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Michael L Steigerwald

Publications and source records attributed to Michael L Steigerwald.

11 recordsLinked to original sources
PubMed2006

Crystalline graphite from an organometallic solution-phase reaction.

We report a chemical method by which graphitic carbon is prepared at reaction temperatures as low as 110 degrees C from readily available molecular reagents. This process requires no forcing conditions such as high pressures, intense light, or electrical discharge but is a simple, catalytic organometallic reaction. The carbon forms in a variety of morphologies including graphene sheets and nanotubes. The mild reaction conditions are similar to those routinely employed in homogeneous catalysis; therefore, they will allow the subtle manipulation of reaction variables to give desired morphologies selectively.

Journal Article↗
PubMed2006

Dependence of single-molecule junction conductance on molecular conformation.

Since it was first suggested that a single molecule might function as an active electronic component, a number of techniques have been developed to measure the charge transport properties of single molecules. Although scanning tunnelling microscopy observations under high vacuum conditions can allow stable measurements of electron transport, most measurements of a single molecule bonded in a metal-molecule-metal junction exhibit relatively large variations in conductance. As a result, even simple predictions about how molecules behave in such junctions have still not been rigorously tested. For instance, it is well known that the tunnelling current passing through a molecule depends on its conformation; but although some experiments have verified this effect, a comprehensive mapping of how junction conductance changes with molecular conformation is not yet available. In the simple case of a biphenyl--a molecule with two phenyl rings linked by a single C-C bond--conductance is expected to change with the relative twist angle between the two rings, with the planar conformation having the highest conductance. Here we use amine link groups to form single-molecule junctions with more reproducible current-voltage characteristics. This allows us to extract average conductance values from thousands of individual measurements on a series of seven biphenyl molecules with different ring substitutions that alter the twist angle of the molecules. We find that the conductance for the series decreases with increasing twist angle, consistent with a cosine-squared relation predicted for transport through pi-conjugated biphenyl systems.

Journal Article↗
PubMed2006

Chemoresponsive monolayer transistors.

This work details a method to make efficacious field-effect transistors from monolayers of polycyclic aromatic hydrocarbons that are able to sense and respond to their chemical environment. The molecules used in this study are functionalized so that they assemble laterally into columns and attach themselves to the silicon oxide surface of a silicon wafer. To measure the electrical properties of these monolayers, we use ultrasmall point contacts that are separated by only a few nanometers as the source and drain electrodes. These contacts are formed through an oxidative cutting of an individual metallic single-walled carbon nanotube that is held between macroscopic metal leads. The molecules assemble in the gap and form transistors with large current modulation and high gate efficiency. Because these devices are formed from an individual stack of molecules, their electrical properties change significantly when exposed to electron-deficient molecules such as tetracyanoquinodimethane (TCNQ), forming the basis for new types of environmental and molecular sensors.

Biosensing Techniques↗
PubMed2006

Organization of acenes with a cruciform assembly motif.

This study explores the assembly in the crystalline state of a class of pentacenes that are substituted along their long edges with aromatic rings forming rigid, cruciform molecules. The crystals were grown from the gas phase, and their structures were compared with DFT-optimized geometries. Both crystallographic and computed structures show that a planar acene core is the exception rather than the rule. In the assembly of these molecules, the phenyl groups block the herringbone motif and further guide the arrangement of the acene core into higher order structures. The packing for the phenyl-substituted derivatives is dictated by close contacts between the C-H's of the pendant aromatic rings and the carbons at the fusions in the acene backbone. Using thiophene substituents instead of phenyls creates cofacially stacked acenes. In thin films, the thiophene-substituted derivative forms devices with good electrical properties: relatively high mobility, high ON/OFF ratios, and low threshold voltage for device activation. An unusual result is obtained for the decaphenyl pentacene when devices are fabricated on its crystalline surface. Although its acene cores are well isolated from each other, this material still exhibits good electrical properties.

Journal Article↗
PubMed2006

Single-molecule circuits with well-defined molecular conductance.

We measure the conductance of amine-terminated molecules by breaking Au point contacts in a molecular solution at room temperature. We find that the variability of the observed conductance for the diamine molecule-Au junctions is much less than the variability for diisonitrile- and dithiol-Au junctions. This narrow distribution enables unambiguous conductance measurements of single molecules. For an alkane diamine series with 2-8 carbon atoms in the hydrocarbon chain, our results show a systematic trend in the conductance from which we extract a tunneling decay constant of 0.91 +/- 0.03 per methylene group. We hypothesize that the diamine link binds preferentially to undercoordinated Au atoms in the junction. This is supported by density functional theory-based calculations that show the amine binding to a gold adatom with sufficient angular flexibility for easy junction formation but well-defined electronic coupling of the N lone pair to the Au. Therefore, the amine linkage leads to well-defined conductance measurements of a single molecule junction in a statistical study.

Journal Article↗
PubMed2006

Enforced one-dimensional photoconductivity in core-cladding hexabenzocoronenes.

Photoconductivity in contorted hexabenzocoronene liquid crystals is found to be exclusively one-dimensional. Spectroscopic measurements and density functional theory support the existence of two pi-systems attributed to a low-energy radialene-core and higher energy out-of-plane alkoxyphenyl rings. Persistent photocurrents, measured as a function of field, channel length, and intensity, fit a stretched exponential characteristic of intracolumnar transport, restricted through the radialene-core by the alkoxyphenyl-cladding. Bimolecular recombination is enhanced with increasing carrier concentration by the system's one-dimensionality.

Journal Article↗
PubMed2005

Formation of catalytic metal-molecule contacts.

We describe a new strategy for the in situ growth of molecular wires predicated on the synthesis of a trifunctional "primed" contact formed from metal-carbon multiple bonds. The ruthenium-carbon pi bond provides structural stability to the molecular linkages under ambient conditions, and density functional calculations indicate the formation of an efficient conduit for charge carriers to pass between the metal and the molecule. Moreover, the metal-carbon pi bond provides a chemically reactive site from which a conjugated molecular wire can be grown in situ through an olefin metathesis reaction.

Journal Article↗
PubMed2005

An organometallic synthesis of TiO2 nanoparticles.

We report the synthesis of TiO2 nanoparticles that uses the low-temperature reaction of low-valent organometallic precursors. Bis(cyclooctatetraene)titanium reacts with dimethyl sulfoxide in organic solution at temperatures as low as room temperature to produce TiO2. In the absence of any supporting ligand, the reaction gives precipitation of amorphous TiO2 powder; however, in the presence of basic ligands such as tributylphosphine, tributylphosphine oxide and trioctylphosphine oxide, the precipitation is arrested, and chemically distinct, isolated, internally crystalline TiO2 nanoparticles are formed.

Crystallization↗
PubMed2004

Attaching organic semiconductors to gate oxides: in situ assembly of monolayer field effect transistors.

This study unveils a new tetracene derivative that forms dense, upright monolayers on the surface of aluminum oxide. These monolayers spontaneously self-organize into the active layer in nanoscale field-effect transistor devices when aluminum oxide is used as the dielectric layer. This method gives high yields of working devices that have source-drain distances that are less than 60 nm, thereby providing a method to electrically probe the monolayer assemblies formed from approximately 10 zeptomoles of material (approximately 104 molecules). Moreover, this study delineates a new avenue for research in thin-film organic transistors where the active molecules are linked to the dielectric surface to form a monolayer transistor.

Journal Article↗
PubMed2004

Reversible surface oxidation and efficient luminescence quenching in semiconductor single-wall carbon nanotubes.

We have investigated reversible single-wall carbon nanotube (SWNT) oxidation by quantitative analysis of the oxide-induced absorption bleaching and luminescence quenching at low pH. These data, in combination with DFT structure calculations, suggest that the nanotube oxide is a 1,4-endoperoxide. At low pH, the endoperoxide protonates to create a hydroperoxide carbocation, introducing a hole in the SWNT valence band. Nanotube luminescence is extremely sensitive to quenching by hole-doping, while the absorption is relatively robust.

Journal Article↗