Un nanoponte in grafene per la radio del futuro?

Un futuro nanomeccanico per gli apparecchi radio viene esplorato alla Columbia University sovrapponendo una sorta di "ponte" in nanografene tra i due elettrodi di un transistor FET polarizzato in continua. Il ricercatore cinese che ha condotto l'esperimento ha applicato un segnale variabile e ha scoperto che a una determinata frequenza il ponte in grafene risuona, facendo "passare" il segnale che può essere misurato campionando la capacità tra il grafene e il gate di controllo del FET. Alla fine di questo percorso appena iniziato, ci potranno essere ricetrasmettitori che filtrano e generano le radiofrequenze direttamente, senza ricorrere a circuiti di oscillazione e discriminatori. L'articolo originale è apparso su Technology Review, che pubblica anche il rimando all'articolo scientifico pubblicato da Yuehang Xu e colleghi.

Here's a simple device. Stretch a sheet of graphene between two electrodes to form a kind of trampoline and then place a third electrode under the trampoline. Finally, send a DC current through the graphene and then add a radio frequency voltage on top to see what happens.

This is exactly the experiment that Yuehang Xu at Columbia University in New York and pals have done. They say the graphene sheet resonates when blasted with a voltage changing at radio frequency signals and that this can be easily measured by monitoring the capacitance between the sheet and the third electrode below the sheet.

That's a significant result. For some time now, physicists have been hoping to find ways of using nanoelectromechanical resonators to filter and generate radio signals directly. At present, this has to be done with various kinds of mixing techniques. The problem is that nanoelectromechanical devices all suffer from parasitic capacitance, which tends to drown out the signals that physicists are interested in at radio frequencies.

Xu and co say their graphene sheet device is immune from this because its design causes the effects of stray capacitance essentially to cancel out. And they prove it by using their device to pick out a radio frequency signal at 33.27 MHz.

Nanofiltri per nanoradio?

Chissà che i ricevitori del futuro - sicuramente i telefoni cellulari - non finiscano per essere basati su dispositivi nanoelettromeccanici (ricordate la radio a nanotubi di carbonio?) più che sul trattamento digitale del segnale. I ricercatori della Purdue University stanno studiando la possibilità di realizzare strutture micro o nanoelettromeccaniche (MEMS) da impiegare come filtri o amplificatori quasi invisibili ma molto efficienti. Quello dei filtri meccanici è un concetto tutt'altro che nuovo. I ricevitori professionali di parecchi decenni fa montavano eccezionali filtri meccanici Collins, rimasti leggendari. Il principio di funzionamento è semplice: si tratta di realizzare una struttura meccanica che abbia una frequenza di risonanza vicina ai segnali in radiofrequenza da filtrare. Questi dispositivi agivano da filtri passa-banda ad altissime prestazioni. Il fatto che un filtro oggi possa essere realizzato via software con l'aiuto di convertitori analogico-digitali-analogico non deve far pensare che l'idea dei filtri MEMS non abbia futuro. L'uso combinato di tecnologie di filtraggio digitali e elettromeccaniche potrebbe anzi essere molto interessante, considerando che al contrario dei vecchi filtri meccanici Collins, quelli studiati alla Purdue University sono sottili come un capello.

Tiny 'MEMS' Devices To Filter, Amplify Electronic Signals

ScienceDaily (Aug. 31, 2009) — Researchers are developing a new class of tiny mechanical devices containing vibrating, hair-thin structures that could be used to filter electronic signals in cell phones and for other more exotic applications.
Because the devices, called resonators, vibrate in specific patterns, they are able to cancel out signals having certain frequencies and allow others to pass. The result is a new type of "band-pass" filter, a component commonly used in electronics to permit some signals to pass through a cell phone's circuitry while blocking others, said Jeffrey Rhoads, an assistant professor of mechanical engineering at Purdue University.
Such filters are critical for cell phones and other portable electronics because they allow devices to process signals with minimal interference and maximum transmission efficiency. The new technology represents a potential way to further miniaturize band-pass filters while improving their performance and reducing power use, Rhoads said.
The device is an example of a microelectromechanical system, or a MEMS, which contain tiny moving parts. Incoming signals generate voltage that produces an electrostatic force, causing the MEMS filters to vibrate.
Researchers have proposed linking tiny beams in straight chains, but Rhoads has pursued a different approach, arranging the structures in rings and other shapes, or "non-traditional coupling arrangements." One prototype, which resembles spokes attached to a wheel's hub, is about 160 microns in diameter, or comparable in size to a grain of sand.
Findings are detailed in a research paper to be presented on Sept. 2 during a meeting of the American Society of Mechanical Engineers' Third International Conference on Micro and Nano Systems. The conference runs from Aug. 30 to Sept. 2 in San Diego. The paper was written by Rhoads and mechanical engineering graduate student Venkata Bharadwaj Chivukula.
In addition to their use as future cell phone filters, such resonators also could be used for advanced chemical and biological sensors in medical and homeland-defense applications and possibly for a new type of "mechanical memory element" that harnesses vibration patterns to store information.
"The potential computer-memory application is the most long term and challenging," Rhoads said. "We are talking about the possibility of creating complex behaviors out of relatively simple substructures, similar to how in cellular biology you can have a relatively complex behavior by combining hundreds or thousands of simple cells."
The band-pass filter design promises higher performance than previous MEMS technology because it more sharply defines which frequencies can pass and which are rejected. The new design also might be more robust than the traditional linear arrangement, meaning devices could contain manufacturing flaws and still perform well.
The devices are made of silicon and are manufactured using a "silicon-on-insulator" procedure commonly used in the electronics industry to make computer chips and electronic circuits. The small, vibrating mechanical structures contain beams about 10 microns in diameter, which is roughly one-tenth the width of a human hair. The beams can be connected mechanically, like tiny springs, or they can be linked using electric fields and magnetic attractions.
"We are in the process of making a second prototype," said Rhoads, who has used simulations and also conducted experiments with the devices to demonstrate that the concept works.
The devices are being fabricated at the Birck Nanotechnology Center in Purdue's Discovery Park through a collaboration with Dimitrios Peroulis, an assistant professor of electrical and computer engineering.
The research is based at a new Dynamic Analysis of Micro/Nanosystems Laboratory at Birck. The lab, managed by Rhoads and mechanical engineering professor Arvind Raman, is equipped with an instrument called a scanning laser Doppler vibrometer, which uses a laser to measure the minute movement of the tiniest structures. The system is housed inside a vacuum chamber sitting on top of a special vibration-absorbing platform critical to making the precise measurements.
Other faculty members and graduate students also use the specialized facility.
The research is funded by the National Science Foundation through an NSF Faculty Early Career Development grant, awarded to outstanding young researchers. So far four Purdue researchers have received the grants this year. The research includes educational components using Purdue's nanoHUB - the Web portal of the Network for Computational Nanotechnology, also NSF-funded and based at Purdue - as well as Purdue's Summer Undergraduate Research Fellowship program.
Rhoads will develop and deploy on the nanoHUB a software tool to simulate the behavior of the resonators, a new K-12 education curriculum on emerging microelectromechanical and nanoelectromechanical systems, and college-level course materials and lectures associated with a new course on the systems.

Nanoradio, preludio di straordinari apparati

La tanto decantata nanotecnologia - leggo nell'articolo che l'ultimo numero di Scientific American dedica all'invenzione della "radio più piccola del mondo", quella basata sui nanotubi di carbonio scoperta a Berkeley nel 2007 - si è concretizzata in una applicazione che può sembrare umile e poco aggiornata, ma che in effetti, secondo i suoi scopritori, potrebbe figliare dispositivi davvero rivoluzionari. Protesi acustiche invisibili, telefonini grandi come un auricolare, oggetti radiocomandati da iniettare nel flusso sanguigno per scopi medici, interfacce per il controllo di funzionalità neuromuscolari... Una lettura interessante, grazie a Gigi Nadali per la segnalazione.

The World's Smallest Radio

A single carbon nanotube can function as a radio that detects and plays songs
By Ed Regis

Nanotechnology is arguably one of the most overhyped “next big things” in the recent history of applied science. According to its most radical advocates, nanotechnology is a molecular manufacturing system that will allow us to fabricate objects of practically any arbitrary complexity by mechanically joining molecule to molecule, one after another, until the final, atomically correct product emerges before our eyes.
The reality has been somewhat different: today the word “nano” has been diluted to the point that it applies to essentially anything small, even down to the “nanoparticles” in commodities as diverse as motor oil, sunscreen, lipstick and ski wax. Who, then, would have expected that one of the first truly functional nanoscale devices—one that would have a measurable effect on the larger, macroscale world—would prove to be ... a radio? But the nanotube radio, invented in 2007 by physicist Alex Zettl and his colleagues at the University of California, Berkeley, performs a set of amazing feats: a single carbon nanotube tunes in a broadcast signal, amplifies it, converts it to an audio signal and then sends it to an external speaker in a form that the human ear can readily recognize. If you have any doubts about this assertion, just visit www.sciam.com/nanoradio and listen to the song “Layla.”
The nanotube radio, its fabricators say, could be the basis for a range of revolutionary applications: hearing aids, cell phones and ­iPods small enough to fit completely within the ear canal. The nanoradio “would easily fit inside a living cell,” Zettl says. “One can envision interfaces to brain or muscle functions or radio-controlled devices moving through the bloodstream.”
(continua)

Il nanodemodulatore che ascolta in AM

A che cosa può servire una radio tanto piccola da essere invisibile? Se lo chiede il New York Times commentando la notizia, apparsa ieri su Nano Letters, rivista della American Chemical Society, della scoperta di una nuova tecnica di demodulazione nanometrica. Meno stupita e più approfondita è la storia riportata da Wired.
Questa ennesima applicazione delle nanotecnologie, in particolare dei nanotubi di carbonio è davvero straordinaria. In pratica Chris Rutherglen e Peter Burke della University of California a Irvine hanno sviluppato una nano-galena, un demodulatore su scala nanometrica capace di estrarre la componente di modulazione (di ampiezza!) da un segnale radio. Immagino che il principio di funzionamento sia quello della risonanza, su cui si basano anche i filtri meccanici. L'audio estratto (l'ampiezza di banda banda è di 100 kHz) viene poi amplificato da un normale amplificatore differenziale. La demodulazione è stata sperimentata con frequenze portanti fino a 1 GHz e chiaramente gli esperimenti sono stati fatti con un iPod Nano. Science Daily ritiene che il nanodemodulatore potrebbe aprire la strada a una nuova industria di nano-apparecchi per telecomunicazioni.

You Thought Your Nano Was Small: Nano-sized Detector Turns Radio Waves Into Music

Science Daily — Researchers in California report development of the world's first working radio system that receives radio waves wirelessly and converts them to sound signals through a nano-sized detector made of carbon nanotubes.
The "carbon nanotube radio" device is thousands of times smaller than the diameter of a human hair. The development marks an important step in the evolution of nano-electronics and could lead to the production of the world's smallest radio, the scientists say.
Peter Burke and Chris Rutherglen developed a carbon nanotube "demodulator" that is capable of translating AM radio waves into sound. In a laboratory demonstration, the researchers incorporated the detector into a complete radio system and used it to successfully transmit classical music wirelessly from an iPod to a speaker several feet away from the music player.
Although other researchers have developed nano-sized radio wave detectors in the past, the current study marks the first time that a nano-sized detector has been demonstrated in an actual working radio system, the scientists say. The study demonstrates the feasibility of making other radio components at the nanoscale in the future and may eventually lead to a "truly integrated nanoscale wireless communications system," they say. Such a device could have numerous industrial, commercial, medical and other applications.
Their findings appeared online October 17 and are scheduled for publication in the Nov. 14 print edition of ACS' Nano Letters.