Pinch Mode Research Articles

Selenium intercalated solid C60

Two new phases based on Se intercalated into C60 were produced in a high temperature reaction of the chalcogen with the fullerene. Based on laser-ablation time-of-flight mass spectroscopy the two phases are proposed to have stoichiometries (Se5)2C60 and (Se5)2(C60)2 in which the latter is proposed to contain dimeric C60. The two species are stable in air. Laser Raman spectroscopy indicated a lowering of the Ag pentagonal pinch mode of the C60 by 5 cm−1 and 11 cm−1 for (Se5)2C60 and (Se5)2(C60)2, respectively, suggesting an electron transfer from the Se5 clusters to the C60 in the range 0.5 to 2 electrons per Se5.

Selenium intercalated solid C 60

Two new phases based on Se intercalated into C 60 were produced in a high temperature reaction of the chalcogen with the fullerene. Based on laser-ablation time-of-flight mass spectroscopy the two phases are proposed to have stoichiometries (Se 5) 2C 60 and (Se 5) 2(C 60) 2 in which the latter is proposed to contain dimeric C 60. The two species are stable in air. Laser Raman spectroscopy indicated a lowering of the A g pentagonal pinch mode of the C 60 by 5 cm −1 and 11 cm −1 for (Se 5) 2C 60 and (Se 5) 2(C 60) 2, respectively, suggesting an electron transfer from the Se 5 clusters to the C 60 in the range 0.5 to 2 electrons per Se 5.

Raman Scattering from Single-Crystal KC60

Raman scattering is used to analyse phase transitions and structures in KC60 crystals. The doping of the crystals was performed at 450 K and studied in situ by observing the Ag(2) pinch mode in the Raman spectrum. The crystals were homogeneously doped at least up to the penetration depth of the laser used. Slow cooling from 450 K to room temperature showed clearly the phase separation of KC60 into C60 and K3C60 at 420 K. With fast cooling rates, dramatic changes in the Raman spectra were observed and interpreted as a phase transition to an orthorhombic structure as previously for RbC60. Quenching experiments for KC60 showed that a fraction of KC60 could be retained, but it dissolved rapidly even at temperatures below room temperature.

Absorption spectrum of C60 in the gas phase: Autoionization via core-excited Rydberg states

The absolute absorption cross section of C60 in the gas phase (830–870 K) was measured as a function of the photon energy (3.5–11.4 eV) (absorption spectrum). Absorption peaks at 7.87, 8.12, 8.29, 9.2 eV and a dip at 8.45 eV observed are assigned as Feshbach resonances in the photoexcitation involving superexcited states. The superexcited states responsible for the 7.87, 8.12, and 9.2 eV peaks are assigned to be core-excited Rydberg states converging to the second, the third and the fourth ionization limits of C60 (8.89, 9.12, 10.82–11.59 eV), respectively. The 8.29 eV peak is considered to originate from vibrational excitation of a totally symmetric pentagonal pinch mode of the superexcited state responsible for the 8.12 eV peak. Further, a relative photoionization quantum yield was estimated from the absorption cross section measured and the relative photoionization cross section reported. The yield increases particularly in the vicinity of 8 eV in accordance with a high efficiency of autoionization of the superexcited states. Ionization efficiency is not high in the vicinity of the first ionization energy, probably because of rapid energy dissipation into its vibrational modes. The spectrum below the ionization energy resemble the absorption spectra of C60 in its solutions.

Raman spectroscopy of C60 solid films.

In spite of the simplicity of the Raman spectrum of ${\mathrm{C}}_{60}$, controversy remains concerning the identification of the Raman spectrum associated with the pure ${\mathrm{C}}_{60}$ solid. In particular, the exact frequency of the pentagonal pinch mode is still an open question. In this paper, the change of the room-temperature Raman spectrum of an oxygen-free ${\mathrm{C}}_{60}$ film as a function of the laser power is reported. As previously reported, two distinct Raman spectra are found: a 1469-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ Raman spectrum under a low incident laser power (${\mathit{P}}_{\ensuremath{\lambda}}$=1 mW) and a 1459-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ Raman spectrum under a moderate incident laser power (${\mathit{P}}_{\ensuremath{\lambda}}$=50 mW). The temperature dependences of these two Raman spectra in a large temperature range 10\char21{}430 K, and under low, moderate, and high incident laser power are analyzed. The most consistent explanation of these dependences is a phototransformation of oxygen-free ${\mathrm{C}}_{60}$ under the laser irradiation featured by a dominant 1459-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ Raman spectrum at room temperature, the 1469-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ Raman spectrum being assigned to the pure oxygen-free ${\mathrm{C}}_{60}$. Our point of view is confirmed by the observation, after a peculiar thermal treatment, of a stable-in-air ${\mathrm{C}}_{60}$ form featured by a 1459-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ room-temperature Raman spectrum. Obviously, this result argues against the interpretation of the 1459-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ spectrum as that of a pure oxygen-free ${\mathrm{C}}_{60}$ solid.

Face-centered-cubic to orthorhombic phase transition in single-crystal RbC60 analyzed by Raman scattering.

Single crystals of ${\mathrm{C}}_{60}$ were doped with rubidium to the phase ${\mathrm{RbC}}_{60}$ by a load and equilibrate technique. No matter which nonequilibrium state of doping was reached, the equilibration procedure always resulted in the Rb monofulleride phase. Raman scattering for the fcc and for the orthorhombic phase is reported and compared with spectra for ${\mathrm{KC}}_{60}$. For rapidly cooled crystals the latter are found to be identical with the spectra of the rudibium compound. A line at 344 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ was found to be most characteristic for the orthorhombic phase in both cases and, together with some other new and very strong spectral features, considered as an indication of covalent bonds in a polymeric chain. The phase transition for the Rb phase was observed to start at 397 K on cooling with a hysteresis of about 17 K between the cooling and the heating cycle. The continuous shift of the pinch mode line during the process of the phase transition is considered as evidence for a growing linear chain during the process of the phase transition.

Pressure induced phase transitions in C 60 single crystals

Raman spectra of C 60 single crystals, in the pentagon pinch mode frequency region, have been measured as a function of hydrostatic pressure up to 7.2 GPa at room temperature. The softening of the A g(2) and H g(8) modes in the pressure region up to 0.4 GPa indicate the orientational ordering phase transition from fcc to sc structure. The pressure dependence of the intramolecular H g(7) and A g(2) phonon frequencies and their integrated intensities shows the existence of an interaction between these modes above 2.5 GPa, which is connected with the phase transition to the rotation-free orientationally ordered simple cubic phase of C 60 occurring at 2.5 GPa at room temperature.

Raman spectroscopy of undoped and doped single crystal C60

Summary form only given. We present high resolution Raman spectra of undoped single crystals at low temperatures using laser exitations in the red and green spectral range. A splitting of the H/sub g/ modes in A/sub g/, E/sub g/ and F/sub g/ modes was found. A maximum of 8 splitted components could be observed. The doping of the single crystals with potassium was performed at temperatures slightly above room temperature and the doping process was studied by using in-situ Raman spectroscopy. Up to the penetration depth of the laser homogeneous single phases of K/sub 1/ C/sub 60/ and K/sub 3/C/sub 60/ could be obtained. The doped phases were identified by the position of the A/sub g/ pinch mode, which is very sensitive relative to the charge transfer. At cooling the high temperature phase K/sub 1/C/sub 60/ was observed to undergo a sharp transition into C/sub 60/ and K/sub 3/C/sub 60/ between 430 K and 410 K. In addition temperature dependent Raman measurements were performed on single phase K/sub 3/C/sub 60/. At liquid nitrogen temperature an unexpected strong splitting and selective line broadening of the H/sub g/ modes was observed.

Structural alteration and chemical stability of heat treated C60 films

Abstract The effects of postdeposition annealing of C 60 films on the structure and chemical stability of the solid-state material are investigated. An initial study of the change of the photoconductive response during a heating cycle reveals an anomalous irreversible increase of the photocurrent. In a subsequent study, non-oxygenated polycrystalline C 60 films are subject to postdeposition heating at 200°C under vacuum. IR analyses reveal no new bond formation, indicating no chemical alteration of the sample. Ultra Violet/Visible absorption spectroscopy shows a decrease in the intensities of the allowed transitions and a significant red-shift of up to 50nm. In Scanning Electron Microscopy, larger grains and island growth are observed. X-ray diffraction shows the presence of mixed phases identified as cubic and hexagonal, which is a closer packed lattice, not seen in unheated films. Time of illumination dependent, low intensity room temperature Raman studies of the pentagonal pinch mode reveal the presence of two spectral components, which are associated with the two phases observed in X-ray diffraction. The intensity of the first component identified with the 1468cm −1 Raman line decreases with time. The intensity of the second spectral component identified with the 1464cm −1 Raman line remains stable under prolonged low intensity illumination. The first component is seen to be highly unstable and undergoes a light induced chemical transformation, while the second appears largely insensitive to prolonged illumination. The results suggest that the effect of film annealing is structural in nature and leads to a repacking of the lattice.

Structure and properties of thermally annealed fullerene films

The effects of postdeposition thermal annealing on the structure and properties of C 60 films are investigated. Non-oxygenated C 60 films are heated at 200°C under vacuum. UV/visible absorption spectroscopy shows a decrease in the intensities of the allowed transitions and a red-shift of up to 50 run. IR analyses show no indication of chemical alteration of the sample. X-ray studies reveal cubic symmetry in the unannealed thin films, while the annealed films show a mixture of face-centred cubic and hexagonal close-packed phases and a higher degree of crystallinity. Room-temperature Raman studies in the region of the pentagonal pinch mode reveal the presence of two components. The first component, at 1468 cm −1, is photochemically unstable, as is the pristine material. The second component, positioned at 1464 cm −1, remains stable under prolonged low-intensity illumination. The effects of thermal annealing of oxygen-free films are contrasted to those of thermal treatment under oxygen and it is concluded that the modifications are structural in nature.

Superconductivity at 40K in cesium doped C 60

We report superconductivity in Cs 3C 60 at 40K using ac susceptibility measurements under hydrostatic conditions up to 15 kbar. Cs 3C 60 was prepared by reaction of C 60 with Cs in liquid ammonia, followed by heating at 150°C. This route circumvents formation of the energetically more stable Cs 1C 60 and Cs 4C 60 phases. We have studied the synthesis and phase formation by measuring the symmetric Ag pentagonal pinch mode of C 60 using Raman spectroscopy. Whereas Raman spectroscopy indicates homogeneous charge transfer of three electrons, x-ray diffraction indicates two phases other than the commonly observed fcc structure. This superconducting transition temperature is considerably higher than for known doped C 60 compounds and intermetallic compounds.

Temperature dependence of the Raman spectrum in C 60 doped with alkali metals

We report on Raman-scattering results of a well characterized K 3C 60 sample and of Rb xC 60 and Cs xC 60 samples. In these three compounds the position of the A g pinch mode at room-temperature is measured at around 1448 cm -1. This last position of the A g pinch mode is known as the Raman signature of a A 3C 60 phase. This result raises the question of the ability to obtain a Cs 3C 60 phase by direct action of Cs vapor on C 60. On the other hand, the temperature dependence of the Raman spectra of the studied A xC 60 samples is reported and a special attention is given on the temperature dependence of the shift and width of the Raman A g pinch mode.

Raman Spectroscopy of C60 Solid Films: A Tale of Two Spectra

Two distinct Raman spectra have been reported for solid C[sub 60]. They differ in the exact position and relative intensities of the spectral lines. Specifically, the frequency of the pentagonal pinch mode occurs at 1469 and 1459 cm[sup [minus]1] in the two spectra. Several explanations have been offered for the existence of a second spectrum, including the influence of interstitial oxygen, a change in phase, the coexistence of singlet C[sub 60] with laser-pumped triplet C[sub 60], and photopolymerization. We show that there are two Raman spectra for solid films of C[sub 60] with varying relative intensities according to the sample temperature even in the absence of oxygen. The reversibility of the two forms of C[sub 60] with temperature argues against photopolymerization. Finally pump-probe experiments designed to detect either triplet absorption or singlet depletion indicate that neither of the two Raman spectra are due to triplet C[sub 60]. We conclude that our previous explanation in terms of two phases of C[sub 60] is still the most consistent with the observations. 28 refs., 10 figs.

Superconductivity at 40K in Cesium Doped C60

ABSTRACTWe report superconductivity in Cs3 C60 at 40K using ac susceptibility measurements under hydrostatic conditions up to 15 kbar. Cs3 C60 was prepared by reaction of C60 with Cs in liquid ammonia, followed by heating at 150°C. This route circumvents formation of the energetically more stable CSl C60 and Cs4 C60 phases. We have studied the synthesis and phase formation by measuring the symmetric Ag pentagonal pinch mode of C60 using Raman spectroscopy. Whereas Raman spectroscopy indicates homogeneous charge transfer of three electrons, x-ray diffraction indicates two phases other than the commonly observed fcc structure. This superconducting transition temperature is considerably higher than for known doped C60 compounds and intermetallic compounds.

24 K magnetism in non-stoichiometric TDAE-C 60

The organic magnetic material TDAE-C 60 is investigated by magnetic susceptibility, ESR and Raman measurements. We find that, depending on preparation technique, the critical temperature (as determined by ESR) can vary from 16 to 24 K, the latter being the highest T c reported so far for this compound or any non-polymeric organic material composed entirely of first-row elements. The ESR linewidths in the magnetic state - an indication of the magnitude of the internal field - vary between 15 and 50 Gauss. Rather unexpectedly, Raman spectra of the 16 K material do not show a clear 6 cm -1 shift of the C 60 pentagonal pinch mode frequency relative to pure C 60 corresponding to transfer of one electron in the ground state. Instead we obtain a sharp line at 1466 cm -1 which would imply only a partial charge transfer from TDAE to C 60 i.e. a resonating molecular charge sharing between C 60 and TDAE molecules. In contrast, the 24 K sample shows a broad set of lines shifted to lower frequency implying that the higher T c materials may be non-stoichiometric.

Raman spectra and electrical resistivity measurements of K xC 60 (x = 0,3 and 6) thin films

We report the Raman spectroscopy and resistivity of C 60 films as a function of exposure to potassium vapour. Thin films of C 60 are prepared under UHV conditions using a resistively heated effusive beam source (400 - 450°C). Stepwise dosing of the C 60 film with potassium results in the formation of mixtures of the three phases: C 60, K 3C 60 and K 6C 60. Subsequent addition of C 60 returns the film to a prior composition. Our spectra indicate a granular solid film with spots of varying composition. No evidence is found of a K 4C 60 phase. As a function of potassium dosing the resistivity decreases steadily to a minimum value of 0.0092 ohm-cm, characteristic of K 3C 60, and then increases steadily to a final intermediate value which changed little with further addition of potassium. We also report surface enhanced Raman spectroscopy (SERS) spectra of C 60 on rough silver films with and without exposure to potassium vapour. We do not observe a K 3C 60 phase on rough silver films; however there is a small red shift of the pentagonal pinch mode upon exposure to potassium vapour in addition to a large red shift induced by the rough silver film.

Raman study of a structural phase transition in C60 crystals.

Raman spectra of purified ${\mathrm{C}}_{60}$ single crystals are measured over a wide temperature range. It is found that above 400 K the high-frequency ${\mathit{A}}_{\mathit{g}}$ mode (pentagonal pinch mode) changes in frequency and becomes sharp compared with the ${\mathit{A}}_{\mathit{g}}$ mode at room temperature. This spectral change is reversible. The results indicate that structural change occurs and the intermolecular interaction for each ${\mathrm{C}}_{60}$ molecule is isotropic above 400 K. A reversible spectral change is observed as the intensity of the excitation laser varies. The similarity between the spectral change at 400 K and the spectral change at high laser intensity levels suggests that the photoinduced spectral change observed in the present study is due to the thermal effects of laser irradiation.

Raman study of photoexcited C 60

The high-energy pentagonal pinch mode of C 60 is known to soften under high incident laser irradiance in an oxygen-free environment. We have studied this phenomenon in C 60 films, single crystals, and solutions in CS 2. Our Raman, optical absorption, and EPR data indicate that the softened Raman peak is related to very long lived states in C 60. The experiments are consistent with the recently proposed photoinduced polymerization of C 60 and also suggest the presence of C 60 anions in the photoexcited material.

Surface-enhanced Raman spectroscopy of electroactive films of C 60 and an Ir-C 60 complex on gold: symmetry lowering, electron localization, and countercation effects

Surface-enhanced Raman (SER) spectra are reported for thin films of C 60 and (η 5-C 9H 7)Ir(CO) (η 2-C 60) on gold electrodes in acetonitrile. The latter spectra display the hallmarks of vibrational symmetry lowering for C 60 arising from metal coordination, involving splitting of the normally degenerate H g modes and the appearance of vibrationally forbidden bands. Only a milder selection rule relaxation is observed for SERS of uncoordinated C 60. Electroreduction of both C 60 and the Ir-C 60 complex yielded similar frequency downshifts in the A g pentagonal pinch mode, indicating that electron addition to the latter is fullerene localized. These spectral shifts are sensitive to the nature of the countercation.

Potassium-doped fullerene K χC 60 with X = 0, 1, 2, 3, 4, and 6

The potassium doping of C 60 to K χC 60 with 0 ≤ χ ≤ 6 was studied by Raman spectroscopy at temperatures slightly above room temperature. A new phase with x = 1 was found. This phase decays at a temperature of 80°C into C 60 and K 3C 60. For potassium concentrations above x = 3 a stoichiometric phase with x = 4 and diluted structures with x ≤ 6 were observed. The x = 6, 3 and 1 structures revealed a strictly linear relation between charge on the C 60 and position of the Raman line for the pinch mode. This suggests that the pinch mode has the frequency 1464.5 cm −1 at 90°C and 1466 cm −1 at room temperature.