Excited-State Relaxation Dynamics of 3 ... - ACS Publications

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J. Phys. Chem. C 2010, 114, 11693–11698

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Excited-State Relaxation Dynamics of 3-Vinylthiophene-Terminated Silicon Quantum Dots Vincent Groenewegen,† Volker Kuntermann,† Dietrich Haarer,‡ Michael Kunz,† and Carola Kryschi*,† Institute of Physical Chemistry, Department of Chemistry and Pharmacy and ICMM, UniVersity of Erlangen, Egerlandstr. 3, D-91058 Erlangen, Germany, and BIMF, UniVersity of Bayreuth, UniVersita¨tsstr. 30, D-95447 Bayreuth, Germany ReceiVed: October 13, 2009; ReVised Manuscript ReceiVed: June 12, 2010

In theory, silicon quantum dots (SiQDs) emit enhanced photoluminescence with a size-tunable spectrum in the visible range. In practice, surface states originating from oxide defect structures or organic ligands are strongly involved in exciton relaxation dynamics because the amplitudes of hole and electron wave functions are nonzero at the SiQD surface. In this study, SiQDs with well-defined surface properties were obtained through a wet-chemistry procedure providing SiQDs with adjustable sizes and oxide-free, 3-vinylthiopheneterminated surfaces. The 3-vinylthiophene-terminated SiQDs have a crystalline spherical 2 nm core and were observed to exhibit blue photoluminescence (∼460 nm) with a quantum yield and lifetime of ca. 23% and 1.3 ns, respectively. The interplay between electronically excited molecular states and conduction band states was examined upon direct monitoring of photoexcited carrier dynamics with femtosecond transient absorption spectroscopy. The 3-vinylthiophene ligands were found to act as surface-bound antennae that mediate ultrafast electron transfer or excitation energy transfer across the SiQD interface. Introduction Bulk crystalline silicon (Si) is an inefficient light emitter and thus inappropriate as a material for light-emitting and optoelectronic technologies.1,2 This is due to its indirect band-gap structure that is associated with a large momentum mismatch between electrons and holes that prohibits their direct radiative recombination.3 However, the momentum conservation rule is relaxed in the quantum confinement regime, where Si crystals have sizes less than the exciton Bohr radius (∼4.2 nm). Further downsizing the crystalline core of a Si quantum dot (SiQD) is predicted to increase both the electronic band gap and the overlap between electron and hole wave functions.3-6 The benefit is largely enhanced photoluminescence (PL) in the visible spectrum, where the PL wavelengths can be adjusted by tuning the QD size.7-13 On the other hand, quantum confinement is associated with nonvanishing carrier wave function amplitudes at the natively oxidized surface of SiQDs. This amorphous SiOx surface layer offers a broad variety of defect structures providing nonradiative recombination centers for the electron-hole pairs14-16 or just localizing electrons and holes at the surface.17 Therefore, high PL quantum yields are only feasible for SiQDs with oxidefree surfaces. This can be achieved through complete removal of the SiOx layer and subsequent saturation of all dangling bonds of the surface Si atoms through covalent Si-C linkages with alkyl or alkenyl groups. In fact, both the steric volume of the passivant group and the surface curvature of the SiQD determine the density of surface passivation.18,19 Recent ab initio calculations suggest that complete alkyl passivation of SiQDs may be accomplished by covalently linking methyl and ethyl groups to Si clusters smaller than 2 nm.19 A virtue of alkyl passivation is the relatively small effect of Si-C bonds on the effective band gap of the SiQD.19 Despite these theoretical efforts, the influence * To whom correspondence should [email protected]. † University of Erlangen. ‡ University of Bayreuth.

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of organic passivation on the electronic structure and optical properties of SiQDs is so far hardly understood. In addition, the PL mechanism and the nature of the radiative states of highly luminescent SiQDs have been controversially discussed.1,15 This implies that a successful synthesis route that produces SiQDs with tailored PL properties has to provide full control of both the size distribution and surface properties. In the recent past, intense research activities have been devoted to the synthesis of stable, monodisperse SiQDs with a reproducible surface chemistry, sizes, shapes, and optical properties.7,20-23 Several synthesis routes for oxide-free functionalized SiQDs have been reported, where monodisperse SiQDs were produced either by laser-driven SiH4 pyrolysis,10 by RF plasma induced dissociation of SiH4,24 or through a liquidphase reduction procedure of SiCl4 via the use of reverse micelles in apolar solvents.25 The wet-chemistry synthesis yielded aminopropyl-terminated SiQDs with rather small sizes (1.4 ( 0.3 nm), whereas the gas-phase synthesis routes led to SiQDs that were subsequently functionalized with 1-alkenes or 1-alkyne derivatives by performing a thermal hydrosilylation reaction in controlled oxygen-free environments.26,27 Because gas-phase synthesized SiQDs are initially H-terminated, they will rapidly oxidize under ambient conditions. Therefore, functionalization of oxidized SiQDs requires a two-step wetchemistry procedure that comprises the removal of the SiOx surface by HF etching and subsequent thermal hydrosilylation with 1-alkene or 1-alkyne derivatives via thermal hydrosilylation.27 As a considerably smarter, timesaving method, Kortshagen et al. developed a two-stage plasma procedure, where monodisperse SiQDs (4.1 nm) were synthesized in a RFpowered flow-through reactor and passivated with dodecane via a plasma-aided in-flight process in a second plasma chamber.26 However, the plasma-grafted SiQDs emit photoluminescence in the NIR (∼800 nm) with a quantum yield smaller than 10%. In a previous work, Kortshagen and coauthors reported equally and smaller-sized, gas-phase synthesized SiQDs, which were

10.1021/jp100266w  2010 American Chemical Society Published on Web 06/22/2010

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thermally hydrosilylated with 1-octadecene in the liquid phase.10 Immediately after thermal hydrosilylation, colloidal 4 nm SiQD samples were observed to exhibit photoluminescence around 789 nm with a quantum yield of 62%, which decreased below 50% after 8 days of air exposure.10 Obviously, this surface coating could not obviate the formation of surfacial SiOx structures. Klimov and coauthors studied the ultrafast PL dynamics of similarly produced, dodecane- and octadecaneterminated SiQDs by carrying out picosecond (ps) resolved photoluminescence measurements and femtosecond (fs) luminescence upconversion experiments on colloidal samples.28 The temporal evolution of the PL spectra indicates the existence of two emitting states (540 and 620 nm) that decay on two distinct time scales: 5 ns and >100 µs. The fast PL decay dynamics were observed to exhibit strong size dependence and were, therefore, attributed to the core states. On the other hand, PL spectra recorded on the microsecond scale were ascribed to radiative decay of surface states that unfortunately could not be structurally identified. In this paper, we examined the PL properties and excitedstate dynamics of 3-vinylthiophene-terminated SiQDs with sizes between 2 and 3 nm. To this end, gas-phase synthesized SiQDs with initial sizes of about 5 nm were functionalized using an advanced two-step wet-chemistry procedure that starts with a size-controlling HF etching process, followed by thermal hydrosilylation of the SiQD surface with 3-ethynylthiophene.27 We obtained chemically stable 3-vinylthiophene-terminated SiQDs that emit blue PL (460 nm) with a moderate quantum yield (∼23%). Experimental Methods Materials. The chemical reagents, 3-ethynylthiophene (Aldrich, 98%), 3-thionyl carboxaldehyde (Aldrich), methyl lithium (Aldrich), aqueous 85% phosphorus acid (Fluka), anhydrous hexane (Aldrich), anhydrous diethyl ether (Aldrich), o-xylene anhydrous (Aldrich, 97%), ethanol (Merck), and aqueous hydrofluoric acid (Riedel-de Hae¨n, 40%), were used as received or were degassed by a series of freeze-pump-thaw cycles as described below. SiQDs with a mean size of 5 nm were fabricated by Evonik Degussa-Creavis Technologies & Innovation in a low-pressure microwave plasma reactor using silane as a precursor.24 Synthesis of 3-Vinylthiophene. 3-Vinylthiophene was synthesized via a 2-step procedure. The first reaction, yielding 5.4 g of 1-(3-thiophenyl) ethan-1-ol, was conducted in a Schlenk line under N2 gas flow at room temperature by adding 0.096 mol of methyl lithium in hexane (60 mL) to 0.089 mol of 3-thionyl carboxaldehyde in anhydrous diethyl ether (60 mL) and by subsequent hydrolyzing with 10 mL of aqueous 32% hydrochloride solution. The volatile components were removed by distillation under atmosphere pressure, and afterward, 1-(3thiophenyl) ethan-1-ol was obtained by distillation under vacuum. In the second reaction step, 3 mL of 1-(3-thiophenyl) ethan-1-ol and 3 mL of aqueous 85% phosphorus acid were mixed in a microdestillation apparatus, and 3-vinylthiophene was distilled as an aqueous azeotrope at 55 °C under vacuum. Subsequent drying over sodium sulfate and filtration yielded 1.3 g of 3-vinylthiophene as a colorless liquid; the H1 NMR data are in agreement with those given in the literature.29,30 Functionalization of the SiQDs. A 1.5 g portion SiQDs was dispersed in 70 mL of ethanol and 10 mL of HF (40%) and stirred for 10 min. The ethanolic HF solution was removed by vacuum distillation. A solution of 3-ethynylthiophene in oxylene (5 g in 70 mL) was degassed by carrying out three

Groenewegen et al. freeze-pump-thaw cycles and then added to the freshly etched, hydride-terminated SiQDs. The suspension was refluxed at 145 °C for 20 h. The functionalized SiQDs were washed over a polyvinylidene fluoride (PVDF) membrane filter (Millipore) with o-xylene and ethanol several times to remove unreacted 3-ethynylthiophene. High-Resolution Transmission Electron Microscopy (HRTEM). The functionalized SiQDs were prepared for HRTEM imaging by dispersing them in water and evaporating in air multiple drops of the suspension onto an ultrathin carbon-coated copper grid. The HRTEM images were recorded using a Phillips CM 300 UltraTwin microscope. The measurements were carried out at an accelerating voltage of 300 kV in the bright-field mode. Steady-State Spectroscopy. The FTIR spectra were measured using a Bruker Equinox 55 spectrometer. The PL spectra were recorded on a Jobin-Yvon FluoroMax-3 spectrofluorometer, and the UV/vis absorption spectra were taken with a PerkinElmer UV/vis spectrometer Lambda 2. All experiments were performed at room temperature, preparing samples either in KBr pellets (FTIR spectroscopy) or as colloids in 10 mm quartz cuvettes. Functionalized SiQDs were dispersed in ethanol at a concentration of 100 µg/mL and 3-vinylthiophene was dissolved in ethanol at concentrations between 1 mM and 10 µM. Time-Resolved Spectroscopy. Femtosecond (fs) transient absorption spectroscopy spectra were recorded on a Clark MXR CPA 2001 fs laser system in conjunction with an Ultrafast Systems detection system, consisting primarily of a glass fiber based spectrometer. The output pulses at 387 nm with a 150 fs pulse and a 1 kHz repetition rate used as pump pulses, were obtained by amplifying and frequency-doubling the 775 nm seeding pulses of the Er3+-doped glass fiber oscillator in a regenerative chirped-pulse titanium-sapphire amplifier and in a nonlinear optical amplifier (NOPA), respectively. All samples were pumped at excitation densities between 6.3 × 107 and 1.9 × 109 W/cm2. The samples consisting of 1 mg of SiQDs dispersed in 1 mL of ethanol or 0.1 mM 3-vinylthiophene in ethanol or pure ethanol were irradiated in a quartz cuvette with a path length of 2 mm. A fraction of the fundamental was simultaneously passed through a sapphire plate to generate the fs white-light continuum between 400 and 1200 nm. The chirp between 400 and 750 nm was approximately 350 fs. Transient absorption spectra of 3-vinylthiophene-terminated SiQDs in ethanol were taken at delay times between -2 ps and 3 ns. They were recorded between 400 and 750 nm. No photochemical degradation was observed after each experiment as subsequent checking of the sample by luminescence spectroscopy revealed. In contrast, the 0.1 mM ethanolic 3-vinylthiophene solution turned out to be photochemically instable. Pump pulse excitation at energy densities of 6.3 × 107 W/cm2 and larger led to an instantaneous formation of a yellowish brown precipitate. The optical density of transient absorption (∆O.D.) is extracted from recording the spectral intensity of the probe beam that impinges on the sample (I0(λ)) and passes through the sample as a function of the delay time relatively to the pump pulse (I(λ,τ)):∆O.D.(λ,τ) ) lg(I0(λ)/I(λ,τ)). For I0(λ) > I(λ,τ) and therewith positive values of ∆O.D., we observe absorption transitions of excited species, the so-called photoinduced absorption (PIA), whereas negative values of ∆O.D. result from I0(λ) < I(λ,τ) and are ascribed to photoinduced bleaching (PB) of the ground-state population density. PL decay profiles of colloidal SiQD-ethanol samples in a 10 mm quartz cuvette were recorded using time-correlated

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Figure 1. Schematic diagram of thermal hydrosilylation of Hterminated SiQDs with 3-ethynylthiophene.

Figure 3. FTIR spectrum of 3-vinylthiophene-terminated SiQDs.

Figure 2. HRTEM image of 3-vinylthiophene-terminated SiQDs highlighted by red circles.

single-photon counting (TCSPC) luminescence spectroscopy. The PL was excited using the 387 nm output pulses of the Clark MXR CPA 2001 laser system, and its temporal evolution was monitored on the Fluorolog-3 (Jobin Yvon) TCSPC spectrometer equipped with a microchannel plate (Hamamatsu, R3809U-50) that provides a time resolution of about 60 ps. Results and Discussion The initially oxidized SiQDs with sizes around 5 nm were first treated by a controlled HF etching procedure that provides complete removal of the SiOx layer, size reduction, and surface hydrogenation. Subsequently, a thermal hydrosilylation reaction with 3-ethynyl-thiophene was performed, which yielded 3-vinylthiophene-passivated SiQDs (Figure 1). As being obvious from the high-resolution transmission electron microscopy (HRTEM) image (Figure 2), both the spherical shape and the single crystallinity of the SiQDs were maintained during the HF etching and thermal hydrosilylation procedures, while the QD sizes were reduced to 2-3 nm. The structural features of the passivation layer were studied using FTIR spectroscopy (Figure 3). The strong absorption peak at 502 cm-1 is attributed to a TO (transversal optical) phonon band of crystalline silicon. The absence of the strong ν(Si-O) stretching mode between 1000 and 1100 cm-1 unambiguously indicates the complete removal of the SiOx shell. On the other hand, the very strong peak at 736 cm-1 and the weak shoulder at 1250 cm-1, as being assigned to the δ(Si-C) bending and ν(Si-C) stretching vibration, respectively, substantiate the formation of covalent Si-C bonds. In addition, the covalent tethering of 3-vinylth-

Figure 4. PL (black solid line) and PLE (black dashed line) spectra of colloidal 3-vinylthiophene-terminated SiQDs as well as fluorescence (black dotted line) and UV/vis absorption (blue solid line) spectra of 3-vinylthiophene in ethanol.

iophene was clearly identified by characteristic peaks ascribed to the thiophene ring stretching, ν(C-C), at 1450 cm-1, the ν(C-H) and ν(C-S) stretching modes at 1216 and 1154 cm-1, the δ(C-H) and δ(C-S) deformation vibrations at 1085 and 634 cm-1, and the vinyl ν(CdC) stretching modes at 1641 cm-1. FTIR spectra taken from 4 and 6 month old samples display exactly the same vibrational bands and, therefore, verify the chemical stability of this surface coverage. The PL and excitation (PLE) spectra of 3-vinylthiopheneterminated SiQDs dispersed in ethanol and the fluorescence and UV/vis absorption spectra of 5 µM 3-vinylthiophene in ethanol are shown in Figure 4. The PL spectrum (black solid line) recorded upon excitation at 390 nm exhibits a slightly asymmetric emission band peaking at 456 nm (2.72 eV). The PLE spectrum (black dashed line), which was obtained by detecting the emission at 540 nm, consists of a Gaussian-shaped band centered at 407 nm (3.04 eV) with an fwhm of 0.54 eV. The fluorescence of 3-vinylthiophene (black dotted line) excited at 260 nm reaches maximum intensity at 313 nm (3.95 eV), whereas the UV/vis absorption spectrum (blue solid line) peaks at 278 nm (4.47 eV). The fluorescence spectrum of 3-vinylthiophene overlaps between 3 and 3.5 with the PLE spectrum of 3-vinylthiophene-terminated SiQDs. This spectral overlap is sufficiently large to facilitate excitation energy transfer between initially excited ligand states and electronic band states of the QD. On the other hand, the 3-vinylthiophene ligands may excitonically interact via their transition dipole moments. In terms of Kasha’s exciton model,31 the most probable alignment configurations for surface-bound 3-vinylthiophene ligands as being parallel and oblique transition dipole moments are expected to either lead to a blue shifted excitation spectrum or

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Figure 5. PL decay dynamics of colloidal 3-vinylthiophene-terminated SiQDs detected at 500 nm.

appear as a splitting of the excitation band. Because the PLE spectrum exhibits no splitting and is largely red shifted in comparison with the absorption spectrum of 3-vinylthiophene, it is attributed to the direct zero-phonon band-to-band transition of the SiQD, which becomes partially allowed due to the quantum confinement effect. The PL spectrum resembles that of colloidal SiQDs that were produced with sizes around 2 nm by microwave plasma induced pyrolysis of silane and subsequent etching with an HF/HNO3 mixture.32 The almost Gaussian shape and relative large fwhm with 0.64 eV of the PL spectrum are explained with inhomogeneous broadening due to a Gaussian distribution in the QD sizes. Using the comparative method by Williams et al.33 with coumarine 102 as a fluorescence standard, we determined the PL quantum yield with a value of 23 ( 3%. This quantum yield and the relatively small Stokes shift of 0.3 eV are consistent with the quantum-confinement model that predicts enhanced probabilities and the blue shift of the optical zero-phonon and phonon-induced radiative transitions for sizes smaller than the Bohr’s exciton radius. This is due to spatial confinement of electrons and holes in Si nanostructures, which is associated with a spread of the carrier wave functions in k space and thereupon with the relaxation of the usual crystal momentum selection rules and widening of the optical band gap.34 The PL decay dynamics were studied on the nanosecond scale using TCSPC luminescence spectroscopy. Pump pulses (150 fs) at 387 nm were used to excite the colloidal SiQD-ethanol samples. The PL emission between 450 and 550 nm was detected with a time resolution of 60 ps. The excited-state dynamics were followed by recording transient absorption spectra with temporally delayed, white-light continuum probe pulses. The apparatus response time of the fs transient absorption experiment is 270 fs. Figure 5 shows the decay dynamics of the PL emission detected at 500 nm. The PL decay curve (navy blue open circles) was analyzed employing a combined procedure that deconvolves the apparatus time response and fits an exponential function with the time constant, τ ) 1.25 ( 0.03 ns, to the PL decay curve (red thick line). This result nicely agrees with those obtained from TCSPC luminescence experiments that were recently conducted on colloidal octadecaneand dodecane-terminated as well as octanethiol-capped SiQD samples.12,28 The authors reported size-dependent PL lifetimes with values between 0.4 and 5 ns. Ultrafast transient absorption measurements were performed to achieve information of the impact of the 3-vinylthiophene passivation layer on excitedstate dynamics in SiQDs. Therefore, colloidal samples were pumped at 387 nm with 150 fs pulses, and the absorption of the excited states of 3-vinylthiophene-terminated SiQDs was

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Figure 6. Temporal evolution of the transient absorption spectra of colloidal 3-vinylthiophene-terminated SiQDs (black solid lines) and ethanol (blue solid line) recorded at different delay times varied between -0 ps and 3 ns.

probed using variably delayed pulses of a white-light fs continuum. Figure 6 depicts the time evolution of the transient absorption spectra for delay times varied between 0 ps and 3 ns (black lines) and the solvent spectrum detected at 0.6 ps (blue line). Upon pump pulse excitation, photoinduced absorption (PIA) peaking at 450 nm (2.8 eV) evolved within 600 fs and had partially decayed after 1 ps, whereas a broad PIA band at 650 nm (1.9 eV) was formed at the expense of the PIA band at 450 nm. Because the solvent response in this spectral range is negligibly small, the maximum at 450 nm is ascribed to PIA transitions emerging from the first excited singlet state (S1) of surface-bound 3-vinylthiophene, which is presumably populated by two-photon absorption. On the other hand, the broad featureless PIA around 650 nm matches absorption transitions of conduction band electrons in SiQDs.16 Hence, the 3-vinylthiophene ligands may act as antennae for pump pulse light. To corroborate the hypothesis that pump pulse excitation at 387 nm creates a population density of the 3-vinylthiophene S1 state via two-photon absorption, transient absorption spectra of 3-vinylthiophene in ethanol were recorded. Although a rather small excitation energy density of 6.3 × 107 W/cm2 was applied, the 3-vinylthiophene solution was observed to degrade instantaneously by the formation of a yellowish brown precipitate. Furthermore, transient absorption spectra of 3-vinylthiopheneterminated SiQDs were recorded at different pump pulse excitation energy densities that were varied between 4.8 × 108 and 1.9 × 109 W/cm2 (Figure 7). The maximum ∆O.D. values of the transient spectra measured at the delay times of 0.6 and 1.6 ps show obviously a nonlinear pump pulse energy dependence. For the excitation density ratio of 1.0:1.6:3.7, the corresponding ratio of maximum ∆O.D. values around 450 nm is 1.0:2.8:13. Both the primarily formed PIA band (0.6 ps) in the transient absorption spectra of 3-vinylthiophene-terminated SiQDs and the absence of any solvent response in this spectral range (Figure 6) as well as the nonlinear excitation energy dependence of the ∆O.D. values at 450 nm substantiate the hypothesis that irradiation of the functionalized SiQDs with 387 nm pump pulses initially excites two-photon absorption transitions of the 3-vinylthiophene ligands. The decay of the PIA maximum at 450 nm and simultaneous rise of the prominent PIA band at 650 nm are explained by interfacial excitation energy transfer or electron transfer that evolve from ligand S1 states to SiQD band states. Because photoexcited 3-vinylthiophene-terminated SiQDs were observed to emit blue PL as arising from radiative recombination of electron-hole pairs in the core, the population of the conduction band states may

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Figure 7. Transient absorption spectra of 3-vinylthiophene-terminated SiQDs recorded at delay times of 0.6 and 1.6 ps in relation to the pump pulse excitation energy density varied between 4.8 × 108 and 1.9 × 109 W/cm2.

Figure 8. Short-time behavior of the transient detected at 650 nm (navy blue open circles); the best fit (red solid line) was calculated using a convolution of the apparatus response function (black solid line) with an exponential rise (τrise ) 0.6 ps) and a biexponential decay function (τdecay 1 ) 0.5 ps, τdecay 2 ) 18 ps).

originate from interfacial excitation energy transfer or be due to electron transfer that is followed by ultrafast backward electron transfer from valence band states to the ligand ground state. Unfortunately, our fs transient absorption spectroscopy experiment is limited in the short-wavelength range to detection in the visible spectrum. Therefore, we could not adduce any spectroscopic evidence for backward electron transfer as being only traceable in the UV range through monitoring bleaching decay dynamics of the S0-to-S1 absorption transition of the ligand. For this reason, we cannot unambiguously distinguish between interfacial excitation energy and electron transfer. Figure 8 shows the short-time behavior of the transient detected at 650 nm (navy blue open circles) in comparison with the Gaussian-shaped apparatus time response (black solid line). The solid red line represents the best fit using a convolution of the Gaussian time response with an exponential rise and a biexponential decay function. This combined fitting procedure yielded the time constants τrise ) 0.6 ps, τdecay 1 ) 0.5 ps, and τdecay 2 ) 18 ps. Because ultrafast photoinduced electron transfer or excitation energy transfer across semiconductor interfaces cannot be described by exponential functions,35,36 the time constant τrise ) 0.6 ps is considered as just representing a time span for electron transfer or excitation energy transfer dynamics. On the other hand, the calculated decay times τdecay 1 ) 0.5 ps and τdecay 2 ) 18 ps of the PIA transient are attributed to electron-LO phonon scattering16,37 and capture of the conduc-

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Figure 9. Long-time behavior of the transient detected at 650 nm (navy blue open circles); the best fit was carried out using a triexponential decay function (τdecay 1 ) 18 ps, τdecay 2 ) 220 ps, τdecay 3 ) 1.25 ns) (red solid line).

tion band electrons by shallow surface states, respectively. The long-time decay behavior of the PIA transient at 650 nm is also nonexponential (Figure 9). To compare its temporal characteristics with that of the PL decay (Figure 5), the PIA transient was simulated by implementing a triexponential decay function with the time constants τdecay 1 ) 18 ps, τdecay 2 ) 220 ps, and τdecay 3 ) 1.25 ns and the amplitude ratio A1:A2:A3 ) 8:5:3 (red solid line). Therewith, the conduction-band electrons were suggested to decay on this time scale via three competing relaxation channels. The two faster channels are associated with electron capture by shallow and deep surface states, respectively, while the slow decay process with τdecay 3 ) 1.25 ns is ascribed to radiative recombination of electron-hole pairs in the SiQDs. Because surface states originate from interfacial defect centers that may emerge in the course of both HF etching and the thermal hydrosilylation procedure, their structural nature could not be elucidated. Conclusions In conclusion, a two-step wet-chemistry synthesis route was employed to produce blue-luminescent 3-vinylthiophene-passivated SiQDs with Gaussian distributed sizes at 2 nm. The PL spectrum at 456 nm was shown to originate from radiative recombination of electron-hole pairs confined in the Si core. As being obvious from the stationary PLE spectrum, the photogeneration of excitons takes place within the Si core by direct excitation of valence-band electrons to conduction band states. In contrast, photoexcitation with 150 fs pump pulses at 387 nm pump pulses creates an initial population density of the 3-vinylthiophene S1 states via two-photon absorption. Subsequent ultrafast excitation energy transfer or electron transfer (∼0.6 ps) to conduction band states of the Si core was observed to take place by monitoring the rise dynamics of transient absorption transitions of conduction-band electrons. Both the rather fast PL decay dynamics (≈1.3 ns) and the PL quantum yield with ca. 23% indicate a dipole-allowed optical interband transition. This is attributed to confinement-induced relaxation of momentum conservation, which opens an additional radiative decay channel via zero-phonon, pseudodirect transitions. Acknowledgment. Support from the Deutsche Forschungsgemeinschaft (Graduiertenkolleg 1161/1,2) is gratefully acknowledged. We thank Dr. Andre´ Ebbers (Evonik-DegussaCreavis Technologies & Innovation) for generously providing us with SiQDs and Prof. Walter Bauer for measuring the H1 NMR of 3-vinylthiophene.

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References and Notes (1) Sa’ar, A. J. Nanophotonics 2009, 3, 032501. (2) Hersam, M. C.; Guisinger, N. P.; Lyding, J. W. Nanotechnology 2000, 11, 70. (3) Efros, A. L.; Rosen, M. Annu. ReV. Mater. Sci. 2000, 30, 475. (4) Canham, L. T. Appl. Phys. Lett. 1990, 57, 1046. (5) Brus, L. Phys. ReV. B 1996, 53, 4649. (6) Allan, G.; Delerue, D. Phys. ReV. B 2002, 66, 233303. (7) Holmes, J. D.; Ziegler, K. J.; Doty, C.; Pell, L. E.; Johnston, K. P.; Korgel, B. A. J. Am. Chem. Soc. 2001, 123, 3743. (8) Ledoux, G.; Gong, J.; Huisken, F. Appl. Phys. Lett. 2001, 79, 4028. (9) Anthony, R.; Kortshagen, U. Phys. ReV. B 2009, 80, 115407. (10) Jurbergs, D.; Rogojina, E.; Mangolini, L.; Kortshagen, U. Appl. Phys. Lett. 2006, 8, 233116. (11) Hao, H. L.; Shen, W. Z. Nanotechnology 2008, 19, 455704. (12) English, D. S.; Pell, L. E.; Yu, Z.; Barbara, P. F.; Korgel, B. A. Nano Lett. 2002, 2, 68. (13) Meier, C.; Gondorf, A.; Lu¨ttjohann, S.; Lorke, A.; Wiggers, H. J. Appl. Phys. 2007, 101, 103112. (14) Wolkin, M.; Jorne, J.; Fauchet, P.; Allan, G.; Delerue, C. Phys. ReV. Lett. 1999, 82, 197. (15) Wen, X.; Dao, L. V.; Hannaford, P.; Cho, E.-C.; Cho, Y. H.; Green, M. A. New J. Phys. 2007, 9, 337. (16) Kuntermann, V.; Cimpean, C.; Brehm, G.; Sauer, G.; Kryschi, C.; Wiggers, H. Phys. ReV. B 2008, 77, 115343. (17) Martin, J.; Cichos, F.; Huisken, F.; von Borczyskowski, C. Nano Lett. 2008, 8, 656. (18) Puzder, A.; Williamson, A. J.; Reboredo, F. A.; Galli, G. Phys. ReV. Lett. 2003, 91, 157405. (19) Reboredo, F. A.; Galli, G. J. Phys. Chem. B 2005, 109, 1072. (20) Lie, L. H.; Duerdin, M.; Tuite, E. M.; Houlton, A.; Horrocks, B. R. J. Electroanal. Chem. 2002, 538, 183.

Groenewegen et al. (21) Hua, F.; Erogbogbo, F.; Swihart, M. T.; Ruckenstein, E. Langmuir 2006, 22, 4363. (22) Rogozhina, E. V.; Eckhoff, D. A.; Gratton, E.; Braun, P. V. J. Mater. Chem. 2006, 16, 1421. (23) Hua, F.; Ruckenstein, E. Langmuir 2005, 21, 6054. (24) Knipping, J.; Wiggers, H.; Rellinghaus, B.; Roth, P.; Konjhodzic, D.; Meier, C. J. Nanosci. Nanotechnol. 2004, 4, 1039. (25) Warner, J. H.; Hoshino, A.; Yamamoto, K.; Tilley, R. D. Angew. Chem. 2005, 117, 4626. (26) Mangolini, L.; Kortshagen, U. AdV. Mater. 2007, 19, 2513. (27) Cimpean, C.; Groenewegen, V.; Kuntermann, V.; Sommer, A.; Kryschi, C. Laser Photonics ReV. 2009, 3, 138. (28) Sykora, M.; Mangolini, L.; Schaller, R. D.; Kortshagen, U.; Jurbergs, D.; Klimov, V. I. Phys. ReV. Lett. 2008, 100, 067401. (29) Tominaga, Y.; Lee, M. L.; Castle, R. N. J. Heterocyclic Chem. 1981, 18, 967. (30) Randazzo, M. E.; Toppare, L.; Fernandez, J. E. Macromolecules 1994, 27, 5102. (31) Kasha, M.; Rawls, H. R.; Ashraf El-Bayoumi, M. Pure Appl. Chem. 1965, 11, 371. (32) Gupta, A.; Swihart, M. T.; Wiggers, H. AdV. Funct. Mater. 2009, 19, 696. (33) Williams, A. T. R.; Winfield, S. A.; Miller, J. N. Analyst 1983, 108, 1067. (34) Kanemitsu, Y. Phys. Rep. 1995, 263, 1. (35) Wang, L.; Willig, F.; May, V. J. Chem. Phys. 2006, 124, 014712. (36) Kondov, I.; Thoss, M.; Wang, H. J. Phys. Chem. A 2006, 110, 1364. (37) Harb, M.; Ernstorfer, R.; Dartigalongue, T.; Hebeisen, C. T.; Jordan, R. E.; Miller, R. J. D. J. Phys. Chem. B 2006, 110, 25308.

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