Comprehensive Pyrometry of Incandescent Multiwalled Carbon Nanotubes and Graphene in the Visible and Near Infrared
Pyrometry via blackbody radiation is used to measure temperature of systems throughout physics. The spectrum, described by Planck's law, depends solely on the temperature T and surface area A of a black source. However, the derivation of Planck's law considers only the limit where wavelength λ << L, the linear dimension of the source. Many nanosystems, however, exist in the opposite limit, λ > L, in the visible and near infrared.
We investigate this subwavelength limit of thermal radiation by bringing multi-walled carbon nanotubes, for which r << λ < L$, to incandescence via joule heating. Their light is measured in an optical microscope, and their geometry is measured with a transmission electron microscope. With complete knowledge of the source geometry and the photon emission, a full characterization of the temperature, thermal conductivity, and electrical conductivity of each nanofilament is possible. We find that the filaments emit highly polarized light with suprablack effective emissivities, indicative of thermal radiation originating in a phase coherent manner from the nanotube's volume as opposed to its surface area, in concordance with classical electromagnetism.
Multiwavelength pyrometry is then performed on graphene, which has linear dimensions in the classical blackbody limit and theoretical gray emission. This pyrometry allows us to measure the number of layers in the graphene, a result confirmed with absorption measurements in agreement with Kirchoff's law of thermal radiation. Light emitted from incandescent graphene's bulk is found to be unpolarized, yet exhibits polarization as high as 20% near the sheet edge in accordance with diffraction theory. However, light polarized to 5% is observed originating from the bulk away from the hot region, which we attribute to anisotropy in the temperature gradient.
We also find that as temperature of a nanotube and graphene increases, the signal in the near infrared becomes suppressed compared to the emission models. As trapped surface states and contaminants on graphene samples shift the Fermi energy away from the Dirac point, long wavelength transitions become disallowed. Thus, this infrared effect owes to the transparency at long wavelengths of carbon nanostructures with a Fermi level shifted away from the Dirac point, implying that broadband optical modulation in the visible and near infrared is attainable through gating and heating of carbon nanotubes and graphene.