Vibronic Excitons and Conical Intersections in Semiconductor Quantum Dots

Abstract

Surface defects and organic surface-capping ligands affect the photoluminescence properties of semiconductor quantum dots (QDs) by altering the rates of competing nonradiative relaxation processes. In this study, broadband two-dimensional electronic spectroscopy reveals that absorption of light by QDs prepares vibronic excitons, excited states derived from quantum coherent mixing of the core electronic and ligand vibrational states. Rapidly damped coherent wavepacket motions of the ligands are observed during hot-carrier cooling, with vibronic coherence transferred to the photoluminescent state. These findings suggest a many-electron, molecular theory for the electronic structure of QDs, which is supported by calculations of the structures of conical intersections between the exciton potential surfaces of a small ammonia-passivated model CdSe nanoparticle.

Origin of Vibronic Coherences During Carrier Cooling in Colloidal Quantum Dots

“…Despite the truncated ligands and small CdSe core employed here, our analysis is in very good agreement with the two-dimensional electronic spectrum (2DES) of the hexadecylamine-capped CdSe QDs measured by the Beck group . In this previous work, rapidly damped coherent oscillations were observed in the off-diagonal cross peaks corresponding to relaxation between excited states.…”

“…With recent advances in nonlinear optical spectroscopy, it is now possible to interrogate the mechanism for carrier dynamics in QDs in unprecedented detail. − Recent broadband multidimensional electronic spectroscopy experiments with 6.8 fs midvisible laser pulses by the Beck group have detected rapidly damped, coherent wavepacket motions involving the vibrational modes of surface-capping ligands following optical excitation of hexadecylamine-passivated colloidal CdSe quantum dots . In short, excitation to an electronic state above the band gap results in the creation of a wavepacket of mixed electronic/vibrational character moving coherently between exciton states on subpicosecond time scales.…”

“…The spectrum corresponding to direct relaxation from the FC point to Min X1 is shown in green. The experimental LPSVD spectrum of the amplitude modulations of the X3-PL off-diagonal peak in the 2DES spectrum of Tilluck et al is shown in orange for comparison.…”

Friction-Induced Clustered Rearrangement at a PbS Quantum Dot Nanocoating via Long-Term Lubrication under an Atmosphere Environment

“…With the rapid development of nanoscience and nanotechnology, a unique type of quasi-zero-dimensional functional nanomaterial named quantum dots (QDs) has attracted a great deal of attention in the scientific community. Based on the remarkable optical and semiconducting properties, − QDs have been widely utilized in excitation-based optoelectronic devices such as colloidal solar cells, , fluorescence probes, , and light-emitting diodes. , Also, QDs have been successfully used in the mechanical engineering field as solid lubricants via the transferred lubricating films generated by physical or chemical reactions of sliding-contact interfaces. Previous tribological studies on QDs primarily focus on those derived from two-dimensional (2D) layered materials such as graphene − QDs, transition metal dichalcogenide QDs, as well as black phosphorus (BP) QDs.…”

Molecular Aggregates Associated THz Coherent Vibrations Contribute to Enhancements of Emission Efficiencies

“…7,14,15 Recently, due to rapid advances in materials science, VC in the solid phase has been investigated, and the impact of VC on optoelectronic devices has received considerable attention. [16][17][18][19][20][21][22][23][24][25] Several studies have proposed that VC may assist photoinduced charge separation in photovoltaic materials, 16,18,25,26 although the correlation between VC-assisted charge separation and device performance is still unknown. 17,27 Despite fundamental and technical difficulties, this emerging field has considerable potential and various unexplored topics.…”