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

Abstract

Chiral nanostructures exhibiting different absorption of right- and left-handed circularly polarized light are of rapidly growing interest due to their potential applications in various fields. Here, we have studied the induction of chirality in atomically thin (0.6–1.2 nm thick) ZnSe and CdSe nanoplatelets grown by a colloidal method and coated with L-cysteine and N-acetyl-L-cysteine ligands. We conducted an analysis of the optical and chiroptical properties of atomically thin ZnSe and CdSe nanoplatelets, which was supplemented by a detailed analysis of the composition and coordination of ligands. Different signs of circular dichroism were shown for L-cysteine and N-acetyl-L-cysteine ligands, confirmed by different coordination of these ligands on the basal planes of nanoplatelets. A maximum value of the dissymmetry factor of (2–3) × 10−3 was found for N-acetyl-L-cysteine ligand in the case of the thinnest nanoplatelets.

Chirality in Atomically Thin CdSe Nanoplatelets Capped with Thiol-Free Amino Acid Ligands: Circular Dichroism vs. Carboxylate Group Coordination

“…Chiral ligands on the surface of the NPL are able to transfer their mirror asymmetry to the excitations in the semiconductor core NPL, thus inducing the chirality of the entire system. This phenomenon has been called chirality transfer and has been actively discussed recently [ 2 , 3 , 26 , 32 ]. The chiroptical properties of NPLs acquired in this way and the efficiency of chirality transfer in the case of chiral amino acid ligands were investigated using CD spectroscopy and demonstrated in Figure 6 c,d.…”

“…The position of the maxima corresponds with the transition bands in the absorption spectra and indicates the excitonic nature of CD transitions. Unlike the sulfur-containing ligands described earlier [26,32], CD bands corresponding to heavy and light holes (around 390 and 370 nm correspondingly) have the same sign, which is similar to the case described for zinc blende with L-cysteine ligand on the surface [24,25]. However, a CD band corresponding to a spin-orbit hole with a positive sign at around 350 nm is visible only for the CdSe394Ala sample, possibly due to better passivation for the L-alanine ligand.…”

“…Thus, to induce chiral excitons, the coordination of ligands on the nanoparticle surface plays an important role [ 30 ]. It was shown that L/D cysteine and N-acetylcysteine ligands induce different signs of circular dichroism at the same absolute configuration for both 0D CdSe quantum dots and 2D CdSe nanoplatelets [ 31 , 32 ]. The change in the sign of the CD is due to a change in ligand coordination when replacing cysteine with acetylcysteine, leading to a change in the orientation of the molecular dipole and the exciton dipole in the core.…”

“…Pointed lines show correspondence between CD and absorbance bands.Chiral ligands on the surface of the NPL are able to transfer their mirror asymmetry to the excitations in the semiconductor core NPL, thus inducing the chirality of the entire system. This phenomenon has been called chirality transfer and has been actively discussed recently[2,3,26,32]. The chiroptical properties of NPLs acquired in this way and…”

Chirality in Atomically Thin CdSe Nanoplatelets Capped with Thiol-Free Amino Acid Ligands: Circular Dichroism vs. Carboxylate Group Coordination

“…[26]. It was shown that L/D cysteine and N-acetylcysteine ligands induce different signs of circular dichroism at the same absolute configuration for both 0D CdSe quantum dots and 2D CdSe nanoplatelets [27,28]. The change in the sign of the CD is due to a change in ligand coordination when replacing cysteine with acetylcysteine, leading to a change in the orientation of the molecular dipole and the exciton dipole in the core.…”