Carbon Nanotube Transistors Could Help Displays Flex
Researchers use a springy ion gel to help carbon nanotube tran-sistors stretch further than previous devices
To make future displays that roll, bend, and stretch, electronics makers need the circuits that control the pixels to be elastic. In particular, they need flexible transistors. Now researchers have built super stretchy transistors from carbon nanotubes (Nano Lett. 2014, DOI: 10.1021/nl403941a).
Michael S. Arnold, a materials scientist at the University of Wisconsin, Madison, thinks carbon nanotubes have amazing properties that make them ideally suited for elastic electronics. “They are some of the best conductors of charge ever discovered—much faster than silicon or organic electronics,” he says. They’re also chemically and mechanically robust: The nanomaterials don’t oxidize when exposed to air, and meshes of the nanotubes can stretch, fold, and withstand tremendous strain without damage.
Given this great stretchiness and conductivity, companies are already looking into using these materials to make flexible electrodes for touchscreens. Making stretchable transistors is harder, Arnold says. Transistors have more parts, and all of them have to stretch in unison. If they cannot move in sync, electrical contacts break, and the transistor can no longer switch on and off.
One particularly tricky transistor part is the dielectric, an insulating material that’s usually made out of unyielding inorganic oxides. The dielectric layer helps ensure that a transistor doesn’t leak current when it is in its off state. It basically ensures the transistor stays off and doesn’t waste power. Feng Xu, a researcher in Arnold’s lab, had the idea to use an ion gel—a squishy polymer embedded with ions—as the dielectric. Ion gels have been used in carbon nanotube transistors before (Nano Lett.2013, DOI: 10.1021/nl3038773). That previous work took advantage of the gels’ compatibility with printing-based fabrication methods, not their ability to stretch.
To make the stretchy transistors, the researchers first constructed a nanotube mesh by dripping a solution of semiconducting nanotubes onto a glass slide, spreading out the solution, and letting the organic solvent evaporate. Next the team stretched a sheet of clear, rubbery polydimethylsiloxane and pressed it onto the glass slide to pick up the carbon nanotube film. The researchers patterned gold electrodes onto the sheet while it was stretched. They then drop-cast the ion gel onto the nanotube mesh surface.
When the team let the rubber substrate relax, the nanotubes and the gold electrodes wrinkled and buckled. This pre-stretching enabled the finished devices to expand without breaking when strained, Arnold says.
The Wisconsin researchers put their stretchy transistors through electrical and mechanical tests. The devices had a high electron mobility, a measure of how fast electrons can move through the transistor, and a large on-off ratio, a measure of the difference in conductance between its on and off states. To test the devices’ physical resilience, the scientists pulled the transistors sitting on polydimethylsiloxane so that the devices were 57% longer than their resting length. After more than 1,000 pulls, the transistors showed no signs of degradation in electrical performance. The previous record for carbon nanotube transistors was a 20% stretch (Nat. Mater. 2013, DOI: 10.1038/nmat3572).
The devices exhibit good stretchiness, says Young Hee Lee, a researcher at Sungkyunkwan University, in South Korea, who developed the previous record-holding stretchy transistors. But he says the devices still need improvements before they are ready for commercialization. For example, transistor arrays for commercial organic light-emitting diode (OLED) displays will need to switch on and off faster than the devices reported, Lee says.
Arnold is continuing to develop his team’s transistors for use with OLED displays. He wants to incorporate a more elastic dielectric that better matches the other ingredients—it’s the ion gel that breaks down when it’s stretched past 57%, not the electrodes or the nanotubes.
Metal-Organic Framework Used For Chiral Chromatography
A column loaded with a MOF can separate enantiomers of pharmaceuticals via high-performance liquid chromatography.
Researchers have demonstrated that a chiral metal-organic framework (MOF) can separate enantiomers of drug molecules using traditional chromatography (Anal. Chem. 2013, DOI: 10.1021/ac403674p). Although the study marks a new application for the materials, some experts warn that the MOFs may not outperform standard materials used for chiral separations.
Separating mixtures of enantiomers is important in the pharmaceutical industry because enantiomers of drug molecules may have different biological effects. Currently, chemists isolate enantiomers using high-performance liquid chromatography (HPLC) through columns packed with a stationary phase that includes chiral materials. As the mixture moves through the column, each enantiomer interacts differently with the stationary phase, causing the molecules to travel at different speeds through the column. As a result, one compound will flow off of the column earlier than the second one.
Bo Tang at Shandong Normal University in China, and his colleagues wondered if a chiral MOF could be used as a stationary phase. MOFs are three-dimensional structures easily assembled using metals and ligands. When chemists use chiral ligands, they produce MOFs with chiral pores. Since individual enantiomers would have different affinities for these asymmetric pores, Tang thought that the chiral MOFs could be used to separate the isomers.
The researchers synthesized a MOF using zinc and a chiral salen ligand functionalized with pyridine. They packed crystals of the MOF into an HPLC column and used the column to separate enantiomers of ibuprofen, phenylethylamine, and benzoin. While the column easily separated those three mixtures, it could not separate enantiomers of ketoprofen and naproxen because the molecules are larger than the diameter of the MOF’s pores.
Though using MOFs for chiral separations is a new application of the materials, says Yoshio Okamoto, an emeritus professor at Nagoya University in Japan and a researcher at Harbin Engineering University in China, wonders if this MOF offers any advantage over the ubiquitous polysaccharide columns currently used in chiral separations. He also notes that, like ketoprofen and naproxen, many drug molecules are larger than the approximate 9.8 Å pore size of this MOF. Yu Ma, a researcher who worked on the project, says the pore size can easily be changed by varying the metal and ligand used to make the MOF.
Engineering Titanium Dioxide To Respond To Visible Light
Photocatalysis: Gently depositing nitrogen atoms on the surface of TiO2 makes the material active across a broader range of wavelengths of light.
Many scientists view titanium dioxide as an attractive, low-cost photocatalyst for a variety of applications, including water purification, water splitting, and solar power. But there is one snag: The material catalyzes reactions only in response to ultraviolet light. Now, researchers in Singapore have found a way to dope TiO2 with nitrogen so that it responds to visible light, drastically increasing its activity in sunlight (J. Phys. Chem. C 2013, DOI: 10.1021/jp408798f).
A material’s photoactivity depends in part on its band gap, the energy needed to kick electrons from a nonconductive state to a conductive one. TiO2 has a band gap of 3 eV, which corresponds to energies of photons in the UV range. Doping TiO2 with nitrogen lowers the band gap below 2 eV, making the material photoactive with visible light. However, up to now, the doping methods used, such as magnetron sputtering and high-energy ion bombardment, have created defects in the bulk TiO2, which reduced the photocatalytic efficiency of the material.
A team of researchers from the Institute of Materials Research & Engineering (IMRE) and the National University of Singapore used a different source of nitrogen atoms that sent low-speed beams of atoms at the TiO2. Because these nitrogen atoms did not penetrate beyond the TiO2 surface, the researchers could replace the top layer of the material with nitrogen and create a defect-free surface, says Junguang Tao, a physicist at IMRE.
The team measured the photocatalytic activity of TiO2 samples with and without a nitrogen-doped surface. The doped TiO2 showed photoactivity when illuminated with visible light, unike the undoped TiO2. What’s more, the surface-doped TiO2 showed greatly enhanced photoactivity under UV illumina-tion, unlike TiO2 doped using previous methods.
Clear Solar Cells Power Up Windows
Materials: Islands of perovskite make light-colored solar cells that are efficient, semitransparent
The vast real estate of windows in office buildings and skyscrapers could be a fruitful field for harvesting solar energy—if light-weight solar cells could be made with high enough efficiency and aesthetic appeal. Now researchers at Oxford University report sem-itransparent solar cells that might do the trick (ACS Nano 2013, DOI: 10.1021/nn4052309).
For use in windows, solar cells need to absorb enough light to produce sufficient energy but also let enough visible light through to be transparent. Organic photovoltaic materials can absorb infrared light and pass visible light, but they have low energy-conversion efficiencies. On the other hand, inorganic semiconductors, such as amorphous silicon, absorb visible light strongly. So films of these materials must be thin to be transparent, thus decreasing the amount of photons they capture. They also tend to give windows a brownish or reddish tint, which architects dislike.
The Oxford team, led by physicist Henry J. Snaith, made solar cells using perovskites. This class of crystalline materials has recently grabbed the attention of photovoltaics researchers. That’s because perovskites have properties similar to inorganic semiconductors, including high sunlight-to-electricity conversion efficiencies.
To make their semitransparent cells, the researchers first deposited a film of the perovskite CH3NH3PbI3-xClx onto glass coated with fluorine-doped tin oxide. They made the film by mixing methylammonium iodide and lead chloride and spin-coating the solution along with a solvent, such as dimethylsulfoxide, onto the glass. They then heated the resulting film to temperatures ranging from 90 to 130 °C.
As the solution cooled, it formed droplets on the glass surface. As the solvent evaporated, the droplets yielded islands of crystalline material with empty spaces in between. The islands absorb photons and convert them to electrons, while the empty spaces let light pass through. The result was a transparent solar cell with a neutral, grayish tint that is more desirable for use in windows.
As the transparency of the film increases, energy-conversion efficiency decreases. The most transparent films, which let through about 30% of incoming light, converted light to electricity with efficiencies of 3.5%. The darkest films, only 7% transparent, had efficiencies near 8%. Snaith says the ideal coating would let through about half the light and have a 5% conversion efficiency. “We think there’s a lot of scope to improve it further,” he says. He’s formed a company, Oxford Photovoltaics, that hopes to commercialize a device by 2017.
Snaith says the next step is to determine the stability of the perovskite material. A practical solar cell should function for several years. But even if it stops generating electricity, the window containing the solar cell should maintain its color and transparency for at least a decade.
The new solar cell is “fantastic technology,” says Yang Yang, head of the organic electronic materials and devices group at the University of California, Los Angeles. Yang, who is also pursuing transparent solar cells for windows, says there are challenges to commercialization for perovskites, such as the use of lead and the cells’ sensitivity to moisture. But he finds the materials promising: “The rapid progress of efficiency is unprecedented in solar-cell technology.”
A Control Knob To Add C–N Bonds
Simple catalyst modifications orchestrate amine formation reactions
A new technique makes it possible to add carbon-nitrogen bonds at different spots in some organic compounds by making simple changes to a catalyst. The technique controls how and where amine formation reactions occur in a compound by altering the catalystligand ratio in a silver-based catalytic system (J. Am. Chem. Soc. 2013, DOI: 10.1021/ja406654y).
Nitrogen-containing groups such as amines are crucial in many pharmaceuticals and other biologically active molecules. Researchers have long used transition-metal catalysts to introduce amines at reactive C–H sites or C=C sites. But the ability to favor one reaction over the other was previously possible only by changing reagent combinations.
Studies suggest that silver catalyst complexes undergo unique alterations in geometry when their composition changes in certain ways. Jennifer M. Schomaker and coworkers at the University of Wisconsin, Madison, wondered whether that feature of silver-based complexes could alter their reactive properties and, thus, be used to control where C–N bonds formed.
They now report that a silver triflate catalyst with a phenanthroline ligand in a 4:5 catalyst-ligand ratio catalyzed almost exclusively the addition of aziridine (a three-membered ring with a nitrogen) to the site of a C=C group in carbamate compounds. But changing the catalyst-ligand ratio to 1:3 transformed the reaction predominantly to amine formation at a C–H bond in the same carbamates.
Schomaker suspects that cutting the catalyst-ligand ratio congests the catalyst’s local steric environment, making the C–H reaction more likely and vice versa. Her group is working on extending this silver-based catalysis technique to chemoselective amination of other organic compounds.
Jennifer L. Roizen, an organic chemist at Duke University, comments that “other examples where ligand stoichiometry is linked to or causes indirect or direct changes in reactivity or chemoselectivity are few and far between.”
Chemical & Engineering News
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