In this presentation, we first discuss the structure of codoped Si QDs based on the data obtained by atom probe tomography. We then discuss the size dependence of the HOMO and LUMO level energies measured from the vacuum level by combining photoemission yield spectroscopy and photoluminescence spectroscopy. Furthermore, we show density of state spectra of single codoped Si QDs obtained by scanning tunneling spectroscopy and discuss the size dependence of the in-gap donor and accepter states, onset of the conduction and valence band edges, and the ionization energies of dopants. From these data, we will identify the origin of the luminescence in B and P codoped Si QDs. We demonstrate that the codoped Si QDs exhibit efficient luminescence in the energy ranges below and above bulk Si band gap. The below bulk band gap luminescence opens up a new application of codoped Si QDs as a phosphor in the second biological window (1000-1300 nm). We also discuss charge transfer between Si QDs and adsorbed molecules. We show charge-transfer-induced reversible enhancement and quenching of photoluminescence of Si QDs and the photocatalytic activity to adsorbed molecules. We can produce a variety of nanocomposites composed of codoped Si QDs and noble metal (Au, Ag and Pt) nanostructures. We discuss the enhanced optical response due to the coupling with the localized surface plasmon resonances of metal nanostructures. By using the colloidal solution of codoped Si QDs, we can produce the monolayer film, the multilayer film and the dense solids. In these films, we discuss the energy transfer between QDs. Finally, we show electrical transport properties of the films and demonstrate the possible application as electronic devices.