4 research outputs found
Diatom Mimics: Directing the Formation of Biosilica Nanoparticles by Controlled Folding of Lysine-Leucine Peptides
Silaffins, long chain polyamines,
and other biomolecules found
in diatoms are involved in the assembly of a large number of silica
nanostructures under mild, ambient conditions. Nanofabrication researchers
have sought to mimic the diatomâs biosilica production capabilities
by engineering proteins to resemble aspects of naturally occurring
biomolecules. Such mimics can produce monodisperse biosilica nanospheres,
but in vitro production of the variety of intricate biosilica nanostructures
that compose the diatom frustule is not yet possible. In this study
we demonstrate how LK peptides, composed solely of lysine (K) and
leucine (L) amino acids arranged with varying hydrophobic periodicities,
initiate the formation of different biosilica nanostructures in vitro.
When L and K residues are arranged with a periodicity of 3.5 the Îą-helical
form of the LK peptide produces monodisperse biosilica nanospheres.
However, when the LK periodicity is changed to 3.0, corresponding
to a 3<sub>10</sub> helix, the morphology of the nanoparticles changes
to elongated rod-like structures. β-strand LK peptides with
a periodicity of 2.0 induce wire-like silica morphologies. This study
illustrates how the morphology of biosilica can be changed simply
by varying the periodicity of polar and nonpolar amino acids
Covalently Attached Organic Monolayers onto Silicon Carbide from 1âAlkynes: Molecular Structure and Tribological Properties
In
order to achieve improved tribological and wear properties at
semiconductor interfaces, we have investigated the thermal grafting
of both alkylated and fluorine-containing ((C<sub><i>x</i></sub>F<sub>2<i>x</i>+1</sub>)â(CH<sub>2</sub>)<sub><i>n</i></sub>â) 1-alkynes and 1-alkenes onto silicon
carbide (SiC). The resulting monolayers display static water contact
angles up to 120°. The chemical composition of the covalently
bound monolayers was studied by X-ray photoelectron spectroscopy (XPS),
infrared reflectionâabsorption spectroscopy (IRRAS), and near-edge
X-ray absorption fine structure (NEXAFS) spectroscopy. These techniques
indicate the presence of acetal groups at the organicâinorganic
interface of alkyne-modified SiC surfaces. The tribological properties
of the resulting organic monolayers with fluorinated or nonfluorinated
end groups were explored using atomic force microscopy (AFM). It was
found that the fluorinated monolayers exhibit a significant reduction
of adhesion forces, friction forces, and wear resistance compared
with non-fluorinated molecular coatings and especially bare SiC substrates.
The successful combination of hydrophobicity and excellent tribological
properties makes these strongly bound, fluorinated monolayers promising
candidates for application as a thin film coating in high-performance
microelectronic devices
Magnetic Field Landscapes Guiding the Chemisorption of Diamagnetic Molecules
It
is shown that the self-assembly of diamagnetic molecule submonolayers
on a surface can be influenced by magnetic stray field landscapes
emerging from artificially fabricated magnetic domains and domain
walls. The directed local chemisorption of diamagnetic subphthalocyaninatoboron
molecules in relation to the artificially created domain pattern is
proved by a combination of surface analytical methods: ToF-SIMS, X-PEEM,
and NEXAFS imaging. Thereby, a new method to influence self-assembly
processes and to produce patterned submonolayers is presented
Effect of Internal Heteroatoms on Level Alignment at Metal/Molecular Monolayer/Si Interfaces
Molecular
monolayers at metal/semiconductor heterointerfaces affect
electronic energy level alignment at the interface by modifying the
interfaceâs electrical dipole. On a free surface, the molecular
dipole is usually manipulated by means of substitution at its external
end. However, at an interface such outer substituents are in close
proximity to the top contact, making the distinction between molecular
and interfacial effects difficult. To examine how the interface dipole
would be influenced by a single atom, internal to the molecule, we
used a series of three molecules of identical binding and tail groups,
differing only in the inner atom: aryl vinyl ether (<b>PhO</b>), aryl vinyl sulfide (<b>PhS</b>), and the corresponding molecule
with a CH<sub>2</sub> groupî¸allyl benzene (<b>PhC</b>). Molecular monolayers based on all three molecules have been adsorbed
on a flat, oxide-free Si surface. Extensive surface characterization,
supported by density functional theory calculations, revealed high-quality,
well-aligned monolayers exhibiting excellent chemical and electrical
passivation of the silicon substrate, in all three cases. Currentâvoltage
and capacitanceâvoltage analysis of Hg/PhX (X = C, O, S)/Si
interfaces established that the type of internal atom has a significant
effect on the Schottky barrier height at the interface, i.e., on the
energy level alignment. Surprisingly, despite the formal chemical
separation of the internal atom and the metallic electrode, Schottky
barrier heights were not correlated to changes in the semiconductorâs
effective work function, deduced from Kelvin probe and ultraviolet
photoemission spectroscopy on the monolayer-adsorbed Si surface. Rather,
these changes correlated well with the ionization potential of the
surface-adsorbed molecules. This is interpreted in terms of additional
polarization at the molecule/metal interface, driven by potential
equilibration considerations even in the absence of a formal chemical
bond to the top Hg contact