Structural and surface correlations to the optical properties of nonthermal plasma-produced silicon nanoparticles

Abstract

Nanomaterials have diverse capabilities to enable new technology and to deepen our understanding of our world, providing exciting prospects for scientists and the public alike in a vast span of uses. In the past decade, however, the potential held by nanotechnology has been reframed in the context of helping to slow global climate change and to alter the ways in which we use our energy to reflect more efficient technology and renewable energy sources. Silicon is a standout material in this new framework: as a nanomaterial, silicon can emit light when exposed to an applied voltage or ultraviolet optical excitation source. Silicon nanocrystals also exhibit size-dependent light emission, due to quantum confinement. This thesis is an exploration of the synthesis and processing parameters that affect the optical performance of silicon nanocrystals produced in a nonthermal plasma reactor. The efficiency of this light emission is sensitive to both synthesis environment and post-synthesis treatment. The work presented here is an attempt to deepen our understanding of the effects of different reactor and treatment parameters on the light emission efficiency from silicon nanoparticles, such that the luminescence behavior of the nanoparticles can be specifically engineered. Being able to fine-tune the structure, surface, and optical characteristics of the silicon nanocrystals is key in maximizing their use in luminescence applications. For all of the experiments described here, a nonthermal plasma flow-through reactor has been used to create the silicon nanoparticles. Silane gas is dissociated in the plasma and fragments come together to form silicon clusters, then grow to create nanoparticles. The nanoparticles were collected from the reactor for further processing, characterization, and experiments. The first discovery in this project was that by adjusting the power to the plasma reactor, the crystallinity of the silicon particles can be tuned: low power results in amorphous silicon nanoparticles, and high power yields crystalline nanoparticles. Even more important, the crystallinity of a nanoparticle ensemble relates directly to the photoluminescence (PL) efficiency, or quantum yield, from the ensemble: crystalline silicon nanoparticle samples, after alkyl functionalization, exhibit PL efficiencies of 40% or greater, while amorphous samples emit light with very poor efficiency (<2%). Additional studies of the plasma reactor revealed the importance of injecting a flow of hydrogen gas into the afterglow of the plasma, which turns out to have dramatic implications for the ultimate PL quantum yields of the nanocrystals. This injection scheme was systematically studied by varying the injected gas and its position. Hydrogen injected directly into the plasma afterglow was found to be vital for achieving high quantum-yield silicon nanocrystals, likely due to a reduction in surface trap states due to additional hydrogen passivation at the nanocrystal surface.Further investigations into the nanocrystal surface and how it relates to PL quantum yield showed that the photoluminescence from silicon nanocrystals is not only dependent on synthesis parameters, but also on processing temperatures and procedures following synthesis. While the highest PL efficiencies are found for silicon nanocrystals capped with alkyl chains, the PL efficiency of a nanocrystal ensemble can also be improved simply by heating the sample to temperatures between 150-200° C. This heating step also leads to a change in the hydride structure at the nanocrystal surface, which appears to be brought about by the effusion of silyl (or disilane) groups. Finally, details of the construction of a silicon-nanocrystal-based LED will be discussed. The LED project is part of a collaboration, and while the majority of device-specific aspects of the project were carried out in the lab of Professor R. Holmes by his Ph.D. student Kai-Yuan Cheng, the processing and alterations made to the nanocrystals used in the LED were all the responsibility of the author. The details of the project and a summary of the results bear discussion here in this thesis, as well as outlining of a novel scheme for deposition of SiNCs for device construction.