<p> Given a rapidly developing world, the need exists for inexpensive renewable energy alternatives to help avoid drastic climate change. Photovoltaics have the potential to fill the energy needs of the future, but significant cost decreases are necessary for widespread adoption. Semiconductor nanocrystals, also known as quantum dots, are a nascent technology with long term potential to enable inexpensive and high efficiency photovoltaics. When deposited as a film, quantum dots form unique nanocomposites whose electronic and optical properties can be broadly tuned through manipulation of their individual constituents. </p><p> The contents of this thesis explore methods to understand and optimize the optoelectronic properties of PbSe quantum dot films for use in photovoltaic applications. Systematic optimization of photovoltaic performance is demonstrated as a function of nanocrystal size, establishing the potential for utilizing extreme quantum confinement to improve device energetics and alignment. Detailed investigations of the mechanisms of electrical transport are performed, revealing that electronic coupling in quantum dot films is significantly less than often assumed based on optical shifts. A method is proposed to employ extended regions of built-in electrical field, through controlled doping, to sidestep issues of poor transport. To this end, treatments with chemical redox agents are found to effect profound and reversible doping within nanocrystal films, sufficient to enable their use as chemical sensors, but lacking the precision required for optoelectronic applications. Finally, a novel doping method employing "redox buffers" is presented to enact precise, stable, and reversible charge-transfer doping in porous semiconductor films. An example of oxidatively doping PbSe quantum dot thin films is presented, and the future potential for redox buffers in photovoltaic applications is examined.</p>

Necessity is the mother of all inventions. With Si hitting the speed bottleneck, newer materials to replace Si are being sought out. The ex-foliation based experiments on graphene by Geim and Novoselov at this point was perfect as many of its physical properties were fascinating from an electronics standpoint and hence it was very soon projected as a Si replacement for logic applications. In addition, graphene is also an attractive alternative to applications such as radio frequency devices, ultra-sensitive mass/chemical sensing, high-speed optoelectronics and transparent conductors for photo-voltaic applications. While the widespread success and utility of Si can be attributed to easy availability of source material and the ability to synthesize large areas of ultra high quality material, chemical vapor deposition (CVD) is the only available method to controllably produce large area monolayer graphene. CVD graphene is however polycrystalline and therefore defective. Hence, in order to promote graphene towards large-scale commercialization, it is necessary to be able to grow spatially homogeneous graphene with tailored defect densities. Transfer of atomic layers of graphene from the substrate on which it is grown, a Cu foil typically, on to an insulating substrate for electrical measurements is typically a major defect inducing step. Hence, a direct transfer-free fabrication of suspended device using graphene grown on thin films of electro-deposited Cu was attempted and successfully reported for the first time. Though it was shown that the fabrication process itself did not introduce any additional defects, the maximum obtained mobility on such fabricated structures was 5200 cm2/V·s. This value is lower than reported values in literature and thus improvements for electronic applications warranted further optimization. However, limitations on ability of electro-deposited Cu films (melting point of 1083 ◦C) to withstand high temperatures, 1000 ◦C, impeded further optimizations. Hence, growth on Cu foils was taken up. On Cu foil, we were able to identify the roles of the growth kinetics and system thermodynamics on the final quality of graphene. Specifically, by carefully altering the conditions during appropriate growth phases, we were able to obtain graphene films of tunable defect densities with motilities ranging from 200 - 20000 cm2/V·s. Using a host of characterization Techniques like electrical transport, Raman spectroscopic measurements, TEM imaging and water permeation studies, we find that the defect densities in graphene are largely concentrated at the boundaries, while the bulk of the graphene grain remains pristine. Further investigations revealed a thermodynamic correlation between the growth conditions and quality of the grain boundary in terms of defect density and structure. In addition to the influence of defects in graphene on charge mobility as seen before, their impact on the device contact resistance and charge transport hysteresis in graphene field effect transistors were also investigated. With a careful control on the film defect density, we were able to demonstrate devices with low contact resistance (1000 Ωµm ) and tunable hysteresis behavior. Finally, alternate substrates for graphene and its impact on the carrier densities were explored. Non-polar substrate SiO2 and polar substrates such AlN and AlGaN were chosen. On AlN, we obtained higher carrier mobility due to reduced phonon-electron scattering and a higher ’P’ doping behavior due to piezo-electric effects. Hence, to leverage the previous observation, novel FET device architecture with a HEMT based substrate using AlGaN was demonstrated.

In this thesis, we report the various aspects of fabricating a Cold Atmospheric Plasma system, which can be used for Plasma Enhanced Chemical Vapour Deposition. The greatest advantage of this system is its vacuum free operation, which provides a cost e effctive alternative over conventional high vacuum systems. We have designed a reactor geometry for such a plasma system, in which, the contamination due to ambient air is kept at a minimum value using a low flow of Ar (500 sccm). Towards this end, we have modeled and simulated the flow pattern of Ar gas entering the reactor geometry and have studied its e effectiveness in removing air from the plasma zone. We have fabricated such a geometry and studied the contamination at different flow rates of Ar by observing the plasma optical emission. Further, the aspect of lamentation in atmospheric pressure plasma has been studied and we have identified a few process parameters which can convert a filamentary discharge to a diffused glow. Subsequently, a complete system was developed, including an in-house built high voltage power supply, to generate a plasma with low contamination and less number of laments. We have also carried out plasma diagnostics, specifically to estimate the Electron Energy Distribution Function (EEDF) of the plasma, by analysing the radiation emitted from an Ar plasma, acquired using an Optical Emission Spectroscope. The peaks in the spectrum were curve flatted with Voigt pro les and their widths and intensities were mapped to the electron number density and the EEDF of the plasma, using the mathematical models for Stark broadening and Corona population respectively. An optimization routine based on Nelder-Mead simplex algorithm was run to estimate the optimal values of these plasma parameters that produced a good match between the simulated spectrum and the experimentally acquired one. This analysis estimated that the value of electron number density in our plasma was in the range 0:82 1017 cm 3 to 3:56 1017 cm 3 and the electron temperature was in the range 0.36 eV -0.39 eV . It also predicted that the EEDF closely approximated a Maxwellian distribution. As a proof of concept, the fabricated reactor was used to deposit thin films of Polyacetylene over microscopic cover glass slides by polymerizing Acetylene gas in the cold plasma. Deposition rates as high as 1 m=min, were obtained during thin lm deposition of the polymer. The polymeric structure of the lm was studied using NMR and FT-IR. XPS measurement revealed 5% O2 inclusion in the samples. XRD showed no distinguishable peak, indicating the amorphous state of the films. The surface morphology investigated using SEM revealed highly porous broid kind of structures, which appeared to be agglomeration of particles with sizes in the order of few micrometers. P-type Polyacetylene lms were fabricated by doping them with 5.3% by atomic concentration of I2 vapours. The UV-Visible spectroscopy study revealed a bandgap of 2.05 eV for undoped and 1.49 eV for the doped Polyacetylene samples. The lms exhibited an increase in conductivity by two orders of magnitude; from 3:6 10 13 1cm 1 to 3:5 10 11 1cm 1 for un-doped and doped Polyacetylene samples respectively.