Since their discovery two decades ago, single walled carbon nanotubes (SWNT) have created an expansion of scientific interest that continues to grow to this day. This is due to a good balance between presence of bandgap, chemical reactivity and electrical conductivity. By interconnection of the individual nanotubes or modulation of the SWNT’s electronic states, electronic devices made with thin films can become candidates for next generation electronics in areas such as memory devices, spintronics, energy storage devices and optoelectronics. My thesis focuses on the modulation of the electronic structure, optical properties and transport characteristics of single walled carbon nanotube films and their application in electronic and optoelectronic devices.

Individual SWNTs have exceptional electronic properties but are difficult to manipulate for use in electronic devices. Alternatively, devices utilize SWNTs in thin films. SWNT thin films, however, may lose some of the properties due to Schottky barriers and electron hoping between metal-nanotube junctions and individual nanotubes within the film, respectively. Until recently, there has been no known route to preserve both conjugation and electrical properties. Prior attempts using covalent chemical functionalization led to re-hybridization of sp2 carbon centers to sp3, which introduces defects into the material and results in a decrease of electron mobility.

As was discovered in Haddon Research group, depositing Group VI transition metals via atomic vapor deposition into SWNT films results in formation of bis-hexahapto covalent bonds. This (η6–SWNT)Metal(η6–SWNT) type of bonding was found to interconnect the delocalized systems without inducing structural re-hybridization and results in a decrease of the thin films electrical resistance. Recently, with the assistance of electron beam deposition, we deposited atomic metal vapor of various lanthanide metals on the SWNT thin films with the idea that they would also form covalent interconnects between nanotube sidewalls. In the case of highly electropositive lanthanides, the possibility of hexahapto bonding combined with ionic character can be evaluated and theorized.

We have reported the first use of lanthanides to enhance the conductivities of SWNT thin films and showed that these metals can not only form bis-hexahapto interconnects at the SWNT junctions but can also inject electrons into the conduction bands of the SWNTs, forming a new type of mixed covalent-ionic bonding in the SWNT network. By monitoring electrical resistance and taking spectroscopic measurements of the Near-Infrared region we are able to show the correlation between enhanced conductivity and suppression of the S11 interband transition of semiconducting SWNTs.

Potential applications of SWNT thin films as electrochromic windows require reversible modulation of the electronic structure. In order to fabricate SWNTs devices which allow for this behavior it is necessary to modulate the electronic structure by physical means such as the application of an electrical potential. We found that ionic solutions can assist with maintaining complete suppression of two Van Hove singularities in the Density of States of semiconducting SWNTs which results in optically transparent windows in the Near-Infrared region, similar to the effect seen with the incorporation of atomic lanthanide metals in thin films. We demonstrate this behavior to provide a route to nanotube based optoelectronic devices in which we use electric fields to reversibly dope the SWNT films and thereby achieve controllable modulation of optical properties of SWNT thin film.

A mechanophore is a molecule which displays sensitivity toward some applied force. Mechanophores show promise for applications in smart materials and nanomachines. Identifying structural motifs in photoactive molecules which enhance pressure sensitivity has been one focus of this research. It was demonstrated that 9-tert-butylanthracene dissolved in Zeonex polymer shows increased rates of back reaction upon applying mild pressures (< 1.5 GPa) in a diamond anvil cell. While strained rings can enhance pressure sensitivity, it is not the most effective handle for tuning mechanophoric properties. This work highlighted that large structural changes in the photoproduct are also required to bring about the desired pressure sensitivity.

Intermolecular interactions cause assemblies of conjugated chromophores to behave distinctly different from the monomer. Delocalized excited states which lead to novel photophysics are examined in systems of varying complexity from covalently linked chromophores to molecular crystals. In covalent dimers of simple organic molecules like terthiophene and anthracene, a sulfur atom is used to link the chromophores, and the oxidation state of the sulfur is found to modulate the electronic coupling leading to photophysics and photochemistry which are controlled by the electronic structure of the linker. An added advantage of using the sulfur linker is that the geometry between molecules remains constant regardless of the oxidation state, allowing purely electronic effects to be isolated, demonstrating that chemical modification of a single atom can dramatically alter the excited state behavior of a molecular assembly. In molecular crystals, delocalized excited states can lead to technologically important multiexciton processes such as fission and fusion. In crystalline rubrene NIR-to-visible upconversion is observed without the use of extrinsic sensitizers, a process which may be facilitated by low energy intermolecular states. The mechanism of singlet fission in crystalline tetracene is also explored particularly regarding the correlated triplet pair state. Evidence of this spin superposition state is manifest by temporal oscillations in the photoluminescence decay of tetracene. Based on the temperature dependence, the triplets diffuse independently throughout the crystal while maintaining spin coherence. These results could have important implications for strategies which seek to use triplet states to enhance device efficiencies.

The parameters of temperature and, particularly, pressure are useful tools for the investigation of the phase transitions and vibrational dynamics of condensed matter, such as molecular crystals. Building upon the design of the Merril-Bassett Diamond Anvil Cell (DAC), a Dynamic DAC was developed and shown to be able to dynamically control the pressure of the sample. Also, the dynamic phase changes of water to solid Ice VI and VII were imaged using the Dynamic DAC.

Using a standard DAC, the hydrogen bonding of both N1,N1,N5,N5,3,3- hexamethylcadaverine HBr (33HMCHBr) and N,N,N',N'-tetramethylputrescine HI (TMPHI) were explored using high pressure Raman spectroscopy. Evidence of a possible phase transition for 33HMCHBr is shown along with the likely vibrational mode of the low barrier hydrogen bond in TMPHI.

New single crystal x-ray crystallographic evidence is shown regarding a new crystal structure of p-terphenyl involving twinning at low temperature as well as x-ray diffraction evidence of the temperature induced phase transition in p-terphenyl.

Lastly, the results of computational studies, using a previously developed forcefield, of the pentacene in p-terphenyl mixed molecular crystal are shown. The polarizations of the calculated modes of the hindered translations of pentacene along the molecular and crystallographic axes and the anharmonicity of the modes are investigated. Also, a discussion on the identification and separation of the intramolecular motions from the calculated hindered rotations spectrum is presented.

Both temperature and pressure control and influence the packing of molecules in crystalline phases. Our molecular simulations indicate that at ambient pressure, the cubic polymorph of tetracyanoethylene, TCNE, is the energetically stable form up to ~ 160 K. The observed transition from the cubic to the monoclinic polymorph occurs however only at temperatures above ~ 318 K due to the large transition barrier. The temperature-induced phase transition in TCNE studied with high-resolution IR spectroscopy is explained in terms of the increased vibrational entropy in the crystals of the monoclinic polymorph.

Based upon the inverted design of the Merril-Bassett Diamond Anvil Cell, an improved, second generation dynamic Diamond Anvil Cell was developed. Based on the fluorescence of ruby crystals, we were able to demonstrate that the pressure variation range can be further increased at least up to 7 kbar and that the dynamic pressure compression of up to 1400 GPa/s can be achieved.

A new class of mechanophoric system, bis-anthracene, BA, and its photoisomer, PI, is shown to respond reversibly to a mild, static pressure induced by a Diamond Anvil Cell as well as to shear deformation based on absorption spectroscopic measurements. The forward reaction occurs upon illumination with light while the back-reaction may be accelerated upon heating or mechanical stress, coupled to a rehybridization on four equivalent carbon atoms. It is an intriguing result as high pressure stabilizes the photodimerized species in related systems. Our molecular volume simulations ruled out significant differences in the volumes between bis-anthracene and its photoisomer. Kinetic absorption measurements at several different pressures reveal a negative volume of activation in the exothermic back-reaction at room temperature. Through a series of temperature-dependent kinetic measurements it is shown that the barrier of activation for the back-reaction is reduced by more than an order of magnitude at several kbar pressure. Results are compared to the photoisomer of 9-anthroic anhydride and to the Dewar photoisomer. A two-step model involving the transition state is proposed for the pressure-induced PI-to-BA conversion, explaining the mechanism of the reaction occurring upon the application of homogeneous stress.

Xerogels are porous glasses formed from the hydrolysis and polycondensation of metal alkoxides. Xerogels are used as insulators, catalysts, hosts in electro-optical devices, as well as solid-state matrices in chemical sensors. An important attribute of xerogels in chemical sensing applications is the porosity of the sol-gel matrix, which results from the formation of the matrix, allows analytes to diffuse into the glass and come in contact with the sensing element. Our main goal, driven in part by the environmental applications, was directed toward incorporating sensing fluorescent chromophores into the sol-gel matrix and characterizing the effects of the matrix on the fluorophores' respective chemistry.

The mechanism by which ions diffuse into the sol-gel matrix, illustrated by measuring of the proton diffusion into the glass, was elucidated by the time-dependent absorbance change of the pH indicator dye, fluorescein. Small pore xerogel glasses were found to possess proton diffusion rates that depend on the direction of the pH change. The fluorescein-doped xerogels showed a slower time response for a decrease in pH and a longer time response for an increase in pH. The difference in these two rates could indicate that the sol-gel matrix provides a kinetic barrier to proton diffusion into the pores of the glass.

Sol-gel clad fiber-optic waveguides were investigated as intrinsic distributed fiber-optic chemical sensors. The porous sol-gel cladding allows diffusion of analytes into the evanescent field region close to the fiber-optic core. Pulsed optical excitation (0.5 ns) and time-resolved emission detection were used to simultaneously monitor several sensor regions along a fiber optic waveguide. Narrow band excitation and spectrally resolved emission provide additional means for discriminating between specific regions offering spatial sensitivity and kinetic-based sensing. A fluorescein-doped silica xerogel clad pH sensor and an undoped xerogel clad lucigenin sensor were demonstrated as intrinsic sol-gel clad fiber-optic sensors.

The field of materials science has produced a variety of valuable products and technologies defining modern day civilization. While still dominated by inorganic materials, the field of organic electronics opens new technological opportunities and the access to a new class of complementary materials. Being carbon based, organic electronic materials are abundant and environment friendly, often made with low temperature processing compatible with existing inorganic materials, and highly tunable for specific optical and electronic properties. These properties have helped enable the use of organic electronic materials throughout a wide range of products. Some of these include organic light emitting diodes (OLEDs) for electronic displays and lighting, organic field effect transistors (OFETs) for device control and computing, and organic photovoltaics (OPVs) for energy generation among other uses. With their wide range of uses, the study of organic electronic materials is of high importance.

In this work, we examine both highly studied and relatively new low-dimensional materials. Since their discovery in 1991, single-walled carbon nanotubes (SWNTs) have seen much interest due to their unique electronic and optical properties. Here we design all-SWNT electrochromic devices capable of modulating incoming light with fast response times (few milliseconds). The mechanism behind the device is explored through the view of an electric double layer capacitor (EDLC) and reveals important clues for the design of fast electrochromic devices.

Also explored here is the substitutional doping of phenalenyl based radical molecular conductors. Through the use of substitutional doping most commonly used in silicon-based semiconductors, we demonstrate the enhancement of the electrical conductivity of some molecular conductors and modification of their magnetic properties. We also explored the possibility of using the substitutional doping to control a hysteretic phase transition and related bistable state in another phenalenyl based radical molecular conductor. Finally, the phenalenyl based radicals are incorporated into an electrochromic device which is able to modulate both visible and short-wave infrared light thus opening a novel pathway for development of a new class electrochromic materials for smart window applications.