Aggregation of Cyanine Dye Molecules in the Near Fields of

Article pubs.acs.org/JPCC

Aggregation of Cyanine Dye Molecules in the Near Fields of Plasmonic Nanoparticles Excited by Pulsed Laser Irradiation Nikita A. Toropov,* Peter S. Parfenov, and Tigran A. Vartanyan* Saint Petersburg National Research University of Information Technologies, Mechanics and Optics, 197101 Saint Petersburg, Russia ABSTRACT: Influence of laser irradiation on the supramolecular structure of thin organic dye layers is shown to depend on the presence of metal nanoparticles supporting localized surface plasmon resonance. Silver nanoparticles with the average size of 13 nm were produced on the sapphire substrate via physical vapor deposition under UHV conditions. The thus obtained granular silver film was spin-coated by monocarbocyanine dye. The samples were characterized by vis-NIR spectroscopy, SEM, and AFM. It is established that in the presence of silver nanoparticles pulsed laser irradiation at the wavelength of 532 nm leads to an increase of the relative concentration of the J-aggregates of the monocarbocyanine dye molecules, while on the bare sapphire substrate only the destruction of the dye molecules was observed.

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INTRODUCTION Metal nanoparticles (NPs) possess unique optical properties that arise from the collective oscillations of their free electrons. External electromagnetic waves effectively excite these oscillations known as localized surface plasmon resonances (LSPR). It is known that the strength and frequency of LSPR depend on the metal NP shape, size, and the dielectric constant of the environment of the nanoparticles.1−4 LSPR are widely used in basic research enabling surface-enhanced Raman scattering spectroscopy5−8 as well as enhancement of absorption and luminescence of organic dyes.9,10 They also find important applications in photovoltaic devices,11 sensors,12 and in biomedicine.13 All of the above applications exploit the enhancement of the nanoparticle near field as compared to the field of the incident electromagnetic wave. Among less explored consequences of the near field enhancement are the photochemical processes that lead to the conformational changes of organic molecules. Below we describe the results of our experiments in this scarcely studied area. As the research subject, we chose 3,3′-diethylthiacarbocyanine iodide, an organic dye with very strong absorption bands in the spectral range of silver NPs absorption. Optical properties of thin films formed by this dye on the bare dielectric support were thoroughly studied by authors.14,15 3,3′Diethylthiacarbocyanine iodide is a monocarbocyanine (MCC) dye. Its molecules consist of two heterocyclic end groups with nitrogen atoms conjugated by the relatively short methine chain (−CHCH−CH=). Thin films of MCC dye consist of 2 isomers (all-trans- and cis-isomers) and molecular aggregates: dimers and J-aggregates. The relative concentrations of these forms are prone to changes due to the rotations of molecular fragments around the double chemical bond. In the ground electronic state, the stereoisomerization and aggregation of molecules are unlikely because there are energetic barriers for © 2014 American Chemical Society

such transitions. On the other hand, excitation of molecules by heat or laser irradiation supplies the system with additional energy that can accelerate such transitions.16 The energetic barriers for conformational changes of MCC layers are expected to be rather large because of the rigidness of its short polymethine chain.16,17 Despite that, we found that in the presence of silver NPs, laser irradiation leads to the conformational change of MCC molecular layers at a comparatively low threshold. This phenomenon may be used in the ultrafast optical memory devices due to the rapid (∼1 ps)14 changes of configurations of the molecules.

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EXPERIMENTAL SECTION Granular metal films were deposited on sapphire substrates by thermal evaporation in the vacuum chamber PVD 75 (Kurt J. Lesker) at the residual pressure of less than 3 × 10−7 Torr. A quartz-crystal oscillator was used to monitor the growth rate and the layer thickness. Prior to deposition, the substrates were cleaned by an ultrasonic and thermal treatment. Silver NPs were obtained by depositing Ag (99.99% purity) at 0.01 nm/s to the thickness of 3 nm, as determined by the quartz-crystal oscillator. The temperature of the substrate measured directly by the thermocouple attached to its surface was 200 °C. Analysis of the chemical composition of obtained nanostructures was performed by energy-dispersive X-ray spectroscopy (Oxford Instruments) and detected no impurities. Sapphire substrates with NPs were coated by organic thin films using a spin-coating technique with rotational speed up to 4000 rpm. 3,3′-Diethylthiacarbocyanine iodide (purchased from Sigma-Aldrich) was dissolved in alcohol, and the obtained Received: May 28, 2014 Revised: July 10, 2014 Published: July 11, 2014 18010

dx.doi.org/10.1021/jp505234j | J. Phys. Chem. C 2014, 118, 18010−18014

The Journal of Physical Chemistry C

Article

Figure 1. (a) FE-SEM image and (b) absorption spectrum of Ag island film deposited on sapphire substrate at 200 °C.

Figure 2. (a) Absorption spectrum of MCC in ethanol. (b) Absorption spectrum of MCC thin film (1) before and (2) after irradiation. Curve 3 corresponds to the difference of (1) and (2). (c) Absorption spectrum of the Ag island film (1), same film coated by MCC (2), and the difference of (1) and (2). (d) Absorption of MCC in near fields of silver NPs (1) before and (2) after irradiation. Curve (3) demonstrates the difference of (1) and (2).

solution was purified by filter paper. We were aware of the possible changes to the structure of the Ag island films upon contact with the solvent. To avoid the influence of such processes on the results of the experiments, the sapphire substrates with deposited Ag NPs were washed in alcohol. This procedure leads to the slight reduction of the NP plasmon band intensity, but the sample treated this way withstands many cycles of spin-coating and washing without damage. Extinction spectra of the organic dye films, the granular silver films, and the composite layers consisting of both parts were measured by the spectrophotometer SF-56 (LOMO). Because of small sizes of silver NPs, scattering is believed to make minor contribution to the extinction spectra. For this reason, extinction differences are used as a measure of the absorption of the corresponding constituents of the composite film. The morphology of Ag NPs was determined by the fieldemission scanning electron microscope (FE-SEM) Hitachi SU8000 using the technique described by authors.18 Before measurements, the samples were mounted on a 25 mm aluminum specimen stub and fixed by conductive graphite tape. Sample morphology was studied under native conditions to exclude metal coating surface effects.19 Images were acquired in a secondary electron mode at a 10 kV accelerating voltage and at a working distance of 3−10 mm. As the organic dye layers were destroyed by high-energy electrons, FE-SEM was used only to obtain images of the granular metal films. To study the morphology of the samples containing dyes, the atomic force microscope (AFM) Solver PRO-M has been used. Microscopic studies were carried out in the tapping mode with the probe resonance frequency of 283 kHz. The probe radius was about 10 nm.

The second harmonic of Nd:YAG-laser (Solar LS) with 10 ns pulse duration was used for irradiation of the samples.

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RESULTS AND DISCUSSION As can be seen in the scanning electron micrograph in Figure 1a, a 3 nm silver layer consists of irregularly shaped nanoislands. Evaluation of this image made by the original software of the Hitachi microscope (SEM Data Manager) leads to the conclusion that the average particle diameter is equal to 13 nm and the interparticle spacing is comparable to the particle size. The absorption spectrum of the uncovered Ag NPs is shown in Figure 1b and features a single band centered at 445 nm. Due to the small sizes of the Ag NPs compared to the wavelength at the band center, the band may be surely ascribed to the dipole LSPR. The width of the band is mainly due to the wide distribution of NPs over their shapes. More oblate particles contribute to the red wing of the LSPR absorption band, while less oblate particles forms the blue wing of the band.20 The wide absorption band of the ensembles of NPs allows us to study the interaction of metal NPs with a number of dyes with various absorption spectra. The absorption spectrum of the MCC in ethanol (63 μm thick cuvette, concentration of dye molecules 1.46 × 10−3 mol/ L) is shown in Figure 2a. Two main contributions to this spectrum are due to the all-trans- and cis- isomers of the dye molecules.15−17 The absorption spectrum of the thin film obtained from this solution is shown in Figure 2b (curve 1). Supposing that the absorption cross section of dye molecules in the film is the same as in solution, the film thickness may be estimated to be about 10 nm. From Figure 2b, it is clear that the absorption spectrum of the molecular dye layer is broader than the absorption spectrum of the same dye in solution. This 18011



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Figure 3. Examples of 3D and 2D AFM images of MCC on the bare sapphire (after irradiation). White line corresponds to 2 μm.

It is to be noted, that the spectral positions of absorption peaks of different components of the MCC dye layer are not affected by the presence of plasmonic NPs. The consequences of the laser irradiation of the pure dye and the composite layers are shown in Figure 2 (panels b and d), respectively. The laser wavelength, set at 532 nm, corresponds to the absorption bands of both all-trans- and cis-isomers of MCC. The MCC layers with silver NPs were irradiated by laser pulses of the 8.4 mJ/cm2 fluence. This value is well below the threshold of laser-induced modifications of silver nanoparticles determined in the spectral hole burning experiment.20,24−26 Gradual changes of the absorption spectra were registered in the course of 3 cycles of irradiation with 10 pulses in each cycle. The final change of the layer absorption is about 1% (Figure 2d). The absorption increased both at the high-frequency edge of the MCC absorption with a peak at 470 nm that corresponds to the band of the dimers and at the low-frequency edge of the MCC absorption with a peak at 620 nm that corresponds to the band of J-aggregates. Thus, we can conclude that at low fluence part of monomers were associated in molecular aggregates. Laser irradiation of the MCC layers without silver NPs at the same fluence that was used in the case of the composite layer does not lead to measurable changes of their absorption spectra. The absorption spectrum of the pure dye layer was changed by the value of 1% only when the laser fluence was increased to 30.5 mJ/cm2 and 100 pulses were delivered to the layer. Contrary to the case of the MCC layer with Ag NPs, laser irradiation of the pure dye layer leads to the reduction of absorption (Figure 2b, curve 3). Taking into account the spectral ranges where the absorption is reduced, one can conclude that the laser irradiation of the pure dye layer leads to the decay of molecular aggregates. The monomers decay as well, but their amount reduces much slower because the decaying aggregates produce monomers. Disregarding the sign of the absorption changes, one can conclude that in the case of pure dye, the total energy expended for 1% change of the layer absorption is 12 times larger than in the case of the composite layer consisting of dye and silver NPs. AFM images of the pure dye and composite layers were obtained before and after laser irradiation. An image of the pure dye layer after laser irradiation is shown in Figure 3. It is clearly seen that the molecular layer of the MCC is rather rough. It consists of quasi islands with large aspect ratios. Typical lateral

broadening is due to the interaction of dye molecules with the sapphire substrate and to the presence in the layer of different isomers and supramolecular aggregates. Separation of the absorption spectrum into components belonging to different isomers and supramolecular aggregates was done following the method developed in refs 15 and 16. It is based on the spectroscopic studies of molecular layers of different thicknesses together with the spectra of the linear dichroism. According to ref 15, broadening is mainly due to two additional contributions that arise in the absorption spectra of thin films besides the all-trans- and cis- isomer contributions known from the spectra of MCC in solution. They are the dimers absorption at the blue edge of the MCC band and the J-aggregates absorption at the wavelengths larger than 600 nm. From Figure 2c, it is obvious that in the near fields of silver NPs, absorption of the MCC layers is substantially altered. In order to reveal the impact of the presence of the Ag NPs on the absorption of the dye, the extinction spectrum of the Ag NPs was subtracted from that of the composite film (curve 3). The difference spectrum features two regions of different nature. Additional absorption at the wavelengths shorter than 390 nm, where the dye itself does not absorb at all, is to be ascribed to the blue shift of the silver nanoparticle plasmon band. Since the frequency of LSPR depends on the refractive index of the surrounding media,1 this shift is due to the changes in the dielectric environment of silver nanoparticles provided by the organic dye coating.9,21 The sign of this shift is in accord with the decreased dielectric constant of the dye layer at the blue edge of its absorption band.22 The second spectral range lying at the wavelengths larger than 420 nm corresponds to the dye molecular absorption. As expected, the absorption of dye molecules in the near field of metal nanoparticles appeared to be larger (36.4%, curve 3 in Figure 2c) than on the bare sapphire substrate (3.9%, curve 1 in Figure 2b). This large, 9-fold, difference in absorption cannot be explained by the difference in the surface concentration of dye molecules that was controlled independently and was found to be the same in both cases within 10% uncertainty. On the other hand, the maximum enhancement estimated with the use of the known optical constants of silver23 reaches 400. The observed enhancement is less than this value because most of the molecules in the layer are located at some distance from the nanoparticles surface and do not experience the field enhancement in full because it is rapidly decaying as the distance to the NP surface increases. 18012



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Education and Science of the Russian Federation (Project 2350 no. 01201459446) and ITMO University (Project no. 414667).

sizes of these islands are 500−700 nm and larger, while their heights are about 10 nm. To quantify the laser action on the morphology of the films, the root-mean-square (RMS) roughness of different samples was computed and presented in Table 1. In the case of pure dye

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Table 1. RMS Roughness of the Films (nm) type of the sample

before irradiation

after irradiation

MCC without Ag NPs MCC with Ag NPs

3.6 5.2

0.8 4.3

samples, laser irradiation leads to the more than 4-fold reduction of the roughness. This observation is in accord with the destruction of supramolecular aggregates revealed by optical means. Both processes may be due to the melting of the pure dye layer at the elevated fluences used in this case. On the other hand, the roughness of the composite film that is somewhat larger than the roughness of the pure dye film does change considerably under laser irradiation. It is not surprising as the laser fluences used in the case of the composite layer were much lower as compared to the case of the pure dye layer. This serves as an additional confirmation of our claim that silver nanoparticles preserve their shape under laser irradiation, and the observed changes are not connected with the morphology changes.

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CONCLUSIONS Laser irradiation of pure dye layers leads to the destruction of molecular aggregates. In accordance with AFM studies, this process is accompanied by the layers flattening that may be due to their melting. Altogether, laser-induced transformation of the pure dye layers looks like a thermally activated process. This conjecture was checked in the special experiment. The layers were heated at 175 °C for 5 min. The changes of the layers optical properties were similar to that observed in the case of laser irradiation of the pure dye layer. In the presence of silver nanoparticles, photoinduced transformations of organic dye layers change drastically. They occurred under much smaller fluences than in the pure dye layers. Moreover, contrary to the case of pure dye layers, laser irradiation leads to the buildup of molecular aggregates, in particular, J-aggregates and dimers. It seems plausible that the laser-induced transformations of organic dye molecules in the enhanced near fields of plasmonic nanoparticles proceed via excited electronic states without excessive heating of the layer as a whole.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +79112895606. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Electron microscopy characterization was performed in the Department of Structural Studies of Zelinsky Institute of Organic Chemistry, Moscow. This work was supported by the RFBR (Grants 14-02-31281 and 12-02-00853), the Government of Russian Federation (Grant 074-U01), Ministry of 18013



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Aggregation of Cyanine Dye Molecules in the Near Fields of

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