  In the present work, some MoS2 and WS2 nanosheets were prepared and
characterized. Depending on the preparation procedures, trigonal prismatic (2H)
or octahedral (1T) coordination of the metal atoms were obtained, exhibiting
metallic (1T) or semiconducting (2H) character. Both MoS2 and WS2 nanosheets
were found exhibiting large nonlinear optical (NLO) response, strongly
dependent on their metallic (1T) or semiconducting (2H) character. So, the
semiconducting character 2H-MoS2 and 2H-WS2 exhibit positive nonlinear
absorption and strong self-focusing, while their metallic character
counterparts exhibit strong negative nonlinear absorption and important
self-defocusing. In addition, the semiconducting MoS2 and WS2 were found
exhibiting important and very broadband optical limiting action extended from
450 to 1750 nm. So, by selecting the crystalline phase of the nanosheets, i.e.,
their semiconduction/metallic character, their NLO response can be greatly
modulated. The results of the present work demonstrate unambiguously that the
control of the crystalline phase of MoS2 and WS2 provides an efficient strategy
for 2D nanostructures with custom made NLO properties for specific
optoelectronic and photonic applications.
