Near-infrared photoluminescence enhancement in Ge/CdS and Ge/ZnS core/shell nanocrystals: Utilizing IV/II-VI semiconductor epitaxy

Ge nanocrystals have a large Bohr radius and a small, size-tunable band gap that may engender direct character via strain or doping. Colloidal Ge nanocrystals are particularly interesting in the development of near-infrared materials for applications in bioimaging, telecommunications and energy conversion. Epitaxial growth of a passivating shell is a common strategy employed in the synthesis of highly luminescent II-VI, III-V and IV-VI semiconductor quantum dots. Here, we use relatively unexplored IV/II-VI epitaxy as a way to enhance the photoluminescence and improve the optical stability of colloidal Ge nanocrystals. Selected on the basis of their relatively small lattice mismatch compared with crystalline Ge, we explore the growth of epitaxial CdS and ZnS shells using the successive ion layer adsorption and reaction method. Powder X-ray diffraction and electron microscopy techniques, including energy dispersive X-ray spectroscopy and selected area electron diffraction, clearly show the controllable growth of as many as 20 epitaxial monolayers of CdS atop Ge cores. In contrast, Ge etching and/or replacement by ZnS result in relatively small Ge/ZnS nanocrystals. The presence of an epitaxial II-VI shell greatly enhances the near-infrared photoluminescence and improves the photoluminescence stability of Ge. Ge/II-VI nanocrystals are reproducibly 1-3 orders of magnitude brighter than the brightest Ge cores. Ge/4.9CdS core/shells show the highest photoluminescence quantum yield and longest radiative recombination lifetime. Thiol ligand exchange easily results in near-infrared active, water-soluble Ge/II-VI nanocrystals. We expect this synthetic IV/II-VI epitaxial approach will lead to further studies into the optoelectronic behavior and practical applications of Si and Ge-based nanomaterials

Methyl Complexes of the Transition Metals

Organometallic chemistry can be considered as a wide area of knowledge that combines concepts of classic organic chemistry, that is, based essentially on carbon, with molecular inorganic chemistry, especially with coordination compounds. Transition-metal methyl complexes probably represent the simplest and most fundamental way to view how these two major areas of chemistry combine and merge into novel species with intriguing features in terms of reactivity, structure, and bonding. Citing more than 500 bibliographic references, this review aims to offer a concise view of recent advances in the field of transition-metal complexes containing M-CH fragments. Taking into account the impressive amount of data that are continuously provided by organometallic chemists in this area, this review is mainly focused on results of the last five years. After a panoramic overview on M-CH compounds of Groups 3 to 11, which includes the most recent landmark findings in this area, two further sections are dedicated to methyl-bridged complexes and reactivity.Ministerio de Ciencia e Innovación Projects CTQ2010–15833, CTQ2013-45011 - P and Consolider - Ingenio 2010 CSD2007 - 00006Junta de Andalucía FQM - 119, Projects P09 - FQM - 5117 and FQM - 2126EU 7th Framework Program, Marie Skłodowska - Curie actions C OFUND – Agreement nº 26722

Crystal structures of three sterically congested disilanes

In the three sterically congested silanes, C24H38Si2 (1) (1,1,2,2-tetraisopropyl-1,2-diphenyldisilane), C24H34Br4Si2 (2) [1,1,2,2-tetrakis(2-bromopropan-2-yl)-1,2-diphenyldisilane] and C32H38Si2 (3) (1,2-di-tert-butyl-1,1,2,2-tetraphenyldisilane), the Si—Si bond length is shortest in (1) and longest in (2), with (3) having an intermediate value, which parallels the increasing steric congestion. A comparison of the two isopropyl derivatives, (1 and 2), shows a significant increase in the Si—C(ipso) distance with the introduction of bromine. Also, in the brominated compound 2, attractive intermolecular Br...Br interactions exist with Br...Br separations ca 0.52 Å shorter than the sum of the van der Waals radii. In compound 2, one of the bromoisopropyl groups is rotationally disordered in an 0.8812 (9):0.1188 (9) ratio. Compound 3 exhibits `whole molecule' disorder in a 0.9645 (7):0.0355 (7) ratio with the Si—Si bonds in the two components making an angle of ca 66°

Ring flipping in heterobimetallic Re-Ir complexes and its effect on structural isomerism: Dynamic NMR and DFT study

Experimental and DFT study on the ring flipping phenomenon from the two fused puckered five–membered rings in complex [PNP(Me)(CH3CN)Ir-ReO3][PF6] (2) and its derivative [PNP(Me)(CNtBu)Ir-ReO3][PF6] (3) is reported here. Dynamic NMR studies from 31P{1H}NMR on ring flipping revealed that complexes 2 and 3 possess ΔG‡ value of 11.2 ± 0.3 and 9.2 ± 0.3 kcal/mol at 298 K respectively. Density Functional Theory (DFT) calculations concurred with the Potential Energy (PE) values of 12.36 and 8.09 kcal/mol respectively for the same phenomena. Also, DFT studies revealed that ring flipping and structural isomerism between 2 and its isomer [PNP(H)Ir-μ(CH2)-μ(O)-Re(O)2][PF6] (1) possess distinct transition states and can occur concurrently as observed experimentally

Trioxorhena(VII)carborane Anion and Its Methyl-Substituted Analogue: Synthesis, Structure, DFT, and Catalytic Studies

Synthesis and characterization of trioxorhena­(VII)­carborane [Bu₄N]­[(η¹-C₂B₉H₁₁)­ReO₃] (<b>1a</b>) and its methyl-substituted analogue [Bu₄N]­[(7,8-Me₂-η¹-C₂B₉H₁₁)­ReO₃] (<b>1b</b>) are reported. The single-crystal X-ray structures of <b>1a</b> and <b>1b</b> display η¹ coordination between Re and the carborane cage. Density functional theory computations at B3LYP/LANL2DZ (Gaussian 09 suite) level show that the strong π donor character of the oxo ligands results in a weak interaction between the d orbitals on Re and the π orbitals of the boron cage, the consequence of which is favoring η¹ Re–B coordination. Complexes <b>1a</b> and <b>1b</b> exhibit reversible one-electron reduction in cyclic voltammetry experiments at <i>E</i>_1/2 potentials of −1.83 and −1.88 V versus Cp₂Fe/Cp₂Fe⁺, respectively. Complex <b>1a</b> catalyzes the hydrosilylation of aldehydes and ketones in excellent yields and with high tolerance to a variety of functional groups. Mechanistic investigation of the hydrosilylation reaction revealed potential involvement of multirhenium–boron cluster