Hongjun Liu

The performance of two-dimensional transition metal dichalcogenide (TMD) heterostructures is profoundly influenced by their interlayer coupling with modulating twist angles. However, it is still a great challenge to achieve thermodynamically unfavorable twisted TMD heterostructures. Herein, WS2/MoS2 heterostructures with unfavorable twist angles from 0° to 100° were directly synthesized by a spatially confined liquid-precursor-assisted CVD growth. Spectroscopic analyses reveal that non-0° twisted heterostructures exhibit weakened interlayer coupling with the redshift of the Raman A1g mode. Low-temperature photoluminescence indicates the resonance energy transfer from the MoS2 B exciton to the WS2 A exciton, enhancing WS2 emission. The thermodynamically unfavorable (20°, 30°, and 90°) twisted heterostructures are found to possess much larger interfacial work function difference and longer exciton lifetimes, indicating more efficient interfacial charge transfer. This work clarifies the fundamental role of twist angle in modulating the built-in electric field and exciton dynamics of WS2/MoS2 heterostructures and underscores their great potential for practical optoelectronic applications.

The bulk photovoltaic effect (BPVE) provides a promising route toward bias-free, high-efficiency photodetection, yet its realization in low-dimensional anisotropic semiconductors remains largely unexplored. Here, we report a pronounced intrinsic BPVE in one-dimensional tellurium (Te) nanowires and exploit it for self-powered photodetection. A photodetector based on a single Te nanowire exhibits broadband photoresponse from 405 to 1550 nm under zero external bias, with a responsivity of 5.9 mA/W, a detectivity of 1.85 × 1011 Jones at 1550 nm, a power of 0.01 mW/cm2, and a polarization anisotropy ratio (PAR) of up to 2.33 (1550 nm, 0.15 mW/cm2). Benefiting from its strong polarization sensitivity, the device further enables polarization-encoded optical communication and polarization-resolved photocurrent imaging without external bias. These results establish Te nanowires as a promising platform for self-powered and highly integrated polarization-sensitive optoelectronics.

The interfacial processes of two-dimensional van der Waals heterostructures (vdWHs) are of critical importance to their ultrafast carrier transfer dynamics and device performances as optoelectronic and valleytronic devices. Herein, the intricate interfacial dynamics and valley polarization behavior in WS2/GaSe vdWHs have been studied. Charge transfer (CT) is suggested to be the major interfacial process governing the photocarrier dynamics in this WS2/GaSe heterostructure (HS) by the control experiments, in which an 11-nm h-BN spacer between WS2 and GaSe effectively confirms the pronounced effect of the CT process on the dynamics of the HS. Considering spectral overlap between WS2 and GaSe excitons and multilayer interference effects, contribution from non-radiative energy transfer cannot be fully excluded. Furthermore, valley-resolved measurements reveal that the CT process drastically shortens the valley polarization lifetime to 151 fs in WS2/GaSe compared to 362 fs in monolayer WS2. This acceleration of valley depolarization may be interpreted by spin-degenerate valence bands in GaSe arising from its weak spin–orbit coupling. Acting as interlayer intermediate states, this valence band could facilitate an indirect carrier scattering pathway of “WS2 K valley → GaSe valence band → WS2 K′ valley.” This valley lifetime partially recovers when CT is blocked by h-BN insertion, directly correlating accelerated valley depolarization with interfacial CT. This work gains deep insights into the relationship between the charge separation and valley polarization dynamics in vdWHs, which is crucial for designing future valleytronic and optoelectronic devices.

Tailoring nonlinear optical (NLO) crystal with a large bandgap and wide transparency extending from ultraviolet (UV) to mid-infrared (IR) wavelengths remains a challenge due to the inherent trade-offs in these properties. Here, we report two novel scandium tellurite NLO crystals, AScTe₂O₆ (A=K, Rb), engineered through a synergistic strategy that optimizes both electron distribution and lattice vibrations. These isostructural crystals feature a unique 2D topological framework analogous to KBBF, built from distorted [TeO₃] pyramids and [ScO₆] octahedra. Notably, RbScTe₂O₆ achieves an ultrawide bandgap of 4.43 eV with a UV cutoff at 236 nm, the shortest reported among known NLO tellurates, coupled with an extended IR transparency beyond 7.0 µm. Moreover, it affords a compelling combination of a high laser-induced damage threshold of 27.8×AgGaS₂, a strong phase-matchable strong harmonic generation response of 2.3×KDP, and a large birefringence of 0.19 at 1064 nm. The highly polarizable [TeO₃] pyramids and well-aligned [ScO₆] octahedra collectively dominate the optical anisotropy and NLO activity, as evidenced by first-principles calculations and dipole moment analyses. These outstanding attributes establish RbScTe₂O₆ as a highly promising candidate for NLO applications in wide wavelength ranges.

Semiconductor heterojunctions composed of stacked two-dimensional (2D) materials hold great promise for electronics and optoelectronic devices. Hexagonal boron nitride (h-BN) serves as the encapsulation layer that enhances the optical properties of heterojunctions by suppressing interfacial charge scattering and enabling quantum-confined energy transfer. Nevertheless, a mechanistic understanding of the modulation effect of interlayer coupling and interfacial charge dynamics in these heterostructures remains insufficient. Here, we report the in situ growth of ultrathin MoS₂ flakes encapsulated within porous h-BN fibers and construct high-quality h-BN/MoS₂ heterojunctions, demonstrating defect-mediated interactions and tunable optoelectronic properties. The cross-sectional architecture with the encapsulating sites in porous h-BN confirmed intimate interlayer contact, and investigations via femtosecond transient reflectance spectroscopy revealed the exciton radiative lifetime and defect-mediated charge transfer in the heterojunctions. The photoexcited carriers transfer between long-lived defect states in h-BN and the MoS₂, which induces the defect-related PL quenching in h-BN and prolonged radiative lifetime of MoS₂ excitons by 3-fold. This was attributed to defect-mediated charge transfer in MoS₂ and the dielectric screening from h-BN. This work elucidates the critical role of defect-engineered interfacial charge transfer in h-BN/MoS₂ heterostructures, offering valuable insights for advancing optoelectronic devices based on 2D heterojunctions.

2D transition metal dichalcogenides (TMDs) have received extensive attention due to their tunable electronic and optical properties. Trivalent rare earth ions doping can be used as luminescence activation centers to significantly improve their luminescence efficiency. Nevertheless, few studies are reported on the investigation of rare‐earth dual‐doped TMDs homojunctions. Herein, Er‐MoS₂/Yb‐MoS₂ dual‐doped homojunctions with different twist angles are constructed by the mechanically transferred stacking. Unlike pure MoS₂ bilayers or other MoS₂ heterojunctions that suffer from photoluminescence (PL) quenching, the constructed dual‐doped MoS₂ homojunctions with specific twist angle exhibit a prominent PL peak with three‐fold increase of intensity and prolonged exciton lifetime. This unique behavior arises from the formation of localized interlayer excitons induced by two synergistic factors including rare‐earth doping modulating effect and specific stacking angle. The introduction of rare‐earth doping increases localized interlayer exciton formation in doped homojunctions and enhances the radiative recombination and exciton lifetimes. The precise control of stacking angles of doped‐MoS₂ homojunction can effectively modulate the interlayer coupling, enabling optimized conditions for exciton localization. This work establishes an efficient strategy to construct localized interlayer excitons in rare‐earth doped 2D materials for the junctions, but also provides fundamental insights into the design of high‐performance light‐emitting devices.

Interfacial carrier behavior is vital for modifying the optoelectronic performance of 2D materials with atomical thickness, yet understanding exciton dynamics within hetero‐phase heterojunctions at the nanoscale remains elusive. Here, carrier dynamics at the interface in molecular beam epitaxy grown 1T PtSe ₂ –1H MoSe ₂ heterostructures are revealed by ultrafast pump‐probe spectroscopy, and the corresponding mechanisms are studied by combining scanning tunneling microscopy/spectroscopy (STM/STS). The difference in exciton lifetimes and signal intensities in the heterostructures at the energies larger and narrower than the bandgap of MoSe ₂ demonstrates both electrons and holes transfer at the interface of PtSe ₂ and MoSe ₂ monolayers. Such bipolar transfer is interpreted through both the type‐I (lateral) and type‐II (vertical) band alignments revealed by STS measurements across the heterojunctions interface. Besides, it also shows that the band alignment modification of in‐plane heterojunctions by a specific conductive substrate can fairly alter the exciton dynamics at the interface. This work provides a comprehensive insight into the carrier dynamics in PtSe ₂ –MoSe ₂ heterostructures with high spatial and temporal resolutions and may be beneficial for their design in optoelectronic devices.

We synthesized centimeter-sized Weyl semimetal layered PtBi ₂ crystals, which indicate that the hydrogen evolution activity of the synthesized Weyl semimetal crystals significantly exceeds that of commercial Pt/C catalysts.