Emission of Photons by Near-Infrared PbS Quantum Dot Nanocrystals for a Large Diameter Range

In pursuit of the functionalization of 3D silicon nanophotonics with optically active building blocks, we investigate, as candidates, near-infrared (NIR) emitting semiconductor quantum dot (QDot) nanocrystals. We present continuous-wave and time-resolved spectral emission of different commercially obtained lead sulfide (PbS) quantum dots with considerable 2-fold variations in diameters, with two different cappingspolyethylene glycol (PEG) and oleic acid (OA)that are suspended in three different solvents: water, chloroform, or toluene. The emission spectra reveal maxima at different photon energies, as set by the nanocrystals' average diameters. Time-resolved emission histograms reveal varying amounts of nonsingle exponential decay, all of which are successfully modeled with a log-normal distribution of decay rates. We observe, for quantum dots with OA surface groups, that the most frequent decay rate varies slightly with photon energy and concomitant quantum dot diameter. The rates agree with a two-level exciton model, especially if we assume the transition dipole moment to vary proportionally to the quantum dot diameter, but not with an atomic-like dipole emitter. For quantum dots with PEG surface groups, the most frequent decay rate decreases remarkably beyond photon energies 8000 cm^-1 or conversely nanocrystal diameters below 4.4 nm. A hypothesis for this observation is that the protonation of the terminal amine groups of the thiol-PEG-NH₂ ligands generates an enhanced local electric field, which enhances the quantum-confined Stark effect (QCSE). This leads to a greater spatial separation of the electron and hole wave functions and thus reduces the radiative rate, particularly in smaller dots. Other mechanisms, such as quenching, solvent polarity, solvent resonance (vibrational or electronic), nanoparticle acoustics, and electron or hole wave function trapping, are also evaluated. As a preliminary concrete step toward silicon photonics, we study quantum dots dip-coated on a silicon surface and observe that the emission rates are markedly increased 5- to 10-fold, which is attributed to the higher dielectric function near the silicon surface compared to suspensions.