GaAs surface passivation by ultra-thin epitaxial GaP layer and surface As-P exchange

The GaAs surface passivation effects of epitaxially grown ultra-thin GaP layers and surface As-P exchange have been investigated. Optical properties of passivated and unpassivated InGaAs/GaAs near-surface quantum wells (QWs) grown by metal organic vapor phase epitaxy (MOVPE) are studied by low-temperature continuous-wave and time-resolved photoluminescence (PL). By optimizing the growth conditions, smooth surface morphologies and significant improvement of optical properties were observed for both passivation methods. Passivation improved the PL intensity more than two orders of magnitude and notably increased the PL decay time.     

Nano-scale mapping of mechanical and chemical surface properties of pigment coated surfaces by torsional harmonic atomic force microscopy

In this study, torsional harmonic atomic force microscopy (TH-AFM, HarmoniX mode) was applied for surface mapping of the mechanical properties of pigment-latex coated paper samples. In addition, topographic images and force maps of adhesive tip-sample interactions were captured concurrently. The spatial distribution of latex binder on the composite surface was distinguished with high resolution. The latex was found to dominate the surface chemistry of the composite coating, despite the fact that latex is a minor component in the coating color formulation. The latex resided as a thick layer between the pigments and as a thin layer on the individual pigments. In addition, the tip-sample thermodynamic work of adhesion of the composite materials on the coated surface was compared to the surface energy values obtained by contact angle measurements. A high tip-sample work of adhesion correlated to high surface energy.     

A Laboratory-Scale Vertical Gravity Separator for Emulsion Characterization

A laboratory-scale vertical gravity separator has been built in order to perform characterization on oil and water based emulsions. The separation rig consists of two positive displacement pumps, a feed separator, and a test separator. Probes for sampling are placed at various heights of the test separator and at different points at the pipes. To induce mixing of the oil and water phases, a needle valve is placed downstream of the pumps. The valve, when choked, increases the pressure drop and thus increases the shear of the oil and water mixture pumped through the system. A differential pressure cell is used to monitor the pressure drop over the needle valve when choked. The differential pressure is used as a reference for the energy put into the mixing process. Calculation of the maximum surviving drop size for relevant pressure drops gave sizes in the range of 93 µm for a pressure drop of 0.25 bars and 29 µm for a pressure drop of 4.5 bars. Droplet sizes were determined with a digital video microscope. The results from the microscope analysis gave droplets with sizes beween 2 and 90 µm with mean diameters in the order of 4–12 µm.     

Dependence on Oil Chain‐Length of the Curvature Elastic Properties of Nonionic Surfactant Films: Droplet Growth from Spheres to a Bicontinuous Network

Using NMR self‐diffusion and ²H‐NMR relaxation experiments, we have investigated growth of oil‐in‐water microemulsion droplets when spontaneous curvature is lowered, starting at emulsification failure boundary. We compare two ternary nonionic microemulsion systems, containing penta‐ethylene glycol dodecyl ether (C₁₂E₅), and decane or hexadecane, at same surfactant‐to‐oil ratio and approximately the same spontaneous curvature. Droplet growth in microemulsions appears in general to be only minor. Quantitative differences between the two systems indicate differences in the curvature elastic constants.     

Energetics and mechanisms for the unimolecular dissociation of protonated trioses and relationship to proton-mediated formaldehyde polymerization to carbohydrates in interstellar environments

We report the unimolecular decomposition of protonated glyceraldehyde, [HOCH(2)CH(OH)CHO]H(+), and protonated dihydroxyacetone, [HOCH(2)C(O)CH(2)OH]H(+). On the basis of mass spectrometric experiments and computational quantum chemistry, we have found that these isomeric ions interconvert freely at energies below that required for their unimolecular decompositions. The losses of formaldehyde and water (the latter also followed by CO loss) are the dominating processes, with formaldehyde loss having the lower energetic threshold. The reverse of the formaldehyde loss, namely, the addition of formaldehyde to protonated glycolaldehyde, appears to be an inefficient reaction at low temperature and pressure in the gas phase, leading to dissociation products. The relevance of these findings to interstellar chemistry and prebiotic chemistry is discussed, and it is concluded that the suggestion made in the literature that successive addition of formaldehyde by proton-assisted reactions should account for interstellar carbohydrates most likely is incorrect.     

Temperature dependence of charge separation and recombination in porphyrin oligomer-fullerene donor-acceptor systems

Electron-transfer reactions are fundamental to many practical devices, but because of their complexity, it is often very difficult to interpret measurements done on the complete device. Therefore, studies of model systems are crucial. Here the rates of charge separation and recombination in donor-acceptor systems consisting of a series of butadiyne-linked porphyrin oligomers (n = 1-4, 6) appended to C(60) were investigated. At room temperature, excitation of the porphyrin oligomer led to fast (5-25 ps) electron transfer to C(60) followed by slower (200-650 ps) recombination. The temperature dependence of the charge-separation reaction revealed a complex process for the longer oligomers, in which a combination of (i) direct charge separation and (ii) migration of excitation energy along the oligomer followed by charge separation explained the observed fluorescence decay kinetics. The energy migration is controlled by the temperature-dependent conformational dynamics of the longer oligomers and thereby limits the quantum yield for charge separation. Charge recombination was also studied as a function of temperature through measurements of femtosecond transient absorption. The temperature dependence of the electron-transfer reactions could be successfully modeled using the Marcus equation through optimization of the electronic coupling (V) and the reorganization energy (λ). For the charge-separation rate, all of the donor-acceptor systems could be successfully described by a common electronic coupling, supporting a model in which energy migration is followed by charge separation. In this respect, the C(60)-appended porphyrin oligomers are suitable model systems for practical charge-separation devices such as bulk-heterojunction solar cells, where conformational disorder strongly influences the electron-transfer reactions and performance of the device.     

An all-photonic molecule-based D flip-flop

The photochromic fluorescence switching of a fulgimide derivative was used to implement the first molecule-based D (delay) flip-flop device, which works based on the principles of sequential logic. The device operates exclusively with photonic signals and can be conveniently switched in repeated cycles.     

Application and Structural Analysis of Triazole‐Bridged Disulfide Mimetics in Cyclic Peptides

Ruthenium‐catalysed azide–alkyne cycloaddition (RuAAC) provides access to 1,5‐disubstituted 1,2,3‐triazole motifs in peptide engineering applications. However, investigation of this motif as a disulfide mimetic in cyclic peptides has been limited, and the structural consequences remain to be studied. We report synthetic strategies to install various triazole linkages into cyclic peptides through backbone cyclisation and RuAAC cross‐linking reactions. These linkages were evaluated in four serine protease inhibitors based on sunflower trypsin inhibitor‐1. NMR and X‐ray crystallography revealed exceptional consensus of bridging distance and backbone conformations (RMSD<0.5 Å) of the triazole linkages compared to the parent disulfide molecules. The triazole‐bridged peptides also displayed superior half‐lives in liver S9 stability assays compared to disulfide‐bridged peptides. This work establishes a foundation for the application of 1,5‐disubstituted 1,2,3‐triazoles as disulfide mimetics.