Infoscience

Thesis

Development of a scanning near-field microscope and investigations of solid C60 luminescence

This thesis presents the development of a scattering scanning near-field microscope (s-SNOM) and investigations on the photoluminescence study of solid C60. The first part concerns setting up of such a scattering SNOM where a tuning fork with an attached AFM tip is used as the force sensor and as a light-scattering probe. The light is coupled into the microscope and detected in a confocal arrangement. The characterization of the system and the force and optical scanning measurement of nanoparticles on glass substrates are presented and discussed. In order to know the tip vibration amplitude of quartz tuning-fork based sensors, we have developed a simple method, requiring only the measurement of a few mechanical properties of the fork (dimensions and Q factor), which can be easily obtained without changing the experimental setup in a matter of minutes. This method uses the (known) electrical energy absorbed by the system and the Q-factor to derive the elasto-mechanical energy stored in the tuning fork, and from this, the amplitude of motion through the elastic constants of the system. In order to improve the sensitivity for controlling the tip-sample distance, we have also made experimental and theoretical investigations on the performance of a different resonator, based on a tuning-fork + optical fiber mechanical scheme for use in shear-force mode. We have found that both the quality-factor and the spring constant play the main role in determining the behavior of such a force sensor. Based on this understanding, we are able to control both of these important parameters and, hence to optimize the force sensor and accurately model its response to an external force. The second part presents a time resolved local spectroscopic study of the photoluminescence of single C60 microcrystals performed with the optical microscope and with time-correlated photon counting techniques. C60 crystals prepared by the vapor sublimation method and the solution evaporation method are discussed. The photoluminescence of C60 crystals prepared by the vapor sublimation method is tentatively assigned to phosphorescence from the lowest two triplet states. The emission quantum efficiency and decay dynamics shows dependence on temperatures for C60 samples made by both methods, while the sample with a less ordered crystal structure made by the solution evaporation has a higher energy barrier for opening a non-radiative decay channel, pointing to quenching effects related to exciton migrations. This work helps clarify the origins of the C60 emitting states.

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