Letter pubs.acs.org/JPCL
Surface-Enhanced Infrared Absorption on Elongated Nickel Nanostructures Donald A. Perry,*,† Reece L. Borchers,† Jon W. Golden,† Aaron R. Owen,† Adam S. Price,† William A. Henry,† Fumiya Watanabe,‡ and Alexandru S. Biris‡ †
Department of Chemistry, University of Central Arkansas, Conway, Arkansas 72035, United States Center for Integrative Nanotechnology Sciences, University of Arkansas at Little Rock, Little Rock, Arkansas 72204, United States
‡
ABSTRACT: There is a need for increasing the number of transition metals that can be used as substrates for surface-enhanced infrared absorption (SEIRA). We present here microscopy and infrared experiments that show oblique-angle deposition of Ni onto CaF2 or BaF2, which result in elongated Ni nanostructures (ENiNSs) that are partially aggregated and exhibit surface plasmon resonances in the midinfrared. SEIRA enhancement factors in the range of 10−20-fold were observed for a monolayer of the pnitrobenzoate ion adsorbed onto the ENiNS. Extending SEIRA to a metal such as Ni would yield different ways of studying Ni thin film and catalysis chemistry. This work also suggests that oblique-angle deposition might be used to create new SEIRA substrates from other metals. SECTION: Physical Processes in Nanomaterials and Nanostructures
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As early as 1986, it was suggested that Ni might be a viable metal for SEIRA.10 However, the small factor of two-fold SEIRA enhancement in this early publication could have been the result of intermittent contact of the metal film with an ATR crystal. More recently, it has been demonstrated that prefabricated microarrays of metals such as nickel with subwavelength apertures have resonances in the infrared that yield SEIRA.9 These microarrays take advantage of the extraordinary optical transmission of subwavelength hole arrays to derive the SEIRA effect.11,12 Huo and co-workers have shown that the SEIRA from a 60 nm gold layer under a thick, 40 nm nickel overlayer exhibited SEIRA. This technique of using underlying gold films to impart enhancement to an otherwise SEIRA-inactive metal overlayer has worked for a number of other metals such as cobalt as long as the metal overlayer is free of pinholes.13,14 We are aware of one paper suggesting the use of Ni nanorods formed by electrodeposition for SEIRA.6 It is important to state here that this paper only explored the SPR of the Ni nanorods and never confirmed SEIRA from molecules in proximity to the Ni nanorods, Here, it will be demonstrated that stand-alone Ni nanostructures (NiNSs) formed in a vacuum can in fact induce SEIRA enhancement in adsorbed organic layers under the right conditions. We show that elongated Ni nanostructures (ENiNSs) formed by oblique-angle deposition of Ni in a vacuum onto either CaF2 or BaF2 substrates can yield SEIRA enhancement factors of at least 20-fold for an adsorbed pnitrobenzoic acid (PNBA) layer. Oblique-angle deposition has been used previously to produce exceptionable Ag substrates for SERS and SEIRA.7,15,16 It was recently shown that BaF2 and CaF2 are superior base substrates for metal nanostructure
here has been much interest in developing new nanostructured substrates for applications involving surface-enhanced Raman spectroscopy (SERS) and surfaceenhanced infrared absorption (SEIRA).1−3 SEIRA is proving to be a strong compliment to the more commonly applied SERS in that SERS enhancement typically occurs in just the monolayer while SEIRA enhancement can be observed in both the monolayer and multilayer.4,5 Although Au and Ag are the most common SEIRA substrates, SEIRA has been demonstrated for a number of metal nanostructures including Cu, Pt, Pd, Sn, Pb, and Ru.2 Several recent papers have shown that Ag and Au nanoantennas have the potential to be superior SEIRA substrates.3,6,7 Ag and Au are famous for their surface plasmon resonances (SPRs) in the visible region, which makes Ag and Au nanoparticles ideal substrates for SERS.1,2 Most SPR theories are based upon the dielectric constant, which is a complex permittivity with a frequency dependence.8 If there is a chance for a SPR to exist at a given frequency, the real portion of the dielectric constant must be negative. These SPR resonances must be long-lived (narrow bands) to be appropriate for conventional SERS and SEIRA studies.9 Ideally, this means that to have narrow resonance conditions, the absolute value of the real portion of the dielectric constant must be greater than the imaginary portion of the dielectric constant in the frequency range in question. These characteristics are demonstrated for a number of metals using various theories including a Lorentz−Drude model.8,9 This resonance condition is satisfied for Ag, Au, and Cu in the visible and infrared regions. Not surprisingly, the coinage metals have become the most popular SERS and SEIRA substrates.1,2 However, other metals such as chromium and nickel have the right resonance conditions satisfied only in the infrared, making these metals candidates for SEIRA substrates.9 © 2013 American Chemical Society
Received: September 27, 2013 Accepted: November 6, 2013 Published: November 6, 2013 3945
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Figure 1. (Left) AFM image of 7 nm of Ni evaporated onto CaF2 at 0°. (Right) AFM of 50 nm of Ni evaporated on CaF2 at 75°.
onto CaF2 at a 75° tilt is less than 10 nm thick. In such thin metal films, the AFM cantilever is long enough to image entire nanostructures. Hence, Figure 1(right) exhibits complete threedimensional images of the ENiNS and not just representations of the surface roughness as is typically observed in AFM images of metal nanorods in thicker films. Scheme 1 helps to partially explain the results in Figure 1(left) and (right). The AFM image in Figure 1(left) shows Ni
formation for SEIRA as compared to other common infrared transparent materials including Ge, AMTIR, KRS-5, and ZnSe.17 Since Harstein’s original experiments that first described SEIRA, PNBA has become somewhat of a standard molecule for comparing the utility of different SEIRA substrates.17,18 Figure 1(left) shows an atomic force microscopy (AFM) image of 7 nm of Ni evaporated onto a CaF2 slide where the CaF2 is oriented perpendicular (0° tilt) with respect to the metal deposition source. When referring to a number such as 7 nm of Ni evaporated, the 7 nm is measured by a quartz crystal microbalance in a geometrically equivalent position to a CaF2 crystal with 0° of tilt. In such a situation, 7 nm of Ni does not refer to a metal film thickness but the amount of deposited Ni. Upon inspecting Figure 1(left), it appears that the Ni has distributed over the CaF2 substrate with little evidence for consistent NiNS formation. Certainly, any structures that might be NiNSs are dispersed far apart on the surface. A 7 nm exposure of Ni at a 0° tilt is already a substantial exposure for which the Ni layer is scattering approximately 60−70% of light in the mid-infrared (measured at 2400 cm−1) that is normally transmitted through the bare CaF2 substrates. It is not a surprise in this circumstance that Ni would spread out over CaF2 instead of forming nanostructures given the strong affinity of Ni for fluoride ions.19 Figure 1(right) is an AFM image of 50 nm of Ni grown on a CaF2 substrate tilted at an oblique angle of 75° with respect to the metal deposition source. It is worth mentioning here that larger metal exposures are necessary in oblique-angle deposition because the metal flux observed at the substrate drops rapidly with increasing substrate tilt.15 Nanostructures observed in Figure 1(right) are called ENiNSs because the average aspect ratio of these structures is only about 2.4. The average length of the structures is approximately 60 nm. It does not make sense to report an average spacing of the ENiNS because of the polydispersed nature of the nanostructures and the large degree of aggregation observed in Figure 1(right). In SEM experiments, a 50 nm exposure of Ni that was evaporated
Scheme 1. Diagram of Experimental Outcomes Depending on the Degree of the Grazing Angle Tilt
spreading out over the CaF2 substrate due to the strong affinity of Ni for fluoride ions. In the upper image of Scheme 1 when the CaF2 substrate is oriented perpendicular to the metal deposition source (0° oblique-angle tilt), the diffuse Ni flux allows plenty of opportunities for Ni to spread out over the surface and interact with fluoride ions in the substrate. At the grazing angle in Figure 2 (oblique-angle tilt of 75− 78°), it is apparent that the ENiNSs are formed. As can be seen in the lower portion of Scheme 1, the averaged Ni flux over time has much less opportunity in terms of the number of 3946
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Figure 3. From bottom to top are the infrared spectrum of a monolayer of PNBA deposited on bare CaF2, 7 nm of Ni/CaF2 at 0°, 50 nm of Ni/CaF2 at 75°, and 70 of nm Ni/CaF2 at 78°.
the formation of ENiNSs that exhibit SPR in the mid-infrared. No PNBA can be observed in this spectrum because the Ni film scatters about 60−70% of the light that would normally transmit through the CaF2 crystal. Hence, A PNBA monolayer can be seen on a bare CaF2 substrate, but a PNBA monolayer is not observable on any 0° Ni films from 2 to 20 nm exposure. In the top two spectra of Figure 3, when a monolayer of PNBA is adsorbed on ENiNSs, a strong adsorbate spectrum is observed. It is well-known that on most transition metals such as Ag and Au, PNBA dissociates into a p-nitrobenzoate ion (PNBI) at least in the monolayer where the carboxylate group of PNBI is bound to the metal nanostructures.1,2,7,16 PNBI adsorption is confirmed on ENiNSs in the top spectra of Figure 3 by the presence of COO symmetric bands centered around 1390− 1400 cm−1. SEIRA enhancement factors of about 10−20-fold are being consistently reproduced for ENiNSs. Infrared enhancement factors are determined in two ways. First, the intensity of the NOO symmetric stretch for PNBA on bare CaF2 is divided into the intensity of the enhanced NOO symmetric stretch for PNBI adsorbed onto ENiNS. A second method is direct comparison of the NOO symmetric stretch mode of PNBI adsorbed on ENiNSs to thick layers of PNBA adsorbed on bare CaF2. Both methods yield similar results. It is believed that several factors contribute to the SEIRA enhancement observed on ENiNSs. First, Figure 1(left) and (right) shows that oblique-angle deposition of Ni on CaF2 and BaF2 leads to a much larger degree of NiNS formation than that when Ni is deposited on CaF2/BaF2 without any oblique-angle tilt. As alluded to previously, the more efficient ENiNS formation that occurs when the CaF2 or BaF2 substrate is set at a grazing angle with respect to the metal deposition source is likely a combination of the shadowing effect observed in oblique-angle deposition7,16 and the fact that the lower metal flux at the grazing angle allows more time for ENiNS formation. ENiNSs formed from oblique-angle deposition are also more likely to have the SPR shifted to the infrared, making them better candidates for SEIRA substrates.6 Partial aggregation of Au nanoparticles has been shown to lead to more SERS enhancement.20 The large degree of aggregation observed in the ENiNS films is conceivably another contributing factor in the resulting SEIRA. It is not believed that the aggregation itself leads to more SEIRA but that incomplete aggregation observed in Figure 1(left) and (right) leaves many ENiNSs with close enough spacing (less than 10
Figure 2. (Top) SEM image of 70 nm of Ni evaporated onto BaF2 at 78°. (Bottom) Infrared spectrum of 70 nm of Ni/BaF2 (black) and 50 nm of Ni/CaF2 (red) showing infrared SPR. Dashed lines help to see the position of the resonance bands.
collisions to interact with fluoride ions due to the shadowing effect observed in grazing angle deposition and instead forms ENiNSs. In the top of Figure 2 is an SEM image of 70 nm of Ni evaporated on BaF2 with a 78° oblique-angle tilt. Here, the average length of the ENiNS was about 26 nm, and the aspect ratio was less than 2. As with Figure 1(right), the ENiNSs are polydispersed with a large degree of observed aggregation. The bottom spectrum of Figure 2 shows the infrared spectra from ENiNSs that correspond to the nanostructure images in Figure 2 on CaF2 as well as the top spectrum of Figure 3 on BaF2, where each spectrum represents the SPR of the ENiNS in the mid-infrared. In each infrared spectrum, the CaF2 or BaF2 background was subtracted out. SPR in the mid-infrared was never observed for Ni films in the range of 2−20 nm of Ni exposure at 0° with no oblique-angle tilt. However, broad-range light scattering in the mid-infrared did occur, which resulted in 0% transmittance at a 20 nm Ni exposure. In Figure 3, from bottom to top, are the infrared transmission spectra of PNBA obtained where the CaF2 substrate had no Ni, 7 nm of Ni at a 0° tilt, 50 nm of Ni at a 75° oblique-angle tilt, and 70 nm of Ni at a 78° oblique-angle tilt. Particular attention should be paid to the symmetric stretch of the nitro PNBA group at ∼1350 cm−1. The bottom spectrum for bare CaF2 is a weak PNBA spectrum without any SEIRA enhancement. In the spectrum second from the bottom (7 nm of Ni at a 0° oblique-angle tilt), no PNBA can be observed in the spectrum. As previously shown in Figure 1(left), 7 nm of Ni adsorbed onto CaF2 at a 0° oblique-angle tilt does not lead to 3947
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(2) Aroca, R. F.; Ross, D. J. Surface-Enhanced Infrared Spectroscopy. Appl. Spectrosc. 2004, 58, 324A−338A. (3) Brown, L. V.; Zhao, K.; King, N.; Sobhami, H.; Norlander, P.; Halas, N. J. Surface-Enhanced Infrared Absorption Using Individual Cross Antennas Tailored to Chemical Moieties. J. Am. Chem. Soc. 2013, 135, 3688−3695. (4) Perry, D. A.; Cordova, J. S.; Spencer, W. D.; Smith, L. G.; Biris, A. S. SERS, SEIRA, TPD, and DFT Study of Cyanobenzoic Acid Isomer Film Growth on Silver Nanostructured Films and Powder. J. Phys. Chem. C 2010, 114, 14953−14961. (5) Posey, K. L.; Viegas, M. G.; Boucher, A. J.; Wang, C.; Stambaugh, K. R.; Smith, M. M.; Carpenter, B. G.; Bridges, B. G.; Baker, S. E.; Perry, D. A. Surface-Enhanced Vibrational and TPD Study of Nitroaniline Isomers. J. Phys. Chem. C 2007, 111, 12352−12360. (6) Sando, G. M.; Berry, A. D.; Owrutsky, J. C. Ultrafast Studies of Gold, Nickel, and Palladium Nanorods. J. Chem. Phys. 2007, 127, 074705/1. (7) Leverette, C. L.; Jacobs, S. A.; Shanmukh, S.; Chaney, S. B.; Dluhy, R. A.; Zhao, Y. P. Aligned Silver Nanorod Arrays as Substrates for Surface-Enhanced Infrared Absorption Spectroscopy (SEIRA). Appl. Spectrosc. 2006, 60, 906−913. (8) Rakić, A. D.; Dijuriŝić, A. B.; Elazar, J. M.; Majewski, M. L. Optical Properties of Metallic Films for Vertical-Cavity Optoelectronic Devices. Appl. Opt. 1998, 37, 5271−5283. (9) Williams, S. M.; Stafford, A. D.; Rodriguez, K. R.; Rogers, T. M.; Coe, J. V. Accessing Surface Plasmons with Ni Microarrays for Enhanced IR Absorption by Monolayers. J. Phys. Chem. B 2003, 107, 11871−11879. (10) Nakao, Y.; Yamada, H. Enhanced Infrared ATR Spectra of Surface Layers Using Metal Films. Surf. Sci. 1986, 176, 578−592. (11) Ebbesen, T. W.; Lezec, H. J.; Ghaemi, H. F.; Thio, T.; Wolff, P. A. Extraordinary Optical Transmission through Sub-Wavelength Hole Arrays. Nature 1998, 391, 667−669. (12) Etou, J.; Ino, D.; Furukawa, D.; Watanabe, K.; Nakai, I. F.; Matsumato, Y. Mechanism of Enhancement in Absorbance of Vibrational Bands of Adsorbates at a Metal Mesh with Subwavelength Hole Arrays. Phys. Chem. Chem. Phys. 2011, 13, 5817−5823. (13) Huo, S.; Xue, X.; Yan, Y.; Li, Q.; Ma, M.; Cai, W.; Xu, Q.; Osawa, M. Extending In Situ Attenuated-Total-Reflection SurfaceEnhanced Infrared Absorption Spectroscopy to Ni Electrodes. J. Phys. Chem. B 2006, 110, 4162−4169. (14) Huo, S.; Wang, J.; Sun, D.; Cai, W. Attenuated Total Reflectance Surface-Enhanced Infrared Absorption Spectroscopy at a Cobalt Electrode. Appl. Spectrosc. 2009, 63, 1162−1167. (15) Fu, J. X.; Collins, A.; Zhao, Y. P. Optical Properties and Biosensor Application of Ultrathin Silver Films Prepared by Oblique Angle Deposition. J. Phys. Chem. C 2008, 112, 16784−16791. (16) Driskell, J. D.; Shanmukh, S.; Liu, Y.; Chaney, S. B.; Tang, X. J.; Zhao, Y. P.; Dluhy, R. A. The Use of Aligned Silver Nanorod Arrays Prepared by Oblique Angle Deposition as Surface Enhanced Raman Scattering Substrates. J. Phys. Chem. C 2008, 112, 895−901. (17) Killian, M. M.; Villa-Aleman, E.; Sun, Z.; Crittenden, S.; Leverette, C. L. Dependence of Surface-Enhanced Infrared Absorption (SEIRA) Enhancement and Spectral Quality on the Choice of Underlying Substrate: A Choice Look at Silver (Ag) Films Prepared by Physical Vapor Deposition (PVD). Appl. Spectrosc. 2011, 65, 272. (18) Badilescu, S.; Ashrit, P. V.; Truong, V.-V.; Badilescu, I. I. Enhanced Infrared ATR Spectra of o-, m-, and p-Nitrobenzoic Acid with Ag Films. Appl. Spectrosc. 1989, 43, 549−552. (19) Mock, J. J.; Norton, S. M.; Chen, S.-Y.; Lazarides, A. A.; Smith, D. R. Electromagnetic Enhancement Effect Caused by Aggregation on SERS-Active Gold Nanoparticles. Plasmonics 2011, 6, 113−124. (20) Priest, H. F. “Anhydrous Metal Fluorides” Inorganic Syntheses; McGraw-Hill: New York, 1950; Vol. 3, pp 171−183. (21) Heaps, D. A.; Griffiths, P. R. Band Shapes in the Infrared Spectra of Thin Organic Films on Metal Nanoparticles. Vib. Spectrosc. 2006, 42, 45−50.
nm) to increase the SEIRA effect. Also observe in the two upper spectra of Figure 3 that the NOO symmetric stretch mode at about 1350 cm−1 has somewhat of an asymmetric or fano-type shape, which is probably due to the partial aggregation of the ENiNSs. Previous authors have discussed the onset of peak asymmetry at the metal nanostructure percolation threshold in SEIRA.21,22 It has been shown that oblique-angle deposition of Ni in a vacuum on CaF2 or BaF2 results in ENiNSs that exhibit SPR in the infrared. SEIRA enhancement factors in the range of 10− 20-fold have been observed for a monolayer of PNBI adsorbed on the ENiNS. Factors contributing to the SEIRA effect include the shadowing effect in oblique-angle depostion, the low Ni metal flux at grazing angles, the SPR shift to the infrared for ENiNSs, and the partial aggregation of the ENiNSs.
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METHODS PNBA (98% pure) was purchased from Aldrich. Solvents used in solution preparation were HPLC or Optima grade (Thermo Fisher or Aldrich). The 25 × 4 mm polished CaF2 or BaF2 windows were purchased from International Crystal Laboratories. The 99.9% pure Ni powder (3−7 μm) was purchased from Johnson Matthey Electronics. SEIRA and attenuated total reflectance Fourier transform infrared spectroscopy spectra were obtained on a Thermo-Nicolet IR100 FTIR spectrometer. All infrared spectra were conducted in transmission mode (2−4 cm−1 resolution; 16 scans were averaged). ENiNSs were grown on CaF2 or BaF2 windows by thermal evaporation of Ni as measured with an Infinicon quartz crystal microbalance in a home-built vacuum chamber with a base pressure better than 1 × 10−6 Torr. BaF2 crystals were polished with a 0.3 μm alumina/methanol suspension, rinsed and sonicated in methanol, and dried in air. CaF2 crystals were polished with a silica suspension, rinsed and sonicated in methanol, and dried in air. A monolayer exposure of PNBA was prepared for SEIRA and SERS studies by pipetting 25 μL of a 50 ppm solution onto a Ni film and allowing the solvent to evaporate. This is approximately a monolayer assuming an experimentally determined average spot size of 4 cm2 for all solvents, resulting in a reasonably uniform layer of about 250 ng/cm2. The monolayer approximation here accounts for the number of PNBA molecules in the given area on a flat surface and does not specifically account for surface roughness.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: (501) 450-5937. Fax (501) 450-3623. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the National Science Foundation Grant Numbers CHE-1008096 and CHE- 1306420 for funding. We also wish to thank Alan Roisen in the Physics Department at the University of Central Arkansas for his help in modifying our deposition apparatus for oblique-angle deposition experiments.
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REFERENCES
(1) Osawa, M. In-Situ Surface-Enhanced Infrared Spectroscopy of the Electrode/Solution Interface. Adv. Electrochem. Sci. Eng. 2006, 9, 269− 314. 3948
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(22) Priebe, A.; Sinther, M.; Fahsold, G.; Pucci, A. The Correlation between Film Thickness and Adsorbate Line Shape in Surface Enhanced Infrared Absorption. J. Chem. Phys. 2003, 119, 4887−4890.
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