Publisher’s Note: Observation of Size-Dependent Thermalization in CdSe Nanocrystals Using Time-Resolved Photoluminescence Spectroscopy [Phys. Rev. Lett.107, 177403 (2011)]

Localization and Dynamics of Long‐Lived Excitations in Colloidal Semiconductor Nanocrystals with Dual Quantum Confinement

“…On longer timescales that exceed the lifetime of primary quantum‐confined direct excitons in semiconductor nanocrystals, the origin of the band‐edge emission remains to be unambiguously established and has been attributed to the detrapping of carriers from dark electronic states into the bright excitonic state of the nanocrystal,32–36 as well as direct radiative relaxation from shallow trap or surface states 37. 38 Further ambiguity is found in the interpretation of the evolution of the emission spectrum on these timescales, where the observed spectral shifts could arise from a number of effects, including energy‐dependent relaxation rates due to a phonon bottleneck,39, 40 inter‐particle energy transfer processes,41 the quantum‐confined Stark effect arising from a build‐up of surface charges in the nanocrystals42–45 or subtle photo‐induced changes in the chemical state of the surfaces of the nanocrystals 34. Although the precise origin of this delayed emission is unclear, these spectral dynamics likely reflect a complex interplay of the nanocrystals with their surrounding environment as well as their own surface and defect states.…”

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

“…16 In such a case the hole is re-excited via Auger energy transfer from excited electrons, and therefore the relaxation takes longer than in the absence of hot electrons (viz., 350 fs). 15 The relaxation from the E ±1 L sublevel to the E ±2 sublevel requires a spin-flip and is therefore relatively slow (0.2−0.4 ps at 300 K, 16 and tens of picoseconds below 10 K) 19 As mentioned above, relaxation of the E ±2 sublevel at low temperatures (<10 K) occurs only radiatively and takes ∼0.2 to 1.4 μs, depending on the QD size.…”

“…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) ).…”

“…Coupling of the electron and/or hole to phonons provides an important energy relaxation pathway, being thus essential to a number of photophysical processes in semiconductor nanostructures (viz., exciton and multiexciton relaxation dynamics, hot carrier cooling, carrier multiplication, interfacial charge separation, etc.). 2,3,10,12,13,15−18 el-ph coupling is also crucial for thermal transport processes 19 and underpins the temperature dependence of both electron transport and nonradiative energy transfer processes in colloidal quantum dot (QD) superlattices. 20−22 Moreover, coupling to acoustic phonon modes determines the homogeneous linewidths of optical transitions and contributes to the resonant Stokes shift, 23 whereas coupling to longitudinal optical (LO) phonon modes has been observed to relax selection rules, yielding momentum-conserving optical transitions (i.e., phonon-assisted transitions or phonon replicas) from the so-called "dark exciton states" in semiconductor QDs and quantum rods.…”

Excited-State Dynamics in Colloidal Semiconductor Nanocrystals

“…The interaction between phonons and excitons in nanoscale semiconductors is expected to differ from that in bulk materials due to both quantum confinement effects on the exciton energy levels and dimensional confinement of phonon modes (i.e., the phonon wavelength cannot be larger than the NC size) [49]. Coupling of photogenerated carriers to phonons provides an important energy relaxation pathway, thus being essential to a number of photophysical processes in semiconductor NCs (e.g., exciton relaxation dynamics, carrier cooling, thermal transport) [42,[50][51][52][53]. Moreover, coupling to acoustic phonon modes determines the homogeneous linewidths of optical transitions [54,55], while coupling to optical phonon modes has been observed to relax selection rules at low temperatures, yielding distinct phonon-assisted transitions (the so-called phonon replicas) [56,57].…”