Probing differential optical and coverage behavior in nanotube-nanocrystal heterostructures synthesized by covalent versus non-covalent approaches.

Double-walled carbon nanotube (DWNT)-CdSe heterostructures with the individual nanoscale building blocks linked together by 4-aminothiophenol (4-ATP) have been successfully synthesized using two different and complementary routes, i.e. covalent attachment and non-covalent π-π stacking.

Carbon nanotube-based heterostructures for solar energy applications.

One means of combining the unique physical and chemical properties of both carbon nanotubes and complementary material motifs (such as metal sulfide quantum dots (QDs), metal oxide nanostructures, and polymers) can be achieved by generating carbon nanotube (CNT)-based heterostructures. These materials can be subsequently utilized as novel and interesting constituent building blocks for the assembly of functional light energy harvesting devices and because of their architectural and functional flexibility, can potentially open up novel means of using and taking advantage of existing renewable energy sources. In this review, we present the reliable and reproducible synthesis of several unique model CNT-based heterostructured systems as well as include an accompanying discussion about the charge transfer and energy flow properties of these materials for their potential incorporation into a range of practical solar energy conversion devices.

Efficient charge separation in multidimensional nanohybrids.

We report unidirectional charge transfer in multidimensional nanohybrids, consisting of a quantum dot, an electronically active molecular linker, and a carbon nanotube. After covalent attachment to the nanotube, only emission consistent with the negatively charged quantum dot exciton ion rather than the neutral exciton is observed, showing nearly monoexponential recombination kinetics and an average lifetime of 3.5 ns.

The evolution of electronic structure in few-layer graphene revealed by optical spectroscopy.

The massless Dirac spectrum of electrons in single-layer graphene has been thoroughly studied both theoretically and experimentally. Although a subject of considerable theoretical interest, experimental investigations of the richer electronic structure of few-layer graphene (FLG) have been limited. Here we examine FLG graphene crystals with Bernal stacking of layer thicknesses N = 1,2,3,.

Carbon nanotube-nanocrystal heterostructures.

The importance of generating carbon nanotube-nanoparticle heterostructures is that these composites ought to take advantage of and combine the unique physical and chemical properties of both carbon nanotubes and nanoparticles in one discrete structure. These materials have potential applicability in a range of diverse fields spanning heterogeneous catalysis to optoelectronic device development, of importance to chemists, physicists, materials scientists, and engineers. In this critical review, we present a host of diverse, complementary strategies for the reliable synthesis of carbon nanotube-nanoparticle heterostructures using both covalent as well as non-covalent protocols, incorporating not only single-walled and multi-walled carbon nanotubes but also diverse classes of metallic and semiconducting nanoparticles (221 references).

X-ray nanocrystallography of individual carbon nanotubes.

By numerical simulations, we show that with the next-generation synchrotron sources one should be able to record within minutes an X-ray diffraction pattern of a single nanotube and from it decipher its detailed atomic structure. In calculated diffractograms we can even discern signatures of presence of a single adatom and locate its position. New synchrotrons will allow the existing method of electron nanocrystallography to be undertaken using X-ray beams, thus facilitating in situ environmental studies.

Measurement of the optical conductivity of graphene.

Optical reflectivity and transmission measurements over photon energies between 0.2 and 1.2 eV were performed on single-crystal graphene samples on a SiO2 substrate.

Measurement of the vector character of electric fields by optical second-harmonic generation.

We present a scheme for the determination of the vector nature of an electric field by optical second-harmonic generation. We demonstrate the technique by mapping the two-dimensional electric-field vector of a biased transmission line structure on silicon with a spatial resolution of ~10mum .

Functionalized carbon nanotubes for detecting viral proteins.

We investigated the biocompatibility, specificity, and activity of a ligand-receptor-protein system covalently bound to oxidized single-walled carbon nanotubes (SWNTs) as a model proof-of-concept for employing such SWNTs as biosensors. SWNTs were functionalized under ambient conditions with either the Knob protein domain from adenovirus serotype 12 (Ad 12 Knob) or its human cellular receptor, the CAR protein, via diimide-activated amidation. We confirmed the biological activity of Knob protein immobilized on the nanotube surfaces by using its labeled conjugate antibody and evaluated the activity and specificity of bound CAR on SWNTs, first, in the presence of fluorescently labeled Knob, which interacts specifically with CAR, and second, with a negative control protein, YieF, which is not recognized by biologically active CAR proteins.

Near-edge X-ray absorption fine structure spectroscopy as a tool for investigating nanomaterials.

We have demonstrated near-edge X-ray absorption fine structure (NEXAFS) spectroscopy as a particularly useful and effective technique for simultaneously probing the surface chemistry, surface molecular orientation, degree of order, and electronic structure of carbon nanotubes and related nanomaterials. Specifically, we employ NEXAFS in the study of single-walled carbon nanotube and multi-walled carbon nanotube powders, films, and arrays, as well as of boron nitride nanotubes. We have focused on the advantages of NEXAFS as an exciting, complementary tool to conventional microscopy and spectroscopy for providing chemical and structural information about nanoscale samples.

Imperfect surface order and functionalization in vertical carbon nanotube arrays probed by near edge X-ray absorption fine structure spectroscopy (NEXAFS).

Probing surface order as well as the degree of structural modification in carbon nanotube systems is of fundamental importance for incorporation of these materials into practical functional devices. The current study pertains to the analysis of the surface order of vertically-aligned single-walled and multi-walled carbon nanotube arrays of varying length and composition by means of near-edge X-ray fine structure spectroscopy (NEXAFS). Both NEXAFS and scanning electron microscopy (SEM) studies concluded that the nanotubes in these samples were oriented vertically to the plane of the surface.

Near-edge X-ray absorption fine structure investigations of order in carbon nanotube-based systems.

Probing order in nanotube systems is of fundamental importance in devising applications of these tubes in field emission applications as well as for components of composite materials. We use near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to qualitatively and quantitatively study the degree of order and alignment in a wide range of carbon nanotube-based systems, including single-walled carbon nanotube (SWNT) powder, SWNT films, and aligned multiwalled carbon nanotubes. The results are compared to analogous data obtained from a highly ordered pyrolytic graphite (HOPG) sample.

Optical spectroscopy of individual single-walled carbon nanotubes of defined chiral structure.

We simultaneously determined the physical structure and optical transition energies of individual single-walled carbon nanotubes by combining electron diffraction with Rayleigh scattering spectroscopy. These results test fundamental features of the excited electronic states of carbon nanotubes. We directly verified the systematic changes in transition energies of semiconducting nanotubes as a function of their chirality and observed predicted energy splittings of optical transitions in metallic nanotubes.

Current-less photoreactivity catalyzed by functionalized AFM tips.

A spatially confined photocatalytic oxidation of a thin film of synthetic textile azo dye (Procion Red MX-5B) using TiO2-functionalized AFM probes is described.

Investigating the structure of boron nitride nanotubes by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy.

Herein, we demonstrate that NEXAFS is a very effective technique at (a) identifying the phases of boron nitride nanotubes with the potential of distinguishing between hexagonal BN and cubic BN, and (b) monitoring the presence of defects and degree of crystallinity in nanoscale samples. Specifically, a prepared sample of boron nitride nanotubes was characterized by NEXAFS. Our results show that the sample consisted of hexagonal BN tubes that were highly crystalline and sp2-hybridized.

Surface chemistry and structure of purified, ozonized, multiwalled carbon nanotubes probed by NEXAFS and vibrational spectroscopies.

Understanding of oxidative processes such as solution-phase ozonolysis in multiwalled carbon nanotubes (MWNTs) is of fundamental importance in devising applications of these tubes as components in composite materials, as well as for development of cutting and filling protocols. We present here an evaluation of various spectroscopic tools to study the structure and composition of functionalized nanotubes. We demonstrate near-edge X-ray absorption fine structure (NEXAFS) spectroscopy as a particularly useful and effective technique for studying the surface chemistry of carbon nanotubes.

Ozonized single-walled carbon nanotubes investigated using NEXAFS spectroscopy.

The use of NEXAFS spectroscopy in studying the electronic structure and chemical composition of pristine, wet-air oxidized, and sidewall-ozonized nanotubes is illustrated.

Electrically induced optical emission from a carbon nanotube FET.

Polarized infrared optical emission was observed from a carbon nanotube ambipolar field-effect transistor (FET). An effective forward-biased p-n junction, without chemical dopants, was created in the nanotube by appropriately biasing the nanotube device. Electrical measurements show that the observed optical emission originates from radiative recombination of electrons and holes that are simultaneously injected into the undoped nanotube.