Measurements of the work function of single-walled carbon nanotubes encapsulated by AgI,AgCl, and CuBr using kelvin probe technique with different kinds of probes

We report the results on the measurements of the work function of single-walled carbon nanotubes encapsulated by Agl (AgI@SWCNT), AgCl (AgCl@SWCNT), and CuBr (CuBr@SWCNT) by the local Kelvin probe technique. We found the values of the work function of tubes encapsulated with AgI and AgCl (Φ(AgI@SWCNT) = 5.08 ± 0.02, Φ(AgCl@SWCNT) = 5.10 ± 0.02 eV) to exceed substantially that of pristine carbon nanotubes, and the value of the work function of carbon nanotubes encapsulated with CuBr is Φ(CuBr@SWCNT) = 4.89 ± 0.03 (eV). The measurements are carried out using different kinds of microscope probes including multi-walled carbon nanotube tips.     

Work function of single-walled carbon nanotubes determined by field emission microscopy

The field emission characteristics of a single micro-bundle of single-walled carbon nanotubes (SWCNTs) were investigated using field emission microscopy (FEM). Fowler–Nordheim plots revealed that the work function of the SWCNTs was reduced with increasing heating temperature, and reached a minimum value around 1000 °C, assuming that the β factor was constant during the heating process. Field emission patterns also demonstrated fine structures that were believed to be images of the cap of a SWCNT, which was in a clean state. The radius of the SWCNT micro-bundle was measured by transmission electron microscopy (TEM), and the β factor was calculated using two empirical formulae. Then, the work function of the SWCNT was determined from the slope, K, of its Fowler–Nordheim plot. The work function values were Φ₁=4.76 eV and Φ₂=4.88 eV, respectively. Received: 26 October 2001 / Revised version: 19 February 2002 / Published online: 6 June 2002     

Surface exciton-plasmons and optical response of small-diameter carbon nanotubes

We study theoretically the interactions of excitonic states with surface electromagnetic modes of small-diameter (≲1 nm) semiconducting single-walled carbon nanotubes. We show that these interactions can result in strong exciton-surface-plasmon coupling. The exciton absorption lineshape exhibits the line (Rabi) splitting ∼0.1–0.3 eV as the exciton energy is tuned to the nearest interband surface plasmon resonance of the nanotube so that the mixed strongly coupled surface plasmon-exciton excitations are formed. We discuss possible ways to bring the exciton in resonance with the surface plasmon. The exciton-plasmon Rabi splitting effect we predict here for an individual carbon nanotube is close in its magnitude to that previously reported for hybrid plasmonic nanostructures artificially fabricated of organic semiconductors deposited on metallic films. We expect this effect to open up paths to new tunable optoelectronic device applications of semiconducting carbon nanotubes.     

X-ray absorption investigation of the electronic structure of the CuI@SWCNT nanocomposite

The Cu 2p, I 3d, and C 1sX-ray absorption spectra of the CuI@SWCNT nanocomposite prepared by filling single-walled carbon nanotubes (SWCNTs) with the CuI melt by the capillary technique have been measured with a high-energy resolution using the equipment of the Russian-German beamline at the BESSY electron storage ring. In order to characterize the electronic structure of the nanocomposite and possible changes in the atomic and electronic structures of CuI and SWCNTs in the CuI@SWCNT nanocomposite, the spectra obtained have been analyzed in the framework of the quasi-molecular approach by comparing with the spectra of the pristine (CuI and SWCNT) and reference (CuO) systems. It has been revealed that the encapsulation of the CuI compound inside SWCNTs is accompanied by changes in the electronic structure of CuI and SWCNTs due to the chemical interaction between the filler and carbon nanotubes and the change in the atomic structure of CuI.     

Comparative X-ray absorption investigation of fluorinated single-walled carbon nanotubes

The C 1s and F 1s X-ray absorption spectra of pristine and fluorinated single-walled carbon nanotubes with different fluorine contents and nanodiamond as a reference compound have been measured with the aim of characterizing single-walled carbon nanotubes and their products formed upon treatment of the nanotubes with molecular fluorine at a temperature of 190°C. The spectra obtained have been analyzed by thoroughly comparing with the previously measured spectra of highly oriented pyrolytic graphite and fluorinated multiwalled carbon nanotubes and the spectrum of nanodiamond. It has been established that the fluorination of single-walled and multiwalled carbon nanotubes leads to similar results and is characterized by the attachment of fluorine atoms to carbon atoms on the lateral surface of the nanotube with the formation of the σ(C-F) bonds due to the covalent mixing of F 2p and C 2p _z π valence electron states.     

Structure of connections of single-walled carbon nanotubes with the use of the combined 5–7 and 4–8 topological defects

The structures of zigzag-zigzag, armchair-zigzag, zigzag-chiral, armchair-armchair, armchair-chiral, and chiral-chiral pair connections produced by single-walled carbon nanotubes 1.5–5.0 diameter with the use of the combined 5–7 and 4–8 topological defects have been calculated by molecular mechanics methods. It has been established that the use of the combined 5–7 topological defect makes it possible to produce connections between pairs of single-walled carbon nanotubes with any conductivities, chiralities, and diameters, whereas the use of the combined 4–8 topological defect provides a means for forming connections between nanotubes only with the same type of conductivity. The angles between the axes of nanotubes connected by the combined 5–7 and 4–8 topological defects lie in the ranges 145°–180° and 112°–178°, respectively. It has been revealed that there are correlations between structural parameters of the connections and the relative arrangement of the simple topological defects in the combined topological defects.     

Non-covalent interaction of benzonitrile with single-walled carbon nanotubes

We have studied the interaction of benzonitrile with as-prepared and purified single-walled carbon nanotubes (SWCNTs). As-prepared SWCNTs, when suspended in benzonitrile, lead to a red colored dispersion which contains fragments composed mostly of amorphous carbon and carbon-coated catalyst, thus suggesting that benzonitrile is a solvent that can be used as one step of the purification process. Optical spectroscopic data (infrared, Raman, absorption) showed that purified carbon nanotubes interact weakly with benzonitrile. These experimental results are confirmed by first principles calculations that predict a very weak adsorption process through π–π interaction instead of through the free electron pair of the nitrile.     

Filling of single-walled carbon nanotubes by CuI nanocrystals via capillary technique

The present study is focused on the synthesis and investigation of the nanocomposite CuI@SWNT obtained by the filling of metallic single-walled carbon nanotubes (SWNTs) (inner diameter 1–1.4 nm) by wide-gap semiconducting CuI nanocrystals using so-called capillary technique. The method is based on the impregnation of pre-opened SWNTs by molten CuI in vacuum with subsequent slow cooling to room temperature. SWNTs and CuI@SWNT nanocomposites were studied by nitrogen capillary adsorption method, EDX microanalysis, HRTEM microscopy and Raman spectroscopy. The changing of electronic properties of CuI@SWNT as compare to row nanotubes was observed.     

Carbon Nanotube/P(VDF-TrFE) Nano-Composite Characterization by Total Reflection X-Ray Fluorescence

In continuation of our research on carbon nanotube/P(VDF-TrFE) nano-composites [1], total x-ray fluorescence (TXRF) is engaged in a novel characterization of these materials regarding their compositions, purities, and structural analysis. Samples such as single-walled carbon nanotubes (SWCNT), multi-walled carbon nanotubes (MWCNT), P(VDF-TrFE) copolymer, SWCNT/P(VDF-TrFE), and MWCNT/P(VDF-TrFE) were analyzed by TXRF. The synthetic quartz used as a substrate was analyzed as reference material for the TXRF measurements. The ethanol and the dimethylformamide (DMF) used as solvents for carbon nanotubes and copolymers respectively were also analyzed by TXRF to determine whether they have an influence or not on the TXRF of the previous material. The preliminary results showed that single-walled and multi-walled carbon nanotubes prepared by the arc-discharge method contain catalytic particles such as Fe, Co, and Ni used to obtain SWCNT while there were no metal or impurities in MWCNT. The TXRF spectrum of CNT/P(VDFTrFE) showed the same results as we found previously with background due to the P(VDF-TrFE) copolymer scattered signal. __________ Published in Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 5, pp. 700–702, September–October, 2005.     

Fabrication of ceramic oxide-coated SWNT composites by sol–gel process with a polymer glue

The functional copolymer bearing alkoxysilyl and pyrene groups, poly[3-(triethoxysilyl)propyl methacrylate]-co-[(1-pyrene-methyl) methacrylate] (TEPM₁₃-co-PyMMA₃), was synthesized via atom transfer radical polymerization. Attributing the π–π interaction of pyrene units with the walls of single-walled carbon nanotubes (SWNTs), this polymer could disperse and exfoliate SWNTs in different solvents through physical interaction as demonstrated by TEM, UV/Vis absorption, and FT-IR analysis. The alkoxysilyl groups functionalized SWNTs were reacted with different inorganic precursors via sol–gel reaction, and, as a results, silica, titania, and alumina were coated onto the surface of SWNTs, respectively via copolymers as a molecular glue. The nanocomposites of ceramic oxides/SWNTs were characterized by SEM analysis. Dependent upon the feed, the thickness of inorganic coating can be tuned easily. This study supplies a facile and general way to coat SWNTs with ceramic oxides without deteriorating the properties of pristine SWNTs.     

Femtosecond luminescence decay due to exciton energy transfer in single-walled carbon nanotube bundles

We investigated luminescence decay kinetics in single-walled carbon nanotubes in large bundles excited by femtosecond pulses. The time constant of luminescence decay becomes longer with decrease in photon energy: 40 fs at 1.2 eV and 380 fs at 0.6 eV. This behavior is explained by exciton energy transfer from an excited to neighboring tubes.     

Electrical conduction of carbon nanotubes due to the migration of protons over their surface

The results of the theoretical calculation of the proton conductivity tensor for single-walled carbon nanotubes are presented. The electronic structure of the objects under investigation is considered within the band model in the approximation of an ideal proton Fermi gas. The temperature dependence of the proton contribution to the static longitudinal conductivity σ of the carbon nanotube is obtained using the Green’s function method. The function σ(T) for all the carbon nanotubes under consideration exhibits a characteristic behavior that is inherent in conductors. The differences in the conductivity curves for zigzagtype carbon nanotubes with the chirality indices both multiple and not multiple of three are predicted.     

Effect of metal oxide and oxygen on the growth of single-walled carbon nanotubes by electric arc discharge

The effect of oxygen on the growth of single-walled carbon nanotubes was studied with Ni–Co alloy powder as catalyst under helium atmosphere of 500 Torr by electric arc discharge. The oxygen included in nickel or (and) cobalt oxides was added in catalyst. The content of oxygen in atmosphere was controlled by changing vacuum degree inside furnace before inputting buffer gas. The examinations of TEM and Raman scattering showed that oxygen in metal oxide as catalyst promotes the nucleation of SWCNT by taking effect on the metal catalyst particles. However, O₂ in atmosphere has the role of oxidizing amorphous particles along with nanotubes. When its molar proportion is higher than 0.22 ppm (Parts per million), the carbon nanotubes produced are oxidized and their purity decreases. The diameter of single-walled carbon nanotube obtained under different condition has a narrow distribution around 1.28 nm.     

Raman scattering of light and IR absorption in carbon nanotubes

Raman light scattering and IR absorption spectra of samples containing multilayer carbon nanotubes in different stages of purification by the selective oxidation technique have been investigated. It was found that the Raman spectra of carbon nanotubes exhibit softening of the mode at 1582 cm⁻¹ corresponding to E _2g vibrations of graphite hexagons and a line at 120 cm⁻¹ due to the radial vibrations of nanotubes. In IR absorption spectra measured in the region of 0.07–0.3 eV, several sets of lines with a spacing of 15 meV (120 cm⁻¹) between lines of each group have been detected. We suggest that each group corresponds to electron transitions generating electron-hole pairs in semiconducting nanotubes and contains a phononless 00-line and its phonon replicas with spacing between them equal to the “breathing” mode energy of 120 cm⁻¹. Measurements of electric conductivity at a frequency of 9300 MHz indicate that, in addition to semiconducting nanotubes, the samples contain nanotubes with properties of a highly disordered semimetal. Zh. éksp. Teor. Fiz. 113, 1883–1891 (May 1998)     

Modeling the binding of peptides on carbon nanotubes and their use as protein and DNA carriers

An in-depth study of a novel functionalization of carbon nanotubes for their application as protein and DNA carriers is presented. First, the optimum conditions for the dispersion of single-walled carbon nanotubes (SWCNTs) with amphiphilic polypeptides were obtained, and the SWCNT–polypeptide complexes were characterized by different techniques (UV–Vis-NIR, CD, and AFM). Based on the properties of the SWCNT–polypeptide complexes, a model that characterizes the adsorption of natural proteins onto SWCNT was described for the first time. This model predicts the adsorption of natural proteins on SWCNTs based on the protein structure and composition, and therefore, allows the design of methods for the preparation of SWCNT–protein complexes. Besides, the use of cationic-designed amphiphilic polypeptides to disperse SWCNTs is applied for subsequent and efficient binding of DNA to carbon nanotubes by a bilayer approach. Therefore, in this article, we develop procedures for the use of SWCNTs as protein and DNA carriers. The systems were delivered into cells showing that the efficiency of delivery is affected by the charge of the complexes, which has important implications in the use of SWCNT as platforms for protein and DNA binding and subsequent use as delivery systems.     

Geometric structure and electronic properties of planar and nanotubular BN structures of the Haeckelite type

Planar and nanotubular structures that are based on boron and nitrogen and consist of tetragons, hexagons, and octagons are considered. By analogy with carbon nanoobjects of the same topology, these structures are referred to as Haeckelites. The geometric, electronic, and energy properties are thoroughly investigated for two variants of the regular mutual arrangement of the polygons. It is established that planar and nanotubular BN structures of the Haeckelite type are dielectrics with a band gap E _g ∼ 3.2–4.2 eV, which is less than the band gap E _g for BN nanotubes consisting only of hexagons. The cohesive energy of the BN nanotubes under investigation exceeds the cohesive energy of BN hexagonal nanotubes by 0.3 eV/atom.     

N-doping and coalescence of carbon nanotubes: synthesis and electronic properties

Self-assembly pyrolytic routes to large arrays (<2.5 cm²) of aligned CN_x nanotubes (15–80 nm OD and <100 μm in length) are presented. The method involves the thermolysis of ferrocene/melamine mixtures (5:95) at 900–1000 °C in the presence of Ar. Electron energy loss spectroscopy (EELS) reveals that the N content varies from 2–10%, and can be bonded to C in two different fashions (double-bonded and triple-bonded nitrogen). The electronic densities of states (DOS) of these CN_x nanotubes, using scanning tunneling spectroscopy (STS), are presented. The doped nanotubes exhibit strong features in the conduction band close to the Fermi level (0.18 eV). Using tight-binding and ab initio calculations, we confirm that pyridine-like (double-bonded) N is responsible for introducing donor states close to the Fermi Level. These electron-rich structures are the first example of n-type nanotubes. Finally, it will be shown that moderate electron irradiation at 700–800 °C is capable of coalescing single-walled nanotubes (SWNTs). The process has also been studied using tight-binding molecular dynamics (TBMD). Vacancies induce the coalescence via a zipper-like mechanism, which has also been observed experimentally. These vacancies trigger the organization of atoms on the tube lattices within adjacent tubes. These results pave the way to the fabrication of nanotube heterojunctions, robust composites, contacts, nanocircuits and strong 3D composites using N-doped tubes as well as SWNTs. Received: 10 October 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

Imaging as-grown single-walled carbon nanotubes originated from isolated catalytic nanoparticles

Discrete catalytic nanoparticles with diameters in the range of 1–3 nm are obtained by placing controllable numbers of metal atoms into the cores of apoferritin. With nanoparticles placed on transmission electron microscope (TEM) grids coated with ultra-thin alumina membranes, isolated single-walled carbon nanotubes are grown by chemical vapor deposition and directly examined by TEM. The characterizations, carried out at single-tube and single-particle level, obtain clear evidence that the diameters of the nanotubes are determined by the diameters of catalytic nanoparticles. For the first time, both ends of an as-grown single-walled nanotube are imaged by TEM, leading to a microscopic picture of the nanotube-growth mechanism. Received: 19 September 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

On the role of interband surface plasmons in carbon nanotubes

We review the properties of collective surface excitations—excitons and interband plasmons—in single-walled and double-walled carbon nanotubes. We show that an electrostatic field applied perpendicular to the nanotube axis can control the exciton-plasmon coupling in individual small-diameter (≲nm) singlewalled nanotubes, both in the linear excitation regime and in the non-linear excitation regime with the photoinduced biexcitonic states formation. For double-walled carbon nanotubes, we report a profound effect of interband surface plasmons on the inter-tube Casimir force at tube separations similar to their equilibrium distances. Strong overlapping plasmon resonances from both tubes warrant their stronger attraction. Nanotube chiralities possessing such collective excitation features will result in forming the most favorable innerouter tube combination in double-walled carbon nanotubes. These findings pave the way for the development of new generation of tunable optoelectronic and nano-electromechanical device applications with carbon nanotubes.     

The role of structural inhomogeneities in the temperature behavior of the thermopower in metallized nanotubes with impurities

High values of the thermopower and its derivative with respect to temperature in metallized carbon nanotubes at low temperatures are shown to result from the following factors: structural inhomogeneities and modification of the electronic structure both due to local structural regions of disrupted long-range order and to electron-electron interaction. The former factor plays an important role in carbon-nanotube bundles, whereas the latter is of substantial significance in single-walled nanotubes. Statistical inhomogeneities and all broken bonds are essential for electron scattering that gives rise to electron transport in metallized carbon nanotubes. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 31–41, March, 2009.