Investigation of heating effects in near-field experiments with luminescent organic semiconductors

We present a study of heating effects in apertured near-field gold-coated probes which makes use of the temperature dependence of the photoluminescence spectra of the polymer semiconductor poly(p-phenylene vinylene) (PPV). The small throughput of metal-coated probes used for aperture-scanning near-field optical microscopy (aperture-SNOM) can induce significant probe heating, even when operated with laser powers of only a few mW. According to recent literature the probe apex can reach several hundred degrees celsius, possibly producing local heating of the specimen. Such a possibility needs to be investigated carefully, since it could affect the results of both near-field spectroscopy or lithography, especially where the photosensitive material happens to be sensitive to high temperatures, as for the PPV precursor. Interestingly, the sensitivity of conjugated polymers optical properties to changes in temperature (blue-shift of either photoluminescence or absorption upon heating), can be used as a convenient tool for investigation of the sample heating. Here, we report a comparison between photoluminescence spectra collected in the far-field but with excitation in either the far- or near-field, which demonstrate the absence of any significant difference, and thus indicate a negligible heating of the polymers during SNOM experiments. In addition, we demonstrate that near-field illumination of PPV precursor films with red light (i.e. with a wavelength longer than the absorption edge of PPV precursor), does not result in any lithographic effect, confirming that thermolithography of the PPV precursor is not significant in our lithography experiments.