Georgy I. Maikov

Anomalous Independence of Multiple Exciton Generation on Different Group IV−VI Quantum Dot Architectures

Multiple exciton generation (MEG) in PbSe quantum dots (QDs), PbSe(x)S(1-x) alloy QDs, PbSe/PbS core/shell QDs, and PbSe/PbSe(y)S(1-y) core/alloy-shell QDs was studied with time-resolved optical pump and probe spectroscopy. The optical absorption exhibits a red-shift upon the introduction of a shell around a PbSe core, which increases with the thickness of the shell. According to electronic structure calculations this can be attributed to charge delocalization into the shell. Remarkably, the measured quantum yield of MEG, the hot exciton cooling rate, and the Auger recombination rate of biexcitons are similar for pure PbSe QDs and core/shell QDs with the same core size and varying shell thickness. The higher density of states in the alloy and core/shell QDs provide a faster exciton cooling channel that likely competes with the fast MEG process due to a higher biexciton density of states. Calculations reveal only a minor asymmetric delocalization of holes and electrons over the entire core/shell volume, which may partially explain why the Auger recombination rate does not depend on the presence of a shell.

Hydrogen-like Wannier–Mott Excitons in Single Crystal of Methylammonium Lead Bromide Perovskite

A thorough investigation of exciton properties in bulk CH3NH3PbBr3 perovskite single crystals was carried out by recording the reflectance, steady-state and transient photoluminescence spectra of submicron volumes across the crystal. The study included an examination of the spectra profiles at various temperatures and laser excitation fluencies. The results resolved the first and second hydrogen-like Wannier-Mott exciton transitions at low temperatures, from which the ground-state excitons binding energy of 15.33 meV and Bohr radius of ∼4.38 nm were derived. Furthermore, the photoluminescence temperature dependence suggested dominance of delayed exciton emission at elevated temperatures, originating from detrapping of carriers from shallow traps or/and from retrapping of electron-hole pairs into exciton states. The study revealed knowledge about several currently controversial issues that have an impact on functionality of perovskite materials in optoelectronic devices.

Emission processes in colloidal PbSe/PbS core-shell quantum dots

The present study describes a thorough investigation of the ground-state exciton emission of PbSe/PbS core-shell colloidal quantum dots (CQDs) with different shell-thickness/core-radius ratio, by recording the photoluminescence (PL) spectra and the corresponding emission decay processes over a temperature range from 1.4 K to 300 K. These spectra were compared with that of the corresponding PbSe core CQDs. All PL spectra showed variation in the emission intensity and peak energy, including two distinct abrupt transitions, upon the increase of the temperature. However, the transitions appeared at different temperatures in the core-shell structures with respect to that of the core. Further on, the PL-decay processes in the core materials showed a single exponent behavior, while the emission of the core-shell CQDs, decayed as a stretched exponent, with an effective lifetime of ~1 μs, at room temperature, slightly longer than that of the corresponding core.

Temperature-Dependent Optical Properties of Colloidal IV-VI Quantum Dots, Composed of Core/Shell Heterostructures with Alloy Components

Colloidal semiconductor nanocrystals attract worldwide scientific and technological interest due the ability to engineer their optical properties by the variation of size, shape, and surface properties.1-3 Recent studies revealed new strategies related to composition control of the properties, including alloying,4-7 doping,8 and in particular the formation of core/shell heterostructures.9-14 Whereas major effort has been devoted to the development of II-VI core/shell structures,12-15 there are only a few reports concerning the heterostructures of IVVI (PbSe, PbS) colloidal quantum dots (CQDs).16-19 PbSe, PbS and PbSexS1-x alloyed CQDs are the focus of widespread interest due to their unique electronic and optical properties, with feasibility of applications in near infra-red (NIR) lasers, photovoltaic solar cells, Qswitches and nano-electronic devices.20 These semiconductors have a simple cubic crystal structure with nearly identical lattice constants 5.93 A and 6.12 A at 300 K, respectively, which facilitates the formation of hetero-structures. Recently, high quality PbSe/PbS core/shell16-19 and completely original PbSe/PbSexS1-x core/alloyed shell CQDs structures19 were produced using a single injection process, offering the potential to tailor the crystallographic and dielectric mismatch between the core and the shell, forming a perfect crystalline hetero-structure. These structures present higher photoluminescence (PL) quantum yield (QY) with respect to those of core CQDs and tunability of the band-edge offset with variation of the shell thickness and composition, eventually controlling the electronic properties of the CQDs.

Chapter 6 - The Significance of Alloy Colloidal Quantum Dots

Abstract Colloidal semiconductor quantum dots attractworldwide scientific and technological interest due the ability to engineer their optical properties by the variation of size, shape, and surface properties. Semiconductor core/shell structures, where core and/or shell constituents have alloyed composition, represent an important new class of composite colloidal nanomaterials. Examples composed from IV-VI semiconductor materials are PbSe x S 1− x /PbSe y S 1− y heterostructures (where y , x may be constant throughout the shell region, or may gradually vary resulting in a formation of a graded structure). The development of these new nanoscale materials was advanced in the recent years and offer significant advantages in wide areas of applications. Here, we report the preparation, characterization and investigation of the optical properties of this class of materials, in comparison with theoretical models describing the electronic band structure.