Origins of Photoluminescence Decay Kinetics in CdTe Colloidal Quantum Dots

Recent experimental studies have identified at least two nonradiative components in the fluorescence decay of solutions of CdTe colloidal quantum dots (CQDs). The lifetimes reported by different groups, however, differed by orders of magnitude, raising the question of whether different types of traps were at play in the different samples and experimental conditions and even whether different types of charge carriers were involved in the different trapping processes. Considering that the use of these nanomaterials in biology, optoelectronics, photonics, and photovoltaics is becoming widespread, such a gap in our understanding of carrier dynamics in these systems needs addressing. This is what we do here. Using the state-of-the-art atomistic semiempirical pseudopotential method, we calculate trapping times and nonradiative population decay curves for different CQD sizes considering up to 268 surface traps. We show that the seemingly discrepant experimental results are consistent with the trapping of the hole at unsaturated Te bonds on the dot surface in the presence of different dielectric environments. In particular, the observed increase in the trapping times following air exposure is attributed to the formation of an oxide shell on the dot surface, which increases the dielectric constant of the dot environment. Two types of traps are identified, depending on whether the unsaturated bond is single (type I) or part of a pair of dangling bonds on the same Te atom (type II). The energy landscape relative to transitions to these traps is found to be markedly different in the two cases. As a consequence, the trapping times associated with the different types of traps exhibit a strikingly contrasting sensitivity to variations in the dot environment. Based on these characteristics, we predict the presence of a sub-nanosecond component in all photoluminescence decay curves of CdTe CQDs in the size range considered here if both trap types are present. The absence of such a component is attributed to the suppression of type I traps

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

Devising an Engineering Procedure for Calculating the Ductility of a Roller Bearing Under a No-central Radial Load

Known theoretical approaches to calculating the ductility of rolling bearings include rather complicated analytical dependences and require cumbersome computation. That makes it a relevant task to undertake a research aimed at the development of an engineering approach to the calculation of radial ductility of bearings.The current study proposes an engineering method for determining radial ductility using cylindrical roller bearings as an example. It accounts for the radial gap, contact deformation of parts, the deformations of bending and misalignment of rings for cases when a bearing is exposed to the action of a central radial load and a radial load with eccentricity. The adopted simplified linear calculation model for determining the angle of rings misalignment is valid for small angles when contact is maintained over the entire length of the roller. Computation of radial ductility of roller bearings under a no-central radial load is based on determining the sum of variable elastic deformations in a contact between rings and the most loaded roller. The values for elastic deformations are determined from known formulae for solving the contact problem in elasticity theory taking into consideration a mismatch between the geometric centers of outer and inner rings.Adequacy of the proposed engineering procedure has been confirmed by results from calculating the specific ductility of the cylindrical roller bearing 2211 with a central radial load. By using the proposed methodology, we have derived values for specific ductility that are 3...4 % lower compared to similar results obtained from a known procedure. By using the cylindrical roller bearing 42726 as an example, we have investigated structural parameters considering a no-central radial load. A decrease in the bearing 42726 ductility with an increase in the number of rollers and rigidity of the outer ring has been shown, as well as with a decrease in the eccentricity of a radial load.The ductility of rolling bearings must be known when constructing dynamic models of certain machines: machine tool spindles, shaft-gears at large-size reducers, crane structures. Therefore, the proposed engineering procedure for determining the ductility of roller bearings at small angles of rings misalignment could be applied in the practice of designing machines and mechanisms for which the elastic characteristics of all their components are important