Electrochemical Reduction Synthesis of Photoluminescent Silicon

pubs.acs.org/Langmuir © 2009 American Chemical Society

Electrochemical Reduction Synthesis of Photoluminescent Silicon Nanocrystals Jonghoon Choi,† Nam Sun Wang,‡ and Vytas Reipa*,† †

Biochemical Science Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, and ‡Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, Maryland 20742 Received January 15, 2009. Revised Manuscript Received April 17, 2009 An efficient synthesizing procedure of photoluminescent silicon nanocrystals is demonstrated by means of ultrasound assisted electrochemical octyltrichlorosilane reduction that produces octane terminated Si nanocrystals in a single step. The described procedure allows one to make Si nanocrystals with alkyl surface termination and is clean, relatively simple, and potentially scalable to industrial quantities. High resolution transmission electron microscopy, energy dispersive X-ray spectroscopy, UV-vis absorbance, Fourier transform infrared spectroscopy, and photoluminescence spectroscopy are employed to characterize the synthesized photoluminescent Si nanocrystals. Resulting octyl termination provides a stable passivation and could serve as a platform for further particle functionalization. Electrochemical chlorosilane reduction potentially could address the requirement for stable photoluminescent Si nanocrystals in diverse applications.

1. Introduction Since the discovery of the intense porous silicon photoluminescence,1 silicon nanocrystals (SiNCs) have been attracting a growing interest for a variety of potential applications.2 Nanocrystal physicochemical properties can be tuned in a wide range by adjusting particle size in the quantum confinement range (d < 5 nm), suggesting novel designs in photonics,3a photovoltaics,4,5a and organic catalysis.5 Moreover, inherent material biocompatibility2h facilitates SiNC use for in vitro biological tagging2a and as a platform for drug delivery.2 Significant quantities of well characterized and stable nanocrystal material would be required for practical applications; however, most current procedures typically provide microgram to milligram quantities of SiNCs (for a recent review, see ref 3b) *Corresponding author. Telephone: 301-975-5056. Fax: 301-330-3447. E-mail: [email protected]. (1) Canham, L. T. Appl. Phys. Lett. 1990, 57, 1046. (2) (a) O’Farrell, N.; Houlton, A.; Horrocks, B. R. Int. J. Nanomed. 2006, 1, 451. (b) Brus, L. Adv. Mater. 1993, 5, 286. (c) Lockwood, D. J. In Light emission in silicon from physics to devices; Academic Press: San Diego, CA, 1998; p 253. (d) Wang, L.; Reipa, V.; Blasic, J. Bioconjugate Chem. 2004, 15, 409. (e) Warner, J. H.; Hoshino, A.; Yamamoto, K.; Tilley, R. D. Angew. Chem., Int. Ed. 2005, 44, 4550. (f ) Choi, J.; Wang, N. S.; Reipa, V. Bioconjugate Chem. 2008, 19, 680. (g) Ding, Z.; Quinn, B. M.; Haram, S. K.; Pell, L. E.; Korgel, B. A.; Bard, A. J. Science 2002, 296, 1293. (h) Canham, L. T. Nanotechnology 2007, 18, 1857041. (i) Prestidge, C. A.; Barnes, T. J.; Lau, C. H.; Barnett, C.; Loni, A.; Canham, L. Expert Opin. Drug Delivery 2007, 4, 101. (j) Park, J. H.; Gu, L.; Maltzahn, G.; Ruoslahti, E.; Bhatia, S. N.; Sailor, M. J. Nat. Mater. 2009, 8, 331. (3) (a) Jaiswal, S. L.; Simpson, J. T.; Withrow, S. P.; White, C. W.; Norris, P. M. Appl. Phys. A: Mater. Sci. Process. 2003, 77, 57. (b) Veinot, J. G. C. Chem. Commun. 2006, 4160. (4) Beard, M. C.; Knutsen, K. P.; Yu, P.; Luther, J. M.; Song, Q.; Metzger, W. K.; Ellingson, R. J.; Nozik, A. J. Nano Lett. 2007, 7, 2506. (5) (a) Stupca, M.; Alsalhi, M.; Al Saud, T.; Almuhanna, A.; Nayfeh, M. H. Appl. Phys. Lett. 2007, 91, 0631071. (b) Vrcek, V.; Slaoui, A.; Muller, J. C. Thin Solid Films 2004, 384, 451. (6) (a) Choi, J.; Tung, S.; Wang, N. S.; Reipa, V. Nanotechnology 2008, 19, 0857151. (b) Choi, J.; Wang, N. S.; Reipa, V. Langmuir 2007, 23, 3388. (c) Belomoin, G.; Therrien, J.; Smith, A.; Rao, S.; Twesten, R.; Chaieb, S.; Nayfeh, M. H.; Wagner, L.; Mitas, L. Appl. Phys. Lett. 2002, 80, 841. (7) (a) Carlisle, J. A.; Dongol, M.; Germanenko, I. N.; Pithawalla, Y. B.; El-Shall, M. S. Chem. Phys. Lett. 2000, 326, 335. (b) Carlisle, J. A.; Germanenko, I. N.; Pithawalla, Y. B.; El-Shall, M. S. J. Electron Spectrosc. Relat. Phenom. 2001, 114-116, 229. (c) Heintz, A. S.; Fink, M. J.; Mitchell, B. S. Adv. Mater. 2007, 19, 3984.

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Published synthesis methods can be classified into broad categories of the top-down (wafer etching,6 laser ablation,7 and ballmilling7c) and bottom-up (inverse micelle based,8 plasma,9 metalloorganic precursor decomposition,2g,10 and solution precursor reduction synthesis11) approaches. Low temperature reductive chlorosilane condensation based synthesis routines typically employ the reduction of the labile Si-Cl bonds and potentially may lead to inexpensive Si nanocrystal mass production.11 These processes typically employ alkali metals or their derivatives as reducing agents for tetrachlorosilane and generate several reaction byproducts. Postreduction treatments include product purification and separate Si nanoparticle surface passivation steps.11b Electrochemical reduction by its very nature does not require chemical reduction agents, and hence minimizes reaction byproducts and allows reaction control via electrode current and potential.12 It is routinely used in organic synthesis reactions, including polysilane condensation from dichloromethylphenylsilane.13 Electroreduction of chlorosilanes requires exceptionally low potentials that can only be realized in select nonaqueous solvents. The electroreductive coupling of organodichlorosilanes with mercury electrode was reported in the 1970s14a as a method (8) (a) Wilcoxon, J. P.; Samara, G. A.; Provencio, P. N. Phys. Rev. B. 1999, 60, 2704. (b) Warner, J. H.; Hoshino, A.; Shiohara, A.; Yamamoto, K.; Tilley, R. D. Proc. SPIE 2006, 6096, 6096071. (9) Mangolini, L.; Thimsen, E.; Kortshagen, U. Nano Lett. 2005, 5, 655. (10) (a) Hua, F.; Swihart, M. T.; Ruckenstein, E. Langmuir 2005, 21, 6054. (b) Hessel, C. M.; Henderson, E. J.; Veinot, J. G. C. Chem. Mater. 2006, 18, 6139. (c) Sacarlescu, L.; Simionescu, M. J. Optoelectron. Adv. Mater. 2008, 10, 649. (d) Hua, F.; Erogbogbo, F.; Swihart, M. T.; Ruckenstein, E. Langmuir 2006, 22, 4363. (11) (a) Baldwin, R. K.; Pettigrew, K. A.; Garno, J. C.; Power, P. P.; Liu, G.; Kauzlarich, S. M. J. Am. Chem. Soc. 2002, 124, 1150. (b) Zou, J.; Baldwin, R. K.; Pettigrew, K. A.; Kauzlarich, S. M. Nano Lett. 2004, 4, 1181. (c) Zou, J.; Kauzlarich, S. M. J. Cluster Sci. 2008, 19, 341. (d) Zhang, X.; Neiner, D.; Wang, S.; Louie, A. Y.; Kauzlarich, S. M. Nanotechnology 2007, 18, 1. (12) Sperry, J. B.; Wright, D. L. Chem. Soc. Rev. 2006, 35, 605. (13) (a) Kogai, Y.; Ishifune, M.; Uchida, K.; Kashimura, S. Electrochemistry 2005, 73, 419. (b) Shono, T.; Kashimura, S.; Ishifune, M.; Nishida, R. J. Chem. Soc., Chem. Commun. 1990, 1160. (c) Ishifune, M.; Kashimura, S.; Kogai, Y.; Fukuhara, Y.; Kato, T.; Bu, H.; Yamashita, N.; Murai, Y.; Murase, H.; Nishida, R. J. Organomet. Chem. 2000, 611, 26. (14) (a) Hengge, E.; Litscher, G. Angew. Chem. 1976, 88, 414. (b) Hengge, E.; Firgoi, H. J. Organomet. Chem. 1981, 212, 155.

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to form disilane, but it was not effective in the preparation of polysilane.14b Only recently, polysilane formation by way of chlorosilane electroreduction has been achieved by Kogai et al.13a using catalytic amounts of anodically dissolved Mg2+. The polymerization of dichloromethylphenylsilane was carried out by using carbon electrodes and a pre-electrolysis technique to give linear polymers of up to 17 000 molecular weight. Reductive condensation of trichlorosilanes form three-dimensional Si networks and can eventually result in Si particles of crystalline structure.15a,15b Silicon electrodeposition from tetrachlorosilane solutions was demonstrated using water-free tetrahydrofuran (THF),16 propylene carbonate,17 and acetonitrile18 as solvents. Nanostructured Si films were recently deposited using ultrapure ionic liquids19 at -2.6 V versus Ag quasi-reference electrode. Silicon single crystals of a broad size range were prepared when trichlorosilane was reduced by metallic sodium inside a high pressure, high temperature bomb over several days.15a Remarkably, the authors have observed that Si crystal size range is limited to 5.5 ( 2.5 nm, when octyltrichlorosilane is included in a reaction precursor mix. In another study, Aihara et al.20 described synthesis of the 3 nm diameter Si nanoparticles using the electrochemical reduction of tetrachlorosilane. The reaction produced amorphous Si particles that demonstrated room temperature photoluminescence and were encapsulated in silicon dioxide. Here, we demonstrate photoluminescent Si nanocrystal synthesis by means of the ultrasound assisted electrochemical octyltrichlorosilane reduction, which generates octyl terminated Si nanocrystals in a single step. We selected octyltrichlorosilane as a reaction precursor, seeking to preserve the octyl termination in the reaction product and at the same time limit Si nanocrystal size.15a Intense ultrasonic agitation during the reduction reaction was projected to facilitate particle crystallization by way of the acoustic cavitation.21 Moreover, it helps to disintegrate the cathode surface passivating layer. The described synthesis route results in nanometer diameter alkyl terminated Si nanocrystals and is clean, relatively simple, and, potentially, scalable to larger quantities.

2. Experimental Section Two pairs of cylindrical Mg electrodes (99.9%, Alfa Aesar, Ward Hill, MA, total immersed surface area = 36 cm-2) alternatively served as cathodes and anodes. See Figure 1 for a schematic of the setup. Electrodes were held by the poly(tetrafluoroethylene) (PTFE) holder, which also covered the reaction cell. A Ti sonication bar was inserted through the opening in the cell cover and was attached to the electrostriction ultrasound converter (model # CL4, Fischer Scientific, Pittsburgh, PA). (Certain commercial equipment, instruments, materials, or companies are identified in this paper to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment are necessarily the best available for the purpose.) Several smaller openings in the cell cover also served for reagent and purging gas delivery. (15) (a) Heath, J. R. Science 1992, 258, 1131. (b) Fischer, J.; Baumgartner, J.; Marschner, C. Science 2005, 310, 825. (16) Gobet, J.; Tannenberger, H. J. Electrochem. Soc. 1988, 135, 109. (17) Nishimura, Y.; Fukunaka, Y. Electrochim. Acta 2007, 53, 111. (18) Lee, C. H.; Kroger, F. A. J. Electrochem. Soc. 1982, 135, 109. (19) (a) Al Salman, R.; Zein El Abedin, S.; Endres, F. Phys. Chem. Chem. Phys. 2008, 10, 4650. (b) Borisenko, N.; Zein El Abedin, S.; Endres, F. J. Phys. Chem. B 2006, 110, 6250. (20) Aihara, S.; Ishii, R.; Fukuhara, M.; Kamata, N.; Terunuma, D.; Hirano, Y.; Saito, N.; Aramata, M.; Kashimura, S. J. Non-Cryst. Solids 2001, 291, 135. (21) Arud Dhas, N.; Raj, C. P.; Gedanken, A. Chem. Mater. 1998, 10, 3278.

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Figure 1. Schematic drawing of the electrochemical reduction setup.

A magnetic stirrer was positioned at the bottom of the cell to facilitate continuous electrolyte motion. The cell was temperature controlled using a water jacket at 4 °C during the electrolysis. The whole setup was located inside a dry nitrogen-filled environmental glovebox (855-AC, Plas Laboratories, Lansing, MI), where water and oxygen-free atmosphere was carefully maintained. Each pair of Mg electrodes was connected to the current source, consisting of a galvanostat (model # 363, EG&G PARC, Oak Ridge, TN) and the waveform generator (model # 29, Wavetek, San Diego, CA) that supplied periodic current pulses. 2.1. Reagent Preparation and Product Purification. A total of 100 mL of anhydrous THF (Sigma-Aldrich, St. Louis, MO) was introduced in the reaction cell, cooled to 4 °C, and purged with N2 for 30 min. Next, 1 g (9.4  10-3 M) of lithium perchlorate (Sigma-Aldrich, St. Louis, MO), and 10 mL (43.2  10-3 M) of octyltrichlorosilane (Sigma-Aldrich, St. Louis, MO) were gradually dissolved in the THF. Following complete dissolution, the colorless liquid was purged with nitrogen for another 30 min, and electrolysis reaction was started by switching on the alternating current (average value ∼ 10 mA/cm2), together with ultrasonic agitation. Electrode polarity alteration and continuous ultrasonic activation were helpful in preventing electrode surface passivation. As the electrochemical reactions progressed, the solution color gradually turned yellow and then brown. The reaction was stopped when 3F (relative to the octyltrichlorosilane content) charge was passed through the cell. Next, dark brown electrolyte was washed with dilute HCl and then purified by way of hexane/methanol extraction. In particular, a 20 mL product fraction was mixed with the 200 mL hexane/methanol solution (1:1 volume ratio) and allowed to equilibrate for 2 h. Si particles were collected from the top hexane layer and filtered through the 0.02 μm inorganic syringe filter (Whatman Inc., Anotop 25). The purification procedure was repeated at least three times. The filtered product was transparent and emitted blue fluorescence when excited by 360 nm UV lamp. 2.2. TEM/EDS and XRD Analysis. A 10 μL SiNC suspension in hexane was deposited on a carbon film copper grid (Ted Pella Inc., Redding, CA) and dried completely in gently flowing nitrogen gas. High resolution transmission electron microscopy (HRTEM) was conducted using a JEOL 2100F field emission instrument (JEOL Ltd., Tokyo, Japan) operated at U = 160 kV. Particle elemental composition analysis was performed using energy dispersive X-ray spectroscopy (EDS, INCAx-sight, Oxford Ins., Oxfordshire, U.K.). The powder X-ray diffraction pattern was recorded with a Bruker C2 Discover X-ray diffractometer, equipped with a Cu KR sealed X-ray tube and Go¨bbel mirror, spanning from 8 to 93°. A HiStar (GADDS) detector was Langmuir 2009, 25(12), 7097–7102

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used for real-time data collection of 2D diffractometry and quality patterns from the dry glass slide supported SiNC films.

2.3. Photoluminescence, Absorption, and Fourier Transform Infrared (FTIR) Measurements. SiNC suspension photoluminescence (PL) was measured using 3 mm optical path length quartz cuvettes (NSG, Inc.) and recorded by using a double monochromator based spectrofluorimeter (model LM800, SLM Inc., Urbana, IL). The excitation wavelength was varied from 260 to 350 nm. Absorption spectra of SiNCs were recorded in 1 cm quartz cuvettes (NSG, Inc.) using a UV/vis spectrophotometer (Lambda 850, Perkin-Elmer, Waltham, MA), with pure solvent as a reference. The quantum efficiency of silicon nanoparticles was calculated by using an integrating sphere method, recently developed at NIST.22 The integrating sphere detector is equipped with an internal cuvette holder so that absorbance measurements can be performed with the cuvette inside the integrating sphere. In addition, the spectrophotometer has a cuvette holder outside the integrating sphere for performing conventional absorbance measurements. It is shown that the fluorescence quantum yield can be obtained from a combination of absorbance measurements of the buffer and the analyte solution inside and outside the integrating sphere detector. Several 50 μL aliquots of SiNC hexane suspension were deposited on KBr crystal based IR cards (ICL, Garfield, NJ) and gently dried in N2 gas stream for the infrared transmission measurements (FTIR). The spectrometer (IFS 66, Bruker Optics Inc., Billerica, MA) resolution was set at 4 cm-1, and the scanner velocity at 10.0 kHz.

3. Results and Discussion Precursor reduction is one of the most attractive ways for Si nanocrystal synthesis, as it affords control of particle size and surface chemistry and at the same time can be scaled up. Previous studies have clearly shown that Si nanoparticles are formed when chlorosilane is reduced by alkali metals or their derivatives;9 however, elevated temperatures and/or pressures were required to form Si nanocrystals. It suggests the electrochemical technique as an appealing alternative to chemical reducing agents, provided adequate reducing potential could be supplied by the electrode. Here, we show that alkyl terminated ultrasmall Si nanocrystals are formed during the low temperature and pressure octyltrichlorosilane electrolysis, when assisted by intense ultrasound activation. Upon application of the sufficiently negative potential, the electrophilic Si-Cl bonds of octyl trichlorosilanes are irreversibly reduced to form polyoctylsilanes, possibly through radical intermediates, and chloride ions are released (Scheme 1). Continuous ultrasound treatment provides localized “hot” spots in the reaction mixture21 due to cavitation. The temperature and pressure in these sonochemical hot spots can transiently reach levels adequate for Si nanocrystal formation from organosilane precursors.15a,21 3.1. Particle Size and Structure. The reaction product was examined using transmission electron microscopy (TEM, Figure 2A), energy dispersive X-ray spectroscopy (EDS, Figure 2C), powder X-ray crystallography (Figure 3C), measurements of the optical properties (Figure 5), and FTIR spectroscopy (Figure 6). Atomic composition of our nanocrystals was assessed with energy dispersive X-ray spectroscopy (EDS). The elemental composition of the reduction product confirms Si as a key component. The EDS spectrum contains a prominent Si feature at 1.74 keV and lower intensity peaks at 0.53 keV (O), 2.62 keV (Cl), and 0.28 keV (C) (Figure 2C). Additional peaks at 0.93 and 8.91 keV are artifacts from the TEM copper grid, as their intensity was not affected by the deposition of the SiNC sample. (22) Gaigalas, A. K.; Wang, L. J. Res. Natl. Inst. Stand. Technol. 2008, 113(1), 17.

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Figure 2. (A) HRTEM image showing pseudospherical, well dispersed SiNCs. (B) Particle size distribution histogram calculated from (A). (C) Energy dispersive X-ray spectra of SiNCs. Scheme 1. Electrochemical Reduction of Octyltrichlorosilane and SiNC Formation

The carbon signal originates from the amorphous carbon film on the copper TEM grid and octyl terminal groups, while oxygen and chlorine provide evidence of particle surface termination with oxide species and residual chlorine moieties. TEM offers direct visualization of nanometer size particles,23a and it has been a primary tool of size measurement for (23) (a) Pyrz, W. D.; Buttrey, D. J. Langmuir 2008, 24, 11350. (b) Santos, D. A. A.; Rocha, A. D. P.; Macedo, M. A. Powder Diffr. 2008, 23, S36. (c) Calvin, S.; Luo, S. X.; Caragianis-Broadbridge, C.; McGuiness, J. K.; Anderson, E.; Lehman, A.; Wee, K. H.; Morrison, S. A.; Kurihara, L. K. Appl. Phys. Lett. 2005, 87, 233102.

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Figure 3. (A) HRTEM image of electrochemically reduced silicon nanocrystals. Scale bar is 5 nm. (B) SAED pattern demonstrating the crystalline structure of the particles. (C) X-ray diffraction pattern of the dried SiNC powder.

various nanoparticles. Furthermore, manual or automated TEM image analysis could be helpful in estimating the nanoparticle sample size distribution. We have analyzed our TEM images (Figure 2A) using ImageJ and obtained a corresponding size distribution histogram (Figure 2B), which was validated by manual examination. The majority of the electrochemically produced Si nanocrystals were pseudospherical and less than 5 nm in diameter (Figure 2B). Limited agglomeration is evident in several TEM images, and could account for the larger diameter (d g 10 nm) components in the size distribution histogram (Figure 2B). A variety of silicon compounds could possibly be formed as a result of the chlorosilane reduction, including polysilanes13 and amorphous silicon;20 therefore, we have examined the crystallographic structure of our reaction product using local area electron diffraction patterns from HRTEM (Figure 3A) and X-ray powder diffraction (Figure 3C). Often, the visible lattice fringes in the HRTEM image that come from the interference of electrons diffracted along the different directions are used to decide the crystallographic structure of the nanocrystal.2 Although it may appear to be a straightforward way to confirm 7100 DOI: 10.1021/la9001829

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the crystalline lattice, their use to determine the crystallographic orientation may be limited. In cases when particles in the TEM image are aggregated, it is impossible to discern fringe direction and, subsequently, the crystallographic orientation. In addition, possible errors from the scale bar will be significantly inflated in the HRTEM and will affect measurement between fringes. For this reason, independent nanocrystal crystalline structure validation by X-ray powder diffraction patterns and/or selected area electron diffraction measurements is very useful. Powder diffraction data are presented as a diffractogram (Figure 3C), where the diffracted intensity I is plotted as a function of the scattering angle 2θ. The most intense features in the diffractogram can be assigned to Si(111), Si(220), and Si(311) structures, while broad reflections at 38 and 48° indicate the presence of the amorphous phase. The diffractogram analysis using Rietveld refinement23b with fundamental approach yielded crystallite sizes of about 20-25 nm. The difference between TEM and XRD size estimates indicates that our sample has substantial polydispersity. As demonstrated by Calvin et al.,23c for a moderately polydisperse nanocrystal sample, XRD becomes dominated by the size distribution of the largest particles. There may also have been occasional large crystallites in the sample that did not appear in our TEM images. As XRD is particularly sensitive to large crystallites, it is feasible that Si nanocrystal size obtained from XRD data would be at or above the top end of the range found via TEM (Figure 2B). The concentric rings in the selected area electron diffraction (SAED) patterns originate from multiple random orientated particles in the selected area, assuming that nanometer size particles are not likely to accommodate the polycrystalline structure. In our case, all three of the above methods confirm the crystalline structure of the electrochemical reaction product (Figure 3), with the Si cubic lattice as a main structural component of the nanocrystalline material. A small fraction of the amorphous phase is detected by XRD. Nanocrystal lattice spacing was estimated using data from two independent techniques: HRTEM image FFT analysis and selected area electron diffraction patterns. Fast Fourier transform (FFT) of the selected area in the HRTEM image (yellow square in Figure 4) produces a diffractogram, shown in Figure 4. The distance between the points in a diffractogram is proportional to the reciprocal of the actual spacing between fringes. Based on the diffractogram scale, we have calculated the lattice spacing to be equal to 3.14 ( 0.04 A˚ that corresponds to the (1 1 1) spacing of the cubic silicon lattice. In addition, the nanocrystal lattice spacing was determined from the SAED patterns (Figure 3B). Since each ring in the diffraction pattern corresponds to the different crystalline structures,24 at least three crystallographic orientations are detected in our sample as illustrated in Figure 3B. According to Madelung et al.,24 the brightest and closest to the center ring is assigned to silicon (1 1 1) crystal structure, since the ring diameter matches the 3.10 ( 0.02 A˚ distance between lattice fringes. The larger diameter ring in the SAED pattern corresponds to the silicon (2 2 0) crystal structure. Finally, a third and somewhat diffused ring could be assigned to the silicon (3 1 1) structure. Several lower intensity diffuse rings possibly originate from the carbon film and/or amorphous structures. 3.2. Optical Properties. UV-vis absorbance and photoluminescence (Figure 5) are used to examine the optical properties (24) Madelung, O., Ro¨ssler, U., Schulz, M., Eds.; In Group IV Elements, IV-IV and III-V Compounds. Part a; Lattice Properties Book Series: Landolt-Bo¨rnstein Group III Condensed Matter Numerical Data and Functional Relationships in Science and Technology; Springer: New York, 2001; Volume 41, p A1.

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Figure 6. FTIR spectrum of electrochemically produced SiNCs with assignment of the characteristic bands.

Figure 4. FFT analysis of the HRTEM image. Yellow box (256  256 pixels) was used for the lattice fringe spacing calculation. The distance between two points in the FFT image (yellow arrow) corresponds to the (3.14 ( 0.04 A˚) distance between lattice fringes.

Figure 5. UV-vis absorption and PL spectra of SiNC in hexane,

λexc = 360 nm. Photograph of SiNC colloid under UV lamp excitation is shown in the inset.

of the reaction product. The absorbance edge is at approximately 360 nm and corresponds to the material band gap Eg = 3.5 eV, a value consistent with the quantum confinement in the single nanometer range Si nanocrystals.6a,6c The Si nanocrystal suspension in hexane emits blue photoluminescence under 360 nm excitation (Figure 5), previously reported for photoluminescent alkyl conjugated nanocrystals.2e,7c,10d,11d The photoluminescence quantum yield of our silicon nanocrystals in hexane is Qsi = 5.8 ( 0.5% at λ em = 450 nm. Several weak sub-bands could be resolved in PL profiles, possibly related to discrete “magic” particle size populations.4 Notably, the PL intensity did not visibly degrade under extended UV exposure, as was reported for polysilanes.26 3.3. Surface Chemical Composition. A FTIR spectrum (Figure 6) recorded using dry Si particles dispersed as a neat film on a KBr crystal displayed distinctive features of the surface alkyl (25) (a) Miller, R. D.; Michl, J. Chem. Rev. 1989, 89, 1359. (b) Bianconi, P. A.; Weidman, T. W. J. Am. Chem. Soc. 1988, 110, 2342. (26) Pawlenko, S. Organosilicon Chemistry; de Gruyter: New York, 1986; p 8.

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groups (CH3: 2955, 2872, 1378 cm-1; CH2: 2924, 2854, 1466 cm-1, and C-C: 888 cm-1) and asymmetric stretches from the surface oxide entities (O-Si-O, 1115 cm-1). A weak feature at 567 cm-1, attributed to Si-Cl stretch, implies that a small number of nanocrystal surface sites retain Cl termination, possibly due to the partial octyltrichlorosilane precursor reduction. However, due to this peak proximity to the instrument working range limit, this assignment requires further validation. The presence of prominent alkyl features in the IR spectrum suggests that SiNCs, produced using electrochemical octyltrichlorosilane reduction, acquire stable organic coverage without additional treatments. Such procedures typically are necessary for Cl terminated products of tetrachlorosilane reduction,11 due to the instability of the Si-Cl bonds. Nanocrystal surface octane termination could be expected, given that Si-octyl linkage in the reaction precursor is less likely to be reduced than Si-Cl.27 However, previously reported high temperature/pressure SiNC synthesis from the octyltrichlorosilane precursor resulted in nanocrystal Si-H termination and elimination of the 1-octene.15a Our FTIR spectra did not contain a characteristic Si-H stretch at ∼ 2100 cm-1. Therefore, the milder conditions of the electrochemical reduction leave the Si-alkyl bond intact in the reaction product. Alkyl linkages to Si provide improved stability and serve as intermediates for further nanocrystal conjugation.2d,2f Alkyl passivation of the Si surface can be achieved using Grignard reagents11a or activation by Pt,27 mechanochemical,7c thermal, or photochemical treatment.28 In this context, a direct synthesis of alkyl terminated SiNCs using electrochemical octyltrichlorosilane reduction is advantageous, since it allows omission of the additional procedures. The total recovered mass (up to 2 g) contained both crystalline and amorphous Si phases, as suggested by powder X-ray diffraction data (Figure 3C). The reaction yield equaled to 20 ( 5%, relative to the reaction precursor (octyltrichlorosilane), as estimated from the nanoparticle concentration measurements using absorbance (K = 1.7  10-4 M-1 cm-1 at λ = 261 nm).29 In view of a relatively straightforward reaction setup, (27) Tilley, R. D.; Warner, J. H.; Yamamoto, K.; Matsui, I.; Fujimori, H. Chem. Commun. 2005, 14, 1833. (28) (a) Li, X.; He, Y.; Swihart, M. T. Langmuir 2004, 20, 4720. (b) Liu, S. M.; Sato, S.; Kimura, K. Langmuir 2005, 21, 6342. (29) Rosso-Vasic, M.; Spruijt, E.; Lagen, B.; De Cola, L.; Zuilhof, H. Small 2008, 4, 1835.

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mild conditions, and a single consumable precursor, the electrochemical reduction together with the ultrasonic activation may open the way to the larger scale production of the alkyl terminated silicon nanocrystals and, possibly, other nanomaterials.

4. Conclusions In summary, photoluminescent silicon nanocrystals in the single nanometer range were prepared at room temperature and ambient pressure using a direct electrochemical reduction of octyltrichlorosilane from the nonaqueous electrolyte. The structure and composition of the SiNCs were examined by HRTEM, EDS, and powder X-ray diffraction. The Si nanocrystal surface

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was predominantly alkyl terminated and contained oxide and Cl terminal groups, as evidenced by FTIR.17 Particle absorbance and PL (QY = 5.8% at λem = 450 nm) profiles were typical for silicon crystals in the single nanometer size range. The octyl termination provides a stable passivation and could serve as a platform for further particle functionalization. A relatively simple and scalable procedure potentially could provide stable Si nanocrystals for various applications. Acknowledgment. The authors thank Dr. L. Salamanca-Riba and Dr. P. Zavalij of Maryland Nanocenter for HrTEM and XRD measurements.

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