CC BY
1The 30th International Conference on Advanced Laser Technologies
ALT'23
LD-I-16
Exciton chirality of atomically thin AIIBVI nanoplatelets with X-type
stereoisomeric ligands
D.A. Kurtina, V.P. Grafova, A.I. Lebedev, R.B. Vasiliev
Lomonosov Moscow State University, 119991, Moscow, Russia
romvas@inorg. chem. msu. ru
Chirality refers to a geometrical property when an object cannot be superimposed onto its mirror image. In addition to its important role in biochemical and chemical processes, chirality leads to different interactions of molecules and structures with right- and left-handed circularly polarized light. Recently, chiral colloidal nanoparticles and nanostructures [1] have been of great interest, demonstrating different absorption of right-and left-handed circularly polarized light (circular dichroism, CD) or rotation of the light polarization plane (optical activity), as well as the emission of photons of a given circular polarization (circularly polarized luminescence). In this repot, we present an analysis of 2D atomically thin nanostructures of cadmium and zinc chalcogenides hybridized with stereoisomeric L-/D-ligands and showed a distinctive CD in excitonic transitions.
We have developed a method for the growth of atomically thin nanosheets (nanoplatelets) of cadmium and zinc chalcogenides with a thickness of less than 1 nanometer (about 2-3 monolayers) in colloidal solutions. 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] 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. For the hybridization of nanostructures with chiral ligands, a protocol was developed for the exchange of native oleic acid ligand for stereoisomers of L-/D-cysteine and N-acetyl- L-/D- cysteine. A detailed study using HRTEM, HAADF-STEM, SAED, XRD methods showed zinc blende structure for CdTe and CdSe and wurtzite structure for ZnSe with the composition followed the ratio [Mn+iEnL2] (M- zinc or cadmium, 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. Narrow (about 8-10 nm) exciton transitions involving the heavy hole HH, light hole LH, and spin-orbit hole SO were observed in the absorption spectrum of nanostructures at room temperatures together with exciton luminescence band in luminescence spectra. It was shown a bathochromic shift of all exciton bands up to 200 meV found for the thinnest nanostructures after ligand exchange supporting strong hybridization of ligand and semiconductor core.
The chiroptical properties of nanostructures hybridized with chiral ligands was studied by CD spectroscopy [3]. CD spectra showed distinct, sign-alternating bands correlated with the LH, HH, and SO transitions in the absorption spectra confirming the exciton nature of the induced CD bands. Change of L-stereoisomer of ligand to D-stereoisomer inverts the sign of the spectrum like mirror image with the same bands spectral position and intensity that supports the effect of the ligand on the induced preference in absorbance of right-or left-hand circularly polarized light. We found a maximum g-factor of dissymmetry of 0.01, which seems to be the highest value reported previously for AIIBVI semiconductor nanoparticles A model is proposed for induced chirality due to Wannier-Mott exciton in helicoidal potential of stereoisomeric ligands that was supported by analysis of effect of solvent polarity, optical rotation and modeling of ligand coordination. 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] Cho, N.H., Guerrero-Martínez, A., Ma, J. et al. Bioinspired chiral inorganic nanomaterials. Nat Rev Bioeng 1, 88-106 (2023).
[2] Kurtina D.A. 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] Kurtina, D.A.; Grafova, V.P.; 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).
