OPTICAL PROPERTIES AND PHOTOINDUCED TRANSFORMATIONS OF ORGANIC DYE MOLECULES IN THE NEAR FIELD OF PLASMONIC NANOSTRUCTURES
T.A. Vartanyan*, N.A. Toropov, A.A. Starovoytov
St. Petersburg National Research University ITMO, St. Petersburg, Russian Federation
*tigran@vartanyan.com
Metal nanoparticles and polymethine dyes being mixed react upon each other through the electromagnetic fields that they scatter.
We report on huge darkening as well as clarification of the hybrid material as compared to a simple composition of the constituents’ optical properties.
Collective electronic excitations in metal nanoparticles give rise to a number of interesting optical phenomena associated with the field enhancement and localization in the immediate vicinity of the particle surface. Besides the well known SERS, an enhanced absorption and fluorescence of different molecules, including organic dyes, have been observed in the near field of the plasmonic nanoparticles [1]. In spite of efforts devoted to researches on the subject the mechanism of these phenomena is not well understood. The problem is especially complicated in the case of granular metal films supported on dielectric substrates. These films are attractive for applications as they are readily obtainable via physical vapor deposition. On the other hand, they suffer of inhomogeneous broadening of plasmon resonances and instability of their characteristics in time.
An additional difficulty in the interpretation of the experimental results with solid substrates is that the optical spectra of supported molecular dye layers differ considerable from that in solution even in the absence of metal nanoparticles. Hence, to interpret correctly the effects of plasmonic nanoparticles one has to study first the dye films on the clean support.
We report here on the experimental results obtained for the polymethine dyes, the molecular layers of that were studied previously [2]. Silver nanoparticles were used because of their superior plasmonic properties. Silver was evaporated onto sapphire substrate in vacuum. The substrate was kept at room temperature. The polymethine dyes were spin-coated on the substrate with granular metal film from alcohol solution. To get stable and reproducible results it was essential to avoid changes in the metal film morphology due to the action of the solution. We found that the as prepared films were prone to the action of the solution. To get rid of that, the granular silver films on sapphire substrate were suspended in the solute for several hours before the first coating with dyes. After this treatment the films stand for many cycles of dye application and washing without noticeable change in their morphology.
For interpretation of the obtained results it was important to establish the nature of broadening of the plasmon bands of the granular silver film. For this purpose a permanent spectral hole burning technique was employed [3]. In these experiments the threshold of the photoinduced transformations of the granular silver film was established. In the experiments on the photoinduced transformation of polymethine dyes the laser intensity was kept below this threshold.
The extinction spectrum of the hybrid material cannot be reduced to the sum extinctions of its constituents [4]. At the short wavelength side of the dye absorption band the hybrid extinction is even lower than the extinction of the metal film without dye.
In the center of the dye absorption band the hybrid extinction is much larger than the sum of extinctions of the dye and the metal film. At the long wavelength side of the dye absorption band extinction is much larger that the metal film extinction although the dye being adsorbed on the dielectric substrate does not absorbe at this wavelengths at all.
The extinction spectrum of the hybrid material was rationalized as a result of mutual interactions between the plasmon oscillations localized in the metal nanoparticles and resonance absorption and refraction of dye molecules. Plasmon resonances are shifted due to the anomalous refraction of dye molecules. Depending on the spectral position of the dye absorption band relative to the inhomogeneously broadened plasmon band this shift may lead to considerable clarification of the sample at particular wavelengths that was observed experimentally. On the other hand, the absorption of dye molecules is enhanced due to the incident field amplification in the near field of metal nanoparticles. Even when the dye absorption band overlaps with the tail of the plasmon band of silver nanoparticles, 3 to 5 times enhancement of the dye absorption was obtained. Besides that a nearly 4-fold increase of cyanine dyes fluorescence intensity in the presence of metal nanoparticles was observed.
The photoinduced transformations of the dye molecules situated in the near field of the metal nanoparticles were studied as well.
The rate of the transformations on the surface of metal nanoparticles was found to increase as compared to that on the surface a dielectric substrate.
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