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Synthesis and Second Harmonic Generation Microscopy of Nonlinear Optical Efficient Hybrid Nanoparticles Embedded in Polymer Films. Evidence for Intra- and Internanoparticles Orientational Synergy Emilie Delahaye,† Nicolas Sandeau,‡ Yi Tao,§ Sophie Brasselet,*,‡,| and Rene´ Cle´ment*,† Laboratoire de Chimie Inorganique, Institut de Chimie Mole´culaire et des Mate´riaux d’Orsay, UMR 8182, Baˆtiment 420, UniVersite´ Paris XI, 91 405 Orsay, France, Laboratoire de Photonique Quantique et Mole´culaire, UMR 8637, Institut d’Alembert (IFR 121), Ecole Normale Supe´rieure de Cachan, 61 aVenue du Pre´sident Wilson, 94 235 Cachan Cedex, France, and Laboratory of AdVance Materials, Fudan UniVersity, Shangai, 200433, China ReceiVed: NoVember 29, 2008; ReVised Manuscript ReceiVed: March 9, 2009
This work describes the synthesis and quadratic nonlinear optical properties of hybrid nanoparticles. These nanoparticles are based on the insertion of the push-pull 4-[2-(4-dimethylaminophenyl)azo]-1-methylpyridinium chromophore in the manganese hexathiohypodiphosphate MnPS3 inorganic matrix. The included chromophores form J-aggregates responsible for second harmonic generation (SHG). The nanoparticles are characterized by infrared and UV-visible spectroscopy and by transmission electronic microscopy. The SHG efficiency of the particles dispersed in a polymer thin film is studied by polarized nonlinear microscopy, which permits probing of both the efficiency of the nanoparticles and their intrinsic molecular order. They are seen to assemble in 100-300 nm size agglomerates whose morphology, depending on the preparation, can lead to a collective optimal one-dimensional arrangement of the nanoparticles resulting from interparticles interactions. Introduction In spite of the tremendous research activity in the field of nanomaterials and nanosciences which has given birth to numerous nano-objects covering a wide range of chemical and physical properties (metallic or semiconducting nanoparticles, composite particles, molecular magnets, molecular wires, quantum dots, etc.), the achievement of nanoparticles exhibiting strong quadratic optical nonlinearities is a recent topic, from which today only very rare examples are known.1-7 Nonlinear optical (NLO) efficient nanoparticles offer a broad range of potential applications from nanoscale photonics to nanoprobes with original optical properties, ranging from fluorescence nanoprobes to second harmonic generation (SHG) nanosources.8 The goal of this article is to report how variations in the synthetic routes of hybrid organic-inorganic particles can affect their nonlinear optical properties as well as those of assemblies of nanoparticles embedded in a polymer film. From the point of view of engineering efficient NLO materials, fabricating NLO active nanoparticles constitutes an alternative approach to the synthesis of multichromophoric architectures, which has been a trend over the past decade.9-11 The main goal is to achieve a synergy through appropriate packing modes of the chromophores, to benefit from an increase of the SHG intensity per unit volume. In particular, the SHG efficiency of single particles is to be optimized using the * To whom correspondence should be addressed. Tel. + 33 1.69.15.61.83; Fax + 33 1.69.15.47.52; E-mail
[email protected] (R.C). Tel. + 33 4.91.28.84.94; Fax + 33 4.91.28.80.67; E-mail
[email protected] (S.B.). † Institut de Chimie Mole´culaire et des Mate´riaux d’Orsay. ‡ Institut d’Alembert. § Fudan University. | Current address: Institut Fresnel-MOSAIC group, Universite´ Saint Je´roˆme, 13 397 Marseille Cedex 20, France.
essential requirement that the chromophores inside the particle possess identical orientations. It is indeed known that the intensity of a coherent second harmonic signal given by an assembly of N chromophores in an excitation volume smaller than the wavelength increases as N2 multiplied by an efficiency factor which depends on the relative orientation of the chromophores.12 The feasibility of synthesizing such optimized nanoparticles has been established in our group recently.3,4 The strategy was based on the ”bottom-up” synthesis of a layered intercalation compound consisting of push-pull organic chromophores 4-[2-(4-dimethylaminophenyl)azo]-1-methylpyridinium (denoted DAZOP, described in the figure below) embedded in a layered manganese hexathiohypodiphosphate (MnPS3) inorganic host lattice. Ten nanometer size nanoparticles dispersed in chloroform yielded an average second order hyperpolarizability β of about 42 000 × 10-30 esu measured by hyper Rayleigh scattering (HRS). This large value has been ascribed to the formation of J-aggregates by the DAZOP chromophores that ensured a ferroelectric-like arrangement of the dipoles.13-15 This early result, however, suffered from the limitation of giving only an average characterization of the particles present in a solution, which is an intrinsic constraint of the HRS technique.3,16 To circumvent this limitation and provide information on single nanoparticles, a preliminary study of isolated particles embedded in a polymer film using SHG microscopy has been performed with the surprising result that imaged nano-objects were found to be much more efficient than expected from the HRS data. An explanation for this high efficiency is the possible formation of ordered agglomerates of particles in the solid phase of a polymer matrix. The present work provides a deeper analysis of the nanoparticles’ behaviors by comparing different preparation techniques and combining SHG efficiency, SHG polarimetric response, and transmission electron microscopy information on isolated nanoparticles. The intra- and interparticles
10.1021/jp8104985 CCC: $40.75 2009 American Chemical Society Published on Web 05/01/2009
Synthesis and Properties of Hybrid Nanoparticles
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Figure 1. Scheme of chromophore DAZOP (4-[2-(4-dimethylaminophenyl)azo]-1-methylpyridinium).
interactions are seen to be a detrimental issue in isolating single efficient structures. The strategy employed to synthesize nanoparticles generating SHG is based on former results obtained in our group. In 1994-1996, some of us had shown that intercalating DAMS (4-[2-(4-dimethylaminophenyl)-R-styryl)]-1-methylpyridinium) or DAZOP push-pull cations (Figure 1) inserted into the layered MPS3 (yet centrosymmetric) host lattice produced SHG active powders that had been characterized by the Kurtz-Perry technique only.17,18 Subsequent research has brought evidence for the formation of J-aggregates extending over large distances along the interlayer galleries.19,20 X-ray diffraction has confirmed that the interlayer distance increased by about 6 Å upon intercalation, a value consistent with a closed packed, edge-on oriented chromophores. An obvious limitation is that intercalation of MPS3 yields intercalated powders, not nanoparticles. However, recent work has shown the possibility of circumventing this difficulty by a complete “one pot bottom-up” construction of MnPS3/DAZOP nanoparticles.3,4 This possibility is a consequence of the fact that the MPS3 layered compounds behave as coordination compounds made up of M2+ ions assembled by P2S64- bridging ligands. Although this is not obvious when one considers the traditional synthesis of the MPS3 family by high temperatures techniques,21,22 this observation has been established by different groups.23 The engineering challenge was therefore to synthesize MPS3 compounds (or rather (M2P2S6)n) by controlling the size of particles formed by the following reaction in aqueous medium:24,25
2M2++P2S46 f M2P2S6
(1)
The strategy designed in 2005 consisted of carrying out the growing process within nanoreactors provided by inverse micelles in cyclohexane.3,4 In the present work, we characterize nanoparticles obtained with two surfactants, as well as nanoparticles obtained using ion exchange resins. All nanoparticles could be dispersed in the presence of polymers and studied by nonlinear microscopy. Experimental Section MnCl2 · 6H2O, PCl3, Na2S · 9H2O, PVP (poly(vinylpyrrolidone), MW 10 000) and PVA (poly(vinyl alcohol), MW 89 000-98 000, 99+% hydrolyzed), resin DOWEX 50 WX2200 (Na) were purchased from Aldrich Chemical Co. Inc. and used as received. Standard deionized water was used for all aqueous solutions. Synthesis of Na4P2S6 · 6H2O. The water-soluble Na4P2S6 · 6H2O sodium salt was prepared following the Falius procedure,26 i.e., by dropwise addition of PCl3 (6.35 mL) into an aqueous solution of Na2S (75 g/100 mL) maintained at 5 °C by an ice/NaCl cooling bath, under vigorous stirring. After addition, the mixture was stirred in air for another hour time and then stored in the fridge overnight. The raw product was filtered, recrystallized from the minimum amount of hot water, and checked by infrared spectroscopy. Synthesis of Nanoparticles. Three procedures were used to obtain nanoparticles that could be dispersed in solution and subsequently embedded in polymer thin films.
Method 1A (Brij 97). Eight grams of poly(oxyethylene)(10) oleyl ether (Brij97) was dissolved in 100 mL of cyclohexane, and 3 mL of hexanol was added. The cloudy solution was filtered. At this stage, two micellar solutions were prepared: the first one was obtained by adding very slowly a mixture of MnCl2 (24 mg) and DAZOPI (7.5 mg) dissolved in 1.5 mL of deionized water into 40 mL of the previous solution of cyclohexane/Brij97. The second one was prepared by adding Na4P2S6 · 6H2O dissolved in 3 mL of deionized water to 50 mL of the above solution of cyclohexane/Brij97. The two resulting solutions were mixed together. After 3 h stirring, a blue solution was obtained. At this stage, 0.1 g of 4-methoxybenzenethiol was added to the mixture. Finally, 10 mL of methanol was added to break the micellar solution, and the mixture was centrifuged. The blue powder collected was washed with 10 mL of methanol and 2*10 mL of ethanol, in order to remove the remaining surfactant. The blue purified powder was further dispersed in 20 mL of MeOH/H2O (2:1) in the presence of poly(vinyl alcohol) (20 mg/mL), yielding a blue solution. This sample will be noted MnPS3/DAZOP/Brij97. Method 1B (AOT). In the case of bis(2-ethylhexyl)sulfosuccinate sodium salt (AOT), the same procedure as above was used. Thus, a first micellar solution was prepared by introduction of an aqueous solution (0.75 mL of deionized water) containing MnCl2 (4.8 mg) and DAZOPI (1.5 mg) in a mixture of heptane/ AOT (4.4 g of AOT and 20 mL of heptane). The second one was prepared by adding an aqueous solution of Na4P2S6 · 6H2O (6.4 mg in 1.5 mL of deionized water) in a mixture of heptane/ AOT (4.4 g of AOT and 20 mL of heptane). This sample will be noted MnPS3/DAZOP/AOT. Method 2. Nanoparticles of MnPS3/DAZOP could also be prepared using a cation exchange resin. A DOWEX 50 WX2200 (Na) (capacity 1.7 meq/mL) resin was treated with a 2 mol/L solution of aqueous NaCl, rinsed with distilled water, then loaded with Mn2+ ions by passing an aqueous solution of MnCl2 (0.3 mol/L), and finally rinsed several times until no chloride ions could be detected by a silver nitrate test. An aqueous solution of Na4P2S6 (50 mg in 25 mL of water) was then added to the top of the column. The column was then slowly eluted with water in an aqueous solution of poly(vinylpyrrolidone) (PVP, 600 mg in 5 mL of water). After the addition of the violet DAZOP iodide (7.5 mg) and stirring over several hours, the colorless solution eluted has turned to intense dark blue. This sample will be noted MnPS3/DAZOP/resin. Measurements. UV-visible spectra were obtained with a Varian Cary 300 spectrometer. Transmission electron microscopy (TEM) was recorded using a Phillips 208 operated at 80 KV. Samples were prepared by evaporating a few drops of solution on a carbon grid (Agar). FTIR spectra of precipitated powders were obtained at room temperature on a Perkin-Elmer Spectrum 1000 spectrometer using KBr pellets. Second harmonic generation (SHG) microscopy was performed using a mode-locked Ti:Sa laser that delivered 150 fs pulses at a fundamental wavelength of 920 nm with a 86 MHz repetition rate. The experimental setup is represented in Figure 2. The laser beam is focused on the sample by a high numerical aperture oil immersion objective (NA 1.4, ×100), leading to lateral and longitudinal resolutions of 300 and 700 nm, respectively. The SHG signal emitted by the sample is collected by the same objective, spectrally filtered around the 460 nm harmonic wavelength. The signal is then redirected to a polarizing beamsplitter behind which two avalanches photodiodes operating in the photon counting regime collect respectively the intensities in the X and Y polarization directions of the (X, Y, Z) analysis
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Figure 2. Schematic nonlinear microscopy setup. APD: silicon avalanche photodiodes.
Figure 3. Infrared spectra of (a) bulk MnPS3/DAZOP, (b) MnPS3/ DAZOP/Brij97, (c) MnPS3/DAZOP/AOT, and (d) MnPS3/DAZOP/ resin.
macroscopic framework. The polarization analysis consists of rotating the incident fundamental polarization in the (X, Y) sample plane, while measuring the optical SHG response on the two X and Y analysis directions. The samples for SHG microscopy were prepared by spin coating, after spreading a few drops of the blue solution obtained at the end of each synthesis on a microscope slide. The typical thickness of the studied films is 500 nm. Results Elemental Analysis. Elemental analysis data are summarized in Table 1. Full elemental chemical analysis of the powder prepared by the “resin method” has been obtained. Data give a composition Mn0.92PS3(DAZOP)0.16 · (H20)1.5, very close to the composition of the parent bulk crystalline compound. The nanopowder contains about 15% of water, significantly more than crystalline organic intercalates (∼3-5%). The powders prepared using Brij/AOT surfactants have been obtained in small amounts, and only the C, N, H content could be determined. From the nitrogen data, the amount of DAZOP is essentially the same in the three types of powders. The slight excess of carbon in the powder prepared from AOT is ascribed to a small amount of AOT remaining as an impurity. Infrared Spectroscopy. The infrared (IR) spectra of the powders collected for compounds MnPS3/DAZOP/Brij97, MnPS3/ DAZOP/AOT, and MnPS3/DAZOP/resin are shown in Figure 3 and compared with the spectrum of bulk MnPS3/DAZOP. The spectra are similar and very close to the spectrum of the bulk compound. Each spectrum exhibits two different regions: the bands above 650 cm-1 are assigned to the DAZOP species, whereas the band over the region 600-550 cm-1 reflects the νP-S asymmetric stretching band. This mode occurs at 570 cm-1 in pure crystalline MnPS327,28 but is systematically split in the
Delahaye et al. related intercalates into two or three components because intercalation renders P-S bonds inequivalent.28-30 The effect is particularly pronounced because manganese vacancies occur in the intercalated host lattice, which results in the occurrence of both terminal and bridging sulfur atoms. Figure 3 deserves two comments: (i) the components of the split PS3 mode are best resolved in the bulk crystalline compound; (ii) from the height of the bands, the amount of DAZOP molecules seems to be higher in the nanomaterials than in the bulk one. UV-Visible Spectroscopy. The UV-visible spectra of compounds MnPS3/DAZOP/Brij97, MnPS3/DAZOP/AOT, and MnPS3/DAZOP/resin redispersed in the solution of MeOH/H2O with polymer (at a concentration in DAZOP of 10-3 mol/L) are shown in Figure 4. In all cases, a strong red shift of the intramolecular charge transfer (ICT) band (from πNMe2 to π*pyridinium) of the chromophore DAZOP occurs. Indeed this ICT band for the DAZOP species in aqueous solution is located at 564 nm. The absorption bands of the nanoparticles are broad and asymmetric; therefore, we have attempted to decompose these curves by analogy with the parents’ compounds (DAZOPI in solution and bulk MnPS3/DAZOP). This decomposition is realized with Gaussian curves, and three parameters (position, width, and peak height of each peak) are used to fit these curves. The result is represented in Figure 4, and the obtained values are reported in Table 2. Each broad spectrum comprises three bands: a first one labeled 1 around 555 nm, a second one labeled 2 around 630 nm, and a third, much narrower one labeled 3 around 665 nm. By comparison with former results,19,31 band 3 can be reasonably attributed to the formation of J-aggregates.32-37 These aggregates are defined by the polymerization of the chromophores in a specific orientation (head to tail orientation of the chromophores). Band 1 is very similar to the free DAZOP species. The assignment of band 2 is not fully clear. Its occurrence demonstrates that a third type of DAZOP occurs, probably “intercalated but no-associated DAZOP” as suggested previously in the parent crystalline materials.19 The value of R, defined as the ratio between the peak height 3 and the sum of peak height 1, 2, and 3, is given in Table 2. This value is seen to be the highest in the case of MnPS3/DAZOP/AOT and the smallest in the case of MnPS3/DAZOP/resin. Transmission Electronic Microscopy (TEM). Representative TEM photographs of the three compounds are shown in Figure 5. In the case of the nanomaterials prepared in Brij97 and AOT, the solid was redispersed in an aqueous solution containing 2% PVA, and a few drops were evaporated on the carbon grid. In the case of the colloidal solution directly obtained with the resin procedure, a few drops of the solution containing PVP (2%) were also vaporized on the microscope grid. All samples display agglomerates (between 50 and 200 nm) made up of smaller particles (5-10 nm). The size of the agglomerates appears significantly larger for the materials prepared in AOT and Brij97 micelles (100-200 nm) than it is for the materials prepared on a resin (
