Letter pubs.acs.org/JPCL
Silicon Nanocrystals Functionalized with Pyrene Units: Efficient Light-Harvesting Antennae with Bright Near-Infrared Emission Mirko Locritani,† Yixuan Yu,‡ Giacomo Bergamini,† Massimo Baroncini,† Jennifer K. Molloy,† Brian A. Korgel,*,‡ and Paola Ceroni*,† †
Department of Chemistry “G. Ciamician”, University of Bologna, Via Selmi 2, 40126 Bologna, Italy Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The University of Texas at Austin, Austin, Texas 78712, United States
‡
S Supporting Information *
ABSTRACT: Pyrene chromophores were attached to silicon nanocrystals (SiNCs) with diameters of 2.6 and 5.0 nm to provide light-harvesting antennae for enhanced optical absorption. Efficient energy transfer from the pyrene moieties to the SiNCs was observed to induce bright visible (2.6 nm) or near-infrared (NIR) (5.0 nm) photoluminescence (PL). The 5.0 nm diameter pyrene-derivatized SiNCs exhibited NIR PL emission that was insensitive to dioxygen, with a 40% quantum yield and long lifetime (hundreds of μs).
SECTION: Physical Processes in Nanomaterials and Nanostructures
S
and blue-emitting SiNCs; Sommer et al.20 observed very fast energy transfer (270 fs−3 ns) from vinylpyridine ligands to SiNCs; and Erogbogbo et al.21 observed 80% enhancement in the brightness of SiNCs by an anthracene-based dye in the hydrophobic core of micelles due to energy transfer. These studies have shown that energy transfer to (and from) SiNCs can indeed occur. Here, we show that photoinduced energy transfer can occur between covalently linked 1(allyloxymethyl)pyrene (Py) and 2.6 nm diameter SiNCs with greater than 95% efficiency. Pyrene-capped SiNCs with 5.0 nm diameter and 40% quantum yield NIR emission at 970 nm exhibited an energy transfer efficiency of 65%. For these studies, hydride-terminated SiNCs were produced by the hydrogen silsequioxane (HSQ) decomposition route of Veinot22 followed by thermal hydrosilylation with alkenes. The synthetic approach yields clean, well-characterized nanocrystals with reliable control over size, surface passivation, emission color, dispersibility, and photostability.4 As described in the Supporting Information and illustrated in Figure 1a, nanocrystals were passivated with a mixed ligand layer of 1-dodecene and 1-(allyloxymethyl)pyrene (Py) using Py/dodecene molar ratios ranging between 0.028 (1:12) and 0.083 (1:36) (SiNC-Py). For comparison, SiNCs were also passivated only with 1dodecene (SiNC). Two SiNC-Py samples with average Si core
ilicon (Si) is an extraordinarily useful semiconductor, employed in integrated circuits, solar cells, and photodetectors; however, it exhibits no significant luminescence at room temperature because of its indirect band gap. Si nanocrystals (SiNCs)or quantum dotson the other hand can be efficient light emitters, with emission wavelength that can be tuned by size from the near-infrared (NIR) into the visible range.1−6 Because Si is nontoxic, SiNCs are also wellsuited for medical imaging applications.7−12 The indirect band gap of Si, however, still makes light absorption relatively weak compared to quantum dots of direct band gap semiconductors, especially at wavelengths near the absorption edge.4 This leads to a large apparent Stokes shift between the excitation and emission wavelengths.13 To enhance the optical absorption of SiNCs while retaining their emissive properties, we explored the addition of photoactive unitscapable of strong light absorption and efficient energy transfer to the SiNCsto create significantly brighter dots. A large body of literature exists detailing photogenerated energy transfer between colloidal semiconductor nanocrystals and various fluorophores,14,15 yet only a few studies have examined energy transfer as a means to induce light emission from SiNCs. Bard’s group16,17 has induced SiNC luminescence by electrochemical charge injection; Liu et al.18 observed energy transfer between a perylene diimide derivative and SiNCs embedded in polymer (PMMA) (molecules were not tethered directly to the SiNCs); Rosso-Vasic et al.19 observed up to 55% efficiency energy transfer between a Ru-based dye © 2014 American Chemical Society
Received: July 23, 2014 Accepted: September 10, 2014 Published: September 15, 2014 3325
dx.doi.org/10.1021/jz501609e | J. Phys. Chem. Lett. 2014, 5, 3325−3329
The Journal of Physical Chemistry Letters
Letter
Supporting Information) and ∼60 Py units per nanocrystal on the 5.0 nm diameter SiNC-Py (with 1:12 Py/dodecene ratio). The PL spectra of the SiNC-Py samples and SiNCs mixed with free pyrene are shown in Figure 2b and d. Pyrene luminescence was observed with peak emission at ∼400 nm both when free in solution and when attached to the SiNCs. There is no evidence of Py excimer emission, which would occur at a longer wavelength close to 500 nm. The 2.6 and 5.0 nm diameter SiNCs exhibit PL emission peaks at 635 and 970 nm, respectively. Pyrene functionalization did not significantly affect the PL peak maxima of the 5.0 nm nanocrystals, whereas the emission of the 2.6 nm nanocrystals was slightly red-shifted to 680 nm after pyrene attachment (see Figure 2b and d and also the Supporting Information, Figure S9b). PL emission spectra were measured with excitation wavelengths of either λex = 378 or 345 nm to selectively photoexcite the SiNC core or the pyrene units. Py does not absorb 378 nm light, and the majority of the 345 nm light is absorbed by pyrene, that is, pyrene absorbs 67% of the 345 nm light in the case of the 2.6 nm diameter SiNC-Py. To gauge the extent of energy transfer, the PL spectra of SiNC-Py samples in Figure 2b and d were measured at the excitation wavelengths of 378 and 345 nm with the same photon absorption by optically matching the SiNC-Py dispersions by dilution. PL spectra were also measured for SiNCs mixed with free Py in the appropriate ratios to match the SiNC-Py absorbance profiles (the green curves in Figure 2b and d). When Py is not attached to the nanocrystals, photoexcitation at 345 nm leads to emission spectra dominated by the 400 nm emission of pyrene, with a lesser contribution of SiNC emission at longer wavelength (green curves in Figure 2b and d). In contrast, photoexcitation of the SiNC-Py dispersions with 345 nm light leads predominantly to emission from the nanocrystals with very little pyrene emission. The Py-related emission band is largely quenched for both the 2.6 and 5.0 nm diameter SiNC-Py samples, indicative of energy transfer. Photoluminescence excitation (PLE) spectra measured by detecting only the long-wavelength emission from the SiNCs are further consistent with energy transfer between the attached Py moieties and the SiNCs in the SiNC-Py samples with the appearance of a peaked pyrene-related absorption band (see the Supporting Information, Figure S12). Energy transfer from pyrene to SiNCs in the SiNC-Py sample is also evidenced in the measurement of the lifetime of the Py-related 400 nm fluorescence. The characteristic lifetime for emission from the free pyrene chromophore is about 18 ns. The 400 nm emission lifetime is much shorter than this for the SiNC-Py samples. The emission intensity decay of SiNC-Py at 400 nm cannot be fitted to a monoexponential function; one component is below our instrumental resolution (95% efficiency.23 (See the Supporting Information for a detailed description of how the energy transfer efficiency is calculated and a schematic representation of the energy transfer process in Figure S13.) Because of the much stronger light absorption of Py compared to that of the SiNCs at 345 nm, Py functionalization led to a nearly 300% brightness enhancement in SiNC PL. Pyrene emission was also strongly quenched (>90%) in the 5.0 nm diameter SiNC-Py sample (red curve, Figure 2d), and the SiNC and SiNC-Py species exhibited PL quantum yields at 970 nm of 45 and 40%, respectively. The 970 nm PL emission was insensitive to dioxygen, and the luminescence decays of the SiNC and SiNC-Py were fit to monoexponential functions with lifetimes of 150 and 190 μs, respectively. The emission quantum yield was high compared to that of dye molecules emitting in the same spectral region,24 for which emission quantum yields higher than 30% have never been reported, to the best of our knowledge, and it is comparable to the value recently reported for PbS and PbSe quantum dots.25 On the basis of the relative PL emission spectra for 5.0 nm diameter SiNC-Py photoexcited with 345 nm (dashed red curve in Figure 2d) and 378 nm light (solid red curve in Figure 2d), energy transfer from adsorbed pyrene to the SiNCs occurred with 65% efficiency. With the three-fold
Table 1. Photophysical Properties of SiNC and SiNC-Py Dispersed in Air-Equilibrated Toluene at 298 K along with the Model Compound Py for Comparison d/nm
λex/nm
λem/nm
Φema
τ/nsb
SiNC SiNC-Py
2.6 2.6
Py SiNC SiNC-Py
5.0 5.0
378 345 378 345 378 345 378
635 400 680 400 970 400 970
0.11 0.005 0.08 0.06 0.45 0.005 0.40
70 × 103
