Category Archives: electronics

Artificial synapse rivals biological synapse in energy consumption

How can we make computers be like biological brains which do so much work and use so little power? It’s a question scientists from many countries are trying to answer and it seems South Korean scientists are proposing an answer. From a June 20, 2016 news item on Nanowerk,

News) Creation of an artificial intelligence system that fully emulates the functions of a human brain has long been a dream of scientists. A brain has many superior functions as compared with super computers, even though it has light weight, small volume, and consumes extremely low energy. This is required to construct an artificial neural network, in which a huge amount (1014)) of synapses is needed.

Most recently, great efforts have been made to realize synaptic functions in single electronic devices, such as using resistive random access memory (RRAM), phase change memory (PCM), conductive bridges, and synaptic transistors. Artificial synapses based on highly aligned nanostructures are still desired for the construction of a highly-integrated artificial neural network.

Prof. Tae-Woo Lee, research professor Wentao Xu, and Dr. Sung-Yong Min with the Dept. of Materials Science and Engineering at POSTECH [Pohang University of Science & Technology, South Korea] have succeeded in fabricating an organic nanofiber (ONF) electronic device that emulates not only the important working principles and energy consumption of biological synapses but also the morphology. …

A June 20, 2016 Pohang University of Science & Technology (POSTECH) news release on EurekAlert, which originated the news item, describes the work in more detail,

The morphology of ONFs is very similar to that of nerve fibers, which form crisscrossing grids to enable the high memory density of a human brain. Especially, based on the e-Nanowire printing technique, highly-aligned ONFs can be massively produced with precise control over alignment and dimension. This morphology potentially enables the future construction of high-density memory of a neuromorphic system.

Important working principles of a biological synapse have been emulated, such as paired-pulse facilitation (PPF), short-term plasticity (STP), long-term plasticity (LTP), spike-timing dependent plasticity (STDP), and spike-rate dependent plasticity (SRDP). Most amazingly, energy consumption of the device can be reduced to a femtojoule level per synaptic event, which is a value magnitudes lower than previous reports. It rivals that of a biological synapse. In addition, the organic artificial synapse devices not only provide a new research direction in neuromorphic electronics but even open a new era of organic electronics.

This technology will lead to the leap of brain-inspired electronics in both memory density and energy consumption aspects. The artificial synapse developed by Prof. Lee’s research team will provide important potential applications to neuromorphic computing systems and artificial intelligence systems for autonomous cars (or self-driving cars), analysis of big data, cognitive systems, robot control, medical diagnosis, stock trading analysis, remote sensing, and other smart human-interactive systems and machines in the future.

Here’s a link to and a citation for the paper,

Organic core-sheath nanowire artificial synapses with femtojoule energy consumption by Wentao Xu, Sung-Yong Min, Hyunsang Hwang, and Tae-Woo Lee. Science Advances  17 Jun 2016: Vol. 2, no. 6, e1501326 DOI: 10.1126/sciadv.1501326

This paper is open access.

Carrying a solar cell on a pencil or glass slide?

Caption: Ultra-thin solar cells are flexible enough to bend around small objects, such as the 1mm-thick edge of a glass slide, as shown here. Credit: Juho Kim, et al/ APL

Caption: Ultra-thin solar cells are flexible enough to bend around small objects, such as the 1mm-thick edge of a glass slide, as shown here. Credit: Juho Kim, et al/ APL

Yes, this is another wearable electronics story and this time, it’s from South Korea. A June 20, 2016 news item on ScienceDaily announces remarkably thin and flexible photovoltaics,

Scientists in South Korea have made ultra-thin photovoltaics flexible enough to wrap around the average pencil. The bendy solar cells could power wearable electronics like fitness trackers and smart glasses. …

A June 20, 2016 American Institute of Physics news release on EurekAlert, which originated the news item, expands on the theme,

Thin materials flex more easily than thick ones – think a piece of paper versus a cardboard shipping box. The reason for the difference: The stress in a material while it’s being bent increases farther out from the central plane. Because thick sheets have more material farther out they are harder to bend.

“Our photovoltaic is about 1 micrometer thick,” said Jongho Lee, an engineer at the Gwangju Institute of Science and Technology in South Korea. One micrometer is much thinner than an average human hair. Standard photovoltaics are usually hundreds of times thicker, and even most other thin photovoltaics are 2 to 4 times thicker.

The researchers made the ultra-thin solar cells from the semiconductor gallium arsenide. They stamped the cells directly onto a flexible substrate without using an adhesive that would add to the material’s thickness. The cells were then “cold welded” to the electrode on the substrate by applying pressure at 170 degrees Celcius and melting a top layer of material called photoresist that acted as a temporary adhesive. The photoresist was later peeled away, leaving the direct metal to metal bond.

The metal bottom layer also served as a reflector to direct stray photons back to the solar cells. The researchers tested the efficiency of the device at converting sunlight to electricity and found that it was comparable to similar thicker photovoltaics. They performed bending tests and found the cells could wrap around a radius as small as 1.4 millimeters.

The team also performed numerical analysis of the cells, finding that they experience one-fourth the amount of strain of similar cells that are 3.5 micrometers thick.

“The thinner cells are less fragile under bending, but perform similarly or even slightly better,” Lee said.

A few other groups have reported solar cells with thicknesses of around 1 micrometer, but have produced the cells in different ways, for example by removing the whole substract by etching.

By transfer printing instead of etching, the new method developed by Lee and his colleagues may be used to make very flexible photovoltaics with a smaller amount of materials.

The thin cells can be integrated onto glasses frames or fabric and might power the next wave of wearable electronics, Lee said.

Here’s a link to and a citation for the paper,

Ultra-thin flexible GaAs photovoltaics in vertical forms printed on metal surfaces without interlayer adhesives by Juho Kim, Jeongwoo Hwang, Kwangsun Song, Namyun Kim, Jae Cheol Shin, and Jongho Lee. Appl. Phys. Lett. 108, 253101 (2016); http://dx.doi.org/10.1063/1.4954039

This paper is open access.

Turning gold into see-through rubber for an updated Rumpelstiltskin story

Rumpelstiltskin is a fairy tale whereby a young girl is trapped by her father’s lie that she can spin straw into gold. She is forced to spin gold by the King under pain of execution when an imp offers to help in exchange for various goods. As she succeeds each time, the King demands more until finally she has nothing left to trade for the imp’s help. Well, there is one last thing: her first-born child. She agrees to the bargain and she marries the King. On the birth of their first child, the imp reappears and under pressure of her pleas makes one last bargain. She must guess his name which she does, Rumplestiltskin. (The full story along with variants is here in this Wikipedia entry.)

With this latest research, we have a reverse Rumpelstiltskin story where gold is turned into something else according to a June 13, 2016 news item on Nanowerk (Note: A link has been removed),

Flexible solar panels that could be rolled up for easy transport and other devices would benefit from transparent metal electrodes that can conduct electricity, are stretchable, and resist damage following repeated stretching. Researchers found that topology and the adhesion between a metal nanomesh and the underlying substrate played key roles in creating such materials. The metal nanomesh can be stretched to three times its length while maintaining a transparency comparable to similar commercial materials used in solar cells and flat panel displays. Also, nanomeshes on pre-stretched slippery substrates led to electrodes that didn’t wear out, even after being stretched 50,000 times (Proceedings of the National Academy of Sciences, “Fatigue-free, superstretchable, transparent, and biocompatible metal electrodes”).

Tuning topology and adhesion of metal nanomeshes has led to super stretchable, transparent electrodes that don’t wear out. The scanning electron microscopy image shows the structure of a gold mesh created with a special lithographic technique that controlled the dimensions of the mesh structure. Optimizing this structure and its adhesion to the substrate was key to achieving super stretchability and long lifetimes in use—nanomeshes on pre-stretched slippery substrates did not show signs of wear even after repeated stretching, up to 50,000 cycles.

Tuning topology and adhesion of metal nanomeshes has led to super stretchable, transparent electrodes that don’t wear out. The scanning electron microscopy image shows the structure of a gold mesh created with a special lithographic technique that controlled the dimensions of the mesh structure. Optimizing this structure and its adhesion to the substrate was key to achieving super stretchability and long lifetimes in use—nanomeshes on pre-stretched slippery substrates did not show signs of wear even after repeated stretching, up to 50,000 cycles.

A June 9, 2016 US Dept. of Energy news release,which originated the news item, provides more detail,

Next-generation flexible electronics require highly stretchable and transparent electrodes. Fatigue, structural damage due to repeated use, is deadly in metals as it leads to poor conductivity and it commonly occurs in metals with repeated stretching—even with short elongations. However, few electronic conductors are transparent and stretchable, even fewer can be cyclically stretched to a large strain without causing fatigue. Now researchers led by the University of Houston found that optimizing topology of a metal nanomesh and its adhesion to an underlying substrate improved stretchability and eliminated fatigue, while maintaining transparency. A special lithographic technique called “grain boundary lithography” controlled the dimensions of the mesh structure. The metal nanomesh remained transparent after being stretched to three times its length. Gold nanomeshes on prestretched slippery substrates impressively showed no wear when stretched 50,000 times. The slippery surface advantageously allowed the structure of the nanomesh to reorient to relax the stress. Such electrically conductive, flexible, and transparent electrodes could lead to next-generation flexible electronics such as advanced solar cells.  The nanomesh electrodes are also promising for implantable electronics because the nanomeshes are biocompatible.

Here’s a link to and a citation for the paper,

Fatigue-free, superstretchable, transparent, and biocompatible metal electrodes by Chuan Fei Guo, Qihan Liu, Guohui Wang, Yecheng Wang, Zhengzheng Shi, Zhigang Suo, Ching-Wu Chu, and Zhifeng Ren. Proceedings of the National Academy of Sciences, vol. 112 no. 40,  12332–12337, doi: 10.1073/pnas.1516873112

This paper appears to be open access.

A new, stable open-shell carbon molecule from Oregon

This discovery could one day make organic solar cells more efficient than silicon ones. Researchers at the University of Oregon announced their discovery in a June 9, 2016 news item on ScienceDaily,

University of Oregon chemists have synthesized a stable and long-lasting carbon-based molecule that, they say, potentially could be applicable in solar cells and electronic devices.

The molecule changes its bonding patterns to a magnetic biradical state when heated; it then returns to a fully bonded non-magnetic closed state at room temperature. That transition, they report, can be done repeatedly without decomposition. It remains stable in the presence of both heat and oxygen.

A June 9, 2016 University of Oregon news release on EurekAlert, which originated the news item, provides more detail,

 

Biradical refers to organic compounds, known as open-shell molecules, that have two free-flowing, non-bonding electrons. Producing them using techniques to control their electron spin, and thus provide semiconducting properties, in a heated state has been hampered by instability since the first synthetic biradical hydrocarbon was made in 1907.

“Potentially our approach could help to make organic solar cells more efficient than silicon solar cells, but that’s probably far in the future,” said UO doctoral student Gabriel E. Rudebusch, the paper’s lead author. “Our synthesis is rapid and efficient. We easily can make a gram of this compound, which is very stable when exposed to oxygen and heat. This stability has been almost unheard of in the literature about biradical compounds.”

The four-step synthesis of the compound — diindenoanthracene, or DIAn — and how it held up when tested in superconducting materials were detailed in a proof-of-principle paper published online May 23 by the journal Nature Chemistry. The UO team collaborated with experts in Japan, Spain and Sweden.

The molecular framework for the new molecule involves the hydrocarbon anthracene, which has three linearly fused hexagonal benzene rings, in combination with two five-membered pentagonal rings.

“The big difference between our new molecule and a lot of other biradical molecules that have been produced is those five-membered rings,” said co-author Michael M. Haley, who holds the UO’s Richard M. and Patricia H. Noyes Professorship in Chemistry. “They have the inherent ability to accept electrons or give up electrons. This means DIAn can move both negative and positive charges, which is an essential property for useful devices such as transistors and solar cells. Also, we can heat up our molecule to 150 degrees Celsius, bring it back to room temperature and heat it up again, repeatedly, and we see no decomposition in its reaction to oxygen. The unique features of DIAn are essential if these molecules are to have a use in the real world.”

Haley’s lab is now seeking to develop derivatives of the new molecule to help move the technology forward into potential applications.

Here’s a link to and a citation for the paper,

Diindeno-fusion of an anthracene as a design strategy for stable organic biradicals by Gabriel E. Rudebusch, José L. Zafra, Kjell Jorner, Kotaro Fukuda, Jonathan L. Marshall, Iratxe Arrechea-Marcos, Guzmán L. Espejo, Rocío Ponce Ortiz, Carlos J. Gómez-García, Lev N. Zakharov, Masayoshi Nakano, Henrik Ottosson, Juan Casado & Michael M. Haley. Nature Chemistry (2016)  doi:10.1038/nchem.2518 Published online 23 May 2016

This paper is behind a paywall.

There is another June 9, 2016 University of Oregon news release by Jim Barlow about this discovery. It covers much of the same material but focuses more closely on Rudebusch’s perspective.

Diamond-based electronics?

A May 24, 2016 news item on ScienceDaily describes the latest research on using diamonds as semiconductors,

Along with being a “girl’s best friend,” diamonds also have remarkable properties that could make them ideal semiconductors. This is welcome news for electronics; semiconductors are needed to meet the rising demand for more efficient electronics that deliver and convert power.

The thirst for electronics is unlikely to cease and almost every appliance or device requires a suite of electronics that transfer, convert and control power. Now, researchers have taken an important step toward that technology with a new way to dope single crystals of diamonds, a crucial process for building electronic devices.

A May 24, 2016 American Institute of Physics (AIP) news release (also on EurekAlert), which originated the news item, provides more detail,

For power electronics, diamonds could serve as the perfect material. They are thermally conductive, which means diamond-based devices would dissipate heat quickly and easily, foregoing the need for bulky and expensive methods for cooling. Diamond can also handle high voltages and power. Electrical currents also flow through diamonds quickly, meaning the material would make for energy efficient devices.

But among the biggest challenges to making diamond-based devices is doping, a process in which other elements are integrated into the semiconductor to change its properties. Because of diamond’s rigid crystalline structure, doping is difficult.

Currently, you can dope diamond by coating the crystal with boron and heating it to 1450 degrees Celsius. But it’s difficult to remove the boron coating at the end. This method only works on diamonds consisting of multiple crystals stuck together. Because such polydiamonds have irregularities between the crystals, single-crystals would be superior semiconductors.

You can dope single crystals by injecting boron atoms while growing the crystals artificially. The problem is the process requires powerful microwaves that can degrade the quality of the crystal.

Now, Ma [Zhengqiang (Jack) Ma, an electrical and computer engineering professor at the University of Wisconsin-Madison] and his colleagues have found a way to dope single-crystal diamonds with boron at relatively low temperatures and without any degradation. The researchers discovered if you bond a single-crystal diamond with a piece of silicon doped with boron, and heat it to 800 degrees Celsius, which is low compared to the conventional techniques, the boron atoms will migrate from the silicon to the diamond. It turns out that the boron-doped silicon has defects such as vacancies, where an atom is missing in the lattice structure. Carbon atoms from the diamond will fill those vacancies, leaving empty spots for boron atoms.

This technique also allows for selective doping, which means more control when making devices. You can choose where to dope a single-crystal diamond simply by bonding the silicon to that spot.

The new method only works for P-type doping, where the semiconductor is doped with an element that provides positive charge carriers (in this case, the absence of electrons, called holes).

“We feel like we found a very easy, inexpensive, and effective way to do it,” Ma said. The researchers are already working on a simple device using P-type single-crystal diamond semiconductors.

But to make electronic devices like transistors, you need N-type doping that gives the semiconductor negative charge carriers (electrons). And other barriers remain. Diamond is expensive and single crystals are very small.

Still, Ma says, achieving P-type doping is an important step, and might inspire others to find solutions for the remaining challenges. Eventually, he said, single-crystal diamond could be useful everywhere — perfect, for instance, for delivering power through the grid.

Here’s an image the researchers have released,

Optical image of a diode array on a natural single crystalline diamond plate. (The image looks blurred due to light scattering by the array of small pads on top of the diamond plate.) Inset shows the deposited anode metal on top of heavy doped Si nanomembrane that is bonded to natural single crystalline diamond. CREDIT: Jung-Hun Seo

Optical image of a diode array on a natural single crystalline diamond plate. (The image looks blurred due to light scattering by the array of small pads on top of the diamond plate.) Inset shows the deposited anode metal on top of heavy doped Si nanomembrane that is bonded to natural single crystalline diamond. CREDIT: Jung-Hun Seo Courtesy: American Institute of Physics

Here’s a link to and a citation for the paper,

Thermal diffusion boron doping of single-crystal natural diamond by Jung-Hun Seo, Henry Wu, Solomon Mikael, Hongyi Mi, James P. Blanchard, Giri Venkataramanan, Weidong Zhou, Shaoqin Gong, Dane Morgan, and Zhenqiang Ma. J. Appl. Phys. 119, 205703 (2016); http://dx.doi.org/10.1063/1.4949327

This paper appears to be open access.

Light-captured energetics (harvesting light for optoelectronics)

Comparing graphene to a tiger is unusual but that’s what researcher Sanfeng Wu does—eventually—in a May 13, 2016 University of Washington news release (also on EurekAlert) about his work,

In the quest to harvest light for electronics, the focal point is the moment when photons — light particles — encounter electrons, those negatively-charged subatomic particles that form the basis of our modern electronic lives. If conditions are right when electrons and photons meet, an exchange of energy can occur. Maximizing that transfer of energy is the key to making efficient light-captured energetics possible.

“This is the ideal, but finding high efficiency is very difficult,” said University of Washington physics doctoral student Sanfeng Wu. “Researchers have been looking for materials that will let them do this — one way is to make each absorbed photon transfer all of its energy to many electrons, instead of just one electron in traditional devices.”

In traditional light-harvesting methods, energy from one photon only excites one electron or none depending on the absorber’s energy gap, transferring just a small portion of light energy into electricity. The remaining energy is lost as heat. But in a paper released May 13 in Science Advances, Wu, UW associate professor Xiaodong Xu and colleagues at four other institutions describe one promising approach to coax photons into stimulating multiple electrons. Their method exploits some surprising quantum-level interactions to give one photon multiple potential electron partners. Wu and Xu, who has appointments in the UW’s Department of Materials Science & Engineering and the Department of Physics, made this surprising discovery using graphene.

There has been intense research on graphene’s electrical properties but the researchers’ discovery adds a new property to be investigated (from the news release),

“Graphene is a substance with many exciting properties,” said Wu, the paper’s lead author. “For our purposes, it shows a very efficient interaction with light.”

Graphene is a two-dimensional hexagonal lattice of carbon atoms bonded to one another, and electrons are able to move easily within graphene. The researchers took a single layer of graphene — just one sheet of carbon atoms thick — and sandwiched it between two thin layers of a material called boron-nitride.

Boron-nitride is a material that has excited a great deal of interest in the last 12 to 18 months (from the news release),

“Boron-nitride has a lattice structure that is very similar to graphene, but has very different chemical properties,” said Wu. “Electrons do not flow easily within boron-nitride; it essentially acts as an insulator.”

Xu and Wu discovered that when the graphene layer’s lattice is aligned with the layers of boron-nitride, a type of “superlattice” is created with properties allowing efficient optoelectronics that researchers had sought. These properties rely on quantum mechanics, the occasionally baffling rules that govern interactions between all known particles of matter. Wu and Xu detected unique quantum regions within the superlattice known as Van Hove singularities.

Here’s an animated .gif illustrating the superlattice in action,

The Moire superlattice they created by aligning graphene and boron-nitride. Credit: Sanfeng Wu.

The Moire superlattice they created by aligning graphene and boron-nitride. Credit: Sanfeng Wu.

The news release goes on to describe the Van Hove singularities within the superlattice and to mention the ‘tiger’,

“These are regions of huge electron density of states, and they were not accessed in either the graphene or boron-nitride alone,” said Wu. “We only created these high electron density regions in an accessible way when both layers were aligned together.”

When Xu and Wu directed energetic photons toward the superlattice, they discovered that those Van Hove singularities were sites where one energized photon could transfer its energy to multiple electrons that are subsequently collected by electrodes— not just one electron or none with the remaining energy lost as heat. By a conservative estimate, Xu and Wu report that within this superlattice one photon could “kick” as many as five electrons to flow as current.

With the discovery of collecting multiple electrons upon the absorption of one photon, researchers may be able to create highly efficient devices that could harvest light with a large energy profit. Future work would need to uncover how to organize the excited electrons into electrical current for optimizing the energy-converting efficiency and remove some of the more cumbersome properties of their superlattice, such as the need for a magnetic field. But they believe this efficient process between photons and electrons represents major progress.

“Graphene is a tiger with great potential for optoelectronics, but locked in a cage,” said Wu. “The singularities in this superlattice are a key to unlocking that cage and releasing graphene’s potential for light harvesting application.”

H/t to a May 13, 2016 news item on phys.org.

Here’s a link to and a citation for the paper,

Multiple hot-carrier collection in photo-excited graphene Moiré superlattices by Sanfeng Wu, Lei Wang, You Lai, Wen-Yu Shan, Grant Aivazian, Xian Zhang, Takashi Taniguchi, Kenji Watanabe, Di Xiao, Cory Dean, James Hone, Zhiqiang Li, and Xiaodong Xu. Science Advances 13 May 2016: Vol. 2, no. 5, e1600002 DOI: 10.1126/sciadv.1600002

This paper is open access.

Oily nanodiamonds

Nanodiamonds if successfully extracted from oil could be used for imaging and communications and the world’s leading program for extracting nanodiamonds (also known as diamondoids) is in California (US). From a May 12, 2016 news item on Nanowerk,

Stanford and SLAC National Accelerator Laboratory jointly run the world’s leading program for isolating and studying diamondoids — the tiniest possible specks of diamond. Found naturally in petroleum fluids, these interlocking carbon cages weigh less than a billionth of a billionth of a carat (a carat weighs about the same as 12 grains of rice); the smallest ones contain just 10 atoms.

Over the past decade, a team led by two Stanford-SLAC faculty members — Nick Melosh, an associate professor of materials science and engineering and of photon science, and Zhi-Xun Shen, a professor of photon science and of physics and applied physics – has found potential roles for diamondoids in improving electron microscope images, assembling materials and printing circuits on computer chips. The team’s work takes place within SIMES, the Stanford Institute for Materials and Energy Sciences, which is run jointly with SLAC.

Close-up of purified diamondoids on a lab bench. Too small to see with the naked eye, diamondoids are visible only when they clump together in fine, sugar-like crystals like these. Photo: Christopher Smith, SLAC National Accelerator Laboratory

Close-up of purified diamondoids on a lab bench. Too small to see with the naked eye, diamondoids are visible only when they clump together in fine, sugar-like crystals like these. Photo: Christopher Smith, SLAC National Accelerator Laboratory

A March 31, 2016 Stanford University news release by Glennda Chui, which originated the news item, describes the work in more detail,

Before they can do that [use nanodiamonds in imaging and other applications], though, just getting the diamondoids is a technical feat. It starts at the nearby Chevron refinery in Richmond, California, with a railroad tank car full of crude oil from the Gulf of Mexico. “We analyzed more than a thousand oils from around the world to see which had the highest concentrations of diamondoids,” says Jeremy Dahl, who developed key diamondoid isolation techniques with fellow Chevron researcher Robert Carlson before both came to Stanford — Dahl as a physical science research associate and Carlson as a visiting scientist.

The original isolation steps were carried out at the Chevron refinery, where the selected crudes were boiled in huge pots to concentrate the diamondoids. Some of the residue from that work came to a SLAC lab, where small batches are repeatedly boiled to evaporate and isolate molecules of specific weights. These fluids are then forced at high pressure through sophisticated filtration systems to separate out diamondoids of different sizes and shapes, each of which has different properties.

The diamondoids themselves are invisible to the eye; the only reason we can see them is that they clump together in fine, sugar-like crystals. “If you had a spoonful,” Dahl says, holding a few in his palm, “you could give 100 billion of them to every person on Earth and still have some left over.”

Recently, the team started using diamondoids to seed the growth of flawless, nano-sized diamonds in a lab at Stanford. By introducing other elements, such as silicon or nickel, during the growing process, they hope to make nanodiamonds with precisely tailored flaws that can produce single photons of light for next-generation optical communications and biological imaging.

Early results show that the quality of optical materials grown from diamondoid seeds is consistently high, says Stanford’s Jelena Vuckovic, a professor of electrical engineering who is leading this part of the research with Steven Chu, professor of physics and of molecular and cellular physiology.

“Developing a reliable way of growing the nanodiamonds is critical,” says Vuckovic, who is also a member of Stanford Bio-X. “And it’s really great to have that source and the grower right here at Stanford. Our collaborators grow the material, we characterize it and we give them feedback right away. They can change whatever we want them to change.”

Squeezing light into extremely thin layers

A May 4, 2016 Rice University (US) news release (also on EurekAlert) describes research on molybdenum disulfide and its light absorption properties,

Mechanics know molybdenum disulfide (MoS2) as a useful lubricant in aircraft and motorcycle engines and in the CV and universal joints of trucks and automobiles. Rice University engineering researcher Isabell Thomann knows it as a remarkably light-absorbent substance that holds promise for the development of energy-efficient optoelectronic and photocatalytic devices.

“Basically, we want to understand how much light can be confined in an atomically thin semiconductor monolayer of MoS2,” said Thomann, assistant professor of electrical and computer engineering and of materials science and nanoengineering and of chemistry. “By using simple strategies, we were able to absorb 35 to 37 percent of the incident light in the 400- to 700-nanometer wavelength range, in a layer that is only 0.7 nanometers thick.”

Thomann and Rice graduate students Shah Mohammad Bahauddin and Hossein Robatjazi have recounted their findings in a paper titled “Broadband Absorption Engineering To Enhance Light Absorption in Monolayer MoS2,” which was recently published in the American Chemical Society journal ACS Photonics. The research has many applications, including development of efficient and inexpensive photovoltaic solar panels.

“Squeezing light into these extremely thin layers and extracting the generated charge carriers is an important problem in the field of two-dimensional materials,” she said. “That’s because monolayers of 2-D materials have different electronic and catalytic properties from their bulk or multilayer counterparts.”

Thomann and her team used a combination of numerical simulations, analytical models and experimental optical characterizations. Using three-dimensional electromagnetic simulations, they found that light absorption was enhanced 5.9 times compared with using MoS2 on a sapphire substrate.

“If light absorption in these materials was perfect, we’d be able to create all sorts of energy-efficient optoelectronic and photocatalytic devices. That’s the problem we’re trying to solve,” Thomann said.

She is pleased with her lab’s progress but concedes that much work remains to be done. “The goal, of course, is 100 percent absorption, and we’re not there yet.”

Here’s a link to and a citation for the paper,

Broadband Absorption Engineering to Enhance Light Absorption in Monolayer MoSby Shah Mohammad Bahauddin, Hossein Robatjazi, and Isabell Thomann. ACS Photonics, Article ASAP DOI: 10.1021/acsphotonics.6b00081
Publication Date (Web): April 27, 2016

Copyright © 2016 American Chemical Society

This paper is behind a paywall.

Dissipating heat with graphene-based film

As the summer approaches here in the Northern Hemisphere I think longingly of frost and snow and so readers may find more than the usual number of stories about ‘cooling’. On that note, Chalmers Technical University (Sweden) is announcing some new research into cooling graphene-based films, from an April 29, 2016 news item on ScienceDaily,

Heat dissipation in electronics and optoelectronics is a severe bottleneck in the further development of systems in these fields. To come to grips with this serious issue, researchers at Chalmers University of Technology have developed an efficient way of cooling electronics by using functionalized graphene nanoflakes. …

“Essentially, we have found a golden key with which to achieve efficient heat transport in electronics and other power devices by using graphene nanoflake-based film. This can open up potential uses of this kind of film in broad areas, and we are getting closer to pilot-scale production based on this discovery,” says Johan Liu, Professor of Electronics Production at Chalmers University of Technology in Sweden.

An April 29, 2016 Chalmers Technical University press release (also on EurekAlert), which originated the news item, describes the work in more detail,

The researchers studied the heat transfer enhancement of the film with different functionalized amino-based and azide-based silane molecules, and found that the heat transfer efficiency of the film can be improved by over 76 percent by introducing functionalization molecules, compared to a reference system without the functional layer. This is mainly because the contact resistance was drastically reduced by introducing the functionalization molecules.

Meanwhile, molecular dynamic simulations and ab initio calculations reveal that the functional layer constrains the cross-plane scattering of low-frequency phonons, which in turn enhances in-plane heat-conduction of the bonded film by recovering the long flexural phonon lifetime. The results suggested potential thermal management solutions for electronic devices.

In the research, scientists studied a number of molecules that were immobilized at the interfaces and at the edge of graphene nanoflake-based sheets forming covalent bonds. They also probed interface thermal resistance by using a photo-thermal reflectance measurement technique to demonstrate an improved thermal coupling due to functionalization.

“This is the first time that such systematic research has been done. The present work is much more extensive than previously published results from several involved partners, and it covers more functionalization molecules and also more extensive direct evidence of the thermal contact resistance measurement,” says Johan Liu.

Here’s a link to and a citation for the paper,

Functionalization mediates heat transport in graphene nanoflakes by Haoxue Han, Yong Zhang, Nan Wang, Majid Kabiri Samani, Yuxiang Ni, Zainelabideen Y. Mijbil, Michael Edwards, Shiyun Xiong, Kimmo Sääskilahti, Murali Murugesan, Yifeng Fu, Lilei Ye, Hatef Sadeghi, Steven Bailey, Yuriy A. Kosevich, Colin J. Lambert, Johan Liu, & Sebastian Volz. Nature Communications 7, Article number: 11281  doi:10.1038/ncomms11281 Published 29 April 2016

This is an open access paper.

Memristor-based electronic synapses for neural networks

Caption: Neuron connections in biological neural networks. Credit: MIPT press office

Caption: Neuron connections in biological neural networks. Credit: MIPT press office

Russian scientists have recently published a paper about neural networks and electronic synapses based on ‘thin film’ memristors according to an April 19, 2016 news item on Nanowerk,

A team of scientists from the Moscow Institute of Physics and Technology (MIPT) have created prototypes of “electronic synapses” based on ultra-thin films of hafnium oxide (HfO2). These prototypes could potentially be used in fundamentally new computing systems.

An April 20, 2016 MIPT press release (also on EurekAlert), which originated the news item (the date inconsistency likely due to timezone differences) explains the connection between thin films and memristors,

The group of researchers from MIPT have made HfO2-based memristors measuring just 40×40 nm2. The nanostructures they built exhibit properties similar to biological synapses. Using newly developed technology, the memristors were integrated in matrices: in the future this technology may be used to design computers that function similar to biological neural networks.

Memristors (resistors with memory) are devices that are able to change their state (conductivity) depending on the charge passing through them, and they therefore have a memory of their “history”. In this study, the scientists used devices based on thin-film hafnium oxide, a material that is already used in the production of modern processors. This means that this new lab technology could, if required, easily be used in industrial processes.

“In a simpler version, memristors are promising binary non-volatile memory cells, in which information is written by switching the electric resistance – from high to low and back again. What we are trying to demonstrate are much more complex functions of memristors – that they behave similar to biological synapses,” said Yury Matveyev, the corresponding author of the paper, and senior researcher of MIPT’s Laboratory of Functional Materials and Devices for Nanoelectronics, commenting on the study.

The press release offers a description of biological synapses and their relationship to learning and memory,

A synapse is point of connection between neurons, the main function of which is to transmit a signal (a spike – a particular type of signal, see fig. 2) from one neuron to another. Each neuron may have thousands of synapses, i.e. connect with a large number of other neurons. This means that information can be processed in parallel, rather than sequentially (as in modern computers). This is the reason why “living” neural networks are so immensely effective both in terms of speed and energy consumption in solving large range of tasks, such as image / voice recognition, etc.

Over time, synapses may change their “weight”, i.e. their ability to transmit a signal. This property is believed to be the key to understanding the learning and memory functions of thebrain.

From the physical point of view, synaptic “memory” and “learning” in the brain can be interpreted as follows: the neural connection possesses a certain “conductivity”, which is determined by the previous “history” of signals that have passed through the connection. If a synapse transmits a signal from one neuron to another, we can say that it has high “conductivity”, and if it does not, we say it has low “conductivity”. However, synapses do not simply function in on/off mode; they can have any intermediate “weight” (intermediate conductivity value). Accordingly, if we want to simulate them using certain devices, these devices will also have to have analogous characteristics.

The researchers have provided an illustration of a biological synapse,

Fig.2 The type of electrical signal transmitted by neurons (a “spike”). The red lines are various other biological signals, the black line is the averaged signal. Source: MIPT press office

Fig.2 The type of electrical signal transmitted by neurons (a “spike”). The red lines are various other biological signals, the black line is the averaged signal. Source: MIPT press office

Now, the press release ties the memristor information together with the biological synapse information to describe the new work at the MIPT,

As in a biological synapse, the value of the electrical conductivity of a memristor is the result of its previous “life” – from the moment it was made.

There is a number of physical effects that can be exploited to design memristors. In this study, the authors used devices based on ultrathin-film hafnium oxide, which exhibit the effect of soft (reversible) electrical breakdown under an applied external electric field. Most often, these devices use only two different states encoding logic zero and one. However, in order to simulate biological synapses, a continuous spectrum of conductivities had to be used in the devices.

“The detailed physical mechanism behind the function of the memristors in question is still debated. However, the qualitative model is as follows: in the metal–ultrathin oxide–metal structure, charged point defects, such as vacancies of oxygen atoms, are formed and move around in the oxide layer when exposed to an electric field. It is these defects that are responsible for the reversible change in the conductivity of the oxide layer,” says the co-author of the paper and researcher of MIPT’s Laboratory of Functional Materials and Devices for Nanoelectronics, Sergey Zakharchenko.

The authors used the newly developed “analogue” memristors to model various learning mechanisms (“plasticity”) of biological synapses. In particular, this involved functions such as long-term potentiation (LTP) or long-term depression (LTD) of a connection between two neurons. It is generally accepted that these functions are the underlying mechanisms of  memory in the brain.

The authors also succeeded in demonstrating a more complex mechanism – spike-timing-dependent plasticity, i.e. the dependence of the value of the connection between neurons on the relative time taken for them to be “triggered”. It had previously been shown that this mechanism is responsible for associative learning – the ability of the brain to find connections between different events.

To demonstrate this function in their memristor devices, the authors purposefully used an electric signal which reproduced, as far as possible, the signals in living neurons, and they obtained a dependency very similar to those observed in living synapses (see fig. 3).

Fig.3. The change in conductivity of memristors depending on the temporal separation between "spikes"(rigth) and thr change in potential of the neuron connections in biological neural networks. Source: MIPT press office

Fig.3. The change in conductivity of memristors depending on the temporal separation between “spikes”(rigth) and thr change in potential of the neuron connections in biological neural networks. Source: MIPT press office

These results allowed the authors to confirm that the elements that they had developed could be considered a prototype of the “electronic synapse”, which could be used as a basis for the hardware implementation of artificial neural networks.

“We have created a baseline matrix of nanoscale memristors demonstrating the properties of biological synapses. Thanks to this research, we are now one step closer to building an artificial neural network. It may only be the very simplest of networks, but it is nevertheless a hardware prototype,” said the head of MIPT’s Laboratory of Functional Materials and Devices for Nanoelectronics, Andrey Zenkevich.

Here’s a link to and a citation for the paper,

Crossbar Nanoscale HfO2-Based Electronic Synapses by Yury Matveyev, Roman Kirtaev, Alena Fetisova, Sergey Zakharchenko, Dmitry Negrov and Andrey Zenkevich. Nanoscale Research Letters201611:147 DOI: 10.1186/s11671-016-1360-6

Published: 15 March 2016

This is an open access paper.