A. Mittelbach

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.

Nonlinear luminescence phenomena in fullerene crystallites

Time-resolved photoluminescence studies in powders and single crystals of C60 are reported. The observed emission spectra, resulting from excitation above the HOMO-LUMO transition energy, are consistent with those reported for steady state conditions which may be associated with an intramolecular process and the observed luminescence decay times are of the order of 1 ns. No long lived decay from the triplet is observed at low intensities. A dramatic increase in the lifetime as well as a redshift and broadening of the spectrum is observed at high excitation densities. The luminescence intensity is seen to increase with the cube of the input intensity. The phenomenon is interpreted as an abrupt onset of emission from the highly populated excited state resulting from an interaction of the intramolecular states, a process which is dependent on a critical excited state density. Furthermore, the phenomenon may be photoexcited at photon energies which lie below the HOMO-LUMO transition energy, under which conditions an intensity dependence of the output luminescence on input to the sixth power is observed. Under “off-resonant” conditions the process is interpreted as two-photon assisted nonlinear emission.

Time-resolved photoluminescence of solid state fullerenes

Abstract Time-resolved photoluminescence studies in films, powders and single crystals of C60 are reported. The observed spectra in all forms are consistent with those reported for steady state conditions and may be associated with an intramolecular process. The spectrum is largely temperature independent and, at low temperatures, the observed luminescence decays with a lifetime of 1.2 ns. No long-lived decay from the triplet is observed at these intensities.In the case of the films, both the emission spectrum and lifetime are independent of excitation intensity. In the powder and single crystallite, a dramatic increase in the lifetime as well as a red-shift and broadening of the spectrum is observed at higher intensities. The luminescence intensity is seen to increase with the cube of the input intensity. The behaviour is interpreted as an abrupt onset of emission from the highly populated triplet state resulting from a banding of the intramolecular triplet states, a process which is dependent on a critical excited state density.

The orientational phase transition in C60 films followed by infrared spectroscopy

Abstract We have measured the infrared of C 60 films sublimed on gold from 25 to 300 K in the frequency range 15–5000 cm −1 . We observed several new lines emerging below 250 K which we assign to the infrared active modes of C 60 in the Pa 3 phase, based on the temperature dependence of these modes around the orientational phase transition. We compare our results with those obtained by other experimental methods and with theoretical predictions

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.

Broadband electroluminescent emission from fullerene crystals

The observation of electroluminescence from crystalline fullerenes is described and discussed for the first time. Using gold and aluminium electrodes as contacts, a broad band emission spectrum, extending from 400 nm to 1100 nm is observed. The spectrum has a primary maximum at 920 nm and a weaker feature centered on 420 nm. The spectral characteristics are independent of the applied field and the longer wavelength region is identical to that measured in the high excitation density photoluminescence spectrum. In addition, the electroluminescent output intensity increases with the cube of the injection current, strengthening the association to the nonlinear phenomena observed in the highly excited state of fullerenes.

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.

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.

Photoluminescence of solid state fullerenes

The synthesis of macroscopic amounts of the fullerenes [1] has stimulated a variety of experimental and theoretical investigations of this new allotropic form of carbon. The photophysical properties in solution have been well characterised. The lowest energy optical transitions are strongly forbidden by symmetry selection rules and no luminescence is observable [2–7]. In the case of the solid state, there is still no conclusive experimental evidence for electronic banding [8], posing questions as to the degree of inter- versus intramolecular interactions. However, in thin films luminescence has been observed, indicating a departure from the behaviour of the isolated molecules [6]. Tn this study, time resolved luminescence of films, powders and crystals of C60 are carried out in an effort to characterise this departure. A strong dependance of the luminescence on excitation intensity is observed in powders and crystals, but not in films. The results are discussed in terms of a transition between intramolecular and banded intermolecular behaviour at high excited state densities, which is dependent on microcrystallite size.