Electronic versus lattice match for metal-semiconductor epitaxial growth: Pb on Ge(111)

Lattice match is important for epitaxial growth. We show that a competing mechanism, electronic match, can dominate at small film thicknesses for metal-semiconductor systems, where quantum confinement and symmetry requirements may favor a different growth pattern. For Pb(111) on Ge(111), an accidental lattice match leads to a √3 × √3 configuration involving a 30° in-plane rotation at large film thicknesses, but it gives way to an incommensurate (1 × 1) configuration at small film thickness. The transformation follows an approximately inverse-film-thickness dependence with superimposed bilayer oscillations.     

Quantitative nanoorganized structural evolution for a high efficiency bulk heterojunction polymer solar cell

We have developed an improved small-angle X-ray scattering (SAXS) model and analysis methodology to quantitatively evaluate the nanostructures of a blend system. This method has been applied to resolve the various structures of self-organized poly(3-hexylthiophene)/C61-butyric acid methyl ester (P3HT/PCBM) thin active layer in a solar cell from the studies of both grazing-incidence small-angle X-ray scattering (GISAXS) and grazing-incidence X-ray diffraction (GIXRD). Tuning the various length scales of PCBM-related structures by a different annealing process can provide a flexible approach and better understanding to enhance the power conversion of the P3HT/PCBM solar cell. The quantitative structural characterization by this method includes (1) the mean size, volume fraction, and size distribution of aggregated PCBM clusters, (2) the specific interface area between PCBM and P3HT, (3) the local cluster agglomeration, and (4) the correlation length of the PCBM molecular network within the P3HT phase. The above terms are correlated well with the device performance. The various structural evolutions and transformations (growth and dissolution) between PCBM and P3HT with the variation of annealing history are demonstrated here. This work established a useful SAXS approach to present insight into the modeling of the morphology of P3HT/PCBM film. In situ GISAXS measurements were also conducted to provide informative details of thermal behavior and temporal evolution of PCBM-related structures during phase separation. The results of this investigation significantly extend the current knowledge of the relationship of bulk heterojunction morphology to device performance.     

Electrochemical impedance characterization and photovoltaic performance of N719 dye‐sensitized solar cells using quaternized ammonium iodide containing polyfluorene electrolyte solutions

This study develops a series of titanium oxide electrode‐based N719 dye‐sensitized solar cells (DSSCs) using quaternized ammonium iodide containing main‐chain and star‐shaped polyfluorene (MPF‐E and SPF‐E) electrolyte solutions. The electrochemical impedance and photovoltaic properties of the polyfluorene electrolyte‐based DSSCs were studied and compared to those of the poly(ethylene oxide) (PEO) electrolyte‐based DSSCs. As with the PEO electrolyte‐based DSSCs, the recombination impedance increased with increase in the polymer content for the MPF‐E electrolyte‐based DSSCs, whereas the photovoltaic performance did otherwise. Nevertheless, the reduction in the photovoltaic properties was not significant for the SPF‐E electrolyte‐based DSSCs. The electrochemical impedance and photovoltaic properties of the different polymer‐based DSSCs are also discussed as a function of the polymer concentration. Copyright © 2010 John Wiley & Sons, Ltd.     

Collapse kinetics of vibrated granular chains

The kinetics of the collapse of the coil state into condensed states is studied with vibrated granular chain composed of N metal beads partially immersed in water. The radius of gyration of the chain, R(g) is measured. For short chains (N < 140), disk-like condensed state is formed and R(g) decreases with time such that the function ΔR(g)(2) (≡ R(g)(2) - R(g)(2)(∞)) = A e(-t/τ), where the relaxation time τ follows a power-law dependence on the chain length N with an exponent γ = 1.9 ± 0.2. For the chains with length N ≥ 300, rod-like clusters are observed during the initial stage of collapse and R(g)(2) = R(g)(2)(0) - Bt(β), with β = 0.6 ± 0.1. In the coarsening stage, the exponential dependence of ΔR(g)(2) on time still holds, however, the relaxation time τ fluctuates and has no simple dependence on N. Furthermore, the time dependence of the averaged radius of gyration of the individual clusters, R(g,cl) can be described by the theory of Lifshitz and Slyozov. A peak in the structure function of long chains is observed in the initial stage of the collapse transition. The collapse transition in the bead chains is a first order phase transition. However, features of the spinodal decomposition are also observed.     

Assessment of dispersion‐improved exchange‐correlation functionals for the simulation of CO2 binding by alcoholamines

In this study, 12 bound complexes were selected to construct a database for testing 15 dispersion‐improved exchange‐correlation (XC) functionals, including hybrid generalized gradient approximation (GGA), modified using the Grimme's pairwise strategy, and double hybrid XC functionals, for specifically characterizing the CO₂ binding by alcoholamines. Bound complexes were selected based on the characteristics of their hydrogen bonds, dispersion, and electrostatic (particularly between the positive charge of CO₂ and the lone pair of N of alcoholamines) interactions. The extrapolated binding energy from the aug‐cc‐pVTZ (ATZ) to aug‐cc‐pVQZ (AQZ) basis set at the CCSD(T)/CBS(MP2+DZ) level was used as the reference for the XC functional comparison. M06‐2X produced the optimal agreement if the optimized geometries at MP2/ATZ level were chosen for all the test bound complexes. However, M06‐L, ωB97X, and ωB97, and were preferred if the corresponding density functional theory (DFT) optimized geometries were adapted for the benchmark. Simple bimolecular reaction between CO₂ and monoethanolamine simulated using polarizable continuum solvation model confirmed that ωB97, ωB97X, and ωB97XD qualitatively reproduced the energetics of MP2 level. The inconsistent performance of the tested XC functionals, observed when using MP2 or DFT optimized geometries, raised concerns regarding using the single‐point ab initio correction combined with DFT optimized geometry, particularly for determining the nucleophilic attack by alcoholamines to CO₂. © 2014 Wiley Periodicals, Inc.     

Construction of spiro-oxaquinolizidinone framework of Upenamide via organoiron complexes

An organoiron approach toward the synthesis of an elaborated tricyclic spiro-oxaquinolizidinone ring has been accomplished. The key intermediate was efficiently prepared through a nitrile addition to the appropriately functionalized cyclohexadienylium-Fe(CO)₃ perchlorate salt to install the requisite quaternary center in the spiro-oxaquinolizidinone ring.     

Diffusion slowdown in the nanostructured liquid Ga‐Sn alloy

The diffusion of gallium in liquid Ga‐Sn alloy embedded into different porous silica matrices was studied by NMR. Spin relaxation was measured for two gallium isotopes, ⁷¹Ga and ⁶⁹Ga, at two magnetic fields. Pronounced rise of quadrupole contribution to relaxation was observed for the nanostructured alloy which increased with decreasing the pore size. The correlation time of atomic mobility was evaluated and found to be much larger than in the relevant bulk melt which evidenced a pronounced diffusion slowdown in the Ga‐Sn alloy under nanoconfinement. It is shown that the diffusion was slower by a factor of 30 for the alloy within 7 nm pores. The spectral densities of electric field gradients at zero frequency were found to double for the finest pores. The Knight shift was found to decrease but slightly for the nanostructured alloy.     

Second-Order Nonlinear Optical Properties of a Polymer Exhibiting Optical Transparency Down to 340 nm

Abstract

The synthesis and characterization of an epoxy-based nonlinear optical (NLO) polymer exhibiting optical transparency down to 340 nm is reported. The synthesized polymers show spectroscopic properties (NMR, IR, UV) in accordance with the proposed structures. A glass transition temperature (T_g) of 92°C and a thermal degradation temperature (T_d) of 322°C were recorded. The poled polymer film exhibits stable second-order nonlinear optical activity (d₃₃ = 4.2 pm/V) over a period of 800 hours as characterized by the temporal response of the second harmonic signal at room temperature.     

Hydroxide Is Not a Promoter of C2+ Product Formation in the Electrochemical Reduction of CO on Copper

Highly alkaline electrolytes have been shown to improve the formation rate of C₂₊ products in the electrochemical reduction of carbon dioxide (CO₂) and carbon monoxide (CO) on copper surfaces, with the assumption that higher OH^? concentrations promote the C?C coupling chemistry. Herein, by systematically varying the concentration of Na⁺ and OH^? at the same absolute electrode potential, we demonstrate that higher concentrations of cations (Na⁺), rather than OH^?, exert the main promotional effect on the production of C₂₊ products. The impact of the nature and the concentration of cations on the electrochemical reduction of CO is supported by experiments in which a fraction or all of Na⁺ is chelated by a crown ether. Chelation of Na⁺ leads to drastic decrease in the formation rate of C₂₊ products. The promotional effect of OH^? determined at the same potential on the reversible hydrogen electrode scale is likely caused by larger overpotentials at higher electrolyte pH.