Małgorzata Zinkiewicz

Lead-free halide double perovskite nanocrystals are promising candidates for optoelectronic applications due to their chemical stability and compositional tunability. Among them, Cs₂(Na,Ag,K)(In,Bi)Cl₆ nanocrystals stand out for high photoluminescence quantum yields, yet the nature of their emissive state remains debated. Here, we study temperature- and magnetic field-dependent photoluminescence dynamics to reveal a fine structure composed of a long-lived dark triplet state and a short-lived bright singlet state, separated by ∼190 meV. This remarkably large dark-bright splitting arises from strong electron-hole exchange between tightly confined carriers. By varying Bi and Na content, we demonstrate that the fine structure and radiative dynamics can be tuned through compositional alloying. Our results reveal that the emissive state is a tightly bound, molecular-like charge-transfer exciton and offer design principles for tailoring the emission properties of lead-free perovskite nanomaterials.

The unusual magnetic properties of van der Waals‐type antiferromagnetic semiconductors such as transition metal thiophosphates make them highly attractive for spintronics and optoelectronics. However, a link between the magnetic and optical properties of these materials, required for practical applications, has not yet been established. A combined experimental and theoretical study of magnetic, optical, and structural properties of CrPS ₄ samples is reported. It is found that the magnetic‐field‐dependent circular polarization degree of the photoluminescence is a direct measure of the net magnetization of CrPS ₄ . Complementary Raman scattering measured as a function of magnetic field and temperature enables the determination of the magnetic susceptibility curve of the material. Experimental results are supported by density functional theory calculations that take as input the lattice parameters determined from temperature‐dependent X‐ray diffraction measurements. This allows the impact of spin ordering on the spectral position of Raman transitions in CrPS ₄ to be explained, as well as the anomalous temperature shifts of some of them. The presented method for all‐optical determination of magnetic properties is highly promising for studies of spin ordering and magnetic phase transitions in single‐ or few‐layer samples of magnetic layered materials, for which a poor signal‐to‐noise ratio precludes reliable neutron scattering or magnetometry measurements.

Optically dark states play an important role in the electronic and optical properties of monolayers (MLs) of semiconducting transition metal dichalcogenides. The effect of temperature on the in‐plane‐field activation of the neutral and charged dark excitons is investigated in a WSe ₂ ML encapsulated in hexagonal BN flakes. The brightening rates of the neutral dark ( X ^D ) and grey ( X ^G ) excitons and the negative dark trion ( T ^D ) differ substantially at particular temperature. More importantly, they weaken considerably by about 3–4 orders of magnitude with temperature increased from 4.2 K to 100 K. The quenching of the dark‐related emissions is accompanied by the two‐order‐of‐magnitude increase in the emissions of their neutral bright counterparts, i . e . neutral bright exciton ( X ^B ) and spin‐singlet ( T ^S ) and spin‐triplet ( T ^T ) negative trions, due to the thermal activations of dark states. Furthermore, the energy splittings between the dark X ^D and T ^D complexes and the corresponding bright X ^B , T ^S , and T ^T ones vary with temperature rises from 4.2 K to 100 K. This is explained in terms of the different exciton–phonon coupling for the bright and dark excitons stemming from their distinct symmetry properties.

The dielectric environment of atomically thin monolayer (ML) of semiconducting transition metal dichalcogenides affects both the electronic bandgap and the excitonic binding energy in the ML. We investigate the effect of the environment on the in-plane magnetic field brightening of neutral and charged dark exciton emissions in the WSe₂ ML. The monolayers placed in three dielectric environments are studied, in particular, the ML encapsulated in hexagonal BN (hBN) flakes, the ML deposited on a hBN layer, and the ML embedded between the hBN flake and SiO₂/Si substrate. We observe that the brightening rates of the neutral and charged dark excitons depend on the dielectric environment, which may be related to the variation of the level of carrier concentration in the ML. Moreover, the surrounding media, characterized by different dielectric constants, weakly influence the relative energies of the neutral and charged dark excitons in reference to the bright ones.

We investigate the origin of emission lines apparent in the low-temperature photoluminescence spectra of n-doped WS₂ monolayer embedded in hexagonal BN layers using external magnetic fields and first-principles calculations. Apart from the neutral A exciton line, all observed emission lines are related to the negatively charged excitons. Consequently, we identify emissions due to both the bright (singlet and triplet) and dark (spin- and momentum-forbidden) negative trions as well as the phonon replicas of the latter optically inactive complexes. The semidark trions and negative biexcitons are distinguished. On the basis of their experimentally extracted and theoretically calculated g-factors, we identify three distinct families of emissions due to exciton complexes in WS₂: bright, intravalley, and intervalley dark. The g-factors of the spin-split subbands in both the conduction and valence bands are also determined.

The properties of dark excitons and dark trions are investigated in a monolayer of WS₂ encapsulated in hexagonal BN.