We provide a long sought-after answer for an open question of the semiconductor nanocrystal community. Namely, we explain what makes seeded CdSe/CdS nanorods emit and absorb linearly polarized light beyond dielectric effects.

The optical anisotropy of CdSe/CdS dot-in-rods has a two-fold origin: (i) a well-known dielectric contribution, similar to that of semiconductor nanorods,[1] and (ii) an electronic one, whose presence was already sensed in early studies[2], but whose origin remains unclear.

Lack of understanding of the electronic contribution has so far limited efficient prediction and engineering of the degree of linear polarization in CdSe/CdS dot-in-rods, which are otherwise excellent nanoemitters with a strong potential for opto-electronic applications. Efforts in the literature are limited to systematic experiments varying geometric parameters of the dot-in-rod to phenomenologically infer the conditions optimizing linear polarization.[3-6]

In the present work, we show theoretically that the electronic origin of linear polarization is related to: (i) an exciton ground state crossover, from heavy-hole (HH) to light-hole (LH) in character, upon growth of the CdS shell around the CdSe core, which is mainly mediated by shear strain. (ii) the fact that, in large core dot-in-rods, LH excitons recombine radiatively faster than HH excitons. Having identified the electronic mechanisms enhancing linear polarization, we next determine the optimal geometrical parameters. Our prediction is that the highest degree of linear polarization is obtained when using large, prolate CdSe cores inside long and thin CdS shells. It turns out these results strongly support the empirical conclusions of Refs.[3-6], while providing solid interpretations.

Our conclusions are obtained by means of a state-of-the-art k·p calculation using 2-band and 6-band wurtzite Hamiltonians for electrons and holes in 3D heterostructures, respectively, and their mutual interaction via self-consistent Coulomb interaction. This allows us to survey how optical anisotropy is influenced by a large number of potentially relevant physical factors, including quantum confinement, spontaneous polarization, strain-induced deformation potential and piezoelectricity, position dependent effective mass, direct and exchange Coulomb interaction.[7]

References

[1] Nano Lett. 2004, 4, 1821.

[2] Nano Lett. 2003, 3, 1677.

[3] ACS Nano 2015, 9, 7992.

[4] Nano Lett. 2011, 11, 2054.

[5] J. Phys. Chem. Lett. 2013, 4, 502.

[6] J. Phys. Chem. Lett. 2014, 5, 85.

[7] J. Phys. Chem. C 2016, 120, 27724.