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Flexible electronics advance boosts performance, manufacturing
December 14, 2006Flexible electronics made with organic, or carbon-based, transistors could enable technologies such as low-cost sensors on product packaging and ''electronic paper'' displays as thin and floppy as a placemat. But the best mass-producible organic transistors so far have only milquetoast performance, and products using them have yet to come to market. In a study published in the Dec. 14 issue of the journal Nature, researchers at Stanford and the University of California-Los Angeles point the way toward manufacturing truly useful flexible electronics with high-performance organic transistors.
''This work demonstrates for the first time that organic single crystals can be patterned over a large area without the need to laboriously handpick and fabricate transistors one at a time,'' says Stanford chemical engineering Associate Professor Zhenan Bao.
The study's lead author is Alejandro Briseno, who was a master's student at UCLA performing part of this research at Stanford. He is now a doctoral student at the University of Washington. The study's other authors are Stefan C. B. Mannsfeld, Mang M. Ling, Shuhong Liu, Colin Reese, Mark E. Roberts and Bao at Stanford, and Ricky J. Tseng, Yang Yang and Fred Wudl at UCLA.
Single-crystal organic transistors are fast-engineers say they have a high ''charge carrier mobility.'' This means that when they are 'switched on,'' electrical current can move through the crystal very quickly. Organic thin-film transistors, carbon-based versions of the kind of transistor commonly found in flat panel computer monitors, have only about a third the charge mobility. Researchers have nevertheless favored the thin-film transistors because they could be manufactured en masse, while single-crystal devices always had to be made by manual selection and placing of individual crystals.
Stamping feat
The trick to being able to manufacture-rather than handcraft-large arrays of single-crystal transistors was to devise a method for printing patterns of transistors on surfaces such as silicon wafers and flexible plastic. The first step is to put electrodes on these surfaces wherever a transistor is desired. Then the researchers make a stamp with the desired pattern out of a polymer called polydimethylsiloxane. After coating the stamp with a crystal growth agent called octadecyltriethoxysilane (OTS) and pressing it onto the surface, the researchers can then introduce a vapor of the organic crystal material onto the OTS-patterned surfaces. The vapor will condense and grow semiconducting organic single crystals only where the agent lies. With the crystals bridging the electrodes, transistors are formed.
In the experiments reported in the paper, the team made arrays out of several different crystal materials including rubrene (it makes the fastest transistors) and even ''buckyballs,'' soccer balls made out of 60 carbon atoms each. In some cases, the researchers were able to make simple grid patterns with crystals in areas as small as 8 hundred-millionths of a square inch (49 square microns). Although not nearly as packed as modern silicon processors or memory chips, with up to 13 million crystals per square inch, the team's patterns could still yield richly functioning circuits and high-resolution displays, Bao says.
In other experiments reported in the paper, the researchers showed that the transistor arrays printed on plastic continue to work well even after significant bending, a key finding for anything that will be used in flexible electronics.
Several further advances will be necessary before the team's progress translates into commercial technologies. Among them is controlling how the crystals line up across the electrodes when the crystals form. Another key step will be ensuring better electrical contact between crystals and electrodes.
Still, the results show that organic single-crystal transistors are now feasible for making a variety of useful devices. ''Until now, the possibility of fabricating hundreds of [organic single-crystal] devices on a single platform [had] been unheard of and essentially impossible from previous methods,'' says lead author Briseno. ''All of this can now be accomplished on an area the size of a human fingernail.''
Stanford University
Single-crystal organic transistors are fast-engineers say they have a high ''charge carrier mobility.'' This means that when they are 'switched on,'' electrical current can move through the crystal very quickly. Organic thin-film transistors, carbon-based versions of the kind of transistor commonly found in flat panel computer monitors, have only about a third the charge mobility. Researchers have nevertheless favored the thin-film transistors because they could be manufactured en masse, while single-crystal devices always had to be made by manual selection and placing of individual crystals.
Stamping feat
The trick to being able to manufacture-rather than handcraft-large arrays of single-crystal transistors was to devise a method for printing patterns of transistors on surfaces such as silicon wafers and flexible plastic. The first step is to put electrodes on these surfaces wherever a transistor is desired. Then the researchers make a stamp with the desired pattern out of a polymer called polydimethylsiloxane. After coating the stamp with a crystal growth agent called octadecyltriethoxysilane (OTS) and pressing it onto the surface, the researchers can then introduce a vapor of the organic crystal material onto the OTS-patterned surfaces. The vapor will condense and grow semiconducting organic single crystals only where the agent lies. With the crystals bridging the electrodes, transistors are formed.
In the experiments reported in the paper, the team made arrays out of several different crystal materials including rubrene (it makes the fastest transistors) and even ''buckyballs,'' soccer balls made out of 60 carbon atoms each. In some cases, the researchers were able to make simple grid patterns with crystals in areas as small as 8 hundred-millionths of a square inch (49 square microns). Although not nearly as packed as modern silicon processors or memory chips, with up to 13 million crystals per square inch, the team's patterns could still yield richly functioning circuits and high-resolution displays, Bao says.
In other experiments reported in the paper, the researchers showed that the transistor arrays printed on plastic continue to work well even after significant bending, a key finding for anything that will be used in flexible electronics.
Several further advances will be necessary before the team's progress translates into commercial technologies. Among them is controlling how the crystals line up across the electrodes when the crystals form. Another key step will be ensuring better electrical contact between crystals and electrodes.
Still, the results show that organic single-crystal transistors are now feasible for making a variety of useful devices. ''Until now, the possibility of fabricating hundreds of [organic single-crystal] devices on a single platform [had] been unheard of and essentially impossible from previous methods,'' says lead author Briseno. ''All of this can now be accomplished on an area the size of a human fingernail.''
Stanford University
Transistor Current Events and Transistor News RSSSmall ... smaller ... smallest? ASU researchers create molecular diode
Recently, at Arizona State University's Biodesign Institute, N.J. Tao and collaborators have found a way to make a key electrical component on a phenomenally tiny scale. Their single-molecule diode is described in this week's online edition of Nature Chemistry.
Discovery Brings New Type of Fast Computers Closer to Reality
Physicists at UC San Diego have successfully created speedy integrated circuits with particles called "excitons" that operate at commercially cold temperatures, bringing the possibility of a new type of extremely fast computer based on excitons closer to reality.
Carbon nanotubes could make efficient solar cells
Using a carbon nanotube instead of traditional silicon, Cornell researchers have created the basic elements of a solar cell that hopefully will lead to much more efficient ways of converting light to electricity than now used in calculators and on rooftops.
Organic electronics a two-way street, thanks to new plastic semiconductor
Plastic that conducts electricity holds promise for cheaper, thinner and more flexible electronics. This technology is already available in some gadgets -- the new Sony walkman that was introduced earlier this summer and the Microsoft Zune HD music player released last week both incorporate organic light-emitting electronic displays.
New material for nanoscale-computer chips
New data from Chinese-Danish collaboration shows that organic nanoscale wires could be an alternative to silicon in computer chips. The discovery has just been published in the respected scientific journal, Advanced Materials.
Scientists break light modulation speed record -- twice
Researchers have constructed a light-emitting transistor that has set a new record with a signal-processing modulation speed of 4.3 gigahertz, breaking the previous record of 1.7 gigahertz held by a light-emitting diode.
Tunable semiconductors possible with hot new material called graphene
Today's transistors and light emitting diodes (LED) are based on silicon and gallium arsenide semiconductors, which have fixed electronic and optical properties.
Boston University biomedical engineers teach bacteria to count
Biomedical engineers at Boston University have taught bacteria how to count. Professor James J. Collins and colleagues have wired a new sequence of genes that allow the microbes to count discrete events, opening the door for a host of potential applications, which could include drug delivery and sensing environmental hazards.
NIST engineers discover fundamental flaw in transistor noise theory
Chip manufacturers beware: There's a newfound flaw in our understanding of transistor noise, a phenomenon affecting the electronic on-off switch that makes computer circuits possible.
Team of researchers achieves major step toward faster chips
New research findings could lead to faster, smaller and more versatile computer chips.
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Transistor by 311 With Sublime and Sugar Ray having made whitewashed reggae a hot commodity on the pop charts, it makes sense that the prime movers of the genre are making a headlong comeback into the fray. With Transistor, 311 goes for the jugular, cramming the disc with over 20 songs, and just as many angles on its melange of rock, hip-hop and Caribbean musical styles. There are hyper rap-metal rehashes of the hit "Down" ("Tune In," "Starshines," and "Borders"), chunky guitar tracks ("Beautiful Disaster"), and lots of frivolous reggae-lite songs ("Light Years," "Stealing Happy Hours"). --Aidin Vaziri |
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