B. M. Ocko

Molecular layering of fluorinated ionic liquids at a charged sapphire (0001) surface

Room-temperature ionic liquids (RTILs) are promising candidates for a broad range of “green” applications, for which their interaction with solid surfaces plays a crucial role. In this high-energy x-ray reflectivity study, the temperature-dependent structures of three ionic liquids with the tris(pentafluoroethyl)trifluorophosphate anion in contact with a charged sapphire substrate were investigated with submolecular resolution. All three RTILs show strong interfacial layering, starting with a cation layer at the substrate and decaying exponentially into the bulk liquid. The observed decay length and layering period point to an interfacial ordering mechanism, akin to the charge inversion effect, which is suggested to originate from strong correlations between the unscreened ions. The observed layering is expected to be a generic feature of RTILs at charged interfaces.

Surface Tension Measurements of Surface Freezing in Liquid Normal Alkanes

Surface tension measurements reveal surface freezing in liquid n-alkanes. A solid monolayer of molecules is found to exist up to 30�C above the bulk freezing point. This surface phase exists only for carbon numbers 14 n ≤ 50. The measured carbon number and temperature dependence of the surface tension is interpreted within a simple thermodynamical model based on known bulk latent heat data and surface energy considerations. The vanishing of the surface phase for n ≤ 14 is a possible transition from surface freezing to surface melting behavior.

Layering of [BMIM]+-based ionic liquids at a charged sapphire interface.

The structure of two model room temperature ionic liquids, [BMIM](+)[PF(6)](-) and [BMIM](+)[BF(4)](-), near the solid/liquid interface with charged Al(2)O(3)(0001) (sapphire) was determined with subnanometer resolution by high energy (72.5 keV) x-ray reflectivity. [BMIM](+)[PF(6)](-) exhibits alternately charged, exponentially decaying, near-surface layering. By contrast, the smaller-anion compound, [BMIM](+)[BF(4)](-), shows only a single layer of enhanced electron density at the interface. The different layering behaviors, and their characteristic length scales, correspond well to the different bulk diffraction patterns, also measured in this study. Complementary measurements of the surface and interface energies showed no significant different between the two RTILs. The combined bulk-interface results support the conclusion that the interfacial ordering is dominated by the same electrostatic ion-ion interactions dominating the bulk correlations, with hydrogen bonding and dispersion interactions playing only a minor role.

Surface Crystallization in a Liquid AuSi Alloy

X-ray measurements reveal a crystalline monolayer at the surface of the eutectic liquid Au82Si18, at temperatures above the alloys melting point. Surface-induced atomic layering, the hallmark of liquid metals, is also found below the crystalline monolayer. The layering depth, however, is threefold greater than that of all liquid metals studied to date. The crystallinity of the surface monolayer is notable, considering that AuSi does not form stable bulk crystalline phases at any concentration and temperature and that no crystalline surface phase has been detected thus far in any pure liquid metal or nondilute alloy. These results are discussed in relation to recently suggested models of amorphous alloys.

Nanoimprint-Induced Molecular Orientation in Semiconducting Polymer Nanostructures

The morphology and orientation of thin films of the polymer poly-3(hexylthiophene)-important parameters influencing electronic and photovoltaic device performance-have been significantly altered through nanoimprinting with 100 nm spaced grooves. Grazing-incidence small-angle X-ray scattering studies demonstrate the excellent fidelity of the pattern transfer, while wide-angle scattering convincingly shows an imprinting-induced π-π reorientation and polymer backbone alignment along the imprinted grooves. Surprisingly, temperature-dependent scattering measurements indicate that the imprinted induced orientation and alignment remain intact even at temperatures where the imprinted topographical features nearly vanish.

STRUCTURE, DISSOLUTION, AND PASSIVATION OF NI(111) ELECTRODES IN SULFURIC ACID SOLUTION: AN IN SITU STM, X-RAY SCATTERING, AND ELECTROCHEMICAL STUDY

Abstract Results of a detailed study of Ni(111) surfaces in air and in sulfuric acid solution (pH 1.0–2.7) by a combination of STM, surface X-ray scattering using synchrotron radiation, and electrochemical techniques are presented. Ni(111) samples, prepared via annealing in H2 and exposure to air at room temperature, are covered by a smooth three to four layers thick NiO(111) film with parallel (NiO[ 1 1 0 ]∣∣Ni[ 1 1 0 ]) and anti-parallel (NiO[ 1 1 0 ]∣∣Ni[ 1 10 ]) in-plane orientation. Electrochemical reduction at potentials ≤−0.40 VAg/AgCl results in the formation of a well-defined, oxide-free surface with large terraces, a low surface mobility, and a (1×1) lattice on the atomic scale. X-ray reflectivity data indicate vertical lattice expansion for the topmost Ni layer and a strongly bound sulfate or oxygen species. Active Ni dissolution commences at potentials ≥−0.25 VAg/AgCl by a step-flow mechanism, followed by the rapid formation of large three-dimensional etch pits, leading to considerable surface roughening. In situ STM observations of the passive film formation show at potentials ≥−0.10 VAg/AgCl the nucleation and growth of an initial ‘grainy’ phase, which is attributed to a Ni hydroxide, followed by a slower restructuring process. According to our combined STM and SXS data, the resulting steady-state passive film exhibits a duplex structure, with a crystalline, inner NiO(111) layer, consisting of exclusively anti-parallel oriented grains (NiO[ 1 1 0 ]∣∣Ni[ 1 10 ]) which are slightly tilted relative to the substrate lattice, and a porous, probably amorphous hydroxide phase on top. The thickness of the crystalline NiO film increases with potential by 14–17 A V−1. In addition, structural changes of the oxide film during immersion of Ni samples into the sulfuric acid solution at potentials in the passive range and after emersion from the electrolyte were observed, which indicate the slow conversion of the air-formed into the passive oxide and the (partial) reformation of the air-formed oxide, respectively.

Bilayer order in a polycarbazole-conjugated polymer

One of the best performing semiconducting polymers used in bulk heterojunction devices is PCDTBT, a polycarbazole derivative with solar-conversion efficiencies as high as 7.2%. Here we report the formation of bilayer ordering in PCDTBT, and postulate that this structural motif is a direct consequence of the polymers molecular design. This bilayer motif is composed of a pair of backbones arranged side-to-side where the alkyl tails are on the outer side. This is in stark contrast to the monolayer ordering found in other conjugated polymers. The crystalline bilayer phase forms at elevated temperatures and persists after cooling to room temperature. The existence of bilayer ordering, along with its high-packing fraction of conjugated moieties, may guide the synthesis of new materials with improved optoelectronic properties.

Postassembly Chemical Modification of a Highly Ordered Organosilane Multilayer: New Insights into the Structure, Bonding, and Dynamics of Self-Assembling Silane Monolayers

Experimental evidence derived from a comprehensive study of a self-assembled organosilane multilayer film system undergoing a process of postassembly chemical modification that affects interlayer-located polar groups of the constituent molecules while preserving its overall molecular architecture allows a quantitative evaluation of both the degree of intralayer polymerization and that of interlayer covalent bonding of the silane headgroups in a highly ordered layer assembly of this type. The investigated system consists of a layer-by-layer assembled multilayer of a bifunctional n-alkyl silane with terminal alcohol group that is in situ converted, via a wet chemical oxidation process conducted on the entire multilayer, to the corresponding carboxylic acid function. A combined chemical-structural analysis of data furnished by four different techniques, Fourier transform infrared spectroscopy (FTIR), synchrotron X-ray scattering, X-ray photoelectron spectroscopy (XPS), and contact angle measurements, demonstrates that the highly ordered 3D molecular arrangement of the initial alcohol-silane multilayer stack is well preserved upon virtually quantitative conversion of the alcohol to carboxylic acid and the concomitant irreversible cleavage of interlayer covalent bonds. Thus, the correlation of quantitative chemical and structural data obtained from such unreacted and fully reacted film samples offers an unprecedented experimental framework within which it becomes possible to differentiate between intralayer and interlayer covalent bonding. In addition, the use of a sufficiently thick multilayer effectively eliminates the interfering contributions of the underlying silicon oxide substrate to both the X-ray scattering and XPS data. The present findings contribute a firm experimental basis to the elucidation of the self-assembly mechanism, the molecular organization, and the modes and dynamics of intra- and interlayer bonding prevailing in highly ordered organosilane films; with further implications for the rational exploitation of some of the unique options such supramolecular surface entities can offer in the advancement of a chemical nanofabrication methodology.

Surface Charge—Induced Ordering of the Au(111) Surface

Synchrotron surface x-ray scattering (SXS) studies have been carried out at the Au(lll)/electrolyte interface to determine the influence of surface charge on the microscopic arrangement of gold surface atoms. At the electrochemical interface, the surface charge density can be continuously varied by controlling the applied potential. The top layer of gold atoms undergoes a reversible phase transition between the (1 x 1) bulk termination and a (23 x √3) reconstructed phase on changing the electrode potential. In order to differentiate the respective roles of surface charge and adsorbates, studies were carried out in 0.1 M NaF, NaCl, and NaBr solutions. The phase transition occurs at an induced surface charge density of 0.07 � 0.02 electron per atom in all three solutions.

Surface structure of liquid metals and the effect of capillary waves: X-ray studies on liquid indium

We report x-ray reflectivity ~XR! and small-angle off-specular diffuse-scattering ~DS! measurements from the surface of liquid indium close to its melting point of 156 °C. From the XR measurements we extract the surface structure factor convolved with fluctuations in the height of the liquid surface. We present a model to describe DS that takes into account the surface structure factor, thermally excited capillary waves, and the experimental resolution. The experimentally determined DS follows this model with no adjustable parameters, allowing the surface structure factor to be deconvolved from the thermally excited height fluctuations. The resulting local electron-density profile displays exponentially decaying surface-induced layering similar to that previously reported for Ga and Hg. We compare the details of the local electron-density profiles of liquid In, which is a nearly free-electron metal, and liquid Ga, which is considerably more covalent and shows directional bonding in the melt. The oscillatory density profiles have comparable amplitudes in both metals, but surface layering decays over a length scale of 3.560.6 A for In and 5.5 60.4 A for Ga. Upon controlled exposure to oxygen, no oxide monolayer is formed on the liquid In surface, unlike the passivating film formed on liquid gallium. @S0163-1829~99!11701-6#