Two-dimensional material shows promise for optoelectronics: LEDs, photovoltaic cells, and light detectors
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March 10, 2014 Source: Massachusetts Institute of Technology Summary: Team creates LEDs, photovoltaic cells, and light detectors using novel one-molecule-thick material. Researchers have used a novel material that's just a few atoms thick to create devices that can harness or emit light. This proof-of-concept could lead to ultrathin, lightweight, and flexible photovoltaic cells, light emitting diodes (LEDs), and other optoelectronic devices, they say.team of MIT researchers has used a novel material that's just a few atoms thick to create devices that can harness or emit light. This proof-of-concept could lead to ultrathin, lightweight, and flexible photovoltaic cells, light emitting diodes (LEDs), and other optoelectronic devices, they say. Their report is one of three papers by different groups describing similar results with this material, published in the March 9 issue of Nature Nanotechnology. The MIT research was carried out by Pablo Jarillo-Herrero, the Mitsui Career Development Associate Professor of Physics, graduate students Britton Baugher and Yafang Yang, and postdoc Hugh Churchill. 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. In addition to the diodes the team has produced, the team has also used the same methods to make p-type and n-type transistors and other electronic components, Jarillo-Herrero says. Such transistors could have a significant advantage in speed and power consumption because they are so thin, he says. Story Source: The above story is based on materials provided by Massachusetts Institute of Technology. The original article was written by David L. Chandler. Note: Materials may be edited for content and length. |
Mapping behavior of charges in correlated spin-orbit coupled materials: Electronic disruption prods Mott insulator's conversion to metallic state
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March 10, 2014 Source: Boston College Summary: Physicists have mapped the inner atomic workings of a compound within the mysterious class of materials known as spin-orbit Mott insulators. The findings confirm the properties that theorists predict could lead to discoveries in superconductivity, the topological phases of matter and new forms of magnetism.In a relatively recently discovered class of materials, known as spin-orbit Mott insulators, theorists have predicted the emergence of new properties at points just beyond the insulating state, when electronic manipulation can transform these compounds into conducting metals. A better understanding of electrons near this transition, theorists have predicted, could allow these new Mott insulators to pave the way to discoveries in superconductivity, new topological phases of matter, and new forms of unusual magnetism. What scientists have lacked is experimental evidence that reveals the microscopic mechanisms that actually drive one of these spin-orbit Mott insulators to become a metal. Now a team of physicists at Boston College report inNature Communications that they manipulated a compound of strontium, iridium and oxygen -- Sr3Ir207 -- with a substitution of ruthenium metal ions, successfully driving the material into the metallic regime, and mapping this previously uncharted transformation as it took place, giving scientists a unique view into the workings of these insulators. Spin-orbit Mott insulators are so named because of their complex electronic properties. Within these novel materials, there is a repulsive interaction between electrons that tends to drive the electrons to a stand still. This tendency is bolstered by the lowering of the electron's energy via a strong interaction between the electron's magnetic field and its orbital motion around the nucleus. This delicate interplay between repulsive action, known as Coulomb interaction, and the coupling between electrons' spin and orbital motion has allowed scientists to define this class of materials as spin-orbit Mott insulators. Boston College Assistant Professor of Physics Stephen D. Wilson said the team succeeded in driving the insulator-to-metal transformation by replacing 40 percent of the iridium ions with ruthenium, thereby creating a metal alloy. That event introduced charge carriers, which have proven successful in destabilizing the so-called Mott phase in the transformation of compounds in this class of insulators. Scanning tunneling microscopy revealed ruthenium effectively created features within the compound that resembled minute metallic puddles, said Wilson, one of the lead researchers on the project. As the amount of additional ruthenium was increased, the puddles began to "percolate," coalescing to form a metal across which charges freely flow, he added. "The addition of ruthenium introduces charge carriers, but at a low ratio of ruthenium to iridium they simply stay put in these little metallic puddles, which are symptoms of strong correlated electrons," Wilson said. "These electrons are stable and wouldn't move much. But when we stepped up the disruption by increasing the amount of ruthenium, the puddles moved together and achieved a metallic state." The behavior in this particular compound parallels what researchers have seen in Mott insulators that play host to such phenomenon as high temperature superconductivity, said Wilson, who will discuss his research at the upcoming annual meeting of the American Physical Society. By pinpointing exactly where this transformation takes place, the team's findings should help to lay the groundwork in the scientific search for new electronic phases within spin-orbit Mott insulators, said Wilson, who co-authored the report with his Boston College Department of Physics colleagues Professor Vidya Madhavan, Professor Ziqiang Wang, and Assoc. Prof. Fr. Cyril P. Opeil, SJ. Story Source: The above story is based on materials provided by Boston College. Note: Materials may be edited for content and length. |
Synthetic biologists shine light on genetic circuit analysis
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March 10, 2014 Source: Rice University Summary: In a significant advance for the growing field of synthetic biology, bioengineers have created a toolkit of genes and hardware that uses colored lights and engineered bacteria to bring both mathematical predictability and cut-and-paste simplicity to the world of genetic circuit design.In a significant advance for the growing field of synthetic biology, Rice University bioengineers have created a toolkit of genes and hardware that uses colored lights and engineered bacteria to bring both mathematical predictability and cut-and-paste simplicity to the world of genetic circuit design. "Life is controlled by DNA-based circuits, and these are similar to the circuits found in electronic devices like smartphones and computers," said Rice bioengineer Jeffrey Tabor, the lead researcher on the project. "A major difference is that electrical engineers measure the signals flowing into and out of electronic circuits as voltage, whereas bioengineers measure genetic circuit signals as genes turning on and off." In a new paper appearing online today in the journal Nature Methods, Tabor and colleagues, including graduate student and lead author Evan Olson, describe a new, ultra high-precision method for creating and measuring gene expression signals in bacteria by combining light-sensing proteins from photosynthetic algae with a simple array of red and green LED lights and standard fluorescent reporter genes. By varying the timing and intensity of the lights, the researchers were able to control exactly when and how much different genes were expressed. "Light provides us a powerful new method for reliably measuring genetic circuit activity," said Tabor, an assistant professor of bioengineering who also teaches in Rice's Ph.D. program in systems, synthetic and physical biology. "Our work was inspired by the methods that are used to study electronic circuits. Electrical engineers have tools like oscilloscopes and function generators that allow them to measure how voltage signals flow through electrical circuits. Those measurements are essential for making multiple circuits work together properly, so that more complex devices can be built. We have used our light-based tools as a biological function generator and oscilloscope in order to similarly analyze genetic circuits." Electronic circuits -- like those in computers, smartphones and other devices -- are made up of components like transistors, capacitors and diodes that are connected with wires. As information -- in the form of voltage -- flows through the circuit, the components act upon it. By putting the correct components in the correct order, engineers can build circuits that perform computations and carry out complex information processing. Genetic circuits also process information. Their components are segments of DNA that control whether or not a gene is expressed. Gene expression is the process in which DNA is read and converted to produce a product -- such as a protein -- that serves a particular purpose in the cell. If a gene is not "expressed," it is turned off, and its product is not produced. The bacteria used in Tabor's study have about 4,000 genes, while humans have about 20,000. The processes of life are coordinated by different combinations and timings of genes turning on and off. Each component of a genetic circuit acts on the input it receives -- which may be one or more gene-expression products from other components -- and produces its own gene-expression product as an output. By linking the right genetic components together, synthetic biologists like Tabor and his students construct genetic circuits that program cells to carry out complex functions, such as counting, having memory, growing into tissues, or diagnosing the signatures of disease in the body. For example, in previous research, Tabor and colleagues designed genetic circuits that allowed bacteria to change their color based on incoming light. The technique allowed the team to create bacterial colonies in Petri dishes that could behave like photo paper and reproduce black and white images. In the new study, Tabor and Olson realized that light could be used to create time-varying gene-expression signals that rise and fall, similar to those used in electronic engineering. "In electronics, two of the key tools are function generators and oscilloscopes," said Olson, a graduate student in applied physics. "The function generator sends a known signal into the circuit being characterized. The oscilloscope is a device with a screen that the engineer uses to see the circuit output. By twisting the knobs on the function generator and viewing the corresponding output on the oscilloscope, the engineer can infer what various parts of the circuit are doing. "The system of fluorescent reporter genes is our version of the oscilloscope," he said. "It lets us view both the circuit's input and output, and because it uses light to report on what's happening, it provides a very clean signal." With their "bioscilloscope" in hand, the team needed a corresponding function generator. Olson, the lead author of the Nature Methods paper, put his electronics skills to work in late 2011 and invented the "light tube array," a programmable, eight-by-eight set of LED lights that will fit under a standard 64-well tray of test tubes. With the addition of some light-blocking foam around each test tube, the team had a way to send individually programmed light signals into each test tube in the array. By varying the signals and measuring the corresponding outputs with their bioscilliscope, the team was able to determine exactly how its test circuit performed. "The precision of light allows us to create exceptionally clean gene expression signals, which we can use to extract far more information about gene circuits than was possible before," Tabor said. "We found there was a seven-minute delay between the gene expression going into and coming out of the genetic circuit," Olson said. "We also found we could program the circuit to follow specific patterns. For example, to rise by a specific amount over a set amount of time, stop and stay at another level for a predetermined length of time and then drop down to a third level for another interval of time." Olson said the light tube array and bioscilliscope will be useful tools for biologists to probe how nature's cells work, as well as for synthetic biologists who want to build and analyze their own circuits and networks. "It's really about having a clean input signal, a clean output signal and the tools required to measure them," Olson said. Tabor added, "You just never see data this clean in biology. It's remarkable." The research was supported by the National Science Foundation, the Office of Naval Research and NASA. Study co-authors include bioengineering graduate students Lucas Hartsough and Brian Landry and former undergraduate Raghav Shroff. Video: http://www.youtube.com/watch?v=74m-wJfaFHA Story Source: The above story is based on materials provided by Rice University. The original article was written by Jade Boyd. Note: Materials may be edited for content and length. |