrophobic media. Here we discuss the example of the PEG-Dextran-water system, which separates into PEG-rich and dextran-rich phases. The interfacial tension between the phases is quite low. Furthermore, many molecules, including polyelectrolytes, partition freely between the two phases. These factors make interfacial structure formation especially challenging. We develop strategies to build membranes from complementary polyelectrolytes in each phase by balancing their rates of transport to the interface. To impart additional functionality, we develop methods to include charged nanoparticles (NPs) in such membranes. Here, nanoparticles can be selected that preferentially partition into one of the phases, facilitating interfacial transport, and creating an osmotic imbalance that leads to spontaneous formation of encapsulated multiple emulsions. These AWE-somes, with internal structures reminiscent of membraneless organelles in cells, provide a rich platform for separation, partitioning, reaction, and transport, suggesting AWE-somes might be developed into capsules that mimic biological-cell functions, or protocell systems.

"Electronic Excitations in the Condensed Phase", Tim Berkelbach, University of Chicago. Host: Pierre Darancet

I will present recent work developing predictive theories and ab initio computational techniques for the description of excited states in nanoscale and condensed-phase materials. First, I will describe a low-energy theory of band gaps and excitons in atomically-thin semiconductors, focusing on the transition-metal dichalcogenides. In particular, the theory is naturally adapted to include environmental effects, which are critically important for such atomically-thin materials. The presented approach can be viewed as a poor-man's GW+BSE, which is a successful suite of techniques for excitations in solids, but one which breaks down for more strongly correlated materials. To address this, I will describe the software development and applications of wavefunction-based quantum chemistry techniques for solid-state problems. In particularly the use of coupled-cluster theory for solids is demonstrated to provide an accurate description of satellite structure in the photoemission of metals, correlation-driven bandwidth narrowing, and high-accuracy band gaps in semiconductors. The formal relation to the GW approximation will be briefly discussed.