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Atomically-thin colloidal CdTe and CdSe nanosheets: effect of spontaneous folding and its impact on optical properties
R. Vasiliev1,2, D. Kurtina2, L. Kozina2, A. Garshev1,2, V. Zaytsev3,I. Vasil'eva4, V. Shubin4, A. Gaskov2
1Lomonosov Moscow State University, Department of Materials Science, Moscow, Russian Federation
2Lomonosov Moscow State University, Department of Chemistry, Moscow, Russian Federation 3Lomonosov Moscow State University, Department of Physics, Moscow, Russian Federation 4Research Center of Biotechnology of the Russian Academy of Sciences, Bach Institute of Biochemistry, Moscow, Russian Federation
Two-dimensional semiconductors have unique electronic and optical properties that are different from those of bulk materials. In present work, we report an effect of spontaneous folding of 2D CdTe and CdSe nanosheets and analyze its impact on a structure and optical properties. CdSe and CdTe nanosheets with a thickness of about 1 nm and extended lateral sizes up to 400 nm were grown by the colloid method [1, 2]. The native stabilizer - oleic acid was exchanged for various thiol ligands, including L- and D-acetylcysteine molecules. 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 initially flat CdTe nanosheets are uniformly rolled up when oleic acid is replaced by thiols. Detailed study shows nanosheet folding along [110] direction forming multiwall scrolllike structures. In contrast, CdSe nanosheets were found initially rolled up into multiwall nanoscrols and retained scroll-like shape after ligand exchange. Folding-unfolding process, however, preserves 2D optical properties.
The optical properties of nanosheets were studied by absorption, photoluminescence and circular dichroism spectroscopy. In the absorption and photoluminescence spectra, well-resolved LH, HH, and SO exciton transitions and a pronounced exciton luminescence band were observed. The exchange of ligands led to a spectral shift of all bands by a value of about 50 nm in the red region. In the spectra of circular dichroism, five pronounced rotational bands were found in the spectral range of 300-500 nm with a half-width of about 20 nm and a high magnitude of rotation with calculated g-factor of 3*10-3. The folding mechanism and its impact on optical properties are discussed.
This work was supported by the Russian Foundation for Basic Research (Nos. 19-03-00481 and 16-29-11694).
References
[1] N. N.Shlenskaya et.al. // Chem. Mater. 2017, 29 (2), pp 579-586.
[2] R. B.Vasiliev et.al. // Chem. Mater. 2018, 30 (5), pp 1710-1717.
