Rajay Kumar

Optical measurement of thermal transport in suspended carbon nanotubes

Thermal transport in carbon nanotubes is explored using different laser powers to heat suspended single-walled carbon nanotubes ∼5μm in length. The temperature change along the length of a nanotube is determined from the temperature-induced shifts in the G band Raman frequency. The spatial temperature profile reveals the ratio of the contact thermal resistance to the intrinsic thermal resistance of the nanotube. Moreover, the obtained temperature profiles allow differentiation between diffusive and ballistic phonon transport. Diffusive transport is observed in all nanotubes measured and the ratio of thermal contact resistance to intrinsic nanotube thermal resistance is found to range from 0.02 to 17.

Surface-enhanced Raman spectroscopy and correlated scanning electron microscopy of individual carbon nanotubes

Raman spectra of individual carbon nanotubes are measured by scanning a focused laser spot (0.5μm diameter) over a large area (100μm2) before and after depositing silver nanoparticles. Local regions exhibiting surface enhanced Raman spectroscopy (SERS) were located with respect to a lithographically patterned grid, allowing subsequent scanning electron microscopy to be performed. The uniquely large aspect ratio of carbon nanotubes enables imaging of the nanoparticle geometry together with the SERS active molecule. By measuring the same individual carbon nanotube before and after metal nanoparticle deposition, the SERS enhancement factor is determined unambiguously. The data reveals SERS enhancement factors up to 134 000, a consistent upshift in the G band Raman frequency and nanoparticle heating in excess of 600°C.

Laser directed growth of carbon-based nanostructures by plasmon resonant chemical vapor deposition.

We exploit the strong plasmon resonance of gold nanoparticles in the catalytic decomposition of CO to grow various forms of carbonaceous materials. Irradiating gold nanoparticles in a CO environment at their plasmon resonant frequency generates high temperatures and strong electric fields required to break the CO bond. By varying the laser power, exposure time, and gas flow rate, we deposit amorphous carbon, graphitic carbon, and carbon nanotubes. The formation of iron oxide nanocrystals catalyzes the growth of carbon nanotubes. Predefined microstructure geometries are patterned by moving the focused laser spot during the growth process, forming suspended single-walled carbon nanotube structures. Raman spectroscopy, energy dispersive X-ray spectroscopy, and transmission electron microscopy are used to characterize the resulting material. The localized nature of the plasmonic heating enables growth of these materials, while the underlying substrate remains at room temperature.

Rapid prototyping of three-dimensional microstructures from multiwalled carbon nanotubes

The authors report a method for creating three-dimensional carbon nanotube structures, whereby a focused laser beam is used to selectively burn local regions of a dense forest of multiwalled carbon nanotubes. Raman spectroscopy and scanning electron microscopy are used to quantify the threshold for laser burnout and depth of burnout. The minimum power density for burning carbon nanotubes in air is found to be 244μW∕μm2. We create various three-dimensional patterns using this method, illustrating its potential use for the rapid prototyping of carbon nanotube microstructures. Undercut profiles, changes in nanotube density, and nanoparticle formation are observed after laser surface treatment and provide insight into the dynamic process of the burnout mechanism.

Top-down lithographic method for inducing strain in carbon nanotubes

We demonstrate a method for inducing strain in carbon nanotubes using standard lithographic techniques. In this work, aligned nanotubes are partially suspended over trenches made by chemical etching. Strain-induced downshifts as high as 31 cm−1 are observed in the Raman spectra, roughly corresponding to 1.1% strain. We also observe significant shifts in the resonant transition energy Eii and an irreversible increase in the D band Raman intensity. The strains demonstrated using this technique are capable of creating significant bandgaps in metallic nanotubes, greater than kBT at room temperature, thereby making it possible to convert metallic nanotubes into semiconductors.