Experimental and numerical analysis of Tm2+excited-states dynamics and luminescence in CaX2(X= Cl, Br, I).

The prospect of using Tm2+-doped halides for luminescence solar concentrators (LSCs) requires a thorough understanding of the temperature dependent Tm2+excited states dynamics that determines the internal quantum efficiency (QE) and thereby the efficiency of the LSC. In this study we investigated the dynamics in CaX2:Tm2+(X= Cl, Br, I) by temperature- and time-resolved measurements. At 20 K up to four distinct Tm2+emissions can be observed. Most of these emissions undergo quenching via multi-phonon relaxation below 100 K. At higher temperatures, only the lowest energy 5d-4f emission and the 4f-4f emission remain. Fitting a numerical rate equation model to the data shows that the subsequent quenching of the 5d-4f emission is likely to occur initially via multi-phonon relaxation, whereas at higher temperatures additional quenching via interband crossing becomes thermally activated. At room temperature only the 4f-4f emission remains and the related QE becomes close to 30%. Possible reasons for the quantum efficiency not reaching 100% are provided.

Analytical model of coincidence resolving time in TOF-PET.

The coincidence resolving time (CRT) of scintillation detectors is the parameter determining noise reduction in time-of-flight PET. We derive an analytical CRT model based on the statistical distribution of photons for two different prototype scintillators. For the first one, characterized by single exponential decay, CRT is proportional to the decay time and inversely proportional to the number of photons, with a square root dependence on the trigger level. For the second scintillator prototype, characterized by exponential rise and decay, CRT is proportional to the square root of the product of rise time and decay time divided by the doubled number of photons, and it is nearly independent of the trigger level. This theory is verified by measurements of scintillation time constants, light yield and CRT on scintillator sticks. Trapping effects are taken into account by defining an effective decay time. We show that in terms of signal-to-noise ratio, CRT is as important as patient dose, imaging time or PET system sensitivity. The noise reduction effect of better timing resolution is verified and visualized by Monte Carlo simulation of a NEMA image quality phantom.

STM study of morphology and electron transport features in cytochrome c and nanocluster molecule monolayers.

The morphology and electron tunneling through single cytochrome c and nanocluster Pt(5)(CO)(7)[P(C(6)H(5))](4) molecules organized as monolayer Langmuir-Blodgett (LB) films on graphite substrate have been studied experimentally using scanning tunneling microscopy (STM) and spectroscopy techniques with sub-nanometer spatial resolution in a double barrier tunnel junction configuration STM tip-monomolecular film-conducting substrate at ambient conditions. STM images of the films revealed globular structures with characteristic diameters (approximately 3.5 nm for the protein molecule and approximately 1.2 nm for the nanocluster). The spectroscopic study by recording the tunneling current-bias voltage (I-V) curves revealed tunneling I-V characteristics with features as steps of different width and heights that are dependent on the STM tip position over the molecule in the monolayer, giving evidence for sequential discrete electron-tunneling effects with the combination of the single electron Coulomb-charging energy and the electronic energy level separation (molecular spectrum) in such immobilized metalloprotein and nanocluster structures that can be of interest for the development of bioelectronic and hybrid functional nanosystems.

Interaction of copper ions with stearic acid Langmuir monolayers and formation of cluster structures in monolayers and Langmuir-Blodgett films.

The interaction of copper ions with a stearic acid Langmuir monolayer resulting in an extremely high level of copper binding to the monolayer in amounts much larger than the number of stearic acid molecules in the monolayer was studied. The shape of the pressure-area isotherm changed drastically upon pH changes from 4 to 6 in the presence of copper ions in the aqueous phase (at concentrations of 10(-5) to 10(-3)(M) or upon addition of copper ions to the aqueous phase under different monolayer compressions. The copper ion concentration changes in the bulk phase, caused by binding to the monolayer, were studied by EPR at the equilibrium after intensive mixing of the bulk phase and were found to depend on pH of the aqueous phase and the extent of monolayer compression. The highest level of binding (up to 100 copper ions per stearic acid molecule, pH 5.6, initial copper concentration 5.10(-4) M) was observed at a surface pressure of about 20 mN/m; further compression of the monolayer and the respective increase in surface pressure caused the reverse growth of aqueous phase copper ion concentration. At the collapse and destruction of the monolayer, the copper ion concentration in the bulk phase was similar to that in the absence of the monolayer. The EPR spectra and SAXS diffractograms of copper-containing stearic acid monolayers confirmed the high copper content in LB films obtained. An STM study of pure stearic acid and the copper-containing monolayer LB films, transferred to graphite wafers from the water subphase surface (pH 5.4) at various copper concentrations, discovered nanosized (about 5 nM) cluster formations on the monolayer surface. The data obtained indicate that the interaction of a charged Langmuir monolayer with copper ions and formation of copper-containing nanostructures depends on monolayer compression and is determined by the arrangement, order, mobility of the monolayer stearic acid molecules and by electrostatics at the interface.