CC BY
7LD-1-5 ALT'22
LASER DIAGNOSTICS AND SPECTROSCOPY
Atomically thin nanosheets of zinc and cadmium chalcogenides: colloidal growth, ligand exchange, and control of exciton properties
D.A. Kurtina, V.P. Grafova, A.V. Knot'ko, A.V. Garshev, R.B. Vasiliev,
Lomonosov Moscow State University, 119991, Moscow, Russia romvas@inorg.chem.msu.ru
Two-dimensional (2D) semiconductors have unique electronic properties due to their atomically thin thickness and two-dimensional electronic structure [1]. In this repot, we consider a new class of 2D semiconductors - atomically thin nanosheets of cadmium and zinc chalcogenides with a thickness of less than 1 nanometer (about 2-3 monolayers), synthesized in colloidal solutions.
A growth method was developed for the synthesis of atomically thin nanosheets of cadmium and zinc chalcogenides in a colloidal system of cadmium (zinc) acetate/octadecene/oleic acid/oleylamine in the temperature range of 110— 250°C [2]. The choice of conditions made it possible to grow nanostructures with given composition, crystal structure, and a precise (with an accuracy of 1 monolayer) thickness in the range of 0.6-1.2 nm. A growth technique has been developed to increase the lateral sizes of nanosheets up to 700 nm with an anisotropy factor up to 1500. A detailed study using HRTEM, HAADF-STEM, SAED, XRD methods showed a perfect crystal structure of nanosheets, allowing us to consider thees nanostructures as atomically thin crystals. It was found zinc blende structure for CdTe and CdSe and wurtzite structure for ZnSe. Using a set of methods, it was established that the composition of nanosheets follows the ratio [Cdn+1EnL2] (E - chalcogen, L - organic ligand, n - number of atomic planes) with integer coefficients.
Correlations between the size, composition, crystal structure of atomically thin nanosheets and their optical properties are established. The two-dimensional nature of the electronic structure and record-breaking narrow exciton bands with a width of about 40 meV at room temperature are shown [3]. The obtained materials with the spectral position of exciton bands specified with an accuracy of 1 nm are of interest for creating light-emitting devices and phosphors.
Approaches to ligand exchange were developed to obtain nanosheets with a variable composition of ligands [Cd-n+1EnL2], where E = Se or Te, for L = hexadecanethiol, thioglycolic acid, or acetylcysteine in the form of thiolates. The nature of the modification of the optical properties depending on the type of ligand, which consists in a bathochromic shift of all exciton bands, has been established, with the maximum shift up to 200 meV found for the thinnest popu-lations.Chiral nanostructures with a chiral ligand acetyl-L- or -D-cysteine, with pronounced exciton bands of circular dichroism with a record dissymmetry factor of 9-10-3 achieved for a thickness of 0.6 nm, have been synthesized.
A new effect of spontaneous folding of atomically thin nanostructures of cadmium chalcogenides has been discovered. A model is proposed for spontaneous folding due to compressive or tensile deformation at the semiconductor/ ligand interface, caused by a mismatch between the available space on the basal cation-terminated (001) plane and the size of the seat of the carboxylate and thiolate ligands. The effect of spontaneous folding on the exciton properties of nanosheets has been revealed
This work was supported by the Russian Science Foundation (grant № 22-13-00101).
[1] Guillemeney, L. et al. Curvature and self-assembly of semi-conducting nanoplatelets. Commun Chem 5, 7 (2022).
[2] 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).
[3] R.B. Vasiliev et.al., High-energy exciton transitions in quasi-two-dimensional cadmium chalcogenide nanoplatelets. Phys. Rev. B, 95, 165414, (2017).
[4] R.B. Vasiliev et.al., Spontaneous Folding of CdTe Nanosheets Induced by Ligand Exchange, Chem. Mater., 30, 1710-1717, (2018).
