Silver Nanostructures on Silicon Based on Galvanic Displacement

Sep 10, 2009 - Corresponding author: Tel +1 (510) 643-3489, Fax +1 (510) 642-4778, e-mail [email protected]. Cite this:J. Phys. Chem. C 113, 39, 1...
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J. Phys. Chem. C 2009, 113, 16939–16944

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Silver Nanostructures on Silicon Based on Galvanic Displacement Process Albert Gutes, Ian Laboriante, Carlo Carraro, and Roya Maboudian* Department of Chemical Engineering, UniVersity of California, Berkeley, California 94720 ReceiVed: June 12, 2009; ReVised Manuscript ReceiVed: July 24, 2009

A simple and versatile process, based on a galvanic displacement reaction, is presented that provides controlled growth of different silver structures on silicon including thin films, nanocrystals, nanoparticles, and more complex structures termed nanodesert rose. Structure selection is achieved by choosing a suitable ratio of AgF:KF in the plating solution, in the absence of any other additive, and by changing immersion times and precursor concentrations. We demonstrate the usefulness of some of these nanostructures as reproducible surface-enhanced Raman spectroscopy substrates. 1. Introduction The integration of metals with semiconductors plays a crucial role in a number of technologies, ranging from integrated circuits to micro-nanosystems technology, with implications in electronic, optoelectronic, electromechanical, and sensing devices. In recent years, the deposition of metals on semiconductors by galvanic displacement (GD) has received renewed interest, as it is a versatile process, well suited to yield films with high purity and substrate adhesion and with substrate selectivity.1-4 In GD reactions, metal ions in the plating bath are reduced by the substrate itself upon immersion, without external current sources or reducing agents in the bath. Controlled galvanic displacement processes have been employed on silicon to deposit a variety of noble metals, such as gold, platinum, and copper, in thin film or nanoparticle forms.1 Silver deposition using galvanic displacement has been reported on other substrates such as Ge, Al, or GaAs.5-7 Electrochemical approaches involving external current8 or complex deposition processes involving patterning have been carried out on silicon,9 but controlled and stable displacement on nonpatterned silicon substrates has not been reported. Galvanic displacement of silver on silicon, carried out most commonly in solutions containing silver nitrate and hydrofluoric acid, tends to result in a fast and uncontrolled etching of the substrate, with formation of silver dendrites on the surface, as shown in Figure 1a. Dendrite removal with HNO3 shows deep etching in the silicon substrate with the formation of Si nanowires arrays as shown in Figure 1b,c, a phenomenon reported previously.10-12 We show in this paper that this uncontrolled process can be changed and metal deposition can be controlled in such a way that silver thin film, nanoparticles, nanocrystals, and other nanostructures can be obtained as desired. These silver structures can then be used for their respective technological applications, for example, in films or nanoparticle forms as catalysts for silicon nanowire growth, while as crystalline nanostructures for surface enhancement Raman spectroscopy or metal-enhanced fluorescence.

Figure 1. (a) Silver dendrites formed via GD from 1 mM AgNO3 in 9 M HF solution; (b) silicon nanowires arrays formed by silicon etching; (c) top view of the SiNW arrays.

2. Experimental Section Silver Nanostructures Preparation. 〈111〉 silicon 5 × 5 mm chips were degreased by sonication in acetone and isopropanol for 10 min and then rinsed with deionized water (18 MΩ) before * Corresponding author: Tel +1 (510) 643-3489, Fax +1 (510) 6424778, e-mail [email protected].

Figure 2. SEM images obtained for different plating concentrations: (a-c) [Ag+] ) 1 mM, [F-] ) 20 mM; (d-f) [Ag+] ) 20 mM, [F-] ) 20 mM. Deposition times: (a, d) 5 s, (b, e) 30 s, (c, f) 60 s. All scale bars refer to 1 µm.

10.1021/jp9055297 CCC: $40.75  2009 American Chemical Society Published on Web 09/10/2009

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Figure 3. AFM images for different plating concentrations: (a-c) [Ag+] ) 1 mM, [F-] ) 20 mM; (d-f) [Ag+] ) 20 mM, [F-] ) 20 mM. Deposition times: (a, d) 5 s, (b, e) 30 s, (c, f) 60 s. In all cases, image area is 10 µm × 10 µm with the z-scale of 400 nm.

drying in gentle N2. Native oxide layer was removed by immersing the chips in concentrated HF for 1 min, rinsed in DI water, and dried in N2. Silver plating solution was prepared by dissolving the appropriate amount of AgF and KF in DI water to the desired final Ag+ and F- concentrations. Silicon chips were immersed right after native oxide etching and left in solution for the desired time. DI rinsing and N2 drying were performed after incubation. Characterization of Ag Nanostructures. X-ray photoelectron spectroscopy (XPS) analysis was performed using an Omicrometer analyzer (EA 125) to confirm metallic Ag nature

of the formed structure. Scanning electron microscopy (SEM) images were taken using a field-effect mySEM microscope (Novel X) operated at 1 kV. Atomic force microscopy (AFM) operating in tapping mode (Digital Instruments Nanoscope IIIa) was used for roughness and film thickness measurements. SERS Sample Incubation and Characterization. Silver nanostructures were immersed in a 5 mM BPE solution in methanol for 24 h, then rinsed in methanol to remove physically adsorbed BPE, and dried in gentle N2 flow. XPS analysis was performed to estimate the surface coverage using C to N ratio as target parameter.

Silver Nanostructures on Silicon

Figure 4. Silver 3d region of X-ray photoelctron spectra obtained on Ag thin film shown in Figure 2a: (a) fresh sample after the galvanic displacement process, (b) after 24 h incubation on a 5 mM BPE solution.

Raman Measurement. Raman spectroscopy (JYHoriba LabRAM) was performed in backscattering configuration with an excitation line provided by a HeNe laser (632.8 nm wavelength, 10 mW at the sample) through an Olympus BX41 100× confocal microscope (numerical aperture ) 0.8). 3. Results and Discussion The process described in this paper is a simple and inexpensive dip-and-rinse galvanic displacement process that enables the synthesis of silver films, nanoparticles, nanocrystals, or more complex structures depending predictably on bath composition and immersion times. Taking into account that the two halfcell reactions involved are

Si(s) + 6F-(aq) f SiF62-(aq) + 4eAg+(aq) + e- f Ag(s) we note that for each single silicon atom that is oxidized four electrons have to be captured by four silver cations; thus, the global kinetics of the reaction is substantially controlled by the diffusion rate of silver cations to the surface that are reduced subsequently by electrons produced in the oxidation of silicon. In the conventional HF-based silver GD processes, silicon is easily oxidized and the oxides are quickly removed. Electrons provided by the oxidation process are then consumed by the

J. Phys. Chem. C, Vol. 113, No. 39, 2009 16941 nearby silver cations in a ratio of 4 silver atoms per oxidized silicon atom. This phenomenon impoverishes the silver cations of the surrounding solution. The reaction evolves via the diffusion of electrons (through the metallic silver previously deposited) that can reach other regions where silver cations are available. Silver dendrite formation enables a long-term chain reaction because of the high porosity of the dendrites that creates a pathway for HF to reach silicon and the conduction of electrons through the dendrites to sustain the two half-cell reactions. The novelty of our process, which departs significantly from the previous work, is in the use of AgF as a source for both silver and fluoride species and in the addition of KF when higher fluoride concentrations are desired. The ratio of concentrations of these salts is varied in order to obtain different deposit morphologies. In all cases, no HF is added directly to the bath, and given the basic behavior of F- ions, the plating solution is alkaline instead of acidic, as commonly used in the past. By controlling the rates of silicon etching as well as the amount of silver that is near the surface, it is possible, as reported in this paper, to grow a variety of silver structures. First, the formation of stable silver thin films is demonstrated. To ascertain the dependence of reaction rate on Ag+ to Fconcentration ratio, two sets of experiments are carried out. The first set involves using a 1 mM AgF with added KF up to 20 mM final concentration, while the second set involves 20 mM AgF and no added KF. Figures 2 and 3 show respective SEM and AFM images obtained on samples from baths consisting of [Ag+] ) 1 mM (left-hand side images) and 20 mM (righthand side images), while maintaining [F-] ) 20 mM in both cases. On freshly prepared samples, the XPS analysis indicates the formation of Ag(0) in all cases. As shown in Figures 2a and 3a, for the 1:20 Ag:F ratio, smooth thin silver films result, while much rougher films, with the presence of silver nanoparticles, are obtained when silver concentration is equal to that of fluoride, with a final concentration of 20 mM. As discussed later, a 1:20 Ag:F ratio but with higher total concentrations leads to the formation of different crystalline structures. Figure 4a shows the Ag 3d region of the X-ray photoelectron spectrum obtained on the freshly prepared Ag film shown in Figure 2a. The Ag 3d5/2 and 3d3/2 observed at 368.3 and 374.3 eV confirm the Ag(0) state. To determine the film thickness, Teflon tweezers are used to scratch the Ag film without affecting the substrate, and the step height between the silver surface and the substrate is measured using AFM, as done previously.13 In all cases, a silver thin film covers the whole surface. As summarized in Table 1, AFM results reveal that both film thickness and rms roughness increase with deposition time for samples prepared from 1:20 Ag:F ratio. A similar trend is observed in thickness for films formed from equal concentration of Ag+ and F-, without systematic change in rms roughness. Apart from the increase in thickness with time, it is observed that for a given concentration condition the number of nanoparticles and their mean diameters increase with time. Comparing films deposited at different concentrations but equal immersion times, higher silver concentrations are found to yield higher density of nanoparticles formed on the film. Silver crystals in the micrometer-size range can also be obtained by Ag GD process with appropriate bath conditions. To this end, a 1:20 Ag+ to F- concentration ratio is again used but at higher absolute concentration values. Figure 5 shows the results obtained from a bath consisting of a Ag+ concentration of 10 mM and a final F- concentration of 200 mM obtained by the addition of the appropriate amount of KF. The figure shows the silver crystals obtained under these conditions when a silicon

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Figure 5. Silver crystals produced on silicon after 24 h of GD with plating concentrations [Ag+] ) 10 mM and [F-] ) 200 mM.

TABLE 1: Average Thicknesses and RMS Roughness Values for the Films Shown in Figure 3a +

[Ag ] ) 1 mM; [F-] ) 20 mM

+

[Ag ] ) 20 mM; [F-] ) 20 mM

immersion time (s)

thickness (nm)

rms (nm)

thickness (nm)

rms (nm)

5 30 60

18.8 57.2 81.4

5.0 15.4 45.4

52.8 84.9 98.4

41.6 36.1 47.4

a The rms values calculated based on a 10 × 10 µm2 image size. Each value is calculated as the average from three different spots on the same sample.

substrate was incubated for 24 h. SEM and XPS characterizations reveal the formation of metallic silver crystallites. XPS also shows the absence of Si signal, indicating the formation of a continuous Ag film in between the Ag crystals. As shown above, a 1:20 Ag+ to F- concentration ratio can lead to Ag thin films or Ag microcrystals with the appropriate nominal concentration values. A higher AgF concentration is then used to a final value of 30 mM, with an increased Fconcentration of 600 mM obtained again by adding the appropriate amount of KF. Figure 6a,b shows the structures achieved by a 24 h immersion of silicon substrates in this plating solution. The formed structures are named nanodesert rose, as they resemble the crystalline gypsum formations found in some deserts, shown as an inset to Figure 6. A ratio of 1:20 for Ag+ and F- concentrations provides again enough fluoride to oxidize the available silicon surface but not fast enough to drive a dendrite-based reaction. This stems from the different pH used in this plating solution (pH ) 7.32) which slows down the overall silicon oxidation rate. XPS and SEM characterizations confirm the formation of crystalline silver flakes. Figure 6c shows the resulting silver plating when no additional fluoride

is added, and thus the final silver and fluoride ions concentrations are 30 mM and the 1:20 ratio is not maintained. In this case, no flakes are formed due to the fast coverage of the surface that takes place in the early stages of reaction. High-quality nanotextured Ag substrates confirm useful applications for example, as surface enhancement Raman spectroscopy substrates. In this regard, since silver is known to be the most effective noble metal for SERS, great efforts are underway for improving the yield and uniformity of SERS hot spots using silver nanostructures as substrates.14 Thus, we have evaluated the SERS activity of the nanodesert rose sample. A high activity might be anticipated on this sample because of the large density of hot spots that the intersecting flakes could provide. Figure 7a shows the Raman spectrum of bulk trans1,2-bis(4-pyridyl)ethylene powder (BPE) (Aldrich, 97% purity), and Figure 7b shows the Raman spectrum obtained on the desert rose substrate after 24 h of incubation and thorough rinsing of the BPE solution. XPS analyses show that this procedure yields about one BPE monolayer on silver, and thus the Raman signal is due to a very small number of molecules compared to the Raman signal from the bulk substance. For the bulk sample, using a 200 µm laser depth and the BPE bulk density of 5 × 1021 molecules cm-3, the number of molecules per unit area probed is ∼1020 molecules cm-2. Assuming the areal density of BPE monolayer adsorbed on the Ag surfaces to be ∼1014 molecules cm-2, the number of molecules present in the laser spot is estimated to be approximately 1/106 of the bulk substance. Despite the large reduction in the number of molecules probed, a much higher Raman signal is achieved from the BPE monolayer adsorbed on Ag nano-desert rose surface than from the bulk BPE sample. In addition, no Raman signal is recorded when using the Ag thin film shown in Figure 2a as the substrate, although XPS indicates a BPE coverage similar to that present on the desert rose structure. In order to check

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Figure 7. (a) Raman shift for the pure BPE powder. (b) SERS BPE spectrum after incubation on the nano-desert rose substrate and rinsing.

Raman signal obtained on the nano-desert rose sample highlights that this structure is a highly effective SERS substrate. 4. Conclusions To conclude, different crystalline silver structures have been produced via silver galvanic displacement using aqueous AgF and additional KF in the absence of HF to control the deposition process. Different nominal concentrations of silver and fluoride ions have been tested maintaining a 1:20 ratio, leading to very different silver structures. Promising applications in SERS have been presented, and other implications of the present studies, for example as catalyst for Si nanowire growth, are presently under investigation.

Figure 6. (a) Silver desert rose produced by a 24 h GD with plating concentrations [Ag+] ) 30 mM and [F-] ) 600 mM. Inset: natural desert rose [wikipedia.org]. (b) Details of the desert rose flakes. (c) Absence of flakes when [Ag+] ) 30 mM and [F-] ) 30 mM.

the purity of Ag after the monolayer formation process, the flat Ag film after BPE incubation, but prior to Raman, is analyzed using XPS. Figure 4b shows the Ag 3d region of the X-ray photoelectron spectrum. The positions of the Ag 3d5/2 and 3d3/2 observed at 368.3 and 374.3 eV confirm that Ag preserves the Ag(0) state after 24 h incubation. The same Ag(0) state is observed on the nano-desert rose structure. The much higher

Acknowledgment. This work was funded by DARPA SERS S&T Fundamental Program under LLNL Subcontract # B573237. Albert Gute´s thanks Comissionat per a Universitats i Recerca (CUR) del Departament d’Innovacio´, Universitat i Empresa de la Generalitat de Catalunya, for funding through the Beatriu de Pino´s postdoctoral program. References and Notes (1) Carraro, C.; Maboudian, R.; Magagnin, L. Surf. Sci. Rep. 2007, 62, 499. (2) Porter, L. A.; Choi, C. H.; Schmeltzer, J. M.; Ribbe, A. E.; Elliott, L. C. C.; Buriak, J. M. Nano Lett. 2002, 2, 1369. (3) Porter, L. A.; Choi, H. C.; Ribbe, A. E.; Buriak, J. M. Nano Lett. 2002, 2, 1067. (4) Carraro, C.; Magagnin, L.; Maboudian, R. Electrochim. Acta 2002, 47, 2583.

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(5) Aizawa, M.; Cooper, A. M.; Malac, M.; Buriak, J. M. Nano Lett. 2005, 5, 815. (6) Brevnov, D. A.; Olson, T. S.; Lpez, G. P.; Atanassov, P. J. Phys. Chem. B 2004, 108, 17531. (7) Sayed, Y. S.; Daly, B.; Buriak, J. M. J. Phys. Chem. C 2008, 112, 12291. (8) Wang, L.; Shaojun, G.; Xiaoge, H.; Shaojun, D. Electrochem. Commun. 2008, 10, 95. (9) Liu, F.-M.; Green, M. J. Mater. Chem. 2004, 14, 1526.

Gutes et al. (10) Peng, K. Q.; Yan, Y. J.; Gao, S. P.; Zhu, J. AdV. Funct. Mater. 2003, 13, 127. (11) Fang, H.; Wu, Y.; Zhao, J.; Zhu1, J. Nanotechnology. 2006, 17, 3768. (12) Peng, K.; Zhu, J. Electrochim. Acta 2004, 49, 2563. (13) da Rosa, C. P.; Maboudian, R.; Iglesia, E. J. Electrochem. Soc. 2008, 155, E70. (14) Sun, Y.; Wiederrecht, G. P. Small 2007, 3, 1964.

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