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Benjamin Abecassis2 1 Santanu Jana1 Marta de Frutos1

2, Laboratoire de Chimie Ecole Normale supérieure de Lyon, Lyon, , France
1, Laboratoire de Physique des Solides, Université Paris Sud, CNRS, Orsay, , France

Colloidal CdSe nanoplatelets (NPLs) are considered to be excellent candidates for many applications in nanotechnology. One of the current challenges is to self-assemble these colloidal quantum wells into large ordered structures to control their collective optical properties. We describe a simple and robust procedure to achieve controlled face-to-face self-assembly of CdSe nanoplatelets into micron-long polymer-like threads made of up to ∼1000 particles. These structures are formed by addition of oleic acid to a stable colloidal dispersion of platelets, followed by slow drying and re-dispersion. We could control the average length of the CdSe nanoplatelet threads by varying the amount of added oleic acid. These 1-dimensional structures are flexible and feature a “living polymer” character because threads of a given length can be further grown through the addition of supplementary nanoplatelets at their reactive ends. [1] We also show that these ribbons of stacked board-shaped NPL twist upon the addition of oleic acid ligand, leading to chiral ribbons that reach several micrometers in length and display a well-defined pitch of ~400 nm. We demonstrate that the chirality originates from surface strain caused by the ligand because isolated NPLs in dilute solution undergo a transition from a flat to a twisted shape as the ligand coverage increases. When the platelets are closely stacked within ribbons, the individual twist propagates over the whole ribbon length. These results show that a ligand-induced mechanical stress can strongly distort thin NPLs and that this stress can be expressed at a larger scale, paving the way to stress engineering in assemblies of nanocrystals. Such a structural change resulting from a simple external stimulus could have broad implications for the design of sensors and other responsive materials. [2]

[1] S. Jana, P. Davidson, B. Abécassis, Angew. Chem. Int. Ed. , 55, 9371, (2016)
[2] S. Jana, M. de Frutos, P. Davidson, B. Abécassis, Ligand-induced twisting of nanoplatelets and their self-assembly into chiral ribbons. Sci. Adv. 3, e1701483 (2017).

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