Steven K. Buratto - US grants

Under this NSF Young Investigator Award Professor Seung Koo Shin will examine photoinitiated reactions in ion-molecule complexes near the transition state. Ionic reaction intermediates weakly held by charge-induced dipole interactions are isolated in an ion trap or are mass-selected with an ion reflector. State-selective laser spectroscopic studies provide details of structural and dynamic information about ion-molecule complex mediated chemical processes such as SN2, charge-transfer, and oriented orbital reactions. %%% With the experimental techniques available to him, Shin will be able to examine on a microscopic level what happens when one molecule, sometimes an electrically charged particle called an ion, reacts with a different neutral molecule. The weak molecular associations which are formed are called complexes. The study of these complexes gives information about how reactions occur, and what products result under given starting conditions. These studies have broad application, especially to atmospheric processes, including those involved in the formation of smog an the destruction of greenhouse gases.

This Faculty Early Career Development (CAREER) project, supported in the Analytical and Surface Chemistry Program, focusses on the lateral structural characterization of organic and inorganic layers in the nanometer length domain. During the tenure of this three- year continuing grant, Professor Buratto and his students at the University of California - Santa Barbara will utilize near-field scanning optical microscopy (NSOM) in conjunction with established spectroscopic techniques to probe the local structure and dynamics of thin-film optical and opto-electronic materials with nanometer resolution and single molecule sensitivity. Component clustering, conformational analysis, molecular ordering, and environment fluctuations of self-assembled monolayer and multilayer films, conducting and semiconducting polymer films, and inorganic semiconductor quantum structures will be examined. Charge carrier transport, diffusion, and trapping in these systems will also be characterized. These studies should provide useful insights into the role of localized chemistry and physics in the performance of these materials. Also during this grant period, Professor Buratto will pursue an educational development plan that includes the broad introduction of laser spectroscopy in undergraduate physical and analytical chemistry courses, and the development of new courses in nonlinear optics and microscopy for advanced undergraduate and graduate students. The ability to characterize optical and opto-electronic materials in the nanoscale regime is essential to understanding the structure-performance relationships of these technologically important systems. Through application of near-field scanning optical microscopy and other complementary optical spectroscopic techniques, this CAREER research project will lead to a fundamental understanding of the local chemistry and physics of these thin-film organic and inorganic materials, as well as insights into relationships between local ord er/disorder and materials performance. The educational development aspects of this CAREER project will enrich the physical and analytical chemistry curriculum available to undergraduate and graduate students at the University of California - Santa Barbara.

9871849 Metiu We plan to synthesize and assemble dilute magnetic semiconductor nanostructures and study their transport and optical properties by using near field optical microscopy. It is well known that by reducing the size of a semiconductor structure we can affect and control the properties of the electrons in it. We know practically nothing about the way in which this size reduction affects the magnetic properties of these systems, even though we expect these effects to be considerable. We will prepare nanocrystalline "dots" of II-VI semiconductor doped with divalent transition metal ions with magnetic properties (Mn(II), Cr(II), Fe(II) and Cu(II) ). We can thus study the effect of confinement on the exchange interactions and the magneto-optical properties of these materials. We plan to use a near field microscope to perform femtosecond and cw studies of the magneto-optical properties of individual dots and to determine how these properties depend on dot sizes. We also plan to create ensembles of such dots so that we can study the collective effects created by the interaction between the dots. One of our most important tools is the near field optical microscope. This is a wonderful tool with many exciting uses, but the quantitative interpretation of the measurements is hampered by the lack of a quantitative theory of the electromagnetic fields produced in the structure being studied. We propose to develop new theoretical methods for solving this problem. %%% The kind of studies proposed here are motivated by the need of making smaller electronic devices and denser computer memories. As the elements of these devices become smaller the properties of the electrons in them are being modified. We cannot design these devices if we cannot anticipate and understand how the electrons in them will behave. Our work hopes to fill this gap in our understanding. The ultimate computer will be a device in which the electrons themselves are computing elements. The main candidates right now are systems in which various spin states are being excited, since they maintain the information imparted to them (coherence) for the longest time. This is one additional reason why the study of spin behavior in small structures may impact future technologies.

This award is being supported by the Office of Multidisciplinary Activities; the Division of Materials Research, Directorate for Mathematical and Physical Sciences; and the Division of Electrical and Communications Systems, Directorate for Engineering.***

Steven Buratto of the University of California at Santa Barbara is supported by the Experimental Physical Chemistry Program for research that aims to probe and manipulate luminescence properties of porous silicon nanoparticles. To avoid deleterious averaging effects, optical properties will be probed on isolated nanoparticles derived from bulk porous silicon, using single molecule spectroscopic techniques. Project goals include obtaining a detailed understanding of the luminescence properties of these nanoparticles and how such properties depend on the surface functionality, the size of the emitting silicon quantum dot (i.e. porosity of the bulk porous silicon) and the dopant concentration of the silicon starting material. Another goal is to correlate more accurately the size of the emitting species with the luminescence wavelength, in order to determine the degree of quantum confinement. Finally, experiments will determine if the single nanoparticles under study consist of a single two-level system by measuring antibunching in the sample emission. Outcomes are expected to enable new understandings of nanoparticle luminescence properties, aid in the development of porous silicon as an optical material, and suggest new directions to maximize the optical properties of porous silicon.

The observation of visible light emission from porous silicon has stimulated tremendous interest over the past several years due to its applications in optoelectronic devices and lasers, and the potential for integration with current silicon processing technologies. Luminescence from porous silicon was first observed over a decade ago, and since that time scientists have struggled to develop a mechanism to describe the photophysical properties of this material. Although a consistent physical description is still lacking, a host of test devices has been produced making use of the tunable luminescence and high surface area of porous silicon. These devices include molecular sensors, light emitting diodes, optical switches, and photovoltaic cells. Successful outcomes for this research are expected to impact the development of further useful devices using porous silicon.

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This award establishes a long-term Partnership for International Research and Education for Electron Chemistry and Catalysis at Interfaces (PIRE-ECCI) between the University of California Santa Barbara (UCSB) and the Dalian Institute for Chemical Physics (DICP) in Dalian, China. The focus area for research is modern heterogeneous catalysis with a multidisciplinary approach that seeks to increase contacts between the fields of surface science, catalysis and chemical dynamics at interfaces, thus striving for a "first principles understanding" of technologically important catalytic systems. Members of the team represent diverse scientific viewpoints, from surface chemical dynamics, to theoretical simulations of surface chemistry, to engineering applications of catalysis.

A major part of the research and education plan involves support for graduate students and postdocs at UCSB and DICP to pursue collaborative research in chemistry. Extended research visits will include Chinese language and cultural sensitivity training for UCSB participants, with similar reciprocal support given to the Chinese participants coming to UCSB. Two summer schools on catalysis will be held at UCSB and a biannual scientific workshop will be held in Dalian. Special educational opportunities concerning the issues of technology transfer in international high-tech business will be organized and provided by the UCSB Technology Management Program.

The Partnerships for International Research and Education Program seeks to create new international collaborations through long-term, large-scale projects that contribute to the development of a diverse, globally-educated U.S. science and engineering workforce. The program is a major initiative in the NSF Office of International Science and Engineering.

In this research supported by the Analytical and Surface Chemistry Program, Professors Buratto, Bowers, and Metiu and their groups will prepare, characterize, and test three new types of nanoscale catalysts, having one feature in common: very small, isolated, well-defined, catalytically active sites. They will prepare and study (a) very small Aun and Agn mass-selected clusters supported on oxide, (b) very small mass-selected, binary clusters such as PdmAun supported on oxides, and (c) very small, mass-selected oxide clusters supported on oxides. A variety of techniques will be used, in a concerted manner, to study these important catalytic processes: model catalytic systems will be prepared by depositing mass-selected clusters on oxide surfaces to ensure atom-by-atom control of catalyst size; all samples will be prepared and studied in ultra-high vacuum by surface science techniques (AES, XPS) as well as by STM/ AFM before, during and after the catalytic chemistry; and density functional theory (DFT) will be used to calculate the structure of the clusters, their XPS spectrum and their chemical activity. Through the work proposed here they will develop a detailed understanding of the catalytic chemistry of these materials and find out how this chemistry depends on size, composition and the nature of the substrate. While the focus of the research is on the catalytic activity of specific nanoscale catalysts, there is a high probability that the results will be applicable to other systems. In addition, it is hoped that the concepts developed through this research will help optimize important industrial processes using these nanoscale catalysts and provide insight into the discovery of new nanoscale catalytic materials.
The research funded by this grant will be interdisciplinary. Graduate students will interact continuously with three different research groups, will have daily contact with other outstanding scientists, and will acquire hands-on experience in a large number of techniques of surface science, gas-phase chemistry, scanned probe microscopy, and high level theory. The research will provide a valuable opportunity for graduate education, found in very few places in the world. Researchers supported by this grant (including PIs) will also be active in outreach to K-12 schools in the Santa Barbara area to present a tutorial on an atomistic view of heterogeneous catalysis and to show an atomically-resolved picture of our model catalyst systems. This will be included in the currently active outreach program in the chemistry department at UCSB. A series of lectures on catalysis by nanostructures will be developed and included as part of a course in nanoscience currently taught in the materials chemistry curriculum.

With support from the Chemical Measurement and Imaging Program in the Division of Chemistry, and co-funding from the Polymers Program in the Division of Materials Research, Professor Steven Buratto and his group at University of California-Santa Barbara are utilizing conductive atomic force microscopy (c-AFM) to image ion conductance and pore connectivity in proton exchange membranes (PEM) used in PEM fuel cells on a nanometer length scale and at the level of a single pore channel. PEM fuel cells, which convert chemical energy into electricity using an electrochemical cell, can be used as efficient power sources, offering high power density and low environmental impact. Critical to fuel cell performance is the polymer electrolyte membrane, which is an efficient proton conductor but an electric insulator. A detailed understanding of proton conduction, in terms of the size and distribution of the chemical domains responsible for transport, is crucial to both a complete understanding of fuel cell performance and a systematic approach to improving the performance. To this end, c-AFM will be used to correlate phase and current images taken on an operating half fuel cell and determine the fraction of electrochemically-active aqueous surface domains. The PI and his group will also image the nanoscale domain morphology and connectivity as a function of (1) the environmental conditions that more accurately reflect those of an operating fuel cell such as high temperature and low relative humidity, (2) the membrane type and composition, and (3) the proximity of the catalyst particles to the ion channels. The results of these experiments will be used to gain a fundamental understanding of ion conductance in PEM fuel cells and provide inspiration and insight into the development of the next-generation membranes materials.

The possibility of producing power with efficiency greater than internal combustion engines, and with environmentally benign byproducts, makes fuel cells an important player in the field of alternative energy and sustainability, especially if the reactants are derived from renewable resources. Students working on this project will be exposed to this alternative source of power, help optimize industrial processes that produce polyelectrolyte membranes, and provide important insight into the discovery of new membrane materials. In addition, students will be trained in state-of-the-art scanned probe microscopy and nano-characterization techniques. Researchers supported by this grant (including PIs) will also be active in outreach to K-12 schools in the Santa Barbara area. Researchers working on this project also plan to develop a demonstration of an operating fuel cell that will be included in the currently active outreach program in the chemistry department here at UCSB. In addition, they will visit high schools in the Santa Barbara and Ventura Counties three times per year to discuss their research and its impact as well as to promote science education.

The Chemical Catalysis Program supports Professors Steven Buratto, Michael Bowers, and Horia Metiu from the University of California at Santa Barbara (UCSB) to prepare, characterize, and test two new types of nanostructured catalysts, having the common feature of being very small, isolated, well-defined catalytically-active sites. The investigators have designed and constructed a unique, highly versatile apparatus which allows production of model catalysts (both metals and metal oxides) by deposition of mass-selected nanoclusters from the gas phase onto single crystal oxide supports. These well-defined nanocluster catalysts are characterized using a normal array of surface science methods and, in addition, utilization of ultrahigh vacuum combined with scanning-tip microscopy (UHV-STM) methods to probe the size and shape of the nanoclusters before and after reaction. The chemistry of such model catalysts is monitored in UHV using temperature-programmed desorption (TPD) and temperature-programmed reaction (TPR) and at elevated pressures using a high pressure batch reactor attached to the surface science chamber. Specifically, the investigators will prepare and study the catalytic activity of very small VxOy mass-selected clusters supported on single crystal titanium dioxide (110) surfaces in the oxidative dehydrogenation of methanol to formaldehyde. They will probe the catalytic activity as a function of both x and y to develop a model for the reaction mechanism. They will also prepare and study the catalytic activity of very small mass-selected, binary clusters such as palladium/gold and platinum/tin alloys of various compositions supported on single crystal titanium dioxide (110) surfaces in the synthesis of vinyl acetate and the selective hydrogenation of alkenes as a function of composition as well as develop a model for the reaction mechanism.

A large number of industrial processes use nanometer-size clusters (both metal and metal oxide) supported on oxide surfaces to perform reactions that would not take place, or would be commercially unsuccessful if performed on the bulk material. In research supported by this grant the investigators will utilize state-of-the-art experimental and theoretical methods to probe the catalytic activity of well-defined nanocluster catalysts in great detail and develop a fundamental understanding of the catalytic chemistry at the atomic level. The concepts developed through this research will help optimize important industrial processes using these nanoscale catalysts and provide valuable insight into the discovery of new nanoscale catalytic materials. Researchers supported by this grant will also be active in outreach to K-12 schools in the Santa Barbara area. They plan to develop a tutorial presentation on an atomistic view of heterogeneous catalysis that will be included in the currently active outreach program in the department at UCSB. In addition, researchers working on this project will visit high schools in the Santa Barbara and Ventura Counties three times per year to discuss their research and its impact as well as to promote science education.

Proposal Number: 1608914 PI: Buratto, S.
Imaging Morphology, Ion Conductance and Degradation Processes in Energy Materials on the Nanometer Scale Using Tunneling Atomic Force Microscopy

Polymer electrolyte membranes (PEMs) are key components in electrochemical devices such as fuel cells and redox flow batteries. These devices are capable of producing clean energy with high efficiency and are useful in tandem with intermittent renewable electricity generation technologies using wind or solar when the wind or sun is not available and also to store electricity in the form of chemical energy for energy storage. PEMs are, in general, composed of a polymer with a hydrophobic backbone and side chains terminated with hydrophilic functional groups. When this polymer is cast in films to make the membrane, phase separation between the hydrophobic and hydrophilic segments results in a random nanoscale network of hydrophilic channels through which ions are transported and hydrophobic domains that give the membrane mechanical strength. Novel membrane chemistries are currently under development, especially alkaline electrolyte membranes, but require feedback from characterization efforts, especially on the nanometer scale, in order to understand ion conductance and its dependence on the pore network structure. Another key question is how the membrane degrades during operation and that impacts the performance of the device. This project addresses this need for fundamental understanding on how degradation occurs in these types of electrochemical devices. Knowledge gained will help to close the feedback loop between designing the membrane's structure and its resultant function. Students working on this research will learn about alternative power sources, help optimize membrane function, and provide valuable insight and inspiration into the development of the next-generation membrane materials.

This project leverages a suite of experimental methods that utilize tapping (or AC) mode atomic force microscopy (AFM) and conductive probe AFM (cAFM) to probe the pore connectivity and ion transport in proton-conducting membranes such as PEMs. The Principal Investigator and his research group have developed these techniques. In this project, the tools are adapted and applied to the study of alkaline electrolyte membranes, which transport hydroxide ions. Under alkaline conditions, the oxygen reduction half-reaction has significantly improved kinetics, obviating the need for precious metal catalysts. This project's transformative feature includes the characterization of PEM membrane morphology and hydroxide-ion conductance on the nanometer length scale to provide a link between morphology and conductance. The project also includes the investigation of the ion domain morphology and connectivity in vanadium redox flow batteries that utilize proton-conducting membranes under drastically different operating conditions than the alkaline fuel cells. These experiments probe the dependence of the pore network structure on the electrolyte concentration, the degree of vanadium ion penetration into the membrane and the prolonged exposure of the membrane surface to water and heat. The ultimate goal is fundamental understanding of ion conduction, in terms of the size and distribution of the chemical domains responsible for the transport in both alkaline fuel cells and vanadium flow cell batteries. In both systems, ion conductance will be studied systematically as a function of (1) the device operating conditions, (2) the membrane type, and (3) the degree of membrane degradation and decomposition.

Chemical catalysis involves a chemical substance, called a catalyst, which lowers energy costs and creates more selective product distributions by providing another pathway for the chemical reaction of interest. Catalysts are often employed to generate environmentally friendly fuels, such as hydrogen which burns cleanly to water, and are also used to produce value-added chemicals, like carbon monoxide and methanol which can be made from sustainable crop sources. Because of the importance, catalyst function is a major driving force of research in the chemistry community. Transition metals and transition metal oxides possess many of the desired chemical properties for catalysts that can activate the bonds in CO2, H2O and CH4, which are particularly promising feedstocks for a more sustainable production of fuels and value-added chemicals. These metals and their oxides can be especially active as small atomic clusters in the 0.00000005 inch or nanometer size range. Nanoclusters exhibit enhanced reactivity due to their unique geometric and electronic characteristics such as under-coordinated surface atoms, modified inter-atomic spacings and large surface to volume ratio. In this project, Drs. Buratto, Bowers and Metiu produce model catalysts of both metals and metal oxides by deposition of well-defined, atomically-precise nanoclusters from the gas phase onto metal and metal oxide supports. These model systems are then tested for their reactivity in the production of hydrogen and methanol, important as both clean-burning fuels and chemical feedstocks. The research team's unique capability to control nanocluster composition atom-by-atom provides the requisite level of detail to understand the chemistry on the atomic level and provides important insight into the development of new catalytic systems. Drs. Buratto, Bowers and Metiu and graduate students supported by this project are active in outreach to high school students in the Santa Barbara and Ventura Counties to discuss their research and its impact as well as promote science education.

With funding from the Chemical Catalysis Program of the Chemistry Division, Drs. Buratto, Bowers and Metiu prepare, characterize, and test two new classes of nanoscale catalysts based on the atom-by-atom assembly of small bimetallic and metal oxide clusters having one feature in common; they have very small, isolated, well-defined, catalytically-active sites and enhanced catalytic activity. The research centers on the preparation of well-defined PdAun and PtSnm clusters supported on single crystal TiO2(110) and well-defined FexOy supported a single crystal Pt(111) in the inverse catalyst geometry. These model systems are prepared by depositing mass-selected clusters from the gas phase onto single crystal surfaces to control catalyst size and composition. Samples are then studied in ultra-high vacuum by x-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) to determine composition and structure. Temperature programmed reaction (TPR) is used to probe the activity to the water gas shift reaction, CO oxidation, and methanol synthesis. Density functional theory (DFT) is used to calculate the structure of the clusters, their XPS spectrum and their chemical activity, and these data are then compared to experiment. The results are used to develop a detailed fundamental understanding of the catalytic chemistry at the atomic level that will in turn help optimize important industrial processes, and improve the performance of the existing catalysts or uncover new ones. The research groups of Drs. Buratto, Bowers and Metiu are committed to K-12 outreach and the promotion of science in general. They are incorporating their research in heterogeneous catalysis into the University of California Santa Barbara's (UCSB's )5th grade outreach program. This program brings 5th grade students from elementary schools in the Santa Barbara area to the UCSB campus for hands-on science activities. The research team is also working with the teachers to develop a lesson in catalysis that is appropriate for the 5th grade curriculum and then incorporate it into the outreach program. In addition, graduate students supported by this project are active in outreach to high school students in the Santa Barbara and Ventura Counties to discuss their research and its impact, as well as to promote science education.