Origin of the large homogeneous line widths obtained from strongly quantum confined PbS nanocrystals at room temperature

We study the temperature dependence of the emission spectrum obtained from a colloidal suspension of PbS nanocrystals with an average diameter of 2.5 nm. The homogeneous component of the spectrum is obtained by employing a deconvolution procedure, with the aid of size selective excitation spectroscopy. The homogeneous line shape is found to be strongly temperature dependent, which is attributed to acoustic phonon broadening. At room temperature, an ultrafast exciton dephasing time of 5 fs is determined from the homogeneous line width, in agreement with a recent prediction. A large temperature-dependent shift of the peak position is also observed. The temperature dependence of the peak shift has the opposite sign to that observed in bulk PbS. This is directly attributed to increased acoustic phonon coupling via enhancement of the deformation potential in strongly quantum confined semiconductors. Both the emission peak shift and large temperature-dependent homogeneous line shape independently point to significantly enhanced acoustic phonon coupling in small, strongly quantum confined, PbS nanocrystals. This conclusion is in agreement with some long-standing theories.