Article pubs.acs.org/JPCC
Properly Structured, Any Metal Can Produce Intense Surface Enhanced Raman Spectra Katherine N. Kanipe,† Philip P. F. Chidester,† Galen D. Stucky,† Carl D. Meinhart,‡ and Martin Moskovits*,† †
Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106-9510, United States Department of Mechanical Engineering, University of California, Santa Barbara, California 93106-5070, United States
‡
S Supporting Information *
ABSTRACT: While silver and gold have been the dominant plasmonic metals used for surface-enhanced Raman spectroscopy (SERS) since the field’s inception. We argue that virtually any metal, when appropriately nanostructured as a grating, has the potential to be an efficient SERS substrate. This conclusion provides the basis for making SERS a general tool for studying surface processes and catalysis and allows SERS substrates to be routinely based on earth-abundant, low-cost, and chemically interesting metals. We illustrate the above premise by producing highly performing SERS substrates using aluminum, nickel, and copper in addition to silver and gold as benchmarks. All five metals were found to yield high SERS intensities. The approximately three orders enhancement variation among the five substrates based on differing metals is ascribed mainly to local field effects associated with individual grating elements. This conclusion is supported by local field calculations. This suggests that the largest contribution to the enhancement is a (radiative) nonlocal grating-based (plasmonic) effect which is approximately equal for all of the gratings we studied regardless of metal from which they were fabricated, so long as the structural details of the gratings were kept constant.
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INTRODUCTION Silver and gold have dominated the surface-enhanced Raman spectroscopy (SERS) literature since the inception of the field due to their favorable and high quality plasmonic properties in the visible region of the spectrum and their high chemical stability. Indeed, the phenomenon was originally observed on roughened silver surfaces1 that are still commonly used as SERS platforms in biological applications,2−4 for detecting low concentration analytes,5 and in other sensing applications.6,7 Not unexpectedly, a great deal of interest has been focused on understanding both theoretically8,9 and experimentally10 what structural characteristics and materials properties lead to a highly effective SERS substrate for analytical applications. Strategies for improving SERS performance have largely centered on optimizing the architecture of SERS substrates, for example, by developing strategies for reproducibly aggregating nanoparticles,11 producing such electromagnetically coupled nanostructures as nanogratings,12 piezoelectrically driving a nanotip toward a metal surface as in tip-enhanced Raman spectroscopy,13 and synthesizing core−shell nanoparticles, in which the plasmon of a nanocavity couples to the plasmon of a nanosphere.14 It has been known from the earliest days of SERS that most metals can sustain plasmons, among which only the alkali metals rival the coinage metals in their plasmonic capabilities.15 Although excellent SERS results were reported with alkali metals under ultrahigh vacuum conditions,16 their lack of stability in air and water make the alkalis impractical for most applications. The plasmonic properties of other high © 2017 American Chemical Society
conductance metals have been explored theoretically, including Cu, Pt, and Pd,17,18 and early SERS results using Al, Pt, and In have been reported.19−23 The plasmonic resonance quality (i.e., the “sharpness” of the resonance) has been discussed by several researchers.24 It is easy to show that the breadth of the plasmon resonance of a sphere whose radius is much smaller than the wavelength increases with increasing electron scattering rate (which is inversely proportional to electrical conductivity) and with increased interband contribution to the material’s dielectric constant in the frequency range of the plasmon (these two properties account for the good plasmonic performance of silver). Also well-known is the fact that the plasmon resonance frequency can be significantly tuned by altering the nanogeometry of the plasmonic entity; for example, one (or more) of the three degenerate plasmonic modes of a Pd nanosphere (in air) whose resonance occurs at ∼230 nm can be shifted to cover the entire visible and near IR spectrum by either elongating the sphere to a prolate spheroid or flattening it to an oblate spheroid.25 Previously, we reported a nanograting architecture that produces both highly enhancing and highly uniform SERS signals over wafer-scale areas.26 The grating consisted of twodimensionally periodic silicon posts coated with silicon dioxide and metal thin films, creating a highly regular square array of Received: March 20, 2017 Revised: June 10, 2017 Published: June 12, 2017 14269
DOI: 10.1021/acs.jpcc.7b02637 J. Phys. Chem. C 2017, 121, 14269−14273
Article
The Journal of Physical Chemistry C
adhesive layer. Copper was deposited using thermal evaporation at 18MΩ (Millipore) water. Thionine was chosen as the analyte for these SERS studies and was prepared by dissolving thionine acetate (SigmaAldrich, used as received) in >18MΩ (Millipore) water to a concentration of 10−3 M. When low concentrations of analyte were used, care was taken to ensure that the substrates has access to a sufficient volume of solution to ensure that at least one full monolayer of molecules could adsorb on the metal surfaces. Substrates were submerged in solution for >18 h to ensure equilibrium coverage with adsorbed thionine and then rinsed with deionized water and blown dry with N2 gas to remove any unadsorbed thionine. SERS spectra were collected in backscattering mode (Horiba JY Aramis Raman microscope equipped with a 633 nm HeNe laser) with an incident laser power of 7.6 μW and exposure time of 0.25 to 2.5 s using a long working distance objective. The spectra obtained for thionine agreed with literature spectra. Multiple spectra were collected using xy-mapping over a 50 × 50 μm area at 5 μm intervals in both directions to determine statistics (i.e., 121 spectra). All reported intensities are averages from mapped areas and are given in counts per milliwatt per second. Details of the electromagnetic field calculations resulting in the data featured in Figure 4 are given in the Supporting Information. The electromagnetic fields scattered by a pair of closely spaced silica core−metal shell nanoparticles whose centers were offset as shown in Figure 4 to mimic approximately the shapes of the elements of our nanogratings. Illuminated by a linearly polarized plane wave of unit magnitude, the scattered fields in the vicinity of the core− shell nanofeatures were simulated using the finite element package COMSOL Multiphysics V5.2a (COMSOL, Stockholm, Se) and the Wave Optics add-on module by solving the following governing equation
core−shell nanoparticles whose average enhancements exceeded most nanoparticle aggregates by two or more orders of magnitude. The key to the strong performance of this architecture, which was reported using a gold metallic layer, is the synergy of local near-field resonances resulting from interactions between neighboring grating elements and longrange grating resonances. The nature of the grating contribution has been capably discussed in the literature;27,28 an important point being that grating resonances are, to first order, a function of the geometrical parameters of the grating independent of the material. Here, we apply such architectural principles to a variety of metals to show that the gratingenabled plasmonic resonances allow a wide range of metals to function effectively as SERS active systems. Expanding the pool of SERS-capable metals beyond silver and gold has important implications. For example, aluminum is earth abundant and therefore inexpensive; nickel, copper, platinum, palladium, and many other transition metals make possible the study of important surface chemical and catalytic processes using SERS. Although few such attempts have appeared, and none discussing the ubiquity of plasmonic materials systematically as we do here, such goals are not altogether new. For example, Weaver29 pioneered studies in the 1980s using palladium-coated gold electrodes, and others followed, motivated by the desire to use SERS for studying chemistries for which gold is not suitable.30−32 Here, we demonstrate that, by using a prescriptive top-down nanofabrication method, a silicon/silica nanograting can serve as a common substrate on top of which essentially any metal can be deposited to create a highly functional SERS substrate. We illustrate this using five metals: gold, silver, copper, aluminum, and nickel. The first two, traditional SERS materials, are the benchmarks against which the performance of the others are compared. Copper was chosen to complete the set of coinage metals and to illustrate the negative effects on enhancement of interband transitions occurring in the vicinity of the plasmon resonance. Aluminum is earth abundant and a good conductor whose dielectric function in the visible region of the spectrum is almost devoid of interband contributions. Nickel is not a particularly good conductor and has strong interband transitions in the visible, but it is a metal with rich surface chemistry.
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EXPERIMENTAL METHODS
Substrate Fabrication. Fabrication follows the previously published methods for gold-coated substrates. Summarizing briefly, a two-dimensional grating pattern is formed in photoresist using laser interference lithography in which two exposures are made at a 90-degree angle to one another with a repeat length of 330 nm. The resulting pattern is transferred into the underlying antireflection coating using reactive ion etching (O2 plasma) and then into the silicon using a modified Bosch process (reactive ion etching with simultaneous flow of SF6, Ar2, and C4F8). Liftoff of residual photoresist is done using warm piranha solution (3:1 sulfuric acid to hydrogen peroxide at approximately 40 °C). The resultant two-dimensional array of silicon pillars is then coated with silica deposited by plasmaenhanced chemical vapor deposition to modulate the thickness of individual posts. Finally, a metal film is deposited using a high-vacuum deposition technique. Gold, silver, aluminum, and nickel were deposited by electron-beam evaporation at
