MRS Spring Day 3: DNA origami, silicon ribbons, self-assembled 3D cubes

by Michael A. Fury, Techcet Group

April 17, 2009 – The second full day of the MRS Spring meeting included impressive talks about new technology in mobile phones; “DNA origami” self-assembly; hyperspectral imaging; single-crystal silicon ribbons; 3D patterned devices made with MEMS and self-assembly techniques; and the utility of “toys” based on electro-osmotic ionic flux.

Taponi Rayhänen of the Nokia Research Center showed some pretty amazing statistics about the proliferation of mobile communication devices. Throughout 2008, Nokia alone produced 13 new communication devices per second, 24/7. By 2010, he predicts 90% global coverage by mobile devices — that’s a 6.6 billion population base. It won’t all be covered by Nokia, of course. But if the Nokia Morph concept phone is any indication, a good portion of it may be. Several videos can be found on YouTube. The technology behind this device includes CNT transistors and circuitry; ZnO sensors, actuators, and photovoltaics for energy harvesting; and flexible, transparent batteries. While many components remain to be developed, Taponi indicated that these will be introduced incrementally in various devices as they become available, and he expects the whole device to be commercialized by 2012.

Paul Rothemund of CalTech demonstrated his DNA Origami technique. Instead of folding paper by hand, you fold DNA strands by spontaneous self-assembly. There may not seem to be much purpose in causing DNA strands to self-assemble into a smiley face, but it is impressive to see. The fact that an arbitrary shape can be fabricated is an essential step in designing the self-assembly of 2D structures. Current materials are not robust enough to be used to transfer images into other materials, but that work is getting underway. The design methodology has already been extended to enable fabrication of encoded DNA ribbons hundreds of microns long. Think of a beaded belt, and you’ll be visualizing one of the examples shown.

Hitoshi Tanaka of Fujitsu gave a prospective view of the FRAM market. It has been 10 years since the introduction of these nonvolatile ferroelectric memory devices, based on the shift between cubic and tetragonal phases; this is the age at which flash memory passed its growth inflexion point. FRAM devices are quite robust, capable of 1010 read and write cycles, equivalent to a transition 30×/sec nonstop for 10 years. The functional phase change materials today are PZT or SBT, with future devices anticipating BiFeO3 or BiTaO3 with gate electrodes evolving to IrO or SRO. FRAMs are fast enough to replace both ROM and RAM functions in several different applications. It is this speed that has positioned FRAM to be a successor to flash in selected markets.

Douglas Bell of the Jet Propulsion Lab demonstrated the concept of hyperspectral imaging, in which each pixel of a digital imaging device functions as an electronically tunable spectrometer. No diffraction grating or other moveable parts are required. Current devices are based on GaN, and can function in the visible range. The new devices employ other III-N compounds and alloys (III = Al, Ga, In). A demonstration device consisting of Au contact/AlGaN/GaN/Si substrate extended the functional range to the near-IR. Further optimization of the composition and configuration are expected to yield devices that span the entire UV-NIR range. Such devices are used extensively in satellite mapping and search missions. This work was done in collaboration with the U. Albany (New York)’s College of Nanoscale Science and Engineering, which is co-located with Albany Nanotech.

John Rogers of the University of Illinois at Urbana-Champaign has developed several variations on a method for fabricating flexible ribbons of single-crystal silicon, to be used for flexible and printed electronics without sacrificing electron mobility. Ribbons 150μm thick can be used for TFT fabrication, with mobilities 1000× higher than organic semiconductors. The ribbons can be fabricated en masse on a silicon wafer; they are then lifted off the template wafer with a rubber stamp that adheres to the ribbon strongly enough to break it free at the tethering points and transfer and affix them to a polymer substrate. In a particular implementation for flexible silicon solar cells, p+ contact regions are defined at one end of an n+ ribbon array. The ribbons are then defined with an etch trench, undercut, and transferred to a polymer substrate. Contact metallization at each end of the ribbon pieces ties them all together into a single-crystal Si solar cell that can flex without the fracture. (An individual PV microcell is 15μm thick × 50μm wide × 1.5mm long.) The same concept can be applied to GaAs PV devices which, coupled with concentrator lenses, could result in affordable high-efficiency PV modules for field applications.

Jeong-Hyun Cho of Johns Hopkins University combined 2D lithographical patterning with MEMS and self-assembly techniques to fabricate 3D cubes at the sub-micron level. The sides of the cube are etched in a flat pattern as if ready to be folded. Special solders are deposited as a hinge glue over the edges to be joined, and the sides are then undercut and freed from the substrate. Upon heating, the solder melts and surface tension causes the sides to fold upward. Video of the self-assembly action made believers of any skeptics in the audience. Boxes ranging from 900nm to 100nm along an edge have been fabricated. Such devices are capable of containing attoliter volumes of reagents or pharmaceuticals.

Suk Tai Chang of NC State demonstrated the utility of the electro-osmotic ionic flux that is generated when an isolated diode is in contact with an ionic liquid. When floated on water and subject to an AC electric field across the liquid vessel, the diode will self-propel under the influence of the field with no moving parts. The velocity is proportional to the field strength, and is independent of the AC frequency and of the diode size (he used 1mm and 3.7mm diodes). If the diode happens to be an LED, it also will emit as it self-propels. If the device is a photodiode, its motion can be stopped by shining a light on it. The utility of these toys comes about by affixing the two diodes to the opposite sides of a microfluidic channel. If the diodes are oriented in the same direction, they function as a fluid pump with no moving parts. If they are oriented in opposing directions, they function as a vortex mixer — which, with appropriate channel design, was shown to be quite effective.

Registered attendance for the 2009 MRS Spring Meeting in San Francisco has been reported as 4120, not counting the exhibitors. That’s about the same as last year’s record attendance (for an MRS Spring Meeting) of around 4100, according to MRS.

Michael A. Fury, Ph.D, is senior technology analyst at Techcet Group, LLC, P.O. Box 29, Del Mar, CA 92014; email [email protected].


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