CC BY
17LD-I-12
Tuning of the optical properties of CdSe atomically thin nanosheets by spontaneous folding: effect of the length and type of
ligands.
D.A. Kurtina1, A.V. Knotko12, A.V. Garshev12, R.B. Vasiliev1'2
1- Department of Chemistry, Lomonosov Moscow State University, 119991, Moscow, Russia 2- Department of Material Science, Lomonosov Moscow State University, 119991, Moscow, Russia
romvas@inorg. chem. msu. ru
Two-dimensional semiconductors have unique electronic and optical properties that are different from those of bulk materials. In this regard, atomically thin nanosheets of cadmium chalcogenides have recently attracted great interest due to the two-dimensional electronic properties and record-narrow bands of exciton transitions [1, 2]. In present work, we report an effect of spontaneous folding of atomically thin CdSe nanosheets and analyze its impact on a structure and optical properties. We studied the effect of carboxylates and thiolates ligands, including analyzed the effect of the length of the hydrocarbon chain of ligand.
Atomically thin CdSe nanosheets with precise thickness of 2.5 and 3.5 monolayers (0.6 and 0.9 nm) were grown by the colloid method [3]. Seed-mediated growth allows extending lateral sizes of nanosheets up to 700 nm. The native stabilizer - oleic acid was exchanged for various ligands, including different thiols and carboxylic acids with different chain lengh. Analysis of the crystal structure, morphology, and composition was carried out using HRTEM, HAADF-STEM, SAED, XRD and FTIR methods in dependence on ligand type.
We show that CdSe nanosheets are uniformly rolled up in multiwall scroll-like nanostructures folding along [110] direction due to spontaneous folding effect [4]. The systematic exchange of ligands for carboxylic acids with different chain lenghs made it possible to change the inter-wall distance in rolled nanostructures, leading to compression or stretching of the nanostructures. Using small-angle diffraction, the dependence of the inter-wall distance on the ligand chain length was established, indicating the formation of a close-packed monolayer of ligands in the inter-wall space.
The optical properties of nanosheets were studied by absorption and photoluminescence spectroscopy. Well-resolved LH, HH, and SO exciton transitions was found and a pronounced exciton luminescence band were observed. The exchange of ligands to thiolates led to a spectral shift of all bands by a value of about 50 nm in the red region. A bathochromic shift in the spectral position of exciton luminescence bands with an increase in the chain length of carboxylic acids on the surface of rolled nanostructures was found. This effect makes it possible to fine tune the energy of the exciton bands. A model of spontaneous folding due to compression or tension deformations at the semiconductor/ligand interface caused by the mismatch of limiting areas the basal cation-rich plane (001) and the size of footprint of carboxylate and thiolate ligands is proposed.
This work was supported by the Russian Foundation for Basic Research (grant № 19-03-00481).
[1] A. Di Giacomo et.al., Colloidal Synthesis of Laterally Confined Blue-Emitting 3.5 Monolayer CdSe Nanoplatelets, Chem. Mater., 32, 9260-9267, (2020).
[2] R.B. Vasiliev et.al., High-energy exciton transitions in quasi-two-dimensional cadmium chalcogenide nanoplatelets. Phys. Rev. B, 95, 165414, (2017).
[3] D.A. Kurtina et.al., Atomically Thin Population of Colloidal CdSe Nanoplatelets: Growth of Rolled-up Nanosheets and Strong Circular Dichroism Induced by Ligand Exchange, Chem. Mater., 31, 9652-9663, (2019).
[4] R.B. Vasiliev et.al., Spontaneous Folding of CdTe Nanosheets Induced by Ligand Exchange, Chem. Mater., 30, 1710-1717, (2018).
