Nanoparticle interaction with 193 nm light: Optical characteristics and particle synthesis.

摘要

Nanoparticles have properties that are distinct and different from bulk materials due to their small size. Understanding the unique nature of nanoparticles can lead to technological innovations ranging from nanoparticle-based environmental monitors to biomedicine. However, evidence suggests that nanoparticles pose potential environmental and human health risks. Control of unintended emission of nanoparticles and the development of viable nanoparticle applications necessitate a thorough characterization of nanoparticle properties and reproducible production of nanostructures.;This thesis investigates optical characteristics and synthesis of nanoparticles, including polystyrene, soot, NaCl, and gold, using 193 nm laser light. These nanoparticles have similar optical characteristics at fluences below 17 J/cm 2: fluorescence signals of the gas phase species produced by UV irradiation are spectrally narrow and self-similar, radiative lifetimes are on the order of the laser pulse duration (∼10 ns), and there is little background signal, indicating that the interaction with 193 nm light is primarily photochemical. At laser fluences exceeding 17 J/cm2, optical breakdown occurs and the emission lasts significantly longer (∼70 ns) with intense white light. With increasing fluence, the fluorescence signals from different particles have linear, intermediate, and saturation regimes, but different rates of the signal increase are observed. A dimensionless parameter, the photon-atom ratio (PAR), is used to evaluate laser-particle interaction energetics. When the signals are normalized with PAR, a similarity in the signals is established regardless of particle materials, sizes, and laser conditions. Above a PAR = 3, where saturation occurs, every particle is fully disintegrated. These optical characteristics are used for in situ, real-time chemical analysis of core-shell nanoparticles.;The photochemical interaction with 193 nm light is exploited to synthesize nanoparticles with a controlled size and morphology. The particles are disintegrated into gas phase species, whose concentration is varied by the laser energy and repetition rate. The photolyzed species undergo nucleation and/or agglomeration to form new smaller nanoparticles at an order of magnitude higher concentration with a more spherical shape than the original particles. The mean diameter and number concentration of polystyrene and soot particles synthesized by a single shot increase with PAR (for PARs < 0.1), whereas significant loss of particle volume at higher PARs by oxidation and the production of stable gaseous species results in the decrease of particle diameter and concentration. In the case of NaCl and gold, increasing PAR leads to increased diameter and concentration of the optically synthesized nanoparticles, as there is no significant particle loss.