Silicon (Si) nanocrystals have been a subject of intensive research for many years as nanoscale luminescence materials. In particular, the research on colloidal form Si nanocrystals are growing rapidly in recent years, because of the versatility in many kinds of applications. Recently, we have developed a new type of all-inorganic Si nanocrystals that can be dispersed in alcohol and water almost perfectly without any surface functionalization processes. The unique property arises from the core-shell structure. The shell is very heavily boron (B) and phosphorus (P) doped crystalline Si, which induces negative potential on the surface and prevents the agglomeration in polar solvents. The B and P codoped Si nanocrystals have several characteristic properties not being observed in usual Si nanocrystals. Among them, an important feature is the insensitiveness of the luminescence property to environment and to chemical treatment of the surface. Because of the insensitiveness, the codoped Si nanocrystals are a very suitable material for the production of a variety of Si nanocrystal-based nanocomposites. In this work, we first discuss structural and optical properties of B and P codoped colloidal Si nanocrystals, and then show the luminescence properties of dense solids made from the colloidal Si nanocrystals. We discuss the effects of the dielectric environment and the energy transfer between nanocrystals on the luminescence properties of Si nanocrystal solids. We also produce composite nanopartciles consisting of codoped Si nanocrystals and gold (Au) nanoparticles or nanorods. We demonstrate the enhancement of the radiative decay rate and quantum yield of Si nanocrystals by tuning the surface plasmon (SP) resonance energy to the luminescence energy. We also show the excitation cross-section enhancement, when the SP resonance energy coincides with the excitation energy.