Tunable Optical Antenna Effect of Palladium-Based Dimeric Nanostructures
Authors: Ikeda Katsuyoshi , Uchiyama Sari , Murakoshi Kei 
Published: February 2020

In photo-energy conversion, photon capture is one of critical issues to determine the overall efficiency. Since the photon density is rather low in sun light, it is indispensable to achieve highly efficient photon capture in the conversion system. Recently, plasmonic metal nanostructures are expected as optical antennas capable of capturing photons efficiently. When localized plasmon modes are excited on a metal nanostructure, photon energy is resonantly trapped on its surface. Indeed, we have recently reported that the efficiency of photocurrent generation on porphyrin-linked molecular monolayers was substantially enhanced by the use of the plasmonic antennas.1-4 Plasmon resonance features of metal nanostructures are characterized by their size and shape. Therefore, it is quite challenging to realize dynamical tuning of plasmon resonances by external stimuli. The external control of the optical antenna effect could lead to development of noble optical devices. Although there have been several reports on hydrogen -induced plasmon resonance variation in Pd nanostructures,5,6such dynamical effect is still small. In the present work, we demonstrate plasmon resonance tuning of Pd-based nanodimer array structures.
Metal nanodimer arrays of various metal species were fabricated on a glass substrate using angle-resolved nanosphere lithography (AR-NSL); a self-assembled monolayer of polystyrene beads with diameter of 350 nm was formed on the substrate as a shadow mask. Then, thin films of Pd or other metals with thickness of 20 nm were formed through the shadow mask by double-angle evaporation with angles of 0 and 23 degrees. Finally, the bead monolayers were removed so that a number of dimers aligned uniaxially were obtained on the substrate. A typical AFM image of the nanodimer array obtained is shown in the right panel of Fig. 1. Transmission spectra of the dimer arrays were measured in a glass cell filled with N2 gas or H2-Ar mixed gas of a specific H2percentage, which was continuously provided from a gas cylinder. Thermal control of the cell was conducted using a transparent thermoplate.
The hydrogen-induced variation of plasmon resonances was investigated in details by measuring in-situ transmission spectra of Pd-Pd homo-dimer arrays. As shown in the left panel of Fig. 2, the dimer array exhibited a broad hybridized plasmon band at around 600 nm. Under hydrogen gas flow, this peak decreased in intensity. The right panel of Fig. 2 shows the degree of the hydrogen-induced variations as functions of hydrogen concentration and temperature. One can clearly see that the hydrogen-induced variation become significantly larger at the specific conditions above 3 % H2concentration and below 30 C°. This region well agrees with that of palladium hydride (b-phase) formation in Pd-H phase diagram. That is, the formation of palladium-hydrogen solid solution (a-phase) does not change plamon resonances much.
On the basis of the hydrogen-induced responses in the Pd-Pd homo-dimer arrays, we have designed various types of sp-metal/Pd hetero- and homo-dimer arrays to increase both of the intensity of plasmon resonance and the ratio of the hydrogen-induced change. We will also discuss a possibility of photo-induced change of the optical antenna effect.
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