Raman Characterization of Nitrogen Doped Multiwalled Carbon Nanotubes
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Raman Characterization of Nitrogen Doped Multiwalled Carbon Nanotubes S. Webster, J. Maultzsch1, C. Thomsen1, J. Liu, R. Czerw, M. Terrones2, F. Adar3, C. John3, A. Whitley3 and D. L. Carroll Center for Nanotechnology, Wake Forest University, Winston-Salem, NC 1 Institut für Festkörperphysik, Technische Universität Berlin, Germany 2 Advanced Materials Department, IPICyT, Venustiano Carranza, San Luis Potosi, Mexico. 3 Jobin Yvon Horiba, Raman Spectroscopy Group, Edison NJ ABSTRACT N-type multi-walled nanotubes were synthesized by nitrogen doping using pyridine and pyridine-melamine mixtures in chemical vapor deposition, and their donor states were verified by Scanning Tunneling Spectroscopy. Tunneling Electron Microscopy reveals small amounts of residual catalyst and Scanning Electron Microscopy show well aligned mats of the Nitrogen doped nanotubes. Nitrogen is present in the lattice of these MWNTs as pyridine structures and CNx structures. Raman scattering measurements were performed as a function of increasing growth temperature and the results compared to previously studied boron doped multiwalled nanotubes. INTRODUCTION Doping of carbon nanotubes can greatly modify their electronic and optical properties paving the way to the creation of novel nanoscale devices. Nitrogen doping of carbon nanotubes (N-MWNTs) creates an electron donor state in the conduction band near the Fermi level.[1] Tailoring the electronic and optical properties by incorporation of dopants will lead to new applications in nano- electronics and photonics. Additionally, introducing defect states in the nanotubes by doping allows us to probe fundamental properties of carbon nanotubes. Previously, boron doping of multiwalled nanotubes (B-MWNTs) was used to investigate the defect induced double resonant process.[2] In this manuscript, Raman spectroscopy of the N-MWNT is used for experimental investigation of the doping level.
EXPERIMENTAL DETAILS Multiwalled carbon nanotubes, both pure (MWNTs) and Nitrogen doped, were grown by chemical vapor deposition (CVD). For comparison, the doped tubes were grown as a function of temperature using melamine and pyridine as different nitrogen sources. Using a growth process similar to Andrews et al. [3] pristine nanotubes were formed. N-MWNTs grown from pyridinemelamine mixtures were synthesized by sublimating melamine (nitrogen source) in a preheater. Additionally, a mechanically injected solution of ferrocene (catalyst), xylene (carbon source) and pyridine (second nitrogen source and additional carbon source) was introduced into the furnace while using hydrogen as a scavenging and carrier gas. Pyridine N-MWNTs were grown with ferrocene as the catalyst and pyridine functioning as both the nitrogen and carbon source. Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) were performed on a Hitachi HD2000 and a Hitachi 4700 respectively. Scanning Tunneling Microscopy (STM) and Spectroscopy (STS) were carried out in ultra high vacuum using a RHK low current STM with a Pt-Ir tip.
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