Activating Dipolar Excitons with Strong Interactions in Two-Dimensional Perovskites.

The results from the electrooptical absorption measurements (EOAM) on the equilibrium ground and excited Franck-Condon state dipole moments of Prodan and Laurdan in 1,4-dioxane are presented. As follows from experiments Prodan and Laurdan in the equilibrium ground and excited Franck-Condon state have two conformers with considerably different dipole moments. The electrical dipole moments and the transition dipole moment, obtained from the short-wavelength region of the absorption spectrum are parallel. The electrical dipole moments measured at the long-wavelength spectral region are parallel to each other but not parallel to the transition dipole moment m a. The angle θ between the transition dipole moment m a and the dipole moment in the equilibrium ground state μ g of the long-wavelength conformer is about 30(0) for both probes. Obtained results evidence that donor-acceptor pairs of the short-wavelength and long-wavelength conformers are not located on the same axis. Two low-energy conformers of Prodan have been found by density functional theory (DFT) calculations, differing in the orientation of the carbonyl group towards the naphthalene system.

The controllable synthesis of complicated nanostructures in advanced two‐dimensional (2D) semiconductors, such as periodic regular hole arrays, is essential and remains immature. Here, we report a green, facile, highly controlled synthetic method to efficiently pattern 2D semiconductors, such as periodic regular hexagonal‐shaped hole arrays (HHA), in 2D‐TMDs. Combining the production of artificial defect arrays through laser irradiation with anisotropic annealing etching, we created HHA with different arrangements, controlled hole sizes, and densities in bilayer WS2. Atomic force microscopy (AFM), Raman, photoluminescence (PL), and scanning transmission electron microscopy (STEM) characterization show that the 2D semiconductors have high quality with atomical clean and sharp edges as well as undamaged crystals in the unetched region. Furthermore, other nanostructures, such as nanoribbons and periodic regular triangular‐shaped 2D‐TMD arrays, can be fabricated. This kind of 2D semiconductors fabrication strategy is general and can be extended to a series of 2D materials. Density functional theory (DFT) calculations show that one WS2 molecule from the edges of the laser‐irradiated holed region exhibits a robust etching activation, making selective etching at the artificial defects and the fabrication of regular 2D semiconductors possible.image

Ultracold polar molecules offer strong electric dipole moments and rich internal structure, which makes them ideal building blocks to explore exotic quantum matter1–9, implement quantum information schemes10–12 and test the fundamental symmetries of nature13. Realizing their full potential requires cooling interacting molecular gases deeply into the quantum-degenerate regime. However, the intrinsically unstable collisions between molecules at short range have so far prevented direct cooling through elastic collisions to quantum degeneracy in three dimensions. Here we demonstrate evaporative cooling of a three-dimensional gas of fermionic sodium–potassium molecules to well below the Fermi temperature using microwave shielding. The molecules are protected from reaching short range with a repulsive barrier engineered by coupling rotational states with a blue-detuned circularly polarized microwave. The microwave dressing induces strong tunable dipolar interactions between the molecules, leading to high elastic collision rates that can exceed the inelastic ones by at least a factor of 460. This large elastic-to-inelastic collision ratio allows us to cool the molecular gas to 21 nanokelvin, corresponding to 0.36 times the Fermi temperature. Such cold and dense samples of polar molecules open the path to the exploration of many-body phenomena with strong dipolar interactions.

The aggregation ability and exciton dynamics of dyes are largely affected by properties of the dye monomers. To facilitate aggregation and improve excitonic function, dyes can be engineered with substituents to exhibit optimal key properties, such as hydrophobicity, static dipole moment differences, and transition dipole moments. To determine how electron donating (D) and electron withdrawing (W) substituents impact the solvation, static dipole moments, and transition dipole moments of the pentamethine indocyanine dye Cy5, density functional theory (DFT) and time-dependent (TD-) DFT calculations were performed. The inclusion of substituents had large effects on the solvation energy of Cy5, with pairs of withdrawing substituents (W-W pairs) exhibiting the most negative solvation energies, suggesting dyes with W-W pairs are more soluble than others. With respect to pristine Cy5, the transition dipole moment was relatively unaffected upon substitution while numerous W-W pairs and pairs of donating and withdrawing substituents (D-W pairs) enhanced the static dipole difference. The increase in static dipole difference was correlated with an increase in the magnitude of the sum of the Hammett constants of the substituents on the dye. The results of this study provide insight into how specific substituents affect Cy5 monomers and which pairs can be used to engineer dyes with desired properties.

The optoelectronic performances of the layered materials are strongly dependent on the thickness of the samples due to the surface effect. As the size of the samples decreases to few nanometers, the surface depletion field and surface defect density are prominent arising from the large surface to volume ratio. For instance, thin two-dimensional (2D) organic-inorganic hybrid perovskite microplates usually exhibit a rather low photoluminescence quantum yield (PLQY), owning to the strong surface effect. Here, we report that the PLQY can be enhanced as large as 28 times in (iso-BA)2Pbl4 (BA = C4H9NH3) 2D perovskite thin microplates encapsulated by graphene, resulting in that the PLQY is more than 18% for the microplate with a thickness of 6.7 nm at 78 K. As the thickness of the 2D perovskite microplate increases, the enhancement is gradually reduced and finally vanishes. This observation is in striking contrast to that in monolayer transition metal dichalcogenides (TMDs), when the PLQY is quenched by covering a layer of graphene due to the efficient charge transfer. The enhancement of PLQY in 2D perovskites can be mainly ascribed to the reduced quantum confined Stark effect (QCSE) due to the reduced surface depletion field after covering graphene flake, resulting in the enhanced radiative recombination efficiency. Our findings provide a cost-effective approach to enhance the luminescence, which may pave the way toward high performance light emitting devices based on 2D perovskites.

Owing to unique electronic, excitonic, and valleytronic properties, atomically thin transition metal dichalcogenides are becoming a promising two-dimensional (2D) semiconductor system for diverse electronic and optoelectronic applications. In an ideal 2D semiconductor, efficient carrier transport is very difficult because of lacking free charge carriers. Doping is necessary for electrically driven device applications based on such 2D semiconductors, which requires investigation of electronic structure changes induced by dopants. Therefore probing correlations between localized electronic states and doping is important. Here, we address the electronic nature of broad bound exciton bands and their origins in exfoliated monolayer (1L) WS2 and MoS2 through monitoring low-temperature photoluminescence and manipulating electrostatic doping. The dominant bound excitons in 1L WS2 vary from donor to acceptor bound excitons with the switching from n- to p-type doping. In 1L MoS2, two localized emission bands appear which are assigned to neutral and ionized donor bound excitons, respectively. The deep donor and acceptor states play critical roles in the observed bound exciton bands, indicating the presence of strongly localized excitons in such 2D semiconductors.

Investigation of light–matter interactions in two-dimensional (2D) semiconductors is essential to understand many-body effects and explore their applications in photonics. 2D semiconductor microcavities can be formed by the integration of 2D semiconductor active media with planar Fabry–Perot resonant cavities. The emerging exciton–photon interactions between the strongly bound excitons in 2D semiconductors and the cavity photons allow exploring on the wealthy photonic and polaritonic physics of bosonic quasiparticles in the 2D limit. This Perspective focuses on recent advances in exciton–photon interactions of 2D semiconductor microcavities and their inspiring applications. First, we picture the research scope of 2D semiconductor microcavities, sort out the historical development from conventional semiconductor microcavities to the emerged 2D semiconductor microcavities, and illustrate mostly employed device structures. Second, we classify exciton–photon interactions in 2D semiconductor microcavities according to their coupling strengths. In the weak coupling regime, control of spontaneous emission and photon lasing are discussed together with the Purcell effect. In the strong coupling regime, we summarize experimental observations and theoretical predictions on Rabi splitting, Bose–Einstein condensation, polariton lasing, superfluidity, and superconductivity. Third, four types of leading-edge applications are outlined, including on-chip coherent light sources, microcavity-enhanced single-photon sources, topological photonics, and other nonlinear optics. Finally, we highlight the remaining challenges and future opportunities for fundamental physics of 2D semiconductor microcavities and their technological applications.

Two-dimensional semiconductors have emerged as a new class of materials for nanophotonics owing to their strong exciton-photon interaction1,2 and their ability to be engineered and integrated into devices3. Here we take advantage of these properties to engineer an efficient lasing medium based on direct-bandgap interlayer excitons in rotationally aligned atomically thin heterostructures4. Lasing is measured from a transition-metal dichalcogenide heterobilayer (WSe2-MoSe2) integrated in a silicon nitride grating resonator. An abrupt increase in the spatial coherence of the emission is observed across the lasing threshold. The work establishes interlayer excitons in two-dimensional heterostructures as a gain medium with spatially coherent lasing emission and potential for heterogeneous integration. With electrically tunable exciton-photon interaction strengths5 and long-range dipolar interactions, these interlayer excitons are promising for application as low-power, ultrafast lasers and modulators and for the study of many-body quantum phenomena6.

Regulating microstructure of organic ammonium cations enables interface defect management and stability improvement in 2D/3D perovskite solar cells

Two-dimensional lead halide perovskites offer numerous attractive features for optoelectronics owing to their soft, deformable lattices and high degree of chemical tunability. While alteration of the metal and halide ions gives rise to significant modification of the bandgap energy, the organic spacer cations offer in-roads to tuning phase behavior and more subtle functionalities in ways that remain to be understood. Here, we study six variations of 2D perovskites changing only the organic spacer cations and demonstrate that these components intrinsically impact material response in important ways such as altering crystallographic structure, temperature-induced phase transitions, and photoluminescence emission. Two-dimensional perovskites containing commonly utilized aliphatic linear spacers, such as butylammonium, undergo phase transitions near room temperature. These transitions and temperature changes induce spacer-dependent variations in the emission spectra. Conversely, 2D perovskites comprising cyclic aliphatic spacers, such as cyclobutylammonium, are found to lack first-order phase transitions. These cyclic molecules are more sterically hindered within the crystal lattice, leading to temperature-induced contraction or expansion along certain crystallographic planes but no other significant thermal effects; additionally, they undergo changes in their emission spectra that cannot be explained by simple thermal expansion. Given the similarities in the dielectric and chemical makeup of this set of six alkylammonium molecules, these results are unexpected and suggest a large structural and thermal phase space via spacer manipulation that could lead to improved 2D perovskite functionalization.

The electronic structure of two-dimensional (2D) semiconductors can be significantly altered by screening effects, either from free charge carriers in the material or by environmental screening from the surrounding medium. The physical properties of 2D semiconductors placed in a heterostructure with other 2D materials are therefore governed by a complex interplay of both intra- and interlayer interactions. Here, using time- and angle-resolved photoemission, we are able to isolate both the layer-resolved band structure and, more importantly, the transient band structure evolution of a model 2D heterostructure formed of a single layer of MoS2 on graphene. Our results reveal a pronounced renormalization of the quasiparticle gap of the MoS2 layer. Following optical excitation, the band gap is reduced by up to ∼400 meV on femtosecond time scales due to a persistence of strong electronic interactions despite the environmental screening by the n-doped graphene. This points to a large degree of tunability of both the electronic structure and the electron dynamics for 2D semiconductors embedded in a van der Waals-bonded heterostructure.

Heterogeneous stacking of two-dimensional (2D) perovskites with other 2D materials is a very effective strategy for designing low-cost and high-performance photovoltaic and optoelectronic devices. The structural, electronic, and optical properties of distinctive all-2D M2XBr4-black phosphorus (BP) [M = (C4H9NH3)+; X = Pb2+, Sn2+, Ge2+] van der Waals (vdW) heterostructures have been studied by first-principle calculations. The M2SnBr4-BP and M2GeBr4-BP heterostructures show type-II band arrangement; however, the M2PbBr4-BP heterostructure exhibits type-I band arrangement. The energy level shift is ascribed to the difference of work function between M2XBr4 monolayer and BP monolayer, driving the movement of carriers spontaneously. Furthermore, the BP layers can enhance the light absorption of the total heterostructures, especially the M2GeBr4-BP heterostructure. These results indicate the all-2D perovskite and BP vdW heterostructures are competitive candidates for low-dimensional photovoltaic and optoelectronic applications.

Strong interactions between excitons are a characteristic feature of two-dimensional (2D) semiconductors, determining important excitonic properties, such as exciton lifetime, coherence, and photon-emission efficiency. Rhenium disulfide ($\mathrm{Re}{\mathrm{S}}_{2}$), a member of the 2D transition-metal dichalcogenide (TMD) family, has recently attracted great attention due to its unique excitons that exhibit excellent polarization selectivity and coherence features. However, an in-depth understanding of exciton-exciton interactions in $\mathrm{Re}{\mathrm{S}}_{2}$ is still lacking. Here we used ultrafast pump-probe spectroscopy to study exciton-exciton interactions in monolayer (1L), bilayer (2L), and triple layer $\mathrm{Re}{\mathrm{S}}_{2}$. We directly measure the rate of exciton-exciton annihilation, a representative Auger-type interaction between excitons. It decreases with increasing layer number, as observed in other 2D TMDs. However, while other TMDs exhibit a sharp weakening of exciton-exciton annihilation between 1L and 2L, such behavior was not observed in $\mathrm{Re}{\mathrm{S}}_{2}$. We attribute this distinct feature in $\mathrm{Re}{\mathrm{S}}_{2}$ to the relatively weak interlayer coupling, which prohibits a substantial change in the electronic structure when the thickness varies. This work not only highlights the unique excitonic properties of $\mathrm{Re}{\mathrm{S}}_{2}$ but also provides novel insight into the thickness dependence of exciton-exciton interactions in 2D systems.

Atomically thin two-dimensional (2D) materials are ideal host systems for quantum defects as they offer easier characterisation, manipulation and read-out of defect states as compared to bulk defects. Here we introduce the Quantum Point Defect (QPOD) database with more than 1900 defect systems comprising various charge states of 503 intrinsic point defects (vacancies and antisites) in 82 different 2D semiconductors and insulators. The Atomic Simulation Recipes (ASR) workflow framework was used to perform density functional theory (DFT) calculations of defect formation energies, charge transition levels, Fermi level positions, equilibrium defect and carrier concentrations, transition dipole moments, hyperfine coupling, and zero-field splitting. Excited states and photoluminescence spectra were calculated for selected high-spin defects. In this paper we describe the calculations and workflow behind the QPOD database, present an overview of its content, and discuss some general trends and correlations in the data. We analyse the degree of defect tolerance as well as intrinsic dopability of the host materials and identify promising defects for quantum technological applications. The database is freely available and can be browsed via a web-app interlinked with the Computational 2D Materials Database (C2DB).

Interlayer excitons have been extensively studied in monolayer transition metal dichalcogenide (TMD) heterobilayers mainly due to the long lifetime, which is beneficial for a wide range of optoelectronic applications. To date, the majority of investigations of interlayer excitons in TMD heterobilayers have been focusing on the geometric arrangement of structures, spin-valley lifetime, and interlayer valley excitons with interlayer hopping rules. Nevertheless, interlayer excitons in TMD heterobilayers strongly depend on the local atomic registry and coupling strength, which increase the complexity of the device fabrication. Here, we report pronounced interlayer exciton emission in two-dimensional (2D) perovskite/monolayer TMD heterostructures without the need of thermal annealing or specific geometric arrangements, and the interlayer exciton emission is rather general among 2D perovskites and monolayer TMDs. Such interlayer exciton emission completely dominates the emission spectrum at 78 K regardless of the stacking sequence, suggesting the robust interlayer coupling in 2D perovskite/monolayer TMD heterostructures. Furthermore, the interlayer exciton emission shows a large blue-shift with increasing laser intensity due to the repulsive dipole-dipole interaction and can persist above 220 K. Importantly, the interlayer exciton emission also possesses robust circular polarization in chiral 2D perovskite/monolayer WSe2 heterostructures, which can be applied to manipulate the valley degree of freedom for valleytronic devices. Our findings would provide a favorable platform to explore interlayer coupling and related physical processes in 2D perovskites and TMDs and further provoke more investigations into the understanding and controlling of excitonic effects and associated optoelectronic applications in van der Waals heterostructures over a broad-range spectral response.