Breaking the Phonon Bottleneck in Semiconductor Nanocrystals via Multiphonon Emission Induced by Intrinsic Nonadiabatic Interactions

Abstract

We observe ultrafast 1P-to-1S intraband relaxation in PbSe and CdSe nanocrystals (NCs) that have distinct energy spectra. While ultrafast dynamics in CdSe NCs has typically been interpreted in terms of electron-hole energy transfer, this mechanism is not active in PbSe NCs because of sparse densities of states in the conduction and valence bands. Our observations of temperature activation and confinement-enhanced relaxation in PbSe NCs can be explained by efficient multiphonon emission triggered by nonadiabatic electron-phonon interactions and are indicative of large, size-dependent, intraband Huang-Rhys parameters.

Excited-state relaxation in PbSe quantum dots

“…Subsequent refined measurements by Schaller et.al. [12] revealed a low-temperature decay time of only ∼1.6 ps for 19Å radius dots and ∼2.7 ps for 35Å radius dots, not much slower than CdSe. These observations were followed by the measurements of Harbold et.…”

“…1 35)], for which the hole level spacing is not known to be particularly large. This "second crisis", created by the experimental absence [3,12,28] of the theoretically expected [28][29][30]32] slow excited-state relaxation in PbSe quantum dots, led to the statement that "this is not understood, and there is a need for a novel mechanism to explain these results." [3] Indeed, rather exotic mechanisms, involving very strong acoustic coupling [3], or multi-phonon emission triggered by nonadiabatic electron-hole interaction [12], were suggested to explain the fast P→S decay in PbSe dots.…”

“…The emergence of PbSe as a novel colloidal-dot material [3,12,[28][29][30][31] has rekindled the hope for a slowdown of excited state relaxation, because PbSe was deemed to be a very different material than the previously-studied CdSe, InP, ZnO quantum dots. Indeed, the hole effective masses of PbSe (longitudinal mass m l =0.07, transverse mass m t =0.034) [11] are light.…”

“…In turn, the expectation of widely spaced hole levels was based on effective-mass-like models [12,30,32] that describe quantum dot states on the basis of very few bulk crystal band-edge states. Close examination [9] of the band structure of bulk PbSe [36] reveals, however, that there are a few valence band extrema near the valence-band maxi-mum (VBM), originating from rather different points (L, Σ, etc.)…”

“…Indeed, in CdSe [7] and PbSe [8][9][10] nanocrystals, the spacing between the first and second electron levels (S and P in Fig. 1a) is 200−400 meV and 100−300 meV respectively, far exceeding the LO phonon energies of ∼30 meV [11] and 17 meV [3,11,12], respectively. Even in much larger, selfassembled InGaAs/GaAs dots, the spacing between the electronic levels (∼50 meV) is larger than the LO phonon energy (∼30 meV).…”