In this work, a systematic study relating the surface morphologies, electrical and structural properties of both doped and undoped InAs and InAsSb epitaxial films grown by metalorganic chemical vapour deposition (MOCVD) was undertaken. A comparative study using TBAs and AsH3 as the group V source in the growth of InAs revealed a considerable improvement, primarily in the electrical properties of InAs grown using TBAs with no significant difference in the surface morphology. InAs layers grown using TBAs, exhibited superior 77 K mobilities of up to 46 000 cm2/Vs, exceeding the best MOCVD data to date. The feasibility of tetraethyl tin (TESn) as an n-type dopant in InAs was to our knowledge investigated for the first time. The incorporation efficiency of this dopant was extensively studied as a function of substrate temperature, V/III ratio, substrate orientation and TESn flow rate. Results from this study show that the doping efficiency is temperature dependent and is not influenced by a variation of the V/III ratio or substrate orientation. Furthermore, Sn doping concentrations could be controlled over 2 orders of magnitude ranging between 2.7 x 1017 and 4.7 x 1019 cm-3 with 77 K mobilities ranging from 12 000 to 1300 cm2/Vs. The electrical properties of zinc doped InAs employing dimethyl zinc (DMZn) as the ptype dopant, were studied as a function of V/III ratio and substrate orientation. The effect of a variation of these parameters on the structural properties and surface morphology of InAs is also reported. The substrate orientation appears to have no influence on the Zn incorporation. An increase in Zn incorporation resulted in a deterioration of both the surface morphology and structural quality of the InAs layers. The incorporation efficiency of DMZn in InAsSb was studied as a function of growth temperature, V/III ratio and DMZn flow rate. A higher Zn incorporation was observed in InAsSb epitaxial layers grown at a lower temperature and V/III ratio as opposed to the layers grown at a higher temperature and V/III ratio. This study also revealed that the use of DMZn caused a dopant memory effect. A two-layer model proposed by Nedoluha and Koch (1952) was used to simulate the Hall measurements of Zn doped InAs and InAsSb in order to correct the shortcomings of conventional Hall measurements in determining the electrical properties exhibited by these materials.

No description available.

Luminescent metal organic frameworks (MOFs) are promising new materials with applications as sensors, photocatalysts, and other luminescent devices. Although MOFs retain the chemical and physical properties of their constituents, the properties of the MOF are often altered from those of its building blocks, making rational design and synthesis difficult. Anthracene is a polyaromatic hydrocarbon whose photophysical properties have been found to be easily tuned through structural modifications. The tunability of anthracene makes it an ideal candidate for use in luminescent devices, such as photoprobes and organic light emitting diodes. MOFs designed with π conjugated molecules like anthracene ligands possess similar photophysical properties such as absorption and fluorescence in the UV and visible spectrum. In hopes of better understanding how the photophysical properties of the organic ligand is altered upon incorporation into a MOF, the spectroscopic properties of anthracenedicarboxylic acids were studied before and after integration into zinc based MOFs. Steady state and time resolved measurements were performed on three anthracenedicarboxylic acids: 9,10-anthracenedicarboxylic acid, 2,6-anthracendicarboxylic acid, and 1,4-anthracenedicarboxylic acid. The position of the carboxylic acid groups on anthracene was found to effect the position and structure of the absorption and emission spectra. The difference in the spectra is attributed to the perturbation by the acid groups on certain electronic transitions with dipole moments across two of the three axes of anthracene. The position of the acid groups had different effects on the fluorescence quantum yields and lifetimes of the three anthracenic acids studied. Two of the linkers were synthesized into MOFs through a solvothermal reaction with zinc nitrate, to form PCN-13, from 9,10-anthracenedicarboxylic acid, and [Zn(C₁₆H₈O₄)(H₂O)]_n, from 2,6-anthracenedicarboxylic acid. The luminescent properties of the two MOFs were studied and compared to those of the free based linker. Incorporation of the luminescent anthracenedicarboxylic acids into Zn based MOFs were found to either increase or decrease the luminescent properties of the ligands. / Master of Science

The utilization of porous materials for electrocatalytic applications has been of high interest due to their high surface area and increase in electrode-electrolyte interface. Metal-organic frameworks (MOFs) are an emerging class of 3-D porous materials consisting of inorganic nodes bound by multidentate organic linkers. MOFs have permanently large surface area, high stability, and the tunability of the structure. The metal source or the organic linker can be swapped to create a material with desirable features. MOFs have been explored for applications in electrocatalysis, conductivity and energy storage. The fundamental charge transfer methods of MOF thin films is discussed and the utilization of these conductive materials for the key reactions in artificial photosynthesis is explored to highlight methods to improve the efficiency of these materials for electrocatalysis. The Morris group has previously shown that charge transfer in MOFs can occur through a redox hopping mechanism in which the charge hops from redox center to redox center through space followed by the movement of a charge balancing ions. In Chapter 2, the charge transfer mechanism within the MOF is further investigated by utilization of spectroelectrochemistry. The incorporation of a redox center, Ru(bpy)2(dcbpy), "RuBPY" where bpy = 2,2′-bipyridine; bpy-(COOH)2 = 5,5′-dicarboxylic acid-2,2′-bipyridine into the UiO-67 framework creates a conductive MOF that is also electrochromic. RuBPY is a deep orange color in the standard RuII state and upon oxidation to RuIII it is pale green. The change in absorption profile of the redox center allows for the rate of oxidation to be determined through absorbance measurements. The material showed minimal change in absorbance upon applying an oxidative potential. The incorporation of a sulfonate group into the backbone of the RuBPY-UiO-67-SO3H MOF allowed for a much higher change in absorbance converting the entire MOF into the oxidized state. The change in level of absorbance indicates that the sulfonate groups improve the conductivity within the pores of the MOF allowing for oxidation of previously electrochemically inaccessible redox centers. The sulfonate groups are thought to break ion pairs of the electrolyte and increase effective electrolyte concentration within the pores. The sulfonate groups' ability to improve the conductivity within the MOF can be further investigated to improve charge transfer through porous materials. The sulfonate groups were again incorporated into the UiO-67 MOF framework for use in electrocatalytic applications by also incorporating the known water oxidation catalyst, RuTPY, Ru(tpy)(dcbpy)H2O. A RuTPY-UiO-67 film had previously shown reactivity as a water oxidation catalyst with improved activity over a monolayer of RuTPY on fluorine-doped tin oxide, FTO. The sulfonate groups were added to create a proton transfer chain that shuttled the generated protons away from the catalytic site to improve reactivity. The incorporation of sulfonate groups again showed improved charge transfer from the MOF materials with the RuTPY-UiO-67-SO3H being 100% electrochemically accessible. The water oxidation capabilities improved giving the material increased oxygen generation upon oxidation of water. The improvement of catalytic activity of RuTPY-UiO-67-SO3H was beyond the increased electrochemical accessibility means the proximal sulfonate groups were aiding in catalysis in some manor. This work highlights the use of multivariate approaches to MOFs to improved efficiency in various applications. The fourth chapter discusses the other half of artificial photosynthesis, CO2 reduction. The known CO2 reduction catalyst, Ni(cyclam), is incorporated into a zirconium-based MOF, VPI-100. The VPI-100 powder was electrochemically deposited onto a glassy carbon electrode and the film was used for electrochemical CO2 reduction into carbon dioxide. The film successfully generated CO as a major product with a faradaic efficiency of 56%. The film was stable under electroreduction conditions and was able to be recycled for continuous production of CO. The final chapter is a review that discusses the utilization of MOFs as photocatalysts for CO2 conversion using only abundant earth metals. While most CO2 catalysts are expensive noble metals, the development of cheap abundant catalytic materials is extremely relevant to a clean energy future. / Doctor of Philosophy / The climate crisis has led to a need to develop more efficient renewable energy sources and novel energy storage devices. Electrochemical systems have shown great promise in providing a clean energy future with developments in lithium-ion batteries, fuel cell technologies, and artificial photosynthesis to produce value added chemicals. The basis of all these electrochemical processes is efficient charge transfer to the necessary components. Metal-organic Frameworks (MOFs) which are crystalline, porous materials made up of inorganic metal nodes connected in a discrete pattern by organic linkers. The three-dimensional nature of the MOFs consisting of connected pore networks has shown great promise to be improved conductive materials and electrocatalysts compared to existing materials. Herein, we exploit the modular nature of the MOFs to incorporate functional groups that improve ion transport and allow catalytic activity for artificial photosynthesis. The work here shows that MOF structures can be further modified to provide even more efficient electrocatalytic behavior to aid in a clean energy future.

Flexible electronics have garnered much interest over the past several decades. Hybrid organic-inorganic materials, such as metal-organic frameworks, offer a unique opportunity to encompass the effective electronic properties of the inorganic material and the flexible nature of the organic with the potential of enhancing other desirable properties, such as the contributing multiferroicity. Using a first principles approach, the goal of this thesis is to serve as a guide for identifying potential ferroelectric and multiferroic metal-organic frameworks. This is done through a screening method of metal-organic frameworks based on their geometry; certain symmetry operators cannot be present in a ferroelectric material. We report the theoretical spontaneous polarization for several dozens of MOFs in which ferroelectricity has not previously been tested, and further we discuss the likelihood that these materials could be engineered to have either increased polarization or added ferromagnetism, the latter of which would lead to multiferroicity.

A growing field of research has evolved around the design and synthesis of a variety of porous metal-organic framework (MOF) materials. Some of the most promising areas for which these materials are potentially useful candidates include gas-separation, heterogeneous catalysis, and gas-storage, and all of these applications involve placing the MOF under pressure. There is clearly a need to understand the structural response of MOFs to applied pressure. Nevertheless, hitherto there are very few published investigations dedicated to determining the behaviour of porous hybrid materials under pressure. Through the use of high-pressure single-crystal X-ray diffraction studies, a series of MOF materials have been studied. Here we present the effect of pressure on a series of MOFs. In chapter 2, the effect of pressure on the prototypical MOF called MOF-5 was studied experimentally from ambient pressure to 3.2 GPa. Here, application of pressure was driven by the hydrostatic medium being forced into the pores of the MOF, which altered the mechanical properties of MOF-5, in particular, medium inclusion delayed the onset of amorphization. Complementary computational analysis was also performed to elucidate further the effect of medium inclusion on compressive behaviour. Detailed structural data was also collected as a function of pressure on the MOF Cu-btc. Application of pressure caused solvent to be squeezed into the pores (like MOF-5) until a phase transition occurred, driven by the sudden compression and expansion of equatorial and axial Cu–O bonds. High-pressure post-synthetic modification of a MOF is reported for the first time. On application of pressure of 0.2 GPa to the Cu-based MOF called STAM-1, a ligand exchange reaction takes place resulting in a change in pore size, shape, and hydrophilicity of the resulting pores. Here, we also demonstrate the ability to force hydrophilic molecules into hydrophobic pores using pressure, counteracting the hydrophobic effect. A high-pressure combined experimental and computational study has been carried to probe the effect of pressure on ‘breathing’ mechanisms in a zeolitic imidazolate framework (or ZIF) called ZIF-8. The penetration of guest molecules and the accommodation of pressure are shown to be inextricably linked to the rotation of methylimidazolate groups in the structure. Finally, the application of pressure to the MOF Sc₂BDC₃ and the nitro functionalized derivative Sc₂(NO₂-BDC)₃ was also studied. Here, the effect of chemical modification of the organic ligand, whilst maintaining framework topology, has been investigated as it pertains to compressibility. Directionality of compression is observed and this is rationalized with respect to the framework topology and medium inclusion/exclusion.

The use of Raman spectroscopy in ultra high vacuum to assess structure and reactivity at the interface of tris-(8-hydroxyquinoline) aluminum (Alq3) with vapordeposited metals is presented. Understanding the structure of the interface between electron transport layer materials such as Alq3 and low work function metals such as Al, Mg and Ca is vital for engineering organic light emitting diodes with high efficiency and low driving voltage. Reactivity at the interface of Al, Mg and Ca with Alq₃ thin films is examined with Raman spectroscopy along with the non-reactive Ag/Alq₃ interface for comparison. Additionally, the effect of a thin LiF barrier layer on reactivity at the Al/Alq₃ and Mg/Alq₃ interfaces is also examined. Raman spectroscopy of post-deposited Ag on Alq3 films confirms preservation of the Alq₃ structure along with evolution of simple surface enhancement of Alq₃ spectral intensities. Changes in key vibrational modes of Alq₃ upon Ag deposition are consistent with weak interaction of Ag with the conjugated ring of the ligand. In contrast, vapor deposition of Al onto Alq₃ films results in the appearance of new Raman modes linked to the formation of an Al-Alq₃ adduct. Additionally, Raman modes associated with graphitic carbon are also noted for the Al/Alq₃ interface and are attributed to partial degradation of the organic film. The Raman spectral results for deposition of Mg onto Alq3 films also indicate formation of a complex interfacial region composed primarily of Mg-Alq₃ adducts and small-grained amorphous or nanocrystalline graphite. Raman spectroscopy of the Ca/Alq₃ interface is also indicative of formation of a Ca-Alq₃ complex; however, the graphitic carbon in this system is noted to be more disordered, sp³-type carbon compared to that observed for Al/Alq₃ and Mg/Alq₃. Examination of the Al/LiF/Alq₃ and Mg/LiF/Alq₃ interfaces illustrates that 5 Å-thick LiF layers partially block reaction chemistry between the metal and organic, while 10 Å thick LiF films completely eliminates reactivity at these interfaces. Implications of the presence of chemical species observed at these metal/organic interfaces on charge transport in devices are also discussed.

No description available.

Metal-organic frameworks (or coordination polymers) are a recently-identified class of porous polymeric materials, consisting of metal ions or clusters linked together by organic bridging ligands. The major advantage of MOFs over other traditional materials, such as zeolites or activated carbons, is that their synthesis methods have provided an extensive class of crystalline materials with high stability, tunable metrics, and organic functionality. The ability to modify the physical environment of the pores and cavities within MOFs allow tuning of the interactions with guest species, and serves as a route to tailor the chemical stability and/or reactivity of the frameworks for specific applications. The classical way to incorporate functional groups into a MOF is the modification of the organic precursor with specific substituents before synthesizing the MOF itself; we call this approach pre-functionalization method. Functionalization of organic precursors is the initial and necessary step to obtaining functionalized isostructural MOFs and also provides the possibility for the post-synthetic modification of MOFs. However, in some cases, the functional groups may interfere with MOF synthesis and alter the topology of desired MOF. The goal of this proposed research is to explore the possibilities of metal-organic frameworks (MOFs) as novel porous structures, to study the effect of functional groups on the topologies and adsorption behavior of MOFs, and to understand how the synthesis conditions affect the phase purity and the in-situ reaction of ligands.

Adsorption in porous materials plays a significant role in industrial separation processes. Here, the host-guest interaction and the pore shape influence the distribution of products. Metal-organic frameworks (MOFs) are promising materials for separation purposes as their diversity due to their building block synthesis from metal corners and organic linker gives rise to a wide range of porous structures. The selectivity differs from MOF to MOF as the size and shapes of their pores are tuneable by altering the organic linkers and thus changing the host-guest interactions in the pores. Using mainly molecular simulation techniques, this work focuses on three types of separations using MOFs. Firstly, the experimental incorporation of calix[4]arenes in MOFs as a linker to create additional adsorption sites is investigated. For a mixture of methane and hydrogen, it is shown that in the calix[4]arene-based MOFs, methane is adsorbed preferentially over hydrogen with much higher selectivities compared to other MOFs in the literature. Remarkably, it was shown that extra voids created by calix[4]arene-based linkers, were accessible to only hydrogen molecules. Secondly, the strong correlation between different pore sizes and shapes in MOFs and their capabilities to separate xylene isomers were investigated for a number of MOFs. Finally, the underlying molecular mechanism of enantioseparation behaviour in a homochiral MOF for a number of chiral diols is presented. The simulation results showed good agreement with experimental enantioselectivity values. It was observed that high enantioselectivity occurs only at high loadings and when a perfect match in terms of size and shape exists between the pore size and the adsorbates. Ultimately, the information obtained from molecular simulations will further our understanding of how network topology, pore size and shape in MOFs influence their performance as selective adsorbents for desired applications.