Lead
halide perovskites (LHPs) exhibit large spin–orbit coupling
(SOC), leading to only twofold-degenerate valence and conduction bands
and therefore allowing for efficient optical orientation. This makes
them ideal materials to study charge carrier spins. With this study
we elucidate the spin dynamics of photoexcited charge carriers and
the underlying spin relaxation mechanisms in CsPbI3 nanocrystals
by employing time-resolved differential transmission spectroscopy
(DTS). We find that the photoinduced spin polarization significantly
diminishes during thermalization and cooling toward the energetically
favorable band edge. Temperature-dependent DTS reveals a decay in
spin polarization that is more than 1 order of magnitude faster at
room temperature (3 ps) than at cryogenic temperatures (32 ps). We
propose that spin relaxation of free charge carriers in large-SOC
materials like LHPs occurs as a result of carrier–phonon scattering,
as described by the Elliott–Yafet mechanism.
Biominerals are organic-inorganic nanocomposites exhibiting remarkable properties due to their unique configuration. Using optical spectroscopy and theoretical modeling, it is shown that the optical properties of a model bioinspired system, an inorganic semiconductor host (Cu 2 O) grown in the presence of amino acids (AAs), are strongly influenced by the latter. The absorption and photoluminescence excitation spectra of Cu 2 O-AAs blue-shift with growing AA content, indicating band gap widening. This is attributed to the void-induced quantum confinement effects. Surprisingly, no such shift occurs in the emission spectra. The theoretical model, assuming an inhomogeneous AA distribution within Cu 2 O-AAs due to compositional disorder, explains the deviating behavior of the photoluminescence. The model predicts that the potential causing the confinement effects becomes a function of the local AA density. It results in a Gaussian band gap distribution that shapes the optical properties of Cu 2 O-AAs. Imitating and harnessing the process of biomineralization can pave the way toward new functional materials.
Spin-dependent properties of lead halide perovskites (LHPs) have recently gained significant attention paving their way toward spin-optoelectronic applications. However, separate measurements of the electron and hole spin relaxation rates are so far missing in LHPs. The knowledge of the electron and hole spin relaxation timescales is necessary to understand the spin-dependent properties of LHPs. Here, we report on the spin polarization dynamics in CsPbI3 nanocrystals (NCs). We employ polarization dependent ultrafast differential transmission spectroscopy (DTS) at room temperature to study the spin polarization dynamics in this system. In the case of pure CsPbI3 NCs, it is not possible to measure separately electron and hole spin relaxation rates from the polarization dependent DTS. Here, we introduce the soluble fullerene derivative PC60BM as an electron acceptor along with CsPbI3 to create an imbalance between the photoexcited electrons and holes in the NCs and, thus, affecting their spin-dependent carrier distribution. CsPbI3:PC60BM blend sample shows a distinct difference in the spin dependent kinetics of the DTS spectra as compared to the NCs-only sample. With the help of a kinetic model for the spin-dependent charge carrier distributions, we separately determine the electron and hole spin relaxation times in CsPbI3 NCs. We find that the room temperature hole spin lifetime ( τh ∼ 5 ps) is ∼13 times longer than the electron spin lifetime ( τe ∼ 0.4 ps). We ascribe the fast electron spin relaxation to the presence of strong spin–orbit coupling in the conduction band, which is ineffective for holes in the s-type valence band.
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