Probing nanoscale defects and wrinkles in MoS2 by tip-enhanced Raman spectroscopic imaging

Abstract

Nanoscale inhomogeneities, such as defects and wrinkles, in atomic layers of transition-metal dichalcogenide (TMDC) semiconductor materials deteriorate the remarkable physical and optoelectronic properties of these materials, precluding their use in optoelectronic devices. Such inhomogeneities can be investigated using vibrational spectroscopic analysis methods, such as Raman spectroscopy, because the deformations in a sample could be identified by the changes in the Raman vibrational energies of the sample. However, it has been challenging to characterize structures localized on the nanoscale in TMDC layers, because such characterization requires a nanoscale high spatial resolution. Here, we present tip-enhanced Raman spectroscopy (TERS) of molybdenum disulfide (MoS2) in the form of monolayers and a few layers to investigate nanoscale inhomogeneities. TERS enables Raman analysis with the nanoscale spatial resolution, using the near-field light generated at a metallic nano-tip through plasmon resonance. TERS imaging, which demonstrates spatially varying Raman spectral features on the nanoscale, allows one to elucidate the existence of nanoscale inhomogeneities. Using this method, defects and wrinkles within MoS2 layers were characterized with a spatial resolution better than 20\u2009nm. This study provides important insights into unique optical and electronic properties of TMDCs for the development of future optoelectronic devices.