Temperature dependent optical properties of PbS nanocrystals

A comprehensive study of the optical properties of PbS nanocrystals (NCs) is reported that includes the temperature dependent absorption, photoluminescence (PL) and PL lifetime in the range of 3300K. The absorption and PL are found to display different temperature dependent behaviour though both redshift as temperature is reduced. This results in a temperature dependent Stokes shift which increases from 75meV at 300K with reducing temperature until saturating at 130meV below 150K prior to a small reduction to 125meV upon cooling from 25 to 3K. The PL lifetime is found to be single exponential at 3K with a lifetime of τ ₁=6.5μs. Above 3K biexponential behaviour is observed with the lifetime for each process displaying a different temperature dependence. The Stokes shift is modelled using a three-level rate equation model incorporating temperature dependent parameter values obtained via fitting phenomenological relationships to the observed absorption and PL behaviour. This results in a predicted energy difference between the two emitting states of 6meV which is close to the excitonic exchange energy splitting predicted theoretically for these systems.