Water-assisted growth of uniform 100 mm diameter SWCNT arrays.

We report a simple method for growing high-quality single-walled carbon nanotube (SWCNT) arrays on 100 mm wafers via the addition of water vapor to highly purified gases during the CNT growth step. We show that adding a small amount of water during growth helps to create a uniform catalyst distribution and yields high-quality (Raman G/D of 26 ± 3), high-density (up to 6 × 10(11) cm(-2)) and uniform SWCNT arrays on 100 mm large wafers. We rationalize our finding by suggesting that the addition of water decreases catalyst mobility, preventing its coarsening at higher temperatures. We also report a new mechanism of catalyst inactivation in wafer-scale growth using ultrapurified gas sources by the formation of large, 5 ± 3 µm iron particles. We found such formations to be common for substrates with large temperature gradients, such as for wafers processed in a typical cold-wall chemical vapor deposition reactor.

Assembling p-type molecules on single wall carbon nanotubes for photovoltaic devices.

We report the design and synthesis of an oligothiophene molecule that noncovalently functionalizes carbon nanotubes to create a hybrid material for photovoltaic devices.

Amine-linked single-molecule circuits: systematic trends across molecular families.

A comprehensive review is presented of single-molecule junction conductance measurements across families of molecules measured while breaking a gold point contact in a solution of molecules with amine end groups. A theoretical framework unifies the picture for the amine-gold link bonding and the tunnel coupling through the junction using density functional theory based calculations. The reproducible electrical characteristics and utility for many molecules is shown to result from the selective binding between the gold electrodes and amine link groups through a donor-acceptor bond to undercoordinated gold atoms. While the bond energy is modest, the maximum force sustained by the junction is comparable to, but less than, that required to break gold point contacts. The calculated tunnel coupling provides conductance trends for all 41 molecule measurements presented here, as well as insight into the variability of conductance due to the conformational changes within molecules with torsional degrees of freedom. The calculated trends agree to within a factor of 2 with the measured values for conductance ranging from 10(-7)G(0) to 10(-2)G(0), where G(0) is the quantum of conductance (2e(2)/h).

Single-molecule devices as scaffolding for multicomponent nanostructure assembly.

We report here a method to integrate discrete multicomponent assembly into molecular electronic devices. We first functionalize a molecule wired between the ends of a single-walled carbon nanotube so that it can be derivatized with a probe molecule. This probe then binds to a complementary biomolecule to form a noncovalent complex. Each step of chemical functionalization and biological assembly can be detected electrically at the single event level. Through this combination of programmed chemical reactions and molecular recognition, we are able to create complex multimeric nanostructures incorporating isolated metallic nanoparticles.

Frustrated ostwald ripening in self-assembled monolayers of cruciform pi-systems.

This study details a scanning tunneling microscopy investigation into the mechanism of chiral grain growth in highly ordered, self-assembled monolayer films composed of cruciform pi-systems. Although the molecules themselves are achiral, when they adsorb from solution onto graphite, they adopt a gear-like conformation that, by virtue of the surface, is chiral. These handed subunits arrange themselves into enantiomeric two-dimensional domains. The unique finding from this study is that Ostwald ripening is frustrated between domain boundaries that are of opposite chirality because direct interconversion between the chiral units on the surface is energetically inhibited.

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.

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.

Covalently bridging gaps in single-walled carbon nanotubes with conducting molecules.

Molecular electronics is often limited by the poorly defined nature of the contact between the molecules and the metal surface. We describe a method to wire molecules into gaps in single-walled carbon nanotubes (SWNTs). Precise oxidative cutting of a SWNT produces carboxylic acid-terminated electrodes separated by gaps of

A molecular switch based on potential-induced changes of oxidation state.

We have measured the conductance of a hepta-aniline oligomer attached to gold electrodes held under potential control in electrolyte. It increases fifteen-fold (to 5.3+/-0.4 nS) on oxidation from the leucoemeraldine form to the emeraldine salt. The single-molecule current-voltage characteristic, linear in toluene, displays negative differential resistance in an acidic electrolyte. The negative differential resistance is accounted for by modification of the local surface potential by the applied bias. These results connect electrochemical data directly to molecular electronic behavior in a two-terminal device.

Chemical reactions with upright monolayers of cruciform pi-systems.

The study below details the synthesis and self-assembly of new cruciform pi-systems and their in situ chemical reactions in monolayer films. Analysis of the packing in the crystal structure of one of these unusually shaped molecules reveals that the terphenyl arm, which is twisted out of conjugation, makes edge-to-face contact with neighboring molecules aligning the conjugated bisoxazole arms in rows. In self-assembled monolayers on metal surfaces, these cruciform pi-systems present reactive groups at the film/air interface. Films that present aldehyde functionality react with aromatic anilines to give surface-bound imines. Dimers that are >4.5 nm in length and contain a conjugated imine linkage can be made in situ on gold substrates through this strategy.

Mechanism of DNA compaction by yeast mitochondrial protein Abf2p.

We used high-resolution atomic force microscopy to image the compaction of linear and circular DNA by the yeast mitochondrial protein Abf2p, which plays a major role in packaging mitochondrial DNA. Atomic force microscopy images show that protein binding induces drastic bends in the DNA backbone for both linear and circular DNA. At a high concentration of Abf2p DNA collapses into a tight nucleoprotein complex. We quantified the compaction of linear DNA by measuring the end-to-end distance of the DNA molecule at increasing concentrations of Abf2p. We also derived a polymer statistical mechanics model that provides a quantitative description of compaction observed in our experiments. This model shows that sharp bends in the DNA backbone are often sufficient to cause DNA compaction. Comparison of our model with the experimental data showed excellent quantitative correlation and allowed us to determine binding characteristics for Abf2p. These studies indicate that Abf2p compacts DNA through a simple mechanism that involves bending of the DNA backbone. We discuss the implications of such a mechanism for mitochondrial DNA maintenance and organization.

Cruciform pi-systems for molecular electronics applications.

This study details a modular and general synthesis of a new class of molecules consisting of cruciform pi-systems. The key to synthesizing these molecules was an unprecedented double Staudinger cyclization. Once formed, these rigid compounds assemble into ordered monolayer films on metal and metal oxide surfaces to orient their conjugated, bis-phenyloxazole subunits upright. This surface orientation is enforced by the external phenyl substituents that are out of the ring plane, thus preventing the prone conformation.