Competing financial interest
7. Control Experiments
We have performed several control experiments in order to verify that upon confocal excitation of single nanofibres their elongated PL signal stems from long-range energy transport and not from direct illumination of the entire fibre. We note that we used excitation intensities between 14 and 24 W/cm2 for the control experiments shown below, i.e. intensities very close to those used for the measurements on isolated nanofibres.
Figure S11. Control experiment I. A: PL image of the laser focus back-reflected from a microscopy cover slip and imaged onto the CCD camera. B: PL image of a single perylene molecule upon confocal excitation. From both images lateral profiles (red and blue curves) were retrieved along the dashed lines in the CCD-images.
Fig. S11A shows an image of the laser light (wavelength: 450 nm) that was focussed by our high-NA objective onto a microscopy cover slip, back-reflected at the glass – air interface, and imaged onto the CCD camera (see Methods Section for details). Fig. S11B depicts a PL image of a single dye molecule (perylene bisimide) upon confocal excitation. We retrieved intensity profiles along the dashed lines from both images. Gaussian fits to the profiles reveal that both the back-reflected laser focus and the single-molecule PL exhibit a radius of
< 350 nm (half width at half maximum, HWHM; both images are slightly asymmetric, we have given the larger value for the radius). This radius is about a factor of two larger than theoretically predicted, r = 0.51∙λ/NA ~ 160 nm,30 which is mainly due to aberrations introduced by the cover slip thickness (the measured thickness is ~ 150 μm, whereas the
objective is corrected for 170 μm).31 However, this radius is still substantially smaller than the shortest nanofibres measured, see Fig. 3c.
In order to exclude direct excitation of a photoluminescent object, that is located substantially off the centre of the confocal excitation spot, we moved a single perylene molecule within the focal plane of the objective laterally across the laser focus in steps of 100 nm by means of the piezo stage of our microscope. For each position we recorded the PL image of the single molecule with the CCD camera. The peak PL signal of the single molecule in each image is displayed in Fig. S12. These data demonstrate that the PL signal drops to ~ 10 % of the maximum value upon moving the molecule by 200 nm away from the centre of the excitation spot.
Figure S12. Control experiment II. Peak PL signal of a single perylene molecule in CCD-images while the molecule is moved through the confocal excitation spot along the two perpendicular directions (red and blue curve) in the focal plane of the objective.
We stress that we did not investigate transport along single nanofibres that are shorter than 1.5 μm (see the histogram in Fig. 3c) in order to reduce the influence of the spatial extend of the excitation spot as much as possible.
Long-Range Energy Transport in Single Supramolecular Nanofibres at Room Temperature
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