Fluorine-intercalated Graphite Fibers Research Articles

Image analysis of TEM pictures of fluorine-intercalated graphite fibers

A digitization of the TEM pictures of fluorine-intercalated graphite fibers has been used to carry out quantitative measurements of the defect structure of this material. Emphasis is given to both the computer analysis technique and to the characterization of the defects. The amount of intercalation-induced disorder increases with increasing fluorine concentration. The fast Fourier transform of the digitized TEM image exhibits two diffuse spots, corresponding to the c-axis repeat distance of the intercalation compound. The length and width of the spots are a measure of the out-of-plane and in-plane disorder present in the fibers. From the fast Fourier transform, the distribution of interlayer repeat distances and the fraction of unintercalated graphite regions throughout the material is obtained. By selecting a small range of repeat distances and carrying out an inverse fast Fourier transform, the spatial distribution of material with a given repeat distance is determined. Regions with the same repeat distance are found to form islands. This particular feature of fluorine graphite intercalation compounds, as well as the nature of the microscopic defects and the staging behavior of fluorine-intercalated graphite fibers, are discussed in connection with the dual covalent and ionic nature of the carbon-fluorine bond in fluorine-intercalated graphite.

Highly Stable Fluorine-Intercalated Graphite Fibers Coated with Graphite Fluoride Film

The stability of fluorine-intercalated graphite fibers were improved by the surface coating of the fibers with graphite fluoride film. The preparation process of the GIC was composed of two steps. In the first step, the pitch-based graphite fibers were pre-treated with elemental fluorine at the temperature range of 300-410°C, so that the surface of the fibers were coated with graphite fluoride which has an excellent water-repellency. Then, in the second step, thus surface-coated fibers were intercalated by F2 below 100°C.The graphite fluoride-coated GICs showed higher stability than the non-coated one from the viewpoints of the change in weight decrease and electrical conductivity as a function of exposure time to air. Furthermore, X-ray diffractions showed no decomposition of the former GICs in air. Where as, the latter decomposed within a few days.

Electron-spin-resonance study of fluorine-intercalated graphite fibers.

We have measured the electron-spin-resonance (ESR) signal of fluorine-intercalated grpahite fibers with fluorine concentrations ranging from ${\mathrm{C}}_{6.6}$F to ${\mathrm{C}}_{3.1}$F. Previous transport measurements have shown that ${\mathrm{C}}_{\mathit{x}}$F fibers are two-dimensional materials that behave as disordered metals for dilute fluorine concentrations and as insulators beyond an approximate threshold ${\mathrm{C}}_{3.6}$F. In this paper, we report the temperature dependence (10--300 K) of the susceptibility, g factor, and ESR linewidth of the ${\mathrm{C}}_{\mathit{x}}$F fibers. We also measured the spin-lattice relaxation time of the fibers at 10 K from the power dependence of the ESR signal. The results are explained by the presence of both localized and conduction spins, and by the shift from an ordered 2D metal to a disordered 2D inhomogeneous insulator as the fluorine concentration is increased.

Influence of magnetic fields on the two-dimensional electron transport in weakly disordered fluorine-intercalated graphite fibers.

We report measurements of the temperature dependence of the resistivity in zero magnetic field and in magnetic fields up to 5 tesla for weakly disordered fluorine-intercalated graphite fibers (${\mathrm{C}}_{\mathit{x}}$F) with various fluorine concentrations (3.6x5.8). These results have been used to determine the importance of weak localization and carrier-carrier interaction in these two-dimensional (2D) systems. We found that both quantum phenomena contribute to the logarithmic increase of resistivity with decreasing temperature, though the interaction effect is generally dominant. Also, the magnitude of the resistivity increase is very sensitive to the fluorine concentration. The temperature dependence of the inelastic scattering rate 1/${\mathrm{\ensuremath{\tau}}}_{\mathit{i}}$, as obtained from magnetoresistance measurements, is well fitted by a polynomial expression of the form ${\mathrm{\ensuremath{\tau}}}_{\mathit{i}}^{\mathrm{\ensuremath{-}}1}$=\ensuremath{\alpha}T+\ensuremath{\beta}${\mathit{T}}^{2}$. The linear term ${\mathrm{\ensuremath{\tau}}}_{\mathit{i}}^{\mathrm{\ensuremath{-}}1}$(T) exhibits a good quantitative agreement with the theoretical prediction for the expected disorder dependence of the carrier-carrier scattering in 2D disordered metals.

Raman-scattering and transmission-electron-microscopy studies of fluorine-intercalated graphite fibers CxF (7.8 >= x >= 2.9).

Raman-scattering and transmission-electron-microscopy (TEM) experiments are reported which provide information on the microstructure of fluorine-intercalated graphite fibers (${\mathrm{C}}_{\mathit{x}}$F) for stoichiometries in the range 7.8\ensuremath{\ge}x\ensuremath{\ge}2.9. Lorentzian fits to our Raman spectra indicate the presence of a doublet near 1600 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and a broad line near 1360 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. An anomalous frequency downshift of the doublet was observed in ${\mathrm{C}}_{\mathit{x}}$F fibers upon increase in the intercalate concentration, unlike the behavior observed in other acceptor graphite intercalation compounds. This downshift in frequency of the doublet can be understood in terms of a decrease in the hole concentration in the graphene layers with increasing fluorine concentration, resulting in an increase in the in-plane lattice constant and a decrease in the in-plane mode frequency. These Raman results are consistent with high-resolution TEM experiments on the same fibers. Lattice fringe images of ${\mathrm{C}}_{\mathit{x}}$F fibers show wavy fringes, and the waviness is identified with semi-ionic (covalent) bonding. Evidence for unintercalated graphite regions is also presented. As the fluorine concentration of the ${\mathrm{C}}_{\mathit{x}}$F fibers increases to high values, the amount of waviness increases.

Magnetotransport at the metal-insulator transition in fluorine-intercalated graphite fibers.

We have investigated the transport properties of high-fluorine-concentration fluorine-intercalated vapor-grown graphite fibers (${\mathrm{C}}_{2.9}$F and ${\mathrm{C}}_{3.0}$F). As previously reported, the transport properties of dilute fluorine-intercalated graphite fibers (${\mathrm{C}}_{\mathit{x}}$F with x\ensuremath{\ge}3.6) exhibit a weak disorder regime with a strong carrer-carrier interaction correction. These corrections to the metallic properties of the ${\mathrm{C}}_{3.6}$F fibers emphasize the presence of intercalation-induced disorder, which is believed to be associated with the semi-ionic bonding of fluorine in carbon. With increasing fluorine concentration (x\ensuremath{\le}3.0), the fibers undergo a transition from a metallic to an insulating regime. The temperature dependence of the resistivity of the fibers in the insulating regime is no longer logarithmic at low temperature as is measured for less concentrated samples (x\ensuremath{\ge}3.6), but rather fits a two-dimensional Mott law quite closely. From the temperature dependence of the resistivity and the nonlinear I-vs-V relationship at high electric fields, a localization length on the order of \ensuremath{\xi}=1000 \AA{} is extracted. This very large localization length shows that the fibers with ${\mathrm{C}}_{2.9}$F and ${\mathrm{C}}_{3.0}$F stoichiometries lie close to the metal-insulator transition. For the ${\mathrm{C}}_{2.9}$F and the ${\mathrm{C}}_{3.0}$F fibers, the transverse magnetoresistance is negative at low fields, but becomes positive at higher fields, whereas the longitudinal magnetoresistance is positive for all fields. Both the transverse and longitudinal magnetoresistance saturate at high field and low temperature. These features are explained by the superposition of an orbital quantum-interference-induced negative magnetoresistance and a spin-polarization-induced positive magnetoresistance.

Electrical properties of room temperature fluorine-intercalated graphite fibers

Fluorine-intercalated graphite fibers have been synthesized at room temperature by direct reaction with elemental fluorine. The host materials were either vapor-grown fibers or pitch-based fibers. The C x F compounds obtained were mostly of the stage-3 type with a 12.65 Å repeat distance. The carbon-to-fluorine atomic ratio ranged from 10 to 14. Resistivity measurements carried out between 4.2 and 300 K show clearly that the fluorination reduces the resistivity values, whatever the type of pristine material. For fluorine-intercalated pitch-based fibers an anomalous increase of resistivity below 30 K may be consistent with a weak electronic localization or with two-dimensional carrier-carrier interaction effects. In fluorine-intercalated vapor-grown fibers the anomalous bump observed around 190 K could result from a phase transition. The room temperature value of the conductivity of this type of material, i.e. σ = 1.6 × 10 5 S cm −1, is one of the highest ever observed for fluorine-intercalated C x F fibers.

Magnetic-field dependence of the hole-hole interaction in fluorine-intercalated graphite fibers.

Magnetoresistance measurements have been performed in high magnetic fields up to 15 teslas in fluorine-intercalated vapor-deposited graphite fibers with concentrations ${\mathrm{C}}_{4.5}$F and ${\mathrm{C}}_{3.6}$F. Previous studies had shown that the intercalation of fluorine, an acceptor in graphite, induces a two-dimensional hole gas in the graphene layers. The fluorine-intercalation process also introduces disorder into the highly ordered pristine fiber, as exhibited by the transport properties. At high magnetic fields, a positive magnetoresistance is observed and is found to vary strongly with temperature. This phenomenon is explained by the effect of the magnetic field on the hole-hole interaction when the spin subbands are split by the magnetic field. A negative magnetoresistance is observed at low fields, and is explained in terms of weak-localization theory. The magnetoresistance data are fitted with the weak-localization contribution in the weak-field regime, then extrapolated to high fields and this contribution is finally subtracted from the measured values to obtain the hole-hole interaction contribution to the magnetoresistance. From a fit to this latter contribution, a value for the screening parameter is found. The relative importance of the weak localization and hole-hole interaction contributions to the logarithmic increase in \ensuremath{\rho}(T) at low temperature is discussed.

Electrical and thermal properties of fluorine-intercalated graphite fibers.

We report measurements on the electrical- and thermal-transport properties of fluorine-intercalated vapor-grown carbon fibers ${\mathrm{C}}_{\mathit{x}}$F with the carbon-to-fluorine-atom ratio x between 4.1 and 5.8. The electrical resistivity is found to be very sensitive to the fluorine concentration. Indeed, while the resistivities of the ${\mathrm{C}}_{5.2}$F and the ${\mathrm{C}}_{5.8}$F compounds are well below that of the pristine graphite fibers, the tendency is reversed when x\ensuremath{\le}4.5 and large resistivity values are obtained. All ${\mathrm{C}}_{\mathit{x}}$F compounds exhibit a logarithmic resistance increase at low temperature, which can consistently be explained by weak-localization and carrier-carrier interaction effects for two-dimensional electron systems. Large negative magnetoresistance values have been measured on the ${\mathrm{C}}_{\mathit{x}}$F compound with x=4.1. At higher temperature, an anomalous resistance-versus-temperature behavior appears for each fluorine-intercalated vapor-grown carbon fiber, possibly resulting from a phase transition. Heat transport in ${\mathrm{C}}_{\mathit{x}}$F (with x=4.1) is almost entirely governed by lattice vibrations between 2.5 and 300 K while the thermoelectric power is positive, and its temperature dependence is similar to that previously observed in other acceptor graphite intercalation compounds. Our results indicate that the high resistivity of the ${\mathrm{C}}_{\mathit{x}}$F compounds (with x\ensuremath{\le}4.5) is due to strong defect scattering rather than a low free-charge-carrier density.

Weak Localization Effects in Fluorineintercalated Graphite Fibers

ABSTRACTThe intercalation of fluorine into graphite introduces defects into the highly crystalline pristine fibers. These defectsare studied using temperature-dependent resistivity and magnetoresistance measurements. A logarithmic increase in resistivity at low temperature is observed, whereas the high temperature behavior is metallic. At weak magnetic fields and low temperatures, a negative magnetoresistance is observed, which becomes positive at high fields. These effects are explainedusing the two theories of weak localization and hole-hole interaction. In the light of TEM pictures of the microstructure of the fluorinated fibers, the origin of the defects in the intercalated fibers is discussed.

Raman Studies of Fluorine-Intercalated Carbon Fibers

ABSTRACTWe report Raman scattering measurements on fluorine-intercalated graphite fibers (CxF) for stoichiometries x =7.8, 4.5 and 2.9. Lorentzian fits to our Raman lineshapes indicate the presence of two lines around 1600 cm−1 and a broad line around 1355 cm−1 . The 1355 cm−1 line is the disorder-induced graphite line and the ratio of the integrated intensity of this line to that of the 1600 cm−1 doublet (R) provides a measure of the intercalationinduced disorder in the CjF fibers. Both TEM and Raman studies indicate the presence of unintercalated graphitic regions in the CxF fiber. The inverse relation between the average crystal planar domain size La, and the Raman intensity ratio R yields La. ,˜52Å for C7.8F fibers and La. , ˜40Å for C4.5,F and C2.9F fibers.

Preparation and electrical conductivity of fluorine-intercalated graphite fibers

Fluorine-graphite intercalation compounds were prepared in the presence of CuF 2, AgF or TiF 4, and their electrical conductivities and stability were examined. Since these metal fluorides are hygroscopic, starting materials were metal powders, which were fluorinated at 200°C in a nickel reactor and successively used for the experiments. After repeating the introduction of fluorine into the reactor and the evacuation 3 times using rotary and diffusion pumps, no absorption due to hydrogen fluoride was confirmed by IR spectrum in every run of experiment in order to avoid cointercalation of hydrogen fluoride and fluorine. When CuF 2 and AgF were used, fluorine was intercalated into graphite and a trace of metal fluoride was detected in GIC. Experimental results suggest that these metal fluorides act as catalysts to aid the intercalation of fluorine. AgF having a lower melting point than CuF 2 was more effective for fluorine- intercalation. On the other hand, highly volatile TiF 4 was cointercalated with fluorine at around 200°C. The electrical conductivity was increased with increasing intercalated fluorine and reached the maximum value at the 3rd stage. The maximum conductivities were 9, 8 and 6 times those of pristine vapor-grown, pitch-based and PAN-based carbon fibers, respectively. The stability of GIC was the reverse order.