Preparation and Characterization of Van der Waals Heterostructures

Author

Horacio Coy Diaz, University of South FloridaFollow

Graduation Year

2016

Document Type

Thesis

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Physics

Major Professor

Matthias Batzill, Ph.D.

Committee Member

Garrett Matthews, Ph.D.

Committee Member

Ivan Oleynik, Ph.D.

Committee Member

Humberto Rodriguez Gutierrez, Ph.D.

Keywords

Graphene, MoS 2, MoTe 2, Angle resolve photoemission spectroscopy

Abstract

In this dissertation different van der Waals heterostructures such as graphene-MoS₂ and MoTe₂-MoS₂ were prepared and characterized. In the first heterostructure, polycrystalline graphene was synthesized by chemical vapor deposition and transferred on top of MoS₂ single crystal. In the second heterostructure, MoTe₂ monolayers were deposited on MoS₂ by molecular beam epitaxy.

Characterization of graphene-MoS₂ heterostructures was conducted by spin and angle resolve spectroscopy which showed that the electronic structure of the bulk MoS₂ and graphene in this van der Waals heterostructures is modified. For MoS₂ underneath the graphene, a band structure renormalization and spin polarization are observed. The band structure of MoS₂ is modified because the graphene induces screening which shifts the Г-point ~150 meV to lower binding compared to the sample without graphene. The spin polarization is explained by the dipole arising from band bending which breaks the symmetry at the MoS₂ surface. For graphene, the band structure at lower binding energy shows that the Dirac cone remains intact with no significant doping. Instead, away from the Fermi level the formation of several gaps in the pi-band due to hybridization with states from the MoS₂ is observed.

For the heterostructures made depositing monolayer of MoTe₂ on MoS₂, the morphology, structure and electronic structure were studied. Two dimensional growth is observed under tellurium rich growth conditions and a substrate temperature of 200 °C but formation of a complete monolayer was not achieved. The obtained MoTe₂ monolayer shows a high density of the mirror-twins grain boundaries arranged in a pseudo periodic wagon wheel pattern with a periodicity of ~2.6 nm. These grain boundary are formed due to Te-deficiency during the growth. The defect states from these domain boundary pin the Fermi level in MoTe₂ and thus determine the band alignment in the MoTe₂-MoS₂ heterostructures.

Scholar Commons Citation

Coy Diaz, Horacio, "Preparation and Characterization of Van der Waals Heterostructures" (2016). USF Tampa Graduate Theses and Dissertations.
https://digitalcommons.usf.edu/etd/6212