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 Not 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 E 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 mu s, 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

Inductively coupled plasma mass spectrometry (ICP-MS) was combined with UV–vis−NIR spectrophotometry and transmission electron microscopy to determine the nanocrystal composition and molar extinction coefficient ϵ of colloidal PbSe quantum dot (Q-PbSe) suspensions. The ICP-MS results show a nonstoichiometric Pb/Se ratio, with a systematic excess of lead for all samples studied. The observed ratio is consistent with a faceted spherical Q-PbSe model, composed of a quasi stoichiometric Q-PbSe core terminated by a Pb surface shell. At high photon energies, we find that ϵ scales with the nanocrystal volume, irrespective of the Q-PbSe size. From ϵ, we calculated a size-independent absorption coefficient. Its value is in good agreement with the theoretical value for bulk PbSe. At the band gap, ϵ is size-dependent. The resulting absorption coefficient increases quadratically with decreasing Q-PbSe size. Calculations of the oscillator strength of the first optical transition are in good agreement with theoretical tight binding calculations, showing that the oscillator strength increases linearly with Q-PbSe size.

This paper presents a systematic evaluation of a commercially available strontium-specific extraction chromatographic resin based on a crown ether (Sr specÔ), for use in applications of Sr isotope ratio analysis dealing with samples displaying a complex and/or Ca-rich matrix composition. A protocol, consisting of (i) loading a sample digest in 7 M HNO 3 onto the resin, (ii) rinsing the resin with 7 M HNO 3 to remove concomitant matrix elements and (iii) rinsing the resin with 0.05 M HNO 3 to strip off the purified Sr fraction, was found to provide the best results. The performance in terms of (i) the purity of the Sr fraction obtained, (ii) the efficiency of Rb/Sr and Ca/Sr separation, (iii) the Sr recovery from samples with a complex and Ca-rich matrix composition and (iv) the Sr isotope ratios obtained using multi-collector ICP-MS, was evaluated for various amounts (250, 500, 750 and 2000 mL) of resin using digests of bone and soil certified reference materials, dental tissues, fluorite and glass samples. Further, it was investigated whether or not the isolation protocol introduces Sr isotopic fractionation. Also the possibility of regenerating the resin after use, allowing multiple use of the resin, was assessed. Finally, the Sr isotopic composition of 2 bone (NIST SRM 1400 Bone Ash and NIST SRM 1486 Bone Meal) and 2 soil (BCR CRM 141 Calcareous Loam Soil and BCR CRM 142 Light Sandy Soil) certified reference materials was determined. The method was shown to be fit-for-purpose for population migration studies and provenancing of archaeological artefacts, and is expected to be suited for a broad range of Sr isotope ratio applications.