Abstract: We revisit the infrared (IR) spectra of C(70) on pure samples as a function of temperature (20-370 K) and pressure (0-10 GPa). Although the rotation of the molecule is not completely free at ambient conditions, the measured spectra are in perfect agreement with previous density functional theory (DFT) calculations on isolated molecules, including the intensities, for 22 out of the possible 31 IR-active modes. We assign the anomalies in the infrared spectra both on decreasing temperature (similar to 280 K) and increasing pressure (similar to 0.8 GPa) to the freezing out of uniaxial rotation; the boundary between uniaxial and quasi-free rotation has a much weaker effect. At similar to 7.5 GPa an irreversible change of the spectrum occurs, which we attribute to room-temperature dimerization. The pressure at which this dimerization occurs adds a new point to the phase diagram presented by Sundqvist.
Abstract: The gelating abilities of enantiopure, racemic, and different enantio-enriched mixtures of 12-hydroxystearic acid (12HSA) have been compared in order to clarify conflicting reports in the literature (1) concerning their ability to gelate organic liquids. Less than 1.0 wt % of optically pure (D)-12HSA was found to gelate mineral oil. The gel matrix was comprised of high aspect ratio fibers in which the 12HSA molecules were organized as head-to-head dimers and the 12-hydroxyl groups formed an H-bonding network along the axis transverse to the longitudinal growth. Below 2 wt %, racemic 12HSA in mineral oil did not reach the percolation threshold. Its organogels were comprised of platelet-like crystals with a molecular arrangement of single, in-plane, hydrogen-bonded acyclic dimers that prevent longitudinal growth and limit the ability of the polar groups to phase separate during nucleation.
Abstract: The liquid crystal morphologies of symmetrical diacy phosphatidylcholine liposomes examined in this research study were found to be dependent on saturated hydrocarbon chain length. Both powder X-ray diffraction and synchrotron mid-IR spectromicroscopy indicate that phosphatidylcholines with short hydrocarbon tails (i.e. ten and twelve carbons) are more likely to form unilamellar liposomes while those with long hydrocarbon tails (i.e. eighteen and twenty carbons) are more likely to form multilamellar liposomes. Hydrocarbon chain lengths of fourteen and sixteen represent a transitional zone between these two liquid crystal morphologies. The FTIR spectra where a shoulder develops on the peak at wavenumber 1750 cm(-1) particularly highlights the change in the packing of adjacent molecules in the transitional zone. (C) 2011 Elsevier B.V. All rights reserved.
Abstract: A description of a coupled electrochemical and spectrometer interface using synchrotron infrared radiation is provided. The interface described allows for the precise and accurate timing needed for time-resolved IR spectroscopic studies of electrochemical systems. The overall interface uses a series of transistor-transistor logic trigger signals generated from the commercial FTIR spectrometer to regulate the recording of control, electrochemical, and IR signals with reproducible and adjustable timing. The instrument has been tested using a thin-layer electrochemical cell with synchrotron light focused through microscope optics. The time-resolved response of the benzoquinone/dihydroxybenzoquinone redox couple is illustrated as an example of the instrument's capability. (C) 2011 American Institute of Physics. [doi:10.1063/1.3624693]
Abstract: A description of a coupled electrochemical and spectrometer interface using synchrotron infrared radiation is provided. The interface described allows for the precise and accurate timing needed for time-resolved IR spectroscopic studies of electrochemical systems. The overall interface uses a series of transistor-transistor logic trigger signals generated from the commercial FTIR spectrometer to regulate the recording of control, electrochemical, and IR signals with reproducible and adjustable timing. The instrument has been tested using a thin-layer electrochemical cell with synchrotron light focused through microscope optics. The time-resolved response of the benzoquinone∕dihydroxybenzoquinone redox couple is illustrated as an example of the instrument's capability.
Abstract: Synchrotron infrared radiation has been successfully coupled through an infrared (IR) microscope to a thin-cavity external reflectance cell to study the diffusion controlled redox of a ferrocyanide solution. Excellent signal-to-noise ratios were achieved even at aperture settings close to the diffraction limit. Comparisons of noise levels as a function of aperture size demonstrate that this can be attributed to the high brilliance of synchrotron radiation relative to a conventional thermal source. Time resolved spectroscopic studies of diffusion controlled redox behavior have been measured and compared to purely electrochemical responses of the thin-cavity cell. Marked differences between the two measurements have been explained by analyzing diffusion in both the axial (linear) and radial dimensions. Whereas both terms contribute to the measured current and charge, only species that originate in the volume element above the electrode and diffuse in the direction perpendicular to the electrode surface are interrogated by IR radiation. Implications for the use of ultramicroelectrodes and synchrotron IR (SIR) to study electrochemical processes in the submillisecond time domain are discussed.
Abstract: Top-off operations is a quasi-continuous injection mode that increases the flux and brightness of a synchrotron source and improves thermal stability of optical components by maintaining a constant current in the storage ring. Although the increased and constant flux is advantageous for FTIR measurements, the frequent injections (about one every 30 seconds in the ALS case) introduce artifacts into the spectrum by creating spikes in the interferogram data. These spikes are caused by brief beam motion during the injection event. Here, we describe our efforts to minimize the effects of top-off generated interferogram spikes on several FTIR spectrometers. They include using a fast feedback mirror system to correct for beam motion and a gating signal to inhibit interferogram collection during a top-off injection.
Abstract: We report the ultrafast THz dynamics and broadband electromagnetic response of few-layer epitaxial graphene. Electrodynamics consistent with a dense Dirac electron plasma and a transient THz response dominated by recombination of excess holes is observed. (C) 2010 Optical Society of America
Abstract: The interaction of O(2) with gold foil and gold nanoparticles grown by thermal deposition on TiO(2)(110) was studied by in situ ambient pressure X-ray photoelectron spectroscopy at room temperature. No spontaneous dissociation of O(2) was observed either on Au foil or oil Au nanoparticles up to 1 Torr of O(2). X-ray irradiation, however, is very effective in promoting gold oxidation on both surfaces in the presence of O(2). Our results help reconcile recent conflicting experimental observations regarding the activation of molecular oxygen, which is a crucial issue in Au catalyzed oxidation reactions.
Abstract: A novel electrochemical setup has been developed for soft X-ray absorption studies of the electronic structure of electrode materials during electrochemical cycling. In this communication we illustrate the operation of the cell with a study of the corrosion behavior of copper in aqueous NaHCO(3) solution via the electrochemically induced changes of its electronic structure. This development opens the way for in situ investigations of electrochemical processes, photovoltaics, batteries, fuel cells, water splitting, corrosion, electrodeposition, and a variety of important biological processes. (C) 2010 Elsevier B.V. All rights reserved.
Abstract: Two different reductive synthetic methods were applied to hydrogenate the sidewalls of HiPCo single-walled carbon nanotubes (SWNTs). In the first one, the reductive agent was melted potassium which doped and exfoliated the nanotube bundles, so that before hydrogenation all of the tubes had been converted to metallic ones. In the second method, doping occurred just before hydrogenation by napthalenide radical anions. The products were characterized by wide range infrared (30-52000 cm(-1)) spectroscopy with special emphasis on the selectivity of the two methods. We found that in the first case the controlling factor is the bandgap, and in the second case the diameter. This difference suggests the importance of the pi-pi interaction between naphthalenide and the nanotube surface. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Abstract: We report the broadband optical conductivity and ultrafast carrier dynamics of few-layer graphene. Equilibrium spectra exhibit significant THz and near-IR absorption, consistent with a model of intra- and interband transitions in a dense Dirac electron plasma. Ultrafast THz transmission changes after photoexcitation are dominated by excess holes, with a 1.2-ps exponential decay that reflects minority-carrier recombination.
Abstract: We report on the fabrication of transistors in a FinFET geometry using isotopically purified silicon-28-on-insulator (28-SOI) substrates. Donor electron spin coherence in natural silicon is limited by spectral diffusion due to the residual (29)Si nuclear spin bath, making isotopically enriched nuclear spin-free (28)Si substrates a promising candidate for forming spin quantum bit devices. The FinFET architecture is fully compatible with single-ion implant detection for donor-based qubits and the donor spin-state readout through electrical detection of spin resonance. We describe device processing steps and discuss results on electrical transport measurements at 0.3 K.
Abstract: We study the broadband optical conductivity and ultrafast carrier dynamics of epitaxial graphene in the few-layer limit. Equilibrium spectra of nominally buffer, monolayer, and multilayer graphene exhibit significant terahertz and near-infrared absorption, consistent with a model of intra- and interband transitions in a dense Dirac electron plasma. Nonequilibrium terahertz transmission changes after photoexcitation are shown to be dominated by excess hole carriers, with a 1.2 ps monoexponential decay that reflects the minority-carrier recombination time.
Abstract: We present optical spectra from the far infrared through the ultraviolet spectral region of freestanding transparent carbon nanotube films: single-, double- and multiwalled, and containing varying amounts of tubes of different chirality. By comparing the spectral features in the far infrared and the near infrared/visible, we can estimate the metallic/semiconducting content. We show by spectroscopic methods that doping the tubes increases both the conductivity and the transparency for visible light. We also discuss the influence of sample preparation and subsequent treatment on application possibilities. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Abstract: We present a study on the structural, spectroscopic, conducting, and magnetic properties of Mg(5)C(60), which is a two-dimensional (2D) fulleride polymer. The polymer phase is stable up to the exceptionally high temperature of 823 K. The infrared and Raman studies suggest the formation of single bonds between the fulleride ions and possibly Mg-C(60) covalent bonds. Mg(5)C(60) is a metal at ambient temperature, as shown by electron spin resonance and microwave conductivity measurements. The smooth transition from a metallic to a paramagnetic insulator state below 200 K is attributed to Anderson localization driven by structural disorder.
Abstract: We present pressure-dependent infrared transmittance measurements on fullerene C-70 at room temperature in the pressure range up to 4 GPa. The vibrational modes of C-70 exhibit nearly linear pressure dependence with a sluggish anomaly at around 0.8 GPa. This anomaly could be attributed to the change in the orientational ordering of the C-70 molecules. The sluggish character of the transition could be either due to the continuous change of the orientational order or the coexistence of two phase in the solid C-70. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Abstract: We introduce a new technique for measurement of the thermal conductivity of ultrathin films of single-walled carbon nanotubes (SWNTs) utilizing IR radiation as heat source and the SWNT film as thermometer. The technique is applied to study the temperature dependence of the thermal conductivity of an as-prepared SWNT film obtained in the electric arc discharge process and a film of purified SWNTs prepared by vacuum filtration. The interplay between thermal and electrical transport in SWNT networks is analyzed in relation to the type of intertube junctions and the possibility of optimizing the thermal and electrical properties of SWNT networks for specific applications is discussed.
Abstract: In this study we present direct sidewall functionalization of single-walled nanotubes, carried out via two different dissolving metal reduction methods. A modified Birch reduction scheme and a novel synthetic route were applied to form carbanion complexes on the surface of the nanotubes in liquid NH3 and tetrahydrofuran, respectively. The carbanion complex was reacted with a series of alkyl and aryl halogenides resulting in the corresponding SWNT derivatives. We determined the chemical composition and the thermal stability of the functionalized nanotubes by thermogravimetry/mass spectrometry and pyrolysis-gas chromatography. Characteristic decomposition peaks, observed in the temperature range of 350-600 degrees C, suggest the formation of covalent derivatives upon functionalization that is also supported by Raman spectroscopic studies. The influence of the functional groups and the type of nanotubes on the thermal stability, as well as the relationship between the functional group content and the intensity of the Raman D peak are discussed.
Abstract: We synthesized a series of MgxC60 samples with nominal composition of x between 1 and 6 via solid state reaction between C-60 and Mg powder. For x = 5 a two dimensional structure was found. The polymer phase is unusually stable up to high temperatures, it remains unchanged at 823 K for 20 minutes as confirmed by ESR. The mid-infrared absorption spectrum suggests that the polymer sheets contain single bonded fulleride ions. Both the spin susceptibility and microwave conductivity measurements show that Mg,C,, is metallic for T > 200 K. The transition from metallic to insulating states around 200 K may be attributed to Anderson localization. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Abstract: C(60)center dot CgHs and C(70)center dot C(8)H(8) are prototypes of rotor-stator cocrystals. We present infrared and Raman spectra of these materials and show how the rotor-stator nature is reflected in their vibrational properties. We measured the vibrational spectra of the polymer phases poly(C(60)C(8)H(8)) and Poly(C(70)C(8)H(8)) resulting from a solid-state reaction occurring on heating. On the basis of the spectra, we propose a connection pattern for the fullerene in Poly(C(60)C(8)H(8)), where the symmetry of the C(60) molecule is D(2h). On illuminating the C(60)-C(8)H(8) cocrystal with green or blue light, a photochemical reaction was observed leading to a product similar to that of the thermal polymerization.
Abstract: Ultrathin, transparent, and pure nanotube films have great importance in the optical studies. The samples are free-standing, which eliminates the difficulties caused by the various substrates in wide-range optical measurements. We prepared uniform homogenous thin films from hole-doped and functionalizated single-wall carbon nanotubes (SWNTs) with a simple process. We examined the spectra from the infrared to the ultraviolet range oil the same sample. The vibrational transitions of the sidewall groups indicated the chemical composition. The changes in the electronic properties were followed simultaneously by Near-infrared/Visible/Ultraviolet (NIR/VIS/UV) spectroscopies.
Abstract: The photoresponse in the electrical conductivity of a single-walled carbon nanotube (SWNT) film is dramatically enhanced when the nanotube film is suspended in vacuum. We show here that the change in conductivity is bolometric (caused by heating of the SWNT network). Electron-phonon interactions lead to ultrafast relaxation of the photoexcited carriers, and the energy of the incident infrared OR) radiation is efficiently transferred to the crystal lattice. It is not the presence of photoexcited holes and electrons, but a rise in temperature, that results in a change in resistance, thus, photoconductivity experiments cannot be used to support the band picture over the exciton model of excited states in carbon nanotubes. The photoresponse of suspended SWNT films is sufficiently high that they may function as the sensitive element of an IR bolometric detector.
Abstract: We present the formation, structure and properties of a new family of molecular crystals of distinct rotating and static components: the derivatives of fullerenes with cubane. The new crystals consist of altemating arrays of the constituents, stabilized by the recognition of the complementary molecular surfaces. The concave cubanes match six rotating fullerenes in octahedral configuration, while the six nearest neighbours of the convex fullerenes may form either octahedral or trigonal prismatic configurations. Fullerenes rotate freely in the 'molecular bearing' of cubanes. Lower symmetry crystals of similar properties form when cubane is replaced by 1,4-diethynyl cubane. The following properties will be discussed: (i) The significant size and shape recognition makes possible the design of the crystal structure of new higher fullerenes and substituted cubane derivatives. (ii) The phase diagram of the rotor-stator crystals can be constructed in terms of the ball diameter of rotating fullerenes. (iii) The molecular geometry of the constituents has an influence on the structure and dynamics of the lower symmetry variants. (iv) At elevated temperatures the decomposition of cubane or diethynyl cubane induces topochemical copolymerization with the surrounding fullerenes.
Abstract: We report on the transmission over a wide frequency range (from the far infrared through the visible) of pristine and hole-doped, free-standing carbon nanotube films at temperatures between 50 and 300 K. Optical constants are estimated by Kramers-Kronig analysis of the transmittance. We see evidence in the far infrared for a gap below 10 meV. Hole doping causes a shift of spectral weight from the first interband transition into the far infrared. Temperature dependence in both the doped and undoped samples is restricted to the far-infrared region.
Abstract: We want to clarify a few issues about using optical absorption spectra for characterizing carbon nanotubes. The common method of using optical density for nanotube counting is reasonable in the near-infrared and visible spectral region, but fails for the mid and far infrared, where metallic tubes can be unambigously detected. By performing Kramers-Kronig transformation on wide-range transmittance spectra of free-standing films, the far-infrared spectral region creates a unique possibility for estimating the amount of metallic tubes in different nanotube networks. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Abstract: We present sidewall functionalization of carbon nanotubes via various dissolving metal reduction methods. Carbanion complexes of lithium were prepared in liquid ammonia and THF and reacted with a series of alkyl and aryl halogenides, as well as methanol resulting in the formations of alkylated, arylated, and hydrogenated nanotube derivatives, respectively. The hydrogenation reactions were also performed on graphite. Thermal stabilities and chemical compositions were determined by thermogravimetry-mass spectrometry (TG-MS). Characteristic decomposition peaks, observed in the range of 350-600 degrees C, suggest the formation of covalent derivatives upon functionalization. The compositions are 0.01-0.25 side group/carbon depending on the materials. NMR and Raman spectroscopic characterizations show the structural changes due to functionalization.
Abstract: Cubane(1) ( C(8)H(8)) and fullerene(2) ( C(60)) are famous cage molecules with shapes of platonic or archimedean solids. Their remarkable chemical and solid- state properties have induced great scientific interest(3 - 5). Both materials form polymorphic crystals of molecules with variable orientational ordering(6 - 9). The idea of intercalating fullerene with cubane was raised several years ago(10) but no attempts at preparation have been reported. Here we show that C(60) and similarly C(70) form high- symmetry molecular crystals with cubane owing to topological molecular recognition between the convex surface of fullerenes and the concave cubane. Static cubane occupies the octahedral voids of the face- centred- cubic structures and acts as a bearing between the rotating fullerene molecules. The smooth contact of the rotor and stator molecules decreases significantly the temperature of orientational ordering. These materials have great topochemical importance: at elevated temperatures they transform to high- stability covalent derivatives although preserving their crystalline appearance. The size- dependent molecular recognition promises selective formation of related structures with higher fullerenes and/ or substituted cubanes.
Abstract: The electronic properties of p-doped single-walled carbon nanotube (SWNT) bulk samples were studied by temperature-dependent resistivity and thermopower, optical reflectivity, and Raman spectroscopy. These all give consistent results for the Fermi level downshift (Delta E(F)) induced by doping. We find Delta E(F) approximate to 0.35 eV and 0.50 eV for concentrated nitric and sulfuric acid doping respectively. With these values, the evolution of Raman spectra can be explained by variations in the resonance condition as E(F) moves down into the valence band. Furthermore, we find no evidence for diameter-selective doping, nor any distinction between doping responses of metallic and semiconducting tubes.
Abstract: We present transmission spectra from the far infrared through the ultraviolet region on freestanding SWNT films at temperatures between 40 and 300 K. Several interesting features are observed in the low-frequency part of the spectrum: the Drude-like frequency dependence of the metallic tubes as well as a (sample-dependent) peak in the conductivity around 0.01 eV. We also studied the accidental nitrate doping of the SWNT samples during purification by nitric acid. As prepared purified samples exhibit increased metallic absorption and decreased interband transitions; these features disappear on heating in vacuum.
Abstract: The Jahn-Teller effect plays a crucial role in the explanation of the insulating character of A(4)C(60) (A = K, Rb, Cs). To detect possible phase transitions arising from the interplay between the molecular Jahn-Teller distortion and the distorting potential field of the counterions, we measured the mid-IR spectra of A(4)C(60) compounds in the temperature range 90-300 K and found significant spectral changes with temperature in all three compounds. We also compare these spectra to that of Na4C60 in its room-temperature polymeric phase, where the distortion is more pronounced and evident from the structure.
Abstract: Here we present preliminary results on Mg4C60, a new fulleride polymer. A series of MgxC60 compositions were prepared by solid state synthesis. While most samples were multiphase, the nominal composition Mg4C60 provided a single phase material. X-ray powder diffraction data revealed that the structure is rhombohedral based on polymeric sheets of C-60 molecules. Charge transfer from Mg to C-60 was estimated by Raman microscopy.
Abstract: We present preliminary results on the preparation and structure Of Mg(4)C(60), a new fulleride polymer. A series of Mg(x)C(60) compositions (0 < x < 6) were prepared by solid state reaction. While most samples were multiphase, a single phase material was obtained at the nominal composition Mg(4)C(60). The X-ray diffraction pattern Of Mg(4)C(60) was successfully indexed and fitted to a rhombohedral structure based on two-dimensional polymeric sheets Of C(60) ions. This result extends the family of fulleride polymers to the salts of multiply charged cations and thus opens a new direction in this field. (C) 2003 Elsevier Ltd. All rights reserved.
Abstract: We have measured the infrared spectra of Cs4C60 in the temperature range 220-450 K. Two anomalies in the low-frequency modes at 270 K and 400 K point to changes in molecular or crystal structure. The most probable explanation is a rotator phase above 400 K and a fully ordered phase below 220 K; the intermediate structure is one where molecular Jahn-Teller distortions compete with crystal field effects.
Abstract: The mid-IR transmission spectra of C-70, M3C70, M4C70 (where M = K and Rb) in inert atmosphere and their air exposed form were measured. Most of the vibrational modes soften with charge, except for a stiffening in the 795 cm(-1) mode. We found the 1430 cm(-1) peak to be a good indicator of molecular charge, giving a 13 cm(-1) shift per electron, in agreement with calculations, M4C70 decomposes on air into M3C70, and M3C70 into C-70. The temperature dependence of the spectra of Rb4C70 was also investigated, but no phase transition was seen between 77 K and 360 K.
Abstract: Optimized geometries, energetics and vibrational spectra of C(59)N, C(59)HN and (C(59)N)(2) have been calculated on the density functional (DFT) level, using B3LYP functional. Several isoelectronic model molecules ((12)C(59)(14)C)(-) and ((12)C(59)H(14)C)(-) have also been considered, for comparison.
