S. Roth

Raman studies of photochemical reactions in fullerene films

Abstract Raman spectroscopy is employed as a structural probe of both oxygen-free and oxygenated thin films of C 60 . Comparison of initial spectra reveals no influence of oxygen inclusion on the Raman frequencies. The amplitudes of the peaks are, however, observed to be sensitive to light exposure. Each of the initially observed peaks degrades on a time scale of tens of seconds while new peaks emerge approximately ten wavenumbers to the low-energy side. In oxygenated films the degradation process is apparently slower. The process is analyzed using a simple rate equation model of a level scheme in which population of the triplet state leads to a sample degradation with a characteristic time τ 2 , competing with the ground state recovery, characterized by the triplet lifetime, τ 1 . The decay of all the Raman peaks in oxygen-free films fits well to such a model with values of 80 ± 5 s and 55 ± 5 ms for τ 2 and τ 1 , respectively. The rise of the new spectral features, corresponding to the degraded product, also fits the model. In oxygenated films, where a similar value of τ 2 is found, the best fits yield a triplet lifetime of 30 ± 5 ms. It is proposed that the role of oxygen is to inhibit the photochemical degradation via triplet quenching.

Photoconductivity of thin film fullerenes; Effect of oxygen and thermal annealing

Abstract The photoconductive response of both oxygen-free and oxygenated thin fullerene films is compared and contrasted. Exposure to oxygen results in a significant decrease in the magnitude of the photoconductivity. The qualitative features of the spectral dependence of the photoconductivity in oxygen-free and oxygenated films are the same, however, and differences in the temperature dependence are small. It is concluded that the influence of oxygen is not the source of subgap responses and anomalous features in the temperature dependence. In addition, the features of the temperature dependence are not directly related to the structural phase transition. Upon heat treatment of the oxygen-free films under vacuum, both the photoconductive response at subgap photon energies, and the anomalous features in the temperature dependence are substantially reduced. The treatment is interpreted as a thermal annealing process. In oxygenated films heat treatment results in an overall increase in the photoconductive response by an order of magnitude, a result of a partial outheating of the absorbed oxygen.

Reversible photochemical processes in fullerenes. A Raman study

Abstract We report the photo-induced depolymerization of the product of the photochemical degradation of C 60 via a metastable excited state intermediate. The Raman scattering signal of C 60 crystallites is monitored as a function of excitation intensity at 514.5 nm. At low intensities, the characteristic 1468 cm −1 C 60 A g “pentagonal pinch” mode degrades quickly to the Raman line at 1459 cm −1 which has been identified as characteristic of an irreversible photochemical product of C 60 The feature at 1459 cm −1 is seen to be stable below an apparent threshold intensity, which is somewhat sample dependent, above which it shifts abruptyly to 1463 cm −1 . The shift of the line to 1463 cm −1 is accompanied by a sharp increase in the Raman intensity and the background luminescence. With increasing intensities, no further shift of the Raman line is seen. Upon a return to low intensities, the 468 cm −1 C 60 A g “pentagonal pinch” mode is seen to reappear. The process is interpreted as a photo-dissociation of the C 60 photo-prod The positioning of the Raman line at 1463 cm −1 is associated with an excited state intermediate, observable only at high excitation densities.

Structure and properties of thermally annealed fullerene films

Abstract The effects of postdeposition thermal annealing on the structure and properties of C60 films are investigated. Non-oxygenated C60 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.

Fullerenes in the highly excited state

Under high intensity illumination, the optical and electronic properties of fullerenes are seen to undergo dramatic, nonlinear changes. The photoluminescence emission is seen to increase with approximately the third power of the input intensity above an apparent threshold intensity. Associated with this nonlinear increase is the emergence of a long lifetime emission component and a redshifting of the emission spectrum. Investigations of the photo-transport properties associate the observed behaviour with a phase transition in the highly excited state. Above an intensity which coincides with the onset of the nonlinear emission, the photoconductive response increases with approximately the cube of the input power. In the highly excited state, the photoconductive response becomes relatively temperature independent compared to the thermally activated behaviour observed at low intensities. The characteristics of the temperature dependence are associated with a high electron mobility phase in the highly excited state and therefore an optically driven insulator to metal transition is proposed as a description of the observed phenomena.

Raman studies of nonlinear phenomena in fullerene crystallites

The resonant Raman scattering of C60 crystallites is monitored as a function of excitation intensity at 514.5 nm. At low intensities, a strong line at 1468 cm−1 is observed. No feature at 1459 cm−1 is observable. With increasing intensities, the 1468 cm−1 line shifts continuously and reversibly to lower frequencies. The mode softening is nonlinearly dependent on the input intensity and is accompanied by a nonlinear increase in the Raman intensity. The spectral changes are discussed in terms of a nonlinear reduction in force constant and increase in bond polarisability as a result of an increased intermolecular delocalisation of the π-electron cloud. The nonlinear changes are associated with the nonlinear luminescence and photoconductive response observed in fullerenes and are contrasted to the irreversible phototransformation recently discussed.

A Raman analysis of C60 at low temperatures: a study of molecular and crystal-field effects

Abstract The Raman spectra in the region of 200–1700 cm−1 of C60 oxygen free crystals and films at low temperatures are found to possess a rich vibrational character. A total of 57 vibrations in the crystal and 47 vibrations in the film are observed. In addition to the strong Raman-active 2Ag and 8Hg modes, 6 silent gerade fundamentals and 10 silent ungerade fundamentals in the crystal and 7 gerade and 7 ungerade in the film can be assigned. The activation of silent fundamentals is observed to be largely independent of the solid-state environment and sample morphology and is attributed to the presence of 13C. A further 40 vibrations in the crystal and 35 in the film are assigned to fundamental combinations and overtones. Notably, 14 modes in the crystal and 12 in the film are observed, which cannot be assigned to the above and these are attributed to the splitting of 10 of the fundamental and combination modes. These splittings are attributed to crystal-field effects. Some differences in splitting between crystals and films are observed and are discussed in terms of the inhomogeneous and polycrystalline character of the latter.

Many-body effects in the highly excited state of fullerenes

The highly excited state of fullerenes is characterised by a luminescence output which is dependent on the cube of the input intensity. This nonlinear emission is red shifted from the low-level emission and has a long, intensity dependent lifetime. Under similar irradiation conditions, the photoconductive response is seen to increase with the cube of the input intensity and the photocurrent in the highly excited state is observed to be largely independent of temperature, contrasting sharply with the thermally activated behaviour at low excitation densities. The degree of nonlinearity of the observed phenomena exclude an interpretation in terms of intra-molecular processes and the temperature dependence of the photoconductive response is suggestive of a Mott-like transition. The nonlinear behaviour is compared to that of indirect band-gap semiconductors in which the origin of similar nonlinear phenomena in the highly excited state luminescence and photoconductivity are explained in terms of electron-hole droplet formation. The similarities of the behaviours leads to a consideration of exchange and correlation energies in fullerenes, which are calculated according to a phenomenological model. Estimates of the contributions are consistent with a Mott-like transition at high excitation densities and an excess exchange/correlation energy in the highly excited state of ∼150 meV.

Conducting Polymers for Molecular Electronics

Basic concepts of molecular switching devices are reviewed and discussed. The applicability of simple molecular systems such as push-pull polyenes to these concepts is considered. Optically excited states are probed by transient spectroscopy and the influence of chemical modification on the lifetime of the excited state is described for bianthrone based systems.

Photophysical and photochemical processes in fullerenes under high-intensity illumination

Abstract The origins and structure of molecular and solid-state fullerenes are reviewed. Comparison of the optical properties of solution and solid state indicates strongly that the molecular nature is preserved in the solid state. Picosecond time resolved photoluminescence, photoconductivity and resonant Raman measurements are performed to investigate the influence of high-intensity illumination on the properties of Fullerene single crystals. A highly non-linear dependence of the luminescence emission efficiency and lifetime is observed on increasing the intensity. This non-linear increase is associated with a dramatic shift to the red of the emission maximum. Under similar conditions, the photoconductive response of the fullerenes is also seen to increase non-linearly with input intensity. Temperature-dependent measurements indicate that the non-linear processes are associated with an insulator-metal phase transition in the material. The transition is reversible and the observed photophysical changes coincide with a reversible shifting of the characteristic fullerene Raman lines to lower energies. At room temperature, in many samples, the shifting becomes irreversible, and a high molecular weight, insoluble material is formed. The photochemical process is proposed to be a polymerisation-like reaction of the fullerene molecules in the triplet excited state. This is supported by the observation that the rate of the reaction is reduced greatly in the presence of oxygen, an efficient triplet quencher. In conclusion, the response of Fullerene crystals to light is divided into three categories. At low intensities the photophysical processes are characteristic of those of a molecular insulator, the electronic wavefunctions being molecularly localised. At higher intensities, the material undergoes an optically-induced Mott-like transition to a semiconductor/metal, in which the electrons become delocalised in three dimensions. Thirdly, the material is found to be photochemically unstable under some conditions but analysis of the temperature and intensity dependence of Raman spectroscopy shows that the photodegradation process can be predicted and therefore controlled.