Exfoliation Behavior of van der Waals Strings: Case Study of Bi 2 S 3 .

The family of crystals constituting covalently bound strings, held together by van der Waals forces, can be exfoliated into smaller entities, similar to crystals made of van der Waals sheets. Depending on the anisotropy of such crystals, and the spacing between their strings in each direction, van der Waals sheets or ribbons can be obtained after the exfoliation process. In this work, we demonstrate that ultra-thin nanoribbons of bismuth sulfide (BiS) can be synthesized via a high power sonication process. The thickness and width of these ribbons are governed by the van der Waals spacings around the strings within the parent crystal. The lengths of the nanoribbons are limited by the dimensions of the starting bulk particles. Interestingly, these nanoribbons change stoichiometry, composition and are elongated when the duration of agitation increases due to Ostwald ripening. An application of the exfoliated van der Waals strings is presented for optical bio-sensing using photoluminescence of BiS nanoribbons, reaching detection limits of less than 10 nM L in response to bovine serum albumin. The concept of exfoliating van der Waals strings could be extend-ed to a large class of crystals for creating bodies ranging from sheets to strings, with optoelectronic properties different from that of their bulk counterparts.