No abstract

Hybrids of two-dimensional (2D) and 0D nanomaterials offer a wider spectrum of properties than their counterparts. Here, we choose a molybdenum disulfide and NaGdF4:Yb3+, Er3+ upconversion nanoparticle (MoS2-UCNP) nanocomposite (NC) on graphene (G)-coated polydimethylsiloxane (PDMS) and silica/silicon (SiO2/Si) substrates as broadband photodetectors (PDs). The band gap (∼680 nm) limited response of pure MoS2 is broadened by the infrared (980 nm) absorbing UCNPs. Identically fabricated PDs on PDMS and SiO2/Si showed the highest photoresponsivity of 26.18 and 84.52 AW–1, respectively, under 661 nm laser illumination at a density of 1 mW/cm2 at 1 V bias. The MoS2-UCNPs/Graphene/SiO2/Si PD (SiO2/Si PD) showed a response time of ∼100 ms compared to ∼3 s for the PDMS-based PD. The PDMS-based PD showed a reasonably stable photocurrent, decaying by ∼39%, under 250 repetitive cycles of 6.25% bending strain; a maximum decrease of ∼40% of the photocurrent was observed under the 11.11% bending strain compared to the as-prepared flat PD. Both devices could detect signals from domestic appliances such as air conditioner remotes, laser pointers, and cellphone flashlights. The flexible PD based on a hybrid of two nanomaterials having complementary ranges of absorption offers the possibility for better wearable sensors.

Graphene and its nanohybrid photodetectors (PDs) offer unique interfacial and synergistic properties in contrast to conventional semiconductor-based PDs. In this work, we compare the performance of identically fabricated PDs based on pure chemical vapor deposition-grown monolayer graphene, demonstrating a positive photoresponse, that exhibits a gain in photoresponsivity (R) with the addition of two-dimensional MoS 2 (chemically exfoliated). However, further addition of plasmonic gold nanorods (AuNRs), on the MoS 2 /graphene PD, results in the flipping of the photoconductivity to negative. The negative photoresponse is explained by an increased dark current via the complex conductive network model and a lower photocurrent due to the photogenerated electron capture by the AuNRs in the AuNRs/MoS 2 /graphene PD. The contrasting shape and nature of the dynamic photoresponses are modeled using theoretical fitting and explained based on carrier transport in individual components. The highest R in the moderately fast graphene PD device is ∼2 × 10 4 A W −1 under 808 nm which could be slightly improved to ∼3 × 10 4 A W −1 by introducing MoS 2 nanoflakes, albeit at the cost of the response time which slows down from ∼90 ms (in the former) to 558 ms in the latter. The introduction of AuNRs did speed up the response time in the AuNRs/MoS 2 /graphene PD to 20.5 ms but produced a strikingly different negative photoresponse attributed to a higher dark current in the material. We proposed a single mechanism to explain the positive and negative photoresponses reported so far for the hybrid PDs.