Spring 2007 Meeting Scene -- Postscript

MRS Meeting Scene
April 9 - 13, 2007
Postscript
Daily dispatch from San Francisco.
Bringing you the very best of MRS.

SCIENCE AS ART COMPETITION WINNERS

First Place Winners

	Matthew Lloyd, Cornell University Sunflowers, "A Bouquet of Anthradithiophene"
	Suresh Donthu, Northwestern University An Early Morning Stroll into Woods: SEM Image of Tin Oxide Nanowires
	Ee Jin Teo, National University of Singapore “Ancient of Days” from classical art to quantum art
	Joel Henzie, Northwestern University Gold Nanopyramids on Silicon Pedestals

Second Place Winners

	Phil Bartlett, University of Southampton Water on a Nanostructured Gold Surface
	Candace Lynch, Air Force Research Laboratory GaAs Sea Creatures
	Samual Shian, Georgia Insitute of Technology Pollen Dawn
	Sarang Ingole, Arizona State University Nano-pop

TECHNICAL TALKS - MATERIALS NEWS

Symposium A: Amorphous and Polycrystalline Thin-Film Silicon Science and Technology

Plasma Synthesis of Silicon and Germanium Nanocrystals
Uwe Kortshagen of the University of Minnesota talked about how silicon nanocrystals can be made by a plasma technique. By tuning the plasma, crystals ranging in size from 2 to 20nm can be made, and these photoluminesce in colors ranging from red to blue. Free electrons in the plasma charge the particles and keep them separated thus avoiding agglomeration. The particles are some 600 C higher in temperature than the plasma which aids in their formation at the rate of 1 um/sec, and 50mg/hr of the dots can be prepared. This latter number is higher than other methods.

The dots, which can be used as bioimaging markers, readily oxidize in air and so an improvement to the fabrication method uses a second plasma that places an organic layer on the dots. Since small silicon particles (blue) are difficult to form by the plasma method, a plasma etching technique has been developed. The blue emission appears to emanate from a silicon defect on the dots.

Symposium J: Nanoscale Magnetics and Device Applications

Nanoscale Design of Perpendicular Recording Media
Andreas Berger of Hitachi 's San Jose Research Center presented a brief tutorial on conventional longitudinal magnetic recording media before describing the perpendicular recording technology that now allows 100 Gb/in 2 areal data densities. Perpendicular recording places the recording media in the center of the read/write head, generating higher magnetic anisotropy due to higher fields. It also has advantages of better high density stability, smaller read-back spacing, and better grain orientation. Disadvantages such as a thicker structure due to the need for a soft magnetic underlayer are more than balanced by the advantages, Berger said. The latest advance is a new structure called “Exchange Spring Media,” in which soft and hard magnetic layers are linked through a coupling layer (CL) which enables tuning of the interaction strength between the hard and soft layers. The CL improves the writeability, increases the signal-to-noise ratio of the read and write procedures, and decreases the bit error rate per areal density.

Symposium O: Organic Thin-Film Electronics--Materials, Processes, and Applications

Several interesting talks were presented in symposium O on Friday, the concluding day of the conference. A few samples are presented here.

The SensorOLED Device
Stefan Sax, of the Institute of Solid State Physics, Graz University of Technology, Graz, Austria, described an optical gas sensor, the SensLED, which consists of an ITO anode on a glass or flexible substrate, the electroluminescent sensor layer ( PtOEP in poly(9-vinylcarbazole) matrix), and a top electrode that is structured for enhanced gas diffusion.With this sensor, which combines the light source with the sensing element in one layer, he is readily able to measure spectral shifts, electroluminescence and lifetime measurements.

Monolayer Transistor using a Highly Ordered Conjugated Polymer as Channel
J. Campbell Scott, of IBM Almaden Research Center, described work on what he called a 40 year old material, polydiacetylene. Using Langmuir-Blodgett deposition techniques and UV radiation (topotactic polymerization) he formed two dimensional crystals to act as the channel in field-effect transistor structures. The only polymer that gives transistor action is pentacosa-10,12-diynoic ethanolamide. It has a wide bandgap of more than 3eV and mobilities that range from 10E-8 to 10E3 cm2/V.sec. The domain size exceeds 100um and it is believed that a high degree of alignment and order in the polymer chains is necessary for transistor action.

Tuning the Contact Resistance in Nanoscale Oligothiophene Field-effect Transistors
Veit Wagner of the School of Engineering and Science, International University Bremen, discussed nanoscale organic field-effect transistors (OFETs) for high frequency applications using oligothiophenes (Dihexyl-n-thiophenes, DHnT) with n, the number of thiophene rings, ranging from four to seven.He made common gate devices on a silicon substrate as gate with thermally grown SiO2 as the gate dielectric and gold contacts and studied the contact resistance of such devices by varying the bandgap of the material. The best performance was obtained for polymers of n-value 7. Contact resistance appears to be a material constant and mobility decreased with channel length.

An Organic Photodetector on a Scanning Probe Cantilever
Kwang Hyup An of the School of Mechanical Engineering, University of Michigan, presented some ingenious work on the construction of an organic photoconductor on the end of a commercial atomic force microscope (AFM) cantilever. The structure has high resolution, high signal-to-noise ratio, easy illumination, compatibility with AFM, and provides simultaneous optical and topological information. The device size is presently 1um by 1um but is expected to be smaller in the future. Applications of this probe will be micropositioning, biological imaging and sensing, and non-destructive semiconductor and optoelectronic device testing.

Role of Charge Interaction in the Behavior of Organic Thin Film Transistors
Christoph Erlen of the Institute for Nanoelectronics, Technische Universität München, Germany , talked about hysteresis in the transfer characteristics of organic thin film transistors. The hysteresis in organic transistors is opposite in nature to that observed in silicon thin film transistors and Chrisoph demonstrated that this was due to the drift of chromium 3+ ions in the polymer. He was able to successfully model the effect using the commercial software tool SENTAURUS(TM).

Symposium Z: Organic and Nanoparticle Hybrid Photovoltaic Devices

Hybrid-nanorod polymer solar cells
In a crowded room, Paul Alivisatos (UC—Berkeley) presented recent work on hybrid-nanorod polymer solar cells with colloidal inorganic crystals as building blocks. While current solar cell technology presents daunting competition, he said there are strong reasons to explore these nanoscale pathways due to opportunities for larger scale fabrication and understanding new physics. Instead of going with bigger and bigger single crystals, which comes with a price, he suggested going as small as is allowed. Based on blends of CdSe nanorods with conjugated polymers such as P3HT formed by solution processing, his talk progressed to look at branched tetrapods having a tetrahedral core with four projecting hexagonal arms. This sparse structure was improved by double branching and then hyperbranching, to start filling space. Next came the dual nanocrystal solar cell, substituting the polymer with a second inorganic nanorod creating a CdSe and CdTe blend.

Finally, he presented dots inside a rod, which reduces interfaces and prevents alternate pathways for photogenerated electrons. To understand the reaction front in the cation exchange involved in this process (substituting cadmium with silver in cadmium sulfide to create silver sulfide), partial cation exchange was examined, with just a third of the Ag ions needed to complete the exchange. Silver sulfide dots form along the length of the CdS rods, and once they are large enough to span the diameter of the rod, the dots repel each other apparently due to a repulsive lattice strain, forming a nearly regular pattern of wells along the length of the rods. The spacing is not yet ideal for communication, and a current challenge is tuning the spacing to bring the dots closer together. Work is also in progress to form contacts to individual arms of the tetrarods.

Symposium EE: Applications of Nanotubes and Nanowires

Properties and Applications of Nanowire Meshes
Semiconducting nanowires are interesting as sensing materials because of their sensitivity and selectivity; Brian Korgel and his colleagues at the University of Texas at Austin are now trying to improve on the manufacturability of sensing devices made with nanowires. In his Symposium EE presentation entitled “Properties and Applications of Nanowire Meshes,” Korgel reported on work done with Si and Ge nanowire meshes to produce chemically sensitive devices in large quantities.

Transistors have already been made using single nanowires, but these do not lend themselves to inexpensive, scalable manufacturing, because in each case a nanowire has to be precisely placed. They also tend to be noisy. Korgel originally considered bundles of nanowires to improve the devices, but alignment of the nanowires would take much care in manufacturing, and the signal-to-noise ratio would be poor. Finally, he hit on the idea placing a large mesh of nanowires onto an interdigitated array of gold electrodes. The nanowire mesh can be made using solution processes to produces quantities of 1 gram at a time in a small laboratory reactor. The process should be scalable to produce hundreds of kilograms per day in a manufacturing setting.

One of the advantages of the nanowire mesh is that AC electric fields can be used to do impedance spectroscopy, allowing a unique “impedance fingerprint” of an adsorbed chemical species to be obtained for each analyte of interest. Korgel’s process includes synthesis of Ge nanowires in a mesh using diphenyl germane and gold in a high pressure supercritical-fluid-liquid-solid (SFLS) process. Passivation of the Ge surface with alkene groups is necessary to prevent solubility in contact with water. These Ge nanomeshes exhibit a p-type gate response. The nanomesh is dropcast from a colloidal suspension onto a prefabricated interdigitated array of gold electrodes. The application of an AC field with varying frequency successfully produced an “impedance fingerprint” when tested with ethanol and hexane analytes. Korgel sees an initial application for these nanowire meshes in radio frequency identification (RFID) devices.

Trace Chemical Vapor Detection Using Single-Walled Carbon Nanotubes
Both the Department of Defense and the Department of Homeland Security have a need for portable detection systems capable of detecting vapors from nerve agents, explosives, and other chemicals in sub-part-per-billion concentrations. According to Eric Snow of the Naval Research Laboratory, this requires a highly surface sensitive approach. He and his coworkers decided to try to use single-walled carbon nanotubes (SWNTs) for this purpose because they are essentially all-surface structures, with the possibility for high electron transport through the tube core.

By fabricating random networks of millions of SWNTs on silica, they produced high-yield devices on a 6-inch silicon wafer in one day. Because low frequency noise is inversely proportional to the number of nanotubes, the average properties of millions of SWNTs on a wafer reduce the noise to acceptable levels. This makes it possible to measure electrical charges at levels as low as 0.01 electron/ SWNT, and also to measure small capacitance changes. Both conductance and capacitance signals were strong in initial tests using acetone as the target molecule, but the capacitance response was unexpectedly large, and had a signal to noise ratio approximately 100 times better than the conductance signal. Further tests using the nerve agent stimulant dimethyl methylphosphonate (DMMP) showed fast, accurate response at levels as low as 100 parts per trillion using the capacitance signal.

Theoretical calculations showed very low expected binding energies of these molecules to pristine SWNTs; this led to the realization that the chemical vapors were sticking to defect sites on the SWNT. “Our new model is that defect sites serve both as low-energy adsorption sites,” Snow said, “and as nucleation sites for additional condensation on the SWNT surface.”

Further developments led to the production of a carbon film composite material composed of 2d graphitic-like carbon 1-nm-thick with lots of defect sites. This composite proved to be even more sensitive to low levels of chemical vapors, with a better signal-to-noise ratio than the original SWNT network on silica. Snow and his colleagues are now working on making a marketable device based on this technology.

Bio-functional Subwavelength Optical Waveguides for Chemical Detection
Donald J Sirbuly, of Lawrence Livermore National Laboratory, gave a very interesting paper on the extension of photonics by confining light into very small volumes. He showed tin dioxide nano-waveguides that were so small that the light traveled on the outside (evanescent behavior). This enables the light to interact with materials placed on the waveguide and is useful in protein screening, sensing, and diagnostics. The waveguides can be readily cleaned using aqua regia (metals) or alcohol (biomaterials). Multiple spectroscopy techniques such as absorbance, fluorescence and surface enhanced Raman spectroscopy (SERS) can be used.

POST-MEETING NOTES

• Spring Meeting Attendees: Did you know that Spring Meeting attendees also receive a 12-month complimentary MRS membership effective July 1? That means access to the Members-Only area of the MRS Website, with over 17,500 online proceedings papers (from the 2000 Spring Meeting to the present) and online access to the MRS Bulletin. Access information, including ID and password, will be sent by e-mail. A complete list of MRS member benefits is available on our Website.
• Proceedings Papers: A number of accepted proceedings papers from the 2007 MRS Spring Meeting are now posted on the web. New papers are posted as they are accepted.
• Purchase Proceedings: Proceedings volumes from the 2007 Spring Meeting can be purchased online.
• Paper Submission: Meeting attendees who have a paper due for submission to the proceedings are encouraged to do so immediately. Contact eproceedings@mrs.org with deadline extension requests.
• Materials Voice: Tell your legislators why support of the physical sciences and science education is necessary . for national security, quality of life and a strong economy. Write your legislators on preselected issues or compose your own message.

• Exhibitors: A list of exhibitors at the Spring Meeting is available online.
• Career Center:  The MRS Spring Career Center would like to thank all the organizations that participated and met with candidates. Résumé Books are now available for purchase to those seeking new employees. For further information please contact Lorri Smiley via e-mail at cc@mrs.org.

• Materials360: We invite you to subscribe to Materials360, our FREE bimonthly e-mail newsletter. It encapsulates current happenings in materials research and professional activities that will give you information at a glance on the rapidly changing world of materials
  
  
• Organize an MRS Symposium: Thinking about submitting a symposium proposal to an MRS Meeting? Perhaps serving as a Symposium Organizer? Here's information on how to go about doing so.

NISE ACTIVITIES VIDEO

     
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© Materials Research Society, 2007