Graphene and Beyond: Electron Transport in Two Dimensional Materials.

Abstract

Owing to their unique energy band structure and the ease of material synthesis, two dimensional nanomaterials, such as graphene, have become the ideal platform for observing novel electron transport phenomena in reduced dimensions. In particular, low-energy quasiparticles in monolayer graphene behave like massless Dirac fermions, which have led to observations of many interesting phenomena, including Klein tunneling, anomalous Quantum Hall effect, etc. In contrast to the monolayer graphene, quasiparticles in bilayer graphene (BLG) are massive chiral fermions due to its parabolic band structure. Thus, BLG also gives a number of intriguing properties which are very different from those of monolayer graphene, including tunable band gap opening and anti-Klein tunneling, arising from chiral characteristics of charge carriers. However, unlike SLG, experimental works on chiral electron transport in BLG have received less attention.

In addition, other two-dimensional atomic layer crystals, such as atomically thin layered transition-metal-dichalcogenides (TMDCs), are also attractive material platform with unique electronic and optical properties, including indirect to direct band gap transition, and valley polarized carrier transport. However, study of the low temperature electron transport in atomic thin layered TMDCs is still in its infancy. One of the major hurdles for electron transport study lies in the large metal/semiconductor junction barrier for carrier injection, which leads to the contact resistance dominated charge transport in short channel nanoscale devices.

In this thesis, I first demonstrated the successful synthesis of wafer scale BLG with high homogeneity by low-pressure chemical vapor deposition (CVD). The bilayer nature of the graphene films were confirmed through a series of characterizations including Raman Spectroscopy, Transmission Electron Microscope, and electrical transports showing field induced bandgap opening. Next, I proceeded to study the importance of chiral electron transport in BLG. I observed electronic cloaking effect with anti-Klein effect as a manifestation of chirality by probing phase coherent transport behavior in CVD bilayer graphene nanostructure. Finally, I studied the electron transport in few-layer TMDCs. I successfully fabricated monolayer MoS2 single electron transistors using low work function metal for the contact electrodes, and observed Coulomb blockade phenomena attributed to single electron charging on a fairly clean quantum dot.