Ultrafast photoresponse in ultraviolet detectors based on zinc oxide nanorods: the effect of a graphene capping layer

This study investigated the influence of the graphene layer on the speed of light detection of ZnO-based photodetectors. Due to their elevated exposed area and antireflective properties, ZnO nanorods are promising materials in photon entrapment and improving the performance of fabricated photodetectors. However, their higher surface resistivity compared to the bulk material reduces the speed of light detection and increases the recombination rate. Graphene thin film was transferred on top of zinc oxide nanorods through the electrophoretic deposition method to overcome this issue. The scanning electron microscopy showed that the graphene entirely covered ZnO nanorods, and their morphology remained intact after the deposition of the capping layer. For studying the effect of graphene on the optical characteristics of ZnO nanorods, the photoluminescence (PL) spectra were investigated before and after transferring of graphene. There was a reduction in the PL intensity of the capped nanostructures due to the slight absorption of light in the graphene layer. Raman spectra of the graphene-containing sample revealed two dominant peaks related to graphene's D and G bands. The intensity of the G band was lower than the D band, showing a reduced amount of structural disorder and defects in the graphene layers. To investigate the optoelectrical characteristics, ultraviolet detectors were designed based on zinc oxide nanorods with and without the presence of graphene. The photonic properties of devices indicated a notable increase in the speed of light detection for the fabricated photodetectors containing a graphene capping layer.