DNA used to assemble carbon nanotube electronics

Citation

(2010), "DNA used to assemble carbon nanotube electronics", Assembly Automation, Vol. 30 No. 3. https://doi.org/10.1108/aa.2010.03330caf.002

DNA used to assemble carbon nanotube electronics

Article Type: Mini features From: Assembly Automation, Volume 30, Issue 3

One of the most pressing but as yet unresolved issues surrounding the fabrication of carbon nanotube-based electronics is how to orientate the CNTs in the required positions. Now, a research team from Caltech, the California Institute of Technology, has demonstrated a unique orientation technique that utilises the self-assembling properties of DNA by employing so-called “DNA origami”. This is a type of self-assembled molecular structure that can be programmed to form almost any shape or pattern at the nanoscale by exploiting the sequence recognition properties of DNA base paring. Invented by Caltech Computer Scientist Paul Rothemund in 2005 (see Nature, 440, pp. 297-302), DNA origami is an example of the burgeoning field of structural DNA nanotechnology. They are created in a saline solution from a long, single strand of viral DNA and a mixture of different, short synthetic DNA strands which bind to and “staple” the viral DNA into the desired shape. This binding arises from the pairing of DNA’s nucleotide bases (A, T, C and G) with those that have complementary sequences (A with T, C with G). A “standard” DNA origami is a rectangle with sides of about 100 nm and with over 200 “pixel” positions where arbitrary DNA strands can be attached.

In the Caltech experiments, two batches of single-walled CNTs labelled by DNA with different sequences were created, which for convenience were dubbed “red” and “blue”, and some DNA strand pixels were labelled anti-red and anti-blue. This marked the positions where the colour-matched CNTs were intended to attach. The rectangular origami template had dimensions of ∼75×96 nm and displayed two lines of single-stranded DNA “hooks” in a cross pattern with a spatial resolution of ∼6 nm. It was designed so that the red-labelled CNTs would cross perpendicular to the blue nanotubes, creating a field-effect transistor (FET) structure, the basic building block of most ICs. This structural configuration was confirmed by atomic force microscopy. Devices were removed from solution, placed on a silicon substrate and palladium/gold electrodes were attached to measure the electrical properties which indeed showed FET-like behaviour. Critically, this is a scalable technology; it is possible to design the origami to construct complex logic units and do this for thousands or even billions of units that self-assemble in parallel. This work was first reported on-line in Nature Nanotechnology (“Self-assembly of carbon nanotubes into two-dimensional geometries using DNA origami templates”. Maune et al., 8 November 2009) and was supported by the National Science Foundation, the Office of Naval Research and the Centre on Functional Engineered Nano Architectonics.

The Caltech group has also collaborated with a ten-strong team from IBM’s Almaden Research Centre and recently reported a technique to create DNA origami-shaped binding sites on electronically useful materials such as silicon dioxide (SiO₂) and diamond-like carbon (Kershner et al. “Placement and orientation of individual DNA shapes on lithographically patterned surfaces”, Nature Nanotechnology, Vol. 4, pp. 557-61). Either electron beam or optical lithography were used with dry oxidative etching to create arrays of binding sites of the necessary size and shape to match those of individual origami structures. Key to the process was the discovery of the template material and deposition conditions which yield high selectivity so that the origami binds only to the patterns of “sticky patches” and nowhere else. To connect the origami to the templates, magnesium ions are added to the saline solution containing the origami. The positively charged ions stick to both the DNA origami and the negatively charged patches on the template. Thus, when the solution is poured over the template, a negative-positive-negative sandwich is created, with the magnesium atoms acting as a “glue” to hold the origami to the patches. It has been shown that the DNA origami bind with high selectivity and good orientation: 70-95 per cent of sites have individual origami aligned with an angular dispersion as low as ±10° on diamond-like carbon and ±20° on SiO₂. In principle, this should allow the fabrication of complex arrangements of nanomaterials such as CNTs, silicon nanowires or quantum dots. Ulimately, these bottom-up, DNA-based technologies may one day allow chipmakers to phase-out their hundreds of million-dollar lithography and other “top-down” fabrication facilities and replace them with far less costly wet processing systems.