Description of Research Programs Underway.

• Modification of Polyanion (LiFePO4)and layered structured (LiCoO2) cathode materials for Lithium ion batteries

Energy storage and conversion devices become more and more important for usage of clean energy generated, for example, from solar radiation and wind. Devices like portable rechargable batteries are supposed to reduce the use of petrolum by powering electric vehecles. Lithium ion batteries (LIBs) are of considerable interests in recent years owning to high power and energy density. One of the most widely used cathode materials for LIBs is LiFePO4. It has high specific energy density, thermal and chemical stability and of low cost. However, its main drawback is low ionic and electric conductivity. Our aim is to overcome this barrier by surface modification of LiFePO4 in order to enhance the electrochemical performance. Another cathode material that's been paid much attention is LiCoO2. Because of its good reversibility and high electrochemical potential, LiCoO2 is the most widely used positive material for today's LIBs. However, it shows relatively larege fading after extended cycling. Our project is aimed to improve its cyclability by coating metal oxides on bare LiCoO2.

• Rare Earth Oxide-based Diluted Magnetic Dielectrics for Spintronic Device Application

    Funding: NSF-Career Award

Recent advances in semiconductor technology have facilitated the realization of a host of new electronic devices with ever-decreasing dimensions. Handheld pocket computers, ultra-thin cell phones with internet, iPods, iPhones and micro-cameras are a few examples that exploit and adopt the advances in the technological development. However, as the typical component dimensions approach the nanometer scale, further miniaturization becomes increasingly difficult. It is believed that any further improvement in device functionality will require a transition from the conventional electronics to an altogether new regime known as "Spintronics." While the electronic devices utilize the charge of electrons, the typical spintronic devices exploit both charge as well as the spin (a magnetic attribute) of an electron. Because of this additional attribute, spintronic devices are expected to be faster, smaller and consume less power than the conventional charge-based electronic devices. However, the practical realization of spintronic devices heavily relies on the development of two new classes of materials namely, Dilute Magnetic Semiconductors (DMS) and Dilute Magnetic Dielectrics (DMD). In this NSF supported project, our efforts are focused on developing new Rare Earth Oxide based DMD systems.

Ref: A. Tiwari et al. “Ferromagnetism in Co doped CeO2-d: Observation of Giant Magnetic Moment with a High Curie Temperature” Applied Physics Letters 88, 142511 (2006).

• Solid State Dye-Sensitized Solar Cells (SS-DSSCs)

    Funding: Petroleum research Fund

DSSCs based on liquid electrolytes have attained an impressive conversion efficiency of ~11%. But a major problem with these DSSCs is the evaporation and possible leakage of the liquid electrolyte from the cell. This limits the application of these cells and poses a serious problem in the scaling up of DSSC technology for practical applications. A current major focus of interest in this field is to fabricate Solid-State DSSCs (SS-DSSC) by using solid phase electrolytes such as molten salts, organic hole transport materials, and polymer electrolytes. However, most SS-DSSCs studied so far have suffered from the problems of short-circuit and mass transport limitations of the ions, resulting in low conversion efficiencies compared with the liquid version. Lately, the use of p-type semiconductors as hole-collectors in DSSCs has been proposed. But, because of the scarcity of suitable p-type semiconductors (having proper band-gap, valence band position and stability) very little progress has been made on SS-DSSCs. In this project we are utilizing a new p-type semiconductor oxide-CuBO2, recently invented in our laboratory, as a hole collector. Preliminary studies have shown that CuBO2 is very stable, has high electrical conductivity and carrier mobility, and at the same time its flat band potential is very suitable for separating electrons and holes in TiO2-based SS-DSSCs. Project has three major tasks: (i) Fabrication of DSSCs using p-type CuBO2 as a hole-collector; (ii) Development of strategies to employ coupled dye mixtures for enhanced light harvesting; and (iii) Understanding the charge injection and recombination dynamics in SS-DSSCs.

Ref: M. Snure and A. Tiwari "CuBO2: A p-type Transparent Oxide" Applied Physics Letters 91, 092123 (2007).

• Transparent Conducting Oxides and Devices based-on

    Funding: Applied Materials Inc

This project is focused on developing materials which are transparent and at the same time show good electrical conductivity. These transparent conducting oxides (TCO) are very important for modern optoelectronic devices such as, light emitting diodes (LED), solar cells, flat panel displays etc. Recently we have shown that by doing a cotrolled doping of Ga in ZnO, we can make the system as conducting as a metal while retaining its high transparency and other optical characteristics.

Ref: M. Snure and A. Tiwari "Structural, Electrical, and Optical Characterization of Epitaxial Zn1-xGaxO Films grown on Sapphire (0001)" Journal of Applied Physics 101, 124912 (2007).

• ZnO based Flexible Solar Cells

    Funding: Technology Commercialization Office and Applied Materials Inc

Dye-sensitized solar cells (DSSC) have received considerable attention as a cost-effective alternative to conventional solar cells. In these solar cells, dye molecules absorb light in the visible region of the electromagnetic spectrum and then “inject” electrons into the electrode consisting of sintered semiconductor nanoparticles. Present-day nanoparticle-based DSSCs rely on trap-limited diffusion for electron transport, a slow mechanism that limits device efficiency, especially at longer wavelengths. In this project we are developing a new version of the dye-sensitized solar cells in which the traditional electrode (nanoparticle film) is replaced by a specially designed ZnO electrode possessing an exotic ‘nanoplant-like’ morphology. It is expected that the direct electrical pathways provided by the nanoplants will ensure the rapid collection of carriers generated throughout the device.

Ref: A. Tiwari and M. Snure, “Methods of Fabricating Nanostructured ZnO Electrodes for efficient Dye Sensitized Solar Cells” US Patent Application # 11/695,393 (April 2, 2007)

• All Oxide Magnetic Spin Valves and Tunnel Junctions

    Funding: Seed grant

Discovery of Giant Magnetoresistance (GMR) in magnetic multilayers, comprising of alternating layers of ferromagnetic and nonferromagnetic metallic materials, has attracted a lot of scientific and technological attention. Interlayer Exchange Coupling (IEC) between the ferromagnetic layers in these multilayers plays the key role in determining the overall magnetic characteristics of the system. Several theoretical and experimental efforts have been made in the past to understand the nature of IEC and GMR in these multilayers. However, most of these studies were focused on metallic multilayers wherein the constituent layers were either metallic elements or their alloys. In this project we are investigating the mechanism of IEC and GMR in Transition Metal (TM) Oxide-based magnetic multilayers. Some of the fundamental scientific issues which are being explored in this program are: (i) how does IEC behave in a system where the nonmagnetic spacer layer undergoes an M-I transition? (ii) how does IEC behave in a multilayer system where the nonmagnetic spacer layer is a semiconductor? (iii) how does IEC behave in a multilayer system where ferromagnetic layers are made of Diluted Magnetic Semiconductors? and (iv) how does the magnetoresistive characteristics of above multilayers correlate with IEC?

Ref: Tiwari et al. “Low-field Giant Magnetoresistance in La0.7Sr0.3MnO3/ZnO Superlattice Structure” Journal of Nanoscience and Nanotechnology 6, 612 (2006).

• Solar-hydrogen using Dispersed Metal-Oxide Photoelectrodes

    Funding: Technology Commercialization Office

Direct water splitting on a particulate photocatalyst using the sun is considered to be a potential way to produce hydrogen at a large scale. Research during the last few years has shown that metal oxide photocatalysts can be very effective for overall water splitting. However, still, most of the metal oxide photocatalysts developed to date only function in the ultraviolet (UV) region due to their large band gaps (>3eV). Although a number of photocatalysts driven by visible light have been proposed as potential candidates for this purpose, a satisfactory material has yet to be devised. Recently in our lab we have invented a new p-type semiconducting oxide CuBO2 which shows great promise for solar hydrogen production. It posses delafossite crystal structure and exhibits a direct bandgap of 3.2 eV and an indirect bandgap of 2.1 eV. Detailed photoelectrochemical measurements on this material showed that its valence band is located at ~5.2 eV below vacuum (0.46 eV vs. SCE), a value very well satisfying the condition for the photo-splitting of water. Inspired by potential of this material in this project, we are trying to utilize CuBO2 (and its nanocomposites with n-type oxides such as ZnO, TiO2, etc) for designing a commercial solar-hydrogen reactor which can produce hydrogen by direct splitting of water.



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