Emerging electronic materials : thin films of asphaltenes and coal nanoparticles for electronic devices and large area layer controlled 2-dimensional semiconductor synthesis

Abstract

Materials for energy and electronic applications is a rich ecosystem. This thesis describes the work I have performed with the help of my colleagues to push the boundaries of this space both in materials processing and unique materials development. In particular, I studied two classes of materials in detail. The first are disordered natural carbonaceous materials which I prepared from source material such as coal and asphaltenes. The second are two dimensional materials in particular transition metal dichalcogenides. Following an introduction to the materials I have studied in chapter 1, in chapters 2 and 3 I will discuss a class of materials which have seen extensive academic interest for energy and electronic applications: carbon materials. Specifically disordered carbon materials, both amorphous and with long-range order (for example, -C:H and reduced graphene oxide), have been used in a variety of optical and electronic applications from conductive additives and contact materials to transistors and photovoltaics. In contrast, the electronic properties of solid natural carbon materials such as coal have not been explored beyond basic bulk electrical conductivity measurements for correlation with combustion and gasification yields. I will discuss a number of exciting results from exploration of this neglected space: 1) Development of a method of fabrication of thin films of coal nanoparticles based on ball milling and solution processing 2) Chemical, electrical, structural, and optical characterization of the properties of coal thin films through Raman, X-ray photoelectron and UV-Vis spectroscopies as well as low temperature charge transport studies. 3) Fabrication of thin film Joule heating devices which compete with or outperform many reported synthetic materials. 4) Exploration of the electrical properties of thin films of asphaltenes and vacuum residuals for photovoltaic applications. Based on these experiments, the solid natural carbon phase space has proven to be rich and promising. Electrical conductivities range over orders of magnitude, and thermal treatment of the resulting films increases the sp2 content, disorder, and tunes the electrical conductivity in excess of 7 orders of magnitude. Optical absorption measurements demonstrate tunable optical gaps from 0 to 1.8 eV, and low-temperature conductivity measurements demonstrate that variable range hopping controls the electrical properties in both as-prepared and thermally treated films. The measured hopping energies further demonstrate electronic properties similar to vacuum deposited amorphous carbon materials and reduced graphene oxide. Next in chapter 4, while very promising work for square inch and larger scale methods of uniform monolayer deposition of 2-dimensional (2D) semiconductors such as MoS 2 has been performed, complete film growth and inhibition of bilayer or thicker nucleation has proven difficult. I will present a divergent growth method for MoS 2 via sulfurization of oxide deposited by both thermal ALD from (tBuN) 2 (NMe2 )2 Mo and O3 and plasma enhanced ALD (PEALD) from (tBuN)2 (NMe2 )2Mo and remote O2 plasma. Large uniform MoS 2 areas were achieved by studying the effects of various growth process conditions and surface treatments to control the nucleation and growth of MoO3 and through a detailed study of the chemistry of the film for varied post-sulfurization temperature profiles. Finally, as discussed in chapter 5, vapor deposition methods are not the only approaches to manufacture of 2D semiconductors. The ultimate in low cost fabrication is based on solution exfoliation of bulk material. After developing a process for depositing materials in this manner, I studied several chemical and thermal methods for removing cholate - a ligand used during the solution exfoliation process - from the films after deposition to improve the purity and quality of the films. I also worked with scotch tape mechanically exfoliated materials to test the properties of heterojunctions of MoS2 and graphene and demonstrate quenching of the MoS2 photoluminescence indicating charge injection into the graphene sheet.