Diana Yanover

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.

Three-pulse femtosecond spectroscopy of PbSe nanocrystals: 1S bleach nonlinearity and sub-band-edge excited-state absorption assignment.

Above band-edge photoexcitation of PbSe nanocrystals induces strong below band gap absorption as well as a multiphased buildup of bleaching in the 1Se1Sh transition. The amplitudes and kinetics of these features deviate from expectations based on biexciton shifts and state filling, which are the mechanisms usually evoked to explain them. To clarify these discrepancies, the same transitions are investigated here by double-pump-probe spectroscopy. Re-exciting in the below band gap induced absorption characteristic of hot excitons is shown to produce additional excitons with high probability. In addition, pump-probe experiments on a sample saturated with single relaxed excitons prove that the resulting 1Se1Sh bleach is not linear with the number of excitons per nanocrystal. This finding holds for two samples differing significantly in size, demonstrating its generality. Analysis of the results suggests that below band edge induced absorption in hot exciton states is due to excited-state absorption and not to shifted absorption of cold carriers and that 1Se1Sh bleach signals are not an accurate counter of sample excitons when their distribution includes multiexciton states.

PbSe/CdSe Thin-Shell Colloidal Quantum Dots

Abstract The present work describes the structural and optical characterization of PbSe and PbSe/CdSe colloidal quantum dots (CQDs), the latter being produced by cation exchange of Pb2+ for Cd2+ ions. The cation exchange occurs on preferred crystallographic facets and results in either non-concentric CdSe shells or PbxCd1–xSe alloyed-shell layers. The obtained heterostuctures are referred to as PbSe/CdSe “thin shell” CQDs. The parent PbSe CQDs are limited to relatively small diameters of 3–4 nm, with absorption edge between 1.0 and 1.3 eV. The steady-state and time-resolved photoluminescence spectra recorded at various temperatures reveal the following properties of the PbSe/CdSe CQDs: (1) the photoluminescence intensity of air-free CQDs is maintained upon their exposure to oxygen; (2) the band-edge exciton lifetime is extended by about a factor of two relative to the parent PbSe CQDs. The experimental results and the effective mass-based calculations suggest the formation of alloyed shells and highlight a pronounced effect of core displacement from the CQD center on the heterostructure optical properties.

Cation Exchange Combined with Kirkendall Effect in the Preparation of SnTe/CdTe and CdTe/SnTe Core/Shell Nanocrystals

Controlling the synthesis of narrow band gap semiconductor nanocrystals (NCs) with a high-quality surface is of prime importance for scientific and technological interests. This Letter presents facile solution-phase syntheses of SnTe NCs and their corresponding core/shell heterostructures. Here, we synthesized monodisperse and highly crystalline SnTe NCs by employing an inexpensive, nontoxic precursor, SnCl2, the reactivity of which was enhanced by adding a reducing agent, 1,2-hexadecanediol. Moreover, we developed a synthesis procedure for the formation of SnTe-based core/shell NCs by combining the cation exchange and the Kirkendall effect. The cation exchange of Sn(2+) by Cd(2+) at the surface allowed primarily the formation of SnTe/CdTe core/shell NCs. Further continuation of the reaction promoted an intensive diffusion of the Cd(2+) ions, which via the Kirkendall effect led to the formation of the inverted CdTe/SnTe core/shell NCs.

Lophine (2,4,5-triphenyl-1H-imidazole)

The title compound, C21H16N2, has been known since 1877. Although the crystal structure of 36 derivatives of lophine are known, the structure of parent compound has remained unknown until now. The three phenyl rings bonded to the imidazole core are not coplanar with the latter, with dihedral angles of 21.4 (3), 24.7 (3), and 39.0 (3)°, respectively, between the phenyl ring planes in the 2-, 4- and 5-positions of the imidazole ring. The molecules are packed in layers running perpendicular to the b axis. Although there are acceptor and donor atoms for hydrogen bonds, no such interactions are detected in the crystal in contrast to other lophine derivatives.

Structural comparisons between methylated and unmethylated nitrophenyl lophines

The lophine derivative 2-(2-nitrophenyl)-4,5-diphenyl-1H-imidazole, C21H15N3O2, (I), crystallized from ethanol as a solvent-free crystal and from acetonitrile as the monosolvate, C21H15N3O2.C2H3N, (II). Crystallization of 2-(4-nitrophenyl)-4,5-diphenyl-1H-imidazole from methanol yielded the methanol monosolvate, C21H15N3O2.CH4O, (III). Three lophine derivatives of methylated imidazole, namely, 1-methyl-2-(2-nitrophenyl)-4,5-diphenyl-1H-imidazole methanol solvate, C22H17N3O2.CH4O, (IV), 1-methyl-2-(3-nitrophenyl)-4,5-diphenyl-1H-imidazole, C22H17N3O2, (V), and 1-methyl-2-(4-nitrophenyl)-4,5-diphenyl-1H-imidazole, C22H17N3O2, (VI), were recrystallized from methanol, acetonitrile and ethanol, respectively, but only (IV) produced a solvate. Compounds (III) and (IV) each crystallize with two independent molecules in the asymmetric unit. Five imidazole molecules in the six crystals differ in their molecular conformations by rotation of the aromatic rings with respect to the central imidazole ring. In the absence of a methyl group on the imidazole [compounds (I)-(III)], the rotation angles are not strongly affected by the position of the nitro group [44.8 (2) and 45.5 (1) degrees in (I) and (II), respectively, and 15.7 (2) and 31.5 (1) degrees in the two molecules of (III)]. However, the rotation angle is strongly affected by the presence of a methyl group on the imidazole [compounds (IV)-(VI)], and the position of the nitro group (ortho, meta or para) on a neighbouring benzene ring; values of the rotation angle range from 26.0 (1) [in (VI)] to 85.2 (1) degrees [in (IV)]. This group repulsion also affects the outer N-C-N bond angle. The packing of the molecules in (I), (II) and (III) is determined by hydrogen bonding. In (I) and (II), molecules form extended chains through N-H...N hydrogen bonds [with an N...N distance of 2.944 (5) A in (I) and 2.920 (3) A in (II)], while in (III) the chain is formed with a methanol solvent molecule as the mediator between two imidazole rings, with O...N distances of 2.788 (4)-2.819 (4) A. In the absence of the imidazole N-H H-atom donor, the packing of molecules (IV)-(VI) is determined by weaker intermolecular interactions. The methanol solvent molecule in (IV) is hydrogen bonded to imidazole [O...N = 2.823 (4) A] but has no effect on the packing of molecules in the unit cell.

Non-poissonian formation of multiple excitons in photoexcited CdTe colloidal quantum qots by femtosecond nonresonant two-photon absorption

Using direct multiexcitonic spectroscopy, we experimentally observe for the first time the non-Poissonian formation of multiple excitons by femtosecond nonresonant two-photon absorption process in semiconductor colloidal quantum dots (QDs). Each of the multiple excitons is individually generated via the absorption of a pair of photons during the femtosecond pulse irradiation. The non-Poissonian distribution of the generated excitons is reflected as a non-quadratic dependence on the pulse intensity of the average number of excitons per QD. This is the main observation of the present work. It is explained by a multiexcitonic formation model that is based on the phenomenon of intrapulse state filling of the few quantum electronic states accessed by the two-photon transitions. The experiments are conducted with 3.9-nm CdTe QDs in room-temperature hexane solution using the femtosecond pump-probe transient absorption technique, where an intense pump pulse generates the excitons and a weak probe pulse measures their number via intraband one-photon absorption.

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.