CARBON NANOTUBE CHARACTERIZATION

Photon etc has designed two narrowband tunable filters for resonance Raman spectroscopy (RRS). These matched tunable passband and notch filters, based on thick volume Bragg gratings, exhibit a bandwidth less than 10cm^-1 and cover a wavelength range of several hundred nanometers.

Raman spectroscopy (RS) is a powerful tool to study the vibrational, optical, and electronic properties of materials in a nondestructive 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.

Resonant Raman Spectroscopy provides a unique tool to characterize the diameter and chirality distribution of a mixed population of carbon nanotubes (CNT). RRS is also a powerful method to monitor in-situ the CNT properties during growth. Thus, it can serve as a diagnosis tool in order to achieve better control of the CNT production.

Experimental Conditions

Using volume Bragg grating (VBG) technology, Photon etc has developed, specifically for RRS, two types of ultra narrow band tunable filters: a laser line filter and a notch filter. The Laser Line Tunable Filter (LLTF) is installed on the Tunable Ti:Saph laser (Figure 2) and blocks the unwanted fluorescence produced by the laser, 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 on the microscope. The TTNF blocks the Rayleigh scattering coming from the material, leaving the Raman signal untouched down to 50 cm^-1 (20cm^-1 has been achieved) with a throughput of up to 80%. The tunable filters are controlled by a computer via USB links, allowing fast wavelength selection.

Results

Stokes and anti-Stokes Raman spectra of single-walled carbon nanotube (SWNT) powder (Figure 1) 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 the nanotube diameter of a given population. Several populations of nanotubes with different diameters can therefore be readily observed and effectively characterized.

MAIN TECHNICAL SPECIFICATIONS

 

---

Excitation Wavelength Range		700-1000 nm  or  800-1100 nm
Average Power at the Sample		5 mW
Microscope Objective and Spatial Resolution		50x (< 1 µm)  and  100x (< 0,8 µm)
Detection Range		from 50 cm-1 and up to CCD detection limit (1100nm)
Detection Resolution @ 715 nm		0.6 cm-1
Detection Resolution @ 1100 nm		0.2 cm-1

 

DETAILED TECHNICAL SPECIFICATIONS

LASER LINE TUNABLE FILTER

---

Bandwidth	@ 715 nm	0.4 nm (8 cm-1)
	@ 1100 nm	0.4 nm (3 cm-1)
Spectral Range		715 - 1000 nm or 800 -1100 nm
Peak Transmission		Up to 60%

---

TUNABLE TOP NOTCH FILTER

---

Bandwidth	@ 715 nm	0.4 nm (8cm-1)
	@ 1100 nm	0.4 nm (3 cm-1)
Spectral Range		715 - 1000 nm or 800 - 1100 nm
Optical Density		4.0
Signal Out-of-Band Throughput		Up to 80%

---

SPECTROMETER

---

Raman Resolution	@ 715 nm	0.6 cm-1
	@ 1100 nm	0.2 cm-1
Raman Range		from 50 cm-1 and up to CCD detection limit (1100 nm)
Monochromator Focal Length		500 nm

DETECTOR

EXCITATION SOURCE Ti : Saphire CW Laser

MICROSCOPE SYSTEM

F.1. Raman spectra of carbon nanotubes using TI:SAPH Laser & Tunable Top Notch Filter
(Courtesy of Prof. R. Martel, U of Montreal)

F.2. Resonant Raman System Configuration