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The detection of light in the longwave infrared (LWIR) region is crucial for many applications such as environmental monitoring, thermal imaging and surveillance. Many commercial LWIR photodetectors involve complex fabrication processes, require cryogenic temperatures or exhibit slow photoresponse. Hence, there is a continuous pursuit of developing room-temperature, on-chip LWIR photodetectors, using simple fabrication processes [1] . Metal-organic charge transfer complexes typically have a narrow bandgap, which allows them to absorb LWIR wavelengths [2] . Here, we report room temperature LWIR photoresponse in one such charge transfer complex, ie. copper 7,7,8,8-tetracyano-2,3,5,6-tetraflouroquinodimethane (CuTCNQF 4 ), achieved via simple synthesis and fabrication processes.

The long-wave infrared photodetector device we study in this work is shown in Fig. 1 (a) and (b). A bull's eye grating collects the incident light from a large area (101 μm diam.), and couples it into surface plasmon polaritons (SPPs) [1] that converge at its center, at which there is an opening in the Au plate on which the grating sits. Within this opening (or aperture), Au nanobar antennas separated by small gaps are arranged in a radial fashion. These enhance the electromagnetic field in the near-field zone, particularly in the gaps, through localized surface plasmon resonances (LSPRs) [2]. This enhances the absorption in the underlying material, here a small circle (6.32 μm diam.) of graphene. The absorption is boosted further by forming an optical cavity under the graphene. We optimized the geometrical parameters to maximise absorption in the graphene at λ = 9.4 μm via finite-difference time-domain electromagnetic simulations. These geometric parameters are listed in Fig. 1 (c)-(f).