Exfoliation Behavior of van der Waals Strings: Case Study of Bi₂S₃

Abstract

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, as well as 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 ultrathin nanoribbons of bismuth sulfide (Bi2S3) 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 initially limited by the dimensions of the starting bulk particles. Interestingly, these nanoribbons change stoichiometry and composition and are elongated when the duration of agitation increases because of Ostwald ripening. An application of the exfoliated van der Waals strings is presented for optical biosensing using photoluminescence of Bi2S3 nanoribbons, reaching detection limits of less than 10 nM L–1 in response to bovine serum albumin. The concept of exfoliating van der Waals strings could be extended to a large class of crystals for creating bodies ranging from sheets to strings, with optoelectronic properties different from that of their bulk counterparts.

Two‐Step Synthesis of Large‐Area 2D Bi2S3 Nanosheets Featuring High In‐Plane Anisotropy

“…Bismuth sulfide (Bi 2 S 3 ) is an archetypal anisotropic layered material from the stibnite family. [7] Bi 2 S 3 exhibits interesting physical properties such as strong spin-orbit coupling which in turn influences its optoelectronic properties. [8] It also has excellent thermoelectric properties due to the small effective carrier masses and low thermal conductivity.…”

“…These strings consist of atoms that are bound covalently in one direction. [7,11] van der Waals forces arise between individual strings, and as such the Bi 2 S 3 crystal exhibits weak vdW bonds along the other two dimensions. [7] This crystal 2D materials with high in-plane anisotropy are rapidly emerging as a tantalizing class of nanomaterials with promising applications in nanoelectronics and optoelectronics since they provide an additional degree of freedom that can be exploited in device design.…”

“…[7,11] van der Waals forces arise between individual strings, and as such the Bi 2 S 3 crystal exhibits weak vdW bonds along the other two dimensions. [7] This crystal 2D materials with high in-plane anisotropy are rapidly emerging as a tantalizing class of nanomaterials with promising applications in nanoelectronics and optoelectronics since they provide an additional degree of freedom that can be exploited in device design. The large-area synthesis of such materials remains however challenging since the anisotropic crystal structure renders identifying a suitable growth substrate to be difficult, while the nanosheets are usually too fragile for the exfoliation and transfer of macroscopic sheets.…”

“…The crystal easily fractures along the vdW gaps, and thus nanosheet synthesis via bulk exfoliation often results in undesired fragmentation due to the brittle nature of the material. [7] For instance, liquid exfoliation of Bi 2 S 3 bulk powders leads to the formation of 1D nanostructures due to exfoliation occurring along both van der Waals gaps within the crystal. [7] Bottom-up synthesis approaches such as chemical vapor deposition (CVD) and molecular beam epitaxy (MBE) are equally challenging since they require carefully matched substrates in order to induce 2D growth while suppressing growth in the third dimension.…”

“…[7] For instance, liquid exfoliation of Bi 2 S 3 bulk powders leads to the formation of 1D nanostructures due to exfoliation occurring along both van der Waals gaps within the crystal. [7] Bottom-up synthesis approaches such as chemical vapor deposition (CVD) and molecular beam epitaxy (MBE) are equally challenging since they require carefully matched substrates in order to induce 2D growth while suppressing growth in the third dimension. [12] The complex crystal structures of high anisotropy 2D materials render substrate selection to be difficult.…”