Fig. 1. Two-dimensional materials covering broad spectrum. (a) Schematics of structure and bandgap of boron nitride, transition metal dichalcogenides and graphene; (b) spectrum range from ultraviolet to far-infrared where optical communication waveband is in near-infrared range

Fig. 2. Graphene-based photodetectors for optical communications. (a) Schematic and scanning electron image (inset) of metal/graphene/metal (MGM) photodetectors with asymmetry metal contacts^[53]; (b) schematic of graphene-based photodetector integrated on silicon optical waveguide with asymmetry contact^[55]; (c) structural diagram of graphene-based photodetector on silicon substrate where silica between electrodes and substrate ensures collected carriers originating only from graphene^[56]; (d) photocurrent and responsivity as functions of power under 1550 nm illumination^[56]; (e) rising-edge photocurrent response as a function of time under 632 nm and 1550 nm illuminations, respectively^[56]; (f) photocurrent as a function of time on graphene nanoribbon-based devices covered with and without hafnium oxide under 1470 nm illumination^[57]

Fig. 3. BP-based photodetectors for optical communications. (a) Responsivity as a function of power density under 532 nm and 1550 nm illumination, respectively^[41]; (b) BP photodetectors integrated on silicon optical waveguide where few-layer graphene is as top gate^[40]

Fig. 4. Tellurium-based photodetectors for optical communications. (a) Structural diagram of tellurium photodetector based on optical cavity for short-wave infrared detection ^[59]; (b) responsivity as a function of wavelength under Al₂O₃ with different thicknesses^[59]; (c) photocurrent as a function of power under 1550 nm illumination where fitting slope is 19.2 mA·W^-1[60]; (d) structural diagram of photodetector based on tellurium^[61]; (e) change rate of photoconductance as a function of time under 1550 nm illumination with power density of 150 mW·cm^-2[61]; (f) rising-edge photoconductance response as a function of time under 1550 nm illumination with falling-edge photoconductance response as a function of time shown in inset^[61]

Fig. 5. Germanium-, bismuth-, and arsenium-based photodetectors for optical communications. (a) Schematic of resonant-photonic crystal germanium photodetector^[62]; (b) measured EQE as a function of wavelength for 30-μm-diameter patterned photodetectors with hole-array (hole diameter of 300 nm) period of 520-560 nm^[62]; (c) 3D schematic of Bi photodetector^[63]; (d) normalized responsivity for Bi photodetector as a function of wavelength^[63]; (e) schematic of As-Si photodetector^[64]; (f) EQE and responsivity for photodetector as functions of wavelength^[64]

Fig. 6. MoS₂- and MoTe₂-based photodetectors for optical communications. (a) 3D schematic of triple-layer MoS₂ photodetector^[65]; (b) responsivity of MoS₂ photodetector as a function of excitation wavelength^[65]; (c) schematic of graphene/MoTe₂/Au photodetector stacked onto silicon waveguide,which is encapsulated by large h-BN flake^[66]; (d) wavelength dependence of responsivity for 19.5 nm thick and 50 nm thick photodetectors^[66]; (e) frequency dependence of responsivity for MoTe₂ photodetector with measurement setup shown in inset ^[66]; (f) schematic of silicon microring resonator-integrated MoTe₂ photodetector (radius of 40 μm, height of 220 nm, width of 500 nm)^[67]; (g) responsivity and EQE as functions of bias voltage for two devices (device 1 with thickness of 40 nm and coverage length of 15 μm and device 2 with thickness of 60 nm and coverage length of 30.7 μm) with magnified responsivity and EQE curves for device 1 shown in inset ^[67]; (h) photoresponse of type II Weyl semimetal MoTe₂ photodetector under different excitation wavelengths^[68]

Fig. 7. Noble metal dichalcogenides-based photodetectors for optical communications. (a) Schematic of palladium selenide-based photodetector integrated on silicon optical waveguide^[69]; (b) responsivity as a function of bias voltage under 1550 nm illumination^[69]; (c) photocurrent as a function of detecting wavelength under light power of 1 mW and bias voltage of 5 V^[69]; (d) schematic of palladium selenide-based photodetector integrated with waveguide^[70]; (e) frequency response curves of three palladium selenide photodetectors under bias of 3 V^[70]; (f) responsivity and external quantum efficiency of three photodetectors as functions of bias voltage^[70]; (g) schematic of palladium selenide photodetector on gold/titanium dioxide optical cavity substrate^[11]; (h) photocurrent response as a function of time under 532 nm and 1550 nm illuminations, respectively^[11]; (i) responsivity on platinum telluride photodetector as a function of wavelength^[71]

Fig. 8. Bismuth selenide-based photodetectors for optical communications^[27]. (a) Schematic of photodetector based on bismuth selenide; (b) photocurrent as a function of time under bias voltage of 1 V and light power of 142.93 mW/cm² ; (c) responsivity and detectivity as functions of temperature; (d) rising time and decay time as functions of temperature

Fig. 9. Mo₂C-based photodetectors for optical communications^[73]. (a) Schematic of photodetectors based on MoS₂/p-Mo₂C hybrid structure under light illumination; (b) wavelength dependence of light-to-dark current ratio for MoS₂ and MoS₂/p-Mo₂C (P=1000 nm) photodetectors; (c) wavelength dependences of responsivity and light-to-dark current ratio for MoS₂ and MoS₂/mp-Mo₂C photodetectors

Fig. 10. Three types of energy band structures of heterojunctions

Fig. 11. vdWs heterojunction-based photodetectors for optical communications. (a) Schematic of device based on BP/MoS₂ vdWs heterojunction^[74]; (b) photocurrent response as a function of time at different bias voltages under incident light wavelength of 1550 nm and power of 96.2 μW^[74]; (c) transfer curves under 1550 nm illumination with different powers^[74]; (d) schematic of interlayer excitation process in MoTe₂/MoS₂ vdWs type II heterojunction^[76]; (e) schematic of type II energy alignment and interlayer transition mechanism in GaTe/InSe vdWs heterojunction where E_g-p and E_g-n are bandgaps of p-GaTe and n-InSe, respectively^[77]; (f) responsivity on GaTe, InSe and GaTe/InSe heterojunction devices as a function of detecting wavelength^[77]; (g) responsivity and detectivity on WSe₂/graphene/MoS₂ heterojunction device as functions of detecting wavelength of 400-2400 nm^[80]; schematics of optical absorption principle and band structure of heterojunction in (h) ultraviolet-visible and (i) infrared ranges^[80]

Table 1. Summary of 2D materials photodetectors for optical communications