8 Modulations of Electronic States in Plasmonic Strong Coupling …
143
420 to 550 nm wavelength which is corresponding to the Soret band of TPPS Jaggregate. The monitor wavelengths for the excitation spectra were set at 718 nm. It
is emphasizing that two shoulder peaks derived from upper and lower branches of the
hybrid states, which are corresponding to the wavelengths of the hybrid states in the
extinction spectrum, can be observed. In the Q-band region, a similar response was
observed (The data are not shown here). Therefore, the excitation spectrum provides
strong evidence that a hybrid state is really formed in the strong coupling regime [6].
8.4 Modal Strong Coupling Systems that Enhances
Photochemical Reactions
Metallic nanostructures showing 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 [16–19]. To construct efficient lightharvesting optical antennae, an optimization of the structural design is one of the most
important research topics in the plasmon-induced light-energy conversions. Coupled
plasmonic systems such as nanogap heptamer, dolmen, metal-insulator-metal nanostructures, and so on are promising as a photoelectrode design because of their strong
near-field enhancement and the wide wavelength responsibility [12–14]. Here, the
modal strong coupling between the waveguide mode and LSPR 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 (TiO 2 ) photoelectrodes. The near-field
spectrum and the photocurrent action spectrum are compared to elucidate the effect
of near-field enhancement on the photocurrent generation.
TiO 2 photoelectrodes having periodic gold nanogratings (AuNGs) with a different
pitch size have been fabricated by deposition of TiO 2 on a glass substrate with a
thickness of 250 nm using an atomic layer deposition (ALD), and subsequent electron beam lithography and lift-off methods. A conventional photoelectrochemical
measurement using three-electrodes was performed for the measurement of photocurrent generation efficiencies. The AuNGs/TiO 2 photoelectrode, platinum wire, and a
saturated calomel electrode (SCE) were employed as working, counter and reference
electrodes, respectively. An aqueous Ar–gas–bubbled KClO 4 (0.1 mol/dm 3 ) solution
was used as a supporting electrolyte solution. A plasmon-induced water oxidation
as a half-reaction of the water-splitting was explored.
Figure 8.9a shows an SEM image of AuNGs/TiO 2 photoelectrode with a pitch size
of 300 nm. Extinction spectra of the AuNGs/TiO 2 photoelectrode with a different
pitch size are shown in Fig. 8.9b. There is only one peak corresponding to the LSPR
band of periodic AuNGs with 200 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
143
420 to 550 nm wavelength which is corresponding to the Soret band of TPPS Jaggregate. The monitor wavelengths for the excitation spectra were set at 718 nm. It
is emphasizing that two shoulder peaks derived from upper and lower branches of the
hybrid states, which are corresponding to the wavelengths of the hybrid states in the
extinction spectrum, can be observed. In the Q-band region, a similar response was
observed (The data are not shown here). Therefore, the excitation spectrum provides
strong evidence that a hybrid state is really formed in the strong coupling regime [6].
8.4 Modal Strong Coupling Systems that Enhances
Photochemical Reactions
Metallic nanostructures showing 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 [16–19]. To construct efficient lightharvesting optical antennae, an optimization of the structural design is one of the most
important research topics in the plasmon-induced light-energy conversions. Coupled
plasmonic systems such as nanogap heptamer, dolmen, metal-insulator-metal nanostructures, and so on are promising as a photoelectrode design because of their strong
near-field enhancement and the wide wavelength responsibility [12–14]. Here, the
modal strong coupling between the waveguide mode and LSPR 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 (TiO 2 ) photoelectrodes. The near-field
spectrum and the photocurrent action spectrum are compared to elucidate the effect
of near-field enhancement on the photocurrent generation.
TiO 2 photoelectrodes having periodic gold nanogratings (AuNGs) with a different
pitch size have been fabricated by deposition of TiO 2 on a glass substrate with a
thickness of 250 nm using an atomic layer deposition (ALD), and subsequent electron beam lithography and lift-off methods. A conventional photoelectrochemical
measurement using three-electrodes was performed for the measurement of photocurrent generation efficiencies. The AuNGs/TiO 2 photoelectrode, platinum wire, and a
saturated calomel electrode (SCE) were employed as working, counter and reference
electrodes, respectively. An aqueous Ar–gas–bubbled KClO 4 (0.1 mol/dm 3 ) solution
was used as a supporting electrolyte solution. A plasmon-induced water oxidation
as a half-reaction of the water-splitting was explored.
Figure 8.9a shows an SEM image of AuNGs/TiO 2 photoelectrode with a pitch size
of 300 nm. Extinction spectra of the AuNGs/TiO 2 photoelectrode with a different
pitch size are shown in Fig. 8.9b. There is only one peak corresponding to the LSPR
band of periodic AuNGs with 200 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