Desenvolvimento e estudo de sondas para microscopia óptica de campo próximo

Abstract

This thesis reports the usage of electron microscopy on the study and development of probes for scanning near-field optical microscopy (SNOM). Two fabrication methods for the production of sharp SNOM probes are described: electrochemical etching of gold wires for making conventional solid gold tips, and lithography for the production of template stripped gold pyramids. The probes were characterized by electron microscopy (SEM and TEM) and other correlated analytical techniques such as EBSD and XEDS. Based on the information obtained from these analyses, we substantially improved the fabrication yield for the pyramid probes. Also, we observed morphological flaws on the surface of the solid gold tips, generally caused by grain boundaries. It is shows that those features are compatible with localized surface plasmon resonance (LSPR) generation that leads to optical absorption, although the transition energy is not controllable in this schema. In order to investigate the plasmonic properties of SNOM probes with high energy and spatial resolutions, we applied electron energy-loss spectroscopy (EELS) combined with scanning transmission electron microscopy (STEM). First, localized absorption channels were observed on EELS spectra acquired at the solid gold tips shafts. Experimental results obtained from four different tips strongly suggest that those absorption channels are consequence of an interference effect on surface plasmon polariton (SPP) which are reflected on the tip apex. In the odder hand, no LSPR absorption was seen on EELS spectra acquired at the vicinity of the apexes of nicely-shaped solid gold tips with no flaws. Finally, we introduce a simple and reproducible route for generating and tuningLSPR in SNOM probes. The method is based on the usage of a focused-ion-beam (FIB) made single groove in the vicinity of the tip apex. The EELS results reveal that the plasmonic properties are linked to the distance L between the groove and the apex of the gold probe, and by varying this single parameter we are able to tune the LSPR energy.Based on the experimental EELS data complemented by discrete dipole approximation simulations, and taking into account the effective wavelength of the plasmon oscillation, we obtained a simple relation to guide the LSPR tuning in FIB-grooved SNOM probes. The protocol was used for production of test probes applied to tip-enhanced Ramanscattering (TERS) experiments which evidenced the improvement of the optical efficiency when operating on SNOM systems in the visible-nIR range.