Uncovering topographically hidden features in 2D MoSe2 with correlated potential and optical nanoprobes

David Moore,Deep Jariwala,Christopher Muratore,Nicholas R Glavin,Christine Nguyen,Kiyoung Jo,Jun Lou

doi:10.1038/s41699-020-00178-w

Abstract

Developing characterization strategies to better understand nanoscale features in two-dimensional nanomaterials is of crucial importance, as the properties of these materials are many times driven by nanoscale and microscale chemical and structural modifications within the material. For the case of large area monolayer MoSe2 flakes, kelvin probe force microscopy coupled with tip-enhanced photoluminescence was utilized to evaluate such features including internal grain boundaries, edge effects, bilayer contributions, and effects of oxidation/aging, many of which are invisible to topographical mapping. A reduction in surface potential due to n-type behavior was observed at the edge of the flakes as well as near grain boundaries. Potential phase mapping, which corresponds to the local dielectric constant, depicted local biexciton and trion states in optically-active regions of interest such as grain boundaries. Finally, nanoscale surface potential and photoluminescence mapping was performed at several stages of oxidation, revealing that various oxidative states can be evaluated during the aging process. Importantly, all of the characterization performed in this study was non-destructive and rapid, crucial for quality evaluation of an exciting class of two-dimensional nanomaterials.

Highlights

The unusual properties of two-dimensional (2D) materials are often generated not from a perfect crystalline lattice, but by the defects, grain boundaries, and other imperfections embedded within the structure
While many researchers are pushing the limits of crystal perfection, the inhomogeneities and defects have proven beneficial in the cases of quantum emission from atomic vacancies[8–10], electronic transport modification at grain boundaries[11,12], and improved catalysis in substoichiometric materials[13]. These features are only observed via low-throughput techniques such as transmission electron microscopy or scanning tunneling microscopy[14–16]
Report, 1D MoS2 nanoribbons were synthesized and were found to emit at higher energies as compared to 2D MoS2 when measured via TEPL32

Summary

INTRODUCTION

The unusual properties of two-dimensional (2D) materials are often generated not from a perfect crystalline lattice, but by the defects, grain boundaries, and other imperfections embedded within the structure. While many researchers are pushing the limits of crystal perfection, the inhomogeneities and defects have proven beneficial in the cases of quantum emission from atomic vacancies[8–10], electronic transport modification at grain boundaries[11,12], and improved catalysis in substoichiometric materials[13]. Often, these features are only observed via low-throughput techniques such as transmission electron microscopy or scanning tunneling microscopy[14–16]. Optical analysis is fundamentally limited by the light diffraction limit (up to hundreds of nanometers for visible frequencies), which has prevented nanoscale mapping of optical signals and correlation of optical or excited state properties with topographic features These limitations can be overcome by combining scanning probe techniques with spectroscopy.

Moore et al 2

RESULTS AND DISCUSSION

METHODS