Conference Report: MRS Spring 2012, Day 5

Blogger Mike Fury reports from the MRS Spring 2012 meeting in San Francisco. Highlights from the fourth day: electronic skin, energy storage with nanowires, printable inks, gas sensing, inkjet printing, semiconductor polymers for organic devices, CNTs, OFETs, touch screen fabrics, and the coffee breaks.

The fifth and final day of the MRS Spring 2012 meeting opened Friday the 13th at Moscone West in San Francisco opened under bright mostly sunny skies following an evening of thunderstorms and torrential rain. Thunderstorms are not a common event in San Francisco, and this one brought some spectacular lightning strikes on the Transamerica pyramid and the Oakland Bay Bridge. The lightning rods worked as advertised. Praise Science!!

K9.1 Ali Javey of UC Berkeley opened with a discussion of printed nanomaterials for artificial electronic skin (which might imply that work is underway on organic electronic skin, but I suspect that this is not actually the case) and its ability to detect and respond to external stimuli. Many types of basic sensing elements can now be integrated into thin film organic electronics. For example, covering an airliner with a strain gauge skin and detecting early signs of structural weakening might reduce the incidence of fuselages peeling off in flight in our aging fleet. Many of his structures are based on CNT TFT on pollyimide, with mobilities of 40-50cm2/Vs. Stretchability up to 10% is achieved by laser ablating holes in the substrate much like honeycomb decoration paper, and placing the TFT devices at substrate locations that have minimum strain when stretched. A pressure sensing honeycomb glove has a tactile sensitivity and response comparable to human skin. Additional work is underway on programmable, reconfigurable materials from which a 3D shape can be self-assembled from a planar substrate by patterning folds and junctions and activating them in the proper sequence — effectively nano origami. Such devices have applications as actuators of all kinds and can respond to temperature, moisture, pH or light. Imagine curtains that can close themselves when the run is bright.

N11.1 Liqiang Mai of the Wutan U (China) – Harvard Joint Nano Key Lab talked about energy storage at several scales using nanowire electrochemical devices. A test device was developed for characterizing the battery electrode properties of a single nanowires with an ion gel electrolyte. Materials studies include VO2, MoO3, FeSe2, MnMoO4, CoMoO4 and Li-doped variants of these. One intriguing variant was CoMoO4 nanowires that were grown on MnMoO4 nanowires that were somewhat larger, resulting in a porcupine structure. The specific area increased to 54.06m2/g compared to <10 for the MnMoO4 substrate itself. Functional battery work remains to be done, but a strategy for increasing electroactivity has been nicely demonstrated.

J15.2 Darin Laird of Plextronics described some printable inks for OPV and OPD (diode detector) devices and their integrated applications. Minimization of dark current is critical for an effective OPD. Current level for a sample Ca/Al electrode device is >100nA/cm2 at 1V, but a proprietary cathode material brings this down to 66±38. The Plextronics PV2000 OPV ink is based on bis-indene, which is a derivatized C60 buckyball that was developed and commercialized in 2006 (contrary to reports elsewhere of first introduction of this material several years later).

BB10.6 Ulrich Simon of Aachen U (Germany) talked about polyol-mediated synthesis and high throughput impedance spectroscopy screening of gas sensing metal oxide nanoparticles. In addition to the familiar list of common industrial and household gases that are sensed to control safety and comfort issues, there are a host of more exotic gases in the medical and security fields that are garnering increased attention. Nanocrystals are desirable for such applications due to their higher active surface area and thus sensitivity. Over 200 metal oxides have been synthesized and screened using a high throughput experimentation protocol developed in his lab. Thick film amorphous oxides are synthesized in parallel on a 64 cell multi-electrode ceramic substrate and fired concurrently, mimicking the use of parallel analysis for drug screening.

K9.7 Craig Arnold of Princeton U demonstrated the utility of laser-induced blister dynamics for actuating inkjet printing of organometallic molecules for electroluminescent devices. His modeling of multicomponent inks shows that inadequate initiation of the drop formation can result in droplet formation of a surface solvent layer without incorporating a representative portion of pigment molecules in the correct proportions. This printing method is particularly suitable for molecules that cannot survive printing with conventional inkjet methods.

Z12.8 Christine Luscombe of U Washington developed a variety of star-shaped and hyperbranched semiconducting polymers for organic electronic devices using a Ni(PPh3)4 ligand exchange protocol to control conformation and molecular weight distribution. A hyperbranched structure is a less regular analog of a dendrimer. Absorbance ~550nm is 2x greater for the star P3HT than for the corresponding linear P3HT, and preliminary but promising OPV performance was shown.

EE10.1 Sung Hun Jin from U Illinois Urbana (John Rogers group) presented a method for aligning SWCNT and subsequently removing the metallic tubes (m-CNT) to leave only semiconducting tubes (S-CNT). After aligning the tubes, they are pinned at each end with an electrode, and the entire CNT array is coated with an organic film. When a current is applied, heating resistance of the m-CNT melts the coating and exposes them to the surface. Plasma ashing then removes the exposed m-CNT; when the coating is stripped, a high performance array of only S-CNT remains. The technique has been dubbed ‘metallic nanotube removal’ or MNR, and was shown to be scalable to a channel nearly 1mm long. A near term objective is to use this S-CNT array to fabricate high frequency RF devices.

EE10.2 Chongwu Zhou of USC explained a method for DNA separation of CNT seeds based on chirality, followed by a catalyst-free CVD cloning technique to extend the seeds from 0.3µm to 30-40µm in length. This chirality-controlled CNT cloning in combination with semiconductor enrichment enabled fabrication of separated nanotube (SN-) TFT devices with 98% yield, 25kΩ/square sheet resistance, 10µA/µm current density and 67cm2/Vs mobility across 3 inch Si/SiO2 wafers. Another demonstration produced a working display element with 500 pixels and 1,000 transistors.

K10.4 Shimpei Ono of the Central Research Institute of the Electric Power Industry (Japan) described a high performance OFET with ultra-thin gate dielectric. The device uses rubrene single crystals with a 50nm ALD HfO2 gate dielectric and Au metal gate. The HfO2 dielectric exhibits a high carrier accumulation of 5×1013cm-2, 5x the number of charge carriers as a comparable SiO2 device. The high current at 2V applied voltage indicates a high quality HfO2 dielectric. Top gate devices are stable in air, while inverted bottom gate devices with the rubrene exposed to air are not.

K10.5 Toshitake Takahashi of UC Berkeley talked about smart skin as one implementation of a CNT active matrix backplane for conformal electronics and sensors using semiconductor-enriched CNT scheme. The CNT network density is controlled by managing the rate of solvent spreading and evaporation as the mixed S-CNT (~99%) and m-CNT source material is applied drop wise to the polyimide substrate. Electrical properties showed no degradation in bending down to a 2.5mm radius. The flexible, stretchable substrate with devices fabricated at the points of minimum substrate strain is the same as described in K9.1 above.  The transistors showed Ion/Ioff ~104 and a mobility of 20cm2/Vs with an ALD Al2O3 gate oxide.

K10.6 Kazuhiro Kudo of Chiba U (Japan) talked about printed common gate vertical channel transistors using nanoimprint patterning. Evaporated devices comprised pentacene channels, 120nm parylene-C gate dielectric, Al gate, and Au S/D. An analogous wet process device deposited all active layers in a planar stack, then formed two vertical channels using imprint to create a depressed channel with a vertical sidewall on each side. SVC-OFETs can be used for active matrix displays and RFID tags with active loop antennae.

E11.6 Umberto Celano of IMEC talked about Cu migration in conductive bridging memories. The CuTe layer exhibited improved homogeneity with the addition of a Ti interlayer. The migration of Cu into the solid electrolyte is confirmed as the driving mechanism for device operation.

K10.8 Hiam Sinno of Linkoping U (Sweden) used a block copolymer as a surface modifier improving the printing of polyelectrolyte gated OFETs. The problem arises because the electrolyte (e.g. PSSH) is typically hydrophilic, and the semiconductor (e.g. P3HT) is hydrophobic. The block copolymer treatment enhances the surface presence of thiophene components in the PSSH. The electrical functionality of the transistor was not impacted. This electrolyte gated OFET device has the best acronym of the week: EGOFET. Freud would be pleased.

K10.9 Peyman Servati of U British Columbia wrapped up the symposium with a presentation on transparent, flexible composite nanofiber electronics for touch screen fabrics as a replacement for ITO. Candidate materials include SWCNT, Ag nanowires and graphene, but each has its down side. The approach taken here is to mimic natural fiber networks by using an electrospinning process on a solution of composite nanofibers (CNF) where the CNF can comprise several different types of materials. The spray head can induce some level of nanotube alignment which is useful for enhancing performance. Following spinning, the fibers are carbonized at ~700°C, producing a conductive CNF mesh. The resulting mesh can be 80% transparent at 100Ω/o. Early OPV devices performed at 2% efficiency vs. 4% for a comparable ITO device, due to surface roughness issues that remain to be addressed.

Managing the schedule for so many concurrent technical sessions is always a challenge, especially given the preponderance of academic speakers in this conference; I refer of course to the notion of herding cats. One element of the conference that always runs like clockwork is the coffee breaks. They start on time, which is a great reliability feature since not all symposia are coordinated to exactly the same break times. Equally as impressive, but with the opposite pleasure polarity, is the fact that they end precisely on time. If Seinfeld were still producing new episodes, I would expect the iconic soup Nazi to be joined before long by these coffee Nazis. Ruthless but precise. Just doing their jobs.

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