W. K. Maser

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.

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.

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.

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.

Nonlinear Optical and Transport Processes in Fullerenes

Abstract Nonlinearities photoluminescence, photoconductivity and resonant Raman measurements of fullerenes at high excitation densities are described and the observations are discussed in terms of an optically induced insulator-metal transition in fullerene crystals. The intensity dependence of the transient photoconductive response is reported to show similar nonlinear behaviour. At low intensities, the response decays exponentially within a time of 15nsec. At increased intensities, a second, delayed component emerges and evolves nonlinearly into a long lived component of lifetime 100-200nsec. The system is fitted with a simple three level model in accordance with a Mott-like transition.

Nonlinear Excited State Phenomena and Electro-luminescence in Fullerene Crystals

Abstract The characteristics of photo and electroluminescent emission from C60 crystals are reported. The photoluminescence emission is seen to increase nonlinearly with the third power of the input intensity above a threshold intensity. Associated with this nonlinear increase is the emergence of a long lifetime emission component and a redshifting of the emission spectrum. Furthermore, above an intensity which coincides with the onset of the nonlinear emission, the photoconductive response increases with the cube of the input power. At high excitation densities, 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. Using gold and aluminium electrodes as contacts, a bro...

Nonlinear optical and transport properties of fullerene crystals

Picosecond time resolved photoluminescence and photoconductivity measurements are performed to investigate the influence of high intensity illumination on the properties of Fullerene crystals. A highly nonlinear dependence of both the photoluminescence characteristics and the photoconductive response of the fullerenes is seen and temperature dependent measurements indicate that the nonlinear processes are associated with an insulator- metal phase transition in the material, and thus that the electronic properties of the excited state are dramatically altered at high excited state densities. Application of a simple phenomenological model to calculate the contribution of exchange and correlation energies supports the feasibility of such an interpretation. A further manifestation of this behavior is the emergence of a broadband electroluminescent emission above a critical injection current density.