Trions, Exciton Dynamics and Spectral Modifications in Doped Carbon Nanotubes: A Singular Defect-Driven Mechanism

Doping substantially influences the electronic and photophysical properties of semiconducting single-wall carbon nanotubes (s-SWNTs). Although prior studies have noted that surplus charge carriers modify optical spectra and accelerate non-radiative exciton decay in doped s-SWNTs, a direct mechanistic correlation of trion formation, exciton dynamics and energetics remains elusive. This work examines the influence of doping-induced non-radiative decay and exciton confinement on s-SWNT photophysics. Using photoluminescence, continuous-wave absorption, and pump-probe spectroscopy, we show that localization of and barrier formation by trapped charges can be jointly quantified using diffusive exciton transport- and particle-in-the-box models, yielding a one-to-one correlation between charge carrier concentrations derived from these models. The study highlights the multifaceted role of exohedral counterions, which trap charges to create quenching sites, form barriers to exciton movement, and host trion states. This contributes significantly to understanding and optimizing the photophysical properties of doped SWNTs

Photoluminescence microscopy and spectroscopy of semiconducting nanotubes

Im Rahmen dieser Dissertation wurden optische Eigenschaften von halbleitenden, einwandigen Kohlenstoffnanoröhren (SWNTs) der (6,5)-Chiralität untersucht. Dies gelang durch Ensemblemessungen aber vor allem durch den Aufbau eines Mikroskops zur Messung an einzelnen SWNTs. Dieses Einzel- SWNT-Mikroskop ermöglichte nebst „normaler“ Bildgebung durch Sammlung und Abbildung der nahinfraroten Photolumineszenz (PL) der (6,5)-SWNTs auch die spektral- und zeitaufgelöste Untersuchung der PL. Durch Verwendung von Dichtegradientenultrazentrifugation (DGU) zur chiralen Aufreinigung des SWNT-Rohmaterials konnten alle Messungen unter Minimierung des störenden Einflusses von Aggregaten oder SWNTs anderer Chiralität durchgeführt werden. Untersucht und bestimmt wurde der Absorptionsquerschnitt und die Exzitonengröße, die PL-Eigenschaften aggregierter SWNTs und der Einfluß der Permittivität auf die PL einzelner SWNTs.Within the course of this work fundamental optical properties of semiconducting single-walled carbon nanotubes (SWNTs) of the (6,5)-chirality were examined by utilizing ensemble measurements and in particular a home-built microscope setup for measurements of individual SWNTs. This single-SWNTmicroscope allowed for „standard“ imaging of the near infrared photoluminescence (PL) signal of the (6,5)-SWNTs as well as for spectrally and timeresolved PL measurements. Facilitating density gradient ultracentrifugation (DGU) for chiral enrichment of the SWNT soot, all measurements were carried out with minimum influence of aggregates or minority species of other SWNT chiralities. The absorption cross section, the exciton size, PL-features of aggregated SWNTs and the influence of permittivity on SWNT-PL have been investigated

Ultrafast non‐linear 2D microspectroscopy reveals coherent phonon‐mediated intra‐ and intervalley exciton interaction in an individual SWCNT

Further developments in molecular electronics, adressing SWCNTs, would benefit strongly from insights in the spatio‐temporal evolution of molecular processes. Ultrafast non‐linear techniques provide tracking of energy transfer pathways e.g., mediated via electron‐phonon coupling [1]. A comprehensive way to observe these dynamics is coherent 2D fluorescence microspectroscopy [2]. This method is a generalization of transient absorption spectroscopy with frequency‐resolved pump and probe steps, combined with spatially‐resolved optical microscopy. This provides, e.g., to observe the phonon‐mediated formation and annihilation dynamics of initially bright and dark‐state excitons due to the strong exciton‐phonon coupling on the femtosecond timescale. Here, we utilize the third‐order 2D signal for monitoring the trapped intra‐ and intervalley exciton interaction in a SWCNT [3,4]. To this end, an transform‐limitted LCD‐shaped four‐pulse sequence is focused on an (6,4) SWCNT and the fluorescence is detected as a function of inter‐pulse time delays and phases. [1] Graham, M. et al., Nano Lett. 12, 813–819 (2012). [2] Goetz, S. et al., Opt. Express 26, 3915–3925 (2018). [3] Secchi, A. et al., Phys. Rev. B 88 (2013). [4] Kislitsyn, D. et al., J. Phys. Chem. Lett. 5, 3138–3143 (2014).</p

Ultrafast non-linear 2D micro-spectroscopy reveals coherent phonon-mediated intra- and intervalley exciton interaction in an individual SWCNT

Poster presentation from  NT19: International Conference on the Science and Application of Nanotubes and Low-Dimensional Materials 21-26 July 2019, Würzburg, Germany We perform nonlinear fluorescence-based two-dimensional spectroscopy(F-2DES) on (6,4) SWCNTs at the single molecule level. We highllight the possibilities with the single molecule setup for fluorescence based experiments at ultra-diluted light levels. We provide evidence for single-molecular investigation by various methods. We identifiy an energetic substructure in the phonon sideband of individual SWCNTs, observed with F-2DES measurements. We evalaute the significance of the peak structure in F-2DES data, by the application of statistical inference methods. We relate our findings of the energetic peak positions to polaron formation, induced by phonon-mediated intervalley dynamics. We explain F-2DES data by Liouville pathways. We further observe vacancy relaxation of laser induced defects in SWCNTs and verify our findings with help of a temperature model </p

Ultrafast polaron dynamics of individual SWCNTs indicating electron transfer related to the optical analogue of the Gunn effect

The Gunn effect [1] has changed daily lives through the use of "transferred-electron" devices. Further developments would benefit from reduced dimensionalities (e.g., single-molecule transistors) with tunable quantized nonlinear electro-optical response. Special challenges are to overcome the "THz gap" and to access quantum effects at room temperature that are limited by ensemble averaging or short lifetime. In this regard an optical Gunn behaviour was shown in Si nanowires and GaAs and proposed in single-walled carbon nanotubes (SWCNTs) [2]. We carried out ultrafast two-dimensional (2D) fluorescence spectroscopy [3] on individual SWCNTs, which provides insight into the spatio-temporal evolution of electron-vacancy-phonon mediated intervalley dynamics during ultrafast polaron decay in individual SWCNTs. In addition we compare the decay over time and power dependence of the fluorescence with simulations, to identify the mechanism for electron transfer and discuss this in the light of the optical analogue of the Gunn effect [1], which meets quantization conditions. [1] J. B. Gunn, Solid State Commun. 1, 88-91 (1963). [2] G. Pennington et al.,Phys. Rev. B 68, 045426 (2003) [3] S. Goetz et al., Optics Express 26, Nr. 4: 3915-25 (2018).</p

Application of Buffon‘s needle on molecular fluorescence probability

Fluorescence of extended and collective quantum systems (SWCNTs) Fluorescence emitter density from fluorescence images Monte Carlo simulations for molecular fluorescence quenching Applications of Buffon's needle </p