Si Nanocrystal Composites with Stable 60

Aug 30, 2017 - Department of Applied Physics, KTH - Royal Institute of Technology, 16440 Stockholm, Sweden. ‡ Department of Micro and Nanosystems, K...
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Light-Converting Polymer/Si Nanocrystal Composites with Stable 60−70% Quantum Efficiency and Their Glass Laminates Aleksandrs Marinins,† Reza Zandi Shafagh,‡ Wouter van der Wijngaart,‡ Tommy Haraldsson,‡ Jan Linnros,† Jonathan G. C. Veinot,§ Sergei Popov,† and Ilya Sychugov*,† †

Department of Applied Physics, KTH - Royal Institute of Technology, 16440 Stockholm, Sweden Department of Micro and Nanosystems, KTH - Royal Institute of Technology, 10044 Stockholm, Sweden § Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada ‡

S Supporting Information *

ABSTRACT: Thiol−ene polymer/Si nanocrystal bulk hybrids were synthesized from alkyl-passivated Si nanocrystal (Si NC) toluene solutions. Radicals in the polymer provided a copassivation of “dark” Si NCs, making them optically active and leading to a substantial ensemble quantum yield increase. Optical stability over several months was confirmed. The presented materials exhibit the highest photoluminescence quantum yield (∼65%) of any solid-state Si NC hybrid reported to date. The broad tunability of thiol−ene polymer reactivity provides facile glass integration, as demonstrated by a laminated structure. This, together with extremely fast polymerization, makes the demonstrated hybrid material a promising candidate for light converting applications. KEYWORDS: Si nanocrystals, polymers, photoluminescence, photovoltaics, hybrids, laminates ilicon nanocrystals (NCs) with diameters below ∼10 nm exhibit properties influenced by the quantum size effect, while at the same time preserving some bulk material properties.1 The resulting luminescence has approximately microsecond lifetime2 and a large Stokes shift,3,4 reflecting large separation between direct and indirect bandgaps of the bulk silicon. These properties clearly distinguish Si NCs from their direct-bandgap or perovskite5 counterparts with approximately nanosecond decays and a small Stokes shift, inherited from the efficient recombination in corresponding bulk materials. These properties are also in contrast to the emission from nanoparticle surface moieties, characterized by narrow, overlapping emission and absorption bands with nanosecond lifetime,6,7 typical of organic fluorophores. The quantum confinement-type of emission from Si NCs is central to some applications, such as for luminescent solar concentrators,8 where the large Stokes shift ensures suppressed reabsorption in the device. Recent progress in chemical synthesis allowed massfabrication of hydride-terminated silicon nanocrystals,9 where exchange to surface alkene groups was suggested to improve the photostability. One of the first such hydrosilylation experiments resulted in up to ∼60% quantum yield (QY) of Si NC colloids,10 although the samples were still unstable, degrading within days of shelf life. Improvements in the thermal hydrosilylation process11 and the use of alternative reaction methods, such as radical-12 or photoinitiation,13 have led to suspensions of Si quantum dots (QDs) with long-term (years)

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stabilities and QYs approaching 60−70% for the best colloid samples.14 At the same time, many practical light-converting applications require nanoparticles to be dispersed in solid matrices.8,15 Therefore, a reliable transfer from liquid to solid phase of Si NCs with quantum-confined emission and high QY is an attractive goal.16 Historically Si NCs encapsulated in a solid phase were first fabricated in silicon-based oxides. The QYs, however, have typically not exceeded 10−20%,17 in best cases reaching ∼30%.18 Surface “dangling” bonds on the Si NCs have been implicated in the luminescence quenching - photoluminescence (PL) and electron paramagnetic resonance (EPR) signals clearly anticorrelate.19 In addition, traps in the silicon dioxide matrix can introduce alternative nonradiative channels. In contrast, encapsulating alkyl-terminated Si NCs in bulk poly(methyl methacrylate) (PMMA) can provide convenient processability and QYs of 30−40%, as was shown for different sizes of Si NCs.20 In this work, a novel hybrid material consisting of an offstoichiometric thiol−ene (OSTE) polymer21 host matrix and alkyl-terminated Si NCs is reported. Thiols have commonly been used for surface termination in direct bandgap nanocrystals, such as CdSe,22 CuInS2,23 and PbS.24 These ligands effectively passivate surface nonradiative centers leading to Received: June 27, 2017 Accepted: August 30, 2017 Published: August 30, 2017 A

DOI: 10.1021/acsami.7b09265 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces improved ensemble QYs,24,25 although quenching and mixed effects were also reported.26 Similar thiol-based functionalization has not received equivalent attention in Si surface modification, presumably because of the weak Si−S bond. Only recently was direct passivation of bulk silicon and Si NC surfaces demonstrated.27,28 However, the QY of the functionalized SiNCs was ∼10% for as-prepared solutions and was not stable.28 Herein, we exploited in situ copassivation of alkyl-terminated Si NCs, concurrent with OSTE polymerization, to prepare photostable QD-OSTE nanocomposites. OSTE is a nontoxic, non-sensitive to oxygen, low-polymerization-shrinkage stress thermoset polymer.21 Thiol−ene step-growth radical-driven UV photopolymerization is based on the addition of a thiol group (-SH) to an ene group (CC) (Scheme S1). The amount of active thiyl and carbon radicals, capable of nanocrystal dangling bond passivation, can be controlled by defining the thiol/allyl ratio at the polymer preparation stage (see Supporting Information). This off-stoichiometry was originally employed to tune mechanical and reactivity properties of the OSTE polymer. For example, the E-modulus can be varied in 0.25−1.8 GPa range by the thiol/allyl ratio with mechanical stability confirmed over years;21 hairpin bends can also be formed without breaking with a strain at break reaching ∼30% for OSTE with thiol excess. The photopolymerization is rapid, requiring only tens of seconds to produce an ∼3 cm3 bulk hybrid; a PMMA thermopolymerization requires many hours to afford an equivalent quantity of material.20 The resulting stable Si NC/OSTE polymer hybrids reveal a positive impact of OSTE radicals on Si NCs QY and show that a combination of alkyl and thiol−ene passivation can lead to stable bulk polymer/Si NC hybrids with 60−70% QY at room temperature. Silicon NCs were fabricated from hydrogen silsesquioxane by annealing, as described elsewhere.9,14 An average size of Si NCs used in this work is ∼5 nm (PL peak position ∼1.6 eV), as deduced from TEM images (Figure S1). One sample with larger ∼9 nm Si NCs (PL peak position ∼1.3 eV) was also used to monitor matrix-related near-infrared absorption bands (Figures S1 and S3). The hydride-terminated nanoparticles were passivated with dodecene using thermal- or radicalinitiated hydrosilylation reaction, where high surface coverage was confirmed by NMR measurements.12 The resulting NCs were dispersed in toluene and stored in ambient conditions. Dispersions were mixed with different OSTE compositions and the Irgacure-184 photoinitiator, cast in a mold, and cured under a UV-lamp resulting in hybrids with Si NC concentration of ∼0.01% by weight (details in the Supporting Information). This is a relatively low value, excluding possible distorting reabsorption effects in quantum yield measurements. Absolute QY measurements were carried out using an integrating sphere with a laser-driven Xe lamp as an excitation source of a tunable wavelength.14 The recorded spectra for the investigated and the reference (only OSTE and toluene) samples were corrected with the system response curve. The spectra were subtracted from each other, and the ratio between the corresponding emission and absorption peaks defined the QY. Relative absorption was measured in the same integrating sphere using white light from the Xe lamp by taking the ratio of the reference and sample curves. Spectrally resolved PL decays were obtained using an avalanche photodiode attached to the output port of a spectrometer under 405 nm modulated laser excitation.

We observed that the QY has markedly increased after incorporation of Si NCs into OSTE for a majority of samples with thiol excess. Figure 1a shows results for a typical hybrid

Figure 1. (a) Absolute QY of a Si NC sample in toluene (black dot) and after incorporation to OSTE (40% thiol excess, red dots) as a function of storage time under ambient conditions (440 nm excitation). (b) Photo of NC/polymer hybrids under a UV lamp (taken after the filter blocking scattered blue light; a scale bar size is 1 cm). (c) Luminescence spectra of a Si NC sample in toluene (black line) and in OSTE (red line), corresponding to the same amount of absorbed light. (d) Ratio between spectra in c, revealing a uniform QY increase.

prepared from a 20−30% QY colloid. Dispersions with low QY values were used here to investigate the polymerization mechanism. Once optimal parameters were established we applied those to colloids with a higher QY to achieve most efficient to date Si NC hybrids, as described below. Importantly, fabricated hybrids remained stable for months under ambient conditions (Figure 1a). To reveal spectral structure of the QY enhancement the luminescence spectra of colloidal Si NCs and the Si NC/OSTE hybrid are plotted in Figure 1c, after normalization to the amount of absorbed light. The ratio between the two curves is shown in Figure 1d, indicating a uniform increase across the spectrum. Similar enhancement and stability effects were confirmed for other samples (Figure S2). The fine structure of luminescence spectra acquired in the integrated sphere reveals information on sample structure/ composition. A fraction of the NC emission gets reabsorbed in the matrix/liquid due to long optical path inside the sphere. Thus, a dip at ∼1.42 eV (∼875 nm) for the black curve in Figure 1c is a toluene absorption band, corresponding to the fourth harmonic of C−H stretching mode in this molecule (Figure S3, top). As expected, this feature is not present in the spectrum of the hybrid (red curve in Figure 1c). Instead a new feature appears at ∼1.37 eV (905 nm, more clearly seen in Figure S2 middle and right), corresponding to an OSTE polymer-specific overtone absorption. This reabsorption is more pronounced for larger NCs (∼9 nm) with luminescence deeper in IR (Figure S3, bottom), yet still represents 0.85.14 Although spectrally resolved curves reflect only a limited spectral range (∼6 nm here), broad homogeneous line width of ligand-passivated Si NCs at room temperature (∼80 nm)32 results in contribution of spectrally adjacent nanoparticles as well. The homogeneous line width broadening originates from the exciton−phonon coupling,32 and it should not be ignored in the decay curve analysis.33 After numerical correction for this homogeneous line width effect, beta values were shown to actually reflect monoexponential decays.14 In other words, the nanoparticles in the colloid were proven to be either bright or dark. The ensemble lifetime τ dispersion is then caused solely by the quantum size effect. In that case the QY simply represents a fraction of the optically active QDs. The results of PL decay measurements for samples noted in Figure 1 are shown in Figure 3 (similar results measured for another sample are shown in Figure S6). Both the decay rate (1/τ) and β remain mostly unchanged after the transfer to the polymer matrix. Importantly, high β values (low dispersion) for the polymer sample imply the lack of new nonradiative

on the formation of thiyl radicals (deprotonated thiols). Those are typically created by light-induced initiator (Irgacure-184) radicals, abstracting a hydrogen radical from the thiol monomers.21 Resulting thiyl radicals, in turn, can produce carbon radicals from allyls. Here we hypothesize that a dangling bond on Si NC surface represents a suitable reaction site for these highly mobile thiyl and carbon radicals.29 One of the possible mechanisms is schematically depicted in Scheme 1. Scheme 1. UV-Light-Induced off-Stoichiometry Thiol−ene Polymerization Process in Parallel with Si NC Dangling Bond Passivation by Thiyl Radicals

When alkyl-terminated Si NCs (some surface oxygen is also present, as evidenced by FTIR shown in Figure S4) get their dangling bond passivated they turn optically active. Indeed, bonding of thiols directly to bulk silicon surface was demonstrated by radical-initiated reactions,30 microwave or thermal treatment,27 and in a flow of supercritical carbon dioxide.31 In all cases, homolysis of the surface Si−H bond resulting in a radical formation was considered as a prerequisite of the surface thiolation. The previously proven dangling bonds on dark NC surface19 justify suggested here passivation reaction mechanisms with OSTE radicals. To verify this conjecture, we performed a control experiment. The thiol off-stoichiometry excess was varied from 0 to 100% (2:1 thiol/allyl group ratio) with the same amount of initiator, and QYs of the resulting batch were measured (Figure 2).

Figure 3. (left) Spectrally resolved luminescence decay parameters of a Si NCs sample in toluene (black squares) and OSTE (red dots). The decay rate 1/τ (left) and the dispersion factor β (right) were extracted from stretched-exponential fittings of the decay curves (example is shown in Figure S6). Transients remain largely unaffected by the transfer from liquid to solid phase.

Figure 2. Effect of the thiol excess in OSTE on NCs/polymer hybrid QY (100% excess refers to 2:1 thiol/allyl group ratio). The gray band indicates original QY with measurement uncertainty in the liquid phase for this sample (all measured under 440 nm excitation). C

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ACS Applied Materials & Interfaces channels introduced by the polymerization. This result signifies that NCs in the ensemble still remain either bright or dark. Then the observed QY enhancement (Figure 1) represents an increase in the fraction of bright particles, consistent with the proposed mechanisms of dangling bond passivation. In Figure 4, absorption curves of the reference (black line) and QD-OSTE (red line) samples are shown. The absorption is

Figure 5. Photo of UV-illuminated OSTE/Si NC layer (∼0.2 mm thick) bonded between two 4 in. fused silica wafers (∼0.5 mm thick each). While being optically transparent, it converts UV radiation to red/NIR, which is then directed to the edges by total internal reflection. Inset shows a magnified side view.

medium-pressure Hg lamp (details in the Supporting Information). To realize chemical bonding of OSTE with glass the wafer surface was pretreated with 1% (3mercaptopropyl)trimethoxysilane ethanol solution. Thanks to close-to-glass refractive index of OSTE (tunable by thiol/allyl ratio in 1.45−1.6 range) and a smooth OSTE/glass interface, the resulting hybrid structure acts as an efficient planar optical waveguide under UV-irradiation. In Figure 5, PL from nanocrystals can be seen efficiently directed to the glass wafer edges (micro-PL images of this film are presented in Figure S9). This shows possibility of using such a structure as a luminescent solar concentrator device. A part of the solar spectrum can be converted into near-infrared emission, collectable by conventional photovoltaic cells at the edges for, for example, building-integrated photovoltaics. Incorporation of large amount of nanoparticles, and of different sizes, to attain stronger absorption in the visible spectrum for more efficient down-conversion will be investigated separately. To summarize, we encapsulated alkyl-terminated Si NCs with quantum-confined luminescence in a solid matrix of the OSTE polymer. The resulting hybrids possessed enhanced quantum yield in relation to the original toluene dispersions. The highest ever, to our knowledge, light conversion efficiency of ∼65% for Si NC solids has been achieved. Dangling bond passivation by OSTE radicals was suggested as a mechanism for the observed increase in the fraction of bright nanocrystals. The curing ability of OSTE, originating from mobile radicals in a polymer with a high chain transfer rate, and the copassivation approach at the polymerization stage in general can be potentially extended to other nanoparticles and host matrices. Finally, successful integration of this nanocrystal−polymer hybrid material with glass was demonstrated, highlighting its low polymerization shrinkage and controllable reactivity.

Figure 4. Absorption curves of OSTE polymer and NCs/polymer hybrid of the same volume. Inset shows independence of the QY on the excitation wavelength and photostability for the sample with ∼65% quantum yield (80% thiol excess, initial QY of Si NCs in toluene was 40−50%).

rather strong for the reference sample in UV and partially blue spectral regions. It mainly reflects the absorption profile of the remaining Irgacure-184 initiator, sensitive to this spectral range. The absorption of QD-OSTE nanocomposite (red line) reflects a growing density of states in Si NCs, stemming from bulk Si energy level structure,3 superimposed with the initiator absorption. At longer wavelengths, the material is essentially absorption free (Figure S7), important for suppression of the emitted light reabsorption in applications. Inset shows an optical stability of ∼65% QY sample prepared from 40 to 50% QY toluene colloid (raw data are shown in Figure S8). The dependence on the excitation wavelength for this QD-OSTE sample is presented, revealing a uniform response. The independence of QY on the excitation wavelength is postulated by the Kasha−Vavilov rule, and verified for Si NCs with quantum confined emission in this energy range.17 The longterm stability was also confirmed for all other Si QD-OSTE samples (Figure 1, Figure S2). For some applications, such as in photovoltaics, parasitic absorption of the visible light by the initiator should be avoided. Therefore, we have fabricated an optically transparent OSTE sample using initiator-free deep UV cure (Figure 5). In this way OSTE can be made optically similar to PMMA, which has been so far the matrix of choice for QD-based luminescent solar concentrators8 because of its transparency above 350 nm and mechanical stability. However, PMMA experiences up to 20% volume shrinkage upon polymerization, which makes it challenging to integrate with other materials. To demonstrate advantageous low shrinkage and controllable chemical reactivity of QD-OSTE we have prepared a laminated glass sample. A monomer blend with Si NCs was sandwiched between two 4 in. fused silica wafers and exposed to nonfiltered UV light from a

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b09265. Experimental details; optical characterization for other samples; near-infrared and infrared absorption spectra PDF) D

DOI: 10.1021/acsami.7b09265 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Aleksandrs Marinins: 0000-0002-0728-6684 Wouter van der Wijngaart: 0000-0001-8248-6670 Jonathan G. C. Veinot: 0000-0001-7511-510X Ilya Sychugov: 0000-0003-2562-0540 Notes

The authors declare the following competing financial interest(s): One of the authors (Tommy Haraldsson) is partly employed by Mercene Labs AB, Stockholm, offering thiol-ene polymer products, such as OSTE used here, for coatings and microsystems.

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ACKNOWLEDGMENTS The authors acknowledge support from Marie Curie ITN project ICONE under Grant 608099, Göran Gustafssons Stiftelse, ÅForsk Foundation, and the Swedish Research Council (VR) through ADOPT Center of Excellence. The University of Alberta and NSERC are also thanked for funding. Regina Sinelnikov is acknowledged for providing some starting material.

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DOI: 10.1021/acsami.7b09265 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX