Fridays at 3:15 to 4:45 p.m. In Building 550 Room 550A
| Jan. 11 | Paul McIntyre Metastability in VLS Growth of Epitaxial Group IV Semiconductor Nanowires Very-low-temperature, Au-catalyzed epitaxial growth of vertical Ge nanowire (NW) arrays on Ge and Si substrates is possible in part because the vapor-liquid-solid (VLS) mechanism appears to be sustained at temperatures as much as 100 C below the bulk binary eutectic melting point. Deep sub-eutectic growth of Ge NWs has been reported by several authors and it has recently been shown to involve a liquid Au-Ge catalyst particle. In comparison to those for Ge, the reported growth temperatures for Si NWs using Au catalysts are substantially higher. In this presentation, I will review new data and literature reports on Si and Ge NW growth kinetics, thermodynamics and surface energies to investigate different mechanisms which may cause this growth temperature discrepancy. The effects on vertical nanowire yield of the thermal history during two-step (nucleation and steady state growth) epitaxy of <111> Ge NWs on Si (111) substrates will also be reported. |
| Jan. 18 | Annelise E. Barron Carefully designed polymer networks for ultrafast DNA sequencing on microfluidic chips: 600 DNA bases in 6.5 minutes Microchannel electrophoresis of DNA molecules through entangled polymer solutions continues to be an important tool for genetic analysis. Through the use of a cross-injection scheme, separations of DNA on microfluidic devices have much higher efficiencies, and thus, the implementation of genotyping and DNA sequencing onto microchips promises to reduce both time and costs for these assays. To fully gain the benefits of the miniaturization of DNA separations, the polymer matrix and wall coating should increase the peak spacing while also reducing the widths of the bands for the greatest improvement in resolution. Most models of the separation mechanism and experimental work on optimizing DNA separations through polymer materials optimization have focused on the size-based mobility of DNA. These studies have tended to minimize the importance of band broadening mechanisms, although biased reptation models have predicted trends in dispersion coefficients that are separation mechanism dependent. Even though band broadening is generally the limiting factor for resolution in DNA separations, systematic studies of band broadening at conditions commonly used in genetic analysis by microchip or capillary electrophoresis are scarce. While the relatively large injection plug widths common to separations in capillaries dominate peak widths within this platform, DNA separations in microchips tend to be limited by analyte-wall interactions and electric field-induced dispersion, since the injector zone is designed to give narrow sample plugs. In this report we will show that different polymer matrix chemistries give rise to vastly different band broadening properties in the system. For example, we show that peaks are generally much wider in linear polyacrylamide matrices (which are ubiquitous in the literature) than in poly(N,N-dimethylacrylamide) matrices. We also demonstrate how both the physical and chemical properties of the polymer matrix along with the electrophoresis conditions affect the widths of the analyte peaks during separation. Specifically, we calculate the effective dispersion coefficients of the DNA fragments for different matrix formulations and under different electrophoresis conditions and discuss how a separation system can be designed to minimize DNA band broadening during electrophoresis. |
| Jan. 25 | Vidya Ramaswamy High temperature behavior of CO2 - selective nanoporous
SiO2 membranes Because of their mechanical durability and thermal and chemical stability, inorganic membranes have the potential to increase the efficiency of industrial processes by enabling efficient separation of process gas streams into their constituents. Numerous industrial processes, including hydrocarbon processing, steam methane reforming, water gas shift, and CO2 capture from power generation systems, would benefit from gas separation membranes that operate at elevated temperatures. Selectivity is a key requirement for membranes. In general, porous membranes are more selective for the smaller or lighter molecules in a gas mixture. However, the mechanism of selective surface transport, due to gas adsorption on the pore walls, can increase the flux of the heavier molecule resulting in a reverse selective membrane. We explore this mechanism using the separation of CO2 from H2 by nanoporous SiO2 membranes as a model system. Nanoporous SiO2 membranes exhibit substantial CO2/H2 selectivity at room temperature. However, gas transport and separation measurements show that with increasing temperature, the CO2 selectivity of SiO2 membranes declines. These results will be discussed within the framework of simple transport models to understand the decrease in CO2 selectivity. Based on the insight gained, guidelines will be established for systematic development of membranes with high temperature CO2 selectivity. |
| Feb. 1 | Dr. Bruce Terris Patterned Nanomagnetic Bits and Devices As conventional magnetic recording technology extends to ever higher areal density, it is possible the often predicted, and constantly increasing, density limit will be reached. The use of nanofabrication to create patterned magnetic elements, or patterned media, is one of the proposed approaches with the promise of delaying the onset of superparamagnetism and thus enabling higher areal density. I will discuss many of the challenges that must be overcome for patterned media to be successful, including fundamental physics and material science issues, new fabrication technologies, nm-scale manufacturing tolerances, and low cost budgets. One of these challenges is to controllably reverse one magnetic element, or bit, without affecting the neighboring elements. A narrow anisotropy distribution will be required. As will be discussed, understanding and controlling the switching properties of magnetic nanostructures is critical not only for patterned media, but for device applications such as MRAM cells and spintronic devices and, for current induced as well as field induced reversal. |
| Feb. 8 | Bruce Dunn Pseudocapacitor Properties of Nanostructured Transition Metal Oxides Pseudocapacitance broadly refers to rapid and reversible Faradaic reactions whose discharge characteristics mimic those of traditional double layer capacitors. The potentially higher energy densities available with pseudocapacitive mechanisms offer the tantalizing possibility of bridging the performance gap between batteries and double layer capacitors. This presentation reviews our work on transition metal oxides which exhibit increased levels of pseudocapacitance when synthesized in a nanostructured form. |
| Feb. 15 | Dr. Ulrich Dahmen Atomic-Scale Imaging of Nanomaterials as a Driving Force for the TEAM Project Advanced electron microscopes show us unprecedented views of materials and their unusual behavior on the nanoscale. It is possible to observe how a nanocrystal grows or melts or changes its structure atom by atom, or to investigate the structure of nanocrystals embedded in microcrystals. However, until now, electron microscopes have remained limited by unavoidable aberrations. The TEAM (Transmission Electron Aberration-corrected Microscope) project was initiated as a collaborative effort to redesign the electron microscope around aberration corrected optics. The underlying vision is the idea of providing a sample space for electron scattering experiments in a tunable electron optical environment by removing some of the constraints that have limited electron microscopy until now. The resulting improvements in spatial, spectral and temporal resolution, the increased space around the sample, and the possibility of exotic electron-optical settings will enable new types of experiments. This talk will present an overview of the TEAM project and its current status, in the context of recent discoveries and future challenges in nanoscale materials science using high resolution imaging and dynamic observations. |
| Feb. 22 | Tobin Marks Materials and Assembly Processes for Unconventional Organic, Organometallic, and Inorganic Electronic Circuitry Chemists are exceptionally skilled at designing and constructing individual molecules with the goal of imbuing them with defined chemical and physical properties. However, the task of rationally assembling them into organized, functional supramolecular structures with precise, nanometer-level control is a daunting challenge. In this lecture, approaches to addressing this problem are described in which the ultimate goal is the fabrication of organic and other unconventional electronic circuitry by high throughput, large area printing techniques. Issues here concern not only the rational design of high-mobility p- and n-type organic and non-organic semiconductors for CMOS electronics, but also modular high-k dielectrics with ultra-high capacitance, low leakage, high breakdown fields, and radiation hardness. It is seen that these approaches are applicable to organic, organometallic, and inorganic semiconducting materials. |
| Feb. 29 | Hongkun Park Single-Nanostructure Electronics, Optoelectronics, and Plasmonics In this presentation, I will discuss several examples of our research efforts, concentrating on electrical, optical, and plasmonic interrogations of device test beds incorporating individual nanostructures. Topics that will be discussed include (1) current-driven phase transitions in oxide and chalcogenide nanostructures, (2) characterization of optoelectronic devices incorporating individual nanostructures and nanostructure/polymer composites, and (3) coupled single-photonic and plasmonic devices that allows the generation and guiding of single photons. |
| Mar. 7 | Peidong Yang Inorganic Nanowires for Photonics & Energy Conversion Nanowires are of both fundamental and technological interest. They represent one of the most important building blocks in the potential nanoscale electronic and photonic device applications. Achieving high level of synthetic control over nanowire growth has allowed us to explore some of their very unique physical properties. Inorganic nanowires can function as self-contained nanoscale lasers, sub-wavelength optical waveguides, photodetector, efficient nonlinear optical mixer and photovoltaic devices. It was also discovered that the thermoconductivity of the silicon nanowires can be significantly reduced when the nanowire size in the 20 nm region, pointing to a very promising approach to design better thermoelectrical materials. |
| Mar. 14 | Francesco Stellacci Supramolecular Nano-Materials and Lithography It is know that specific molecules can spontaneously arrange on various surfaces forming two-dimensional poly-crystalline mono-molecular layers called self-assembled monolayers (SAMs). We will show that when mixed SAMs are formed on surfaces with a radius of curvature smaller than 20 nm they spontaneously phase-separate in highly ordered phases of unprecedented size. In the specific case of mixed SAMs formed on the surface of gold nanoparticles, the molecular ligands separate into 5 Å wide phases of alternating composition that encircle or spiral around the particle metallic core. This new family of nano-structured nano-materials shows properties solely due to this unique morphology, both in terms of fundamental properties such as surface energy and in terms of complex interaction with biological materials such as proteins and cells. Additionally, it will be shown how patterned DNA SAMs can be used as masters for a novel printing technique for organic materials called Supramolecular NanoStamping (SuNs). This method, like the DNA/RNA information transfer, uses the reversible assembly of DNA double strands as a way of transferring patterns from a surface onto another. One of the main advantages of SuNs is that multiple DNA strands (each encoding different information) can be printed at the same time, thus allowing for a complex chemical pattern to be formed, much like Gutenberg movable type. |