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Tip-enhanced near-field optical microscopy of single-walled carbon nanotube/polymer conjugates and improvements of the image contrast
Tip-enhanced near-field optical microscopy of single-walled carbon nanotube/polymer conjugates and improvements of the image contrast
Semiconducting single-walled carbon nanotubes (s-SWCNTs) are regarded as a promising candidate for a wide range of applications. Particularly, tailored blends of single-walled carbon nanotubes with polymers are ideally suited for organic solar cells (OSC) due to their inherent stability, high carrier mobility and the tunability of optical band gaps. In the past, the mixing of semiconducting and metallic SWCNTs and their aggregation into bundles strongly hindered their practical applications. More recently, many techniques have been developed to separate SWCNTs into chirality-pure single carbon nanotube samples. Nowadays, the most common approach is non-covalent polymer or surfactant wrapping. However, the characterization of these SWCNT samples is usually limited to a macroscopic level and few studies have looked into the interaction between SWCNTs and the wrapping agent on the single nanotube level. A comparison of the effects of different wrapping agents could provide useful guideline for device fabrication. Tip-enhanced near-field optical microscopy (TENOM) achieves 10 - 20 nm spatial resolution and substantial signal enhancement by employing an optical antenna such as a gold tip to localize and enhance light-matter interactions. In the first part of the thesis, we utilized TENOM to study the photoluminescence (PL) of single SWCNTs in different sample materials and investigated their optical heterogeneity. We introduced a statistical parameter to evaluate the intensity fluctuation of the near-field PL, which yields information on the occurrence of exciton trapping sites and quenching defects. We compared the results from different wrapping agents and post-synthesis treatment, and found that CVD synthesized SWCNTs, without further treatment together with PFO-BPy as wrapping agent exhibits the highest level of optical homogeneity. For TENOM and other types of scanning near-field optical microscopy (SNOM) as well, the presence of a far-field background from laser illumination of the sample lowers the signal to background ratio and decreases detection sensitivity. The second part of the thesis aims at suppressing the far-field background. We implemented a new tuning fork configuration in which the tip oscillates normal to the sample surface in a tuning-fork based TENOM setup, and drove an oscillation amplitude larger than the near-field decay length. A switch with a tunable threshold was used to extract the far-field part, which was subsequently subtracted from the total signal. Theoretical calculations were first performed to find the optimal tip modulation depth and switch threshold. The new configuration was finally applied to SWCNT PL imaging and was proven to significantly enhance the signal to background ratio.
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Shi, Xian
2017
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Shi, Xian (2017): Tip-enhanced near-field optical microscopy of single-walled carbon nanotube/polymer conjugates and improvements of the image contrast. Dissertation, LMU München: Faculty of Chemistry and Pharmacy
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Abstract

Semiconducting single-walled carbon nanotubes (s-SWCNTs) are regarded as a promising candidate for a wide range of applications. Particularly, tailored blends of single-walled carbon nanotubes with polymers are ideally suited for organic solar cells (OSC) due to their inherent stability, high carrier mobility and the tunability of optical band gaps. In the past, the mixing of semiconducting and metallic SWCNTs and their aggregation into bundles strongly hindered their practical applications. More recently, many techniques have been developed to separate SWCNTs into chirality-pure single carbon nanotube samples. Nowadays, the most common approach is non-covalent polymer or surfactant wrapping. However, the characterization of these SWCNT samples is usually limited to a macroscopic level and few studies have looked into the interaction between SWCNTs and the wrapping agent on the single nanotube level. A comparison of the effects of different wrapping agents could provide useful guideline for device fabrication. Tip-enhanced near-field optical microscopy (TENOM) achieves 10 - 20 nm spatial resolution and substantial signal enhancement by employing an optical antenna such as a gold tip to localize and enhance light-matter interactions. In the first part of the thesis, we utilized TENOM to study the photoluminescence (PL) of single SWCNTs in different sample materials and investigated their optical heterogeneity. We introduced a statistical parameter to evaluate the intensity fluctuation of the near-field PL, which yields information on the occurrence of exciton trapping sites and quenching defects. We compared the results from different wrapping agents and post-synthesis treatment, and found that CVD synthesized SWCNTs, without further treatment together with PFO-BPy as wrapping agent exhibits the highest level of optical homogeneity. For TENOM and other types of scanning near-field optical microscopy (SNOM) as well, the presence of a far-field background from laser illumination of the sample lowers the signal to background ratio and decreases detection sensitivity. The second part of the thesis aims at suppressing the far-field background. We implemented a new tuning fork configuration in which the tip oscillates normal to the sample surface in a tuning-fork based TENOM setup, and drove an oscillation amplitude larger than the near-field decay length. A switch with a tunable threshold was used to extract the far-field part, which was subsequently subtracted from the total signal. Theoretical calculations were first performed to find the optimal tip modulation depth and switch threshold. The new configuration was finally applied to SWCNT PL imaging and was proven to significantly enhance the signal to background ratio.