Ultrafast Charge Separation at the CdSe Quantum Dot/Methylviologen Interface: Dependence on Electron Acceptor Concentration

Monitoring the electric field in CdSe quantum dots under ultrafast interfacial electron transfer via coherent phonon dynamics

Coherent phonon dynamics in CdSe quantum dots (QD) under an ultrafast electron transfer (ET) reaction of the (1S_e-1S_3/2) exciton quenched by methyl viologen (MV²⁺) adsorbed onto the QD surface was studied by ultrafast pump-probe spectroscopy. Frequency and amplitude modulations (FM, AM) of the transient absorption ΔA(ω_probe,t) in the pure CdSe and coupled CdSe/MV²⁺ QDs were identified in the bleach band dynamics of the red-edge exciton. The fast Fourier transform (FFT) and continuous wavelet transform analysis of the FM and AM oscillations revealed peaks at 0.51-0.58 THz (17-19 cm^-1) and 6.06-6.27 THz (202-209 cm^-1) attributed to the longitudinal acoustic (LA) and longitudinal optical (LO) phonons, respectively. The electron transfer to MV²⁺ proceeded non-exponentially with effective time constants of 164 fs (∼30%) and 540 fs (∼70%). The quantum yield of MV˙⁺ radical formation was 40 ± 5%. It implies a fast route for the electron-hole pair [h⁺…MV˙⁺] recombination that can be rationalized in accordance with the adiabatic ET mechanism at the semiconductor surface. In the coupled CdSe/MV²⁺ QDs, the amplitude of the FM oscillations rose considerably with time despite the natural attenuation of the phonon amplitude due to decoherence processes. A kinetic model explaining the increase of FM oscillations is proposed. The surprising growth of FM oscillations is elucidated by the kinetic model taking into account the relatively slow damping of LO phonon oscillations (∼1.5 ps), the ultrafast ET to MV²⁺, and the quantum yield of charge separation [h⁺…MV˙⁺] (∼40%). The fast formation of the charge-separated pair [h⁺…MV˙⁺] suggests the appearance of an electric field F with a strength of ∼3 × 10⁶ V cm^-1. The MV²⁺ reduction substantially increased the magnitude of LA phonon oscillations. Since the ET time is shorter than the period of LA phonon oscillations (∼1.8 ps), the MV²⁺ reduction substantially increased the magnitude of LA phonon oscillations due to the inverse piezoelectric effect. The CdSe nanocrystals exposed to the electric field F exhibit the quantum-confined Stark and Franz-Keldysh electro-absorption effects. The proposed kinetic model gives consideration to the dynamic Stark shift of the red-edge exciton and to the increased amplitude of LO phonon oscillations in the bleach band dynamics.

A thin CdSe shell boosts the electron transfer from CdTe quantum dots to methylene blue

CdTe core and CdTe/CdSe core/shell quantum dots (QD) are investigated with steady state and time-resolved spectroscopic methods. The coating of the CdTe core with a 0.7 nm thick CdSe shell shifts the lowest exciton absorption band to the red by more than 70 nm making the CdTe/CdSe QD an interesting candidate for application in solar energy conversion. Femtosecond transient absorption measurements are applied to study the photoinduced electron transfer (ET) to the molecular acceptor methylene blue (MB). ET times after single excitation of the QD are determined for different MB : QD ratios. The ET reaction is significantly faster in the case of the MB-CdTe/CdSe QD complexes, indicative of an altered charge distribution in the photoexcited heterostructure with a higher electron density in the CdSe shell. As a result of the efficient absorption of incoming light and the faster ET reaction, the amount of reduced MB in the time resolved experiments is higher for CdTe/CdSe QD compared to CdTe QD.