Nathan Saul Lewis - US grants

This Presidential Young Investigator Award project is in the general area of analytical and surface chemistry and in the subfield of interfacial charge transfer. The objective of this research is to develop a quantitative, predictive description of the microscopic variables which control photoeffects at the semiconductor/liquid junction. Specific efforts include exploration of new semiconductor/liquid junctions, development of quantitative theories for describing behavior of known systems, and development of chemical and physical probes for the characterization of surface recombination sites at these junctions. This action provides the $25,000 base plus $37,500 in fully matched funds for the fifth and final year of this Presidential Young Investigator Award which was originally granted to Professor Lewis (CHE-8352151) when he was a faculty member at Stanford University. The studies which are enabled by this award are providing insights into how photoelectrochemical processes at semiconductor/liquid interfaces can be controlled and are pointing the way toward the rational design of new and improved photoelectrochemical systems.

This project is in the general area of analytical and surface chemistry and in the subfields of semiconductor electro- chemistry and photochemistry. Mechanistic aspects of interfacial electron transfer at the semiconductor-electrolyte interface will be investigated experimentally in order to test current theories of junction behavior. In addition, chemical control of electron-hole recombination sites will be investigated. The reactivity of the electrically important surface trapping sites at etched silicon and gallium arsenide surfaces will be studied by investigating the chemistry and resultant physical properties of semiconductor surfaces in reactions with substitution-labile transition metal complexes. Spectroscopic studies of the semiconductor-complex interaction will be performed to elucidate the metal oxidation state, coordination shell, and electronic properties of the modified surface. Photoelectrochemical passivation of silicon surfaces will be studied. Electron transfer kinetics under high injection conditions in crystalline and amorphous silicon will be also be investigated. Experiments with carefully prepared semiconductor surfaces will be performed in order to deduce relations between the surface concentration of majority carriers and the rate of interfacial electron transfer. Chemical information regarding electrically important surface sites will be obtained. These results will provide information basic to the understanding of the processing of semiconductors for electronic devices and solar energy conversion.

This award provides 60% funding for the acquisition of a combined atomic force and scanning tunneling microscope system that will be installed and operated in the Beckman Institute Materials Resource Center at the California Institute of Technology. Funding from the National Science Foundation will be shared equally between the divisions of Chemistry and Earth Sciences. Caltech is committed to providing the remaining 40% funding required. The microscope system will be capable of imaging surface topography of materials at the resolution of individual atoms. It will be applied in a large number of research projects at Caltech where surface analysis is critical; for example the interaction of aqueous species at solid-liquid interfaces.

9354453 Lewis The project will develop high-end workstation quality computer graphics to aid in the visualization of concepts taught throughout an undergraduate chemistry curriculum. The goal of the work is to focus on visual presentations, and real-time visual manipulations, of a variety of concepts that are included in the current chemistry curriculum. Specific projects include 3-dimensional animation sequences of atomic and molecular orbitals, 3-D views of polymer structure and stereochemistry, videos that introduce basic stereochemical concepts in organic chemistry, and animated sequences of crystal structures and Miller index planes. Additional projects include other basic organic transformations, periodic trends, and hybridization. The specific goals of this project are to develop materials that can be used in courses throughout the U.S. and can be readily distributed on computer disks, laser disks, and video tapes, so that the efforts of this project can have a broad impact on the national undergraduate and high school chemistry curriculum. ***y

This award is made by the Divisions of Chemistry, Materials Research and Electrical and Communications Systems under the Materials Synthesis and Processing Initiative. The research will apply ring-opening metathesis polymerization to the synthesis of block copolymers having polyene or other electroactive microdomains. A goal will be the synthesis of quantum dots by morphology control of conjugated co-polymer blends. Stretch-orientation of quantum dots is expected to form quantum lines with useful anisotropic optical properties. These may find application as molecule-based optical gratings. Interfaces between inorganic semiconductors and doped conjugated polymers will also be evaluated as junctions and Schottky barrier devices. %%% Ring opening metathesis routes to conjugated polymers will provide new materials with novel mechanical, physical, optical and electrical properties.

Professor Nathan S. Lewis of California Institute of Technology is supported by the Macromolecular, Supramolecular, and Nanochemistry (MSN) Program of the Division of Chemistry to study the changes in the reactivity of technologically important semiconductor surfaces that accompany the transition from small molecules to nanocrystals and two-dimensional materials, as well as from nanocrystals and two-dimensional materials to bulk crystalline solids. The aim is to gain fundamental insight into the transition in behavior from materials comprised of individual bonds to materials formed by extended one-, two-, and ultimately three-dimensional bonding. The project promotes the progress of science by developing novel reaction pathways that exploit the size-dependent changes in surface reactivity. Si surfaces, small-molecule models of Si surfaces, and Si nanocrystals are selected as a critical example to provide insight into the chemical continuum from the nanoscale to the macroscale. The novel reaction pathways being developed may enable a new generation of solar cells, new interfaces for sensors and electronic devices and new approaches to other related Si device constructs. Graduate and undergraduate students from diverse backgrounds are involved in the project. In addition, the research results are incorporated into Freshmen chemistry course material, integrated in outreach program towards high schools, such as Juice from Juice and Project SEAL hands-on science modules, and communicated to non-professional audiences at all levels and through multiple media outlets.

In this project, new classes of reactions that are enabled by changes in the electronic structure of semiconductors that result from changes in the size and dimensionality of the material are explored. The project is focused on determining: 1) which classes of reactions are influenced by the underlying electronic structure of the solid, 2) whether such reactions can be used to systematically achieve beneficial electronic coupling between nanoparticles, and, 3) whether such reactions can be used to covalently link three-dimensional materials, such as bulk crystals, to two-dimensional materials while also providing control over the electronic coupling of the structurally dissimilar materials. The scope of the project includes reactions on Si surfaces, small-molecule models of Si surfaces, and Si nanocrystals. The project also includes reactions on two-dimensional materials such as graphene, hexagonal boron nitride, and transition-metal chalcogenides anchored to Si surfaces. This project is also developing methods to covalently functionalize two-dimensional materials, such as graphene, to allow robust electronic connections and strong interactions between layers and in heterojunction stacks. Functionalized two-dimensional materials are being attached to linkers covalently bonded to the Si surface, and more layers are being added to the stack using sequential addition of layers and linkers until the behavior approximates that of the bulk material on Si, as determined by XPS and optical characterization methods. This work is developing an understanding of the point at which stacks of two-dimensional materials begin to behave in the same was as bulk materials.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

The focus of this proposal will be to exploit the vapor detection technology developed recently at Caltech that forms the basis for a low power, simple, manufacturable, "electronic nose". In this technology, an array of sensors responds to essentially all vapors, but produces a distinguishable response pattern for each separate type of analyte or mixture, much like the mammalian olfactory sense produces such diagnostic patterns and then transmits them to the brain for processing and analysis. Pattern recognition algorithms and/or neural network hardware are used on the output signals arising from the electronic nose to classify, identify, and where necessary quantify the vapor or odors of concern and to associate them with certain disease states associated with volatile biomarkers in the breath or other headspace samples. Due to the present high sensitivity of our electronic nose to biogenic amines (detection levels of 1-10 ppt in a few seconds in room air), which far exceeds that of humans for this class of compounds, we plan to initially explore the use of the sensor arrays to screen for bacterial vaginosis, which has been positively associated with the presence of "fishy" odors that arise from volatile biogenic amines in the headspace above vaginal swabs. In addition, we will advance the science and technology of the electronic nose sensors to obtain still improved sensitivity and time response for other biomarkers, so as to open up further medical application areas and to respond to potential confounding interferences identified by the initial small-scale clinical studies on the targeted, initial demonstration application of the technology.

Chemistry (12)

The Caltech Chemistry Animation Project combines the technical skills of faculty and students at Caltech with Emmy-award winning sound editors, Oscar-winning sound supervisors, Emmy-winning film editors, professional narrators, film score writers, computer graphics personnel from the Hollywood special effects industry, and advanced computer workstations to prepare instructional materials that are being used worldwide, at the high school and college levels, to allow students and teachers to understand better the fundamental concepts in their chemical world. The initial stages of this project have been highly successful, with distribution of materials currently proceeding in seven countries to audiences in excess of one million students. The continuation of this project is allowing us to complete a series of approximately 15 video tapes, comprising a library of fundamental concepts in the chemical sciences, for use by teachers and students worldwide. New videos being produced include: Atoms, Molecules, and Moles; Chemical Thermodynamics; Spectroscopy and Molecular Motion; Point Groups, Symmetry and Group Theory; Binary Crystals; and DNA/RNA/Proteins. These will complement the existing video titles covering the topics of Atomic Orbitals, VSEPR, Crystals, Stereochemistry, Nucleophilic Substitution, the Diels-Alder Reaction, Periodic Trends, and Molecular Orbitals.

This proposal was received in response to NSE, NSF-0019. This NIRT project focuses on the study of charge transport in molecular systems. An interdisciplinary team from physics and chemistry, working in the areas of nanoscale synthesis, high-precision structural characterization, electrochemistry, DNA chemistry, microfluidic techniques, and the use of novel nanoscale materials will be engaged in the effort. There are two principal experimental thrusts to the work: 1) the systematic exploration of electronic tunneling through solvents and other small, well-characterized molecules and 2) a rigorous study of charge transport through DNA. The award is jointly funded by the Divisions of Physics and Chemistry.

Project Summary In 2004,the 90 nm node for CMOS-based Si integrated circuits was commercialized.90
nm refers to the 'half-pitch ' between the most closely spaced metal lines the actual pitch of those lines is 180 nm..Assuming that the current scaling trends continue,technology nodes within the 10-15 nm range would be commercialized around the timeframe 2020 or so.It has previously not been possible to even explore circuitry at these dimensions,since no patterning method for creating such ultra-high density semiconductor circuitry existed.However,the SNAP (superlattice nanowire pattern transfer)method has been recently demonstrated as capable of producing relatively large scale,highly conducting Si nanowire circuits at these dimensions.
The work proposed herewill utilize these circuits,and will focus on addressing some of the most fundamental,chemical,and materials issues that are associated with scaling semiconductor computational circuitry to near molecular dimensions.The intellectual merit of this work will be to establish whether or not it is even possible to scale CMOS circuitry to such extremes.The broader impact is that,regardless of what computational paradigm follows the current one,a high levelof manufacturing perfection at the atomic scale is likely to be necessary.
The work described in this proposal will lay much of the foundation for achieving such perfection.
In the spirit of the RFA,certain approaches described here require manufacturing at a near atomic level of
control,although parallel fabrication approaches for achieving such perfection are proposed,rather than atom by atom assembly approaches.Also,in the spirit of the RFA,architectural approaches for novel omputational schemes,such as those that can take advantage of highly regular circuit structures,or that can bridge length scales from the nano-scale of the logic circuits to the sub-micron scale of standard lithography,will be exploited.
In fabricating and utilizing ultra-dense silicon circuitry,several chemical and materials issues become im-
portant.For example, as Si wire widths are reduced to a few nm,the role that surface states play in the conductivity characteristics of the nanowires becomes increasingly important.Since oxide passivation of Si reduces the mobility of charge carriers near the surface,we want to replace the oxide with an atomically perfect (and very thin)surface passivant.We propose to explore the use of methyl termination of Si(111)for applications to these circuits,an alternative that has been demonstrated to be air-stable with atomically complete passivation that dramatically reduces the surface charge carrier recombination velocities.
Silicon conductors with a thin,high-k gate dielectrics and metal gate electrodes are envisioned to become
important by decade 's end.Equally important for more extreme scaling,will be low-k dielectrics that serve to electronically isolate one nanowire from its nearest neighbor,so that the field-gating can be localized to individual nanowires within a high density logic circuit.These issues will be addressed by combining theoretical modeling to determine effective dielectric constants of ultra-thin materials and molecular films with experimental studies incorporating atomic-layer deposition of high-k gate dielectrics (i.e.HfO2)coupled with the incorporating low-k dielectrics for separating the Si nanowire conductors.
Finally,ultra-high density patterning methods will likely be limited in terms of the physical complexity
achievable in a circuit design.This requires the incorporation of novel approaches for bridging the length scales between the sub-micron world of lithography and the nanometer world of ultra-high density circuits.It also requires novel architectural concepts to take advantage of highly-or quasi-regular patterning methods.Architectural approaches that solve these issues will provide a driver for much of the fundamental science described herein.

With this Chemical Bonding Center (CBC) Phase I, Step II award, the Division of Chemistry and the Office of Multidisciplinary Activities of the Mathematical and Physical Sciences Directorate jointly support the research of Harry B. Gray, of the California Institute of Technology, who will lead a collaborative effort involving researchers from Caltech and MIT. This CBC will address one of the most important scientific challenges of the 21st century - the efficient, and ultimately economical, storage of solar energy in the form of chemical bonds. The focus of this research will be on the use of sunlight to split water into its higher energy building blocks: hydrogen and oxygen. This CBC seeks to provide the basic science needed to permit future generations to use sunlight as a renewable and environmentally benign energy source. The research will have a broad impact on society in the science that it produces, and also on the students, professionals and public that it educates. National and international energy policies depend on the outcome of these studies. Raising public awareness of the importance of the renewable energy problem and the nature of the scientific challenges required to address it will be a priority of this CBC.

Chemical Bonding Centers are designed to focus innovative collaborative efforts that address a "big problem" which will lead to a major advance in chemistry or at the interface of chemistry and other sciences and will have the potential to attract broad scientific and public interest.

This is a project to develop sensor arrays for vapor detection using chemically sensitive resistors and luminescent polymers together with biologically inspired algorithms to analyze and interpret the data. The development of a low power circuitry to decode odor patterns from the sensors is a significant component with important technological implications. The work can lead to a general purpose, trainable sensor.

Part 1: Non-Technical Summary
Controlled patterning of materials is important for semiconductor integrated circuit fabrication, 3-D electronics, advanced functional biomaterials, optical materials, and catalyst supports. In this project, jointly supported by the Solid State and Materials Chemistry and Electronic and Photonic Materials Programs in the Division of Materials Research, Professor Nathan S. Lewis of the California Institute of Technology seeks to develop and exploit a fundamental understanding of inorganic phototropic growth. In conventional lithography, materials grow where light is present, whereas in phototropic growth, the materials grow toward the light source. By manipulating the wavelength, intensity, and polarization of the light beams provided by sources as simple as ordinary light bulbs or LEDs, inorganic phototropic growth can spontaneously produce complex, self-organized nanoscale patterns that can be controlled in three dimensions in real time. Such scientific research is foundational to national competitiveness in materials research, information, optical-communications, nanotechnology, and chemical-sensing.

To meet the objective of this project, the Lewis Group is exploring the range of materials that exhibit inorganic phototropic growth, examining the generality of the phenomenon by expanding phototropic reactivity to light-induced controlled etching of materials, and using phototropic growth to create materials with unique three-dimensional properties. The experimental work is being integrated with modeling and simulation to develop a mechanistic understanding of inorganic phototropic growth. The research will be integrated with solar energy and materials-discovery outreach programs led by Caltech, specifically the Juice from Juice and Project SEAL hands-on science modules for use at the high-school level throughout the country, with special emphasis on school districts comprising underrepresented groups and diverse student populations.

Part 2: Technical Summary
In this project, Professor Nathan S. Lewis of the California Institute of Technology is examining the mechanisms underlying inorganic phototropic growth, i.e., the production of spontaneous, self-organized mesostructures aligned along the direction of a polarized, incoherent, uniform intensity light beam during electrodeposition of semiconductors. The morphologies are determined by the inherent optical response of the electronic processes within semiconductors stimulated by the tunable properties (e.g. wavelength, polarization, and direction) of the light present during the electrodeposition. The process is adaptive: if the light is moved or otherwise changed, subsequent growth adapts to the changed conditions.

To date, this phenomenon has been demonstrated experimentally only for amorphous or polycrystalline chalcogen and chalcogenide materials. This project will investigate whether inorganic phototropic growth can be extended to other materials and how lattice structures and optoelectronic properties influence the mechanisms of inorganic phototropic growth. The work will also investigate how substrate-electrolyte interfaces affect the early-stage development of the ordered nanostructures by studying nucleation of Se-Te alloys on substrates with varied electronic properties. In addition, phototropic growth will be exploited to design and synthesize complex three-dimensionally structured materials with desired functionality, such as chiral metamaterials and plasmonic materials with tailored optoelectronic properties. The experimental observations will be used in conjunction with development of a mechanistic simulation tool that combines full-wave electromagnetic calculations with Monte-Carlo-based mass addition to predict the structures produced by phototropic growth of a variety of semiconductors under arbitrary optical inputs. This project is jointly supported by the Solid State and Materials Chemistry and Electronic and Photonic Materials Programs in the Division of Materials Research.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.