Brainy Quote of the Day

Friday, July 22, 2016

Semiconductor Defects...

Configuration coordinate diagram, showing important energies and optical transitions. For this example, Etherm gives the acceptor level relative to the CBM.

Citation: J. Appl. Phys. 119, 181101 (2016);
Topics: Education, Nanotechnology, Semiconductor Technology, STEM

Point defects affect or even completely determine physical and chemical properties of semiconductors. Characterization of point defects based on experimental techniques alone is often inconclusive. In such cases, the combination of experiment and theory is crucial to gain understanding of the system studied. In this tutorial, we explain how and when such comparison provides new understanding of the defect physics. More specifically, we focus on processes that can be analyzed or understood in terms of configuration coordinate diagrams of defects in their different charge states. These processes include light absorption, luminescence, and nonradiative capture of charge carriers. Recent theoretical developments to describe these processes are reviewed.

Every material contains defects; perfect materials simply do not exist. While it may cost energy to create a defect, configurational entropy renders it favorable to incorporate a certain concentration of defects, since this lowers the free energy of the system.1 Therefore, even in equilibrium, we can expect defects to be present; kinetic limitations sometimes lead to formation of additional defects. Note that all of these considerations also apply to impurities that are unintentionally present in the growth or processing environment. Of course, impurities are often intentionally introduced to tailor the properties of materials. Doping of semiconductors with acceptors and donors is essential for electronic and optoelectronic applications. In the following, we will use the word “defect” as a generic term to cover both intrinsic defects (vacancies, self-interstitials, and antisites) and impurities.

Since defects are unavoidable, we must consider the effects they have on the properties of materials. These effects can be considerable, to the point of determining the functionality of the material, as in p- or n-type doping. Point defects play a key role in diffusion: virtually all diffusion processes are assisted by point defects. Defects are often responsible for degradation of a device. Even in the absence of degradation, defects can limit the performance of a device. Compensation by native point defects can decrease the level of doping that can be achieved. Defects with energy levels within the band gap can act as recombination centers, impeding carrier collection in a solar cell or light emission from a light-emitting diode. Sometimes, these effects can be used to advantage: luminescence centers in wide-band-gap materials can be used to emit light at specified wavelengths; or single-spin centers (such as the nitrogen–vacancy (NV) center in diamond) can act as an artificial atom and serve as a qubit in a quantum information system.2,3 Finally, sometimes, one deliberately wants to grow materials with many defects. Examples are materials for ultrafast optoelectronic switches or semiconductors used to optically generate THz pulses, where defect densities should be large enough so that carrier lifetimes are as short as a few picoseconds.4

Journal of Applied Physics:
Tutorial: Defects in semiconductors—Combining experiment and theory
Audrius Alkauskas1, Matthew D. McCluskey2 and Chris G. Van de Walle3,a)

Thursday, July 21, 2016

Simpler, Faster, Cheaper...

To prevent cores of single-wall carbon nanotubes from filling with water or other detrimental substances, the NIST researchers advise intentionally prefilling them with a desired chemical of known properties. Taking this step before separating and dispersing the materials, usually done in water, yields a consistently uniform collection of nanotubes, especially important for optical applications.
Credit: Fagan/NIST
View hi-resolution image
Topics: Carbon Nanotubes, Electrical Engineering, Nanotechnology, Semiconductor Technology

Just as many of us might be resigned to clogged salt shakers or rush-hour traffic, those working to exploit the special properties of carbon nanotubes have typically shrugged their shoulders when these tiniest of cylinders fill with water during processing. But for nanotube practitioners who have reached their Popeye threshold and “can’t stands no more,” the National Institute of Standards and Technology (NIST) has devised a cheap, quick and effective strategy that reliably enhances the quality and consistency of the materials—important for using them effectively in applications such as new computing technologies.

To prevent filling of the cores of single-wall carbon nanotubes with water or other detrimental substances, the NIST researchers advise intentionally prefilling them with a desired chemical of known properties. Taking this step before separating and dispersing the materials, usually done in water, yields a consistently uniform collection of nanotubes. In quantity and quality, the results are superior to water-filled nanotubes, especially for optical applications such as sensors and photodetectors.

The approach opens a straightforward route for engineering the properties of single-wall carbon nanotubes—rolled up sheets of carbon atoms arranged like chicken wire or honey combs—with improved or new properties.

“This approach is so easy, inexpensive and broadly useful that I can’t think of a reason not to use it,” said NIST chemical engineer Jeffrey Fagan.

Simpler, Faster and Cheaper: A Full-filling Approach to Making Carbon Nanotubes of Consistent Quality, Mark Bello

Wednesday, July 20, 2016

Wearable Photovoltaics...

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
Topics: Consumer Electronics, Electrical Engineering, Materials Science, Photovoltaics, Solar Power

WASHINGTON, D.C., June 20, 2016 -- 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. The researchers report the results in the journal Applied Physics Letters, from AIP Publishing.

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.

AIP: Ultra-thin Solar Cells Can Easily Bend Around a Pencil, Catherine Meyers

Tuesday, July 19, 2016

Exciton Condensate...

Figure 1: A Coulomb drag experiment measures the interactions between charges in two closely spaced layers. The experiment entails running a current through the “drive” layer (here, the top layer) and measuring the resulting flow of charge in the “drag” layer (the bottom layer). The panels indicate three (of many) possible drag scenarios associated with two sheets of bilayer graphene (grey). At left, exciton pairs form between holes (red) in the drive layer and electrons (green) in the drag layer, giving rise to a large drag effect. At center, holes drag electrons in the same direction (positive drag) because of momentum transfer between the charges in different sheets. At right, holes drag electrons in the opposite direction (negative drag), an observation in bilayer graphene that is yet to be explained.

Topics: Atomic Physics, Bose-Einstein Condensate, Condensed Matter Physics, Quantum Mechanics

Superfluids (fluids with zero viscosity) and superconductors (materials with zero resistance) have a common ingredient: bosons. These particles obey Bose-Einstein statistics, allowing a collection of them at low temperatures to collapse into a single quantum-mechanical state, or Bose-Einstein condensate. Bosons in superconductors consist of two paired electrons, but the pairing is weak and only occurs at low temperatures. In a quest to build devices that carry electricity with low dissipation at higher temperatures, researchers have therefore explored the possibility of engineering electrical condensates [1] out of strongly bound pairs of electrons and holes, or excitons. Now, two research groups have, independently, fabricated and characterized a graphene-based device that is thought to be a promising platform for realizing an exciton condensate [2, 3]. Neither group has yet found evidence for such a condensate—the ultimate goal of such experiments. But their measurements lay the groundwork for future searches.

Excitons form in semiconductors and insulators. The binding energy between the exciton’s electron and hole can be quite strong, greatly exceeding their thermal energy at room temperature. Unfortunately, excitons recombine quickly, too fast to allow a condensate to form. Although excitons coupled to light confined within a cavity can form hybrid particles (exciton-polaritons) that do live long enough to condense [4], such condensates require a continuous input of light.

APS Viewpoint: Chasing the Exciton Condensate
Michael S. Fuhrer, Alex R. Hamilton

Monday, July 18, 2016

A New Migration...

Topics: Climate Change, History, Octavia Butler, Politics, Science Fiction

It's been a breathtaking seven days that puts into context what a president has to do: gather information, calm fears for now the second police shooting - the first generated by Alton Sterling and Philando Castile's executions; a terrorist attack by truck in Nice, France in the backdrop of two political conventions poised to pick this president's successor in a volatile world. This election will be a reflection of our fears and our character, beyond our own self-deluding mythology, who we really are.

Some context: "The Great Migration" was of approximately six million African Americans from the rural south to northern cities for opportunities in the budding industrial revolution and (hopefully) AWAY from the De Jure and De Facto segregation, Jim Crow and racial terrorism they were all fleeing. Notable ex-patriots: The ancestors of First Lady Michelle Obama (documented in "The Warmth of Other Suns" by Isabel Wilkerson); James Lee Boggs, deceased husband of Grace Lee Boggs and author of "The American Revolution: Pages from a Negro Worker's Notebook," in which he predicted the impacts of automation and what he referred to at the time "cybernation" that we recognize as the advent of computers in what were once jobs done by humans and less robotics or apps.

Note the plot synopsis from "Parable of the Sower" written by Octavia Butler in 1993:

Set in a future where government has all but collapsed, Parable of the Sower centers on a young woman named Lauren Olamina who possesses what Butler dubbed hyperempathy – the ability to feel the perceived pain and other sensations of others – who develops a benign philosophical and religious system during her childhood in the remnants of a gated community in Los Angeles. Civil society has reverted to relative anarchy due to resource scarcity and poverty. When the community's security is compromised, her home is destroyed and her family murdered. She travels north with some survivors to try to start a community where her religion, called Earthseed, can grow. Wikipedia

Now look at the plot of the US as it relates to a heating climate (I'm sure the same applies overseas as well):

The previous migration was a drive for opportunity and fairness; the next one will be for the first level of Maslow's hierarchy: comfort. The strain on resources will split humanity along tribal and factional lines like never before. Those who "have" will hoard and build up walled cities; defended castles to maintain their bounty from the hungered herds of "have not's." For those youth that will still be around (I'm not anticipating I will), as 2050 approaches they will see how far we've actually migrated...from the caves.

Scientific American: U.S Cities Are Getting Dangerously Hot [Graphic]
A dramatic rise in “danger days” is underway, Mark Fischetti

Friday, July 15, 2016



Topics: History, Physics, Philosophy, Science

Scientism: It's an old word, so old it has to be added to your online dictionary almost everywhere you might type it. It also at first glance sounds reasonable, and in my own oft-used urban descriptor: "science-y."

This description at the beginning of the article from The American Association for the Advancement of Science is instructive and concise:

Historian Richard G. Olson defines scientism as “efforts to extend scientific ideas, methods, practices, and attitudes to matters of human social and political concern.” (1) But this formulation is so broad as to render it virtually useless. Philosopher Tom Sorell offers a more precise definition: “Scientism is a matter of putting too high a value on natural science in comparison with other branches of learning or culture.” (2) MIT physicist Ian Hutchinson offers a closely related version, but more extreme: “Science, modeled on the natural sciences, is the only source of real knowledge.” (3) The latter two definitions are far more precise and will better help us evaluate scientism’s merit.

A History of Scientism

The Scientific Revolution

The roots of scientism extend as far back as early 17th century Europe, an era that came to be known as the Scientific Revolution. Up to that point, most scholars had been highly deferent to intellectual tradition, largely a combination of Judeo-Christian scripture and ancient Greek philosophy. But a torrent of new learning during the late Renaissance began to challenge the authority of the ancients, and long-established intellectual foundations began to crack. The Englishman Francis Bacon, the Frenchman Rene Descartes, and the Italian Galileo Galilei spearheaded an international movement proclaiming a new foundation for learning, one that involved careful scrutiny of nature instead of analysis of ancient texts.

Descartes and Bacon used particularly strong rhetoric to carve out space for their new methods. They claimed that by learning how the physical world worked, we could become “masters and possessors of nature.”(4) In doing so, humans could overcome hunger through innovations in agriculture, eliminate disease through medical research, and dramatically improve overall quality of life through technology and industry. Ultimately, science would save humans from unnecessary suffering and their self-destructive tendencies. And it promised to achieve these goals in this world, not the afterlife. It was a bold, prophetic vision.

From the seeds of this formed the basis for utopia: H.G. Wells was the first science fiction writer to tackle it; Utopia was written I think before the genre was invented by Mary Shelly ("Frankenstein," fairly dystopian to say the least). Star Trek and the proclivities of Gene Roddenberry (an atheist) embodied it in Mr. Spock and the planet Vulcan: human contact with an entire species of beings supposedly led fully by logic and reason. The Earth - post Armageddon - surviving its own hubris and learning to cooperate beyond borders, languages, religions and the previous things that separated the human tribe and made "Mutually Assured Destruction" (M.A.D.) possible in a hopefully fictional Trek timeline.

New Thought: It apparently started in the 19th century originating from Phineas Parkhurst Quimby - imitated ad nauseum by opportunistic others, branching into several realms via modern communications (radio, television, Internet) from faith healers, prosperity gospel, pseudoscience and general quackery. As the link indicates, the enduring appeal is humans feeling empowered in an unpredictable and often cruel cosmos. Many traditional, non denominational, modern and/or New Age gurus have cashed in on this uncertainty quite lucratively. You can see its sustained and prosperous modern incarnations with a simple exercise of channel-surfing.

I would say scientism in its modern expression would be (a representative off-the-top-of-my-head trio) Richard Dawkins, Sam Harris and Neil deGrasse Tyson. They ARE scientists, but have made a lucrative living speaking and writing about the virtues of science; how if we all thought more rationally we wouldn't have to wait for heaven on Earth: we could design it ourselves. Sociologist Jeffrey Guhin in New Scientist challenged the idea that Tyson forwarded of a nation totally run by logic, reason and science (sounds familiar? \\//_). He posits the very simple question that gives one pause: what does "rational" mean? Things that "sounded" rational and science-y like Eugenics was used for wholesale discriminatory behavior by Hitler's Third Reich (you know: concentration camps and gas chambers). If we just "follow-the-data" of standardized test scores, then the often debunked thesis behind "The Bell Curve" sounds rational, because one does not have to take into account generations of poverty vis-à-vis slavery; sharecropping (a word that is a contradiction in terms on its own); racial terrorism; Jim Crow; De Facto and De Jure segregation; bank red lining; differentiated education (for me, torn and outdated books supplemented by xeroxed copies my teachers purchased at their own expense) and no career opportunities to climb the economic ladder to a better life. The better correlation is wealth of parents and guardians to academic achievement, most of which happens to be the dominant culture.

The National Science Foundation (I think) was right to commission a study on Science Literacy and the public good, as more than anything that will determine the outcome of nations as we share and contest resources on this Earth, or prepare as a species to inhabit other worlds to extend us beyond the fate of the dinosaurs.

The broad brush of "all we need is science" is the proportional equivalent to its antithesis: "all we need is (fill in the blank): Buddha, Chia Pets, Gaia, Jesus, Odin, Mood Rings, Mood Rocks, Positive Thinking, Possibility Thinking, Prayer Cloths, Quantum Physics (since we travel < c, highly doubtful), Holy Water; Thor."

What we could all use is a return to actually teaching civics to our respective populations, and leave proselytizing to family units. Methinks both camps need to step back and consider a true "separation of church and state" (& science). It will benefit both camps better to stay in their lanes, without either one harmfully denigrating the other. We need to survive together as a species, or in the words of Dr. King "perish together as fools." The Earth does not need us to circumnavigate the sun, and the universe if we were so foolish wouldn't blink at our hubris...or departure.

Thursday, July 14, 2016

Neural Networks and H2O...

Schematic showing water molecules in the denser water phase (left) and the ice phase. (Courtesy: Tobias Morawietz)
Topics: Artificial Intelligence, Computer Engineering, Computer Science

Artificial neural networks have been used to simulate interactions between water molecules and provide important clues about the remarkable properties of this live-giving substance. The study has been carried out by physicists in Germany and Austria, who used the networks to perform simulations 100,000 times faster than possible with conventional computers. Their work offers explanations for two key properties of water – its maximum density at 4 °C and its melting temperature – but the technique could be expanded to include other aspects of this ubiquitous substance.

Physicists and chemists have long found water's unusual properties difficult to explain. Its density, for example, peaks at around 4 °C, which means that frozen water floats on liquid water – a property that is vital for aquatic creatures that have to survive in cold climates. Massive computer simulations have shown that hydrogen bonds between water molecules play a key role, but these simulations do not tell the whole story.

One key challenge is understanding the role of van der Waals interactions, which arise from quantum fluctuations in the electrical polarizations of water and other molecules. Van der Waals interactions have traditionally been hard to include in computer simulations, but Tobias Morawietz and colleagues at the Ruhr-Universität Bochum and the University of Vienna have now used artificial neural networks (ANNs) to model them in water. ANNs are computer algorithms that "learn" how to perform a specific task by being fed data related to that task. An ANN could, for example, learn how to recognize an individual's face by being fed photographs of people and being told which images are of the target person.

Physics World: Neural networks provide deep insights into the mysteries of water
Hamish Johnston