Universal Role of Discrete Acoustic Phonons in the Low-Temperature Optical Emission of Colloidal Quantum Dots

Abstract

Multiple energy scales contribute to the radiative properties of colloidal quantum dots, including magnetic interactions, crystal field splitting, Pauli exclusion, and phonons. Identification of the exact physical mechanism which couples first to the dark ground state of colloidal quantum dots, inducing a significant reduction in the radiative lifetime at low temperatures, has thus been under significant debate. Here we present measurements of this phenomenon on a variety of materials as well as on colloidal heterostructures. These show unambiguously that the dominant mechanism is coupling of the ground state to a confined acoustic phonon, and that this mechanism is universal.

Exciton lifetimes of CdTe nanocrystal quantum dots in high magnetic fields

“…Below 4 K (the temperature used for the B-dependent measurements), the lifetime is constant and equal to the lifetime of the dark exciton ground state. This overall size-and temperature-dependent behavior is consistent with earlier reports 19,22,29 for CdTe NQDs at B = 0, and is remarkably similar to the behavior of CdSe NQDs. 11,18,[20][21][22] …”

Enhanced Exciton–Phonon Coupling in Colloidal Type-II CdTe-CdSe Heteronanocrystals

“…26,30,65,68,69,71 As discussed in section 3.2. above, the temperature-and magnetic field-dependence of the exciton lifetimes of CdSe QDs can be well understood by considering the 1S (e) 1S 3/2(h) exciton fine-structure. 26,27,30,65,68,69,71 As the temperature decreases, the population of the lower lying E ±2 state ("dark exciton") increases at the expenses of the higher lying E ±1 L state ("bright exciton"), leading to progressively longer exciton lifetimes, which eventually become equal to the E ±2 radiative lifetime. Similarly, magnetic fields lead to mixing of the E ±2 and E ±1 L states, and therefore the exciton radiative lifetime decreases with increasing magnetic fields.…”

“…The intra-and inter-band relaxation in CdSe QDs has been investigated in detail and is wellunderstood. 12,13,15,16,19,30,69,81 Relaxation from the higher exciton states to the lowest exciton state (1S (e) 1S 3/2(h) ) is very fast (viz., 150−500 fs). 12,13,15,16,81 State specific pump− probe transient absorption measurements have determined that the 1P e → 1S e electron relaxation is size dependent (155 fs for 4 nm diameter and 250 fs for 6.4 nm diameter CdSe QDs), while the hole relaxation is size independent (250 fs for the 2S 3/2(h) → 1S 3/2(h) ).…”

“…65 At low temperatures (below 10 K) the population of the "bright exciton" (E ±1 L state) is negligible, and emission will thus take place primarily from the "dark exciton" (E ±2 state), leading to long exciton lifetimes (viz., ∼0.2 to 1.4 μs, depending on the QD size). 30,68,69 Within the EMA framework the E ±2 lifetime should be size independent and infinite. 65 However, semiempirical tight-binding calculations have successfully reconciled experiment and theory by showing that triplet ("dark exciton") radiative lifetimes in the microsecond range can be obtained for CdSe QDs by incorporating interactions with higher exciton states and explicitly taking into account spin−orbit interactions, which lead to mixing of singlet character into nominally triplet states (i.e., the E ±2 state or "dark exciton").…”

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].…”