Shockwave Spectroscopy of Nanoparticles
This dissertation examines the effects of laser-generated shockwaves on colloidally prepared nanocrystals. Their microscopic structure is first examined utilizing gold nanoparticles. The properties under scrutiny in the first study are used to then initiate a polymorphic phase transition in CdSe quantum dots. In the final study, hollow CdS particles are used to show the utility of nanoparticles as possible energy attenuation materials.
In gold nanoparticles it is shown, for the first time, a direct observation of the effect of density increase on the optical properties of gold nanoparticles. The splitting of the plasmon resonance into two distinct peaks illustrates the result of a uniaxial compression characteristic of a shock wave.
The behavior of CdSe nanocrystals shocked to stresses of 2&ndash3.75 GPa has been studied. Above 3 GPa a near-complete disappearance of the first excitonic feature and broadening of the low-energy absorption edge were observed, consistent with a wurtzite to rocksalt structural transformation. The transformation pressure is reduced relative to hydrostatic compression in a diamond anvil cell. Also, the rate of the phase transition increases. These effects are attributed to shock induced shear stress along the reaction coordinate. The especially rapid rate observed for a 3.75 GPa shock suggests multiple nucleation events per particle.
Hollow CdS nanospheres have been fractured under the action of laser-induced shock waves. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) have been used to image the recovered fragmented particles. Additionally, time-resolved measurements of the transmission of the shock wave through a polymer layer containing hollow nanospheres have been carried out. The hollow nanospheres can attenuate the transmitted shock above a threshold stress. At the highest shock stresses measured, the shock attenuation layer acts as a composite shocked above its elastic limit.
To these ends we show that nanoparticles are useful in studying and altering the properties of shockwaves. Shockwaves are also shown to be useful in high time-resolution studies of the morphological processes in solids, using nanocrystals as the archetypical, single-domain sample.