Combined Computational and Experimental Study of CdSeS/ZnS Nanoplatelets: Structural, Vibrational, and Electronic Aspects of Core-Shell Interface Formation.

In the past few years, core-shell nanoparticles have opened new perspectives for the optoelectronic applications of semiconductor quantum dots. In particular, it has become possible to localize electrons in either part of these heterostructures. Understanding and controlling this phenomenon require a thorough characterization of the interfaces. In this study, we prepared quasi-2D CdSeS/ZnS core-shell nanoplatelets (NPLs) by colloidal atomic layer deposition. This technique allows fine control over the quantum confinement, the surfaces, and the interfaces. The layer-by-layer formation of a the ZnS shell around the CdSeS core was monitored using UV-vis absorption, XRD, and Raman spectroscopy. The measured band gaps and structural distortions were compared with results obtained from density functional theory (DFT) calculations. Modeling has also shown that 34% of the photoexcited electrons are delocalized into the ZnS shell. The herein presented combined modeling and experimental characterization strategy is of general interest since it can be applied to a large choice of layered semiconductor heterostructures in optoelectronics. The present approach paves the way for the synthesis of nanocrystals with precisely engineered properties for light-emitting diodes and solar cells.

Investigation of the bulk and surface properties of CdSe: insights from theory.

CdSe, in the form of quantum dots, is a semiconductor material extensively used for photovoltaic applications. We have performed a comprehensive density functional theory investigation of the geometrical and electronic properties of CdSe bulk crystals (for both the wurtzite and zinc blende phases) as well as their wurtzite (10-10) surface. Several combinations of Gaussian-type orbital basis sets and exchange-correlation functionals have been tested with a periodic formalism. The computational method for further studies was selected on the basis of the comparison of computed geometrical and electronic properties to available experimental data for the bulk crystals of CdSe, as well as to additional projector-augmented wave calculations. In a second step, the so-defined protocol was used to successfully simulate CdSe wurtzite nanocrystals exposing their nonpolar (10-10) surface. Most importantly, we have shown that the presented computational protocol is accurate to model not only bulk crystals but also surfaces, thus providing a powerful theoretical tool to simulate the light harvesting components of quantum dot sensitized solar cells.

Aspects of determining the molecular weight of cyclodextrin polymers and oligomers by static light scattering.

Methodic issues affecting the use of static light scattering to determine the average molecular weight of cyclodextrin poly- and oligomers are discussed. The critical features which enable accurate measurement such as aggregation behavior and segment-segment interaction are elucidated. Static light scattering data supported with globule size and aggregate state analysis (as determined by dynamic light scattering) allowed the aggregate-free state of the samples to be justified and the ideal globular conformation of the macromolecules to be corroborated. These molecular characteristics were demonstrated for uncharged, charged and fluorescent randomly crosslinked water soluble cyclodextrin poly- and oligomers.