Biomedical subjects

J P Rabe

Publications and source records attributed to J P Rabe.

9 recordsLinked to original sources

PubMed2006

Evidence for temperature-dependent electron band dispersion in pentacene.

Evidence for temperature-dependent electron band dispersion in a pentacene thin film polymorph on graphite is provided by angle- and energy-dependent ultraviolet photoelectron spectroscopy. The bands derived from the highest occupied molecular orbital exhibit dispersion of approximately 190 meV at room temperature, and approximately 240 meV at 120 K. Intermolecular electronic coupling in pentacene thin films is thus confirmed to be dependent on temperature and possibly crystal structure, as suggested by additional infrared absorption measurements.

Journal Article↗

PubMed2006

Blowing DNA bubbles.

We report here experimental observations which indicate that topologically or covalently formed polymer loops embedded in an ultrathin liquid film on a solid substrate can be "blown" into circular "bubbles" during scanning force microscopy (SFM) imaging. In particular, supercoiled vector DNA has been unraveled, moved, stretched, and overstretched to two times its B-form length and then torn apart. We attribute the blowing of the DNA bubbles to the interaction of the tapping SFM tip with the ultrathin liquid film.

DNA↗

PubMed2005

The effect of oxygen exposure on pentacene electronic structure.

We use ultraviolet photoelectron spectroscopy to investigate the effect of oxygen and air exposure on the electronic structure of pentacene single crystals and thin films. It is found that O(2) and water do not react noticeably with pentacene, whereas singlet oxygen/ozone readily oxidize the organic compound. Also, we obtain no evidence for considerable p-type doping of pentacene by O(2) at low pressure. However, oxygen exposure lowers the hole injection barrier at the interface between Au and pentacene by 0.25 eV, presumably due to a modification of the Au surface properties.

Electrochemistry↗

PubMed2004

Prototypical single-molecule chemical-field-effect transistor with nanometer-sized gates.

A prototypical single-molecule chemical-field-effect transistor is presented, in which the current through a hybrid-molecular diode is modified by nanometer-sized charge transfer complexes covalently linked to a molecule in an STM junction. The effect is attributed to an interface dipole which shifts the substrate work function by approximately 120 meV. It is induced by the complexes from electron acceptors covalently bound to the molecule in the gap and electron donors coming from the ambient fluid. This proof of principle is regarded as a major step towards monomolecular electronic devices.

Journal Article↗

PubMed2001

Supramolecular staircase via self-assembly of disklike molecules at the solid-liquid interface.

A series of soluble hexabenzocoronene (HBC) derivatives with pendant optically active (S)-3,7-dimethyloctanyl and (R,S)-3,7-dimethyloctanyl (mixture of stereoisomers) hydrocarbon side chains with and without a phenylene spacer were assembled into differently ordered arrays at the interface between a solution and the basal plane of highly oriented pyrolytic graphite (HOPG). Molecularly resolved scanning tunneling microscopy (STM) images revealed that all derivatives self-assemble into oriented crystals in quasi-two dimensions. However, while for the alkyl-substituted HBCs (1,4) all of the single aromatic cores within a monolayer exhibit the same contrast in the STM, the single aromatic cores with a phenylene group between the alkyl side chains and the aromatic core (2a,2b,3) exhibit different contrasts within a monolayer. For the disks carrying racemic branched or n-alkyl side chains (2b,3) a random distribution of the two different contrasts within the 2D-crystal is observed, while the optically active phenylene-alkyl-substituted HBC (2a) exhibits a periodical distribution of three contrasts within the monolayer. We attribute the different contrasts of the aromatic cores in the presence of the phenylene groups to a loss of the planarity of the whole molecule and different conformations, which allow the conjugated disks to attain different equilibrium positions above the surface of HOPG. In the case of the optically active side chains a regular superstructure with three distinctly different positions such as in a staircase is attained. The self-assembly processes are governed by the interplay of intramolecular as well as intermolecular and interfacial interactions. In the present case, the interactions may induce both the molecules to acquire well distinct positions along the z axis and to adopt different conformations. The reported results open new avenues of exploration. For instance, the different couplings of conjugated molecules with the substrate at different separations can be investigated by means of scanning tunneling spectroscopy (STS). Furthermore, experiments on the STM tip-induced switching of single molecules embedded in a monolayer appear feasible.

Journal Article↗

PubMed2000

Synthesis and self-assembly of functionalized hexa-peri-hexabenzocoronenes.

Monolayers of hexa-alkyl substituted derivatives of hexa-peri-hexabenzocoronene (HBC) 1b have previously been investigated by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). It is expected that different functional groups (electron donating or withdrawing) connected to the aromatic core will influence the packing pattern and possibly the current-voltage characteristics as well. In order to provide suitable model systems, a new synthetic approach to synthesize functionalized HBC derivatives has been developed. This was accomplished by [4 + 2]-cycloaddition of suitably bromo-substituted diphenylacetylenes and 2,3,4,5-tetraarylcyclopenta-2,4-dien-1-ones followed by an oxidative cyclodehydrogenation with iron(III) chloride/nitromethane. Using this strategy three different substitution patterns were synthesized: 2-bromo-5,8,11.14,17-pentadodecylhexa-pecri-hexabenzocoronene (2a), 2,5-dibromo-8,11,14,17-pentadodecylhexa-peri-hexabenzocoronene (2b), and 2,11-dibromo5,8,14,17-pentadodecylhexa-peri-hexa-benzocoronene (2c). These bromo-substituted HBC derivatives were subjected to palladium catalyzed coupling reactions to give donor (alkoxy, amino) as well as acceptor (ester, cyano) substituted derivatives. The self-assembly of these new HBC derivatives was studied in the bulk as well as at an interface. DSC, optical microscopy, and X-ray diffraction revealed the existence of columnar mesophases. The bulk structure in the mesophase is largely insensitive to changes of the substitution pattern; however, in situ scanning tunneling microscopy at the solid-fluid interface between an organic solution of the HBC derivative and highly oriented pyrolytic graphite reveals different packing patterns of the first adsorbed monolayer.

Journal Article↗

PubMed1996

Vertical dimension of hydrated biological samples in tapping mode scanning force microscopy.

The vertical dimensions of the well-characterized test samples tobacco mosaic virus, T4 bacteriophage polyhead, purple membrane, and hexagonally packed intermediate (HPI) layer were investigated by tapping mode scanning force microscopy (SFM) in solution. Purple membrane and HPI layer were imaged in both contact mode and tapping mode SFM for direct comparison. All vertical dimensions match the known heights. The practical implications of the absence of frictional forces in tapping mode are discussed.

Bacteriophage T4↗

PubMed1992

Molecules at interfaces: STM in materials and life sciences.

The scanning tunneling microscope (STM) enables us, for the first time, to directly observe individual molecules both at surfaces under ultrahigh-vacuum conditions as well as at interfaces with fluids or soft solids. In suitable systems, structure and dynamics of single molecules or monomolecular layers can be investigated at a resolution down to the atomic length scale and the time scale of milliseconds. The structure of individual biopolymers and biological membranes on solid supports can be studied on the nanometer scale. Furthermore, the STM may be viewed as a tool to address individual molecules, e.g., for molecular electronics studies or the manipulation of individual molecular structures. The paper reviews selected STM results on organic conductors, as well as small organic molecules and large biopolymers, which have been chemi- or physisorbed to conducting substrates. Finally, some prospects for future work are discussed.

Bacterial Outer Membrane Proteins↗

PubMed1987

Images of a lipid bilayer at molecular resolution by scanning tunneling microscopy.

The molecular structure of a fatty acid bilayer has been recorded with a scanning tunneling microscope operating in air. The molecular film, a bilayer of cadmium icosanoate (arachidate), was deposited onto a graphite substrate by the Langmuir-Blodgett technique. The packing of the lipid film was found to be partially ordered. Along one axis of the triclinic unit cell the intermolecular distance varied randomly around a mean of 5.84 A with a SD of 0.24 A. Along the other axis the mean distance was 4.1 A and appeared to vary monotonically over several intermolecular distances, indicating that a superstructure of longer range may exist. The molecular density was one molecular per 19.4 A2. The surprising ability of the scanning tunneling microscope to image the individual molecular chains demonstrates that electrons from the graphite can be transferred along the molecular chains for a distance of 50 A.

Arachidonic Acid↗