Xylene-Capped Luminescent Silicon Nanocrystals: Evidence of

Jun 14, 2012 - Applied Material Science Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700 064, West Bengal, India. J. Phys. C...
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Xylene-Capped Luminescent Silicon Nanocrystals: Evidence of Supramolecular Bonding Arun Kumar Mandal,† Mallar Ray,*,† Indrajith Rajapaksa,‡ Smita Mukherjee,§ and Alokmay Datta§ †

School of Materials Science and Engineering, Bengal Engineering and Science University, Shibpur, Howrah 711103, West Bengal, India ‡ Department of Electrical Engineering and Computer Science, University of California, Irvine, California 92697, United States § Applied Material Science Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700 064, West Bengal, India

ABSTRACT: We report capping of silicon (Si) nanocrystals (NCs) via xylene attachment to the surface of oxide etched, luminescent, core/shell nanostructures of Si/Si-oxide in colloidal suspension. The core/shell nanostructures of Si/Si-oxide are formed by controlled oxidation of mechanically milled crystalline Si, which is subsequently etched in aqueous hydrofluoric acid to remove the oxide shell. Xylene attachment is confirmed by peak splitting and shifting in the Fourier transform infrared spectra for the xylene-treated samples in colloidal suspensions as well as for samples deposited on solid substrates like ZnSe. Structural investigations and spectroscopic evidence suggest that capping of Si NCs is associated with the formation of weak supramolecular bonds with the antibonding electrons of xylene. Therefore, we successfully achieve the much desired attachment of methyl groups onto the surface of luminescent Si NCs by a very weak physical bond with minimal modification of the surface chemistry of the NCs.

1. INTRODUCTION Bulk Si has an indirect band gap, which makes it unsuitable for use in light-emitting devices. Hence, discovery of light emission from nanostructured porous Si has drawn considerable research interest,1 as this opened up the scope for new Si-based lightemitting devices that may be potentially integrated with Si electronics.2−12 Also, Si nanocrystals (NCs) are widely believed to be nontoxic and environmentally benign,13 making Si quantum structures highly attractive for large-scale, environmental-friendly nanomanufacturing as well as for potential novel applications in biology and medicine. It is generally believed that the improvement of the optical properties of Si NCs is due to a combination of effects: the enhanced recombination rate of electrons and holes due to the increased overlap of the electron and hole wave function confined in the NC14 and the reduction of the rate of nonradiative defect mediated and three-body Auger recombination events.15 NCs, in general, are unstable against aggregation because of their large surface free energy. In colloidal solutions, this tendency of aggregation is balanced by thermal dissipative forces exerted by the solvent molecules, but once the solvent is removed, the NCs have to be stabilized through some capping © 2012 American Chemical Society

agent, preferably organic, which reduces the surface free energy of the NCs and creates a barrier against the aggregation.16 The capping may attach through chemical, that is, molecular,17 or physical, that is, supramolecular, bonds18 to the NC surface. Well-developed methods for capping surfaces of Si NCs with terminal alkyl groups or alkenes are extremely wellreported.19−23 Such processes usually involve the formation of stable Si−C covalent bonds. The starting point of such functionalization is usually achieved by halogen,24 or hydrogentermination of Si NCs (which are unstable and prone to oxidation under ambient exposure),25,26 followed by stabilization reaction with alkyl-lithium salts,24 or terminal alkenes27 to provide very stable Si−C linkages. Of course, a chemical bond has the tendency to modify drastically the surface chemistry, which consequently affects most properties of the NC,28 and this, under most circumstances, is not desirable. A supramolecularly bonded capping causes much less significant changes,29 while succeeding in their role as stabilizers. Received: February 21, 2012 Revised: May 11, 2012 Published: June 14, 2012 14644

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3. RESULTS AND DISCUSSION Figures 1 and 2 show TEM micrographs of HF-etched luminescent Si NCs (the luminescent properties and the

Therefore the evidence of a supramolecular bond between a capping material and light-emitting Si NCs is of importance technologically as well as from the point of view of basic understanding of how such a capping may affect the surface electronic states and hence the photonic properties of a Si NC. In this article, we present Fourier transform infrared (FTIR) spectroscopic evidence of weak, supramolecular bonding of xylene molecules as capping agent to hydrofluoric acid (HF)etched luminescent Si NC surface. This is the first step toward a future program of capping the nanoparticles with higher homologues of xylene, such as methyl stearyl benzenes, that are amphiphilic, which opens up the possibility of building multilayers of the NCs through the simple Langmuir−Blodgett (LB) deposition technique.30

2. EXPERIMENTAL SECTION Si/Si-oxide core/shell nanostructures were synthesized by chemical and thermal oxidation of mechanically milled Si powders (Alfa Aesar, 99.99% purity) following a method described elsewhere.31,32 In an attempt to remove the oxide shell, we treated ∼30 mg of the oxidized Si NCs with 10% aqueous buffered HF solution for ∼20 min with intermittent stirring. HF was then allowed to evaporate at ∼70 °C in an inert (nitrogen) atmosphere until dry deposits of Si NCs (with H termination as well as exposed unsaturated and dangling bonds) were obtained. Slow heating at ∼70 °C was continued for ∼30 min even after the powder appeared dry to ensure removal of trapped moisture. Analytical grade meta-xylene (mxylene) solution was subsequently added to dry deposits of the etched Si NCs as soon as the evaporation process was completed. The Si NC solution in m-xylene was subsequently centrifuged at 3000 rpm for 30 min, and the clear supernatant was taken out so that the larger particles and agglomerates were eliminated. This was followed by magnetic stirring under the exposure of intense UV light (365 nm, 60 W Hg lamp), and this step was repeated five times and the stirring duration was varied between 2 to 3 h. The exposure of the sample solution to intense UV was carried out because UV photodissociates the native oxide shell on the NC surface and facilitates the formation of an organic passivation.19 The samples were finally stored as colloids in clean bottles under ambient conditions. All chemicals and reagents used were of analytical grade, purchased from Merck, Germany, and used without further purification. Transmission electron microscopy (TEM) of the samples was carried out by depositing the samples in carbon-coated copper grids, and the bright-field images and diffraction patterns were recorded by a JEOL 3010 TEM, operating at 300 kV. Tapping mode atomic force microscopy (AFM) of the as-prepared and the xylene-treated samples were performed using an Asylum Research MFP-3D scanning probe microscope. Silicon tips with radius of curvature 6 nm, spring constant 3 N/m, and frequency 62 kHz procured from AppNano were used for recording the AFM images. Samples for AFM were prepared by dropcasting the samples on freshly cleaved mica after thorough ultrasonication. Mica was preferred as the substrate for AFM imaging because it provided atomically flat surface that enabled visualizing ultrasmall (≤20 Å) NCs. FTIR spectroscopy (Perkin-Elmer Spectrum GX) of capped Si NCs deposited on quartz were performed in transmission and attenuated total reflection (ATR) modes at a resolution of 4 cm−1.

Figure 1. TEM images of as-synthesized, HF etched Si NCs. (a) Bright-field image of the sample prior to xylene treatment showing the presence of Si NCs that appear as dark spots. The inset panel a is the photograph showing intense visible luminescence from the Si NCs under 350 nm UV excitation. (b) Corresponding SAED clearly reveals the (111), (220), and (311) planes of Si, as marked in the Figure. (c) Two partially overlapped Si NCs with discernible fringes of the atomic planes of Si along with the formation of Moiré fringes at the region of overlap that is indicated with an arrow. The boundaries marked by white rings in the images (a) and (c) are guides to the eye.

Figure 2. TEM images of Si NC after xylene treatment. (a) Extended agglomerates of xylene-attached Si NCs. The inset shows a photograph of blue luminescence from the xylene-treated samples when irradiated by 350 nm radiation. (b) Corresponding SAED of xylene-treated Si NCs reveals the presence of Si NCs. The white rings in panel a are to guide the eye.

possible mechanism of luminescence are discussed elsewhere)32,33 prior to and after xylene attachment, respectively. It is clear from Figure 1a that there are abundant Si NCs with sizes ranging from 2 to 6 nm that appear as dark spots in the bright-field image, but Figure 2a shows that xylene-treated NCs tend to form agglomerates, and it is not possible to locate welldispersed Si NCs. The existence of Si NCs shown in Figure 1a is further confirmed by the selected area electron diffraction (SAED) pattern, shown in the Figure 1b, where the (111), (220), and (311) planes are clearly discernible. The nearly continuous rings in the SAED pattern suggest random orientation of the neighboring crystallites, and the diffused halo indicates an amorphous background that may be due to either amorphous Si or Si oxide, which are expected to remain in small quantities in the milled samples exposed to the 14645

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Figure 3. AFM images of the samples deposited on cleaved mica substrate. (a) Topographic image showing well-dispersed Si NCs prior to xylene treatment, (b) the phase image corresponding to (a) revealing core/shell structures of Si NC/Si-oxide, (c) xylene-attached Si NCs forming extended agglomerates indicating interaction between the benzene rings present in xylene, and (d) the corresponding phase image showing the encapsulation of the etched Si NCs with xylene.

ambient.32,33 Figure 1c further reveals two partially overlapped Si NCs with distinctly identifiable crystalline fringes. The formation of Moiré fringes at the region of overlap is indicated with an arrow. All attempts to locate well-dispersed Si NCs for the xylenetreated samples proved to be unsuccessful. Figure 2a is a representative TEM for this sample, showing the formation of extended agglomerates and indicating the possibility of entrapment of the Si NCs by xylene, which forms a methylated capping over the NCs and consequently hinders imaging the individual NCs, which are identifiable in the untreated Si NC sample shown in Figure 1a. However, the SAED shown in Figure 2b does reveal the presence of Si NCs, which provides support to the proposition of possible xylene encapsulation of the Si NCs thereby forming a thick xylene shell over the Si NCs. In this stage of investigation, it is difficult to comment about the size or the size distribution of the shell covering the NC, but we can tentatively infer that the xylene-attached Si NCs have sizes well below 50 nm, as the agglomerates themselves appear to be in the size regime of 50 nm or below. As previously mentioned, the disappearance of fringes, which are clearly visible for the untreated Si NCs (Figure 1c), suggests that the xylene probably encapsulates the Si NCs, which on one hand hinders visualizing the NCs and on the other hand promotes agglomeration due to interaction between the benzene rings present in xylene, as shown in Figure 2a. Topographic and phase-contrast AFM images of the assynthesized Si NCs and the xylene-attached Si NCs are shown in Figure 3a−d. Nearly spherical and well-distributed particles with sizes (estimated from the height profiles) varying from 2

to 8 nm are clearly visible in Figure 3a. The corresponding phase image shown in Figure 3b reveals that what appears as a single particle in the topography is actually made of two different phases that manifest themselves in the distinct difference in contrast. Phase imaging provides a map of stiffness variation on the sample surface such that a stiffer region appears brighter, thereby providing a means for differentiating phases with different elastic moduli.34 Recent works attribute contrast variations in AFM images with the changes in viscoelasticity.35 It may be asserted here that whatever be the reason of contrast variation, the image shown in Figures 3b clearly reveals the existence of two distinct phases, the darker core representing crystalline Si, which is verified by the TEM images (Figure 1a), and the brighter shell representing amorphous Si oxide. The oxide shell is expected to exist even for the HF-etched samples as the H-passivated Si− H bonds are preferentially replaced by Si−O−Si bonds on exposure to the atmosphere.33 Because deposition of the HFetched samples on the substrate and subsequent imaging were carried out under ambient conditions, it is obvious that we get Si NC/Si-oxide core/shell structures as previously evidenced.32 However, in this case, the size of the oxide shell estimated from the line profile varies from 1 to 8 nm (and the core size from 1 to 4 nm), which is significantly smaller than the thickness previously reported32 for samples that were not HF-etched. Moreover, the formation of this oxide layer is prevented for the xylene-treated samples because the etched NCs are not allowed to come in contact with ambient oxygen. It is interesting to note that the well-dispersed particles as seen in Figure 3a become extended agglomerates when treated with xylene, as shown in Figure 3c. In the phase image (Figure 3d), we see 14646

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Figure 4. Convoluted and deconvoluted ATR-FTIR spectra: (a,b) in the range of 660 to 785 cm−1 and (c,d) in the range of 1260 to 1340 cm−1, showing benzene ring vibrational modes. The two regions of the spectra are shown separately for clarity. Compared with the spectra of bare xylene shown in panels a and c, respectively, panels b and d clearly depict the distinct appearance of peak splitting and shifting for the xylene-treated Si NCs.

Figure 5. Convoluted and deconvoluted FTIR spectra recorded in the transmission mode in the range of 670 to 900 cm−1, showing benzene ring vibration and distinct peak combining, splitting, and shifting for the xylene-treated Si NCs deposited on ZnSe in panel b compared with bare xylene shown in panel a.

the IR-active vibrational modes of xylene-treated NCs. We obtained some specific variations in the FTIR spectra in the range of 660 to 1340 cm−1, which in all possibility is due to the interaction of Si NCs with xylene, as shown in Figure 4. For the sake of clarity, two regions of interest of the entire spectral region spanning from 660 to 1340 cm−1 have been separately presented, where Figure 4a,b highlights the region ranging from 660 to 785 cm−1 and Figure 4c,d is for the spectral region varying from 1260 to 1340 cm−1 respectively.

darker spots representing crystalline Si surrounded by smeared and ill-defined thick layer, which presumably is xylene. The formation of such extended agglomerates as evidenced in Figure 3c,d almost mimics the findings of TEM shown in Figure 2 and is indicative of some long-range supramolecular interaction of the benzene rings present in xylene that is consistent with the spectroscopic evidence discussed later and also with some previous findings.36 FTIR studies of the as-synthesized, HF-etched, and xyleneattached samples and of pure xylene were performed to study 14647

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The spectrum for the region 785 to 1260 cm−1 is not shown here because there is no resolvable peak shift/variation for bare xylene and xylene-attached Si NCs over this region. The Figure 4a−d depicts several small but significant variations of absorbance peaks of xylene-treated Si NC colloids from that of bare xylene. Comparing Figure 4a,b, we see that the absorbance peaks at 688 and 701 cm−1 for bare xylene are split into two peaks at 686 and 692 cm−1 and at 698 and 709 cm−1, respectively, for the xylene-treated NCs. The bands centered around 688 cm−1 and 701 cm−1 have been assigned to the outof-plane bending of the aromatic ring in xylene molecules.37 While comparing Figure 4c,d, we see that the two absorbance peaks at 1283 and 1290 cm−1 in xylene tend to combine with each other to form a single absorbance peak at 1291 cm−1 in xylene-attached Si NC colloids. Also, the peak at 1329 cm−1 for bare xylene split into two peaks at 1324 and 1331 cm−1 for the xylene-treated sample. Both peak splitting and combining in this range of frequency refer to the C−C skeletal vibration of the benzene ring in xylene molecules, as noted in Figure 4a,b.37 To confirm the variation of the absorbance peak in FTIR spectra of bare xylene and xylene-attached Si NCs, both samples were dropcast on ZnSe substrate, which is a widely used standard substrate for FTIR studies because it is practically transparent over the entire mid-IR region.38 We recorded the FTIR spectra in transmission mode of both samples in the region 650 to 4000 cm−1. No significant variation of the spectral features between bare xylene and the treated NCs is observed in the spectral range of 900 to 4000 cm−1 and hence is not presented here. However, within the range of 650 to 900 cm−1, we could see definite indications of peak merging as well as splitting, which are shown in Figure 5a,b. Comparison of Figure 5a,b reveals that most of the Gaussian-fitted absorbance peaks present in bare xylene combine with each other and tend to form a single peak in the case of xylene-treated NCs. From the peak positions in the absorption spectra, we see that the peak shifts are on the order of 10 cm−1 or less. Therefore, we see that the FTIR spectra of the colloidal samples recorded in the ATR mode and that recorded for samples deposited on ZnSe substrate and recorded in the transmission mode are in good agreement with each other. Both show signatures of small peak shifts (∼10 cm−1) along with peak splitting and peak merging, thereby indicating similar mechanism. Such small shifts in the IR absorption peaks are indicative of extremely weak physical bonding (∼0.0286 kcal/mol) between Si NC and the aromatic C present in the benzene ring. It is known that benzene is chemisorbed on Si(001) surface by di-σ bonding with the two Si−Si dimer dangling bonds,39 and on the Si(111) surface benzene behaves as an electron donor with respect to the dangling bond sites.40 With the addition of methyl groups on the benzene ring, such as toluene, the bonding is enhanced for the Si(001) surface,41 whereas it is reduced for the Si(111) surface.40 The features observed in the FTIR spectra of the xylene-treated Si NCs suggest that although there is some weak interaction between Si NCs and xylene it is unlikely to represent the formation of a strong or chemical bond. In fact the energy associated with this bond is about two orders of magnitude lower than the hydrogen bond and falls in the range of weak dipolar or higher order multipolar bond. We propose that for the Si NCs the facets are (111), and whereas the benzene ring may still act as an electron donor to the dangling bonds on the NC surface, the presence of the two methyl groups in xylene is inhibiting a charge-transfer complex from being formed, and the charge localizations on the ring and

the dangling bonds form a supramolecular physical bond through weak dipolar (or multipolar) interactions, as schematically presented in Figure 6.

Figure 6. Schematic diagram illustrating the formation of a supramolecular bond between the dangling bonds on the surface of a Si NC and antibonding electrons present in the benzene ring of a xylene molecule. Although a large number of xylene molecules attach to a Si NC, only a part of a Si NC with few xylene molecules is shown for the sake of clarity. Both of the structures of Si NC with H termination and dangling bonds and the xylene molecules are individually energy-optimized, but the entire structure is a simple schematic and not drawn to scale. It is assumed in the above schematic that after HF etch of the Si/Si-oxide core/shell nanostructure the entire oxide is removed and mostly replaced by H or left as unsaturated dangling bonds.

The proposition of steric encapsulation of light-emitting Si NCs by xylene via supramolecular bonds not only explains the FTIR absorption and the features seen in the TEM and AFM images but also lends support to the fact that the blue emission from the ultrasmall Si NCs under UV excitation (as shown in insets of Figures 1a and 2a) is almost unaffected after xylene treatment. Albeit the fact that so far we have not been able to isolate individual nanospheres of xylene-capped Si NCs, as they always tend to form agglomerates, this method provides a novel and simple route of forming methyl-group-encapsulated nanostructures of luminescent Si, which is expected to render these exciting nanoscale objects amenable for multifarious applications.

4. CONCLUSIONS In summary, we have successfully achieved the so-far elusive methyl group attachment onto the surface of luminescent Si NCs by capping them with xylene. The xylene molecules attach to the unsaturated or dangling bonds of HF-etched Si/Si-oxide core/shell nanostructures. Structural investigations reveal that a relatively thick xylene shell surrounds the Si NC core. FTIR characteristics of the xylene-treated NCs suggest that the nonbonded electrons present in the HF-etched Si NCs interact weakly with the antibonding electrons present in the benzene rings of xylene, thereby forming a supramolecular bond. Such methyl-group-encapsulated nanostructures of luminescent Si have many potential applications in nanoscale science and nanotechnology. 14648

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +91 33 2668 8140. Fax: +91 33 2668 9126. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is partially supported by Department of Science and Technology, India (No.DST/INT/AUS/PROJ/T-2/08). M.R. acknowledges the support of Indo-US Science and Technology Forum, DST. We all thank Prof. H. K. Wickramasinghe, UC, Irvine, and Dr. Robert F. Klie, UIC, for assisting in AFM and TEM imaging, respectively.

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