Au Nanorod Arrays Tailored for Surface-Enhanced Raman Spectroscopy

Abstract

IntroductionFor applying local field enhancement arising from local plasmons present in noble metal nanoparticles to biochemical sensors, such as surface-enhanced Raman scattering (SERS) sensors, it is important to control both the shape and the arrangement of the nanoparticles.1 It is well known that wellordered 2D periodic arrays of triangular nanoparticles prepared by nanosphere lithography (NSL) show plasma resonance within a rather narrow wavelength region, and the sharp corners of the triangles result in strong SERS.2 Although NSL enables the size of the nanoparticles to be tuned, the number density of the nanoparticles is geometrically fixed. Some applications, such as Raman imaging, may require more SERS-active spots to improve the space resolution. In addition, it is difficult to tune the lowest order of the local plasma resonance to the near-infrared (NIR) region, which is appropriate for biological applications. 3A drastic enhancement in the local field is considered to also occur at the ends of elongated nanoparticles. 4 In fact, for light in the visible to NIR region, SERS has been observed on arrays of elongated Ag nanoparticles, or so-called nanorods, whose widths are in the range of tens of nanometers and whose aspect ratios are 2 -5. 5-7 Liao et al. 5 prepared in-line aligned nanorod arrays on lithographically produced templates, while Martínes et al. 6 and Wachter et al. 7 produced side-by-side aligned nanorod arrays by oblique deposition. Taking into account the gap enhancement in the local field, 8 in-line alignment of the nanorods is more desirable than side-by-side alignment. However, even today, it is difficult to produce sub-micrometer patterns over large areas at low cost.Gold nanorods are also attracting much interest, since chemically stable Au is considered to be preferred to Ag for the material of the SERS substrates. In addition to lithographically produced Au nanorod arrays, 9 random aggregates of Au nanorods produced by chemical routes [10][11][12] are extensively investigated. However, it is not easy to immobilize the Au nanorods uniformly in isolation with each other.Recently, we demonstrated the direct formation of Ag nanorods with quasi-parallel major axes on a template layer of SiO2 having a strongly anisotropic surface morphology by using a dynamic oblique deposition (DOD) technique. 13 These Ag nanorods show excellent SERS properties. 14,15 In this study, we report that physically self-assembled Au nanorod arrays show fairly highly sensitive SERS properties that are comparable with those of Ag. ExperimentalThe preparation method of Au nanorod arrays is basically the same as that of Ag nanorod arrays reported in our previous papers.14,15 Briefly, template layers of SiO2 were prepared on a glass substrate by the serial bideposition (SBD) technique, which is a version of the vacuum evaporation technique. During SBD, the deposition angle, αSiO 2 , measured from the surface normal was fixed to 78.6˚, while the azimuth was rapidly changed by 180˚ with each deposition of a 10-nm-t...

Optical properties and microstructures of inorganic wave plates prepared by serial bideposition

“…In the case of serial bideposition, the shadowing area lies alternately in front of and behind the columns, and, accordingly, the intercolumn gaps are widened, and bundles are formed more visibly. 6 Therefore, the degree of anisotropy of the surface morphology is considerably higher than that of a conventional obliquely deposited film.…”

“…Several researchers have reported solutions 2,4 for improving the transmittance, which depends on the smaller birefringent, Δn, and, occasionally, on film haze. 1 Several studies have reported that the retardation films prepared by the serial bideposition technique [4][5][6] have nanostructural growth perpendicular to the substrate, and can therefore be used for increasing Δn. Film thickness could decrease at high values of Δn.…”

Practical applications of thin films nanostructured by shadowing growth

“…Au nanorods were aligned on a SiO 2 template layer (called the "shape control layer") having an anisotropic surface morphology prepared by OAD. 6 The shape control layer of SiO 2 as the template for Au nanorods is prepared by a serial bideposition (SBD) technique. The glass substrate is set obliquely at a deposition angle of about 80 deg.…”

“…As a result, the surface of the shape control layer prepared by SBD is corrugated anisotropically. 6,7 On the anisotropic shape control layer, Au is also evaporated obliquely in vacuum. The amount of deposited Au is only around 10 nm thick on average.…”

“…Although the advanced functionalities of OAD films are well understood in academia, there seems to be prejudice in the industry that OAD thin films are neither durable nor reproducible. However, in the past few years, very fortunately, new products produced by the OAD technique have appeared on the market, including a substrate for surface-enhanced Raman spectroscopy [6][7][8] and a low-reflectivity wire-grid polarizer. 9,10 In this article, we discuss the practicability of OAD thin films and examples of the products produced by the OAD technique.…”