CARBON NANOTUBE CHARACTERIZATION

F1 : Experimental setup for Raman characterization

F2: Raman spectra of CNT using Ti:Saph laser and TTNF Courtesy of Prof. Richard Martel (U. Montreal)

Resonance Raman Spectroscopy on Carbon Nanotubes

Raman spectroscopy (RS) is a powerful tool to study vibrational, optical, and electronic properties of materials in a non-destructive manner. Raman signals are typically orders of magnitude lower than the intensity of the excitation laser line. However, it is possible to significantly increase Raman signals by choosing an excitation wavelength that corresponds to an optical transition of the material under investigation.

In the case of carbon nanotube (CNT), the nanostructures will determine transition energies, and therefore the wavelength of the excitation laser line should be continuously tuned to match optical transitions of the material. Resonant Raman Spectroscopy (RRS) provides a unique tool to characterize the size distribution and chirality of a mixed population of nanotubes. RRS is also a powerful method to monitor in-situ the CNT composition during growth. Thus, it can serve as a diagnosis tool in order to achieve better control on the CNT production for applications such as nanotube conductive layers or transistors.

Using the technology of volume Bragg grating (VBG), Photon etc. has developed, specifically for RRS, two types of ultra narrow bandwidth tunable filters: a laser line filter and a notch filter. The Laser Line Tunable Filter (LLTF) is installed just after the Tunable Laser Source (TLS) (Fig. 1), and blocks the unwanted fluorescence produced by the TLS, leaving the excitation laser line untouched. Two steering mirrors (M1 and BS) send the laser line into a standard microscope where the laser beam is focused on the sample of interest. The second filter, a Tunable Top Notch Filter (TTNF), is installed after the microscope used to collect Raman signals generated by the sample. The TTNF blocks the Rayleigh scattering coming from the material, leaving the Raman signal untouched down to 50 cm-1 with a throughput higher than 60%. After the TTNF, a standard spectrometer can be used to analyse low frequency Raman signals that should normally require the use of a triple spectrometer.

Since the tunable filters are controlled by a computer via USB links, changing the wavelength of operation of the entire system only takes a few minutes. Stokes and anti-Stokes Raman spectra of single-walled carbon nanotube (SWNT) powder presented in fig. 2 were measured within less than one hour using a standard spectrometer. Each peak corresponds to a Radial Breathing Mode (RBM) and the center frequency of a given peak is inversely proportional to nanotube diameter of a given population. Several populations of nanotube with different diameters can therefore be readily observed and effectively characterized.