Sequential Carrier Transfer Can Accelerate Triplet Energy Transfer from Functionalized CdSe Nanocrystals

Area of Science:

• Materials Science
• Photochemistry
• Nanotechnology

Background:

• Triplet-fusion upconversion converts long-wavelength light to higher-energy photons.
• Nanocrystal (NC)-sensitized upconversion is an emerging strategy for this conversion.
• Understanding energy transfer dynamics in functionalized NCs is crucial for optimizing performance.

Purpose of the Study:

• To investigate the photophysics of energy transfer from tailor-functionalized cadmium selenide (CdSe) NCs to surface-anchored transmitter ligands.
• To elucidate the mechanisms of energy transfer, distinguishing between correlated exciton transfer and sequential carrier hops.
• To determine how NC size and energy barriers influence the kinetics and dominant pathways of energy transfer.

Main Methods:

• Fabrication of tailor-functionalized CdSe NCs with varying sizes.
• Spectroscopic analysis to probe energy transfer pathways (exciton transfer vs. carrier hops).
• Transient photoluminescence spectroscopy to study quenching dynamics of band-edge and trap states.

Main Results:

• Energy transfer rate (k_quench) increased significantly (0.0096 to 0.064 ns^-1 ligand^-1) when the hole-first sequential transfer barrier decreased from 100 ± 25 meV to 50 ± 25 meV.
• This acceleration was 5.1 times greater than expected from carrier wave function leakage alone, indicating a shift in dominant mechanism.
• Both NC band-edge and trap states were efficiently quenched (up to ~98% for sequential transfer) with matched long-term dynamics (>300 ns).

Conclusions:

• Sequential carrier transfer becomes kinetically dominant over correlated exciton transfer when the energy barrier is sufficiently lowered.
• Functionalization effectively quenches both band-edge and trap states in CdSe NCs.
• A dynamic quasi-equilibrium facilitates efficient photoexcitation extraction, even from initially trapped states, crucial for upconversion applications.