Photochemical dynamics of laser-irradiated semiconductor nanostructures

Thesis (Ph.D.)--University of Washington, 2016-08

Unique and novel applications of nanoscale materials are continuously being found. Applications span across many fields, including medicine, physics, chemistry, and engineering. Properties of nanoparticles are commonly studied through laser-mediated experiments, but the intense fields generated by laser light will often influence temperatures of the system under study, which may or may not be the intention of the experimentalist. Understanding and quantifying these thermal effects is beneficial, if not crucial, to the researcher. Our studies have focused on the interplay between intrinsic properties of a material, its dimensions, and its environment. The main method utllized in the Pauzauskie group for studying individual nanoparticles is the optical trap, where a highly focused laser is used to isolate a particle from any supporting substrate. Although temperature studies in a laser trap were actively being pursued by the group, it was quickly realized that chemical reactions could be initiated by properly selecting the correct material. In chapter 2, experiments detailing the generation of reactive singlet oxygen species 1O2 from trapped semiconducting nanowires will be described. The implications of potential reactions between the trapped structure and the 1O2 molecules are then discussed. In chapter 3, the nonlinear effects produced by the intense fields within an optical trap are addressed. Potassium niobate (KNbO3) nanowires are selected as an ideal material for the demonstration of second harmonic and sum frequency generation through the co-alignment of near-infrared lasers. It is shown that the unexpected heating found while trapping these supposedly unabsorbing particles can be attributed to the many crystallographic defects produced in the growth process as evidenced by high resolution transmission electron microscopy. Final experiments were based on laser cooling of cadmium sulfide nanoribbons. Chapter 4 describes the implementation of a custom Raman system with a 532 nm source, necessary for the anti-Stokes excitation of the 2.42 eV band gap (~ 510 nm) for CdS. The ability to acquire Raman scattering spectra has been extremely beneficial to the group for a wide variety of experiments. As an example, the results of a high pressure-induced phase transition in silicon nanowires are given and discussed in terms of pressure vs.\ temperature as driving factors, where analytical theory developed in our lab was written into Python to simulate temperatures of nanowires under Raman excitation. Finally, chapter 5 gives the details for initial, trapping experiments of CdS nanoribbons and results from an analytical solution to the heat equation. Although the combination of experimental results and analytical/numerical calculations prove useful in determining a nonlinear absorption coefficient for the nanostructures, it was ultimately decided that laser cooling of CdS suspensions was unlilkely due to the material's instability in aqueous media. The experiments were altered to monitor the resonance frequencies of cantilevered nanoribbons in vacuum to provide an orthogonal method for measuring temperatures of CdS nanoribbons under 532 nm excitation, compared to pump-probe techniques found in the literature. Results from temperature calibrations of nanoribbons show a Young's modulus temperature coefficient of ~150 ppm/K which should allow for the detection of ∆T=-40K as claimed by others; however, measurements made in our lab have, so far, only shown heating. Future efforts would benefit from determination of the quantum yields of individual nanoribbons. If the assertions made that this is the first semiconductor to demonstrate laser cooling can be corroborated by independent techniques, it would provide clear evidence that the refrigeration is physically occurring; but without external verification, researchers will likely remain skeptical.