Fig. 1. (a) Metal-assisted chemical etching process for fabrication of Si nanomotors. (b–c) Scanning electron microscopy images of Si nanomotors. (d) Schematic of system setup. Image credit: Zexi Liang. Reproduced with permission from: Liang and Fan, Science Advances 4 (2018) eaau0981.
Fig. 2. Snapshots of reconfigurable operation of an ultrasmall nanomotor gated by light. Reproduced with permission from: Liang and Fan, Science Advances 4 (2018) eaau0981.How tiny, simple nanomotors move in an electric field can be controlled remotely using light, according to researchers from the University of Texas at Austin [Liang and Fan, Science Advances 4 (2018) eaau0981].
The ability to regulate the motion or activity of nanomaterials in response to external stimuli could pave the way for a new generation autonomous devices. This kind of intelligent device could be useful for drug delivery, sensing, communication, microfluidic, and separation technologies. However, in practical terms, it has proved challenging to switch quickly between different operational modes of individual nanomotors remotely using magnetic, electric, optical, acoustic fields, or chemical reactions. But now researchers Donglei (Emma) Fan and Zexi Liang have shown that tiny silicon nanorod motors in an electric field can be switched from one mode of motion to another instantly and reversibly using light.
“We have demonstrated a highly original and facile way to reconfigure the operation of nanomotors by simply exposing semiconductor nanomotors in an external electric field to light,” explains Fan. “This is the first [demonstration] of its kind in terms of both working principle and device."
Depending on the intensity of the light to which the silicon nanomotors are exposed and the AC field frequency, the mechanical motion can be altered, accelerating or decelerating the speed of rotation, or reversing the orientation.
“The ability to alter the behavior of nanodevices in this way – from passive to active – opens the door to the design of autonomous and intelligent machines at the nanoscale,” says Fan. “Our technique for reconfiguring the operation modes of rotary nanomotors is highly efficient, simple, and low cost.”
The nanomotors comprise silicon nanowires made from undoped or lightly doped silicon wafers and are typically 500 nm in diameter and 5 microns long (Fig. 1). When the nanomotors are placed in an electric field, internally generated forces make the rods rotate. Exposure to light modulates the electrical conductivity of the nanorods so that their interaction with the electric field is altered (Fig. 2).
Light-controlled nanomotors could be in valuable for a range of different applications, but the researchers demonstrated how the phenomenon can be used to differentiate between semiconducting and metallic nanoparticles.
“We were able to distinguish semiconductor and metal nanomaterials just by observing their different mechanical motions in response to light with a conventional optical microscope,” explains Fan. “This distinction was made in a non-contact and nondestructive manner compared to the prevailing destructive contact-based electric measurements.”
The researchers say that the simple approach, which requires only a light projector and a kilohertz AC electric field, should be applicable to all light-sensitive materials, including both solid-state and polymer semiconductors, as well as two-dimensional materials. The approach could even be used to control the release rate of active agents from nanoscale drug carriers.
Thomas E. Mallouk of Pennsylvania State University believes that the finding represents a significant advance in the field.
“Rotary micromotors are useful for a variety of established and emerging applications,” he says. “Motors that can respond to two different kinds of energy inputs (here rotating electric fields and light) are less common and are particularly interesting because they can be powered by one energy source and then switched on/off by the other. This provides a higher level of control over motor movement and can be used to selectively propel some particles in the presence of others.”
The use of light as a controlling input facilitates the possibility of wavelength-selective actuation, phototaxis (movement towards or away from the light source), and other kinds of controlled movement, Mallouk adds.
This article was originally published in Nano Today 23 (2018) 5-6.