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0Chiral atomically thin AIIBVI nanostructures: colloidal growth and chiroptical properties of 2D excitons
D.A. Kurtina, A.I. Lebedev, R.B. Vasiliev*
Lomonosov Moscow State University, Leninskie Gory119991, Moscow, Russia
* romvas@inorg.chem.msu.ru
Chirality refers to a geometrical property of mirror asymmetry when an object cannot be superimposed onto its mirror image. Chiral semiconductor nanostructures have attracted tremendous interest due to a variety of intriguing properties [1]. Due to the chiral shapes of the nanostructures and/or the chirality of the molecular orbitals forming excited states in semiconductor nanostructures, excitons in such nanostructures acquire mirror asymmetry, which leads to different interaction with left- and right-handed circularly polarized photons. Such nanostructures with chiral excitation are promising candidates for biosensing, stereoselective reactions, and enantioselective separation. In this repot, we present a study of chiroptical properties of 2D atomically thin organic-inorganic nanostructures of cadmium and zinc chalcogenides hybridized with enantiomeric L-/D-cysteine derivative ligands and showed a distinctive circular dichroism (CD) of 2D excitons.
We used a colloidal method for the growth of atomically thin nanostructures (nanosheets) of cadmium and zinc chalcogenides with a thickness of less than 1 nanometer (only 2 or 3 monolayers in thickness) and lateral sizes up to 1 micron (anisotropy factor up to 1000). A system of cadmium (zinc) acetate/octadecene/oleic acid/oleylamine was used for the nanostructure growth in the temperature range of 110-250°C [2-4] and we achieved the formation of nanostructures with a precise (with an accuracy of 1 monolayer) thickness in the range of 0.6-0.9 nm. Ligand exchange protocols were developed for the exchange of native oleic acid ligand for enantiomers of L-/D-cysteine derivative: L-/D-cysteine, N-acetyl-L-/D-cysteine and N-oleoyl-L-/D-cysteine. A detailed study using TEM, HAADF-STEM, SAED and XRD methods showed the composition of nanostructures followed the ratio [Mn+1EnL2] (M - zinc or cadmium, E - chalcogen, L - organic ligand, n - number of atomic planes) with n = 2 and 3.
The optical and chiroptical properties of nanostructures were studied by optical absorption and luminescence spectroscopy, CD spectroscopy, magnetic CD spectroscopy, and polarimetry. CD spectra of nanostructures showed distinct, sign-alternating bands related to 2D excitons with high g-factor of dissymmetry up to 0.03. The combination of chiroptical measurements in dependence on type of ligand and DFT calculations of ligand coordination showed that high dissymmetry factor arises from helicoidal distortion of crystal structure and high rotational strength of 2D excitons. To prove the pseudomagnetic nature of the splitting of circular dichroism bands, measurements of the magnetic circular dichroism of samples of nanostructures were carried out when a magnetic field was applied. Based on the dependence of the value of the circular dichroism of the HH transition on the field strength, the value of the internal pseudomagnetic field was found to be on the order of 2 T. Specific optical rotation for the thinnest nanostructures was [a] = + 805 ± 76 grad, which is more than 40 times higher than the specific rotation of the individual ligand. We believe that our work opens up new possibilities for creating artificial chiral nanostructures and will be useful for polarization-enabled applications in photonics.
This work was supported by the Russian Science Foundation (grant № 22-13-00101).
[1] N.H. Cho, A. Guerrero-Martínez, J. Ma, et al, Bioinspired chiral inorganic nanomaterials. Nat Rev Bioeng 1, 88-106 (2023).
[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] D.A. Kurtina, et al, Induction of Chirality in Atomically Thin ZnSe and CdSe Nanoplatelets: Strengthening of Circular Dichroism via Different Coordination of Cysteine-Based Ligands on an Ultimate Thin Semiconductor Core, Materials, 16, 1073 (2023).
[4] D.A. Kurtina, et al, Chirality in Atomically Thin CdSe Nanoplatelets Capped with Thiol-Free Amino Acid Ligands: Circular Dichroism vs. Carboxylate Group Coordination, Materials 17, 237 (2024),
