Posted on

‘Designless’ nanoscale logic circuits resemble Darwinian evolution and neural networks

Illustration of a nanoparticle network (about 200 nanometers in diameter). By applying electrical signals at the electrodes (yellow), and using artificial evolution, this disordered network can be configured into useful logic circuits. (credit: University of Twente)

Researchers at the University of Twente in The Netherlands have designed and demonstrated working electronic logic circuits produced using methods that resemble Darwinian evolution and neural networks like the human brain.

In a radical “designless” approach, the researchers used a 200-nanometer-wide cluster of 20-nanometer gold nanoparticles. They applied a series of voltages to eight electrodes and determined the resulting set of 16 different two-input Boolean logic gates.

Artificial evolution

Instead of designing logic circuits with specified functions, as with conventional transistors, this approach works around — or can even take advantage of — any material defects.

To do this, the researchers used an artificial evolution model — one that runs in less than an hour, rather than millions of years. “Natural evolution has led to powerful ‘computers’ like the human brain, which can solve complex problems in an energy-efficient way,” the researchers note. “Nature exploits complex networks that can execute many tasks in parallel.”

“This is the first time that scientists have succeeded in realizing robust electronics with small dimensions that can compete with commercial technology.”

Schematic device layout of the disordered network of gold nanoparticles, separated by ~1 nm 1-octanethiols, in between eight titanium-gold electrodes, as shown in the scanning electron micrograph (top inset). The gold nanoparticles act as single-electron transistors (SETs) at low temperature (<15 K). (credit: S. K. Bose, et al./Nature Nanotechnology)

Conventional transistors are limited to a handful of atoms. It would a major challenge to produce chips in which the millions of transistors required have the same characteristics, according to the researchers from the Twente MESA+ Institute for Nanotechnology and the CTIT Institute for ICT Research. Current transistor designs are also limited by their energy consumption, which is reaching unacceptable levels.

According to University of Twente prof. Wilfred van der Wiel, the logic circuits they discovered currently have limited computing power. “But with this research we have delivered a proof of principle. By scaling up the system, real added value will be produced in the future. This type of circuitry uses much less energy, both in production and use.” The researchers anticipate a wide range of applications — for example, in portable electronics and in the medical world.

“By choosing a smaller nanoparticle diameter, and scaling down the electrode geometry accordingly, our network would not only further reduce area, but room-temperature operation would come into sight as well,” the researchers note in a paper in the journal Nature Nanotechnology.

Mimicking brain-like systems

The researchers also contrast their “designless” reconfigurable approach with massively parallel (but still design-constrained) architectures such as IBM’s TrueNorth brain-inspired chip.

“An especially interesting avenue to explore is the suitability of this system for advanced functionality that is hard (or expensive) to realize in a conventional architecture, such as pattern recognition by mimicking brain-like systems, or simulations of complex physical systems,” the researchers note in the paper. “Our evolutionary approach works around device-to-device variations at the nanoscale and the accompanying uncertainties in performance, which is becoming more and more a bottleneck for the miniaturization of conventional electronic circuits.”

Abstract of Evolution of a designless nanoparticle network into reconfigurable Boolean logic

Natural computers exploit the emergent properties and massive parallelism of interconnected networks of locally active components. Evolution has resulted in systems that compute quickly and that use energy efficiently, utilizing whatever physical properties are exploitable. Man-made computers, on the other hand, are based on circuits of functional units that follow given design rules. Hence, potentially exploitable physical processes, such as capacitive crosstalk, to solve a problem are left out. Until now, designless nanoscale networks of inanimate matter that exhibit robust computational functionality had not been realized. Here we artificially evolve the electrical properties of a disordered nanomaterials system (by optimizing the values of control voltages using a genetic algorithm) to perform computational tasks reconfigurably. We exploit the rich behaviour that emerges from interconnected metal nanoparticles, which act as strongly nonlinear single-electron transistors, and find that this nanoscale architecture can be configured in situ into any Boolean logic gate. This universal, reconfigurable gate would require about ten transistors in a conventional circuit. Our system meets the criteria for the physical realization of (cellular) neural networks: universality (arbitrary Boolean functions), compactness, robustness and evolvability, which implies scalability to perform more advanced tasks. Our evolutionary approach works around device-to-device variations and the accompanying uncertainties in performance. Moreover, it bears a great potential for more energy-efficient computation, and for solving problems that are very hard to tackle in conventional architectures.

Posted on

First optical ‘rectenna’ converts light to DC current

This schematic shows the components of the optical rectenna developed at the Georgia Institute of Technology (credit: Thomas Bougher, Georgia Tech)

Using nanometer-scale components, Georgia Institute of Technology researchers have demonstrated the first optical rectenna, a device that combines the functions of an antenna and a rectifier diode to convert light directly into DC current.

Based on multiwall carbon nanotubes and tiny rectifiers fabricated onto them, the optical rectennas could provide a new technology for energy harvesters, including photodetectors that would operate without the need for cooling, convert waste heat to electricity,  and ultimately, efficiently capture solar energy.

In the new devices, the carbon nanotubes act as antennas to capture light from the Sun or other sources. As the waves of light hit the nanotube antennas, they create an oscillating charge that moves through rectifier devices attached to them. The rectifiers switch on and off at record high petahertz speeds, creating a small direct current.

The efficiency of the devices demonstrated so far remains below one percent, but the researchers hope to boost that output by using billions of rectennas in an array, which could produce significant current. They believe a rectenna with commercial potential may be available within a year.

“We could ultimately make solar cells that are twice as efficient at a cost that is ten times lower, and that is to me an opportunity to change the world in a very big way” said Baratunde Cola, an associate professor in the George W. Woodruff School of Mechanical Engineering at Georgia Tech. “As a robust, high-temperature detector, these rectennas could be a completely disruptive technology if we can get to one percent efficiency. If we can get to higher efficiencies, we could apply it to energy conversion technologies and solar energy capture.”

A carbon nanotube optical rectenna converts green laser light to electricity in the laboratory of Baratunde Cola at the Georgia Institute of Technology (credit: Rob Felt, Georgia Tech)

“A rectenna is basically an antenna coupled to a diode, but when you move into the optical spectrum, that usually means a nanoscale antenna coupled to a metal-insulator-metal diode,” Cola explained. “The closer you can get the antenna to the diode, the more efficient it is. So the ideal structure uses the antenna as one of the metals in the diode – which is the structure we made.”

The rectennas fabricated by Cola’s group are grown on rigid substrates, but the goal is to grow them on a foil or other material that would produce flexible solar cells or photodetectors.

Cola sees the rectennas built so far as simple proof of principle. “We think we can reduce the resistance by several orders of magnitude just by improving the fabrication of our device structures,” he said. “Based on what others have done and what the theory is showing us, I believe that these devices could get to greater than 40 percent efficiency.”

The research, supported by the Defense Advanced Research Projects Agency (DARPA), the Space and Naval Warfare (SPAWAR) Systems Center and the Army Research Office (ARO), is reported September 28 in the journal Nature Nanotechnology.

* Developed in the 1960s and 1970s, rectennas have operated at wavelengths as short as ten micrometers, but for more than 40 years researchers have been attempting to make devices at optical wavelengths. There were many challenges: making the antennas small enough to couple optical wavelengths, and fabricating a matching rectifier diode small enough and able to operate fast enough to capture the electromagnetic wave oscillations. But the potential of high efficiency and low cost kept scientists working on the technology.

Fabricating the rectennas begins with growing forests of vertically-aligned carbon nanotubes on a conductive substrate. Using atomic layer chemical vapor deposition, the nanotubes are coated with an aluminum oxide material to insulate them. Finally, physical vapor deposition is used to deposit optically-transparent thin layers of calcium then aluminum metals atop the nanotube forest. The difference of work functions between the nanotubes and the calcium provides a potential of about two electron volts, enough to drive electrons out of the carbon nanotube antennas when they are excited by light.

In operation, oscillating waves of light pass through the transparent calcium-aluminum electrode and interact with the nanotubes. The metal-insulator-metal junctions at the nanotube tips serve as rectifiers switching on and off at femtosecond intervals, allowing electrons generated by the antenna to flow one way into the top electrode. Ultra-low capacitance, on the order of a few attofarads, enables the 10-nanometer diameter diode to operate at these exceptional frequencies.


Georgia Tech | Using nanometer-scale components, researchers have demonstrated the first optical rectenna, a device that combines the functions of an antenna and a rectifier diode to convert light directly into DC current.

Abstract of A carbon nanotube optical rectenna

An optical rectenna—a device that directly converts free-propagating electromagnetic waves at optical frequencies to direct current—was first proposed over 40 years ago, yet this concept has not been demonstrated experimentally due to fabrication challenges at the nanoscale. Realizing an optical rectenna requires that an antenna be coupled to a diode that operates on the order of 1 pHz (switching speed on the order of 1 fs). Diodes operating at these frequencies are feasible if their capacitance is on the order of a few attofarads, but they remain extremely difficult to fabricate and to reliably couple to a nanoscale antenna. Here we demonstrate an optical rectenna by engineering metal–insulator–metal tunnel diodes, with a junction capacitance of ∼2 aF, at the tip of vertically aligned multiwalled carbon nanotubes (∼10 nm in diameter), which act as the antenna. Upon irradiation with visible and infrared light, we measure a d.c. open-circuit voltage and a short-circuit current that appear to be due to a rectification process (we account for a very small but quantifiable contribution from thermal effects). In contrast to recent reports of photodetection based on hot electron decay in a plasmonic nanoscale antenna, a coherent optical antenna field appears to be rectified directly in our devices, consistent with rectenna theory. Finally, power rectification is observed under simulated solar illumination, and there is no detectable change in diode performance after numerous current–voltage scans between 5 and 77 °C, indicating a potential for robust operation.

Posted on

How to make 3-D objects totally disappear

This image shows a 3-D illustration of a metasurface skin cloak made from an ultrathin layer of nanoantennas (gold blocks) covering an arbitrarily shaped object. When activated, light reflects off the cloak (red arrows) as if it were reflecting off a flat mirror. (credit: Image courtesy of Xiang Zhang group, Berkeley Lab/UC Berkeley)

An ultra-thin invisibility “skin” cloak that can conform to the shape of an object and conceal it from detection with visible light has been developed by scientists at Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley.

Working with blocks of gold nanoantennas, the Berkeley researchers created a “skin cloak” just 80 nanometers in thickness that was wrapped around a three-dimensional object about the size of a few biological cells and shaped with multiple bumps and dents. The surface of the skin cloak was meta-engineered to reflect light waves, making the object invisible to optical detection when the cloak is activated.

“This is the first time a 3D object of arbitrary shape has been cloaked from visible light,” said Xiang Zhang, director of Berkeley Lab’s Materials Sciences Division. “Our ultra-thin cloak now looks like a coat. It is easy to design and implement, and is potentially scalable for hiding macroscopic objects.”

Zhang, who holds the Ernest S. Kuh Endowed Chair at UC Berkeley and is a member of the Kavli Energy NanoSciences Institute at Berkeley (Kavli ENSI), is the corresponding author of a paper describing this research in the journal Science.

Click to see an animation of an invisibility cloak that makes a 3-D object disappear (credit: Zhang group)

It is the scattering of light from its interaction with matter that enables us to detect and observe objects. The rules that govern these interactions in natural materials can be circumvented in metamaterials, whose optical properties arise from their physical structure rather than their chemical composition.

For the past ten years, Zhang and his research group have been pushing the boundaries of how light interacts with metamaterials, managing to curve the path of light or bend it backwards, phenomena not seen in natural materials, and to render objects optically undetectable. In the past, their metamaterial-based optical carpet cloaks were bulky and hard to scale up, and entailed a phase difference between the cloaked region and the surrounding background that made the cloak itself detectable, though what it concealed was not.

Now you see it, now you don’t

In the new study, the researchers used red light to illuminate an arbitrarily shaped 3-D sample object about 1,300 square micrometers in area and wrapped in the gold nanoantenna skin cloak. The light reflected off the surface of the skin cloak was identical to light reflected off a flat mirror, making the object underneath it invisible even by phase-sensitive detection. The cloak can be turned “on” or “off” simply by switching the polarization of the nanoantennas.

“A phase shift provided by each individual nanoantenna fully restores both the wavefront and the phase of the scattered light so that the object remains perfectly hidden,” says co-lead author Zi Jing Wong, also a member of Zhang’s research group.

According the researchers. the ability to manipulate the interactions between light and metamaterials offers future prospects for technologies such as high-resolution optical microscopes and superfast optical computers, and for hiding the detailed layout of microelectronic components or for security encryption purposes.

At the macroscale, among other applications, invisibility cloaks could prove useful for 3D displays, they add.

Abstract of An ultrathin invisibility skin cloak for visible light

Metamaterial-based optical cloaks have thus far used volumetric distribution of the material properties to gradually bend light and thereby obscure the cloaked region. Hence, they are bulky and hard to scale up and, more critically, typical carpet cloaks introduce unnecessary phase shifts in the reflected light, making the cloaks detectable. Here, we demonstrate experimentally an ultrathin invisibility skin cloak wrapped over an object. This skin cloak conceals a three-dimensional arbitrarily shaped object by complete restoration of the phase of the reflected light at 730-nanometer wavelength. The skin cloak comprises a metasurface with distributed phase shifts rerouting light and rendering the object invisible. In contrast to bulky cloaks with volumetric index variation, our device is only 80 nanometer (about one-ninth of the wavelength) thick and potentially scalable for hiding macroscopic objects.

Posted on

First brain-to-brain ‘telepathy’ communication via the Internet

University of Washington graduate student Jose Ceballos wears an electroencephalography (EEG) cap that records brain activity and sends a response to a second participant over the Internet (credit: University of Washington)

The first brain-to-brain telepathy-like communication between two participants via the Internet has been performed by University of Washington researchers.*

The experiment used a question-and-answer game. The goal is for the “inquirer” to determine which object the “respondent” is looking at from a list of possible objects. The inquirer sends a question (e.g., “Does it fly?) to the respondent, who answers “yes” or “no” by mentally focusing on one of two flashing LED lights attached to the monitor. The respondent is wearing an electroencephalography (EEG) helmet.

By focusing on the “yes” light, the EEG device generates send a signal to the inquirer via the Internet to activate a magnetic coil positioned behind the inquirer’s head, which stimulates the visual cortex and causes the inquirer to see a flash of light (known as a “phosphene”). A “no” signal works the same way, but is not strong enough to activate the coil.

Remote brain-to-brain communication process (credit: A. Stocco et al./PLoS ONE)

The experiment, detailed today in an open access paper in PLoS ONE, is the first to show that two brains can be directly linked to allow one person to guess what’s on another person’s mind. It is “the most complex brain-to-brain experiment, I think, that’s been done to date in humans,” said lead author Andrea Stocco, an assistant professor of psychology and researcher at UW’s Institute for Learning & Brain Sciences.

The experiment was carried out in dark rooms in two UW labs located almost a mile apart and involved five pairs of participants, who played 20 rounds of the question-and-answer game. Each game had eight objects and three questions. The sessions were a random mixture of 10 real games and 10 control games that were structured the same way.*

Participants were able to guess the correct object in 72 percent of the real games, compared with just 18 percent of the control rounds. Incorrect guesses in the real games could be caused by several factors, the most likely being uncertainty about whether a phosphene had appeared.

uw_brain2brain_interface_1

UW team’s initial experiment in 2013: University of Washington researcher Rajesh Rao, left, plays a computer game with his mind. Across campus, researcher Andrea Stocco, right, wears a magnetic stimulation coil over the left motor cortex region of his brain. Stocco’s right index finger moved involuntarily to hit the “fire” button as part of the first human brain-to-brain interface demonstration. (credit: University of Washington)

The study builds on the UW team’s initial experiment in 2013, which was the first to demonstrate a direct brain-to-brain connection between humans, using noninvasive technology to send a person’s brain signals over the Internet to control the hand motions of another person. Other scientists had previously connected the brains of rats and monkeys, and transmitted brain signals from a human to a rat, using electrodes inserted into animals’ brains.

The new experiment evolved out of research by co-author Rajesh Rao, a UW professor of computer science and engineering, on brain-computer interfaces that enable people to activate devices with their minds. In 2011, Rao began collaborating with Stocco and Prat to determine how to link two human brains together.


University of Washington | Team links two human brains for question-and-answer experiment

“Brain tutoring” next

In 2014, the researchers received a $1 million grant from the W.M. Keck Foundation that allowed them to broaden their experiments to decode more complex interactions and brain processes. They are now exploring the possibility of “brain tutoring,” transferring signals directly from healthy brains to ones that are developmentally impaired or impacted by external factors such as a stroke or accident, or simply to transfer knowledge from teacher to pupil.

The team is also working on transmitting brain states — for example, sending signals from an alert person to a sleepy one, or from a focused student to one who has attention deficit hyperactivity disorder, or ADHD.

“Imagine having someone with ADHD and a neurotypical student,” Prat said. “When the non-ADHD student is paying attention, the ADHD student’s brain gets put into a state of greater attention automatically.”

“Evolution has spent a colossal amount of time to find ways for us and other animals to take information out of our brains and communicate it to other animals in the forms of behavior, speech and so on,” Stocco said. “But it requires a translation. We can only communicate part of whatever our brain processes.

“What we are doing is kind of reversing the process a step at a time by opening up this box and taking signals from the brain and with minimal translation, putting them back in another person’s brain,” he said.

* “Telepathy-like” is KurzweilAI’s wording, meaning that no action by the subject outside of the brain were required in the communication. As noted above, the first experiment (known to KurzweilAI) to demonstrate a direct brain-to-brain connection between humans via the Internet, the UW team’s initial experiment in 2013, used involuntary finger movements on a keyboard. Proponents of “telepathy” or “psychic” experiments using the Internet as a link, if any, might counter this.

The researchers took steps to ensure participants couldn’t use clues other than direct brain communication to complete the game. Inquirers wore earplugs so they couldn’t hear the different sounds produced by the varying stimulation intensities of the “yes” and “no” responses. Since noise travels through the skull bone, the researchers also changed the stimulation intensities slightly from game to game and randomly used three different intensities each for “yes” and “no” answers to further reduce the chance that sound could provide clues.

The researchers also repositioned the coil on the inquirer’s head at the start of each game, but for the control games, added a plastic spacer undetectable to the participant that weakened the magnetic field enough to prevent the generation of phosphenes. Inquirers were not told whether they had correctly identified the items, and only the researcher on the respondent end knew whether each game was real or a control round.

UPDATE Sept. 9, 2015: Footnote expanded to clarify “telepathy-like.”

Posted on

A new distance record for quantum teleportation via photons

This graphic describes how researchers at the National Institute of Standards and Technology (NIST) have “teleported” or transferred quantum information carried in light particles over 100 kilometers (km) of optical fiber, four times farther than the previous record. (credit: K. Irvine/NIST)

Researchers at the National Institute of Standards and Technology (NIST) have “teleported” (transferred) quantum information carried in photons over 100 kilometers (km) of optical fiber — four times farther than the previous record.

The experiment confirmed that quantum communication is feasible over long distances in fiber, according to the researchers. Other research groups have teleported quantum information over longer distances in free space (wirelessly), but fiber-optic cables offer more options for network design, the NIST researchers note.

Teleportation is useful in both quantum communications and quantum computing, which allow advancements in unbreakable encryption and code-breaking, respectively.

The new record, described in an open-access paper in Optica, involved the transfer of quantum information from one photon (its specific time slot in a sequence) to another photon* over 102 km of spooled fiber in a NIST laboratory in Colorado.

The achievement was made possible by NIST’s advanced single-photon detectors.

“Only about 1 percent of photons make it all the way through 100 km of fiber,” NIST’s Marty Stevens says. “We never could have done this experiment without these new detectors, which can measure this incredibly weak signal.”

Quantum internet

The new NTT/NIST teleportation technique could be used to make devices called quantum repeaters that could resend data periodically, extending network reach, perhaps enough to eventually build a “quantum internet.”

Previously, researchers thought quantum repeaters might need to rely on atoms or other matter, instead of light, a difficult engineering challenge that would also slow down transmission.*

* Various quantum states can be used to carry information; the NTT/NIST experiment used quantum states that indicate when in a sequence of time slots a single photon arrives. That teleportation method is novel in that four of NIST’s photon detectors were positioned to filter out specific quantum states. (See graphic for an overview of how the teleportation process works.) The detectors rely on superconducting nanowires made of molybdenum silicide. They can record more than 80 percent of arriving photons, revealing whether they are in the same or different time slots each just 1 nanosecond long. The experiments were performed at wavelengths commonly used in telecommunications.

Because the experiment filtered out and focused on a limited combination of quantum states, teleportation could be successful in only 25 percent of the transmissions at best. Thanks to the efficient detectors, researchers successfully teleported the desired quantum state in 83 percent of the maximum possible successful transmissions, on average. All experimental runs with different starting properties exceeded the mathematically significant 66.7 percent threshold for proving the quantum nature of the teleportation process.

Abstract of Quantum teleportation over 100  km of fiber using highly efficient superconducting nanowire single-photon detectors

Quantum teleportation is an essential quantum operation by which we can transfer an unknown quantum state to a remote location with the help of quantum entanglement and classical communication. Since the first experimental demonstrations using photonic qubits and continuous variables, the distance of photonic quantum teleportation over free-space channels has continued to increase and has reached >100  km. On the other hand, quantum teleportation over optical fiber has been challenging, mainly because the multifold photon detection that inevitably accompanies quantum teleportation experiments has been very inefficient due to the relatively low detection efficiencies of typical telecom-band single-photon detectors. Here, we report on quantum teleportation over optical fiber using four high-detection-efficiency superconducting nanowire single-photon detectors (SNSPDs). These SNSPDs make it possible to perform highly efficient multifold photon measurements, allowing us to confirm that the quantum states of input photons were successfully teleported over 100 km of fiber with an average fidelity of 83.7±2.0%.

Posted on

First all-optical chip memory

Illustration of all-optical data memory: ultra-short light pulses (left) make a bit in the Ge2Sb2Te5 (GST) material change from crystalline to amorphous (or the reverse), and weak light pulses (right) read out the data (credit: C. Rios/Oxford University)

The first all-optical chip memory has been developed by an international team of scientists. It is capable of writing data to memory at a speed of up to a gigahertz or more and may allow computers to work more rapidly and more efficiently.

The memory is non-volatile (similar to flash memory), and the new memory can store data even when the power is removed, and may persist for decades, the researchers believe.

The scientists, from Oxford, Exeter, Karlsruhe and Münster universities, used a “phase-change material,” Ge2Sb2Te5 (GST). Phase-change materials radically change their optical properties depending on their phase state (arrangement of the atoms) — crystalline (regular) or amorphous (irregular) — initiated by ultrashort light pulses. For reading out the data, weak light pulses are used.

Light is ideally suited for ultra-fast high-bandwidth data transfer (via optical-fiber cables), but until now, it has not been possible to store large quantities of optical data directly on integrated chips. The memory is also compatible with latest processors, the researchers note.

Permanent all-optical on-chip memories promise to considerably increase the speed of computers and reduce their energy consumption. Together with all-optical connections, on-chip memories might also reduce latencies (transmission delays, which can make long-distance two-way communication difficult, for example). In addition, energy-intensive conversion of optical signals into electronic signals and vice versa would no longer be required, reducing bulk and cost.

The research is published in Nature Photonics.

Abstract of Integrated all-photonic non-volatile multi-level memory

Implementing on-chip non-volatile photonic memories has been a long-term, yet elusive goal. Photonic data storage would dramatically improve performance in existing computing architectures by reducing the latencies associated with electrical memories and potentially eliminating optoelectronic conversions. Furthermore, multi-level photonic memories with random access would allow for leveraging even greater computational capability. However, photonic memories have thus far been volatile. Here, we demonstrate a robust, non-volatile, all-photonic memory based on phase-change materials. By using optical near-field effects, we realize bit storage of up to eight levels in a single device that readily switches between intermediate states. Our on-chip memory cells feature single-shot readout and switching energies as low as 13.4 pJ at speeds approaching 1 GHz. We show that individual memory elements can be addressed using a wavelength multiplexing scheme. Our multi-level, multi-bit devices provide a pathway towards eliminating the von Neumann bottleneck and portend a new paradigm in all-photonic memory and non-conventional computing.

Posted on

3-D printing lightweight, flexible multiple materials in real time, including electronic circuits

Multiple-materials printer. Each fluid enters the mixing chamber through a separate inlet and is mixed in a narrow gap by an impeller rotating at a constant rate. Optical image (left) and schematic illustration (right) of impeller-based mixing nozzle. (credit: Thomas Ober, Harvard SEAS/Wyss Institute)

Harvard researchers have designed new printheads for 3-D printers that can simultaneously handle multiple materials with different properties, allowing for 3-D printing wearable devices, flexible electronics, and soft robots.

To print a flexible device, including the electronics, a 3-D printer must be able to seamlessly transition from a flexible material that moves with the wearer’s joints for wearable applications, to a rigid material that accommodates the electronic components. It would also need to be able to embed electrical circuitry using multiple inks of varying conductivity and resistivity, and precisely switching between them while changing composition and geometry.  And do it all in real time.

How this will change 3-D printing

The researchers say they have designed a new multimaterial printhead that do all of the above. It can handle a wide range of complex fluids by using a rotating impeller inside a microscale nozzle, seamlessly printing combinations of materials and processes that were not formerly possible:

  • Mixed conductive and resistive inks to embed electrical circuitry inside 3D printed objects.
  • Multiple inks within a single nozzle, eliminating the structural defects that often occur during the start-and-stop process of switching materials.
  • Silicone elastomers, with gradient architectures composed of soft and rigid regions.
  • Reactive materials, such as two-part epoxies, which typically harden quickly when the two parts are combined (think: Krazy glue).

The research was led by Jennifer A. Lewis, the Hansjörg Wyss Professor of Biologically Inspired Engineering at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and a Core Faculty Member at the Wyss Institute for Biologically Inspired Engineering at Harvard. The work was published in an open-access paper in Proceedings of the National Academy of Sciences (PNAS). It was supported by the Department of Energy Energy Frontier Research Center on Light-Material Interactions in Energy Conversion, the Intelligence Community Postdoctoral Fellowship program, and the Society in Science Branco-Weiss Foundation.

“The recent work by the Lewis Group is a significant advancement to the field of additive manufacturing,” said Christopher Spadaccini, Director of the Center for Engineered Materials, Manufacturing and Optimization at Lawrence Livermore National Lab. “By allowing for the mixing of two highly viscous materials on the fly, the promise of mixed material systems with disparate mechanical and functional properties becomes much more realistic.  Before, this was really only a concept.  This work will be foundational for applications which [require] integrated electrical and structural materials.”

Abstract of Active mixing of complex fluids at the microscale

Mixing of complex fluids at low Reynolds number is fundamental for a broad range of applications, including materials assembly, microfluidics, and biomedical devices. Of these materials, yield stress fluids (and gels) pose the most significant challenges, especially when they must be mixed in low volumes over short timescales. New scaling relationships between mixer dimensions and operating conditions are derived and experimentally verified to create a framework for designing active microfluidic mixers that can efficiently homogenize a wide range of complex fluids. Active mixing printheads are then designed and implemented for multimaterial 3D printing of viscoelastic inks with programmable control of local composition.

Posted on

How to catch a molecule

With a nano-ring-based toroidal trap, cold polar molecules near the gray shaded surface approaching the central region may be trapped within a nanometer-scale volume (credit: ORNL)

In a paper published in Physical Review A, Oak Ridge National Laboratory and University of Tennessee physicists describe conceptually how they may be able to trap and exploit a molecule’s energy to advance a number of fields.

“A single molecule has many degrees of freedom, or ways of expressing its energy and dynamics, including vibrations, rotations and translations,” said Ali Passian of Oak Ridge National Lab. “For years, physicists have searched for ways to take advantage of these molecular states, including how they could be used in high-precision instruments or as an information storage device for applications such as quantum computing.”

It’s a trap!

Catching a molecule with minimal disturbance is not an easy task, considering its size — about 1 nanometer — but this paper proposes a method that may overcome that obstacle.

When interacting with laser light, the ring toroidal nanostructure can trap the slower molecules at its center. That’s because the nano-trap, which can be made of gold using conventional nanofabrication techniques, creates a highly localized force field surrounding the molecules. The team envisions using scanning probe microscopy techniques, which can measure extremely small forces, to access individual nano-traps.

“Once trapped, we can interrogate the molecules for their spectroscopic and electromagnetic properties and study them in isolation without disturbance from the neighboring molecules,” Passian said.

Previous demonstrations of trapping molecules have relied on large systems to confine charged particles such as single ions. Next, the researchers plan to build actual nanotraps and conduct experiments to determine the feasibility of fabricating a large number of traps on a single chip.

“If successful, these experiments could help enable information storage and processing devices that greatly exceed what we have today, thus bringing us closer to the realization of quantum computers,” Passian said.

Abstract of Toroidal nanotraps for cold polar molecules

Electronic excitations in metallic nanoparticles in the optical regime that have been of great importance in surface-enhanced spectroscopy and emerging applications of molecular plasmonics, due to control and confinement of electromagnetic energy, may also be of potential to control the motion of nanoparticles and molecules. Here, we propose a concept for trapping polarizable particles and molecules using toroidal metallic nanoparticles. Specifically, gold nanorings are investigated for their scattering properties and field distribution to computationally show that the response of these optically resonant particles to incident photons permit the formation of a nanoscale trap when proper aspect ratio, photon wavelength, and polarization are considered. However, interestingly the resonant plasmonic response of the nanoring is shown to be detrimental to the trap formation. The results are in good agreement with analytic calculations in the quasistatic limit within the first-order perturbation of the scalar electric potential. The possibility of extending the single nanoring trapping properties to two-dimensional arrays of nanorings is suggested by obtaining the field distribution of nanoring dimers and trimers.

Posted on

A thermal invisibility cloak that actively redirects heat

Active thermal cloak hides a circular object in conductive heat flow by “pumping” heat from hot end to cold end (credit: Xu & Zhang/NTU)

A new thermal cloak that can render an object thermally invisible by actively redirecting incident heat has been developed by scientists at the Nanyang Technological University (NTU) in Singapore. It’s similar to how optical invisibility cloaks can bend and diffract light to shield an object from sight and specially fabricated acoustic metamaterials can hide an object from sound waves.

The system has the potential to fine-tune temperature distribution and heat flow in electronic and semiconductor systems for applications that require efficient heat dissipation (cooling) and homogenous (even) thermal expansion, such as high-power engines, magnetic resonance imaging (MRI) instruments, thermal sensors, and clothing, said Prof. Baile Zhang of NTU.

Zhang and colleagues previously designed a metamaterial thermal cloak that passively guided conductive heat around a hidden object, with no way to control heat flow and direction. The researchers decided to look into controlling thermal cloaking electrically by actively “pumping” heat from one side of the hidden object to the other side, using thermoelectric modules, as described in an open-access paper and on the cover of Applied Physics Letters, from AIP Publishing.

Building the thermal cloak

Design of active thermal cloak. (a) Multiple thermoelectic components are arranged around the air hole with equal distance on the Carbon Steel plate. Blue components absorb incident thermal flux while red ones release heat back to the plate. (b) The side view of the marked region in (a), illustrating the working mechanism of TE components when functioning as heat absorber/emitter. An applied voltage causes a directional motion of charge carriers in positive/negative blocks, resulting in a heat flux in the designed direction. The bottom/top orange arrow indicates the absorption/release of heat by TE components. The dashed arrow indicates the heat transfer through a constant-temperature heat “reservoir” which is a large copper bulk in the experiment. (credit: Dang Minh Nguyen et al./Applied Physics Letters)

To construct their active thermal cloak, the researchers deployed 24 small thermoelectric modules, which are semiconductor heat pumps controlled by an external input voltage, around a 62-millimeter diameter air hole in a carbon steel plate just 5 mm thick. The modules operate via the Peltier effect, in which a current running through the junction between two conductors can remove or generate heat.

When many modules are attached in series, they can redirect heat flow. The researchers attached the bottom and top ends of the modules to hot and cold surfaces at 60° C and 0° C respectively to generate diffusive heat flux.

When the researchers applied a variety of specific voltages to each of the 24 modules, the heat falling on the hot-surface side of the air hole was absorbed and delivered to a constant-temperature copper heat reservoir attached to the modules. The modules on the cold-surface side released the same amount of heat from the reservoir into the steel plate. This prevented heat from diffusing through the air hole, a technique, the researchers say, that can be used to shield sensitive electronic components from heat dissipation.

The researchers found that their active thermal cloaking was not limited by the shape of the object being hidden.

Zhang and his colleagues plan to apply the thermal cloaks in electronic systems, improve the efficiency of heat transfer, and develop an intelligent control system for the cloak.

Abstract of Active thermal cloak

Thermal cloaking, as an ultimate thermal “illusion” phenomenon, is the result of advanced heat manipulation with thermal metamaterials—heat can be guided around a hidden object smoothly without disturbing the ambient thermal environment. However, all previous thermal metamaterial cloaks were passive devices, lacking the functionality of switching on/off and the flexibility of changing geometries. In this letter, we report an active thermal cloaking device that is controllable. Different from previous thermal cloaking approaches, this thermal cloak adopts active thermoelectric components to “pump” heat from one side to the other side of the hidden object, in a process controlled by input electric voltages. Our work not only incorporates active components in thermal cloaking but also provides controllable functionality in thermal metamaterials that can be used to construct more flexible thermal devices.

Posted on

Transparent photonic coating cools solar cells to boost efficiency

Stanford engineers have invented a transparent material that improves the efficiency of solar cells by radiating thermal energy (heat) into space (credit: Stanford Engineering)

Stanford engineers have developed a transparent material that improves the efficiency of solar cells by radiating thermal energy (heat) into space, even in full sunlight.

The invention may solve a longstanding problem for the solar industry: the hotter solar cells become, the less efficient they are at converting sunlight to electricity. The Stanford solution is based on a thin, patterned silica material laid on top of a traditional solar cell. The material is transparent to the visible sunlight that powers solar cells, but captures and emits thermal radiation, or heat, cooling the solar cell and thus allowing it to convert more photons into electricity.

The work by Shanhui Fan, a professor of electrical engineering at Stanford, research associate Aaswath P. Raman and doctoral candidate Linxiao Zhu is described in the current issue of Proceedings of the National Academy of Sciences.

The same inventors previously developed an ultrathin material, covered on KurzweilAI, that radiated infrared light (in the thermal “long” infrared atmospheric transparency window of ~7 to 30 microns) from the Sun directly back toward space without warming the atmosphere (thus avoiding the greenhouse effect). They presented that work in Nature, describing it as “radiative cooling” because it shunted thermal energy directly into the deep, cold void of space. It was specifically intended for cooling buildings.

A silica photonic crystal radiator

In their new paper, the researchers applied that work to improve solar array performance.

Silica photonic crystal radiates far-infrared light (heat) into space (credit: Linxiao Zhu et al./PNAS)

The Stanford team tested their technology on a custom-made solar absorber — a device that mimics the properties of a solar cell without producing electricity — covered with a silica photonic crystal (a micrometer-scale pattern) designed to maximize the capability to dump heat, in the form of infrared light (in 8 to 30 microns range) into space. Their experiments showed that the overlay allowed visible light to pass through to the solar cells, but that the pattern also cooled the underlying absorber by as much as 23 degrees Fahrenheit.

For a typical crystalline silicon solar cell with an efficiency of 20 percent, 23 F of cooling would improve absolute cell efficiency by more than 1 percent, a figure that represents a significant gain in energy production.

The researchers said the new transparent thermal overlays work best in dry, clear environments, which are also preferred sites for large solar arrays. They believe they can scale things up so that commercial and industrial applications are feasible, perhaps using nanoprint lithography, which is a common technique for producing nanometer-scale patterns.

Cooler cars

Zhu said the technology has significant potential for any outdoor device or system that demands cooling but requires the preservation of the visible spectrum of sunlight for either practical or aesthetic reasons.

“Say you have a car that is bright red,” Zhu said. “You really like that color, but you’d also like to take advantage of anything that could aid in cooling your vehicle during hot days. Thermal overlays can help with passive cooling, but it’s a problem if they’re not fully transparent.” That’s because the perception of color requires objects to reflect visible light, so any overlay would need to be transparent, or else tuned such that it would absorb only light outside the visible spectrum.

“Our photonic crystal thermal overlay optimizes use of the thermal portions of the electromagnetic spectrum without affecting visible light,” Zhu said, “so you can radiate heat efficiently without affecting color.”

Abstract of Radiative cooling of solar absorbers using a visibly transparent photonic crystal thermal blackbody

A solar absorber, under the sun, is heated up by sunlight. In many applications, including solar cells and outdoor structures, the absorption of sunlight is intrinsic for either operational or aesthetic considerations, but the resulting heating is undesirable. Because a solar absorber by necessity faces the sky, it also naturally has radiative access to the coldness of the universe. Therefore, in these applications it would be very attractive to directly use the sky as a heat sink while preserving solar absorption properties. Here we experimentally demonstrate a visibly transparent thermal blackbody, based on a silica photonic crystal. When placed on a silicon absorber under sunlight, such a blackbody preserves or even slightly enhances sunlight absorption, but reduces the temperature of the underlying silicon absorber by as much as 13°C due to radiative cooling. Our work shows that the concept of radiative cooling can be used in combination with the utilization of sunlight, enabling new technological capabilities.