We study the optical properties of excitons in one-dimensional (1D) nanostructures at low temperatures. In single $\mathrm{C}\mathrm{d}\mathrm{S}\mathrm{e}/\mathrm{Z}\mathrm{n}\mathrm{S}$ core-shell nanorods we observe a fine structure splitting and explain it by exchange interaction. Two peaks are observed with different degrees of linear polarization of $\mathrm{D}\mathrm{L}\mathrm{P}<0.85$ and $\mathrm{D}\mathrm{L}\mathrm{P}>0.95$. For small nanorod radii $R\le a_{\mathrm{B}}/2$, an increase in the photoluminescence decay time is found when the temperature increases from 10 to 80 K. The observations are explained by a radius-dependent change in the symmetry of the 1D-exciton ground state which transforms from a dark state into bright states below a critical radius of $R_{\mathrm{c}\mathrm{r}\mathrm{i}\mathrm{t}}\approx 3.7\mathrm{n}\mathrm{m}$.

• Received 13 July 2004

DOI:https://doi.org/10.1103/PhysRevLett.94.016803