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Raman radiation studies of nanocarbon materials
Raman radiation studies of nanocarbon materials
Raman spectroscopy is a valuable characterization method applicable to a broad range of materials. Vibrational information enclosed in the Raman spectrum provides access to various material properties. Because of the strong dependence of vibrations on the material architecture, Raman spectroscopy is an excellent technique for defect investigation. More detailed insight about the analyzed substance is gained by coupling Raman spectroscopy to back focal plane imaging, an experimental method that allows to angularly resolve radiation. This work mainly aims at understanding the physical processes occuring during the interaction of either electromagnetic or acoustic waves with nanocarbon material. The first part reports on the angular distribution of Raman G and 2D scattering from graphene situated on a glass substrate recorded by back focal plane imaging. The Raman G emission can be seen as the superposition of two orthogonal incoherent point-dipoles, oriented in the sample plane, while the 2D Raman signal can be described as a sum of three incoherent point-dipoles each rotated by 120°. Parameter-free calculations of the G and 2D intensities are in excellent agreement with the experimental radiation patterns. The 2D polarization ratio depends on the numerical aperture of the microscope objective due to the depolarization of the emission and excitation light at the air-dielectric interface and tight focusing. Because of this, the polarization contrast decreases substantially for increasing collection angle. This also influences the 2D/G intensity ratio, a crucial quantity for the determination of single-layer graphene. The results are thus important for the quantitative analysis of the Raman intensities in confocal microscopy. First steps towards the measurement of antenna-enhanced Raman radiation patterns are shown. The pattern of the enhanced Raman G signal reflects the orientation of the optical antenna. The second part addresses the Raman defect study of single-walled carbon nanotubes(SWCNTs) processed by a material-efficient dispersion method. This procedure is based on the recycling of the precipitate in multiple steps. SWCNTs from the recycling process exhibit longer mean lengths than SWCNTs extracted by comparable standard dispersion methods. The Raman defect density of the recycled SWCNTs is determined by recording the Raman D/G intensity ratio. The statistical Raman analysis shows a clear increase in defect density with rising sonication time. However, there is no increase in defect density with each recycling step. SWCNTs extracted in a late recycling step are localized in the center of a SWCNT aggregate protecting them from sonication.
Graphene, Carbon Nanotubes, Raman
Budde, Harald
2016
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Budde, Harald (2016): Raman radiation studies of nanocarbon materials. Dissertation, LMU München: Faculty of Chemistry and Pharmacy
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Abstract

Raman spectroscopy is a valuable characterization method applicable to a broad range of materials. Vibrational information enclosed in the Raman spectrum provides access to various material properties. Because of the strong dependence of vibrations on the material architecture, Raman spectroscopy is an excellent technique for defect investigation. More detailed insight about the analyzed substance is gained by coupling Raman spectroscopy to back focal plane imaging, an experimental method that allows to angularly resolve radiation. This work mainly aims at understanding the physical processes occuring during the interaction of either electromagnetic or acoustic waves with nanocarbon material. The first part reports on the angular distribution of Raman G and 2D scattering from graphene situated on a glass substrate recorded by back focal plane imaging. The Raman G emission can be seen as the superposition of two orthogonal incoherent point-dipoles, oriented in the sample plane, while the 2D Raman signal can be described as a sum of three incoherent point-dipoles each rotated by 120°. Parameter-free calculations of the G and 2D intensities are in excellent agreement with the experimental radiation patterns. The 2D polarization ratio depends on the numerical aperture of the microscope objective due to the depolarization of the emission and excitation light at the air-dielectric interface and tight focusing. Because of this, the polarization contrast decreases substantially for increasing collection angle. This also influences the 2D/G intensity ratio, a crucial quantity for the determination of single-layer graphene. The results are thus important for the quantitative analysis of the Raman intensities in confocal microscopy. First steps towards the measurement of antenna-enhanced Raman radiation patterns are shown. The pattern of the enhanced Raman G signal reflects the orientation of the optical antenna. The second part addresses the Raman defect study of single-walled carbon nanotubes(SWCNTs) processed by a material-efficient dispersion method. This procedure is based on the recycling of the precipitate in multiple steps. SWCNTs from the recycling process exhibit longer mean lengths than SWCNTs extracted by comparable standard dispersion methods. The Raman defect density of the recycled SWCNTs is determined by recording the Raman D/G intensity ratio. The statistical Raman analysis shows a clear increase in defect density with rising sonication time. However, there is no increase in defect density with each recycling step. SWCNTs extracted in a late recycling step are localized in the center of a SWCNT aggregate protecting them from sonication.