We report measurements of excitation and emission spectra of single, polymer-embedded, perylene dye molecules at room temperature. From these measurements, we can derive the Stokes shift for each single molecule. We determined the distribution of excitation and emission peak energies and, thus, the distribution of single molecule Stokes shifts. Single molecule Stokes shifts have not been recorded to date, and the Stokes shift has often been assumed to be constant in single molecule studies. Our data show that the observed spectral heterogeneity in single molecule emission originates not only from synchronous energetic shifts of the excitation and the emission spectra but also from variations in the Stokes shift, speaking against the assumption of constant Stokes shift.

We report on the localized deposition of nanoparticles and proteins, nano-objects commonly used in many nanodevices, by the liquid nanodispensing (NADIS) technique which consists in depositing droplets of a solution through a nanochannel drilled at the apex of an AFM tip. We demonstrate that the size of spots can be adjusted from microns down to sub-50 nm by tuning the channel diameter, independently of the chemical nature of the solute. In the case of nanoparticles, we demonstrated the ultimate limit of the method and showed that large arrays of single (or pairs of) nanoparticles can be reproducibly deposited. We further explored the possibility to deposit different visible fluorescent proteins using NADIS without loss of protein function. The intrinsic fluorescence of these proteins is characteristic of their structural integrity; the retention of fluorescence after NADIS deposition demonstrates that the proteins are intact and functional. This study demonstrates that NADIS can be a viable alternative to other scanning probe lithography techniques since it combines high resolution direct writing of nanoparticles or biomolecules with the versatility of liquid lithography techniques.

SummaryWe report a single molecule detection scheme to investigate excitation spectra of single emitters at room temperature. We demonstrate the potential of single emitter photoluminescence excitation spectroscopy by recording excitation spectra of single CdSe nanocrystals over a wide spectral range of 100 nm. The spectra exhibit emission intermittency, characteristic of single emitters. We observe large variations in the spectra close to the band edge, which represent the individual heterogeneity of the observed quantum dots. We also find specific excitation wavelengths for which the single quantum dots analyzed show an increased propensity for a transition to a long-lived dark state. We expect that the additional capability of recording excitation spectra at room temperature from single emitters will enable insights into the photophysics of emitters that so far have remained inaccessible.

This article describes the patterning of alkene-terminated perylenes on thiolated surfaces. A photochemical thiol-ene click reaction was locally induced (i) in the contact area between the stamp and the substrate, (ii) on surfaces, which were modified with thiolated nanolithographic patterns, immersed in solution and (iii) using a laser scanning confocal microscope over a thiolated substrate immersed in solution. The chemoselective attachment of the alkene-terminated perylenes was demonstrated. All patterning methods yielded homogeneous fluorescent patterns; while, the solutionbased methods limit the presence of aggregates of perylenes. The patterns were characterized using atomic force microscopy, fluorescence spectroscopy and fluorescence lifetime measurements revealing monolayers of oriented perylenes in high coverages.

In imaging applications the blinking of quantum dots is an ambiguous phenomenon. Although unwanted in conventional imaging, blinking is of significant relevance for super-resolution microscopy. Recent studies report on excitation-wavelength dependent quantum dot blinking, while other studies do not observe this effect. To investigate this disagreement we have systematically studied the effect of the excitation wavelength on blinking of CdSe/ZnS core-shell quantum dots that are commonly used in imaging applications. We recorded single quantum dot intensity traces for 14 different excitation wavelengths and determined for each excitation wavelength the probability to find short-(,0.5 s) and long-(.0.5 s) lasting bright state periods. This represents a systematic study of a larger set of excitation wavelengths than heretofore reported in the literature. For the quantum dots analyzed, we find minor variations in blinking with excitation wavelength. These variations do not follow a trend and do not correlate with features in the absorbance spectrum of the quantum dots. Our results show that in practice changing the excitation wavelength for CdSe/ZnS quantum dots does not allow optimization of blinking for e.g. super-resolution microscopy and that the effect of blinking as a function of excitation wavelength is not a salient parameter in imaging applications.