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The presence of coherence phenomenon due to the interference of probability amplitude terms, is one of the most important features of quantum mechanics theory. Recent experiments show the presence of quantum processes whose coherence provided over suddenly large interval-time. In particular, photosynthetic mechanisms in light-harvesting complexes provide oscillatory behaviors in quantum mechanics due to quantum coherence. In this work, we investigate the coherent quantum transfer energy for a single-excitation and nonlocal correlation in a dimer system modelled by a two-level atom system with and without time-dependent coupling effect. We analyze and explore the required conditions that are feasible with real experimental realization for optimal transfer of quantum energy and generation of nonlocal quantum correlation. We show that the enhancement of the probability for a single-excitation transfer energy is greatly benefits from the combination of the energy detuning and time-dependent coupling effect. We investigate the presence of quantum correlations in the dimer using the entanglement of formation. We also find that the entanglement between the donor and acceptor is very sensitive to the physical parameters and it can be generated during the coherent energy transfer. On the other hand, we study the dynamical behavior of the quantum variance when performing a measurement on an observable of the density matrix operator. Finally, an interesting relationship between the transfer probability, entanglement and quantum variance is explored during the time evolution in terms of the physical parameters.

A simple two-terminal device containing a semiconducting single-walled-nanotube thin-film is reported that generates an extremely high density of holes when it comes in contact with an ionic liquid, inducing strong electromodulation of the interband and excitonic transitions as a result of the position-dependent shift of the single-walled-nanotube Fermi level.

The concept of optical protein-based memory has been of interest since the early 1970s. Yet, no commercially available protein-based memory devices exist. This review presents an analysis of the main challenges associated with the practical implementation of such devices. In addition, the discussion includes details on the potential of using the unparalleled properties of photochromic proteins by creating an optical data storage disk drive with unmatched features and, particularly, record-high data densities and rates.

Low-temperature (LT) buffer-layer techniques were employed to improve the crystalline quality of ZnO films grown by molecular-beam epitaxy (MBE). Photoluminescence (PL) spectra show that CdO, as a hetero-buffer layer with a rock-salt structure, does not improve the quality of ZnO film grown on top. However, by using ZnO as a homo-buffer layer, the crystalline quality can be greatly enhanced, as indicated by PL, atomic force microscopy (AFM), x-ray diffraction (XRD), and Raman scattering. Moreover, the buffer layer grown at 450°C is found to be the best template to further improve the quality of top ZnO film. The mechanisms behind this result are the strong interactions between point defects and threading dislocations in the ZnO buffer layer.

The problems posed by the synthesis and purification of single-walled carbon nanotubes (SWNTs) have inhibited progress in the field. In this article, we review the methods available for measuring the purity of SWNTs and the current status of processes designed to purify them.We emphasize the hierarchy of the purification steps that must be developed in order to obtain high-quality material suitable for the full range of advanced applications that are envisioned for the ultimate carbon nanofiber.We review two strategies for SWNT purification, the assessment of SWNT purity by use of near-IR spectroscopy and its application to the thermal oxidation of thin films of SWNTs, as well as recent advances in the separation of metallic and semiconducting SWNTs. While substantial progress has been made in the purification and separation of SWNTs, we note the need for quality control and quality assurance within the industry. Much work remains before pure SWNTs of specific lengths, diameters, and chirality can be made available for applications.

We have subjected single-walled carbon nanotube materials (SWNTM's) to a variety of organic functionalization reactions. These reactions include radioactive photolabeling studies using diradical and nitrene sources, and treatment with dichlorocarbene and Birch reduction conditions. All of the reactions provide evidence for chemical attachment to the SWNTM's, but because of the impure nature of the staring materials, we are unable to ascertain the site of reaction. In the case of dichlorocarbene we are able to show the presence of chlorine in the SWNT bundles, but as a result of the large amount of amorphous carbon that is attached to the tube walls, we cannot distinguish between attachment of dichlorocarbene to the walls of the SWNT's and reaction with the amorphous carbon.

The difference in magnetic susceptibility of graphite and diamond prompted Raman to postulate the flow of currents around the ring system of graphite in response to an applied magnetic field. The discovery of new carbon allotropes, the fullerenes, has furthered our understanding of this phenomenon and its relationship to aromatic character. C60 and the other fullerenes exhibit both diamagnetic and paramagnetic ring currents, which exert subtle effects on the magnetic properties of these molecules and provide evidence for the existence of π-electrons mobile in three dimensions.

α-hexathienyl (α–6T) is a highly promising material for application in thin film transistor devices. Recently, record high mobilities, together with record high current on/off ratios, have been reported.1 Thus far, structural information on this exciting material is sketchy. The crystal structures of several such hexamers have been investigated, but only with powder samples, since the crystal growth has proven exceedingly difficult.2-5 Powder Rietveld refinements on these materials are severely hampered by the large number of overlapping reflections, preferred orientation, ambiguities in symmetry, etc. Here, we present a crystal structure of the high-temperature polymorph of α–6T (α–6T/HT), as determined from a single-crystal structure analysis. In this polymorph, the hexamer crystallizes in the smallest unit cell so far reported for this material, but the molecule is flat. Extended Hückel theory (EHT) band structure calculations show that α–6T/HT is an indirect gap semiconductor, with the conduction band minimum at Y and the valence band maximum at Γ. The conduction and valence bands both show a remarkable degree of dispersion along X and Y for a molecular crystal. The electronic band structure of this material is strikingly similar to that of the two-dimensional organic superconductors based on bis(ethylenedithio)tetrathiafulvalene (ET), such as κ−(ET)2 Cu(NCS)2.

The irradiation of sublimed fullerene (C60 and C70) thin films with ultraviolet light in an oxygen-rich ambient has been found to lead to a substantially increased cohesive energy in the fullerene solid. The decreased solubility and lower vapor pressure of the phototransformed material enables wet (organic solvents) or dry (thermal or photon-induced sublimation) development of photo-defined negative images. One micrometer wide lines with good edge definition are demonstrated. X-ray, infrared, optical absorption, and high performance liquid chromatography reveal that photo-oxygenated C60 retains its fcc crystal structure but with a substantial fraction of the C60 molecules modified with carbonyl (C=O) bonds.

IT has been demonstrated1–6 that solid C60 can be readily intercalated with group IA alkali metals to give metallic or insulating compounds, depending on the dopant concentration. The metal atoms diffuse into tetrahedral and octahedral interstitial sites of the C60 lattice with little disturbance to the face-centred cubic (f.c.c.) packing7. The A3C60f.c.c. phases (A is K, Rb, Cs and mixtures of these) exhibit superconductivity with a transition temperature that increases with lattice constant8. At higher dopant concentration a body-centred tetragonal A4C60 phase9 and an insulating, body-centred cubic A4C60 phase10 are found. Here we report that the divalent group IIA intercalant calcium can be intercalated into the f.c.c. sites of C60 to form a solid solution, and that, near a Ca:C60 ratio of 5:1, a phase transformation occurs to a simple cubic phase. Measurements of microwave loss, magnetic susceptibility and Meissner effect show that the simple cubic phase becomes superconducting below 8.4 K.

Polyphosphazenes present a range of unique and fascinating physical properties, including thermotropic behaviour, which are highly dependent both on the type of substitution at the P atom in the inorganic backbone, and the thermal history of the material. Recent technological advances have allowed us to study this group of polymers using vibrational spectroscopy. Fourier transform (FT) Raman spectra have been recorded from samples of poly[bis(4-methoxy-phenoxy)phosphazene] with different thermal histories. With the aid of spectral subtraction methods, differences observed between the spectra have been related to the crystalline and mesophase content of the materials.

Conventional fuel cell architecture on one side of the membrane electrode assembly consists of a carbon backing layer, hydrophobic microporous layer (MPL), and a catalyst layer, which is in contact with the solid proton exchange membrane. Pt nanoparticles are deposited onto multi-walled carbon nanotubes (Pt/MWNTs) and a free-standing film of Pt/MWNTs is fabricated to act as the MPL and the catalyst layer in hydrogen fuel cells. The free-standing film of Pt/MWNTs condenses two functions into one bifunctional layer that simplifies the fuel cell fabrication procedure. Fuel cell polarization performance improves when using the free-standing film of Pt/MWNTs without the MPL resulting in a higher peak performance of 1.2 W/cm2 in comparison with 1.0 W/cm2 when in the presence of a MPL.

A WIDE variety of alkali-metal fullerides (AxC60) have been prepared, with stoichiometries varying between 1⩽=x⩽=11 (refs 1,2). Most of the compounds with x = 3 are superconductors with unusually high transition temperatures (Tc) which increase with the size of the face-centred cubic unit cell3. The increase in Tc on lattice expansion is attributed to a decrease in the conduction band width, and therefore one ultimately expects a saturation or downturn of Tc followed by a metal–insulator transition. We have sought to explore this regime by expanding the A3C60 structure through intercalation of neutral molecules capable of solvating the A+ ions. Here we report that the reaction of ammonia with Na2CsC60 (Tc = 10.5K; lattice constant α = 14.132 A) produces the compound (NH3)4Na2CsC60, which has an expanded unit cell (a = 14.473 A) and an increased Tc of 29.6 K. The structure contains the Na(NH3)+4 cation on the octahedral site.

INTERCALATING solid C60 with dopant atoms yields materials with a remarkable range of properties1. Alkali metal atoms, for example, readily form charge-transfer compounds2,3, AxC60 (where A is an alkali metal), which can be metallic, superconducting or insulating depending on the dopant concentration4–9. In all cases, the superconducting phase has a face-centred cubic (f.c.c.) structure and stoichiometry A3C60, which suggests a common mechanism for superconductivity dependent, at least in part, on the external coordination number of the C60 molecules. More recently, it has been shown10 that the alkaline earth metal calcium can also be intercalated with fulleride to form a superconducting phase, again with a f.c.c.-derived structure, near a Ca:C60 ratio of 5:1. Here we report the intercalation of fulleride with barium, in which a pure body-centred cubic phase with a lattice constant of 11.171 A is realized near a stoichiometry of Ba6C60. This phase is also superconducting (with a transition temperature of 7 K), suggesting that the mechanism of superconductivity is related to an intrinsic property of the C60 molecules, rather than the external coordination number.

SUPERCONDUCTING K3C60 (ref. 1) has been shown2 to have an intercalated face-centred cubic (f.c.c.) structure, and other A3C60 compounds (where A is K, Rb or mixtures of K, Rb and/or Cs) form an isostructural series with superconducting transition temperatures up to 31.3 K (ref. 3). Recently we reported413C NMR studies of KxC60 for x

BULK C60 (fullerite) is a solid composed of extremely hard pseudospherical molecules bonded by weak van der Waals interactions. We expect that pressurization will probe this diversity as the intermolecular distance is reduced. Here we present experimental measurements of the room-temperature pressure–volume equation of state of C60 fullerite to pressures of 20 GPa. The face-centred cubic phase remains stable under hydrostatic compression to this pressure, with an atmospheric-pressure isothermal bulk modulus of K0 = 18.1 ± 1.8 GPa and dK0/dP = 5.7 ± 0.6, characteristic of isotropic van der Waals intermolecular bonding. X-ray diffraction intensity changes are consistent with the compressibility of the C60 molecule being significantly lower than that of the bulk fullerite solid. Under non-hydrostatic compression, a transition to a crystallographic structure of lower symmetry is observed at 16±1 GPa.

THE recent syntheses1,2 of macroscopic quantities of C60 have suggested possible applications in host–guest and organic chemistry, tribology, electrochemistry and semiconductor tech-nology. Here we report the preparation of alkali-metal-doped films of C60 and C70 which have electrical conductivities at room temperature that are comparable to those attained by n-type doped polyacetylene. The highest conductivities observed in the doped films are: 4Scm−1 (Cs/C60), 100 (Rb/C60), 500 (K/C60), 20 (Na/C60), 10 (Li/C60), 2 (K/C70). The doping process is reversed on exposure of the films to the atmosphere. At high doping levels, the films become more resistive. We attribute the conductivity induced in these films to the formation of energy bands from the π orbitals of C60 or C70, which become partially filled with carriers on doping. The smaller alkali metal ions should be able to fit into the interstices in the lattice without disrupting the network of contacts between the carbon spheroids. In the case of C60, this would allow the development of an isotropic band structure, and we therefore propose that these materials may constitute the first three-dimensional 'organic' conductors.

The electronic structure of the two organic, layered metals κ-ET2Cu(SCN)2 and K-ET 2Cu[N(CN)2]Br has been studied using a combination of soft X-ray emission (SXE) and soft X-ray absorption (SXA) spectroscopy. These techniques are powerful probes of the site and angular-momentum resolved partial density of states (PDOS) for both occupied and unoccupied states. Therefore these spectroscopies are particularly suited for an analysis of the density of states of multi-atomic, complex materials as the ET-based organic conductors, allowing site-specific electronic structure to be measured. Furthermore, for certain materials, the electronic structure of specific layers can be measured. We present a preliminary picture of the electronic structure of κ-ET 2Cu(SCN)2 and κ-ET 2Cu[N(CN)2]Br as measured by SXE and SXA performed at the C-Is and N- Is core levels.

The effect that the presence or absence of interfacial C60 layers has on the nucleation and subsequent growth of overlying thin metal films has been studied using in situ resistivity measurements. Comparisons are made for Al and Cu films grown on quartz and yttria stabilized zirconia (YSZ) substrates. Our results indicate that electron donation across M/C60 (M=Al, Cu) interfaces reduces the percolation threshold for conductivity while simultaneously giving rise to an increased resistance in the metal due to electron depletion. Additional physical processes that are taken into account include the effect of different interfacial energies at the M/C60 and M/substrate boundaries and the effect of substrate tunneling between percolating metallic islands during the initial stages of film growth.

As developed by Senturia and co-workers [1], microdielectrometry is a technique for measuring complex permittivity utilizing microfabrication technology to incorporate both the interdigital sensing electrodes and associated circuitry on the same microchip. By covering a microdielectrometer chip with a thin layer of C60, it is possible to dynamically monitor the film's frequency response and dielectric properties as a function of doping with selected gases and other species.It is known that solid C60 has a substantial amount of interstitial volume. The presence of mobile ions in these spaces compromises the breakdown voltage and makes pure C60 unsuitable for applications requiring high-quality dielectric films. However, various immobile ions or neutral species (e.g. oxygen) can be made to fill the interstitial volume, changing the characteristics of the C60 films and, in some cases, improving the dielectric properties. In-situ microdielectric measurements of pristine and modified C60 films were performed for frequencies ranging from 0.005 Hz to 100 kHz. Based on the low-frequency behavior of the dielectric constant, a model is proposed for the mechanism of oxygen diffusion into the interstitial spaces of the fullerene material.

The phonon spectra of pristine fullerene, superconducting K3C60 and saturation-doped Rb6C60 measured by inelastic neutron scatteringin the energy range 2.5 - 200 meV at low temperatures reveal substantial broadening of five-fold degenerate Hg intramolecular vibrational modes both in the low-energy radial and the high-energy tangential part of the spectrum. This provides strong evidence for a traditional phonon-mediated mechanism of superconductivity in the fullerides but with an electron-phonon coupling strength distributed over a wide range of energies (33-195 meV) as a result of the finite curvature of the fullerene spherical cage.

Carbon-13 NMR spectra of powder samples of pure C60 and of a mixture of C60 and C70 provide information about molecular motions in the solid state. At room temperature, C60 molecules rotate rapidly and isotropically. The transition from a rapidly rotating to a stationary (on the time scale of the experiment) system occurs over a temperature range from 120 K to 60 K, suggesting a distribution of activation energies. C70 molecules also rotate rapidly at room temperature, but more anisotropically.

We have grown crystals of the carbon structure C60 by sublimation. In contrast to solution-grown crystals, the sublimed crystals have long range order with no evidence of solvent inclusions. Sublimed C60 forms three dimensional, faceted crystals with a close-packed, face-centered cubic unit cell. We have refined a crystal structure using the “soccer ball” model of the C60 molecule. The results indicate that the C60 molecule has the expected spherical shape, however the data are not sufficiently accurate to unambiguously determine atomic positions.

In a recent Scientific Correspondence, Liu et al.1 reported observing carbon toroids. They suggested that these objects could be considered as quantum wires and that they would exhibit interesting transport properties. Here I provide theoretical support for this proposition.

INTERCALATION of C60 (buckminsterfullerene1,2) by alkali metals3 leads to superconducting compounds of stoichiometry A3C60 (refs 4–6) with transition temperatures Tc as high as 33 K (ref. 7). These transition temperatures are considerably higher than those for alkali-metal-intercalated graphite (

THE discovery of conductivity1 in AxC60 (where A represents an alkali metal) and superconductivity2 in KxC60 has been followed by reports of superconductivity in other alkali-metal-doped full-erides with transition temperatures as high as 33 K (ref. 3). Elucidation of phase diagrams and understanding the relationship between structure and superconducting properties is essential to a detailed understanding of superconductivity in these systems. So far, structural data have been reported only for the non-conducting, intercalated body-centred cubic (b.c.c.) structures A6C60 (where A is K or Cs; ref. 4), a body-centred tetragonal structure for A6C60 (where A is K, Rb, Cs; ref. 5) and the superconducting, intercalated face-centred cubic (f.c.c.) material K3C60 (ref. 6). Here we report the preparation of a series of single-phase, isostructural f.c.c. superconductors with composition A3C60 (where A is K, Rb, Cs or a mixture of these), and show that Tcc increases monotonically with the size of the unit cell. Extended Hiickel band-structure calcula-tions also show a monotonic increase in the density of states at the Fermi level, N(EF), with lattice parameter. The primary impli-cation of these results is that all A3C60 superconductors have the same structure and that changes in Tc can be accounted for by changes in N(EF).

THE magnetic susceptibility of C60 and the possibility of magnetic-field-induced π-electron ring currents in this carbon spheroid have been of interest since the initial experiments on carbon clusters1. If the molecule is regarded as a sphere with a radius of 3.5 A, on which 60 electrons are free to move, the Pauling ring-current model predicts a ring-current diamagnetic susceptibility 41 times the π-electron ring-current magnetic susceptibility of benzene with the field normal to the plane of the six-membered ring2,3. London theory predicts, however, that the π-electron ring currents in C60 should be weakly paramagnetic or diamagnetic, depending on the relative bond strengths used in the calculation2,3. With the availability of macroscopic quantities of C60 (ref. 4), it is now possible to study experimentally the magnetic properties of the molecule. Here we report on such measurements. We find that the diamagnetism of C60 is small, a result that we attribute to excited-state paramagnetic contributions to the π-electron ring-current magnetic susceptibility. Thus C60 seems to be an aromatic molecule with a vanishingly small π-electron ring-current magnetic susceptibility. We have performed similar measurements on C70, which indicate an appreciable π-electron diamagnetism, consistent with theoretical calculations. We attribute the differences in magnetic properties of these two molecules to their different fractions of five-membered ring structures. The fullerenes may thus constitute a class of compounds of 'ambiguous' aromatic character, traditional measures of which will not provide an adequate classification.

Nature 352, 701-703 (1991). THE positions of the alkali atoms, originally reported as (0,0.5, z) with z = 0.28, should read (x,0.5,0) with x = 0.22. These are the positions used in the calculations for Table 1 and the positions illustrated in Fig. 2.

I WISH to draw attention to the potential of odd alternant hydrocarbons (OAHs) in the design of organic metals and superconductors which gives reason to suppose that they may prove equal or even superior to present systems. There is now strong evidence for the existence of the metallic state in certain organic charge transfer salts, and a suggestion of low temperature superconducting fluctuations in the case of tetrathiofulvalene–tetracyanoquinodimethane (TTF–TCNQ); present interest is now focused on the stabilisation of these states at greater (critical) temperatures1. In contrast to the molecular crystals usually formed by conjugated organic molecules (which are held together by van der Waals' forces and characterised by a relatively weak intermolecular interaction), these salts crystallise as segregated one-dimensional stacks of cation and/or anion radicals in which the charge transfer process and thus the electronic mobility within a stack is greatly enhanced by the abnormally high overlap of the π-electron wave functions of nearest neighbours in the stack and by the ability of the constituent molecules to stabilise an ionic fluctuation.

Smalley and co-workers1 have speculated that an icosahedral C60 molecule might have unusual magnetic properties. Here we report that the π-electron ring-current susceptibility of C60 is unusually small and sensitively dependent on the relative strengths of the two inequivalent bonds of the molecule. For equal bond strengths the susceptibility is only -0.21 that of benzene (with field normal to the plane of the six-membered ring). A 2% change in relative bond strengths is enough to change the sign of the susceptibility with the result that C60 is probably weakly diamagnetic. The shielding effect of the π-electrons is calculated to be less than 1 p.p.m. for an atom placed at the centre of C60. A larger effect is expected for C606-, which is calculated to be strongly diamagnetic.