title: 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.
authors.affiliation: Texas A&M University, College Station, TX 77843, USA.|Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China.
authors.forename: Yingchao
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: Texas A&M University, College Station, TX 77843, USA.|Baylor University, Waco, TX 76798, USA.
authors.forename: Dmitri V
authors.initials: DV
authors.lastname: Voronine
authors.affiliation: Texas A&M University, College Station, TX 77843, USA.|Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China.
authors.forename: Shangran
authors.initials: S
authors.lastname: Qiu
authors.affiliation: Texas A&M University, College Station, TX 77843, USA.
authors.forename: Alexander M
authors.initials: AM
authors.lastname: Sinyukov
authors.affiliation: Baylor University, Waco, TX 76798, USA.
authors.forename: Mary
authors.initials: M
authors.lastname: Hamilton
authors.affiliation: Texas A&M University, College Station, TX 77843, USA.|Baylor University, Waco, TX 76798, USA.
authors.forename: Zachary
authors.initials: Z
authors.lastname: Liege
authors.affiliation: Texas A&M University, College Station, TX 77843, USA.|Baylor University, Waco, TX 76798, USA.
authors.forename: Alexei V
authors.initials: AV
authors.lastname: Sokolov
authors.affiliation: Baylor University, Waco, TX 76798, USA.
authors.forename: Zhenrong
authors.initials: Z
authors.lastname: Zhang
authors.affiliation: Texas A&M University, College Station, TX 77843, USA.|Baylor University, Waco, TX 76798, USA.|Princeton University, Princeton, New Jersey 08544, USA.
authors.forename: Marlan O
authors.initials: MO
authors.lastname: Scully