Letter pubs.acs.org/NanoLett
Au Nanocrystal Array/Silicon Nanoantennas as Wavelength-Selective Photoswitches Yu-Kai Lin,† Heng-Wen Ting,† Chun-Yuan Wang,‡ Shangjr Gwo,‡ Li-Jen Chou,† Cho-Jen Tsai,† and Lih-Juann Chen*,† †
Department of Materials Science and Engineering and ‡Department of Physics, National Tsing Hua University, No. 101, Section 2, Kuang-Fu Road, Hsinchu 30013, Taiwan 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. 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. The wavelength selectivity of photocurrent enhancement contributed by LSPR induced local field amplification was confirmed by simulated near-field distribution. In addition, it can be integrated to well-developed Si-based manufacturing process. These characteristics make Au nanocrystal array/Si nanoantennas promising as low power-consumption photoswitches and nano-optoelectronic and photonic communication devices. KEYWORDS: Localized surface plasmon resonance, photoresponce, metal nanocrystal array, metal-assisted chemical etching, degree of immersion
P
lasmonic nanoantennas1−4 composed of metallic nanostructures have been receiving a tremendous amount of interest as a medium for localization and intensification of light in free space to subwavelength volumes and converting light to electric power. The strong near-field around metallic nanostructures derived from the localized surface plasmon resonance (LSPR) will be coupled to the surrounding absorber layer and increase its absorption. The LSPR-derived amplification of local electromagnetic field and optical absorption enhancement were widely exploited for various applications including surface-enhanced Raman scattering (SERS) biosensors,5,6 nanophotonic switches,7,8 and nanolasers.9,10 The enhancement of optical absorption results from high near-field intensities derived from the LSPR response of metallic nanostructures is proportional to the electromagnetic field intensities.11,12 A high local electromagnetic field leads to increased light absorption and effective generation of energetic (“hot”) electron−hole pairs. These hot electron−hole pairs derived from plasmon decay will surmount potential barriers, inject to surrounding semiconductors, and contribute to photocurrent.1,13−15 For more efficient light absorption and photocurrent generation, intensification of the local electromagnetic field near metallic nanostructures is imperative. The LSPR responses of optical nanoantennas have been conventionally engineered through adjustment of material, size, and shape of the structure by photo- or e-bean lithography.16−18 Herein, we demonstrate a facile method to control the LSPR responses and photocurrent enhancement of optical nanoantennas through tuning the depth of immersion of © XXXX American Chemical Society
metallic nanostructures in silicon. Furthermore, because of the immersion of metallic nanostructures into silicon, the local electromagnetic field induced by LSPR can be significantly enhanced (to increase hot spot) and contribute to wavelengthselective photocurrent enhancement. In this Letter, we present Au nanocrystal array/silicon optical nanoantennas exhibiting wavelength-selective photocurrent enhancement for the first time. These nanoantennas comprising hexagonal close-packed single crystalline Au nanocrystal arrays inlaid in silicon substrates were fabricated by a facile, inexpensive, and Si manufacturing-compatible process combining a colloidal lithography (CL) and metal-assisted chemical etching (MaCE) method. The wavelength-selective photocurrent enhancement characteristics, achieved by tuning the depth of immersion of Au nanocrystal arrays in silicon through MaCE process, 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 and photoswitches characteristics can be achieved in Au nanocrystal array/Si nanoantennas by tuning the depth of immersion of Au nanocrystal arrays in silicon. Compared to conventional Au particles on Si, the high near-field enhancement increases with Received: March 10, 2013 Revised: May 5, 2013
A
dx.doi.org/10.1021/nl400896c | Nano Lett. XXXX, XXX, XXX−XXX
Nano Letters
Letter
Figure 1. Fabrication process of Au nanocrystal array/Si structures: (a) hexagonal close-packed monolayer polystyrene (PS) spheres on SiO2/Si substrate after O2 plasma etching, (b) SiO2 honeycomb cells after deposition of SiO2 thin film and removal of PS spheres, (c) hexagonal close-packed Au nanocrystal arrays after annealing of deposited Au thin film on SiO2 honeycomb cells, and (d) Au nanocrystal inlaid Si fabricated by metalassisted chemical etching of Si under uniform electrical field normal to surface of samples.
Figure 2. (a) Top-view SEM image of hexagonal close-packed of Au nanocrystal arrays on Si with an average nanocrystal size of 85 nm after the removal of SiO2 honeycomb cells. The inset shows SAED pattern of single crystalline Au nanocrystal examined by TEM. The scale bar represents 1 μm. Tilted SEM images of Au nanocrystal arrays/Si nanoantennas with varied degree of immersion (DOI, defined as the immersed depth into Si divided by the diameter of Au nanocrystals) of (b) 0, (c) 0.25, (d) 0.5, (e) 0.75, (f) 1. All of the scale bars represent 100 nm.
the fraction of their volume in intimate contact with the substrate in the Au nanocrystal array inlaid Si structure.19 On the other hand, LSPR responses, which are extremely sensitive to dielectric properties of metal and the surrounding environment,20,21 can be tuned by the depth of immersion of Au nanocrystal array on/in silicon. The wavelength maximum of LSPR scattering (λmax) 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 (>70%) operated under low voltage (
