Ag Nanostructures on a Silicon Nanowire Template: Preparation and

Etching Behavior of Silicon Nanowires with HF and NH4F and Surface Characterization by Attenuated Total Reflection Fourier Transform Infrared Spectros...
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Chem. Mater. 2002, 14, 2519-2526

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Ag Nanostructures on a Silicon Nanowire Template: Preparation and X-ray Absorption Fine Structure Study at the Si K-edge and Ag L3,2-edge X. H. Sun,† R. Sammynaiken,‡ S. J. Naftel, Y. H. Tang, P. Zhang, P.-S. Kim, and T. K. Sham* Department of Chemistry, University of Western Ontario, London, Ontario, N6A 5B7 Canada

X. H. Fan, Y.-F. Zhang, C. S. Lee, and S. T. Lee Center Of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong, Hong Kong SAR, China

N. B. Wong COSDAF and Department of Biology and Chemistry, City University of Hong Kong, Hong Kong SAR, China

Y.-F. Hu and K. H. Tan Canadian Synchrotron Radiation Facility, Synchrotron Radiation Center, University of WisconsinsMadison, Madison, Wisconsin 53589 Received October 29, 2001. Revised Manuscript Received February 12, 2002

We use X-ray absorption fine structure (XAFS) spectroscopy at the Si K-edge and the Ag L3,2-edge to monitor the surface chemistry of silicon nanowires. We found that Si nanowires prepared by laser ablation and thermal evaporation are coated with a thick layer of oxides that can be readily removed with a HF solution. HF-refreshed silicon nanowires become a moderate reducing agent and can be used as a nanostructure template upon which silver nanoaggregate deposited reductively from a Ag+ aqueous solution, reoxidizing the surface in the process. These results are compared with those of porous silicon.

I. Introduction In recent years, pseudo one-dimensional quantum wires of silicon and related materials have attracted much attention because of their exhibition of quantum confinement effects and potential technological applications.1-11 These materials are expected to play a vital role both as interconnects and functional components * To whom correspondence should be addressed. † Visiting scholar from the City University of Hong Kong. ‡ Present address: Structural Science Centre, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 5E5 Canada. (1) Zhang, Y. F.; Tang, Y. H.; Wang, N.; Yu, D. P.; Lee, C. S.; Bello, I.; Lee, S. T. Appl. Phys. Lett. 1998, 72, 1835. (2) Morales, A. M.; Lieber, C. M. Science 1998, 279, 208. (3) Yu, D. P.; Bai, Z. G.; Ding, Y.; Hang, Q. L.; Zhang, H. Z.; Wang, J. J.; Zou, Y. H.; Qian, W.; Xiong, G. C.; Zhou, H. T.; Feng, S. Q. Appl. Phys. Lett. 1998, 283, 3458. (4) Wang, N.; Zhang, Y. F.; Tang, Y. H.; Lee, C. S.; Lee, S. T. Phys. Rev. B 1998, 58, 16024. (5) Tang, Y. H.; Zhang, Y. F.; Wang, N.; Lee, C. S.; Han, X. D.; Bello, I; Lee, S. T. J. Appl. Phys. 1999, 85, 7981. (6) Au, C. K. F.; Wong, K. W.; Tang, Y. H.; Zhang, Y. F.; Bello, I.; Lee, S. T. Appl. Phys. Lett. 1999, 75, 1700. (7) Volz, S. G.; Chen, G. Appl. Phys. Lett. 1999, 75, 2056. (8) Lee, S. T.; Wang, N.; Zhang, Y. F.; Tang, Y. H. Mater. Res. Soc. Bull. 1999, 24, 36. (9) Cui, Y.; Duan, X.; Hu, J; Lieber, C. M. J. Phys. Chem. B 2000, 104, 5213. (10) Zhang, Y. F.; Liao, L. S.; Chan, W. H.; Lee, S. T.; Sammynaiken, R.; Sham, T. K. Phys. Rev. B 2000, 61, 8296. (11) Sun, X. H.; Peng, H. Y.; Tang, Y. H.; Shi, W. S.; Wong, N. B.; Lee, C. S.; Lee, S. T.; Sham, T. K. J. Appl. Phys. 2001, 89, 6396.

in future mesoscopic electronic optical devices. For example, the so-called bottom-up approach for fabricating a molecular computer has been to date under rapid development.12 Because silicon is of great technological importance in microelectronics, Si nanowires (SiNWs) have attracted great attention lately. Many successful synthesis strategies to obtain bulk quantities of SiNWs have now been developed using both gas-phase and condensed-phase procedures.1-9 While a number of properties such as the morphology, structure, photoluminescence, electron field emission, and thermal conductivity of SiNWs have been studied,5-7 information on the surface chemical properties of SiNWs is relatively lacking. Clearly, the chemical properties of SiNWs surfaces are crucial to their application in mesoscopic electronic devices in terms of stability and transport properties, among others. Recently, we reported the redox surface chemistry of SiNWs in connection with the fabrication of metallic nanostructures using a nanostructure template by studying the reaction of SiNWs with a number of metal ions such as silver and copper in solution.11 We found that the HF-etched SiNW surface is hydrogen-passivated and can readily reduce silver and copper ions to metal nanostructures at room temperature, becoming reoxidized in the process. In a (12) Dagani, R. Chem. Eng. News 2000, Oct 16, 27.

10.1021/cm011548n CCC: $22.00 © 2002 American Chemical Society Published on Web 04/24/2002

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recent study, the reaction products from the SiNWmetal ion reaction were characterized by TEM, SEM, EDX, EELS, and XPS.11 Here, we report further investigation of the electronic properties and local structure of the SiNWs and its interaction with Ag ions using X-ray absorption fine structure (XAFS). This approach is analogous to recent studies of the surface chemistry of porous silicon (PS),13-16 a spongelike interconnecting network of nanowires with nodules and pillars of the size of nanometers.17 The surface chemical behavior of SiNWs and PS and their implications will be noted. XAFS refers to the modulation of the absorption coefficient above an absorption threshold (edge) of a core level of an element in a chemical environment.18 Because the initial state is atomic and the excitation probes the surrounding of the absorbing atom, XAFS is sensitive to the local environment (structure and bonding) of the absorbing atom. When the photon energy increases from below to above the edge, the absorption coefficient often exhibits an abrupt and dramatic increase at the threshold (edge jump). At the Si K-edge, for example, the dipole transition (∆l ) (1, ∆j ) 0, (1) of the Si 1s electron to the unoccupied states of p character in the conduction band and to continuum states turns on. Thus, the Si K-edge edge jump arises from the excitation of a 1s electron into the bottom of the conduction band.19 Similarly, the Ag L3,2 -edge (p3/2 and p1/2 level) probes the unoccupied states of both d and s character above the Fermi level with the former heavily weighted in transition probability.20 The absorption coefficient sometimes exhibits sharp spikes at the edge (called whitelines) followed by spectral resonance in the vicinity of the edge (≈40-50 eV above the edge, often called X-ray absorption near-edge structures, XANES) and oscillations as much as 1000 eV above the edge (extended X-ray absorption fine structure, EXAFS). These spectral features are intimately related to the extended local environment (several shells of neighboring atoms) and contain a wealth of chemical information about the local environment of the absorbing atom.18 In this study, we use XAFS to monitor the Ag+(aquo)SiNW interaction in two types of silicon nanowire specimens. One exhibits a smooth, pseudo-one-dimensional morphology with no nodules, henceforth denoted type A or normal SiNW; the other is an approximately 50%/50% mixture of normal SiNW and nanoparticlechain (pearl-necklace-like) Si nanowires, henceforth denoted type B. Type A is typically produced using a laser ablation technique1-4 while type B is often the result of thermal evaporation5 at higher temperatures. Figure 1 shows the TEM image of the nanoparticlechain SiNW in coexistence with the normal type. The (13) Andsager, A.; Hillard, J.; Hetrick, J. M.; Abu-Hassin, L. H.; Plisch, M.; Nayfeh, N. H. J. Appl. Phys. 1993, 74, 4783. (14) Sham, T. K.; Coulthard, I.; Lorimer, J. W.; Hiraya, A.; Watanabe, M. Chem. Mater. 1994, 6, 2085. (15) Coulthard, I.; Sham, T. K. Appl. Surf. Sci. 1998, 126, 287. (16) Coulthard, I.; Sammynaiken, R.; Naftel, S. J.; Zhang, P.; Sham, T. K. Phys. Status Solidi 2000, 182, 157. (17) Cullis, A. G.; Canham, T. L.; Calcott, P. D. J. J. Appl. Phys. 1997, 82, 909. (18) See for example Kroningsberger, D. C., Prins, R., Eds.; X-ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS and XANES; Wiley: New York, 1988. (19) Sham, T. K.; Coulthard, I.; Jiang, D. T.; Lorimer, J. W.; Feng, X. H.; Tan, K. H.; Frigo, S. P.; Rosenberg, R. A.; Houghton, D. C.; Bryskiewicz, B. Nature (London) 1993, 363, 331. (20) Coulthard, I.; Sham, T. K. Phys. Rev. Lett. 1996, 77, 4842.

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Figure 1. TEM image of silicon nanowires containing nanoparticle chains.

beads (nanoparticles) in the nanoparticle chains are clearly visible. II. Experimental Section SiNWs were synthesized by laser ablation and thermal evaporation techniques as described previously.1-5,8 The assynthesized SiNWs are literally long (millimeters in length), free-standing wires with a diameter of several nanometers to tens of nanometers, depending on the preparation conditions, and are usually encapsulated by a silicon oxide layer. Two type-A (normal) SiNWs specimens with nominal diameters of 25 and 6 nm (revealed by TEM) and a type-B (normal and nanoparticle-chain mixture) specimen with a nominal diameter of ≈10 nm were used in this study. The as-prepared SiNW are relatively inert because of the stability of the Si oxide layer. Our main interest here is the surface chemistry of hydrogenpassivated SiNWs resulting from the treatment of the asprepared SiNWs with a HF solution. In the experiments, all SiNWs were treated by HF solution of various concentrations and different duration. XAFS was employed to monitor the effect of HF following each stage of the treatment and subsequent chemical processes. The HF concentration, treatment duration, and number of treatments were determined by the amount of oxide signal (a characteristic resonance at ≈1848 eV, see below) remaining after each treatment. The process continued until the silicon oxide signal was no longer observable at the Si K-edge XAFS. For example, in a typical sequence, the SiNW specimen was immersed in a 1% HF solution for 2 min, for another 4 min, and then in 5% HF for 2 min. After HF etching, Ag deposition was carried out by exposing the SiNW to droplets (≈0.1 mL) of a silver nitrate aqueous solution of concentrations of 1 × 10-4 M or 5 × 10-5 M for 30 s. XAFS measurements were made at each step of the procedure. For comparison, a similar experimental procedure was also carried out with a porous silicon (PS) specimen. The PS was prepared by electrochemical etching of a Si(100) wafer (n-type) at room temperature.15 The preparation condition of a current density of 100 mA/cm2 and a 30-min duration was used. XAFS measurements were carried out at the Double Crystal Monochromator (DCM) beamline of the Canadian Synchrotron Radiation Facility (CSRF) located at the Synchrotron Radiation Center (SRC), University of WisconsinsMadison. SRC is

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Figure 2. TEM of Ag deposited on SiNWs. an 800 MeV/1 GeV, second-generation storage ring operating at ≈250-mA injection current. The Canadian DCM at SRC with InSb(111) crystals and a post-mirror at 3.5-keV cutoff provides an excellent match of optics and source for Si K-edge measurements21 and is ideally suited for Si K-edge as well as Ag L3,2-edge studies of Ag/SiNW. Both total electron yield (TEY) using the sample current and fluorescence yield (FLY) using a channel plate detector were used to monitor the absorption.22 In the soft X-ray energy region, as was the case in this study, TEY has short sampling depths and provides information above the surface and near-surface region of the sample (of the order of nanometer to 10 nm), while FLY, due to the much longer attenuation length of the fluorescence photons in condensed matters, probes the “bulk” (of the order of 10-102 nm) of the sample of interest.22

III. Results and Discussion A. Morphology and Surface Chemistry. The morphology and chemistry of SiNW with Ag and Cu ions from solution have been recently reported.11 The wires are often encapsulated with a layer of thick oxide, the presence of which strongly suggests an oxide-assisted linear growth mechanism.8 The oxides are removed in the experiments with HF prior to the reaction with silver ions. Upon the reaction with HF, the surface forms silicon hydrides readily. It has been established that hydrogen atom-silicon interaction leads to the formation of surface hydrides (from monohydride to trihydride).23 It is entirely conceivable that similar hydrides are formed in a HF solution. Thus, it is the Si-H bond that is intimately involved in the reaction yielding H2 (gas), Ag metal, and SiO2. A typical image (21) Yang, B. X.; Middleton, H. F.; Olson, B. G.; Bancroft, G. M.; Chen, J. M.; Sham, T. K.; Tan, K. H.; Wallace, D. J. Nucl. Instrum. Methods 1992, 316, 422. (22) Kasrai, M.; Lennard, W. N.; Brunger, R. W.; Bancroft, G. M.; Bardwell, J. A.; Tan, K. H. Appl. Surf. Sci. 1996, 99, 303. (23) Lu, Z. H.; Griffiths, K.; Norton, P. R.; Sham, T. K. Phys. Rev. Lett. 1992, 68, 1343.

of Ag on SiNW is shown in Figure 2. The reductive metal ion-silicon interaction, particularly the ions of noble metals, has been observed in porous silicon.13-16 It has been proposed that the following mechanism leads to the formation of metallic silver nanostructure on porous silicon templates:16

Ag+(aq) + 4Si-H(surface) w Ag(s) + H2(g) + 4Si(surface) + 2H+(aq) (1a) Ag+(aq) + Si(surface) + 2H2O(aq) w Ag(s) + H2(g) + SiO2 + 2H+(aq) (1b) According to this scheme, the silver ion was first reduced by the surface hydride, exposing the silicon atom (eq 1a), that in the presence of water further reduces the silver ion forming SiO2 and suboxide (eq 1b). Evidence supporting this mechanism includes the observations of the evolution of gas, formation of metallic Ag, SiO2, and silicon suboxide, and a noted increase in acidity of the residue solution. We believe that the same mechanism is taking place for the Ag+-SiNW interaction described here. B. Si K-Edge Studies of Si Nanowires. Figure 3a,b shows the TEY and FLY Si K-edge XAFS of a normal SiNW specimen (≈25 nm) following stages of treatment: from as-deposited to after reaction with a silver nitrate solution. All spectra have been normalized to unity edge jump and shifted vertically for clarity. The near-edge region is shown in the inset. The Si K-edge XAFS of a clean Si(100) wafer is also shown. As noted above, the as-prepared SiNWs are encapsulated by a relatively thick oxide layer and are generally chemically inert. To study the oxide-free Si surface of the SiNWs, a controlled HF treatment was used to remove the Si oxide layer. The surface thus-obtained is passivated

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Figure 3. Si K-edge XAFS of normal SiNWs (≈25 nm) with a series of chemical treatments: (a) TEY (left panel) and (b) FLY (right panel). The XANES region is shown in the inset.

with the formation of surface silicon hydrides that not only greatly inhibit surface oxidation but also become a moderate reducing agent. It can be seen from Figure 3 that the as-prepared sample exhibits an intense resonance at ≈1848 eV in the TEY. It is still noticeable in the FLY but with a much-reduced intensity. This peak is characteristic of a Si 1s to 3p (t2 orbital in a Td symmetry) transition of SiO2 and is given rise by the outer silicon oxide layer of the SiNW. The shoulder at ≈1845 eV can be clearly seen in the inset and is characteristic of silicon suboxide that is most certainly at the silicon oxide and crystalline silicon interface. The first resonance at ≈1840 eV arises from the unoxidized crystalline Si core of the nanowire. Close examination reveals that the doublet that is clearly observable in Si(100) is blurred in SiNW. This feature is associated with the band structure of crystalline Si (long-range order). The observation of a blurred Si K-edge whiteline in SiNW indicates a deterioration of long-range order in these nanowires. This observation is in good agreement with previous observations in porous silicon and SiNW.10 We now examine the FLY results (Figure 3b). The whiteline in the XANES region is generally less intense and noticeably broader because of the thickness effect (self-absorption),18 although the positions of all the peaks are the same. Thus, FLY is less reliable in oscillators strength (transition rate) estimates. However, FLY is bulk-sensitive and probes the inside of the nanowire. When used together with TEY, FLY provides valuable information about the structure inside the nanowire noninvasively. For example, a much-reduced oxide signal in FLY relative to that of the unoxidized silicon strongly indicates that the oxide is on the surface of the nanowire.

From Figure 3, we also see that, despite the variation in the XANES region (inset), the XAFS region (1860 eV and beyond) for all the SiNW spectra is essentially the same in its oscillatory pattern as that of Si(100) in both TEY and FLY, even in the as-prepared sample. In the latter case, the number of Si atoms in the surface oxide layer is still small compared to those in the core and Si-O EXAFS is weaker than that of Si-Si. This observation indicates that only the surface of these SiNW specimens is involved in the chemistry; the core of the SiNW is crystalline silicon albeit with some disorder (reduction in EXAFS amplitude) and remains intact under our experimental conditions. After HF treatment, the oxide peak disappears, indicating that the surface oxide has been removed. The surface silicon hydride peak cannot be clearly identified because of the small backscattering amplitude of hydrogen compared to Si and a huge silicon background. The slope of the rising edge of the HF-treated sample becomes less steep. This may indicate the presence of hydrides as the result of a chemical shift. More details of this issue will be dealt with elsewhere. We now return to the inset of Figure 3a. Close inspection of the XANES reveals that, upon Ag deposition (10-4 M), there is a slight increase in the oxide signal and in the intensity (relative to the whiteline) of a resonance at ≈1853 eV. This observation hints that the surface becomes reoxidized, even at this low concentration, and there exists Ag-Si interaction at the interface. We will return to these issues below. The TEY and FLY spectra of another normal SiNW sample of a significantly smaller nominal diameter (6 nm) are shown in Figure 4a,b. The size of the SiNW in this specimen is particularly interesting in that the surface oxide layer is almost as thick as the core, and

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Figure 4. Si K-edge XAFS of normal SiNWs (≈6 nm) with a series of chemical treatments: (a) TEY (left panel) and (b) FLY (right panel). The XANES region is shown in the inset.

after HF treatment, the diameter of the remaining unreacted core is ≈3 nm, in the regime of quantum confinement behavior. The aspect ratio is high, ≈20% surface atoms. Thus, the contribution of the surface Si atoms will be noticeable in both TEY and FLY. The as-prepared sample clearly shows a very intense SiO2 resonance relative to Si in the XANES (inset) in both TEY and FLY and compared to the 25-nm specimen (Figure 3). It shows a less intense suboxide shoulder however. This observation is consistent with the size of the specimen as discussed above and the presence of suboxide at the interface. The EXAFS is noticeably different from that of the clean Si and shows some beating of more than one type of EXAFS oscillations, indicating the presence of a significant surface oxide EXAFS component. The EXAFS oscillating pattern characteristic of crystalline Si returns, however, upon HF treatment, similar to the behavior of the 25nm specimen. These observations are expected from a small diameter SiNW specimen encapsulated by a relatively thick oxide layer. The most interesting feature is seen in the Agdeposited sample. Both Si K-edge TEY and FLY show a three-peak pattern in the XANES, uncharacteristic of crystalline silicon. The resonance at ≈1848 eV is due to SiO2, which results from the redox reaction involving the reductive deposition of Ag+ and the oxidation of the surface Si-H. The most noticeable feature is the broadening and skewing of the Si whiteline that shifts slightly to lower photon energy and the presence of an intense resonance at 1853 eV. This behavior is characteristic of many metal silicides (Si gains charge through redistribution).24 The EXAFS are very noisy, indicating some specimen inhomogenuity. We attribute the XANES features to either a texture effect that is sensitive to (24) Naftel, S. J.; Coulthard, I.; Sham, T. K.; Xu, D.-X.; Das, S. R. Phys. Rev. B 1998, 57, 9179.

the polarization of the photon or the presence of a strong Ag-Si interaction at the interface, forming an interface silicide. We shall come back to this in the Ag L3,2-edge discussion. We now turn to the type-B SiNW specimen that is a mixture of normal nanowires and chains containing nanoparticles connected by normal nanowires (Figure 1) with a nominal diameter of 10 nm. Figure 5a,b shows, respectively, the TEY and FLY Si K-edge XANES recorded for the successive treatment of the SiNW starting from the as-prepared and then 1% HF etching for 2 min, 1% HF etching for another 4 min, and 5% HF etching for 2 min and finally reacting with 1 × 10-4 M AgNO3 solution. All spectra have been normalized to the incident photon flux and a unity edge jump. The resonance characteristic of crystalline Si and SiO2 in SiNW are at ≈1841 and 1848 eV, respectively. The intensities of these resonances can be used to monitor the relative amount of surface oxide.10,22 It is interesting to note that the as-prepared SiNW exhibits a very strong SiO2 peak at ≈1848 eV, relative to the Si K-edge jump (whiteline) of the crystalline Si. It is due to a large surface Si (oxide) atom to bulk (wire) Si atom ratio and that TEY is more sensitive to the surface oxide than the internal Si. This is supported by the FLY results that show that the oxide signal is still quite large. Compared with the normal SiNWs with a diameter of 25 and 6 nm (Figures 3 and 4), this type-B specimen appears to exhibit a significantly larger Si in oxide to crystalline Si ratio. This observation indicates that the beads (nanoparticles) in the chains are likely encapsulated entirely by a thick layer of silicon oxide as suggested by TEM.5 Hence, the specimen has a relatively large surface area compared to other SiNWs. With HF etching, the crystalline Si K-edge jump increases gradually. This is accompanied by the reduction and finally the disappearance of the SiO2 resonance. More extensive HF treatment is required here to

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Figure 6. Si K-edge XAFS of porous silicon with a series of chemical treatments: (a) TEY (top panel) and (b) FLY (bottom panel).

Figure 5. Si K-edge XAFS of nanoparticle-chain nanowires (≈10 nm) with a series of chemical treatments: (a) TEY (top panel) and (b) FLY (bottom panel).

completely remove the surface oxide compared to the required treatment for the other SiNW specimens. When the surface oxide is removed, the type-B SiNW exhibits the same Si K-edge XANES as pure crystalline Si as were the case in other SiNW specimens and in porous silicon. The surface is hydrogen-terminated.10 As noted above, Si-H XAFS is difficult to detect on the huge background of crystalline silicon, the slightly reduced whiteline intensity, and the blurring of finer details hints at the presence of surface hydrides. The enhanced resonance at ≈1853 eV may also be a sign of Si-H interaction. These spectroscopic issues will be dealt with elsewhere. After immersion into the silver nitrate solutions, the SiO2 peak at 1848 eV reappears with a more visible shoulder. This shoulder is attributed to the suboxide at the interface. The silicon oxide results from surface reoxidization of HF-etched SiNWs by the silver ions in the solution. It confirms the XPS results that also show the reappearance of the oxides.11 FLY results show very similar features except that the intensity ratio of Si to SiO2 is greater than that in TEY because the FLY is a more “bulk” sensitive technique. The Si K-edge XAFS of porous silicon (PS) are shown in Figure 6. PS is a spongelike silicon nanowire network with nodules and pillars of the size of nanometers. It

has been shown that PS is crystalline albeit with considerable disorder.10,17 It can be seen from Figure 6 that the behavior of PS and SiNWs is essentially the same: the as-prepared sample is covered with a thick oxide that can be removed by HF; the surface is reoxidized by silver ion in the redox process. Thus, the redox process in SiNWs, hence surface chemical property, is very similar to that of porous silicon. Returning to the SiNWs, there is one feature that is noteworthy in the Si K-edge XAFS collectively. We observed that the as-prepared SiNWs show considerable amorphous oxide features in the spectra; after HF etching, SiNWs exhibit the pronounced crystalline silicon feature. Thus, the as-prepared SiNW was covered with a thick amorphous oxide layer. However, the reoxidation of the SiNW surface by Ag ion shows that the thickness of the surface oxide is substantially less under these experimental conditions than that in the as-prepared sample. This observation reveals that the amorphous oxide layer on the surface of the SiNW must be formed in the process of SiNW growth rather than due to exposure to air after growth. This result provides additional evidence to the oxide-assistant growth mechanism of SiNW.8 Finally, there appears to be very little difference in the chemistry of all the Si nanowires specimens (other than the amount of surface oxide) and the chemical behavior between silicon nanowires and porous silicon. The common factor appears to be the dimension of the nanostructure and the associated surface area. C. Ag L3,2-Edge Studies of Ag+(aq)-SiNW Interaction. Figure 7 shows the TEY Ag L3,2-edge XANES

Ag Nanostructures on a Silicon Nanowire Template

Figure 7. Ag L3,2-edge XANES (TEY) of Ag/SiNWs (normal: ≈25 and ≈6 nm) compared with that of Ag metal.

Figure 8. Ag L3,2-edge XANES (TEY) of Ag/SiNWs (chain: ≈10 nm) compared with that of Ag metal.

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shown. All spectra have been normalized to the incident photon flux and subtracted with a pre-edge background so that the edge jump is related to the amount of Ag within the TEY sampling depth (limited by the electron escape depth). At high concentration, Ag exhibits a greater edge jump than that for low-concentration deposits. Thus, Ag+ is the limiting reagent in the reaction. The XANES in all spectra exhibit identical oscillatory patterns characteristic of face-center-cubic Ag.20 This observation immediately indicates that the deposits are metallic silver. Close examination reveals that the Ag deposits exhibit a small threshold shift (0.5 to 1.5 eV) to higher photon energy depending on the concentration and there is some broadening in the spectrum of the low-concentration deposits that exhibit the largest shift. (Figures 7 and 8, inset). The blue shift likely arises from the size hence surface effect of Ag aggregates. In the TEM image (Figure 2),11 the Ag particles of 5-50 nm in diameter formed in the reaction with a 1 × 10-4 M AgNO3 solution were observed on the SiNW. It has been established that the electronic structure of the surface atoms differs from that of bulk even in clean Ag single crystals.25 This is due to the fact that the s-p-d rehybridization of the valence orbitals of the Ag surface atom is different from that of the bulk as a result of lattice truncation (lower coordination number). We believe that the situation here is more complex due to the presence of surface oxidation, adsorption, and Ag-Si interactions. These factors will collectively have an effect on the charge redistribution within a nanostructure. The fact that a positive edge shift is observed yet a dominantly metallic fcc Ag EXAFS remains indicates a strong surface/interface interaction that pulls electron charge slightly away from the core of the particle, resulting in a small blue shift in threshold energy. Some evidence for Si-Ag interaction is seen in the 6-nm SiNW spectra where surface Si atom contribution becomes very important since the HF treatment thins the wire down to a couple of nanometers. The corresponding Ag L3,2-edge XANES (Figure 7 inset, marked with an arrow) indeed shows some additional features not found in metallic Ag. Ag on porous silicon shows similar metallic fcc features as those of 6- and 25-nm SiNW. However, behavior similar to 6-nm SiNW has been observed in porous silicon when excess Ag+ (0.1 M) was used in the reaction.16 IV. Summary and Conclusion

Figure 9. A detailed comparison of the Ag L3-edge threshold among different Ag structures.

of Ag on normal SiNW (≈25 and ≈6 nm) and Figure 8 shows that of Ag on the nanoparticle-chain-type SiNW (≈10 nm) and porous silicon. The Ag L3,2-edge XANES arises from the excitation of the 2p3/2 and 2p/2 core level electron into the unoccupied states of d and s character above the Fermi level. An Ag metal spectrum is also

We have reported a detailed study of aqueous Ag+ ion-Si nanowire interaction using X-ray absorption fine structure spectroscopy at both the Si K- and the Ag L3,2edge. We confirm previous findings that the surface of hydrogen-passivated Si nanostructure, such as HF etched SiNWs and porous silicon, are moderate reducing agents that are capable of reducing the metal ions into metallic nanophase aggregates. Si K-edge results show that the core of the wire is crystalline silicon and remains intact throughout the process. The surface oxide layer was the result of the preparation process as noted previously for an oxide-assisted growth mechanism, not a native oxide as normally found on silicon (25) Johanson, B.; Martensson, N. Phys. Rev. B 1980, 21, 4427.

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wafers. The oxide layer can be removed with HF under controlled conditions, resulting in a reactive hydride surface that becomes reoxidized upon reacting with Ag+ ion in solution. The Ag L3,2-edge results show that the reaction of Ag+ with a hydrogen-passivated surface produces metallic Ag nanostructures. Thus, SiNWs act both as a template and a reducing agent. The results also show that under favorable conditions (small wires) there is evidence of Ag-Si interaction at the metalsilicon interface, as was the case in porous silicon. This observation suggests a plausible chemical route to the synthesis of metal silicide nanowires.

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Acknowledgment. Research at the University of Western Ontario was supported by the Natural Science and Engineering Research Council (NSERC) of Canada. CSRF is supported by NSERC through a MFA grant and the National Research Council (NRC) of Canada. SRC is supported by the U.S. National Science Foundation under Grant DMR-00-84402. Research at the City University of Hong Kong was supported by the Research Grant Council (RGC) of Hong Kong. CM011548N