Experimental study of the strain patterned in 2D materials using thermal nanoimprint lithography

Since straining semiconductors can enhance the carriers’ mobility and thus improve optoelectronic properties, this project aims at determining whether t-NIL can be used in order to strain thin flakes of MoS2 out of the plane. In this approach, the thin flakes are placed between the stamp of the nanoimprinter and the soft polymeric resist into which the imprinting is performed. Due to the fineness of the 2D material, the imprinting is not hindered, and, thanks to Van der Waals forces, the thin layer would remain against the imprinted resist upon stamp withdrawal, effectively building strain by the out-of-plane stretching. The strain built in the thin flakes is estimated through the redshift of the Raman spectra of the two peaks corresponding to two of the vibrational modes of the lattice of the MoS2 crystal. The characteristics of the imprints being highly dependent on the stamps’ features’ profiles, preliminary investigations useful for the future design of efficient stamps have been directed. Using a stamp with features exhibiting trapeziform cross-sections with leg slopes up to 8%, corresponding in theory to 0.32% strain, redshifts obtained for the E12g and A1g peaks reached 0.27 cm⁻¹ and 0.24 cm⁻¹ respectively. Another imprint using a stamp with leg slopes up to 170 % broke the 2D flakes, the theoretical strain exceeding the yield strength by far. In order to increase the strain achieved in the thin flakes, the effect of the stamp profiles ought now to be studied further. A hypothesis would be that the slope of the sides of the profile of the stamps’ features could be related to the strain built into the 2D material.