Structural characterization of InAlAsSb/InGaAs/InP heterostructures for solar cells

In this work, we have characterized by transmission electron microscopy techniques the structural properties of InAlAsSb/InGaAs/InP heterostructures, with target applications in high efficiency solar cells. Previous photoluminescence (PL)1 analysis suggested the existence of compositional fluctuations in the active layer of these heterostructures. 220 bright field (BF)2 diffraction contrast micrographs have revealed strong strain contrast in the InGaAs buffer layer, related to the existence of these compositional fluctuations. The effect of a decomposed buffer on the growth of the InAlAsSb layer has been analyzed through the simulation of the strain fields in the heterostructure using the finite elements method (FEM).3 These simulations have shown that the strain in the buffer layer due to the compositional fluctuations only affects the first few nm of the InAlAsSb layer. The analysis by aberration corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM)4and electron energy loss spectroscopy (EELS)5of the composition of the InAlAsSb layer reveals that any compositional fluctuation is only observed as an average effect, rather than in the form of clustering or atomically sharp transitions. The limitations of these techniques for the detection of small 3D compositional fluctuations are discussed

Quantum dots coordinated with conjugated organic ligands: new nanomaterials with novel photophysics

CdSe quantum dots functionalized with oligo-(phenylene vinylene) (OPV) ligands (CdSe-OPV nanostructures) represent a new class of composite nanomaterials with significantly modified photophysics relative to bulk blends or isolated components. Single-molecule spectroscopy on these species have revealed novel photophysics such as enhanced energy transfer, spectral stability, and strongly modified excited state lifetimes and blinking statistics. Here, we review the role of ligands in quantum dot applications and summarize some of our recent efforts probing energy and charge transfer in hybrid CdSe-OPV composite nanostructures

Synthesis and prospective study of the use of thiophene thiosemicarbazones as signalling scaffolding for the recognition of anions

A family of phenyl-thiosemicarbazone dyes have been prepared and their interactions with anions monitorized via UV-Vis, fluorescence and 1H NMR titrations. Additionally quantum chemical calculations and electrochemical studies completed the studies carried out. The phenyl-thiosemicarbazone dyes show a modulation of their hydrogen-bonding and electron-donating capabilities as a function of the chemical groups attached and display two different chromo-fluorogenic responses towards anions in acetonitrile solutions. The more basic anions fluoride and cyanide are able to induce the dual coordination-deprotonation processes for all the receptors studied, whereas acetate only interacts with receptors 2, 3, 6, 7, 8, 9 and dihydrogen phosphate displays sensing features only with the more acidic receptors 6. Coordinative hydrogen bonding interactions is indicated by a small bathochromic shift, whilst deprotonation results in the appearance of a new band at ca. 400-450 nm corresponding to a colour change from colourless-yellow to yellow-red depending on the receptor. In the emission fluorescence, hydrogen bonding interaction is visible through the enhancement of the emission band, whereas deprotonation induced the growth of a new red-shifted emission. The chromo-fluorogenic behaviour could be explained on the basis of the deprotonation tendency of the binding sites and the proton affinity of the anions. PM3 and 1H NMR calculations are in agreement with the existence of the dual complexation-deprotonation process, whereas both studies are in discrepancy in relation to which is the proton involved in the deprotonation. Electrochemical studies carried with receptor 3 showed a quite complex redox behaviour and anodic shifts of the reduction peaks in the presence of the basic anions fluoride, cyanide and acetate.Fundação para a Ciência e a Tecnologia (FCT