Nonlinear optical characteristics of monolayer MoSe2

In this study, we utilized picosecond pulses from an Nd:YAG laser to investigate the nonlinear optical characteristics of monolayer MoSe₂. Two‐step growth involving the selenization of pulsed‐laser‐deposited MoO₃ film was employed to yield the MoSe₂ monolayer on a SiO₂/Si substrate. Raman scattering, photoluminescence (PL) spectroscopy, and atomic force microscopy verified the high optical quality of the monolayer. The second‐order susceptibility χ⁽²⁾ was calculated to be ～50 pm V^?1 at the second harmonic wavelength ～810 nm, which is near the optical gap of the monolayer. Interestingly, our wavelength‐dependent second harmonic scan can identify the bound excitonic states including negatively charged excitons much more efficiently, compared with the PL method at room temperature. Additionally, the MoSe₂ monolayer exhibits a strong laser‐induced damage threshold ～16 GW cm^?2 under picosecond‐pulse excitation_. Our findings suggest that monolayer MoSe₂ can be considered as a promising candidate for high‐power, thin‐film‐based nonlinear optical devices and applications.     

The SN3–SN2 spectrum. Rate constants and product selectivities for solvolyses of benzenesulfonyl chlorides in aqueous alcohols

Rate constants for a wide range of binary aqueous mixtures and product selectivities (S) in ethanol–water (EW) and methanol–water (MW) mixtures, are reported at 25 °C for solvolyses of benzenesulfonyl chloride and the 4‐chloro‐derivative. S is defined as follows using molar concentrations: S = ([ester product]/[acid product]) × ([water solvent]/[alcohol solvent]). Additional selectivity data are reported for solvolyses of 4‐Z‐substituted sulfonyl chlorides (Z = OMe, Me, H, Cl and NO₂) in 2,2,2‐trifluoroethanol–water. To explain these results and previously published data on kinetic solvent isotope effects (KSIEs) and on other solvolyses of 4‐nitro and 4‐methoxybenzenesulfonyl chloride, a mechanistic spectrum involving a change from third order to second order is proposed. The molecularity of these reactions is discussed, along with new term ‘S_N3–S_N2 spectrum’ and its connection with the better established term ‘S_N2–S_N1 spectrum’. Copyright © 2009 John Wiley & Sons, Ltd.     

Memory characteristics of platinum nanoparticle-embedded MOS capacitors

The capacitance characteristics of platinum nanoparticle (NP)-embedded metal–oxide–semiconductor (MOS) capacitors with gate Al₂O₃ layers are studied in this work. The capacitance versus voltage (C–V) curves obtained for a representative MOS capacitor exhibit flat-band voltage shifts, demonstrating the presence of charge storages in the platinum NPs. The counterclockwise hysteresis and flat-band voltage shift, observed from the C–V curves imply that electrons are stored in a floating gate layer consisting of the platinum NPs present between the tunneling and control oxide layers in the MOS capacitor and that these stored electrons originate from the Si substrate. Moreover, the charge remains versus time curve for the platinum NP-embedded MOS capacitor is investigated in this work.     

Two-leg spin ladder model for Ag2VOP2O7 from mapping analysis based on first principles density functional calculations

The magnetic properties of Ag₂V OP₂O₇ were examined by evaluating its spin exchange interactions in terms of spin dimer analysis based on tight binding calculations and mapping analysis based on first principles density functional theory calculations. Both calculations show that a strong spin exchange interaction occurs through the super-superexchange path J₁ with the V…V distance of 5.293 Å. This strong antiferromagnetic interaction forms isolated spin dimer units, which are coupled antiferromagnetically by the spin exchange path J₃ to form a two-leg spin ladder that has no spin frustration. The inter–dimer interaction J₂ is found to be ferromagnetic, and does not lead to spin frustration.     

Use of pH electrode for precipitation titration analysis: Theory and practice

A theoretical model for the potentiometric analysis of precipitation titrations using a pH electrode has been developed and tested. The new analytical method is possible by introducing a mediator which must be a weak acid and must be able to form an insoluble salt with a cation (a titrant). Theoretical expressions of pH, as well as the concentrations of all other species, were derived and solved numerically using bisection method. Among the various factors that influence pH during the titration, concentrations of the mediator and initial values of pH were proved to be very critical. The experimental potentiometric titration curves agree well with those predicted from the theoretical model. Crossing point method was adopted to determine an equivalence point from the titration curves. The method is tested using a known system of chloride determination. Among the several mediators tested (bisulfite, chromate, phosphate, cyanide, arsenate and EDTA), phosphate yielded the best results with an error of 0.1%; bisulfite, chromate and arsenate yielded comparably good results with an error of 0.3∼0.5%, but cyanide and EDTA yielded unsatisfactory results. The optimum mediator concentration found to be in a range of 1–2 mM.     

Pseudorotation-driven dynamical structure of the tropyl radical

Despite intensive studies of the neutral tropyl radical, none of its structure, energetics, and vibrational modes are still clear. This system has puzzled scientists for over a decade since one vibrational mode frequency sharply varies from imaginary number 3000i cm-1 to the real number 6000 cm-1, depending on the calculation methods employed. We find that the origin of this peculiar mode is due to the pseudorotation (omegairot) involved in the interconversion of two nearly isoenergetic Jahn-Teller configurations (elongated structure 2B1 and compressed structure 2A2 with C2v symmetry). Here, we first report that this interconversion is not via D7h or C2v symmetry configuration but via Cs symmetry (i.e., by changing the C2v axis). This interconversion barrier is found negligibly small. Thus, the two conformers are considered to be not two different structures but a dynamically identical structure with partial quantum statistical distributions on the potential energy surface. Owing to the nearly barrierless pseudorotation, the overall structure in a short time scale (less than femtosecond) would be Cs-like between 2A2 and 2B1 configurations with small fluctuation of bond distances. However, the dynamical transitions between the 2B1 and 2A2 configurations via 14 different pseudorotation pathways would make the tropyl radical have the effective D7h structure in either a nonshort time scale (greater than femtosecond) or at nonlow temperatures, which explains the high temperature electron spin resonance experiments.     

Doped colloidal photonic crystal structure with refractive index chirping to the [111] crystallographic axis

A three-dimensionally ordered array of close-packed colloidal spheres, a photonic crystal structure in which the refractive index of the medium interstitial lattice in a colloidal crystal spatially changes in the [111] crystallographic axis, is demonstrated. The colloidal photonic crystal structure with refractive index chirping was produced by infiltration of a monomer and organic dopants with a high refractive index into a silica opal, followed by interfacial gel polymerization. The resulting photonic crystal structure has a gradually varying stop band at each different (111) plane in the face-centered cubic (fcc) crystal structure at a normal incidence. This novel structure exhibited optical characteristics that have band-gap broadening by the superposition of stop bands at each plane of the crystal with different dielectric functions. Moreover, the refractive index perturbation in the [111] fcc opal also showed a defect state within a pseudo-photonic band gap. This new type of photonic crystal structure should be useful for the band-gap engineering of photonic-band-gap materials.     

Downward self‐polarization of lead‐free (K0.5Na0.5)(Mn0.005Nb0.995)O3 ferroelectric thin films on Nb:SrTiO3 substrate

Spontaneously appearing macroscopic polarization (self‐polarization) in ferroelectrics without an electrode or an external electric field would enable the freedom to design many ferroelectric heterostructures and devices. The (K_0.5Na_0.5)(Mn_0.005Nb_0.995)O₃ (KNMN) thin film was grown on Nb:SrTiO₃ single‐crystal substrate and the resultant self‐polarization behavior due to strain relaxation was investigated. The lattice mismatch and difference in TEC between the KNMN thin film and the Nb:SrTiO₃ substrate creates a compressive strain. The compressive strain gradient may be the main cause for the observed downward self‐polarization. The downward self‐polarization of the KNMN thin film can be explained by the strong inhomogeneous compressive strain caused by strain relaxation. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Acentric 2-D ensembles of D-br-A electron-transfer chromophores via vectorial orientation within amphiphilic n-helix bundle peptides for photovoltaic device applications

We show that simply designed amphiphilic 4-helix bundle peptides can be utilized to vectorially orient a linearly extended donor-bridge-acceptor (D-br-A) electron transfer (ET) chromophore within its core. The bundle's interior is shown to provide a unique solvation environment for the D-br-A assembly not accessible in conventional solvents and thereby control the magnitudes of both light-induced ET and thermal charge recombination rate constants. The amphiphilicity of the bundle's exterior was employed to vectorially orient the peptide-chromophore complex at a liquid-gas interface, and its ends were tailored for subsequent covalent attachment to an inorganic surface, via a "directed assembly" approach. Structural data, combined with evaluation of the excited state dynamics exhibited by these peptide-chromophore complexes, demonstrate that densely packed, acentrically ordered 2-D monolayer ensembles of such complexes at high in-plane chromophore densities approaching 1/200 ?(2) offer unique potential as active layers in binary heterojunction photovoltaic devices.