Bend me, shape me: flexible electronics perform under punishing conditions
Molecule-thick electronics still work after being wrapped around a toothpick.
This research involved simpler electronics than these, but only a single molecule thick.
by Matthew Francis - June 5 2012, 12:00pm CDT
"Who doesn't want thin and lightweight electronic devices? From flexible computer displays to printable solar cells to medical equipment, the possibilities are myriad. However, testing the durability of nanoscale electronics under strain—twisting, bending, folding, etc.—provides a challenge. The substrate (the layer upon which electronic components are assembled) and the electronics must endure stresses, meaning the entire device needs to perform adequately under the same adverse conditions.
Sungjun Park et al. constructed single-molecule thick electronic components on a flexible substrate. By testing the charge-transport properties under a variety of stresses and repetitive bending, they determined that their devices maintain stability, even after more than 1000 bending and straightening cycles. While the simple electronics they tested are not equivalent to complex modern devices, they demonstrate in principle that flexible electronics are workable.
The researchers' work builds on previous efforts in molecular electronics, including prior work by the same group involving transistors on a rigid substrate. They deposited alkyl, or aromatic thiol molecules, large organic molecules that produce self-assembled monolayers (SAMs) with well-studied electronic properties. For this particular experiment, transistors were not practical, since the experimenters couldn't produce them in large enough numbers; instead, they fabricated two-terminal devices (analogous to diodes).
The particular substrate (polyimide or PI) was chosen because it is both flexible and tolerates high temperatures, required for depositing the SAMs. The researchers constructed 512 devices on transparent squares of PI, 3 centimeters on the side. To test flexibility, they wrapped the devices around cylinders of known radii (5 and 10mm) as well as over a toothpick (less than 1mm radius of curvature). They also twisted the devices through 15° and 30° and rolled them into a helix. (The images in the paper that show the electronics being tortured remind me of a scene cut from the first Superman film where Lex Luthor tries and fails to kill Superman in a variety of ways.) Finally, they performed mechanical bending, flexing and unflexing the devices over 1000 repetitions.
In all these cases, the researchers tested the electric current output as they applied a variable DC voltage. They found the SAMs performed with no degradation under all the stresses and strains, a remarkable outcome given the tortures they inflicted. Neither the substrate nor the electronic components fractured or exhibited loss of electrical performance.
While two-terminal devices are not as useful as three-terminal devices like transistors, the researchers have proven in principle that truly flexible electronics made of single molecular layers are possible. Further tests should lead to fully integrated circuit electronics that can be contorted in a wide variety of ways without losing their ability to perform.
Nature Nanotechnology, 2012. DOI: 10.1038/nnano.2012.81