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.

Optical and Electronic Properties of Nonconcentric PbSe/CdSe Colloidal Quantum Dots.

Lead chalcogenide colloidal quantum dots are attractive candidates for applications operating in the near infrared spectral range. However, their function is forestalled by limited stability under ambient conditions. Prolonged temperature-activated cation-exchange of Cd(2+) for Pb(2+) forms PbSe/CdSe core/shell heterostructures, unveiling a promising surface passivation route and a method to modify the dots' electronic properties. Here, we follow early stages of an-exchange process, using spectroscopic and structural characterization tools, as well as numerical calculations. We illustrate that preliminary-exchange stages involve the formation of nonconcentric heterostructures, presumably due to a facet selective reaction, showing a pronounced change in the optical properties upon the increase of the degree of nonconcentricity or/and plausible creation of core/shell interfacial alloying. However, progressive-exchange stages lead to rearrangement of the shell segment into uniform coverage, providing tolerance to oxygen exposure with a spectral steadiness already on the formation of a monolayer shell.

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-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.

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.

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.

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.

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

The title compound, C(21)H(16)N(2), 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 mol-ecules are packed in layers running perpendicular to the b axis. Although there are acceptor and donor atoms for hydrogen bonds, no such inter-actions are detected in the crystal in contrast to other lophine derivatives.