Residential and commercial buildings account for about 40% of energy use and about 30% of energy-related carbon emissions in the United States1. To decrease this energy demand, materials are needed that help to regulate the heating and lighting requirements of buildings by responding to environmental changes. In particular, electrochromic window materials, which change colour and/or transparency when subjected to an electric field, could significantly reduce energy consumption in buildings.
Brian A. Korgel is in the Department of Chemical Engineering, Center for Nano- and Molecular Science and Technology, Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, USA.
Adapting conventional printing technology, researchers have developed a way to rapidly and inexpensively make uniform arrays of high-performing transistors out of carbon nanotubes on flexible plastic sheets. The process could eventually lead to a tool for manufacturing large-area, low-power sensor arrays and displays.
Thin-film transistors made from carbon nanotubes are attractive for these types of applications because they are robust and mechanically flexible, and they can be much more energy efficient than silicon transistors. They can also be applied as a solution, or “ink,” and can be processed at relatively low temperatures, making them compatible with plastic substrates.
Researchers using techniques more like those used in conventional integrated-circuit fabrication have previously made prototypes of nanotube transistor arrays on flexible plastic, including responsive sensor networks called “electronic skin” (see “Electronic ‘Skin’ Emits Light When Pressed”). Carbon nanotube transistors have also been printed before, but their performance has lagged for one reason or another, says Ali Javey, a professor of electrical engineering and computer science at the University of California, Berkeley. Javey led the new work, which he says is the first demonstration of fully printed carbon nanotube transistor arrays that also consistently show very high performance—an important step toward roll-to-roll manufacturing of such devices.
In a paper published earlier this month in Nano Letters, Javey and his colleagues report using their multistep process to produce transistor arrays with much higher electron mobility than printed carbon nanotube transistors demonstrated previously.
Higher mobility makes the transistors more efficient and is crucial for displays, says Javey, because it means less voltage is required to supply the current necessary for running organic light-emitting diodes (OLEDs). The method his group has demonstrated “holds a lot of promise for very large-area displays—covering an entire wall with a display or a sensor array, for example,” he says. “If you are dealing with such large areas, in terms of manufacturing cost it’s just not feasible to use conventional-based processing.”
Javey uses a lab version of a well-known manufacturing process called gravure printing. In his setup, the plastic substrate is mounted to a cylindrical drum, which rolls it over a flat surface that serves as a mask, patterned with holes filled with inks made of the desired materials. In a roll-to-roll setup, a second roll would serve as the mask.
For now, Javey’s group will focus on refining this gravure method to further improve transistor performance and make the arrays more uniform. Eventually, the researchers hope to print more complicated integrated circuits that would include sensors and display components.
This article is written by Mike Orcutt Ref: http://www.technologyreview.com/news/518126/a-printing-process-to-make-wall-sized-displays/
Doping cadmium telluride with copper increases solar cell efficiency from 8 to 11.5 percent
Flexible thin film solar cells that can be produced by roll-to-roll manufacturing are a highly promising route to cheap solar electricity.
Now scientists from research institute Empa, the Swiss Federal Laboratories for Materials Science and Technology, have made significant progress in paving the way for the industrialisation of flexible, light-weight and low-cost cadmium telluride (CdTe) solar cells on metal foils.
They succeeded in increasing their efficiency from below eight to 11.5 percent by doping the cells with copper, as reported in the current issue of Nature Communications.
(a) Scanning electron micrograph and schematic of the cross section of a CdTe solar cell in the substrate configuration which allows the use of opaque substrates like metal foils. (b) Photo of CdTe solar cell
In order to make solar energy widely affordable scientists and engineers all over the world are looking for low-cost production technologies. Flexible thin film solar cells have a huge potential in this regard because they require only a small amount of material and can be manufactured in large quantities by roll-to-roll processing. One such technology relies on CdTe to convert sunlight into electricity.
Scientists have made a carbon nanotube pencil that can draw gas sensors straight on to paper. This cheap and extremely quick prototyping method could spur huge advances in gas sensors, both for public health and in something as simple as toilet ventilation. The stability of the solid pencil composite also means it could be suitable for 3D printing, making gas sensors on demand and to the customer’s specification.
Volatile organic compounds (VOCs) are a huge health concern in public spaces and in the workplace: some VOCs contribute to photochemical smog and factory operators must be extremely careful that employees are not exposed to health-damaging vapours. For this reason gas sensors are increasingly in demand to monitor such molecules. According to lead author Tim Swager at the Massachusetts Institute of Technology, US, the gas sensor market is already worth $3 billion (£1.9 billion) a year and he expects that figure to grow significantly over the next few years.
Swager’s gas sensors are based on the concept of a chemiresistor – a resistor that registers a change in conductivity as it interacts with a chemical. To create the chemiresistor PENCIL (process enhanced nanocarbon for integrated logic) the team mixed nanostructured carbon – graphite or carbon nanotubes – with a selector molecule, in this case naphthalene with a hexafluoroisopropyl moiety, which reacts selectively with different chemical vapours. They then ball milled the two solids together and compressed the PENCIL mixture into either a pellet or into the shape of a conventional pencil’s lead. Finally, the team drew the gas sensor onto cellulose paper that had gold electrodes at either end. The sensor was able to detect and distinguish between acetone, tetrahydrofuran and dimethyl-methylphosphonat vapour by changes in resistance when they interacted with the selector molecule. And because the production process is so simple a gas sensor with new selector molecules can be created in just 15 minutes.
According to Swager, the speed and ease of this method allows gas sensor prototypes to be tested very quickly, meaning scientists can look at combinations they never had the time to before. ‘We are reporting a discovery tool as much as a fabrication tool and because it’s such a rapid method we can start working with three compound mixtures.’
Such advanced sensors could have a huge impact. They could be used to test whether food has gone off – fish release amines when they go bad, for example. Another example of their potential importance is dynamic ventilation – these low-power, wireless sensors could be placed around a building and linked to the ventilation system. Air conditioning could then be turned on, for example, when carbon dioxide levels started rising in a lecture theatre, rather than being left on all the time.
Radislav Potyrailo, principal scientist at the GE Global Research Center in Niskayuna, US, is impressed by the work, and agrees that the applications of such gas sensors could be huge. ‘Solvent-free blending of chemically active ingredients with transducing ingredients (such as carbon nanotubes and graphite) is really innovative and far-reaching.’ He adds that the sensors could be used in areas ‘ranging from chemical leak detection, to food safety, homeland security [and] environmental testing’.
Nanomaterials have offered the tantalizing possibility of lifelike artificial skin. Now researchers at the Georgia Institute of Technology have developed a use for zinc-oxide (ZnO) nanowires to serve as tiny LEDs whose emission intensity is dependent on the local strain put on them.
The Georgia Tech researchers believe that this work offers a new approach to imaging force and could lead to a new approach for human-machine interfaces.
“You can write with your pen and the sensor will optically detect what you write at high resolution and with a very fast response rate,” said Zhong Lin Wang, Regents’ professor and Hightower Chair in the School of Materials Science and Engineering at Georgia Tech, in a press release. “This is a new principle for imaging force that uses parallel detection and avoids many of the complications of existing pressure sensors.”
The research, which was published in the journal Nature Photonics (“High resolution electroluminescent imaging of pressure distribution using a piezoelectric nanowire-LED array”), builds on Wang’s previous work in applying ZnO nanowires to uses that can exploit their piezoelectric properties.
The ZnO nanowires used in this latest device exploit a phenomenon Wang has dubbed piezo-phototronics because they operate on the same principal as piezoelectric materials, but emit different light intensities based on the level of pressure applied to them.
“When you have a zinc oxide nanowire under strain, you create a piezoelectric charge at both ends which forms a piezoelectric potential,” Wang explained in the press release. “The presence of the potential distorts the band structure in the wire, causing electrons to remain in the p-n junction longer and enhancing the efficiency of the LED.”
Wang and his team fabricated the devices by growing the nanowires on a gallium nitride thin film substrate with the c-axis pointing upward. A polymer is then added, filling the space between the nanowires. A nickel-gold electrode is then attached to the gallium-nitride film to form an ohmic contact, and a transparent indium-tin oxide (ITO) film is deposited on top of the array to serve as a common electrode.
When pressure is applied to the device, some of the nanowires are compressed along their axial directions, resulting in a negative piezo-potential; the uncompressed nanowires have no potential.
The researchers demonstrated that when they used an ordinary pen to write on the material, light is emitted from the bottom of the material that corresponds to the letter written on the top.
“The response time is fast, and you can read a million pixels in a microsecond,” said Wang. “When the light emission is created, it can be detected immediately with the optical fiber.”
While the light turning on and off is completed in a mere 90 milliseconds, Wang believes that the spatial resolution, which currently stands at 2.7 micrometers, can be improved. He thinks the team can achieve this by reducing the diameter of the nanowires, making it possible to fit more of them into a given area.
Photo: Georgia Institute of Technology
With the introduction of Apple’s iPhone and then all the other smart phones, and then the introduction of Apple’s iPad followed by all the other tablets, touch screen displays have experienced enormous growth over the last six years. However, from the beginning of that growth, concern was developing about what could be done about the relatively scarce resource of indium-tin oxide (ITO) that these devices need to operate.
ITO is used as a transparent conductor to control display pixels. What was a clear challenge and concern for display manufacturers actually served as a new ray of hope for nanomaterial producers. Companies like Cambrios Technologies, which had been launched back in 2002 with the aim of getting man-made viruses to pattern inorganic materials for a host of electronic applications, finally saw an application that was driven by “market pull” rather than “technology push”.