Impact of substrate on tip-enhanced Raman spectroscopy: A comparison between field-distribution simulations and graphene measurements

Abstract

Tip-enhanced Raman spectroscopy (TERS) has reached nanometer spatial resolution for measurements performed at ambient conditions and subnanometer resolution at ultrahigh vacuum. Super-resolution (beyond the tip apex diameter) TERS has been obtained mostly in the gap mode configuration, where a conductive substrate localizes the electric fields. Here we present experimental and theoretical TERS to explore the field distribution responsible for spectral enhancement. We use gold tips of 40 +/- 10 nm apex diameter to measure TERS on graphene, a spatially delocalized two-dimensional sample, sitting on different substrates: (i) glass, (ii) a thin layer of gold and (iii) a surface covered with 12 nm diameter gold spheres, for which 6 nm resolution is achieved at ambient conditions. The super-resolution is due to the field configuration resulting from the coupled tip-sample-substrate system, exhibiting a nontrivial spatial surface distribution. The field distribution and the symmetry selection rules are different for nongap versus gap mode configurations. This influences the overall enhancement which depends on the Raman mode symmetry and substrate structure.