Date on Master's Thesis/Doctoral Dissertation

5-2017

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Physics and Astronomy

Degree Program

Physics, PhD

Committee Chair

Sumanasekera, Gamini

Committee Co-Chair (if applicable)

Jasinski, Jacek

Committee Member

Jasinski, Jacek

Committee Member

Jayanthi, Chakram

Committee Member

Yu, Ming

Committee Member

Liu, Jinjun

Author's Keywords

phosphorene; graphene; CVD; characterization; semiconductors; renewable energy

Abstract

Recently, high-pressure science and technology has flourished and rapidly advanced to impact a wide domain of materials and physical sciences. One of the most substantial technological developments is the integration of samples at ultrahigh pressure with a wide range of in-situ probing techniques. Applications of extreme pressure have significantly enriched our understanding of the electronic, phonon, and doping effects on the newly emerged two-dimensional (2D) materials. Under high pressure, materials’ atomic volume radically decreases, and electronic density rises, which will lead to extraordinary chemical reaction kinetic and mechanisms. The promising capability of high pressure combine with the significance of novel emerging 2D materials in energy-related research was the main motivation of this dissertation. Firstly, the application of high pressure to enable the direct synthesis of α-AgGaO2 through a reaction of Ag2O and Ga2O3 is demonstrated. The synthesized samples were extensively characterized, and their crystal phase and chemical composition were confirmed. Especially, the rhombohedral delafossite crystal phase of the prepared sample was verified by electron diffraction. The vibrational phonon modes were investigated using a combination of ab initio density functional theory (DFT) and experimental Raman measurement. In addition, using a modified DFT to calculate the electronic band structure of α-AgGaO2 reported a more accurate valu of theoretical[1] band gap than those have been reported previously. Two-dimensional (2D) materials with efficient ion transport between the layers and the large surface areas have demonstrated promise for various energy-related applications. Few-layer black phosphorus (phosphorene), as a novel two-dimensional (2D) material, is gaining researchers’ attention due to the exceptional properties, including puckered layer structure, widely tunable band gap, strong in-plane anisotropy, and high carrier mobility. Phosphorene application expanded from energy storage and conversion devices to thermoelectrics, optoelectronic and spintronic to sensors and actuators. Several recent theoretical studies have indicated that strain engineering can be a viable strategy to tune the electronic structure of phosphorene. Although several theoretical studies have predicted an electronic phase transition such as direct-indirect bandgap and semiconductor-metal transitions, there is not experimental study to indicate the transition. Next, in this dissertation, a systematic experimental study of in situ high-pressure Raman and PL spectroscopy of phosphorene was reported. Furthermore, short transport growth of bulk black phosphorus and also, liquid-phase exfoliation technique to preparing few-layer black phosphorus was described. The study motivated by a better understanding of high-pressure effects on optical properties and band structure of this material system. This study help to verify theoretical predictions and to enhance fundamental understanding of relationships between strain and electronic band structure, enabling rational strain engineering towards additional functionalities and device applications of phosphorene and few-layer BP. In situ characterization techniques are invaluable for a fundamental understanding of materials, their processing, and functionalities. Three-dimensional architecture of graphene has also attracted considerable attentions as an effective way to utilize the unique inherent properties of graphene sheets in practical applications. Three-dimensional graphene-based materials offer an easy and versatile platform for functionalization and integration into devices. Furthermore, the interlocking of graphene sheets into 3D structures solve the restacking issue and make 3D graphene-based materials more compatible with conventional material processing. Finally, in the dissertation, we report a novel, inexpensive, and highly scalable, approach of fabricating a three-dimensional graphene network (foam) via pyrolysis of organic materials as the source of carbon. A template-assisted method to prepare and tune the properties of a high-quality 3D graphene network was described. In this simple method, the 3D graphene foam is synthesized in a controlled environment by thermal decomposition of the organic materials in the presence of Ni foam which plays a dual role of catalyst and 3D template. This technique can efficiently facilitate and control the in situ nitrogen doping of 3D graphene structure by adjusting the growth parameters and choosing the right organic materials (i.e. nitrogen-containing organic acids). In this work, inexpensive organic materials including caffeine (C8H10N4O2), urea (CH4N2O) and acetaminophen (C8H9NO2) were used with a citric acid solution as the source of carbon and nitrogen. Nitrogenation of 3D graphene foam create an effective improvement of properties which is suitable for an extensive range of new energy and environmental applications. Our Raman analysis indicated an improvement of graphene network quality with an increase of synthesis temperature between 650 °C and 1000 °C. Both Raman and TEM study (HRTEM, SAED, and EELS) showed uniform coverage and high crystallinity of multilayered graphitic shells formed in samples synthesis at 1000 °C. The motivation for this 3D graphene research is to use in-situ high-pressure measurements to study fundamental properties of these materials including its vibrational structures, doping and functionalization. With its distinct Raman signatures dependent on the quality and structure, defect distribution, as well types of dopants and their concertation, 3D graphene seems well-suited for high-pressure in-situ Raman studies. These type of measurements are proposed as part of the future, follow-up research. [1] Performed by Dr. Madhu Menon (Center for Computational Sciences at the University of Kentucky).

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