Silicon Nanocrystals

Size-Dependent Oxidation of Monodisperse Silicon Nanocrystals with Allylphenylsulfide Surfaces Julia Rinck,* Dirk Schray, Christian Kübel, Annie K. Powell, and Geoffrey A. Ozin*

The

synthesis and characterization of size-separated silicon nanocrystals functionalized with a heteroatom-substituted organic capping group, allylphenylsulfide, via photochemical hydrosilylation are described for the first time. These silicon nanocrystals form colloidally stable and highly photoluminescent dispersions in nonpolar organic solvents with an absolute quantum yield as high as 52% which is 20% above that of the allylbenzene analogue. Solutions of the size-separated fractions are characterized over time to monitor the effect of aging in air by following the change of their photoluminescence and absolute quantum yields, supplemented by transmission electron microscopy.

1. Introduction Luminescent, colloidally stable silicon nanocrystals are a new class of materials with a wide range of applications from medical imaging[1,2] to optoelectronics such as light emitting diodes (LEDs)[3–15] and solar cells.[16,17] In contrast to II–VI and III–V

Dr. J. Rinck Karlsruhe Institute of Technology Center for Functional Nanostructures Wolfgang-Gaede Str. 1a, Karlsruhe 76131, Germany E-mail: [email protected] Prof. G. A. Ozin University of Toronto Lash Miller Chemical Laboratories Department of Chemistry 80 St George Street, Toronto, Ontario M5S 3H6, Canada E-mail: [email protected] Dr. D. Schray, Prof. A. K. Powell Institute of Inorganic Chemistry Engesserstr. 15, Karlsruhe 76131, Germany Dr. C. Kübel, Prof. A. K. Powell Karlsruhe Institute of Technology Institute of Nanotechnology Hermann-von-Helmholtz-Platz 1 Eggenstein-Leopoldshafen 76344, Germany Dr. C. Kübel Karlsruhe Institute of Technology Karlsruhe Nano Micro Facility Hermann-von-Helmholtz-Platz 1 Eggenstein-Leopoldshafen 76344, Germany DOI: 10.1002/smll.201401965 small 2014, DOI: 10.1002/smll.201401965

semiconductor nanocrystals, the advantage of silicon, the element that dominates the microelectronics and photovoltaic industry, is its nontoxicity,[18–20] earth abundance and low cost. Although bulk silicon is an indirect bandgap semiconductor, silicon nanocrystals with a size smaller than the Bohr exciton radius of silicon show photoluminescence with a high quantum yield[21,22] in the size range of 1–5 nm due to spatial confinement effects.[23,24] Semiconductor nanocrystals with sizes between 1 and 100 nm consist of several tens to several thousand atoms. Their physical and chemical properties are determined by quantum size effects and differ from those of the corresponding bulk material. The energy levels of the electrons are quantized and the effect of decreasing size is to widen the electronic band gap, causing their photoluminescence to shift to smaller wavelength from the red towards the blue spectral region when excited within their optical absorption edge.[25] The chemical and, consequently, optical properties of the nanocrystals can be altered by attachment of organic capping groups onto the surface atoms of the nanocrystals to provide them with colloidal stability and protect them from oxidation,[26] which is important for advanced materials and biomedical applications. Thus, control over both the size and the surface of silicon nanocrystals is highly desirable. Since optoelectronics, and in particular LEDs, are currently attracting great interest, the search for improvements in performance and especially for the discovery of environmentally friendly systems is an important goal. We showed previously[27] that LEDs with high external quantum efficiencies and low turnon voltages can be built using silicon nanocrystals as the active light emitting material. Using size-separated ncSi, we

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found that not only the color can be tuned, but also that the lifetime of the device can be significantly improved. Indeed, the efficiency of the device can in principle be further improved by modifying the ligand system, for example by introducing a heteroatom next to the aromatic node it should be possible to increase the electron density within the aromatic system and thus to alter the influence of the ligand on the photoluminescence (PL). This raises the question as to whether allylphenylsulfide (APS) can lead to an increased Photoluminescence absolute quantum yield (PL AQY) compared with its allylbenzene (AB) analogue. Various synthetic routes to silicon nanocrystals have been developed in recent years[28–31] and amongst these pyrolysis of SiH4 is a favored method which can be carried out thermally[32] or using microwave irradiation[33] or else within a high-frequency plasma.[34] In our study we applied the method published by Henderson et al.,[35–37] in which the hydrolysis of trichlorosilane is used to obtain a sol-gel. This was then dried prior to reduction to the crystalline silicon nanocrystals (ncSi) which are encapsulated within a matrix of silicon dioxide which can be etched away using aqueous hydrogen fluoride, resulting in hydrophobic hydride terminated nanocrystals of ncSi:H. These could then be functionalized with allylphenyl sulfide (APS) in a light driven hydrosilylation reaction to yield ncSi:APS. A survey of the different ways of functionalizing ncSi:H has been provided by Veinot.[38] Despite the many different approaches used to functionalize silicon nanocrystals, these mostly result in a broad size distribution, leading to averaging effects of the measured size-dependent properties. This is mostly clearly exemplified by the absolute quantum yield (AQY), which shows a distinct size effect and which can only be understood when monodisperse samples are studied. As a result, recent efforts have turned to post-synthetic size-separation out of which fractionation by density gradient ultracentrifugation (DGU)[39] and simple size-selective precipitation using an anti-solvent[40] have proved successful means for obtaining size-separated functionalized silicon nanocrystals and thus enabling the study of quantum size effects in the NIR- and visible range. Such approaches can be used because the nanocrystals are capped with organic ligands which enable colloidal stability in non-polar solvents. This approach was used to investigate nanocrystals of silicon. The stepwise addition of an anti-solvent, such as methanol, was found to result in an aggregation of larger nanocrystals in solution and permits a stepwise precipitation according to size. We find that in addition to the desirable property of a narrow size for applications in optoelectronic devices, the chemical stability of the silicon nanocrystals is an issue. For example, adventitious oxidation will result in a change of the optical properties and, since the surface to volume ratio and also the surface curvature changes with size, is expected to be size dependent, the subject of the work reported herein.

2. Results and Discussion To investigate the quantum size effects and the size-dependent aging of the chosen APS-capped silicon nanocrystals, the

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polydisperse ensemble (sample 1) was dissolved in a small amount of toluene and size-separated by size-selective precipitation into 15 fractions by stepwise addition of methanol as an anti-solvent. This resulted initially in the precipitation of the largest nanocrystals. These nanocrystals were collected by centrifugation and re-dissolved in dry toluene for characterization. The photoluminescence (PL) of each fraction was measured at an excitation wavelength of 350 nm and the curves were normalized. The PL maximum of each fraction (increasing fractionation step) shifted continuously to a smaller wavelength as the band gap widens with decreasing size. Each fraction shows two distinct emission peaks, one in the red and one in the blue spectral region. With decreasing nanocrystal size the red peak shifts to shorter wavelength and its intensity slowly decreases while the intensity of the blue peak correspondingly increases but without any discernible shift. Overall, with decreasing nanocrystal size, a continuous shift of the measured optical extinction maxima to a shorter wavelength was observed, ranging from 374 nm (fraction 5) to 325 nm (fraction 10). The organic capping groups of the nanocrystals do not only enhance the colloidal stability of the nanocrystals in solution, but they also protect the surface of the nanocrystals from oxidation, which would, in turn, lead to a change of the chemical and optical properties of the nanocrystals. With increasing oxidation, the band gap of the nanocrystals increases and the PL maxima shift to smaller wavelength into the blue spectral region as the silicon core shrinks in size. This was investigated for the different fractions. The first four fractions emit in the NIR beyond the range of the spectrometer and were not used. Thus starting from fraction 5 the emission was in the visible spectral region and was investigated in detail. These fractions emit in the range 682 to 578 nm with colors ranging from deep red to pale yellow. For the initial measurement (day 0) all samples were transferred into a quartz glass cuvette in a glove box under argon atmosphere and the cuvette was sealed with a plastic lid to prevent contact with air. The spectra were recorded immediately thereafter. For these ten fractions the PL maxima shifts from 682 (fraction 5) to 578 nm (fraction 15) in steps of approximately 10 nm, the first steps being somewhat larger. We assume that the fresh samples are only minimally oxidized but then age to form a surface oxide that was monitored by FT-IR spectroscopy. Thus as the samples age, the overall shift between fractions becomes smaller starting at 105 nm for the initial spectra and reducing to 71 nm after five days (Figure 1). The full width at half maximum (FWHM) continuously decreases from around 140 nm to almost half of this value, indicating a more effective size separation with decreasing particle size. This is not surprising taking into account the larger volume of the first fractions which steadily decreases with each step. After the measurement the sample was transferred into a glass beaker, allowing contact with air. The slow oxidation of the nanocrystals was monitored over two weeks, re-measuring the samples after 1, 2, 5, 7, and 13 days. Each fraction shows a monotonic shift of the emission maxima to smaller wavelength with an average of 30 nm over these 13 days. For most

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small 2014, DOI: 10.1002/smll.201401965

Size-Dependent Oxidation of Silicon Nanocrystals with Allylphenylsulfide Surfaces

Figure 2. Evolution of time-dependent photoluminescence spectra and intensity for Sample 1 Fraction 9 of size separated ncSi:APS after exposure to air.

Figure 1. Size depend PL spectra normalized to 1 of the freshly prepared ncSi sample (Sample 1) (top) and after exposure to air after 5 days (bottom).

samples almost no PL change was observed after one day but beyond this time the oxidation progressed slowly. Fraction 9 was chosen to demonstrate the decrease of the intensity of the red peak that occurs over 13 days as well as the shift to smaller wavelength, in this particular case from 618 to 591 nm and a total shift of 27 nm, as the nanocrystals age (Figure 2 and Table S1). As the surface of the silicon nanocrystals begins to oxidize, the silicon core shrinks in size causing the PL shift to the blue. This behavior was observed for all fractions although the effect is more distinct when looking at the fractions with the largest and smallest nanocrystals. The total blue shift of fractions 6–11 is significantly smaller than the shift of the fractions with larger or smaller nanocrystals (Figure S1) (and this trend is reproducible). Several proposals have been put forward to explain the origin of the blue PL although a quantum size effect is unlikely as the blue PL does not shift with increasing surface oxidation and decreasing size of the silicon nanocrystal core. From earlier studies it is known that silicon dioxide shows a strong blue luminescence due to recombination of self-trapped excitons.[41] Therefore we prefer to suggest the surface oxidation of the silicon nanocrystals as the more likely explanation of the origin of the blue PL. This assumption is supported by Fourier-transform infrared spectroscopy (FTIR). The spectra of freshly etched nanocrystals show a significant Si–H vibration band at 2070 cm−1 and only weak small 2014, DOI: 10.1002/smll.201401965

Si–O, Si–O–Si and Si–OH vibration modes at 805, 1028–1067, and 3450 cm−1, respectively. After exposure to air for two weeks a significant increase of the Si–O, Si–O–Si and Si–OH bands is observed and the Si–H peak vanishes, indicating that the remaining surface hydride atoms have been replaced by oxide in the form of either oxide or hydroxide (Figure S2). Transmission electron microscopy (TEM) was used to investigate the size distribution of three selected fractions, namely fractions 6, 9, and 14. With increasing fraction number, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) clearly shows a decrease in the nanocrystal size with mean diameters of 2.53 ± 0.45 nm for fraction 6, 2.20 nm ± 0.36 nm for fraction 9, 1.61 nm ± 0.34 nm for fraction 14 (Figure 2). This decrease in the size of the nanocrystals can be correlated with the increasing fraction number and assigned to the PL peak values of 667, 618 and 584 nm. This supports the previous statements suggesting a blue shift to smaller wavelength with decreasing particle size. Photoluminescence absolute quantum yield (PL AQY) measurements were performed on size-separated ncSi:APS dispersed in dry toluene (sample 2) that were transferred into a quartz glass ampoule in air prior to the measurement. With an excitation wavelength of 353 the highest AQY of 51.5% was measured for the bright red emitting nanocrystals with a PL maximum of 706 nm. The AQY dropped with decreasing nanocrystal size concomitant with a shift of the PL to smaller wavelength. For nanocrystals with a PL maximum of 671 nm, the AQY dropped to 35.8% and with a PL maximum of 637 nm the AQY dropped abruptly to 15.3% and at 626 nm only 11.6% and less were recorded. Also for larger particle with a PL of 760 nm the AQY drops to 36.7% and particles emitting in the NIR (958 nm) show an AQY of only 9.6% (Figure 3). There could be the possibility that between the fraction that shows the highest AQY and the neighboring fractions in which the AQY already dropped by a third (to roughly 36%), that there is an additional fraction lying somewhere between these values with an even higher AQY than 51.5%. With an AQY of 51.5% this material is highly interesting for optical applications, e.g., in light-emitting diodes and it proves that

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Figure 3. HAADF-STEM images and histograms of monodisperse ncSi (Sample 1) Fractions 6 (a), 9 (b), and 14 (c).

introduction of a heteroatom changes the electronic properties towards a higher AQY more than 20%. For the AB analogue an AQY for red emitting particles of 43% has been reported.[21] The initial increase of the PL AQY from the infrared to the visible region with decreasing size of the particles could indicate the transition from bulk properties to quantum confinement. To investigate how oxidation affects the AQY, the fractions of sample 2 were stored in air and their AQY was re-measured after some time. A steady decrease for all fractions (IR and VIS-region) was observed (Figure 4, Table S2). In fraction 2 particles started to precipitate over time and were filtered off. Although the PL shifted 23 nm (from 763 to 740 nm) towards the region where we initially measured the highest AQY (706 nm), a decrease and not an increase of ∼24% was observed. For fraction 1 a PL shift of 46 nm and an

AQY loss of ∼8% was measured. For fractions 2, 4 and 5 with the highest initial AQYs the decrease is more pronounced although the shift of the PL to smaller wavelength is less pronounced compared to the fractions with smaller particles. For fractions 4–7 the PL shift was 25, 17, 31 and 46 nm and the AQY decreased to ∼16, 5, 3, and 3%, corresponding to a loss of 20, 18, 12, and 9%, respectively (Table S2). Values below 10% are only approximate values. Unfortunately the solvent of fraction 3 evaporated over time (aging). After re-suspension of the precipitated particles in toluene by ultra-sonication the absorption was too low to obtain data of their AQY. Comparison with the TEM results allows us to give an approximation for the size of the nanocrystals of sample 2 fraction 4 with an AQY of 35.8% and PL of 671 nm. We find these are in the same size regime as the nanocrystals of sample 1 fraction 6 for which a PL of 667 nm and an average particle size of 2.53 ± 0.45 nm was determined. The particles with the highest measured AQY of 51.5% and a bright red PL of 706 nm are thus larger in size.

3. Conclusion

Figure 4. Wavelength-dependent photoluminescence AQY of fresh (black) and aged (gray) of Fraction 1–7 of ncSi:APS (Sample 2).

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It has been shown that the introduction of a heteroatom into the ligand system is a suitable method to change the electronic properties of the silicon nanocrystals resulting in an increase of the AQY. Using allylphenylsulfide as ligand instead of allylbenzene the AQY can be increased more than 20% to an overall AQY of 51.5% making this system extremely interesting for optoelectronic applications like LEDs especially as size-selective precipitation allows one to tune the color. This increase of the AQY can be explained by the lower frequency vibrations associated with the C-S bond in APS compared to C–C in AB, that reduces the non-radiative vibrational relaxation in the former relative to the latter

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small 2014, DOI: 10.1002/smll.201401965

Size-Dependent Oxidation of Silicon Nanocrystals with Allylphenylsulfide Surfaces

and thereby enhances the PL AQY of ncSi:APS relative to ncSi:AB. The time dependent study of the size-dependant PL and AQY shows their stability towards oxidation in air which is an important result for applications such as LEDs. It seems that for the largest particles (emitting in the IR region) we already start to observe the transition from quantum confinement to bulk properties, resulting in a strong decrease of the PL AQY. The highest AQY was observed for ncSi:APS particles emitting in the range of 650–600 nm. These turned out to also be the most stable particles towards oxidation in air showing a smaller blue shift of their PL after aging. This could be a result of closer packing of the capping organics on the surface of the larger particles that emit in the visible region. The converse is true for the smaller particles where the higher surface curvature results in more open packing of the organic capping groups, which results in faster oxidation of the more accessible surface, resulting in a more rapid loss of the PL and decrease of the AQY.

4. Experimental Section Synthesis Procedures: Due to the air sensitivity of the materials to oxidation and/or hydrolysis all procedures were carried out under an atmosphere of dry nitrogen or argon using Schlenk techniques on a high vacuum line capable of being evacuated to 10−3 Torr and an inert gas supply or using a glovebox from BRAUN company. Nitrogen was used from the building supply and cleaned from traces of oxygen and water by letting it flow over a Cu-catalyst from BASF company loaded with hydrogen (at 140 °C), bubbling it through H2SO4, a mixture of pumice and P4O10 and blue silica gel prior to use. Toluene and methanol were dried prior to use. Toluene was dried using sodium and adding benzophenone, methanol was refluxed with magnesia and iodine for 2 h before distillation. Synthesis of the Ligand Allylphenylsulfide (APS): To a stirred solution of 25 mL (0.27 g, 0.24 mol) thiophenol and 100 mL dry diethyl ether, cooled to 0 °C with an ice bath, 100 mL (0.25 mol) of a 2.5 molar butyl lithium solution in pentene solution was added dropwise over one hour. The mixture was stirred at room temperature for one hour. To remove the white precipitate the mixture was filtered with a G4 glass frit before the solvent was removed by distillation. The formed lithium thiophenol was dissolved in 100 mL dry toluene and 24 mL (0.27 mol) allyl bromide was added dropwise within two hours. The mixture was stirred for 12 h at room temperature before it was filtered in air and the solvent was distilled off under vaccum. Allyl phenyl sulfide was distilled at 130 °C and 10−2 bar to yield a clear colourless liquid with a yield of 70% (26 g). 1H NMR (300 MHz, CD CN, δ): 7.24 (m, 5H, Ar H), 5.95 (m, 1H, 3 CH), 5.10 (m, 2H, CH2), 3.72 (m, SCH2); 13C NMR (300 MHz, CD3CN, δ): 136.12, 129.1, 126.10 (Ar C), 133,98 (-CH = ), 117.17 ( = CH2), 37.0 (-CH2-); IR (KBr): ν = 3062 (w), 3027 (w), 2958 (w), 2922 (s), 2862 (w), 1700 (vw), 1653 (vw), 1560 (s), 1496 (w), 1483 (s), 1453 (w), 1419 (s), 1375 (w), 1358 (w), 1229 (w), 1086 (vs), 917 (w), 743 (vs), 698 (s), 668 (vw), 623 cm−1 (vw); Anal. calcd for C9H10S: C 71.94, H 6.70, N 0, S 21.34; found: C 72.12, H 6.75, N 0.15, S 21.13. Synthesis of the Functionalized Silicon Nanocrystals (ncSi:APS): 20 mL HSiCl3 were transferred into a beaker that was cooled with an isopropanol/CO2(s) bath to −78 °C. 80 mL of deionised water were added quickly and a white precipitate formed immediately. small 2014, DOI: 10.1002/smll.201401965

The acetic mixture was stirred with a plastic spatula and was allowed to warm to room temperature. After 30 min the precipitate was transferred into a Büchner funnel and washed with approximately 2 L of deionised water until the pH was almost neutral. The product was transferred into a Schlenk flask and dried in vacuo, heating it to 100 °C in a water bath. The fine white powder (HSiO1.5)n was stored under nitrogen to prevent unwanted oxidation and hydrolysis of the Si-H. The sol-gel was heated to 1100 °C in a tube furnace flushing it with 5% H2/95% Ar with 3 L/min. The temperature was kept for 1 h before slowly cooling to room temperature. 0.3 g of the brown product was ground in an agate mortar and transferred into a plastic beaker. 5 mL ethanol and 10 mL HF (48%) were added. The mixture was stirred for 1 h and 45 min to etch the SiO2 matrix and gradually decrease the size of the nanoparticle core, before 15 mL dry toluene was added. The mixture was stirred for a few minutes to allow the hydrophobic hydrate-terminated nanocrystals (ncSi:H) to transfer into the solvent phase before the solution was separated into a Schlenk flask under inert gas. After the addition of 12 mL allylphenylsulfide the stirred mixture was exposed to UV-light (254 nm, 9 Watt, Pen-Ray Hg lamp) at room temperature for 12 h. The solvent and any excess of the ligand were distilled in vacuo and increasing heat resulting in a dry residue of functionalized nanocrystals (ncSi:APS). Size separation of the functionalized nanoparticles: Under inert gas the functionalized particles were re-dissolved in 3 mL dry toluene and gradually precipitated by stepwise addition of 2 mL dry methanol.[21] The precipitate was centrifuged for 3 min (9000 rpm), separated and re-dissolved in dry toluene. Characterization: The photoluminescence of the particles dissolved in toluene was collected with a Varian Cary Eclipse Fluorometer. The emission was collected in the region of 375–800 nm with an excitation of 350 nm. The cuvettes were made of Quartz glass. For the determination of AQY ϕ, an absolute PL quantum yield measurement system from Hamamatsu Photonics was used. The system consisted of a photonic multichannel analyzer PMA-12, a model C99200–02G calibrated integrating sphere and a monochromatic light source L9799–02 (150 W Xe- and Hg-Xe-lamps). Data analysis was done with the PLQY measurement software U6039–05, provided by Hamamatsu Photonics. TEM analysis was performed using an image-corrected FEI Titan 80−300 microscope operated at an acceleration voltage of 300 kV HAADF-STEM imaging was performed with a nominal probe size of 0.15 nm. TEM sample preparation of the ncSi:APS was done by diluting the material in toluene and dispersing it on a carbon-coated Au grid (Quantifoil) under inert gas atmosphere. The infrared spectra were collected with a Perkin Elmer Spectum GX spectrometer in the region of 400 cm−1 to 4000 cm−1 in transmission mode using 4 scans to a resolution of 4 cm−1. The samples were prepared as KBr-discs by grinding the dry sample with a small amount of KBr in an approx. ratio of 1:50 and then pressed to a KBr disc of 1 cm in diameter under the pressure of 10·104 N.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

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Acknowledgements GAO is a Canada Research Chair in Materials Chemistry and Nanochemistry. He thanks the KIT for his Distinguished Research Fellowship and the Natural Sciences and Engineering Research Council NSERC and the Ministry of Innovation MRI for strong and sustained support of his research. JR thanks the Center for Functional Nanostructures. We thank the Karlsruhe Nano Micro Facility (KNMF) for the electron microscopic analysis and Cynora GmbH in Eggenstein-Leopoldshafen for the measurements of the absolute quantum yield.

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Received: July 4, 2014 Revised: August 8, 2014 Published online:

small 2014, DOI: 10.1002/smll.201401965

Size-dependent oxidation of monodisperse silicon nanocrystals with allylphenylsulfide surfaces.

The synthesis and characterization of size-separated silicon nanocrystals functionalized with a heteroatom-substituted organic capping group, allylphe...
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