Synthesis and structure of pristine and alkali-metal-intercalated single-walled carbon nanotubes

8 (M=K or Cs). No crystalline structure was observed in the reacted materials by X-ray diffraction. In situ metal deposition, TEM, and electron energy loss spectroscopy (EELS) measurements were performed on individual SWNT bundles at 300 K. The results showed that alkali metals can be reversibly intercalated into the SWNT bundles. Although intercalation induced structural disorder, individual nanotubes and to a large extent the bundles maintained their structural integrity after intercalation and de-intercalation. Received: 10 February 1998     

Hydrogen adsorption in microporous alkali-doped carbons (activated carbon and single wall nanotubes)

Adsorption of hydrogen gas was tested in microporous doped carbons: activated carbon (1600 m²/g) and single wall carbon nanotubes (SWNTs). The isotherms of adsorption of LiC₁₈ and KC₂₄ doped microporous activated carbons were determined in the range [0–30 bar] at room temperature and 77 K. The chemisorption ratio observed at room temperature increases with increasing the alkali/carbon rate. The isotherm profiles of doped activated carbon at 77 K show no clear enhancement of the sorption ratio compared to the raw activated carbon.The adsorption sites of potassium doped SWNTs with closed end were determined by neutron diffraction experiment using deuterium gas. The K-doped SWNTs were found only slightly intercalated by K ions so that empty cavities are preserved in between the tubes. At room temperature, the chemisorption of deuterium was not observed in doped SWNTs bundles, but only in the KC₈ graphite intercalation compound impurities. At low temperature, the isotherms analysis and neutron diffraction experiments have shown that D₂ molecules are physisorbed in the free interstitial voids in between the tubes within the bundles.     

Electrical resistivity of intercalated molybdenum disulfide

The electrical resistivity of hexagonal synthetic single crystals and natural crystals of MoS₂ intercalated with alkali metals (K, Rb and Cs) has been measured. The average room-temperature values is 0.004 ω-cm, and the best single crystal data on K_0.4MoS₂ showed at T^1.5 temperature behaviour characteristic of acoustical phonon scattering for a quasi-isotropic metal. The metallic behaviour and scattering mechanism are discussed.     

Alkali displacements in intercalated 1T-TaSe2

Single crystals of 1T-TaSe₂ have been intercalated with different alkali metals by deposition in ultra-high vacuum onto in situ-cleaved (0001) surfaces. In a second step a different alkali metal, or Cl₂, has been deposited on top. The interactions among the deposited species and the substrate have been investigated using soft x-ray photoelectron spectroscopy (SXPS). Li and Na appear to compete in the intercalation process. Li replaces Na, by pushing it deeper into the crystal. Cs on Li:TaSe₂ does not intercalate, but stays on top, repelling intercalated Li⁺ deeper inside. For Li on Cs:TaSe₂ an exchange reaction takes place, and Cs is deintercalated. The same effect is induced by Na deposited on intercalated Cs. 1T→2H phase transition for TaSe₂ has been observed only for Li deposition. Cl₂ deposition on Na intercalated substrate induced deintercalation of Na. The experimental results are discussed in relation to thermodynamic, electronic and electrostatic effects. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

X-ray study of the order—disorder transition in high-stage alkali metal intercalated graphite single crystals

Graphite intercalation compounds display a variety of structural properties because of their composite nature (graphite + intercalate) and their layered arrangement. Alkali metal intercalated graphite compounds undergo order-disorder phase transitions when the temperature is varied in the range 300–10 K. The disordered state shows true two-dimensional character, whereas three-dimensional coupling takes place on ordering. Results of single-crystal X-ray diffractometric and photographic studies of stage-2 KC₂₄ single crystals are presented. The positional and orientational correlations of the modulated liquid phase have been studied from 300 K down to the temperature transition T_u = 123.5° K. At the transition, the hexagonal incommensurate solid structure of the alkali metal is modulated by the graphite potential. This transition is discussed in terms of the relaxed-close packed structure model (Dicenzo, 1982). At low temperature a second transition takes place at T_L ≈ 95 K. It is found to correspond to the breaking of the 2D hexagonal symmetry of the K layer.     

Alkali metal intercalation into SnS2

The intercalation of sodium and potassium into the layered semiconductor SnS₂ has been investigated by ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and ion scattering spectroscopy (ISS). After deposition of the alkali metals onto (0001) cleavage planes of SnS₂ in ultra high vacuum (UHV), semiconducting intercalation phases were formed. They seem to be homogeneous and disordered under the given experimental conditions. The valence electrons of the alkali metals are transferred into electronic states of the host lattice, whose valence band density of states changes significantly during intercalation. The underlying changes of the binding properties of the host lattice are discussed. The course of intercalation can be separated into three phases. During an induction period the concentration of the alkali metal on the surface remains very small, the electronic states of the substrate are shifted by band bending. During an intercalation period the topotactic reaction proceeds. After reaching saturation compositions of the intercalation phase at the surface, the alkali metal diffuses into the bulk. Crystal or surface defects seem to have a significant influence on the kinetics of intercalation and on the stoichiometry of the intercalation compounds.     

Highly-angle-resolved ultraviolet photoelectron spectroscopy of C8Cs

Highly-angle-resolved ultraviolet photoelectron spectroscopy was carried out for a C₈Cs single crystal to study the electronic charge transfer in alkali metal graphite intercalation compounds. The dispersive π¹-band at the K̃ point in the Brillouin zone was observed for the first time. The electron occupation in the π¹-band was estimated to be 0.45±0.05 unit electronic charge. This strongly suggests that a substantial part of an interlayer band exists below the Fermi level at the γ point, forming a spherical Fermi surface on the center of the Brillouin zone.     

Thin graphite overlayers: Graphene and alkali metal intercalation

Using LEED and angle resolved photoemission for characterisation we have prepared graphite overlayers with down to monolayer thickness by heating SiC crystals and monitored alkali metal intercalation for the multilayer films. The valence band structure of the monolayer is similar to that calculated for graphene though downshifted by around 0.8 eV and with a small gap at the zone corner. The shift suggests that the transport properties, which are of much present interest, are similar to that of a biased graphene sample. Upon alkali metal deposition the 3D character of the π states is lost and the resulting band structure becomes graphene like. A comparison with data obtained for ex situ prepared intercalation compounds indicates that the graphite film has converted to the stage 1 compounds C₈K or C₈Rb. Advantages with the present preparation method is that the graphite film can be recovered by desorbing small amounts of alkali metal and that the progress of compound formation can be monitored. The energy shifts measured after different deposits indicate that saturation is reached in three steps. Our interpretation is that in the first the alkali atoms are dispersed while the final steps are characterized by the formation of first one and then a second (2 × 2) ordered alkali metal layer adjacent to the uppermost carbon layer.     

Electronic structure of ordered Cs and K overlayers on graphite: Direct observation of complete charge transfer

To elucidate further the electronic changes occurring during the initial stages of intercalation, we have made the first studies of ordered Cs and K overlayers on single crystal graphite using angle-resolved ultraviolet photoelectron spectroscopy (ARUPS). In this Communication, we clearly demonstrate that in the case of a (2 × 2)R0° overlayer of Cs or K, the outer alkali metal s electron is completely transferred to the graphite π¹-conduction bands. We find no evidence for alkali-like s states at the Fermi level.     

Spektroskopische Untersuchung unelastischer Stöße zwischen Ionen oder metastabilen Atomen der Edelgase und Alkalimetallatomen bei thermischen Stoßenergien

The present work deals with the spectroscopic investigation of Penning ionization and nonresonant charge exchange due to thermal-energy collisions of metastable atoms or ions of the inert gases He, Ne with atoms of alkali metals K, Rb, Cs in a low temperature plasma of the corresponding binary mixtures. The rate constants of these reactions and partial rate constants of exciting several alkali metal ion levels were determined by the kinetics of the decay of ionized and excited particles in the afterglow of a pulsed discharge. Measured Penning and charge exchange constants are in order of 10^10-9 cm³ sec^?1. Partial Penning constants in the He? Cs system exhibit a small dependence of the energy defect of reaction, whereby an important part of collisions results in excited Cs⁺-states. On the contrary, energy transfer by charge exchange in He? Rb, Cs systems distinguishes by a significant energy defect dependence. Charge exchange from He⁺ to Rb₀ leads above all to He(2³S₁)-atoms and normal Rb⁺-ions. It follows from the metal ion spectrums, that Penning ionization and charge exchange of the investigated systems under utilized conditions represent as processes of selective excitation of alkali metal ions in afterglow and stationary discharge.     

Intercalation of graphite and hexagonal boron nitride by lithium

Although graphite and hexagonal form of BN (h-BN) are isoelectronic and have very similar lattice structures, it has been very difficult to intercalate h-BN while there are hundreds of intercalation compounds of graphite. We have done a comparative first principles investigation of lithium intercalation of graphite and hexagonal boron nitride to provide clues for the difficulty of h-BN intercalation. In particular lattice structure, cohesive energy, formation enthalpy, charge transfer and electronic structure of both intercalation compounds are calculated in the density functional theory framework with local density approximation to the exchange-correlation energy. The calculated formation enthalpy of the considered forms of Li intercalated h-BN is found to be positive which rules out h-BN intercalation without externally supplied energy. Also, the Li(BN)₃ form of Li-intercalated h-BN is found to have a large electronic density of states at the Fermi level and an interlayer state that crosses Fermi level at the zone center; these properties make it an interesting material to investigate the role of interlayer states in the superconductivity of alkali intercalated layered structures. The most pronounced change in the charge distribution of the intercalated compounds is found to be charge transfer from the planar σ states to the π states.     

Intercalation of C60 fullerene crystals with alkali and alkaline-earth metals through self-propagating high-temperature synthesis

A method for rapidly intercalating C₆₀ fullerene crystals has been implemented using self-propagating high-temperature synthesis. The method has been used to intercalate C₆₀ fullerene crystals with alkali (K, Rb) and alkaline-earth (Ca, Ba) metals. The superconducting transition temperatures of the prepared compounds have been measured. The C₆₀ fullerene crystals intercalated with calcium have been investigated using X-ray diffraction.     

Absorption spectrum of thin K<Subscript>2</Subscript>CdI<Subscript>4</Subscript> films

The absorption spectrum of thin films of a new compound, K₂CdI₄, was studied. It was established that this compound belongs to direct-bandgap dielectrics and that its low-frequency electronic and excitonic excitations are localized in CdI ₄ ^2? structural blocks of the crystal lattice. It was found that, in M₂CdI₄ compounds (M = K, Rb, Cs), the bandgap width E _g grows and the spin-orbit splitting of the valence band top decreases with increasing ionic radius of the alkali metal.     

Low energy photoelectron spectroscopy on alkali graphite intercalation compounds

The conduction band of various stages of alkali graphite intercalation compounds has been studied by low energy photoelectron spectroscopy (hv ? 6.55 eV). The dissimilar behaviour of the width β of the conduction band peak as a function of photon energy for C₆Li and C₈M (M = K, Rb, and Cs) is discussed in terms of different band types in the vicinity of the Fermi level. The stage dependence of β is measured and interpreted for the system C_xK (for stages 1, 2, 4, and 5).     

The effect of a large SWNT diameter distribution on cesium intercalation

Vapor-phase intercalation of a single-walled carbon nanotube sample with Cs was carried out and monitored in situ by Raman spectroscopy. Results indicate that the endpoint of the intercalation was limited by small interstitial gaps in the nanotube bundles. These small-diameter gaps are present because of the significant number of small-diameter nanotubes (0.9-1.0 nm, as calculated from Raman radial breathing mode frequencies) present in the sample. It is not possible to determine from our Raman spectra whether the early endpoint is the result of diffusion limitation or the equilibrium energetics at the endpoint, although some diffusion limitation is observed near the beginning of the reaction. A simple geometric model for expansion of the nanotube bundles under intercalation is presented; this model reproduces, reasonably well, measured expansions reported by others and explains both diffusion- and equilibrium-limited mechanisms in terms of the larger lattice expansion required for smaller-diameter nanotubes. Staging of the intercalation process, in analogy with the staged intercalation of graphite intercalation compounds, is not observed. Instead, the transverse mode peaks undergo a gradual decrease in intensity and a gradual charge transfer- and electronic coupling-induced downshift.     

Characterization of carbon nanotubes exposed to Na or bombarded with Na at room temperature

We report an electron spectroscopy study on low energy Na⁺ ion implantation and Na atom intercalation in single walled and multi walled carbon nanotube mat samples. Our results show that these two different methods yield quite different dopant spatial distribution since implanted sodium atoms remain on the surface while intercalated alkali metal particles readily diffuse in the bulk.     

Spectroscopic (FT‐IR,FT‐Raman,UV, 1H,and 13C NMR) and theoretical studies of m‐anisic acid and lithium,sodium, potassium,rubidium, and caesium m‐anisates

In order to understand the nature of the interactions of biologically important ligands, it is necessary to carry out the physico‐chemical studies of these compounds with their biological targets (e.g., receptors in the cell or important cell components). Results of this study make it possible to predict some properties of a molecule, such as its reactivity, durability of complex compounds, and kinship to enzymes. In this paper the effect of alkali metal cations (Li, Na, K, Rb, and Cs) on the electronic structure of m‐methoxybenzoic acid (m‐anisic acid) was studied. The experimental IR (in solid state and solution), Raman, UV (in solid state and solution), ¹H, and ¹³C NMR spectra of m‐methoxybenzoic acid, and its salts were registered, assigned, and analyzed. Some of the obtained results were compared with published data for o‐anisic acid and o‐anisates. The structures of anisic acid and Li, Na, and K m‐anisates were optimized at the B3LYP/6‐311++G** level. The IR, ¹H, and ¹³C NMR spectra and NPA, ChelpG, and MK atomic charges were calculated. The change of metal along with the series: Li → Na → K → Rb → Cs caused: (1) the change in the electronic charge distribution in anisate anion that is seen via the occurrence of the systematic shifts of several bands in the experimental and theoretical IR and Raman spectra of anisates; (2) systematic ¹H and ¹³C NMR chemical shifts; (3) hypsochromic shifts in UV spectra of salts as compared to ligands. Copyright © 2009 John Wiley & Sons, Ltd.     

Aspects of crystal growth within carbon nanotubes

The comparative crystallisation and HRTEM imaging properties of simple binary halides formed by the alkali iodides MI (M = Li, K, Na, Rb and Cs) within single walled carbon nanotubes (SWNTs) are described. The most common structure type observed within SWNTs is the rocksalt archetype, although CsI was observed to form both bcc and rocksalt structure types. In SWNTs forming in the 1.2–1.6 nm diameter range, all of the incorporated halides showed preferred orientation, with the 〈100〉 growth direction predominating for rocksalt-type packing and 〈112〉 so far observed exclusively for bcc packing. Crystals with dimensions spanning 2–6 atomic layers thickness in projection invariably exhibited lattice expansions that were attributed predominantly to a net reduction in coordination at the crystal-carbon interface. The crystallisation behaviour of UCl₄–KCl and AgI–AgCl eutectic melts was compared in carbon nanotubes of different diameters and a pronounced ordering influence over the normally glassy melts was observed in narrower capillaries. HgI₂ crystallised within nanotubes with ultra-narrow (i.e., 0.8 nm) capillaries were observed to form helical 2 ×1 layer crystals. To cite this article: J. Sloan et al., C. R. Physique 4 (2003).     

Metallization and demetallization of clean and oxygen-covered ultrathin alkali metal films on GaAs(1 0 0)

The structure and the electronic valence state occupation of ultrathin K, Rb, and Cs films grown on a GaAs(1 0 0)-(4×2) surface have been studied by means of metastable He atom scattering (MHAS), He atom scattering (HAS), and low-energy electron diffraction (LEED) at temperatures ranging from 150 to 400 K. From the survival probability of the scattered He^* atoms, detailed information on the coverage-dependent filling of the alkali metal valence states and their emptying upon subsequent exposure to oxygen were derived. These data reveal for K and Rb a nearly linear band filling with increasing coverage starting at about 0.5 ML whereas a more rapid filling is observed for Cs which is almost completed at about 0.7 ML. Subsequent oxygen adsorption causes a demetallization of the metallic alkali metal monolayers. In case of Cs, a distinct minimum of the He^* signal appears at an oxygen exposure of about 0.8 L, presumably indicating the onset of subsurface oxidation.     

Photoelectric and optical properties of thin films of ternary chalcogenides of the form MeISbX 2 VI

We have studied the photoconductivity and optical absorption of thin amorphous films of ternary chalcogenide compounds of the type Me^ISbX ₂ ^VI (where Me is Na, K, Rb and X^VI is either S or Se) produced by thermal evaporation in a vacuum. The band gaps were determined by various methods. A regular increase in band gaps in the films was noted as the atomic number of the alkali metal increased.