Ernesto Joselevich

Covalently functionalized nanotubes as nanometre- sized probes in chemistry and biology

Carbon nanotubes combine a range of properties that make them well suited for use as probe tips in applications such as atomic force microscopy (AFM). Their high aspect ratio, for example, opens up the possibility of probing the deep crevices that occur in microelectronic circuits, and the small effective radius of nanotube tips significantly improves the lateral resolution beyond what can be achieved using commercial silicon tips. Another characteristic feature of nanotubes is their ability to buckle elastically,, which makes them very robust while limiting the maximum force that is applied to delicate organic and biological samples. Earlier investigations into the performance of nanotubes as scanning probe microscopy tips have focused on topographical imaging, but a potentially more significant issue is the question of whether nanotubes can be modified to create probes that can sense and manipulate matter at the molecular level. Here we demonstrate that nanotube tips with the capability of chemical and biological discrimination can be created with acidic functionality and by coupling basic or hydrophobic functionalities or biomolecular probes to the carboxyl groups that are present at the open tip ends. We have used these modified nanotubes as AFM tips to titrate the acid and base groups, to image patterned samples based on molecular interactions, and to measure the binding force between single protein–ligand pairs. As carboxyl groups are readily derivatized by a variety of reactions, the preparation of a wide range of functionalized nanotube tips should be possible, thus creating molecular probes with potential applications in many areas of chemistry and biology.

Stacking and Registry Effects in Layered Materials: The Case of Hexagonal Boron Nitride

The interlayer sliding energy landscape of hexagonal boron nitride (h-BN) is investigated via a van der Waals corrected density functional theory approach. It is found that the main role of the van der Waals forces is to anchor the layers at a fixed distance, whereas the electrostatic forces dictate the optimal stacking mode and the interlayer sliding energy. A nearly free-sliding path is identified, along which band gap modulations of ∼0.6  eV are obtained. We propose a simple geometric model that quantifies the registry matching between the layers and captures the essence of the corrugated h-BN interlayer energy landscape. The simplicity of this phenomenological model opens the way to the modeling of complex layered structures, such as carbon and boron nitride nanotubes.

Guided growth of millimeter-long horizontal nanowires with controlled orientations.

Long, horizontal gallium nitride nanowires are controllably grown on different faces of a sapphire substrate. The large-scale assembly of nanowires with controlled orientation on surfaces remains one challenge preventing their integration into practical devices. We report the vapor-liquid-solid growth of aligned, millimeter-long, horizontal GaN nanowires with controlled crystallographic orientations on different planes of sapphire. The growth directions, crystallographic orientation, and faceting of the nanowires vary with each surface orientation, as determined by their epitaxial relationship with the substrate, as well as by a graphoepitaxial effect that guides their growth along surface steps and grooves. Despite their interaction with the surface, these horizontally grown nanowires display few structural defects, exhibiting optical and electronic properties comparable to those of vertically grown nanowires. This paves the way to highly controlled nanowire structures with potential applications not available by other means.

Torsional electromechanical quantum oscillations in carbon nanotubes

Carbon nanotubes1,2 can be distinctly metallic or semiconducting depending on their diameter and chirality3. Here we show that continuously varying the chirality by mechanical torsion4 can induce conductance oscillations, which can be attributed to metal–semiconductor periodic transitions. The phenomenon is observed in multiwalled carbon nanotubes, where both the torque5 and the current are shown to be carried predominantly by the outermost wall6,7. The oscillation period with torsion is consistent with the theoretical shifting8 of the corners of the first Brillouin zone of graphene across different sub-bands allowed in the nanotube. Beyond a critical torsion, the conductance irreversibly drops due to torsional failure, allowing us to determine the torsional strength of carbon nanotubes. Carbon nanotubes could be ideal torsional springs for nanoscopic pendulums4,9,10, because electromechanical detection of motion could replace the microscopic detection techniques used at present. Our experiments indicate that carbon nanotubes could be used as electronic sensors of torsional motion in nanoelectromechanical systems11.

Carbon Nanotube Synthesis and Organization

The synthesis, sorting and organization of carbon nanotubes are major challengestoward future applications. This chapter reviews recent advances in these topics,addressing both the bulk production and processing of carbon nanotubes, and theirorganization into ordered structures, such as fibers, and aligned arrays on surfaces.The bulk synthetic methods are reviewed with emphasis on the current advancestoward mass production and selective synthesis. New approaches for the sorting ofcarbon nanotubes by structure and properties are described in the context of thespecific physical or chemical interactions at play, and referring to the characterizationmethods described in the contribution by Jorio et al. Recent advances in theorganization of carbon nanotubes into fibers are reviewed, including methodsbased on spinning from solution, from dry forests, and directly from the gasphase during growth. The organization of carbon nanotubes on surfaces,as a critical prerequisite toward future applications in nanoelectronics, isreviewed with particular emphasis given to the synthesis of both vertically andhorizontally aligned arrays. Vertically aligned growth has been recently boosted bythe development of highly efficient catalytic processes. Horizontally alignedgrowth on surfaces can yield a whole new array of carbon-nanotube patterns,with interesting physical properties and potential applications. Differentmechanisms of horizontally aligned growth include field- and flow-directedgrowth, as well as recently developed methods of surface-directed growth onsingle-crystal substrates by epitaxial approaches. The proposed mechanismspertinent to each technique are discussed throughout this review, as well astheir potential applications and critical aspects toward future progress.

Field-Effect Transistors Based on WS2 Nanotubes with High Current-Carrying Capacity

We report the first transistor based on inorganic nanotubes exhibiting mobility values of up to 50 cm(2) V(-1) s(-1) for an individual WS2 nanotube. The current-carrying capacity of these nanotubes was surprisingly high with respect to other low-dimensional materials, with current density at least 2.4 × 10(8) A cm(-2). These results demonstrate that inorganic nanotubes are promising building blocks for high-performance electronic applications.

Functionalization of carbon nanotube AFM probes using tip-activated gases

Abstract Multiwalled carbon nanotube (MWNT) probe microscopy tips have been functionalized with gases, activated in a transient arc produced at the tip ends. Adhesion measurements between these tips and hydroxyl-terminated monolayers versus pH reveal that MWNT tips reacted in O 2 , H 2 and N 2 display acidic, pH-independent and basic properties, respectively. MWNT tips derivatized in O 2 /N 2 and H 2 /N 2 mixtures demonstrate the greater reactivity of carbon nanotubes towards O 2 and H 2 , respectively. Moreover, the chemical reactivity of tips functionalized in N 2 suggests termination in aromatic nitrogen heterocycles. Tip-activated gas functionalization of MWNTs provides facile generation of chemically sensitive nanoprobes.

Guided growth of horizontal ZnO nanowires with controlled orientations on flat and faceted sapphire surfaces.

The large-scale integration of nanowires into practical devices is hindered by the limited ability to controllably assemble these nanoscale objects on surfaces. Following our first report on the guided growth of millimeter-long horizontal nanowires with controlled orientations, here we demonstrate the generality of the guided growth approach by extending it from GaN nanowires to ZnO nanowires. We describe the guided growth of horizontally aligned ZnO nanowires with controlled crystallographic orientations on eight different planes of sapphire, including both flat and faceted surfaces. The growth directions, crystallographic orientation, and faceting of the nanowires are constant for each surface plane and are determined by their epitaxial relation with the substrate, as well as by a graphoepitaxial effect that guides their growth along surface steps and grooves. These horizontal ZnO nanowires exhibit optical and electronic properties comparable to those of vertically grown nanowires, indicating a low concentration of defects. While the guided growth of ZnO nanowires described here resembles the guided growth of GaN nanowires in its general aspects, it also displays notable differences and qualitatively new phenomena, such as the controlled growth of nanowires with vicinal orientations, longitudinal grain boundaries, and thickness-dependent orientations. This article proves the generality of the guided growth phenomenon, which enables us to create highly controlled nanowire structures and arrays with potential applications not available by other means.

Ultrahigh Torsional Stiffness and Strength of Boron Nitride Nanotubes

We report the experimental and theoretical study of boron nitride nanotube (BNNT) torsional mechanics. We show that BNNTs exhibit a much stronger mechanical interlayer coupling than carbon nanotubes (CNTs). This feature makes BNNTs up to 1 order of magnitude stiffer and stronger than CNTs. We attribute this interlayer locking to the faceted nature of BNNTs, arising from the polarity of the B-N bond. This property makes BNNTs superior candidates to replace CNTs in nanoelectromechanical systems (NEMS), fibers, and nanocomposites.

Modulating the electronic properties along carbon nanotubes via tube-substrate interaction.

We study single wall carbon nanotubes (SWNTs) deposited on quartz. Their Raman spectrum depends on the tube-substrate morphology, and in some cases, it shows that the same SWNT-on-quartz system exhibits a mixture of semiconductor and metal behavior, depending on the orientation between the tube and the substrate. We also address the problem using electric force microscopy and ab initio calculations, both showing that the electronic properties along a single SWNT are being modulated via tube-substrate interaction.