Organic Spacers in 2D Perovskites: General Trends and Structure-Property Relationships from Computational Studies

Addition of large organic molecules to halide perovskites has been shown to provoke dimensionality reduction and formation of two-dimensional phases that demonstrate improved long-term stabilities. Optoelectronic properties of the resulting 2D layered perovskites are strongly influenced by the chemical nature of the additive molecules, which opens immense possibilities for preparation of materials with tailored properties. However, given the huge chemical space of possible organic spacers, a systematic and exhaustive search for optimal compounds is impossible and general structure–property relationships that could guide a rational design are still largely absent. Here, we provide an overview of a series of recent computational studies from our group on different types of spacers. We first develop a simplified universal monovalent cation model to map out approximate structural stability maps as a function of the van der Waals radius and the magnitude of dispersion interactions to monitor the possible emergence of 2D phases. We further provide structural and photophysical insights from classical and first-principles molecular dynamics simulations and density functional theory calculations on 2D hybrid perovskites based on a wide range of spacers with different chemical nature and varying conformational properties. Our computational predictions are validated through comparison with powder diffraction, conductivity and optical measurements. Such comparative study allows for providing some general structure–property correlations, which can serve as design guidelines in the search for optimal 2D and mixed 2D/3D perovskite photovoltaic materials.     

Monitoring the emission of volatile organic compounds from flowers of Jasminum sambac using solid-phase micro-extraction fibers and gas chromatography with mass spectrometry detection

Solid-phase micro-extraction (SPME) was studied as a solvent free alternative method for the extraction and characterization of volatile compounds in intact and plucked flowers of Jasminum sambac at different day time intervals using gas chromatography (GC-FID) and gas chromatography-quadrupole mass spectrometry. The analytes identified included alcohols, esters, phenolic compounds, and terpenoids. The main constituents identified in the flower aroma using different fibers were cis-3-hexenyl acetate, (E)-beta-ocimene, linalool, benzyl acetate, and (E,E)-alpha-farnesene. The benzyl acetate proportion decreased from morning to afternoon and then increased in evening collections. PDMS fiber showed a high proportion of (E,E)-alpha-farnesene in jasmine floral aroma. Among other constituents identified, cis-3-hexenyl acetate, linalool, and benzyl acetate were major aroma contributors in plucked and living flowers extracts using PDMS/DVB, Carboxen/PDMS, and DVB/Carboxen/PDMS fibers. PDMS/DVB recorded the highest emission for benzyl acetate while the (E)-beta-ocimene proportion was highest in DVB/Carboxen/PDMS when compared with the rest. The highest linalool content, with increasing proportion from morning to noon, was found using mixed coating fibers. Almost negligible volatile adsorption was recorded for the polyacrylate fiber for intact flower aroma, whereas it was most effective for benzyl acetate, followed by indole under plucked conditions. Moreover, the highest amounts extracted, evaluated from the sum of peak areas, were achieved using Carboxen/PDMS, and DVB/Carboxen/PDMS. Introduction of a rapid, and solvent free SPME method for the analysis of multicomponent volatiles can be successfully employed to monitor the extraction and characterization of flower aroma constituents.     

meso-Aryl triphyrin(2.1.1)

Synthesis, spectral, and single-crystal X-ray structural analysis of meso-aryl triphyrin(2.1.1) featuring three pyrrole rings and four meso-aryl rings are described. The title compound represents the first example of a ring-contractedmeso-aryl β-unsubstituted free-base triphyrin containing only pyrrole rings reported to date and generates 2-D supramolecular assembly in the solid state.     

Isothermal crystallization kinetics of poly(trimethylene terephthalate)/multiwall carbon nanotubes composites

The isothermal crystallization kinetics of poly(trimethylene terephthalate) (PTT) in the presence of varying amounts of multiwall carbon nanotubes (MWCNT) have been investigated using differential scanning calorimetry (DSC) and analyzed using Avrami and secondary nucleation theory. Polarized light microscopy (PLM) was used to study the crystal morphology of PTT/MWCNT composites. The results showed that the presence of MWCNTs in PTT acted as an effective nucleating agents and lead to the spherulitic morphology. The decrease in the spherulites size on MWCNT addition was observed by polarized light microscopy. Using values of transport parameters ( $ U* = 1500{\text{ cal mol}}^{ - 1} , \, \Updelta T =T_{\text{g}} - 30\, $ °C) together with experimentally determined values of equilibrium melting temperature [ $ T^{\text{o}}_{\text{m}} $ (245.2 °C)] and glass transition temperature [ $ T_{\text{g}} $ (45 °C)], the nucleation parameter, $ K_{\text{g}} $ and $ \sigma_{\text{e}} $ were determined for PTT and PTT/MWCNT composites according to Lauritzen–Hoffman theory. The decrease in the values of these parameters on MWCNT addition is in agreement with the fact that the rate of crystallization of PTT increased in the presence of MWCNTs.     

Reactivity of Diruthenium and Dirhodium Analogues of Pentaborane(9): Agostic versus Boratrane Complexes

A series of novel Cp*‐based (Cp*=η⁵‐C₅Me₅) agostic, bis(σ‐borate), and boratrane complexes have been synthesized from diruthenium and dirhodium analogues of pentaborane(9). The synthesis and structural characterization of the first neutral ruthenadiborane(6) analogue are also reported. This new route offers a very efficient method for the isolation of bis(σ‐borate) and agostic complexes from diruthenapentaborane(9).     

Raman study of NiFe2O4 nanoparticles,bulk and films: effect of laser power

Nanoparticles, bulk and thin films of NiFe₂ O₄ compounds are studied by micro‐Raman spectroscopy. The effect of varying the incident laser power up to 40 mW was studied in all forms of the samples. The spectra showed a large magnitude of red shift and line broadening as a result of high incident laser power. It is shown that the inverse spinel structure remains robust, and no trace of laser‐induced oxidation was observed. The low‐temperature study of the bulk and nanoparticles has also been carried out for elucidating thermal effects due to the high incident laser power. The rise in temperature for maximum incident laser power of 40 mW was estimated to be ∼625 °C. Copyright © 2010 John Wiley & Sons, Ltd.     

Generation and structural analysis of reactive empty particles derived from an icosahedral virus

Chemical and genetic modifications on the surface of viral protein cages confer unique properties to the virus particles with potential nano and biotechnological applications. The enclosed space in the interior of the virus particles further increases its versatility as a nanomaterial. In this paper, we report a simple method to generate a high yield of stable cowpea mosaic virus (CPMV) empty capsids from their native nucleoprotein counterparts by removing the encapsidated viral genome without compromising the integrity of the protein coat. Biochemical and structural comparison of artificially generated empty particles did not reveal any distinguishable differences from CPMV particles containing viral RNA. Preliminary results on the use of artificially produced empty CPMV capsids as a carrier capsule are described.     

Physical controls on directed virus assembly at nanoscale chemical templates

Viruses are attractive building blocks for nanoscale heterostructures, but little is understood about the physical principles governing their directed assembly. In situ force microscopy was used to investigate organization of Cowpea Mosaic Virus engineered to bind specifically and reversibly at nanoscale chemical templates with sub-30 nm features. Morphological evolution and assembly kinetics were measured as virus flux and inter-viral potential were varied. The resulting morphologies were similar to those of atomic-scale epitaxial systems, but the underlying thermodynamics was analogous to that of colloidal systems in confined geometries. The 1D templates biased the location of initial cluster formation, introduced asymmetric sticking probabilities, and drove 1D and 2D condensation at sub-critical volume fractions. The growth kinetics followed a t(1/2) law controlled by the slow diffusion of viruses. The ability of poly(ethylene glycol) (PEG) to induce the lateral expansion of virus clusters away from the 1D templates suggests a significant role for weak interactions.     

Rietveld refinement and impedance spectroscopy of calcium titanate

Single phase perovskite CaTiO₃ has been synthesized by conventional solid state reaction technique. The ceramic was characterized by XRD at room temperature and its Rietveld refinement inferred orthorhombic crystal structure with the space group Pbnm. The field dependence of dielectric relaxation and conductivity was measured over a wide frequency range from room temperature to 673 K. Analysis of Nyquist plots of CaTiO₃ revealed the contribution of many electrically active regions corresponding to bulk mechanism, distribution of grain boundaries and electrode processes. The dc conductivity depicted a semiconductor to metal type transition. Frequency dependence of dielectric constant (ε′) and tangent loss (tan δ) show a dispersive behavior at low frequencies and is explained on basis of Maxwell-Wagner model and Koop's theory. Both conductivity and electric modulus formalisms have been employed to study the relaxation dynamics of charge carriers. The variation of ac conductivity with frequency at different temperatures obeys the universal Jonscher's power law (σ_ac α ω^s). The values of exponent ‘s’ lie in the range 0.13 ≤ s ≤ 0.33, which in light of CBH model suggest a large polaron hopping type of conduction mechanism.     

Simultaneous First Derivative Spectrophotometric Determination of Palladium and Nickel Using 2-(2-Thiazolylazo)-5-Dimethylaminobenzoic Acid as an Analytical Reagent

 A simple, rapid, selective, sensitive and economical method has been developed for the simultaneous determination of trace amounts of palladium and nickel in aqueous methanolic medium using 2-(2-thiazolylazo)-5-dimethylam inobenzoic acid as an analytical reagent by first derivative spectrophotometr y. Palladium is determined by measuring base to peak distance at λ=695.0 nm while nickel is estimated by zero crossing method in the mixture. The linearity is maintained between 0.12–1.75 μg mL⁻¹ for palladium and 0.07–1.60 μg mL⁻¹ for nickel in the pH range 2.8–7.2 and 3.4–8.8 respectively. Seven replicate determinations of 1.0 μ g mL⁻¹ of palladium and 0.8 μg mL⁻¹ of nickel in a mixture give a mean signal height of 0.391 for Pd and 0.541 for Ni with relative standard deviations of 0.9% and 1.2%, respectively. The sensitivity of the proposed method is 0.391 (dA/dλ)/(μg mL⁻¹) for palladium and 0.685 (dA/dλ)/(μg mL⁻¹) for nickel. Various parameters have been optimised for the simultaneous determination of palladium and nickel in various complex samples. Received March 30, 1999. Revision November 25, 1999.