Spatial variation of the electrochemical potential due to nanoscopic scatterers in epitaxial graphene investigated by scanning tunnelling potentiometry
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Date
2023
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Doctoral Thesis
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Abstract
As electric devices continue to shrink in size, it has become increasingly important to understand nanoscopic transport for future technological advancements. One technique that has shown promise in this area is scanning tunneling potentiometry (STP), which enables the imaging of local deviations in electrochemical potential (ECP) on a device in which a charge current is flowing while simultaneously measuring its topography at nanometer resolution. This approach provides direct correlations between atomic structures and electron transport, offering valuable insight into phenomena that take place at length scales that cannot be resolved through conventional transport measurements. Graphene, as the first two-dimensional material, has shown promising potential for future applications due to its unique massless Dirac fermions. However, intrinsic or extrinsic defects and the substrate have been found to strongly deteriorate the charge transport properties of graphene. Li intercalation is proposed as a means to decouple the graphene from the substrate and significantly increase the carrier density. The objectives of this thesis are to further improve STP programs and investigate the local electron transport around defects, as well as the effects of Li intercalation on epitaxial graphene grown on the SiC substrate.
In the first part, we present improvements and comparisons of STP programs implemented on two different scanning tunneling microscope (STM) operation platforms: R9 and Nanonis. The tip scan stability is enhanced at high lateral current density on the R9 platform by implementing a line-by-line current reversal protocol, which increases the signal-to-noise ratio for the ECP measurements. A Nanonis STP program is developed, exhibiting lower noise levels; however, it suffers from a longer overhead time. The differences between R9 and Nanonis are due to different handling of time critical commands as well as their overall ADC/DCAC architecture. For both platforms, we can readily achieve rms noise levels down to a few μv.
In the second part, growth recipes for monolayer and bilayer graphene on SiC(0001) are developed using in situ direct heating procedure. The graphene sheets are characterized by Raman spectroscopy, atomic force microscopy, and STM. Different parameters in the annealing curves for the growth are investigated, including the temperature and annealing time. Furthermore, their influence on the number of graphene layers is studied in detail. In the optimized growth condition, we obtained reproducible growth of mono- and bilayer graphene on the entire SiC surface.
In the third part, we conduct local STP measurements around intrinsic defects, including domain boundaries in bilayer graphene, monolayer-bilayer interfaces, and pits, which are discontinuities in the graphene layer. Interestingly, dipole-like scattering potential profiles appear around the pits, with their amplitudes dependent on the pit size. Particularly, we demonstrate that naturally occurring pits act as strong scattering centers in the intermediate regime between diffusive and ballistic transport.
In the fourth part, the influence of extrinsic defects is investigated through Li intercalation on epitaxial graphene. The intercalation processes are characterized by STM at room temperature, revealing two distinct Li intercalation phases with different charge transfer states. The sheet resistance and step resistance around the Li-intercalated graphene are thoroughly characterized by STP measurements. By correlating doping effects with the electron transport properties directly, our findings highlight the critical role of charge transfer from Li in modifying the graphene properties via intercalation.
Finally, we present efforts toward gateable graphene systems, such as graphene/hBN heterostructures and exfoliated graphene flakes on a prepatterned circuit. Despite technical challenges, these measurements show promising potential for studies of local electron transport on gateable graphene, where carrier density can be flexibly modulated.
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Examiner : Gambardella, Pietro
Examiner: Stepanow, Sebastian
Examiner: Kröger, Martin
Examiner : Schuler, Bruno
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ETH Zurich
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Subject
Charge transport; Graphene; scanning tunnelling potentiometry
Organisational unit
03986 - Gambardella, Pietro / Gambardella, Pietro
Notes
Funding
163225 - Local detection of spin accumulation by scanning probe microscopy (SNF)