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Stacking atomically thin films enables artificial construction of van der Waals heterostructures with exotic functionalities such as superconductivity, the quantum Hall effect, and engineered light-matter interactions. In particular, heterobilayers composed of monolayer transition metal dichalcogenides have attracted significant interest due to their controllable interlayer coupling and trapped valley excitons in moiré superlattices. However, the identification of twist-angle-modulated optical transitions in heterobilayers is sometimes controversial since both momentum-direct (K–K) and -indirect excitons reside on the low energy side of the bright exciton in the monolayer constituents. Here, we attribute the optical transition at ∼1.35 eV in the WS2/WSe2 heterobilayer to an indirect Γ–K transition based on a systematic analysis and comparison of experimental photoluminescence spectra with theoretical calculations. The exciton wavefunction obtained by the state-of-the-art GW-Bethe-Salpeter equation approach indicates that both the electron and hole of the excitons are contributed by the WS2 layer. Polarization-resolved k -space imaging further confirms that the transition dipole moment of this optical transition is dominantly in-plane and is independent of the twist angle. The calculated absorption spectrum predicts that the so-called interlayer exciton peak coming from the K–K transition is located at 1.06 eV, but with a much weaker amplitude. Our work provides new insight into the steady-state and dynamic properties of twist-angle-dependent excitons in van der Waals heterostructures. [ABSTRACT FROM AUTHOR]

Monolayer transition‐metal dichalcogenides grown by chemical vapor deposition (CVD) always contain certain types of defects that dramatically affect their electronic and optical properties. For CVD‐grown hexagonal WS2 monolayer, complex photoluminescence (PL) patterns are commonly observed, but the defect‐related optical mechanisms are still not well understood. Here, by combining the optical and structural characterizations and ab initio calculations, the correlation between the patterned PL emission and the details of defects in CVD‐grown hexagonal WS2 monolayer are revealed. The temperature‐dependent PL spectra show the correlation between the defect‐trapped and band‐edge exciton emission. The high‐resolution scanning transmission electron microscopy identifies the positive correlation between the density of WSx‐vacancy and PL intensity. In the end, the ab initio calculations and molecule adsorption PL spectra show that the coexistence of p‐ and n‐doping effects, caused by the W and S complex vacancy, weakens the modulation of molecular adsorption on PL intensity. This work gives new insights into the origin of the inhomogeneous PL distribution in WS2 monolayer, which provides important guidance in the regulation of electronic and optical properties of transition‐metal dichalcogenides via defect engineering. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) transition metal dichalcogenide (TMD) has emerged as an effective optoelectronics material due to its novel optical properties. Understanding the role of defects in exciton kinetics is crucial for achieving high-efficiency TMD devices. Here, we observe defects induced anomalous power dependence exciton dynamics and spatial distribution in hexagonal heterogeneous WS2. With transient absorption microscopy study, we illustrate that these phenomena originate from the competition between radiative and defect-related non-radiative decays. To understand the physics behind this, a decay model is introduced with two defect-related channels, which demonstrates that more excitons decay through non-radiative channels in the dark region than the bright region. Our work reveals the mechanisms of anomalous exciton kinetics by defects and is instrumental for understanding and exploiting excitonic states in emerging 2D semiconductors. [ABSTRACT FROM AUTHOR]

Surface plasmon is promising for applications in 2-D semiconductor photodetector due to their great potential to improve light absorption of 2-D semiconductor, where the poor light absorption of 2-D semiconductor limits the performance of 2-D photodetector to some extent. However, high-energy deposition processes of metal surface plasmon damage the surface crystal lattice of 2-D semiconductor, which hampers the current transport in 2-D semiconductor. Here, we report the cladded surface-plasmon-enhanced black-phosphorus (BP) photodetector with damage-free metal–semiconductor interface. Without the adverse effect of the direct deposition process for metal surface plasmon, the cladded surface-plasmon-enhanced BP photodetector exhibits a superior photoresponse enhancement by up to 1000%, which exceeds other conventional surface-plasmon-enhanced 2-D semiconductor photodetectors. Our results provide guidelines to design advanced surface-plasmon-enhanced 2-D semiconductor photodetector for practical application. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) semiconductors have the potential to revolutionize future electronics. However, large Schottky barriers exist between 2D semiconductors and metals due to the Fermi level pinning effect, which seriously limits their applications. Here, we demonstrate ohmic contacts to MoS2 by utilizing transparent ZnO:Al (AZO) conductors without forming any contact barriers. Benefiting from the creation of a van der Waals junction in which the atomic precursors react with the physically adsorbed H2O on the MoS2 surface, a nondestructive interface between AZO and MoS2 can be made without any interfacial chemical interactions and disorder. An ideal AZO–MoS2 ohmic junction, which overcomes the Schottky–Mott limit, is achieved by work function matching for the first time. More importantly, this ideal ohmic junction, to our knowledge, yields so far the highest mobility (26.9 cm2 V−1 s−1), fastest switching speed (0.07 V dec−1) and largest optical average transmittance (nearly 90% throughout the visible range) for monolayer MoS2 (grown by chemical vapor deposition) transistors. Thus, our study not only experimentally validates an ideal metal–semiconductor ohmic contact, but also provides a novel material integration method to fabricate high-performance transparent 2D electronic devices. [ABSTRACT FROM AUTHOR]

2D materials have aroused tremendous attention in the last decade, and magnetic materials of the 2D scale are rising to prominence in recent years, which may revolutionize current optical and electronic applications. Photoconductivity is a well‐known optical and electrical property, which should be positive as the conductivity of material increases typically upon the absorption of electromagnetic radiation. Here, a controllable switch fromnegative to positive photoconductivity effect of FePS3 nanosheets is enabledby the modulation of the excitation wavelengths. These effects originate from an ultrafast process of hot carrier trapping, as visualized and investigated by the transient absorption microscopy at a quantitative level. This hot carrier trapping process is about 1.25 ps resulting in a lower diffusion constant state (≈3.5 cm2 s−1), which provides deep insight into the intrinsic properties of 2D FePS3 for further study. The results demonstrate experimentally the possibility to tune the electronic properties of 2D magnetic materials by an optical way, which not only induces the change in the magnitude but also in the sign, leading to more understandings and possibilities in 2D optoelectronic devices with many unexpected and multifunctional properties. [ABSTRACT FROM AUTHOR]

Monolayer transition‐metal dichalcogenides grown by chemical vapor deposition (CVD) always contain certain types of defects that dramatically affect their electronic and optical properties. For CVD‐grown hexagonal WS2 monolayer, complex photoluminescence (PL) patterns are commonly observed, but the defect‐related optical mechanisms are still not well understood. Here, by combining the optical and structural characterizations and ab initio calculations, the correlation between the patterned PL emission and the details of defects in CVD‐grown hexagonal WS2 monolayer are revealed. The temperature‐dependent PL spectra show the correlation between the defect‐trapped and band‐edge exciton emission. The high‐resolution scanning transmission electron microscopy identifies the positive correlation between the density of WSx‐vacancy and PL intensity. In the end, the ab initio calculations and molecule adsorption PL spectra show that the coexistence of p‐ and n‐doping effects, caused by the W and S complex vacancy, weakens the modulation of molecular adsorption on PL intensity. This work gives new insights into the origin of the inhomogeneous PL distribution in WS2 monolayer, which provides important guidance in the regulation of electronic and optical properties of transition‐metal dichalcogenides via defect engineering. [ABSTRACT FROM AUTHOR]