Broadband optical absorption enhancement in Au-coated ZnO nanotips

The enhancement of broadband optical absorption in zinc oxide (ZnO) nanotip (NT) arrays coated with evaporated gold (Au) on fluorine-doped SnO2 (FTO)/glass by a simple hydrothermal growth and subsequent Au evaporation is reported. As the core of ZnO NT arrays is gradually coated with evaporated Au, the reflectance is slightly increased at lambda > approximately 800 nm while the transmittance is decreased at lambda approximately 400-1800 nm. For both FTO/glass and ZnO NT arrays on FTO/glass, the coating of Au improves the light absorption due to the antireflective geometry compared to the flat Au films and the absorptance is also enhanced by increasing the nominal thickness of Au with evaporation time. For the ZnO NT arrays with an Au evaporated for 600 s at 0.5 A/s, a high absorptance of >72% is achieved over the wavelength range of 250-2000 nm, indicating a significant increase due to the enhanced antireflection property as well as the increased surface area compared to the Au-coated FTO/glass without ZnO NT arrays.