Lithography-free thin-titanium-nanocone metamaterial perfect absorbers using ZnO nanostructures

In this work, thin Ti nanocones are deposited on top of the arrays of ZnO nanopagodas, and the whole structure works as an efficient nanostructured metamaterial perfect absorber (MPA) without using lithography and dry etching. In this design, ~1µm long ZnO nanopagoda arrays are grown on a 100 nm ZnO buffer layer over the silicon/glass substrate by a treatment with an aqueous solution of L-ascorbic acid. Growth direction and the degree of lamination in the ZnO nanostructures can be easily controlled by adjusting the concentration of L-ascorbic acid. Afterward, these ZnO nanopagodas are coated with a 30nm thin top and a 500nm thick bottom layer of Ti to achieve the proposed nanocone resonant cavity structure with electromagnetic wave field penetration. The overall structure encapsulates three physical concepts, namely, field penetration, adiabatic coupling and cavity resonance, which contribute the broadband perfect absorption. The entire process is carried out at a low temperature (<90°). We believe the proposed tapered Ti nanocones MPA structure facilitates ultra-broadband perfect spectral absorption with promising nature of low-cost, large-area, and lithography-free.