SciCombinator

Discover the most talked about and latest scientific content & concepts.

Journal: Langmuir : the ACS journal of surfaces and colloids

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In-situ functionalization of polar (c-plane) and nonpolar (a-plane) gallium nitride (GaN) was performed by adding (3-bromopropyl)bromopropyl) phosphonic acid or propyl phosphonic acid to a phosphoric acid etch. The target was to modulate GaN’s emission properties and oxide formation, which was explored through surface characterization with atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), photoluminescence (PL), inductively coupled plasma-mass spectrometry (ICP-MS) and water contact angle. The use of (3-bromopropyl) phosphonic acid and propyl phosphonic acid in phosphoric acid demonstrated lower amounts of gallium oxide formation and greater hydrophobicity for both sample sets, while also improving PL emission of polar GaN samples. In addition to crystal orientation, growth related factors such as defect density in bulk GaN versus thin GaN films residing on sapphire substrates were investigated as well as their responses to in-situ functionalization. Thin nonpolar GaN layers were the most sensitive to etching treatments due in part to higher defect densities (stacking faults and threading dislocations), which accounts for large surface depressions. High quality GaN (both free-standing bulk polar and bulk nonpolar) demonstrated increased sensitivity to oxide formation. Room temperature PL stands out as an excellent technique to identify nonradiative recombination as observed in the spectra of heteroepitaxially grown GaN samples. The chemical methods applied to tune optical and physical properties of GaN provide a quantitative framework for future novel chemical and biochemical sensor development.

Concepts: Etching, Density, Gallium, Sapphire, Spectroscopy, Gallium nitride, X-ray photoelectron spectroscopy, Phosphoric acid

28

Epitaxial graphene is expected to be the only way to obtain large-area sheets of this two-dimensional material for applications on an industrial scale. So far, there are different recipes for epitaxial growth of graphene, using either intrinsic carbon, such as the selective desorption of silicon from a SiC surface, or using extrinsic carbon, as via the chemical vapor deposition (CVD) of simple hydrocarbons on transition metal surfaces. In addition, even liquid precursor deposition (LPD) provides well-ordered graphene monolayers. It will be shown that graphene formation on transition metal surfaces by LPD synthesis is a very robust mechanism that even works if carbon is provided in a quite undefined way, namely by using a human fingerprint as a liquid precursor. Graphene growth from fingerprints provides well-ordered monolayers with the same quality as LPD grown graphene using ultrapure synthetic single precursors. The reliability of the self-assembly process of graphene growth on transition metals by LPD therefore offers a simple and extremely robust synthesis route for epitaxial graphene and may give access to production pathways for substrates for which the CVD method fails.

Concepts: Transition metal, Periodic table, Iron, Silicon carbide, Carbon, Chemical vapor deposition

28

The complexity of molecular recognition and assembly of biotic-abiotic interfaces at a scale of 1 to 1000 nm can be understood more effectively using simulation tools along with laboratory instrumentation. We discuss current capabilities and limitations of atomistic force fields and explain a strategy to obtain dependable parameters for inorganic compounds that has been developed and tested over the last decade. Parameter developments include several silicates, aluminates, metals, oxides, sulfates, and apatites that are summarized in what we call the INTERFACE force field. The INTERFACE force field operates as an extension of common harmonic force fields (PCFF, COMPASS, CHARMM, AMBER, GROMACS, and OPLS-AA) by employing the same functional form and combination rules to enable simulations of inorganic-organic and inorganic-biomolecular interfaces. The parameterization builds on in-depth understanding of physical-chemical properties at the atomic scale to assign each parameter, especially atomic charges and van-der-Waals constants, as well as on the validation of macroscale physical-chemical properties for each compound in comparison to measurements. The approach eliminates large discrepancies between computed and measured bulk and surface properties up to two orders of magnitude using other parameterization protocols and increases the transferability of the parameters by introducing thermodynamic consistency. As a result, a wide range of properties can be computed in quantitative agreement with experiment, including densities, surface energies, solid-water interface tensions, anisotropies of interfacial energies of different crystal facets, adsorption energies of biomolecules, thermal, and mechanical properties. Applications include insight into the assembly of inorganic-organic multiphase materials, recognition of inorganic facets by biomolecules, growth and shape preferences of nanocrystals and nanoparticles, as well as thermal transitions and nanomechanics. Limitations and opportunities for further development are described.

Concepts: Organic compound, Energy, Parametrization, Parameter, Molecular dynamics, CHARMM, Force field implementation, Oxygen

28

The phase behavior of two types of poly(ethylene oxide)/ poly(propylene oxide) (PEO/PPO) copolymers in aqueous solutions was studied by light scattering, viscometry and infrared spectroscopy. Both the reverse poloxamer (Pluronic 10R5) and the star type poloxamine (Tetronic 90R4) have practically the same PEO/PPO ratio with their hydrophobic blocks (PPO) located in the outer part. The temperature-composition phase diagrams show that both 10R5 and 90R4 tend to form aggregates in water. Up to four different phases can be detected in the case of Tetronic 90R4 for each temperature: unimers, random networks, micellar networks and macrophase separation. Viscometric and infrared measurements complemented the results obtained by light scattering and visual inspection.

Concepts: Near infrared spectroscopy, Triple point, Solutions, Polymers, Concentration, Spectroscopy, Poloxamer 407, Poloxamer

28

Herein we report the surfactant-triggered assembly of fullerene (C60) into three dimensional (3D) flower-like microcrystals at liquid-liquid interface. C60 crystals were grown using a liquid-liquid interfacial precipitation (LLIP) method by layering surfactant solution in butanol with saturated solution of C60 in benzene. In the LLIP method, it is suggested that the crystal formation mechanism is driven by supersaturation related to the low C60 solubility in alcohol. We found that the dimensions of the synthesized C60 flowers depend on concentration and surfactant type. In the absence of surfactant, i.e., in butanol/benzene system, 1D C60 nanowhiskers (nanorods) and C60 nanotubes (diameter 400 nm  2 m and length 5  20 m) are obtained. However, when surfactants are incorporated in the system flower-like microcrystals consisting of C60 nanotubes are observed; for instance, crystals grown at the interface of 0.01% diglycerol monolaurate (C12G2) nonionic surfactant in butanol with benzene leads to the formation of flower-shaped microcrystals of average sizes in the range of 10  35 m. To the best of our knowledge, this is the first example of surfactant-assisted assembly of C60 crystals. X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements have shown that the fullerene flowers have hexagonal structure with cell dimensions a = 2.539 nm and c = 1.021 nm, which differs from the pristine C60. Mixtures of flower-shaped C60 crystals and freely standing C60 nanotubes are found in the 0.1% C12G2/butanol system. On the other hand, clusters or giant aggregates of nanowhiskers lacking any specific shape are observed in the 1% C12G2/butanol system although these crystals exhibit hexagonal close packed structures. Flower-shaped C60 microcrystals are also observed with anionic surfactants cetyltrimethylammonium bromide (CTAB) and cetyltrimethylammonium chloride (CTAC) systems. C60 flowers obtained from the 0.01% CTAB and 0.01% CTAC also exhibit hexagonal structures with cell dimensions a = 2.329 nm and c = 1.273 nm, a = 2.459 nm and c = 0.938 nm, respectively. Our C60 flowers exhibit intense photoluminescence (PL) and blue shifted PL intensity maximum compared to pristine C60, demonstrating the potential to control the optoelectronic properties of fullerene-based nanostructures.

Concepts: Pulmonary surfactant, Hexagonal crystal system, Transmission electron microscopy, Solution, Surfactant

28

Formation of amyloid fibrils is often associated with intriguing far-from-equilibrium phenomena such as conformational memory effects or flow-driven self-assembly. Insulin is a model amyloidogenic polypeptide forming distinct structural variants of fibrils which self-propagate through seeding. According to infrared absorption, fibrils from bovine insulin ([BI]) and LysB31-ArgB32 human insulin analog ([KR]) cross-seed each other and imprint distinct structural features in daughter fibrils. In the absence of preformed [KR] amyloid seeds, bovine insulin agitated at 60o C converts into chiral amyloid superstructures exhibiting negative extrinsic Cotton effect in bound thioflavin T. However, when agitated bovine insulin is simultaneously cross-seeded with [KR] amyloid, daughter fibrils reveal a positive extrinsic Cotton effect. Our study indicates that dramatic changes in global properties of amyloid superstructures may emerge from subtle conformational-level variations in single fibrils (e.g. alignment and twist of β-strands) that are encoded by memory effects.

Concepts: Chirality, Effect, Amino acid, Seed, Eli Lilly and Company, Amyloid, Insulin analog, Insulin

28

New fluorine-containing terminal alkynes were synthesized and self-assembled onto Si(111) substrates to obtain fluorine-containing organic monolayers. The monolayers were analyzed in detail by ellipsometry, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS), static water contact angle measurements (CA), and atomic force microscopy (AFM). The SAMs exhibit excellent hydrophobicity, with static water contact angles of up to 119° and low critical surface tensions of 5-20 mN/m depending on the number of F atoms per molecule. IRRAS confirmed the formation of highly ordered monolayers, as indicated by the antisymmetric and symmetric stretching vibrations of the CH(2) moieties at 2918-2920 and 2850-2851 cm(-1), respectively. Upon increasing the number of fluorine atoms in the alkyne chains from 0 to 17, the adhesion of bare silica probes to the SAMs in air decreases from 11.6 ± 0.20 mJ/m(2) for fluorine-free (F0) alkyne monolayers to as low as 3.2 ± 0.03 mJ/m(2) for a heptadecafluoro-hexadecyne (F17)-based monolayer. Likewise, the friction coefficient decreases from 5.7 × 10(-2) to 1.2 × 10(-2). The combination of high ordering, excellent hydrophobicity, low adhesion, and low friction makes these fluoro-hydro alkyne-derived monolayers highly promising candidates for use in high-performance microelectronic devices.

Concepts: Scientific techniques, Atom, Infrared spectroscopy, Angle, Force, X-ray photoelectron spectroscopy, Contact angle, Spectroscopy

28

Long lasting anticorrosive coatings for steel have been developed based on halloysite nanotubes loaded with three corrosion inhibitors: benzotriazole, mercaptobenzothiazole and mercaptobenzimidazole. The inhibitors' loaded tubes were admixed at 5-10 wt % to oil based alkyd paint providing sustained agent release and corrosion healing in the coating defects. Slow 20-30 hour release of the inhibitors in defect points caused remarkable improvement in anticorrosion efficiency of the coatings. Further time expansion of anticorrosion agents release has been achieved by formation of release stoppers at nanotube-ends with urea-formaldehyde copolymer and copper-inhibitor complexation. Corrosion protection efficiency was tested on ASTM A366 steel plates in 0.5 M NaCl solution with microscanning of corrosion current development, by microscopy inspection and studying paint adhesion. The best protection was found using halloysite / mercaptobenzimidazole and benzotriazole inhibitors. Stopper formation with urea-formaldehyde copolymer provided additional increase in corrosion efficiency due to longer release of inhibitors.

Concepts: Sodium chloride, Agent, Coating, Formulations, Paint, Passivation, Coatings, Corrosion

28

The present work demonstrates a rational designing and synthesis of cholesterol based amino acid containing hydrogelators with the aim to improve the biocompatibility of these amphiphilic molecules. A thorough structure-property correlation of these hydrogelators was carried out by varying the hydrophilic terminal from neutral amine to quaternized ammonium chloride. The amphiphiles having cationic polar head as hydrophilic domain and cholesterol as hydrophobic unit showed better water gelation efficiency (minimum gelation concentration (MGC) ~0.9-3.1 %, w/v) than the analogous free amines. Presumably the additional ionic interactions for quaternized amphiphiles might have played the crucial role in gelation as counterions also got involved in hydrogen bonding with solvent molecules. Hence the attainment of desired hydrophilic-lipophilic balance (HLB) of hydrophobic cholesterol in combination with appropriate hydrophilic terminal led to the development of efficient hydrogels. Microscopic investigations revealed the formation of various supramolecular morphologies of hydrogels due to the variation in the molecular structure of the amphiphile. Spectroscopic investigations showed the involvement of hydrogen bonding, hydrophobic and π-π interaction in self-assembled gelation. Importantly, biocompatibility of all the cholesterol based hydrogelators tested against Human hepatic cancer derived HepG2 cells showed increased cell viability than the previously reported alkyl chain based amphiphilic hydrogelators. In order to incorporate broad spectrum antibacterial properties, silver nanoparticles (AgNP) were synthesized in situ within the hydrogels using sunlight. The amphiphile-AgNP soft nanocomposite exhibited notable bactericidal property against both Gram positive and Gram negative bacteria.

Concepts: Bacteria, Amino acid, Oxygen, Cholesterol, Amine, Gram-negative bacteria, Ammonia, Amphiphile

28

This study uses molecular dynamics (MD) modeling to examine the interlayer microstructures of montmorillonite intercalated with single chain QACs. Three types of QACs-tetramethylammonium (TMA), decyltrimethylammonium (DTMA), and hexadecyltrimethylammonium (HDTMA)-were selected to synthesize the organoclay complex, and the surfactant arrangement was analyzed quantitatively in systems in the absence of water. A series of arrangement patterns of interlayer QAC surfactant were observed, including lateral monolayers, lateral bilayers, pseudotrilayers, and paraffin monolayers, in agreement with previous experimental results. The effects of increasing one carbon chain length and amount of loading of QAC on the resultant QAC arrangement are summarized, yielding a model that provides insight into the prediction of synthesized QAC-clay microstructure and engineering behavior in practice.

Concepts: Atom, Scientific method, Fabric softener, Cocamidopropyl betaine, Salt, Surfactant, Quaternary ammonium cation, Ammonium