Selvasundaram, Pranauv Balaji (2020)
Near Infrared Photocurrent Spectroscopy on Carbon Nanotube Devices.
Technische Universität Darmstadt
doi: 10.25534/tuprints-00015395
Dissertation, Erstveröffentlichung, Verlagsversion

Kurzbeschreibung (Abstract)

Single-walled carbon nanotubes (SWCNTs) constitute an allotrope of carbon with a two dimensional lattice structure rolled up to a seamless cylinder. Owing to their one dimensional structure, they possess unique optical properties including the ability to absorb light from visible to infrared regime. Additionally, they behave as metals or semiconductors depending upon their structure or the direction in which they are rolled up with respect to the graphene lattice. With solution processing techniques, it is now possible to fabricate devices consisting of carbon nanotubes with tailored properties for a variety of applications including optical detectors, optical emitters and other organic electronics. This thesis work is focused on achieving for photocurrent generation in carbon nanotube transistor devices working under ambient conditions. The transistors with split gate geometry were fabricated with solution - processed carbon nanotubes as the transport channel and characterized using photocurrent spectroscopy. The nanotubes were integrated to form transistor devices dielectrophoretically, by depos-iting semiconducting previously sorted CNTs using polymer assisted size exclusion chromatography (SEC) in toluene. Also, a new method to measure the length distributions of the nanotubes using Analytical Untracentrifugation (AUC) was explored for the first time for non-aqueous suspensions. It was realized that the sedimentation behavior of monochiral suspensions studied in toluene deviated strongly from prior works carried out for aqueous suspensions, but a new and a rather simpler model allows to explain the observation. On characterizing the transistor devices with photocurrent spectroscopy, it is realized that for the device geometry used, the photocurrent spectrum correlates well with the absorption spectrum of the deposit-ed nanotubes. Also, the results display signatures from the substrates in the off-resonant regions of the spectrum. It was realized that only insulating substrates could provide a clean photoresponse specific to the nanotubes alone and that CNTs are sensitive to light absorption by the underlying substrate. Furthermore, electric - field assisted measurements were carried out by applying a gate voltage on the split gates and measuring the short - circuit (source to drain) photocurrent signal. Based on the trends observed in the results, it is possible to identify the mechanism behind the generated photocurrent signal for a particular measurement scheme. Also, the results indicate that the Schottky barrier at the nanotube and the metal electrode interface dominates the pn-junction formed by the split gates. Never-theless, the (n, m) specific relative photocurrent contribution could be tailored with the electrostatic field from the split gates. Lastly, transistors were fabricated with silicon contact with solution processed few chiral and monochiral carbon nanotubes as the transport channel. Photocurrent spectroscopy was carried out on these transis-tors as well. Results show photocurrent signals originating from the substrate as well as from the smaller diameter tubes however, in opposite polarity, but not from the larger diameter tubes due to unfavorable energy level positions primarily for the few chiral suspensions. However, devices fabricated with a monochiral suspension, revealed that the photocurrent signatures resemble the signatures of the few chiral suspensions, indicating that the signatures are rather an effect from the substrate and not from the nanotube channel itself.

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