Anisotropic optical emission of single CdSe/CdS tetrapod heterostructures: Evidence for a wavefunction symmetry breaking

Semiconductor tetrapods are expected to exhibit isotropic optical properties due to their tetrahedral symmetry. We have investigated the optical polarization properties of individual $\mathrm{Cd}\mathrm{Se}∕\mathrm{Cd}\mathrm{S}$ tetrapods of high structural quality. In contrast to the light absorption behavior, a pronounced anisotropy is observed in the optical emission. This unexpected result can only be explained by assuming a symmetry breaking in the electronic wavefunction. Calculations reveal that slight differences between the arm diameters induce the polarized emission and a decrease in the photoluminescence intensity.

Figure 1

(Color online) (a) TEM image of a single $\mathrm{Cd}\mathrm{Se}∕\mathrm{Cd}\mathrm{S}$ tetrapod. The average value for the diameter of the core is estimated to be in the range of $4–6\mathrm{nm}$. The arm length can vary from $14\text{to}20\mathrm{nm}$. The inset shows a lower magnification image of a tetrapod oriented with one arm perpendicular to the surface. (b) Room temperature absorption (solid black line) and PL (dashed blue line) spectra of a dilute solution of tetrapods in toluene. The inset shows the absorption band $\left(630\mathrm{nm}\right)$ ascribed to the CdSe core. Orange (light gray) columns represent a histogram of the emission energy peaks obtained from the room temperature PL spectra of 170 single tetrapods.

Figure 2

(Color online) Polarization anisotropy histograms in (a) excitation, (b) emission at low temperature $(T=5\mathrm{K})$, and (c) emission at room temperature $(T=300\mathrm{K})$. The insets in (a) and (b) show the examples of polarization anisotropy traces for two different tetrapods in excitation and emission, respectively. The red (dark gray) lines represent fitting of the experimental data (black dots) with a ${\cos }^2\theta$ function. The extracted anisotropy values are 0.15 and 0.75 for excitation and emission, respectively.

Figure 3

(Color online) Color contour plots for the normalized electron and hole wavefunction distribution probability for [(a) and (b)] symmetric and (c) and (d) asymmetric tetrapods, respectively. The asymmetry is induced by increasing the diameter of one arm by $1\mathrm{nm}$. The wavefunction distribution is plotted on a plane parallel to two arms and containing the center of the CdSe core.

Figure 4

(Color online) (a) Plot of the calculated squared wavefunction overlap integral ${\mid ⟨{\varphi }_e\mid {\varphi }_h⟩\mid }^2$ against the increase in the diameter of one arm. The inset shows a cross section of the electron and hole wavefunctions in the conduction and valence bands along the modified arm for increases of 0 (red dashed curve) and $1\mathrm{nm}$ (black curve) in the diameter. (b) Measured scatter plot of PL intensity as a function of polarization anisotropy for 90 single tetrapods (circles); the squares are obtained after averaging single tetrapods with a binning of 0.1 in polarization anisotropy.