Public defence in Engineering Physics, M.Sc. Md Abu Taher Khan

Title of the thesis: Electrical properties and transport characteristics of single-walled carbon nanotube bundles

Thesis defender: Md Abu Taher Khan 
Opponent: Principal Researcher Don Norimi Futaba, National Institute of Advanced Science and Technology (AIST), Japan
Custos: Professor Esko Kauppinen, Aalto University School of Science

Single-walled carbon nanotubes (SWCNTs) are one-dimensional members of the carbon family with tremendous potential for a wide range of applications, including field-effect transistors, integrated circuits, wearable sensors, medical diagnostic devices, conductive films, solar energy systems, and next-generation EUV pellicles.

This doctoral study focuses on the fundamental electrical properties of as-grown SWCNT bundles. A novel one-step direct deposition method, from reactor to chip, was developed to measure the pristine electrical properties of SWCNTs. Previous solvent-based deposition methods often contaminated the nanotubes, affecting their pristine electrical properties. Using our method, we achieved electrical performance 3.3 times better than the solution-based approach.

In floating catalyst chemical vapor deposition (FC-CVD), SWCNTs naturally form bundles that contain a mixture of semiconducting and metallic nanotubes. Since one-third of as-grown nanotubes are metallic, one might expect most transistors to behave like metals, with high off-state currents and low on-off ratios. However, our results showed a higher fraction of semiconducting devices than the as-grown semiconducting SWCNTs. This is because metallic tubes are shorter and less effective when embedded in bundles.

We also observed that charge carrier mobility increases in larger bundles, and that tube-to-tube junction resistance is the main factor limiting mobility in thin films. A single junction can reduce mobility by two to four times compared to a single-bundle transistor.

Additionally, this study achieved a high-purity semiconducting SWCNT fraction of 94%, the highest reported for as-synthesized SWCNTs.

In the future, these findings can help mitigate the effects of metallic nanotubes in devices by controlling bundle formation. This study can guide future research toward achieving even higher SWCNT purity (up to 99.9999%) without post-processing, moving closer to the ultimate goal of high-performance SWCNT-based transistors.

Keywords: FC-CVD, SWCNT bundle, field-effect transistor, thin film transistor, high fraction, semiconducting, metallic, mobility, on-off ratio, tube-to-tube junction.

Contact information: 
md.1.khan@aalto.fi 
Linkdin: https://www.linkedin.com/in/abu-taher-khan-b31a59219/ 
Research Group: https://www.aalto.fi/en/department-of-applied-physics/nanomaterials-nmg 

Thesis available for public display 7 days prior to the defence at Aaltodoc.