'Nanowire' measurements could improve computer memory

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(PhysOrg.com) -- A recent study at the National Institute of Standards and Technology may have revealed the optimal characteristics for a new type of computer memory now under development. The work, performed in collaboration with researchers from George Mason University (GMU), aims to optimize nanowire-based charge-trapping memory devices, potentially illuminating the path to creating portable computers and cell phones that can operate for days between charging sessions.

The nascent technology is based on silicon formed into tiny wires, approximately 20nanometersin diameter. These"nanowires"form the basis ofmemorythat is non-volatile, holding its contents even while the power is off—just like the flash memory in USB thumb drives and many mp3 players. Such nanowire devices are being studied extensively as the possible basis for next-generationcomputer memorybecause they hold the promise to store information faster and at lower voltage.

Nanowirememory devicesalso hold an additional advantage over flash memory, which despite its uses is unsuitable for one of the most crucial memory banks in a computer: the local cache memory in the central processor.

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"Cache memory stores the information a microprocessor is using for the task immediately at hand,"says NIST physicist Curt Richter."It has to operate very quickly, and flash memory just isn't fast enough. If we can find a fast, non-volatile form of memory to replace what chips currently use as cache memory, computing devices could gain even more freedom from power outlets—and we think we've found the best way to helpsiliconnanowires do the job."

While the research team is by no means the only lab group in the world working on nanowires, they took advantage of NIST's talents at measurement to determine the best way to design charge-trapping memory devices based on nanowires, which must be surrounded by thin layers of material called dielectrics that store electrical charge. By using a combination of software modeling and electrical device characterization, the NIST and GMU team explored a wide range of structures for the dielectrics. Based on the understanding they gained, Richter says, an optimal device can be designed.

"These findings create a platform for experimenters around the world to further investigate the nanowire-based approach to high-performance non-volatile memory,"says Qiliang Li, assistant professor of Electrical and Computer Engineering at GMU."We are optimistic that nanowire-based memory is now closer to real application."

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'Computer synapse' analyzed at the nanoscale

Researchers at Hewlett Packard and the University of California, Santa Barbara, have analysed in unprecedented detail the physical and chemical properties of an electronic device that computer engineers hope will transform computing.

Memristors, short for memory resistors, are a newly understood circuit element for the development of electronics and have inspired experts to seek ways of mimicking the behaviour of our own brains' activity inside a computer.

Research, published today, Monday, 16 May, in IOP Publishing'sNanotechnology, explains how the researchers have used highly focused x-rays to map out the nanoscale physical and chemical properties of theseelectronic devices.

It is thought memristors, with the ability to 'remember' the total electronic charge that passes through them, will be of greatest benefit when they can act likesynapseswithinelectronic circuits, mimicking thecomplex networkof neurons present in the brain, enabling our own ability to perceive, think and remember.

Mimicking biological synapses - the junctions between two neurons where information is transmitted in our brains– could lead to a wide range of novel applications, including semi-autonomous robots, if complex networks of neurons can be reproduced in an artificial system.

In order for the huge potential of memristors to be utilised, researchers first need to understand the physical processes that occur within the memristors at a very small scale.

Memristors have a very simple structure– often just a thin film made of titanium dioxide between two metal electrodes– and have been extensively studied in terms of their electrical properties.

For the first time, researchers have been able to non-destructively study the physical properties of memristors allowing for a more detailed insight into the chemistry and structure changes that occur when the device is operating.

The researchers were able to study the exact channel where the resistance switching of memristors occurs by using a combination of techniques.

They used highly focusedx-raysto locate and image the approximately one hundred nanometer wide channel where the switching of resistance takes place, which could then be fed into a mathematical model of how the memristor heats up.

John Paul Strachan of the nanoElectronics Research Group, Hewlett-Packard Labs, California, said:"One of the biggest hurdles in using these devices is understanding how they work: the microscopic picture for how they undergo such tremendous and reversible change in resistance.

"We now have a direct picture for the thermal profile that is highly localized around this channel during electrical operation, and is likely to play a large role in accelerating the physics driving the memristive behavior."

This research appears as part of a special issue on non-volatile memory based on nanostructures.

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Sharpening the nanofocus: Researchers use nanoantenna to enhance plasmonic sensing

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(PhysOrg.com) -- Such highly coveted technical capabilities as the observation of single catalytic processes in nanoreactors, or the optical detection of low concentrations of biochemical agents and gases are an important step closer to fruition. Researchers with the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab), in collaboration with researchers at the University of Stuttgart in Germany, report the first experimental demonstration of antenna-enhanced gas sensing at the single particle level. By placing a palladium nanoparticle on the focusing tip of a gold nanoantenna, they were able to clearly detect changes in the palladium's optical properties upon exposure to hydrogen.

"We have demonstrated resonant antenna-enhanced single-particle hydrogen sensing in the visible region and presented a fabrication approach to the positioning of a single palladium nanoparticle in the nanofocus of a gold nanoantenna,"says Paul Alivisatos, Berkeley Lab's director and the leader of this research."Our concept provides a general blueprint for amplifying plasmonic sensing signals at the single-particle level and should pave the road for the optical observation of chemical reactions and catalytic activities in nanoreactors, and for local biosensing."

Alivisatos, who is also the Larry and Diane Bock Professor of Nanotechnology at the University of California, Berkeley, is the corresponding author of a paper in the journalNature Materialsdescribing this research. The paper is titled"Nanoantenna-enhanced gas sensing in a single tailored nanofocus."Co-authoring the paper with Alivisatos were Laura Na Liu, Ming Tang, Mario Hentschel and Harald Giessen.

One of the hottest new fields in technology today is plasmonics– the confinement of electromagnetic waves in dimensions smaller than half-the-wavelength of the incident photons in free space. Typically this is done at the interface between metallic nanostructures, usually gold, and a dielectric, usually air. The confinement of the electromagnetic waves in these metallic nanostructures generates electronic surface waves called"plasmons."A matching of the oscillation frequency between plasmons and the incident electromagnetic waves gives rise to a phenomenon known as localized surface plasmon resonance (LSPR), which can concentrate the electromagnetic field into a volume less than a few hundred cubic nanometers. Any object brought into this locally confined field– referred to as the nanofocus - will influence the LSPR in a manner that can be detected via dark-field microscopy.

"Nanofocusing has immediate implications for plasmonic sensing,"says Laura Na Liu, lead author of theNature Materialspaper who was at the time the work was done a member of Alivisatos' research group but is now with Rice University."Metallic nanostructures with sharp corners and edges that form a pointed tip are especially favorable for plasmonic sensing because the field strengths of theelectromagnetic wavesare so strongly enhanced over such an extremely small sensing volume."

Plasmonic sensing is especially promising for the detection of flammable gases such as hydrogen, where the use of sensors that require electrical measurements pose safety issues because of the potential threat from sparking. Hydrogen, for example, can ignite or explode in concentrations of only four-percent. Palladium was seen as a prime candidate for the plasmonic sensing of hydrogen because it readily and rapidly absorbs hydrogen that alters its electrical and dielectric properties. However, the LSPRs of palladiumnanoparticlesyield broad spectral profiles that make detecting changes extremely difficult.

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This is a scanning electron microscopy image showing a palladium nanoparticle with a gold antenna to enhance plasmonic sensing. Image courtesy of Alivisatos group

"In our resonant antenna-enhanced scheme, we use double electron-beam lithography in combination with a double lift-off procedure to precisely position a single palladium nanoparticle in the nanofocus of a gold nanoantenna,"Liu says."The strongly enhanced gold-particle plasmon near-fields can sense the change in the dielectric function of the proximal palladium nanoparticle as it absorbs or releases hydrogen. Light scattered by the system is collected by a dark-field microscope with attached spectrometer and the LSPR change is read out in real time."

Alivisatos, Liu and their co-authors found that the antenna enhancement effect could be controlled by changing the distance between the palladium nanoparticle and the gold antenna, and by changing the shape of the antenna.

"By amplifying sensing signals at the single-particle level, we eliminate the statistical and average characteristics inherent to ensemble measurements,"Liu says."Moreover, our antenna-enhanced plasmonic sensing technique comprises a noninvasive scheme that is biocompatible and can be used in aqueous environments, making it applicable to a variety of physical and biochemical materials."

For example, by replacing the palladium nanoparticle with other nanocatalysts, such as ruthenium, platinum, or magnesium, Liu says their antenna-enhanced plasmonic sensing scheme can be used to monitor the presence of numerous other important gases in addition to hydrogen, including carbon dioxide and the nitrous oxides. This technique also offers a promising plasmonic sensing alternative to the fluorescent detection of catalysis, which depends upon the challenging task of finding appropriate fluorophores. Antenna-enhanced plasmonic sensing also holds potential for the observation of single chemical or biological events.

"We believe our antenna-enhanced sensing technique can serve as a bridge between plasmonics and biochemistry,"Liu says."Plasmonic sensing offers a unique tool for optically probing biochemical processes that are optically inactive in nature. In addition, since plasmonic nanostructures made from gold or silver do not bleach or blink, they allow for continuous observation, an essential capability for in-situ monitoring of biochemical behavior."

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