Revealing the Exciton Fine Structure of PbSe Nanocrystal Quantum Dots Using Optical Spectroscopy in High Magnetic Fields

We measure the photoluminescence lifetime $\tau$ of excitons in colloidal PbSe nanocrystals (NCs) at low temperatures to 270 mK and in high magnetic fields to 15 T. For all NCs, $\tau$ increases sharply below 10 K but saturates by 500 mK. In contrast to the usual picture of well-separated “bright” and “dark” exciton states (found, e.g., in CdSe NCs), these dynamics fit remarkably well to a system having two exciton states with comparable—but small—oscillator strengths that are separated by only $300–900\mu \mathrm{eV}$ depending on NC size. Importantly, magnetic fields reduce $\tau$ below 10 K, consistent with field-induced mixing between the two states. Magnetic-circular dichroism studies reveal exciton $g$ factors from 2–5, and magnetophotoluminescence shows $>10\%$ circularly polarized emission.

Figure 1

(a) PL spectra from PbSe NC ensembles of different particle size. (b) PL decays. The lifetime $\tau$ increases as temperature $T$ decreases. (c) Left axis: $\tau$ vs $T$ from 270 mK to 300 K. Line is a fit of the low-$T$ data to a two-level model (see inset). Right axis: The total PL emission.

Figure 2

(a)–(c) $\tau$ vs $T$ for three PbSe NC sizes at $B=0\mathrm{T}$ (black points). Lines are fits to a two-level model (see text). Open symbols show $\tau (T)$ at $B=15\mathrm{T}$. High fields reduce $\tau$ to its value near 10 K. (d) The energy splitting $\Delta$ between these two lowest exciton levels versus NC radius.

Figure 3

(a) Optical absorption (right axis) and corresponding MCD (left axis) from $r=2.1\mathrm{nm}$ PbSe NCs. (b) Zeeman splitting of the $1S$ absorption feature, for different NCs. (c) The exciton $g$ factor $|g_{\mathrm{ex}}|$ vs $1S$ energy.

Figure 4

$\tau$ vs $B$ at 1.5 K, for both (a) large and (b) small PbSe NCs. Lines are fits to the model of 22.

Figure 5

(a) Circularly polarized infrared PL emission from PbSe NCs at $B=7\mathrm{T}$ and $T=2.5\mathrm{K}$. (b) PL polarization versus $B$. (c) PL polarization versus $T$ at $B=7\mathrm{T}$.