Breaking the Phonon Bottleneck in PbSe and CdSe Quantum Dots: Time-Domain Density Functional Theory of Charge Carrier Relaxation

Abstract

Spatial confinement can create relaxation bottlenecks by mismatch between electronic and vibrational frequencies. This hypothesis motivated discovery of multiple excitons, which could greatly enhance the efficiency of quantum dot (QD) solar cells. Surprisingly, recent experiments showed no bottleneck. Our time-domain ab initio study of the electron-phonon dynamics rationalizes the fast relaxation in PbSe and CdSe QDs, which have substantially different electronic properties. Atom fluctuations and surface effects lift degeneracies and create dense distributions of electronic levels at all but the lowest energies, while confinement enhances the electron-phonon coupling. The analysis applies to nanomaterials in general, modifying the fundamental view on the electron-phonon interaction.

Visualization of nanocrystal breathing modes at extreme strains

“…Here the damping reflects both scattering from the induced disorder as well as coupling of the nanocrystal to the surrounding matrix. A 500-fs decay time for the applied stress is in agreement with prior studies of the ultrafast relaxation of photoexcited carriers in CdSe nanocrystals of this size: electronhole-hole Auger processes lead to sub-picosecond timescale coupling to the lattice typically through hole relaxation without phonon bottleneck effects 38,41,42 .…”

Calculation of non-adiabatic coupling vectors in a local-orbital basis set

“…The surface hopping method has been implemented within DFT 20,33,34 for calculations of systems on the order of 100 atoms and with durations of up to a few picoseconds. In particular, surface hopping simulations using a time-dependent KS (TDKS) approach 20 have been used to analyze non-adiabatic dynamics in a number of relatively large systems; [42][43][44] this TDKS approach 20 has been compared with the more accurate linear-response time-dependent DFT (LR-TDDFT) method 34 by performing non-adiabatic simulations for different systems. 45 The results show that the simpler and computationally more efficient TDKS approach yields a very reasonable description of the systems analyzed, the agreement with the more accurate method being better for larger systems.…”

Phonon Spectra of Small Colloidal II-VI Semiconductor Nanocrystals

“…No trace of 2TO or 2SO band was detected, what can be the result of the dominance of Fröhlich mechanism in electron-phonon coupling in II-VI compounds. The resonant phonon Raman spectrum of NCs smaller than 2 nm is shown to be dominated by a broad feature similar to the SO mode of larger NCs or phonon density of states of a bulk crystal.The understanding of the phonon spectrum of semiconductor nanocrystals (NCs), electron-phonon coupling (EPC), dependence of the phonon spectra on the NC size, surfacebound moieties, and extended environment is of a large fundamental and application significance [1][2][3][4][5][6][7][8][9][10][11] due to the determinant role phonons play in the optical and electrical properties of semiconductor nanostructures [12,13]. Though a number of in-depth studies have addressed the phonon spectra of colloidal [14][15][16][17][18][19][20], glass-embedded [21][22][23] NCs, and epitaxial nanostructures [24][25][26], a significant divergence of the results exists concerning both the nature of the modes and their response to such factors as size, surface conditions, properties of the hosting medium and so forth.…”