Search

We studied the interlayer coupling and decoupling of bilayer graphene (BLG) by using spatially resolved electron energy loss spectroscopy (EELS) with a monochromated electron source. We correlated the twist-angle-dependent energy band hybridization with Moire superlattices and the corresponding optical absorption peaks. The optical absorption peak originates from the excitonic transition between the hybridized van Hove singularities (vHSs), which shifts systematically with the twist angle. We then proved that the BLG decouples when a monolayer of metal chloride is intercalated in its van der Waals (vdW) gap, and results in the elimination of the vHS peak.

Cs and ethylene were co-intercalated to form Cs-ethylene-graphite intercalation compounds (GIC) using multi-layer graphene (MLG) flakes prepared from the cleavage of highly oriented pyrolytic graphite (HOPG). The air stability of the resultant structure was examined by Raman scattering, for possible future applications such as large-scale integrated circuit (LSI) interconnects and transparent conductive electrodes. Compared with Cs- or FeCl3-intercalated MLG, the Cs-ethylene-MLG has remarkably higher structural air stability, and its initial stage 2 structure was maintained after four weeks of air exposure. In contrast, major de-intercalation of FeCl3 occurred in FeCl3-MLG. In Cs-MLG, some de-intercalation also occurred, but the stage 5 structure was maintained. The Cs-ethylene-MLG and Cs-MLG with good air stability in their structure are expected to have an n-type conduction system.

Ternary graphite intercalation compounds (GICs) incorporating two kinds of metal chlorides, namely, AlCl3-FeCl3-GIC and AlCl3-CuCl2-GIC, were synthesized from two types of graphite sheets, PGS and GRAFOIL. Their electrical conductivities and air-stabilities were evaluated in an attempt to identify highly electrically conductive and air-stable GICs. Their electrical conductivities were found to be higher than those of binary GICs such as FeCl3- and CuCl2-GICs. Especially, the AlCl3-CuCl2-GIC prepared from a PGS graphite sheet was found to have a high conductivity and high air-stability; the highest electrical conductivity was in excess of 1.0 × 105 Scm−1. Most AlCl3-CuCl2-GICs have a mixed structure of AlCl3-CuCl2-ternary GIC domains and/or AlCl3-GIC domains. The proportion of each domain was dependent on the reaction time and the type of the host graphite. AlCl3-FeCl3-GICs prepared from PGS were found to consist only of FeCl3-GICs, and their electrical conductivity and air-stability were slightly inferior to those of AlCl3-CuCl2-GICs. We concluded that an AlCl3-CuCl2-ternary GIC prepared from a PGS graphite sheet is highly electrically conductive and highly air-stable. Therefore, this is a promising candidate as a highly conductive and air-stable GIC for use in practical applications as a conducting material.

In this study, four graphite intercalation compounds (GICs) of metal chlorides, such as FeCl3, CuCl2, MoCl5, and SbCl5, were synthesized from two graphite sheets, and their electrical conductivity and air stability were accurately measured under standardized experimental conditions. After the exposure of these GICs to air, changes in their electrical conductivities and structures were recorded as a function of time. Three decomposition modes were observed. CuCl2-GICs did not decompose, and MoCl5-GICs prepared from a graphite sheet manufactured from a polyimide film (PGS) were also highly stable in air; on the other hand, immediate, extensive degradation was observed for FeCl3-GICs, while the SbCl5-GICs transformed to higher-stage (diluted) compounds. Thus, the air stability of these GICs varied with the metal chloride and host graphite used; however, their electrical conductivities prior to decomposition varied only with respect to the host graphite. Among the highly air stable, conductive compounds prepared herein, the MoCl5-GICs prepared from PGS demonstrate the highest potential for practical applications. This study reveals that highly conductive, air stable metal chlorides GICs can be synthesized.

Potassium–graphite intercalation compounds (K–GICs) prepared from flexible graphite sheets are unusually stable in air and their high electrical conductivity can be maintained for long periods. To investigate the reasons for this air-stability, we evaluated the changes in the electrical conductivity as a function of electrical carrier densities and mobilities, upon exposure of the GICs to air. These electrical carrier parameters were estimated using the measured electrical conductivity, magnetoresistance, and Hall coefficient. The decomposition processes varied depending on the characteristics of both the host graphite and the intercalated materials. In the case of K–GICs prepared from flexible graphite sheets, the electron density was decreased by the de-intercalation of K atoms. However, the electrical conductivity hardly changed, because the electron mobility was increased sufficiently to compensate for the decrease in the density. The recovery of the electron mobility was thought to be caused by the flexibility of the graphite layers. Therefore, it was found that the air-stability of GICs could be determined by considering the degree of de-intercalation as well as the flexibility of the graphite layers. Furthermore, the perfect graphite crystal is not suitable to prepare air-stable and high conductive GICs.

n-Type alkali metal-graphite intercalation compounds (GICs) and p-type metal chloride-GICs were prepared from commercially available graphite sheets, namely PGS® and GRAFOIL®. Their thermoelectric properties, electrical conductivity, thermal conductivity, and Seebeck coefficient were measured, and their thermoelectric performance was estimated in terms of the figure of merit and power factor. The electrical conductivity (103 S cm−1 to 104 S cm−1) and thermal conductivity (40 W m−1 K−1 to 200 W m−1 K−1) of these GICs are much higher than those of other, conventional thermoelectric materials, whereas their absolute Seebeck coefficients (±30 μV K−1) are lower. Therefore, although the figures of merit of the GICs are somewhat lower (∼105 K−1) than those of other thermoelectric materials, their power factors (∼103 W m−1 K−2) are sufficiently high. The thermoelectric properties of these GICs mainly depend on the host graphite type and slightly on the intercalated species. However, the Seebeck coefficient is independent of both, and the thermoelectric performance of the GICs is strongly governed by their high electrical conductivity. The power factors of almost all the GICs prepared from PGS were greater than 10−3 W m−1 K−2, which is a critical threshold value for use of thermoelectric materials in practical applications. Furthermore, the dilute K-GIC with stage-7 structure had a large Seebeck coefficient (−58 μV Κ−1), which improved the power factor to more than 10−2 W m−1 K−2. Considering the advantages of GICs, this study confirms their significant potential as thermoelectric materials.

We have reported that metal chloride-graphite intercalation compounds (GICs) prepared from flexible polyimide film-derived graphite sheets, such as CuCl2-, MoCl5-, and AlCl3-CuCl2-GICs, demonstrate excellent air stability. In this work, the high-temperature thermal stability and gas release properties of those GICs were investigated. MoCl5- and AlCl3-CuCl2-GICs were unstable at high temperatures, exhibiting weight losses at 150 °C and 100 °C, respectively, which accompanied the release of HCl and Cl2 gases. In contrast, the CuCl2-GIC demonstrated relatively high thermal stability in inert atmospheres at temperatures below 400 °C, showing neither weight loss nor gas release. After heat treatment at 600 °C at a heating rate of 5 °C/min, only a slight decrease in the electrical conductivity was observed.

Magnetoresistance and Hall coefficient of air-stable potassium-intercalated graphite sheets (hereafter abbreviated as K-PGS) were determined at room temperature. The magnitude of the magnetoresistance and the absolute value of Hall coefficient of K-PGS decreased with increasing potassium content of K-PGS, n K / n C . Two-carrier model was used for calculating carrier density and mobility. The electron density increased with increasing n K / n C : 3.07×10 20 cm −3 ( n K / n C =0.005), 5.67×10 20 cm −3 ( n K / n C =0.008) and 6.40×10 20 cm −3 ( n K / n C =0.011). The value of the electron density of K-PGS with n K / n C =0.011 (nominal composition KC 91 ) was about 80% of the reported value, 7.8×10 20 cm −3 , for KC 48 ( n K / n C =0.021) prepared from HOPG (highly oriented pyrolytic graphite). The mobility decreased with increasing n K / n C : 2.11×10 3 cm 2 V −1 s −1 ( n K / n C =0.005), 1.42×10 3 cm 2 V −1 s −1 ( n K / n C =0.008) and 1.34×10 3 cm 2 V −1 s −1 ( n K / n C =0.011). The value of the mobility of K-PGS with n K / n C =0.011 was about 60% of the reported value (2300 cm 2 V −1 s −1 ) for KC 48 prepared from HOPG.

Potassium–graphite intercalation compounds with chemical compositions of KC 84 , KC 48 , KC 36 and KC 8 were prepared from PGS graphite sheet and then treated with 6 M-HCl aqueous solution. After washing with water and drying under open air, chemical composition, stage structure and electrical conductivity of the resulting air-stable compounds were determined. It was found that about 40% of potassium in the starting potassium–graphite intercalation compounds remained in the matrix after treated with 6 M-HCl aqueous solution for KC 84 , KC 48 and KC 36 , while 15% for KC 8 . The room temperature electrical conductivity of an air-stable compound, containing potassium of 0.008 in molar ratio of potassium to carbon, was determined to be 1.3×10 5 Scm −1 . This is comparable to that of iron (1.0×10 5 Scm −1 ) and about 1/3 of that of aluminum (3.6×10 5 Scm −1 ).

The practical use of graphite intercalation compounds (GICs) as industrial materials has been considered to be unfeasible because their air stability is fairly low, notwithstanding their high electrical conductivities and low densities. On the other hand, some air-stable GICs that exhibit p-type conduction have been reported. In this study, air-stable and electrically conductive GICs that exhibited n-type conduction were prepared from commercially available flexible graphite sheets (PGS graphite sheets). We observed that K-GIC and Cs-GIC were air-stable enough to be used in practical applications and were highly electrically conductive. The air stability of Cs-GIC was slightly higher than that of K-GIC. In addition, the stage-2 structure of Cs-GIC was more stable in that it maintained ten times higher electrical conductivity than that of the host graphite after 10 years of exposure to air. Moreover, Cs-ethylene-GIC was the most stable and exhibited a blue colored surface after 10 years of exposure to air. We assumed that the significantly improved air stability of the alkali-metal-GICs prepared from PGS graphite sheets is caused by the large crystalline size and the bending graphite layer structure of the host PGS graphite.

The host graphite material of choice for most CaC6 studies is highly oriented pyrolytic graphite (HOPG), which can affect the resulting properties of prepared graphite intercalation compounds (GICs). In order to gain a better understanding of the superconductivity of CaC6, we examined CaC6 prepared from Grafoil and PGS. To confirm their superconductivity, the dependences of the magnetic moment on temperature under a constant magnetic field were measured over 2–20 K. The resulting CaC6 transitioned to a superconducting state at around 11 K, independent of purity and homogeneity, while slight differences that depended on the host materials were observed. The transition to a superconducting state at 11.5 K occurred provided that a small amount of CaC6 was present. The co-existence of LiC6 and other GICs, and the residual Li atoms in the CaC6 had little effect on the transition temperature. The transition temperature of the CaC6 prepared from PGS was slightly lower than that of the CaC6 prepared from HOPG and Grafoil.

Hydrogen-sorption isotherms of alkali metal-doped carbons at 77 K were determined for promoting application of these materials as hydrogen-recovery and isotope-separation agent. The hydrogen-sorption behavior of rubidium-doped Grafoil, with composition of RbC24, showed high sorption ability against hydrogen at low pressure. Taking into account the fact that sorption-desorption was fast and reversible, and the equilibrium pressure at half coverage was very low, i.e., 40 Pa, RbC24 prepared from Grafoil is promising as a recovery agent for hydrogen gas at low pressure. The hydrogen (H2)/deuterium(D2)-sorption isotherms of potassium-doped carbons with composition of KC10, prepared from multi wall carbon nanotube (MWCNT) and carbons derived from petroleum cokes with heat-treatment temperatures of 1000 and 1500 degrees C, were also determined. Isotope separation coefficient was estimated from those isotherms. A very large isotope effect was found for KC10 prepared from MWCNT, comparable to those prepared from carbons with heat-treatment temperatures of 1000 or 1500 degrees C. However, a severe problem was found for KC10 (MWCNT) that repetition of the sorption-desorption cycles resulted in the decrease of the sorbed amount of H2 and D2.

Calcium graphite intercalation compounds (Ca-GICs; the composition is CaC6) and GICs containing CaC6 are synthesized from Grafoil and PGS graphite sheets, and the Seebeck coefficients are measured. The absolute values of the Seebeck coefficient of these GICs are very small; in some cases, the values are positive. This phenomenon observed in the Seebeck coefficient of GICs containing CaC6 is very different from that of general GICs such as alkali-metal GICs. The temperature dependence of the Seebeck coefficient of these GICs is to that of general GICs; that is, the absolute values of the Seebeck coefficient decrease with a decrease in the temperature.

Graphite intercalation compounds (GICs) have high electrical conductivities, large Seebeck coefficients, and low thermal conductivities as compared with their host graphite materials. Due to these properties, GICs are expected to act as effective thermoelectric materials. The thermoelectric figure of merit of the GICs is not as high as that of other thermoelectric materials; this is because of the small Seebeck coefficient and high thermal conductivity of GICs. However, the thermoelectric power factor of GICs is sufficiently high at present. In previous works, it was suggested that an increase in concentration of the intercalated species improves the thermoelectric performance of GICs. To better understand the thermoelectric properties of GICs, the dependence of thermoelectric properties with the electrical carrier density and mobility were investigated through the measurements of galvanomagnetic properties. As a result, the electrical conductivity of GICs slightly increases with the carrier density and the thermal conductivity increases with the carrier mobility. Furthermore, the carrier mobility decreases with an increase in the carrier density. In conclusion, the thermoelectric performance of GICs is suggested to be improved by an increase in the carrier density, that is, by an increase in the intercalate concentration. [doi:10.2320/matertrans.E-M2009813]

Alkali metal-graphite intercalation compounds with the composition of MC{sub 24} (M=K, Rb, Cs) were prepared by heating a mixture of MC{sub 8} (saturated compound) and graphite sheet (Grafoil) at 350-450 deg. C. The resistivity perpendicular to the layer planes ({rho} {sub c}) of the resulting compounds was determined by the two-terminal method. The anisotropy factor of the resistivity, ({rho} {sub c}/{rho} {sub a}), of KC{sub 24} prepared from Grafoil was {approx}130, being about 1/6-1/10 in magnitude compared with that of KC{sub 24} prepared from highly oriented pyrolytic graphite. The resistivity change during sorption of hydrogen (at 90 K), ethylene (at 194 K) and acetylene (at 194 K) was determined. The resistivity of MC{sub 24} increased with increase of the sorbed amount of H{sub 2}. The magnitude of the increase was in the order KC{sub 24}>RbC{sub 24}>CsC{sub 24}. This resistivity increase was considered to be due to the expansion along c-direction which reduces the charge-transfer interaction between the carbon layers and potassium ions, resulting in the decrease of the density of the conduction electron. The resistivity of MC{sub 24} increased extensively during sorption of C{sub 2}H{sub 4} and C{sub 2}H{sub 2}. It was discussed in connection with the in-plane structural transitionmore » and chemical interaction between alkali metal ions and sorbed molecules. - Graphical abstract: The resistivity of MC{sub 24} increased with increase of the sorbed amount of H{sub 2}. The magnitude of the increase was in the order KC{sub 24}>RbC{sub 24}>CsC{sub 24}. This resistivity increase was considered to be due to the expansion along c-direction which reduces the charge-transfer interaction between the carbon layers and potassium ions.« less

The thermoelectric properties of cesium–graphite intercalation compounds (Cs-GICs) and some other GICs were studied. The electrical conductivities of the Cs-GICs prepared from Grafoil were around 106 � � 1 m� 1, about 10 times higher than that of the host graphite, and the thermal conductivities were 50–100 Wm � 1 K � 1 , about 1/4–1/2 of that of the host graphite. While the Seebeck coefficient of the host graphite was nearly zero, those of the Cs-GICs were found to be around � 30 m VK � 1 . These properties were determined also for the Cs-GICs prepared from well-oriented PGS graphite sheets. Consequently, the figures of merit of the Cs-GICs were of the order of 10 � 5 K � 1 , approximately 10 5 times higher than that of the host graphite. The power factor of the Cs-GICs reached the order of 10 � 3 Wm � 1 K � 2 , with the best datum at 6:5 � 10 � 3 Wm � 1 K � 2 . The carrier density of the GICs increased as the concentration of intercalated species increased, whereas their lattice thermal conductivities decreased. It is suggested that highly concentrated GICs might be promising candidate thermoelectric materials.

Samples of CsC 24 were prepared from carbon materials (derived from petroleum cokes) with different graphitization degree and were characterized by X-ray diffraction (XRD), electron spin resonance (ESR) and Raman spectroscopy. The XRD measurement showed that all the host carbon samples heat treated from 1300 to 2800°C being contacted with CsC 8 planar specimen allowed the intercalation of Cs. Correspondingly, the resulting CsC 24 samples were able to sorb hydrogen molecules, which is a clear evidence of the formation of the nanospace. The broad ESR signal due to conduction electron observed for carbon samples (heat-treatment temperature, HTT above 2200°C) disappeared after intercalation of Cs because of the spin–orbit interaction caused by the intercalated Cs. The Raman G-band of the CsC 24 samples (HTT above 1750°C) shifted from 1584 cm −1 of the host carbon to higher wave number (∼1602 cm −1 ) in agreement with the reported data for CsC 24 . In addition, it was confirmed that the D-band signal disappeared for the CsC 24 samples (HTT above 2000°C).

Intercalation reaction of benzene (C6H6) into CsC24 and the characteristics of resulting ternary graphite intercalation compound (GIC) have been investigated, in comparison with the similar CsC24–ternary GICs with ethylene (C2H4) and acrylonitrile (C2H3CN), and the behavior of these organic molecules within the interlayer nanospace has been discussed. The stability of these ternary GICs under exposure in air, in both stage structure and electrical conductivity was found to be in the order of C2H4–, C6H6– and C2H3CN–GICs. This result was reasonably explained as follows: the C2H4 molecules in the interlayer space oligomerize considerably and form network preventing diffusion of the Cs atoms in the interlayer, while the C6H6 molecules form only dimers or trimers and unable to form network, and the C2H3CN molecules, in spite of their strong tendency to polymerize, apparently do not form oligomers in the interlayer space, and its ternary GIC tends to degrade easily. By the electrical and galvanomagnetic measurements, it is revealed that the dominant carrier of these ternary GICs is electrons, even when the electrical conductivity decreases considerably after prolonged exposure in air, while the host graphite, Grafoil, shows two-carrier conduction. This fact indicates that the degradation of the ternary GICs occurs mainly at the periphery or surface layer of the crystallite, and at the inside of the crystallite, the ternary stage structure is retained unchanged.

Electrical conductivities of Cs-unsaturated organic molecule-ternary GIC decrease gradually or rapidly in air, while their thermoelectric powers remain unchanged. For investigating their conductive carriers, electrical conductivity (σ), Hall coefficient (RH) and magnetoresistance (Δρ/ρ) of CsC24 (C2H4) x, CsC24 (C6H6) x and CsC24 (C2H3CN) x were measured, and their carrier concentrations (n) and mobilities (μ) were estimated. The values of RH were negative for all the GICs investigated, thus their dominant carriers were considered to be electrons. On the other hand, thevalues of Δρ/ρ of CsC24 (C2H4) 1.4 were too small to be detected, indicating that its electrical conduction is only throughelectrons, while other GICs have electrons and much less number of holes. These results indicate that all of the ternary GICs investigated maintain a certain fraction of Cs atoms between graphite layers, even after a long time of exposureto air, although some degradation of stage structure are found by XRD as the case of CsC24 (C6H6) 2.1. This consideration corresponds well with the fact that their thermoelectric powers kept unchanged. By observing the change of thegalvanomagnetic properties of the ternary GICs with time, we found that the releasing of part of Cs atoms from theinterlayer nanospace occurred exclusively in the first 30 days.

Raman scattering and X-ray diffraction measurements were made to identify stage structure of CsC24(C2H4)n and CsC24(C2H3CN)n, which were exposed to air for a long time. Their electric conductivities were also measured during the long exposure to air. Raman spectra suggest that the gradual degradation of their electric conductivities in air is caused by the slow decomposition of the ternary GIC's from the periphery region, although results of X-ray diffraction for all samples indicate the retention of the initial stage structure. Among the samples, CsC24(C2H4)1.4 which absorbed large amount of ethylene, was found to be exceptionally stable both in its stage structure and electric conductivity.

Carbon materials (A-1, A-2 and A-3 derived from pitch cokes) with different graphitization degree, were allowed to react with potassium. The hydrogen-sorption behavior at 77 K, electrical resistivity. ESR and Raman spectra of the resulting compounds with the composition of KC60 were determined. The sorbed amount at saturation, (n H2/n K)sat, was 0.55 and 1.43 for KC60s prepared from A2 (d 002 = 0.3377 nm) and A3 (d 002 = 0.3361 nm), respectively. No H2 sorption was observed for KC60 from A-1 (d 002=0.342 nm). Temperature dependence of the resistivity of KC60s from A-2 and A-3 showed metallic behavior, contrary to semiconductive one for the host materials. However, KC60 (A-1) showed semiconductive temperature dependence, similarily to the host material. Raman spectra of KC60s from A-2 and A-3 showed doublet structure, similarly to that of K-GICs from HOPG, characteristic for graphite intercalation compounds with stage n > 2. On the contrary, KC60 (A-1) gave single peak at around 1597 cm−1. Those f...