Effect of head-group chemistry on surface-mediated molecular self-assembly

Surface molecular self-assembly is a fast advancing field with broad applications in sensing, patterning, device assembly, and biochemical applications. A vast number of practical systems utilize alkane thiols supported on gold surfaces. Whereas a strong Au-S bond facilitates robust self-assembly, the interaction is so strong that the surface is reconstructed, leaving etch pits that render the monolayers susceptible to degradation. By using different head group elements to adcust the molecule-surface interaction, a vast array of new systems with novel properties may be formed. In this paper we use a carefully chosen set of molecules to make a direct comparison of the self-assembly of thioether, selenoether, and phosphine species on Au(111). Using the herringbone reconstruction of gold as a sensitive readout of molecule-surface interaction strength, we correlate head-group chemistry with monolayer (ML) properties. It is demonstrated that the hard/soft rules of inorganic chemistry can be used to rationalize the observed trend of molecular interaction strengths with the soft gold surface, that is, P>Se>S. We find that the structure of the monolayers can be explained by the geometry of the molecules in terms of dipolar, quadrupolar, or van der Waals interactions between neighboring species driving the assembly of distinct ordered arrays. As this study directly compares one element with another in simple systems, it may serve as a guide for the design of self-assembled monolayers with novel structures and properties.     

Preparation of onium salts of a reduced anthracenone crown ether macrocycle: a reactivity series involving pyridine,phosphine, thiophene,nitrile and primary amide nucleophiles

Reaction of mineral acids with a cyclic macromolecule containing a secondary alcohol produces ammonium, phosphonium, thiophene, and amide adducts via a carbocation intermediate. X‐ray crystallography confirms the structures of the products, including those when two competing nucleophiles are present. A reactivity series that mirrors the nucleophilicity index, where reactivity decreases in the order thiophene > pyridine > primary amides > alkyl nitriles >> aromatic nitriles (unreactive), results. Addition of metal ions to ammonium adducts dissolved in acetonitrile produces secondary amides via the Ritter amide synthesis. Copyright © 2012 John Wiley & Sons, Ltd.     

Dynamics and spectroscopy of hydrogen atoms on Pd{111}

Chemisorption of hydrogen on Pd{111} is a relatively simple, yet important surface chemical process. By using low-temperature scanning tunneling microscopy, tip-induced motion of adsorbed atomic hydrogen at 4 K has been observed at low coverage. The motion has been ascribed to excitation of vibrational modes that decay into translational modes; vibrational spectroscopy via inelastic electron tunneling corroborates this assignment, and the barrier to hydrogen atom motion has been determined. At higher coverages, tip-induced motion of vacancies in the hydrogen overlayer is observed, and the associated barrier has also been determined.     

Untersuchung und Beurtheilung von Essig

Ohne Zusammenfassung     

Physics of spherical tokamaks

Spherical tokamaks are a limiting case of conventional tokamaks, combining simple design with attractive physical characteristics. Being of potential importance for the controlled nuclear fusion program in their own right, spherical tokamaks also contribute much to our understanding of the physics of conventional tokamaks. For instance, they are contributing to the modeling of confinement scalings for ITER and to research on general plasma properties such as energy confinement and MHD processes. Results obtained for the first generation of spherical prototype tokamaks are reviewed and the main trends for further research are indicated. Zh. Tekh. Fiz. 69, 50–57 (September 1999)     

BODIPY chromophores as efficient green light sensitizers for lanthanide-induced near-infrared emission

A new boron dipyrromethene (BODIPY) modified 8-hydroxylquinoline ligand (8-HOQ-BODIPY) is synthesized for the sensitization of near-infrared emission of lanthanide(III) ions. The BODIPY unit, as revealed by single-crystal X-ray diffraction analysis, aligns almost perpendicularly to the 8-HOQ unit. The ligand exhibits strong absorption at ~506 nm and fluorescence at 510 nm in organic solvents with quantum yields ranging from ~0.45 in dichloromethane to 0.015 in ethanol. It forms stable ytterbium(III), erbium(III) and neodymium(III) complexes with 3:1 ligand-to-metal molar ratios. Upon excitation (~522 nm), the neodymium(III) and erbium(III) complexes emit weakly at 1060 and 1382 nm, respectively, whereas the ytterbium(III) complex exhibits strong emission at 976 and 1003 nm. The results demonstrate the potential of BODIPY dyes as efficient and robust visible light sensitizers for lanthanide-based NIR emitters in medical diagnosis.     

Controllable restructuring of a metal substrate: Tuning the surface morphology of gold

Organic thin films composed of naphtho[2,3-a]pyrene, a very promising multifunctional polyaromatic hydrocarbon for use in optoelectronic devices, are demonstrated to restructure the pristine Au(111) 22 × √3 surface. The perturbation of the herringbone reconstruction was used to gauge the interaction strength of the organic molecule with the substrate through a series of controlled adsorption and annealing treatments. The overall behavior of the system is interpreted as an interplay between thermodynamic and kinetic factors, dictated by the temperature of the sample and the surface coverage of the molecules. The restructuring mechanism involves the exchange of Au atoms from the herringbone reconstruction to the step edges. Our results reveal that the molecular and substrate equilibrium structure is only achieved after annealing the system to > 470 K. This infers that caution should be taken when organic–metal interfaces are constructed by low or room temperature deposition, as they may not correspond to the equilibrium structures, and therefore, may misrepresent the adlayer structure which exists at the realistic working conditions of devices.     

Benzene on Au[111] at 4 K: monolayer growth and tip-induced molecular cascades

Low-temperature scanning tunneling microscopy has been used to characterize the various structures of submonolayer and near-monolayer coverages of benzene (C6H6) on Au[111] at 4 K. At low coverage, benzene is found to adsorb preferentially at the top of the Au monatomic steps and is weakly adsorbed on the terraces. At near-monolayer coverage, benzene was found to form several long-range commensurate overlayer structures that depend on the regions of the reconstructed Au[111] surface, namely a (radical 52 x radical 52)R13.9 degrees structure over the hcp regions and a (radical 133 x radical 133)R17.5 degrees "pinwheel" structure over the fcc regions. Time-lapse imaging revealed concerted cascade motion of the benzene molecules in the (radical 133 x radical 133)R17.5 degrees pinwheel overlayer. We demonstrate that the observed cascade motion is a result of concerted molecular motion and not independent random motion.