STM observation of surface phases of Sn/Cu(0 0 1)

Geometrical structures of the Sn-adsorbed Cu(0 0 1) surfaces are studied with scanning tunneling microscopy. There are four phases in the Sn coverage range between 0.2 and 0.5 mono-atomic layer (ML). On the basis of the observed atomic images in this range, we propose structural models for the phases with 0.33 and 0.375 ML of Sn. All the phases consist of embedded Sn atoms in the Cu surface, forming two-dimensional surface alloy structures. On the surface with ∼0.4 ML of Sn, a novel one-dimensional structure is observed.     

Anionic copolymerization of 1,1-diphenylethylene and 2,3-dimethylbutadiene

1,1-Diphenylethylene (M₂) and 2,3-dimethylbutadiene (M₁) were copolymerized with n-butyllithium in tetrahydrofuran. The rate of consumption of each monomer was followed by the change of high resolution NMR spectra of the reaction mixture. The copolymerization proceeded alternately, if the ratio of initial monomer concentrations, [M₂]₀/[M₁]₀, was sufficiently larger than unity. By assuming the rate constant k₂₂ to be zero, the constants k₂₁ were obtained from the consumption rates of the monomers. In the alternating copolymerization, 2,3-dimethylbutadiene was incorporated into the copolymer only as the 1,4-structure, while the 1,2-structure was predominant in homopolymerization.     

Influence of the carbon nanotube probe tilt angle on the effective probe stiffness and image quality in tapping-mode atomic force microscopy

Previous studies have shown that when using carbon nanotubes (CNTs) as tapping-mode AFM probes, their tilt angle with respect to vertical (denoted phi) must be close to 0 degrees to obtain high-quality images and that very poor images are obtained for phi > 30 degrees . Here we present a quantitative theoretical investigation of the effect of phi on tapping-mode AFM imaging for single-wall and multiwall nanotube (SWNT and MWNT, respectively) probes of diameters 3.4-5.5 nm and aspect ratio 7.5, which have been found ideal for imaging via TEM. Using molecular and classical dynamics, we investigate the effect of phi on CNT probe stiffness (quantified through the maximum gradient of the tip-sample interaction force) and show that it decreases linearly with increasing phi, becoming negligible at around phi approximately 40 degrees , thus confirming the conclusions of previous studies. We find that MWNT probe stiffness is proportional to the number of walls, but that the difference in stiffness between SWNTs and MWNTs also decreases linearly with increasing phi and becomes negligible at around phi approximately 40 degrees . The simulated cross-sectional scans of a sample SWNT using two different values of phi show that the image can be distorted and shifted laterally when phi is large, in some cases giving measured heights appreciably greater than the sample dimensions. We show analytically that the tip-sample forces that occur during imaging can be significantly lower when CNT probes are used instead of conventional probes, even in the absence of buckling, and that they can be further reduced by increasing phi. On the basis of this result, we propose the design of free-standing kinked probes for the characterization of sensitive samples, whereby the probe approaches the sample at a vertical orientation and possesses a tilted section that regulates the tip-sample interaction forces.     

Reactivity of methacrylates in anionic copolymerization with methyl methacrylate by n-BuLi

Monomer reactivity ratios, r₁ and r₂ were determined in the anionic copolymerizations of methyl methacrylate (MMA, M₁) with ethyl (EtMA), isopropyl (i-PrMA), tert-butyl (t-BuMA), benzyl (BzMA), α-methylbenzyl (MBMA), diphenylmethyl (DPMMA), α,α-dimethylbenzyl (DMBMA), and trityl (TrMA) methacrylates (M₂) by use of n-BuLi as an initiator in toluene and THF at -78°C. The order of the reactivity of the monomers towards MMA anion was DPMMA > BzMA > MMA > EtMA > MBMA > i-PrMA > t-BuMA > TrMA > DMBMA in toluene and TrMA > BzMA > MMA > DPMMA > EtMA > MBMA > i-PrMA > DMBMA > t-BuMA in THF. Except for the extremely low reactivity of TrMA and DPMMA in toluene due to steric hindrance, the order was explained in terms of the polar effect of the ester groups. A linear relationship was found between log (1/r₁) and Taft's σ* values of the ester groups, where the ρ* value was 1.1. The plots of log (1/r₁) vs. the ¹H_a (cis to the carbonyl) and ¹³C_ß chemical shifts of the monomers were also on straight lines. The polymer obtained in the copolymerization of MMA with TrMA in toluene by n-BuLi at -78°C was a mixture of poly-MMA and a copolymer, suggesting that there exist two kinds of growing centers.     

Finite element approximation of -surfaces

In this paper a piecewise linear finite element approximation of -surfaces, or surfaces with constant mean curvature, spanned by a given Jordan curve in is considered. It is proved that the finite element -surfaces converge to the exact -surfaces under the condition that the Jordan curve is rectifiable. Several numerical examples are given.

     

Effects of strain on the dissociation dynamics of O2 on Si(001)

There are two distinctive channels in the dissociation reaction of O2 on Si(001)-(2 x 1): a trapping-mediated channel and a direct-activated channel. Externally applied tensile strain along the <110> direction on the (001) surface is found to suppress the dissociation via a direct-activated channel and to enhance that via a trapping-mediated channel in the temperature range between 200 and 300 K. It has been demonstrated that the dissociation dynamics involving elementary processes such as inelastic scattering and trapping, desorption and/or dissociation from a trapping precursor, and direct dissociation are sensitively influenced by the strain to change the branching ratio of the dissociation reaction.     

Interaction of Natronobacterium pharaonis phoborhodopsin (sensory rhodopsin II) with its cognate transducer probed by increase in the thermal stability

Pharaonis phoborhodopsin (ppR, also called Natronobacterium pharaonis sensory rhodopsin II) and its transducer protein, pharaonis halobacterial transducer of ppR (pHtrII), form a signaling complex, and light signals are transmitted from the sensor to the transducer by the protein-protein interaction. A truncated pHtrII(1-159) consisting of intramembrane helices (expressing amino acid residues from the first to the 159th position) and ppR form the complex in a solution containing 0.1% n-dodecyl-beta-D-maltoside. At 75-85 degrees C, the time-dependent color loss of ppR was caused by denaturation. We found that pHtrII(1-159) retarded the denaturation rate of ppR. This increase in the thermal stability was used as a probe for the binding ability in the dark. Tyr199 of ppR and Asn74 of pHtrII(1-114) were proposed as amino acid residues interacting with each other through hydrogen bonding. Then,ppR and pHtrII(1-159) mutants at these positions were prepared to examine the effect on the binding in the dark. The wild-type and Y199F mutant can bind pHtrII(1-159), suggesting that the hydrogen bonding between these specific amino acid residues may not be the only cause of the binding, but the hydrophobic interaction via phenyl ring of ppR may contribute dominantly.     

Phase structures of the N-acetyl-L-glutamic acid oligomeric benzyl esters (exact residue numbers 4, 6, 8, and 12)–dioxane systems and their optical properties

A series of N-acetyl-L-glutamic acid oligomeric benzyl esters with exact residue numbers (4, 6, 8, 10, and 12) has been synthesized by a stepwise procedure. It has been found that the phase maps of these oligomer-dioxane systems consist of three regions (I : an isotropic solution, II : a liquid crystalline phase, and III : a two-phase (I and II) solution). In particular, for the samples with residue numbers 6, 8, and 12, selective light scattering of coloured regions in the transparent II and III (bottom-layer) regions have been investigated by the use of UV-visible absorption spectra, leading to the conclusion that there exists a helical axis in the structure of the supramolecular aggregates formed by these oligomeric molecules.