Time-Resolved Photoluminescence and Elastic White Light Scattering Studies of Individual Carbon Nanotubes and Optical Characterization of Oxygen Plasma Treated Graphene

In the course of this work the excited state dynamics of individual
single-walled carbon nanotubes (SWCNTs) were studied by a
combination of confocal PL spectroscopy and time correlated single
photon counting (TCSPC) measurements. Nonradiative decay channels
dominate the excited state dynamics of SWCNTs leading to low
photoluminescence (PL) quantum yields and PL decay times on the
picosecond timescale. Knowledge about the microscopic nature of
these decay channels is crucial to improve the material properties.
The measurements on the single nanotube level revealed large
tube-to-tube variations of PL decay times, which could be
attributed to different defect densities for different tubes. For
the present SWCNT material the PL decay times only depend weakly on
the nanotube length. SWCNT material synthesized by using a
cobalt-molybdenum catalyst (CoMoCAT) systematically display short
monoexponential PL decays, while the PL decay dynamics of SWCNTs
produced high pressure decomposition of carbon monoxide process
(HiPco) is either mono or biexponential depending on the respective
local environment of the nanotube. The transition from a bi- to
monoexponential PL decay can be explained by synthesis-dependent
differences in defect densities. This defect related nonradiative
decay channels reduce the amplitude of one decay component below
the experimental detection limit. It is further shown, that
photo-induced defects and gold atoms adsorbed to the sidewalls of
SWCNTs are shown to alter the PL properties of individual SWCNTs.
Additional low-energy PL satellite bands arise in the spectra.
Their origin can be attributed to emission from nominally dark
excitons which are ”brightened” due to defect facilitated mixing of
intrinsic states with different parity/spin. The role of defects in
the brightening process was investigated by time-resolved PL
measurements and complementary Raman spectroscopy. Based on its
energy separation and the unusually slow PL decay dynamics the
lowest energy satellite band can be assigned to the radiative
recombination of a triplet exciton. In a second project a
common-path interference scattering approach (iSCAT) utilizing a
conventional inverted laser scanning confocal microscope combined
with a photonic crystal fibre as a supercontinuum white light
source is successfully tested for its capabilities for elastic
scattering imaging and spectroscopy of individual SWCNTs. Finally,
it is shown that single layer graphene can selectively be turned
luminescent upon exposure to a mild oxygen plasma. The treatment
leads to a strong and spatially uniform PL which is characterized
by a single, broad PL band extending from the visible to the near
infrared spectral region. The analysis of the defect related Raman
I(D)/I(G) intensity ratio indicates the formation of nanometer
sized islands for which the sp2 conjugated lattice of graphene is
still preserved. Emission of quantum confined states within these
islands is discussed as a possible origin of the PL.