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

Nonlinear fractional differential equations in nonreflexive Banach spaces and fractional calculus

The aim of this paper is to correct some ambiguities and inaccuracies in Agarwal et al. (Commun. Nonlinear Sci. Numer. Simul. 20(1): 59-73, 2015; Adv. Differ. Equ. 2013: 302, 2013, doi:10.1186/1687-1847-2013-302) and to present new ideas and approaches for fractional calculus and fractional differential equations in nonreflexive Banach spaces

Theoretical studies on the structure of M<SUP>+</SUP>BF<SUP>-</SUP><SUB>4</SUB> ion pairs M = Li<SUB>+</SUB>, NH<SUP>+</SUP><SUB>4</SUB>: the role of electrostatics and electron correlation

A systematic investigation of the M<SUP>+</SUP>BF<SUB>4</SUB><SUP>-</SUP> (M=Li or NH<SUB>4</SUB>) ion-pair conformers has been carried out using an electrostatic docking model based on the molecular electrostatic potential topography of the free anion. This method provides a guideline for the subsequent ab initio molecular orbital calculations at the Hartree-Fock (HF) and second-order Moller-Plesset perturbation theory (MP2) levels. It has been demonstrated that the model presented here yields more than 75% of the HF interaction energy when Li<SUP>+</SUP> is the cation involved and more than 90% for the case of NH<SUB>4</SUB><SUP>+</SUP>. Inclusion of MP2 correlation in the HF-optimized geometries leads to stationary point geometries with different numbers of imaginary frequencies and in some places where the energies of two adjacent conformers are very close, the energy rank order is altered. The HF lowest-energy minima for the Li<SUP>+</SUP>BF<SUB>4</SUB><SUP>-</SUP>and NH<SUB>4</SUB> <SUP>+</SUP>BF<SUB>4</SUB><SUP>-</SUP> show a bidentate and tridentate coordinating cation, respectively, whereas at the MP2 level, this ordering is reversed

Electronic Structure, NMR, Spin–Spin Coupling, and Noncovalent Interactions in Aromatic Amino Acid Based Ionic Liquids

Noncovalent interactions accompanying phenylalanine (Phe), tryptophan (Trp), and tyrosine (Tyr) amino acids based ionic liquids (AAILs) composed of 1-methyl-3-butyl-imidazole and its methyl-substituted derivative as cations have been analyzed employing the dispersion corrected density functional theory. It has been shown that cation–anion binding in these bioionic ILs is primarily facilitated through hydrogen bonding in addition to <i>lp---π</i> and CH<i>---π</i> interactions those arising from aromatic moieties which can be probed through ¹H and ¹³C NMR spectra calculated from the gauge independent atomic orbital method. Characteristic NMR spin–spin coupling constants across hydrogen bonds of ion pair structures <i>viz</i>., Fermi contact, spin–orbit and spin–dipole terms show strong dependence on mutual orientation of cation with the amino acid anion. The spin–spin coupling mechanism transmits spin polarization via electric field effect originating from <i>lp---π</i> interactions whereas the electron delocalization from lone pair on the carbonyl oxygen to antibonding C–H orbital is facilitated by hydrogen bonding. It has been demonstrated that indirect spin–spin coupling constants across the hydrogen bonds correlate linearly with hydrogen bond distances. The binding energies and dissected nucleus independent chemical shifts (NICS) document mutual reduction of aromaticity of hydrogen bonded ion pairs consequent to localization of π-character. Moreover the nature and type of such noncovalent interactions governing the <i>in-plane</i> and <i>out-of-plane</i> NICS components provide a measure of diatropic and paratropic currents for the aromatic rings of varying size in AAILs. Besides the direction of frequency shifts of characteristic CO and NH stretching vibrations in the calculated vibrational spectra has been rationalized

CO₂ Absorption Using Fluorine Functionalized Ionic Liquids: Interplay of Hydrogen and σ‑Hole Interactions

Use of ionic liquids (ILs) for CO₂ capture offers certain advantages over currently used methodologies and is of growing interest. With this perspective, ILs composed of <i>S</i>-ethyl-<i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetramethylthiouronium ([ETT]) and 1-hexyl-3-methylimidazolium ([Hmim]) cations and tris­(pentafluoroethyl)­trifluorophosphate ([FEP]) anion have been investigated. The present work unravels the noncovalent interactions accompanying CO₂ capture by these ILs. Electronic structure of ion pairs and their CO₂ absorbed [ETT]­[FEP]·<i>n</i>(CO₂) and [Hmim]­[FEP]·<i>n</i>(CO₂) (<i>n</i> up to 30) complexes are derived. The anisotropy in molecular electrostatic potential dictates the binding of CO₂ through the interplay of (i) halogen bonding (O···F) between electron deficient σ-holes on fluorines, (ii) electrostatic C···F interactions between electron deficient carbons of CO₂ and the electron-rich fluorine atoms, and the (iii) hydrogen bonding (O···H) interactions from the cation. The manifestations of these interactions on binding energies, polarizabilities, and vibrational spectra of CO₂ absorbed complexes are presented. Consequent “frequency shift” accompanying hydrogen and halogen bonding exhibit complementary characteristics in the infrared spectra of CO₂ absorbed complexes. Correlation of binding energies to absorbed CO₂ molecules further demonstrate that [Hmim] based ILs are more efficient for CO₂ capture applications

Hydrogen Bonding, ¹H NMR, and Molecular Electron Density Topographical Characteristics of Ionic Liquids Based on Amino Acid Cations and Their Ester Derivatives

Amino acid ionic liquids (AAILs) have attracted significant attention in the recent literature owing to their ubiquitous applications in diversifying areas of modern chemistry, materials science, and biosciences. The present work focuses on unraveling the molecular interactions underlying AAILs. Electronic structures of ion pairs consisting of amino acid cations ([AA⁺], AA = Gly, Ala, Val, Leu, Ile, Pro, Ser, Thr) and their ester substituted derivatives [AAE⁺] interacting with nitrate anion [NO₃^–] have been obtained from the dispersion corrected M06-2x density functional theory. The formation of ion pair is accompanied by the transfer of proton from quaternary nitrogen to anion facilitated via hydrogen bonding. The [Ile], [Pro], [Ser], and [Thr] and their esters reveal relatively strong inter- as well as intramolecular hydrogen-bonding interactions. Consequently, the hierarchy in binding energies of [AA]­[NO₃] ion pairs and their ester analogues turns out to be [Gly] > [Ala] > [Ser] ∼ [Val] ∼ [Ile] > [Leu] ∼ [Thr] > [Pro]. The work underlines how the interplay of intra- as well as intermolecular hydrogen-bonding interactions in [AA]- and [AAE]-based ILs manifest in their infrared and ¹H NMR spectra. Substitution of −OCH₃ functional group in [AA]­[NO₃] ILs lowers the melting point attributed to weaker hydrogen-bonding interactions, making them suitable for room temperature applications. As opposed to gas phase structures, the presence of solvent (DMSO) does not bring about any proton transfer in the ion pairs or their ester analogues. Calculated ¹H NMR chemical shifts of the solvated structures agree well with those from experiment. Correlations of decomposition temperatures in [AA]- and [AAE]-based ILs with binding energies and electron densities at the bond critical point(s) in molecular electron density topography, have been established

Radiation chemical oxidation of aniline derivatives

The reactions of OH, O- and N3 with chloro- and hydroxy-anilines were studied by pulse radiolysis. The rates of the OH radical reaction are higher (k ∼ 5 × 109 dm3 mol-1 s-1) than those found for the O- and N3 reactions (k ∼ 2 × 109 dm3 mol-1 s-1). Neither the position of the substituent nor the introduction of an additional Cl to monochloroanilines has any significant effect on the rates of the OH reaction. The intermediates formed in all the aniline derivatives studied herein have λmax values around 310-320 and 350-380 nm. The OH radical reacts both by addition and direct H abstraction giving rise to OH adducts (350-380 nm) and anilino radicals (310-320 nm). The extent of these two reactions depends on the position of the substituent, the former being more predominant in the meta than in the ortho and para isomers. The initially-formed OH adducts subsequently undergo dehydration, leading to anilino radicals in the case of chloroanilines and phenoxyl radicals with hydroxyanilines. The OH attack at the carbon bonded to Cl in all three monochloroanilines is not significant (≤15%). Semi-empirical quantum calculations using the PM3 method were carried out to evaluate the possible sites of the OH radical attack. The charge distribution and the heats of formation data reveal that the OH attack extends over more than one carbon center. The relative stabilities of the isomeric OH adducts formed from the attack at the unsubstituted carbons of chloro- and hydroxy-anilines are nearly the same, their respective heats of formation being approximately -70 and -230 kJ mol-1

Theoretical Appraisal of Cyclopropenone: Aggregation and Complexes with Water

Cyclopropenone (HCCOCH, “CPN”) is an exotic quasi-aromatic cyclic carbene that abounds in the interstellar medium (ISM). Astronomical observations suggest that (i) stagnate CPN exhibits a tendency to polymerize and that (ii) interactions may occur between CPN and water that is also ubiquitous in the ISM. In this light, density functional theory investigations reveal cooperative hydrogen bonding, which leads to stable polymeric conformations of (CPN)n, tracked up to n = 14. Stable agglomerations with water, however, constitute at best only two CPN and two water molecules, signifying that while CPN exhibits remarkable cooperativity for “cohesive” clustering via hydrogen bonding, this tendency is markedly diminished for “hetero”-interactions. Multifaceted data are employed to probe cogent molecular descriptors, such as structure and energetics of various conformers, vibrational spectroscopic response, molecular electrostatic potential (MESP), effective atomic charges: all these, in unison, describe the evolution of the characteristics upon cluster formation. Salient stretching frequency shifts, as well as charge redistribution gleaned from MESP morphology, have a direct bearing on variegated hydrogen bonding patterns: linear, nonlinear, as well as bifurcated. In particular, characteristic C–H, CO stretching, and O–H vibrations in the water complexes reveal a “softening” (downshift) of frequencies. While small conformers have markedly distinct MESP variations, the differences become less pronounced with incremental clustering, an effect substantiated by corresponding emergent atomic charges