Showing papers by "Mildred S. Dresselhaus published in 1982"

Raman studies of benzene-derived graphite fibers

Abstract: Carbon fibers prepared from thermal decomposition of benzene at \ensuremath{\sim} 1100\ifmmode^\circ\else\textdegree\fi{}C are studied by Raman spectroscopy as a function of heat-treatment temperature. The structural ordering at each heat-treatment temperature is monitored by observation of both the Raman-allowed ${E}_{2{g}_{2}}$ mode at 1580 ${\mathrm{cm}}^{\ensuremath{-}1}$ and the disorder-induced lines at \ensuremath{\sim} 1360 ${\mathrm{cm}}^{\ensuremath{-}1}$ in the first-order spectra and at 2730 and 2970 ${\mathrm{cm}}^{\ensuremath{-}1}$ in the second-order spectra. Raman and resistivity results indicate three characteristic heat-treatment temperatures relevant to the establishment of in-plane and interplanar ordering. Using fibers heat treated to the maximum available temperature of 2900\ifmmode^\circ\else\textdegree\fi{}C, Raman spectroscopy shows that single-staged fibers can be prepared by acceptor intercalation, in agreement with direct Debye-Scherrer x-ray measurements. Resistivity measurements on pristine fibers previously heat treated to 2900\ifmmode^\circ\else\textdegree\fi{}C show a metallic temperature dependence with $\ensuremath{\rho}=70$ \ensuremath{\mu}\ensuremath{\Omega} cm at 300 K and a residual resistance ratio of 1.5. Upon intercalation, a resistivity $\ensuremath{\rho}=7$ \ensuremath{\mu}\ensuremath{\Omega} cm at 300 K and a residual resistance ratio of 5 is achieved. Raman-spectroscopy and temperature-dependent resistivity measurements demonstrate that the benzene-derived fibers exhibit the highest degree of ordering achieved in fibers and provide an attractive host material for intercalation.

Structural characterization of ion-implanted graphite

Abstract: Ion implantation of graphite is characterized with respect to lattice damage and the distribution of implanted ions. Both the depth profile of the damage and of the implanted ions are shown to follow the models previously developed for ion-implanted semiconductors. Raman spectroscopy is used in a variety of ways to monitor different aspects of the lattice damage while Auger spectroscopy is used to monitor the implantation profile. Both first- and second-order Raman spectra are reported as a function of ionic mass and ion energy. The surface damage is examined by scanning electron microscopy while the microcrystalline regions in an amorphous background are observed by scanning transmission electron microscopy.

Commensurate-Incommensurate Transition in Bromine-Intercalated Graphite: A Model Stripe-Domain System

Abstract: An x-ray scattering study is reported of the commensurate-incommensurate transition (CIT) in single-crystal ${\mathrm{C}}_{28}$${\mathrm{Br}}_{2}$ in which the ${\mathrm{Br}}_{2}$ stage-4 intercalant has an in-plane $\sqrt{3}\ifmmode\times\else\texttimes\fi{}7$ superlattice structure. The CIT, which occurs only in the sevenfold direction, is a solid-to-solid transition with incommensurability $\ensuremath{\epsilon}\ensuremath{\sim}{(T\ensuremath{-}342.20)}^{0.50\ifmmode\pm\else\textpm\fi{}0.02}$. The relative displacements of successive harmonics are accurately predicted by a sharp domain-wall model with $\frac{4\ensuremath{\pi}}{7}$ phase shifts. The in-plane coherent ${\mathrm{Br}}_{2}$ regions exceed 10 000 \AA{} in both commensurate and incommensurate phases.

Commensurate-Incommensurate and Melting Transitions in Bromine-Intercalated Single Crystal Kish Graphite

Abstract: We summarize the results of an extensive x-ray scattering study of the phase transitions in intercalated graphite-Br2 compounds. The experiments are aimed at understanding the intraplanar and interplanar correlations of the intercalate bromine as a function of temperature. Using in-situ high resolution x-ray scattering techniques, we have studied stage-4 graphite-Br2 under equilibrium conditions, where the intercalant is in the commensurate phase, incommensurate phase, and fluid phase. We demonstrate that the transitions between these phases are model examples of phase transitions in quasi two-dimensional systems. The coherently ordered in-plane bromine regions exceed 10000 A in size in both commensurate and incommensurate phases. A commensurate-incommensurate transition in the 7–fold direction is observed from a centered (√3 × 7) phase to a stripe domain phase. The domain wall density exhibits a power-law behavior in temperature with an exponent of 0.50 ± 0.02. We have also observed power-law lineshapes for bromine superlattice peaks in the incommensurate phase due to the rigorous absence of true long range order. The intercalate layer appears to exhibit an anisotropic melting transition from an incommensurate solid to a novel fluid phase.

Shubnikov—de Haas measurements in alkali-metal—graphite intercalation compounds

Abstract: Shubnikov\char22{}de Haas (SdH) oscillations are reported for well-characterized (single stage) encapsulated potassium (stages $n=4,5,8$) and rubidium ($n=2,3,5,8$) graphite intercalation compounds. Shapes of the Fermi surface (FS) are deduced from the dependence of the FS cross sections on the angle between the $c$-axis of the sample and $\stackrel{\ensuremath{\rightarrow}}{H}$. The temperature dependence ($1.4lT\ensuremath{\le}25$ K) of the amplitudes of the SdH oscillations has been studied to find cyclotron effective masses for specific FS cross sections. A simple phenomenological energy-band model, based on the $\ensuremath{\pi}$ bands of pristine graphite and $c$-axis zone folding, is used to calculate SdH frequencies as a function of Fermi energy and is applied to interpret the stage- and intercalant-dependent experimental SdH frequencies and effective masses. The good agreement between the observed and predicted effective masses and the FS cross sections confirms the general validity of this model.

Intercalation of Acceptors and Donors in Highly Ordered Graphite Fibers

Abstract: The intercalation of various acceptors and donors into graphite fibers, prepared from benzene-derived precursor materials is investigated by Raman spectroscopy, x-ray diffraction, electron diffraction, lattice fringing, and electrical resistivity measurements. Evidence for formation of well-staged acceptor compounds is provided by Debye-Scherrer x-ray diffraction which probes the bulk fiber and by Raman spectroscopy which probes an optical skin depth (< 0.1 µm). Lattice fringing measurements provide direct observation of large regions (up to 50 A × 400 A) of defectfree single-staged regions. Values for the c-axis repeat distance Ic are obtained by indexing (00l) lines of the x-ray diffraction pattern. Raman results show characteristic upshifted modes for stage 1 acceptor compounds with a sharpening in linewidth as compared to the E2g2 mode of the pristine fiber. The room temperature electrical conductivity is increased about an order of magnitude upon intercalation and exhibits a metallic dependence on temperature. The highest air-stable room temperature conductivity 1.4 × 105 (Ω-cm)−1 ever reported for an intercalated fiber has been achieved.