Tiny rods and paddle make new thermal sensor

How the new sensor works (click to view diagram)

Researchers in the US have developed a new type of thermal sensor by combining micron-sized "paddles" with nanometre-sized support rods. The device is made using standard silicon-fabrication techniques and is extremely sensitive to infrared radiation – without the need for cooling. As a result, the team claims that it could be used as a thermal and infrared imager that is integrated into electronic devices.

A torsional resonator is a rectangular paddle-shaped device with two thin support rods connecting it to the rest of a chip structure. When a force is applied to the paddle, it vibrates at a certain characteristic resonant frequency. There are two main ways in which the resonator vibrates. These are the torsional mode where the paddle twists back and forth along the rod axis and the flexural mode where the paddle vibrates up and down like a trampoline perpendicular to the rod axis.

Michael Roukes and colleagues at the California Institute of Technology have now made thermal sensors from such resonators. The researchers began by coating the surface of a micron-sized silicon paddle with titanium nitride – a material that absorbs infrared radiation. Next, they applied a time-varying voltage between the paddle and the substrate it was on. This generates a force that sets the paddle vibrating at its resonant frequency.

Resonance-frequency shifts

When the paddle is exposed to infrared radiation, it heats up and this shifts its resonant frequency. "By tracking this change in frequency, we are able to determine the intensity of radiation that hits the device," explains team member Edward Myers. This change is measured using an optical interference set-up that involves reflecting laser light from the surface of the paddle (see figure).

The paddle supports are made of silicon nanowires and measure just 1 µm long and 50–100 nm in diameter. Thanks to these tiny supports, the paddle is extremely well isolated from its environment. This means that only a small amount of infrared radiation is needed to heat the device by a measurable amount.

The sensor was made from standard semiconductor and metal materials using standard industrial techniques – so in principle, Roukes and colleagues could make large arrays of these paddles for use as thermal imagers.

Portable applications

"Ultimately, we believe that these sensors can perform as well as certain standard infrared sensors that require cooling to cryogenic temperatures," Myers says. "Our devices can operate without cooling, which makes them potentially useful for portable applications, such as night-vision goggles, home surveillance and perhaps even the next generation of smartphone cameras."

The researchers now plan to work on making the device smaller and improving the materials used to make it – something that should further improve its thermal characteristics. "We are also looking at scaling up from one device to an array of devices," reveals Myers. "As part of this plan we hope to integrate these sensors with on-chip CMOS electronics, which will make for easier control and readout of many thousands of elements at once."

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Why water prefers the single life!?

Double life of water?

The idea that liquid water can exist in two different forms has been controversial since it was first raised more than 20 years ago. But now the existence of the "liquid–liquid phase transition" in water has been emphatically challenged by two researchers at the University of California at Berkeley, who say that their extensive search for it in computer simulations has revealed no such thing. Instead, they say, the earlier claims stemmed from the common problem of interpreting simulations before they have reached their equilibrium state.

 

The possibility that liquid water has two different phases dates back to 1992 when Gene Stanley at Boston University and colleagues investigated the metastable region of deeply supercooled water. Using computer simulations rather than experiments, which are hard to do as the water tends to freeze to ice, Stanley and colleagues found that below about –75 °C and at pressures of several thousand atmospheres, metastable liquid water can spontaneously separate into two forms.

The phase boundary between them ends in a critical point, where the two types of water become indistinguishable.

Order in chaos?

The idea that a liquid – essentially a dense jumble of disordered molecules – could have two different forms was surprising as it is hard to see how there can be two distinct kinds of disorder.

Water molecules, however, are different, as they link into a constantly changing 3D network via hydrogen bonds. Each molecule has (on average) four near neighbours in a tetrahedral arrangement, making the local structure of the liquid relatively orderly.

However, the hydrogen bonds tend to hold the molecules "at arm's length", keeping them further apart than they would otherwise be. In ice this creates a rather open crystal structure, but in liquid water many of the hydrogen bonds are deformed or broken, allowing the molecules to come closer.

Stanley and colleagues saw the liquid–liquid transition as a reflection of two opposing tendencies: on the one hand, the molecules want to maintain a fairly open structure through hydrogen bonding, but on the other hand they achieve denser, random packing when bonds are broken. One of the putative metastable forms of water was therefore a "low-density liquid" (LDL) and the other a "high-density liquid" (HDL).

Several other liquid–liquid transitions have also since been found, both in simulations and in some experiments in liquids that have a similar tetrahedral coordination to water, such as silicon and phosphorus. Indeed, Stanley has suggested that many of water's famous anomalies – for example, the fact that its density peaks at 4 °C – are an echo of the two distinct liquid phases far inside the metastable region.

No distinctions

But now David Limmer and David Chandler at Berkeley claim their computer simulations show only one liquid phase in the metastable region, which eventually freezes to ice. Their suggestion has been provoking vigorous debate ever since their first paper on the matter was published in 2011. The new work is more exhaustive, but still finds no evidence for the liquid–liquid state.

So why have others seen the transition? Limmer and Chandler say that, in a simulation, it is not enough to wait for the density of the system to settle down to a steady state, which might be taken as a sign that the system has reached its equilibrium state.

They say one needs to wait long enough for equilibration of a second parameter, which distinguishes an amorphous phase from a crystalline one. But the latter, they say, can take thousands of times longer to equilibrate than the density because it involves the reorientation of large domains during the transition from liquid to ice.

"On the way to crystallization, [the second parameter] changes imperceptibly on a timescale where the density will fluctuate many times between the higher-density liquid and the lower-density partially formed crystal," Chandler explains. "The error others have made has been in thinking that those density fluctuations represent transitions between distinct liquid phases." Stanley, though, is not persuaded. "All simulations of realistic water potentials are consistent with the liquid–liquid transition hypothesis," he says.

Vigorous debate

Pablo Debenedetti, a chemical physicist at Princeton University, New Jersey, also remains convinced that the liquid–liquid transition is real, and has recently reported simulations showing the two liquid phases using one of the same water potentials for which Limmer and Chandler report only a single phase. Stanley thinks that the Berkeley duo looked only outside the region of the phase diagram where the two liquids are metastable. Chandler, however, denies this claim. "Of course we look in the same regions of phase space," he says.
Debenedetti presented further results favouri

ng the transition at the American Physical Society meeting in Baltimore last week. "I provided clear computational evidence of two coexisting metastable liquid phases and a stable crystal, all at the same temperature and pressure," he says. "Our calculations are completely inconsistent with David Chandler's – we clearly see two, not one, liquid phases."


Stanley also points out that "liquid–liquid transitions are unambiguously present in systems other than water." But while Chandler agrees that "the transition for phosphorus appears to be the real thing, and I believe liquid sulphur does something similar,” he also says that “this phenomenon is nothing like the putative liquid–liquid transition in water, which cannot be directly observed."
At this point the argument seems to be approaching stalemate. "There is clearly need for an independent assessment of this topic," says Chandler.

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Graphene loudspeaker could rival commercial speakers and earphones!!!

Graphene sounds

Researchers in the US have made a graphene loudspeaker that has an excellent frequency response across the entire audio frequency range (20 Hz–20 kHz). While the speaker has no specific design, it is already as good as, or even better than, certain commercial speakers and earphones in terms of both frequency response and power consumption.

Loudspeakers work by vibrating a thin diaphragm. These vibrations then create pressure waves in surrounding air that produce different sounds depending on their frequency. The human ear can detect frequencies of between 20 Hz (very low pitch) and 20 KHz (very high pitch), and the quality of a loudspeaker depends on how flat its frequency response is – that is, the consistency of the sound it produces over the entire 20 Hz–20 KHz range.

"Thanks to its ultralow mass, our new graphene loudspeaker fulfils this important requirement because it has a fairly flat frequency response in the human audible region," says team leader Alex Zettl of the University of California, Berkeley. He told physicsworld.com that the fact that "graphene is also an exceptionally strong material means that it can be used to make very large, extremely thin film membranes that efficiently generate sound".

Because the graphene diaphragm is so thin, the speaker does not need to be artificially damped (unlike commercial devices) to prevent unwanted frequency responses, but is simply damped by surrounding air. This means that the device can operate at just a few nano-amps and so uses much less power than conventional speakers – a substantial advantage if it were to be employed in portable devices, such as smartphones, notebooks and tablets.

High-fidelity sound

The Berkeley researchers made their loudspeaker from a 30 nm thick, 7 mm wide sheet of graphene that they had grown by chemical vapour deposition (CVD). They then sandwiched this diaphragm between two actuating perforated silicon electrodes coated with silicon dioxide to prevent the graphene from accidentally shorting to the electrodes at very large drive amplitudes.

When power is applied to the electrodes, an electrostatic force is created that makes the graphene sheet vibrate, so creating sound. By changing the level of power applied, different sounds can be produced. "These sounds can easily be heard by the human ear and also have high fidelity, making them excellent for listening to music, for example," says Zettl.

The researchers have already tested their device against high-quality commercial earphones of a similar size (Sennheiser® MX-400) and found that its frequency response over the 20 Hz to 20 kHz range is comparable, if not better.

The Berkeley team says that its CVD technique for fabricating the speaker is very straightforward and could easily be scaled up to produce even larger-area diaphragms and thus bigger speakers. "The configuration we describe could also serve as a microphone," adds Zettl.

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