FLEISCHER ET AL. VOL. 5 ’ NO. 4 ’ 2570–2579 ’ 2011 www.acsnano.org 2570 March 14, 2011 C 2011 American Chemical Society Gold Nanocone Near-Field Scanning Optical Microscopy Probes Monika Fleischer, †, * Alexander Weber-Bargioni, ‡ M. Virginia P. Altoe, ‡ Adam M. Schwartzberg, ‡ P. James Schuck, ‡ Stefano Cabrini, ‡ and Dieter P. Kern † † Institute for Applied Physics, University of T € ubingen, Auf der Morgenstelle 10, 72076 T € ubingen, Germany and ‡ Molecular Foundry, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States M etallic nanoparticles are of great interest for their ability to sustain localized surface plasmon reso- nances when interacting with a resonant electromagnetic field. 1-4 The particles can act as antennas for the incoming light, where the external field induces collective oscillations of the conducting electrons. 5-7 These plasmons are accompanied by an evanescent near-field exponentially decay- ing within tens of nanometers of the metal surface. When the resonance frequency of the optical antenna corresponds to the frequency of excitation, the near-field can be enhanced by several orders of magni- tude. The optical near-field is effectively localized to the dimension of the small particle, thus beating the diffraction limit. In this way, light can be directed and con- trolled at the nanometer scale. Gold particles with diameters below 100 nm exhibit plasmon resonance frequencies in the visible light spectrum. Making use of such plasmonic particles paves the way to exciting applications in opto-electronic cir- cuits, light harvesting for solar cells, nano- lithography, data storage, high sensitivity biosensing, and not least of all, high resolu- tion near-field scanning optical microscopy (NSOM). 8-10 NSOM enables simultaneous high-resolution topographical and subdif- fraction limited optical mapping of surfaces. The present work focuses on the fabrication of NSOM scanning probes. To demonstrate their functionality, the near-field enhance- ment is harnessed for tip-enhanced Raman spectroscopy (TERS). 11-13 TERS can be ap- plied for chemical mapping down to the single molecule level. 14 Both the resolution and the near-field enhancement depend strongly on the properties of the scanning probe. The lateral topographical and optical resolutions directly correlate with the probe tip radius. The plasmon resonance fre- quency of the NSOM probe depends on the tip geometry, material, dimensions, and radius of curvature, as well as the dielectric properties of the immediate surroundings, including the sample material. 15 Therefore, the quality of the experimental results de- pends critically on well-suited probe tips. Recently, significant effort has gone into the design and specific engineering of opti- mized scanning probes. Most commonly, apertureless scanning probes consist in elec- trochemically etched gold wires 16 or scanning probes covered with a thin metal layer. To create a more well-defined, strongly localized light-source at the probe tip, single gold or silver spheres or rods have been placed on cantilevers and glass fiber tips, or metallic films have been etched into shape by focused ion beam milling. 17-27 Alternatively, bowtie antennas have been added at the apex of cantilever tips. 28,29 However, the ultimate goal of reproducibly fabricating specifically engi- neered, high purity optical antennas with ultrahigh topographic resolution on scanning probes has not been achieved yet. * Address correspondence to monika.fleischer@uni-tuebingen.de. Received for review August 29, 2010 and accepted February 24, 2011. Published online 10.1021/nn102199u ABSTRACT Near-field scanning optical microscopy enables the simultaneous topographical and subdiffraction limited optical imaging of surfaces. A process is presented for the implementation of single individually engineered gold cones at the tips of atomic force microscopy cantilevers. These cantilevers act as novel high-performance optical near-field probes. In the fabrication, thin-film metallization, electron beam induced deposition of etch masks, and Ar ion milling are combined. The cone constitutes a well-defined highly efficient optical antenna with a tip radius on the order of 10 nm and an adjustable plasmon resonance frequency. The sharp tip enables high resolution topographical imaging. By controllably varying the cone size, the resonance frequency can be adapted to the application of choice. Structural properties of these sharp-tipped probes are presented together with topographical images recorded with a cone probe. The antenna functionality is demonstrated by gathering the near-field enhanced Raman signature of individual carbon nanotubes with a gold cone scanning probe. KEYWORDS: near-field scanning optical microscopy . nanostructures . gold nanocones . electron beam induced deposition . ion milling . near-field enhancement . tip-enhanced Raman spectroscopy ARTICLE