Probing slow dynamics by ultrafast process: Sol–gel transition detected by transient absorption spectroscopy of quantum dots

Abstract

Sol–gel transition is a second-order phase transition depending on the creation of the macroscopic solute network, which confines the solvent in its pores. The phase transition from the sol state to the gel state takes place, when small clusters of molecules join together and create one huge cluster, which fills most of the volume of the sample. The aim of this paper is to study the phase transition process using as detectors the semiconductor PbS nanocrystals, called quantum dots, embedded in the solution. As a material we are using the aqueous solution of gelatin, which creates the gel network due to hydrogen bonds between monomers belonging to different chains. The lifetimes of excitons generated in quantum dots by the light pulse appear to be very sensitive to the polymer surrounding. These lifetimes have been measured by use of the transient absorption spectroscopy. In order to show the dynamics of the gelation process the measurements were performed with appropriate time lags in train of the process duration. It appears that dynamics of the exciton lifetime significantly changes during the sol–gel transition, what allows the indication of the gel point in the system and the appropriate universal critical exponents.