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

Abstract

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.

Temperature and Magnetic-Field Dependence of Radiative Decay in Colloidal Germanium Quantum Dots

“…A similar T-dependent PL behavior was observed previously in QDs of CdSe, 35,36 CdTe, 38 InAs, 38 Si, 39 and PbSe 40,41 and attributed to the presence of an exciton fine structure that comprises closely spaced states with different oscillator strengths. These effects are especially prominent in CdSe QDs, 35 in which low-temperature PL decay is dominated by slow recombination of a dipole-forbidden "dark" exciton (the projection of the total angular momentum along the hexagonal c-axis |J| = 2), while an increase in the temperature leads to thermal activation of a dipole-allowed "bright" exciton (|J| = 1), resulting in a dramatic, orders-of-magnitude increase in the decay rate.…”

Exciton lifetimes of CdTe nanocrystal quantum dots in high magnetic fields

“…11,15,[23][24][25] Similar high-field data are scarce for NQDs with a crystal lattice different from wurtzite. 26 In this paper, we report on the radiative lifetimes of excitons in zinc-blende CdTe NQDs as a function of strong magnetic fields (B), as determined from the PL decay times. We have found that the exciton lifetime strongly decreases with increasing magnetic field, as well as with increasing temperature and size, which is similar to the behavior of wurtzite CdSe NQDs.…”

Excited-State Dynamics in Colloidal Semiconductor Nanocrystals

“…These effects are beyond the scope of this review. The interested reader is referred to a number of publications addressing this topic in detail [36][37][38][39][40][41][42][43][44][45][46][47][48]. Phonons (i.e., lattice vibrations) have a pervasive role in semiconductors, and therefore coupling of charge carriers and excitons to phonons plays a decisive role in a wide range of properties [49].…”