Distributed Feedback Lasing from a Composite Poly(phenylene

Oct 20, 2009 - Department of Chemistry, UniVersity of Toronto, 80 St. George Street, Toronto ... UniVersity of Bristol, Cantock's Close, Bristol BS8 1...
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NANO LETTERS

Distributed Feedback Lasing from a Composite Poly(phenylene vinylene)-Nanoparticle One-Dimensional Photonic Crystal

2009 Vol. 9, No. 12 4273-4278

Daniel P. Puzzo,†,⊥ Francesco Scotognella,‡,⊥ Margherita Zavelani-Rossi,§ Maria Sebastian,† Alan J. Lough,† Ian Manners,| Guglielmo Lanzani,§ Riccardo Tubino,‡ and Geoffrey A. Ozin*,† Department of Chemistry, UniVersity of Toronto, 80 St. George Street, Toronto M5S 3H6, Canada, Dipartimento di Scienza dei Materiali, UniVersita` di Milano Bicocca, Via R. Cozzi 53, 20125 Milano, Italy, Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy, and School of Chemistry, UniVersity of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K. Received August 3, 2009; Revised Manuscript Received September 30, 2009

ABSTRACT Nanoparticle one-dimensional photonic crystals exhibit intense, broadband reflectivity coupled with a unique mesoporosity. The latter property allows for infiltration of the one-dimensional photonic crystal with functional materials, such as emitting polymers, which in turn can lead to the fabrication of composites whereby the emitter’s emission can be modulated by the photon density of states of the photonic crystal. We exploit this interaction in order to produce efficient distributed feedback lasing from a composite poly(phenylene vinylene)-infiltrated nanoparticle one-dimensional photonic crystal.

During the past decade, significant attention has been directed toward the use of organic materials in photonics, due to the ability to customize their optical properties. Conjugated polymers, such as poly(phenylene vinylene) (PPV) and poly(fluorene) in particular, represent a class of organic materials commonly exploited for such applications. In fact, enormous progress over the past two decades has been achieved in the field of electroluminescence, leading to the development of high performance organic light-emitting diodes (LEDs).1 On the other hand, with regards to conjugated polymer-based lasers, progress has been comparatively slow, which is rather unexpected since lasing from optically pumped small-molecule dye-doped oxides as well as optically pumped organic molecular crystals was developed over 30 years ago.2-4 It was only in 1992 that lasing was demonstrated from solutions containing conjugated polymers and not until 1996 that laser-like emission in a solid conjugated polymer was demonstrated.5,6 * To whom correspondence should be addressed. E-mail address: [email protected]. † University of Toronto. ‡ Universita` di Milano Bicocca. § Politecnico di Milano. | University of Bristol. ⊥ These authors equally contributed to this work. 10.1021/nl902516t CCC: $40.75 Published on Web 10/20/2009

 2009 American Chemical Society

In general, a laser must possess at the very least two components. The first is an active emissive material, which should exhibit a good stimulated emission cross-section, and the second is a structure that supplies a feedback mechanism. For the feedback mechanism, diffractive resonators such as photonic crystals (PCs),7,8 opportunely doped with active materials, are commonly employed to fabricate distributed feedback (DFB) lasers.9,12 Such are commonly planar structures in which optical feedback is provided by a nanopatterned surface, either as a substrate or directly embossed in the active layer.13 Recently, interest in vertical one-dimensional (1D) PCs or distributed Bragg reflectors (DBRs) has surfaced within the materials chemistry community in an attempt to exploit the intense broadband reflectivity such materials offer for a variety of applications, both conventional and unconventional. Recent reports have focused on the fabrication of “functional” DBRs by employing building blocks ranging from nanoparticles (NPs) to clays to polyelectrolytes and zeolites.14-16 In such systems, the photonic structures exhibit the common DBR characteristics of intense and broadband reflectivity but, in addition, are also imparted the added functionality of their respective functional building blocks. For example, for DBRs fabricated purely from NPs of SiO2 and TiO2, significant mesoporosity

Figure 1. (A) Reaction employed for the synthesis of PPV. (B) The molecular structure of the monomer to the polyelectrolyte PPV precursor, p-xylene-bis(tetrahydrothiophenium chloride), with displacement ellipdoids drawn at the 30% probability level.

as well as the rich and versatile surface chemistry of SiO2 and TiO2 become available. In addition, for a DBR with individual layers fabricated from zeolites, the microporosity of the zeolites can now be exploited for the selective adsorption of analytes, which subsequently manifests itself in a shift in the reflectivity of the DBR. Our group has been particularly active in this area, as we recently reported on the stimulated emission from a small-molecule dye-doped NP DBR. The mesoporosity of the NP DBR structure was pivotal to the study, as it allowed for the uptake of a sufficient quantity of laser dye and also ensured that the dye was spatially well dispersed in order to prevent fluorescence selfquenching.17 Here we report on lasing from a PPV-infiltrated NP 1D PC. The traditional “precursor route” was selected as the synthetic method of choice to the emissive polymer, the scheme of which is shown in Figure 1. The monomer p-xylene-bis(tetrahydrothiophenium chloride) was obtained via reaction of dichloro-p-xylene with excess tetrahydrothiophene. The monomer was then precipitated from acetone and allowed to stand for 3 days at 4 °C, yielding large needle-like crystals. To the authors knowledge, the crystal structure of p-xylene-bis(tetrahydrothiophenium chloride) has yet to be reported, and thus it is also included in Figure 1B. The PPV polyelectrolyte precursor was then subsequently obtained upon treatment of p-xylene-bis(tetrahydrothiophenium chloride) with NaOH solution. Following neutralization, the precursor polyelectrolyte was purified by dialysis and subsequently used as acquired from the dialysis tube for infiltrating the 1D PC. 4274

PPV is a versatile emissive and semiconducting polymer, as it has found widespread use in organic electronics, in devices such as heterojunction solar cells,18 polymer LEDs,19 and transistors.20 However, the fully conjugated and unsubstituted structure lacks solubility in commonly used solvents, and so to ensure solution processability of PPV, one of two approaches are usually adopted. The first simply involves employing an alkoxy-substituted derivative of PPV, such as poly(2-methoxy-5-[2′-ethylhexyloxy]-p-phenylenevinylene) (MEH-PPV), which of course allows for processing from organic solvents. The second involves the preparation of a polyelectrolyte precursor that can be processed from aqueous solution and subsequently thermally converted to the desired conjugated polymer. For the lasing application described herein, either route is suitable; however, we opted for the polyelectrolyte precursor method for reasons highlighted below. For example, an attractive feature associated with generating a composite PPV/NP 1D PC whereby the PPV is obtained via the precursor route stems from the PPV’s subsequent insolubility following thermal conversion. Once infiltrated and thermally converted, the resulting composite (PPV/NP 1D PC) is quite robust and highly resistant to common solvents. In addition, this particular polymer was specifically chosen because its respective precursor is cationic, thereby rendering it ideally suited to interact favorably with the negatively charged surface of the NPs, thereby providing a high polymer loading of the porous photonic structure. The properties of PPV generated from the precursor route are governed primarily by the conjugation length of the polymer, which in turn is highly dependent on the purity of sample and the conditions used for the thermal conversion (i.e., temperature and atmosphere).21 A decreased conjugation length is common with PPV obtained via the precursor route, which is certainly undesired for electronic applications where high carrier mobility is often sought, but can actually be beneficial for optical applications such as lasing. A decreased conjugation length decreases the interaction between emitting centers and so minimizes deleterious mechanisms of fluorescence self-quenching (i.e., excited state absorption). Typically, PPV obtained from the precursor route emits green-yellow light, has good photostability, and exhibits approximate photoluminescence efficiencies of ∼22%.22 To the authors’ knowledge, DFB lasing from PPV obtained from the precursor route has not been demonstrated, but lasing from such a polymer embedded in a microcavity architecture was shown in 1996.6 The 1D PC employed in this study, the scanning electron microscopy (SEM) image of which is included in Figure 2A, consists of periodically alternating layers of SiO2 and TiO2 NPs prepared by the spin-coating method previously reported in the literature by our group and that of Miguez.15 This methodology affords a rapid and simple materials chemistry synthetic pathway to the self-assembly of high optical quality 1D PCs. To prepare the mesoporous 1D PCs, colloidal dispersions of SiO2 and TiO2 NPs were alternately spincoated until the desired number of bilayers is deposited. Following the deposition of each bilayer, the multilayer Nano Lett., Vol. 9, No. 12, 2009

Figure 3. Absorption (black line) and emission (red line) spectra of a 300 nm thick pure film of PPV.

Figure 2. (A) SEM image of an 11-bilayer SiO2/TiO2 NP Bragg reflector (scale bar ) 1 µm). (B) A cross-sectional confocal microscopy image of the PPV-infiltrated NP 1D PC (scale bar ) 1 µm); note that the thickness correlates well with the thickness of the SEM in A. (C) Photograph of the PPV/NP 1D PC composite under UV excitation.

structure was subjected to a brief thermal treatment at 450 °C for 15 min. Complete details of the method as well as the TiO2 NP preparation are provided in the Experimental Section. To maximize the PC effect, PCs consisting of 11 bilayers were fabricated with good structural and optical quality over an area of 2 cm × 2 cm. The effective refractive index of the SiO2 layers and the TiO2 layers were determined by spectroscopic ellipsometry to be 1.21 and 1.80 at 598 nm, respectively. From the effective refractive indices as well as layer thicknesses, the porosities of the SiO2 and TiO2 layers were determined to be 38% and 34%, respectively. Having prepared the NP-based 1D PCs by the spin-coating method followed by a mild sintering post-treatment, precursor PPV, prepared by the synthetic scheme provided in Figure 1, was drop-cast or spin-cast onto the DBR in order to effectively infiltrate the pores of the DBR. To the authors knowledge, only one other report at present describes polymer infiltration of such NP DBRs.23 Nevertheless, regardless of the method of PPV infiltration, an undesired polymer overlayer was always present, which was easily removed by gently scraping the overlayer with a glass slide. Here the polymer is easily removed, as it is significantly softer than the polymer/NP composite, and thus the latter remains intact suffering minimal degradation. Once the PC is infiltrated and the overlayer removed, the precursor was thermally converted to the desired conjugated structure by heating at 225 °C for 3 h under N2. A cross-sectional confocal optical microscopy image is provided in Figure 2B, effectively demonstrating that the polymer uniformly permeates the entire photonic structure, which is of course again made possible by the mesoporosity of the NP 1D PC. It is the porosity of such a structure that truly enables the inclusion Nano Lett., Vol. 9, No. 12, 2009

of the emitting polymer into the pores, which in turn ultimately affords the capability of fabricating a composite emitting polymer/NP 1D PC laser. The photograph of Figure 2C, which is of the PPV/NP 1D PC composite under UV excitation, is a true testament to the aforementioned attributes. The absorption and spontaneous emission spectrum of a pure 300 nm thick PPV film is shown in Figure 3. The absorption has a maximum of ∼400 nm, and thus this wavelength was selected as the pump wavelength. The spontaneous emission of the thermally converted polymer is broad extending over a range of 500-690 nm, which is indeed advantageous for this application, as it does not impose severe restrictions on the position of the 1D PC Bragg reflectance. The reflectance spectrum of a PPV-infiltrated NP 1D PC is shown in Figure 4A (black line). The broadband between 530 and 630 nm, with a peak reflectance at 570 nm, corresponds to the photonic stopgap of the NP 1D PC. A comparison of reflectance and the spontaneous emission reveal a significant overlap, which is of course a prerequisite for the development of the desired band edge laser. The optical setup used to acquire photoluminescence measurements is described in detail in the Experimental Section. For such measurements, the DFB was pumped by 100 fs pulses at 400 nm at a repetition rate of 1 kHz, obtained from the second harmonic of a Ti:Sapphire amplified laser. The beam, incident at an angle of ∼20 degrees to the surface normal, was focused on a circular excitation area (1.1 × 10-4 cm2), and the emission was measured with an optical multichannel analyzer (OMA, 1.2 nm resolution). As mentioned previously, the spontaneous emission of PPV exhibited good overlap with the Bragg peak of the constructed 1D PC. The function of the PC in this application is a modulation of the photon density of states (DOS) of the composite. In this 1D PC system, the DOS is high at the band edges and tends to zero in the bandgap. Thus, for an emitting species confined within the NP PC, provided that there exists good overlap (as is the case in our system) with the emission of the emitter and the Bragg peak of the DBR, it is expected that emission wavelengths overlapping the band edges will be enhanced, and those overlapping the stopgap will be suppressed. At high enough excitation energy density, the gain within the system should exceed the losses and 4275

Figure 4. (A) Reflectance spectrum of a PPV-infiltrated NP 1D PC (blue) along with the emission spectrum of the composite above threshold. (B) Input-output (i.e., excitation energy-output intensity) characteristics of the emission of the composite. Inset: transmission spectrum of the NP 1D PC (black line) and laser peak of the PPV-infiltrated NP 1D PC.

lasing be observed. It is worth noting that the ideal system would be one whereby the blue edge of the stopgap of the NP PC exactly overlaps the gain maximum of the emitting polymer. Such a system is very challenging to construct, however, owing to the difficulty in precision controlling of the stopgap position as well as the sensitivity of the polymer conjugation length (and thus the gain spectrum) on the fabrication conditions. Nevertheless, although the aforementioned condition certainly represents the ideal, its realization is not absolutely necessary to obtain stimulated emission, as we achieved this situation by simply ensuring sufficient overlap of polymer emission with the blue edge of the PC. As shown in Figure 4, above an excitation energy density of 550 µJ/cm2, a narrow peak (full width at half-maximum (fwhm) ∼ 3 nm) centered at 520 nm can be observed as shown in Figure 4. The observed lasing wavelength correlates well with the blue edge of the PC stopgap, and thus the vertical DFB photonic structure provides an efficient feedback to light inside the cavity. In Figure 4b we plot the input-output (i.e., excitation energy-output intensity) characteristics of the emission. It shows the typical signatures of laser action: a clear threshold at approximately 100 nJ, with concomitant line narrowing (from ∼40 to ∼3 nm) and a linear increase for higher excitation energies. Note that the linear increase is obtained over a very large range of input energies (from 100 to 1400 nJ), which is worth emphasizing, as this effect is seldom 4276

achieved with organic lasers, as usually saturation and degradation play a major role at higher excitation conditions. The threshold corresponds to an energy density of 550 µJ/ cm2, of absorbed light, as the exciting laser beam is focused on an area of 1.1 × 10-4 cm2. This value is in good agreement with thresholds for other organic DFB lasers reported in the literature.24 It is expected that the threshold can be lowered by increasing the number of layers of the DFB structure, as this would augment the feedback mechanism. The improvement achieved by employing the porous photonic matrix can also be deduced by analyzing the emission of a pure PPV film as a function of the excitation energy (Figure S1 in the Supporting Information). The emission of the pure PPV film as a function of the excitation energy is very broad with no apparent line width narrowing upon increasing excitation energy. It is finally noteworthy that this device shows very good stability, and no degradation was observed after many hours of operation. In conclusion, we demonstrated lasing (fwhm ∼ 3 nm) from a composite material consisting of PPV infiltrated into a NP 1D PC. It is the porosity of the NP 1D PC that is key to the preparation, as it allows for the polymer to be introduced into the photonic structure after it has been fabricated and thermally stabilized. Our device is robust under optical pumping, and does not show saturation even at fluence 1 order of magnitude above the threshold. The threshold and fwhm of the laser are good, although improvements can be made if a larger number of bilayers are introduced into the 1D PC, if a better matching between photonic band edge and gain maximum of PPV are established, or if the active polymer were to be incorporated into a microcavity architecture (since this structure exhibits higher photonic DOS at the defect).25 Note that the porosity of the medium, which is essential for construction of the laser device, also provides other functionalities. The use of “functional” NP 1D PCs14-16 that show a shift in the PBG by selective absorption of analytes paves the way to a possible realization of laser-based switches and sensors. Experimental Section. Materials. SiO2 NPs were purchased from Aldrich (Ludox SM-30). TiO2 NPs used in this work were prepared according to literature protocol.26 Briefly, titanium ethoxide was added dropwise to 0.1 M HNO3 at room temperature under vigorous stirring. The suspension was then peptized by stirring at 80 °C for 8 h. The resulting dispersion was filtered to remove any agglomerates and subsequently diluted to the desired concentration. Preparation of PPV Precursor. The PPV precursor was prepared according to the literature.27 The monomer pxylene-bis(tetrahydrothiophenium chloride) was obtained via the reaction of dichloro-p-xylene (0.75 M) with tetrahydrothiophene (2.25 M) at 50 °C in a 80:20% by volume methanol/water solution under N2 overnight. The product was subsequently obtained by evaporation of the methanol followed by precipitation from cold acetone. Precipitation was not immediate, and so the solution was stored for 3 days at 4 °C, after which needle-like crystals of the desired monomer were obtained. The solid was collected by filtration and dried under vacuum on a Schlenk line. The PPV Nano Lett., Vol. 9, No. 12, 2009

Table 1. Crystal Data and Structure Refinement for p-Xylene-bis(tetrahydrothiophenium chloride) empirical formula formula weight temperature wavelength crystal system space group unit cell dimensions volume Z density (calculated) absorption coefficient F(000) crystal size theta range for data collection index ranges reflections collected independent reflections completeness to theta ) 27.46° absorption correction max and min transmission refinement method data/restraints/parameters goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff. peak and hole

C16 H32 Cl2 O4 S2 423.44 150(1) K 0.71073 Å monoclinic P21/n a ) 7.0933(2) Å b ) 10.1351(5) Å c ) 14.8789(6) Å 1048.03(7) Å3 2 1.342 Mg/m3 0.526 mm-1 452 0.18 × 0.18 × 0.14 mm3 2.79 to 27.46°. -9 e h e 9, -13 e k e 13, -19 e l e 19 6538 2376 [R(int) ) 0.0389] 99.2% semiempirical from equivalents 0.933 and 0.845 full-matrix least-squares on F2 2376/0/125 1.088 R1 ) 0.0381, wR2 ) 0.0856 R1 ) 0.0549, wR2 ) 0.0958 0.362 and -0.336 e.Å-3

polyelectrolyte precursor was then subsequently prepared by reacting 2.145 g of monomer with an equimolar quantity of sodium hydroxide (0.4 M) at 0 °C for 1 h under N2. The total volume of NaOH solution was added dropwise over 20 min. The reaction was stopped by simply neutralizing with 0.4 M HC1 to a pH of 6.8. At this point, the solution became relatively viscous. The polyelectrolyte was separated from residual monomer and NaOH by dialysis against deionized water for 3 days. The dialysis tubing used had a molecular weight cutoff of 3500 Da. Preparation of Oxide NP-Based 1D PCs. Piranha- and air-plasma-treated glass slides and silicon wafers were used as the substrates for the NP DBRs. Prior to spin-coating each dispersion was stirred thoroughly, sonicated for 10 min, and filtered through a 0.45 µm pore syringe filter to remove any aggregates. Following each bilayer deposition, the SiO2/TiO2, NP DBRs were thermally treated at 450 °C for 30 min. Instrumentation. The spin coater was a Laurell WS-400A6NPP/LITE. Transmission spectra were collected with a spectrophotometer Cary Varian 50 (bandwidth 1.5 nm). High-resolution SEM (HRSEM) was performed using a Hitachi S-5200 (10-15 kV, 15 mA). Laser confocal microscopy was performed on a Leica TCS SP2 model with a HeNe laser operating at 1.2 mW under 50× magnification. Ellipsometry was performed with a Sopra GES5E ellipsometric porosimeter. To measure the laser emission of the sample, we used the second harmonic of an amplified Ti: Sapphire tunable laser (Coherent Mira). The selected wavelength was 800 nm, in order to have the second harmonic at 400 nm. The laser emission was collected normal to the substrate by an OMA (1.2 nm resolution). X-ray data were collected on a Bruker-Nonius Kappa-CCD diffractometer using monochromated Mo KR radiation and were measured using a combination of φ scans and ω scans with κ offsets, to fill the Ewald sphere. The data were processed using the Nano Lett., Vol. 9, No. 12, 2009

R ) 90°. β ) 101.544(3)°. γ ) 90°.

Denzo-SMN package.28 Absorption corrections were carried out using SORTAV.29 The structure was solved and refined using SHELXTL V6.130 for full-matrix least-squares refinement based on F2. All H atoms bonded to C atoms were included in calculated positions and allowed to refine in riding-motion approximation with U∼iso∼ tied to the carrier atom. H atoms bonded to the O atoms were refined independently with isotropic displacement parameters. Acknowledgment. G.A.O. is the Government of Canada Research Chair in Materials Chemistry and Nanochemistry. He is deeply grateful to the Natural Sciences and Engineering Research Council of Canada NSERC for generous and sustained funding of his research. The authors acknowledge Ilia Gourevich for aiding with microscopy. F.S. and R.T. acknowledge the European Commission (Marie-Curie RTN NANOMATCH, Grant No. MRTN-CT-2006-035884) for financial support. Supporting Information Available: Crystal data and structure refinement for p-xylene-bis(tetrahydrothiophenium chloride), and emission spectra of a pure PPV film as a function of the excitation energy. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bredas, J. L.; Logdlund, M.; Salaneck, W. R. Nature 1999, 397 (6716), 121. (2) Soffer, B. H.; McFarland, B. B. Appl. Phys. Lett. 1967, 10, 266. (3) Karl, N. Phys. Status Solidi A 1972, 13, 651. (4) Avanesjan, O. S.; Benderskii, V. A.; Brikenstein, V. K.; Broude, V. L.; Korshunov, L. I.; Lavrushko, A. G.; Tartakovskii, I. I. Mol. Cryst. Liq. Cryst. 1974, 29, 165. (5) Moses, D. Appl. Phys. Lett. 1992, 60, 3215. (6) Tessler, N.; Denton, G. J.; Friend, R. H. Nature 1996, 382, 695. (7) John, S. Phys. ReV. Lett. 1987, 58, 2486. (8) Yablonovitch, E. Phys. ReV. Lett. 1987, 58, 2059. (9) Samuel, I. D. W.; Turnbull, G. A. Chem. ReV. 2007, 107, 1272. 4277

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NL902516T

Nano Lett., Vol. 9, No. 12, 2009