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Posted: Jun 19, 2009
Another step in advancing nanotechnology fabrication techniques
(Nanowerk Spotlight) One of the outstanding challenges in nanotechnology generally, and in the exploitation of so-called 'bottom-up' assembly of basic nanoscale building blocks such as nanowires, is the development of techniques for assembling large numbers of such nanostructures into more complex systems and precisely specified patterns in an accurate, deterministic manner. For instance, it is possible to build transistors, optical devices, and sensors with very specific properties – such as alloy composition or physical dimension – using nanowires. Thus many useful applications of nanowires will depend on the ability to take these building blocks and organize them in some deterministic way in order to ultimately construct and interface with a nanowire-based system.
New work by scientists at the University of California, San Diego, demonstrates the basic capability for, and elucidates some of the guiding principles in, the use of dielectrophoretic (DEP) behavior to direct the placement of large numbers of nanowires on complex, pre-patterned structures as might be required for integration of nanowires with, for example, silicon-based microelectronic circuitry. They show that a high level of placement precision can be achieved by paying careful attention to the signal frequency as well as the macroscopic electrode architecture that is employed.
"In looking at nanotechnology we saw the problem of assembly to be a significant and challenging hurdle that will need to be overcome to realize the benefits of nanowire based devices," Sourobh Raychaudhuri tells Nanowerk. "In considering a number of different approaches we believed that DEP offers a good compromise between the benefits of being able to tailor devices and being able to place them in a deterministic, scaleable way. Prior work in the field has already suggested that DEP can be a viable option. Our work takes a careful look at the physics associated with DEP and identifies the importance of frequency and electrode geometry in order to place nanowires on very narrow electrodes – the kind of electrodes that one might envision to be very important when building a complex nanowire based system."
In their work, the team demonstrates the ability to place and arrange a number of individual nanowires with a very high level of precision using AC electric fields.
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)
"Specifically" says Raychaudhuri, " we have examined the well known dielectrophoretic theory in order to determine the importance of various electric field parameters including amplitude, frequency, field direction and field gradient in order to influence and orient specific axes of the nanowire and thus allow for the precise placement of nanowires at specified locations."
The high geometric aspect ratio of a nanowire results in a situation where the electric field will act on the long and short axis of the nanowire differently. A thorough understanding of how nanowires will behave in various electric field geometries and conditions will help system designers construct electrode structures that will allow for the placement of nanowires in a precise and deterministic way.
The major focus of the UCSD researchers was to explore how a nanowire can be placed on very narrow electrodes; electrodes that are of similar widths to that of the nanowire. This is a challenging problem, but one that could help unlock the ability to make fairly complex systems. They also believe that it should be possible to apply electric potential to these fingers individually at different times, allowing sequential nanowire placement. This is significant because it allows for the possibility of placing different types of nanowires next to each other in the same system. The ability to integrate a variety of specifically tailored devices into a single system will help realize one of the significant advantages that a nanowire-based design can have over conventional planar electronics.
"There are a number of scenarios in which our work could be directly relevant, and in fact provide a key enabling capability" says Yu. "For example, it might be necessary to place large numbers of nanowire-based devices, e.g., photodetectors or light emitters, in contact with pre-patterned electrodes and control circuitry for an imaging or display system. The control circuitry and electrodes would almost certainly need to be fabricated using conventional, well-established planar microelectronic processing techniques. However, integration of nanowires with the resulting chip would then require either direct synthesis of the nanowires on the chip, which may not be compatible with thermal or other process constraints, or highly precise and directed placement of the nanowires at specific locations on the chip, which our work now would enable."
This work would also enable nanowires or other nanostructures with very different properties, e.g., light detection or emission at different wavelengths, or sensitivity to different chemical stimuli, to be integrated in a straightforward manner within a single chip or system – something that would most likely be much more challenging with a direct synthesis approach.
Generally, these findings could be useful in a wide variety of nanowire-based systems, basically in any design that requires the precise and deterministic placement of a specific nanowire. One example that Raychaudhuri gives is a high resolution chemical sensing system – using many individual nanowire devices to achieve great sensitivity and resolution for a given stimulus – as well as using variety of engineered nanowires to sense a number of different stimuli in the same system.
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.