Enhancement of Raman Scattering for Silver Nanoparticles Located on Electrolessly Roughened Silicon Yen-Chen Maggie Liou,a Jiann-Yeu Chen,b Jyisy Yanga,* a b
Department of Chemistry, National Chung-Hsing University, Taichung 402, Taiwan, Republic of China Center of Nanoscience and Nanotechnology, National Chung-Hsing University, Taichung 402, Taiwan, Republic of China
To study the effect of roughness of a supporting substrate to Raman enhancement, silver nanoparticles (AgNPs) were prepared on Si with different degrees of roughness. To roughen the surface of silicon, electroless displacement was used first to grow AgNPs on smooth Si. By chemically removing the resulting AgNPs, an electrolessly roughened Si surface can be exposed. A second electroless displacement then was performed to grow new AgNPs on the roughened Si crystal to form surface-enhanced Raman scattering substrates. Another approach, called the protecting method, also was proposed and demonstrated to structure AgNPs on surface-roughened Si. In this second method, electroless displacement also was used to grow AgNPs on the Si crystal. The resulting AgNPs then were protected by thio compounds to control removal of the outer layer of AgNPs, thereby exposing the underlying AgNPs located directly on the electroless roughened Si surface. Results indicate that the structure of AgNPs on roughened Si surfaces provides approximately two orders of magnitude higher enhancement than AgNPs on non-roughened Si, and the substrates prepared in this work are highly sensitive, with enhancement factors reaching 108. Index Headings: Raman scattering; Surface enhancement; Silver nanoparticles; Electroless displacement; Surface-enhanced Raman scattering substrate; SERS.
INTRODUCTION Since the observation almost 40 years ago of the large increase in Raman scattering for molecules adsorbed on a roughened Ag electrode,1 surface-enhanced Raman scattering (SERS) has been studied extensively and has gradually improved as an extremely sensitive approach for measuring molecular information at the singlemolecule level. The large enhancements result partly from induced localized electromagnetic fields generated on the surface of the nanoscale metal particles2–10 and partly from chemical effects. Numerous studies have been focused on the influence of the size,2–4 shape,5–8 and composition of the metallic nanoparticles produced by a wide variety of methods, such as vapor,11,12 electrochemical,13,14 and chemical deposition.15–19 In recent years, attention has shifted to preparation of SERS-active substrates based on nanostructures such as nanocavity and nanoporous structures.20–22 For instance, Au nanocavities have been prepared lithographically, with an enhancement factor (EF) of 108 being reported.20,21 With deposition of a thin layer of Ag on the nanocavities, the EF increased seven-fold compared Received 29 May 2013; accepted 11 September 2013. * Author to whom correspondence should be sent. E-mail: jyisy@ dragon.nchu.edu.tw. DOI: 10.1366/13-07162
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with Au nanocavity substrates alone.22 Porous silicon materials have been prepared by anodization and, after coating with a thin layer of Ag,23–29 Au,29 or Cu,30 these SERS substrates are able to detect analytes with an identifiable concentration of 100 pM. Similarly, porous alumina membranes31 or porous gallium nitride substrates32 have been prepared and used for deposition of a thin layer of silver nanoparticles (AgNPs). The resulting SERS substrates also provide EFs larger than 108. It is clear from the discussion above that SERS substrates with porous structures significantly improve detection sensitivity. To provide improved alternative methods for preparing this type of SERS substrate, a two-step electroless displacement method is proposed and demonstrated in this work. The concept is shown schematically in Fig. 1. First, the literature-reported electroless displacement methods33–37 are adapted and used to grow AgNPs on smooth Si crystals. During this step, the surface of Si is naturally roughened during electroless displacement by Ag ions. The resulting AgNPs on the Si surface can be dissolved, thereby exposing and cleaning the roughened Si surface. In the second step, electroless displacement again is used to grow AgNPs on the roughened Si. Another approach, called the protecting method in this work, also is proposed and demonstrated for comparison. In the protecting method, electroless displacement is used to grow AgNPs on Si crystal. Due to etching during the displacement reaction, the resulting AgNPs already are located on a roughened Si surface. If the outer layer of AgNPs can be removed gently, i.e., without disrupting the enhancement factor of the AgNPs nearer to the surface, the underlying structure of AgNPs on roughened Si crystal can be exposed with a resulting increase in Raman intensity. To expose the rough surface of Si, a thio compound is used to form monolayers on AgNPs, thus allowing better control in removing the outer layer of AgNPs. During soaking of the protected substrates in an oxidizing acid solution, the AgNPs are partially removed and the structure of AgNPs on roughened Si crystal can be exposed. This concept of preparation by the thiol-protecting method is shown schematically in Fig. 1.
EXPERIMENTAL Reagents. Silver nitrate was purchased from ProChem (Rockford, MI). Para-hydroxythiophenol (pHTP) and para-mercaptobenzoic acid (pMBA) were obtained from TCI (Tokyo, Japan). Para-methylthiophenol (pMTP) was obtained Aldrich (St. Louis, MO). Sodium nitrite and
0003-7028/14/6802-0172/0 Q 2014 Society for Applied Spectroscopy
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FIG. 1. Schematic diagram for preparation of AgNPs on roughened Si substrates.
hydrofluoric acid were obtained from Showa (Tokyo, Japan). Sulfuric acid (H2SO4) was purchased from Merck (Darmstadt, Germany). High-performance liquid chromatography-grade methanol was purchased from Tedia (Fairfield, OH). Phosphoric acid (Scharlau, Barcelona, Spain), H2SO4 (Merck), nitric acid (Scharlau), and nitrous acid (fresh prepared) were used as cleaning agents to remove AgNPs. Due to the instability of HNO2, it was prepared on-site as necessary by mixing sodium nitrite (Aldrich) with an equal number of moles of H2SO4 (Merck). Silicon disks were purchased from Lattice Materials Corp. (Bozeman, MT). Aluminum oxide abrasive with a nominal diameter of 50 nm was purchased from LECO Corp. (St. Joseph, MI) and used for polishing the Si disks/wafers. Equipment. The SERS spectra of probe molecules on the novel substrates were collected using a Triax 320 Raman system (Jobin-Yvon, Inc., Longjumeau, France), equipped with a 35 mW, 632.8 nm He/Ne laser (JDS Uniphase Corporation, Milpitas, CA) and a liquid nitrogen cooled Ge charge-coupled device array detector (Jobin-Yvon) at a spectral resolution of 0.06 nm. The spectral acquisition time was 1 s. A field-emission scanning electron microscope ([SEM] JSM-7600F, JEOL, Ltd., Tokyo, Japan) equipped with an energy-dispersive X-ray spectrometer was used to analyze the SERS substrates. Preparation of SERS Substrates. The method used was modified from a description in the literature.33 In brief, Si substrates were polished, cleaned, and airdried. These Si crystals were soaked into a solution containing 7.5 mM silver nitrate (AgNO3) and 0.6 M hydrofluoric acid in an ice-water bath. To study the effect of AgNO3 concentration, this solution also was diluted
with deionized water to have 5 and 2.5 mM AgNO3 in the solution. After designated reaction times, the substrates were removed from the reaction vessel and soaked in deionized water for 5 min to terminate the reaction. To remove the unprotected and protected AgNPs from the Si substrates, they were soaked in HNO2, prepared as described above, for different lengths of time. To simplify the description of substrates prepared by the two-step method, the substrate prepared with a mM AgNO3 for b min is named Aga,bSi. A substrate further prepared by cleaning with c mM HNO2 for d h is named Cc,dAga,bSi. After resoaking substrates of Cc,dAga,bSi in e mM AgNO3 for f min, the resulting substrate is named Re,fCc,dAga,bSi. To incorporate the protecting method in the name, the thio compound-protected Aga,bSi substrates are named SAga,bSi.
RESULTS AND DISCUSSION Characterization of Aga,bSi Substrates. The roughness of the Si surface is mainly controlled by the first electroless displacement. Therefore, Aga,bSi substrates were first prepared by soaking polished Si crystal in 2.5, 5, and 7.5 mM AgNO3 solutions for different lengths of time to form substrates designated Ag2.5,bSi, Ag5,bSi, and Ag7.5,bSi, respectively. These substrates were probed with thio compounds, whereas SERS spectra were acquired. Examples of the resulting spectra are shown in Fig. 2a for Ag2.5,40Si soaking in 0.1 mM methanoic solutions of pHTP, pMBA, and pMTP for 1 h. By comparing SERS spectra of these probe compounds reported in the literature,38–43 we found that the band features of the SERS spectra obtained with our novel substrates showed no significant differences. The new
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FIG. 2. SERS spectra of pHTP, pMBA, and pMTP on Ag2.5,40Si (a), R2.5,20C15,10Ag2.5,40Si (b), and C15,1Ag2.5,40Si (c).
substrates also were scanned by SEM to observe the morphologies of the AgNPs. Significant stacking of 100 nm spherical AgNPs was observed, as shown in Figs. 3a and 3b for substrates designated Ag2.5,40Si and Ag7.5,40Si, respectively. These results indicate that electroless displacement can form useful AgNPs on Si crystal for SERS measurements. Efficiency in Removing AgNPs from Aga,bSi Substrates by Acids. To remove the AgNPs formed on Aga,bSi, they were etched with 15 mM HNO3, H2SO4, phosphoric, and HNO2. This tested the relative performance of the acids for dissolving/cleaning AgNPs. After soaking Ag2.5,40Si substrates in these acids for different lengths of time, the resulting substrates were imaged with SEM to observe the surface condition and detect any residual AgNPs on the Si surface. Results indicate that nitric, sulfuric, and phosphoric acid cannot effectively remove AgNPs. Only significant aggregation of AgNPs were observed after treatment of these acids, represented by the two examples shown in Figs. 3c and 3d for soaking in H2SO4 and HNO3 for 10 h. With nitrous acid, AgNPs can be removed effectively by soaking for 1 h. Most of the
FIG. 3. SEM images of Ag2.5,40Si (a), Ag7.5,40Si (b), C15,10Ag2.5,40Si by H2SO4 (c), C15,10Ag2.5,40Si by HNO3 (d), C15,1Ag2.5,40Si by HNO2 (e), C15,10Ag2.5,40Si by HNO2 (f), C15,10Ag5,40Si by HNO2 (g), C15,10Ag7.5,40Si by HNO2 (h), R2.5,5C15,10Ag2.5,40Si (i), R2.5,20C15,10Ag2.5,40Si (j), R2.5,5C15,10Ag5,40Si (k), R2.5,5C15,10Ag7.5,40Si (l), C15,2SAg2.5,40Si (m), C15,2Ag7.5,40Si (n), C7.5,2SAg2.5,40Si (o), and C30,2SAg2.5,40Si (p).
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FIG. 4. Raman intensities for pHTP chemisorbed on substrates Ag2.5,xSi (.), R2.5,xC15,10Ag2.5,40Si ( R2.5,xC15,10Ag7.5,40Si (m).
AgNPs were removed (see Fig. 3e). With 10 h of soaking, no AgNPs remained on the Si surface (see Fig. 3f). To further examine the roughness of the Si surface, substrates Ag5,40Si and Ag7.5,40Si were cleaned by 15 mM nitrous acid for 10 h. The resulting substrates C15,10Ag5,40Si and C15,10Ag7.5,40Si were scanned by SEM, and the results shown in Figs. 3g and 3h, respectively. These images show that the roughness of the Si surface increased as higher concentrations of AgNO3 were used in first electroless displacement. The sharp edges on the roughened structure in these images indicate that Si surface structures are not destroyed, or even damaged significantly, by nitrous acid. Regrowth of AgNPs on Electrolessly Roughened Si Substrates. To examine the SERS performance of AgNPs on roughened Si surfaces, substrates C15,10Ag2.5,40Si, C15,10Ag5,40Si, and C15,10Ag7,40Si were prepared. These substrates exhibit different degrees of roughness, as discussed above. To grow AgNPs on these roughened Si substrates, electroless displacement was used with 2.5 mM AgNO3. After preparation, these substrates were probed with pHTP, and the observed Raman intensities are shown in Fig. 4. This figure shows that large increases of the SERS intensities are observed for AgNPs on roughened Si. The higher degree of roughness, e.g., the substrate C15,10Ag7,40Si, the larger is the SERS intensity that can be obtained. Based on the SERS intensities for regrowth times of 20 min, a further enhancement of 15 times was observed. This indicates that the structure of AgNPs on roughened Si surfaces provides a high sensitivity for SERS measurements. Surface images also were obtained for these substrates by SEM (Figs. 3i, 3j, 3k, and 3l). These images show that the AgNPs resulting from a second electroless displacement on the roughened
),
R2.5,xC15,10Ag5,40Si (&), and
surface have a smaller distribution of sizes, but the average size of AgNPs remains ; 100 nm. Exposing the Structure of AgNPs on Porous Si to Study the SERS Effect. Figures 3e, 3f, 3g, and 3h show that roughened Si surfaces can be obtained by dissolving away AgNPs from the surface layers. However, based on the images of AgNPs on substrates Ag2.5,40Si and Ag7.5,40Si (refer to Figs. 3a and 3b), thickly stacked AgNPs were observed that limit the laser beam interaction with the deeper AgNPs. In contrast, if AgNPs can be partially and accurately removed, the structure of AgNPs on roughened surfaces can be exposed to significantly enhance SERS intensity. To prove this concept is viable, the following experiments were designed to partially remove stacked AgNPs, thereby exposing the active structure and providing AgNPs on rough surface of Si for SERS studies. To achieve partial removal of the AgNPs, thio compounds were used to control dissolution. To examine the feasibility of this protecting method, pHTP was first used to protect substrates Ag2.5,40Si, Ag5,40Si, and Ag7.5,40Si to form substrates SAg2.5,40Si, SAg5,40Si, and SAg7.5,40Si. After protection, these substrates were soaked in 15 mM nitrous acid for different lengths of time before acquisition of SERS spectra. The results are shown in Fig. 5, where plotted is the Raman intensity of pHTP against the soaking time for removing AgNPs. This figure shows at least an order of magnitude increase in Raman intensity results from partially removing the AgNPs with nitrous acid. Meanwhile, the concentration of AgNO3 used in preparation of the substrates affects the increases of the Raman intensity as the maximal intensities follow the order of SAg2.5,40Si . SAg5,40Si . SAg7.5,40Si. This behavior can be explained by a second coagulation of the AgNPs during dissolution with nitrous acid as
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), C
FIG. 5. Raman intensities of pHTP on substrates of C15,xSAg2.5,40Si (
evidenced by comparing the SEM images of substrates C15,2SAg2.5,40Si and C15,xSag7.5,40Si in Figs. 3m and 3n. To further examine the effect of nitrous acid in controlling the morphologies of the residual AgNPs, substrates SAg2.5,40Si were exposed to 7.5, 15, and 30 mM nitrous acid for different lengths of time. The resulting SERS intensities are plotted in Fig. 6, showing that the concentration of nitrous acid mainly affects the speed of removal of AgNPs from the surface of Si. The resulting substrates have similar morphologies, as can be seen by comparing the images in Figs. 3m, 3o, and 3p. Evaluation of Enhancement Factors. To estimate EFs provided by the new AgNPs on roughened Si
15,x
surfaces, substrates Ag2.5,40Si and R2.5,20C15,10Ag2.5,40Si were prepared. After deposition of controlled amounts of probe molecules of pHTP, pMBA, and pMTP on these substrates, the resulting Raman intensities were compared with the Raman intensity of 5% (w/v) probe molecule in methanol. The EF is defined as EF ¼ ISERS 3 NBulk =IRaman 3 Nads where ISERS is the Raman intensity of the probe molecule deposited on SERS substrate, IRaman is the Raman intensity of 5% probe molecule, Nads is the number of probe molecule deposited on SERS substrate, and NBulk is the number of probe molecule in the methanol solution
), C
FIG. 6. Raman intensities of pHTP on substrates of C7.5,xSAg2.5,40Si (
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SAg5,40Si (&), and C15,xSAg7.5,40Si (m).
15,x
SAg2.5,40Si (&), and C30,xSAg2.5,40Si (m).
exposed to the laser. Based on the intensities of the band located nearby 1000 cm1 for all the examined molecules, the calculated EFs were 3.1 (60.4) 3 105, 3.9 (60.6) 3 105, and 1.7 (60.1) 3 105 for pHTP, pMBA, and pMTP deposited on substrate Ag2.5,40Si, respectively. With substrate R2.5,20C15,10Ag2.5,40Si, the calculated EFs were 6.4 (60.8) 3 106, 3.5 (61.0) 3 106, and 1.9 (60.2) 3 106 for pHTP, pMBA, and pMTP, respectively. Clearly, the enhancement effect is improved at least 15-fold for AgNPs on electrolessly roughened Si surfaces. In the protecting method, the residual amounts of pHTP on the substrates should be estimated to allow for more accurate calculation of the EF. Therefore, substrates were scanned by energy-dispersive X-ray (EDX) spectrometer. The EDX data indicate that the weight percentages of Ag on substrates C15,1Ag2.5,40Si and C15,2Ag 2.5,40Si were 14.7 6 3.4 and 5.4 6 1.8%, respectively. By assuming that the residual amount of pHTP chemisorbed on the AgNPs is related to the weight percentage of AgNPs, the EFs can be corrected accordingly, with the results of 2.7 (60.3) 3 107 and 6.0 ( 62.1) 3 10 7 for substrates C 15,1 Ag 2 .5,40 Si and C15,2Ag2.5,40Si, respectively.
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CONCLUSIONS In this study, the Raman enhancement effect provided by AgNPs on roughened Si surfaces was explored. For preparation of these structured materials, two methods were proposed, demonstrated, and examined: a two-step method and a protecting method. In the two-step method, roughened surfaces are prepared by a substitution reaction electroless displacement, with a cleaning step to remove all of the resulting AgNPs with nitrous acid. This leaves a roughened surface, as the electroless displacement is not uniform over the surface. A second electroless displacement reaction then is used to form a new layer of AgNPs on the roughened surface. This approach provides a 15-fold increase in Raman intensity compared to AgNPs formed on smooth Si crystals. In the protecting method, the underlying AgNPs on a roughened Si surface were exposed by etching with nitrous acid. With appropriate removal conditions, the enhancement factors observed for the remaining AgNPs were close to 108. These results indicate that highly sensitive SERS substrates can be prepared by electroless displacement methods and that the roughness of the substrate crystal significantly influences the enhancement factor.
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35. H. Miyake, S. Ye, M. Osawa. ‘‘Electroless Deposition of Gold Thin Films on Silicon for Surface-Enhanced Infrared Spectroelectrochemistry’’. Electrochem. Commun. 2002. 4(12): 973-977. 36. L. Scudieroa, A. Fasasi, P.R. Griffiths. ‘‘Characterization of a Controlled Electroless Deposition of Copper Thin Film on Germanium and Silicon Surfaces’’. Appl. Surf. Sci. 2011. 257(9): 4422-4427. 37. P.R. Brejna, P.R. Griffiths. ‘‘Electroless Deposition of Silver onto Silicon as a Method of Preparation of Reproducible SurfaceEnhanced Raman Spectroscopy Substrates and Tip-Enhanced Raman Spectroscopy Tips’’. Appl. Spectrosc. 2010. 64(5): 493-499. 38. L. Xia, J. Wang, S. Tong, G. Liu, J. Li, H. Zhang. ‘‘Design and Construction of a Sensitive Silver Substrate for Surface-Enhanced Raman Scattering Spectroscopy’’. Vib. Spectrosc. 2008. 47(2): 124128. 39. H.M. Lee, M.S. Kim, K. Kim. ‘‘Surface-Enhanced Raman Scattering of ortho- and para-Mercaptophenols in Silver Sol’’. Vib. Spectrosc. 1994. 6(2): 205-214. 40. Z. Lu, Y. Gu, J. Yang, Z. Li, W. Ruan, W. Xu, C. Zhao, B. Zhao. ‘‘SERS-Active Ag Substrate from the Photo-Active Decomposition of Electrodeposited Divalent Silver Oxide’’. Vib. Spectrosc. 2008. 47(2): 99-104. 41. M. Rycenga, M.H. Kim, P.H.C. Camargo, C. Cobley, Z.-Y. Li, Y. Xia. ‘‘Surface-Enhanced Raman Scattering: Comparison of Three Different Molecules on Single-Crystal Nanocubes and Nanospheres of Silver’’. J. Phys. Chem. A. 2009. 113(16): 3932-3939. 42. S.W. Han, I. Lee, K. Kim. ‘‘Patterning of Organic Monolayers on Silver via Surface-Induced Photoreaction’’. Langmuir. 2002. 18(1): 182-187. 43. B.O. Skadtchenko, R. Aroca. ‘‘Surface-Enhanced Raman Scattering of p-Nitrothiophenol: Molecular Vibrations of Its Silver Salt and the Surface Complex Formed on Silver Islands and Colloids’’. Spectrochim. Acta, Part A. 2001. 57(5): 1009-1016.
Department of Chemistry, National Chung-Hsing University, Taichung 402, Taiwan, Republic of China Center of Nanoscience and Nanotechnology, National Chung-Hsing University, Taichung 402, Taiwan, Republic of China
To study the effect of roughness of a supporting substrate to Raman enhancement, silver nanoparticles (AgNPs) were prepared on Si with different degrees of roughness. To roughen the surface of silicon, electroless displacement was used first to grow AgNPs on smooth Si. By chemically removing the resulting AgNPs, an electrolessly roughened Si surface can be exposed. A second electroless displacement then was performed to grow new AgNPs on the roughened Si crystal to form surface-enhanced Raman scattering substrates. Another approach, called the protecting method, also was proposed and demonstrated to structure AgNPs on surface-roughened Si. In this second method, electroless displacement also was used to grow AgNPs on the Si crystal. The resulting AgNPs then were protected by thio compounds to control removal of the outer layer of AgNPs, thereby exposing the underlying AgNPs located directly on the electroless roughened Si surface. Results indicate that the structure of AgNPs on roughened Si surfaces provides approximately two orders of magnitude higher enhancement than AgNPs on non-roughened Si, and the substrates prepared in this work are highly sensitive, with enhancement factors reaching 108. Index Headings: Raman scattering; Surface enhancement; Silver nanoparticles; Electroless displacement; Surface-enhanced Raman scattering substrate; SERS.
INTRODUCTION Since the observation almost 40 years ago of the large increase in Raman scattering for molecules adsorbed on a roughened Ag electrode,1 surface-enhanced Raman scattering (SERS) has been studied extensively and has gradually improved as an extremely sensitive approach for measuring molecular information at the singlemolecule level. The large enhancements result partly from induced localized electromagnetic fields generated on the surface of the nanoscale metal particles2–10 and partly from chemical effects. Numerous studies have been focused on the influence of the size,2–4 shape,5–8 and composition of the metallic nanoparticles produced by a wide variety of methods, such as vapor,11,12 electrochemical,13,14 and chemical deposition.15–19 In recent years, attention has shifted to preparation of SERS-active substrates based on nanostructures such as nanocavity and nanoporous structures.20–22 For instance, Au nanocavities have been prepared lithographically, with an enhancement factor (EF) of 108 being reported.20,21 With deposition of a thin layer of Ag on the nanocavities, the EF increased seven-fold compared Received 29 May 2013; accepted 11 September 2013. * Author to whom correspondence should be sent. E-mail: jyisy@ dragon.nchu.edu.tw. DOI: 10.1366/13-07162
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with Au nanocavity substrates alone.22 Porous silicon materials have been prepared by anodization and, after coating with a thin layer of Ag,23–29 Au,29 or Cu,30 these SERS substrates are able to detect analytes with an identifiable concentration of 100 pM. Similarly, porous alumina membranes31 or porous gallium nitride substrates32 have been prepared and used for deposition of a thin layer of silver nanoparticles (AgNPs). The resulting SERS substrates also provide EFs larger than 108. It is clear from the discussion above that SERS substrates with porous structures significantly improve detection sensitivity. To provide improved alternative methods for preparing this type of SERS substrate, a two-step electroless displacement method is proposed and demonstrated in this work. The concept is shown schematically in Fig. 1. First, the literature-reported electroless displacement methods33–37 are adapted and used to grow AgNPs on smooth Si crystals. During this step, the surface of Si is naturally roughened during electroless displacement by Ag ions. The resulting AgNPs on the Si surface can be dissolved, thereby exposing and cleaning the roughened Si surface. In the second step, electroless displacement again is used to grow AgNPs on the roughened Si. Another approach, called the protecting method in this work, also is proposed and demonstrated for comparison. In the protecting method, electroless displacement is used to grow AgNPs on Si crystal. Due to etching during the displacement reaction, the resulting AgNPs already are located on a roughened Si surface. If the outer layer of AgNPs can be removed gently, i.e., without disrupting the enhancement factor of the AgNPs nearer to the surface, the underlying structure of AgNPs on roughened Si crystal can be exposed with a resulting increase in Raman intensity. To expose the rough surface of Si, a thio compound is used to form monolayers on AgNPs, thus allowing better control in removing the outer layer of AgNPs. During soaking of the protected substrates in an oxidizing acid solution, the AgNPs are partially removed and the structure of AgNPs on roughened Si crystal can be exposed. This concept of preparation by the thiol-protecting method is shown schematically in Fig. 1.
EXPERIMENTAL Reagents. Silver nitrate was purchased from ProChem (Rockford, MI). Para-hydroxythiophenol (pHTP) and para-mercaptobenzoic acid (pMBA) were obtained from TCI (Tokyo, Japan). Para-methylthiophenol (pMTP) was obtained Aldrich (St. Louis, MO). Sodium nitrite and
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FIG. 1. Schematic diagram for preparation of AgNPs on roughened Si substrates.
hydrofluoric acid were obtained from Showa (Tokyo, Japan). Sulfuric acid (H2SO4) was purchased from Merck (Darmstadt, Germany). High-performance liquid chromatography-grade methanol was purchased from Tedia (Fairfield, OH). Phosphoric acid (Scharlau, Barcelona, Spain), H2SO4 (Merck), nitric acid (Scharlau), and nitrous acid (fresh prepared) were used as cleaning agents to remove AgNPs. Due to the instability of HNO2, it was prepared on-site as necessary by mixing sodium nitrite (Aldrich) with an equal number of moles of H2SO4 (Merck). Silicon disks were purchased from Lattice Materials Corp. (Bozeman, MT). Aluminum oxide abrasive with a nominal diameter of 50 nm was purchased from LECO Corp. (St. Joseph, MI) and used for polishing the Si disks/wafers. Equipment. The SERS spectra of probe molecules on the novel substrates were collected using a Triax 320 Raman system (Jobin-Yvon, Inc., Longjumeau, France), equipped with a 35 mW, 632.8 nm He/Ne laser (JDS Uniphase Corporation, Milpitas, CA) and a liquid nitrogen cooled Ge charge-coupled device array detector (Jobin-Yvon) at a spectral resolution of 0.06 nm. The spectral acquisition time was 1 s. A field-emission scanning electron microscope ([SEM] JSM-7600F, JEOL, Ltd., Tokyo, Japan) equipped with an energy-dispersive X-ray spectrometer was used to analyze the SERS substrates. Preparation of SERS Substrates. The method used was modified from a description in the literature.33 In brief, Si substrates were polished, cleaned, and airdried. These Si crystals were soaked into a solution containing 7.5 mM silver nitrate (AgNO3) and 0.6 M hydrofluoric acid in an ice-water bath. To study the effect of AgNO3 concentration, this solution also was diluted
with deionized water to have 5 and 2.5 mM AgNO3 in the solution. After designated reaction times, the substrates were removed from the reaction vessel and soaked in deionized water for 5 min to terminate the reaction. To remove the unprotected and protected AgNPs from the Si substrates, they were soaked in HNO2, prepared as described above, for different lengths of time. To simplify the description of substrates prepared by the two-step method, the substrate prepared with a mM AgNO3 for b min is named Aga,bSi. A substrate further prepared by cleaning with c mM HNO2 for d h is named Cc,dAga,bSi. After resoaking substrates of Cc,dAga,bSi in e mM AgNO3 for f min, the resulting substrate is named Re,fCc,dAga,bSi. To incorporate the protecting method in the name, the thio compound-protected Aga,bSi substrates are named SAga,bSi.
RESULTS AND DISCUSSION Characterization of Aga,bSi Substrates. The roughness of the Si surface is mainly controlled by the first electroless displacement. Therefore, Aga,bSi substrates were first prepared by soaking polished Si crystal in 2.5, 5, and 7.5 mM AgNO3 solutions for different lengths of time to form substrates designated Ag2.5,bSi, Ag5,bSi, and Ag7.5,bSi, respectively. These substrates were probed with thio compounds, whereas SERS spectra were acquired. Examples of the resulting spectra are shown in Fig. 2a for Ag2.5,40Si soaking in 0.1 mM methanoic solutions of pHTP, pMBA, and pMTP for 1 h. By comparing SERS spectra of these probe compounds reported in the literature,38–43 we found that the band features of the SERS spectra obtained with our novel substrates showed no significant differences. The new
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FIG. 2. SERS spectra of pHTP, pMBA, and pMTP on Ag2.5,40Si (a), R2.5,20C15,10Ag2.5,40Si (b), and C15,1Ag2.5,40Si (c).
substrates also were scanned by SEM to observe the morphologies of the AgNPs. Significant stacking of 100 nm spherical AgNPs was observed, as shown in Figs. 3a and 3b for substrates designated Ag2.5,40Si and Ag7.5,40Si, respectively. These results indicate that electroless displacement can form useful AgNPs on Si crystal for SERS measurements. Efficiency in Removing AgNPs from Aga,bSi Substrates by Acids. To remove the AgNPs formed on Aga,bSi, they were etched with 15 mM HNO3, H2SO4, phosphoric, and HNO2. This tested the relative performance of the acids for dissolving/cleaning AgNPs. After soaking Ag2.5,40Si substrates in these acids for different lengths of time, the resulting substrates were imaged with SEM to observe the surface condition and detect any residual AgNPs on the Si surface. Results indicate that nitric, sulfuric, and phosphoric acid cannot effectively remove AgNPs. Only significant aggregation of AgNPs were observed after treatment of these acids, represented by the two examples shown in Figs. 3c and 3d for soaking in H2SO4 and HNO3 for 10 h. With nitrous acid, AgNPs can be removed effectively by soaking for 1 h. Most of the
FIG. 3. SEM images of Ag2.5,40Si (a), Ag7.5,40Si (b), C15,10Ag2.5,40Si by H2SO4 (c), C15,10Ag2.5,40Si by HNO3 (d), C15,1Ag2.5,40Si by HNO2 (e), C15,10Ag2.5,40Si by HNO2 (f), C15,10Ag5,40Si by HNO2 (g), C15,10Ag7.5,40Si by HNO2 (h), R2.5,5C15,10Ag2.5,40Si (i), R2.5,20C15,10Ag2.5,40Si (j), R2.5,5C15,10Ag5,40Si (k), R2.5,5C15,10Ag7.5,40Si (l), C15,2SAg2.5,40Si (m), C15,2Ag7.5,40Si (n), C7.5,2SAg2.5,40Si (o), and C30,2SAg2.5,40Si (p).
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FIG. 4. Raman intensities for pHTP chemisorbed on substrates Ag2.5,xSi (.), R2.5,xC15,10Ag2.5,40Si ( R2.5,xC15,10Ag7.5,40Si (m).
AgNPs were removed (see Fig. 3e). With 10 h of soaking, no AgNPs remained on the Si surface (see Fig. 3f). To further examine the roughness of the Si surface, substrates Ag5,40Si and Ag7.5,40Si were cleaned by 15 mM nitrous acid for 10 h. The resulting substrates C15,10Ag5,40Si and C15,10Ag7.5,40Si were scanned by SEM, and the results shown in Figs. 3g and 3h, respectively. These images show that the roughness of the Si surface increased as higher concentrations of AgNO3 were used in first electroless displacement. The sharp edges on the roughened structure in these images indicate that Si surface structures are not destroyed, or even damaged significantly, by nitrous acid. Regrowth of AgNPs on Electrolessly Roughened Si Substrates. To examine the SERS performance of AgNPs on roughened Si surfaces, substrates C15,10Ag2.5,40Si, C15,10Ag5,40Si, and C15,10Ag7,40Si were prepared. These substrates exhibit different degrees of roughness, as discussed above. To grow AgNPs on these roughened Si substrates, electroless displacement was used with 2.5 mM AgNO3. After preparation, these substrates were probed with pHTP, and the observed Raman intensities are shown in Fig. 4. This figure shows that large increases of the SERS intensities are observed for AgNPs on roughened Si. The higher degree of roughness, e.g., the substrate C15,10Ag7,40Si, the larger is the SERS intensity that can be obtained. Based on the SERS intensities for regrowth times of 20 min, a further enhancement of 15 times was observed. This indicates that the structure of AgNPs on roughened Si surfaces provides a high sensitivity for SERS measurements. Surface images also were obtained for these substrates by SEM (Figs. 3i, 3j, 3k, and 3l). These images show that the AgNPs resulting from a second electroless displacement on the roughened
),
R2.5,xC15,10Ag5,40Si (&), and
surface have a smaller distribution of sizes, but the average size of AgNPs remains ; 100 nm. Exposing the Structure of AgNPs on Porous Si to Study the SERS Effect. Figures 3e, 3f, 3g, and 3h show that roughened Si surfaces can be obtained by dissolving away AgNPs from the surface layers. However, based on the images of AgNPs on substrates Ag2.5,40Si and Ag7.5,40Si (refer to Figs. 3a and 3b), thickly stacked AgNPs were observed that limit the laser beam interaction with the deeper AgNPs. In contrast, if AgNPs can be partially and accurately removed, the structure of AgNPs on roughened surfaces can be exposed to significantly enhance SERS intensity. To prove this concept is viable, the following experiments were designed to partially remove stacked AgNPs, thereby exposing the active structure and providing AgNPs on rough surface of Si for SERS studies. To achieve partial removal of the AgNPs, thio compounds were used to control dissolution. To examine the feasibility of this protecting method, pHTP was first used to protect substrates Ag2.5,40Si, Ag5,40Si, and Ag7.5,40Si to form substrates SAg2.5,40Si, SAg5,40Si, and SAg7.5,40Si. After protection, these substrates were soaked in 15 mM nitrous acid for different lengths of time before acquisition of SERS spectra. The results are shown in Fig. 5, where plotted is the Raman intensity of pHTP against the soaking time for removing AgNPs. This figure shows at least an order of magnitude increase in Raman intensity results from partially removing the AgNPs with nitrous acid. Meanwhile, the concentration of AgNO3 used in preparation of the substrates affects the increases of the Raman intensity as the maximal intensities follow the order of SAg2.5,40Si . SAg5,40Si . SAg7.5,40Si. This behavior can be explained by a second coagulation of the AgNPs during dissolution with nitrous acid as
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), C
FIG. 5. Raman intensities of pHTP on substrates of C15,xSAg2.5,40Si (
evidenced by comparing the SEM images of substrates C15,2SAg2.5,40Si and C15,xSag7.5,40Si in Figs. 3m and 3n. To further examine the effect of nitrous acid in controlling the morphologies of the residual AgNPs, substrates SAg2.5,40Si were exposed to 7.5, 15, and 30 mM nitrous acid for different lengths of time. The resulting SERS intensities are plotted in Fig. 6, showing that the concentration of nitrous acid mainly affects the speed of removal of AgNPs from the surface of Si. The resulting substrates have similar morphologies, as can be seen by comparing the images in Figs. 3m, 3o, and 3p. Evaluation of Enhancement Factors. To estimate EFs provided by the new AgNPs on roughened Si
15,x
surfaces, substrates Ag2.5,40Si and R2.5,20C15,10Ag2.5,40Si were prepared. After deposition of controlled amounts of probe molecules of pHTP, pMBA, and pMTP on these substrates, the resulting Raman intensities were compared with the Raman intensity of 5% (w/v) probe molecule in methanol. The EF is defined as EF ¼ ISERS 3 NBulk =IRaman 3 Nads where ISERS is the Raman intensity of the probe molecule deposited on SERS substrate, IRaman is the Raman intensity of 5% probe molecule, Nads is the number of probe molecule deposited on SERS substrate, and NBulk is the number of probe molecule in the methanol solution
), C
FIG. 6. Raman intensities of pHTP on substrates of C7.5,xSAg2.5,40Si (
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SAg5,40Si (&), and C15,xSAg7.5,40Si (m).
15,x
SAg2.5,40Si (&), and C30,xSAg2.5,40Si (m).
exposed to the laser. Based on the intensities of the band located nearby 1000 cm1 for all the examined molecules, the calculated EFs were 3.1 (60.4) 3 105, 3.9 (60.6) 3 105, and 1.7 (60.1) 3 105 for pHTP, pMBA, and pMTP deposited on substrate Ag2.5,40Si, respectively. With substrate R2.5,20C15,10Ag2.5,40Si, the calculated EFs were 6.4 (60.8) 3 106, 3.5 (61.0) 3 106, and 1.9 (60.2) 3 106 for pHTP, pMBA, and pMTP, respectively. Clearly, the enhancement effect is improved at least 15-fold for AgNPs on electrolessly roughened Si surfaces. In the protecting method, the residual amounts of pHTP on the substrates should be estimated to allow for more accurate calculation of the EF. Therefore, substrates were scanned by energy-dispersive X-ray (EDX) spectrometer. The EDX data indicate that the weight percentages of Ag on substrates C15,1Ag2.5,40Si and C15,2Ag 2.5,40Si were 14.7 6 3.4 and 5.4 6 1.8%, respectively. By assuming that the residual amount of pHTP chemisorbed on the AgNPs is related to the weight percentage of AgNPs, the EFs can be corrected accordingly, with the results of 2.7 (60.3) 3 107 and 6.0 ( 62.1) 3 10 7 for substrates C 15,1 Ag 2 .5,40 Si and C15,2Ag2.5,40Si, respectively.
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CONCLUSIONS In this study, the Raman enhancement effect provided by AgNPs on roughened Si surfaces was explored. For preparation of these structured materials, two methods were proposed, demonstrated, and examined: a two-step method and a protecting method. In the two-step method, roughened surfaces are prepared by a substitution reaction electroless displacement, with a cleaning step to remove all of the resulting AgNPs with nitrous acid. This leaves a roughened surface, as the electroless displacement is not uniform over the surface. A second electroless displacement reaction then is used to form a new layer of AgNPs on the roughened surface. This approach provides a 15-fold increase in Raman intensity compared to AgNPs formed on smooth Si crystals. In the protecting method, the underlying AgNPs on a roughened Si surface were exposed by etching with nitrous acid. With appropriate removal conditions, the enhancement factors observed for the remaining AgNPs were close to 108. These results indicate that highly sensitive SERS substrates can be prepared by electroless displacement methods and that the roughness of the substrate crystal significantly influences the enhancement factor.
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35. H. Miyake, S. Ye, M. Osawa. ‘‘Electroless Deposition of Gold Thin Films on Silicon for Surface-Enhanced Infrared Spectroelectrochemistry’’. Electrochem. Commun. 2002. 4(12): 973-977. 36. L. Scudieroa, A. Fasasi, P.R. Griffiths. ‘‘Characterization of a Controlled Electroless Deposition of Copper Thin Film on Germanium and Silicon Surfaces’’. Appl. Surf. Sci. 2011. 257(9): 4422-4427. 37. P.R. Brejna, P.R. Griffiths. ‘‘Electroless Deposition of Silver onto Silicon as a Method of Preparation of Reproducible SurfaceEnhanced Raman Spectroscopy Substrates and Tip-Enhanced Raman Spectroscopy Tips’’. Appl. Spectrosc. 2010. 64(5): 493-499. 38. L. Xia, J. Wang, S. Tong, G. Liu, J. Li, H. Zhang. ‘‘Design and Construction of a Sensitive Silver Substrate for Surface-Enhanced Raman Scattering Spectroscopy’’. Vib. Spectrosc. 2008. 47(2): 124128. 39. H.M. Lee, M.S. Kim, K. Kim. ‘‘Surface-Enhanced Raman Scattering of ortho- and para-Mercaptophenols in Silver Sol’’. Vib. Spectrosc. 1994. 6(2): 205-214. 40. Z. Lu, Y. Gu, J. Yang, Z. Li, W. Ruan, W. Xu, C. Zhao, B. Zhao. ‘‘SERS-Active Ag Substrate from the Photo-Active Decomposition of Electrodeposited Divalent Silver Oxide’’. Vib. Spectrosc. 2008. 47(2): 99-104. 41. M. Rycenga, M.H. Kim, P.H.C. Camargo, C. Cobley, Z.-Y. Li, Y. Xia. ‘‘Surface-Enhanced Raman Scattering: Comparison of Three Different Molecules on Single-Crystal Nanocubes and Nanospheres of Silver’’. J. Phys. Chem. A. 2009. 113(16): 3932-3939. 42. S.W. Han, I. Lee, K. Kim. ‘‘Patterning of Organic Monolayers on Silver via Surface-Induced Photoreaction’’. Langmuir. 2002. 18(1): 182-187. 43. B.O. Skadtchenko, R. Aroca. ‘‘Surface-Enhanced Raman Scattering of p-Nitrothiophenol: Molecular Vibrations of Its Silver Salt and the Surface Complex Formed on Silver Islands and Colloids’’. Spectrochim. Acta, Part A. 2001. 57(5): 1009-1016.
