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 exciton's 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.

Tuning of electronic properties in IV-VI colloidal nanostructures by alloy composition and architecture.

Colloidal lead chalcogenide (IV-VI) quantum dots and rods are of widespread scientific and technological interest, owing to their size tunable energy band gap at the near-infrared optical regime. This article reviews the development and investigation of IV-VI derivatives, consisting of a core (dot or rod) coated with an epitaxial shell, when either the core or the shell (or both) has an alloy composition, so the entire structure has the chemical formula PbSexS1-x/PbSeyS1-y (0 ≤ x(y) ≤ 1). The article describes synthesis procedures and an examination of the structures' chemical and temperature stability. The investigation of the optical properties revealed information about the quantum yield, radiative lifetime, emission's Stokes shift and electron-phonon interaction, on the variation of composition, core-to-shell division, temperature and environment. The study reflected the unique properties of core-shell heterostructures, offering fine electronic tuning (at a fixed size) by changing their architecture. The optical observations are supported by the electronic band structure theoretical model. The challenges related to potential applications of the colloidal lead chalcogenide quantum dots and rods are also briefly addressed in the article.

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.

Composition-tunable optical properties of colloidal IV-VI quantum dots, composed of core/shell heterostructures with alloy components.

Colloidal quantum dots (CQDs) attract worldwide scientific and technological attention due to the ability to engineer their optical properties by the variation of their size. However, several important applications, such as biological tagging and photovoltaic cells, impose a limit on their size yet demand tunability and thermal stability of the optical band edge. This work introduces a new class of heterostructures, composed of PbSe or PbSe(y)S(1-y) cores, coated by PbS or PbSe(x)S(1-x) shells, with different core-radius/shell-width division, with a radial gradient composition (with 0 < y < 1, 0 < x < 1), which offer a control of the band edge properties by varying the CQDs' composition. Continuous-wave and transient photoluminescence measurements over a wide temperature range (1.4-300 K) revealed a distinct behavior of the heterostructures with respect to that of pure PbSe cores: (i) increase of the emission quantum yield; (ii) red-shift of the absorption edge but a decrease of the emission Stokes shift; (iii) alleviation of a dark exciton recombination, viz., a reduction of an exchange interaction; (iv) tuning of the radiative lifetime with shell width and composition; (v) reduction of the band edge temperature coefficient, dE/dT, viz., induction of thermal stability. The k·p envelope function calculation, considering abrupt or smooth alloying continuation of the potential at the core-shell interface, revealed a delocalization of the hole wave function over the entire volume of the CQDs, as a partial explanation for the marked tunability, nonetheless preserving a desired size.

Thermally activated photoluminescence in lead selenide colloidal quantum dots.

The thermal activation processes in PbSe colloidal quantum dots and their influence on the ground-state exciton emission are discussed. Activation of a dark exciton occurs at 1.4-7 K, assisted by an acoustic phonon coupling. Activation of a bright exciton occurs at 100-200 K, which appears as a sudden change in the photoluminescence band intensity, energy, and full width at half maximum. This activation overcomes the dark-bright-state splitting, when the activation temperature increases with the decrease of the dots' size. The dark exciton lifetime is found to be approximately 6-12 micros at 1.4 K, while the bright exciton lifetime at 300 K evaluated as 450 ns varies slightly with the change in the size of the dots. In addition, the emission quantum yield of these dots, measured at a variety of temperatures when dissolved in various solvents, reveals information about the influence of the environment on the recombination processes.