MOTION CONTROL: Nanoscroll unrolls to become actuator

Motion control in laboratory optics and photonics setups typically means using a screw or micrometer for motions in the millimeters-to-microns range, with piezoelectric actuators used for micron-to-nanometer movements. But with the development of micro- and nanophotonics devices that require motions with ever finer resolution (for example, adjustable coupling of light into microring resonators), true nanoscale motion-control devices are needed. Here, nanoscale applies not just to the motion itself but to the size of the positioning device as well.

While many schemes are being investigated, one approach being pursued by researchers at Brown University (Providence, RI), the Institute of High Performance Computing (IHPC; Singapore), and Politecnico di Torino (PdT; Torino, Italy) is especially simple in concept. It relies on the rolling and unrolling of a carbon "nanoscroll" (CNS), which is a single-layer sheet of graphene whose amount of furling and unfurling depends on the level of an applied electric field.

Previously, the group had experimentally demonstrated that a suspended, furled CNS could be partially loosened in place via an applied electric field, causing the inner core size to expand, although not unroll with a substantial motion in one direction.¹ A separate group also made some progress on fabricating CNSs on solid substrates.²

The importance of the inner core

In this latest development, the researchers at Brown, IHPC, and PdT have performed both a theoretical study and molecular dynamics simulations of a CNS with one end of its sheet anchored to a flat graphite substrate and subjected to a variable electric field.³ What they found was that, unfortunately, the core began unrolling in place, just as it did in the case where the CNS was suspended.

However, when they placed a carbon nanotube (which is a closed tube and thus cannot unfurl) in the center of the CNS, everything changed: The core of the CNS did not enlarge at all, but instead the CNS unrolled itself across the substrate, just as a rug can be unrolled across a floor (see figure). The reason for this is that the inner core of the CNS was attracted to the nanotube, fixing the radius of the inner core.

The theoretical study, in addition to the numerical simulation, depended on three types of interactions: CNS-to-nanotube, CNS-to-substrate, and CNS-to-CNS. While the first two interactions did not vary over time, the CNS-to-CNS interaction could be tuned to different levels by changing a parameter related to the intensity of the applied electric field.

Next, a nanoactuator

The researchers discovered that the energy release per unit area of the CNS was on the order of 0.06 to 0.08 nN/nm. From this, the driving force of a nanoactuator could be calculated: A 100-nm-wide CNS could provide about 6 to 8 nN of force (the force is linearly proportional to the width of the CNS actuator).

Further steps on the path to practical CNS-based motion include fabricating a nanotube-containing CNS on a substrate, getting the device to unroll predictably as a function of an applied voltage, and–very important–figuring out how the CNS would move an external object. Would it push the object, or pull it? Could the central nanotube be used as an axle to which an object could be attached? Such details could well depend on the application; what is significant about this type of motion device is that even as nanophotonics approaches the molecular level, nanoactuation may be on its way too, ready to help.

REFERENCES

1. X.H. Shi et al., Appl. Phys. Lett. 95, 1631133 (2009).
2. X. Xie et al., Nano Lett. 9, 2565 (2009).
3. Xinghua Shi et al., Appl. Phys. Lett. 96, 053115 (2010).