Methods for Obtaining One Single Larmor Frequency, Either v1 or v2, in the Coherent Spin Dynamics of Colloidal Quantum Dots

Abstract

The coexistence of two spin components with different Larmor frequencies in colloidal CdSe and CdS quantum dots (QDs) leads to the entanglement of spin signals, complicating the analysis of dynamic processes and hampering practical applications. Here, we explored several methods, including varying the types of hole acceptors, air or anaerobic atmosphere and laser repetition rates, in order to facilitate the obtention of one single Larmor frequency in the coherent spin dynamics using time-resolved ellipticity spectroscopy at room temperature. In an air or nitrogen atmosphere, manipulating the photocharging processes by applying different types of hole acceptors, e.g., Li[Et3BH] and 1-octanethiol (OT), can lead to pure spin components with one single Larmor frequency. For as-grown QDs, low laser repetition rates favor the generation of the higher Larmor frequency spin component individually, while the lower Larmor frequency spin component can be enhanced by increasing the laser repetition rates. We hope that the explored methods can inspire further investigations of spin dynamics and related photophysical processes in colloidal nanostructures.

Anomalous Laser-Fluence Dependence of Electron Spin Excitation in CdS Colloidal Quantum Dots: Surface Effects

“…The e → T HH+LH – transition energy is red-shifted compared with the first neutral exciton absorption energy (marked by the blue dashed line in Figure whose value is obtained from the second derivative of the absorption spectrum, as shown in Figures c and d) because of the Coulombic interaction and carrier induced Stark shift. As demonstrated previously, the g 2 electron is short-lived in an air atmosphere, and there are no accumulation effects from previous laser pulses. In the two beam pump–probe measurements in this work, in order to get negative photocharging for spin excitation, there should be a very fast hole transfer from the QD to surface ligands (the hole transfer time is shorter than the laser pulse duration and/or neutral exciton spin relaxation time).…”

“…Colloidal CdS QDs with native ligands or OT hole acceptors in an air atmosphere usually have a pure spin component of the g 2 electron in the pump–probe measurements with low laser repetition rates (typically ≤1 kHz), where g 1 = 1.85 and g 2 = 1.94 for the QD sample in this work (Supporting Information). Unless otherwise stated, the data shown below are obtained from the g 2 electron in QD solutions in an air atmosphere.…”

“…Time-resolved ellipticity spectroscopies with degenerate or nondegenerate pump−probe wavelengths are used to measure spin dynamics of the QD solution with diverse surface conditions. Circularly polarized pump pulses generate Colloidal CdS QDs with native ligands or OT hole acceptors in an air atmosphere usually have a pure spin component of the g 2 electron in the pump−probe measurements with low laser repetition rates (typically ≤1 kHz), 18 where g 1 = 1.85 and g 2 = 1.94 for the QD sample in this work (Supporting Information). Unless otherwise stated, the data shown below are obtained from the g 2 electron in QD solutions in an air atmosphere.…”

Enhancement of Photoluminescence in CsPbBr3 Nanocrystals with 1-Octanethiol by Light Illumination for Potential Emitting Applications

“…On the other hand, the addition of some organic molecules, such as electron or hole acceptors, into CsPbX 3 NCs will weaken their PL intensities and lead to a charge separation state, − which is crucial for photocatalytic and photovoltaic applications. 1-Octanethiol (OT) is a well-known hole-acceptor in II–VI colloidal NCs, − which can sustain stable negative photocharging in CdSe quantum dots in both air and nitrogen (N 2 ) atmospheres . However, systematic investigations of carrier dynamics and PL performance in perovskite NCs with OT molecules are still lacking.…”

Coherent Spin Dynamics of Electrons in CdSe Colloidal Nanoplatelets

“…Spin studies of CdSe nanocrystals started more than 20 years ago [26][27][28] and still attract attention of researchers [29][30][31][32]. A very unusual effect has been found, i.e., the manifestation of surface spins, which strongly polarize exciton spins in CdSe NPLs at low temperatures [33].…”