Magdalena Grzeszczyk

Research on single photon sources in layered materials has been limited so far to transition metal dichalcogenides (TMDs) and hexagonal boron nitride (hBN) as hosting platforms. These quantum emitters exhibit advantages due to the distinct semiconducting and insulating characteristics of the two classes of materials, which enable their integration with van der Waals heterostructures and devices. Here, we report single photon emission in ZnPS₃, which belongs to the MPX₃ family characterized by stronger electronic correlations than those observed intrinsically in TMDs or hBN. We provide a comprehensive characterization of the vibrational and optical properties of nonmagnetic ZnPS₃ crystals, focusing on unraveling the mechanisms responsible for the single photon emission. Using polarization-resolved Raman scattering spectroscopy, we identify key phonon modes and uncover strong metal-ligand interactions that influence both phonon dynamics and defect-bound excitonic states. Low-temperature photoluminescence spectroscopy reveals stable and narrow optical transitions localized at defect sites, while second-order correlation measurements confirm the quantum nature of the emission. We complement our experimental analysis with ab initio density functional and GW many-body perturbation theory calculations to investigate the characteristics of the bulk and defect-related electronic structure. Our theoretical analysis reveals that phosphorus vacancies introduce midgap states, enabling optical transitions occurring at the energy range consistent with the experimentally observed emission lines. This joint approach identifies P-vacancies as the likely origin of single photon emitters in ZnPS₃. Furthermore, we anticipate that similar behavior should be present in other MPX₃ compounds, offering a framework for exploring defect-based quantum emitters with intrinsic magnetic tunability.

The authors initiate the correction to the original publication.1

Monolayers (MLs) of semiconducting transition metal dichalcogenides emit light very efficiently and display rich spin-valley physics, with gyromagnetic ( g ) factors of about − 4 . Here, we investigate how these properties can be tailored by alloying. Magneto-optical spectroscopy is used to reveal the peculiar properties of excitonic complexes in Mo x W 1 − x Se 2 MLs with different metal concentrations. We show that the alloys feature extremely high g factors for neutral excitons, that change gradually with the composition up to reaching values of the order of − 10 for x ≈ 0.2 . First-principles calculations quantitatively identify the alloy-induced mixing between different conduction band valleys as the underlying mechanism originating the anomalous composition dependence of the neutral exciton g factor. The theoretical framework also suggests a high strain sensitivity of the alloys, making them promising candidates for tailor-made optoelectronic devices.

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 effect of pressure on Raman scattering (RS) in bulk HfS2 is investigated under hydrostatic and non-hydrostatic pressure conditions. The RS line shape does not change significantly in the hydrostatic regime up to P = 9.6 GPa, showing a systematic blueshift of the spectral features. In a non-hydrostatic environment, seven peaks appear in the spectrum at P = 7 GPa, which dominate the RS line shape up to P = 10.5 GPa. The change in the RS line shape manifests a pressure-induced phase transition in HfS2. The simultaneous observation of both low-pressure (LP) and high-pressure (HP) related RS peaks suggests the coexistence of two different phases over a wide pressure range. It is found that the HP-related phase is metastable and persists during the decompression cycle down to P = 1.2 GPa, while the LP-related features eventually recover at even lower pressure. The angle-resolved polarized RS performed under P = 7.4 GPa revealed a strong in-plane anisotropy of both the LP-related A1g mode and the HP peaks. The anisotropy is related to the possible distortion of the structure induced by the non-hydrostatic component of the pressure. The results are explained in terms of a distorted Pnma phase as a possible pressure-induced phase of HfS2.

A key advantage of utilizing van‐der‐Waals (vdW) materials as defect‐hosting platforms for quantum applications is the controllable proximity of the defect to the surface or the substrate allowing for improved light extraction, enhanced coupling with photonic elements, or more sensitive metrology. However, this aspect results in a significant challenge for defect identification and characterization, as the defect's properties depend on the the atomic environment. This study explores how the environment can influence the properties of carbon impurity centers in hexagonal boron nitride (hBN). It compares the optical and electronic properties of such defects between bulk‐like and few‐layer films, showing alteration of the zero‐phonon line energies and their phonon sidebands, and enhancements of inhomogeneous broadenings. To disentangle the mechanisms responsible for these changes, including the atomic structure, electronic wavefunctions, and dielectric screening, it combines ab initio calculations with a quantum‐embedding approach. By studying various carbon‐based defects embedded in monolayer and bulk hBN, it demonstrates that the dominant effect of the change in the environment is the screening of density–density Coulomb interactions between the defect orbitals. The comparative analysis of experimental and theoretical findings paves the way for improved identification of defects in low‐dimensional materials and the development of atomic scale sensors for dielectric environments.

Ferromagnetism in van der Waals systems, preserved down to a monolayer limit, attracted attention to a class of materials with general composition CrX₃ (X=I, Br, and Cl), which are treated now as canonical 2D ferromagnets. Their diverse magnetic properties, such as different easy axes or varying and controllable character of in‐plane or interlayer ferromagnetic coupling, make them promising candidates for spintronic, photonic, optoelectronic, and other applications. Still, significantly different magneto‐optical properties between the three materials have been presenting a challenging puzzle for researchers over the last few years. Herewith, it is demonstrated that despite similar structural and magnetic configurations, the coupling between excitons and magnetization is qualitatively different in CrBr₃ and CrI₃ films. Through a combination of the optical spin pumping experiments with the state‐of‐the‐art theory describing bound excitonic states in the presence of magnetization, we concluded that the hole‐magnetization coupling has the opposite sign in CrBr₃ and CrI₃ and also between the ground and excited exciton state. Consequently, efficient spin pumping capabilities are demonstrated in CrBr₃ driven by magnetization via spin‐dependent absorption, and the different origins of the magnetic hysteresis in CrBr₃ and CrI₃ are unraveled.