Exciton dynamics and energies in single carbon nanotubes
Final Report Abstract
This project focused on the experimental investigation of the exciton energies and decay dynamics in semiconducting single-walled carbon nanotubes (SWCNTs). To gain detailed insight into these properties and the factors influencing them we studied single SWCNTs to avoid averaging over different species (n,m) and within the same species. We first implemented a scheme for recording elastic white-light scattering spectra of single SWCNTs on glass embedded in immersion oil and detected their E22 transitions together with their photoluminescence (PL) spectrum for unambiguous (n,m) assignment. By careful tuning of the refractive indices of microscope glass cover slide and immersion oil we were able to control the signal contrast and to increase it up to 23 % for single (6,5) SWCNTs. Due to the limited coherence properties of the photonic crystal fiber used to generate the broadband white-light continuum, however, we were not able to record clear signals in the E11 spectral regime even after the contrast optimization mentioned above. We thus could not follow the E11 exciton dynamics in the implemented pump-probe scheme until know. Relaxation from the E22 state occurs within less than 20 fs and is too fast to be observed by the available pulse length of 150 fs. At the end of this project we obtained access to a broadband laser source that allowed us to observe E11 transitions for SWCNTs on a thin SiO2 / gold layer structure. For this sample we observe an extremely large scattering contrast in the E11 region and we are now studying the angular distribution of the scattered light by back-focal plane imaging. Preliminary data indicates a strong interaction between excitons in the SWCNT and propagating surface plasmons in the metal film. Based on these results we continue our efforts on the pump-probe experiments on single SWCNTs. The exciton energies and decay dynamics were then studied in detail by timeresolved PL spectroscopy. We achieved the following key results. First, we clarified the origin of the conflicting reports on the excited state dynamics of single SWCNTs for the same type of sample material in collaboration with the group of Prof. Lounis at the University of Bordeaux and traced it to an unsuitable instrument response function. Second, we studied the influence of the nanotube environment and growth methods and refined the three state model used to describe the decay dynamics of SWCNTs. Third, we showed for the first time that new low-energy PL bands can be created in the spectra of semiconducting SWCNTS by intense pulsed excitation. The new bands were attributed to PL from different nominally dark excitons that are brightened because of a defect-induced mixing of states with different parity and/or spin. The lowest-energy dark state has longer lifetimes and is not in thermal equilibrium with the bright state. We showed for the first time that the PL intensity and decay dynamics of SW- CNTS can be efficiently controlled by surface acoustic waves (SAWs). PL quenching is caused by electric fields that are associated with SAW leading to a decrease of the radiative exciton recombination rate by up to 25 %. The field dependence of the radiative rate reduction can be explained by a quadratic Stark effect. In a collaboration with the Universities of Cambridge and Manchester we investigated the excited state energies and decay dynamics of oxygen-plasma treated graphene. These were the first reports on spatially extended and homogeneous PL emission from a graphene based material.
Publications
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“Excited state dynamics of individual single-walled carbon nanotubes”, PhD thesis (2009), University of Pisa, Italy
H. Harutyunyan
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“Making Graphene Luminescent by Oxygen Plasma Treatment”, ACS Nano 3, 3963 (2009)
T. Gokus, R. R. Nair, A. Bonetti, M. Bohmler, A. Lombardo, K. S. Novoselov, A. K. Geim, A. C. Ferrari, A. Hartschuh
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“Defect induced photoluminescence from dark excitonic states in individual single - walled carbon nanotubes”, Nano Lett. 9, 2010 (2009)
H. Harutyunyan, T. Gokus, A. Green, M. Hersam, M. Allegrini, A. Hartschuh
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“Mono and biexponential luminescence decays of individual single walled carbon nanotubes”, J. Phys. Chem. C 114, 14025 (2010)
T. Gokus, L. Cognet, J.G. Duque, M. Pasquali, A. Hartschuh, B. Lounis
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“Time-Resolved Photoluminescence and Elastic White Light Scattering Studies of Individual Carbon Nanotubes and Optical Characterization of Oxygen Plasma Treated Graphene”, PhD thesis (2011), LMU München, Germany
T. Gokus
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“Controlling exciton decay dynamics in semiconducting single-walled carbon nanotubes by surface acoustic waves”, Chem. Phys. (2012)
M. E. Regler, H. J. Krenner, A. A. Green, M. C. Hersam, A. Wixforth, A. Hartschuh
