PbS Nanocrystal Emission Is Governed by Multiple Emissive States.

Lead chalcogenide colloidal nanocrystals (NCs) are promising materials for solution processable optoelectronics. However, there is little agreement on the identity and character of PbS NC emission for different degrees of quantum confinement-a critical parameter for realizing applications for these nanocrystals. In this work, we combine ensemble and single NC spectroscopies to interrogate preparations of lead sulfide NCs. We use solution photon correlation Fourier spectroscopy (S-PCFS) to measure the average single NC linewidth of near-infrared-emitting PbS quantum dots and find it to be dominated by homogeneous broadening. We further characterize PbS NCs using temperature-dependent linear and time-resolved emission spectroscopy which demonstrate that a kinetically accessed defect state dominates room temperature emission of highly confined emitting NCs. These experiments, taken together, demonstrate that the linewidth and Stokes shift of PbS NCs are the result of emission from two states: a thermally accessed defect-with an energetically pinned charge carrier-and an inhomogeneously broadened band-edge state.