Thermal luminescence quenching of amine-functionalized silicon quantum dots: a pH and wavelength-dependent study

Understanding and resolving the mechanisms that affect the photoluminescence (PL) of Si QDs are of great importance because of their strong potential for optoelectronic and solar cell materials. In this article, the intrinsic exciton dynamics of water-dispersed allylamine-functionalized silicon quantum dots (Si QDs) have been explored as a function of temperature by means of steady-state and time-resolved PL spectroscopy. Significant PL quenching of Si QDs has been observed with increase in temperature from 278 K to 348 K. This thermal quenching is found to be a reversible process. The mechanism involves nonradiative reversible relaxation of conduction band electrons through the thermally-created temporary trap states. These temporary trap states arise due to the displacement of surface atoms from their regular positions at elevated temperature. Upon cooling, these surface irregularities relax back to their equilibrium positions with retrieval of the original PL intensity. It has been observed that the quenching mechanism is strongly influenced by the pH and excitation wavelength (λex). At pH 3.5, the quenching mechanism involves nonradiative relaxation of conduction band electrons through the thermally-created temporary trap states. However, at pH 7.4, the unprotonated surface amine groups introduce permanent nitrogen-related surface defects inside the bandgap of Si QDs. At elevated temperature, the conduction band electrons get trapped in these nitrogen-related surface defects through the involvement of thermally-created temporary trap states. Subsequent exciton recombination of these nitrogen-related defect states results in red-shifted green color luminescence. By using the Arrhenius equation we have estimated the activation energy of this nonradiative thermal relaxation process and it was found to be 138 and 139 meV at pH 3.5 and pH 7.4, respectively.