in optical radar, night vision, military surveillance, and water-quality inspection applications. [1-3] The state-of-theart infrared photodetectors, especially those working in MIR regimes, are fabricated using HgCdTe alloys, [4] InSb, [5] and quantum-wells, [6] which inevitably suffer from strict operation demands, high-cost, and environmental toxicity, thus limiting their widespread usage. [7] Alternatively, two-dimensional (2D) materials have been emerging as ideal candidates for MIR photodetection due to their unique optoelectronic properties and easy integrability. [8] One of the key advantageous features is that the out-of-plane van der Waals (vdW) interaction between layered structures without surface dangling bonds can effectively lower noise from generation-recombination by using the layered materials as main absorbers in MIR regions. [9] For instance, semi-metallic graphene with the ability to absorb light from visible to terahertz enables the design of novel graphene photonic devices that can operate well in mid-wave infrared (MWIR, 3-5 µm), and even in long-wave infrared (LWIR, 8-14 µm) spectral ranges. [10] Unfortunately, the low optical absorption and gapless nature of graphene result in the poor photoresponsivity and large dark current. [11] Although a number of alternative approaches such as integrating quantum dots (QDs), [12] introducing defective states, [13] effective surface doping, [14] and patterning nanoribbon arrays, [15] have been intensively employed to enhance the device performance, they are mainly dominated by uncontrolled processing techniques with time-consuming and complex fabrication procedures. [8] A lately rediscovered black phosphorus (BP) with a large bandgap tunability is widely used for the fabrication of high-sensitivity MIR photodetectors, but its poor air stability leads to the degradation of device performance. Meanwhile, both theoretical and experimental analyses reveal a short cutoff wavelength of ≈3.7 µm for BP-based photodetectors due to its bulk bandgap of ≈0.3 eV, which is far below the second atmospheric window of LWIR photodetection. [16] The existing dilemma stimulates the research community to explore a promising alternative with wide absorption, high air stability, and considerable carrier mobility toward longer-wavelength MIR photodetection. Mid-infrared (MIR) photodetection, covering diverse molecular vibrational regions and atmospheric transmission windows, is vital to civil and military purposes. Versatile use of MIR photodetectors is commonly dominated by HgCdTe alloys, InSb, and quantum superlattices, which are limited by strict operation demands, high-cost, and environmental toxicity. Despite the rapid advances of black phosphorus (BP)-based MIR photodetectors, these are subject to poor stability and large-area integration difficulty. Here, the van der Waals (vdW) epitaxial growth of a wafer-scale 2D platinum ditelluride (PtTe 2) layer is reported via a simple tellurium-vapor transformation approach. The 2D PtTe 2 layer possesses a unique mosaic-like c...

A MoSe2/Si heterojunction photodetector is constructed by depositing MoSe2 film with vertically standing layered structure on Si substrate. Graphene transparent electrode is utilized to further enhance the separation and transport of photogenerated carriers. The device shows excellent performance in terms of wide response spectrum of UV–visible–NIR, high detectivity of 7.13 × 1010 Jones, and ultrafast response speed of ≈270 ns, unveiling the great potential for the heterojunction for high‐performance optoelectronic devices.

Broadband photodetectors are of great importance for numerous optoelectronic applications. Two-dimensional (2D) tungsten disulfide (WS2), an important family member of transition-metal dichalcogenides (TMDs), has shown great potential for high-sensitivity photodetection due to its extraordinary properties. However, the inherent large bandgap of WS2 and the strong interface recombination impede the actualization of high-sensitivity broadband photodetectors. Here, we demonstrate the fabrication of an ultrabroadband WS2/Ge heterojunction photodetector through defect engineering and interface passivation. Thanks to the narrowed bandgap of WS2 induced by the vacancy defects, the effective surface modification with an ultrathin AlO x layer, and the well-designed vertical n–n heterojunction structure, the WS2/AlO x /Ge photodetector exhibits an excellent device performance in terms of a high responsivity of 634.5 mA/W, a large specific detectivity up to 4.3 × 1011 Jones, and an ultrafast response speed. Significantly, the device possesses an ultrawide spectral response spanning from deep ultraviolet (200 nm) to mid-wave infrared (MWIR) of 4.6 μm, along with a superior MWIR imaging capability at room temperature. The detection range has surpassed the WS2-based photodetectors in previous reports and is among the broadest for TMD-based photodetectors. Our work provides a strategy for the fabrication of high-performance ultrabroadband photodetectors based on 2D TMD materials.

2D transition metal dichalcogenides are promising candidates for high‐performance photodetectors. However, the relatively low response speed as well as the complex transfer process hinders their wide applications. Herein, for the first time, the fabrication of a few‐layer MoTe2/Si 2D–3D vertical heterojunction for high‐speed and broadband photodiodes by a pulsed laser deposition technique is reported. Owing to the high junction quality, ultrathin MoTe2 film thickness, and unique vertical n–n heterojunction structure, the photodiode exhibits excellent device performance in terms of a high responsivity of 0.19 A W−1 and a large detectivity of 6.8 × 1013 Jones. The device is also capable of detecting a broadband light with wavelength spanning from 300 to 1800 nm. More importantly, the device possesses an ultrahigh response speed up to 150 ns with a 3‐dB electrical bandwidth approaching 0.12 GHz. This work paves the way toward the fabrication of novel 2D–3D heterojunctions for high‐performance, ultrafast photodetectors.