Identification of aggregated 2D cobalt tellurides using a spatial self-phase modulation technique

We report a previously unreported application of the spatial self-phase modulation (SSPM) technique for recognizing solute-solvent interaction in a suspension of 2D material. Broadband optical absorption of the 2D Co₂Te₃ leads to a nonlinear optical (NLO) susceptibility for the monolayer, i.e., χ M o n o(3) of 1.5 ×10^-9 (3.3 ×10^-9) esu at 532 (632) nm, which is 1-2 orders higher than for the 2D CoTe and CoTe₂. The fine structure of the SSPM patterns is analyzed to explore the foundations of the observed NLO effects. With increasing polarity of the liquid media, a change of 2D Co₂Te₃ from homophonous dispersion to aggregation occurs, as confirmed from in situ optical microscopy and UV-vis absorption spectroscopy. As a result, type-I (thickest outer ring) SSPM ring patterns are converted to type-II (thinnest outer ring) SSPM ring patterns. Therefore, using SSPM with a CW laser as an optical tool to identify solvent-polarity-induced aggregation in 2D materials is possible.