applications, [4,[17][18][19] and therefore have aroused enthusiasm in researchers. Due to their high crystal structure symmetry, graphene, which has been well studied, and most TMDCs exhibit an inplane isotropic feature. However, a few 2D materials with a surprisingly low lattice symmetry, such as BP, tin selenide (SnSe), [20][21][22][23] gallium telluride (GaTe), [24] and rhenium disulfide (ReS 2 ), [17,[25][26][27][28] can also show significant anisotropic in-plane optical, electrical, and thermal properties. [17,20,[23][24][25]28] For instance, the charge carrier mobility, [18,29] photoemission, [30] and thermoelectric figure of merit (ZT) [31][32][33] of BP along the armchair direction are larger than those in the zigzag direction. In addition to anisotropy, BP also has a tunable thickness-dependent direct bandgap ranging from 0.3 to 1.5 eV, [12,13] filling the space between zero-gap graphene [1,2] and large-gap (1-2.5 eV) TMDCs. [4,34] Additionally, BP has a high carrier mobility (≈10 3 cm 2 V −1 s −1 ) [18] compared with TMDCs (10-200 cm 2 V −1 s −1 ). [4] Therefore, BP has been viewed as an alluring and ideal candidate for applications in field-effect transistors (FETs), [18,35] fast-response optical switches, [36] photovoltaic devices, [37] mid-infrared polarizers, and polarization sensors, [29] owing to its distinguished physical properties. Although BP exhibits great potential for various anisotropic optical, electronic, and optoelectronic high-performance devices, it degrades within a short time when exposed to oxygen and water vapor in air, causing difficulties in practical applications. Consequently, it is highly important to explore BP-like materials with appropriate properties including a narrow bandgap, high carrier mobility, air stability, and low cost.Regarded as a promising alternative to BP, SnSe also consists of a puckered honeycomb layered crystal structure similar to that in BP, exhibiting highly anisotropic valence bands, [23] a crystal-orientation-dependent high charge carrier mobility (≈10 3 cm 2 V −1 s −1 ), [38] and linear optical absorption. [39] Additionally, due to the narrow-bandgap semiconductor nature, the indirect bandgap of SnSe is ≈0.9 eV, [39][40][41][42] whereas its direct bandgap is ≈1.3 eV, [40,42] leading to optical transitions of SnSe The deceptively simple tin selenide (SnSe) film has emerged as an appealing 2D material with a narrow bandgap, high charge carrier mobility, and significant thermoelectric figure of merit. In particular, compared with most commonly investigated 2D materials, SnSe with a puckered honeycomb structure possesses a lower lattice symmetry, resulting in prominent in-plane anisotropy. Herein, with polarization-dependent Raman spectroscopy and polarization-dependent nonlinear absorption measurements, pronounced polarization-dependent nonlinear optical properties of a SnSe flake are demonstrated originating from the anisotropic optical transition probability of SnSe, which is confirmed by ultrafast polarization-dependent pump-probe experiments. Furt...

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Electron-phonon scattering is the key process limiting the efficiency of modern nanoelectronic and optoelectronic devices, in which most of the incident energy is converted to lattice heat and finally dissipates into the environment. Here, we report an acoustic phonon recycling process in graphene-WS 2 heterostructures, which couples the heat generated in graphene back into the carrier distribution in WS 2. This recycling process is experimentally recorded by spectrally resolved transient absorption microscopy under a wide range of pumping energies from 1.77 to 0.48 eV and is also theoretically described using an interfacial thermal transport model. The acoustic phonon recycling process has a relatively slow characteristic time (>100 ps), which is beneficial for carrier extraction and distinct from the commonly found ultrafast hot carrier transfer (~1 ps) in graphene-WS 2 heterostructures. The combination of phonon recycling and carrier transfer makes graphene-based heterostructures highly attractive for broadband high-efficiency electronic and optoelectronic applications.

Van der Waals (vdWs) heterostructures based on in-plane isotropic/anisotropic 2D-layered semiconducting materials have recently received wide attention because of their unique interlayer coupling properties and hold a bright future as building blocks for advanced photodetectors. However, a fundamental understanding of charge behavior inside this kind of heterostructure in the photoexcited state remains elusive. In this work, we carry out a systematic investigation into the photoinduced interfacial charge behavior in type-II WS2/ReS2 vertical heterostructures via polarization-dependent pump–probe microscopy. Benefiting from the distinctive (ultrafast and anisotropic) charge-transfer mechanisms, the photodetector based on the WS2/ReS2 heterojunction displays more superior optoelectronic properties compared to its constituents with diverse functionalities including moderate photoresponsivity, polarization sensitivity, and fast photoresponse speed. Additionally, this device can function as a self-driven photodetector without the external bias. These results of our work tangibly corroborate the intriguing interlayer interaction in in-plane isotropic/anisotropic heterostructures and are expected to shed light on designing balanced-performance multifunctional optoelectrical devices.

Engineering 2D‐structured lead halide perovskites leads to both excellent optoelectronic performance and intriguing photophysics. Here, the underlying mechanisms of exciton interaction and carrier transfer in 2D/3D hybrid perovskites are studied. The investigated perovskites are in the form of self‐assembled microplatelets, which are naturally composed of multiple perovskite phases, with n being 1, 2, 3, or 4 or nanocrystals in bulk phase (n ≈ ∞). Excitonic energy transfer and charge separation between different phases are found to coexist in this hybrid system, which occur at an ultrafast timescale (<100 fs) followed by a relatively slow channel (2–15 ps). The experimental results reveal that this hybrid perovskite naturally forms a series of “heterostructures,” with excitons generated in different phases, showing Coulomb interactions across the interface. This interlayer Coulomb coupling should account for the aforementioned ultrafast carrier transfer. This work provides an accurate and thorough explanation for the remarkable charge transfer rate, which is extremely beneficial for their applications in photovoltaic and optoelectronic devices, even with the apparent interfacial scattering, defect trapping, and disorder‐induced exciton localization in 2D/3D hybrid perovskites.