Nanoscale optoelectronic characterization of graphene-based heterostructures
by Francesca Falorsi
Date of Examination:2025-03-07
Date of issue:2025-06-11
Advisor:Prof. Dr. Thomas Weitz
Referee:Prof. Dr. Thomas Weitz
Referee:Prof. Dr. Martin Wenderoth
Persistent Address:
http://dx.doi.org/10.53846/goediss-11307
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Abstract
English
The ongoing downscaling of electronic devices requires innovative approaches to probe and manipulate electronic behavior at the nanoscale. Two-dimensional (2D) materials offer a remarkable opportunity to directly explore transport processes at this scale and to precisely modify their optoelectronic properties by changing their environment. In particular, graphene is an ideal candidate for such applications due to its remarkable optoelectronic properties, which can be directly tuned by external parameters (i.e. gating), and its combination with other 2D materials to form so-called van der Waals (vdW) heterostructures (HSs). In this thesis, we studied different types of graphene-based HSs with two purposes: to investigate the origin of the resistivity on a nanoscopic level and to manipulate the properties of graphene by combining it in HSs with a new class of materials, known as covalent-organic frameworks (COFs) and metal-organic frameworks (MOFs). These materials are crystalline polymers with customizable structure and electronic properties. In the first part of the thesis, we investigated the mechanisms dominating the Ohmic resistance on a nanoscopic level. The theoretical framework addressing this topic was developed by Landauer in 1957. Landauer predicted that the voltage drop across a conductor would not be uniform. Instead, it would be localized around fixed scatterers due to a dipolar charge distribution that forms around defects as electronic flow occurs, known as Landauer resistivity dipoles (LRDs). The long-range dipolar charge distribution is too weak to be probed in 3D, thus the first experimental evidence was observed only after the isolation of graphene and other 2D materials. However, only a few experimental studies are available due to the need for a non-invasive local probe. In our study, we utilize the nanometer lateral resolution of near-field photocurrent imaging to thoroughly analyze the formation of LRDs in terms of total carrier density and drain current at a monolayer-bilayer graphene interface, which constitutes an ideal one-dimensional defect. We observed that the photocurrent at the interface has the same polarity as the applied drain current for Fermi energy values close to the charge neutrality point (i.e., at low hole or electron doping). This signature is consistent with the carrier accumulation induced by the LRDs and disappears at higher carrier densities, in agreement with the numerical calculations performed. We could thus demonstrate that photocurrent nanoscopy can be used as a non-invasive technique to study local dissipation at hidden interfaces, and we foresee that this method can be applied to study the electronic flow of other 2D systems. In the second part of the thesis, we investigated the combination of graphene in vdW HSs with 2D COFs/MOFs. The integration of these novel 2D materials with vdW crystals remains mainly unexplored, although it could be a fruitful avenue of exploration. COFs/MOFs offer highly tunable chemical and electrical properties, as they can be synthesized in all mathematically possible Bravais lattices. In a first study, we thoroughly investigated the optoelectronic properties of HSs composed of graphene and an insulating rectangular 2D polyimide (2DPI). Using theoretical density functional theory calculations and several experimental techniques, we found an interlayer charge exchange between the 2DPI and graphene, which induces hole doping in the graphene layer. This interlayer charge exchange can be tuned with the thickness of the 2DPI layer. In particular, the doping increases with polymer thickness. In a second study, aimed at increasing the interlayer interaction, we investigated the magnetotransport properties of an HS composed of graphene and a 2D MOF with paramagnetic copper. While we observed intriguing behaviors in magnetic fields and resistance, further investigation is needed to understand their origin. In general, the findings of these works highlight the unique ability to tailor functionalities in 2D COF/MOF-based HSs. This could open avenues for the development of optoelectronic devices with precisely engineered properties and stimulate further exploration of the diverse phenomena accessible through tailored designs of the 2D COFs/MOFs. In conclusion, we analyzed different types of graphene-based HSs. The low-disorder graphene hBN encapsulated HSs allowed us to gain valuable insights into fundamental aspects of electronic transport, while the COF/MOF-graphene HSs enabled us to tailor the graphene's properties. Thus, twenty years after its isolation, graphene continues to serve as an extraordinary platform for understanding fundamental transport mechanisms and remains a possible key component for modern electronics due to the great tunability of its properties.
Keywords: Graphene; Nanoscale resistivity; Photocurrent; Landauer resistivity dipoles; Electronic transport; Heterostructures; Scanning near-field optical microscopy; Interlayer charge transfer; Covalent organic frameworks; Low-temperature magnetotransport
