Substrate-mediated intermolecular interactions: a quantitative single molecule analysis

Long-range intermolecular interactions mediated by the surface are believed to be responsible for many effects in surface science, including molecular ordering, formation of nanostructures, and aligning reactive intermediates in catalysis. Here, we use scanning tunneling microscopy to probe the weak substrate-mediated interactions in benzene overlayers on Au{111} at 4 K. Using an automated procedure to monitor single molecule motion, we are able to quantify the substrate-mediated interaction strength. We explain quantitatively both the kinetics of the benzene motion and the thermodynamics that determine the packing structures benzene adopts in this system in light of these substrate-mediated interactions.     

Enhanced mechanical stability of microtubules polymerized with a slowly hydrolyzable nucleotide analogue

Atomic force microscopy (AFM) has been used to investigate the local mechanical and structural properties of microtubules polymerized using guanylyl-alpha-beta-methylene diphosphonate (GMPCPP), a slowly hydrolyzable analogue of guanosine triphosphate. Using a combination of AFM imaging and local force spectroscopy, GMPCPP-polymerized microtubules have been qualitatively and quantitatively compared to paclitaxel-stabilized microtubules. GMPCPP-polymerized microtubules qualitatively display a greater resistance to destruction by the AFM probe tip during imaging and during deformation measurements and maintain structural details after indentation. In addition, using force spectroscopy taken during the indentation and collapse of individual microtubules with the AFM probe tip, an effective spring constant of the microtubule wall (kMT) for both types of microtubules was determined. The average kMT of GMPCPP-polymerized microtubules, 0.172 N/m, is more than twice that of paclitaxel-stabilized microtubules. These results complement previously reported measurements of bending experiments on GMPCPP-polymerized and paclitaxel-stabilized microtubules.     

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.     

Control of alkanethiolate monolayer structure using vapor-phase annealing

We describe an annealing procedure for self-assembled monolayers (SAMs) that uses vapor-phase molecules to modify the local domain structure. Existing SAMs of decanethiolate on Au{111} were annealed using vapor-phase dodecanethiol molecules, so that the original and newly introduced molecules could be distinguished using scanning tunneling microscopy (STM). Molecules deposited from the vapor phase inserted at existing monolayer defect sites and domain boundaries, and at substrate step edges forming discrete network-like domains. The SAM molecular lattice can be preserved across molecular terrace boundaries between the decanethiolate and dodecanethiolate domains. Candidate molecular electronic component molecules were inserted from solution in the decanethiolate matrix as isolated molecules. These inserted molecules could then be surrounded by dodecanethiolate molecules introduced from the vapor phase, thus demonstrating a method for controlling the local environment of inserted molecules.     

Two-Scale Simulation of a Disc With a Crack by X-FEM and Cell Model

The two-scale simulation of a linear-elastic orthotropic disc with a central crack under mode-I loading may be used to verify the extended finite element method implementation of orthotropic enrichment functions into finite element codes such as FEAP. The stress distribution on the finer scale is simultaneously resolved by the high fidelity generalized method of cells called at each integration point of the macro elements. (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)