Resonant tip-enhanced Raman scattering by CdSe nanocrystals on plasmonic substrates.

I A Milekhin,A G Milekhin,D R T Zahn,T A Duda,K V Anikin,V M Dzhagan,M Rahaman,B M Saidzhonov,A V Latyshev,E E Rodyakina,R B Vasiliev

doi:10.1039/d0na00554a

Abstract

Tip-enhanced Raman scattering (TERS) has recently emerged as a powerful technique for studying the local properties of low dimensional materials. Being a plasmon driven system, a dramatic enhancement of the TERS sensitivity can be achieved by an appropriate choice of the plasmonic substrate in the so-called gap-mode configuration. Here, we investigate the phonon properties of CdSe nanocrystals (NCs) utilizing gap-mode TERS. Using the Langmuir–Blodgett technique, we homogeneously deposited submonolayers of colloidal CdSe NCs on two different nanostructured plasmonic substrates. Amplified by resonant gap-mode TERS, the scattering by the optical phonon modes of CdSe NCs is markedly enhanced making it possible to observe up to the third overtone of the LO mode reliably. The home-made plasmonic substrates and TERS tips allow the analysis of the TERS images of CdSe phonon modes with nanometer spatial resolution. The CdSe phonon scattering intensity is strongly correlated with the local electromagnetic field distribution across the plasmonic substrates.

Highlights

Technological progress in nanotechnology faces a challenge to reduce the size of active elements down to the nanoscale
We extend our investigations by demonstrating gap-mode AFM-Tip-Enhanced Raman Scattering (TERS) imaging for the local analysis at the nanometer scale of submonolayers of colloidal CdSe nanocrystals deposited on home-made plasmonic substrates consisting of an array of Au nanodisks and for comparison on a commercial SERS substrate
Using transmission electron microscopy (TEM), we found that CdSe NCs predominantly have a zinc-blende crystalline structure and an average size of 5–6 nm (Fig. 1)

Summary

Introduction

The TERS enhancement reaches values up to 107 for various organic materials, nanocarbons including carbon nanotubes,[3,4,5] one-dimensional carbyne,[5] graphene,[5,6] and other 2D materials,[7,8] while the spatial resolution steadily improves from tens down to sub-nanometers.[9,10,11,12] The recent advances in TERS imaging are discussed in several comprehensive reviews.[9,11,12,13]. The STM-TERS experiments require special experimental conditions such as well-de ned conductive substrates, which are free of carbon contamination, fabrication of ultra-sharp metal STM tips, nitrogen atmosphere[19] or ultra-high vacuum conditions[20] needed for cryogenic temperatures to be achieved.[21] AFM-TERS, on the other hand, can be realized under ambient conditions or even in a liquid environment.[22,23] Note that, under ambient conditions a thin water layer is formed on the surface of most hydrophilic samples It may play an important role in the surface diffusion of analyte molecules or cause undesirable effects such as the transition of molecules to the tip or molecule decomposition, leading to contamination of the tip. We extend our investigations by demonstrating gap-mode AFM-TERS imaging for the local analysis at the nanometer scale of submonolayers of colloidal CdSe nanocrystals deposited on home-made plasmonic substrates consisting of an array of Au nanodisks and for comparison on a commercial SERS substrate

Methods

Results and discussion

Conclusions