Colloidal Crystal Lasers from Monodisperse Conjugated Polymer

Colloidal Crystal Lasers from Monodisperse Conjugated Polymer Particles via Bottom-Up Coassembly in a Sol−Gel Matrix Annabel Mikosch,† Sibel Ciftci,† and Alexander J. C. Kuehne* DWI−Leibniz Institute for Interactive Materials, RWTH Aachen University, Forckenbeckstraße 50, 52076 Aachen, Germany S Supporting Information *

ABSTRACT: The potential of colloidal crystals for applications in optics and photonics has been recognized since the description of spontaneous self-assembly of monodisperse colloids into periodic opaline geometries. Provided with a laser gain medium, these direct assemblies generate optical feedback and have prospective use as lasers or frequency converters; however, problems associated with the colloidal crystal integrity and low loading fractions of the gain medium in the self-assembled resonator structure have prevented their realization to date. Here, we circumvent these problems by synthesizing monodisperse conjugated polymer colloids, which consist entirely of gain medium. We coassemble these colloids together with a sol−gel precursor to achieve encapsulated photonic crystals, which can be applied via inkjet printing. These conjugated polymer photonic crystals exhibit single line laser emission upon optical pumping. This technique circumvents time-consuming micro- and nanofabrication steps as well as error-prone backfilling and etching procedures, providing an effortless way to generate laser geometries. KEYWORDS: conjugated polymers, sol−gel, self-assembly, photonic crystal, laser printing technology,9,10 allowing for fast and easy implementation into scalable manufacturing processes. Provided with a laser gain medium, these direct assemblies have prospective use as lasers or frequency converters;11 however, problems associated with mechanical stability of the colloidal crystals and low loading capacities of the gain medium in the opal resonator structure have prevented their realization to date. Current approaches apply a self-assembled photonic crystal from dielectric particles with diameters between 200 nm and 1 μm, which are backfilled with a laser gain material,12 followed optionally by removal of the original opal material.13,14 This leads to inverse opals, which exhibit laser emission;12,15,16 however, the amount of gain medium which can be incorporated into these structures is limited. Backfilling into the interstitial spaces of a self-assembled colloidal crystal with gain medium only facilitates loading capacities of 26%, versus 74% in direct opal, hexagonally packed geometries. The low amount of gain medium per volume results in impractically high laser thresholds, whereas backfilling often entails defects in

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onochromatic light sources with long-ranged coherence and high intensity are powerful tools for telecommunication, sensors, and spectroscopy.1 To achieve these characteristics in a light source, an active gain medium is placed inside a resonator structure, providing feedback and amplification of the stimulated emission. On the macroscale, this mechanism can be facilitated by using two parallel aligned mirrors, whereas on the microscale, the resonator often comprises a periodic Bragg grating structure, which provides resonance and effectively reduces the size of the laser.1 This geometry allows integration of lasers into lab-onchip devices2 and their implementation on optical computing platforms.3 However, such grating structures require costly and time-consuming nanofabrication, laser interference, or holographic lithography, complicating the scalability of the manufacturing process.4 Colloidal self-assembly elegantly circumvents these drawbacks. Monodisperse particles associate spontaneously into colloidal crystals. These colloidal crystals are periodic and allow confinement of light and optical feedback in three dimensions. The potential of colloidal crystals for applications in optics and photonics has been recognized since the description of spontaneous self-assembly of monodisperse colloids into opaline geometries.5−7 This bottom-up approach is compatible with roll-to-roll8 and inkjet © 2016 American Chemical Society

Received: August 16, 2016 Accepted: October 18, 2016 Published: October 18, 2016 10195

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Figure 1. Preparation of monodisperse conjugated polymers. (a) Synthetic equation and dispersion polymerization mechanism for the generation of monodisperse conjugated polymer particles. As monomers, a mixture of m-/p-divinylbenzene and fluorene with R = octyl or dodecyl is applied. The conjugated polymer chains phase-separate when reaching a critical molecular weight and then grow via condensation of polymers subsequently reaching the critical molecular weight (cf. Supplementary Movie S1). (b) Schematic of the BTES sol−gel coassembly process. Deposition of the liquid (left), self-assembly of the particles, and concurrent evaporation of the solvent and condensation and formation of the sol−gel network (middle) to obtain crystalline and fully encapsulated photonic crystals (right). (c) Photographs of the reaction mixture before (left) and dispersion (right) after polymerization. (d) Relation of the particle diameter versus monomer concentration. Gray squares represent particles with the didodecylfluorene monomer, and cyan circles represent particles with dioctylfluorene monomers. The error bars represent the dispersity of the particles as measured by DLS or SEM image analysis of more than 100 particles. (e) SEM of monodisperse particles. (f) Confocal microscopy image of conjugated polymer particles showing uniform fluorescence. The scale bars represent 10 μm.

RESULTS AND DISCUSSION We start by using diiododialkylfluorene and divinylbenzene as commodity monomers. Fluorene-containing conjugated polymers are known to exhibit lasing,18 and divinylbenzene is an inexpensive compound, which comes as a mixture of m- and pisomers.19 The resulting conjugated fluorenedivinylbenzene copolymers (PFDVB) exhibit amplified spontaneous emission ASE at low threshold (192 nJ/cm2).20 We apply previously established conditions with triethylamine as base, palladium(II)diacetate as catalyst, as well as poly(vinylpyrrolidone-covinyl acetate) and Triton X-45 as stabilizers in 1-propanol.21,22 All reagents are dissolved in the solvent and then heated to start the polymerization. At a critical molecular weight, the produced PFDVB becomes insoluble and phase-separates from solution. Subsequently produced polymers condensate onto the nucleated polymer chains, and the particles grow uniformly following a dispersion polymerization mechanism (see Figure 1a,c).22 This dispersion polymerization is easily scalable, and we produce the particles in batches on the liter scale with yields on the gram scale, while retaining monodispersity (see Supplementary Movie S1). We apply dioctyl- or didodecyl-functionalized diiodofluorene monomers; however, we do not see an effect of the side chain length with respect to the resulting particle size (see Figure 2d). We gain control over the particle size by varying the monomer concentration. We can reproducibly access monodisperse sizes between 400 nm and 2 μm (see Figure 1d). The particles are precisely monodisperse

the inverse opal structure. Furthermore, the thickness of the structures is limited to a few crystal cells in height, and lack of encapsulation leads to mechanical instability and fast photooxidation of the inverse opal structures. Colloidal crystals assembled from monodisperse particles consisting entirely of conjugated polymer would allow the generation of direct opal geometries, enabling applications such as organic microlasers and printable frequency converters. The availability of these materials would allow for all-organic devices in optical computing, small disposable devices for point-of-care medicine, and the consumer electronics market. However, monodisperse laser gain particles in an encapsulated photonic crystal geometry are not available, and current technologies and materials have drawbacks in their processability, cost, and scalability. There is a strong requirement for microlasers with high fill factors of gain medium, which facilitate high fidelity resonators combined with facile processing. Here, we present a direct and scalable approach to synthesize conjugated polymer particles using atom-economic17 Heck coupling and low-price commodity monomers. The particles are monodisperse; they self-assemble into colloidal crystals and exhibit gain for lasing. We process these particles together with a silica sol−gel precursor to obtain self-assembled and encapsulated photonic crystals in a purely additive fashion. This procedure represents a simple way for creating a laser. We show how these materials can be processed using inkjet printing, and we investigate the laser characteristics of such self-assembled devices. 10196

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Figure 2. Coassembly of particles with a sol−gel matrix into encapsulated photonic crystals and their optical performance. (a) Cross section of a coassembled photonic crystal deposited from 1-propanol. The scale bar represents 5 μm. (b) Ar-ion polished cross section to identify the size of the colloidal crystallites in the assembly. The scale bar represents 10 μm. (c) Cross section of a coassembled photonic crystal deposited from DMSO. The scale bar represents 10 μm. (d) Ar-ion polished cross section to identify the size of the colloidal crystallites at the air− crystal interface. The scale bar represents 10 μm. The inset shows a tilted view exposing the deposit surface and the cross section. The scale bar represents 5 μm. (e) Angle-dependent reflection spectrum of coassemblies from d = 500 nm particles in a glassy matrix. The black line represents the theoretical reflection maximum according to the modified Bragg equation. The dashed lines represent reflection signals. (f) Absorption (gray line) and fluorescence (turquoise circles) of the PFDVB material. The 0° reflection spectrum (black squares) of a coassembly (900 nm particles, reflection perpendicular to the substrate direction) overlaps with the (0 → 1) fluorescence transition. The filled turquoise data represent the laser spectrum of the crystalline coassembly for above threshold excitation. (g) Refractive indices of the conjugated polymer np (turquoise line) and the sol−gel matrix nm (gray line) determined on thin films by variable angle spectroscopic ellipsometry. The effective refractive index (black line) is calculated for the visible spectrum assuming ϕ = 0.74. (h) Normalized emission spectra below (blue, turquoise) and above threshold (beige). The mode at 532 nm is double frequency stray light from the Nd:YAG pump source. The inset shows the full width at half-maximum (fwhm) of a laser peak with a value of 0.13 nm. (i) Excitation pulse power (at 475 nm) versus emission intensity to identify the threshold of 0.5 μJ. The bending at high pulse powers is due to saturation. The inset shows laser thresholds Eth versus the excitation wavelength. Gray squares represent lasing thresholds of coassembled particles with the didodecylfluorene monomer, and the turquoise circles represent data from particles with the dioctylfluorene monomer. Best lasing results are obtained from the dioctylfluorene-containing particles with excitation at 475 nm. (j) Time study of a lasing PFDVB coassembly driven above threshold for over 144 000 laser pulses. Over this number of pulses, the output power drops to 64%.

coassemblies of our particles completely encapsulated by the BTES-derived matrix when we apply the formulation from either 1-propanol or DMSO. The formulations are deposited onto glass substrates by dispensing defined volumes using micropipettes or by inkjet printing. While the solvents evaporate, the monodisperse conjugated polymer particles assemble into a colloidal crystal and BTES hydrolyzes around the self-assembled particles to form an encapsulating glass matrix around the conjugated polymer photonic crystal (see Figure 1b and Figure 2a−d). For the deposition from 1propanol, we observe crystalline assemblies close to the substrate and less-ordered structures when moving toward the surface of the deposit, where the sol−gel forms an encapsulating layer over the colloidal assembly. For deposits assembled from DMSO, we observe reversed properties with crystalline order at the deposit−air interface and decreasing

with standard deviations smaller than 5%, as determined by dynamic light scattering (DLS) and scanning electron microscopy (SEM) analysis, and the particles show fluorescence in the blue−green spectrum (see Figure 1e,f). Upon deposition of these dispersions, the particles self-assemble into crystalline colloidal arrays (see Figure 1e). These assemblies are very fragile, prone to cracking and delamination from the substrate. To enhance the colloidal crystallinity and the mechanical robustness of the colloidal crystal and to protect the conjugated polymer from atmospheric oxidants, we encapsulate the colloidal crystal in a glassy matrix derived from a sol−gel precursor. We add 1,2-bis(triethoxysilyl)ethane (BTES) to our particle dispersion, which hydrolyzes into an amorphous organo−silica hybrid glass.23 To activate the hydrolysis of BTES to form the silica matrix, we add 0.01 M aqueous hydrochloric acid solution. We obtain crystalline 10197

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entirely of laser gain medium facilitating the amplification of light. We test this geometry for its photonic potential by varying the excitation wavelength across the PFDVB absorption tail (band edge) and find a minimum threshold Eth at 475 nm for laser emission for the dioctyl-functionalized PFDVB particles (see inset in Figure 2i). Consistently, over this range, the particles featuring the dioctylfluorene monomer units have thresholds lower than those of the didodecylfluorene homologues. We deduce from this experiment that the octyl periphery supplies sufficient chromophore separation to prevent aggregation quenching. The higher thresholds in the conjugated polymer particles with dodecyl periphery will be the result of reduced gain because the alkyl periphery does not contribute to gain. We continue with the dioctylfluorenecontaining particles and an excitation wavelength of 475 nm. We incrementally increase the excitation power to determine the laser threshold. The colloidal coassemblies exhibit typical laser behavior, showing fluorescence at low excitation intensities and the appearance of a narrow laser line when moving across the threshold excitation power (see Figure 2h). Above the threshold, the laser peak has a full width at halfmaximum (fwhm) of 0.13 nm when fitted with a Lorentzian oscillator (see inset in Figure 2h). The resolution limit of the spectrometer is 0.12 nm, which means that our fwhm value will be limited by the resolution of the spectrometer. At the threshold, we see a deviation of the linear relationship between emission and excitation intensity (see Figure 2h,i). This change represents the laser threshold Eth at 0.5 μJ per pulse. Note that the output increase beyond threshold is not as steep as expected for state-of-the-art conjugated polymer laser gain materials. In spite of the low threshold, optical gain is low in PFDVB. This might be the result of the broad absorption profile of PFDVB extending into the spectral emission range of the laser, effectively introducing loss.25 Considering that we decided against gain-optimized materials20 and opted for polymer particles produced from commodity monomers, the low laser threshold is encouraging. Application of gainoptimized materials will facilitate higher emission efficiencies in future studies.26,27 At high excitation intensities, we observe a leveling of the excitation−emission relation known as gain saturation, which is typical for organic laser sources. Without the sol−gel matrix, the colloidal crystal also shows laser emission, however, only over very few excitation pulses because photo-oxidation gradually impairs lasing. Furthermore, the attractive van der Waals forces between particles in the colloidal crystal are relatively weak, and the photonic crystal disintegrates over time, with particles being ablated by the excitation beam. To investigate whether the sol−gel encapsulation protects the colloidal PFDVB gain medium from photo-oxidation, we pump the coassembly above threshold continuously over 2 h, adding up to 144 000 pulses (see Figure 2j). Over this time, the coassembly continues to lase with no spectral shift, and the output intensity decreases slowly to level at about 64% of the initial value. The sol−gel matrix effectively provides a barrier to oxidation and holds the particles together, preventing disintegration and delamination from the substrate. To showcase the potential of this material system for upscaling, we apply the same formulation in an inkjet printing process. We deposit the formulation as ∼200 pL droplets onto cleaned glass substrates. We print an array of 12 × 20 pixels, resulting in encapsulated photonic crystal arrays with pixel diameters of ∼100 μm (see Figure 3a). These deposits show a variety of iridescence colors when illuminated with diffuse white

order toward the substrate. This behavior is governed by the relative density of PFDVB and the dispersing medium. PFDVB has a value between the density of DMSO (1.1 g/cm3) and 1propanol (0.803 g/cm3), resulting in interface-confined selfassembly at the solvent−air or solvent−substrate interface, respectively (see Figure 2a,c). To identify the size of the crystallites and therefore the quality of the self-assembly process, we investigate cross sections of the coassembled photonic crystal deposits. We polish the cross section using an argon-ion beam to gain insight into the morphology of the deposits. For both types of deposition, we obtain at least 15 crystalline hexagonal layers parallel to the substrate due to the constriction of the interfaces during assembly (see Figure 2b,d). This direction is the most useful for future integrated laser resonators when considering a printing process directly onto optical pump sources, corresponding, for example, to the geometry of VCSEL technology. We investigate the optical performance of the coassembled photonic crystals only in the (111) direction of the hexagonal assembly, which is vertical to the substrate. When we look at such coassembled photonic crystals prepared from particles with a diameter of 500 nm, we obtain angle-dependent reflection from the ordered colloidal crystal as well as surface reflections from the corrugated surface of the deposit and the glass substrate (see Figure 2d and inset and Figure 2e and Supplementary Movie S2). The obtained reflection from the self-assembled photonic crystal corresponds well with the theoretical reflection line calculated using the modified Bragg equation: mλ = 8/3d 2(neff 2 − sin 2 θ ) , with m as the Bragg order, λ the wavelength, d the particle diameter, neff the effective refractive index, and θ the angle of incidence (see black line in Figure 2e). Assuming a fill factor of ϕ = 0.74, we can determine neff from the refractive indices of the particles np and the matrix nm, which we determine using variable angle spectroscopic ellipsometry (see Figure 2g). The particles exhibit fluorescence with a maximum at 477 nm and well-resolved vibronic structure (see turquoise circles in Figure 2f). The absorption of the particles largely overlaps with the 0 → 0 transition, which is usually detrimental to laser emission and leads to emission from the 0 → 1 vibronic transition (gray line in Figure 2f).20 These characteristics are independent from the dioctyl or didodecyl substitution of the fluorene unit. We prepare photonic crystals made from PFDVB particles with a diameter of 900 nm, which facilitate overlap with the fluorescence spectrum for m = 5. For these coassemblies encapsulated by the BTES-derived matrix, we obtain a reflection band with a maximum at λmax = 511 nm and lower order reflections and those from other crystal planes of similar direction above 550 nm (see black squares in Figure 2f). We determine the normalized stop-bandwidth Δλ/λmax as a measure for the colloidal crystallinity of the photonic crystal. Δλ/λmax has a value of 0.063 and is reasonably close to the theoretical value of Δλ/λmax = 0.014, which was calculated along the (111) axis for a perfect colloidal face-centered cubic crystal of our materials using the scalar wave approximation.24 The deviation from the theoretical value is probably caused by crystal defects or surface roughness. The 0 → 1 fluorescence transition coincides with the reflection band (see Figure 2f). When we excite the coassembly with a pulsed laser source (10 ns at 20 Hz) and increase the excitation intensity, we observe narrow laser emission at 509 nm (see Figure 2f). Effectively, upon self-assembly, the conjugated polymer particles have formed their own resonator. Furthermore, the particles consist 10198

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and coassembled pixel (see Figure 3d). Under UV illumination, the pattern exhibits green−blue fluorescence across all printed pixels (see Figure 3c). The fluorescence is independent of the size of the employed particles, while the iridescent colors depend on the illumination and viewing angle as well as the particle size. The inkjet-printed coassembled photonic structures exhibit lasing under optical pumping conditions. Depending on the particle diameter, the laser emission shifts in accordance with Bragg’s law. Individual pixels of the coassembled photonic crystal lasers exhibit the same laser emission wavelength as long as they are produced using the same particle batch and sol−gel fraction. The narrow line width laser emission and threshold behavior are on the same orders of magnitude as the larger directly deposited photonic crystals discussed before. The lowest threshold obtained from an inkjetprinted assembly is at 0.34 μJ (see Figure 3g). We can therefore assume equivalent quality of the assembly process after inkjet printing compared to the directly deposited coassemblies (cf. Figure 2h,i). The slightly increased fwhm of the inkjet-printed photonic crystals results from their small size. Multiple inkjetprinted pixels are excited by the excitation beam while the curvature of the deposited assemblies entails detection of emission from slightly misaligned crystallites leading to limited line broadening (see Figure 3h and inset in Figure 3g).

CONCLUSIONS In summary, we present a straightforward method to produce a laser in a purely additive process. The development completely avoids etching, backfilling, and time-consuming nanofabrication techniques. The materials we use are available as commodity monomers with low cost and good optical performance. The presented technology allows integration of high fidelity photonic structures using a facile printing process, which reduces cost and makes this technology highly scalable. Low cost and self-assembled active photonic crystals could find application as printed security labels to improve forgery protection on means of payment. Furthermore, they could be applied directly on top of inorganic pump light-emitting diodes and microdisplays28 for integrated microlasers in microfluidic point-of-care chips and optical computation.

Figure 3. Micrographs of inkjet-printed arrays of coassembled photonic crystals. (a) Array of 12 × 20 pixels of coassembled photonic crystal deposits produced via inkjet printing. The scale bar represents 500 μm. The inset represents a close-up view of a single pixel showing iridescence, reflecting the crystalline geometry inside these deposits. The scale bar represents 50 μm. (b) Coassembly exhibiting blue, green, and red iridescence as a response to white light exposure under different angles. The scale bar is 1 mm. (c) Inkjet-printed QR code under UV illumination showing fluorescence for all pixels in the blue−green spectrum. Each square of the QR code is composed of 2 × 2 inkjet-printed pixels of coassembled particles. The scale bar represents 5 mm. (d) Micro-QR code, where each square of the QR code is represented by one inkjet-printed pixel. The scale bar represents 1 mm. (e) Side view electron micrograph of a coassembled deposit revealing crystalline colloidal domains at the surface. The scale bar represents 5 μm. (f) Micrograph of the inkjet-printed QR code in (c) under ambient illumination. (g) Excitation pulse power (at 475 nm) versus emission intensity to identify the threshold of an inkjetprinted photonic laser crystal (Eth = 0.34 μJ). The inset shows the laser spectrum fitted using a Lorentzian oscillator giving a fwhm of 0.5 nm. (h) Array of (on-the-fly) inkjet-printed photonic crystal under laser excitation. The scale bar represents 50 μm.

MATERIALS AND METHODS Materials. All reagents and solvents were purchased from SigmaAldrich, Merck Chemicals, or VWR. The commercially available materials were used without additional purification. In the following section, the purity and properties of the used materials are listed. 1Bromooctane (purity: 99%) and 1-bromododecane (97%), benzyltrietylammonium chloride (99%), fluorene (97%), iodine (≥98%), palladium(II)acetate (98%), periodic acid (≥99%), poly(1-vinylpyrrolidone-co-vinyl acetate) (average MW 50 kDa, molar ratio of VP/VA is 1.0:0.77), styrene (≥99%), triethylamine (≥99%), Triton X45, and tri(o-tolyl)phosphine (97%) were all purchased from SigmaAldrich. Divinylbenzene (65%) (mixture of para- and meta-isomers) was purchased from Merck Chemicals; 1-propanol (99.7%) was purchased from VWR. Dimethylsulfoxide, 1,2-bistriethoxysilyl ethane (96%), and hydrochloric acid (4 M) were purchased from SigmaAldrich. Nuclear Magnetic Resonance (NMR). NMR spectra were measured with a Bruker AVX400 (400 MHz). The coupling constants J are stated in hertz. For the multiplicities, the following abbreviations are used: s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet, br = broadened. Synthesis of 2,7-Diiodofluorene. A 250 mL four-neck flask was charged with 20 g (0.12 mol) of fluorene and a 200 mL solution of concentrated acetic acid, water, and concentrated sulfuric acid in a

light (see inset in Figure 3a). When illuminated with a collimated white light source, the coassemblies exhibit angledependent reflections of pure spectral colors (see Figure 3b). The inkjet-printed coassemblies exhibit large crystalline domains on the surface of the deposit, much the same way as we presented for the larger photonic coassemblies deposited from micropipettes (cf. Figures 3e and 2a−d). To display the versatility of this approach, we inkjet print information-carrying matrix bar codes in the form of a QR code. Each pixel of the matrix consists of 4 (2 × 2) single coassemblies with a diameter of ∼100 μm, resulting from individually deposited droplets (see Figure 3c,f). To miniaturize this code further, we print a microQR code where each matrix point consists of only one printed 10199

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Figure 4. Droplet formation at the inkjet printer nozzle over time. The droplet formation is highly reproducible, even if the first droplet splits into a primary droplet with satellites; these readily recombine to form one large droplet which reaches the substrate. ratio of 100:20:3. This suspension was heated under reflux to 100 °C until a clear solution was obtained. After the reaction mixture was cooled to 80 °C, 33 g (0.14 mol) of periodic acid was added to the mixture. The solution turned dark yellow. Additionally, 36.6 g (0.14 mol) of iodine was added in portions to the reaction. The mixture was stirred at 80 °C for 4 h and then cooled to room temperature. The obtained solid was filtered and washed with an aqueous Na2S2O3 solution. The solids were dissolved in chloroform and extracted with an aqueous Na2S2O3 solution until the organic solution turned colorless. The organic layer was then dried over magnesium sulfate, concentrated, and recrystallized in cyclohexane/ethyl acetate (3:2). The product was obtained as a bright orange solid (18.08 g, 36%): 1H NMR (CDCl3) [ppm] δ 7.86 (2H, s, ArH), 7.70 (2H, dd, J = 8.0 and 1.7, ArH), 7.48 (2H, d, J = 8.0, ArH), 3.82 (2H, s, CH). Synthesis of 2,7-Diiodo-9,9-dioctylfluorene. 2,7-Diiodofluorene and triethylbenzylammonium chloride were dissolved in DMSO at 60 °C. Subsequently, a sodium hydroxide (12.5 M) solution was added, and the mixture was stirred for 20 min. Next, 2.5 equiv of 1bromo-octane was added to the reaction, and the solution was stirred at 90 °C for 12 h. After the reaction mixture was cooled to room temperature, the mixture was diluted with chloroform and extracted three times with 1 M hydrochloric acid and three times with H2O. The organic layer was dried over magnesium sulfate, concentrated, and purified via column chromatography: 1H NMR (CDCl3) [ppm] δ 7.64−7.66 (4H, m, ArH), 7.40 (2H d, J = 7.8, ArH), 1.92−1.85 (4H, m, CH), 1.28−1.00 (24H, m, CH), 0.83 (6H, t, J = 7.1, CH3). Synthesis of 2,7-Diiodo-9,9-didodecylfluorene. The synthesis 2,7-diiodo-9,9-didodecylfluorene was conducted analogously to the synthesis of 2,7-diiodo-9,9-dioctylfluorene. 2,7-Diiodofluorene and triethylbenzylammonium chloride were dissolved in DMSO at 60 °C. Sodium hydroxide (12.5 M) solution was added, and the mixture was stirred for 20 min. Next, 2.5 equiv of 1-bromododecane was added to the reaction mixture, and the solution was stirred at 90 °C for 12 h. Purification was conducted the same way as mentioned above: 1H NMR (CDCl3) [ppm] δ 7.63−7.66 (4H, m, ArH), 7.40 (2H, d, J = 7.8, ArH), 1.94−1.82 (4H, m, CH), 1.36−0.95 (44H, m, CH), 0.87 (6H, t, J = 6.8, CH3). Particle Synthesis Using the Example of the Dioctylfluorene Monomer. The monomers diiododioctylfluorene (2.4 g, 3.7 mmol) and divinylbenzene 527 mL, 3.7 mmol) and the stabilizers (90 g PVPVA, 100 g Triton X-45) were dissolved in 800 mL of 1-propanol while stirring. The catalyst solution was prepared by dissolving palladium acetate (125 mg, 0.55 mmol) and the ligand triphenyl-σtolyl phosphine (750 mg, 2.4 mmol) in 125 mL of 1-propanol under stirring until the mixture turned clear yellow. The catalyst solution was kept under argon atmosphere. After all materials were dissolved in 1propanol, both mixtures were combined and the solution was filtered through a syringe filter (to remove potential dust) into a 2000 mL flask charged with a stir bar. After the solution was degassed for 2 min by bubbling with argon and stirring, the reaction mixture was heated to 80 °C. After reaching the desired temperature, 175 mL of a degassed NEt3 solution in 1-propanol (0.1 M) was added to start the polymerization. The polymerization was considered complete after 3 h. Particle Formulations for Printing/Preparation of the Ink. The solid content of the purified particle dispersions was determined using thermogravimetric analysis. A mixture of HCl (0.01 M, 6.4 μL)

and dimethylsulfoxide (11.6 μL) was added to 2.6 mg of particles to obtain the typical loading of particles for all printing experiments. After addition of the sol−gel precursor 1,2-bis-triethoxysilyl ethane (BTES, 5 μL, 200 vol % with respect to the particles), the mixture was vortexed and ultrasonicated to completely mix and redisperse the particles. The mixture was left to hydrolyze (precondensation) for 1 h before deposition via pipet. For the formulation for inkjet printing, a mixture of HCl (0.01 M, 353 μL) and dimethylsulfoxide (642 μL) was added to 25 mg of particles. After addition of BTES (24.1 μL, 400 vol % with respect to the particles), the particles were thoroughly redispersed in the solvents by ultrasonication and a vortex machine. The ink was then hydrolyzed for 1 h before inkjet printing. Conditions for Pipet Deposition. Cover glasses were used as substrates, and they were applied as purchased. The formulations were deposited as 1 and 5 μL droplets using a micropipette. The deposits were dried at 30 °C in a convection oven. Typical Inkjet Printing Conditions. As substrates we used either cover glasses or microscope slides, which were cleaned before use with acetone and then dried in a stream of nitrogen. For inkjet printing, we used an Autodrop compact system MD-P-82x from Microdrop Technologies. The printer comprises a piezo head with a glass capillary and an orifice of 68 μm in diameter. A stable droplet formation for printing was achieved at a piezo voltage of 164 V and a pulse length of 34 μs. The frequency was chosen to be 1100 Hz, and the orifice was kept at room temperature. The stable droplet formation is shown in Figure 4. Argon-Ion Polishing Conditions. A Hitachi IM4000 ion-milling system was used to polish the cross sections of the pipetted deposits. The acceleration voltage of the Ar beam was 3.8 kV, the discharge voltage was 1.5 kV, and the gas flow was 0.07 cm3/min. The cross sections were exposed to the Ar-ion beam for 5 h before sputtering and transfer into the electron microscope. Electron Microscopy. Electron microscopy was performed on a Hitachi UHR FE-SEM SU9000 and on a Hitachi S-4800 field emission microscope, operating between 2.0 and 5.0 kV. The samples were deposited onto glass substrates, dried in air at 30 °C, and sputtered with a 1−3 nm thick layer of gold. The images were analyzed using ImageJ image analysis software. Reflectance and Laser Spectroscopy. The photonic crystals encapsulated with the (BTES-derived) hybrid silica matrix were characterized on an AIQTEC microscopic imaging spectrometer (MIS1000) at a magnification of 40×. The spectrometer is equipped with a white light source and a monochromator for wavelength- and angle-dependent reflectance measurements as well as a frequencytripled Nd:YAG laser source with an optical parametric oscillator to tune the excitation wavelength for laser spectroscopic measurements. The pulse duration was 10 ns at a repetition rate of 10 or 20 Hz. For detection, the MIS1000 is equipped with a spectrometer/monochromator with gratings of 300, 600, and 1200 lines and a cooled intensified charge-coupled device detector. Spectra were obtained by accumulating 100 pulses. The samples subjected to reflection and laser spectroscopy were measured at ambient conditions (room temperature and air). The excitation intensity was varied by neutral density filters and measured simultaneously with a thermoelectric detector. The photoluminescence was collected perpendicular to the substrate with the deposit facing the incoming laser beam and focused onto the entrance slit of the spectrometer. 10200

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(12) Yamada, H.; Nakamura, T.; Yamada, Y.; Yano, K. ColloidalCrystal Laser Using Monodispersed Mesoporous Silica Spheres. Adv. Mater. 2009, 21, 4134−4138. (13) Tétreault, N.; Mihi, A.; Míguez, H.; Rodríguez, I.; Ozin, G. A.; Meseguer, F.; Kitaev, V. Dielectric Planar Defects in Colloidal Photonic Crystal Films. Adv. Mater. 2004, 16, 346−349. (14) Deutsch, M.; Vlasov, Y. A.; Norris, D. J. Conjugated-Polymer Photonic Crystals. Adv. Mater. 2000, 12, 1176. (15) Kim, S.-H.; Kim, S.-H.; Jeong, W. C.; Yang, S.-M. LowThreshold Lasing in 3D Dye-Doped Photonic Crystals Derived from Colloidal Self-Assemblies. Chem. Mater. 2009, 21, 4993−4999. (16) Shkunov, M. N.; Vardeny, Z. V.; DeLong, M. C.; Polson, R. C.; Zakhidov, A. A.; Baughman, R. H. Tunable, Gap-State Lasing in Switchable Directions for Opal Photonic Crystals. Adv. Funct. Mater. 2002, 12, 21−26. (17) Trost, B. M. Atom EconomyA Challenge for Organic Synthesis: Homogeneous Catalysis Leads the Way. Angew. Chem., Int. Ed. Engl. 1995, 34, 259−281. (18) Xia, R.; Heliotis, G.; Hou, Y.; Bradley, D. D. C. Fluorene-Based Conjugated Polymer Optical Gain Media. Org. Electron. 2003, 4, 165− 177. (19) Jin, Y.; Kim, J. Y.; Park, S. H.; Kim, J.; Lee, S.; Lee, K.; Suh, H. Syntheses and Properties of Electroluminescent Polyfluorene-Based Conjugated Polymers, Containing Oxadiazole and Carbazole Units as Pendants, for LEDs. Polymer 2005, 46, 12158−12165. (20) Lee, T.-W.; Park, O. O.; Choi, D. H.; Cho, H. N.; Kim, Y. C. Low-Threshold Blue Amplified Spontaneous Emission in a Statistical Copolymer and Its Blend. Appl. Phys. Lett. 2002, 81, 424. (21) Kuehne, A. J. C.; Gather, M. C.; Sprakel, J. Monodisperse Conjugated Polymer Particles by Suzuki−Miyaura Dispersion Polymerization. Nat. Commun. 2012, 3, 1088. (22) Ciftci, S.; Kuehne, A. J. C. Monodisperse Conjugated Polymer Particles via Heck CouplingA Kinetic Study to Unravel Particle Formation in Step-Growth Dispersion Polymerization. Macromolecules 2015, 48, 8389−8393. (23) Hatton, B.; Mishchenko, L.; Davis, S.; Sandhage, K. H.; Aizenberg, J. Assembly of Large-Area, Highly Ordered, Crack-Free Inverse Opal Films. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 10354− 10359. (24) Iṅ anç Tarhan, I.̇ ; Watson, G. Analytical Expression for the Optimized Stop Bands of Fcc Photonic Crystals in the Scalar-Wave Approximation. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 7593−7597. (25) Mansuripur, T. S.; de Naurois, G.-M.; Belyanin, A.; Capasso, F. Lasers with Distributed Loss Have a Sublinear Output Power Characteristic. Optica 2015, 2, 48−55. (26) Kim, H.; Schulte, N.; Zhou, G.; Müllen, K.; Laquai, F. A High Gain and High Charge Carrier Mobility Indenofluorene-Phenanthrene Copolymer for Light Amplification and Organic Lasing. Adv. Mater. 2011, 23, 894−897. (27) Rose, A.; Zhu, Z.; Madigan, C. F.; Swager, T. M.; Bulović, V. Sensitivity Gains in Chemosensing by Lasing Action in Organic Polymers. Nature 2005, 434, 876−879. (28) Wu, M.; Gong, Z.; Kuehne, A. J.; Kanibolotsky, A. L.; Chen, Y. J.; Perepichka, I. F.; Mackintosh, A. R.; Gu, E.; Skabara, P. J.; Pethrick, R. A.; Dawson, M. D. Hybrid GaN/organic Microstructured LightEmitting Devices via Ink-Jet Printing. Opt. Express 2009, 17, 16436− 16443.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b05538. Movie S1: particle formation (AVI) Movie S2: angle-dependent reflections of the selfassembled deposits (AVI)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Author Contributions †

A.M. and S.C. contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank Eva Paffenholz for support with the particle synthesis, and Dennis Go for formatting the video. We thank Sabrina Mallmann for help and advice for milling the organic− inorganic composites, and Dr. Wojciech Ogieglo for support with ellipsometry measurements. (All of the above are at DWI.) We thankfully acknowledge funding from the DFG (Grant No. KU 2738/3-1) and the BMBF in form of the AktiPhotoPol research group (Grant No. 13N13522). This work was performed in part at the Center for Chemical Polymer Technology CPT, which was supported by the EU and the federal state of North Rhine-Westphalia (Grant EFRE 30 00 883 02). REFERENCES (1) Samuel, I. D. W.; Turnbull, G. A. Organic Semiconductor Lasers. Chem. Rev. 2007, 107, 1272−1295. (2) Vannahme, C.; Klinkhammer, S.; Lemmer, U.; Mappes, T. Plastic Lab-on-a-Chip for Fluorescence Excitation with Integrated Organic Semiconductor Lasers. Opt. Express 2011, 19, 8179−8186. (3) Meerholz, K.; Volodin, B. L.; Sandalphon; Kippelen, B.; Peyghambarian, N. A Photorefractive Polymer with Optical Gain and Diffraction Efficiency near 100%. Nature 1994, 371, 497−500. (4) Kuehne, A. J. C.; Gather, M. C. Organic Lasers: Recent Developments on Materials, Device Geometries, and Fabrication Techniques. Chem. Rev. 2016, DOI: 10.1021/acs.chemrev.6b00172. (5) Ozin, G. A.; Yang, S. M. The Race for the Photonic Chip: Colloidal Crystal Assembly in Silicon Wafers. Adv. Funct. Mater. 2001, 11, 95−104. (6) Pusey, P. N.; van Megen, W. Phase Behavior of Concentrated Suspensions of Nearly Hard Colloidal Spheres. Nature 1986, 320, 340−342. (7) Cheng, Z.; Russel, W. B.; Chaikin, P. M. Controlled Growth of Hard-Sphere Colloidal Crystals. Nature 1999, 401, 893−895. (8) Huebner, C. F.; Carroll, J. B.; Evanoff, D. D.; Ying, Y.; Stevenson, B. J.; Lawrence, J. R.; Houchins, J. M.; Foguth, A. L.; Sperry, J.; Foulger, S. H. Electroluminescent Colloidal Inks for Flexographic Rollto-Roll Printing. J. Mater. Chem. 2008, 18, 4942. (9) Ko, H. Y.; Park, J.; Shin, H.; Moon, J. Rapid Self-Assembly of Monodisperse Colloidal Spheres in an Ink-Jet Printed Droplet. Chem. Mater. 2004, 16, 4212−4215. (10) Burkert, K.; Neumann, T.; Wang, J.; Jonas, U.; Knoll, W.; Ottleben, H. Automated Preparation Method for Colloidal Crystal Arrays of Monodisperse and Binary Colloid Mixtures by Contact Printing with a Pintool Plotter. Langmuir 2007, 23, 3478−3484. (11) Soljačić, M.; Joannopoulos, J. D. Enhancement of Nonlinear Effects Using Photonic Crystals. Nat. Mater. 2004, 3, 211−219. 10201

DOI: 10.1021/acsnano.6b05538 ACS Nano 2016, 10, 10195−10201