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Hydrogen plasma etched graphene nanoribbons

Kalyoncu, Yemliha Bilal. Hydrogen plasma etched graphene nanoribbons. 2018, Doctoral Thesis, University of Basel, Faculty of Science.

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Official URL: http://edoc.unibas.ch/diss/DissB_12928

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Abstract

Magnetic ordering in low dimensional semiconducting structures is an important element for spin-based electronics. Zigzag graphene nanoribbons which sustain ferromagnetic edge states and ordered nuclear magnetic states in GaAs 2DEGs are two such examples which represent potentials for spintronics applications. In order to be realized, both of these phenomena require certain conditions. For the magnetic ordering at the zigzag edges of graphene nanoribbons, fabrication of high quality edges is a prerequisite. Additionally, for the nuclear magnetic ordering in GaAs, the 2D system needs to reach below miliKelvin temperatures.
Here, we study hydrogen plasma etching in graphite and graphene flakes in order to fabricate graphene nanoribbons with zigzag edges. We study the distance and pressure dependence of the etching process in graphite flakes, and define two distinct plasma regimes. The direct plasma regime contains high density of H radicals and energetic ions, which continuously induce defects on the surface, and results in perforated surfaces. On the other hand, the remote plasma regime includes only H radicals, giving the opportunity to take the etching process under control to fabricate graphene structures.
The underlying substrate plays an important role in the etching process. For single layer flakes on hBN, the etching is highly anisotropic and creates hexagonal etch-pits, whereas on SiO2, the etching is isotropic. For bilayer flakes, the process is anisotropic on both subtrates.
Atomic resolution atomic force microscopy reveals that the edges of the etched hexagons on graphite are parallel with the zigzag crystallographic direction and the absence of D-peak intensity in Raman spectra represents the high quality of the edges, since only armchair and disordered edges can result in D-peak intensity. However, for single layer samples on hBN, the high D-peak intensity shows that even tough the edges are along the zigzag direction, they are interrupted by symmetric armchair segments. Analysis on the polarization dependent Raman measurements result in the ratio of 40% for armchair segments along a zigzag edge.
The picture of poor quality graphene edges is further confirmed by the low temperature transport measurements. None of the features predicted for electronic band structure of zigzag graphene nanoribbons are observed in the experiments. The findings are also supported by the tight-binding simulations for a disordered zigzag edge.
In the second part of the thesis, we applied the well-established adiabatic nuclear demagnetization technique on a coulomb blockade thermometer in order to reach electron temperatures of below 1mK. In this experiment, each measurement lead is cooled by its own nuclear refrigerator made of 2mols of copper. The nuclei and charge carriers inside the coulomb blockade thermometer are cooled by large copper fins deposited on the chip. In order to investigate the efficiency of the process, we tested two different ramp-rates and precooling durations. Electron temperatures of 1.8mK and 2.7mK in the coulomb blockade thermometer are measured in two cool-downs, resulting in efficiencies of about 10%. A simple thermal model is given in the context of the measurements considering the heat leaks and thermal dynamics within the system, also comparing the results with previous experiments.
Advisors:Zumbühl, Dominik M. and France, J.
Faculties and Departments:05 Faculty of Science > Departement Physik > Physik > Experimentalphysik Quantenphysik (Zumbühl)
Item Type:Thesis
Thesis Subtype:Doctoral Thesis
Thesis no:12928
Thesis status:Complete
Bibsysno:Link to catalogue
Number of Pages:1 Online-Ressource (150 Seiten)
Language:English
Identification Number:
Last Modified:24 Jan 2019 11:07
Deposited On:24 Jan 2019 11:06

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