Introducing a few impurity atoms in semiconductor quantum dots (QDs) drastically changes the electronic property and provide new functionalities towards the photonic applications. Controlled formation of localized impurity states in the energy gap extends the freedom to engineer the luminescence properties exceeding the range that can be achieved by size and shape control. In fact, superior luminescence properties have been demonstrated in copper or silver-doped cadmium-chalcogenide QDs and manganese-doped zinc-chalcogenide QDs. In addition to compound semiconductors, silicon (Si) is earth abundant and highly biocompatible material and thus colloidal Si QDs emerge as excellent luminescent materials for optoelectronic and biophotonic applications. However, doping in Si QDs is more difficult than II-VI QDs due to the small solid solubility of typical impurity atoms and resultant strong self-purification effects.
In this work, we introduce donor and acceptor levels in the band gap of Si QDs by simultaneously doping phosphorus and boron. It is known that codoping of n- and p-type impurities reduces the self-purification effects due to the charge compensation and makes very high concentration doping possible. Furthermore, charge carrier-induced Auger processes, which reduces the luminescence quantum yield significantly, can be avoided by codoping, In this work, we first discuss the distribution of impurity atoms in phosphorus and boron codoped Si QDs from the atom-probe tomography analyses . 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 quantitatively. From these data, we will identify the origin of the luminescence in impurity codoped Si QDs. In parallel, we perform comprehensive studies to maximize the luminescence quantum yield of codoped Si QDs in a wide wavelength range. We demonstrate that the codoped Si QDs exhibit efficient luminescence in the energy range below bulk Si band gap. This opens up a new application of codoped Si QDs as a phosphor in the second biological window (1000-1350 nm).
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