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

Mono- and Biexponential Luminescence Decays of Individual Single-Walled Carbon Nanotubes

We have studied the exciton recombination dynamics of individual (6,4) and (6,5) single-walled carbon nanotubes embedded in aqueous gels or deposited on glass surfaces. CoMoCat nanotubes systematically display short monoexponential photoluminescence (PL) decays presumably due to defects introduced during their synthesis. In contrast HiPco nanotubes can either display mono- or biexponential PL decays depending on the environmental conditions. Transition from bi- to monoexponential decays can be reproduced by a simple three level model taking into account defect-dependent nonradiative decay mechanisms

Defect Induced Photoluminescence from Dark Excitonic States in Individual Single-Walled Carbon Nanotubes

We show that new low-energy photoluminescence (PL) bands can be created in semiconducting single-walled carbon nanotubes by intense pulsed excitation. The new bands are attributed to PL from different nominally dark excitons that are "brightened" due to defect-induced mixing of states with different parity and/or spin. Time-resolved PL studies on single nanotubes reveal a significant reduction of the bright exciton lifetime upon brightening of the dark excitons. The lowest energy dark state has longer lifetimes and is not in thermal equilibrium with the bright state.Comment: 4 pages, 3 figure

Exciton decay dynamics in individual carbon nanotubes at room temperature

We studied the exciton decay dynamics of individual semiconducting single-walled carbon nanotubes at room temperature using time-resolved photoluminescence spectroscopy. The photoluminescence decay from nanotubes of the same (n,m) type follows a single exponential decay function, however, with lifetimes varying between about 1 and 40 ps from nanotube to nanotube. A correlation between broad photoluminescence spectra and short lifetimes was found and explained by defects promoting both nonradiative decay and vibronic dephasing

Visualizing the Local Optical Response of Semiconducting Carbon Nanotubes to DNA-Wrapping

We studied the local optical response of semiconducting single-walled carbon nanotubes to wrapping by DNA segments using high resolution tip-enhanced near-field microscopy. Photoluminescence (PL) near-field images of single nanotubes reveal large DNA-wrapping-induced red shifts of the exciton energy that are two times higher than indicated by spatially averaging confocal microscopy. Near-field PL spectra taken along nanotubes feature two distinct PL bands resulting from DNA-wrapped and unwrapped nanotube segments. The transition between the two energy levels occurs on a length scale smaller than our spatial resolution of about 15 nm

Anisotropic Strain Induced Soliton Movement Changes Stacking Order and Bandstructure of Graphene Multilayers

The crystal structure of solid-state matter greatly affects its electronic properties. For example in multilayer graphene, precise knowledge of the lateral layer arrangement is crucial, since the most stable configurations, Bernal and rhombohedral stacking, exhibit very different electronic properties. Nevertheless, both stacking orders can coexist within one flake, separated by a strain soliton that can host topologically protected states. Clearly, accessing the transport properties of the two stackings and the soliton is of high interest. However, the stacking orders can transform into one another and therefore, the seemingly trivial question how reliable electrical contact can be made to either stacking order can a priori not be answered easily. Here, we show that manufacturing metal contacts to multilayer graphene can move solitons by several $\mu$m, unidirectionally enlarging Bernal domains due to arising mechanical strain. Furthermore, we also find that during dry transfer of multilayer graphene onto hexagonal Boron Nitride, such a transformation can happen. Using density functional theory modeling, we corroborate that anisotropic deformations of the multilayer graphene lattice decrease the rhombohedral stacking stability. Finally, we have devised systematics to avoid soliton movement, and how to reliably realize contacts to both stacking configurations