Harold W. Kroto

Controlled production of aligned-nanotube bundles

Carbon nanotubes might be usefully employed in nanometre-scale engineering and electronics. Electrical conductivity measurements on the bulk material, on individual multi-walled and single-walled nanotubes and on bundles of single-walled nanotubes have revealed that they may behave as metallic, insulating or semiconducting nanowires, depending on the method of production—which controls the degree of graphitization, the helicity and the diameter. Measurements of Youngs modulus show that single nanotubes are stiffer than commercial carbon fibres. Methods commonly used to generate nanotubes—carbon-arc discharge techniques, catalytic pyrolysis of hydrocarbons and condensed-phase electrolysis—generally suffer from the drawbacks that polyhedral particles are also formed and that the dimensions of the nanotubes are highly variable. Here we describe a method for generating aligned carbon nanotubes by pyrolysis of 2-amino-4,6-dichloro-s-triazine over thin films of a cobalt catalyst patterned on a silica substrate by laser etching. The use of a patterned catalyst apparently encourages the formation of aligned nanotubes. The method offers control over length (up to about 50 μm) and fairly uniform diameters (30–50 nm), as well as producing nanotubes in high yield, uncontaminated by polyhedral particles.

Multiferroic Behavior Associated with an Order−Disorder Hydrogen Bonding Transition in Metal−Organic Frameworks (MOFs) with the Perovskite ABX3 Architecture

Multiferroic behavior in perovskite-related metal-organic frameworks of general formula [(CH(3))(2)NH(2)]M(HCOO)(3), where M = Mn, Fe, Co, and Ni, is reported. All four compounds exhibit paraelectric-antiferroelectric phase transition behavior in the temperature range 160-185 K (Mn: 185 K, Fe: 160 K; Co: 165 K; Ni: 180 K); this is associated with an order-disorder transition involving the hydrogen bonded dimethylammonium cations. On further cooling, the compounds become canted weak ferromagnets below 40 K. This research opens up a new class of multiferroics in which the electrical ordering is achieved by means of hydrogen bonding.

Electronic spectra and transitions of the fullerene C60

Abstract Absorption spectra of C60 have been measured in the ranges (a) 190–700 nm in n-hexane solutions at 300 K, (b) 390–700 nm in n-hexane and in 3-methylpentane solutions at 77 K. 40 vibronic bands were observed. They exhibit a large range of bandwidths and intensities, whose significance is discussed. Assignment of electronic transitions has been carried out using the results of theoretical calculations. Vibronic structures have been analyzed within the framework of theories of electronic transitions of polyatomic molecules applied to the Ih symmetry group. Nine allowed 1T1u−1Ag transitions have been assigned in the 190–410 nm region. Observed and calculated allowed transition energies and oscillator strengths are compared. Detailed vibronic analyses of the 1 1T1u−1 1Ag and 2 1T1u−1 1Ag transitions illustrate the role of Jahn-Teller couplings. Orbitally forbidden singlet-singlet transitions are observed between 410 and 620 nm. Their vibronic structures were analyzed in terms of concurrent Herzberg-Teller and Jahn-Teller vibronic interactions. The 77 K spectra provided useful information on hot bands and on other aspects of the analyses. Vibronic bands belonging to triplet←singlet transitions were detected between 620 and 700 nm.

Preparation and UV / visible spectra of fullerenes C60 and C70

We have studied electronic states of various fragments of

The production and structure of pyrolytic carbon nanotubes (PCNTs)

C_{60}

Order-disorder antiferroelectric phase transition in a hybrid inorganic-organic framework with the perovskite architecture.

within the Pariser-Parr-Pople (PPP) model and have obtained structural, magnetic and spectral properties of these molecules. The fragments studied include corannulene, fluoranthene and pyracylene. Pyracylene is studied using the exact valence bond (VB) approach while fluoranthene and corannulene are studied using a novel restricted CI technique which employs molecular orbitals for constructing the VB functions. The electronic excitations, bond order and ring currents are calculated for these systems. From these studies, the wide range of absorptions in

Pyrolytic carbon nanotubes from vapor-grown carbon fibers

C_{60}

Efficient route to large arrays of CNx nanofibers by pyrolysis of ferrocene/melamine mixtures

can be viewed as those localized on pyracylene units or on the corannulene/ fluoranthene units. The bond orders and ring currents show the hexagons to be similar to benzene rings. The pentagon---hexagon bonds are also found to be longer than the hexagon---hexagon bonds.

Pyrolytic production of aligned carbon nanotubes from homogeneously dispersed benzene-based aerosols

Abstract The structures of pyrolytic carbon nanotubes (PCNTs), produced by thermal decomposition of hydrocarbon vapor (benzene at ca 1100°C) have been studied. Electron microscope images of the PCNTs “as-formed” and after various degrees of subsequent heat treatment are presented and discussed on the basis of elongated giant fullerene structures and a possible primary growth mechanism. The images indicate that some PCNTs show evidence of thickening due to secondary pyrolytic deposition whereas others appear to be essentially exposed nanotubes (without further carbon deposition) or have sections with negligible secondary deposition. The nanotubes appear initially to grow as ultra-thin tubes with a central hollow core (diameters ca2 nm or more). No catalytic particles are observed at the tips of the tubes shown by high resolution transmission electron microscope (HRTEM) studies to be composed of concentric graphite cylinders with consecutive radii increasing by a factor close to that of the standard graphite interlayer spacing. The further pyrolytic carbon thickening occurs at roughly the same time as longitudinal nanotube growth. This phenomenon can cause the diameters of the tubes to increase to micron sizes. Heat treatment of the “as-formed” PCNTs results in encapsulation of the hollow cores by polyhedral hemispherical fullerene-like caps. Single or multi-layered nodes are also generated, yielding “bamboo-like” structures. The PCNTs are similar to those obtained by d.c. arc techniques using graphite electrodes.

Electrolytic formation of carbon nanostructures

[(CH3)2NH2]Zn(HCOO)3, 1, adopts a structure that is analogous to that of a traditional perovskite, ABX3, with A = [(CH3)2NH2], B = Zn, and X = HCOO. The hydrogen atoms of the dimethyl ammonium cation, which hydrogen bond to oxygen atoms of the formate framework, are disordered at room temperature. X-ray powder diffraction, dielectric constant, and specific heat data show that 1 undergoes an order-disorder phase transition on cooling below 156 K. We present evidence that this is a classical paraelectric to antiferroelectric phase transition that is driven by ordering of the hydrogen atoms. This sort of electrical ordering associated with order-disorder phase transition is unprecedented in hybrid frameworks and opens up an exciting new direction in rational synthetic strategies to create extended hybrid networks for applications in ferroic-related fields.