Si Nanocrystal Bulk Hybrids with Tunable

Aug 30, 2016 - Solid polymer/Si nanocrystal bulk nanocomposites were fabricated from solutions of alkene- and hydride-terminated silicon nanocrystals ...
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Letter pubs.acs.org/journal/apchd5

Photostable Polymer/Si Nanocrystal Bulk Hybrids with Tunable Photoluminescence Aleksandrs Marinins,† Zhenyu Yang,‡,¶ Hongzheng Chen,§ Jan Linnros,† Jonathan G. C. Veinot,‡ Sergei Popov,† and Ilya Sychugov*,† †

Materials and Nano Physics Department, ICT School, KTH Royal Institute of Technology, 16440 Kista, Sweden Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada § State Key Laboratory of Silicon Materials, MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: Solid polymer/Si nanocrystal bulk nanocomposites were fabricated from solutions of alkene- and hydride-terminated silicon nanocrystals (NCs) in toluene. The photoluminescence peak position of hydride-terminated SiNCs before polymerization was tuned by photoassisted hydrofluoric acid etching. Optical properties of obtained PMMA/NC hybrids, such as quantum yield, luminescence lifetime, and dispersion factor, were evaluated over time. Photostability of these transparent bulk polymer/SiNC hybrids over months was confirmed. The emission covers the visible to near-infrared range with a quantum yield of ∼30− 40% for yellow-red nanocomposites.

KEYWORDS: silicon nanocrystals, photoluminescence, polymers, poly(methyl methacrylate), nanocomposites

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the above-mentioned applications.21−24 In this work, we focus on the preparation of a SiNCs/poly(methyl methacrylate) (PMMA) hybrid that combines the favorable SiNC optical response and solid matrix of the PMMA (Figure 1). Unlike polymer/nanocrystal hybrid thin films and nanoparticles studied previously,25−28 bulk nanocomposites allow repetitive quantum yield measurements in an integrating sphere.19 In combination with decay transient measurements we have monitored and analyzed changes in optical performance of a series of PMMA/SiNC hybrids. We show that stable and transparent luminescent bulk nanocomposites exhibiting 30−40% absolute quantum yield (external quantum efficiency) and tunable emission may be prepared by this approach. SiNCs were prepared through high-temperature thermal processing of hydrogen silsesquioxane (HSQ), resulting in an NC-rich SiO2 matrix20 (see detailed description in the Supporting Information). The hydride-terminated SiNCs were released upon HF etching of the obtained brown powder, and then the hydrophobic NCs were extracted to toluene. These Si−H bonds are prone to oxidation by oxygen or moisture, leading to precipitation of SiNCs and degradation/ evolution of their optical properties within days after fabrication. Functionalization with ligands is needed to stabilize SiNC surfaces. An effective approach is to passivate SiNCs with

s a result of their exquisitely tunable optical properties, quantum dots (QDs) are attractive for a wide range of applications. The rapidly expanding fields of solid-state lighting,1 solar energy harvesting,2,3 and LED displays4 require inexpensive, efficient, and environment-friendly active materials. Prototypical colloidal QDs consisting of II−VI semiconductors (e.g., CdSe) that contain heavy metals have been incorporated in solid matrices and have promising characteristics.5,6 Unfortunately, the established cytotoxicity of cadmium has seen it regulated in numerous jurisdictions and limits its widespread use.7,8 In this regard, there is substantial effort being made to identify and develop nontoxic alternatives.9,10 Si nanocrystals (NCs) possess many of the benefits of QDs and are also biocompatible11 as well as biodegradable.12 Silicon is also an abundant material, second only to oxygen in Earth’s crust, in contrast to rare-earth elements commonly used today in white light emitting diodes as phosphors13−15 or to sparse In used in III−V QDs. Silicon oxide encapsulated SiNCs have been prepared in small quantities via annealing of nonstoichiometric silicon oxide thin films;16 however, these materials exhibit limited photoluminescent quantum efficiencies (∼10%),17 similar to porous Si.18 In contrast, ligand-passivated Si NCs can be prepared in relatively large quantities as colloids and exhibit quantum yields (QYs) up to 60%.19,20 Clearly, the efficient conversion of highly luminescent colloidal SiNCs to a solid phase while preserving their attractive optical properties is an important advance for © 2016 American Chemical Society

Received: July 11, 2016 Published: August 30, 2016 1575

DOI: 10.1021/acsphotonics.6b00485 ACS Photonics 2016, 3, 1575−1580

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For alkene-terminated nanocrystals the solidification did not affect PL spectra of the studied samples (Figure 2, top).

Figure 1. (Top) SiNC/polymer transparent solid composites under ambient light and (bottom) 365 nm UV lamp illumination. The leftmost block is a reference pure PMMA sample.

alkenes using hydrosilylation approaches.29,30 Here we investigated the encapsulation of both the alkene- and hydride-terminated particles into a PMMA matrix. For the first set of experiments we used dodecene-passivated SiNCs that are photostable as a toluene colloid for more than 2 years (Scheme 1). Here dodecene-passivated SiNCs in toluene were Scheme 1. Polymer/SiNC Nanocomposite Synthesis from Dodecene-Passivated NCs

Figure 2. (Top) PL spectra of two dodecene-terminated SiNC samples with different PL peaks, which remain unchanged upon transition from liquid to solid phase. Inset shows stretched exponential transients for these samples. (Bottom) Evolution of photoluminescence parameters in transition from toluene to PMMA: QY, lifetime, and decay dispersion factor β.

Different peak positions for these two samples reflect different average sizes of NCs in the ensemble, and the broad line width is a result of size-dispersion and a wide homogeneous line width of SiNCs in a polymer matrix.32 Average SiNC size was adjusted by predetermined HSQ anneal step conditions (see Supporting Information). To analyze PL decay transients (inset in Figure 2), a stretched exponential function I = I0 exp(−(t/τ)β) was used, where τ is the lifetime and β is the dispersion factor, indicating a variation of individual lifetimes in the ensemble. A decrease in τ and β values indicates the appearance of new nonradiative processes, comparable in strength with the radiative one, leading to the reduction of the internal quantum efficiency.19 Thus, these parameters, together with the absolute QY, are a good measure of sample photostability. These values were measured as a function of time after the hybrid preparation (Figure 2, bottom). After an initial drop immediately following polymerization these parameters return to the level nearly equivalent to that of the colloid phase after several weeks for the orange sample. A similar trend was observed for the red sample, although the final values are somewhat lower compared to the toluene colloid. This sample had a higher concentration of SiNCs (see Supporting Information), and it took longer for it to form the PMMA/

added to the monomer MMA solution with polymerization initiator AIBN, and heat was applied to increase the polymerization reaction rate, a process described elsewhere31 (see Supporting Information for further details). Finally, the solid sample was released from the glass reaction vial and characterized without further purification. Samples are named according to their color under UV irradiation (Figure 1). The concentration of SiNCs in these two samples was estimated to be roughly 1014−15 cm−3 (see Figures S2 and S3 and calculation details in the Supporting Information). The photoluminescence (PL) spectra, absolute QY, and PL decay transients of SiNCs were evaluated before and after encapsulation in PMMA and repeatedly after the encapsulation (see Supporting Information for measurement details) to evaluate nanocomposite photostability. 1576

DOI: 10.1021/acsphotonics.6b00485 ACS Photonics 2016, 3, 1575−1580

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sample to sample, possibly reflecting the variation in the quantity of residual HF in the solution. After the desired PL spectrum peak shift SiNCs were isolated by centrifuging at 13 000 rpm for 25 min to remove remaining HF and then encapsulated in a bulk PMMA matrix (Scheme 2, bottom). Considering that the same radical initiator (AIBN) is used for polymerization and for hydrosilylation reaction, both surface hydrosilylation and radical driven polymerization may coexist here. This scenario is supported by FTIR characterization (Figure S4). It reveals the vanishing of the Si−H band in the transition from the liquid to solid phase accompanied by the appearance of polymer-related peaks. The same optical characterization as for PMMA-encapsulated dodecyl-SiNCs was carried out for samples derived from HSiNCs (Figure 4). N1 and N2 are yellow-emitting samples from

SiNC hybrid (∼24 h). In any case these results indicate that the spectrum and the QY (as well as internal quantum efficiency) are very little affected by the polymerization procedure and stable nanocomposites can be relatively easily fabricated. The passivation with alkenes is known to stabilize optical properties of SiNCs, but also to limit the PL emission to the red-NIR range, since the oxygen influence cannot be totally excluded.33 The size-dependent quantum confinement effect is readily observed from larger SiNCs in oxide,34 yet becomes less pronounced for smaller NCs presumably due to oxygen-related surface trap states,35 which limits the spectral tunability.36 Hydride-terminated particles, on the other hand, show emission across the full visible range in the absence of oxygen.37 So, encouraged by the stability of ligand-terminated NC-based hybrids, we performed a similar experiment for as-prepared hydride-passivated SiNCs (Scheme 2), resulting in yellow and blue samples shown in Figure 1. Scheme 2. Polymer/SiNC Hybrid Synthesis from HydrideTerminated NCs

To control the size of hydride-terminated NCs, we used photoassisted etching, relying on the trace amount of HF remaining after the SiNCs release from the annealed HSQ.38 We illuminated hydride-terminated toluene-dispersed SiNCs with a 575 nm LED light and measured the emission spectra as a function of irradiation time (Figure 3). As can be seen, the PL peak blue-shifts with a constant rate, consistent with a reduction of the NC average size. The exact rate varies from

Figure 4. (Top) Spectra of hydride-passivated SiNCs in toluene and in a PMMA matrix. Inset: SEM image of a cleaved hybrid surface. (Bottom) Time stability of photoluminescence parameters for another hydride-terminated SiNC in a PMMA sample.

different batches. In contrast to their dodecyl counterparts, spectra of hydride-terminated NCs blue-shifted after polymerization; this can be tentatively ascribed to the effects of additional etching with HF residuals and/or NC oxidation during the MMA polymerization process. Once NCs were embedded in PMMA, however, the PL spectrum was stabilized and did not change markedly over ∼10 weeks. This further supports the proposed substitution of hydride-terminated surfaces to AIBN-induced MMA/PMMA passivation since it facilitates optical stability, similarly to the mechanism described in ref 27. The QY reduced from ∼60% to ∼45% within the first 2−3 weeks, while both the lifetime and dispersion factor did not change. This is consistent with the appearance of dark NCs

Figure 3. Photoluminescence spectrum tuning via photoassisted (575 nm) HF etching of SiNCs in toluene. Inset: Peak position blue shifting as a function of etch time. 1577

DOI: 10.1021/acsphotonics.6b00485 ACS Photonics 2016, 3, 1575−1580

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temporal emission characteristics were reported for pure oxide nanoparticles,40 where a defect pair consisting of a dioxasilyrane and silylene was associated with this band. So, it could be oxide defect luminescence taking into account that all reactions were performed under an unprotected atmosphere, and the presence of Si−O bond was shown previously for such dodecenepassivated samples.19 At the same time some authors do suggest a quantum confinement mechanism for the blue-UV luminescence from small Si nanoparticles.41,42 Finally, we turn to one remarkable property of such nanocomposites. As seen in Figure 1, top, this material appears transparent and colorless under ambient light. This is different from direct band-gap QDs (e.g., CdSe) and organic dye molecules, which typically exhibit a specific color even at low concentration. The reason is a very small Stokes shift of direct band-gap QDs and organic dye molecules, where absorption bands are located right next to the emission band for those fluorophores.5 Silicon, however, is an indirect band-gap material, and the optical transitions for the levels next to the emission state are weak, resulting in a large Stokes shift.43 At higher energies due to intermixing of direct- and indirect-like states in NCs the absorption becomes stronger, allowing efficient excitation with blue-UV light.43 In conclusion, we have demonstrated the preparation of a bulk PMMA/SiNC hybrid material with tunable, efficient, and stable photoluminescence in the yellow-red emission range. The resulting QYs are relatively high and may possibly be improved further by optimization of the process. Blue-emitting hybrids were also obtained, although with less efficient and less stable optical performance. We have also shown that hydrideterminated SiNCs can be directly transferred to the solid phase without a ligand-passivation step for facile synthesis of transparent and luminescent bulk nanocomposites.

that do not contribute to the decay curve, yet absorb excitation light, thereby reducing the absolute quantum yield. It is reasonable to suggest that at this point some of the hydrideterminated NCs, which were not effectively passivated with surface ligands, degrade similarly to hydride-terminated colloids described above. During the ensuing 7−8 weeks the QY reduced slightly to ∼40%, accompanied by the slight decrease of the lifetime and beta values. At this stage the surface suboxide on SiNCs may form, leading to nonradiative channels, whose strength is similar to the radiative ones, reducing the internal quantum efficiency.19 Thus, although hydride-terminated SiNCs show some degradation after polymerization, the resulting polymer passivation eventually does stabilize the QY values similar to alkene-terminated samples (∼30−40%). To evaluate structural properties of the prepared PMMA/ SiNC hybrids, samples were cleaved into millimeter size fragments and coated with a thin (∼2 nm) conductive carbon layer for scanning electron microscopy (SEM) characterization (inset in Figure 4). The image reveals a uniform structure without any visible porosity at the imaging resolution. The present long defect grooves originate from the cleavage during sample preparation and are consistent with these hybrids behaving as a monolithic solid rather than a porous sponge-like structure. These results confirm that PMMA can act as a host matrix capable of protecting the NC optical properties by passivation and limiting their interaction with the environment. To investigate the possibility of further spectral tuning to the blue-green spectral range, we fabricated hybrids by extended light-assisted etching of H-terminated NCs (cf. Figure 3). The resulting blue-emitting sample is shown in Figure 1, the spectrum and decay curve of which are given in Figure 5. One

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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/acsphotonics.6b00485. Additional information (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (I. Sychugov). Present Address ¶

Department of Electrical and Computer Engineering, University of Toronto, 10 King’s College Road, Toronto, Ontario, M5S 3G4, Canada. Notes

Figure 5. Spectrum of blue-emitting hydride-passivated SiNCs in PMMA. Inset: Stretched exponential decay curve in the nanosecond range.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Marie Curie ITN project ICONE under grant no. 608099, Göran Gustafssons Stiftelse, and the Swedish Research Council (VR) through ADOPT Center of Excellence. The University of Alberta and NSERC are thanked for funding.

can immediately notice substantially different optical properties compared to the previous samples. The QY is poor (