Two phosphonic acid (PA) self-assembled monolayers (SAMs) are studied on three aluminum oxide surfaces: the C and R crystallographic planes of single crystal alpha-alumina (sapphire) and an amorphous vapor-deposited alumina thin film. SAMs are either fully hydrogenated CH3(CH2)17PO3H2 or semifluorinated CF3(CF2)7(CH2)11PO3H2. Atomic force microscope (AFM) topographic imaging reveals that the deposited films are homogeneous, atomically smooth, and stable for months in the laboratory environment. Static and advancing contact angle measurements agree with previous work on identical or similar films, but receding measurements suggest reduced coverage here. To enable reproducible nanotribology measurements with the AFM, a scanning protocol is developed that leads to a stable configuration of the silicon tip. Adhesion for the semifluorinated films is either comparable to or lower than that for the hydrogenated films, with a dependence on contact history observed. Friction between each film and the tips depends strongly upon the type of molecule, with the fluorinated species exhibiting substantially higher friction. Subtle but reproducible differences in friction are observed for a given SAM depending on the substrate, revealing differences in packing density for the SAMs on the different substrates. Friction is seen to increase linearly with load, a consequence of the tip's penetration into the monolayer.

Use of phase transfer catalysts such as 18-crown-6 enables ionic, linear conjugated poly[2,6-{1,5-bis(3-propoxysulfonicacidsodiumsalt)}naphthylene]ethynylene (PNES) to efficiently disperse single-walled carbon nanotubes (SWNTs) in multiple organic solvents under standard ultrasonication methods. Steady-state electronic absorption spectroscopy, atomic force microscopy (AFM), and transmission electron microscopy (TEM) reveal that these SWNT suspensions are composed almost exclusively of individualized tubes. High-resolution TEM and AFM data show that the interaction of PNES with SWNTs in both protic and aprotic organic solvents provides a self-assembled superstructure in which a PNES monolayer helically wraps the nanotube surface with periodic and constant morphology (observed helical pitch length ) 10 ( 2 nm); time-dependent examination of these suspensions indicates that these structures persist in solution over periods that span at least several months. Pump-probe transient absorption spectroscopy reveals that the excited state lifetimes and exciton binding energies of these well-defined nanotube-semiconducting polymer hybrid structures remain unchanged relative to analogous benchmark data acquired previously for standard sodium dodecylsulfate (SDS)-SWNT suspensions, regardless of solvent. These results demonstrate that the use of phase transfer catalysts with ionic semiconducting polymers that helically wrap SWNTs provide well-defined structures that solubulize SWNTs in a wide range of organic solvents while preserving critical nanotube semiconducting and conducting properties.KEYWORDS Single chain, helical wrapping, ionic poly(aryleneethynylene), phase transfer catalyst, SWNTs, organic solvent S ingle wall carbon nanotubes (SWNTs) 1-4 possess a wide range of uncommon mechanical, 5-9 optical, 1,10-12 electrical, 13-17 magnetic, 18-21 and thermal 22,23 properties that fuel multidisciplinary efforts aimed at developing neoteric nanoscale, microscale, and bulkphase materials. 2 Strong van der Waals interactions between SWNTs 24,25 are an anathema to SWNT solubilization. 13 Water-solubilized SWNTs dominate the literature; there exists no general method to disperse SWNTs in nonaqueous media. Potential dispersion approaches are truncated severely by the fact that preservation of significant SWNT electrooptic properties require solubilization via agents that noncovalently interact with the nanotube surface. 24,25 It has thus been a long-standing goal to develop a universal SWNT solubilization strategy that (i) relies on noncovalent interactions for nanotube dispersion, (ii) provides a high yield of individualized tubes, (iii) enables dispersion in a wide range of dielectric media, and yet (iv) gives rise to suspended SWNT structures having a constant morphology, regardless of solvent.Ultrasonication 11,26 and high-speed vibrational milling techniques 27 are commonly utilized to drive exfoliation of nanotube ropes and bundles in the presence of surfactants, small molecules, and polymers. 28 The huge library o...

Friction converts kinetic energy at sliding interfaces into lattice vibrations, but the detailed mechanisms of this process remain unresolved. Atomic force microscopy measurements reveal that changing the mass of the terminating atoms on a surface, and thus their vibrational frequencies, affects nanoscale friction substantially. We compared hydrogen- and deuterium-terminated single-crystal diamond and silicon surfaces, and in all cases the hydrogenated surface exhibited higher friction. This result implies that the lower natural frequency of chemisorbed deuterium reduces the rate at which the tip's kinetic energy is dissipated. This discovery is consistent with a model describing energy transfer to adsorbates from a moving surface.