Polarization-sensitive photodetection in a broad spectrum range is highly desired due to the great significance in military and civilian applications. Palladium diselenide (PdSe2), a newly explored air-stable, group 10 two-dimensional (2D) noble metal dichalcogenide with a puckered pentagonal structure, holds promise for polarization-sensitive photodetection. Herein, we report a highly polarization-sensitive, broadband, self-powered photodetector based on graphene/PdSe2/germanium heterojunction. Owing to the enhanced light absorption of the mixed-dimensional van der Waals heterojunction and the effective carrier collection with graphene transparent electrode, the photodetector exhibits superior device performance in terms of a large photoresponsivity, a high specific detectivity, a fast response speed to follow nanosecond pulsed light signal, and a broadband photosensitivity ranging from deep ultraviolet (DUV) to mid-infrared (MIR). Significantly, highly polarization-sensitive broadband photodetection with an ultrahigh polarization sensitivity of 112.2 is achieved, which represents the best result for 2D layered material-based photodetectors. Further, we demonstrated the high-resolution polarization imaging based on the heterojunction device. This work reveals the great potential of 2D PdSe2 for high-performance, air-stable, and polarization-sensitive broadband photodetectors.

We report on the simple fabrication of monolayer graphene (MLG)/germanium (Ge) heterojunction for infrared (IR) light sensing. It is found that the as-fabricated Schottky junction detector exhibits obvious photovoltaic characteristics, and is sensitive to IR light with high Ilight/Idark ratio of 2 × 10(4) at zero bias voltage. The responsivity and detectivity are as high as 51.8 mA W(-1) and 1.38 × 10(10) cm Hz(1/2) W(-1), respectively. Further photoresponse study reveals that the photovoltaic IR detector displays excellent spectral selectivity with peak sensitivity at 1400 nm, and a fast light response speed of microsecond rise/fall time with good reproducibility and long-term stability. The generality of the above results suggests that the present MLG/Ge IR photodetector would have great potential for future optoelectronic device applications.

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 new Schottky junction ultraviolet photodetector (UVPD) is fabricated by coating a free-standing ZnO nanorod (ZnONR) array with a layer of transparent monolayer graphene (MLG) film. The single-crystalline [0001]-oriented ZnONR array has a length of about 8-11 μm, and a diameter of 100∼600 nm. Finite element method (FEM) simulation results show that this novel nanostructure array/MLG heterojunction can trap UV photons effectively within the ZnONRs. By studying the I-V characteristics in the temperature range of 80-300 K, the barrier heights of the MLG film/ZnONR array Schottky barrier are estimated at different temperatures. Interestingly, the heterojunction diode with typical rectifying characteristics exhibits a high sensitivity to UV light illumination and a quick response of millisecond rise time/fall times with excellent reproducibility, whereas it is weakly sensitive to visible light irradiation. It is also observed that this UV photodetector (PD) is capable of monitoring a fast switching light with a frequency as high as 2250 Hz. The generality of the above results suggest that this MLG film/ZnONR array Schottky junction UVPD will have potential application in future optoelectronic devices.

The high-performance broadband photodetectors have attracted intensive scientific interests due to their potential applications in optoelectronic systems. Despite great achievements in two-dimensional (2D) materials based photodetectors such as graphene and black phosphorus, obvious disadvantages such as low optical absorbance and instability preclude their usage for the broadband photodetectors with the desired performance. An alternative approach is to find promising 2D materials and fabricate heterojunction structures for multifunctional hybrid photodetectors. In this work, 2D WS 2 /Si heterojunction with a type-II band alignment is formed in situ. This heterojunction device produced a high I on /I off ratio over 10, 6 responsivity of 224 mA/W, specific detectivity of 1.5 × 10 12 Jones, high polarization sensitivity, and broadband response up to 3043 nm. Furthermore, a 4 × 4 device array of WS 2 /Si heterojunction device is demonstrated with high stability and reproducibility. These results suggest that the WS 2 /Si type-II heterojunction is an ideal photodetector in broadband detection and integrated optoelectronic system.