Enhancement Visible Light Sensitivity of the PbS Quantum-Dot/Graphene Hybrid Photodetector Using Ag Meta-Pattern Substrate

Graphene is known to have excellent various characteristics such as electrical, mechanical, and optical properties. In particular, in terms of optical characteristics, graphene is transparent, has the advantage of light absorption in a wide range up to ultra-violet (UV) - infrared (IR), and has a constant light absorption rate of 2.3% at an electric wavelength. The reason why it could not be used as an optical application despite its wonderful optical characteristics is that the life-time of the optical carrier is very short at several ps, so it has a limitation in its use as an optical application. Accordingly, in order to overcome the optical limitations of graphene, hybrid structure with quantum-dot (QD) was produced to increase sensitivity and detection. This method dramatically increased the performance of the graphene-based photodetector (PD), but there are several problems. First, because organic QD is utilized, the effect of toxicity according to the components of QD cannot be avoided, and the absolute range is fixed. In addition, the second is the problem of the substrate. When light is irradiated, absorption and reflection occur simultaneously on the substrate, and the sensing characteristics of the photodetector are not maintained constant due to absorption into the substrates. Therefore, in this study, we utilized an Ag meta-surface (AMS), which can lead to maintaining the sensing characteristics of the QD-graphene-based photodetector and improving the sensing property. AMS produced a replica through nanoimprint lithography, used a method of depositing and detaching Ag on the replica, and could be ripped without damaging the AMS. After that, in order to manufacture a QD-graphene device, a flat layer is manufactured through spin coating of PVA (polyvinyl alcohol) on AMS. After that, patterned graphene is transferred on PVA/AMS, and electrodes are deposited using a shadow mask to manufacture devices. In addition, the PbS QD used in this study used commercially available PbS QD, and coating was coated using a widely known method. Before checking the characteristics of the manufactured PD, the effect of the PVA layer on the graphene layer was confirmed, and it was confirmed that the n-doping effect of PVA layer under the graphene through the Dirac point negative shift. In addition, when observing the change in the optical characteristics of graphene according to the presence or absence of AMS, it was observed that the current changes depending on the on/off of light, although the reactivity to light was not shown on the SiO2 substrate. Since it was confirmed that AMS can positively affect the optical characteristics of graphene, PbS QD was coated to improve the optical sensing characteristics of graphene-based PD. And a QD/graphene device was manufactured with SiO2 and AMS substrates to compare characteristics. Comparing the characteristics of PDs manufactured using the two substrates, it was confirmed that the current that changes in response to light at the IV curve was up to 3 times, and the current that changes at laser on/off was also 3 times higher than SiO2. In addition, it was confirmed that PD on the AMS substrate still showed similar current levels over time, but PD on SiO2 showed a characteristic that the current level gradually decreased. Therefore, we were able to present in this study that the use of AMS in PD or optical applications can improve optical properties. And the laser used in this study was a 532 nm wavelength laser. In conclusion, meta-surface and QD have the disadvantage that wavelength absorbed or reflected is fixed depending on the structure, size, and components, but on the contrary, it can be used in specific fields by increasing sensitivity to specific wavelengths, and it is expected that high-performance and high-sensitivity photodetector can be produced by combining them with graphene.