Graduation Year

2018

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Electrical Engineering

Major Professor

Rüdiger Schlaf, Ph.D.

Committee Member

Alberto Sagüés, Ph.D.

Committee Member

Shengqian Ma, Ph.D.

Committee Member

Alex Volinsky, Ph.D.

Committee Member

Jing Wang, Ph.D.

Committee Member

Arash Takshi, Ph.D.

Keywords

Density of States, Electronic Structures, Energy Band Diagrams, Molecular Electronics, Photoemission Spectroscopy, Thin Films

Abstract

Metal-organic frameworks stand at the frontiers of molecular electronic research because they combine desirable physical properties of organic and inorganic components. They are crystalline porous solids constructed by inorganic nodes coordinated to organic ligands to form 1D, 2D, or 3D structures. They possess unique characteristics such as ultrahigh surface area crystal lattices up to 10000 m2 g-1, and tunable nanoporous sizes ranging from 0.2 to 50 nm. Their unprecedented structural diversity and flexibility beyond solid state materials can lead to unique properties such as tailorable electronic and ionic conductivity which can serve as interesting platforms for a wide range of electronic applications from photonics, sensors, and energy harvesting/storage devices such as photovoltaics, thermoelectrics, supercapacitors to data storage systems like memristors.

Despite the significant growth of MOF materials during the past two decades, the fundamental understanding of the resultant electronic and ionic structures at the interface of these hybrid materials are still largely unexplored, and the lack of these properties are the basic requirements for elucidating the physics and chemistry of the devices—The exquisite role of physical electronic properties is crucial for the construction of energy band diagrams of MOF thin films, and is key to achieve further advancement in the development of elaborated devices that require tunability and control over functionality. With this motivation, powerful surface science techniques (e.g. direct and inverse photoemission spectroscopy) have been engaged in this dissertation, which provided useful guidelines to access and study the electronic and chemical structures (e.g. valence and conduction bands, and core electrons) at the internal interface of the individual hybrid constituents of MOF films. This is achieved using a combination of low intensity X-ray photoelectron spectroscopy (LIXPS), ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and inverse photoelectron spectroscopy (IPES). Furthermore, the density of states obtained from DFT calculations agreed well with the photoemission spectra measurements of the fabricated 2D MOF thin film. Energy level alignment was achieved by judicious selection of various organic ligands in 2D MOF architectures, whereas further incorporation of various pillaring linkers in the 3D MOFs have modified the HOMO and LUMO energy levels as suitable conducting medium for hole or electron transport.

The fundamental study delivered in this dissertation gives a unique feedback by tailor-designing the electronic properties of the fabricated 2D to 3D MOF thin films. These crucial properties at the interface offer very important understanding and breakthrough in MOFs as functional and tunable electronic materials.

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