X-ray photoemission investigations of binary and ternary C60 fullerides of Na, K, Rb, and Cs

Direct observation of charge separation in an organic light harvesting system by femtosecond time-resolved XPS

The ultrafast dynamics of photon-to-charge conversion in an organic light-harvesting system is studied by femtosecond time-resolved X-ray photoemission spectroscopy (TR-XPS) at the free-electron laser FLASH. This novel experimental technique provides site-specific information about charge separation and enables the monitoring of free charge carrier generation dynamics on their natural timescale, here applied to the model donor-acceptor system CuPc:C60. A previously unobserved channel for exciton dissociation into mobile charge carriers is identified, providing the first direct, real-time characterization of the timescale and efficiency of charge generation from low-energy charge-transfer states in an organic heterojunction. The findings give strong support to the emerging realization that charge separation even from energetically disfavored excitonic states is contributing significantly, indicating new options for light harvesting in organic heterojunctions.

Nanometer-size cluster formation in alkali-metal-doped fullerene layers

Kinetic Monte Carlo methods have been used to simulate structural transformations in fullerene layers during electrochemical intercalation with alkali-metal ions (A). Special attention is paid to the thermodynamic stability of the A(x)C(60) phases. The calculations point out a phase separation in the doped fullerene layer into alkali-metal-rich and alkali-metal-depleted areas at room temperature. The final state is represented by two phases which coexist as a stable fine mixture of nanoscale particles. The instability of homogeneous layers has potentially critical impact on their electrical properties and can explain the formation of nanostructures (20-50 nm) at the fullerene-electrolyte interface. Rb(3)C(60) clusters are predicted to be larger than K(3)C(60) ones for equal mean alkali-metal concentrations. Experimental data on electrochemical metal deposition on alkali-metal-doped fullerene substrates-in particular, atomic force microscopy measurements-are also consistent with the model proposed.

The LUMO-derived band of the [Formula: see text] phases

The LUMO-derived band of the [Formula: see text] compound was investigated by means of x-ray and UV photoemission and photoelectric yield spectroscopies. A double-peak structure is found, with peak maxima at 1.1 eV and 0.35 eV. The relative magnitude of the two peaks is strongly temperature dependent: a large transfer of spectral weight from the lower- to the higher-binding-energy peak takes place upon cooling to 110 K. The peaks are tentatively attributed to the dimer and the polymeric phase respectively. However the behaviour as a function of temperature does not agree with the metastability of the dimer phase at room temperature. The position of the Fermi level is compatible with a correlated system on the border of the metal - insulating transition.