Revealing the Exciton Fine Structure in PbSe Nanocrystal Quantum Dots

TL;DR

This study investigates the exciton fine structure in PbSe nanocrystals by measuring photoluminescence lifetimes and magnetic effects, revealing closely spaced exciton states with magnetic field-induced mixing and circular polarization.

Contribution

It provides the first detailed measurement of exciton fine structure in PbSe nanocrystals, showing two nearly degenerate states with small oscillator strengths and magnetic field effects.

Findings

Exciton lifetimes increase sharply below 10 K and saturate around 500 mK.

Two exciton states are separated by 300-900 μeV with comparable oscillator strengths.

Magnetic fields induce mixing between exciton states, reducing lifetimes and enabling circularly polarized emission.

Abstract

We measure the photoluminescence (PL) 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 (1.3-2.3~nm radii), $\tau$ 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 $\mu$eV. 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 magneto-PL shows $>$10\% circularly polarized emission.