Rapid Synthesis of One-Dimensional Nanostructures via Induction Heating

The synthesis platform was first demonstrated for the bulk synthesis of carbon nanotubes (CNTs). By rapidly heating the heavily-doped, catalyst-coated silicon specimen to 700-1000degC for just one minute, it was possible to synthesize aligned, base-growth, multi-walled carbon nanotubes with an average diameter of 6.8nm. Growth rates as high as 200μm/min were achieved, leading to experimental turn-around times as low as two minutes. A 15 second heating of the substrate to 900degC to nucleate the catalyst was observed to dramatically increase the CNT density, while higher temperatures were observed to decrease the CNT diameter.

The versatility of the induction heating synthesis platform was extended through the development of a method for the localized synthesis of nanostructures on MEMS. CNTs were successfully synthesized near the center of the induction coil through induced Joule heating of ring-shaped MEMS structures. An analytical model, developed to observe the dependence of the induced-EMF in the rings, was used to determine the favorable locations within the coil for low-power synthesis, and the trends were validated experimentally. Several two-terminal connections were synthesized enabling IV measurements of the metallic carbon nanotubes, and the extraction of the CNT resistance.

Using the induction heating synthesis technique, the discovery was made of a uniquely-shaped, titanium dioxide nanostructure that we are calling a “nanosword.” The broad, flat, high-quality, single-crystalline nanoswords of rutile phase taper to a sharp tip with a 2-3nm radius of curvature. The catalyst-free synthesis, in bulk or locally on MEMS, occurred in as little as one minute with just a thin film of source titanium, making this technique one of the fastest and simplest methods for the synthesis of titania nanostructures. Two different crystallographic orientations were observed. The first grows along the [100]crystallographic direction, with the tip tapered at 90deg enclosed by two {110} planes and exposing a large (001) plane. The second structure has a twin-plane along the (101) and grows along the [101]crystallographic direction, with the tip tapered at 80.1deg enclosed by four {110} planes and exposing a large (101) plane. Temperatures near 900degC were needed for synthesis, and increasing source material thickness increased the nanosword density. Average lengths and widths were measured to be 6.5μm and 542nm, while thicknesses were 55-62nm.

To characterize the material properties of the induction-heating-synthesized nanoswords, a simple technique for the manipulation and integration of faceted, nanostructured materials was developed by exploiting van der Waals forces.The adhesive forces between the nanoswords and various different substrates were measured. Using this technique to create test specimens, the elastic modulus of the nanoswords was measured by contact-mode atomic force microscopy to be 177GPa, while electrical resistivity of the nanoswords averaged 0.192Ω-cm. Photoluminescence testing revealed a dominant peak at 436nm. Moreover, an ultraviolet light sensor was fabricated from the nanoswords, illustrating an increase in current of 25% to 6W UV illumination at 1V bias.

Finally, the induction-heating-grown nanoswords were integrated into a tunable plasmonic antenna device. Two-photon photoluminescence tests on isolated nanoswords revealed enhancements as large as 251-fold at the nanosword tip, while closing the gap of the MEMS-integrated nanosword antenna showed a repeatable, plasmonically coupled increase in the localized optical field between the tips. Though several strategies will need to be implemented for furtherenhancement at the tips, it is believed that this device may provide versatilefunctionality for surface-enhanced Raman scattering analysis.