Seed Research

Seed #1

Molecular Logic for Nanoelectronics

Fraser Stoddart, chemistry

Following the successful demonstration (Nature 2007, 445, 414–417) of a working defect-tolerant 160,000 bit molecular memory composed of a Langmuir-Blodgett (LB) derived monolayer of amphiphilic, bistable rotaxane molecules and fabrication in a crossbar architecture with nanowires (15 nm wide polysilicon underneath and 15 nm wide Ti/Al on top sandwiching approximately 200 molecules) at a density (10¹¹ bits cm^-2) not predicted, according to the 2005 International Technology Roadmap for Semiconductors (2005 ITRS), to be reached until 2020 at the earliest, the aim of this research project is to design and synthesize, by template-directed protocols that depend upon the operation of molecular recognition and self-assembly processes, bistable rotaxanes, which are amphiphilic or functionalized for carrying out Huisgen/Sharpless-style ‘click chemistry’ with matching electrode surfaces, and undego a change in their dipole moments in response to an electrochemical stimulus that causes relative mechanical motions to occur within the bistable rotaxane molecules. Monolayers of these molecules will then be assessed in a device setting which involves a two-terminal molecular switch tunnel junction (MSTJ) to establish whether or not they can be switched electrically between high and low capacitance states, and hence, in principle at least, can serve as active reconfigurable channels in logic circuits. The research objectives will be reached by controlling the nature and location of the charged components in these nanoelectromechanical systems (NEMS) where control of the dielectric properties of monolayers of these bistable molecules (see figure below) will be achieved through dipole induction and/or charge-storage processes. The compounds that are designed to address reconfigurable molecular logic will also feature a unique collection of recognition units which could be employed to expand the available chemical space for a much wider range of applications addressable by artificial molecular machinery.

The principal mode of operation for a new bistable [2]rotaxane designed for molecular logic. An electron-poor cationic station (blue) is encircled by an electron-rich macrocycle (purple). Reduction of the blue station to its neutral state (blue with orange stripes) causes the macrocycle to move to the secondary electron-poor station (red). Returning the system to zero bias reoxidizes the blue station, creating a high dipole, charge- separated metastable state.

Seed #2

Porous Ceramics for Energy Applications

Katherine Faber, materials science and engineering

Solid oxide fuel cells (SOFCs) are efficient devices for producing electricity from a variety of gaseous fuels, including hydrogen, methane, and propane through a clean solid-state reaction.  A typical SOFC consists of a porous nickel + yttria-stabilized zirconia (Ni-YSZ) support layer and anode, YSZ electrolyte, and lanthanum strontium manganate (LSM) cathode.  The efficiency of the SOFC depends in part on the morphology of the pore network, which serves as the conduit for fuel to reach the electrolyte and reaction products to escape, and the number of triple phase boundaries (TPBs) between pore, electrolyte, and anode phases.  In particular, the tortuosity of the pore network limits transport and should be minimized while the number of TPBs should be maximized.  Thermoreversible gelcasting (TRG) provides a convenient pathway to producing net-shaped, porous bodies such as SOFC supports.  The pore networks of SOFC supports produced with this technique are evaluated using mercury intrusion porosimetry and X-ray computed tomography in order to optimize pore size and pore network morphology and tortuosity.

Thermoelectric generators provide the ability to convert waste heat from industrial processes and transportation into electricity.  Oxide-based thermoelectric generators have advantages over their more common metal counterparts due to their better temperature and environmental stability.  Calcium cobaltite-based materials have high thermoelectric figures of merit at high temperatures.  Using TRG, the material can be aligned in a chosen direction during component processing, producing anisotropic properties that improve the thermoelectric figure of merit in that direction.

X-ray computed tomography image of fully interconnected pore network in gelcast material. Approx. 30% porosity.  450 x 450 x 450 micron³.

Seed #3

Shaping Plasmonic Nanomaterials by Chemical Synthesis

Jiaxing Huang, materials science and engineering

Metal nanoparticles (e.g., Au, Ag, Cu, Al) are essential components in the toolbox of plasmonic nanostructures. Bottom-up chemical synthesis offers the potential for scaling up the materials production, as well as tailoring optical properties by fine tuning size, shape and surface beyond the conventional photolithography limit. We aim to develop rational synthesis strategies for producing nanoparticles with desired morphologies, and for building up a knowledge base of shape-properties relationships. Some examples are shown in the following figures including chemically synthesized Au nanowires with single crystalline surface (Figure a, b), Au nanocubes (Figure c) and Au nano square cuboids (Figure d). Such nanoparticles can be used as the building blocks in many areas such as plasmonic circuits, surface enhanced Raman scattering (SERS), bioimaging and even cancer therapy.

Seed #4

Analysis and Design of Genetic and Metabolic Control Systems of Bacterial Cells

Adilson E. Motter, physics
John F. Marko, biochemistry, molecular biology & cell biology/physics (joint)

Molecular biology permits custom programming of cells to carry out specific tasks, such as synthesis of specific biomolecules or modification of molecules in the extracellular environment. Further examples include formation of multicellular structures or sending chemical or optical signals in response to detection of specific molecules.

The bacterium Escherichia coli, for which there exists a staggering array of genetic engineering methods and genomic data, is well suited for such tasks. Efforts at bioengineering of E. coli depend on understanding unresolved basic questions of the mechanisms underlying control of gene expression (i.e., modulation of which genes are expressed at any given time, largely through DNA-protein interactions on the chromosome) and metabolic processes (i.e., the biochemical reaction pathways through which the basic molecules of a cell are processed).

The overall objective of this joint project between the Motter and Marko labs is to biologically engineer bacterial cells, and design them to perform specific tasks and to produce specific materials. Two complementary projects focus on understanding mechanistic aspects of gene expression and metabolic processes in E. coli. Key strengths of the project are the complementary abilities of groups in the areas of theoretical study of metabolic and genetic networks of E. coli (Motter), study of information in genetic sequences (Motter), theoretical and experimental study of protein-DNA interactions at the single-molecule level (Marko), experimental study of chromosome structure and function (Marko), and theoretical and experimental study of soft materials including supramolecular assemblies of biomolecules (Marko).

Components of the genetic and metabolic system of the bacterium E.coli explored in this project. Cyan arrows highlight protein-gene interactions (D->c), protein-protein interactions (E<->F), and the enzymatic catalysis of metabolic reactions (B->R1, G->R5). In this example, A...G represent proteins encoded by genes a...g, respectively, and R1...R6 are enzyme-catalyzed metabolic reactions interconnected by metabolic compounds (red arrows).

Seed #5

Novel Magnetotransport Techniques for Multifunctional Oxides and Oxide Heterostructures

Matthew Grayson, electrical engineering and computer science

The goal of this Seed project is to develop new contacting techniques for magnetotransport of multifunctional oxides, oxide heterostructures, and highly interacting semiconductor layers.  Conducting films are often buried below an insulating layer as with oxide heterostructures, are difficult to contact ohmically as with oxide semiconductors at low temperature, or have topological boundary effects which affect their conductivity as with high mobility layers in the quantum Hall effect regime.  In all cases, standard ohmic contacts might not reveal the transport parameter of interest.  This seed project will consist of two efforts.  The first is to develop a capacitive contact technique that will enable magnetoresistance characterizations (R_xx and R_xy) of films for which ohmic contact measurements are not feasible. The second task is to investigate other magnetotransport phenomena where contact geometry is critical to understanding the electrical properties, such as contacts to sharp and smooth boundaries of quantum Hall systems. The two techniques are developed on 2-dimensional conducting films in semiconductor quantum wells, with the goal of applying them to the more complex oxide systems thereafter.  The instrument used in the Seed project is a variable temperature cryostat allowing resistance and Hall measurements in magnetic fields up to 17 T, and temperatures from 300 K down to 1.5 K. Initial experiments include a collaboration with IRG 1 to characterize a Fe₃O₄ magnetite film. The magnetic oxides and ferroelectrics can maximize functionality in heterostructures leading to striking transport properties, which require characterization over the full range of temperatures above and below ferroelectric and ferromagnetic Curie temperatures.

Seed #6

Chemical Control of the Optical and Electronic Properties of Semiconductor Nanocrystals

Emily Weiss, Chemistry

Semiconductor quantum dots (QDs) are crystalline clusters of atoms with diameters (1 – 15 nm) on the order of the size of the exciton (photoexcited electron-hole pair) in the corresponding bulk material. A QD has the photostability and bright photoluminescence (PL) of a semiconductor, and the solution-processability and synthetic tunability of an organic molecule. Depending on their elemental composition and size, QDs have lowest-energy (“band-edge”) optical absorptions ranging from the ultraviolet to the mid-infrared, and can be mixed to form multi-size or multi-material solid state films for broad-spectrum light absorption and charge transport. These properties make QDs exciting materials for electronic and energy conversion materials. Their ultimate potential in these applications has yet to be realized, however, because, even as crystalline solid state arrays, films of QDs are insulators. This issue is addressable by modifying the surface chemistry of the QDs, and by synthesizing QD-organic complexes that rapidly (on the femtosecond to picosecond timescale) separate charge. As synthesized, the surfaces of QDs (which contain up to 80% of their atoms in the QD) are passivated by a monolayer of coordinated molecules that prevent aggregation, impart solubility to the QDs in a variety of solvents, and donate electrons to (or accept electrons from) dangling bonds of incompletely coordinated metal ions. The objective of this work is to tailor the surfactant molecules to perform specified functions, such as passivation, charge conduction, and interfacial charge transfer. By executing fundamental studies of the QD-organic complexes in solution and in the solid state via time-resolved optical spectroscopy, PL quenching studies, and electrical conductivity measurements, we will deduce the mechanisms of interaction between ligands and the core states of the QDs, and use this knowledge to design organic molecules that have the binding chemistry, redox characteristics, and electronic structure to serve as optimal ligands for electronic and energy conversion applications.

Selected Research Highlights:

X-ray Computed Microtomography of Porous Ceramics:
Models for SOFC Anode Supports
Noah O. Shanti, Scott A. Barnett, and Katherine T. Faber
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Monitoring Chromosome Dynamics in a Living Bacterial Cell
Nastaran Hadizadeh, John F. Marko
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Network-based Design of Microbial Strains for the Production of Biomaterial
A.E. Motter, T. Nishikawa, N. Gulbahce , E. Almaas , and A.-L. Barabasi
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Moldable Supraspheres Made of Metal Nanoparticles
R. Klajn, K.J.M. Bishop, M. Fialkowski, M. Paszewski, C.J. Campbell, T.P. Gray, B.A. Grzybowski
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More Highlights on Seed Projects:
Energy, Biologically-Relevant Materials, Art Conservation and Charged Systems