University of California, San Diego researchers demonstrated the ability to precisely control the alignment and placement of large numbers of InAs nanowires from solution onto very narrow, prepatterned electrodes using dielectrophoresis.
An understanding of dielectrophoretic behavior associated with such electrode geometries is essential to development of approaches for assembly of intricate nanowire systems. The influence of signal frequency and electrode design on nanowire manipulation and placement is examined. Signal frequencies in the range of 10 MHz are found to yield high percentages of aligned nanowires on electrodes with dimensions similar to that of the nanowire. Strategies for further improvement of nanowire alignment are suggested and analyzed.
Dielectrophoresis (or DEP) is a phenomenon in which a force is exerted on a dielectric particle when it is subjected to a non-uniform electric field. This force does not require the particle to be charged. All particles exhibit dielectrophoretic activity in the presence of electric fields. However, the strength of the force depends strongly on the medium and particles’ electrical properties, on the particles’ shape and size, as well as on the frequency of the electric field. Consequently, fields of a particular frequency can manipulate particles with great selectivity. This has allowed, for example, the separation of cells or the orientation and manipulation of nanoparticles and nanowires. Dielectrophoresis can be used to manipulate, transport, separate and sort different types of particles. Since biological cells have dielectric properties Dielectrophoresis is used in medicine. Prototypes that separate cancer cells from healthy cells are already made.
Optical microscope images of electrode arrays after DEP alignment. The top and bottom rows show images of chips with 100 alignment sites and 50 alignment sites, respectively. Sites with perfect alignment are indicated by a rectangle while unaligned wires are indicated by a line. Each image is 500 x 500 µm. (Reprinted with permission from American Chemical Society)
This technique can also be used to integrate III-V nanowires with existing silicon based circuitry to act as light sources and detectors for on chip optical interconnects. Furthermore, because nanowires devices are placed and positioned on a host substrate after fabrication it is possible to create high quality devices and then place them on exotic substrates allowing for the possibility of stretchable or flexible electronics.
Yu points out that there are many challenges facing nanowire based circuits and systems. “It is still necessary to improve the quality of individual devices and the yield associated with them. While integration schemes are improving, further refinement is still necessary. As a field we must not only show that we are able to construct nanowire based devices and complex systems based on nanowire components – but that there is a significant advantage to using nanowire based systems given the different complexities and costs associated with such systems.”
Yu and Raychaudhuri also note that, with further development, there are certain applications for which nanowires and related nanostructures are likely to provide either improved performance or new functionality that cannot be realized using more conventional material or fabrication technologies. In particular, nanowire-based systems are likely to be particularly well suited to those applications that require the harnessing different inherent material properties into a single system – such as CMOS circuitry with on chip optical interconnects, or applications involving unconventional substrates – such as flexible sensor arrays, displays, or energy harvesting systems.