Changing Spin and Orbital Ground State Symmetries in Colloidal Nanoplatelets with Magnetic Fields

Abstract

The symmetry of the electronic ground state is of paramount importance in determining the magnetic, optical, and electrical properties of semiconductor nanostructures. Here, it is shown theoretically that nontrivial spin and orbital symmetries can be induced in colloidal nanoplatelets (NPLs) by applying out‐of‐plane magnetic fields. Two scenarios are presented. The first one deals with two electrons confined inside a platelet. Here, the strong electron–electron exchange interaction reduces the interlevel energy spacing set by lateral quantum confinement. As a result, relatively weak magnetic fields suffice to induce a singlet‐to‐triplet spin transition. The second one deals with type‐II core/crown NPLs. Here, the crown has doubly connected topology, akin to that of quantum rings. As a result, the energy levels of carriers within it undergo Aharonov–Bohm (AB) oscillations. This implies changes in the ground state orbital symmetry, which switch the exciton and trion optical activity from bright to dark.

2D II–VI Semiconductor Nanoplatelets: From Material Synthesis to Optoelectronic Integration

“…The same phenomenon of magnetic spin polarons is reported in QDs; these dangling bond spins modify the spin splitting of the dark excitonic state and recombination. , Depending on the sample preparation conditions, the surface spins may interact ferromagnetically or antiferromagnetically with the dark exciton, resulting in vastly divergent sample behavior with respect to the degree of circular polarization of emitted photons . This complicates the picture of exciton fine structure obtained from low-temperature, high magnetic field measurements (Figure a), which show that magnetic fields increase the fraction of exciton emission (as opposed to emission from trions) as the rate of emission increases due to enhanced excitation from a lowest dark exciton. , The interaction between bright and dark, charged and neutral, trion states has also been addressed computationally, showing singlet–triplet crossing of the ground states at magnetic fields <10 T . Even at room temperature, spin coherence has been detected by measuring the time-resolved ellipticity of the band edge absorption .…”

“…379 This complicates the picture of exciton fine structure obtained from low-temperature, high magnetic field measurements (Figure 31a), which show that magnetic fields increase the fraction of exciton emission (as opposed to emission from trions) 218 as the rate of emission increases due to enhanced excitation from a lowest dark exciton. 252,255 The interaction between bright and dark, charged and neutral, trion states has also been addressed computationally, showing singlet−triplet crossing of the ground states at magnetic fields <10 T. 391 Even at room temperature, spin coherence has been detected by measuring the timeresolved ellipticity of the band edge absorption. 392 In such situations, it is possible to distinguish between electron and hole trapping signals, as core/shell CdSe/CdS NPLs suppress hole trapping but not electron trapping.…”