(Invited) Plasmon-Induced Photocurrent Generation for Exploring the Near-Field Ofstrongly Coupled Plasmonic Systems
Authors: Ueno Kosei , Guo Jingchun , Shi Xu , Oshikiri Tomoya , Misawa Hiroaki 
Published: February 2020

[Introduction]
Metallic nanostructures which show localized surface plasmon resonances (LSPRs) have received considerable attention as a light harvesting optical antenna for light-energy conversion systems such as solar cells as well as artificial photosynthesis [1,2]. To construct efficient light harvesting optical antennae, an optimization of structural design is one of the most important research topics in plasmon-induced light energy conversions. Coupled plasmonic systems such as nanogap heptamer, dolmen, metal-insulator-metal nanostructures, and so on is promising as a photoelectrode design because of their strong near-field enhancement and wide wavelength responsibility. In this study, a coupled plasmonic system based on a waveguide-LSPR coupling system is employed to investigate whether the photocurrent response extends over a wide wavelength range and is promoted by near-field enhancement in the plasmon-induced photocurrent generation using gold nanostructured titanium dioxide (TiO2) photoelectrodes. Near-field spectrum and photocurrent action spectrum are compared to elucidate the effect of near-field enhancement on the photocurrent generation.
[Experimental]
TiO2 photoelectrodes supporting periodic gold nanogratings (AuNGs) with different pitch sizes were fabricated by a deposition of TiO2 on a glass substrate with a thickness of 250 nm using an atomic layer deposition (ALD) reactor, and subsequent electron beam lithography and lift-off processes. A conventional photoelectrochemical measurement was performed. The AuNGs/TiO2 photoelectrode, a platinum wire and a saturated calomel electrode (SCE) were employed as working, counter and reference electrodes, respectively. An aqueous Ar-gas-bubbled KClO4 (0.1 mol/dm3) solution was used as a supporting electrolyte solution. A plasmon-induced water oxidation as a half reaction of water splitting was explored [3].
[Results and Discussion]
Figure 1(a) shows a scanning microscope (SEM) image of AuNGs/TiO2 photoelectrode with a pitch size of 300 nm. Extinction spectra of the AuNGs/TiO2 photoelectrode with a different pitch size are shown in Fig. 1(b). There is only one peak corresponding to the LSPR mode of periodic Au-NGs with 200 nm and 225 nm pitch sizes. Starting with a pitch size of 250 nm, three peaks can be observed, and the peaks show a spectral shift with increasing the pitch size. The two outer peaks can be assigned as coupled modes (P+ and P-) deriving from the symmetric waveguide mode Ew
and LSPR mode Ep
as shown in the illustration of the waveguide-LSPR coupling system (Fig. 1(c)). The center peak can be considered as a coupling mode between the asymmetric waveguide mode and the waveguide-LSPR coupling system.
Internal quantum efficiency (IQE) spectra and simulated near-field spectra of the AuNGs/TiO2 photoelectrodes with 300 nm and 350 nm pitch sizes are shown in Fig. 2. The near-field intensity in the spectra was calculated by monitoring at the interface between the AuNGs and the TiO2 film by the electromagnetic simulations using a finite-difference time-domain (FDTD) method. It was clearly elucidated that IQE spectrum has successfully reproduced the near-field spectrum under the coupling conditions. This indicates that the photocurrent response extended over a wide wavelength range utilizing the coupled plasmonic systems and near-field enhancement effects promoted the plasmon-induced water oxidation because IQE values increased responding to the near-field spectra [4].

References

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Guo, J., Ueno, K., Shi, X., Oshikiri, T., Misawa, H. et al., J. Phys. Chem. C
2017, 121, 21627−21633.

Figure 1