Vapor-Phase Growth of Two-Dimensional Molecular Crystals and Their Structure-Property Relationships

The wafer-scale synthesis and patterning of thin films forms the foundation of modern technologies. As we continue to scale down the size of electronic systems, we find ourselves pushing the limits of conventional thin-film materials. For this reason, two-dimensional (2D) materials, which exist as stable films that are only few atoms thick, are becoming the center of intense research. For many years, 2D materials simply presented exciting opportunities for exploring exotic physical phenomena. However, similar to how silicon technologies are enabled by the ability to grow large crystals of silicon, scalable synthetic techniques are necessary to take the next step towards implementation of 2D materials in real electronic systems. In this thesis, we present our approaches towards the synthesis of 2D crystals and films of inorganic and organic semiconductors, using gas-phase deposition techniques. Chapter 1 will introduce the unique, anisotropic structure of 2D materials, and use this as a context for understanding the growth mechanics of 2D systems. Chapter 2 will focus on the controllable synthesis of semiconducting 2D transition-metal dichalcogenides (TMDs), such as MoS2, WS2, and WSe2, using kinetically-limited metal-organic chemical vapor deposition. We will discuss the conditions necessary to scale up from single crystals to wafer-scale monolayer films, as well as provide a detailed understanding of the role of each reactant in the growth. Chapter 3 will elaborate more on challenges in producing high-quality films with reproducible physical, mechanical, and surface properties. Chapter 4 will describe how these TMD films can be used as substrates for the physical vapor deposition of highly-crystalline 2D molecular films in the thermodynamic limit. Chapter 5 will present specific studies on the growth mechanisms of perylene-based 2D molecular crystals. Chapter 6 will illustrate how intermolecular forces can be modified through molecular functionalization to grow crystals with different structural characteristics. These structural differences manifest as unique optical properties, specifically related to responses to polarized light. Chapter 7 will present a number of ideas and proof-of-concept demonstrations for integrating these nanometer-thick hybrid films into functional systems, with a particular focus on electrochromic devices