Quantum limit in subnanometre-gap tip-enhanced nanoimaging of few-layer MoS2

TL;DR

This paper demonstrates nanoscale optical imaging of few-layer MoS2 using tip-enhanced Raman scattering with a subnanometre-gap configuration, revealing quantum effects that limit signal enhancement and enable new surface mapping contrast mechanisms.

Contribution

It introduces a quantum limit in subnanometre-gap TERS of MoS2, achieving 20 nm resolution and exploring quantum quenching and Schottky-Ohmic transition effects.

Findings

Achieved ~20 nm spatial resolution in TERS imaging of MoS2.

Identified quantum quenching behavior at subnanometre gaps.

Observed Schottky-Ohmic transition in gap-mode enhancement.

Abstract

Two-dimensional (2D) materials beyond graphene such as transition metal dichalcogenides (TMDs) have unique mechanical, optical and electronic properties with promising applications in flexible devices, catalysis and sensing. Optical imaging of TMDs using photoluminescence and Raman spectroscopy can reveal the effects of structure, strain, doping, defects, edge states, grain boundaries and surface functionalization. However, Raman signals are inherently weak and so far have been limited in spatial resolution in TMDs to a few hundred nanometres which is much larger than the intrinsic scale of these effects. Here we overcome the diffraction limit by using resonant tip-enhanced Raman scattering (TERS) of few-layer MoS2, and obtain nanoscale optical images with ~ 20 nm spatial resolution. This becomes possible due to electric field enhancement in an optimized subnanometre-gap resonant tip-substrate configuration. We investigate the limits of signal enhancement by varying the tip-sample gap with sub-Angstrom precision and observe a quantum quenching behavior, as well as a Schottky-Ohmic transition, for subnanometre gaps, which enable surface mapping based on this new contrast mechanism. This quantum regime of plasmonic gap-mode enhancement with a few nanometre thick MoS2 junction may be used for designing new quantum optoelectronic devices and sensors.