Characterizing unusual metal substrates for gap‐mode tip‐enhanced Raman spectroscopy

Abstract

In this article, the electromagnetic (EM) field in gap‐mode tip‐enhanced Raman spectroscopy (TERS) is investigated theoretically and experimentally for a range of commonly used and unusual metal and nonmetal substrates. By approaching a metal tip to a substrate, both form a coupled system that confines the EM field created at the tip apex. The influence of the substrate onto the EM field enhancement is observed in a top‐illumination gap‐mode TERS setup for different metal substrates. These include Au, the most commonly used substrate, and also a wide range of rarely or previously unused TERS substrates (Cu, Ag, Al, Pd, Pt, Ni, Ti, Mo, W, stainless steel, Al2O3, SiO2). Self‐assembled monolayers of thiols and brilliant cresyl blue thin film samples are investigated experimentally on nine metal substrates, all showing considerable TERS enhancement. With finite difference time domain and finite element simulations used, the article provides a good estimate of the EM field enhancement for a wide range of substrates for users to estimate how well a substrate of choice will perform in a gap‐mode TERS experiment. The reduction in EM field strength |E2| compared with Au is less than an order of magnitude for many metals (Calculations: Cu 92%, Ag 81%, Ni 53%). This article experimentally shows that a wide variety of conductive substrates can be used, when one is willing to trade a fraction of the EM field enhancement. TERS was seen on all metal substrates including stainless steel, yet quantification was not always possible. These qualitative results were complemented with intensities from calculations. The wider variety of substrates will increase the applicability of TERS and evolve it one step further towards use in standard analytics. Copyright © 2012 John Wiley & Sons, Ltd.

Si@Cu@Au AFM tips for tip-enhanced Raman spectrum

“…The calculated normalized signal-tonoise ratios of the Raman peaks were dependent on the specific vibrational modes. This was consistent with previous results [2,25,26,33,34].…”

“…The absolute and relative peak positions of the PhS TER spectrum in Figure 3 were matched to those in the normal Raman spectrum of bulk PhS and those in previously reported TER spectra (Table 1) [25,27,30]. Because of the complete PhS coverage, the S-H band at 914 cm −1 was not observed in the TER spectrum, consistent with Xu et al [31].…”

“…BCB is another mostly used sample for TERS enhancement evaluation for different tips and instrumentation [2,20,25,26,33]. AFM-TERS of BCB with the Si@Cu@Au tip was Figure 3 Raman signals of PhS acquired with Si@Cu@Au tip in (the upper) and with the tip retracted (the below).…”

“…Bimetallic materials with core-shell structures exhibit excellent SERS performances [22][23][24][25]. Here, we report a new bimetallic tip structure that includes a thin Cu primer layer between the silicon (Si) substrate and the Au coating.…”

Electrochemical design of plasmonic nanoantennas for tip-enhanced optical spectroscopy and imaging performance

“…To date, most TERS experiments are performed with gold/silver deposited atomic force microscopy (AFM) [16,17] or scanning tunneling microscopy (STM) probe [18]. For reinforcing the light confinement between the tip apex operating in tapping mode and a specimen of interest, a substrate is often deposited with a plasmonic material for exciting gap modes [19,20].…”