Complex Doping Structures in 2D Heterostructures of Mo2C, MoS2 and Graphene
Restricted (Penn State Only)
- Author:
- Binion, Anna
- Graduate Program:
- Physics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 23, 2022
- Committee Members:
- Jun Zhu, Major Field Member
Eric Hudson, Chair & Dissertation Advisor
Joan Redwing, Outside Unit & Field Member
Jorge Sofo, Major Field Member
Nitin Samarth, Program Head/Chair - Keywords:
- 2D Heterostructure
Mo2C
MoS2
Graphene
STM
AFM
Phase Seperation
Doping - Abstract:
- Nanoscale phase separation can have a profound impact on a material’s electronic, chemical, and mechanical properties. The factors driving that separation, crucial for understanding and potentially controlling the separation, are, however, not always apparent. In this dissertation I present a scanning probe microscopy investigation of intense charge segregation in CVD-grown Mo2C. Previously reported in pristine samples, we sought to investigate the stability of the phase separation and better understand the forces driving it by taking Mo2C through a sulfurization process. Through carefully understanding and analyzing STM and AFM topography and spectroscopy of the sample as it undergoes sulfurization, we are better able to understand the underlying structure of the material. This particular project is the culmination of a series of research projects I performed during my time as a doctoral student, each of which required the development of tools that I ultimately utilized in the study of Mo2C. For example, looking at surface roughness through a phase transition in LaVO3 through AFM demonstrates a use case for AFM in the study surface structure, and I use a novel method for improving STM spectroscopic maps to examine confinement states in impurities in BN-doped graphene. This same method can also be applied in analyzing the sulfurized Mo2C I then return to the sulfurized Mo2C, showing that doping patterns are clearly visible in the Mo2C both before and after sulfurization. After sulfurization the domain size of the doping pattern increases significantly, from an average area of 4±3 nm2 to 61±93 nm2. I also demonstrate that the top of the material is atomically flat, and that a moiré pattern reveals that up to two layers of graphene are present on the surface of the material. Looking at the electronics of the material, I show that there is a shift in the Dirac point, from 312±16 mV to 375±22 mV, consistent with a significant shift in doping. Additionally, these doped regions result in confinement states, and I look at how the energies associated with the states relate to the unusual shapes of the doping structures. After sulfurization, there emerge edge states at both step edges and the transitions between the doped and undoped areas. The edge states appear at a range of energies for the step edges, but those that appear around the pits are energy dependent, emerging between 300 mV and 400 mV. We find that the segregation pattern is surprisingly resilient in the face of the large structural changes driven by the sulfurization process. However, changes in the size and density of charged regions, coupled with the emergence of electronic states along their edges, give us insight into their source.