Category Archives: Flexible Displays

Trans-Lux Corporation has released a new series of modular court side LED displays under its premier TL Vision brand. The units provide a high-performance LED display in a sturdy steel frame optimized for reduced weight to allow the systems to be easily stored, moved and set up.

“With a lightweight, modular design, these new innovative court side LED displays provide a high degree of versatility while increasing fan engagement, and facilitating sponsor-driven advertising revenues,” said J.M. Allain, President and CEO, Trans-Lux Corporation. “These new Court Side LED systems are the perfect extension of our TL Vision LED display solutions for sports venues, and ideally complement our comprehensive portfolio of FairPlay by Trans-Lux LED scoreboard solutions.”

TL Vision Court Side LED Displays are available in 8- and 10-foot configurations with either 6mm or 10mm pitches with pixel counts ranging from 64×224 pixels to 112×480 pixels depending on their length and pitch selection. To provide customers with even greater flexibility, each modular unit can be custom-ordered as a stand-alone scoreboard, TL Vision scoreboard, TL Vision display, rear-illuminated sign or non-illuminated sign. Multiple units can be connected end-to-end to create LED displays tailored to specific events.

Additional features of the new TL Vision court side LED displays include an adjustable display angle of 0, 3 or 6 degrees; six casters for even weight distribution and ease of mobility; and an internal cable tray to hide and protect cables and wires. Rather than requiring equipment to be stacked on the table top or within a separate rack unit, TL Vision court side LED Displays incorporate rack-mount space that is hidden from view to reduce clutter and protect equipment.

The folding table tops incorporated into the TL Vision court side LED Displays are designed to avoid damage during setup and maximize storage space. To provide safety the unit features padded tops and ends, which can be easily removed to access the cable tray or knobs for adjusting both display angle and alignment.

A number of professional and Division I college basketball teams have already implemented TL Vision Court Side LED Displays in their arenas.

University of Utah engineers have developed a new type of carbon nanotube material for handheld sensors that will be quicker and better at sniffing out explosives, deadly gases and illegal drugs.

A carbon nanotube is a cylindrical material that is a hexagonal or six-sided array of carbon atoms rolled up into a tube. Carbon nanotubes are known for their strength and high electrical conductivity and are used in products from baseball bats and other sports equipment to lithium-ion batteries and touchscreen computer displays.

Ling Zang, a University of Utah professor of materials science and engineering, holds a prototype detector that uses a new type of carbon nanotube material for use in handheld scanners to detect explosives, toxic chemicals and illegal drugs. Zang and colleagues developed the new material, which will make such scanners quicker and more sensitive than today's standard detection devices. Ling's spinoff company, Vaporsens, plans to produce commercial versions of the new kind of scanner early next year. Credit: Dan Hixon, University of Utah College of Engineering.

Ling Zang, a University of Utah professor of materials science and engineering, holds a prototype detector that uses a new type of carbon nanotube material for use in handheld scanners to detect explosives, toxic chemicals and illegal drugs. Zang and colleagues developed the new material, which will make such scanners quicker and more sensitive than today’s standard detection devices. Ling’s spinoff company, Vaporsens, plans to produce commercial versions of the new kind of scanner early next year.
Credit: Dan Hixon, University of Utah College of Engineering.

Vaporsens, a university spin-off company, plans to build a prototype handheld sensor by year’s end and produce the first commercial scanners early next year, says co-founder Ling Zang, a professor of materials science and engineering and senior author of a study of the technology published online Nov. 4 in the journal Advanced Materials.

The new kind of nanotubes also could lead to flexible solar panels that can be rolled up and stored or even “painted” on clothing such as a jacket, he adds.

Zang and his team found a way to break up bundles of the carbon nanotubes with a polymer and then deposit a microscopic amount on electrodes in a prototype handheld scanner that can detect toxic gases such as sarin or chlorine, or explosives such as TNT.

When the sensor detects molecules from an explosive, deadly gas or drugs such as methamphetamine, they alter the electrical current through the nanotube materials, signaling the presence of any of those substances, Zang says.

“You can apply voltage between the electrodes and monitor the current through the nanotube,” says Zang, a professor with USTAR, the Utah Science Technology and Research economic development initiative. “If you have explosives or toxic chemicals caught by the nanotube, you will see an increase or decrease in the current.”

By modifying the surface of the nanotubes with a polymer, the material can be tuned to detect any of more than a dozen explosives, including homemade bombs, and about two-dozen different toxic gases, says Zang. The technology also can be applied to existing detectors or airport scanners used to sense explosives or chemical threats.

Zang says scanners with the new technology “could be used by the military, police, first responders and private industry focused on public safety.”

Unlike the today’s detectors, which analyze the spectra of ionized molecules of explosives and chemicals, the Utah carbon-nanotube technology has four advantages:

  • It is more sensitive because all the carbon atoms in the nanotube are exposed to air, “so every part is susceptible to whatever it is detecting,” says study co-author Ben Bunes, a doctoral student in materials science and engineering.
  • It is more accurate and generates fewer false positives, according to lab tests.
  • It has a faster response time. While current detectors might find an explosive or gas in minutes, this type of device could do it in seconds, the tests showed.
  • It is cost-effective because the total amount of the material used is microscopic.

Rice University scientists who want to gain an edge in energy production and storage report they have found it in molybdenum disulfide.

The Rice lab of chemist James Tour has turned molybdenum disulfide’s two-dimensional form into a nanoporous film that can catalyze the production of hydrogen or be used for energy storage.

The versatile chemical compound classified as a dichalcogenide is inert along its flat sides, but previous studies determined the material’s edges are highly efficient catalysts for hydrogen evolution reaction (HER), a process used in fuel cells to pull hydrogen from water.

Tour and his colleagues have found a cost-effective way to create flexible films of the material that maximize the amount of exposed edge and have potential for a variety of energy-oriented applications.

Molybdenum disulfide isn’t quite as flat as graphene, the atom-thick form of pure carbon, because it contains both molybdenum and sulfur atoms. When viewed from above, it looks like graphene, with rows of ordered hexagons. But seen from the side, three distinct layers are revealed, with sulfur atoms in their own planes above and below the molybdenum.

This crystal structure creates a more robust edge, and the more edge, the better for catalytic reactions or storage, Tour said.

“So much of chemistry occurs at the edges of materials,” he said. “A two-dimensional material is like a sheet of paper: a large plain with very little edge. But our material is highly porous. What we see in the images are short, 5- to 6-nanometer planes and a lot of edge, as though the material had bore holes drilled all the way through.”

The new film was created by Tour and lead authors Yang Yang, a postdoctoral researcher; Huilong Fei, a graduate student; and their colleagues. It catalyzes the separation of hydrogen from water when exposed to a current. “Its performance as a HER generator is as good as any molybdenum disulfide structure that has ever been seen, and it’s really easy to make,” Tour said.

While other researchers have proposed arrays of molybdenum disulfide sheets standing on edge, the Rice group took a different approach. First, they grew a porous molybdenum oxide film onto a molybdenum substrate through room-temperature anodization, an electrochemical process with many uses but traditionally employed to thicken natural oxide layers on metals.

The film was then exposed to sulfur vapor at 300 degrees Celsius (572 degrees Fahrenheit) for one hour. This converted the material to molybdenum disulfide without damage to its nano-porous sponge-like structure, they reported.

The films can also serve as supercapacitors, which store energy quickly as static charge and release it in a burst. Though they don’t store as much energy as an electrochemical battery, they have long lifespans and are in wide use because they can deliver far more power than a battery. The Rice lab built supercapacitors with the films; in tests, they retained 90 percent of their capacity after 10,000 charge-discharge cycles and 83 percent after 20,000 cycles.

“We see anodization as a route to materials for multiple platforms in the next generation of alternative energy devices,” Tour said. “These could be fuel cells, supercapacitors and batteries. And we’ve demonstrated two of those three are possible with this new material.”

Co-authors of the paper are Rice graduate students Gedeng Ruan and Changsheng Xiang. Tour is the T.T. and W.F. Chao Chair in Chemistry as well as a professor of materials science and nanoengineering and of computer science.

The Peter M. and Ruth L. Nicholas Postdoctoral Fellowship of Rice’s Smalley Institute for Nanoscale Science and Technology and the Air Force Office of Scientific Research Multidisciplinary University Research program supported the research.

At first glance, the static, greyscale display created by a group of researchers from the Hong Kong University of Science and Technology, China might not catch the eye of a thoughtful consumer in a market saturated with flashy, colorful electronics. But a closer look at the specs could change that: the ultra-thin LCD screen described today in a paper in The Optical Society’s (OSA) journal Optics Letters is capable of holding three-dimensional images without a power source, making it a compact, energy-efficient way to display visual information.

Liquid crystal displays (LCDs) are used in numerous technological applications, from television screens to digital clock faces. In a traditional LCD, liquid crystal molecules are sandwiched between polarized glass plates. Electrodes pass current through the apparatus, influencing the orientation of the liquid crystals inside and manipulating the way they interact with the polarized light. The light and dark sections of the readout display are controlled by the amount of current flowing into them.

The new displays ditch the electrodes, simultaneously making the screen thinner and decreasing its energy requirements. Once an image is uploaded to the screen via a flash of light, no power is required to keep it there. Because these so-called bi-stable displays draw power only when the image is changed, they are particularly advantageous in applications where a screen displays a static image for most of the time, such as e-book readers or battery status monitors for electronic devices.

“Because the proposed LCD does not have any driving electronics, the fabrication is extremely simple. The bi-stable feature provides a low power consumption display that can store an image for several years,” said researcher Abhishek Srivastava, one of the authors of the paper.

The researchers went further than creating a simple LCD display, however—they engineered their screen to display images in 3D. Real-world objects appear three-dimensional because the separation between your left eye and your right creates perspective. 3D movies replicate this phenomenon on a flat screen by merging two films shot from slightly different angles, and the glasses that you wear during the film selectively filter the light, allowing one view to reach your left eye and another to fall on your right to create a three-dimensional image.

However, instead of displaying multiple images on separate panels and carefully aligning them—a tedious and time-consuming process—the researchers create the illusion of depth from a single image by altering the polarization of the light passing through the display. They divide the image into three zones: one in which the light is twisted 45 degrees to the left, another in which it is twisted 45 degrees to the right, and a third in which it is unmodified. When passed through a special filter, the light from the three zones is polarized in different directions. Glasses worn by the viewer then make the image appear three-dimensional by providing a different view to each eye.

This technology isn’t ready to hit the television market just yet: it only displays images in greyscale and can’t refresh them fast enough to show a film. However, Srivastava and his colleagues are in the process of optimizing their device for consumer use by adding color capabilities and improving the refresh rate. The thin profile and minimal energy requirements of devices could also make it useful in flexible displays or as a security measure on credit cards.

Until now, if you want to print a greeting card for a loved one, you can use colorful graphics, fancy typefaces or special paper to enhance it. But what if you could integrate paper-thin displays into the cards, which could be printed at home and which would be able to depict self-created symbols or even react to touch? Those only some of the options computer scientists in Saarbrücken can offer.

They developed an approach that in the future will enable laypeople to print displays in any desired shape on various materials and therefore could change everyday life completely.

For example: A postcard depicts an antique car. If you press a button, the back axle and the steering wheel rim light up in the same color. Two segments on a flexible display, which have the same shape as those parts of the car, can realize this effect. Computer scientists working with Jürgen Steimle printed the post card using an off-the-shelf inkjet printer. It is electro-luminescent: If it is connected to electric voltage, it emits light. This effect is also used to light car dashboards at night.

Steimle is leader of the research group “Embodied Interaction” at the Cluster of Excellence “Multimodal Computing and Interaction”. Simon Olberding is one of his researchers. “Until now, this was not possible”, explains Olberding. “Displays were mass-produced, they were inflexible, they always had a rectangular shape.” Olberding and Steimle want to change that. The process they developed works as follows: The user designs a digital template with programs like Microsoft Word or Powerpoint for the display he wants to create. By using the methods the computer scientists from Saarbrücken developed, called “Screen Printing” and “Conductive Inkjet Printing”, the user can print those templates. Both approaches have strengths and weaknesses, but a single person can use them within either a few minutes or two to four hours. The printing results are relatively high-resolution displays with a thickness of only 0.1 millimeters. It costs around €20 to print on a DIN A4 page; the most expensive part is the special ink. Since the method can be used to print on materials like paper, synthetic material, leather, pottery, stone, metal and even wood, two-dimensional and even three-dimensional shapes can be realized. Their depiction can either consist of one segment (surface, shape, pattern, raster graphics), several segments or variously built-up matrixes. “We can even print touch-sensitive displays”, says Olberding.

The possibilities for the user are various: displays can be integrated into almost every object in daily life – users can print not only on paper objects, but also on furniture or decorative accessories, bags or wearable items. For example, the strap of a wristwatch could be upgraded so that it lights up if a text message is received. “If we combine our approach with 3D printing, we can print three-dimensional objects that display information and are touch-sensitive”, explains Steimle.

Wearable electronics are going from geek to chic, as new smartwatches from the likes of Apple and Samsung have set a new standard for technological bling.

At IFA 2014 in Berlin last month, the European consumer electronics show highlighted new smartwatches meant to entice consumers with more fashion-forward designs. Smartwatch makers hope to eventually legitimize wearable products as a category by improving their usability, and the secret sauce in this effort is an upgrade in design centered on the use of flexible displays. 

The display panel market for all types of wearable electronic items is forecast to enjoy very rapid growth in the years to come. From a projected $300 million this year, industry revenue will climb more than 80 percent annually for at least four more years as high resolution and color displays are increasingly adopted in devices. By 2023, the market will be worth some $22.7 billion, as shown in the attached figure.

In terms of shipments, the market will surge to 800 million units in 2023, up from 54 million in 2014.

Samsung, LG, Sony, Asus and Motorola were on hand at IFA to introduce proprietary offerings—ostensibly to get a head start on Apple, which unveiled its own smartwatches a few days later after the show, in which it does not participate. 

Samsung introduced the Gear S smart watch, which features a curved screen and a 2-inch super active-matrix organic light-emitting diode (AMOLED) flexible display that is large enough to accommodate a keyboard for the smartwatch.

For its part, LG introduced the G Watch R that flaunts a completely circular screen. With a 1.3-inch diameter, this round display has 57 percent more area than a square screen. The sleek P-AMOLED panel is less than 0.6mm thick and features 320 x 320 resolution, 100-percent color gamut, 300-nits peak luminance and unlimited contrast ratio, typical of an organic light-emitting diode (OLED) display.

LG Display recently started mass production of its revolutionary circular plastic P-OLED screen, made possible by the company’s development of a circular mask and new production processes that improve deposition efficiency and employ highly precise laser cutting. LG Display’s power-save mode, which enables the screen to retain its resolution without a power supply, has also contributed to longer battery life for the watch.

Like the G Watch R, Motorola’s Moto 360 also comes with an attractive round screen. Both the LG and Motorola models are powered by Android Wear as extensions of the Android smartwatch ecosystem. Meanwhile, the Samsung Gear S employs Samsung’s Tizen operating system.

After months of rumors, Apple finally introduced the Apple Watch—fashionably late but highly anticipated. Set to be available at the beginning of 2015 with a starting price of $349, Apple Watch will use a square display. Detailed specs about the display are still not available, but the wearable timepieces will employ a flexible Retina display. According to Apple, the display is “not just a display but the focal point of the whole experience.” Its advertised flexibility, high-energy efficiency and very-high contrast mean it likely will use an OLED display.

And just like the iPhone, Apple Watch will have the solid advantage of application support from its entrenched ecosystem fully behind the product. 

Imperatives for wearable displays

Developments in flexible displays have opened up new opportunities for wearable devices, enabling the kind of design innovations seen in the latest group of smartwatch products at IFA.

“Wearables are best viewed as functional fashion accessories rather than as electronic goods,” said Sweta Dash, senior director for research and display at IHS. But because the fashion accessory market is determined by design rather than by simple function, wearable products such as smartwatches must be adaptable to various forms including squares, circles or even ovals.”

Displays used in wearables need three essential elements, Dash noted. These include outdoor visibility, low power consumption and flexibility in form factor and design. New forms of display, such as stretchable panels that are expected to come in the near future, can meet even more demanding designs in wearables, creating possibilities for exotic shapes and forms.

Also of significance in future wearables will be efficient, low-power flexible displays with longer battery lives that enable increased functionality in smaller form factors. Expected to dominate the wearable display market with improved capability and reduced costs is OLED, a self-emissive display technology with no backlight, excellent flexibility, faster response time and great video quality.

Most of the next wave of wearable products will come from smartwatch computing, Dash remarked. This field of wearable technology will be diverse, ranging from gaming, to infotainment, to health monitoring.

On the downside, most current products—including smartwatches and smartglasses from Google and others—are not completely ready for mainstream consumer adoption. The smartwatch models shown at IFA and Apple’s offerings alike are all expensive and lack the kind of affordable pricing to make them universally appealing. Moreover, a clear value proposition is needed before consumers fully accept the design and available applications provided by these new timepieces to replace the trusty traditional watches of old.

Wearable devices will need to strike the correct combination of price, performance, form factor and usability to reach the consumer mainstream market, IHS believes. Until then, actual wearable products like smartwatches may take longer to gain traction before the market can take off. 

These findings can be found in the Displays research service of IHS Technology.

Printed, flexible and organic electronic (PFOE) sensors can offer flexible form factors, larger area, lower cost, lower power, and better disposability compared to conventional sensors, key attributes for wearable applications. These attributes will allow them to grow into a $244 million market in wearables, according to Lux Research.

“With players from Apple to Intel to Kickstarter-funded start-ups launching devices, wearables are getting hot, but they still need to add functionality while trimming cost and size to really go mainstream,” said Jonathan Melnick, Lux Research Senior Analyst and lead author of the report titled, “Dial-Up Sensors: Printed, Flexible and Organic Sensors for the Things in the Internet of Things.”

“Printed, flexible, and organic electronic sensors can play a key enabling role for wearables — though many technology developers still need to improve performance, reliability and lifetime,” he added.

Lux Researchers analyzed the market for PFOE sensors across a wide variety of connected applications on the “Internet of Things” (IoT), include wearables, retail, transportation, and buildings. Among their findings:

  • Wearable, retail sensors drive growth. Wearable sensors, particularly for health and fitness, will be the biggest segment for PFOE sensors, but retail sensors — with a $117 million market in 2024 — will clock the fastest growth, a compound annual growth rate (CAGR) of 50% through the next decade.
  • Transportation, buildings remain small. Automotive and buildings, which have accounted for a lot of IoT hype, will be a bust for most PFOE sensors due to performance and reliability disadvantages and a limited addressable market.
  • PFOE sensors face opportunities and challenges. Six types of IoT sensors may be suited for PFOE technologies: motion, pressure, gas, temperature, electromagnetic and optical. For each, the value proposition comes down to manufacturing, form factor or size in each target application.

The report, titled “Dial-Up Sensors: Printed, Flexible and Organic Sensors for the Things in the Internet of Things,” is part of the Lux Research Printed, Flexible, and Organic Electronics Intelligence service.

Flexible LEDs


September 24, 2014

Flexible light-emitting diode (LED) displays and solar cells crafted with inorganic compound semiconductor micro-rods are moving one step closer to reality, thanks to graphene and the work of a team of researchers in Korea.

Currently, most flexible electronics and optoelectronics devices are fabricated using organic materials. But inorganic compound semiconductors such as gallium nitride (GaN) can provide plenty of advantages over organic materials for use in these devices — including superior optical, electrical and mechanical properties.

One major obstacle that has so far prevented the use of inorganic compound semiconductors in these types of applications was the difficulty of growing them on flexible substrates.

In the journal APL Materials, from AIP Publishing, a team of Seoul National University (SNU) researchers led by Professor Gyu-Chul Yi describes their work growing GaN micro-rods on graphene to create transferrable LEDs and enable the fabrication of bendable and stretchable devices.

“GaN microstructures and nanostructures are garnering attention within the research community as light-emitting devices because of their variable-color light emission and high-density integration properties,” explained Yi. “When combined with graphene substrates, these microstructures also show excellent tolerance for mechanical deformation.”

Why choose graphene for substrates? Ultrathin graphene films consist of weakly bonded layers of hexagonally arranged carbon atoms held together by strong covalent bonds. This makes graphene an ideal substrate “because it provides the desired flexibility with excellent mechanical strength — and it’s also chemically and physically stable at temperatures in excess of 1,000°C,” said Yi.

It’s important to note that for the GaN micro-rod growth, the very stable and inactive surface of graphene offers a small number of nucleation sites for GaN growth, which would enhance three-dimensional island growth of GaN micro-rods on graphene.

To create the actual GaN microstructure LEDs on the graphene substrates, the team uses a catalyst-free metal-organic chemical vapor deposition (MOCVD) process they developed back in 2002.

“Among the technique’s key criteria, it’s necessary to maintain high crystallinity, control over doping, formation of heterostructures and quantum structures, and vertically aligned growth onto underlying substrates,” Yi says.

When the team put the bendability and reliability of GaN micro-rod LEDs fabricated on graphene to the test, they found that “the resulting flexible LEDs showed intense electroluminescence (EL) and were reliable — there was no significant degradation in optical performance after 1,000 bending cycles,” noted Kunook Chung, the article’s lead author and a graduate student in SNU’s Physics Department.

This represents a tremendous breakthrough for next-generation electronics and optoelectronics devices — enabling the use of large-scale and low-cost manufacturing processes.

“By taking advantage of larger-sized graphene films, hybrid heterostructures can be used to fabricate various electronics and optoelectronics devices such as flexible and wearable LED displays for commercial use,” said Yi.

Researchers from the University of Texas at Austin and Northwestern University have demonstrated a new method to improve the reliability and performance of transistors and circuits based on carbon nanotubes (CNT), a semiconductor material that has long been considered by scientists as one of the most promising successors to silicon for smaller, faster and cheaper electronic devices. The result appears in a new paper published in the journal Applied Physics Letters, from AIP Publishing.

These are optical images of individual SWCNT field-effect transistors. Credit: S. Jang and A. Dodabalapur/University of Texas at Austin

These are optical images of individual SWCNT field-effect transistors.
Credit: S. Jang and A. Dodabalapur/University of Texas at Austin

In the paper, researchers examined the effect of a fluoropolymer coating called PVDF-TrFE on single-walled carbon nanotube (SWCNT) transistors and ring oscillator circuits, and demonstrated that these coatings can substantially improve the performance of single-walled carbon nanotube devices. PVDF-TrFE is also known by its long chemical name polyvinyledenedifluoride-tetrafluoroethylene.

“We attribute the improvements to the polar nature of PVDF-TrFE that mitigates the negative effect of impurities and defects on the performance of semiconductor single-walled carbon nanotubes,” said Ananth Dodabalapur, a professor in the Cockrell School of Engineering at UT Austin who led the research. “The use of [PVDF-TrFE] capping layers will be greatly beneficial to the adoption of single-walled carbon nanotube circuits in printed electronics and flexible display applications.”

The work was done in collaboration between Dodabalapur’s group at UT Austin and Mark Hersam’s group at Northwestern University as part of a Multi-University Research Initiative (MURI) supported by the Office of Naval Research.

A potential successor to silicon chips

Single-walled carbon nanotubes (SWCNT) are just about the thinnest tubes that can be wrought from nature. They are cylinders formed by rolling up a material known as graphene, which is a flat, single-atom-thick layer of carbon graphite. Most single-walled carbon nanotubes typically have a diameter close to 1 nanometer and can be twisted, flattened and bent into small circles or around sharp bends without breaking. These ultra-thin carbon filaments have high mobility, high transparency and electric conductivity, making them ideal for performing electronic tasks and making flexible electronic devices like thin film transistors, the on-off switches at the heart of digital electronic systems.

“Single-walled carbon nanotube field-effect transistors (FETs) have characteristics similar to polycrystalline silicon FETs, a thin film silicon transistor currently used to drive the pixels in organic light-emitting (OLED) displays,” said Mark Hersam, Dodabalapur’s coworker and a professor in the McCormick School of Engineering and Applied Science at Northwestern University. “But single-walled carbon nanotubes are more advantageous than polycrystalline silicon in that they are solution-processable or printable, which potentially could lower manufacturing costs.”

The mechanical flexibility of single-walled carbon nanotubes also should allow them to be incorporated into emerging applications such as flexible electronics and wearable electronics, he said.

For years, scientists have been experimenting with carbon nanotube devices as a successor to silicon devices, as silicon could soon meet its physical limit in delivering increasingly smaller, faster and cheaper electronic devices. Although circuits made with single-walled carbon nanotube are expected to be more energy-efficient than silicon ones in future, their drawbacks in field-effect transistors, such as high power dissipation and less stability, currently limit their applications in printed electronics, according to Dodabalapur.

A new technique to improve the performance of SWCNTs devices

To overcome the drawbacks of single-walled carbon nanotube field-effect transistors and improve their performance, the researchers deposited PVDF-TrFE on the top of self-fabricated single-walled carbon nanotube transistors by inkjet printing, a low-cost, solution based deposition process with good spatial resolution. The fluoropolymer coated film was then annealed or heated in air at 140 degrees Celsius for three minutes. Later, researchers observed the differences of device characteristics.

“We found substantial performance improvements with the fluoropolymer coated single-walled carbon nanotube both in device level and circuit level,” Dodabalapur noted.

On the device level, significant decreases occur in key parameters such as off-current magnitude, degree of hysteresis, variation in threshold voltage and bias stress degradation, which, Dodabalapur said, means a type of more energy-efficient, stable and uniform transistors with longer life time.

On the circuit level, since a transistor is the most basic component in digital circuits, the improved uniformity in device characteristics, plus the beneficial effects from individual transistors eventually result in improved performance of a five-stage complementary ring oscillator circuit, one of the simplest digital circuits.

“The oscillation frequency and amplitude [of the single-walled carbon nanotube ring oscillator circuit] has increased by 42 percent and 250 percent respectively,” said Dodabalapur. The parameters indicate a faster and better performing circuit with possibly reduced power consumption.

Dodabalapur and his coworkers attributed the improvements to the polar nature of PVDF-TrFE.

“Before single-walled carbon nanotube field-effect transistors were fabricated by inkjet printing, they were dispersed in an organic solvent to make a printable ink. After the fabrication process, there could be residual chemicals left [on the device], causing background impurity concentration,” Dodabalapur explained. “These impurities can act as charged defects that trap charge carriers in semiconductors and reduce carriers’ mobility, which eventually could deteriorate the performance of transistors.”

PVDF-TrFE is a polar molecule whose negative and positive charges are separated on different ends of the molecule, Dodabalapur said. The two charged ends form an electric bond, or dipole, in between. After the annealing process, the dipoles in PVDF-TrFE molecules uniformly adopt a stable orientation that tends to cancel the effects of the charged impurities in single-walled carbon nanotube field-effect transistors, which facilitated carrier flow in the semiconductor and improved device performance.

To confirm their hypothesis, Dodabalapur and his coworkers performed experiments comparing the effects of polar and non-polar vapors on single-walled carbon nanotube field-effect transistors. The results support their assumption.

The next step, Dodabalapur said, is to implement more complex circuits with single-walled carbon nanotube field-effect transistors.

Toward optical chips


September 19, 2014

Chips that use light, rather than electricity, to move data would consume much less power — and energy efficiency is a growing concern as chips’ transistor counts rise.

Of the three chief components of optical circuits — light emitters, modulators, and detectors — emitters are the toughest to build. One promising light source for optical chips is molybdenum disulfide (MoS2), which has excellent optical properties when deposited as a single, atom-thick layer. Other experimental on-chip light emitters have more-complex three-dimensional geometries and use rarer materials, which would make them more difficult and costly to manufacture.

In the next issue of the journal Nano Letters, researchers from MIT’s departments of Physics and of Electrical Engineering and Computer Science will describe a new technique for building MoS2 light emitters tuned to different frequencies, an essential requirement for optoelectronic chips. Since thin films of material can also be patterned onto sheets of plastic, the same work could point toward thin, flexible, bright, color displays.

The researchers also provide a theoretical characterization of the physical phenomena that explain the emitters’ tunability, which could aid in the search for even better candidate materials. Molybdenum is one of several elements, clustered together on the periodic table, known as transition metals. “There’s a whole family of transition metals,” says Institute Professor Emeritus Mildred Dresselhaus, the corresponding author on the new paper. “If you find it in one, then it gives you some incentive to look at it in the whole family.”

Joining Dresselhaus on the paper are joint first authors Shengxi Huang, a graduate student in electrical engineering and computer science, and Xi Ling, a postdoc in the Research Laboratory of Electronics; associate professor of electrical engineering and computer science Jing Kong; and Liangbo Liang, Humberto Terrones, and Vincent Meunier of Rensselaer Polytechnic Institute.

Monolayer — with a twist

Most optical communications systems — such as the fiber-optic networks that provide many people with Internet and TV service — maximize bandwidth by encoding different data at different optical frequencies. So tunability is crucial to realizing the full potential of optoelectronic chips.

The MIT researchers tuned their emitters by depositing two layers of MoS2 on a silicon substrate. The top layers were rotated relative to the lower layers, and the degree of rotation determined the wavelength of the emitted light.

Ordinarily, MoS2 is a good light emitter only in monolayers, or atom-thick sheets. As Huang explains, that’s because the two-dimensional structure of the sheet confines the electrons orbiting the MoS2 molecules to a limited number of energy states.

MoS2, like all light-emitting semiconductors, is what’s called a direct-band-gap material. When energy is added to the material, either by a laser “pump” or as an electrical current, it kicks some of the electrons orbiting the molecules into higher energy states. When the electrons fall back into their initial state, they emit their excess energy as light.

In a monolayer of MoS2, the excited electrons can’t escape the plane defined by the material’s crystal lattice: Because of the crystal’s geometry, the only energy states available to them to leap into cross the light-emitting threshold. But in multilayer MoS2, the adjacent layers offer lower-energy states, below the threshold, and an excited electron will always seek the lowest energy it can find.

Mind the gap

So while the researchers knew that rotating the layers of MoS2 should alter the wavelength of the emitted light, they were by no means certain that the light would be intense enough for use in optoelectronics. As it turns out, however, the rotation of the layers relative to each other alters the crystal geometry enough to preserve the band gap. The emitted light is not quite as intense as that produced by a monolayer of MoS2, but it’s certainly intense enough for practical use — and significantly more intense than that produced by most rival technologies.

The researchers were able to precisely characterize the relationship between the geometries of the rotated layers and the wavelength and intensity of the light emitted. “For different twisted angles, the actual separation between the two layers is different, so the coupling between the two layers is different,” Huang explains. “This interferes with the electron densities in the bilayer system, which gives you a different photoluminescence.” That theoretical characterization should make it much easier to predict whether other transition-metal compounds will display similar light emission.

“This thing is something really new,” says Fengnian Xia, an assistant professor of electrical engineering at Yale University. “It gives you a new model for tuning.”

“I expected that this kind of angle adjustment would work, but I didn’t expect that the effect would be so huge,” Xia adds. “They get quite significant tuning. That’s a little bit surprising.”

Xia believes that compounds made from other transition metals, such as tungsten disulfide or tungsten diselenide, could ultimately prove more practical than MoS2. But he agrees that the MIT and RPI researchers’ theoretical framework could help guide future work. “They use density-functional theory,” he says. “That’s a kind of general theory that can be applied to other materials also.”