Solution Synthesis of Ultrastable Luminescent Siloxane-Coated

Department of Chemistry, University of California, One Shields Avenue, Davis, California 95616 ...... FaramusRegina SinelnikovXiyu ZhangJob BoekhovenJ...
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NANO LETTERS

Solution Synthesis of Ultrastable Luminescent Siloxane-Coated Silicon Nanoparticles

2004 Vol. 4, No. 7 1181-1186

Jing Zou, Richard K. Baldwin, Katherine A. Pettigrew, and Susan M. Kauzlarich* Department of Chemistry, UniVersity of California, One Shields AVenue, DaVis, California 95616 Received February 16, 2004; Revised Manuscript Received April 27, 2004

ABSTRACT Silicon nanoparticles (NPs) of ∼4.5(1.10) nm from a room-temperature solution route are terminated by a silanization method for the first time. Energy-selected emission is observed, consistent with the distribution of sizes obtained by this route. The NPs are photochemically stable in nonpolar organic solvents and when exposed to air/water under ambient conditions for up to 1 year. The nanoparticles were characterized by TEM, HRTEM, EDX, SAED, FTIR, 1H/13C NMR, UV−vis, and photoluminescence (PL) spectroscopy.

Currently, one of the main research interests in the field of semiconductor nanoclusters is their optical properties.1 It is expected that these semiconductor nanoclusters or quantum dots (QDs) will greatly contribute to the fields of biology and biochemistry by taking advantage of their attractive luminescence properties: broad excitation-narrow emission photophysics, size-tunable emission, longer fluorescence lifetime, and negligible photobleaching.2,3 Monitoring living cells in vivo can be realized by tracking the luminescence from the QDs bioconjugated to target species.4 Silicon, the mainstay semiconductor of the modern microelectronic industry, plays a relatively minor role in optoelectronics because of inefficient light emission.5 It was not until Canham6 connected emission from porous Si to quantum confinement that numerous studies were initiated on both porous Si5,7-9 and Si nanocrystals.10-19 Si nanoparticles may have benefits over other quantum dots for optical and electronic applications.5,7-20 For instance, the well-established microprocessor technique and the well-understood silicon chemistry make it possible to update existing silicon-based devices to their corresponding nanostructured versions. Additionally, their better biocompatibility over other QDs opens the opportunity for a technological union between biochemistry and electronics. However, problems such as a lack of synthetic control and poor stability and solubility have slowed the NPs’ practical applications.19,21 In this paper, we focus on the issue of stability improvement for silicon nanoparticles prepared by the reduction of silicon tetrachloride as well as the characterization of surface passivation. Previous passivation methods for silicon nanoparticles prepared by a solution route * Corresponding author. E-mail: [email protected]. 10.1021/nl0497373 CCC: $27.50 Published on Web 06/04/2004

© 2004 American Chemical Society

such as hydrogenation16 or organic passivation in situ10,18 for Si NPs have some shortcomings. Although the Si-H bonds found on the surface of porous Si have been utilized to prepare alkyl-terminated surfaces via hydrosilation, which are significantly more stable than oxide-terminated surfaces,22 similar chemistry for hydrogen-terminated nanoparticles prepared via reduction has not yet been reported. In situ organic passivation allows for limited further functionalization. We have reported a novel reduction synthesis of Si nanoparticles at room temperature and pressure, giving rise to NPs with a chemically accessible surface.23,24 Alkoxyterminated NPs were produced in situ by replacing the surface chlorides with alkoxyl groups.24 However, alkoxyterminated nanoparticles have unsatisfactory stability and eventually decompose, consistent with what is observed for alcohols attached to porous Si and Si surfaces.22 We have also found that the alkyl-terminated nanoparticles prepared by this route are not as stable as those that we have synthesized from Zintl salts.17,25 This paper presents the silanization of silicon nanoparticles giving siloxane-coated Si NPs (the same as silanized Si NPs or silanization-terminated Si NPs hereafter). Silanization has been widely used to alter the surface characteristics of various kinds of nanoparticles, such as CdSe NPs and Au NPs, with good success.4,26,27 Silanization has also been used to modify the surface of nanostructured Si and Si NPs.28-30 These reactions utilize the fact that the surface of the silicon already has an oxide coating. This is the first time that a silanization method has been applied in situ to Si NPs with a nonoxidized surface that are synthesized via a room-temperature solution route. This treatment significantly enhances the stability of nanoparticles and has a negligible effect on the optical properties of the silicon core.

Scheme 1. Three-Step Termination Route with the Proposed Silanization Process

Synthesis and Termination. A suggested mechanism to explain the termination process is shown in Scheme 1. The Cl-terminated Si NPs a were produced via the reduction of silicon tetrachloride with sodium(naphthalide) according to a published method24 and appear as a clear, dark-brown solution in 1,2-dimethoxyethane (glyme), with NaCl (s) and naphthalene as byproducts. An excess amount of dried methanol (relative stoichiometries refers to those provided in Scheme 1) was added to the reaction mixture of a, NaCl, and naphthalene, and the color of the solution changed from dark-brown to orange immediately. The reaction mixture was stirred for 12 h, and then the solids were allowed to settle for several hours. The orange supernatant of the reaction mixture, which contains methoxy-terminated Si NPs b, was transferred to another flask via a cannula. The solids were washed several times with additional solvent, and the supernatant was combined with the first. After the removal of the solvent and the sublimation of the remaining byproduct, naphthalene, a pale-yellow, slightly oily product, b was obtained. Octoxy-terminated Si NPs24 were also prepared using octanol rather than methanol in termination step I (Scheme 1) for a stability comparison because the octoxyterminated Si NPs are much less reactive than the methoxyterminated Si NPs. Freshly dried, distilled, degassed glyme was added to the flask containing b. HPLC-grade H2O, with a mole ratio double that expected to give 100% hydroxyterminated Si NPs (Scheme 1), was degassed and injected into the flask containing b to give the postulated intermediate, hydroxy-terminated Si NPs c, and was stirred for 30 min. We noted that the addition of H2O directly to a usually resulted in the oxidation of the particles and the formation of a gel-like product. This is consistent with the fact that substitution of chloride with hydroxyl is a faster process and more difficult to control than the substitution of methoxy.31 An excessive amount of H2O is avoided to prevent possible large-scale aggregation between particles due to surface hydroxyl groups and the further formation of silica gel. The next step is the addition of octyltrichlorosilane (OTCS) to the reaction system with gentle heating (∼60 °C) for 30 min, and the product is stirred for another 12 h at room temperature. The resultant silanized product d was obtained by the removal of solvent and then dissolved in chloroform. The chloroform solution was purified by extraction with a water/hexane mixture. The hexane extract was further centrifuged to acquire a clear solution, d. d can be dried to 1182

yield a waxy, light-yellow solid that can be resuspended in many nonpolar organic solvents, such as chloroform and hexane, to obtain a light-yellow solution. A proposed model for d is provided in Scheme 1. The synthesis of polymerized OTCS, a possible byproduct of the synthesis, was carried out to compare its optical properties to those of the silanization-terminated Si NPs. The synthesis was performed by adding the same amount of degassed HPLC-grade water to freshly dried, distilled, degassed glyme and then injecting the same amount of OTCS as that used in the Si NPs’ silanization termination. The reaction mixture was also gently heated (∼60 °C) for 30 min and stirred for another 12 h. The solvent and excess reactants were removed in vacuo. A clear oil of the hydroxylation-condensation product from OTCS was obtained and will be referred to as “poly-OTCS” herein. In the termination route I f II f III, the addition of water converts methoxy-terminated Si NPs into hydroxy-terminated Si NPs c with the release of HCl.31 Mild heating aids in OTCS reacting with surface hydroxyl groups forming siloxane bonds (Si-O-Si) and releasing water molecules. It is possible that the hydroxylation process in step II does not result in NPs with a fully hydroxylated surface. If this were correct, then some of the unreacted H2O would then hydrolyze OTCS to octyltrihydroxylsilane ((HO)3SiC8H17) as shown in step II′. This intermediate will react with unsubstituted methoxy groups on the Si NPs’ surface, also bringing about the final product d, as shown in step III′. Transmission Electron Microscopy (TEM), HighResolution TEM (HRTEM), Selected-Area Electron Diffraction Pattern (SAED), and Energy-Dispersive X-ray Spectroscopy (EDX). These results were obtained on a Phillips CM-12 with a 100-keV accelerating voltage, except for HRTEM taken on a Philips CM-200 with a 200-keV accelerating voltage. Parts a and b of Figure 1 show a brightfield (BF) and dark-field (DF) image of Si NPs at a magnification of 45K×. These are typical TEM images for samples prepared by this route. In Figure 1a, the particles are isolated from each other without significant overlap or agglomeration, which suggests that the silanization occurs on the Si NPs’ surface and does not result in particle aggregations. Most of the silicon nanoparticles are spherical or hexagonal in shape. In Figure 1b, the DF image, it can be clearly discerned that the nanoparticles are crystalline because the crystals with the correct orientation to meet Nano Lett., Vol. 4, No. 7, 2004

Figure 1. (a) Bright-field TEM micrograph of well-dispersed silanized Si nanoparticles at a magnification of 45K×. (b) Dark-field TEM micrograph of silanized Si nanoparticles in the same region as in part a at a magnification of 45K×. (c) Bright-field TEM micrograph of silanized Si nanoparticles at a higher magnification of 160K×. (d) SAED pattern of the silanized Si NPs. The inset in part a is the histogram of the size distribution of 990 silanized Si NPs. The inset in part b is the HRTEM showing the lattice fringes of the {220} Si crystal planes.

Bragg’s condition are bright. The inset in Figure 1b shows a high-resolution TEM image of a single Si NP with lattice fringes consistent with the {220} planes of crystalline silicon. Figure 1c shows a typical TEM micrograph at a higher magnification (160K×). The shapes of nanoparticles can be observed more clearly in this image. The SAED pattern (Figure 1d) shows four of the diffraction rings corresponding to the Miller indices [111], [200], [220], and [311] of the diamond cubic lattice of silicon. The rings have varying intensities arising from the contribution from larger nanocrystals to the diffraction pattern. EDX confirms the presence of Si, C, and O. Particles (990) from different regions of the TEM grid were randomly chosen (from 6 TEM micrographs) for the determination of the size distribution and are shown as a histogram inset in Figure 1a using the commercial software Analysis. The average diameter of the nanoparticles is 4.51 nm, with a standard derivation of 1.10 nm. This is similar to the reported size of NPs prepared by the same synthetic method.24 Using simple geometric arguments, we find that a 4-nm Si particle is composed of approximately 3600 silicon atoms and 900 surface sites that can be bound to terminal groups. With the proposed silanized Si NP model shown in Scheme 1d and this assumption, a formula of Si4000(O2SiC8H17)1000 or Si4(O2SiC8H17) for d is therefore postulated.25 Assuming that the dried product is all 4-nm silanized Si nanoparticles, the percentage yield for the reaction is 30.0%. Nano Lett., Vol. 4, No. 7, 2004

Figure 2. FTIR spectrum of silanized Si NPs with the assignment of characteristic peaks.

FTIR Spectroscopy. A Mattson Galaxy 3000 FTIR spectrophotometer was employed to identify and characterize the terminal moieties on the particle surface. The FTIR spectrum of the sample is similar to that of the OTCS.32 Four types of organic groups can be assigned to the peaks in the spectrum as shown in Figure 2.33 The strong peaks in the stretching region from 2980-2850 cm-1 and the medium peaks in the bending/scissoring region from 1450-1350 cm-1 indicate the presence of both methylene and methyl groups. These vibrations are attributed to the alkyl chain, -(CH2)7CH3, in the terminal groups. In addition, a strong vibration at 1050 cm-1 (Si-O-Si), which is absent in the 1183

Figure 3. (a) 1H NMR spectrum of silanized Si NPs. (b) 13C APT NMR spectrum of silanized Si NPs.

IR spectrum of OTCS, is present. This is due to either the hydroxylation-condensation of OTCS or the surface linkage between OTCS and the Si NPs. A weak, broad band assigned to -OH is also found in the FTIR spectrum of the sample. It is unclear whether the hydroxyl groups are on the surface of the Si nanoparticles c, from excess H2O, or are derived from the formation of (HO)3SiC8H17. Small peaks shown in the aromatic region are assigned to the residual amount of naphthalene or its derivatives in the sample. 1H and 13C NMR Spectra. 1H NMR (400 MHz) and 13C NMR spectra (75 MHz) of samples in CDCl3 were recorded on a Varian Inova 400 NMR spectrometer. The spectra are presented in Figure 3. Major sharp peaks and some broad resonances are observed in the 1H and 13C NMR spectra (Figure 3a and b, respectively), correlated to the protons and carbons in various organic groups within the product.33 Usually, the better the solute is solvated in the solvent, the better the signals that can be obtained from the solution NMR. Therefore, the protons/carbons on the smallest nanoparticles of a sample with a size distribution make the greatest contribution to the solution NMR of this study.34 Moreover, when protons/carbons are attached more closely to the NP core, their rotation in solution becomes more hindered, and the fwhm of their NMR peaks becomes broader.35-37 Therefore, the fwhm of NMR peaks is usually regarded as a qualitative parameter in determining this effect. This information has been taken into account to provide peak assignments in this study. In the1H NMR spectrum, the four resonances in the alkyl-proton region (2-0 ppm) can be assigned to Hb (1.405 ppm; medium, broad, and overlapped with the peak of Hc), Hc (1.278 ppm; sharp, strong), Hd (0.893 ppm; sharp, strong), and Ha (0.634 ppm; broad, medium), respectively. (Denominations of the protons are shown in 1184

the structure inset in the NMR spectrum.) Compared to those in the 1H NMR spectrum of the terminal silane reagent (OTCS),32 the chemical shifts of Hb, Hc, and Hd are almost identical; however, the δ (Ha) upshift from 1.48 to 0.65 ppm is close to the δ (Ha) in the spectrum of (CH3O)3SiC8H17. This information is strong evidence that Ha experiences a change in its chemical environment from strong electronwithdrawing atoms (-CH2-Si-Cl) in its surroundings to weaker electron-withdrawing atoms (-CH2-Si-O-Si NPs) in its surroundings. Meanwhile, line broadening in peaks Ha and Hb is also observed, especially in peak Ha. Obviously, the line width of Ha is more sensitive to the environment than other protons because it is on the carbon closest to the silicon core, which results in restricted conformations. There are two other resonance regions present in the 1H NMR spectrum: an aromatic region (7.1-6.9 ppm) and an ether region (4-3 ppm). They are in the same positions as the molecules naphthalene and glyme, but the lines are much broader. On the basis of their chemical shifts and broadened lines, they can be regarded as peaks of naphthalenecontaining and glyme-containing derivatives, respectively. The relative proton integral ratio of Inaphthalene-Iglyme-IOTCS (0.04:0.10:1) shows that the majority of the product is the silanization-terminated Si species. Additionally, corresponding resonances are observed in the 13C APT NMR spectrum. Their chemical shifts (at 1035 ppm) are consistent with the expected presence of an alkyl chain on terminated groups attached to silicon nanoparticles. The negative signal in 13C APT NMR is attributed to the methyl, and seven positive signals are attributed to methylene groups. There are two small peaks at 33.36 and 20.99 ppm that are assigned to the glyme-containing derivatives. Optical Properties. Figure 4 shows the photoluminescence spectrum (PL, Jobin Yvon Fluromax-P spectrophotometer) and photobleaching behavior of the silanized Si NPs excited at 360 nm. In Figure 4a, the emission from naphthalene, poly-OTCS, and silanized Si NPs is presented. Compared to our silanized NPs at a comparable concentration, naphthalene and poly-OTCS can be regarded as PL transparent when excited at 360 nm. Figure 4b shows the PL from Si NPs with different surface terminations, and Figure 4c shows the PL from Si NPs in different organic solvents at the same concentration and excitation wavelength. As expected, the photophysics of the Si NPs is maintained regardless of variations in termination groups and solvents. Moreover, the NPs show no significant photobleaching (∼25% reduction) over 4000 s (Figure 4d) compared to organic fluorophores and polysilanes. (For example, a common organic dye, rhodamine 6G, completely bleaches in ∼10 min,26 and polysilane derivatives photobleach quickly upon exposure to UV light.38) These results are consistent with the luminescence from inorganic quantum dots rather than that from some unidentified fluorophores. The optical properties of silanization-terminated Si nanoparticles are further investigated by UV-vis spectroscopy (HP 8452A diode array spectrophotometer) and PL spectroscopy, shown in Figure 5a. A UV-vis absorption onset is at approximately 380 nm for the silanized sample in hexane Nano Lett., Vol. 4, No. 7, 2004

Figure 4. (a) Photoluminescence comparison of silanized Si NPs, poly-OTCS, and naphthalene in chloroform; emission vs wavelength. (b) Photoluminescence emission vs wavelength of Si NPs with various terminations. (c) Comparison of the photoluminescence emission spectra of Si NPs in various nonpolar solvents. (d) Photobleaching curve of silanized Si NPs and intensity of the emission at 403 nm under continuous irradiation as a function of time. All of the PL spectra are obtained from excitation at 360 nm.

Figure 5. (a) Normalized UV-vis absorption and PL spectra of silanized Si NPs in hexane. The maximum emission of NPs is at 392 nm with excitation at 330 nm. (b) Long-term stability plot of different terminated samples; PL intensity at 392 nm with excitation at 330 nm plotted as a function of time.

and extends toward lower wavelengths. This blue-shift absorbance from the band gap of bulk Si is considered to be the result of quantum confinement. The absorbance onset is close to that of other Si NPs synthesized by solution routes,16-18,24,25,39,40 which suggests that the silanization Nano Lett., Vol. 4, No. 7, 2004

treatment does not significantly change the photophysics of the particles. The emission PL spectra were collected with excitation wavelengths from 320 to 390 nm in 10-nm increments. The maximum intensity emission spectrum is centered at 392 nm (λmax) with an excitation wavelength of 330 nm. Because the sample has a distribution of sizes, a size-dependent luminescence property is expected in the PL spectra,40 as shown. Quite a few Si nanoparticles synthesized by various methods are air-sensitive, which undoubtedly hinders their practical application. A passivated surface is extremely important to the realization of functionalization and bioconjugation processes in subsequent steps.4,9 Silanizationterminated Si nanoparticles not only exhibit remarkable stability but also afford the potential of easy manipulation for more complex functionalizations. The silanized Si NPs maintain their optical stability under various excitations very well. The example that we present in this paper (Figure 5b) is the stability curve determined at an excitation of 330 nm. The samples measured are in chloroform solutions with the same initial concentration stored in sealed UV cuvettes. The silanized Si NPs were compared to the Cl-terminated Si nanoparticles and the octoxy-terminated Si NPs. All three differently terminated NPs luminesce in the same region. However, being susceptible to hydrolysis and subsequent decomposition, the Cl-terminated Si NPs degrade and lose their photoluminescence over time. The octoxy-terminated silicon NPs, although more stable than the Cl-terminated NPs, degrade over a slightly longer time (Figure 5b). The photoluminescence intensity of the silanized Si NPs sample remained approximately constant for 2 months (Figure 5b) with only a slight decrease of 5.7%, and they have shown 1185

photochemical stability for up to 1 year. Thus, the enhancement of photochemical stability in the silanized Si NPs is of significance to future Si NPs applications. Further silanization-termination studies on the Si NPs, such as terminations to give NPs with functional ending groups (amine, carboxyl acid, etc.), are underway and will be reported separately. The realization of those groups on Si surfaces will be key to developing water-soluble Si NPs and subsequent bioconjugation as well as the visualization of biospecies in vivo. Acknowledgment. This work was supported by an NSF grant (NIRT-0210807). R.K.B. received support from a NIST ATP grant via Evergreen Solar. We thank Jackie GervayHague, Gang-yu Liu, Margie Longo, Angelique Louie, and Satya Dandekar for useful discussions. Work at the National Center for Electron Microscopy (NCEM) was performed under the auspices of the Director, Office of Energy Research, Office of Basic Energy Science, Materials Science Division, U.S. Department of Energy under contract DEAc-03-76XF00098. References (1) Nirmal, M.; Brus, L. Acc. Chem. Res. 1999, 32, 407-414. (2) Michalet, X.; Pinaud, F.; Lacoste, T. D.; Dahan, M.; Bruchez, M. P., Jr.; Alivisatos, A. P.; Wiess, S. Single Mol. 2001, 4, 261-276. (3) Chan, W. C. W.; Maxwell, D. J.; Gao, X.; Bailey, R. E.; Han, M.; Nie, S. Curr. Opin. Biotechnol. 2002, 13, 40-46. (4) Parak, W. J.; Gerion, D.; Pellegrino, T.; AZanchet, D.; Micheel, C.; Williams, S. C.; Boudreau, R.; Le Gros, M. A.; Larabell, C. A.; Alivisatos, A. P. Nanotechnology 2003, 14, R15-R27. (5) Brus, L. J. Phys. Chem. 1994, 98, 3575-3581. (6) Canham, L. T. Appl. Phys. Lett. 1990, 57, 1046-1048. (7) Sailor, M. J.; Lee, E. J. AdV. Mater. 1997, 9, 783-793. (8) Cullis, A. G. J. Appl. Phys. 1997, 82, 909-965. (9) Stewart, M. P.; Buriak, J. M. AdV. Mater. 2000, 12, 859-869. (10) Heath, J. R. Science 1992, 258, 1131-1133. (11) Littau, K. A.; Szajowshki, P. J.; Muller, A. J.; Kortan, A. R.; Brus, L. E. J. Phys. Chem. 1993, 97, 1224. (12) Fojtik, A.; Henglein, A. Chem. Phys. Lett. 1994, 221, 363-367. (13) Schuppler, S.; Friedman, S. L.; Marcus, M. A.; Adler, D. L.; Xie, Y.-H.; Ross, F. M.; Chabal, Y. J.; Harris, T. D.; Brus, L. E.; Brown, W. L.; Chaban, E. E.; Szajowshki, P. F.; Christman, S. B.; Citrin, P. H. Phys. ReV. B 1995, 52, 4910-4925. (14) Bley, R. A.; Kauzlarich, S. M. J. Am. Chem. Soc. 1996, 118, 1246112462. (15) Dhas, N. A.; Raj, C. P.; Gedanken, A. Chem. Mater. 1998, 10, 32783281. (16) Wilcoxon, J. P.; Samara, G. A.; Provencio, P. N. Phys. ReV. B: Condens. Matter 1999, 60, 2704-2714.

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Nano Lett., Vol. 4, No. 7, 2004