As conventional electronic materials approach the device scaling limits, new types of materials and structures have been examined for potential use in future electronic and optoelectronic applications including transistors, light emitting diodes, and solar cells. In recent years, atomically thin or two-dimensional (2D) transition metal dichalcogenide (TMD) materials have emerged as attractive candidates to meet this need based on the intriguing electronic and optical properties that they display. Moreover, these properties coupled with the mechanical flexibility and elasticity that these materials exhibit make them excellent options for wearable and flexible technologies, as well. And finally, the unique physical phenomena, such as superconductivity and ferromagnetism, that many materials in this family exhibit in the 2D limit make this system an interesting option for applications in topological quantum computing and optics. This dissertations centers around the transition metal dichalcogenides, a subset of this larger 2D materials family, and explores the role that complex interfacial structure have on the resultant electronic, optical, and thermal properties in these materials. First, a number of vapor phase methods used to synthesize these 2D materials are discussed. This includes both a discussion on various chemical vapor deposition and physical vapor deposition techniques along with details on the specific conditions used for synthesizing various 2D sulfides, selenides, and oxides. The advantages and disadvantages associated with each technique are also addressed. Subsequently, the idea of interfacing multiple 2D materials together in a single atomic plane is discussed. This type of heterostructure offers a pathway for manipulating the electronic structure in an effort to achieve properties and device metrics that exceed those of the individual constituents. This investigation examines the role lateral interfaces in 2D materials play on the resultant electronic and optoelectronic properties. In the case of these monolayer materials, however, the traditional metal contacts used to probe elec- tronic and optical properties add complications. Namely, the atomic orbitals in these metal typically hybridize with the atomic orbitals in the neighboring TMD layer, which leads to the formation of metal/semiconductor barriers opposing charge transport. This is especially troublesome in the case where composite material systems or heterostructures are being examined because these barriers radically modify the measured properties. This makes it impossible to disentangle the properties of the heterostructure from the external environment. To resolve this issue, we applied a new approach to achieve low-resistance contacts by inserting a monolayer thick h-BN tunneling barrier between the metal and a monolayer TMD heterostructure consisting of MoS2 and WS2. By using this insulating layer, the metal-TMD interaction is eliminated and the metal/semiconductor Schottky barrier is replaced with an ultra-thin tunnel barrier. The importance of this new method- ology is highlighted by the fact that the results we obtain with this advanced approach are wildly different than those obtained and reported by others using the traditional methodology. Additionally, the heat dissipation properties in these materials is a critical issue due to the reduced dimensions in this system. These properties are accessible through scan- ning thermal microscopy (SThM), a specialized atomic force microscopy technique that pairs topographical sample information with local temperature measurements. We used this method to examine the role that various interfaces in 2D TMDs, such as lateral heterojunctions and grain boundaries, have on the heat transport and the temperature rise distribution in 2D systems while under operation. Our results indicate that inter- faces between two dissimilar materials, such as MoS2 and WS2, do not appreciably cause localized heating. On the contrary, grain boundary interfaces that form between two sim- ilar materials clearly localize heat and give rise to non-uniform current densities in the material. Based on these findings, the role that grain boundaries play in terms of atomistic dynamics within these materials is explored through in situ high-resolution transmission electron microscopy. This technique allows us to directly ascertain how interfacial atoms respond to external stimuli. For this study, we constructed a setup for applying a lateral electric field across a 2D material sample that allows for concurrent TEM imaging and as- sociated techniques. Using this platform, we monitor the evolution of the grain boundary structure as a function of electric field. We find that grain boundaries in these materials are commonly populated with micron-scale atomic voids. Moreover, when subjected to an electric field, the voids present at grain boundaries coalesce to induce structural defor- mation and form large networks of intercorrelated void channels. This feature gives rise to non-uniform current conduction across these boundaries and explains the presence of local hot spots at grain boundary interfaces in these systems. Using a combination of electrical transport and electron microscopy, the impact of interfaces on the memristive electrical performance in an analogous 2D system is then discussed. Finally, some related areas for future exploration are discussed along with a broader look at research topics prime for investigation within the field of 2D materials. A discussion of collaborative studies that were conducted is provided in the Appendix.

Creator

• MURTHY, AKSHAY ARUN

DOI

• https://doi.org/10.21985/n2-e5gm-bj98

Subject

• Materials Science

Language

• en

Alternate Identifier

• etdadmin_upload_730816
• http://dissertations.umi.com/northwestern:15038

Keyword

• electron microscopy
• heterostructures
• electronic properties
• grain boundaries
• 2d materials
• interfaces

Date created

• 2020-01-01

Resource type

• Dissertation

Rights statement

• In Copyright