Tag Archives: semiconductors

IBM, the Cognitive Era, and carbon nanotube electronics

Leave a reply

IBM has a storied position in the field of nanotechnology due to the scanning tunneling microscope developed in the company’s laboratories. It was a Nobel Prize-winning breakthough which provided the impetus for nanotechnology applied research. Now, an Oct. 1, 2015 news item on Nanowerk trumpets another IBM breakthrough,

IBM Research today [Oct. 1, 2015] announced a major engineering breakthrough that could accelerate carbon nanotubes replacing silicon transistors to power future computing technologies.

IBM scientists demonstrated a new way to shrink transistor contacts without reducing performance of carbon nanotube devices, opening a pathway to dramatically faster, smaller and more powerful computer chips beyond the capabilities of traditional semiconductors.

While the Oct. 1, 2015 IBM news release, which originated the news item, does go on at length there’s not much technical detail (see the second to last paragraph in the excerpt for the little they do include) about the research breakthrough (Note: Links have been removed),

IBM’s breakthrough overcomes a major hurdle that silicon and any semiconductor transistor technologies face when scaling down. In any transistor, two things scale: the channel and its two contacts. As devices become smaller, increased contact resistance for carbon nanotubes has hindered performance gains until now. These results could overcome contact resistance challenges all the way to the 1.8 nanometer node – four technology generations away. [emphasis mine]

Carbon nanotube chips could greatly improve the capabilities of high performance computers, enabling Big Data to be analyzed faster, increasing the power and battery life of mobile devices and the Internet of Things, and allowing cloud data centers to deliver services more efficiently and economically.

Silicon transistors, tiny switches that carry information on a chip, have been made smaller year after year, but they are approaching a point of physical limitation. With Moore’s Law running out of steam, shrinking the size of the transistor – including the channels and contacts – without compromising performance has been a vexing challenge troubling researchers for decades.

IBM has previously shown that carbon nanotube transistors can operate as excellent switches at channel dimensions of less than ten nanometers – the equivalent to 10,000 times thinner than a strand of human hair and less than half the size of today’s leading silicon technology. IBM’s new contact approach overcomes the other major hurdle in incorporating carbon nanotubes into semiconductor devices, which could result in smaller chips with greater performance and lower power consumption.

Earlier this summer, IBM unveiled the first 7 nanometer node silicon test chip [emphasis mine], pushing the limits of silicon technologies and ensuring further innovations for IBM Systems and the IT industry. By advancing research of carbon nanotubes to replace traditional silicon devices, IBM is paving the way for a post-silicon future and delivering on its $3 billion chip R&D investment announced in July 2014.

“These chip innovations are necessary to meet the emerging demands of cloud computing, Internet of Things and Big Data systems,” said Dario Gil, vice president of Science & Technology at IBM Research. “As silicon technology nears its physical limits, new materials, devices and circuit architectures must be ready to deliver the advanced technologies that will be required by the Cognitive Computing era. This breakthrough shows that computer chips made of carbon nanotubes will be able to power systems of the future sooner than the industry expected.”

A New Contact for Carbon Nanotubes

Carbon nanotubes represent a new class of semiconductor materials that consist of single atomic sheets of carbon rolled up into a tube. The carbon nanotubes form the core of a transistor device whose superior electrical properties promise several generations of technology scaling beyond the physical limits of silicon.

Electrons in carbon transistors can move more easily than in silicon-based devices, and the ultra-thin body of carbon nanotubes provide additional advantages at the atomic scale. Inside a chip, contacts are the valves that control the flow of electrons from metal into the channels of a semiconductor. As transistors shrink in size, electrical resistance increases within the contacts, which impedes performance. Until now, decreasing the size of the contacts on a device caused a commensurate drop in performance – a challenge facing both silicon and carbon nanotube transistor technologies.

IBM researchers had to forego traditional contact schemes and invented a metallurgical process akin to microscopic welding that chemically binds the metal atoms to the carbon atoms at the ends of nanotubes. This ‘end-bonded contact scheme’ allows the contacts to be shrunken down to below 10 nanometers without deteriorating performance of the carbon nanotube devices.

“For any advanced transistor technology, the increase in contact resistance due to the decrease in the size of transistors becomes a major performance bottleneck,” Gil added. “Our novel approach is to make the contact from the end of the carbon nanotube, which we show does not degrade device performance. This brings us a step closer to the goal of a carbon nanotube technology within the decade.”

Every once in a while, the size gets to me and a 1.8nm node is amazing. As for IBM’s 7nm chip, which was previewed this summer, there’s more about that in my July 15, 2015 posting.

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

End-bonded contacts for carbon nanotube transistors with low, size-independent resistance by Qing Cao, Shu-Jen Han, Jerry Tersoff, Aaron D. Franklin†, Yu Zhu, Zhen Zhang‡, George S. Tulevski, Jianshi Tang, and Wilfried Haensch. Science 2 October 2015: Vol. 350 no. 6256 pp. 68-72 DOI: 10.1126/science.aac8006

This paper is behind a paywall.

TRIUMF accelerator used by US researchers to visualize properties of nanoscale materials

Leave a reply

The US researchers are at the University of California at Los Angeles (UCLA) and while it’s not explicitly stated I’m assuming the accelerator they mention at TRIUMF (Canada’s national laboratory for particle and nuclear physics) has something special as there are accelerators in California and other parts of the US.

A July 15, 2015 news item on Nanotechnology Now announces the latest on visualizing the properties of nanoscale materials,

Scientists trying to improve the semiconductors that power our electronic devices have focused on a technology called spintronics as one especially promising area of research. Unlike conventional devices that use electrons’ charge to create power, spintronic devices use electrons’ spin. The technology is already used in computer hard drives and many other applications — and scientists believe it could eventually be used for quantum computers, a new generation of machines that use quantum mechanics to solve complex problems with extraordinary speed.

A July 15, 2015 UCLA news release, which originated the news item, expands on the theme and briefly mentions TRIUMF’s accelerator (Note: A link has been removed),

Emerging research has shown that one key to greatly improving performance in spintronics could be a class of materials called topological insulators. Unlike ordinary materials that are either insulators or conductors, topological insulators function as both simultaneously — on the inside, they are insulators but on their exteriors, they conduct electricity.

But topological insulators have certain defects that have so far limited their use in practical applications, and because they are so tiny, scientists have so far been unable to fully understand how the defects impact their functionality.

The UCLA researchers have overcome that challenge with a new method to visualize topological insulators at the nanoscale. An article highlighting the research, which was which led by Louis Bouchard, assistant professor of chemistry and biochemistry, and Dimitrios Koumoulis, a UCLA postdoctoral scholar, was published online in the Proceedings of the National Academy of Sciences.

The new method is the first use of beta‑detected nuclear magnetic resonance to study the effects of these defects on the properties of topological insulators.

The technique involves aiming a highly focused stream of ions at the topological insulator. To generate that beam of ions, the researchers used a large particle accelerator called a cyclotron, which accelerates protons through a spiral path inside the machine and forces them to collide with a target made of the chemical element tantalum. This collision produces lithium-8 atoms, which are ionized and slowed down to a desired energy level before they are implanted in the topological insulators.

In beta‑detected nuclear magnetic resonance, ions (in this case, the ionized lithium-8 atoms) of various energies are implanted in the material of interest (the topological insulator) to generate signals from the material’s layers of interest.

Bouchard said the method is particularly well suited for probing regions near the surfaces and interfaces of different materials.

In the UCLA research, the high sensitivity of the beta‑detected nuclear magnetic resonance technique and its ability to probe materials allowed the scientists to “see” the impacts of the defects in the topological insulators by viewing the electronic and magnetic properties beneath the surface of the material.

The researchers used the large TRIUMF cyclotron in Vancouver, British Columbia.

According to the UCLA news release, there were also researchers from the University of British Columbia, the University of Texas at Austin and Northwestern University *were* involved with the work.

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

Nanoscale β-nuclear magnetic resonance depth imaging of topological insulators by Dimitrios Koumoulis, Gerald D. Morris, Liang He, Xufeng Kou, Danny King, Dong Wang, Masrur D. Hossain, Kang L. Wang, Gregory A. Fiete, Mercouri G. Kanatzidis, and Louis-S. Bouchard. PNAS July 14, 2015 vol. 112 no. 28 doi: 10.1073/pnas.1502330112

This paper is behind a paywall.

*’were’ added Jan. 20, 2016.

Replace silicon with black phosphorus instead of graphene?

Leave a reply

I have two black phosphorus pieces. This first piece of research comes out of ‘La belle province’ or, as it’s more usually called, Québec (Canada).

Foundational research on phosphorene

There’s a lot of interest in replacing silicon for a number of reasons and, increasingly, there’s interest in finding an alternative to graphene.

A July 7, 2015 news item on Nanotechnology Now describes a new material for use as transistors,

As scientists continue to hunt for a material that will make it possible to pack more transistors on a chip, new research from McGill University and Université de Montréal adds to evidence that black phosphorus could emerge as a strong candidate.

In a study published today in Nature Communications, the researchers report that when electrons move in a phosphorus transistor, they do so only in two dimensions. The finding suggests that black phosphorus could help engineers surmount one of the big challenges for future electronics: designing energy-efficient transistors.

A July 7, 2015 McGill University news release on EurekAlert, which originated the news item, describes the field of 2D materials and the research into black phosphorus and its 2D version, phosperene (analogous to graphite and graphene),

“Transistors work more efficiently when they are thin, with electrons moving in only two dimensions,” says Thomas Szkopek, an associate professor in McGill’s Department of Electrical and Computer Engineering and senior author of the new study. “Nothing gets thinner than a single layer of atoms.”

In 2004, physicists at the University of Manchester in the U.K. first isolated and explored the remarkable properties of graphene — a one-atom-thick layer of carbon. Since then scientists have rushed to to investigate a range of other two-dimensional materials. One of those is black phosphorus, a form of phosphorus that is similar to graphite and can be separated easily into single atomic layers, known as phosphorene.

Phosphorene has sparked growing interest because it overcomes many of the challenges of using graphene in electronics. Unlike graphene, which acts like a metal, black phosphorus is a natural semiconductor: it can be readily switched on and off.

“To lower the operating voltage of transistors, and thereby reduce the heat they generate, we have to get closer and closer to designing the transistor at the atomic level,” Szkopek says. “The toolbox of the future for transistor designers will require a variety of atomic-layered materials: an ideal semiconductor, an ideal metal, and an ideal dielectric. All three components must be optimized for a well designed transistor. Black phosphorus fills the semiconducting-material role.”

The work resulted from a multidisciplinary collaboration among Szkopek’s nanoelectronics research group, the nanoscience lab of McGill Physics Prof. Guillaume Gervais, and the nanostructures research group of Prof. Richard Martel in Université de Montréal’s Department of Chemistry.

To examine how the electrons move in a phosphorus transistor, the researchers observed them under the influence of a magnetic field in experiments performed at the National High Magnetic Field Laboratory in Tallahassee, FL, the largest and highest-powered magnet laboratory in the world. This research “provides important insights into the fundamental physics that dictate the behavior of black phosphorus,” says Tim Murphy, DC Field Facility Director at the Florida facility.

“What’s surprising in these results is that the electrons are able to be pulled into a sheet of charge which is two-dimensional, even though they occupy a volume that is several atomic layers in thickness,” Szkopek says. That finding is significant because it could potentially facilitate manufacturing the material — though at this point “no one knows how to manufacture this material on a large scale.”

“There is a great emerging interest around the world in black phosphorus,” Szkopek says. “We are still a long way from seeing atomic layer transistors in a commercial product, but we have now moved one step closer.”

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

Two-dimensional magnetotransport in a black phosphorus naked quantum well by V. Tayari, N. Hemsworth, I. Fakih, A. Favron, E. Gaufrès, G. Gervais, R. Martel & T. Szkopek. Nature Communications 6, Article number: 7702 doi:10.1038/ncomms8702 Published 07 July 2015

This is an open access paper.

The second piece of research into black phosphorus is courtesy of an international collaboration.

A phosporene transistor

A July 9, 2015 Technical University of Munich (TUM) press release (also on EurekAlert) describes the formation of a phosphorene transistor made possible by the introduction of arsenic,

Chemists at the Technische Universität München (TUM) have now developed a semiconducting material in which individual phosphorus atoms are replaced by arsenic. In a collaborative international effort, American colleagues have built the first field-effect transistors from the new material.

For many decades silicon has formed the basis of modern electronics. To date silicon technology could provide ever tinier transistors for smaller and smaller devices. But the size of silicon transistors is reaching its physical limit. Also, consumers would like to have flexible devices, devices that can be incorporated into clothing and the likes. However, silicon is hard and brittle. All this has triggered a race for new materials that might one day replace silicon.

Black arsenic phosphorus might be such a material. Like graphene, which consists of a single layer of carbon atoms, it forms extremely thin layers. The array of possible applications ranges from transistors and sensors to mechanically flexible semiconductor devices. Unlike graphene, whose electronic properties are similar to those of metals, black arsenic phosphorus behaves like a semiconductor.

The press release goes on to provide more detail about the collaboration and the research,

A cooperation between the Technical University of Munich and the University of Regensburg on the German side and the University of Southern California (USC) and Yale University in the United States has now, for the first time, produced a field effect transistor made of black arsenic phosphorus. The compounds were synthesized by Marianne Koepf at the laboratory of the research group for Synthesis and Characterization of Innovative Materials at the TUM. The field effect transistors were built and characterized by a group headed by Professor Zhou and Dr. Liu at the Department of Electrical Engineering at USC.

The new technology developed at TUM allows the synthesis of black arsenic phosphorus without high pressure. This requires less energy and is cheaper. The gap between valence and conduction bands can be precisely controlled by adjusting the arsenic concentration. “This allows us to produce materials with previously unattainable electronic and optical properties in an energy window that was hitherto inaccessible,” says Professor Tom Nilges, head of the research group for Synthesis and Characterization of Innovative Materials.

Detectors for infrared

With an arsenic concentration of 83 percent the material exhibits an extremely small band gap of only 0.15 electron volts, making it predestined for sensors which can detect long wavelength infrared radiation. LiDAR (Light Detection and Ranging) sensors operate in this wavelength range, for example. They are used, among other things, as distance sensors in automobiles. Another application is the measurement of dust particles and trace gases in environmental monitoring.

A further interesting aspect of these new, two-dimensional semiconductors is their anisotropic electronic and optical behavior. The material exhibits different characteristics along the x- and y-axes in the same plane. To produce graphene like films the material can be peeled off in ultra thin layers. The thinnest films obtained so far are only two atomic layers thick.

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

Black Arsenic–Phosphorus: Layered Anisotropic Infrared Semiconductors with Highly Tunable Compositions and Properties by Bilu Liu, Marianne Köpf, Ahmad N. Abbas, Xiaomu Wang, Qiushi Guo, Yichen Jia, Fengnian Xia, Richard Weihrich, Frederik Bachhuber, Florian Pielnhofer, Han Wang, Rohan Dhall, Stephen B. Cronin, Mingyuan Ge1 Xin Fang, Tom Nilges, and Chongwu Zhou. DOI: 10.1002/adma.201501758 Article first published online: 25 JUN 2015

© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

This paper is behind a paywall.

Dexter Johnson, on his Nanoclast blog (on the Institute for Electrical and Electronics Engineers website), adds more information about black phosphorus and its electrical properties in his July 9, 2015 posting about the Germany/US collaboration (Note: Links have been removed),

Black phosphorus has been around for about 100 years, but recently it has been synthesized as a two-dimensional material—dubbed phosphorene in reference to its two-dimensional cousin, graphene. Black phosphorus is quite attractive for electronic applications like field-effect transistors because of its inherent band gap and it is one of the few 2-D materials to be a natively p-type semiconductor.

One final comment, I notice the Germany-US work was published weeks prior to the Canadian research suggesting that the TUM July 9, 2015 press release is an attempt to capitalize on the interest generated by the Canadian research. That’s a smart move.

Just how bendy are the new organic semiconductors?

Leave a reply

In all the excitement about flexible electronics, an interesting question about performance, which seems to have been overlooked until now (how bendy are they?), is being answered by scientists, according to a May 5, 2015 University of Massachusetts at Amherst news release (also on EurekAlert),

A revolution is coming in flexible electronic technologies as cheaper, more flexible, organic transistors come on the scene to replace expensive, rigid, silicone-based semiconductors, but not enough is known about how bending in these new thin-film electronic devices will affect their performance, say materials scientists at the University of Massachusetts Amherst.

…

They are the first to apply inhomogeneous deformations, that is strain, to the conducting channel of an organic transistor and to understand the observed effects, says Reyes-Martinez [Marcos Reyes-Martinez], who conducted the series of experiments as part of his doctoral work.

As he explains, “This is relevant to today’s tech industry because transistors drive the logic of all the consumer electronics we use. In the screen on your smart phone, for example, every little pixel that makes up the image is turned on and off by hundreds of thousands or even millions of miniaturized transistors.”

“Traditionally, the transistors are rigid, made of an inorganic material such as silicon,” he adds. “We’re working with a crystalline semiconductorcalled rubrene, which is an organic, carbon-based material that has performance factors, such as charge-carrier mobility, surpassing those measured in amorphous silicon. Organic semiconductors are an interesting alternative to silicon because their properties can be tuned to make them easily processed, allowing them to coat a variety of surfaces, including soft substrates at relatively low temperatures. As a result, devices based on organic semiconductors are projected to be cheaper since they do not require high temperatures, clean rooms and expensive processing steps like silicon does.”

Until now, Reyes-Martinez notes, most researchers have focused on controlling the detrimental effects of mechanical deformation to atransistor’s electrical properties. But in their series of systematic experiments, the UMass Amherst team discovered that mechanical deformations only decrease performance under certain conditions, and actually can enhance or have no effect in other instances.

“Our goal was not only to show these effects, but to explain and understand them. What we’ve done istake advantage of the ordered structure of ultra-thin organic single crystals of rubrene to fabricate high-perfomance, thin-film transistors,” he says. “This is the first time that anyone has carried out detailed fundamental work at these length scales with a single crystal.”

Though single crystals were once thought to be too fragile for flexible applications, the UMass Amherst team found that crystals ranging in thickness from about 150 nanometers to 1 micrometer were thin enough to be wrinkled and applied to any elastomer substrate. Reyes-Martinez also notes, “Our experiments are especially important because they help scientists working on flexible electronic devices to determine performance limitations of new materials under extreme mechanical deformations, such as when electronic devices conform to skin.”

They developed an analytical model based on plate bending theoryto quantifythe different local strains imposed on the transistor structure by the wrinkle deformations. Using their model they are able to predict how different deformations modulate charge mobility, which no one had quantified before, Reyes-Martinez notes.

These contributions “represent a significant step forward in structure-function relationships in organic semiconductors, critical for the development of the next generation of flexible electronic devices,” the authors point out.

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

Rubrene crystal field-effect mobility modulation via conducting channel wrinkling by Marcos A. Reyes-Martinez, Alfred J. Crosby,  & Alejandro L. Briseno. Nature Communications 6, Article number: 6948 doi:10.1038/ncomms7948 Published 05 May 2015

This is an open access paper.

Transition metal dichalcogenides (molybdenum disulfide and tungsten diselenide) rock the graphene boat

Leave a reply

Anyone who’s read stories about scientific discovery knows that the early stages are characterized by a number of possibilities so the current race to unseat graphene as the wonder material of the nanoworld is a ‘business as usual’ sign although I imagine it can be confusing for investors and others hoping to make their fortunes. As for the contenders to the ‘wonder nanomaterial throne’, they are transition metal dichalcogenides: molybdenum disulfide and tungsten diselenide both of which have garnered some recent attention.

A March 12, 2014 news item on Nanwerk features research on molybdenum disulfide from Poland,

Will one-atom-thick layers of molybdenum disulfide, a compound that occurs naturally in rocks, prove to be better than graphene for electronic applications? There are many signs that might prove to be the case. But physicists from the Faculty of Physics at the University of Warsaw have shown that the nature of the phenomena occurring in layered materials are still ill-understood and require further research.

….

Researchers at the University of Warsaw, Faculty of Physics (FUW) have shown that the phenomena occurring in the crystal network of molybdenum disulfide sheets are of a slightly different nature than previously thought. A report describing the discovery, achieved in collaboration with Laboratoire National des Champs Magnétiques Intenses in Grenoble, has recently been published in Applied Physics Letters.

“It will not become possible to construct complex electronic systems consisting of individual atomic sheets until we have a sufficiently good understanding of the physics involved in the phenomena occurring within the crystal network of those materials. Our research shows, however, that research still has a long way to go in this field”, says Prof. Adam Babinski at the UW Faculty of Physics.

A March 12, 2014 Dept. of Physics University of Warsaw (FUW) news release, which originated the news item, describes the researchers’ ideas about graphene and alternative materials such as molybdenum disulfide,

“It will not become possible to construct complex electronic systems consisting of individual atomic sheets until we have a sufficiently good understanding of the physics involved in the phenomena occurring within the crystal network of those materials. Our research shows, however, that research still has a long way to go in this field”, says Prof. Adam Babiński at the UW Faculty of Physics.

The simplest method of creating graphene is called exfoliation: a piece of scotch tape is first stuck to a piece of graphite, then peeled off. Among the particles that remain stuck to the tape, one can find microscopic layers of graphene. This is because graphite consists of many graphene sheets adjacent to one another. The carbon atoms within each layer are very strongly bound to one another (by covalent bonds, to which graphene owes its legendary resilience), but the individual layers are held together by significantly weaker bonds (van de Walls [van der Waals] bonds). Ordinary scotch tape is strong enough to break the latter and to tear individual graphene sheets away from the graphite crystal.

A few years ago it was noticed that just as graphene can be obtained from graphite, sheets a single atom thick can similarly be obtained from many other crystals. This has been successfully done, for instance, with transition metals chalcogenides (sulfides, selenides, and tellurides). Layers of molybdenum disulfide (MoS2), in particular, have proven to be a very interesting material. This compound exists in nature as molybdenite, a crystal material found in rocks around the world, frequently taking the characteristic form of silver-colored hexagonal plates. For years molybdenite has been used in the manufacturing of lubricants and metal alloys. Like in the case of graphite, the properties of single-atom sheets of MoS2 long went unnoticed.

From the standpoint of applications in electronics, molybdenum disulfide sheets exhibit a significant advantage over graphene: they have an energy gap, an energy range within which no electron states can exist. By applying electric field, the material can be switched between a state that conducts electricity and one that behaves like an insulator. By current calculations, a switched-off molybdenum disulfide transistor would consume even as little as several hundred thousand times less energy than a silicon transistor. Graphene, on the other hand, has no energy gap and transistors made of graphene cannot be fully switched off.

The news release goes on to describe how the researchers refined their understanding of molybdenum disulfide and its properties,

Valuable information about a crystal’s structure and phenomena occurring within it can be obtained by analyzing how light gets scattered within the material. Photons of a given energy are usually absorbed by the atoms and molecules of the material, then reemitted at the same energy. In the spectrum of the scattered light one can then see a distinctive peak, corresponding to that energy. It turns out, however, that one out of many millions of photons is able to use some of its energy otherwise, for instance to alter the vibration or circulation of a molecule. The reverse situation also sometimes occurs: a photon may take away some of the energy of a molecule, and so its own energy slightly increases. In this situation, known as Raman scattering, two smaller peaks are observed to either side of the main peak.

The scientists at the UW Faculty of Physics analyzed the Raman spectra of molybdenum disulfide carrying on low-temperature microscopic measurements. The higher sensitivity of the equipment and detailed analysis methods enabled the team to propose a more precise model of the phenomena occurring in the crystal network of molybdenum disulfide.

“In the case of single-layer materials, the shape of the Raman lines has previously been explained in terms of phenomena involving certain characteristic vibrations of the crystal network. We have shown for molybdenum disulfide sheets that the effects ascribed to those vibrations must actually, at least in part, be due to other network vibrations not previously taken into account”, explains Katarzyna Gołasa, a doctorate student at the UW Faculty of Physics.

The presence of the new type of vibration in single-sheet materials has an impact on how electrons behave. As a consequence, these materials must have somewhat different electronic properties than previously anticipated.

Here’s what the rocks look like,

Molybdenum disulfide occurs in nature as molybdenite, crystalline material that frequently takes the characteristic form of silver-colored hexagonal plates. (Source: FUW)

Molybdenum disulfide occurs in nature as molybdenite, crystalline material that frequently takes the characteristic form of silver-colored hexagonal plates. (Source: FUW)

I am not able to find the published research at this time (March 13, 2014).

The tungsten diselenide story is specifically application-centric. Dexter Johnson in a March 11, 2014 post on his Nanoclast blog (on the IEEE [Institute of Electrical and Electronics Engineers] website) describes the differing perspectives and potential applications suggested by the three teams that cooperated to produce papers united by a joint theme ,

The three research groups focused on optoelectronics applications of tungsten diselenide, but each with a slightly different emphasis.

The University of Washington scientists highlighted applications of the material for a light emitting diode (LED). The Vienna University of Technology group focused on the material’s photovoltaic applications. And, finally, the MIT [Massachusetts Institute of Technology] group looked at all of the optoelectronic applications for the material that would result from the way it can be switched from being a p-type to a n-type semiconductor.

Here are some details of the research from each of the institutions’ news releases.

A March 10, 2014 University of Washington (state) news release highlights their LED work,

University of Washington [UW] scientists have built the thinnest-known LED that can be used as a source of light energy in electronics. The LED is based off of two-dimensional, flexible semiconductors, making it possible to stack or use in much smaller and more diverse applications than current technology allows.

“We are able to make the thinnest-possible LEDs, only three atoms thick yet mechanically strong. Such thin and foldable LEDs are critical for future portable and integrated electronic devices,” said Xiaodong Xu, a UW assistant professor in materials science and engineering and in physics.

…

The UW’s LED is made from flat sheets of the molecular semiconductor known as tungsten diselenide, a member of a group of two-dimensional materials that have been recently identified as the thinnest-known semiconductors. Researchers use regular adhesive tape to extract a single sheet of this material from thick, layered pieces in a method inspired by the 2010 Nobel Prize in Physics awarded to the University of Manchester for isolating one-atom-thick flakes of carbon, called graphene, from a piece of graphite.

…

In addition to light-emitting applications, this technology could open doors for using light as interconnects to run nano-scale computer chips instead of standard devices that operate off the movement of electrons, or electricity. The latter process creates a lot of heat and wastes power, whereas sending light through a chip to achieve the same purpose would be highly efficient.

“A promising solution is to replace the electrical interconnect with optical ones, which will maintain the high bandwidth but consume less energy,” Xu said. “Our work makes it possible to make highly integrated and energy-efficient devices in areas such as lighting, optical communication and nano lasers.”

Here’s a link to and a citation for this team’s paper,

Electrically tunable excitonic light-emitting diodes based on monolayer WSe2 p–n junctions by Jason S. Ross, Philip Klement, Aaron M. Jones, Nirmal J. Ghimire, Jiaqiang Yan, D. G. Mandrus, Takashi Taniguchi, Kenji Watanabe, Kenji Kitamura, Wang Yao, David H. Cobden, & Xiaodong Xu. Nature Nanotechnology (2014) doi:10.1038/nnano.2014.26 Published online 09 March 2014

This paper is behind a paywall.

A March 9, 2014 University of Vienna news release highlights their work on tungsten diselinide and its possible application in solar cells,

… With graphene as a light detector, optical signals can be transformed into electric pulses on extremely short timescales.

For one very similar application, however, graphene is not well suited for building solar cells. “The electronic states in graphene are not very practical for creating photovoltaics”, says Thomas Mueller. Therefore, he and his team started to look for other materials, which, similarly to graphene, can arranged in ultrathin layers, but have even better electronic properties.

The material of choice was tungsten diselenide: It consists of one layer of tungsten atoms, which are connected by selenium atoms above and below the tungsten plane. The material absorbs light, much like graphene, but in tungsten diselenide, this light can be used to create electrical power.

The layer is so thin that 95% of the light just passes through – but a tenth of the remaining five percent, which are absorbed by the material, are converted into electrical power. Therefore, the internal efficiency is quite high. A larger portion of the incident light can be used if several of the ultrathin layers are stacked on top of each other – but sometimes the high transparency can be a useful side effect. “We are envisioning solar cell layers on glass facades, which let part of the light into the building while at the same time creating electricity”, says Thomas Mueller.

Today, standard solar cells are mostly made of silicon, they are rather bulky and inflexible. Organic materials are also used for opto-electronic applications, but they age rather quickly. “A big advantage of two-dimensional structures of single atomic layers is their crystallinity. Crystal structures lend stability”, says Thomas Mueller.

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

Solar-energy conversion and light emission in an atomic monolayer p–n diode by Andreas Pospischil, Marco M. Furchi, & Thomas Mueller. Nature Nanotechnology (2014) doi:10.1038/nnano.2014.14 Published online 09 March 2014

This paper is behind a paywll.

Finally, a March 10, 2014 MIT news release details their work about material able to switch from p-type (p = positive) to a n-type (n = negative) semiconductors,

The material they used, called tungsten diselenide (WSe2), is part of a class of single-molecule-thick materials under investigation for possible use in new optoelectronic devices — ones that can manipulate the interactions of light and electricity. In these experiments, the MIT researchers were able to use the material to produce diodes, the basic building block of modern electronics.

Typically, diodes (which allow electrons to flow in only one direction) are made by “doping,” which is a process of injecting other atoms into the crystal structure of a host material. By using different materials for this irreversible process, it is possible to make either of the two basic kinds of semiconducting materials, p-type or n-type.

But with the new material, either p-type or n-type functions can be obtained just by bringing the vanishingly thin film into very close proximity with an adjacent metal electrode, and tuning the voltage in this electrode from positive to negative. That means the material can easily and instantly be switched from one type to the other, which is rarely the case with conventional semiconductors.

In their experiments, the MIT team produced a device with a sheet of WSe2 material that was electrically doped half n-type and half p-type, creating a working diode that has properties “very close to the ideal,” Jarillo-Herrero says.

By making diodes, it is possible to produce all three basic optoelectronic devices — photodetectors, photovoltaic cells, and LEDs; the MIT team has demonstrated all three, Jarillo-Herrero says. While these are proof-of-concept devices, and not designed for scaling up, the successful demonstration could point the way toward a wide range of potential uses, he says.

“It’s known how to make very large-area materials” of this type, Churchill says. While further work will be required, he says, “there’s no reason you wouldn’t be able to do it on an industrial scale.”

In principle, Jarillo-Herrero says, because this material can be engineered to produce different values of a key property called bandgap, it should be possible to make LEDs that produce any color — something that is difficult to do with conventional materials. And because the material is so thin, transparent, and lightweight, devices such as solar cells or displays could potentially be built into building or vehicle windows, or even incorporated into clothing, he says.

While selenium is not as abundant as silicon or other promising materials for electronics, the thinness of these sheets is a big advantage, Churchill points out: “It’s thousands or tens of thousands of times thinner” than conventional diode materials, “so you’d use thousands of times less material” to make devices of a given size.

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

Optoelectronic devices based on electrically tunable p–n diodes in a monolayer dichalcogenide by Britton W. H. Baugher, Hugh O. H. Churchill, Yafang Yang, & Pablo Jarillo-Herrero. Nature Nanotechnology (2014) doi:10.1038/nnano.2014.25 Published online 09 March 2014

This paper is behind a paywall.

These are very exciting, if not to say, electrifying times. (Couldn’t resist the wordplay.)

Artificial graphene?

Leave a reply

I’m not sure I ever want to hear the word ‘revolutionary’ or its cousin’ revolution’ in relationship to science and/or technology ever again and I don’t think anyone’s going to be paying attention to this heartfelt plea: please, please, please find another word for a couple of years at least.  That said, artificial graphene does sound exciting as it’s described in a Feb. 17, 2014 news item on Azonano,

A new breed of ultra thin super-material has the potential to cause a technological revolution. “Artificial graphene” should lead to faster, smaller and lighter electronic and optical devices of all kinds, including higher performance photovoltaic cells, lasers or LED lighting.

For the first time, scientists are able to produce and have analysed artificial graphene from traditional semiconductor materials. Such is the scientific importance of this breakthrough these findings were published recently in one of the world’s leading physics journals, Physical Review X. A researcher from the University of Luxembourg played an important role in this highly innovative work.

The University of Luxembourg Feb. 14, 2014 news release (also on EurekAlert), which originated the news item, describes both graphene and artificial graphene

Graphene (derived from graphite) is a one atom thick honeycomb lattice of carbon atoms. This strong, flexible, conducting and transparent material has huge scientific and technological potential. Only discovered in 2004, there is a major global push to understand its potential uses. Artificial graphene has the same honeycomb structure, but in this case, instead of carbon atoms, nanometer-thick semiconductor crystals are used. Changing the size, shape and chemical nature of the nano-crystals, makes it possible to tailor the material to each specific task.

…

University of Luxembourg researcher Dr. Efterpi Kalesaki from the Physics and Materials Science Research Unit is the first author of the article appearing in the Physical Review X . Dr. Kalesaki said: “these self‐assembled semi-conducting nano-crystals with a honeycomb structure are emerging as a new class of systems with great potential.” Prof Ludger Wirtz, head of the Theoretical Solid-State Physics group at the University of Luxembourg, added: “artificial graphene opens the door to a wide variety of materials with variable nano‐geometry and ‘tunable’ properties.”

I’m going to provide two links and two citations to the paper as its publishing journal is currently beta testing a new website and the paper is available on both,

Dirac Cones, Topological Edge States, and Nontrivial Flat Bands in Two-Dimensional Semiconductors with a Honeycomb Nanogeometry by E. Kalesaki, C. Delerue, C. Morais Smith, W. Beugeling, G. Allan, and D. Vanmaekelbergh. Phys. Rev. X 4, 011010 (2014) [12 pages] DOI: 10.1103/PhysRevX.4.011010

Dirac Cones, Topological Edge States, and Nontrivial Flat Bands in Two-Dimensional Semiconductors with a Honeycomb Nanogeometry by E. Kalesaki, C. Delerue, C. Morais Smith, W. Beugeling, G. Allan, and D. Vanmaekelbergh. Phys. Rev. X 4, 011010 – Published 30 January 2014 DOI: http://dx.doi.org/10.1103/PhysRevX.4.011010

The second link to the paper will take you to the journal’s beta site. I have to give the designers a big thumbs up on the new design. To contextualize my review, I’m not a fan of changing website designs as functionality is too often sacrificed for ‘good looks’. Sadly, I do have a bit more work cutting and pasting with the new version but I’m hugely relieved that I did not have to spend several minutes trying to find the information.

Both versions of the paper are open access.

Love, hate, and the whole damn thing affect batteries, semiconductors, and electronic memory

Leave a reply

A Jan. 24, 2013 news item on ScienceDaily features love triumphing over hate where tetracationic rings are concerned,

Northwestern University graduate student Jonathan Barnes had a hunch for creating an exotic new chemical compound, and his idea that the force of love is stronger than hate proved correct. He and his colleagues are the first to permanently interlock two identical tetracationic rings that normally are repelled by each other. Many experts had said it couldn’t be done.

On the surface, the rings hate each other because each carries four positive charges (making them tetracationic). But Barnes discovered by introducing radicals (unpaired electrons) onto the scene, the researchers could create a love-hate relationship in which love triumphs.

The Jan. 24, 2013 Northwestern University news release by Megan Fellman, which originated the news item, probes into the nature of the problem and its solution (Note: A link has been removed),

Unpaired electrons want to pair up and be stable, and it turns out the attraction of one ring’s single electrons to the other ring’s single electrons is stronger than the repelling forces.

The process links the rings not by a chemical bond but by a mechanical bond, which, once in place, cannot easily be torn asunder.

The study detailing this new class of stable organic radicals will be published Jan. 25 [2013] by the journal Science.

“It’s not that people have tried and failed to put these two rings together — they just didn’t think it was possible,” said Sir Fraser Stoddart, a senior author of the paper. “Now this molecule has been made. I cannot overemphasize Jonathan’s achievement — it is really outside the box. Now we are excited to see where this new chemistry leads us.”

…

The rings repel each other like the positive poles of two magnets. Barnes saw an opportunity where he thought he could tweak the chemistry by using radicals to overcome the hate between the two rings.

“We made these rings communicate and love each other under certain conditions, and once they were mechanically interlocked, the bond could not be broken,” Barnes said.

Barnes’ first strategy — adding electrons to temporarily reduce the charge and bring the two rings together — worked the first time he tried it. He, Stoddart and their colleagues started with a full ring and a half ring that they then closed up around the first ring (using some simple chemistry), creating the mechanical bond.

When the compound is oxidized and electrons lost, the strong positive forces come roaring back — “It’s hate on all the time,” Barnes said — but then it is too late for the rings to be parted. “That’s the beauty of this system,” he added.

Most organic radicals possess short lifetimes, but this unusual radical compound is stable in air and water. The compound tucks the electrons away inside the structure so they can’t react with anything in the environment. The tight mechanical bond endures despite the unfavorable electrostatic interactions.

The two interlocked rings house an immense amount of charge in a mere cubic nanometer of space. The compound, a homo[2]catenane, can adopt one of six oxidation states and can accept up to eight electrons in total.

“Anything that accepts this many electrons has possibilities for batteries,” Barnes said.

“Applications beckon,” Stoddart agreed. “Now we need to spend more time with materials scientists and people who make devices to see how this amazing compound can be used.”

For anyone interested in the details of the work, here’s a citation and link to the paper published in Science,

A Radically Configurable Six-State Compound by Jonathan C. Barnes, Albert C. Fahrenbach, Dennis Cao, Scott M. Dyar, Marco Frasconi, Marc A. Giesener, Diego Benítez, Ekaterina Tkatchouk, Oleksandr Chernyashevskyy, Weon Ho Shin, Hao Li, Srinivasan Sampath, Charlotte L. Stern, Amy A. Sarjeant, Karel J. Hartlieb, Zhichang Liu, Raanan Carmieli, Youssry Y. Botros, Jang Wook Choi, Alexandra M. Z. Slawin, John B. Ketterson, Michael R. Wasielewski, William A. Goddard III, J. Fraser Stoddart. Science 25 January 2013: Vol. 339 no. 6118 pp. 429-433 DOI: 10.1126/science.1228429

This is paper is behind a paywall.

Insomniac iron oxide (rust) electrons and environmentally friendly semiconductors

Leave a reply

The Sept. 7, 2012 news item by Lynn Yarris for physorg.com highlights some research on rust being conducted (pun intended) at Lawrence Berkeley National Laboratory (Berkeley Labs).

Rust – iron oxide – is a poor conductor of electricity, which is why an electronic device with a rusted battery usually won’t work. Despite this poor conductivity, an electron transferred to a particle of rust will use thermal energy to continually move or “hop” from one atom of iron to the next. Electron mobility in iron oxide can hold huge significance for a broad range of environment- and energy-related reactions, including reactions pertaining to uranium in groundwater and reactions pertaining to low-cost solar energy devices.  …

“We believe this work is the starting point for a new area of time-resolved geochemistry that seeks to understand chemical reaction mechanisms by making various kinds of movies that depict in real time how atoms and electrons move during reactions,” says Benjamin Gilbert, a geochemist with Berkeley Lab’s Earth Sciences Division and a co-founder of the Berkeley Nanogeoscience Center who led this research. “Using ultrafast pump-probe X-ray spectroscopy, we were able to measure the rates at which electrons are transported through spontaneous iron-to-iron hops in redox-active iron oxides. Our results showed that the rates depend on the structure of the iron oxide and confirmed that certain aspects of the current model of electron hopping in iron oxides are correct.”

The news item provides a wealth of detail about electron hopping and iron oxide but I was most intrigued by future applications,

Katz [Jordan Katz, the lead author, now with Denison University]  is excited about the application of these results to finding ways to use iron oxide for solar energy collection and conversion.

“Iron oxide is a semiconductor that is abundant, stable and environmentally friendly, and its properties are optimal for absorption of sunlight,” he says. “To use iron oxide for solar energy collection and conversion, however, it is critical to understand how electrons are transferred within the material, which when used in a conventional design is not highly conductive. Experiments such as this will help us to design new systems with novel nanostructured architectures that promote desired redox reactions, and suppress deleterious reactions in order to increase the efficiency of our device.”

I find rust quite attractive although, admittedly, very irritating at times. I have never before considered the possibility it might prove useful nor had I realized that it never rests (sleeps).

Thermal bottleneck opens up at US Dept.of Energy

Leave a reply

Heat is always an issue with electronics and as the devices get smaller and smaller, it becomes a more pressing problem. From the March 13, 2012 news item on Nanowerk,

For decades, engineers have sought to build more efficient electronic devices by reducing the size of their components. In the process of doing so, however, researchers have reached a “thermal bottleneck,” said Argonne [US Dept. of Energy, Argonne Laboratory] nanoscientist Anirudha Sumant.

In a thermal bottleneck, the excess heat generated in the device causes undesirable effects that affect its performance. “Unless we come-up with innovative ways to suck the heat off of our electronics, we are pretty much stuck with this bottleneck,” Sumant explained.

Diamond films have excited interest in the scientific community as a solution to thermal bottlenecks, from the news item,

The unusually attractive thermal properties of diamond thin films have led scientists to suggest using this material as a heat sink that could be integrated with a number of different semiconducting materials. However, the deposition temperatures for the diamond films typically exceed 800 degrees Celsius—roughly 1500 degrees Fahrenheit, which limits the feasibility of this approach.

Reducing the deposition temperature to 400 degrees Celsius would allow for integration of diamond materials with a whole range of semiconductor materials.  A new technique that allows just that thing has been developed (from the news item),

By using a new technique that altered the deposition process of the diamond films, Sumant and his colleagues at Argonne’s Center for Nanoscale Materials were able to both reduce the temperature to close to 400 degrees Celsius and to tune the thermal properties of the diamond films by controlling their grain size. This permitted the eventual combination of the diamond with two other important materials: graphene and gallium nitride.

According to Sumant, diamond has much better heat conduction properties than silicon or silicon oxide, which were traditionally used for fabrication of graphene devices. As a result of better heat removal, graphene devices fabricated on diamond can sustain much higher current densities.

In the other study, Sumant used the same technology to combine diamond thin films with gallium nitride, which is used extensively in high-power light emitting devices (LED). After depositing a 300 nm-thick diamond film on a gallium nitride substrate, Sumant and his colleagues noticed a considerable improvement in the thermal performance. Because a difference within an integrated circuit of just a few degrees can cause a noticeable change in performance, he called this result “remarkable.”

There are two published papers on the technique, one focusing on the graphene application and the other on the gallium nitride application. The first is in Nano Letters, 2012, 12 (3), pp 1603–1608, DOI: 10.1021/nl204545q, (“Graphene-on-Diamond Devices with Increased Current-Carrying Capacity: Carbon sp2-on-sp3Technology”, and the other is in Advanced Functional Materials, first published online: 1 FEB 2012, DOI: 10.1002/adfm.201102786,  (“Direct Low-Temperature Integration of Nanocrystalline Diamond with GaN Substrates for Improved Thermal Management of High-Power Electronics”).