Abstract
Au-nanocrystal-array/silicon nanoantennas exhibiting wavelength-selective photocurrent enhancement were successfully fabricated by a facile and inexpensive method combining colloidal lithography (CL) and a metal-assisted chemical etching (MaCE) process for the first time. These nanoantennas comprise Au nanocrystal arrays inlaid in silicon substrates with controllable degree of immersion. The localized surface plasmon resonance (LSPR) response and wavelength- selective photocurrent enhancement characteristics were achieved by tuning the depth of immersion of Au nanocrystal arrays in silicon through a MaCE process. Compared to conventional Au particles on Si, the high near-field enhancement increases with the fraction of their volume in intimate contact with the substrate in the Au nanocrystal array inlaid Si structure. On the other hand, LSPR responses, which are extremely sensitive to dielectric properties of metal and the surrounding environment, can be tuned by the depth of immersion of Au nanocrystal array on/in silicon. The wavelength selectivity of photocurrent enhancement contributed by LSPR induced local field amplification was confirmed by simulated near-field distribution. The wavelength maximum of LSPR scattering (max) exhibits sensitivity to the surrounding environment and shows consistence with the simulated results obtained by the finite-difference time-domain (FDTD) method. The wavelength-selective photocurrent enhancement characteristics were measured under illumination of lasers of different wavelengths and under dark conditions. In addition, the repeatability of wavelength-selective photocurrent enhancement was also tested by multiple ON/OFF cycles and can be exploited as photoswitches. The wavelength-selective photocurrent enhancement (>70 %) operated under low voltage (<200 mV) was achieved under laser illumination coincident to its LSPR max. In addition, the wavelength-selective photocurrent enhancement can be elucidated by the FDTD simulations of the near-field enhancements (|E|^2), which can intensify local electromagnetic field and optical absorption. The good tunability over LSPR responses and wavelength-selective photocurrent enhancement characteristics can be exploited as low power-consumption photoswitches and nano-optoelectronic and photonic communication devices. Furthermore, it can be integrated into the well-developed Si-based manufacturing process.