DOI: 10.25549/usctheses-c40-340546
title: Optoelectronic and thermoelectric properties of two-dimensional materials and heterostructures

description: In this paper, I will discuss the 2D layered nanomaterials including: graphene, transition metal dichalcogenides (MoS₂ and WSe₂), bismuth telluride, misfit layered compounds (SnSe)n(TiSe₂)n (n=1,3,4,5) and black phosphorous. The topic will cover from growth to characterization, from material to device, and potential thermoelectric and optoelectronic applications of these nanomaterials and their hetero-structures. ❧ Chapter 1 is a brief introduction as the foundation for this dissertation. What is 2D materials, why people are interested in those materials, and new opportunities. This chapter provides a brief overview of properties of graphene, TMDCs and other related material in following chapters, their crystalline structure, as well as electronic band structures. The background knowledge needed to understand each topic is also covered within the relevant chapter. ❧ Chapter 2 reports studies performed on anisotropic etching of CVD graphene. Because of the gapless feature of graphene, graphene nano-ribbons is introduced to open an finite gap of graphene for better electric performance. A top-down approach to get graphene nano-ribbon is to etch the graphene film. In this chapter, a simple, clean, and highly anisotropic hydrogen etching method is reported for chemical vapor deposited (CVD) graphene etching, catalyzed by the copper substrate. ❧ In Chapter 3 and 4, we are going to talk about studies of transition metal dichalcogenides (TMDCs), including MoS₂, WSe₂. The few layer and monolayer TMDCs is exfoliated by using the Scotch tape method. While bulk MoS₂ and WSe₂ is an indirect band gap semiconductor with a band gap of 1.3eV, quantum confinement converts monolayer MoS₂ and WSe2 into a direct band gap material with a band gap of 1.85eV. It makes MoS₂ and WSe₂ potential candidates for optoelectronics devices. In these atomically thin films, interfaces are very important, Chapter 3 includes the interface related study performance on MoS₂. Surface passivation of MoS₂ by using ionic liquid, and the hot electron injection from metal to MoS₂. In Chapter 4, a very simple technique is found to perturbing the atomic structure of few layer MoS₂/WSe₂. The remote oxygen plasma treatment can pull the atomic layers in this material system apart, leading to a dramatic change in optical properties, such as a transition to a direct band gap for MoS₂. For WSe₂, it can serve as a self-saturated surface oxidization technique. ❧ Chapter 5 introduces the opticoelectronic devices fabricated base on graphene and TMDCs, including photo detector based on the monolayer graphene/metal Schottky junction. THz detector of bilayer graphene on h-boron nitride, and a highly Efficient, high speed photodiodes with a vertical structure of few-layer MoS₂. ❧ Chapter 6 is on another layered material, bismuth telluride, which is well known as an efficient thermoelectric material for refrigeration or power harvesting at room temperature. Recently, Bi₂Te₃ is re-discovered as a topological insulator, and thus exhibits many thickness-dependent physical properties, including the figure of merit of a thermoelectric material. More than twenty year ago, researchers have shown that theoretically, if the thickness of a Bi₂Te₃ film can be reduced to nm scale, its thermoelectric performance will be improved significantly. In this chapter, we are going to related measurement on Bi₂Te₃ nanoplates. ❧ Besides Bi₂Te₃, misfit layered compounds has also shown their great potential in thermoelectric filed. Chapter 7 presents the thermoelectric device fabricated based on a misfit layered compound (SnSe)n(TiSe₂)n (n=1,3,4,5). For the first time, we measured the cross-plane Seebeck coefficient and thermal conductivity of this misfit layered compound. The results shows that due to the anisotropic nature and the material and different behavior between in-plane and cross-plane electron transport, the cross-plane Seebeck coefficient and thermal conductivity of the materials is very different from the in-plane results. ❧ Chapter 8 is focused on a new rising star in two-dimensional layered material field, black phosphorous, which is discovered a hundred years ago, and now pulled back to the scientific community and become one of the most popular topics in experimental condensed matter physics. There are many appealing features of this material, anisotropy transport, high mobility, and tunable band-gap from twenty to forty meV to 1eV. However, thin black phosphorous film will degrade very quickly in atmosphere, which is a deadly drawback of this materials. In this chapter we demonstrate an analysis of degradation of BP in air, together with a passivation method to protect the device from moister and air.