Jenya Tilchin

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.

PbSe-Based Colloidal Core/Shell Heterostructures for Optoelectronic Applications

Lead-based (IV–VI) colloidal quantum dots (QDs) are of widespread scientific and technological interest owing to their size-tunable band-gap energy in the near-infrared optical region. This article reviews the synthesis of PbSe-based heterostructures and their structural and optical investigations at various temperatures. The review focuses on the structures consisting of a PbSe core coated with a PbSexS1–x (0 ≤ x ≤ 1) or CdSe shell. The former-type shells were epitaxially grown on the PbSe core, while the latter-type shells were synthesized using partial cation-exchange. The influence of the QD composition and the ambient conditions, i.e., exposure to oxygen, on the QD optical properties, such as radiative lifetime, Stokes shift, and other temperature-dependent characteristics, was investigated. The study revealed unique properties of core/shell heterostructures of various compositions, which offer the opportunity of fine-tuning the QD electronic structure by changing their architecture. A theoretical model of the QD electronic band structure was developed and correlated with the results of the optical studies. The review also outlines the challenges related to potential applications of colloidal PbSe-based heterostructures.

Quantum Confinement Regimes in CdTe Nanocrystals Probed by Single Dot Spectroscopy : From Strong Confinement to the Bulk Limit

Sufficiently large semiconductor nanocrystals are a useful model system to characterize bulk-like excitons, with the electron and hole bound predominantly by Coulomb interaction. We present optical characterization of excitons in individual giant CdTe nanocrystals with diameters up to 25.5 nm at 4.2 K under varying excitation power and magnetic field strength. We determine values for the biexciton binding energy, diamagnetic shift constant, and Landé g-factor, which approach the bulk values with increasing nanocrystal size.

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.