Highly Stable, Protected Plasmonic Nanostructures for Tip Enhanced Raman Spectroscopy Carlos A. Barrios, Andrey V. Malkovskiy, Alexander M. Kisliuk, Alexei P. Sokolov, and Mark D. Foster* Department of Polymer Science, The UniVersity of Akron, 170 UniVersity AVenue, Akron, Ohio 44325-3909 ReceiVed: NoVember 06, 2008; ReVised Manuscript ReceiVed: February 12, 2009 An ultrathin aluminum oxide (Al 2 O 3 ) coating improves significantly the stability of plasmonic structures used in tip enhanced Raman spectroscopy (TERS). The coating does not alter the favorable optical properties of metallic structures yet improves wear resistance and inhibits degradation, so that signal enhancement remains constant over 40 days for structures with a 3 nm thick protective coating. Most unprotected structures show substantial losses in enhancement over periods as short as 10 days when stored and used in ambient conditions. Introduction Developing noble metal nanostructures with stable plasmonic activity is essential to achieve high resolution chemical imaging. Resonant plasmon excitations in noble metal structures allow the localization and amplification of light in very small volumes and are the basis of surface enhanced spectroscopies, including tip enhanced Raman spectroscopy (TERS). 1 Unfortunately, since noble metal nanostructures are fragile, mechanical, chemical, and morphological degradation processes make them unstable. Protection against mechanical deformation, chemical degrada- tion, and laser heating is important for designing stable plasmonic devices. The coating of metallic substrates with dielectric thin films for surface enhanced Raman spectroscopy (SERS) has already received some attention. Walls and Bohn 2 showed that sputtering SiO 2 films thicker than 3 nm on top of 5 nm silver islands imparted resistance to chemical attack of the metal structures. Lacy et al. 3 have shown that evaporative deposition of SiO at 9 × 10 -6 Torr results in complete and uniform SiO 2 5 nm thick films able to completely cover 4.5 nm thick Ag islands. In the case of alumina (Al 2 O 3 ), Murray and Allara 4,5 studied the effect on SERS of separating an analyte molecule from a silver substrate by layers of Al/Al 2 O 3 and a polymer thin film. They suggested some synergistic effect of the aluminum with the silver increased the enhancement. Van Duyne et al. 6 found that placing even a 0.2 nm layer of aluminum oxide by atomic layer deposition (ALD) on metal structures created for catalysis applications provided heat resistance and improved the preser- vation of sharp plasmonic structures. Recently our group 7 has demonstrated experimentally and Zenobi et al. 8 have demon- strated theoretically that a SiO x layer can protect metallized tips for TERS. Furthermore, it has recently become known that alumina ultrathin layers have excellent gas barrier properties. Park et al. 9 showed in 2005 that the volume expansion that occurs upon oxidation of aluminum generates a coating so dense that it is impermeable even to hydrogen. Hakim and co-workers 10,11 then showed that highly conformal Al 2 O 3 layers could be deposited on titania and that iron nanoparticles coated by alumina using atomic layer deposition (ALD) are resistant to oxidation, even at high temperatures. Very recently, publications for alumina gas barrier coatings on plastics have appeared 12 because thin alumina layers are optically transparent and very hard (Vickers Hardness 2600 MPa). In this article we demon- strate that controlled physical vapor deposition (PVD) of aluminum can be used to create an ultrathin (∼2-3 nm) Al 2 O 3 coating that improves significantly the stability and wear resistance of plasmonic Ag nanostructures on tips for aperture- less near-field optics without substantial degradation of their advanced optical properties. TERS combines scanning probe microscopy with Raman spectroscopy, taking advantage of apertureless near-field optics, 13 and has already achieved a remarkable lateral resolution of ∼10-20 nm. 14,15 Bottom, top, and side illumination optical schemes have all been proposed. 16-18 The key element of TERS, a plasmonic structure on a very sharp tip, provides a high amplification (∼10 3 -10 4 ) of the Raman signal by surface plasmon resonance. Tip characteristics such as roughness, 19 shape, 20 radius, and material under the plasmonic structure 21 determine both the spatial resolution and the contrast. Contrast, which relates the total signal measured when the tip is in contact to the signal measured when the tip is withdrawn, is the key parameter controlling one’s ability to image with the tip. 18,22 Maintaining the morphological, mechanical, and chemical integrity of these structures during scanning, i.e., keeping their optical properties unaltered, is crucial for obtaining undistorted optical images with nanometer scale resolution. This is espe- cially problematic for silver-based structures that usually provide the best enhancements, but mechanically and chemically degrade very quickly. Gold-based structures are chemically more stable, but suffer mechanical degradation to an extent similar to that seen for silver-based structures. A metallized apertureless tip usually consists of a silicon or silicon nitride tip covered with a thin (20-50 nm), rough layer of a metal such as gold (Au) or silver (Ag). Abrasive friction forces between the metal layer and the surface under analysis are the main reason for degradation of the metal structure during “contact” or “tapping” mode scanning. While noncontact mode imaging is also possible, it is more challenging to implement robustly. Wearing is a well-known problem for silicon or silicon nitride tips in conventional SPM imaging. Even more challeng- ing problems are expected with a metallized tip because the hardness of the metal layer (Vickers Hardness: Ag 251 MPa, Au 216 MPa) is substantially lower that that of the bare tip (Vickers Hardness: Si 1415 MPa, Si 3 N 4 2040 MPa). Metallized tips can also deteriorate irreversibly due to exposure to adverse environmental conditions 23 or intense light. 24 SPM tips with Ag * Corresponding author. E-mail: mfoster@uakron.edu. J. Phys. Chem. C 2009, 113, 8158–8161 8158 10.1021/jp8098126 CCC: $40.75  2009 American Chemical Society Published on Web 04/17/2009