Strongly Inhibited Spontaneous Emission of PbS Quantum Dots Covalently Bound to 3D Silicon Photonic Band Gap Crystals

Abstract

We present an optical study of the spontaneous emission of lead sulfide (PbS) nanocrystal quantum dots in 3D photonic band gap crystals made from silicon. The nanocrystals emit in the near-infrared range to be compatible with 3D silicon nanophotonics. The nanocrystals are covalently bound to polymer brush layers that are grafted from the Si-air interfaces inside the nanostructure by using surface-initiated atom transfer radical polymerization. The presence and position of the quantum dots were previously characterized by synchrotron X-ray fluorescence tomography. We report both continuous wave emission spectra and time-resolved, time-correlated single photon counting. In time-resolved measurements, we observe that the total emission rate greatly increases when the quantum dots are transferred from suspension to the silicon nanostructures, likely due to quenching (or increased nonradiative decay) that is tentatively attributed to the presence of Cu catalysts during the synthesis. In this regime, continuous wave emission spectra are known to be proportional to the radiative rate and thus to the local density of states. In spectra normalized to those taken on flat silicon outside the crystals, we observe a broad and deep stop band that we attribute to a 3D photonic band gap with a relative bandwidth of up to 26%. The shapes of the relative emission spectra match well with the theoretical density of states spectra calculated with plane-wave expansion. The observed inhibition is 4-30 times, similar to previously reported record inhibitions, yet for coincidental reasons. Our results are relevant to applications in photochemistry, sensing, photovoltaics, and efficient miniature light sources.