Determining Carbon Nanotube Properties from Raman. Scattering Measurements

Transcription

1 Determining Carbon Nanotube Properties from Raman Scattering Measurements Ying Geng 1, David Fang 2, and Lei Sun The Institute of Optics, Electrical and Computer Engineering, Laboratory for Laser Energetics Rochester, NY Abstract Carbon nanotubes (CNT) have fascinating optical, electrical and mechanical properties which stem from their unique geometry. Current fabrication techniques are limited in their control of CNT size. Therefore, a post-process characterization technique such as Resonant Confocal Raman Spectroscopy (RCRS) becomes useful for highlighting nanotubes with properties of interest. In our study, we grow carbon nanotubes of varying diameters using a chemical vapor deposition (CVD) process and subsequently determine chirality and electronic transport properties using RCRS. Introduction Research in carbon nanotubes (CNT) has progressed rapidly since their discovery in 1991 by Iijima from the NEC laboratory in Tsukuba, Japan [1]. CNTs are quasi-one-dimensional structures with a length on the order of microns and diameter of 1-2 nanometers. A single-walled Nanotube (SWNT) is comprised of a single layer of carbon (graphene) with its ends folded to form a seamless hollow cylinder. Multi-walled nanotubes are constructed from multiple SWNT of varying radii arranged concentrically. Nanotubes attract a great amount of interest because of their size and unique physical, electrical, and optical properties [2]. A significant barrier in nanotube research, however, stems from limited control of nanotube size distribution and chirality (n,m) during fabrication. Therefore, post-fabrication characterization techniques become critical. Resonant Confocal Raman Spectroscopy (RCRS) is one such technique used to characterize CNTs [3]. In this study, we determine the size, chirality, and electrical transport properties of several CNTs using RCMRS.

2 Nanotube preparation A chemical vapor deposition (CVD) system was used in the synthesis of CNTs for our study. A quartz substrate is pre-baked in the oven at 500 C at 10-5 mbar for 12 hours to eliminate moisture which can hinder the growth process. A methanol solution containing the catalyst bis(acetylacetonato)-dioxomolybdenum(vi) and cobalt acetylacetonate (0.1% each by weight) was placed on the substrate and then baked under atmospheric pressure for 20 minutes at 500 C causing the cobalt to oxidize. Next, the sample was placed into the tube furnace which was evacuated to 10-5 mbar and purged with Ar to bring the pressure inside the furnace to 10 mbar. Pressure at 10 mbar was maintained with hydrogen gas flow at a rate of 200 sccm. The sample was then heated to 800 C reducing the Cobalt Oxide to metal Cobalt. In the final step, ethanol gas was flown into the tube at a rate of 1000 gm/min at 8 mbar for 30 mintues. At 800 C, ethanol is decomposed into carbon which reacts with the cobalt catalyst to form CNTs. The sample is allowed to cool for several hours under Ar flow before Raman measurements were taken. Raman scattering spectra CNTs are described by their tube diameter and chirality. The chirality is defined r r r by the vector C h = na1 + ma2 ( n, m), where a r 1 and a r 2 are unit vectors of the hexagonal lattice. This vector connects two crystallographically equivalent sites O and A on a two-dimensional plane shown in Figure 1. Figure 1 The chirality vector describing an unrolled CNT

3 The diameter of a (n,m) nanotube can be determined by d = C / π = 3a ( m + mn n ) 1/ /π, where a C-C is the nearest-neighbor center t h C C + to center distance (1.42 Å for graphite) and C h is the length of the chiral vector C r h. SWNT are classified as metallic or semiconducting based on their chiral indices (n, m). Metallic tubes have n m = 3q and semiconducting CNTs have n m 3q where q is an integer. Raman measurements were performed using a He-Ne laser operating at a wavelength of nm (1.96 ev) and input power of ~300 μw at the sample surface. Figure 2 shows a schematic of the confocal microscope experimental setup used for Raman signal acquisition. Sample on XY Translation Stage Oil immersion NA = 1.4 (60X) nm ~300 μw Linearly Polarized He-Ne 10/90 BS APD FM LP, BP Filter Notch filter Spectrometer Figure 2 Confocal microscope experimental setup Figure 3 shows a 40 μm x 40 μm scanning confocal microscope image of a sample area. The encircled areas indicate selected areas where the laser excitation was in resonance with the CNT structures. Figure 5 shows a typical Raman spectrum from a

4 semiconducting nanotube. The peaks at 1300, 1592 and 2600 cm -1 correspond to the D-band (defect), G-band and G -band, respectively. The D-band is a result of a photon-defect interaction. The G-band is due to the longitudinal and to a lesser extent the circumferential, stretching of the nanotube. The G -band stems from a photon-second phonon interaction. These aforementioned peaks are characteristic of both CNTs and graphite sheets. 5 μm Figure 3 Confocal microscope image with selected nanotubes circled. The contrast is acquired by integrating over the intensity of the G-band in the Raman spectrum. A nm He-Ne laser was used for the source. The peak at 300 cm -1 corresponds to the radial breathing mode (RBM) and is unique to CNTs. The RBM also provides information on chirality and thus the electronic properties of the nanotube. Because a single excitation energy was used in our experiment, only nanotubes resonant with this particular energy will demonstrate a peak at the RBM frequency. The nanotube diameter can be determined by ν RBM = 248/ d t [3], where ν RBM is the Raman frequency shift of the RBM in cm -1. The ratio of the D-band to G-band intensity I D /I G is used as a measure of the defect density of a particular nanotube and can provide information on the crystalline quality of the CNTs. Because the nanotubes were grown at low temperature by CVD, it is

5 possible that some of the structures did not fully crystallize due to insufficient thermal energy during the annealing process. It is also possible that stray atoms may have infiltrated the carbon lattice thus creating a defect in the periodic structure of the nanotube. Pixel RBM (cm -1 ) n m d (Å) ID / IG Class SC M N/A M N/A M M M M M M SC 11 N/A N/A N/A N/A N/A M/SC M SC Occurrence Nanotube Size Distribution Diameter (Angstroms) Table 1 CNT properties of thirteen samples Figure 4 Histogram showing CNT diameter distribution The diameter and (n, m) chirality of the 13 CNT samples are listed in Table 1. Characteristic CNT properties were determined based on the frequency of the RBM and laser excitation energy. Our experimental values were compared with measurements performed by Maultzsch et. al to determine nanotube chirality and diameter [4]. A distribution of CNT diameter is shown in Figure 4. Because only thirteen samples were probed, no conclusions can be made about the general distribution of nanotube diameter of this growth process. From literature, it is predicted that nanotube size distributions approach a Gaussian [2]. Figure 5 and Figure 6 demonstrate typical spectra from semiconducting and metallic nanotubes, respectively. We note that the G-band lineshape is different for these two samples as well as the position of the RBM frequency.

6 Figure 5 Raman spectra of semiconducting sample Figure 6 Raman spectra of metallic sample Summary Fabrication of carbon nanotubes was carried out using the CVD method and the structure of CNTs identified using Raman scattering spectra. Both semiconducting and metallic nanotubes were probed. Of the thirteen samples, four could be classified as semiconducting. In general, the defect density of our samples was low (< %25). Acknowledgements This project could not have been possible without the guidance of Graham Marsh (fabrication) and Neil Anderson (Raman measurements).

7 References [1] S. Iijima, Helical microtubules of graphitic carbon, Nature (London) 354: 56, [2] M. S. Dresselhaus, G. Dresselhaus, Ph. Avouris, Carbon Nanotubes: Synthesis, Structure, Properties and Applications, Springer [3] A. Jorio, R. Saito, J. H. Hafner, et al, Structural ( n, m) Determination of Isolated Single-Wall Carbon Nanotubes by Resonant Raman Scattering, Phys Rev Lett. 86: 6, [4] A. Maultzsch, H. Telg, S. Reich, C. Thomsen, Radial breathing mode of single-walled carbon nanotubes: Optical transition energies and chiral-index assignment, Phys Rev B. 72: , 2005.