Blinking Suppression in Highly Excited CdSe/ZnS Quantum Dots by Electron Transfer under Large Positive Gibbs (Free) Energy Change

Abstract

Semiconductor quantum dots with stable photoluminescence are necessary for next generation optoelectronic and photovoltaic devices. Photoluminescence intensity fluctuations of cadmium and lead chalcogenide quantum dots have been extensively investigated since the first observation of blinking in CdSe nanocrystals in 1996. In a quantum dot, blinking originates from stochastic photocharging, nonradiative Auger recombination, and delayed neutralization. So far, blinking is suppressed by defect passivation, electron transfer, and shell preparation, but without any deep insight into free energy change of electron transfer. We report real-time detection of significant blinking suppression for CdSe/ZnS quantum dots exposed to N, N-dimethylaniline, which is accompanied by a considerable increase in the time-averaged photoluminescence intensity of quantum dots. Although the Gibbs (free) energy change (Δ G = +2.24 eV), which is estimated electrochemically and from density functional theory calculations, is unfavorable for electron transfer from N, N-dimethylaniline to a quantum dot in the minimally excited (band-edge) state, electron transfer is obvious when a quantum dot is highly excited. Nonetheless, Δ G crosses from the positive to negative scale as the solvent dielectric constant exceeds 5, favoring electron transfer from N, N-dimethylaniline to a quantum dot excited to the band-edge state. Based on single-molecule photoluminescence and ensemble electron transfer studies, we assign blinking suppression to the transfer of an electron from N, N-dimethylaniline to the hot hole state of a quantum dot. In addition to blinking suppression by electron transfer, complete removal of blinking is limited by short-living OFF states induced by the negative trion.

Hot electron transfer in Zn–Ag–In–Te nanocrystal–methyl viologen complexes enhanced with higher-energy photon excitation

“…Charge separation dynamics with the use of various electron acceptors has been reported in many papers. [5][6][7][8][9][10][11][12][13][14][15][16][17] For example Banin et al reported that hybrid nanostructures of Au-tipped CdS nanorods with a larger Au size exhibited a higher probability for charge separation of multiple excitons formed in the CdS nanocrystals, improving the photocatalytic activity for H 2 generation. 15 The rate of photoinduced electron transfer from CdSe NCs to methyl viologen (MV 2+ ) was shown to be dependent on the particle morphology: plate-like CdSe NCs exhibited a slower rate than that of spherical or rod-shaped CdSe NCs due to the weak electronic coupling along the short and long axes in the lateral dimension.…”

Influence of Shape Anisotropy on the Emission of Low-Dimensional Semiconductors

“…The rates of processes such as intraband relaxation, multiple exciton generation, nonradiative Auger recombination, and emission from biexcitons ,,,− increase with decreasing size in the quantum-confined regime . In strongly confined spherical QDs, sub-100 ps Auger-type annihilation of biexcitons and recombination of trions gives both of these species a much lower PL quantum yield than single excitons in the same material, such that emission from these species is generally only observable when their radiative rates are enhanced ( e.g ., by an external field) or when the Auger rate is slowed at low temperature. − …”

Photoluminescent Nanomaterials for Medical Biotechnology

“…One of the downsides of QDs is the intermittent nature of their PL (blinking) that occurs when one or both components of an exciton (electron and hole) hit the surface of the particle, which leads to the appearance of a charge on the particle and quenching of the PL as a result of nonradiative recombination [ 49 ]. In order to overcome this drawback, several methods have been designed that provide complete or partial blinking suppression [ 50 , 51 ].…”