לוח שנה משולב

The isotopes of C and O are not randomly distributed within single phases such as CO2 gas and carbonates—in particular, the heavy isotopes of C and O tend to bond preferentially (‘clump’) at lower temperatures. Consequently, the measurement of the deviation from a random distribution of C and O isotope distributions in a single phase can be used as a thermometer, which is independent of other constraints (such as the isotopic composition of water). The 'clumped' isotope geothermometer has been used to measure paleotemperatures as far back as the Ordovician, the body temperature of dinosaurs, and the temperature of African grasslands during human evolution. As with other geothermometers based on homogeneous or heterogeneous equilibria, the clumped-isotope thermometer is susceptible to resetting (e.g., if the phase is reheated or experiences slow cooling). Thus, clumped-isotope “temperatures” of phases that have experienced complex thermal histories may, in fact, be closure temperatures, the interpretation of which requires quantification of the kinetics of redistribution of C and O isotopes as a function of temperature. To better constrain these kinetics, we performed experiments on natural optical calcite and carbonate-bearing apatite in rapid quench furnaces at a variety of temperatures and times and created to models to quantify the rates. Additionally, we measured clumping in cogenetic apatite and calcite from three carbonatites; these intrusive rocks cooled slowly from igneous temperatures and thus comparison of the apatite and calcite closure temperatures provides constraints on the differences in kinetics of resetting of clumping in these two phases. Using the results of our models and measurements we are able to calculate activation energies and estimate expected changes in apparent temperatures of samples for geologically plausible scenarios. As such, this study has relevance for our interpretation of ancient samples that have thermal histories involving any significant heating. Additionally, it provides the groundwork for using apatites and carbonates together in paleotemperature studies to determine if thermal overprinting could be present.

Studying the transition of nanostructures as they develop from the zero-dimensional to the one-dimensional regime is significant for unveiling the modifications that occur in the electronic structure of the particle as its length to width aspect ratio is increased. Such understanding can lead to better design and control of the particle properties, making them attractive for a wide spectrum of technological applications. The ongoing improvements in the control of shape and morphology of nanoparticles in colloidal synthesis, which allows forming structures of similar composition but of different dimensionalities and shapes, open the way for probing such dimensionality effects.
In this talk, I will present several effects involving the 0D to 1D transition in CdSe/CdS core/shell nano heterostructures of different morphologies including “sphere in a sphere”, “sphere in a rod” and “rod in a rod”.
Using these systems, I’ll demonstrate and discuss the changes which occur in the optical properties, and in particular in emission and absorption polarizations, as the dimensionality of the particles and of their cores changes. I’ll also discuss the use of these particles as donors in energy transfer processes, and show how the efficiency of the energy transfer and the donor’s survival probability depend on the dimensions and dimensionalities of the particles, and may lead to systems with enhanced FRET