Conical intersections shed light on hot carrier cooling in quantum dots

Area of Science:

• Materials Science
• Quantum Chemistry
• Spectroscopy

Background:

• Ultrafast dynamics of hot carrier cooling in semiconductor nanocrystals are crucial for optoelectronic applications.
• Vibronic coherences in electronically excited quantum dots (QDs) provide insights into these dynamics.
• Previous work identified relaxation via conical intersections in amine-passivated QDs.

Purpose of the Study:

• To demonstrate the generality of the conical intersection framework for vibronic coherences in QDs with carboxylate ligands.
• To investigate the impact of different ligands on hot carrier cooling dynamics.
• To elucidate the mechanisms of ligand-core coupling influencing relaxation.

Main Methods:

• Broadband multidimensional spectroscopy was used to observe vibronic coherences.
• A theoretical model involving a cascade of conical intersections was applied.
• Comparison of QDs with different surface ligands (amine vs. carboxylate, acetate vs. formate) was performed.

Main Results:

• The conical intersection model accurately reproduced observed vibronic coherence frequencies for carboxylate-passivated QDs.
• Electronic coupling between the QD core and ligands was less prominent in carboxylate-passivated QDs compared to amine-passivated QDs.
• Truncating ligand alkyl chains (acetate vs. formate) altered the predicted relaxation behavior.

Conclusions:

• The cascade of conical intersections is a general mechanism for vibronic coherence in excited QDs, applicable to various ligand types.
• Ligand-specific electronic and vibrational coupling mechanisms significantly influence hot carrier cooling dynamics.
• Ligand structure, including alkyl chain length, plays a role in tuning relaxation pathways.