Shallow impurity doping is an efficient route to tailor optical and electronic features of semiconductor quantum dots (QDs). However, the effect of doping is often smeared by the size, shape, and composition inhomogeneities. In this paper, we study optical properties of almost monodispersed spherical silicon (Si) QDs that are heavily doped with boron (B) and phosphorus (P). The narrow size distribution achieved by a size-separation process enables us to extract doping-induced phenomena clearly. The degree of doping-induced shrinkage of the optical band gap is obtained in a wide size range. Comparison of the optical band gap with theoretical calculations allow us to estimate the number of active donor−acceptor pairs in a QD. Furthermore, we found that the size and detection energy dependence of the luminescence decay rate is significantly modified below a critical diameter, that is ∼5.5 nm. In the diameter range above 5.5 nm, the luminescence decay rate is distributed in a wide range depending on the detection energy even in size-purified Si QDs. The distribution may arise from that of donor−acceptor distances. On the other hand, in the diameter range below 5.5 nm the detection energy dependence of the decay rate almost disappears. In this size range, which is smaller than twice of the effective Bohr radius of B and P in bulk Si crystal, the donor−acceptor distance is not a crucial factor to determine the recombination rate.

We developed a self-limited self-assembly process to produce red-to-near-infrared luminescent supraparticles made from biocompatible silicon (Si) quantum dots (QDs) for fluorescence bioimaging. A starting material is a methanol solution of boron (B) and phosphorus (P) codoped all-inorganic Si QDs. The Si QDs have a heavily B and P codoped amorphous shell, and the shell induces negative potential on the surface, which prevents agglomeration of QDs in polar solvents. By adding toluene to the methanol solution, controlled agglomeration of Si QDs occurs and spherical supraparticles around 100 nm in diameter with a narrow size distribution are grown. The average diameter of supraparticles was controlled by the growth parameters. We also developed a process to stabilize the supraparticles by coating the surface by polyvinylpyrrolidone (PVP) and then by silica. The photoluminescence spectra of PVP-and silica-coated Si QD supraparticles were very similar to those of Si QDs dispersed in solution.