Li, Wenshan (2017)
Advanced Dielectrophoresis for High-Performance Single-Walled Carbon Nanotube Electronics.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

Single walled carbon nanotubes (SWCNTs) are cylindrical tubes with nanometer diameter, which can be thought of a wrapped-up mono-layer graphene sheet equivalently. Benefiting from their unique one dimensional structure, SWCNTs possess many unique electronic properties, such as high electrical conductivity, high charge mobility, low field emission threshold, high current capacity, etc. Additionally, SWCNTs can behave as either metals or semiconductors depending on their electronic band structures. These features make them one of the most promising materials for novel electronics. The present thesis focuses on the fabrication of high performance electronics based on semiconducting SWCNT (s-SWCNT) using dielectrophoresis (DEP) techniques. There are two key requirements that have to be satisfied for efficient DEPs, i.e. 1) using SWCNT suspensions with the highest possible semiconducting content and 2) depositing s-SWCNT with high packing-density in good alignment. Recently, toluene-based, polymer-assisted size exclusion chromatography was reported to yield over 99.7% s-SWCNT content in suspensions, which is a promising candidate for high-performance nanotube devices. Therefore, such suspensions were widely used in this thesis. In order to make dense and well-aligned s-SWCNT deposition by DEPs, the key issue is how to enhance the polarizability of carbon nanotubes more effectively. Basically, three aspects can be taken into considerations, i.e. the s-SWCNT itself, the solvent for suspending s-SWCNTs in and the applied electric field during the DEPs. By populating excitonic excitations, thereby pumping up free charges of SWCNT themselves using high-power laser, s-SWCNT deposition was improved using laser-coupled DEP technique, as confirmed by scanning electron microscope and electrical properties of the corresponding transistors. The achievement was attributed to the field-dependent exciton relaxations of s-SWCNTs. In terms of DEP solvents, media with low dielectric properties were verified to be productive for enhancing the polarizability of s-SWCNTs. In addition, through analyzing orientations of nanotubes in such media under an external electric field, an efficient method was also proposed to obtain the chirality-resolved nanotube length distribution. Furthermore, it was evident that the low-k solvent based DEPs perhaps supports a new approach towards the selective deposition of s-SWCNTs rather than their metallic counterparts when considering the difference of geometry features in SWCNTs. Effects of electric fields on the SWCNT DEP deposition were studied using proper finite element simulations in combination with experimental characterizations, which further clarified the underlying mechanism of solution-nanotube based DEPs in presence of a direct current (DC) or alternating current (AC) field. As a consequence, low-frequency bias was demonstrated to be beneficial for pronouncing the polarizability of s-SWCNTs compared to high-frequency one. Furthermore, the low-conductive media based DC-DEPs, making use of both low-k solvent and DC bias, was demonstrated to be the most efficient way to perform the s-SWCNT DEPs under the scope of this thesis. Lastly, s-SWCNT transistors with a hole mobility up to 297 cm^2V^-1s^-1 and On/Off ratio as high as 2×10^8 were achieved using DC based DEPs of toluene dispersed, polymer-wrapped SWCNTs. Moreover, by replacing the Si/SiO2 based back-gate with weakly oxidized Al top-gate, a sub-threshold swing of 95 mV/decade of SWCNT transistors was realized.

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