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Research Alert: March 10, 2015

Graphene meets heat waves

In the race to miniaturize electronic components, researchers are challenged with a major problem: the smaller or the faster your device, the more challenging it is to cool it down. One solution to improve the cooling is to use materials with very high thermal conductivity, such as graphene, to quickly dissipate heat and thereby cool down the circuits.

At the moment, however, potential applications are facing a fundamental problem: how does heat propagate inside these sheets of materials that are no more than a few atoms thick?

In a study published in Nature Communications, a team of EPFL researchers has shed new light on the mechanisms of thermal conductivity in graphene and other two-dimensional materials. They have demonstrated that heat propagates in the form of a wave, just like sound in air. This was up to now a very obscure phenomenon observed in few cases at temperatures close to the absolute zero.Their simulations provide a valuable tool for researchers studying graphene, whether to cool down circuits at the nanoscale, or to replace silicon in tomorrow’s electronics.

Quasi-Lossless Propagation

If it has been difficult so far to understand the propagation of heat in two-dimensional materials, it is because these sheets behave in unexpected ways compared to their three-dimensional cousins. In fact, they are capable of transferring heat with extremely limited losses, even at room temperature.

Generally, heat propagates in a material through the vibration of atoms. These vibrations are are called “phonons”, and as heat propagates though a three-dimensional material,, these phonons keep colliding with each other, merging together, or splitting. All these processes can limit the conductivity of heat along the way. Only under extreme conditions, when temperature goes close to the absolute zero (-200 0C or lower), it is possible to observe quasi-lossless heat transfer.

A wave of quantum heat

The situation is very different in two dimensional materials, as shown by researchers at EPFL. Their work demonstrates that heat can propagate without significant losses in 2D even at room temperature, thanks to the phenomenon of wave-like diffusion, called “second sound”. In that case, all phonons march together in unison over very long distances. “Our simulations, based on first-principles physics, have shown that atomically thin sheets of materials behave, even at room temperature, in the same way as three-dimensional materials at extremely low temperatures” says Andrea Cepellotti, the first author of the study. “We can show that the thermal transport is described by waves, not only in graphene but also in other materials that have not been studied yet,” explains Cepellotti. “This is an extremely valuable information for engineers, who could exploit the design of future electronic components using some of these novel two-dimensional materials properties.”

UT Dallas technology could make night vision, thermal imaging affordable

Engineers at The University of Texas at Dallas have created semiconductor technology that could make night vision and thermal imaging affordable for everyday use.

Researchers in the Texas Analog Center of Excellence (TxACE) in the University’s Erik Jonsson School of Engineering and Computer Science created an electronic device in affordable technology that detects electromagnetic waves to create images at nearly 10 terahertz, which is the highest frequency for electronic devices. The device could make night vision and heat-based imaging affordable.

Presently, night vision and thermal imagers are costly, in part because they are made with specialty semiconductor devices or need isolation from the environment.

The UT Dallas device is created using Schottky diodes in Complementary Metal-Oxide Semiconductor (CMOS) technology. CMOS is used to make affordable consumer electronic devices such as personal computers, game consoles and high-definition TVs. In addition to being affordable, these devices could be more easily incorporated into smartphones.

“There are no existing electronic detection systems operating in CMOS that can reach above 5 terahertz,” said Zeshan Ahmad, lead author of the work, electrical engineering doctoral candidate and a research assistant in TxACE. “We designed our chip in such a way that it can be mass produced inexpensively, has a smaller pixel and operates at higher frequencies.”

Dr. Kenneth O, professor of electrical engineering in the Jonsson School and director of TxACE, noted the time it took for the field to reach this frequency in CMOS.

“This is a truly remarkable accomplishment,” said Dr. O, holder of the Texas Instruments Distinguished Chair.

“Twenty years ago, we were struggling to build CMOS circuits operating at 1 gigahertz. Now we are building circuits working at frequencies that are 10,000 times higher.”

The device could eventually be used for imaging animals near a road while driving at night; imaging intruders in darkness; providing light for night hiking; and estimating how many people are in a room to better control heating, air conditioning and light. It also could be used for other tasks such as finding pipes covered by concrete or walls.

“This technology could provide a very superior means to use the infrared portion of the spectrum,” said Dr. Robert Doering, research strategy manager at Texas Instruments.” Electronic control of generating infrared directly from CMOS integrated circuits will enable a wide variety of important new applications.”

The next step in the research is to realize CMOS devices that can reach even higher frequencies, up to 40 terahertz.

Breakthrough in OLED technology

Organic light emitting diodes (OLEDs), which are made from carbon-containing materials, have the potential to revolutionize future display technologies, making low-power displays so thin they’ll wrap or fold around other structures, for instance.

Conventional LCD displays must be backlit by either fluorescent light bulbs or conventional LEDs whereas OLEDs don’t require back lighting. An even greater technological breakthrough will be OLED-based laser diodes, and researchers have long dreamed of building organic lasers, but they have been hindered by the organic materials’ tendency to operate inefficiently at the high currents required for lasing.

Now a new study from a team of researchers in California and Japan shows that OLEDs made with finely patterned structures can produce bright, low-power light sources, a key step toward making organic lasers. The results are reported in a paper appearing this week on the cover of the journal Applied Physics Letters, from AIP Publishing.

The key finding, the researchers say, is to confine charge transport and recombination to nanoscale areas, which extends electroluminescent efficiency roll off the current density at which the efficiency of the OLEDs dramatically decreases — by almost two orders of magnitude. The new device structures do this by suppressing heating and preventing charge recombination.

“An important effect of suppressing roll-off is an increase in the efficiency of devices at high brightness,” said Chihaya Adachi of Kyushu University, who is a co-author of the paper. “This results in lower power to obtain the same brightness.”

“For years scientists working in organic semiconductors have dreamed of making electrically-driven organic lasers,” said Thuc-Quyen Nguyen of the University of California, Santa Barbara, another co-author. “Lasers operate in extreme conditions with electric currents that are significantly higher than those used in common displays and lighting. At these high currents, energy loss processes become stronger and make lasing difficult.

“We see this work, which reduces some loss processes, as one step on the road toward realizing organic lasers,” Nguyen added.

OLEDs operate through the interaction of electrons and holes. “As a simple visualization,” Adachi said, “one can think of an organic semiconductor as a subway train with someone sitting in every seat. The seats represent molecules and the people represent energetic particles, i.e., electrons. When people board the train from one end, they have extra energy and want to go to the relaxed state of sitting. As people board, some of the seated people rise and exit the train at the other end leaving empty seats, or ‘holes,’ for the standing people to fill. When a standing person sits, the person goes to a relaxed state and releases energy. In the case of OLEDs, the person releases the energy as light.”

Production of OLED-based lasers requires current densities of thousands of amperes per square centimeter (kA/cm2), but until now, current densities have been limited by heating. “At high current densities, brightness is limited by annihilation processes,” Adachi said. “Think of large numbers of people on the train colliding into each other and losing energy in ways other than by sitting and releasing light.”

In previous work, Adachi and colleagues showed OLED performance at current densities over 1 kA/cm2 but without the necessary efficiency required for lasers and bright lighting. In their current paper, they show that the efficiency problem can be solved by using electron-beam lithography to produce finely-patterned OLED structures. The small device area supports charge density injection of 2.8 kA/cm2 while maintaining 100 times higher luminescent efficiency than previously observed. “In our device structure, we have effectively confined the entrance and exit to the middle of the train. People diffuse to the two less crowded ends of the train and reduce collisions and annihilation.”

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