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【原创】IMRC 2009----DAYS 3
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DAY 3 TUESDAY, August 18 The XVIII International Materials Research Congress 2009 (IMRC 2009) entered its third day of activities in Cancun including the third plenary session of the congress by Ullrich Scherf, the evening poster session, and the continuing exhibit. The conference is being held at the CasaMagna Marriott and the JW Marriott in Cancun, Mexico, which are adjoining sister properties, with sessions held at both venues. There is easy walkway access between the two.  CONTENTS Plenary Lecture - Ullrich Scherf Technical Sessions  PLENARY LECTURE -ULLRICH SCHERF Functional Oligomers and (Co)Polymers for Electronics Applications  There has been significant recent interest in conducting organics and polymers, and for good reason. The 2000 Nobel Prize in Chemistry was awarded to Heeger, MacDiarmid and Shirakawa for their discovery of a conducting conjugated polymer using doping, and there has been an explosion of research in this arena. In the third plenary talk of the conference, Ullrich Scherf (Bergische Universität Wuppertal, Germany) presented a rational and realistic overview of new polymers being developed for electronics applications. Scherf is a polymer chemist and he approached his talk through the prism of polymer synthesis. First he overviewed the basics of conjugated and semiconducting polymers. He then described various approaches to the synthesis of conjugated polymers, including several from his own group, including novel synthetic approaches. Processing of conjugated polymers for applications is also an important factor and he discussed processing issues giving a specific example of nanostructured polymer blends based on semiconducting polymer nanoparticles, and the development of an organic light emitting diode (OLED) based on semiconducting polymer nanoparticles. In the second part of his talk, Scherf considered four applications areas in devices - polymer light emitting diodes, polymer field effect transistors, polymer lasers, and polymer solar cells - and gave specific examples for each. Finally, he discussed his most recent work with all-conjugated amphiphilic rod-rod block copolymers, and the hierarchical self assembly of micellar, vesicular and lamellar nanoscale structures. Clearly, this is a fertile research area of materials science. In the follow-up Q&A session, there was discussion on the cost aspects of these new conducting polymers with an audience member suggesting that the prices of these new polymers are high. Scherf indicated that with scale of production, prices are bound to go lower.   Credit: Ullrich Scherf TALKS ENERGY FORUM Materials Challenges In Photovoltaic Solar Energy Conversion Rueben Collins (Colorado School of Mines, USA) presented on behalf of the Renewable Energy Materials Research Science and Engineering Center (funded by the US NSF), the only one specific to renewables. The world currently consumes 15 TW of energy annually, and this consumption will double by 2050. Renewables contributed a mere 7% of US energy consumption in 2007. Solar could produce up to 125,000 TW, but cost is the key issue. The price of a photovoltaic (PV) module is $3-4 per watt-peak (Wp) panel, but this cost increases to $6-8/Wp when including installation, maintenance, etc. Considering a 1 Wp panel running for 20 years (a typical PV lifetime), operated 4.5 hrs per day (such as the average amount of daily sunlight in Denver) will cost $0.21/kWh; unfortunately, this is more than five times the wholesale price of coal-produced electricity. Collins outlined a variety of PV technologies, their costs, and current state-of-the-art research. Crystalline and multi-crystalline Silicon (c-Si and p-Si, respectively) currently make up more than 90% of the PV market at $3.50/Wp. The main challenge in c-Si and p-Si PV is to use less Si, with current efforts towards this goal including thinner PV cells. Thin-film technologies are currently at $1/Wp (although First Solar recently announced breaking the $1/Wp manufacturing cost barrier), and mostly focused on polycrystalline chalcogenides (CdTe). Record lab efficiency is 16.5%, but panel efficiency lags at 9-12%. Main materials challenges in thin-films include thin CdS/interdiffusion, controlling defect doping, improving open-circuit voltages (weak diodes, defect recombination), and understanding the roles of impurities and grain boundaries. In Cu(in,Ga)Se2, the record lab efficiency is 20%, with panel efficiencies similar to CdTe. Amorphous Si (a-Si) has a lower cost of deposition, but the major issue here is light-induced instabilities, known as the Stabler-Wronski effect. Thin-film polycrystalline Si issues include optimizing the seed layer, reducing defect density, reducing/eliminating high-temperature processing steps, and utilizing low-cost substrates. Organic PVs (oPVs) could produce energy with costs as low as $0.20/Wp, but are much further from being commercialized than inorganic-based cells, mostly because roll-to-roll processing must be realized to employ economies of scale and drive down the cost. As efficiencies increase, interface properties (morphology, ordering, defects, energetics) become more critical for exciton solar cell operation. Much of the basic research in this area are focused on bulk heterojunction approaches, employing quantum dots or nanowires, where the PV action occurs at the interfaces between the nano-scale player and the bulk matrix. Quantum confinements leads to significant efficiency enhancements in these systems, including effects such as: slower carrier cooling (phonon bottlenecks), variation from traditional selection rules, increased carrier confinements, and nonequilibrium processes. These technologies could push costs down to $0.20/Wp and with improved efficiency over oPVs.  The exhibit Importance of Nanomaterials to Energy Conversion and Storage Hydrogen, as a source of electricity converted directly from chemical energy by hydrogen oxidation, has three times the chemical energy per mass compared to that of other chemical fuels, is a clean fuel source, and is the most abundant element on Earth. Hydrogen storage, however, represents a significant technology barrier. Current storage techniques are, in general, compressed H2 gas (requires high pressure), liquid hydrogen (requires low temperature), and solid-state materials. Solid-state hydrogen storage can be accomplished by either physisorption or by chemisoprtion. While many solid-state materials are capable of storing hydrogen, in many cases this storage is not reversible within the required range of temperatures and pressures for commercial applications. Nanomaterials may aid this barrier by increase storage kinetics, capacity, and reversibility, as well as to enable rapid heat transfer involved in phase transitions. Sam Mao's (University of Californic, Berkeley, USA) interest is in developing nanomaterials for controlled solid-state hydrogen storage material synthesis and testing, and to develop storage composite nanomaterials based on ultra-low density physisorption network structures. Free-standing nanostructured films are strong candidates for controlled storage materials. Whereas traditional thin-film technology has complications due to the effects of substrates, free-standing sandwiched nanostructures, provide more reliable determination of hydrogen soprtion properties, reduce oxidation, support loose agglomerates, and eliminate the possibility of substrate-induced nucleation as an origin of phase change. To make these types of structures, Mao deposits metal by vapor deposition onto glass substrates, and then delaminates the film by heating. Mao finds that the H sorption capability of the film increases with temperature, H sorption kinetics are increased, and that a free-standing film absorbs 20% more H than typical ball-milled powders, and does so at temperatures more amenable to commercial application. One possibility for a storage composite is ultra-low density oxide networks. Mao has fabricated silica aerogels, made of silica nanoparticles 3 nm in diameter that form a network of nanopores (10 nm, although the size is easily tuned by modifying synthesis parameters). These aerogels have extraordinary physical properties, such as high specific surface area, very low bulk density, and very high porosity, and can act as a high-surface area support matrix for carbon. Finally, Mao focused on the designs of nanostructures for solid-state lighting applications of nanomaterials. Solid-state lighting is made of layered semiconductor devices, and has many advantages. The entire device is on the order of 300 nm thick and is operated with 2-10 V. In addition, oLEDs have particular advantages, such as the potential for large-area illumination, tailored emission spectra, low operating voltages, low-temperature processing; even though significant challenges remain regarding charge injection barriers, decreasing both device efficiency and lifetime. Mao was able to improve efficiency at the barrier by nanostructuring, which increased the local electric field at the cathode-organic interface, and showed promising results from texturing by imprint lithography, soft lithography, metal nanoparticles, carbon nanotubes, and metal-doped carbon nanotubes.  The Exhibit Biogas Production from Coffee Waste  Carlos Cano of the Instituto de Ciencias, Mexico, presented a poster on the process of generating methane gas from the anaerobic digestion of coffee residue. Pretreated coffee residues were mixed with water and catalyzed by cow feces in a heated bio-reaction vessel. The amount of methane produced was a function of pH, reaction temperature, and time of digestion. The vessel was able to yield gas at a rate of 50 ml/min, in 10 minute intervals. FTIR analysis of the gas generated revealed not only the presence of methane, but also of CO2, CO, H2O, and C3H8 among other constituents. The study concluded that over a period of 5 months, methane production was optimized under neutral conditions and a steady state temperature of 45°C. Symposium 1. Nanostructured Materials and Nanotechnology Electron Phonon Interactions In Carbon Nanotubes Studied By Raman Spectroscopy Electron-phonon coupling in carbon nanotubes (CNTs) can affect both electronic and thermal properties along a single CNT through coupling of electron and phonon migration, and has important implications in CNT electronics and optoelectronics-- for example, phonon-assisted photoluminescence, phonon-assisted tunneling, and light emission from hot electrons. CNTs have periodic boundary conditions in radial directions, causing discretization in reciprocal space, leading to discrete energy bands and sharp van Hove singularities in the density of states. Depending on the morphology, a CNT may be metallic or semiconducting. The G-band phonon in the Raman spectrum has different peak shapes, depending on the electronic structure, and Jing Kong's group at the Massachusetts Inst. Tech., USA, is investigating the origin of the shape difference. The Kohn Anomaly describes the kink in the CNT phonon dispersion diagram, which is caused by electronic screening. The bandgap lowers energy of valence electrons, resulting in softening of the G-band phonon, altering the shape of the phonon peak. The G-band frequency can be used to assess the doping level in individual CNTs. Kong's group synthesizes aligned CNTs by chemical vapor deposition and studies electron-phonon coupling in individual CNTs by Raman spectroscopy. They found that the G-band peak shape evolved with gate voltage in metallic CNTs. A sharp contrast is observed between G+ and G- bands: the G- peak is significantly broadened and downshifted, whereas the G+ peak has slight linewidth variation and frequency shift. The fermi level dependence of the metallic G-band agrees well with the Kohn anomaly. In semiconducting CNTs, the Kohn Anomaly is not applicable and the electron-phonon coupling downshifts the phonon energies. Phonon energy is renormalized, but much less than that in metallic CNTs, and no line broadening is observed. Electron-phonon coupling can also occur with the radial breathing mode (RBM), and this work was compared to the trends found in G-band. Finally, electromechanical coupling describes the mechanical response of individual CNTs in response to excess charges, which has importance in bulk actuators. Kong's group finds that metallic CNTs have a different response than semiconductor CNTs to excess charge.  Poster session Symposium 2. Theory and Computer Simulation of Materials Predicting Solid-Aqueous Equilibria for Optimized Energy Storage Materials  Pourbaix diagrams map the stability of bulk metal complexes as a function of pH and electric field, and are particularly helpful for electrochemical processing in alkaline battery applications. Kristin Persson (Lawrence Berkeley National Laboratory, USA) has modified traditional Pourbaix diagrams to understand the effects of nanoparticle-size on catalyst stability in fuel cells, as well as to predict the stability of these catalysts in the presence of other cations, for the improvement of fuel cell efficiencies through wiser choices of catalysts and catalyst baths. Size effects on the stability of Pt nanoparticles in aqueous solutions, such as those in fuel cells, were investigated. Persson employed Pourbaix diagrams, modified by simulations, to understand how Pt nanoparticles coarsen to determine if decreased cell efficiencies were due to Pt dissolution. Density-functional method calculations described potential bulk and nano-phases. A grand canonical potential was constructed, including different absorbants and nanoparticle sizes, to calculate phase stability as a function of chemical potential. Simulation results for the bulk were shown to match experimentally observed values and trends, validating the method. LiFePO4, a common Li battery cathode material, was investigated to understand the evolution of particle morphology with pH. In reducing conditions, the particle is completely covered in hydrogen. As pH was varied from reducing to oxidizing, the particle evolved to be completely covered in H2O and the (010) facet became stable. Li was found to dissolve anisotropically at a rate depending upon the chemical potential of each facet, and the (010) facet became unstable. As oxidizing conditions are introduced, the particle is coated in H2O and OH, and finally at strongly oxidizing conditions the particle is entirely covered in O and has a rectangular shape. Persson concludes that the shape most amenable to fuel cell activity, that with the largest (010) surface, is found in acidic pH, as confirmed by experimental observations in the literature. This methodology was then extrapolated to high-throughput screening of the aqueous stability of oxides, to correlate the energetics of all solids and ions to understand their stabilities in water. Symposium 3. LASMAC and Archeological & Arts Issues in Materials Science History of Conservation and Natural Sciences Applied to Study Cultural Heritage of Ancient Mexico  Luis Alejandro Torres Montes of Instituto de Investigaciones Antropológicas, UNAM, Ciudad Universitaria, Mexico, is one of the pioneers of archaelogical conservation in Mexico. It was very appropriate that he opened and gave the first talk in symposium 3 on Archaelogical and Art Issues in Materials Science. He began in a philosophical mode and asked what comes first, the heritage or its conservation? He then reviewed conservation of archaeological artifacts of ancient Mexico during all the periods of its history, since epochs before the arrival of the Spaniards, finishing around the beginning of the 21st century. He divided time into various periods including prehispanic times, the conquest period of 1521-1555, the colonial era of 1555-1770, formation of the nation from 1770-1864, and on to more modern times. He indicated that conservation efforts in Mexico came into their own only in the 1950s and 1960s, with a few specific people including himself who went abroad and attained the training needed, returned to Mexico and then set up appropriate institutions here. In fact, other conservators from Latin America were able to come to Mexico and get their own training. He gave several specific example of artifacts in Mexico subject to conservation efforts. Pottery And Adobe Walls At Casa Grande Ruins National Monument: Technology, Conservation And Public Access  Pam Vandiver of the University of Arizona described a study on pottery as well as adobe walls found at the Casa Grande National Monument located in southern Arizona. The pieces were obtained from the "Great House" which is a three story adobe building with 5 rooms and dates back to A.D. 1200-1450. This investigation included compositional and microstructural characterization of both ceramic and adobe samples analysis using scanning electron microscopy and energy dispersive x-ray analysis of inorganic phases in conjunction with micro-Raman spectroscopy of organics. Pottery firing temperature range was determined for decorated and undecorated ceramics using differential thermal analysis, refiring experiments and scanning electron microscopy. In addition, the adobe walls, which were found to be remarkably well preserved were also investigated. The study revealed five to seven types of pottery formed in bands, strips, and coils, that were fired to 700-900 C, were decorated with iron oxide and burnished with painted plant resins. The architectural walls with 800 year staying power suggest an unusually durable composition with a greater amount of caliche than in the soil. Adobe walls encased with mixtures of adobe and various polymeric consolidants and tested over a 20 year period all failed. Possible construction methods for the walls are currently being tested to determine how a standardized amount of material was laid into place. They have begun testing the mechanical and chemical properties of several natural mixtures of local materials, including variations in coarse fraction, clay, lime and plant gums that have been proposed as possible protective surface layers and encasements for fragile adobe walls. Symposium 4. Materials Characterization Synthesis and Characterization of AlN Nanorods Produced By Mechanothermal Process In a poster presentation, R. Esparza presented AlN nanorods produced by a mechanothermal process. High-purity AlN powders were milled by SiN mixing balls for 30 hrs, and the resulting nanocrystals were then annealed for 10 min at 1200 deg C. X-ray diffraction patterns taken between processing steps showed gradual line broadening through the processing. AlN nanorods were characterized x-ray diffraction and transmission electron microscopy (TEM). These nanorods were found to be 30 nm in diameter and 100 nm in length, with a growth direction of [001], as determined by selected-area diffraction and high-resolution TEM. Energy dispersive spectroscopy confirms the presence of Al and N. Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) isosurfaces were simulated for the AlN nanorods.  Poster session Symposium 6. New Trends in Polymer Chemistry The Preparation of Biopolymers Derived from Sucrose by Means of Thiol-ene Photopolymerization Ricardo A. Ortiz, of Centro de Investigacion en Quimica Aplicada, presented a novel method for photopolymerizing functionalized sucrose monomers. The benefits of using this Thiol-ene based method are high cure rate, high regioselectivity under mild conditions, and lack of O2 inhibition. The researchers first synthesized allyl sucrose monomers with a minimum of 2 functional groups. A total of seven sucrose derivatives were investigated for this presentation. The polymerization process begins with the generation of a thiol radical that reacts with the double bonds of the sucrose monomer, which generates a secondary free radical. The free radical chain reaction from this step is terminated when two radicals interact. For this study, two different thiols, PETKMP and TMPTMP and two different photoiniatiors, (DMPA) and benzophenone were used. The effects of the photoinitiator and thiol type on the storage modulus, kinetics of photopolymerization, and the viscoelastic properties of the polymer were examined. The effects of different substitution groups on thermal stability, reactivity and contact angle were also explored. Ortiz concluded that all of the derivatives examined had good thermal stability, and the most reactive monomer had the least substitution. However monomers with greater substitution had better viscoelastic properties due to crosslink density.  Registration area Symposium 12. Technological Innovation and its Influence on Materials Processing Improving of Ni-base Seamless Rings Manufacturing for Gas-Oil Applications Using Statistical Tools FRISA aerospace (Santa Catarina NL, México), established in 2003 specializes in state-of-the-art rolling processes, and has employed students to test a new software to develop efficient experiments for the optimization of Ni-based alloys for oil-gas applications, aircraft engines, etc, as described by O. Covarrubias. The multi-step production of these Ni-based rings leaves many variables for opimization: raw materials are forged, rolled, pressed, fed into a ring-rolling machine, heat treated, and finally finished in the machine shop. Alloy development is expensive-- in one example, the development cost was $600,000-- so Minitab software was utilized to efficiently optimize production parameters. Covarrubias focused on Alloy 600 and 800H, producing 15 samples from each alloy for testing. Testing metrics included tensile properties, grain size, and hardness, obtained as a function of processing time and temperatures. Statistical tools, such as regression analysis, were used to determine the effects of process parameters on the properties of interest and ensure the validity of the findings. Processing steps were optimized by this way, in one case representing a 45% savings. A Model For The Hot Deformation Of Fe-Ni-Co Alloys For The Aerospace Industry  FRISA Aerospace produces rings from Ni-based alloys, Ti-based alloys, stainless steel, and other specialty alloys for sale to aerospace companies. Due to the expensive nature of the raw materials and the high quality requirements imposed by the aerospace industry customers, the processing of each alloy must be individually optimized. Each alloy goes through a variety of processing steps, including forging, piercing, and ring-rolling. Superalloys refer to Fe-Ni or Ni-Co alloys used at temperatures above 550 deg C, and represent a particularly interesting class of heat-resistance stainless steels. In collaboration with Martha Guerrero-Mata's students from the Universidad Autonoma de Neuvo Leon, Mexico, superalloys were evaluated for the effects of temperature and stress due to the forging and piercing processes with Abaqus/Explicit software. In particular, friction coefficients for the different tools and the contact method were investigated for the forging process, and damage theory for the piercing process. This information allows for a deeper understanding of the production process, aiding in the optimization of the steps.  Poster session Symposium 13. Advances in Semiconducting Materials Magnesium-Doped GaN Grown By MOCVD C. Guarneros and co-workers investigated the doping of MOCVD-grown GaN films with Mg for the development of optoelectronic devices, as summarized in this poster. Low-pressure MOCVD was carried out in a horizontal chamber at 76 Torr at 900 deg C on Al2O3 substrates. Bis-cyclopentadienyl magnesium was employed as a Mg-precursor, and flowed at a constant rate with TMGa and NH3, employing H2 as a carrier gas. Following the growth, the GaN:Mg film was annealed in N2. XRD confirmed GaN in a wurtzite structure. The sample was excited by 325 nm light at 10K and the photoluminescence collected, showing a dominant emission peak at 366 nm corresponding to the near band-edge (NBE) emission, as well as donor-acceptor pair (DAP) emission at 380 nm, as well as its LO phonon replicas at 390 and 400 nm. As the doping concentration was increased, the NBE peak was observed to disappear, whereas the DAP emission was strengthened and a peak at 413 nm emerged, which the authors attribute to a deep acceptor level, a Mg complex, or a DAP-type transition from a donor to a shallow Mg acceptor. Annealing the sample did not noticeably alter the PL spectrum.  Poster session CVD Preparation and Photoluminiscence Of ZnS Nanowires ZnS nanowires were grown by Au-assisted chemical vapor deposition (CVD) from ZnS powders onto Si (111) substrates, as summarized in this poster presented by J. Manuel Juarez. The CVD was a hot-walled horizontal chamber. Characterization by SEM shows that the nanowires have diameters within the range of 10-20 nm and lengths of several microns. Room-temperature photoluminescence (PL) was performed by excitation with a Hg lamp. The PL spectra shows a variety of peak emission wavelengths of the nanowires, ranging from 520 to 600 nm, with a central cumulative wavelength of 560 nm. Improving Electrical and Photoluminescence Properties of Undoped GaN Layers by Deuteration Non-intentionally doped (NID) GaN layers are found in a variety of devices. Due to a lack of intrinsic substrates, these films are grown on foreign substrates and are thus highly defective. These defects cause reduced device performance, delaying the commercialization of GaN film technology. Carrier mobilities in GaN drops significantly at 10^17 carriers per cm3. Hydrogen is known to passivate a variety of deep and shallow energy levels in most semiconducting materials, which can improve electrical properties. Deuterization is a common technique to introduce hydrogen, and has been widely reported with consistently favorable results in p-Si solar cells, MOSFETs, and a-Si (to tie up dangling bonds). Hydrogen passivation is not well-understood in GaN, with most reports concluded that deuterization does not appear to passivate the electrical activity of threading dislocations. J. Mimila-Arroyo (Centro de Invetigacion y de Estudios Avanzados del IPN) has investigated the effects of deuterization on the electrical properties of NID GaN. GaN layers (3.5 microns thick) were grown by LP-MOCVD on c-sapphire and ohmic contacts made with Ti/Al. A final annealing step at 800 deg C for 2 min allowed for the activation of the H. He found that controlling plasma power during deuterization controls the concentration of H impinging the GaN surface. SIMS profiles suggest that diffusion occurs by two means: one that is abnormally fast, likely through the highly disordered material around the dislocations; and another that pins the diffusion profile, decreasing the defect concentration. Mimila-Arroyo concludes that deuterized H decorates the defects, thus acting as a probe in these regions. Deuterated GaN shows increased PL intensity five times over as-grown GaN, with no observed shifts or line broadening in the peak. Mimila-Arroyo posits that H atoms travel quickly along defects until the annealing step, at which point the H diffuses away from the defects, along the way passivating any electronic states created by defects that previously acted as donors. Once ionized, H- has a large activation energy for thermal diffusion (3.4 eV), and so remains stationary at the defect. Another annealing step completely removed H, and Mimila-Arroyo showed that PL properties return back to their as-grown behavior, showing that increased PL intensity is indeed an effect of the deuterization. Finally, he observes a homogenization of physical properties across the wafer due to deuterization.  Poster session Symposium 18. Solar-Hydrogen and Biofuels Death Rate Analysis for Coliforms in a Green House Effect Device for Water Disinfection A presentation on the research of Alvarez Gutierrez et al. gave an insightful look into the use of the green house phenomenon to disinfect drinking water. Waterborne pathogens, such as Escherichia coli, Shigella, and Vibrio cholera continue to present a hazard to the water supply of Mexico. Gram-negative, non-spore forming bacterial pathogens, such as E. coli, are classified as coliforms and their activity level is used as an indicator of water quality. The goal of the Green House Effect Device (GHED) is to reduce the levels of coliforms to standards set by Normas Oficial Mexicana (NOM). The GHED operates by heating the water supply to temperatures greater than 60°C, which is deemed sufficient to eliminate most waterborne pathogens. A study on a contaminated water sample showed the GHED was able to heat the water to disinfection temperatures under a number of experimental conditions. The results showed a log reduction of coliforms with time, and after 2 hours there were no living bacteria present. The speaker concluded that the GHED is a low cost, quick, simple way of disinfecting water supplies to a level that meets NOM 112-SSA standards.  Poster session Oil from Thevetia Grown in Yucatan Peninsula: Extraction and Characterization Thevetia peruviana, a tropical plant native to Mexico was the subject of an investigation conducted by Lopez Dominguez et al. to evaluate Thevetia oil as an alternative feedstock for biodiesel production, and to characterize its physiochemical properties. The study began with the collecting, drying, and removal of Thevetia seeds. The oil was then manually extracted, with 33.4% oil recovery. The physicochemical properties examined were fatty acid composition, saponification value, specific gravity, and viscosity among others. Oil from the Jatropha curcas plant was used as a comparison. The results of the study indicated that the Thevetia oil had sufficient physiochemical properties for use a biodiesel fuel and that if Thevetia seeds are to be used extensively in the future, 1st generation seeds should be avoided, to maintain the food source.  Poster session Symposium 19. Photovoltaics, Solar Energy Materials and Thin-Films Structural and Electrical Properties of ZnO Ultrathin Films Deposited By Atomic Layer DepositionZnO is a wide, direct bandgap semiconductor and a promising candidate for optoelectronic applications, especially as a replacement to ITO as a transparent electronic contact. Atomic layer deposition (ALD) uses precursor gases to deposit individual atoms to form high-quality films over large areas, and can be performed at low temperatures. In this work by Jose Flores (C+CIIDIT-UANL), in a class-100 clean room, a Si(100) substrate was cleaned by HF-dip and then coated by hydroxyl groups in an ALD chamber, which were reacted with diethyl zinc (DEZ) to deposit a layer of Zn atoms. Water vapor was pulsed into the chamber to contribute a layer of oxygen, and the cycle continued to form alternating layers of Zn and O atoms. Uniformity was ensured by the saturation mechanism and the thickness controlled by the number of reaction cycles. Pressures within the chamber were also closely monitored. Substrate temperature was varied to study the effect on film quality. X-ray diffraction confirmed polycrystalline ZnO films, and the substrate temperature was found to have a profound effect on the crystalline orientation of the films. Carbon contamination was detected by surface-chemistry XPS, which may have been introduced during pre- or post-ALD processing or from DEZ residues during ALD. AFM characterization and Hall effect measurements indicated consistent results across all samples. Optical transmittance show 73% transmittance of the visible range, similar to that of ITO. The room-temperature photoluminescence spectrum is dominated by near band-edge (NBE) emission at 390 nm, with the blueshift from the bulk NBE contributed to the presence of point-defects in the form of vacancies. Reality Check on Optical Ways to Improve Photovoltaics: Elegance, Science, and Reduction to Practice  In a poster presentation, D. Cahen (Weizmann Institute of Science, Israel) described the employment of dichoric mirrors to efficiently realize solar spectrum splitting with minimal optical concentration. A photovoltaic (PV) material is limited in the energies of photons it can absorb by its electronic bandgap, and efficiency improvements may be made by the employment of tandem solar cells and/or spectral splitting. Theoretically, an infinite number of junctions could achieve efficiencies of 68%. Cahen suggests the use of cheap, readily-available dichoric mirrors made of TiO2 or SiO2 to split the light and direct higher energies to a large bandgap material and lower energies to a small bandgap material, and shows a 50% improvement in efficiency over a solo polycrystalline Si PV. He concludes that these results "point to the urgent need for efforts towards cheap high bandgap, high voltage gap solar cells."  Walkway between the CasaMagna Marriott and JW Marriott ABOUT THE MEETING SCENE The Meeting Scene e-mails are compiled and edited by Dr. Gopal Rao, Web Science Editor, Materials Research Society. Contributors include Megan Brewster (Massachusetts Institute of Technology) and Tara Washington (University of Florida).    Megan Brewster and Tara Washington, under the Apprentice Science Reporter program, are supported by the IMI Program of the National Science Foundation under Award No. DMR04-09848, managed by the International Center for Materials Research, University of California, Santa Barbara, USA. You have received this as a subscriber to the Meeting Scene or as a member of MRS-Mexico. To Unsubscribe, please e-mail braughler@mrs.org. Archived Meeting Scene Issues are available online. View all free MRS e-newsletters and alertsand subscribe. |
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