Semiconducting p–n heterojunctions, serving as the basic unit of modern electronic devices, such as photodetectors, solar-energy conversion devices, and light-emitting diodes (LEDs), have been extensively investigated in recent years. In this work, high performance self-powered broad-band photodetectors were fabricated based on vertically stacked p–n heterojunctions though combining p-type WSe2 with n-type Bi2Te3 via van der Waals (vdW) epitaxial growth. Devices based on the p–n heterojunction show obvious current rectification behaviors in the dark and superior photovoltaic characteristics under light irradiation. A maximum short circuit current of 18 nA and open circuit voltage of 0.25 V can be achieved with the illumination light of 633 nm (power density: 26.4 mW/cm2), which are among the highest values compared with the ever reported 2D vdW heterojunctions synthesized by chemical vapor deposition (CVD) method. Benefiting from the broad-band absorption of the heterostructures, the detection range can be expanded from the visible to near-infrared (375–1550 nm). Moreover, ascribing to the efficient carriers separation process at the junction interfaces, the devices can be further employed as self-powered photodetectors, where a fast response time (∼210 μs) and high responsivity (20.5 A/W at 633 nm and 27 mA/W at 1550 nm) are obtained under zero bias voltage. The WSe2/Bi2Te3 p–n heterojunction-based self-powered photodetectors with high photoresponsivity, fast photoresponse time, and broad spectral response will find potential applications in high speed and self-sufficient broad-band devices.

Two-dimensional (2D) heterostructures have aroused widespread attentions due to the fascinating properties originating from the interfaces and the derived potential applications in modern electronics and optoelectronics. The interfacial band alignment engineering of 2D heterostructures would open up promising routes toward the flexible design and optimization of the electronic and optoelectronic properties. Herein, we report a one-step chemical vapor deposition method for the growth of band alignment continuously modulated WS-WSSe (0 < x ≤ 1) monolayer lateral heterostructures, with atomically sharp interfaces at the junction area. Local photoluminescence (PL) and Raman measurements demonstrate the position-dependent composition and band gap information on the as-grown nanosheets. Kelvin probe force microscopy (KPFM) investigations further confirm the tunable band alignments in the heterostructures, where a continuously decreased Fermi level difference between the core and the shell regions is observed with the x value varied from 1 to 0. The direct growth of high-quality atomic-level junctions with controllable band alignment marks an important step toward the potential applications of 2D semiconductors in integrated electronic and optoelectronic devices.

2D layered 3-rhombohedral (3R) phase transition metal dichalcogenides (TMDs) have received significantly increased research interest in nonlinear optical applications due to their unique crystal structures and broken inversion symmetry. However, controlled growth of 2D 3R phase TMDs still remains a great challenge. In this work, a direct growth of large-area WS 2 and WSe 2 atomic layers with controllable crystal phases via a developed temperature selective physical vapor deposition route is reported. Large-area triangular 3R phase layers are synthesized at a lower deposition temperature. Steady state and timeresolved photoluminescence spectroscopy and Raman spectroscopy are used to study the unique properties of 3R phase layers due to different layer stacking and interlayer coupling. More importantly, with broken inversion symmetry, 3R phase layers show a quadratically increased second harmonic generation (SHG) intensity with respect to layer numbers. Furthermore, by polarizationresolved SHG, a uniform polarization preference is observed in bilayer and trilayer 3R phase WS 2 , which could be a benefit for practical applications. The results not only contribute to the controlled growth of 2D TMDs layers with different phases but also pave the way to promising nonlinear optical devices.

No abstract

2D vertical van der Waals (vdW) heterostructures with atomically sharp interfaces have attracted tremendous interest in 2D photonic and optoelectronic applications. Band alignment engineering in 2D heterostructures provides a perfect platform for tailoring interfacial charge transfer behaviors, from which desired optical and optoelectronic features can be realized. Here, by developing a two‐step chemical vapor deposition strategy, direct vapor growth of monolayer PbI 2 on monolayer transition metal dichalcogenides (TMDCs) (WS 2 , WSe 2 , or alloying WS 2(1− x ) Se 2 x ), forming bilayer vertical heterostructures, is demonstrated. Based on the calculated electron band structures, the interfacial band alignments of the obtained heterostructures can be gradually tuned from type‐I (PbI 2 /WS 2 ) to type‐II (PbI 2 /WSe 2 ). Steady‐state photoluminescence (PL) and time‐resolved PL measurements reveal that the PL emissions from the bottom TMDC layers can be modulated from apparently enhanced (for WS 2 ) to greatly quenched (for WSe 2 ) compared to their monolayer counterparts, which can be attributed to the band alignment–induced distinct interfacial charge transfer behaviors. The band alignment nature of the heterostructures is further demonstrated by the PL excitation spectroscopy and interlayer exciton investigation. The realization of 2D vertical heterostructures with tunable band alignments will provide a new material platform for designing and constructing multifunctional optoelectronic devices.