Influence of Core-Shell Interfecial Strain on Electronic and Optical Properties of IV-VI Nanocrystals

Physical and Analytical Chemistry Seminar

Lecturer: Anna Rubin-Brusilovski

09 Aug 2016 @ 12:30

Location: Faculty Seminar Room

Abstract: IV-VI colloidal nanocrystals (NCs) have been of great scientific interest during the past two decades, due to their tunable optical properties over a wide spectral range in the IR spectral regime. However, IV-VI NCs show high sensitivity to oxidation in ambient conditions, a process that degrades their optical properties and makes device fabrication challenging. Inorganic coverage of cores, in the form of core/shell heterostructures leads to higher chemical stability and quantum efficiency compared to corresponding cores. Shell growth is pseudomorphic, leading to a strain at the core-shell interface, due to a crystallographic mismatch between the constituents. The effect of the core-shell interfacial strain on the electronic and optical properties in PbSe/PbS NCs with a 3% lattice mismatch is discussed. The strain profile in PbSe/PbS quantum dots (QDs) is simulated using the classical linear elasticity model, under the assumption of isotropic materials. Then, the strain profile is incorporating into a band structure calculation by the k.p method, in order to evaluate a picture for the core/shell energy levels in the vicinity of the L point in Brillouin zone. The calculated results are compared to the experimentally measured absorption, continues-wave and transient emission of the core/shell QDs before and after annealing. It is shown that the annealing process relaxes the core-shell interfacial strain by the ionic inter-diffusion and formation of graded alloying composition near the core-shell interface. The strain relaxation was expressed as a red-shift of the absorption energy and an extended excitonic lifetime with respect to the non-annealed samples.