Ultrafast Charge Transfer at a Quantum Dot/2D Materials Interface Probed by Second Harmonic Generation

TL;DR

This study demonstrates ultrafast charge transfer between quantum dots and monolayer WS2 using time-resolved second harmonic generation, revealing strong electronic coupling and coherent phonon modulation at the interface.

Contribution

It introduces a femtosecond SHG microscopy method to probe ultrafast charge transfer and vibrational dynamics in QD/TMD heterostructures, highlighting tunable electronic coupling.

Findings

Charge transfer occurs within 50 fs to 1 ps.

Oscillations in SHG signal indicate coherent phonon modulation.

Strong electronic coupling at the QD/TMD interface.

Abstract

Hybrid quantum dot (QD) / transition metal dichalcogenide (TMD) heterostructures are attractive components of next generation optoelectronic devices, which take advantage of the spectral tunability of QDs and the charge and exciton transport properties of TMDs. Here, we demonstrate tunable electronic coupling between CdSe QDs and monolayer WS$_2$ using variable length alkanethiol ligands on the QD surface. Using femtosecond time-resolved second harmonic generation (SHG) microscopy, we show that electron transfer from photoexcited CdSe QDs to single-layer WS$_2$ occurs on ultrafast (50 fs - 1 ps) timescales. Moreover, in the samples exhibiting the fastest charge transfer rates ($\leq$ 50 fs) we observed oscillations in the time-domain signal corresponding to an acoustic phonon mode of the donor QD, which coherently modulates the SHG response of the underlying WS$_2$ layer. These results reveal surprisingly strong electronic coupling at the QD/TMD interface and demonstrate the usefulness of time-resolved SHG for exploring ultrafast electronic-vibrational dynamics in TMD heterostructures.