A study on the properties of graphene nanoribbons for electronics

Graphene has the potential to lead the construction of next-generation electronics due to its high mobility, high thermal conductivity, and atomically thin two-dimensional structure. However, graphene’s intrinsic zero bandgap property restricts its use in a wide range of applications. Thus, the controllable engineering of the band structure of graphene is both important and necessary for many future applications. Nano-lithography has facilitated research into band structure engineering of graphene, as it has been shown that a size-dependent bandgap opens for graphene nanoribbons with widths below around 40 nm. To date, photo- and electron-beam lithography (PL & EBL, respectively) have routinely been used to etch graphene to fabricate graphene nanoribbons with sizes down to around 15 nm. However, these processes are complex and time-consuming, involve the use of solvents, resists, metals, and often oxygen plasma, and they result in devices with potentially significant amounts of residues and chemical contamination. In this thesis, we explore AFM-based scanning probe lithography (SPL) as an alternative method for the minimization of graphene-based field effect transistors (G-FETs), as it is single-step, potentially higher resolution, and does not involve the use of any chemicals other than water, so is essentially a direct-write process. We investigated three kinds of AFM-based SPL, namely (i) mechanical, (ii) DC voltage and (iii) AC voltage, and demonstrated their capabilities for patterning graphene at the micro- and nano-scale. Finally, we developed the process to such a degree that we were able to demonstrate a sub-10nm G-FET device by AFM-based SPL with AC voltage, which signifies the narrowest graphene nanoribbon (GNR) fabricated by a top-down lithography method. In addition, we have observed that the charge neutrality point (CNP) shifts as the width of these GNRs reduces below 80nm. This is in good agreement with previous studies on the CNP shift in G-FET deices fabricated by EBL in our group. To explore the mechanisms underlying this phenomenon, we performed simulations using Cambridge Serial Total Energy Package (CASTEP) to compute the band structure of GNRs with different widths and both with and without H-passivation at the edges, as a first step towards taking edge effects into account. Combined with our experimental findings, we propose that the effect of dangling bonds at the edges is to lower the energy levels of bands in GNRs, resulting in a shift of the CNP. Our investigations contribute to a better understanding of edge states in graphene nanodevices.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2024

Author dc:creator

• Liu, Xiao

Advisor dc:contributor.advisor

• Durkan, Colm

• graphene
• lithography
• Density Function Theory

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/076ec8cc-bd0f-49ae-ae0a-0cfafa7cfaf6/download
• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.113774

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/376604