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February 05, 2009

Nanotech Roundup: Nanolasers, Room Temperature Nanocars, Fast Nanotube Memory, Nanowire Building


This plasmonic whispering gallery microcavity consists of a silica interior that is coated with a thin layer of silver. It improves on the quality of current plasmonic microcavities by better than an order of magnitude and paves the way for plasmonic nanolasers. (Credit: Image courtesy of DOE/Lawrence Berkeley National Laboratory)

1. The principle behind whispering galleries – where words spoken softly beneath a domed ceiling or in a vault can be clearly heard on the opposite side of the chamber – has been used to achieve what could prove to be a significant breakthrough in the miniaturization of lasers. Ultrasmall lasers, i.e., nanoscale, promise a wide variety of intriguing applications, including superfast communications and data handling (photonics), and optical microchips for instant and detailed chemical analyses.

Cavities are the confined spaces in lasers where light amplification takes place and this new micro-sized metallic cavity for plasmons improves on the quality of current plasmonic cavities by better than an order of magnitude.

Whereas previous plasmonic microcavities achieved a best Q factor below 100, the whispering gallery plasmonic microcavity allows Q factors of 1,376 in the near infrared for SPP modes at room temperature.

“This nearly ideal value, which is close to the theoretical metal-loss-limited Q factor, is attributed to the suppression and minimization of radiation and scattering losses that are made possible by the geometrical structure and the fabrication method,” said Min, who believes that there is still room for plasmonic Q-factor improvement by geometrical and material optimizations.

Min said one of the first applications of the whispering gallery plasmonic microcavity is likely to be the development of a plasmonic nanolaser.

“To build a working laser, it is essential to have both the laser cavity (or resonator) and the gain media,” Min said. “Therefore, we need a good, high-Q plasmonic microcavity to make a plasmonic nanolaser. Our work paves the way to accomplish the demonstration of a real plasmonic nanolaser. In addition, fundamental research can also be pursued with this plasmonic cavity, such as the interaction of a single light emitter with plasmons.”






2. From New Scientist, although carbon nanotubes have long been believed to be perfect for making faster, smaller computer memory prototype devices have so far proved too sluggish for practical use. Now a new design that is 100,000 times faster than previous efforts has blasted through that barrier, paving the way for nanotube flash memory to be a part of future electronic and computing devices.

Most previous carbon nanotube devices used silicon dioxide as the insulating layer. But loading that material with charge takes several milliseconds, an age in memory terms. Existing flash memory takes just microseconds to perform the same operation.

But the new device, developed by Päivi Törmä at Helsinki University of Technology and colleagues from the University of Jyväskylä, both in Finland, has closed that gap by using a different insulating material.

They coated the gate electrode in a thin layer of hafnium oxide, which is very sensitive to changes in voltage and has a porous structure that helps it to capture charge.

In tests, the new device could store and erase data in just 100 nanoseconds – a dramatic improvement over previous prototypes and even faster than commercial flash memory.

Faster to come?
"It's pretty amazing considering it has not gone through any optimisation or refining process," says Törmä. "What actually sets the 100 nanosecond limit is not the nanotube memory, but our experimental setup, so it might be able to work at even higher speeds – we just don't know yet."

The device managed to withstand 18,000 operations, which is a reasonable lifetime for a memory device, she adds.

3. One of the great nanotechnology challenges has been solved by chemists who have worked out how to place individual nanowires onto silicon chips reliably and accurately.



Christine Keating and colleagues at Pennsylvania State University in University Park developed their technique using nanowires made of rhodium. They first took a silicon wafer and carved an array of microwells into it, for the nanowires to sit in. A pair of electrodes added to each well enables a powerful electric field to form across its length.

The team then divided the rhodium nanowires into groups and coated each group with strands of DNA designed to bind to a specific disease marker.

With the silicon chip immersed in ethanol, they released one group of DNA-coated nanowires into the fluid and switched on the electric fields in some of the microwells. The interplay between the dielectric properties of ethanol and the nanowires creates a force that pushes the nanowires towards the wells (see diagram). "Our DNA-coated rhodium nanowires then snap into place due to the higher field strength there," says Keating.

The team rinses the chip, then releases another batch of nanowires coated with DNA strands that bind to a different disease marker. This time, they electrify a different set of microwells, and so on. In this way, the team places nanowires designed to detect certain diseases at specific sites on the chip. In tests, the team coated nanowires in DNA sequences that bind to nucleic acids from the genomes of hepatitis B, hepatitis C and HIV (Science, vol 323, p 352).

By labelling the nanowires with fluorescent groups, the team could see where they ended up. They found that 99 per cent of the nanowires settled precisely where they were meant to be, with electrostatic forces holding them firmly in place.


4. The drivers of Rice University's nanocars were surprised to find modified versions of their creation have the ability to roll at room temperature.
While practical applications for the tiny machines may be years away, the breakthrough suggests they'll be easier to adapt to a wider range of uses than the originals, which had to be heated to 200 degrees Celsius before they could move across a surface.

Tour's original single-molecule car had buckyball wheels and flexible axles, and it served as a proof-of-concept for the manufacture of machines at the nanoscale. A light-activated paddlewheel motor was later attached to propel it, and the wheels were changed from buckyballs to carboranes. These were easier to synthesize and permitted the motor to move, because the buckyball wheels trapped the light energy that served as fuel before the motor could turn. Since then, nanotrucks, nanobackhoes and other models have been added to the Rice showroom.




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