Spin-Polarized ${\mathrm{Mn}}^{2+}$ Emission from Mn-Doped Colloidal Nanocrystals

We report magnetophotoluminescence studies of strongly quantum-confined 0D diluted magnetic semiconductors (DMS), realized in ${\mathrm{Mn}}^{2+}$-doped ZnSe/CdSe core-shell colloidal nanocrystals. In marked contrast to their 3D (bulk), 2D (quantum well), 1D (quantum wire), and 0D (self-assembled quantum dot) DMS counterparts, the ubiquitous yellow emission band from internal $d\mathrm{\text{−}}d$ (${}^{4}T_1\to {}^{6}A_1$) transitions of the ${\mathrm{Mn}}^{2+}$ ions in these nanocrystals is not suppressed in applied magnetic fields and does become circularly polarized. This polarization tracks the ${\mathrm{Mn}}^{2+}$ magnetization, and is accompanied by a sizable energy splitting between right- and left-circular emission components that scales with the exciton-Mn $sp\mathrm{\text{−}}d$ coupling strength (which, in turn, is tunable with nanocrystal size). These data highlight the influence of strong quantum confinement on both the excitation and the emission mechanisms of magnetic ions in DMS nanomaterials.

Figure 1

(a),(b) Magneto-PL from bulk ZnMnSe at 4 K, showing conventional DMS behavior. The yellow ${}^{4}T_1\to {}^{6}A_1$ ${\mathrm{Mn}}^{2+}$ PL at $\sim 2.1\mathrm{eV}$ is suppressed by magnetic fields and remains unpolarized, while 2.8 eV exciton PL (scaled down $15\times$) increases. Similar behavior is found in 3D, 2D, 1D, and 0D DMS grown by MBE. (c),(d) The contrasting magneto-PL from $\mathrm{Mn}\mathrm{\text{:}}\mathrm{ZnSe}/\mathrm{CdSe}$ nanocrystals ($⟨n_{\mathrm{Mn}}⟩=2.6$ Mn/core; $T=1.8\mathrm{K}$). The ${\mathrm{Mn}}^{2+}$ PL is not suppressed by magnetic fields, and does develop a sizable circular polarization (and, exciton PL never appears). The dashed line shows the NC absorption.

Figure 2

(a) The circular polarization (CP) of the ${\mathrm{Mn}}^{2+}$ PL from $\mathrm{Mn}\mathrm{\text{:}}\mathrm{ZnSe}/\mathrm{CdSe}$ nanocrystals versus field and temperature (lines; left axis). CP tracks the Brillouin-like ${\mathrm{Mn}}^{2+}$ magnetization (independently measured by MCD; points, right axis). (b) $\mathrm{CP}(B)$ for different $⟨n_{\mathrm{Mn}}⟩$ (all NCs have similar CdSe shells). (c) $\mathrm{CP}(B)$ for NCs with different CdSe shell thickness (all cores have $⟨n_{\mathrm{Mn}}⟩=1.6$).

Figure 3

(a) Normalized ${\sigma }^{\pm }$ ${\mathrm{Mn}}^{2+}$ PL from $\mathrm{Mn}\mathrm{\text{:}}\mathrm{ZnSe}/\mathrm{CdSe}$ NCs at 6 T, showing an energy splitting $\Delta E$. (b) $\Delta E$ (lines; left axis) scales with the ${\mathrm{Mn}}^{2+}$ magnetization in this sample (points, right axis, from MCD). (c) Using NCs with identical Mn:ZnSe cores but increasing CdSe shell thickness to $6\pm 2\AA$, $\Delta E$ tracks the $1S$ exciton Zeeman splitting (from MCD), indicating that $\Delta E$ scales with the net $sp\mathrm{\text{−}}d$ coupling strength. (d) A similar comparison, in the $⟨n_{\mathrm{Mn}}⟩=1.6$ Mn/core NCs.