Colloidal Nanoantennas for Hyperspectral Chemical Mapping Tyler J. Dill, Matthew J. Rozin, Stephen Palani, and Andrea R. Tao* NanoEngineering Department, University of California, San Diego 9500 Gilman Drive MC 0448, La Jolla, California 92093-0448, United States S Supporting Information *
ABSTRACT: Tip-enhanced Raman spectroscopy enables access to chemical information with nanoscale spatial resolution and single-molecule sensitivities by utilizing optical probes that are capable of confining light to subwavelength dimensions. Because the probes themselves possess nanoscale features, they are notoriously difficult to fabricate, and more critically, can result in poor reproducibility. Here, we demonstrate high-performance, predictable, and readily tunable nanospectroscopy probes that are fabricated by self-assembly. Shaped metal nanoparticles are organized into dense layers and deposited onto scanning probe tips. When coupled to a metal surface, these probes behave like nanoantenna by supporting a strong optical resonance, producing dramatic Raman field enhancements in the range of 108−109 with sub-50 nm spatial resolution. In contrast to other nanospectroscopy probes, our colloidal probes can be fabricated in a scalable fashion with a batch-tobatch reproducibility of ∼80% and serve as an important demonstration of bottom-up engineering. KEYWORDS: tip-enhanced Raman spectroscopy, TERS, nanoantenna, fabrication, chemical imaging, self-assembly, colloidal nanoparticles appropriate size and shape such that it exhibits maximum field confinement of light near the incident wavelength. In addition, tips must be resilient to both mechanical damage incurred by contact with the sample surface and thermal damage incurred by laser irradiation.14,15 There are currently two approaches to TERS probe fabrication. The first approach uses methods such as electrochemical etching of an all-metal wire or physical vapor deposition (PVD) of metals such as silver or gold onto an atomic force microscopy (AFM) probe.16,17 These methods tend to produce tips with arbitrary nanoscopic features, leading to the difficulties in characterizing or reproducing optical measurements for which TERS is notorious. While recent studies have shown improvements in etching Au tips with consistent shapes at the micro- and nanoscale,18,19 they have not led to consistent optical field enhancements. The second approach uses top-down fabrication techniques such as focused ion beam milling20,21 and induced deposition mask lithography22 to generate engineered nanoantenna with specific plasmonic properties.23 However, top-down methods encounter difficulties in producing reliable features
