The material for such an ultrabroadband photodetector could be graphene, but the absorption is small. So, we have fabricated an interdigitated electrode on a SiO2/Si substrate and deposited graphene ink on top. The ink consists of monolayer and bilayer graphene flakes dispersed in water and obtained from the company Nanointegris.

We have functionalized the graphene ink with nanoparticles of gold and silver to increase the yield of the photodetection process. The nanoparticles are arranged mainly on the contours of the graphene flakes. To produce more electrons via scattering, it is essential to use nanoparticles of the correct size. Nanoparticles with unsuitable diameters could have an adverse effect by acting as defects or parasitic contacts. Finally, we have "sealed" our photodetector with a bovine serum albumin (BSA) solution.

Extensive and difficult measurements were performed in dc mode, by illuminating our graphene ink photodetector with various lamps to cover the spectrum 200–2500 nm. To our surprise, we have seen significant current changes compared with the dark current for all regions of the spectrum from UV up to IR. The calculated responsivity was about 6–8 mA/W for UV and visible light and 13.7 mA/W for IR.

The detection of light over such a large bandwidth using only drops of pure or functionalized graphene ink opens the door to inexpensive broadband photodetection of light.

This work was published in the journal Nanotechnology.