The behavior of semiconductor nanocrystals under intense ultraviolet irradiation and shock wave compression.

摘要

The behavior of semiconductor nanocrystals under the influence of strong optical and mechanical perturbations has been studied, near the limits of the structural stability of the nanocrystals with regard to the effects of those perturbations.;Multielectron ionization of CdSe nanocrystals has been achieved using intense femtosecond UV light, and a maximum ionization yield of >30 electrons ejected from each nanoparticle was observed within the range of excitation fluences studied. The time-dependence of CdSe nanoparticle transient absorption, and transient absorption of the water-solvated ejected electrons, indicated a resonant two-photon ionization mechanism. At the highest excitation fluences used, melting of rod-shaped CdSe nanocrystals was observed, due to thermalization of the large population of electron-hole pairs generated from photoexcitation. An attempt to time resolve the melting dynamics of CdSe and CdTe nanocrystals, using second harmonic scattering as a probe of the centrosymmetry loss upon melting, led to the discovery of enhanced second harmonic scattering from photoexcited nanocrystals. A saturation in the relative enhancement per excited electron indicated the presence of a dense electron-hole plasma in the nanocrystals, which exhibited a more harmonic response to the laser field with increasing carrier density. The rapid excitation of a high density of electron-hole pairs also resulted in the generation of coherent acoustic phonons in CdSe spherical and rod-shaped nanocrystals. The phonon frequencies were not dependent upon the incident intensity, indicating the stability of the nanocrystal elastic constants during strong excitation. The phonon modes created were determined to be radial, not longitudinal, based on the lack of rod length dependence of the phonon frequency, while a frequency dependence on particle radius was observed.;The wurtzite to rocksalt polymorphic phase transformation in CdSe nanocrystals was studied using laser-induced shock waves. Under shock wave compression, the nanocrystals transformed under the influence of an applied pressure of 3.2GPa, a significantly lower pressure than the 7GPa required to transform nanocrystals under hydrostatic compression in diamond anvil cell, based on experiments performed previously.[1] Additionally, the transformation was complete within 100 picoseconds, exhibiting significantly faster kinetics than observed under hydrostatic compression. This is approaching the timescale for individual particle transformation, determined from recent simulations to be 7-50ps.[2, 3, 4] The kinetics became faster with increasing shock pressure, indicating a faster nucleation rate with higher instantaneous stress. Laser-induced shock waves were also used to induce the fracture of hollow CdS nanospheres. Hollow nanospheres exhibited a greater degree of fragmentation when subjected to a higher applied shock stress. A time resolved experiment was performed to measure the attenuation of an incident shock wave by a layer of hollow particles, which was determined to be 0.5GPa for a 2.5GPa shock. This was consistent with the fracture stress determined by a single-particle hollow sphere compression experiment.[5]