Optimizing the near‐field and far‐field properties of tips in tip‐enhanced Raman scattering

Abstract

The process of tip-enhanced Raman scattering (TERS) depends critically on the morphology near the apex of the tip used in the experiment. Many tip designs have focused on optimization of electromagnetic enhancement in the near-field, which is controlled to a large extent by subtle details at the nanoscale that remain difficult to reproduce in the tip fabrication process. The use of focused ion beams (FIB) permit modification of larger features on the tip in a reproducible manner, yet this approach cannot produce sub-20-nm structures important for optimum near-field enhancement. Nonetheless, FIB milling offers excellent opportunities for improving the far-field radiation properties of the tip-antenna, a feature that has received relatively little attention in the TERS research community. In this work, we use finite-difference time-domain (FDTD) simulations to study both the near-field and far-field radiation efficiency of several tip-antenna systems that can be constructed with FIB techniques in a feasible manner. Starting from blunt etched tips, we find that excellent overall enhancement of the TERS signal can be obtained with pillar-type tips. Furthermore, by applying vertical grooves on the tip's shaft, the overall efficiency can be improved even more, producing TERS signals that are up to 10-fold stronger than signals obtained from an ideal (unmodified) sharp tip of 10-nm radius. The proposed designs constitute a feasible route toward a tip fabrication process that not only yields more reproducible tips but also promises much stronger TERS signals. K E Y W O R D S finite-difference time-domain simulations, tip-enhanced Raman scattering

Tribute to Derek Long: An instant snapshot of the development of Raman spectroscopy and its application in the fields of instrumentation and methodology, solid‐state materials, cultural heritage, DFT modeling and applications in biology, microbiology, and medicine

“…The first contribution by Pienpinijtham et al ( Electric field analysis, polarization, excitation wavelength dependence, and novel applications of tip‐enhanced Raman scattering (TERS) , Toyota Physical and Chemical Research Institute, Japan) [ 11 ] reviews the theoretical and physical foundations of Tip Enhancement Raman Spectroscopy (TERS), in particular electric field analysis, polarization, and excitation wavelength dependence and uses examples concerning “classic” (epitaxial graphene and graphene oxide) and chemically modified tips (e.g., nanoscale pH measurement and eniantiometric discrimination) to conclude on the prospects for evolution. The second contribution of Sifat and Potma ( Optimizing the near‐field and far‐field properties of tips in tip‐enhanced Raman scattering , University of California, USA) [ 12 ] is concerned with the design of tips (especially the morphology near the apex). The use of focused ion beams (FIB) permits the modification of larger features on the tip in a reproducible manner, for improving the far‐field radiation properties of the tip‐antenna, a feature that has received relatively little attention in the TERS research community thus far.…”

Progress of tip-enhanced Raman scattering for the last two decades and its challenges in very recent years

“…[1][2][3][4][5][7][8][9]11 High electric field enhancement of the tip provides a better capability of TERS to investigate a sample with a smaller size or a weaker Raman scattering coefficient. Therefore, theoretical calculations on the geometry of the tip to achieve high electric field enhancement have widely been noted, [67][68][69] and there have been several studies which have tried to prepare TERS tips with high enhancement using various strategies such as focused ion beam (FIB) milling, 70 a silver nanowire attachment, [71][72][73][74][75] electrodeposition, 76,77 or automated electrochemical etching. 78 Different types of tip prepared by several strategic methods and their characteristics are summarized in Table 1.…”