Giant Optical Activity of Quantum Dots, Rods, and Disks with Screw Dislocations

Abstract

For centuries mankind has been modifying the optical properties of materials: first, by elaborating the geometry and composition of structures made of materials found in nature, later by structuring the existing materials at a scale smaller than the operating wavelength. Here we suggest an original approach to introduce optical activity in nanostructured materials, by theoretically demonstrating that conventional achiral semiconducting nanocrystals become optically active in the presence of screw dislocations, which can naturally develop during the nanocrystal growth. We show the new properties to emerge due to the dislocation-induced distortion of the crystal lattice and the associated alteration of the nanocrystal's electronic subsystem, which essentially modifies its interaction with external optical fields. The g-factors of intraband transitions in our nanocrystals are found comparable with dissymmetry factors of chiral plasmonic complexes, and exceeding the typical g-factors of chiral molecules by a factor of 1000. Optically active semiconducting nanocrystals-with chiral properties controllable by the nanocrystal dimensions, morphology, composition and blending ratio-will greatly benefit chemistry, biology and medicine by advancing enantiomeric recognition, sensing and resolution of chiral molecules.

Figure 1

Cylindrical nanocrystals made of cubic semiconductor (a) without defects, (b) with a screw dislocation and Eshelby twist, and (c) with a screw dislocation and without Eshelby twist. Dislocation axis coincides with the cylinder axis z and b = 2a.

Figure 2. Chiral isosurfaces of real (upper panels) and imaginary (lower panels) parts of wave function ψ₁₈₃ in cylindrical nanocrystals with right-handed (left panels) and left-handed (right panels) screw dislocations b = ±5a, L = 150a, and R = 30a.

Figure 3. CD strengths (left panels) and CD spectra (right panels) of intraband transitions in ZnS nanocrystals.

Pronounced CD is observed for (a) quantum dot, (b) quantum rod, and (c) quantum disk with right-handed screw dislocations b = a. All transitions occur from the ground state (101) of the nanocrystals and are represented by the spectral lines with FWHMs of 0.8 meV; negative and positive CD strengths are the solid and dashed lines, respectively; scaling factors are the same for all spectra. The volumes of the dot and disk are equal to the volume V₀ of the rod.

Figure 4. CD strengths of intraband transitions in ZnS quantum rod shown in Fig. 3.

Transitions of the first and second groups are represented by (a,c), transitions of the third group are shown in (b,d). Solid and dashed guides for eyes mark negative and positive CD strengths, respectively. The signs of the CD strengths highlighted in green are opposite to the signs determined by the respective guides for eyes.