Site-Selective Etching of Silver Nanocubes on TiO2 By Plasmon-Induced Charge Separation
Authors: Saito Koichiro , Tanabe Ichiro , Tatsuma Tetsu 
Published: February 2020

Metal nanoparticles exhibit strong light absorption and scattering, which originate from localized surface plasmon resonance (LSPR). The resonance intensity and wavelength can be controlled by changing the particle size and shape and local refractive index around the nanoparticles. Silver nanocubes on a high refractive index substrate exhibit two separated LSPR modes, i.e. “distal mode”, in which electric field is localized at the top of the nanocube, and “proximal mode”, in which electric field is localized at around the Ag-TiO2 interface.1 We have discovered “asymmetric light scattering” of silver nanocubes on a TiO2 thin film,2 in which blue scattering light based on the distal mode is observed from the front side, and yellow or red scattering based on the proximal mode from the back side (Figure a). On the other hand, a plasmonic nanoparticle on a semiconductor could exhibit plasmon-induced charge separation (PICS), which involves electron transfer from the nanoparticle to the semiconductor.3 Therefore, silver nanoparticles on semiconducting TiO2 is oxidatively dissolved and its morphology and color are changed by relatively strong light excitation under humid conditions.4-6 Since PICS is known to occur preferentially at the site with strong localized electric field,5,6 site-selective etching of silver nanocubes should also be possible and could lead to sophisticated photofunctoinalities. Here we demonstrate site-selective etching and its application to the asymmetric scattering.
First, for exclusive excitation of the proximal mode, silver nanocubes (~100 nm) on TiO2 were irradiated by 620‒700 nm light. The red scattering based on the proximal mode was suppressed and blue scattering was retained after the light irradiation. In SEM observation, we found that the bottom of the nanocube was selectively etched (Figure b). We calculated the scattering spectra by means of a finite-differential time-domain (FDTD) method. As a result, the scattering peak of proximal mode is blueshifted largely whereas the distal mode peak is almost unchanged, in good accordance with the experimental observation.
Next, silver nanocubes on TiO2 were irradiated by 420 nm light for excitation of the distal mode. As a result, the blue scattering of distal mode was suppressed. Many nanocubes were rounded at the top (Figure c). In FDTD simulation, increased curvature radius at the top edges of the nanocube selectively changed the distal mode scattering peak. Some nanocubes were etched not only at the top but also at the bottom, likely because electric field was localized also at the bottom of the nanocubes to some extent. Thus, we revealed that site-selective etching based on PICS is possible by proximal mode excitation and to some extent by distal mode excitation.
We applied the site-selective etching to the asymmetric scattering. Silver nanocubes (~80 nm) on a TiO2 thin film (40 nm thick) were irradiated by 580 nm light. After irradiation, the change in the scattering spectrum for the front side, which is chiefly due to the distal mode, was small. On the other hand, the proximal mode peak for the back side largely blueshifted and decreased. The color alteration from yellow to blue-green is visible only from the backside. By use of this phenomenon, we drew translucent images which can be recognized only from the backside (Figure d).

References

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(2) K. Saito and T. Tatsuma, Adv. Opt. Mater., 2015, 3, 8835.
(3) Y. Tian and T. Tatsuma, J. Am. Chem. Soc., 2005, 127, 7632.
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(5) E. Kazuma, N. Sakai and T. Tatsuma, Chem. Commun., 2011, 47, 5777.
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Figure 1