Hole-Acceptor-Manipulated Electron Spin Dynamics in CdSe Colloidal Quantum Dots

Abstract

Electron spin dynamics in CdSe quantum dots with hole acceptors are investigated by time-resolved ellipticity spectroscopy. Two types of hole acceptors, Li[Et 3 BH] and 1-octanethiol, result in distinctly different electron spin dynamics. The differences include electron g factors, spin dephasing/relaxation times, and mechanisms. In CdSe quantum dots with Li[Et 3 BH], the electron spin dephasing and relaxation are dominated by electron−nuclear hyperfine interactions in zero and weak magnetic fields. In contrast, hyperfine interactions, electron carrier lifetimes, and exchange interactions between electrons and holes or surface dangling bond spins control the electron spin dynamics in CdSe quantum dots with 1octanethiol. Inhomogeneous dephasing limits the spin coherence time in larger transverse magnetic fields for both hole acceptor cases, but with distinct different g-factor inhomogeneity. These findings manifest that surface conditions play an important role in the spin dynamics and that thereby the surface and its surroundings can be exploited to control the spin in colloidal nanostructures.

Hyperfine-Induced Electron-Spin Dephasing in Negatively Charged Colloidal Quantum Dots: A Survey of Size Dependence

“…The coherent spin dynamics typically show two spin components with two different Larmor precession frequencies, namely, two different g factor values. CdSe QDs with addition of Li­[Et 3 BH] only show the component with a smaller g factor value, whose size dependence is in agreement with previously reported results. , The smaller g factor is attributed to the photodoped electron in the dot core. , Without the pump pulses for spin excitation, the photodoped electron has equal possibilities for spin-up (+ 1 / 2 ) and spin-down (− 1 / 2 ) states. Under the circularly polarized light excitation, say, a σ + pump laser will excite the + 1 / 2 electron to a + 3 / 2 trion whose spin is defined by the hole spin and relaxes fast, generating a net spin-down polarization in the photodoped electron (Figure S4 in the Supporting Information).…”

“…We note that the value of T 2,1 * , i.e., corresponding to the Gaussian decay envelope, is adopted for 6.5 nm CdSe n -QDs due to the requirement of parallel comparison between different QD sizes. Also, the proportion of the exponential decay envelope is much less than that of the Gaussian decay envelope in 6.5 nm QDs . As shown from Figure d, by increasing the transverse magnetic field, the electron spin dephasing time decreases for all samples due to inhomogeneous dephasing mechanism caused by the g factor inhomogeneity of QD ensemble with nonuniform sizes.…”

“…Also, surface effects should depend strongly on the QD size. We note that although hyperfine-related spin dynamics have been measured in various QDs, such as epitaxial and colloidal II–VI and III–V materials, ,,,,− there is still lack of experimentally determined size-dependence information on hyperfine-induced spin dephasing time. To the best of our knowledge, one only measurement of size-dependent hyperfine-related spin dynamics have been carried out in epitaxial InGaAs QDs in which the large lateral diameter size of ∼20–100 nm might also involve strong influence of spin–orbit coupling …”

“…The ellipticity signals measured in different transverse magnetic fields for all samples except 6.5 nm CdSe n -QDs can be well fitted by where v L is the Larmor precession frequency, A and T 2 * are the initial amplitude of the spin signal and spin dephasing time, respectively. However, the sample with a diameter of 6.5 nm can be well fitted by where A 1 and T 2,1 * depict the amplitude and spin dephasing time for the Gaussian decay envelope, respectively. A 2 and T 2,2 * refer to the amplitude and spin dephasing time for the exponential decay envelope, respectively.…”

“…Long-lived room temperature electron spin coherence that persists up to a few nanoseconds has been observed in both CdSe and CdS colloidal QDs. − There exist typically two distinct spin components in CdSe and CdS colloidal QDs which have different g factor values, ,− where spin relaxation processes are also strongly different. , In large transverse magnetic fields, the electron spin dynamics are controlled by g -factor inhomogeneity-induced dephasing, which is a result from the QDs’ size and shape distribution. The electron spin dynamics in zero or weak magnetic fields, however, are hard to be analyzed due to the entanglement of the aforementioned two spin components typically with different spin relaxation mechanism.…”