Recent advances in tip-enhanced Raman spectroscopy probe designs

Tip-enhanced Raman spectroscopy (TERS) imaging is a super-resolution imaging technique that features the merits of both surface-enhanced Raman spectroscopy (SERS) and scanning probe microscopy (SPM), such as the high chemical sensitivity from the former and the nanoscale spatial resolution from the latter. These advantages make TERS an essential nano-spectroscopic characterization technique for chemical analysis, materials science, bio-sensing, etc. TERS probes, the most critical factor determining the TERS imaging quality, are expected to provide a highly confined electromagnetic hotspot with a minimized scattering background for the generation of Raman signals with high spatial resolution. After two decades of development, numerous probe design concepts have been proposed and demonstrated. This review provides a comprehensive overview of the state-of-the-art TERS probe designs, from the working mechanism to the practical performance. We start with reviewing the recent development of TERS configurations and the corresponding working mechanisms, including the SPM platforms, optical excitation/collection techniques, and probe preparation methods. We then review the emerging novel TERS probe designs, including the remote-excitation probes, the waveguide-based nanofocusing probes, the metal-coated nanofocusing probes, the nanowire-assisted selective-coupling probes, and the tapered metal-insulator-metal probes. Our discussion focuses on a few critical aspects, including the surface-plasmon-polariton (SPP) hotspot excitation technique, conversion efficiency, working frequency, and controllability. In the end, we review the latest TERS applications and give a perspective on the future of TERS.