David De Muynck

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.

Development of a new method for Pb isotopic analysis of archaeological artefacts using single-collector ICP-dynamic reaction cell-MS

A new methodology has been developed for Pb isotopic analysis of different kinds of archaeological artefacts. The method has been optimized for bone tissue, soil, amphorae and lead objects dating from the Roman Era, and consists of 3 key steps: (i) sample digestion with quantitative Pb recovery, optimized using the certified reference materials NIST SRM 1400 Bone Ash, BCR CRM 141 Calcareous Loam Soil and BCR CRM 142 Light Sandy Soil, (ii) quantitative isolation of the pure Pb fraction and (iii) isotope ratio measurement with a quadrupole-based ICP-mass spectrometer, equipped with a dynamic reaction cell (DRC). For Pb isolation, an extraction chromatographic column, containing a resin with a Pb-selective crown ether (4,4′(5′)-di-tert-butylcyclohexane-18-crown-6) purchased from Eichrom Technologies, was used. The Pb fraction can be obtained in a pure form and in a quantitative way after a relatively fast process, the columns can be reused and no Pb isotopic fractionation occurs on the column. The isolation process has been validated using NIST SRM 981 Common Lead, and by application to samples with a known isotopic composition. For isotope ratio measurements, the use of Ne as a collision gas in the DRC allowed an external precision below 0.17% RSD for the XPb/204Pb ratios (where X = 206, 207, 208) and below 0.09% RSD for the 207Pb/206Pb, 208Pb/206Pb and 208Pb/207Pb ratios to be obtained. The accuracy of the measurement protocol was validated by comparing the results thus obtained with the corresponding MC-ICP-MS and TI-MS values. Pb isotope ratios for the certified reference materials used are given. The method was also demonstrated to work for a totally different kind of (complex) sample (lichen), and thus it is expected that the method can be used for Pb isotope ratio analysis of a wide variety of materials.

From volcanic rock powder to Sr and Pb isotope ratios: a fit-for-purpose procedure for multi-collector ICP–mass spectrometric analysis

Geochemical research into volcanic rocks often involves isotopic analysis of whole rock powders. The method of Deniel and Pin (Anal. Chim. Acta, 2001, 426, 95–103) for simultaneous isolation of strontium and lead using extraction chromatography via Sr spec™ resin was therefore adapted into a straightforward procedure for Sr and Pb isotope ratio determination by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The main focus was to reduce their rather extensive and costly cleaning procedures for resin and columns without negatively affecting data quality. It was furthermore demonstrated that non-quantitative Sr and Pb recoveries do not compromise the quality of the isotope data obtained and that no Pb isotopic fractionation occurs on the Sr spec™ resin. The accuracy of the analyses was assessed by monitoring rock reference materials. GSJ basalt JB-2, commonly regarded as the most homogeneous Pb isotopic rock reference material, hereby produced one anomalous Pb isotopic composition out of eight analyses, suggesting that JB-2 might also be affected by nugget contamination.

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.

Comment on 'Size-Dependent Composition and Molar Extinction Coefficient of PbSe Semiconductor Nanocrystals'

We are happy to read that Dai et al. find that PbSe quantum dots (Qdots) are nonstoichiometric, thereby reproducing the results we published 2 years ago. However, we cannot agree with their conclusion that “Moreels et al. (. . .) reported different molar extinction coefficients for PbSe semiconductor nanocrystals”. Treating our data according to the procedure proposed by Dai et al. shows that both data sets agree (Figure 1). In addition, the key point of our work is that PbSe Qdots have the same absorbance as bulk PbSe at short wavelengths like 400 nm. Therefore, Qdot concentrations can be determined very accurately from a single point absorbance measurement, regardless of size dispersion, and the normalization procedure reintroduced by Dai et al. is unnecessary for PbSe Qdots. We also regret that Dai et al. stick to an integration or normalization on a wavelength scale. Integration of the first exciton transition on an energy scale yields an extinction coefficient directly proportional to the oscillator strength of the transition, which is the more fundamental materials property. In conclusion, in contrast to the contribution of Dai et al., we believe that the message to researchers using extinction coefficients to determine Qdot concentrations should be to use size-independent (bulk) values whenever applicable and, otherwise, energy integrated extinction coefficients.