Improving resolution in quantum subnanometre-gap tip-enhanced Raman nanoimaging

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, edge states, and surface functionalization from materials to bioscience. 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 with a wide range of applications.

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

“…30 Superior TERS images of a single CoTPP molecule on a Cu (100) surface with subnanometer-scale resolution resolving the patterns of normal modes were also obtained using an ultrahigh-vacuum STM setup at 6 K. 21 For inorganic nanostructures, signicant progress has been achieved for the local gap-mode TERS analysis of 2D semiconductors. 31 Recently we employed gap-mode AFM-TERS for determining the local strain in mechanically transferred monolayers of MoS 2 on Au nanostructures. 8,26,32 The biaxial strain in an ultrathin MoS 2 layer induces a shi of the E 2g phonon mode, which is probed by TERS spectroscopy with a spatial resolution of less than 25 nm.…”

Quantum plasmonic control of trions in a picocavity with monolayer WS 2

“…Quantum plasmonics plays an important role when the dimensions of plasmonic nanostructures reach a critical subnanometer size ( 13 ), as, for example, in the picoscale cavity formed by the plasmonic scanning probe and metal substrate ( 14 ). Few-layer MoS 2 in the picocavity showed interesting tunneling-induced photoluminescence (PL) and Raman quenching effects ( 14 ).…”

“…Quantum plasmonics plays an important role when the dimensions of plasmonic nanostructures reach a critical subnanometer size ( 13 ), as, for example, in the picoscale cavity formed by the plasmonic scanning probe and metal substrate ( 14 ). Few-layer MoS 2 in the picocavity showed interesting tunneling-induced photoluminescence (PL) and Raman quenching effects ( 14 ). However, although the classical plasmonic modulation of excitons in MoS 2 has been achieved ( 15 , 16 ), the quantum yield of exciton generation is low in few-layer compared to monolayer 2D materials, and the quantum plasmonic control of trions in monolayer TMDs was not yet explored.…”

Microscale Spectroscopic Mapping of 2D Optical Materials

“…Vibrational modes and peak width of MoS 2 depended on the layer number, and the influences of substrates on redshift were also observed. Besides, TERS was proven strong plasmonic imaging tool in revealing the quantum coupling and inhomogeneous structural features of few‐layer MoS 2 on a gold substrate . The shear and breathing modes in related TMDs like MoSe 2 and ReSe 2 , and the spatial excitation features were also observed, expanding the suitability of hyperspectral Raman imaging in few‐layer TMDs …”