Biomedical subjects

Steven L Bernasek

Publications and source records attributed to Steven L Bernasek.

14 recordsLinked to original sources

PubMed2006

Competition and coadsorption of di-acids and carboxylic acid solvents on HOPG.

The self-assemblies of di-acids HOOC-(CH(2))(n)-COOH (n = 20, 18, 16, 14, 12, 10) in three solvents hexanoic acid, octanoic acid, and decanoic acid on highly oriented pyrolytic graphite (HOPG) were studied with scanning tunneling microscopy (STM). In the solvent hexanoic acid, solvent molecules coadsorb with HOOC-(CH(2))(n)-COOH (n = 20, 18, 16) via formation of hydrogen bonds. The solvent octanoic acid coadsorbs with HOOC-(CH(2))(n)-COOH (n = 20, 18). Decanoic acid only coadsorbs with HOOC-(CH(2))(20)-COOH. In each solvent, the trend of coadsorption between solvent molecules and di-acid molecules decreases with decreasing chain-length of di-acid molecules. These systematic investigations suggest that coadsorption of solvent molecules with di-acid solute molecules is mainly dependent on the relative hydrogen-bond densities in the formed monolayer. This is consistent with the maximization of adsorption heat of the self-assembled monolayers of di-acids dissolved in solvents of carboxylic acids.

Journal Article↗

PubMed2006

Characterization of self-assembled organic films using differential charging in X-ray photoelectron spectroscopy.

Differential charging is often regarded as a problem in X-ray photoelectron spectroscopic studies, especially for insulating or partially conducting samples. Application of a positive bias can reduce the effect of differential charging by attracting stray electrons from the system, thereby compensating for the electron loss. On the other hand, differential charging effect can be enhanced by the application of a negative bias to the sample during spectrum acquisition. The successful use of the differential charging technique to distinguish between multi- and monolayer organophosphonate films on oxide-covered silicon has been reported. A detailed description of this technique is now presented which shows how differential charging can be used as an important tool for the characterization of self-assembled films deposited on various surfaces. The dependence of this technique on the conductivity of the substrate has been investigated by studying the spectral behavior of the deposited films of phosphonic acid on conducting, semiconducting, and insulating samples (stainless steel, silicon, and glass). Application of either positive or negative dc electrical bias affects the carbon core-level (C1s) line shape and intensity, which is dependent on the atom's physical location above the surface.

Electric Conductivity↗

PubMed2006

Self-assembly and odd-even effects of cis-unsaturated carboxylic acids on highly oriented pyrolytic graphite.

The self-assembly of several cis-unsaturated carboxylic acids of the structure cis-CH3(CH2)p-1CH=CH(CH2)m-1COOH on highly oriented pyrolytic graphite (HOPG) was studied. The impact of the interior cis-CH=CH group and the molecular chain length on their self-assembled structures was considered. Due to the cis conformation of the -HC=CH- group in the interior of these molecules, they display self-assembled structures significantly different from saturated acids with all-trans configurations. As an example of the class of molecules cis-CH3(CH2)p-1CH=CH(CH2)2n-1COOH (p not equal 2n) (p=8, n=7), cis-CH3(CH2)7CH=CH(CH2)13COOH self-assembles into two kinds of enantiomer domains with opposite 2-D chirality. Due to the steric restriction of the interior cis-HC=CH group, all chains with acid groups are packed at the same side of a lamella, a head-to-head arrangement which is different from the head-to-tail packing of saturated all-trans acids. However, cis-CH3(CH2)7CH=CH(CH2)8COOH, considered as one example of the group cis-CH3(CH2)p-1CH=CH(CH2)2n-2COOH (p not equal 2n-1) (p=8, n=5), does not form any stable self-assembled domain, consistent with the molecular arrangement model. This difference in self-assembly behavior between cis-CH3(CH2)p-1CH=CH(CH2)2n-1COOH (p not equal 2n) and cis-CH3(CH2)p-1CH=CHC2n-2COOH (p not equal 2n-1) shows an odd-even chain-length effect of cis-CH3(CH2)p-1CH=CH(CH2)m-1COOH (p not equal m, m=2n or 2n-1). For another category of molecules, cis-unsaturated acids with equal numbers of all-trans carbon atoms on both sides of the cis-CH=CH group, cis-CH3(CH2)m-1CH=CH(CH2)m-1COOH (m=2n or 2n-1), display another odd-even effect. cis-CH3(CH2)7CH=CH(CH2)7COOH, one example of cis-CH3(CH2)2n-1-CH=CH(CH2)2n-1COOH (n=4), is predicted to form both an enantiomer and a nonchiral racemic structure, which is in accordance with the experimental observation of its self-assembled monolayer. However, cis-CH3(CH2)2n-2CH=CH(CH2)2n-2COOH does not form a stable self-assembled domain due to the same steric repulsion as that seen in the cis-CH3(CH2)7CH=CH(CH2)8COOH structure. These odd-even effects demonstrate that molecular self-assembly can be significantly tailored by slightly changing the molecular chain length.

Journal Article↗

PubMed2005

Two-dimensional self-assembly of a two-component molecular system: formation of an ordered and homogeneous molecular mesh.

The formation of nanoscaled objects often relies on the two-dimensional self-assembly of organic molecules on solid substrates, leading to a number of interesting structures with nanometer dimensions. Assembly of single-component systems driven by chain-chain van der Waals interactions, hydrogen bonding, and dipolar interactions governs the structures typically formed. The two-dimensional self-assembly of a two-component molecular system is described here, where the structure involves mixing of the components at the molecular level. A mixture of 5-octadecyloxyisophthalic acid and octanoic acid forms an ordered stoichiometric array of homogeneous nanometer-sized openings of dimension 8.5 A x 13.5 A x1.8 A, verified by atomic resolution scanning tunneling microscopy. Assembly in the structure is driven by van der Waals and hydrogen bonding interactions between the molecular components.

Caprylates↗

PubMed2005

Advanced surface modification of indium tin oxide for improved charge injection in organic devices.

A new method is described for surface modification of ITO with an electroactive organic monolayer. This procedure was done to enhance hole injection in an electronic device and involves sequential formation of a monolayer of a pi-conjugated organic semiconductor on the indium tin oxide (ITO) surface followed by doping with a strong electron acceptor. The semiconductor monolayer is covalently bound to the ITO, which ensures strong adhesion and interface stability; reduction of the hole injection barrier in these devices is accomplished by formation of a charge-transfer complex by doping within the monolayer. This gives rise to very high current densities in simple single layer devices and double layer light emitting devices compared to those with untreated ITO anodes.

Journal Article↗

PubMed2005

Measuring the surface roughness of sputtered coatings by microgravimetry.

A novel application of quartz crystal microgravimetry is described to enable determination of the roughness of native and sputtered oxide coatings. The technique is applicable to coatings that may be too rough for accurate atomic force microscopic imaging where measurements may be limited by the dimensions of the probe tip.

Journal Article↗

PubMed2005

Chirality in supramolecular self-assembled monolayers of achiral molecules on graphite: formation of enantiomorphous domains from arachidic anhydride.

The molecular arrangement and chirality of the self-assembled arachidic anhydride monolayer on graphite were investigated using scanning tunneling microscopy (STM). This molecule has two identical alkyl chains, linked by an anhydride group in the middle. In its extended form, one alkyl chain is shifted, with respect to the other, along the molecular backbone. Upon adsorption on graphite, this achiral anhydride spontaneously forms two types of homogeneous domains (denoted as m and m') with mirror symmetry. The angle from the molecular chain to the row-packing direction is 98.0 degrees +/- 0.5 degrees and 82.0 degrees +/- 0.5 degrees for domains m and m', respectively. Domain m is the mirror image of m'. The molecular arrangement of this self-assembled monolayer shows that domains m and m' are two-dimensional enantiomers with opposite chiralities. This new molecular packing motif is confirmed by line-profile analyses along the molecule-chain and the row-packing directions. This finding demonstrates the spontaneous formation of highly ordered homogeneous enantiomorphous domains on graphite resulting only from weak van der Waals forces between the achiral arachidic anhydride molecules.

Adsorption↗

PubMed2005

Structures formed by the chiral assembly of racemic mixtures of enantiomers: iodination products of elaidic and oleic acids.

The self-assembled monolayer structure of the products of elaidic acid iodination (the racemic mixture of 9,10-(9S,10R)-diiodooctadecanoic acid and 9,10-(9R,10S)-diiodooctadecanoic acid) and the products of oleic acid iodination (the racemic mixture of 9,10-(9R,10R)-diiodooctadecanoic acid and 9,10-(9S,10S)-diiodooctadecanoic acid) are studied by high-resolution scanning tunneling microscopy. For the iodination products of elaidic acid, the separation of enantiomers into distinct chiral domains during the formation of the 2-D crystal on the highly ordered pyrolytic graphite (HOPG) surface is not observed. Instead, within the diiodooctadecanoic acid SAM, each row of molecules is composed of opposite racemates. The two opposite racemates pack alternately inside a row, using different faces to adsorb on the surface. The unit cell is composed of a pair of opposite racemates, forming a heterochiral structure. For the iodination products of oleic acid, the racemic mixture is observed to exhibit quasi-phase separation during the formation of the 2-D crystal on the HOPG surface. Each row is composed of homochiral acid molecules, either the 9,10-(9R,10R)-diiodooctadecanoic acid (R) or the 9,10-(9S,10S)-diiodooctadecanoic acid (S). The R row and the S row pack alternately, with a unit cell composed of four molecules. Two of the molecules in the unit cell are the 9,10-(9R,10R)-diiodooctadecanoic acid (R) molecules; two are the 9,10-(9S,10S)-diiodooctadecanoic acid (S) molecules. In the unit cell, the two molecules that have the same chirality pack antiparallel inside the homochiral row, using different faces to adsorb on the surface. These results suggest that several different types of chiral assembly are possible. Enantiomers with opposite chirality exhibit many chiral assembly patterns, forming heterochiral structures on the surface in addition to separation to form macroscopic chiral domains. By using different conformations, similar enantiomers with opposite chirality will display many chiral assembly patterns to form heterochiral structures on the surface.

Fatty Alcohols↗

PubMed2005

Direct measurement of surface complex loading and surface dipole and their effect on simple device behavior.

Tin complexes of phenoxide ligands having a range of dipole moments were prepared on the surface of indium-tin oxide (ITO). Surface complex loadings and stoichiometries were measured by quartz crystal microgravimetry. Work functions of ITO substrates treated with these various surface complexes were measured using a Kelvin probe. Surface complex dipole moments were then calculated based on measured surface loadings. Changes in the ITO work function effected by surface phenoxide complex introduction correlate with these surface complex dipole moments and with total surface dipole per unit area, and current densities in simple hole-only diode devices also correlate with these total surface dipoles.

Journal Article↗

PubMed2005

Scanning tunneling microscopy studies of the self-assembly of carboxylic esters on graphite: linear distortion and multiple adsorption structures.

Self-assembled monolayers of carboxylic esters (stearic acid palmityl ester, lauric acid palmityl ester, and lauric acid behenyl ester) on graphite were investigated using scanning tunneling microscopy. All three esters, which are bent at the carboxylic group in the gas phase, are distorted into a straight-chain shape upon self-assembly on graphite. This results from optimizing the adsorption energy by matching the adsorbate molecular chain with the graphite substrate lattice periodicity. In all the formed lamellae, the long alkyl chain of the ester always aligns with the long chain of the adjacent molecule. Steric repulsion of the carbonyl group pointing perpendicularly to the neighboring molecule weakens the interaction of the ester molecule with the substrate. The ester molecules then easily self-assemble into multilamellae with molecular chain-trough angles of 73, 61, and 49 degrees in addition to the 90 degrees angle typical of n-alkane monolayers. This results from a shifting of 1/2, 1, or 3/2 units from the adjacent molecule in a lamella. The relatively weak interaction between ester molecules and substrate lattice also results in the formation of zigzag patterns with domain-domain angles of 145, 133, and 122 degrees , respectively. The structures of esters adsorbed on HOPG indicate, contrary to what might be expected, that physisorbed molecular adsorbates do not necessarily have the same geometry as in the gas phase.

Journal Article↗

PubMed2004

Adsorption-induced asymmetric assembly from an achiral adsorbate.

Symmetry breaking in the self-assembled monolayer (SAM) structure of 1-octadecanol on highly ordered pyrolytic graphite (HOPG) is observed. Due to the slight mismatch of the octadecanol molecule with the graphite lattice, the alkane chain undergoes distortion upon adsorption on the surface. The asymmetric distortion of the octadecanol SAM unit cell pair is observed by scanning tunneling microscopy at the liquid/solid interface. Asymmetric distortion is due to the requirement for planarity of the hydrogen bond connecting the two octadecanol molecules in the chevron-shaped unit cell. This very simple structure provides the first example of an adsorption-induced distortion to form a supramolecular asymmetric structure, which is formed by achiral molecules adsorbed on an achiral surface. What makes this system interesting and different from other examples of adsorption-induced chirality is that the adsorbate itself undergoes asymmetric distortion due to the existence of the substrate and the adsorbate conformation is different from the molecule in solution.

Journal Article↗

PubMed2003

Chiral pair monolayer adsorption of iodine-substituted octadecanol molecules on graphite.

High-resolution scanning tunneling microscopy has been used to examine the adsorbate structures formed when a racemic mixture of (9R,10R)-9,10-diiodooctadecan-1-ol and (9S,10S)-9,10-diiodooctadecan-1-ol is adsorbed at the basal plane of highly ordered pyrolytic graphite. The herringbone structure characteristic of the adsorption of long-chain molecules on graphite is observed. Close examination of the micrographs indicates a unique structure in which the chiral molecules adsorb in pairs, with one enantiomer filling half of the unit cell, and the other enantiomer filling the other half. Instead of forming separate chiral domains, as is sometimes observed when a racemic mixture adsorbs on an achiral surface, chiral pairs are formed and the pairs form an ordered monolayer, exposing opposite faces of the same molecule. An achiral racemic mixture is observed to form a chiral structure on an achiral surface in the regions of the surface examined here.

Journal Article↗

PubMed2002

High-resolution TOF-SIMS study of varying chain length self-assembled monolayer surfaces.

A high-resolution time-of-flight secondary ionization mass spectrometer (TOF-SIMS) has been used to investigate chain length effects in hydrocarbon seff-assembled monolayer (SAM) surfaces on gold substrates. A wide range of n-alkanethiols was used to make homogeneous SAM surfaces, which included both odd and even hydrocarbon chain length thiols. Variations in coverage, extent of oxidation, and high-mass cluster formation as a function of hydrocarbon chain length of the alkanethiol SAM surfaces were investigated. Long-short chain length effects were observed for the relative coverage of the SAM surfaces, which directly influences the extent of oxidation for the thin films. The formation of gold-sulfur and gold-adsorbate cluster ions was also observed, since the mass range of the TOF-SIMS made it possible to monitor all of the cluster ions that were formed following the high-energy ion/surface interactions.

Journal Article↗

PubMed2002

Controlling the work function of indium tin oxide: differentiating dipolar from local surface effects.

Indium tin oxide (ITO) reacts with tetra(tert-butoxy)tin to give surface bound alkoxytin species. Ligand exchange occurs in these surface bound species by reaction with substituted phenols. The speciation of surface phenoxides was measured in ultrahigh vacuum by X-ray photoelectron spectroscopy, as was the work function for the surface modified ITO. It is shown that the molecular dipole moment of the phenol correlates strongly with the measured ITO work function change; no such correlation exists between the acidity of the phenol and this measured change in work function. Results are consistent therefore with an electrostatic model and not with one involving electronegativities of the ligated phenoxide oxygens.

Journal Article↗