The de Haas-van Alphen effect of graphite intercalation compounds with SbCl5 and HNO3

The de Haas-van Alphen (dHvA) effect of SbCl₅-graphite intercalation compounds of stage 2, 3 and 4, and residual HNO₃-compound of stage 3 has been studied. The dHvA spectra are stage dependent, and no combination frequency relations are found, which are in disagreement with Batallan et al.'s report. The amount of charge transfer per intercalant estimated on the basis of the rigid band model is 0.44, 0.49 and 0.43 for stage 4, 3 and 2 SbCl₅-compounds and is 0.14 for stage 3 HNO₃-compound.     

Quantum oscillatory phenomena in graphite intercalated with AsF5

Both Shubnikov-de Haas and de Haas-van Alphen oscillations are reported for C_8nAsF₅ (n=1,2, is the stage number). The observed oscillations are interpreted in terms of a tight-binding model for the band structure, and good agreement with both band parameters and effective carrier masses is found. In addition, the effective charge transfer per AsF₅ molecule (f=.37 for stage 1, =.41 for stage 2) is in basic agreement with previous determinations.     

Variable charge transfer and band structure of graphite intercalation compounds: C8nAsF5

Using measurements of oscillatory magnetoresistance, the charge transfer in stage 2 AsF₅-graphite is found to vary over the range f ? 30–45%. By following the change of Fermi surface areas with f, a more precise determination of the band structure is possible. The band parameters are nearly identical to those of pristine graphite.     

De haas-van alphen effect in stage 2 graphite-AsF5

We have observed dHvA frequencies in several samples of stage 2 graphite-AsF₅ intercalation compound. A consistent interpretation is given in terms of two nested hole surfaces located at each of the 6 corners of the 2D graphite zone. A fractional ionization f = 0.42 is obtained, consistent with spin susceptibility, with the Blinowski-Rigaux analysis of reflectivity, and with recent chemical results.     

Metallic reflectance of AsF5-graphite intercalation compounds

Room temperature measurements of the 0.07–2.0 eV optical reflectance of carefully prepared stage 1–4 AsF₅-graphite intercalation compounds have been performed. Stages 1–3 show simple metallic behavior with a well-defined plasma edge. Curve fits to the data give good agreement between dc and optical conductivities for stage 1 and 2. Comparison between stage 2 data for AsF₅ and HNO₃ compounds suggests that the higher conductivity of the former arises from a longer carrier relaxation time rather than from a greater carrier density.     

Conduction electron spin resonance in acceptor-type graphite intercalation compounds

An initial survey of the conduction electron spin resonance is presented for a series of graphite compounds intercalated with acceptor molecules: stages 1–3 AsF₅, stages 2–5 HNO₃, and stage 2 Br₂ and ICl. The g-values and lineshapes were studied as functions of temperature and concentration. The results suggest metallic behavior but with very small density of states at the Fermi energy: N(E_F) ～10²⁰/cm³ eV. The temperature dependence of the linewidth is dominated by an order-disorder transition of the intercalant layers, implying that the conduction electrons are not entirely confined to the graphite portion of the crystal. The decrease in g-value anisotropy upon intercalation can be understood in terms of Elliott's theory.     

Raman scattering in low stage compounds of graphite intercalated with AsF5, HNO3 and SbCl5

Raman measurements on low stage compounds of graphite intercalated with AsF₅, HNO₃ and SbCl₅ are reported. The spectrum of the stage 1 C₈AsF₅ acceptor compound is found to be in sharp contrast with that reported for the donor stage 1 C₈M(M=K, Rb, Cs) compounds. Whereas the stage 1 donor compounds have been found to exhibit a characteristic broad, asymmetric Breit-Wigner line shape, the spectrum of C₈AsF₅ contains a single Lorentzian line in this frequency region. Temperature studies of the bounding layer mode lineshape parameters in C₈AsF₅ showed no evidence of the order-disorder transition in the adjacent intercalate layer. The bounding layer mode frequencies of AsF₅, HNO₃ and SbCl₅ graphite are reported, but no intercalate layer modes were observed.     

Optical determination of the charge transfer in AsF5-graphite intercalation compounds

Optical reflectance experiments are performed on well-characterized compounds C_8nAsF₅ (n = 1, 2), in the temperature range 10–300 K. The theoretical analysis of the reflectivity spectra within the 2-D model of graphite independent subsystems [1] provides an optical determination of E_F and the charge transfer coefficient which is found independent of temperature.     

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.     

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.     

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).     

Charge transfer and islands in metal halides-graphite intercalation compounds: New evidence from x-ray diffraction of intercalated Mn Cl2

Single crystal structure studies at room temperature have been made for the first stage Mn Cl₂ intercalated graphite. The nominal composition was C_5·6 Mn Cl_2·4, as deduced from chemical analyses and X-ray diffraction intensities. The data are in agreement with the island model, already proposed for Ni Cl₂ intercalation. From the decrease in C-C bond length and comparison with As F₅ compound, it is shown that the charge transfer is determined by chlorine in excess with respect to free metal halide.     

Infrared and Raman spectroscopy of graphite-ferric chloride

We present the first detailed study of the stage dependence of the IR- and Raman-active optic graphitic modes in a graphite acceptor intercalation compound. The general frequency upshift observed with increasing FeCl₃ concentration for all optic modes is interpreted to indicate an in-plane compression within the graphitic layers. An identification of the IR-active modes with bounding and interior graphite layers is made. A lineshape analysis of the IR spectra implies IR dipole moments corresponding to ～70% of the effective charge in the graphite bounding layers, independent of stage, and ～30% distributed among the graphite interior layers for stage n?3 compounds.     

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.     

Room temperature electrical conductivity of a highly two dimensional synthetic metal: AsF5-graphite

We report here studies at room temperature of the electrical conductivity of AsF₅-graphite, a lamellar intercalation compound. Compounds with composition C_8nAsF₅ have been synthesized where n is the stage. Preliminary measurements of basal plane electrical conductivities indicating values comparable with OFHC copper have been confirmed. Associated anisotropy ratios $\text{α ≡}\text{σ}{}_{\mathrm{<mtext>a</mtext>}}\text{σ}{}_{\mathrm{<mtext>c</mtext>}}\text{> 10}{}^6$ are observed for n ≤ 3. Data for both the a-axis and c-axis conductivities as a function of stage for low stage compounds is reported.     

TDPAD studies of graphite intercalated by AsF5 vapour

The TDPAD technique has been used via the¹⁹F(p,p¹γ)¹⁹F reaction to study the graphite intercalation compound (GIC) formed in AsF₅ vapour. The spectra are dominated by the quadrupole interaction corresponding to the formation of C-F bonds, withv _q~58 MHz, η=0 and δ=0.04. However, peaks are also observed which can be ascribed tentatively to the presence of AsF₃. The strong textural features of the spectra can be related to the direction of the electric field gradient (efg) with respect to the incident beam direction and the detector plane. This suggests that potentially TDPAD can be useful for the characterization of GIC's.     

Resonant Raman scattering of graphite intercalation compounds KC8, KC24, and KC36

Graphite intercalation compounds, due to charge transfer between layers of graphite and intercalants, have a strongly shifted Fermi level. Potassium is known to give its electron leading to a large charge transfer f_c close to for stage 1 (KC₈) and for stage 2 (KC₂₄). The question is more subtle in stage 3 (KC₃₆) for which the graphene layers are not equivalent. For stage 3, two Raman G bands are clearly visible, corresponding to the interior layer and the boundary layers, respectively. By varying the excitation energy from UV to infrared, we observe that the intensity of the boundary layers G band versus that of the interior layer is maximum at 2.5 eV, leading to a sharp resonance profile at room temperature. Using first‐principle calculation, we associate this transition to π → π^∗ of the bounding layers. Copyright © 2014 John Wiley & Sons, Ltd.     

Nuclear magnetic resonance and static magnetic susceptibility of AsF5 - intercalated graphite

NMR and static magnetic susceptibility (χ) measurements on stage 1 AsF₅ intercalated graphite (C₈AsF₅) are presented. The relaxation times, T_i and T₂, of the F¹⁹ nuclear magnetic resonance were measured over the temperature range 136K – 295K; χ was measured over the range 80K – 295K. The NMR results indicate a motionally narrowed line with gradual ordering of the intercalant as the temperature is decreased. The magnetic susceptibility is independent of temperature with no observable Curie law controbution. The absence of localized moment behavior for the intercalant is interpreted either in terms of chemical disproportionation of the AsF₅ to closed shell ions or in terms of the Anderson model of localized moments in metals.     

C 1s NEXAFS study of rare-earth’s (La,Eu)-graphite intercalation compounds

Electronic structure of unoccupied states of Eu- and thin surface layer of La-intercalation compounds was studied by light polarization dependent NEXAFS at the C 1s threshold in a bulk sensitive (E_kin=1–2 eV) and a more surface sensitive (E_kin=265 eV) partial electron yield mode. It was shown that the C 1s spectra in both cases are mainly characterized by the π*- and σ*-symmetry graphite-derived features. For both systems the π*-derived peak was found at similar energies of exciting photons as for pristine graphite. A decrease of relative intensity of the π*-originated structure in intercalation compounds can be understood by partial occupation of the π*-derived states upon intercalation due to a charge transfer from rare-earth (RE) atoms. NEXAFS features found on both sides of the π* response may be related to pd hybrids forming as a result of chemical interaction between RE atoms and graphite layers.