We study the band-edge exciton fine structure and in particular its bright-dark splitting in colloidal semiconductor nanocrystals by four different optical methods based on fluorescence line narrowing and time-resolved measurements at various temperatures down to 2 K. We demonstrate that all these methods provide consistent splitting values and discuss their advances and limitations. Colloidal CdSe nanoplatelets with thicknesses of 3, 4 and 5 monolayers are chosen for experimental demonstrations. The bright-dark splitting of excitons varies from 3.2 to 6.0 meV and is inversely proportional to the nanoplatelet thickness. Good agreement between experimental and theoretically calculated size dependence of the bright-dark exciton slitting is achieved. The recombination rates of the bright and dark excitons and the bright to dark relaxation rate are measured by time-resolved techniques.

In heterostructures consisting of different transition-metal dichalcogenide monolayers, a staggered band alignment can occur, leading to rapid charge separation of optically generated electron-hole pairs into opposite monolayers. These spatially separated electron-hole pairs are Coulomb-coupled and form interlayer excitons. Here, we study these interlayer excitons in a heterostructure consisting of MoSe2 and WSe2 monolayers using photoluminescence spectroscopy. We observe a non-trivial temperature dependence of the linewidth and the peak energy of the interlayer exciton, including an unusually strong initial redshift of the transition with temperature, as well as a pronounced blueshift of the emission energy with increasing excitation power. By combining these observations with time-resolved photoluminescence measurements, we are able to explain the observed behavior as a combination of interlayer exciton diffusion and dipolar, repulsive exciton-exciton interaction.In recent years, two-dimensional crystal structures have garnered a lot of scientific attention. Using simple techniques such as mechanical exfoliation, a plethora of different materials is readily available as a twodimensional sheet [1], including large-gap insulators, superconductors, and semiconductors. Due to quantum confinement effects, the electronic structure of these atomically thin layers can be very different from that of their corresponding bulk crystals. MoS 2 and related transition-metal dichalcogenides (TMDCs) such as WSe 2 and MoSe 2 are among the most promising systems: while they are indirect-gap semiconductors in the bulk, a transition to a direct band gap occurs as they are thinned down to a single layer [2][3][4]. The peculiar band structure of the TMDC monolayers, combined with a strong spin-orbit interaction, leads to a coupling of spin and valley degrees of freedom [5,6], making these materials highly interesting for potential valleytronic applications. Due to the strictly two-dimensional confinement of electrons and holes and the weak dielectric screening, excitons in these monolayer TMDCs are stable even at room temperature and exhibit large binding energies of about 0.5 eV [7][8][9][10][11]. While various TMDCs show qualitatively similar features, they are characterized by different absolute values of band gap and band offsets with respect to the vacuum level [12]. Therefore, several combinations of TMDCs were predicted to yield a staggered band alignment [12,13] when they are combined into a heterostructure, leading to spatial separation of optically generated electron-hole pairs. The development of various transfer techniques [14,15] for building Van der Waals heterostructures [16] by stacking twodimensional crystals made it possible to fabricate proofof-concept devices such as light-emitting diodes and solar cells using TMDCs [17][18][19], and to experimentally verify the predictions regarding band alignment for different TMDC combinations [20][21][22][23]. Remarkably, photoluminescence (PL) spectra of TMDC heterostructures...

The optical properties of colloidal cesium lead halide perovskite (CsPbBr) nanocrystals are examined by time-resolved and polarization-resolved spectroscopy in high magnetic fields up to 30 T. We unambiguously show that at cryogenic temperatures the emission is dominated by recombination of negatively charged excitons with radiative decay time of 300 ps. The additional long-lived emission, which decay time shortens from 40 down to 8 ns and in which the decay time shortens and relative amplitude increases in high magnetic fields, evidences the presence of a dark exciton. We evaluate g-factors of the bright exciton g = +2.4, the electron g = +2.18, and the hole g = -0.22.

Monolayers of semiconducting transition metal dichalcogenides exhibit intriguing fundamental physics of strongly coupled spin and valley degrees of freedom for charge carriers. While the possibility of exploiting these properties for information processing stimulated concerted research activities towards the concept of valleytronics, maintaining control over spin–valley polarization proved challenging in individual monolayers. A promising alternative route explores type II band alignment in artificial van der Waals heterostructures. The resulting formation of interlayer excitons combines the advantages of long carrier lifetimes and spin–valley locking. Here, we demonstrate artificial design of a two-dimensional heterostructure enabling intervalley transitions that are not accessible in monolayer systems. The resulting giant effective g factor of −15 for interlayer excitons induces near-unity valley polarization via valley-selective energetic splitting in high magnetic fields, even after nonselective excitation. Our results highlight the potential to deterministically engineer novel valley properties in van der Waals heterostructures using crystallographic alignment.

Transition-metal dichalcogenides can be easily produced as atomically thin sheets, exhibiting the possibility to optically polarize and read out the valley pseudospin of extremely stable excitonic quasiparticles present in these 2D semiconductors. Here, we investigate a monolayer of tungsten disulfide in high magnetic fields up to 30 T via photoluminescence spectroscopy at low temperatures. The valley degeneracy is lifted for all optical features, particularly for excitons, singlet and triplet trions, for which we determine the g factor separately. While the observation of a diamagnetic shift of the exciton and trion resonances gives us insight into the real-space extension of these quasiparticles, magnetic field-induced valley polarization effects shed light onto the exciton and trion dispersion relations in reciprocal space. The field dependence of the trion valley polarizations is in line with the predicted trion splitting into singlet and triplet configurations.