Vibrational properties and exciton-phonon interactions in lead sulfide nanocrystals

This thesis concerns the vibrational properties and exciton-phonon interactions in lead sulfide (PbS) nanocrystals, also known as PbS quantum dots (QDs). The measurements in this thesis were all performed on 3 nm PbS nanocrystals, characterized by electron microscopy and x-ray fluorescence measurements. The optical absorption spectra of these nanocrystals show discrete exciton states and a large blue-shift of the band gap. The energies of the exciton states agree well with calculated values. Raman scattering and far-infrared absorption measurements were used to determine the true vibrational modes of PbS nanocrystals. The experimental results are consistent with theoretical calculations, which account correctly for the mechanical boundary conditions at the nanocrystal-host interface, and include the first observation of the mixed or coupled vibrational modes predicted by the theory. The exciton-phonon coupling strength of PbS nanocrystals was investigated using resonant Raman scattering. The strength of this coupling is four orders of magnitude greater than that predicted theoretically for the intrinsic states of the nanocrystal, and is consistent with one charge carrier localized at the surface of the nanocrystal. Coherent acoustic phonons in PbS nanocrystals were observed using femtosecond optical techniques. This is the first observation of coherent acoustic phonons in a semiconductor nanocrystal, the phonons are generated through the deformation-potential coupling to the quantum dot exciton. The unusually weak damping of the acoustic mode facilitates its observation. Interestingly, in the time-domain experiments we also find extremely weak coupling to the optical mode. This coupling is two orders of magnitude weaker than that measured using resonant Raman scattering. These conclusions have important consequences for the vibronic nature of the exciton transition in the quantum dot and its dephasing.