Strongly inhibited spontaneous emission of PbS quantum dots inside 3D silicon photonic crystals

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 are covalently bonded to polymer brush layers that are grafted from the Si-air interfaces inside the 3D nanostructure using surface-initiated atom transfer radical polymerization (SI-ATRP). The presence and position of the quantum dots was previously characterized by X-ray fluorescence tomography. We report both continuous wave emission spectra and time-resolved time-correlated single photon counting of the quantum dots. 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 that is tentatively attributed to the presence of Cu-catalyst during synthesis. In this regime, continuous wave (cw) emission spectra are known to be proportional to the radiative rate, hence to the local density of states. In spectra normalized to those taken on flat Si, we observe a broad and deep stop band that we attribute to a 3D photonic band gap with a relative bandwidth up to 26%. The observed inhibition is 5 to 30 times enhanced, similar to previously reported band gap inhibitions, but for completely coincidental reasons. Our results are relevant to applications in photochemistry, sensing, photovoltaics, and to efficient miniature light sources.