SEARCH · Engineering Papers
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A facile halide and cation exchange reaction to co-dope d- and f-block elements in cesium lead halide nanoparticles at room temperature.
The kinetics of light-induced Pb 0 formation in mixed-cation, mixed-halide perovskite films are studied using sub-bandgap absorption measurements. Oxygen is shown to prevent Pb 0 formation and modulate the optoelectronic performance decline.
A theoretical study of singlet state cations of polycyclic aromatic hydrocarbons is performed. Appropriate symmetry suitable for further calculations is chosen for each of the systems studied. The excitation states of such species are obtained by the time dependent density functional theory (TD-DFT) method. The computations are performed using both Pople and electronic response properties basis sets. The results obtained with the use of different basis sets are compared. The electronic transitions are described and the relationships for the lowest-lying transitions states of different species are found. The properties of in-plane and out-of-plane transitions are also delineated. The TD-DFT results are compared with the experimental data available.
The binary mutual neutralization (MN) of a series of 17 cations (O{sub 2}{sup +}, NO{sup +}, NO{sub 2}{sup +}, CO{sup +}, CO{sub 2}{sup +}, Cl{sup +}, Cl{sub 2}{sup +}, SO{sub 2}{sup +}, CF{sub 3}{sup +}, C{sub 2}F{sub 5}{sup +}, NH{sub 3}{sup +}, H{sub 3}{sup +}, D{sub 3}{sup +}, H{sub 2}O{sup +}, H{sub 3}O{sup +}, ArH{sup +}, ArD{sup +}) with 3 halide anions (Cl{sup −}, Br{sup −}, I{sup −}) has been investigated in a flowing afterglow-Langmuir probe apparatus using the variable electron and neutral density attachment mass spectrometry technique. The MN rate constants of atom-atom reactions are dominated by the chemical nature of the system (i.e., the specific locations of curve crossings). As the number of atoms in the system increases, the MN rate constants become dominated instead by the physical nature of the system (e.g., the relative velocity of the reactants). For systems involving 4 or more atoms, the 300 K MN rate constants are well described by 2.7 × 10{sup −7} μ{sup −0.5}, where the reduced mass is in Da and the resulting rate constants in cm{sup 3} s{sup −1}. An upper limit to the MN rate constants appears well described by the complex potential model described by Hickman assuming a cross-section to neutralization of 11 000 Å{sup 2} at 300 K, equivalent to 3.5 × 10{sup −7} μ{sup −0.5}.
Vibrational spectra of Xe-tagged cationic silicon oxide clusters Si{sub n}O{sub m}{sup +} with n = 3–5 and m = n, n ± 1 in the gas phase are obtained by resonant infrared multiple photon dissociation (IRMPD) spectroscopy and density functional theory calculations. The Si{sub n}O{sub m}{sup +} clusters are produced in a laser vaporization ion source and Xe complexes are formed after thermalization to 100 K. The clusters are subsequently irradiated with tunable light from an IR free electron laser and changes in the mass distribution yield size-specific IR spectra. The measured IRMPD spectra are compared to calculated linear IR absorption spectra leading to structural assignments. For several clusters, Xe complexation alters the energetic order of the Si{sub n}O{sub m}{sup +} isomers. Common structural motifs include the Si{sub 2}O{sub 2} rhombus, the Si{sub 3}O{sub 2} pentagon, and the Si{sub 3}O{sub 3} hexagon.
The synthesis of a cationic cellulose copolymer from cellulose macro-initiator (MCC-BiB) and quaternary compound monomer (METMA) via atom transfer radical polymerization (ATRP) was studied. By using dimethylformamide (DMF), the optimum condition for successful synthesis was at the mole ratio of MCC-BIB:Catalyst:METMA = 1:1:26. The highest copolymer recovery was 93.2 % for 6 h and at 40°C. The copolymer was insoluble in weak polar solvents such as THF and DMF but soluble in methanol and water. The chemistry of cellulose copolymer was confirmed by the FTIR and TGA in which the METMA monomer was used as a reference. The absence of C C bond in the CiB-g-METMA spectrum indicated that graft copolymerization occurred.
Time-dependent fluorescence shift (TDFS) of Laurdan embedded in phospholipid bilayers reports on hydration and mobility of the phospholipid acylgroups. Exchange of H{sub 2}O with D{sub 2}O prolongs the lifetime of lipid-water and lipid-water-lipid interactions, which is reflected in a significantly slower TDFS kinetics. Combining TDFS measurements in H{sub 2}O and D{sub 2}O hydrated bilayers with atomistic molecular dynamics (MD) simulations provides a unique tool for characterization of the hydrogen bonding at the acylgroup level of lipid bilayers. In this work, we use this approach to study the influence of fluoride anions on the properties of cationic bilayers composed of trimethylammonium-propane (DOTAP). The results obtained for DOTAP are confronted with those for neutral phosphatidylcholine (DOPC) bilayers. Both in DOTAP and DOPC H{sub 2}O/D{sub 2}O exchange prolongs hydrogen-bonding lifetime and does not disturb bilayer structure. These results are confirmed by MD simulations. TDFS experiments show, however, that for DOTAP this effect is cancelled in the presence of fluoride ions. We interpret these results as evidence that strongly hydrated fluoride is able to steal water molecules that bridge lipid carbonyls. Consequently, when attracted to DOTAP bilayer, fluoride disrupts the local hydrogen-bonding network, and the differences in TDFS kinetics between H{sub 2}O and D{sub 2}O hydrated bilayers are no longer observed. A distinct behavior of fluoride is also evidenced by MD simulations, which show different lipid-ion binding for Cl{sup −} and F{sup −}.
We simulate electron dynamics following ionization in 2-phenyl-ethyl-amine and 2-phenylethyl-N,N-dimethylamine as examples of systems where 3 coupled cationic states are involved. We study two nuclear effects on electron dynamics: (i) coupled electron-nuclear motion and (ii) nuclear spatial delocalization as a result of the zero-point energy in the neutral molecule. Within the Ehrenfest approximation, our calculations show that the coherent electron dynamics in these molecules is not lost as a result of coupled electron-nuclear motion. In contrast, as a result of nuclear spatial delocalization, dephasing of the oscillations occurs on a time scale of only a few fs, long before any significant nuclear motion can occur. The results have been rationalized using a semi-quantitative model based upon the gradients of the potential energy surfaces.
Cobalt ferrite nanoparticles were synthesized by co-precipitation method and structural properties was investigated by X-ray diffraction (XRD) at room temperature. X-ray diffraction data was used to determine lattice parameter, X-ray density, distributions of cations among tetrahedral and octahedral sites, site radii, ionic radii and bond length of inverse spinel cobalt ferrite. XRD analysis revealed crystallinity and high intense peak correspond to cubic inverse spinel structure with average crystalline size measured by X-ray line profile fitting was found to be 13nm for most intense peak (311). The surface morphology and microstructural feature was investigated by TEM analysis which revealed that particle size varying from 12-22 nm with selected electron diffraction pattern (SAED).
Polyaniline (PANI) and its composites PANI-ZnO (Zinc oxide) and PANI-ZnO-GO (Graphene oxide) were successfully constructed. These materials were characterized by electron spin resonance (ESR) technique and ultraviolet visible spectrometry. The parameters such as line width, g-factor and spin concentration were deduced from ESR spectra, from the results the radical cation stabilization of PANI, PANI-ZnO and PANI-ZnO-GO composites were compared by the polaron and bipolaron formation. The absorption features obtained in the UV absorption spectra reveal the band gap of these modified PANI composites and also predicted the information of increasing and decreasing features of signal intensity and spin concentration.
Herein we report preparation, crystal structure and ion exchanging properties of a new phosphate of tetravalent cerium, K{sub 2}Ce(PO{sub 4}){sub 2}. A monoclinic structure having framework type arrangement of Ce(PO{sub 4}){sub 6} units formed by C2O{sub 8} square-antiprism and PO{sub 4} tetrahedra is assigned for K{sub C}e(PO{sub 4}){sub 2}. The K{sup +} ions are occupied in the channels formed by the Ce(PO{sub 4})6 and provide overall charge neutrality. The unique channel type arrangements of the K+ make them exchangeable with other cations. The ion exchanging properties of K2Ce(PO4)2 has been investigated by equilibrating with solution of 90Sr followed by radiometric analysis. In optimum conditions, significant exchange of K+ with Sr2+ with Kd ~ 8000 mL/g is observed. The details of crystal structure and ion exchange properties are explained and a plausible mechanism for ion exchange is presented.
Ab initio methods in conjunction with a large basis set are used to compute the potential energy surfaces of the 12 lowest electronic states of the HNS{sup +} and HSN{sup +} isomeric forms. These potentials are used in discussions of the metastability of these cations and plausible mechanisms for the H{sup +}/H + SN{sup +}/SN, S/S{sup +} + NH{sup +}/NH, N/N{sup +} + SH{sup +}/SH ion-molecule reactions. Interestingly, the low rovibrational levels of HSN{sup +}(1{sup 2}A″) and HNS{sup +}(1{sup 2}A″) electronically excited ions are predicted to be long-lived. Both ions are suggested to be a suitable candidate for light-sensitive NO{sup ⋅} donor in vivo and as a possible marker for the detection of intermediates in nitrites + H{sub 2}S reactions at the cellular level. The full spin rovibronic levels of HNS{sup +} are presented, which may assist in the experimental identification of HNS{sup +} and HSN{sup +} ions and in elucidating their roles in astrophysical and biological media.
Nd 1-x Sr x MnO 3 (NSMO) is a well-known manganite due to close connection between structure, transport, magnetism, and chemistry. Thus, it would be an ideal system to study modification of physical properties by external stimuli including control of stoichiometry in growth. In this work, we show that abrupt change of electronic and magnetic properties can be achieved by subtle change of oxygen partial pressure in pulsed laser deposition. Interestingly, the pressure indeed modulates cation stoichiometry. We clearly observed that the films grown at 150 mTorr and higher showed clear insulator to metal transition and stronger magnetism, commonly found in less hole doping, while the films grown at 130 mTorr and lower showed insulating behavior and weak magnetism. From soft x-ray spectroscopic methods, we clearly observed the compositional difference in those thin films. This result is further supported by scattering of lighter elements in high oxygen partial pressure but not by anion deficiency in growth.
The bandgap of ZnGeN 2 changes with the degree of cation site disorder and is sought in light emitting diodes for emission at green to amber wavelengths. By combining the perspectives of carrier localization and defect states, we analyze the impact of different degrees of disorder on electronic properties in ZnGeN 2 , addressing a gap in current studies, which largely focus on dilute or fully disordered systems. The present study demonstrates changes in the density of states and localization of carriers in ZnGeN 2 calculated using bandgap-corrected density functional theory and hybrid calculations on partially disordered supercells generated using the Monte Carlo method. We use localization and density of states to discuss the ill-defined nature of a bandgap in a disordered material and identify site disorder and its impact on the structure as a mechanism controlling electronic properties and potential device performance. Decreasing the order parameter results in a large reduction of the bandgap. The reduction in bandgap is due, in part, to isolated, localized states that form above the valence band continuum associated with nitrogen coordinated by more zinc than germanium. The prevalence of defect states in all but the perfectly ordered structure creates challenges for incorporating disordered ZnGeN 2 into optical devices, but the localization associated with these defects provides insight into the mechanisms of electron/hole recombination in the material.
Vibronic spectra of lutetium oxide (LuO) seeded in supersonic molecule beams are investigated with mass-analyzed threshold ionization (MATI) spectroscopy and second-order multiconfigurational quasi-degenerate perturbation (MCQDPT2) theory. Six states of LuO and four states of LuO + are located by the MCQDPT2 calculations, and an a 3 Π(LuO + ) ← C 2 Σ + (LuΟ) transition is observed by the MATI measurement. Finally, the vibronic spectra show abnormal vibrational intervals for both the neural and cation excited states, and the abnormality is attributed to vibrational perturbations induced by interactions with neighboring states.
Using muon spin relaxation measurements on formamidinium lead iodide [FAPbI3, where FA denotes HC(NH2)2], we show that, among the five structurally distinct phases of FAPbI3 exhibited through two different temperature hysteresis, the reorientation motion of FA molecules is quasi-static below ≈50 K over the time scale of 10−6 s in the low-temperature (LT) hexagonal (Hex-LT, <160 K) phase, which has a relatively longer photo-excited charge carrier lifetime (τc∼10−6 s). In contrast, a sharp increase in the FA molecular motion was found above ≈50 K in the Hex-LT phase, LT-tetragonal phase (Tet-LT, <140 K), the high-temperature (HT) hexagonal phase (Hex-HT, 160–380 K), and the HT-tetragonal phase (Tet-HT, 140–280 K), where τc decreases with increasing temperature. More interestingly, the reorientation motion is further promoted in the cubic phase at higher temperatures (>380/280 K), while τc is recovered to comparable or larger than that of the LT phases. These results indicate that there are two factors that determine τc, one related to the local reorientation of cationic molecules that is not unencumbered by phonons and the other to the high symmetry of the bulk crystal structure.
Although it plays a critical role in the photophysics and catalysis of lanthanides, spin–orbit coupling of electrons on individual lanthanide atoms in small clusters is not well understood. The major objective of this work is to probe such coupling of the praseodymium (Pr) 4f and 6s electrons in Pr 2 O 2 and Pr 2 O 2 +. The approach combines mass-analyzed threshold ionization spectroscopy and spin–orbit multiconfiguration second-order quasi-degenerate perturbation theory. The energies of six ionization transitions are precisely measured; the adiabatic ionization energy of the neutral cluster is 38 045 (5) cm –1 . Most of the electronic states involved in these transitions are identified as spin–orbit coupled states consisting of two or more electron spins. The electron configurations of these states are 4f 4 6s 2 for the neutral cluster and 4 f 46 s for the singly charged cation, both in planar rhombus-type structures. Finally, the spin–orbit splitting due to the coupling of the electrons on the separate Pr atoms is on the order of hundreds of wavenumbers.
Modeling chemical reactions with quantum chemical methods is challenging when the electronic structure varies significantly throughout the reaction and when electronic excited states are involved. Multireference methods, such as complete active space self-consistent field (CASSCF), can handle these multiconfigurational situations. However, even if the size of the needed active space is affordable, in many cases, the active space does not change consistently from reactant to product, causing discontinuities in the potential energy surface. The localized active space SCF (LASSCF) is a cheaper alternative to CASSCF for strongly correlated systems with weakly correlated fragments. The method is used for the first time to study a chemical reaction, namely the bond dissociation of a mono-, di-, and triphenylsulfonium cation. LASSCF calculations generate smooth potential energy scans more easily than the corresponding, more computationally expensive CASSCF calculations while predicting similar bond dissociation energies. Furthermore, our calculations suggest a homolytic bond cleavage for di- and triphenylsulfonium and a heterolytic pathway for monophenylsulfonium.