Showing papers by "James M. Tour published in 2008"

Diazonium Functionalization of Surfactant-Wrapped Chemically Converted Graphene Sheets

Abstract: Surfactant-wrapped chemically converted graphene sheets obtained from reduction of graphene oxide with hydrazine were functionalized by treatment with aryl diazonium salts. The nanosheets are characterized by X-ray photoelectron spectroscopy, attenuated total reflectance infrared spectroscopy, Raman spectroscopy, atomic force microscopy, and transmission electron microscopy. The resulting functionalized nanosheets disperse readily in polar aprotic solvents, allowing alternative avenues for simple incorporation into different polymer matrices.

Mesoporous silicon particles as a multistage delivery system for imaging and therapeutic applications

Abstract: Many nanosized particulate systems are being developed as intravascular carriers to increase the levels of therapeutic agents delivered to targets, with the fewest side effects. The surface of these carriers is often functionalized with biological recognition molecules for specific, targeted delivery. However, there are a series of biological barriers in the body that prevent these carriers from localizing at their targets at sufficiently high therapeutic concentrations. Here we show a multistage delivery system that can carry, release over time and deliver two types of nanoparticles into primary endothelial cells. The multistage delivery system is based on biodegradable and biocompatible mesoporous silicon particles that have well-controlled shapes, sizes and pores. The use of this system is envisioned to open new avenues for avoiding biological barriers and delivering more than one therapeutic agent to the target at a time, in a time-controlled fashion.

Electronics: the fourth element.

Abstract: Almost four decades since its existence was first proposed, a fourth basic circuit element joins the canonical three. The 'memristor' might herald a step-change in the march towards ever more powerful circuitry.

Simultaneous measurements of electronic conduction and Raman response in molecular junctions.

Abstract: Electronic conduction through single molecules is affected by the molecular electronic structure as well as by other information that is extremely difficult to assess, such as bonding geometry and chemical environment. The lack of an independent diagnostic technique has long hampered single-molecule conductance studies. We report simultaneous measurement of the conductance and the Raman spectra of nanoscale junctions used for single-molecule electronic experiments. Blinking and spectral diffusion in the Raman response of both p-mercaptoaniline and a fluorinated oligophenylyne ethynylene correlate in time with changes in the electronic conductance. Finite difference time domain calculations confirm that these correlations do not result from the conductance modifying the Raman enhancement. Therefore, these observations strongly imply that multimodal sensing of individual molecules is possible in these mass-producible nanostructures.

Reversible photo-switching of single azobenzene molecules in controlled nanoscale environments.

Abstract: We drive reversible photoinduced switching of single azobenzene-functionalized molecules isolated in tailored alkanethiolate monolayer matrices on Au{111} We designed molecular tethers to suppress excited-state quenching from the metal substrate and formed rigid assemblies of single tethered azobenezene molecules in the domains of monolayer to limit steric constraints and tip-induced and stochastic switching effects Single molecules were reversibly photoisomerized between trans and cis conformations by cycling exposure to visible and UV light Trans and cis conformations were imaged as high (21 ± 03 A) and low (07 ± 02 A) protrusions in STM images and were assigned to the on and off states of the molecule, respectively

Electronic two-terminal bistable graphitic memories.

Abstract: Transistors are the basis for electronic switching and memory devices as they exhibit extreme reliabilities with on/off ratios of 104–105, and billions of these three-terminal devices can be fabricated on single planar substrates. On the other hand, two-terminal devices coupled with a nonlinear current–voltage response can be considered as alternatives provided they have large and reliable on/off ratios and that they can be fabricated on a large scale using conventional or easily accessible methods. Here, we report that two-terminal devices consisting of discontinuous 5–10 nm thin films of graphitic sheets grown by chemical vapour deposition on either nanowires or atop planar silicon oxide exhibit enormous and sharp room-temperature bistable current–voltage behaviour possessing stable, rewritable, non-volatile and non-destructive read memories with on/off ratios of up to 107 and switching times of up to 1 μs (tested limit). A nanoelectromechanical mechanism is proposed for the unusually pronounced switching behaviour in the devices. Carbon-based structures are being intensively investigated for their use in electronic devices. A pronounced non-volatile switching is now observed in two-terminal devices made from graphitic sheets. The highly reliable switching mechanism is explained by the local breaking and rejoining of atomic bonds in the sheets.

Silicon/molecule interfacial electronic modifications.

Abstract: Electronic structures at the silicon/molecule interface were studied by X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, inverse photoemission spectroscopy, and Kelvin probe techniques The heterojunctions were fabricated by direct covalent grafting of a series of molecules (−C6H4−X, with X = NMe2, NH2, NO2, and Mo6 oxide cluster) onto the surface of four types of silicon substrates (both n- and p-type with different dopant densities) The electronic structures at the interfaces were thus systematically tuned in accordance with the electron-donating ability, redox capability, and/or dipole moment of the grafted molecules The work function of each grafted surface is determined by a combination of the surface band bending and electron affinity The surface band bending is dependent on the charge transfer between the silicon substrate and the grafted molecules, whereas electron affinity is dependent on the dipole moment of the grafted molecules The contribution of each to the work

Structure-dependent reactivity of semiconducting single-walled carbon nanotubes with benzenediazonium salts.

Abstract: The addition of diazonium salts to single-walled carbon nanotubes (SWCNTs) in aqueous surfactant suspensions quenches the intrinsic near-infrared fluorescence of semiconducting SWCNTs through sidewall chemical reactions. Spectrally resolved fluorescence spectroscopy of mixed SWCNT samples has been used to measure structure-dependent relative reactivities in the initial stages of these reactions. For several 4-substituted benzenediazonium salts, Ar−R (Ar = N2+−C6H4 and R = Cl, NO2, OMe), reactivities at pH 10 were found to be greatest for SWCNTs having the largest band gaps. The magnitude of this band gap dependence varies according to the R-group of the salt, with R = OMe showing the strongest variation. For R = OH, acidification of the sample to pH 5.5 results in reversal of the structural trend, as smaller band gap SWCNTs show slightly greater reactivities. The derivatization reactions observed here proceed concurrently, although at different rates, for semiconducting and metallic SWCNT species. These r...

In vitro cytotoxicity of single-walled carbon nanotube/biodegradable polymer nanocomposites

Abstract: Injectable nanocomposites made of biodegradable poly(propylene fumarate) and the crosslinking agent propylene fumarate-diacrylate as well as each of three forms of single-walled carbon nanotubes (SWNTs) were evaluated for their in vitro cytotoxicity. Unreacted components, crosslinked networks, and degradation products of the nanocomposites were investigated for their effects on cell viability using a fibroblast cell line in vitro. The results did not reveal any in vitro cytotoxicity for purified SWNTs, SWNTs functionalized with 4-tert-butylphenylene, and ultra-short SWNTs at 1- 100 microg/mL concentrations. Moreover, nearly 100% cell viability was observed on all crosslinked nanocomposites and cell attachment on their surfaces was comparable with that on tissue culture polystyrene. The degradation products of the nanocomposites displayed a dose-dependent adverse effect on cells, which was partially due to increased osmolarity by the conditions of accelerated degradation and could be overcome at diluted concentrations. These results demonstrate that all three tested nanocomposites have favorable cytocompatibility for potential use as scaffolds for bone tissue engineering applications.

Molecular Dynamics of Surface-Moving Thermally Driven Nanocars.

Abstract: We developed molecular models describing the thermally initiated motion of nanocars, nanosized vehicles composed of two to four spherical fullerene wheels chemically coupled to a planar chassis, on a metal surface. The simulations were aimed at reproducing qualitative features of the experimentally observed migration of nanocars over gold crystals as determined by scanning tunneling microscopy. Coarse-grained-type molecular dynamics simulations were carried out for the species "Trimer" and "Nanotruck", the simplified versions of the experimentally studied nanomachines. Toward this goal, we developed a version of the rigid body molecular dynamics based on the symplectic quaternion scheme in conjunction with the Nose-Poincare thermostat approach. Interactions between rigid fragments were described by using the corrected CHARMM force field parameters, while several empirical models were introduced for interactions of nanocars with gold crystals. With the single adjusted potential parameter, the computed trajectories are consistent with the qualitative features of the thermally activated migration of the nanocars: the primary pivoting motion of Trimer and the two-dimensional combination of translations and pivoting of Nanotruck. This work presents a first attempt at a theoretical analysis of nanocars' dynamics on a surface by providing a computationally minimalist approach.

Synthesis and photoisomerization of fullerene- and oligo(phenylene ethynylene)-azobenzene derivatives.

Abstract: The presence of fullerenes and oligo(phenylene ethynylene)s (OPEs) in azobenzene derivatives have a large effect on the photoisomerization behavior of the molecules. Fullerenes reduce the photoisomerization yield for cis isomers, and the OPEs, when directly attached to the azobenzenes, have a similar yet smaller effect when compared with the fullerenes. While these effects have not been previously considered for fullerene− and OPE−azobenzene derivatives, they were clearly detected in our work using NMR and UV–vis spectroscopy methods. The intramolecular electronic energy transfer between the fullerene and azobenzene moiety was examined in two cases in which separation of the two functional groups was small, as in 1, or large, as in 2. Almost no photoisomerization was observed for 1, while significant photoisomerization was observed for 2, apparently due to the effective isolation and blocking of electronic communication between the two functional groups.

Selective Photochemical Functionalization of Surfactant-Dispersed Single Wall Carbon Nanotubes in Water

Abstract: Ultraviolet (UV) irradiation of single wall carbon nanotubes (SWCNTs) individually dispersed in surfactants leads to diameter and type-selective photohydroxylation of the nanotubes. Photohydroxylation of first semiconductor and then small diameter metallic SWCNTs was confirmed after 254 nm UV irradiation in acidic, neutral, and basic aqueous solutions at ambient and elevated temperatures. The increased oxygen content of the SWCNTs after UV irradiation, as detected by X-ray photoelectron spectroscopy, suggests that SWCNTs were hydroxylated by reaction with water. Attenuated total reflectance Fourier transform infrared analysis provides evidence of hydroxyl functional groups on their surface. This photochemical reaction is impeded by molecular oxygen and appears to involve a reactive intermediate generated in the vicinity of semiconducting SWCNTs. This represents a noncontaminating selective reaction in the liquid phase that uses an intrinsic property of the tubes.

Nanotrains and self-assembled two-dimensional arrays built from carboranes linked by hydrogen bonding of dipyridones

Abstract: The strong hydrogen bonding ability of 2-pyridones were exploited to build nanotrains on surfaces. Carborane wheels on axles difunctionalized with 2-pyridone hydrogen bonding units were synthesized and displayed spontaneous formation of linear nanotrains by self-assembly on SiO2 or mica surfaces. Imaging using atomic force microscopy confirmed linear formations with lengths up to 5 µm and heights within the range of the molecular height of the carborance-tipped axles.

Energy level alignment at organic semiconductor/metal interfaces: Effect of polar self-assembled monolayers at the interface

Abstract: We determined the shifts in the energy levels of approximately 15 nm thick poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] films deposited on various substrates including self-assembled monolayer (SAM) modified Au surfaces using photoelectron spectroscopy. As the unmodified substrates included Au, indium tin oxide, Si (with native oxide), and Al (with native oxide), a systematic shift in the detected energy levels of the organic semiconductor was observed to follow the work function values of the substrates. Furthermore, we used polar SAMs to alter the work function of the Au substrates. This suggests the opportunity to control the energy level positions of the organic semiconductor with respect to the electrode Fermi level. Photoelectron spectroscopy results showed that, by introducing SAMs on the Au surface, we successfully increased and decreased the effective work function of Au surface. We found that in this case, the change in the effective work function of the metal surface was not reflected as a shift in the energy levels of the organic semiconductor, as opposed to the results achieved with different substrate materials. Our study showed that when a substrate is modified by SAMs (or similarly by any adsorbed molecules), a new effective work function value is achieved; however, it does not necessarily imply that the new modified surface will behave similar to a different metal where the work function is equal to the effective work function of the modified surface. Various models and their possible contribution to this result are discussed.

Etching-dependent reproducible memory switching in vertical SiO2 structures

Abstract: Vertical structures of SiO2 sandwiched between a top tungsten electrode and conducting nonmetallic substrate were fabricated by dry and wet etching methods. Both structures exhibit similar voltage-controlled memory behaviors, in which short voltage pulses (1 μs) can switch the devices between high- and low-impedance states. Through the comparison of current-voltage characteristics in structures made by different methods, filamentary conduction at the etched oxide edges is most consistent with the results, providing insights into similar behaviors in metal/SiO/metal systems. High ON/OFF ratios of over 104 were demonstrated.

Etching-dependent reproducible memory switching in vertical SiO2 structures

Abstract: Vertical structures of SiO$_{2}$ sandwiched between a top tungsten electrode and conducting non-metal substrate were fabricated by dry and wet etching methods. Both structures exhibit similar voltage-controlled memory behaviors, in which short voltage pulses (1 $\mu$s) can switch the devices between high- and low-impedance states. Through the comparison of current-voltage characteristics in structures made by different methods, filamentary conduction at the etched oxide edges is most consistent with the results, providing insights into similar behaviors in metal/SiO/metal systems. High ON/OFF ratios of over 10$^{4}$ were demonstrated.

Surface Grafting of Ferrocene-Containing Triazene Derivatives on Si(100)

Abstract: Ferrocene-containing molecules have been grafted to Si(100) surfaces to form monolayers and multilayers via a triazene derivative and its subsequent diazonium chemistry. The growth of the ferrocene-containing films was controlled by molecular concentrations and reaction times. Results from ellipsometry showed that the film thicknesses were in the range of subnanometer to several nanometers. X-ray photoelectron spectroscopy has confirmed the structural integrity of ferrocene in the films. Electrochemical studies of the ferrocene-containing multilayer have shown a reversible one-electron wave of the ferrocene/ferrocenium couple. The multilayer coverage was found to be 2.8 × 10−9 mol cm−2. The calculated electron transfer rate constant was 164 s−1.

Synthesis and Self-Assembly of Thio Derivatives of Calix[4]arene on Noble Metal Surfaces

Abstract: Self-assembled monolayers (SAMs) provide a simple route to functionalize electrode surfaces with organic molecules. Herein we use cavity-containing derivatives of calix[4]arenes in SAMs. Bound to noble metal surface, the assembled molecules are candidates to serve as molecular sieves for H2 molecules and H+ ions, which could have relevance for fuel cell applications. Tetra-O-alkylated calix[4]arenes with thiolacetate and thiolamide wide-rim anchoring groups in cone and partial-cone conformations were designed, synthesized and self-assembled onto Au, Pt, and Pd surfaces. The resulting SAMs were systematically examined. Single crystal X-ray diffraction of 5,11,17,23-tetrakis(thioacetyl)-25,26,27,28-tetra-i-propoxycalix[4]arene confirmed the cone conformation and revealed the cavity dimensions of the SAMs that were formed by immersing noble metal substrates (Au, Pt and Pd deposited on Si-wafers) in solutions of calix[4]arenes. Surface characterization techniques including ellipsometry, cyclic voltammetry (CV...

Experimental and theoretical identification of valence energy levels and interface dipole trends for a family of (oligo)phenylene-ethynylenethiols adsorbed on gold

Abstract: Metal−molecule−metal junctions composed of organic molecular wires formed via self-assembly are of relevance in the empirical evaluation of single-molecule electronics. Key to understanding the effects of these monolayer structures on the transport through single molecules, however, is discerning how the molecular electronic levels evolve under the influence of the metal substrate and intermolecular interactions. We present a joint experimental and computational investigation of the electronic structure and electrostatic properties of a series of self-assembled donor- and acceptor-substituted (oligo)pheneylene-ethynylenethiols (OPEs) on gold. Photoemission spectroscopy is employed to determine the energy-level alignment for these monolayers. Isolated molecule and small cluster calculations are performed to assess changes in geometry, electronic structure, and charge distribution upon chemisorption. The calculated densities of electronic states allow assignment of the higher-lying occupied states mapped by...

Electronic switching, memory, and sensor devices from carbon sheets on dielectric materials

Abstract: Electronic devices comprising a dielectric material, at least one carbon sheet, and two electrode terminals are described herein. The devices exhibit non-linear current-versus-voltage response over a voltage sweep range in various embodiments. Uses of the electronic devices as two- terminal memory devices, logic units, and sensors are disclosed. Processes for making the electronic devices are disclosed. Methods for using the electronic devices in analytical methods are disclosed.

Chemical Reactions in Monolayer Aromatic Films on Silicon Surfaces

Abstract: The formation of thin films on solid surfaces can be limited by the structures of the molecules and protocols used to form the films. These limitations can be mitigated by performing additional che...

Controlling transistor threshold voltages using molecular dipoles

Abstract: We develop a theoretical model for how organic molecules can control the electronic and transport properties of an underlying transistor channel to whose surface they are chemically bonded. The influence arises from a combination of long-ranged dipolar electrostatics due to the molecular head-groups, as well as short-ranged charge transfer and interfacial dipole driven by equilibrium band-alignment between the molecular backbone and the reconstructed semiconductor surface atoms.

Vertically-stacked electronic devices having conductive carbon films

Abstract: Vertically-stacked electronic devices having conductive carbon films are disclosed. The vertically-stacked devices exhibit non-linear current-versus-voltage response over a voltage sweep range in various embodiments. The vertically-stacked devices may be assembled into arrays where the vertically-stacked devices may be electrically addressed independently of one another. Uses of the vertically-stacked electronic devices and arrays as two-terminal memory devices, logic units, and sensors are disclosed. Crossbar arrays of vertically-stacked electronic devices having conductive carbon films and nanowire electrodes are disclosed.

Synthesis of a New Photoactive Nanovehicle: A Nanoworm.

Abstract: A nanovehicle with a new photoactive moiety has been synthesized. The incorporation of the azobenzene chassis allows for potential wormlike movement accompanying the rolling behavior of the wheels. Two versions, the fullerene-wheeled and carborane-wheeled nanoworms, were synthesized to examine the solution-based photoisomerization yields of the chassis.