Alavi Eshkaftaki, Seyed Khalil: Optical spectroscopy and applications of atomically precise graphene nanoribbons: from light emission to photodetection. - Bonn, 2020. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
Online-Ausgabe in bonndoc:
author = {{Seyed Khalil Alavi Eshkaftaki}},
title = {Optical spectroscopy and applications of atomically precise graphene nanoribbons: from light emission to photodetection},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2020,
month = nov,

note = {Atomically precise graphene nanoribbons (GNRs) are narrow stripes of graphene with a width of a few atoms and a well-defined edge structure. On-surface synthesis of GNRs from molecular precursors enables atomic precision in producing GNRs with identical width and edge topology. This allows synthesizing GNRs with tunable electronic and optical properties that hold promise for exploration of fundamental materials physics and applications in the next generation of nanoscale electronic and optoelectronic devices. In the present work, the optical properties of such GNRs are explored and they are applied to demonstrate nanoscale photodetectors.
The first study of this work was dedicated to exploring the photophysics of single layers of aligned seven-atom wide armchair GNRs (7–AGNRs) in order to answer the question of whether such GNRs are fluorescent. For this purpose, polarized photoluminescence (PL) and Raman microspectroscopy were carried out in the visible spectral range. It is found that the GNRs are intrinsically dim as a result of non-radiative transitions. This is analogous to their sister materials, carbon nanotubes (CNTs). I discovered that the GNRs could be rendered bright through a photo-induced brightening mechanism in which localized defects are created in the GNRs under laser exposure in ambient conditions. The process of defect formation is activated by continuous low-power blue laser illumination. This mechanism facilitates controlled local modification of the PL emission of GNRs, which enables patterning of microscopic emitting structures. This is in contrast to chemically-driven methods to induce defects, which are commonly used to render CNTs bright. Extinction spectroscopy was applied to gain additional insight into the role of defects on the PL brightening of GNRs. For this purpose, I measured the extinction spectrum of a single-atom-thick layer of the GNRs. The extinction is anisotropic in accordance with the structure and alignment of the GNRs. The results reveal the dominant presence of excitonic effects in optical response, which is consistent with earlier work. In pristine GNRs, one of these features spectrally coincides with the peak of PL emission (at 1.8 eV) in defected GNRs. Observation of a decrease in the intensity of this feature upon formation of defects, and its relation with PL increase, suggests that this originates from a quenching state in the energy structure of pristine GNRs. The absorption of GNRs is significantly higher than that of graphene due to excitonic effects, making them interesting for photodetection.
In the final part of the work, I harness the merit of atomically precise GNRs for photodetection in the visible range. The fabricated photodetectors consist of a single layer of aligned 7–AGNRs placed between source and drain electrodes. The channel length of preliminary devices is larger than the length of GNRs. Thus the electrical transport is mediated by charge hopping between GNRs, being distinct with respect to transport in graphene. The dark current of the photodetectors in photoconductive operational mode is low in comparison to graphene photodetectors while the responsivity is similar.
This thesis presents the photophysical properties of semiconducting AGNRs and successful application of them to demonstrate atomically-thin photodetectors. With this work, I introduce the application of GNRs in optoelectronic devices.},

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