Fast and Highly Sensitive Ionic‐Polymer‐Gated WS₂–Graphene Photodetectors

Abstract

been shown to operate as high-speed photodetectors [3] with response times comparable to conventional silicon-based devices, but the absence of a bandgap and lack of significant gain mechanism limits their use for ultrasensitive light detection. Hybrid structures of graphene with semiconductor materials such as quantum dots, [4][5][6] chlorophyll molecules, [7] and MoS 2 [8][9][10] have been shown to enhance light absorption and provide an internal gain mechanism. However, these implementations typically have a limited operational bandwidth of less than 10 Hz which hampers their use in real world applications.Slow response times in these systems are produced by the long-lived trapping of charges, often manifested as hysteresis in gate-voltage sweeps. This has been observed in organic, carbon nanotubes, graphene, and more recently in transitionmetal dichalcogenide (TMD) field-effect transistors and is typically attributed to unavoidable intrinsic and/or extrinsic charge traps, e.g., SiO 2 surface states [11][12][13][14] and atmospheric contamination. [12,13,[15][16][17] To reduce the impact of such traps, various solutions have been explored including gate-voltage pulses, [11,18,19] vacuum annealing, [20,21] and ionic-liquid gating. [22,23] Although ionic-liquid gating has been utilized in WS 2 phototransistors [24] and MoTe 2 -graphene photodetectors, [25] the beneficial effect of poly mer gating on the performance of photodetectors consisting of atomically thin heterostructures has not yet been explored.In this work, we report the first study of WS 2 -graphene heterostructure photodetectors with an ionic-polymer gate. We demonstrate a gate-tunable responsivity up to 10 6 A W −1 , which is comparable with other heterostructure devices, [4][5][6][7]9,10] and surpasses that of graphene or TMD photodetectors by at least four orders of magnitude. Our devices reach a −3 dB bandwidth of 1.5 kHz, without the need for gatevoltage pulses, leading to sub-millisecond rise and fall times. The observed 10 3 -fold increase of photodetection bandwidth, when compared to other heterostructure photodetectors, is enabled by the enhanced screening properties of the mobile ions in our ionic polymer top gate, which act to compensate the charge traps limiting the speed of previous devices. Our devices have a detectivity of D* = 3.8 × 10 11 Jones, which is approaching that of single-photon counters, and are able to operate on a broad spectral range (400-700 nm). These properties make ionic-polymer-gated WS 2 -graphene photodetectors highly suitable for video-frame-rate imaging applicationsThe combination of graphene with semiconductor materials in heterostructure photodetectors enables amplified detection of femtowatt light signals using micrometer-scale electronic devices. Presently, long-lived charge traps limit the speed of such detectors, and impractical strategies, e.g., the use of large gatevoltage pulses, have been employed to achieve bandwidths suitable for applications such as video-frame-rate imaging. Here, atomically thin gr...

Design strategies for two‐dimensional material photodetectors to enhance device performance

“…Some relatively 2D materials also act as main absorbing layer and trapping layer at the same time. Mehew et al reported a all 2D materials photogating photodetectors with an ionic‐polymer gate to achieve a photoresponsivity of 10 6 A/W …”

“…For achieving high interfacial charge transfer efficiency, a series thickness of C 8 ‐BTBT less than exciton diffusion length are investigated in Figure D with a responsivity of 1.57 × 10 4 A/W (a gain of 10 8 ). The detection range of single layer enhanced device always limited by absorption spectrum of active layer mostly range from 400 to 700 . Therefore, the NIR sensitive materials in combination with 2D materials has raised a huge wave.…”

“…Incorporating light-absorbing semiconductors with 2D materials can realize high G photodetectors. At earlier time, most works focus on single light-sensitive layer, including QDs, [73][74][75] 2D materials, 70,76,77 organic polymers/small molecules, 57,78,79 single crystals, 80 perovskites, [81][82][83] SWNTs, 84 and topological insulator. 73 The single layer enhanced graphene photoconductors consist of one semiconductor light absorbing layer.…”

Asymmetrically Encapsulated Vertical ITO/MoS2/Cu2O Photodetector with Ultrahigh Sensitivity

“…Such low‐cost material deposition, coupled with excellent light absorption and moderate carrier transport properties, makes these layered TMD materials promising candidates for sensitive and inexpensive photodetection applications. While large responsivity has been demonstrated in layered material based photodetectors, in general, there are two important bottlenecks that limit the practical applicability of these devices. First, the response of many of the 2D devices is slow owing to long lived charge trapping in the active material as well as in the substrate.…”

Role of defect states in functionalized graphene photodetectors

“…[5][6] [7] However, in these extrinsic strategies the spectral range of the photodetector is limited by the resonance of the plasmonic structure, or the absorption edge of the semiconductor which is typically in the visible, hindering their use in broadband photodetection applications. [3] Intrinsic strategies also exist whereby a band gap is opened in graphene through functionalization with adatoms, such as fluorine and hydrogen.…”