I. Alexandrou

High open-circuit voltage photovoltaic devices from carbon-nanotube-polymer composites

Organic photovoltaic devices based on the bulk heterojunction concept, containing a blend of single-wall carbon nanotubes (SWNTs) and soluble polythiophenes (P3OT) were studied. The open circuit voltage Voc of the devices was found to be 0.75 V, which is larger than the theoretical limit calculated by the metal–insulator–metal (MIM) model. In order to investigate the origin of this unusually high Voc, we have prepared P3OT–SWNT based devices with different metal negative electrodes. The Voc measured is only very weakly dependent on the work function of the metal, suggesting that the MIM model does not apply in this case. From the analysis of the current–voltage characteristics and electron microscopy imaging of the composite structure, it is proposed that the photovoltaic response of these devices is based on the introduction of internal polymer/nanotube junctions within the polymer matrix, which due to a photoinduced electron transfer from the polymer to the nanotube contribute to enhanced charge separat...

Properties of carbon onions produced by an arc discharge in water

A simple method to fabricate high-quality nanoparticles including spherical carbon onions and elongated fullerene-like nanoparticles similar to nanotubes in large quantities without the use of vacuum equipment is reported. The nanoparticles are obtained in the form of floating powder on the water surface following an arc discharge between two graphite electrodes submerged in water. High-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy images confirm the presence of spherical carbon onions with diameters ranging from 4 to 36 nm. The specific surface area of the floating powder was found to be very large, 984.3 m2/g, indicating that the material is promising for gas storage. From the surface area measurements, the mean particle diameter was calculated to be 3.7 nm. This value is close to the lower limit of the carbon onions observed in HRTEM. However, closer HRTEM observations also reveal that some carbon onions are not well crystallized. The large specific surface area c...

Characterisation of carbon nano-onions using Raman spectroscopy

Characteristics of the Raman spectrum from carbon onions have been identified in terms of the position of the G peak and appearance of the transverse optic phonon peaks. Five new peaks were observed in the low wavenumber region, at about 1100, 861, 700, 450 and 250 cm−1. The origins of these peaks are discussed in terms of the phonon density of states (PDOS) and phonon dispersion curves of graphite. The curvature of the graphene planes is invoked to explain the relaxation of the Raman selection rules and the appearance of the new peaks. The Raman spectrum of carbon onions is compared with that of highly oriented pyrolytic graphite (HOPG). The strain of graphene planes due to curvature has been estimated analytically and is used to account for the downward shift of the G peak.

Temperature selective growth of carbon nanotubes by chemical vapor deposition

Randomly oriented and vertically aligned carbon nanotubes were synthesized by chemical vapor deposition varying the growth temperature between 550 °C and 850 °C. Transmission electron microscopy (TEM) was used to investigate the growth process and in particular the role of the Ni catalyst. It was found that a catalytic particle was always situated at the tip of the tube thus implying a tip growth mechanism. The TEM analysis revealed a strong correlation between the size and shape of the Ni particle encapsulated at the tip of the tubes and the growth temperature, suggesting that temperature is an influential process parameter in determining an optimum size and shape for the catalytic particle and hence select the diameter, and the wall thickness of the growing tubes.

Carbon films with ansp2network structure

Laser-Arc evaporation of a graphite target has been used to deposit carbon films that exhibit high hardness (45 GPa) and elastic recovery (85%). High Resolution Electron Microscopy (HREM) and Electron Energy Loss Spectroscopy (EELS) were subsequently used to study the microstructure and bonding of the resultant layers. The structure of the films from HREM is seen to consist of a dense array of parallel curved graphene sheet segments packed in various orientations. EELS reveals that the films are comprised of mainly sp2 bonded carbon. The results suggest that a new form of carbon thin film with fullerene-like structure can be realised. In order to explain how a predominantly sp2 bonded material can exhibit such a high hardness, a simple model is proposed to correlate the excellent mechanical properties with the observed structure.

Polymer–nanotube composites: Burying nanotubes improves their field emission properties

Composite structures of single walled carbon nanotubes (SWNTs) and poly(3-octylthiophene) polymer were tested as field emitters. The structures studied were formed on n+-Si by mixing the two materials after they had been dissolved in chloroform. Three different structures were tested: (a) a SWNT mat, (b) a SWNT mat on a SWNT (30 wt %)–polymer composite and (c) a SWNT mat on pure polymer. Although the conductivity of the composite is very much lower than that of the nanotube mat, experiments show that the field emission threshold is lower and the emission current higher from structure (b) compared to the pure SWNT mat on n+-Si. A comparison between the field enhancement factors and the expected aspect ratios shows that enhancement due to an additional field enhancement mechanism other than that due to the geometrical shape of the SWNTs must exist. We propose that the field due to the surface potential modification at the triple junction between SWNTs, polymer and vacuum is adequate to create additional fie...

Fabrication of inorganic molybdenum disulfide fullerenes by arc in water

Closed caged fullerene-like molybdenum disulfide (MoS2) nano-particles were obtained via an arc discharge between a graphite cathode and a molybdenum anode filled with microscopic MoS2 powder submerged in de-ionized water. A statistical study of over 150 polyhedral fullerene-like MoS2 nano-particles in plan view transmission electron microscopy revealed that the majority consisted of 2–3 layers with diameters of 5–15 nm. We show that the nano-particles are formed by seamless folding of MoS2 sheets. A model based on the agglomeration of MoS2 fragments over an extreme temperature gradient around a plasma ball in water is proposed to explain the formation of nano-particles. 2002 Elsevier Science B.V. All rights reserved.

Generation and deposition of fullerene- and nanotube-rich carbon thin films

We report the generation of fullerenes and nanotubes using an arc discharge on graphite in a high-pressure nozzle. The vapour from the arc is quenched via collisions with helium gas, forming carbon clusters within a localized highpressure region. The carbon molecules are entrained in the gas jet as it expands into the vacuum and deposited onto a silicon substrate in a low-pressure (6 mT) environment. Mass spectroscopy measurements of the plasma reveal the presence of C molecules in the expanding plume. Microstructural examination of films deposited using this method revealed clustered regions of larger fullerenes and nanotubes surrounded by an amorphous matrix. Films containing fullerenes and nanotubes were found to be significantly harder and more elastic than amorphous carbon films deposited under identical parameters but without conditions for fullerene and/or nanotube formation.

Structural investigation of MoS2 core-shell nanoparticles formed by an arc discharge in water

The intense environment of an electric arc has been used to create MoS2 core–shell particles. Instead of running the arc in a vacuum chamber under reduced pressure conditions, an arc discharge in water was employed. The arc was run between a carbon cathode and a Mo hollow rod anode whose interior was packed with MoS2 powder. High-resolution electron microscopy and image simulations have been used to characterize the structure of the resultant material which was seen to consist of agglomerates of polyhedral MoS2 cages with filled cores. A correlation between the growth conditions and the observed nanostructure is presented.

Field emission properties of nanocomposite carbon nitride films

A modified cathodic arc technique has been used to deposit carbon nitride thin films directly on n+ Si substrates. Transmission electron microscopy showed that clusters of fullerene-like nanoparticles are embedded in the deposited material. Field emission in vacuum from as-grown films starts at an electric field strength of 3.8 V/μm. When the films were etched in an HF:NH4F solution for 10 min, the threshold field decreased to 2.6 V/μm. The role of the carbon nanoparticles in the field emission process and the influence of the chemical etching treatment are discussed.