The growth of Co:ZnO/ZnO core/shell colloidal quantum dots: changes in nanocrystal size, concentration and dopant coordination.

We report a synthesis route for the growth of Co:ZnO/ZnO core/shell quantum dots. This procedure consists of successive steps, comprising the addition of diluted precursor salt solutions, and heat treatment at 50 degrees C. By deriving a relation between the extinction coefficient at 250 nm and the nanocrystal diameter, we are able to monitor changes in quantum dot concentration during shell growth. We found that a mechanism based on the nucleation of new particles after salt addition and subsequent Ostwald ripening during the heat treatment is responsible for the shell growth. Based on ligand-field absorption spectroscopy, we demonstrate that the Co(2+) ions adsorbed at the surface of Co:ZnO quantum dots are incorporated inside the ZnO shells. Finally, EPR spectroscopy indicates that the surface-adsorbed Co(2+) ions can be incorporated as substitutional as well as interstitial Co(2+) ions.

Lead isotopic analysis of infant bone tissue dating from the Roman era via multicollector ICP-mass spectrometry.

Archaeological samples originating from a cemetery of a Roman settlement, Pretorium Agrippinae (1st-3rd century A.D.), excavated near Valkenburg (The Netherlands) have been subjected to Pb isotopic analysis. The set of samples analysed consisted of infant bone tissue and possible sources of bone lead, such as the surrounding soil, garum, and lead objects (e.g., water pipes). After sample digestion with quantitative Pb recovery and subsequent quantitative and pure isolation of lead, the Pb isotopic composition was determined via multicollector ICP-mass spectrometry. The Pb isotope ratio results allowed distinction of three groups: bone, soil, and lead objects + garum. The 208Pb/206Pb ratio ranges were between 2.059 and 2.081 for the soils, between 2.067 and 2.085 for the bones, and between 2.087 and 2.088 for the lead objects. The garum sample is characterised by a 208Pb/206Pb ratio of 2.085. The bone group is situated on the mixing line between the soil and lead object groups, allowing the statement that diagenesis is not the main cause of the Pb found in the bones.

Size-dependent optical properties of colloidal PbS quantum dots.

We quantitatively investigate the size-dependent optical properties of colloidal PbS nanocrystals or quantum dots (Qdots), by combining the Qdot absorbance spectra with detailed elemental analysis of the Qdot suspensions. At high energies, the molar extinction coefficient epsilon increases with the Qdot volume d(3) and agrees with theoretical calculations using the Maxwell-Garnett effective medium theory and bulk values for the Qdot dielectric function. This demonstrates that quantum confinement has no influence on epsilon in this spectral range, and it provides an accurate method to calculate the Qdot concentration. Around the band gap, epsilon only increases with d(1.3), and values are comparable to the epsilon of PbSe Qdots. The data are related to the oscillator strength f(if) of the band gap transition and results agree well with theoretical tight-binding calculations, predicting a linear dependence of f(if) on d. For both PbS and PbSe Qdots, the exciton lifetime tau is calculated from f(if). We find values ranging between 1 and 3 mus, in agreement with experimental literature data from time-resolved luminescence spectroscopy. Our results provide a thorough general framework to calculate and understand the optical properties of suspended colloidal quantum dots. Most importantly, it highlights the significance of the local field factor in these systems.