Revealing the exciton fine structure of PbSe nanocrystal quantum dots using optical spectroscopy in high magnetic fields.
We measure the photoluminescence lifetime τ of excitons in colloidal PbSe nanocrystals (NCs) at low temperatures to 270 mK and in high magnetic fields to 15 T. For all NCs, τ 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  μeV depending on NC size. Importantly, magnetic fields reduce τ 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.
Schaller
R D
RD
Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA.
Crooker
S A
SA
Bussian
D A
DA
Pietryga
J M
JM
Joo
J
J
Klimov
V I
VI