California NanoSystems Institute
CNSI
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March 11, 2008

Mark Lundstrom
Don and Carol Scifres Distinguished Professor of Electrical and Computer Engineering
Purdue University


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The Ultimate Transistor: Device Physics, Limits, and New Possibilities
After forty years of progress in integrated circuit technology, microelectronics is undergoing a transformation to nanoelectronics. Present-generation MOSFETs have channel lengths of about 30 nm, and billion transistor logic chips have arrived. An intensive effort is underway across the world to push MOSFET channel lengths as short as possible. What are the practical and fundamental limits to MOSFET scaling? What kinds of digital switches have the potential to out-perform the silicon MOSFET? To address these questions, we need a sound, conceptual understanding of the nanoscale MOSFET backed up by rigorous, fully quantum mechanical simulations. In this talk, I will describe a very simple but very sound way to understand the MOSFET. I'll examine the practical limits of MOSFETs, as well as the fundamental limits. I'll then discuss the question of whether changing from silicon to some of other channel material would help, the benefits of novel transistors structures, such as nanowire and nanotube MOSFETs, and, finally, I'll look at the potential for fundamentally new concepts for switching, such as spintronic to examine the question: "Is there anything fundamentally (or practically) better than a silicon MOSFET for digital electronics?"

February 19, 2008

David Reinhoudt
Laboratory for Supramolecular Chemistry and Technology
MESA+ Institute for Nanotechnology
University of Twente


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Molecular Printboards; From Supramolecular Chemistry to Nanofabrication
Like in microelectronics, the fabrication of nanomaterials and -devices will most likely start with the patterning of surfaces, at the nanoscale. Individual nanostructures can be manufactured most easily when confined to a surface.

We have developed host-guest systems that make use of supramolecular chemistry in water. Nanopatterning on self-assembled monolayers of beta-cyclodextrine receptors (molecular printboards) can be achieved by supramolecular nano-imprint lithography or DPN nanolithography. Using the concept of multivalency, molecules can be anchored permanently or in a dynamic regime. Several examples of the immobilization of biomolecules at such surfaces will be discussed.

A second way to pattern surfaces uses dynamic covalent chemistry. This methodology has the same advantages as supramolecular patterning, but can easily be combined with irreversible confinement, e.g. by reduction of an imine linker. We will discuss the patterning of surfaces with proteins that selectively recognize cancer cells. In this work we have discovered that under conditions of μCP or DPN lithography covalent reactions are much faster than from solution. This observation has been used in click chemistry at surfaces and applied for the generation of DNA arrays.

References:
1) J. Am. Chem. Soc., 2007, 129, 11593
2) Small, 2006, 2, 1192
3) Angew. Chem. Int. Ed., 2007, 46, 4104 and 2006, 45, 5292

February 12, 2008

C. Jeffrey Brinker
Professor of Chemical Nuclear Engineering
Sandia National Laboratory/University of New Mexico


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Directing the assembly of nanostructured films with living cells
This talk describes our recent discovery of the ability of living cells to organize extended nanostructures and nano-objects in a manner that creates a unique, highly biocompatible nano/bio interface (Science 313, 337-340, 2006). We find that, using short chain phospholipids to direct the formation of thin film silica mesophases during evaporation-induced self-assembly, the introduction of cells (yeast, bacteria, and mammalian cells) alters profoundly the inorganic self-assembly pathway. Cells actively organize around themselves an ordered, multilayered lipid-membrane that interfaces coherently with a lipid-templated silica mesophase. This bio/nano interface is unique in that it withstands drying (even evacuation) without cracking or the development of tensile stresses - yet it maintains accessibility to molecules, proteins/antibodies, plasmids, etc - introduced into the 3D silica host. Additionally cell viability is preserved for days to months in the absence of buffer, making these constructs useful as standalone cell-based sensors. The bio/nano interfaces we describe do not form 'passively' - rather they are a consequence of the cell's ability to sense and actively respond to external stimuli. During EISA, solvent evaporation concentrates the extracellular environment in smolytes. In response to this hyperosmotic stress, the cells release water, creating a gradient in pH, which is maintained within the adjoining nanostructured host and serves to localize lipids, proteins, plasmids, lipidized nanocrystals, and a variety of other components at the cellular surface. This active organization of the bio/nano interface can be accomplished during ink-jet printing or selective wetting -processes allowing patterning of cellular arrays - and even spatially-defined genetic modification. These constructs are useful in understanding both how cells can direct nanostructure formation and how nanostructuring and nanoconfinement can influence cellular behavior.

February 05, 2008

Cyrus Safinya
Professor of Materials & Physics
UC Santa Barbara


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Nanoscale self-assembly in biological systems: From cytoskeletal proteins to curvature stabilizing lipids
In this post-genomics proteomics era there is renewed interest in elucidating collective interactions between cellular proteins and associated biomolecules leading to supramolecular structures, with the ultimate goal of relating structure to function. The lecture will show examples of our research consisting of learning from, and building upon, the many illuminating examples of (out-of-equilibrium) assembly occurring in vivo. For example, the nerve cell cytoskeleton provides a rich variety of highly ordered bundles and networks of interacting neurofilaments, microtubules (MT), and filamentous actin, where the nature of the interactions, structures, and structure-function correlations remain poorly understood. In recent work in far simpler reconstituted protein systems in the presence of counter-ions, using a combination of synchrotron x-ray diffraction, electron microscopy, and optical imaging, we have demonstrated how 3D MT bundles and 2D network-like bundles may be assembled in vitro revealing unexpected structures not predicted by current electrostatic theories of polyelectrolyte bundling. In another set of experiments we will describe recent work on lipid-protein and lipid nanotubes through membrane shape evolution processes involving protein templates and curvature stabilizing lipids. Indeed, similar membrane shape changes, occurring in vivo for the purpose of specific cellular functions, are often induced by protein scaffold recruitment through interactions between membranes and proteins. Supported by DOE, NSF and NIH.

January 29, 2008

Victor Klimov
Team Leader, Los Alamos National Laboratory


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Carrier Multiplication in Semiconductor Nanocrystals from the Perspective of Solar Energy Conversion
Solar cells represent an important clean source of energy. However, in order to make them competitive with traditional energy sources, the cost-to-efficiency ratio of photovoltaics must be reduced appreciably. Cost considerations have been a strong driver for the development of non-silicon devices that are instead based on, e.g., polymers (plastic cells) and dye-sensitized porous metal oxides (Graetzel cells). Increases in efficiency have typically relied on iterative improvements of material quality (for both Si and non-Si systems) and/or device engineering aspects including, e.g., the use of tandem architectures. There exist, however, approaches that can potentially lead to a leap in photovoltaic performance through the use of new principles for harvesting solar energy and converting it into charge carriers. One such approach involves the use of carrier multiplication, which is a process in which absorption of a single photon produces not one but multiple electron-hole pairs (excitons). In 2004, we demonstrated that colloidal semiconductor nanocrystals, in distinction from bulk semiconductors, undergo this process very efficiently within the range of solar photon energies (Phys. Rev. Lett. 92, 186601, 2004). More recently, we observed generation of up to seven excitons per absorbed photon (Nano Lett. 6, 424, 2006), which corresponds to the ultimate limit allowed by energy conservation for the excitation energy used in these measurements. This talk reviews our recent follow-up work on this carrier multiplication phenomenon, which addresses issues such as its mechanism (Nature Phys. 1, 189, 2005; Phys. Rev. B 76, 125321, 2007), statistics of carrier populations produced via carrier multiplication (Phys. Rev. Lett. 96, 097402, 2006), activation thresholds (Nano Lett. , 2007), and implications of this process in photovoltaics and photocatalisis (Appl. Phys. Lett. 89, 123118, 2006).
January 15, 2008

David Nelson
Professor of Physics and Applied physics
Arthur K. Solomon Professor of Biophysics
Harvard University


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Nanoscience in Biology: Virus Buckling and Folding of Pollen Grains
The difficulty of constructing ordered states on spheres was recognized by J. J.Thomson, who discovered the electron and then attempted regular tilings of the sphere in an ill-fated attempt to explain the periodic table. We discuss how protein packings in virus shells solve a related "Thomson problem", and the remarkable modifications in the theory necessary to account for thermal fluctuations in amorphous shells of spider silk proteins. We then apply related ideas to the folding strategies and shapes of pollen grains during dehydration when they are released from the anther after maturity. The grain can be modeled as a pressurized high-Young-modulus sphere with a weak sector and a nonzero spontaneous curvature. In the absence of such a weak sector, these shells crumple irreversibly under pressure via a strong first order phase transition. The weak sectors (both one and three-sector pollen grains are found in nature) eliminate the hystersis and allow easy rehydration at the pollination site, somewhat like the collapse and subsequent reassembly of a folding chair.