ARTICLE

Size-independent organosilane functionalization of siliconnanocrystals using Wilkinson’s catalystZhenyu Yang, Maike H. Wahl, and Jonathan G.C. Veinot

Abstract: A low-temperature size-independent method for modifying the surface chemistry of the silicon nanocrystal (SiNC)surface is reported. Wilkinson’s catalyst has been applied to accelerate the dehydrogenative coupling reaction between organosi-lane molecules and hydride-terminated SiNCs. Two strategies and multiple organosilanes were evaluated to study surfacemodification efficiency. During the investigations, it was determined that surface functionalization efficiency showed somedependence upon the organic modifier in question. The comparatively low reactivity of octadecyldimethyl silanes may resultfrom the formation of “Si–Rh–Si” intermediates hindered by the steric bulk of the silanes when reacted with activated SiNCs.Quenching of the SiNC-based photoluminescence is observed and the origin of this phenomenon has been attributed to thepresent of trace rhodium on SiNCs detected using X-ray photoelectron spectroscopy.

Key words: silicon nanocrystals, size-independent functionalization, Wilkinson’s catalyst, surface modification, dehydrocoupling.

Résumé : Une méthode a basse température indépendante de la taille servant a modifier la chimie de surface de nanocristauxde silicium (NCSi) est décrite dans le présent article. Le catalyseur de Wilkinson a été employé pour accélérer la réaction decouplage déshydrogénant entre les molécules d’organosilane et les NCSi composés d’hydrures terminaux. Deux stratégies et denombreux organosilanes ont été évalués pour étudier le rendement de modification de la surface. Au cours de cette étude, il aété déterminé que le rendement de fonctionnalisation de la surface présentait une certaine dépendance a l’agent modificateuren question. La réactivité relativement faible des octadécyldiméthylsilanes peut résulter de la formation d’intermédiaires« Si–Rh–Si », entravée par l’encombrement stérique des silanes lorsqu’ils réagissent avec les NCSi activés. L’extinction de laphotoluminescence associée aux NCSi est observée et l’origine de ce phénomène est attribué a la présence de traces de rhodiumsur les NCSi détectées a l’aide de la spectrométrie photoélectronique X. [Traduit par la Rédaction]

Mots-clés : nanocristaux de silicium, fonctionnalisation indépendante de la taille, catalyseur de Wilkinson, modification de lasurface, déshydrocouplage.

IntroductionSince their discovery 30 years ago, semiconductor nanocrystals,

or quantum dots (QDs), have received unprecedented attentionbecause of their unique physical and chemical properties.1–4 Ourunderstanding and the practical applications of these materialsare well developed.5–9 Despite their great potential, restrictionson the use of “status quo” QDs (e.g., CdSe) exist because of thewell-established cytotoxicity of heavy metal ions.10–12 These regu-lations are significant barriers to the widespread integration ofthese QDs into consumer products. Thus, the development ofalternative nontoxic QDs and methods for their derivatizationare of paramount importance. Silicon nanocrystals (SiNCs) arefavourable CdSe QD replacements because they exhibit compara-ble optoelectronic responses and silicon is abundant as well ascost effective and, most importantly, biocompatible.13–16

Numerous synthetic approaches providing well-defined SiNCshave been reported;17–22 each method has its own benefits anddrawbacks. The majority of these procedures involve formation ofhydride-terminated SiNCs that are prone to oxidation that canlead to unpredictable properties. To address this issue, varioussurface hydrosilylation (i.e., thermal, photochemical, and catalytic)procedures have been developed to modify particle surfaces.23–29

The resulting robust Si–C linkages render the surfaces reasonably

resistant to oxidation. While hydrosilylation provides effectivepassivation of SiNC surfaces,30 limitations do exist: example.g.,photo-induced hydrosilylation reactivity is size dependent31 andthermally induced hydrosilylation is limited to reagents with suit-ably high boiling points and can lead to formation of surfacebonded multilayers (i.e., oligomers).32 In this context, it is usefulto explore new methods for SiNC modification. Catalyst-mediateddehydrogenative coupling of silanes offers a potentially new ap-proach. Despite showing promise in the modification of bulk andporous silicon surfaces, similar procedures have not been appliedto the modification of freestanding SiNCs.33 Herein, we describefunctionalization of SiNC surfaces via silane dehydrocouplingusing Wilkinson’s catalyst.

Experimental section

Reagents and materialsHydrogen silsesquioxane (trade name Fox-17) was purchased

from Dow Corning Corporation (Midland, Michigan). Electronics-grade hydrofluoric acid (49% aqueous solution) was purchasedfrom J.T. Baker. Ethanol (reagent grade), toluene (reagent grade),phenylsilane (97%), octadecylsilane (97%), and tris(triphenylphos-phine) rhodium(I) chloride (i.e., Wilkinson’s catalyst) were pur-chased from Sigma Aldrich. Octylsilane (98%), dodecylsilane (98%),

Received 29 January 2014. Accepted 28 April 2014.

Z. Yang and J.G.C. Veinot. Department of Chemistry, University of Alberta, Edmonton, AB T6G 2G2, Canada.M.H. Wahl. Department of Chemistry, Technische Universität München, Lichtenbergstraße 4, 85748 Garching, Germany.Corresponding author: Jonathan G.C. Veinot (e-mail: jveinot@ualberta.ca).This article is part of a Special Issue dedicated to Professor Barry Lever in recognition of his contributions to inorganic chemistry across Canada and beyond.

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Can. J. Chem. 92: 1–7 (2014) dx.doi.org/10.1139/cjc-2014-0048 Published at www.nrcresearchpress.com/cjc on 1 May 2014.

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and octadecyldimetylsilane (98%) were purchased from Gelest Inc.All of the chemicals were used as received.

Preparation of oxide-embedded SiNCs (d = 3 nm)The solvent was removed from the stock hydrogen silsesquiox-

ane solution under vacuum to yield white solid powders. Hydro-gen silsesquioxane solid (approximately 4 g) was placed in aquartz reaction boat and transferred to a Lindberg Blue tube fur-nace and heated from ambient to a peak processing temperatureof 1100 °C at 18 °C min–1 in a slightly reducing atmosphere (5% H2 −95% argon). The sample was maintained at the peak processingtemperature for 1 h. Upon cooling to room temperature, the re-

sulting amber solid was ground into a fine brown powder using atwo-step process. First, the solid was crushed using an agate mor-tar and pestle to remove large particles. Further grinding wasachieved using a Burrell Wrist Action Shaker upon shaking withhigh-purity silica beads for 5 h. The resulting SiNC−SiO2 compos-ite powders were stable for extended periods and stored in stan-dard glass vials.

Preparation of oxide-embedded SiNCs (d = 5, 8, and 12 nm)Following mortar and pestle grinding (vide supra), 0.5 g of SiNC−

SiO2 composite containing 3 nm SiNCs was transferred to ahigh-temperature furnace (Sentro Tech Corp.) for further thermalprocessing under argon atmosphere. This procedure leads to par-ticle growth while maintaining relatively narrow particle size dis-tributions. In the furnace, the SiNC−SiO2 composite was heated toappropriate peak processing temperatures at 10 °C min to achievethe target particle size (i.e., 1200 °C for 5 nm NCs, 1200 °C for 5 nmNCs, and 1400 °C for 12 nm NCs). Samples were maintained at thepeak processing temperature for 1 h. After cooling to room tem-perature, the brown composites were ground using proceduresidentical to those noted above.

Liberation of SiNCsHydride-terminated SiNCs were liberated from the SiNC−SiO2

composites using hydrofluoric acid etching. An amount of 0.2 g ofthe ground SiNC/SiO2 composite was transferred to a polyethyl-ene terephthalate beaker equipped with a Teflon-coated stir bar.Ethanol (3 mL) and water (3 mL) were added under mechanicalstirring to form a brown suspension followed by 3 mL of 49%hydrofluoric acid aqueous solution. After 1 h of etching in sub-dued light, the suspension appeared orange−yellow. Hydride-terminated SiNCs were subsequently extracted from the aqueouslayer into approximately 30 mL of toluene by (i.e., 3 × 10 mL). TheSiNC toluene suspension was transferred into test tubes and theSiNCs were isolated by centrifugation at 3000 rpm.

Organosilane functionalization of SiNCs (Strategy A)After initial isolation (vide supra), hydride-terminated SiNCs

were resuspended in fresh toluene followed by a second centrifu-gation at 3000 rpm. After this second toluene wash, the particleswere isolated by centrifugation and the toluene supernatantwas discarded. Subsequently, the hydride-terminated SiNCs wereredispersed into approximately 20 mL of fresh toluene and

Fig. 1. FTIR spectra of 3 nm freshly etched hydride-terminatedSiNCs (I), reacted with Wilkinson’s catalyst (II), with octadecylsilane(III and VI), with octylsilane (IV and VII), with dodecylsilane (V andVIII), with octadecyldimethylsilane (IX), and with phenylsilane (X).Samples III, IV, and V were prepared following Strategy A, whilesamples VI, VII, VIII, IX, and X were prepared following Strategy B.

Fig. 2. XRD patterns of Si−SiO2 composites formed with theindicated prossessing temperature. Broad peaks at 2� = 22° may befrom the oxide and amorphous silicon.

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transferred into a dry 100 mL Schlenk flask equipped with a stirbar and attached to an argon-charged Schlenk line. Wilkinson’scatalyst (4.0 mg, 0.0043 mmol) was dissolved in 3.0 mL of drytoluene and transferred into the Schlenk flask containing theSiNCs. The reaction mixture was subjected to three freeze−pump−thaw cycles and subsequently maintained under an argon atmo-sphere. The mixture was stirred at room temperature for 20 minand the cloudy suspension gradually changed colour from orangeto brown.

Silane solutions were prepared upon dissolution of the silane ofchoice in 2.0 mL of toluene. The concentration of this solution waschosen such that the molar ratio of silane to catalyst in the finalreaction mixture would be 150:1. The resulting clear and colorlesssilane solution was added to the Schlenk flask containing thecatalyst−SiNC mixture and stirred for 5 min. Finally, the reactionmixture was heated to 50 °C and stirred for a minimum of 15 h. Toevaluate the scope of the reaction, five representative organosilanes(octylsilane, dodecylsilane, octadecylsilane, octadecyldimetylsilane,and phenylsilane) were investigated.

Organosilane functionalization of SiNCs (Strategy B)The pretreatment of hydride-terminated SiNCs was similar to

that described for Strategy A; however, the sequence of the pro-cedure differs. After initial isolation of hydride-terminated SiNCs,particles were redispersed into approximately 15 mL of fresh tol-uene and transferred into a dry 100 mL Schlenk flask equippedwith a stir bar attached to an argon-charged Schlenk line. In anargon-filled glovebox, two solutions were prepared. Solution 1:4.0 mg (0.0043 mmol) of catalyst was dissolved in 3.0 mL of dry

toluene. Solution 2: the silane of choice (0.64 mmol) was dissolvedin 2.0 mL of dry toluene. The two solutions were combined withstirring and the mixture gradually turned from dark red to lightyellow. The catalyst−silane solution was added to the SiNC suspen-sion in an argon atmosphere. The temperature was increased to50 °C and the reaction mixture was stirred for a minimum of 15 hafter which the reaction mixture was transparent and orange−brown depending upon the silane of choice.

Purification and separation of SiNCsFollowing functionalization (both strategies), the reaction mix-

ture was divided equally amongst four centrifuge tubes. Approx-imately 35 mL of ethanol was added into each tube to yield a paleyellow precipitate that was isolated by centrifugation in a high-speed centrifuge at 14 000 rpm for 0.5 h. The supernatant wasdecanted and the precipitate redispersed in a minimum amountof toluene. The centrifugation/decanting procedure was repeatedtwice. Finally, the purified functionalized SiNCs were redispersedin toluene, filtered through a 0.45 �m PTFE syringe filter, andstored in vials.

Material characterization and instrumentationTransmission electron microscopy (TEM) and energy dispersive

X-ray (EDX) analyses were performed using a JEOL-2010 (LaB6filament) electron microscope with an accelerating voltage of200 keV. TEM samples of SiNCs were drop-cast onto a holeycarbon-coated copper grid (SPI Supplies) and the solvent wasevaporated in a vacuum. X-ray powder diffraction (XRD) was per-formed using an INEL XRG 3000 X-ray diffractometer equipped

Fig. 3. Bright-field TEM images of octadecylsilane-functionalized SiNCs of various sizes: (a) 3 nm, (b) 5 nm, (c) 8 nm, and (d) 12 nm.

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with a Cu-K� radiation source (� = 1.54 Å). Photoluminescence (PL)spectra were obtained upon irradiating a quartz vial containingthe toluene solution of SiNCs in question with a 441 nm line of aGaN laser. Emitted photons were collected with a fiber optic con-nected to an Ocean Optics USB2000 spectrometer. FTIR spectros-copy was performed on powder samples using a Nicolet Magna750 IR spectrophotometer. X-ray photoelectron spectroscopy (XPS)was acquired in energy spectrum mode at 210 W using a KratosAxis Ultra X-ray photoelectron spectrometer. Samples were pre-pared as films drop-cast from solution onto a copper foil sub-strate.

Results and discussionFor the present study, five organosilanes (i.e., octylsilane, dodecylsi-

lane, octadecylsilane, octadecyldimethylsilane, and phenylsilane)were evaluated as substrates for Wilkinson’s catalyst mediateddehydrocoupling of SiNC surfaces. Two different surface modifi-cation strategies were employed: Strategy A: catalyst activation ofSiNC surfaces following addition of the silane in question andStrategy B: addition of hydride-terminated SiNCs to activated mo-lecular silanes. The following discussion will address observationsmade for these two approaches.

All Strategy A reactions involving primary alkyl silanes yieldedtransparent brown−orange solutions after 1.5 h of heating. Follow-ing purification and isolation, FTIR analyses of the products fromthese reactions showed spectral signatures consistent with theformation of primary silane functionalized SiNC surfaces (Fig. 1).A typical FTIR spectrum of hydride-terminated SiNCs shows twodistinctive signals at approximately 2100 and 850 cm−1 that arereadily assigned to Si–Hx stretching and scissoring, respectively.31

These signals were not present when evaluating the FTIR spec-trum of SiNCs activated with Wilkinson’s catalyst. Instead, com-paratively weak signals are noted at 2850–3100, 1450–1500, and1170 cm−1, which are attributable to phenyl ring C–H stretching,C–C skeletal vibration, and P–C vibration between the phos-phorus atom and the phenyl rings of the catalyst. Following reac-tion with organosilanes, intense absorptions characteristic ofalkyl chains (i.e., 2650–2900 and 1420 cm−1) and Si–H stretching onsilyl groups (i.e., 2170 and 920 cm−1) appear. These observationssupport the proposal that successful surface modification hasbeen achieved. In addition, absorptions are noted at approxi-mately 1100 cm−1, which are readily attributed to Si–O–Si stretch-ing. The intensity of these Si–O–Si-based absorptions dependsupon the silane used and is consistent with some surface oxida-tion. While the origin of these oxidized surfaces is surely complex,one source may be Si–Cl bonds arising from the reductive elimi-nation from the rhodium catalyst that subsequently react withethanol, trace water, or air.34

In contrast, even after 24 h of heating, reaction mixtures con-taining phenyl or octadecyldimethyl silane remained cloudy anddark brown or orange, respectively. FTIR analysis of these samplesfollowing purification only showed intense oxide signals, suggest-ing that SiNC surfaces were not modified. To our knowledge, ter-tiary silanes have not been investigated for the modification ofsilicon surfaces; phenylsilane and its derivatives have been re-ported to attach via surface dehydrocoupling.33 Having confirmedthe formation of Si–Rh linkages as the first step of Strategy A (videsupra), it is reasonable that the larger steric bulk of the tertiarysilanes significantly influences the formation of the necessary“Si–Rh–Si” intermediate structure.

To evaluate this proposal, we implemented Strategy B in whichorganosilanes are mixed with Wilkinson’s catalyst at room tem-perature under inert argon atmosphere before addition to SiNCs.Experimental observations differ substantially from those madefor reactions performed by Strategy A. The catalyst solution intoluene appeared dark red; addition of molecular silanes affordedan immediate colour change to persistent light yellow, consistent

with the formation of a silane-containing catalyst adduct.34 Inaddition, a colourless gas (H2) is released when primary silanes areemployed. Upon addition of the activated silane to a SiNC suspen-sion followed by heating at 50 °C, the appearance of the reactionmixture shifted from a dark yellow cloudy suspension to a darkred transparent solution over 1.5–2 h. To our surprise and consis-tent with effective surface modification, all silane−SiNC dehy-drocoupling reactions performed using Strategy B afforded

Fig. 4. (a) EDX and (b and c) XPS data of SiNCs functionalized withoctadecylsilane. Original and fitted data are indicated by the blackcurve and circles, respectively. Fitting components are only shownfor the silicon spectrum with Si 2p3/2 signals. The Si 2p1/2 signalshave been removed for clarity.

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transparent solutions. The FTIR analysis of the product of thereaction with phenyl silane clearly showed phenyl ring C–H andC–C vibrations at 2800–3150 and 1300–1600 cm−1 (Fig. 1b, X).Similarly, the FTIR spectrum of purified octadecyldimethylsilyl-functionalized particles (Fig. 1b, IX) shows no evidence of Si–Hvibrations in the 2000–2200 cm−1 region and the appearance ofC–H vibrations at 2850–3100 cm−1 indicating complete reaction ofsurface hydrides and the removal of excess silane. A broad signalat approximately 3150–3450 cm−1 might be assigned to residualethanol. As one might anticipate from other surface modificationprotocols, we note more intense Si–O–Si signals in the FTIR anal-ysis of SiNCs bearing bulky surface groups (i.e., phenyl andoctadecyldimethyl), suggesting that the steric bulk of thesegroups leads to inefficient surface passivation. To investigate anypossible SiNC size dependence of this reaction, we functionalizedSiNCs of d = 5, 8, and 12 nm (see Figs. 2 and 3 for XRD and TEManalysis) using Strategies A and B and observed identical reactivitytrends.

EDX analysis (Fig. 4a) shows that purified octadecylsilane-functionalized SiNC samples only contain silicon, carbon, andoxygen. Neither phosphorus nor rhodium was detected at thesensitivity of EDX, suggesting no, or at least limited, catalyst con-tamination. However, XPS spectra (see Fig. 4) show the presence ofvery low rhodium concentrations (i.e., approaching the detectionlimit of the method). The low-intensity signal located at approxi-mately 304–308 eV is slightly lower energy that than characteris-tic of the 3d5/2 emission from rhodium in pure Wilkinson’scatalyst,35 suggesting the possible presence of “Rh−Si” adduct(s)that could reasonably be anchored on SiNC surfaces. Multicompo-nent silicon 2p emissions are found between 99 and 104 eV(Fig. 4c). The emission noted at 99.3 eV is attributed to elementalsilicon, while other features (i.e., 100.3, 101.3, 102.4, and 103.4 eV)are assigned to surface suboxides (SiOx, 0 < x ≤ 2) and Si–C bond-ing. Previous reports regarding alkyl-functionalized SiNCs32 indi-cated that suboxide and Si–C features dominate the silicon 2pregion of the SiNC XP spectrum; this is also expected for thepresent SiNCs in light of the presence of unsaturated R−Si(Hx)–SiNC surface linkages as well as unsaturated surface silicon atoms.

General hydrosilylation approaches have proven effective inpreserving SiNC PL.36 To evaluate the impact of the presentWilkinson’s catalyst mediated surface dehydrocoupling on SiNCPL, we compared the response of these SiNCs with that of alkyl-modified SiNCs obtained from thermal hydrosilylation directly.Unfortunately, the PL of the SiNCs was quenched after dehy-drocoupling modification (Fig. 5), while identical 3 nm dodecyl-functionalized SiNCs obtained from thermal hydrosilylationprocedures show characteristic red luminescence (� = 734 nm) (seeFig. 5c). Similar quenching was also noted for larger particles (notshown), indicating that this effect is not size dependent. In light ofthe trace rhodium-containing surface adducts detected by XPS, itis reasonable that these species quench the SiNC PL.

In the context of the observations herein, we propose two inde-pendent mechanisms of dehydrocoupling on the SiNC surfacefollowing Strategies A and B (see Schemes 1 and 2). In Strategy A,Si–H bonds on SiNC surfaces react with Wilkinson’s catalyst torelease triphenylphosphine to form surface Si–Rh–Cl intermedi-ates that further react with molecular silanes forming “Si–Rh–Si”structures.34 Subsequently, solution-borne PPh3 attacks the inter-mediate surface, liberating HRh(PPh3)3 and forming a new Si–Sibond. The freed HRh(PPh3)3 then reenters the catalytic cycle.34

Because the Si–Rh intermediates are bonded to the SiNC surfaces,the progress of the reaction relies upon the accessibility of theSi–Rh–Si surface species. As the reaction proceeds, attack of PPh3

to liberate HRh(PPh3)3 is hindered by adjacent surface groupsleading to incomplete modification and residual surface rhodiumthat account for the PL quenching. Similarly, bulky molecularsilanes such as octadecyldimethyl silanes cannot access the Rh–Clbond to form the necessary “Si–Rh–Si” intermediate that leads tosurface linking. The limited surface modification upon reactionwith phenylsilane is more complex because of known redistribu-tion reactions that are expected to produce Ph2SiH2 and Ph3SiHthat could limit surface attachment.37

In contrast, for Strategy B, Si–Rh intermediates are formedupon activation of molecular silanes. Dehydrocoupling reactionsof organosilanes using Wilkinson’s catalyst have been widelystudied and our experimental observations (e.g., elimination of

Fig. 5. (a and b) Images of toluene dispersion of 3 nm dodecyl-functionalized and dodecylsilyl-functionalized SiNCs under visible light and365 nm UV irradiation and (c) their corresponding PL spectra.

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colourless gas, solution colour change) are consistent with forma-tion of organosilane oligomers upon addition of the catalyst. Si-multaneously, Rh–silyl complexes will form (see B in Scheme 2)that can afford subsequent surface modification (vide infra). Theaddition of hydride-terminated SiNCs facilitates the dehydrocouplingreaction between surface hydrides and solution-borne activatedsilane monomers/oligomers yielding silyl-functionalized SiNCs. Itis reasonable that formation of oligomers facilitates reaction withphenylsilane-based species leading to surface modification. Simi-lar to the proposal for Strategy A, some Si−Rh−Si species willremain and will quench the SiNC PL.

An important question remains: why does a tertiary silane reactwith the particle surface? Numerous reports have demonstratedthat the –SiH2 group is essential to dehydrocoupling reactionscatalyzed by Wilkinson’s catalyst,37–43 and few examples of ter-tiary silane coupling may be found for molecular systems.42

However, Ojima et al. found that the dehydrocoupling reactionproceeds slowly when a mixture of disilane and trisilane is em-ployed.40 For the present system, freshly etched SiNCs have mul-tiple surface hydrides (i.e., Si–Hx where x = 1, 2, 3) as evidencedby the Si−H stretching region in the FTIR. Hence, the hydride-terminated SiNC surface can be reasonably considered as the ensem-ble of tertiary, secondary, and primary silanes. In this context, thesurface hydride/tertiary silane dehydrocoupling reaction is expectedto proceed, albeit more slowly, and it would only lead to partialpassivation, which is supported by the comparatively intense oxidesignal in the FTIR spectrum for the octadecyldimethylsilane-modified SiNCs.

ConclusionWe have reported a facile low-temperature size-independent

route for modifying air-sensitive hydride-terminated SiNCs byorganosilane dehydrocoupling reactions using Wilkinson’s cata-lyst. Multiple organosilanes were investigated and two strategieswere applied. It was found that surface modification efficiencydepended upon the silane structure and strategy employed. Wepropose that the comparatively low reactivity of octadecyldimethyl-silane arises because the formation “Si–Rh–Si” intermediates ishindered by the steric bulk of the silane when reacted with acti-vated SiNCs. In addition, we propose that steric interactions ofsurface groups limit release of surface-bonded rhodium speciesthat quench the SiNC PL. Despite the lack of PL response, thepresent SiNCs hold promise in non-light-emitting applications(e.g., battery electrodes, printable electronics, and photovoltaics).Ongoing work is aimed at recovering the SiNC PL response and

investigating potential applications of the organosilane-functionalizedparticles.

AcknowledgementsThe authors acknowledge funding from the Natural Sciences

and Engineering Research Council of Canada, Canada Foundationfor Innovation, Alberta Science and Research Investment Pro-gram, and University of Alberta Department of Chemistry. Wewould like to thank W.C. Moffat, M. Skjel, and M. Hoyle for assis-tance with FTIR spectroscopy analysis. G. Popowich is thanked forassistance with TEM and selected area electron diffraction analy-sis. M.H.W. is greatful for the assistance from the exchange pro-gram operated by Technische Universität München Departmentof Chemistry and University of Alberta International Office. Thestaff at the Alberta Centre for Surface Engineering and Sciences isthanked for XPS analysis. All Veinot Team members are alsothanked for useful discussion.

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Scheme 1. Proposed mechanism of SiNC surface modification with organosilanes following Strategy A.

Scheme 2. Proposed mechanism of SiNC surface modification with organosilanes following Strategy B.

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