Abstract
The lead halide perovskites demonstrate huge potential for optoelectronic applications, high energy radiation detectors, light emitting devices and solar energy harvesting. Those materials ...exhibit strong spin-orbit coupling enabling efficient optical orientation of carrier spins in perovskite-based devices with performance controlled by a magnetic field. Here we show that elaborated time-resolved spectroscopy involving strong magnetic fields can be successfully used for perovskites. We perform a comprehensive study of high-quality lead halide perovskite CsPbBr
3
crystals by measuring the exciton and charge carrier
g
-factors, spin relaxation times and hyperfine interaction of carrier and nuclear spins by means of coherent spin dynamics. Owing to their ‘inverted’ band structure, perovskites represent appealing model systems for semiconductor spintronics exploiting the valence band hole spins, while in conventional semiconductors the conduction band electrons are considered for spin functionality.
We report a time-resolved study of the photoluminescence of CdSe colloidal nanoplatelets with two different thicknesses. By studying the exciton recombination dynamics we assess the exciton fine ...structure in these systems. The splitting between bright and dark excitons is enhanced compared to epitaxial quantum well structures as result of dielectric confinement. Despite of strong variations in the absolute magnitude, by comparison with literature data we find a relatively slightly varying bright–dark exciton lifetime ratio in very different CdSe-based colloidal nanostructures, regardless of growth technique and of core and shell properties such as materials, dimensions, etc. This finding points to a universal mechanism in the dark exciton recombination.
The versatile potential of lead halide perovskites and two-dimensional materials is merged in the Ruddlesden–Popper perovskites having outstanding optical properties. Here, the coherent spin dynamics ...in Ruddlesden–Popper (PEA)2PbI4 perovskites is investigated by picosecond pump–probe Kerr rotation in an external magnetic field. The Larmor spin precession of resident electrons with a spin dephasing time of 190 ps is identified. The longitudinal spin relaxation time in weak magnetic fields measured by the spin inertia method is as long as 25 μs. A significant anisotropy of the electron g-factor with the in-plane value of +2.45 and out-of-plane value of +2.05 is found. The exciton out-of-plane g-factor of +1.6 is measured by magneto-reflectivity. This work contributes to the understanding of the spin-dependent properties of two-dimensional perovskites and their spin dynamics.
We investigate the charge separation dynamics provided by carrier surface trapping in CdSe/CdS core/shell nanoplatelets by means of a three-laser-beam pump–orientation–probe technique, detecting the ...electron spin coherence at room temperature. Signals with two Larmor precession frequencies are found, which strongly differ in their dynamical characteristics and dependencies on pump power and shell thickness. The electron trapping process occurs on a time scale of about 10 ns, and the charge separation induced thereby has a long lifetime of up to hundreds of microseconds. On the other hand, the hole trapping requires times from subpicoseconds to hundreds of picoseconds, and the induced charge separation has a lifetime of a few nanoseconds. With increasing CdS shell thickness the hole trapping vanishes, while the electron trapping is still detectable. These findings have important implications for understanding the photophysical processes of nanoplatelets and other colloidal nanostructures.
The class of Ruddlesden–Popper type (PEA)2PbI4 perovskites comprises 2D structures whose optical properties are determined by excitons with a large binding energy of about 260 meV. It complements the ...family of other 2D semiconductor materials by having the band structure typical for lead halide perovskites, that can be considered as inverted compared to conventional III–V and II–VI semiconductors. Accordingly, novel spin phenomena can be expected for them. Spin‐flip Raman scattering is used here to measure the Zeeman splitting of electrons and holes in a magnetic field up to 10 T. From the recorded data, the electron and hole Landé factors (g‐factors) are evaluated, their signs are determined, and their anisotropies are measured. The electron g‐factor value changes from +2.11 out‐of‐plane to +2.50 in‐plane, while the hole g‐factor ranges between ‐0.13 and ‐0.51. The spin flips of the resident carriers are arranged via their interaction with photogenerated excitons. Also the double spin‐flip process, where a resident electron and a resident hole interact with the same exciton, is observed showing a cumulative Raman shift. Dynamic nuclear spin polarization induced by spin‐polarized holes is detected in corresponding changes of the hole Zeeman splitting. An Overhauser field of the polarized nuclei acting on the holes as large as 0.6 T can be achieved.
The spin properties of electrons and holes in 2D (PEA)2PbI4 perovskite structure are studied by means of spin‐flip Raman scattering in strong magnetic fields up to 10 T. The Lande g‐factors including their anisotropy are measured. The nuclear spin system is dynamically polarized and its effect on the hole Zeeman splitting is measured.
Rydberg Series of Dark Excitons in Cu2O Farenbruch, Andreas; Fröhlich, Dietmar; Yakovlev, Dmitri R ...
Physical review letters,
11/2020, Volume:
125, Issue:
20
Journal Article
Peer reviewed
We demonstrate the Rydberg series of dark excitons, known as paraexcitons, up to the principal quantum number n=6 for the yellow exciton series in Cu2O, using second harmonic generation. Each of ...these states is optically inactive to all orders, but their observation becomes possible by application of a magnetic field which leads to mixing with the quadrupole-allowed bright excitons, called orthoexcitons, of the same n. The dark parastates are generally located below the bright orthostates, whose energies are increased by the electron-hole exchange interaction, except for n=2, where this order is reversed. This inversion occurs due to band mixing, namely, of the 2Sy,o orthoexciton of the yellow series with the 1Sg,o orthoexciton of the green exciton series.
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The low-temperature emission spectrum of CdSe colloidal nanoplatelets (NPLs) consists of two narrow lines. The high-energy line stems from the recombination of neutral excitons. The origin of the ...low-energy line is currently debated. We experimentally study the spectral shift, emission dynamics, and spin polarization of both lines at low temperatures down to 1.5 K and in high magnetic fields up to 60 T and show that the low-energy line originates from the recombination of negatively charged excitons (trions). This assignment is confirmed by the NPL photocharging dynamics and associated variations in the spectrum. We show that the negatively charged excitons are considerably less sensitive to the presence of surface spins than the neutral excitons. The trion binding energy in three-monolayer-thick NPLs is as large as 30 meV, which is 4 times larger than its value in the two-dimensional limit of a conventional CdSe quantum well confined between semiconductor barriers. A considerable part of this enhancement is gained by the dielectric enhancement effect, which is due to the small dielectric constant of the environment surrounding the NPLs.
We report on a time-resolved study of the photoluminescence of core/shell CdSe/CdS dot-in-rod colloidal nanocrystals having various geometries. By studying the exciton recombination dynamics, we ...unveil a quadratic dependence of the bright–dark exciton energy splitting and the dark exciton radiative recombination rate on the inverse CdS rod width, regardless of the CdSe core size. We also evidence a strong dependence of the spin-flip rate between bright and dark exciton states on the shell thickness that suggests an acoustic phonon bottleneck. This work highlights the possibility to fully control and tune the optical properties of colloidal nanocrystals by shape engineering of the CdS shell.
We report on a temperature-, time-, and spectrally resolved study of the photoluminescence of type-I InP/ZnS colloidal nanocrystals with varying core size. By studying the exciton recombination ...dynamics we assess the exciton fine structure in these systems. In addition to the typical bright–dark doublet, the photoluminescence stems from an upper bright state in spite of its large energy splitting (∼100 meV). This striking observation results from dramatically lengthened thermalization processes among the fine structure levels and points to optical-phonon bottleneck effects in InP/ZnS nanocrystals. Furthermore, our data show that the radiative recombination of the dark exciton scales linearly with the bright–dark energy splitting for CdSe and InP nanocrystals. This finding strongly suggests a universal dangling bonds-assisted recombination of the dark exciton in colloidal nanostructures.
The spin dynamics in CsPbBr3 lead halide perovskite nanocrystals are studied by picosecond pump–probe Faraday rotation in an external magnetic field. Coherent Larmor precession of electrons and holes ...with spin dephasing times of ∼600 ps is detected in a transversal magnetic field. The longitudinal spin relaxation time in weak magnetic fields reaches 80 ns at a temperature of 5 K. In this regime, the carrier spin dynamics is governed by nuclear spin fluctuations characterized by an effective hyperfine field strength of 25 mT. The Landé factors determining the carrier Zeeman splittings are g e = +1.73 for electrons and g h = +0.83 for holes. A comparison with a CsPbBr3 polycrystalline film and bulk single crystals evidences that the spatial confinement of electrons and holes in the nanocrystals only slightly affects their g factors and spin dynamics.
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