Direct Conversion of Hydride- to Siloxane ... - ACS Publications

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Direct Conversion of Hydride- to Siloxane-Terminated Silicon Quantum Dots Ryan T. Anderson,† Xiaoning Zang,‡ Roshan Fernando,§ Michael J. Dzara,†,§ Chilan Ngo,§ Meredith Sharps,§,⊥ Rebecca Pinals,§,# Svitlana Pylypenko,†,§ Mark T. Lusk,‡ and Alan Sellinger*,†,§,∥ †

Materials Science Program, ‡Department of Physics, and §Department of Chemistry, Colorado School of Mines, Golden, Colorado 80401, United States ∥ National Renewable Energy Laboratory (NREL), Golden, Colorado 80401, United States ⊥ Department of Chemistry, Emory University, Atlanta, Georgia 30322, United States # Department of Chemical and Biochemical Engineering, Brown University, Providence, Rhode Island 02912, United States S Supporting Information *

ABSTRACT: Peripheral surface functionalization of hydrideterminated silicon quantum dots (SiQD) is necessary in order to minimize their oxidation/aggregation and allow for solution processability. Historically thermal hydrosilylation addition of alkenes and alkynes across the Si−H surface to form Si−C bonds has been the primary method to achieve this. Here we demonstrate a mild alternative approach to functionalize hydride-terminated SiQDs using bulky silanols in the presence of free-radical initiators to form stable siloxane (∼Si−O−SiR3) surfaces with hydrogen gas as a byproduct. This offers an alternative to existing methods of forming siloxane surfaces that require corrosive Si−Cl based chemistry with HCl byproducts. A 52 nm blue shift in the photoluminescent spectra of siloxane versus alkyl-functionalized SiQDs is observed that we explain using computational theory. Model compound synthesis of silane and silsesquioxane analogues is used to optimize surface chemistry and elucidate reaction mechanisms. Thorough characterization on the extent of siloxane surface coverage is provided using FTIR and XPS. TEM is used to demonstrate SiQD size and integrity after surface chemistry and product isolation.

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INTRODUCTION In recent years quantum dot materials have become the subject of intense research interest in a number of areas including photovoltaics,1,2 optoelectronics,3−5 and biological sensing.3,6,7 Quantum dots have been synthesized from a variety of semiconductor constituents, but those of silicon are of special interest due to silicon’s natural abundance, relatively low cost, ease of processing, and nontoxic and biological compatibility.8 Silicon quantum dots (SiQDs) can be synthesized by a number of routes, including top-down methods requiring the decomposition of bulk silicon or hydrofluoric acid (HF) etch treatment of silicon-rich oxides. Other methods include bottom-up approaches via plasma enhanced chemical vapor deposition (PECVD) and solution methods from smallmolecule precursors.6,9−13 A common and relatively simple top-down method involves the reductive thermal decomposition of hydridosilicate glasses (produced from the cocondensation of trichlorosilane and water) and subsequent HF etching of the SiOx matrix with an ethanolic solution to yield a dispersion of hydride-terminated SiQDs.14−16 While these hydride-terminated SiQDs are luminescent, their usefulness can be greatly enhanced by surface modifications to increase their ambient stability,17 solubility,3,17,18 and quantum yield10,19 or make them suitable for biological applications.6 © XXXX American Chemical Society

One surface modification of interest that has been studied and applied for a number of years on bulk silicon surfaces is passivation with siloxane moieties.20,21 More recently, this approach has been applied to SiQDs17,22 and has shown promise for optoelectronic applications.23 In addition, siloxane termination of SiQDs increases solubility for solution processing and may reduce nonradiative processes,24 potentially making them particularly attractive for photovoltaic applications. Siloxane termination is also more hydrolytically stable than the corresponding silicon−oxygen−carbon bond derived from alcohols22,25 that are the active functional groups for sol− gel processing and thus very susceptible to hydrolysis. To date, existing methods for the passivation of SiQDs with siloxanes involve the use of toxic and highly reactive reagents,11,22 producing reactive intermediate products, such as silanol17,26 or silyl halide11,22,27 terminated SiQDs and generating toxic, corrosive hydrogen halide gas as a byproduct. We report here a mild process to prepare siloxane-functionalized SiQDs directly from silanols and hydride-terminated Received: August 5, 2016 Revised: October 18, 2016 Published: October 20, 2016 A

DOI: 10.1021/acs.jpcc.6b07930 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Scheme 1. Model Reaction Products (3 and 6), a Control Dodecyl-SiQD (8), Direct Hydride to Triethylsiloxy-SiQD (9), and a Mixed Product Having Both Dodecyl- and Siloxy-Functionalized Surface (10)

Model Reactions. Compound 3 (Scheme 1) was prepared in a manner similar to that described elsewhere.28 Into a 50 mL Schlenk flask (in a nitrogen-filled glovebox) was charged triethylsilanol (2) (0.5 mL, 3.3 mmol), triphenylsilane (1) (0.87 g, 3.3 mmol), Pd/C (10% on dry support, 85 mg), and anhydrous mesitylene (10 mL). The mixture was removed from the glovebox and attached to a Schlenk line, heated under argon at 150 °C for 44 h, and purified by silica column in hexanes:dichloromethane (9:1) to give 3 as a clear, colorless oil (0.49 g, isolated yield 37%). 1H NMR (CD2Cl2): δ 0.62 (q, 6 H, CH2), 0.93 (t, 9 H, CH3), 7.44 (m, 9 H, ArH), 7.64 (d, 6 H, ArH). 13C NMR (CD2Cl2): δ 6.40 (s, 3 C, CH3), 6.77 (s, 3 C, CH2), 127.67 (s, 6 C), 129.68 (s, 3 C), 134.97 (s, 6 H), 136.32 (s, 3 C). GC-MS: (m/z) found 363.2, calcd 390.2 (difference due to loss of ethyl group in MS). Compound 6 was prepared in a manner similar to 3. Into a two-necked round-bottom flask (in a nitrogen-filled glovebox) was charged 4 (0.53 g, 0.55 mmol), tert-butyldimethylsilanol (5) (0.1 mL, 0.60 mmol), Pd/C (10% on dry support, 90 mg), and anhydrous mesitylene (10 mL). The mixture was removed from the glovebox, attached to a Schlenk line, and refluxed

SiQDs bypassing the toxic and corrosive conditions used previously.

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EXPERIMENTAL SECTION Materials. Triphenylsilane (97%), toluene (99.8%, anhydrous), tert-butyldimethylsilanol (99%), methanol (ACS reagent, 99.9%), Pd/C (10% on dry carbon), 1,1′-azobis(cyclohexanecarbonitrile) (ACHN) (98%), acetonitrile (HPLC grade), and hexanes (ACS reagent) were used as purchased from Sigma-Aldrich. Triethylsilanol (>97%) was used as purchased from Gelest, and 1-dodecene (>95%) was used as purchased from TCI America. Monohydridoheptaphenyl POSS was purchased from Hybrid Plastics and used as received. Dichloromethane (>99.9%, ChromaSolv) was used as purchased from Macron. 1,3,5-Trimethylbenzene (mesitylene) (98%) was purchased from Sigma-Aldrich, purified by fractional distillation, dried over molecular sieves, and sparged with argon prior to use to remove oxygen. Oxide-embedded silicon quantum dots were prepared in our laboratories as described elsewhere.14 B


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under argon via a syringe and continued stirring at 150 °C for 1 day. After cooling to room temperature, the reaction mixture was filtered to remove insoluble ACHN byproducts. Then the filtrate was concentrated by rotary evaporation, and the dots were precipitated in acetonitrile, centrifuged, washed several times with acetonitrile until supernatant was clear, and dried at 50 °C under vacuum for overnight. The product (10) was obtained as a light brown solid (78 mg). Numerous attempts to synthesize compound 9 using a Pd/C catalyst were attempted as follows. To a 25 mL Schlenk flask (in a nitrogen-filled glovebox) was charged 7 (approximately 20 mg), silanol (0.8 g, 7 mmol), Pd/C (10% on dry support, 40 mg), and anhydrous mesitylene (9 mL). The dots were dispersed by sonication, and the mixtures were heated under argon at 160 °C for 72 h. While there was some evidence to indicate that the desired products were produced, the reactions yielded only very small amounts of product, and the reactions were not reproducible with any degree of reliability. This method was abandoned in favor of the ACHN free radical method described above. Characterization. Carbon and proton nuclear magnetic resonance (NMR) spectra were obtained with a JEOL 500 MHz instrument. Fourier transform infrared (FT-IR) spectra were recorded with a Thermo Scientific Nicolet iS50 FT-IR spectrometer using an ATR attachment. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were recorded in chloroform solutions with a Horiba NanoLog spectrofluorometer, equipped with a single-channel photomultiplier tube (PMT) detector (FL-1073) and a low-pass, 375 nm filter. Ultraviolet/visible (UV/vis) spectra were recorded in chloroform solution using a Beckman Coulter DU800 spectrophotometer. Mass spectra were obtained with a Varian CP-3800 gas chromatograph, equipped with a 1200L quadrupole MS/MS detector and a Restek 5% diphenyl/95% dimethylpolysiloxane column. Thermal gravimetric analysis (TGA) was done using a TA Instruments Q500. XPS analysis was performed on a Kratos Nova X-ray photoelectron spectrometer using a monochromatic Al Kα source operated at 300 W. Survey and high-resolution spectra of C 1s, O 1s, N 1s, and Si 2p were acquired for a minimum of three areas per sample at pass energy of 160 and 20 eV using charge neutralizer. Linear background subtraction, smoothing, charge referencing, and curve fitting were performed using CasaXPS software. Each Si peak contains 2p3/2 and 2p1/2 components constrained to have ratio of 0.67 and separation by 0.6 eV. The fitting parameters are provided in the Supporting Information. Transmission electron microscopy (TEM) and energydispersive spectroscopy (EDS) analyses were performed on an FEI Talos F200X TEM with ChemiSTEM detector. Measurements of particle size were made via ImageJ. Computational Approach. All calculations were carried out using density functional theory (DFT) as implemented in the DMOL package.29 An all-electron approach was used with exchange and correlation effects accounted for by the generalized gradient approximation (GGA) parametrized by Perdew, Burke, and Ernzerhof (PBE)30 for SiQDs and hybrid B3LYP for organic molecules.31 A real-space, double numeric plus polarization (DNP) basis was used along with an octupole expansion specify the maximum angular momentum function.29 Systems were optimized under the following convergence criteria: the energy difference between successive configurations was less than 2 × 10−5 Ha, the maximum force on any atom

under argon at 130 °C for 48 h. The product was purified by precipitation in methanol to give 6 as a white solid (0.41 g, yield 68%). 1H NMR (CD2Cl2): δ 0.00 (s, 6 H, SiCH3), 0.79 (s, 9 H, SiC(CH3)3), 7.34 (m, 14 H, ArH), 7.41 (t, 7 H, ArH), 7.72 (t, 14 H, ArH). 13C NMR (CD2Cl2): δ −3.56 (s, 2 C, SiCH3), 17.91 (s, 1 C, SiC(CH3)3, 25.40 (s, 3 C, SiC(CH3)3, 127.80 (s 14 C), 130.19 (s, 7 C), 130.74 (s, 7 C), 134.20 (s, 14 C). Etching of SiQDs in from SiO2 Matrix. Oxide-embedded silicon quantum dots were prepared as described elsewhere,14 ground to a fine powder in a mortar and pestle, and etched (in the dark) in 4:1 48% HF:ethanol for 5 h. The resulting hydrideterminated SiQDs (7) were extracted into toluene, centrifuged, washed three times with methanol (until the supernatant was pH-neutral), dried at 60 °C under vacuum for 12 h, and stored in a nitrogen glovebox. Synthesis of Dodecyl-Terminated SiQDs (8). For use as a reference material, 8 was prepared as follows. Hydrideterminated SiQDs (7) (50 mg) were suspended in anhydrous toluene (10 mL) by mild sonication for 60 min. In a nitrogenfilled glovebox, this suspension was transferred into a 50 mL Schlenk flask, and 1-dodecene (4 mL/3.03 g/18 mmol), ACHN (700 mg, 2.86 mmol), and anhydrous toluene (10 mL) were added inside the glovebox. The mixture was removed from the glovebox and attached to a Schlenk line and heated under argon at 100 °C for 3 days. After cooling to room temperature, the reaction mixture was filtered to remove insoluble ACHN byproducts. Then the filtrate was concentrated by rotary evaporation, and the dots were precipitated in acetonitrile, centrifuged, washed several times with acetonitrile until supernatant was clear, dried at 50 °C under vacuum overnight, and then stored in a N2-filled glovebox. The product (8) was obtained as a brown paste (182 mg). Synthesis of Triethylsiloxy-Terminated SiQDs (9). This was synthesized in a similar procedure to the synthesis of 8 using the same batch of hydride-terminated dots. Hydrideterminated SiQDs (7) (50 mg) were suspended in anhydrous mesitylene (10 mL) by mild sonication for 60 min. In a nitrogen-filled glovebox, this suspension was transferred into a 50 mL Schlenk flask, and triethylsilanol (8 mL (6.91 g, 52.3 mmol), ACHN (700 mg, 2.86 mmol), and anhydrous mesitylene (10 mL) were added inside the glovebox. The mixture was removed from the glovebox and attached to a Schlenk line and heated under argon at 150 °C for 3 days. After cooling to room temperature, the reaction mixture was filtered to remove insoluble ACHN byproducts. Then the filtrate was concentrated by rotary evaporation, and the dots were precipitated in acetonitrile, centrifuged, washed several times with acetonitrile until supernatant was clear, and dried at 50 °C under vacuum overnight. The product was obtained as a light brown solid (75 mg). Synthesis of Triethylsiloxy/Dodecyl-Terminated SiQDs (10). This was synthesized in a similar procedure to the synthesis of 8 using the same batch of hydride-terminated dots. Hydride-terminated SiQDs (7) (50 mg) were suspended in anhydrous mesitylene (10 mL) by mild sonication for 60 min. In a nitrogen-filled glovebox, this suspension was transferred into a 50 mL Schlenk flask, and triethylsilanol (2) (8 mL (6.91 g, 52.3 mmol), ACHN (700 mg, 2.86 mmol), and anhydrous mesitylene (10 mL) were added inside the glovebox. The mixture was removed from the glovebox and attached to a Schlenk line and heated under argon at 150 °C for 3 days. Then 1-dodecene (2 mL/9 mmol) was added to the reaction mixture C


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The Journal of Physical Chemistry C was less than 2 × 10−3 Ha/Å, and the maximum atom displacement was less than 5 × 10−3 Å. A ΔSCF method32−34 was used for calculating the optical HOMO and LUMO levels. The ΔSCF method was also used to calculate reorganization energies,35 and these show good agreement with both experiment and other more sophisticated computational methods, such as quantum Monte Carlo (QMC) and timedependent density functional theory (TDDFT). For an Nelectron system, instead of using the lowest N orbitals to construct density as in DFT, ΔSCF uses the lowest N − 1 orbitals combined with the (N + 1)th orbital to construct density. In this way, it can estimate the excitation energy with the relatively low computational cost of DFT. In all calculations, the SCF density was deemed to be converged when the norm of the density of two successive SCF iterations was less than 1 × 10−5. After performing the ΔSCF analysis, HOMO and LUMO orbitals of the system were used to calculate electron−hole coulomb interaction:

unambiguous experimental optical gaps in their vapor phase. This approach was taken since there are no experimental measurements available for the small SiQDs sizes of our computation. The optical gap of benzyl chloride is 4.60 eV (37 119.5 cm−1),37 and we obtained 4.57 eV (36 839.4 cm−1) using ΔSCF. The optical gap of fluorobenzene is 4.69 eV (37 818.8 cm−1),38 and we obtained 4.68 eV (37 715.4 cm−1) using ΔSCF. Finally, the optical gap of chlorobenzene is 4.59 eV (37 052.0 cm−1),38 and we obtained 4.74 eV (38 233.8 cm−1) using ΔSCF.

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RESULTS AND DISCUSSION Synthesis and Characterization. In an effort to determine the optimal conditions for the silyl hydride to siloxy transition in the SiQDs, two model reactions (Scheme 1) were conducted, and the resulting products were characterized by NMR, GC-MS, and FT-IR (see Supporting Information). The two silyl hydride model reactants selected were triphenylsilane (Scheme 1, compound 1) and monohydridoheptaphenyl POSS (Scheme 1, compound 4). These were chosen primarily for their hindered hydrides that may mimic the hydrides on the SiQD surface. While the triphenylsilane reactant does provide a hindered hydride, it is also capable of participating in reaction mechanisms (such as an SN2 inversion of stereochemistry) that would be impossible at the SiQD surface. For this reason, monohydridoheptaphenyl POSS was also chosen as a model reactant that more closely mimics the surface of a SiQD, provides a hindered hydride, and only allows for reaction at a single site with no chance for backside SN2 inversion. Two silanol passivation agents, triethylsilanol and tert-butyldimethylsilanol (Scheme 1, compounds 2 and 5), were selected for reaction with both the model substrates and with the SiQDs in order to test the viability of the methods at different reaction temperatures and with different amounts of steric hindrance as well as to ascertain what effect, if any, the identity of the passivation group has on the optical properties of the passivated SiQDs. As a result of the existing limitations and challenges for the passivation of SiQDs with silanols, we attempted reactions using three different existing methods: (1) homogeneous Pd catalysts,28 (2) Lewis acid catalysts,39−41 and (3) heterogeneous palladium on carbon (Pd/C) catalysts.42 The first method was unsuccessful for all compounds, and the second method was successful only with compound 3. The third method which worked with both small molecules, however, was unsuccessful with SiQDs. The fourth, and most successful, method attempted utilized the thermal free radical initiator 1,1azobis(cyclohexanecarbonitrile) (ACHN).9,43 This method requires the simple, low-temperature heating of hydrideterminated SiQDs with silanols (or alcohols) in the presence of a stoichiometric amount of ACHN, resulting in siloxaneterminated SiQDs and hydrogen gas as a byproduct. This method has the additional advantages of a simple purification process (filtering, followed by precipitation and washing of the products with acetonitrile), toleration of SiQDs passivation by less-stable Si−O−C moieties (minimizing hydrolysis or exchange to methyl groups),22,25 and elimination of the possibility of contamination by residual palladium. Reactions utilizing several other catalytic systems, as well as the two previously mentioned model reactions, were also performed; while these reaction systems were not successful in the passivation of SiQDs, they have provided useful insight into the reaction mechanisms and limitations at the SiQD surface.

2

J=

∬ dr dr′ ψH*(r)ψH(r) |r −e r′| ψL*(r′)ψL(r′)

The exciton binding energy was then estimated to be

Eb =

J ε

where ε is the dielectric constant of SiQDs as estimated using the generalized Penn model.36 It was further assumed that the dielectric constant does not change for dots of the same size but with different terminations. The energy shift, ΔE, was calculated as follows: EOPTGap = E LUMO − E HOMO ,

E TGap = EOPTGap + E b

ΔE = EOPTGap ,OSiH3 −EOPTGap,H = ΔE TGap − ΔE b

Here ETGap is the transport gap, EOPTGap is the optical gap, and Eb is the exciton binding energy. SiQDs were computationally created by carving spheres from bulk diamond silicon and then removing any singly bonded Si atoms. The dots were then hydrogen passivated and subjected to geometry optimization. All SiQDs after optimization had Td symmetry. The siloxane-terminated QDs were created by replacing a fraction of H with SiH3 to obtain a surface coverage of approximately 50% and then geometry optimizing once again. Full coverage (all H replaced by SiH3) is impossible, and the larger the coverage, the stronger the effect of siloxane termination. The SiQDs are extremely small, and their electro-optical properties are extremely sensitive to dot diameter. We calculated an effective radius as the average of the surface Si atoms from the dot center without considering H or SiH3. The standard deviation in these radii was used to create the error bars for the uncertainty in dot diameter that are shown in Figures 6−8. This was deemed to be the most appropriate measure of computational error. In comparison, the numerical error of ΔSCF is set by the convergence criteria for energy: 2 × 10−5 Ha. This is 4 orders of magnitude less than the transport and optical gap measures, 3 orders of magnitude less than the exciton binding energy, and 2 orders of magnitude less than the smallest energy shift. We also considered the inherent accuracy of our ΔSCF analysis by examining several small organic molecules that have D


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The Journal of Physical Chemistry C NMR and GC-MS results indicate that the model reactions produced the desired products (Scheme 1, compounds 3 and 6; see Supporting Information for characterization spectra). The FT-IR for compound 6 (and Supporting Information Figure S5) indicate stretching modes for C−H bonds around 2800− 3000 cm−1 and a distinctive stretch mode for Si−O−Si bonds around 1050 cm−1.26 The magnitude of the Si−O−Si stretch mode for 6 is larger than that of 3, as would be expected for a POSS cage material with multiple sets of Si−O−Si bonds. The FT-IR spectra of SiQDs (Scheme 1, compounds 7−10) are presented in Figure 1 and Figure S6. The hydride-terminated SiQD (compound 7) shows strong Si−Hx (x = 1−3) stretching modes between 2030 and 2160 cm−1.

appears that is likely due to the triethylsilanol reacting with Si− H2 and Si−H3 functional groups to form Et3SiO−Si−H and (Et3SiO)2Si−H moieties. Furthermore, a strong broad band is observed in the range 1000−1190 cm−1 due to the stretching mode of the new Si−O−Si bonds.22,26 A weaker peak at 1239 cm−1 is observed for Si−CH2−CH3 ethyl stretching mode in the case of siloxane-terminated compounds 9 and 10. Absence of the O−H stretching band and CC stretching around 1640 cm−1 (for dodecyl/siloxane mixed compound 10) indicate that final products are pure. In ambient conditions we observed that dodecyl-SiQDs (8) were more stable than triethylsiloxy-SiQDs (9). This was perhaps expected, as the FTIR results showed the triethylsiloxy coverage to be much less than dodecyl by virtue of larger residual Si−H peaks. The residual Si−H groups then react with ambient water to form surface silanol groups that co-condense with one another to form insoluble materials. In an effort to stabilize the triethylsiloxy-SiQDs, we further reacted them with dodecene to prepare a hybrid dodecyl/triethylsiloxy-SiQD (10). From Figure 1, in going from 9 to 10 one can observe the reduction in the peak intensity of 2200 cm−1 versus 2100 cm−1. This implies the hydrosilylation of dodecene is preferentially reacting with −(O−Si)1−2−H sites as opposed to −C1−2Si−H, as is expected based on literature reports.46 Optical Properties. Photoluminescence (PL) spectra for 8−10 are shown in Figure 2, and photoluminescence excitation

Figure 1. Normalized FTIR spectra of Si−H region for SiQD compounds 7−10 from Scheme 1.

Upon reaction of 7 with dodecene and triethylsilanol, the intensity of the Si−Hx peaks are reduced considerably (Figure S6). However, the reaction does not proceed to completion as evidenced by residual Si−Hx stretching peaks in purified and isolated products. This is quite common due to steric hindrance and buried Si−H groups on the SiQD surface that are not accessible to the reactants. For the dodecyl-terminated SiQDs 8, in addition to the Si−Hx peak magnitude being reduced, the peak maximum is slightly shifted to 2091 cm−1 while another smaller peak arises centered at 2232 cm−1. We propose that the 2091 cm−1 peak is due to the environment of the residual Si−H peak changing as a result of the newly formed Si−C bonds and surface oxidation that has occurred. For example, Si−H2 and Si−H3 are known to be more reactive toward hydrosilylation than Si−H bonds. Thus, these species preferentially react first to form C−Si−H and C2−Si−H bonds that give rise to the slightly shifted Si−H stretch. The new peak arising at 2232 cm−1 we propose is due to the oxidation of Si−H2− and Si−H3 functional groups to O−Si−H and O2−Si−H that has likely occurred,44,45 as the hydride-terminated SiQDs are very susceptible to oxidation. The oxidation is also confirmed from the XPS data detailed in the following sections. The SiQDs (8) also showed strong C−H stretching around 2800− 3000 cm−1, indicating the presence of dodecyl groups. The absence of CC stretching around 1640 cm−1 confirmed that the starting material 1-dodecene has been completely removed by the purification process. For the siloxane-terminated SiQDs (compounds 9 and 10), the Si−H stretching region also develops a similar bimodal distribution. For triethylsiloxy-SiQD (9), a relatively large Si−H stretch peak centered at 2233 cm−1

Figure 2. Comparison of PL spectra for dodecyl-SiQDs (8), triethylsiloxy-SiQDs (9), and dodecyl/triethylsiloxy-SiQDs (10) from excitation at 295 nm (absorption intersection of all three solutions was 0.081 at 295 nm).

(PLE) and absorption spectra are presented in the Supporting Information (Figures S7−S9). The PL peak at 672 nm (1.85 eV) for the dodecyl-SiQDs (8) indicates a QD size of approximately 3.2 nm.47 Compared to dodecyl SiQDs, the PL maximum of the triethylsiloxy-SiQDs (9) is significantly blueshifted 52 nm to 620 nm. A more detailed analysis of this observation is explained using computation later in this article. These results are comparable to those from siloxy-terminated SiQDs prepared by previously published methods.17,22,26 In the mixed dodecyl/triethylsiloxy SiQDs (10), only a small shift of 7 nm to 665 nm was observed compared to the dodecyl-SiQD control. In order to compare relative PL yields for compounds 8−10, we determined a solution absorption intersection for all three materials at 295 nm (Figure S7). On the basis of this, we calculated the relative PL quantum yield for each material using E


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Figure 3. High-resolution XPS C 1s (a) and Si 2p (b) spectra of (1) triethylsiloxy-SiQD (9) and (2) dodecyl-SiQD (8).

Figure 4. (a) High-angle annular dark field (HAADF) scanning transmission electron micrograph (STEM) showing siloxane-terminated SiQDs as bright, high-contrast particles. EDS measurements confirm that the particles are Si. (b) Bright-field TEM image of triethylsiloxy-SiQD (9) on ultrathin C support film. (c) High-resolution TEM (HR-TEM) image of a single triethylsiloxy-SiQD, showing three twin planes indicated by the arrows. (d) Histogram showing particle size distribution as measured from (a). Average size from these measurements was 3.3 nm, with a mode of 3.5 nm and minimum/maximum of 2.4 and 4.4 nm.

the area under the curves in Figure 2 and normalized the dodecyl-SiQD PL area to 1. Using this method, we determined

the triethylsiloxy-SiQD (9) and dodecyl/triethylsiloxy-SiQD (10) PL areas of 1.64 and 1.14, an increase of 64% and 14%, F


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provides solid confirmation of triethylsiloxy modification of SiQDs due to the presence of Si−O and Si−C bonds, where Si−O is the proposed tethering bond and the Si−C species are part of the tethered ligand. Figure 4 demonstrates typical size and morphology of triethylsiloxy-SiQD. A size count histogram (Figure 4d) for 272 dots reveals an average size of 3.3 nm and mode of 3.5 nm, which agrees well with the estimation of 3.2 nm from PL. It is noted that some particles/agglomerations larger than 5−10 nm were seen via HRTEM and were excluded from the measurements that were made via the HAADF image. Air and Thermal Stability. In order to understand air stability of the SiQDs, static TGA measurements were taken at different temperatures for a period of 30 h (12 h each at 30 and 60 °C and 1 h each at 70, 80, 90, 100, 110, and 120 °C), under a flow of dry air at a rate of 40 mL/min. Results are presented in Figure S10. Dodecyl-SiQD (8) showed no change in mass throughout the 30 h, indicating good air stability. Both siloxaneSiQDs 9 and 10 are stable at 30 and 60 °C for 12 h each and start to incur mass increase at 70 °C. However, 10 did not show a similar behavior, instead showing a decrease in mass at higher temperatures. This may be due to the removal of residual solvents. We believe mass increase above 70 °C for the siloxane-SiQDs is due to oxidation of unreacted Si−H groups. Efforts in our research group to achieve enhanced surface coverage and thus more stable siloxane-terminated SiQDs are ongoing. Computation. An interesting observation in the PL measurements was that the triethylsiloxy-SiQDs were blueshifted versus their dodecyl-modified analogues. One possible explanation for the blue shift is that it is the result of quantum confinement, as has been posited for oxidized dots.49 The reasoning is that the HOMO and/or LUMO are compacted into the SiQD by the oxide so that the optical response is effectively that of a smaller dot. A density functional theory analysis of hydrogen and trihydridosiloxy (a simplified ligand versus triethylsiloxy) terminated dots indicates that just the opposite is true as shown in Figure 5. The effect of trihydridosiloxy on the electronic state is to pull the HOMO out toward the dot surface with no qualitative change to the LUMO. Reduced confinement by itself would result in a red shift. However, because only the HOMO is pulled outward, the effect is to reduce the Coulombic attraction between the occupied (negative) LUMO and unoccupied (positive) HOMO, i.e., to reduce the excitonic binding energy (Figure 5, panels c1 and c2). This increases the optical gap and would, by itself, lead to a blue shift. The strength of the competing effects of confinement and exciton binding energy vary with dot size as quantified in Figures 5−8. The emissive optical gap, binding energy, and transport gap were calculated for hydride- and trihydridosiloxyterminated SiQDs for five SiQD sizes. Figure 6 quantifies the anticipated red-shift in the quasi-particle gap due to siloxane treatment. This shift decreases with dot size and is effectively zero for dots larger than 3 nm in diameter. Figure 7, on the other hand, shows that trihydridosiloxy treatment decreases the excitonic binding energy. Figure 8 shows the net effect on the emissive optical gap due to trihydridosiloxya small blue-shift for dots larger than 2 nm. The data do not vary smoothly with size because the results are sensitive to small changes in dot surface structure and the positioning of ligands on these highly curved surfaces.

respectively, over dodecyl-SiQD (8). The qualitative differences can be seen in images of dilute solutions with and without 365 nm excitation, provided in Figure S9. XPS and TEM. SiQDs were further evaluated by X-ray photoelectron spectroscopy (XPS). Triethylsiloxy-SiQDs (9) were composed of ∼57% C, 14% O, 28% Si, and 1% N (Supporting Information Table S1). Analysis of dodecyl-SiQDs (8) was included as a point of comparison and showed that the sample contained ∼75% C, 6% O, and 19% Si (Table S1). A portion of the detected carbon and oxygen can be attributed to adventitious carbon and QD surface oxidation. The presence of N in the siloxane-modified SiQDs also indicates that small amounts of N-containing compounds used in the synthesis (ACHN free radical initiator) and processing of the SiQDs may still be present in the sample. No measurable amount of N was seen in the dodecyl-modified sample, confirming its presence is due to a condition or material present only in the preparation of 9. A much higher O/C and Si/C ratio was achieved in 9, as is expected based on the chemical structure of triethylsiloxy moiety as compared to that of dodecyl. It is not expected that either sample would show exact stoichiometric ratios, due in part to the presence of contaminants and small levels of oxidation, and the possibility of sampling a noninteger amount of SiQDs as the XPS sampling depth is greater than the particle size yielding composition from both surface and bulk. The C 1s and Si 2p regions were deconvoluted for each sample, as shown in Figure 3.5,48 The fitting parameters and quantification are available in Tables S2 and S3. For each SiQD sample, the C 1s region contained a C−Si, C−O, and C−C/ C−H peak. The sp3-hybridized carbon bonding (285 eV) is a much higher percentage of the total C 1s signal in 8 as the dodecyl ligand contains more C−C and C−H species than the triethylsiloxy ligand. Species at the highest binding energy (BE) ∼ 286−286.5 eV, attributed to C−O groups present in both samples, are most likely due to presence of adventitious carbon and oxidation. The peak at 285.6 eV is present only in 9 and is in the region typically attributed to C−N bonding. Some of this signal is likely due to C−N; however, with only 1 at. % N, the species cannot be solely assigned to C−N as it is 24.5% of the total C 1s signal (13.9 at. %). We propose that species contributing to this peak are C−O−Si bonds that are likely present due to oxidation of the triethylsiloxy ligands. Importantly, both samples show a peak at 284.4 eV. In the case of dodecyl-modified sample this peak confirms formation of a bond between Si−C, and in the triethylsiloxy-modified sample this peak is due to presence of Si−C bonds in the ligand itself. For the Si 2p region, the species present are due to Si−Si bonding, and varieties of species formed between Si and either O, C, or both. The majority of the Si 2p signal for each sample is due to Si−Si bonding in the QD. In 8, Si−C bonding anchors the ligand to the SiQD and is responsible for the peak at 100.6 eV (2p3/2 component).5,48 It is expected that the Si−O species in 9 would appear at a higher BE than Si−C species in 8, as O is a more electron-withdrawing element than C. Indeed, the peak at 100.8 eV (2p3/2 component) is shifted to a higher BE as compared to that observed in 8, confirming the presence of Si− O. Material 9 also has a peak at 101.6 eV (2p3/2 component) that is not seen in the dodecyl sample. This is likely due to the presence of Si−O−C bonding; these species were also detected in the C 1s spectra. On both samples, higher binding energy signal is due to oxidized Si, where Si−Ox peaks represent Si species with multiple bonds to O.5,48 Thus, XPS analysis G


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Figure 7. Plot of exciton binding energy versus SiQD diameter. Error bars quantify the uncertainty in dot radii as detailed in the Computational Approach section.

Figure 5. Visualization of HOMO (red) and LUMO (blue) of SiQD with 353 Si atoms. a1 and b1 are the HOMO and LUMO distribution of H-terminated SiQD, respectively. a2 and b2 are the HOMO and LUMO distribution of OSiH3-terminated SiQDs, respectively. c1 and c2 are idealized HOMO and LUMO distributions of H- and OSiH3terminated SiQDs (calculation details are provided in the Supporting Information).

Figure 8. Plot of emissive optical gap versus dot size. Error bars quantify the uncertainty in dot radii as detailed in the Computational Approach section.

published methods.6,11,16 Computational work is presented that may explain the observed PL spectral differences between siloxane- and alkyl-terminated SiQDs. It is expected that this method will prove useful in a number of fields and will provide a much-needed simpler, safer, and “greener” synthetic route to siloxane-terminated soluble SiQDs as well as a greater understanding of the involved reaction mechanisms, product properties, and underlying electronic interactions. Further work is currently underway to refine the presented methods for enhanced surface coverage, to apply the methods to different SiQD/silanol/alcohol systems, and to characterize the resultant materials in order to determine their stability and use in photovoltaics, biological sensing, and other applications.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 6. Plot of transport gap versus SiQD diameter for hydride and trihydridosiloxy terminations. Error bars quantify the uncertainty in dot radii as detailed in the Computational Approach section.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b07930. NMR, GC-MS, UV−vis, PLE, PL, and FTIR spectra, TGA (air stability), and XPS data tables (PDF)

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CONCLUSIONS We have described a mild approach for reacting silyl hydride groups on the surface of silicon quantum dots with silanols to produce siloxy-terminated SiQDs without the use of highly toxic or corrosive materials or intermediates. The characterization data for the resulting siloxane-terminated SiQDs are shown to be comparable to those produced by previously

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; phone 303-384-2586. Notes

The authors declare no competing financial interest. H


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ACKNOWLEDGMENTS This research is supported by the Renewable Energy Materials Research Science and Engineering Center (REMRSEC) under Award DMR-0820518 and by startup funds from Colorado School of Mines (CSM). M.S. and R.P. were undergraduate researchers in the REMRSEC Research Experiences for Undergraduates (REU) programs (DMR-1063150 and DMR1461275). We acknowledge the Golden Energy Computing Organization at the Colorado School of Mines for the use of resources acquired with financial assistance from the National Science Foundation and the National Renewable Energy Laboratories (NREL). We thank Dr. Nathan Neale from NREL for very helpful scientific discussions.

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