Atomic Structures and Doping Effects in Intercalated Few-Layer Graphene

• Main Author: Bonacum, Jason P
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2020
• Subjects: Condensed matter physics; Materials science
• ISBN: 9798662598324

Intercalation, the process of inserting atoms or molecules into host materials, has been extensively researched in graphite for energy storage and reaction catalysis applications. The recent ability to isolate graphite with controlled number of carbon layers has also led to a surge of interest in intercalated few-layer graphene (FLG) and bilayer graphene (BLG). Few-layer graphene intercalated with either single atomic species or molecules has been used to explore the physics of graphene with large numbers of free charge carriers (high doping) and for potential applications in next-generation battery electrode and flexible transparent conductors. The atomic structures, and the resulting doping effects those structures, in intercalation compounds are a key part of predicting the properties and applications of these materials. However, most of the data for atomic structures of intercalation compounds have been by x-ray diffraction, Raman spectroscopy, and other spatially averaged techniques. These techniques do not directly provide information about local atomic structures of intercalants. In this dissertation, we describe methods of fabricating intercalated FLG to be studied with aberration-correct scanning transmission electron microscopy, and we present atomic-resolution images of BLG and FLG intercalated with FeCl3. In BLG we discover two distinct intercalated structures that we identify as monolayer-FeClIntercalation, the process of inserting atoms or molecules into host materials, has been extensively researched in graphite for energy storage and reaction catalysis applications. The recent ability to isolate graphite with controlled number of carbon layers has also led to a surge of interest in intercalated few-layer graphene (FLG) and bilayer graphene (BLG). Few-layer graphene intercalated with either single atomic species or molecules has been used to explore the physics of graphene with large numbers of free charge carriers (high doping) and for potential applications in next-generation battery electrode and flexible transparent conductors. The atomic structures, and the resulting doping effects those structures, in intercalation compounds are a key part of predicting the properties and applications of these materials. However, most of the data for atomic structures of intercalation compounds have been by x-ray diffraction, Raman spectroscopy, and other spatially averaged techniques. These techniques do not directly provide information about local atomic structures of intercalants. In this dissertation, we describe methods of fabricating intercalated FLG to be studied with aberration-correct scanning transmission electron microscopy, and we present atomic-resolution images of BLG and FLG intercalated with FeCl3. In BLG we discover two distinct intercalated structures that we identify as monolayer-FeCl3 and monolayer-FeCl2. The two structures exhibit atomically sharp boundaries between intercalated and unintercalated regions, and both structures induce large but different free-carrier densities in the adjacent graphene layers (7.1 × 1013 cm−2 and 7.8 × 1013 cm−2 respectively) measured by resonance-Raman spectroscopy. In FLG, we observe multiple FeCl3 layers stacked in a variety of possible configurations with respect to one another. Finally, we convert the FeCl3 monolayer into FeOCl monolayers in a rectangular lattice using the microscope’s electron beam. These results demonstrate the need for atomically resolved microscopy, combined with density-functional-theory calculations and image simulations, to identify intercalated structures and reveal intercalated BLG to be a useful vessel for creating novel 2D materials and monolayer-FeCl2. The two structures exhibit atomically sharp boundaries between intercalated and unintercalated regions, and both structures induce large but different free-carrier densities in the adjacent graphene layers (7.1 × 1013 cm−2 and 7.8 × 1013 cm−2 respectively) measured by resonance-Raman spectroscopy. In FLG, we observe multiple FeCl3 layers stacked in a variety of possible configurations with respect to one another. Finally, we convert the FeCl3 monolayer into FeOCl monolayers in a rectangular lattice using the microscope’s electron beam. These results demonstrate the need for atomically resolved microscopy, combined with density-functional-theory calculations and image simulations, to identify intercalated structures and reveal intercalated BLG to be a useful vessel for creating novel 2D materials.