Silyl Radical Abstraction in the Functionalization of ... - ACS Publications

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Silyl Radical Abstraction in the Functionalization of Plasma-Synthesized Silicon Nanocrystals Lance M. Wheeler, Nicholas C Anderson, Peter K. B. Palomaki, Jeffrey L. Blackburn, Justin C. Johnson, and Nathan R. Neale Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03309 • Publication Date (Web): 10 Sep 2015 Downloaded from http://pubs.acs.org on September 11, 2015

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Chemistry of Materials

Silyl Radical Abstraction in the Functionalization of Plasma-Synthesized Silicon Nanocrystals Lance M. Wheeler, Nicholas C. Anderson, Peter K. B. Palomaki, Jeffrey L. Blackburn, Justin C. Johnson, and Nathan R. Neale* Chemistry & Nanoscience Center, National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States * E-mail: [email protected] (N.R.N.)

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ABSTRACT Many silicon nanostructures have exhibited favorable optical properties following surface functionalization with molecular groups through a silicon-carbon bond. Here we show the mechanism of functionalization of silicon nanocrystals synthesized in a nonthermal radiofrequency plasma is fundamentally different than in other silicon systems. In contrast to hydrosilylation, where homolytic cleavage of Si–H surface bonds is typically a prerequisite to functionalization, we demonstrate the dominant initiation step for plasma-synthesized silicon nanocrystals is abstraction of a silyl radical, •SiH3, to generate a radical at the silicon nanocrystal surface. We experimentally trap the abstracted silyl radical and show this initiation mechanism occurs for both radical- and thermally-initiated reactions of alkenes using complimentary photoluminescence quantum yield as well as FTIR and 1H NMR spectroscopies. These data additionally indicate that silylsilylation, or addition of a Si–SiH3 group across an unsaturated hydrocarbon, competes with hydrosilylation. We also present a new empirical sizing curve as a convenient method to determine Si NC size from photoluminescence emission energy.

TOC GRAPHICS


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INTRODUCTION Nanocrystals (NCs) comprised of the group IVB elements (Si, Ge, Sn) represent a class of non-toxic and earth-abundant semiconductor building blocks with promising properties for a diverse set of applications including thermoelectrics1, batteries2, next-generation solar cells3,4, LEDs5-7, and bioimaging8. The successful implementation of group IV NCs in these applications depends upon the development of simple and scalable routes for both synthesizing the NCs and tailoring their surface chemistry. Silicon NCs have drawn the most attention of the group IV materials due to the abundance of Si and its compatibility with current optoelectronic technology based on bulk Si. A variety of methods have been explored for preparing diamond cubic Si NCs such as laser-9 and thermal-induced10 pyrolysis of silanes, reduction of silicon(IV) halides11-13, thermal decomposition of diphenylsilane in supercritical fluid14, mechanochemical synthesis15, oxidation of sodium silicide with ammonium bromide16, hightemperature precipitation from silicon sub-oxides17, and decomposition of silane in a nonthermal, radiofrequency (RF) plasma18. Of these approaches, nonthermal plasma synthesis is a high-yield method that has delivered the most impressive optical properties such as high photoluminescence quantum yield (PLQY)19,20 and a near-ideal onset for multiple exciton generation (MEG)4. The optical properties of Si NCs are intimately linked to the surface chemistry. Many of the synthetic methods yield a surface terminated with silicon hydrides (*SiHx) either transiently or as stable species. Reaction of these silicon hydrides via hydrosilylation – adding a Si–H group across an unsaturated hydrocarbon bond – appears to be critical for passivating defect states that enable high PLQY20. The functionalization of *SiHx with hydrocarbon groups is well-established for both planar21,22 and porous23,24 silicon for a number of different chemical moieties25-30 and has provided a foundation for similar reaction chemistries on Si NC surfaces. Functionalization with both alkenes and alkynes can be



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initiated by thermal, radical, photolytic, metal-catalyzed, and chemomechanical approaches31-38, and the mechanisms have recently been reviewed39. The most widely accepted mechanism for the majority of these functionalizations is hydrosilylation in which homolytic cleavage of a surface Si–H bond yields a hydrogen radical H• and a Si surface radical Si• (dangling bond). The Si surface radical reacts with an alkene to produce a covalent Si–C bond and a radical at the alkyl β-carbon. This radical is satisfied by abstracting a neighboring surface hydride, re-generating the Si surface radical, and propagating the reaction along the Si surface31,39. Recently, light-assisted mechanisms involving plasmons or excitons that generate a positive charge on the surface that is subsequently attacked by an electron-rich alkene (i.e., without homolytic cleavage of Si–H bonds) have also been described32,39. Interestingly, Anthony et al. have suggested that an increased fraction of trihydride groups (*SiH3) on as-synthesized Si NC surfaces leads to more complete passivation of surface states after reaction with alkenes resulting in higher PLQYs19. This observation suggests that *SiH3 has an impact on the functionalization chemistry, but a rigorous investigation of the mechanism in plasma-synthesized Si NCs has not been conducted. Here, we study alkyl functionalization of plasma-synthesized Si NCs using thermal- and radical-initiated routes and reveal a new mechanism of alkyl attachment via abstraction of a silyl radical, •SiH3. EXPERIMENTAL SECTION All solvents were purchased from Aldrich or Fischer, ACS grade or better unless otherwise noted. Toluene and 1-dodecene were distilled from sodium under nitrogen. Tetrachloroethylene (TCE) was dried over calcium chloride, and acetonitrile dried over calcium hydride, followed by distilling under nitrogen. Benzene-d6 and toluene-d8 were dried over calcium hydride followed by vacuum transfer and stored in the glove box. All reactions were performed in sealed glass vials in a nitrogen-filled glove


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box. Potassium bromide was ground in a mortar and pestle and dried by heating to 200 °C under vacuum (~10 mTorr) for at least 12 h. Gas Phase Synthesis of Si NCs. Si NCs were prepared using a custom-built RF plasma reactor, the details of which have been described elsewhere18. Briefly, 30–60 standard cubic centimeters per minute (sccm) of 10% silane (SiH4) in helium was passed through a capacitively-coupled plasma at a pressure of 3.00 Torr in a quartz reactor tube with 7 mm inner diameter and 9 mm outer diameter. Argon (30–80 sccm) and hydrogen (0–60 sccm) flows were adjusted to modify the residence time and tune particle size and surface hydrogen coverage. A forward power of 75 W at 13.56 MHz was applied via an Advanced Energy Cesar 136 generator through an Advanced Energy VM1000 matching network (tuned to give a reflected power of 0–1 W) to a copper ring electrode. A grounded electrode was positioned downstream and separated by a 1.5 cm tall ceramic spacer. An Advanced Energy Z’Scan device was used to dynamically monitor the plasma conditions. Using the 1.5 cm spacing between the two electrodes as an estimate for the length of the plasma zone, the estimated range of particle residence times varied between 0.91–2.28 ms for total gas flow rates ranging from 150–60 sccm, respectively, based on an internal reaction tube diameter of 7 mm. NCs are created in the plasma through electron impact dissociation of SiH4 and subsequent clustering of the fragments. Hydrogen-terminated Si NCs were collected downstream from the plasma on a 400-mesh stainless steel filter and transferred via loadlock to an inert-atmosphere glove box for collection. A photograph of as-prepared Si NC samples is included in Fig. S1. Table 1 provides a summary of reaction parameters.



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Table 1. RF plasma reaction parameters for Si NC samples. A forward power of 75 W and a pressure of 3.00 Torr were used for all samples. Size was determined from a sizing curve with error bars from the PL peak FWHM energies fit to the sizing curve as described in the Results and Discussion. Size (nm)

Ar (sccm)

10% SiH4 in He (sccm)

H2 (sccm)

Residence Time (ms)

3.7 ± 0.30

60

30

60

0.91

4.7 ± 0.29

45

30

30

1.30

7.4 ± 0.24

30

30

0

2.28

X-ray diffraction (XRD). XRD measurements were performed on a Bruker D8 Discover X-ray Diffraction system with a 2.2 kW sealed Cu X-ray source. Spectra were acquired by depositing Si NC powder onto a glass slide and scanning over 2θ using a beam voltage and current of 40 kV and 35 mA, respectively. Thermally-Initiated Reaction. As-prepared hydrogen-terminated Si NCs (50–100 mg) were subjected to reactions in an inert-atmosphere glove box by combining with neat 1-dodecene (0.5–1.0 mL) in a vial (empty volume 15 mL) sealed with a Teflon-lined cap and heating to reflux (215 °C) for ~3 h. Successful reaction was observed by formation of a deeply colored translucent solution after 10– 15 min of heating. Once cool, toluene (5–10 mL) was added and the solution filtered through a 0.2 µm syringe filter to remove insoluble material. Acetonitrile (5–20 mL; 5 mL for the 7.4 nm sample, 20 mL for the 3.7 nm sample) was added resulting in a cloudy suspension, which was centrifuged at 12,000 × g for 3–10 min. Alcohols were avoided as the anti-solvent owing to the known reactivity of these species with silicon hydrides40. After decanting the supernatant, the NC solid was re-dissolved in toluene (5–10 mL) and the procedure was repeated once more. The resulting dark brown solid was dried under vacuum and stored under inert atmosphere.


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Radical-Initiated Reaction. Radical-initiated reactions were carried out in a manner identical to the thermal reaction with the following modifications: In addition to neat ligand, radical initiator 1,1'azobis(cyclohexanecarbonitrile) (ABCN, 5–10 mg) was added to the reaction mixture and heated to 100 °C for 18 h (for samples functionalized with 1-dodecene and 1-octene) or 75 °C for 26 h (for 1-hexene). These longer reaction times were employed since the 3 h used for thermally initiated reaction did not result in colloidal solutions. Purification was performed identically to the thermally initiated reaction. Control experiments revealed that solubility was not achieved with solely the ABCN initiator (heptadecane solvent) or neat 1-dodecene (no ABCN) at these temperatures. Transmission Electron Microscopy. High-resolution transmission electron microscopy (HRTEM) studies were performed in an FEI ST30 TEM operated at 300 kV. Samples were prepared by dropping dilute hexanes solutions of dodecyl-capped Si NCs onto ultrathin carbon film/holey carbon, 400 mesh copper TEM grids (Ted Pella Prod. No. 01824) coated with graphene (Electron Microscopy Sciences Prod. No. GF1200). Photoluminescence. Emission spectra were made from TCE solutions of alkyl-functionalized Si NCs in 1-cm quartz cuvettes with an optical density at the excitation wavelength of 0.2–0.4. Two systems were used to record emission spectra. Wavelengths from 535–1067 nm were recorded on a modified Horiba Jobin-Yvon Fluorolog 3 fluorescence system. Excitation at 420 nm was accomplished using a 450 W Xenon lamp, with wavelength selection provided by a double-grating spectrometer (Grating specs: 1200 grooves/mm; blazed at 500 nm), and fluorescence from the samples was detected with a liquid nitrogen-cooled CCD detector coupled to a single-grating iHR320 Imaging Spectrometer (Grating specs: 150 grooves/mm; blazed at 500 nm). Data from 750–1250 nm were collected on a QuantaMaster PTI fluorescence spectrophotometer using excitation from a Hg–Xe lamp (436 nm or 546 nm lines) passed through a monochromator and a band-pass filter. The excitation and emitted light were routed to



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a monochromator. A cooled InGaAs photodiode (750–1250 nm) was used for detection, and the resulting signal was amplified using a lock-in amplifier referenced to a chopper driver. Spectra were collected by scanning the wavelength range using a monochromator. For samples with emission that spans the ranges of the two detectors, the spectra were correlated at approximately 930 nm. All spectra were corrected for the response of the detection systems. Photoluminescence quantum yield data were obtained similarly with 546 nm Xe lamp excitation, but in this case the sample was placed inside an integrating sphere. Transmitted and emitted light was passed through a liquid light guide (Lumatec, Series 2000) before being routed to the CCD detector. Both the emission and excitation spectra for the Si NC sample as well as the excitation spectrum of a TCE blank in a matched 1-cm cuvette were obtained. The areas under the emission and excitation spectra were determined by fitting with a Gaussian using the Igor Pro software package. PLQY was calculated using the formula: Area(sample emission)/[Area(blank excitation) – Area(sample excitation)]. Samples were measured at three different concentrations to obtain average values with standard deviations. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). DRIFTS measurements on the 3.7 nm and 4.7 nm Si NC samples were performed on a Thermo-Nicolet 6700 FTIR spectrometer with a liquid nitrogen cooled MCT detector fit with high pressure/high temperature DRIFTS cell. A Thermo Spectra Tech Collector II (P/N 700-0042) adapter was used to modify the spectrometer for reflection measurements, and the DRIFTS sample holder was a Thermo Spectra Tech High Temperature/Vacuum Chamber (P/N 0030-103) with a ZnSe window. The DRIFTS spectra displayed in this article were acquired using 256 scans for the background and 128 scans for the sample scan from 500-6000 cm–1 with a resolution of 4 cm–1. Samples for DRIFTS were ground with dry KBr (

Silyl Radical Abstraction in the Functionalization of ... - ACS Publications

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