Spatial resolution of tip-enhanced Raman spectroscopy – DFT assessment of the chemical effect

Federico Latorre,Thomas Bocklitz,Stefanie Gräfe,Steffen Trautmann,Volker Deckert,Stephan Kupfer,Daniel Kinzel

doi:10.1039/c6nr00093b

Abstract

Experimental evidence of extremely high spatial resolution of tip-enhanced Raman scattering (TERS) has been recently demonstrated. Here, we present a full quantum chemical description (at the density functional level of theory) of the non-resonant chemical effects on the Raman spectrum of an adenine molecule mapped by a tip, modeled as a single silver atom or a small silver cluster. We show pronounced changes in the Raman pattern and its intensities depending on the conformation of the nanoparticle-substrate system, concluding that the spatial resolution of the chemical contribution of TERS can be in the sub-nm range.

Highlights

We present the results of purely quantum chemical calculations based on density functional theory (DFT) to evaluate the spatial resolution of a metal tip acting on adenine by investigating the effect of minute lateral variations of the tip with respect to an individual sample molecule
Starting from a distance of z = 10 Å between the silver atom and the molecular plane of the pre-optimized adenine molecule, the Raman intensity pattern along the grid has been calculated for each vibrational normal mode using eqn (1)
The simulated Raman spectra at 10 Å along the grid points share a uniform intensity pattern, which is almost identical to the z-polarized Raman spectrum of an isolated adenine molecule

Summary

Introduction

Several recent TERS experiments have strongly indicated spatial resolutions of 1 nm or smaller (i.e., single molecule resolution).[35,36,37,38,39,40,41] Such high resolutions are unexpected considering the dimensions of the commonly used plasmonic nanoparticles.[42,43] While a high resolution was predicted by Elfick and coworkers for tips with radii of 1 nm,[44] this tip size is generally difficult to achieve, and radii of 10–20 nm are considered more realistic.[45,46,47,48]. We present the results of purely quantum chemical calculations based on density functional theory (DFT) to evaluate the spatial resolution of a metal tip acting on adenine by investigating the effect of minute lateral variations of the tip with respect to an individual sample molecule. Such position effects are generally not considered in the estimation of the chemical effects in SERS because a thermodynamically governed association of the metal particle and molecule (an optimized minimum energy geometry) can be assumed. A comparison between the effects of a single metal atom and a 20-atom metal cluster mimicking a metal tip will be shown, providing insight into second layer effects, e.g., how strongly the atoms “behind” the front-most atom influence the molecular structure and the vibrational spectra

Computational details

One-silver-atom model

Silver-cluster model

Conclusions