Nanoscale Identification of MoS2 Layers and Defects

Layers in MoS2 can be identified by photoluminescence or Raman spectroscopy, both of which suffer from diffraction limit of far-field optics. Here we demonstrate that visible PiFM can detect number of layers as well as defect associated PiFM signals with exceptional sensitivity and spatial resolution. The sample below is exfoliated MoS2 film transferred onto a quartz substrate. The PiFM measurements were performed with excitation wavelength of 488 nm.

Even at this scale, the topography is quite messy due to exfoliation whereas PiFM image maintains its excellent SNR. The difference between the average signal of the flake is 0.15 mV higher than the average signal of the background, which indicates that most likely, the flake is one layer thicker than the surrounding region. The hot spots in PiFM image is assumed to be associated with the defects in the MoS2.