Carbon nanotube pencils
While carbon nanotubes offer a powerful new way to detect harmful gases in the environment, the methods typically used to build carbon nanotube sensors are hazardous and not suited for large-scale production. Because of this, MIT chemists have created a new fabrication method that is as simple as drawing a line on a sheet of paper.

MIT postdoc Katherine Mirica has designed a new type of pencil lead in which graphite is replaced with a compressed powder of carbon nanotubes. The lead, which can be used with a regular mechanical pencil, can inscribe sensors on any paper surface.
The sensor detects minute amounts of ammonia gas, an industrial hazard – but the sensors could be adapted to detect nearly any type of gas.

MIT chemists designed a new type of pencil lead consisting of carbon nanotubes, allowing them to draw carbon nanotube sensors onto sheets of paper. (Source: MIT)

Nanoparticle crack avoidance
Making uniform coatings is a common engineering challenge and when working at the nanoscale even the tiniest cracks or defects can be a big problem so researchers from the University of Pennsylvania have developed a new way of avoiding cracks when depositing thin films of nanoparticles.

To generate a nanoparticle film, the desired particles are suspended in a suitable liquid, which is then thinly and evenly spread over the surface through a variety of physical methods. The liquid is then allowed to evaporate, but, as it dries, the film can crack like mud in the sun.

One method for preventing cracking is modifying the suspension’s chemistry by putting binding additives in there but that is essentially adding a new material to the film, which may ruin its properties.

This dilemma is highlighted in the case of electrodes, the contact points in many electrical devices that transfer electricity. High-end devices, like certain types of solar cells, have electrodes composed of nanoparticle films that conduct electrons, but cracks in the films act as insulators. Adding a binder to the films would only compound the problem.

The binders are usually polymers, which are insulators themselves and if they are used, the targeted conductivity will not be reached. Engineers can prevent cracks with alternative drying methods, but these involve ultra-high temperatures or pressures and thus expensive and complicated equipment. A cheap and efficient method for preventing cracks would be a boon for any number of industrial processes.

The ubiquity of cracking in this context, however, means that researchers know the “critical cracking thickness” for many materials

The method the researchers used to make the films is known as “spin-coating.” A precise amount of the nanoparticle suspension — in this case, silica spheres in water — is spread over the target surface. The surface is then rapidly spun, causing centrifugal acceleration to thin the suspension over the surface in a uniform layer. The suspension then dries with continued rotation, causing the water to evaporate and leaving the silica spheres behind in a compacted arrangement.

But to make a second layer over this first, another drop of liquid suspension would need to be placed on the dried nanoparticles, something that would normally wash them away. However, the researchers were surprised when the dried layers remained intact after the process was repeated 13 times; the exact mechanism by which they remained stable is something of a mystery.

They believe that the nanoparticles are staying on the surface because covalent bonds are being formed between them even though they are not being exposed high temperatures

Nanoparticle films crack at certain thicknesses (left). By adding layers of thinner films, cracking can be avoided (right). (Source: University of Pennsylvania)

–Ann Steffora Mutschler