A. K. Cheetham

Science and technology of nanomaterials: current status and future prospects

The science and technology of nanomaterials has created great excitement and expectations in the last few years. By its very nature, the subject is of immense academic interest, having to do with very tiny objects in the nanometer regime. There has already been much progress in the synthesis, assembly and fabrication of nanomaterials, and, equally importantly, in the potential applications of these materials in a wide variety of technologies. The next decade is likely to witness major strides in the preparation, characterization and exploitation of nanoparticles, nanotubes and other nanounits, and their assemblies. In addition, there will be progress in the discovery and commercialization of nanotechnologies and devices. These new technologies are bound to have an impact on the chemical, energy, electronics and space industries. They will also have applications in medicine and health care, drug and gene delivery being important areas. This article examines the important facets of nanomaterials research, highlighting the current trends and future directions. Since synthesis, structure, properties and simulation are important ingredients of nanoscience, materials chemists have a major role to play.

Evidence for the likely occurrence of magnetoferroelectricity in the simple perovskite, BiMnO3

Investigation of polycrystalline samples synthesized at high pressures as well as thin films deposited by nebulized spray pyrolysis suggest that BiMnO 3 is ferromagnetic with a T C of 105 K and ferroelectric with a Curie temperature of around 450 K. It remains ferroelectric down to low temperatures through the ferromagnetic transition.

B-C-N, C-N AND B-N NANOTUBES PRODUCED BY THE PYROLYSIS OF PRECURSOR MOLECULES OVER CO CATALYSTS

Abstract B–C–N, C–N and B–N nanotubes, prepared by the pyrolysis of the appropriate precursor compounds around 1000°C in an atmosphere of argon, have been examined by electron microscopy and other techniques. The compositions of the nanotubes have been analysed by electron energy loss spectroscopy and X-ray photoelectron spectroscopy. There is significant compositional variation in B–C–N and C–N nanotubes. Properties of the B–C–N and C–N nanotubes have been compared with those of carbon nanotubes, particularly with respect to the I – V characteristics as obtained from UHV–STM studies. The near absence of B–N nanotubes on pyrolysing the appropriate precursor compound, and other observations made in the present study, indicate the crucial role played by carbon in the initial nucleation and growth of nanotubes.

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

We report a high-yield route to two-dimensional arrays (<400×400 μm2) of aligned C49Nx (x ⩽ 1) nanofibers (<100 nm o.d.; <60 μm length), by pyrolyzing mixtures of ferrocene and melamine at 950–1050 °C under an Ar flow. The fibers exhibit unusual interlinked stacked-cone morphologies, ascribed to the presence of nitrogen. High-resolution electron energy-loss spectroscopy of the individual fibers reveals a 2% nitrogen content with ionization energies mainly at ∼400.9 eV, corresponding to N bonded to three C atoms within a hexagonal framework. The nanofibers may be useful for the economic fabrication of field emission sources and robust composites.

Charge ordering in the rare earth manganates: the experimental situation

Charge-ordered phases of rare earth manganates are novel manifestations arising from interactions between the charge carriers and phonons, giving rise to the localization of carriers at specific sites in the lattice below a certain temperature. Accompanying this phenomenon, the Mn 3+ (e g ) orbitals and the associated lattice distortions also exhibit long range ordering (orbital ordering). What makes the manganates even more interesting is the occurrence of complex spin ordering related to anisotropic magnetic interactions. In this article, we discuss the emerging scenario of charge-ordered rare earth manganates in the light of specific case studies and highlight some of the new experimental findings related to spin, orbital and charge ordering. We also examine features such as the charge stripes and phase separation found experimentally in these materials, and discuss the factors that affect charge-ordering such as the size of A-site cations and magnetic and electric fields, as well as isotopic and chemical substitutions.

On the Nature of Water Bound to a Solid Acid Catalyst

The nature of the species formed when water interacts with Brønsted acid sites in a microporous solid acid catalyst, HSAPO-34, was studied by powder neutron diffraction and infrared spectroscopy. Previous infrared and computational studies had failed to unambiguously establish whether water is protonated to form hydronium (H3O+) ions or is merely hydrogen-bonded to acid sites inside the zeolite. Our experiments clearly showed that both species are present: An H3O+ ion is found in the eight-ring channel of the zeolite and a second water molecule is hydrogen-bonded to an acid site on the six-ring.

Preparation of aligned carbon nanotubes catalysed by laser-etched cobalt thin films

Abstract Pyrolysis of 2-amino-4,6-dichloro-s-triazine over laser-etched thin films of cobalt deposited on an inverted silica substrate generates aligned bundles and films of carbon nanotubes of uniform dimensions. Scanning electron microscopy, high resolution transmission electron microscopy, energy dispersive X-ray analyses, electron energy loss spectroscopy and electron diffraction studies reveal that the aligned tubes, which usually grow perpendicular to the substrate surface, are mainly straight (length ⩽100 μm; 30–50 nm OD), contain traces of nitrogen (

Giant Magnetoresistance in Transition Metal Oxides

Some materials exhibit large changes in electrical resistance in the presence of a magnetic field, and this change can be used in applications from sensor technology to magnetic data storage. In their Perspective, Rao and Cheetham discuss magnetoresistance in perovskite manganates, where the effect is unusually strong. Much has been learned about these materials, and this understanding is driving the search for new materials with even more impressive properties.

A quantitative description of the active sites in the dehydrated acid catalyst HSAPO-34 for the conversion of methanol to olefins

A neutron diffraction study of the deuterated form of a microcrystalline specimen of the H+ SAPO-34 catalyst for converting methanol to light olefins reveals two distinct bridging hydroxyl Brønsted sites. These sites are also identified by IR spectroscopy which distinguishes their acid strength from the shift in O-H frequencies following adsorption of CO at 77 K. Protons attached to the O(4) bridging oxygen (OH distance 0.95 Å) exhibit greater acidity than those attached to O(2) (OH distance 0.91 Å).

Carbon-assisted synthesis of inorganic nanowires

Nanowires of a variety of inorganic materials such as metal oxides, sulfides, nitrides and carbides have been synthesized and characterized in the last three to four years. Among the several strategies developed for the synthesis of these materials, the carbothermal route is noteworthy since it provides a general method for preparing crystalline nanowires of many of these materials which include oxides such as ZnO, Al2O3 and Ga2O3, nitrides such as AlN and Si3N4, and carbides such as SiC. The method itself is quite simple and involves heating a mixture of an oxide with carbon in an appropriate atmosphere. The method has enabled the synthesis of crystalline nanowires of both silica and silicon. In the case of GaN, it has been possible to dope it with Mn, Mg and Si to bestow useful optical and magnetic properties. In this article, highlights of the recent results on the carbon-assisted synthesis of inorganic nanowires are presented.