Magnetic and optical functionalities ascribable to nanostructure and sub-nanostructure of elemental metals and metal oxides are described. It is known that localized surface plasmon resonance takes place in a nano-structured metal and induces enhanced electric fields around the metal, which can interact strongly with incident light. This phenomenon can be applied to a random laser system comprising metal nanoparticles and fluorescent species. It is demonstrated that the threshold of laser oscillation can be controlled by tuning the separation between a metal nanoparticle and a fluorescent molecule. Also, a drastic change of magnetic and magneto-optical properties based on the modification of sub-nanostructure of metal oxides is exemplified. ZnFe₂O₄, the stable phase of which adopts the normal spinel structure and is antiferromagnetic with a Néel temperature as low as 10 K, can be ferrimagnetic with large magnetization and transition temperature higher than room temperature when a random spinel structure is achieved. EuTiO₃, the stable phase of which is antiferromagnetic, can be converted to a ferromagnet by lattice volume expansion or amorphization of the crystalline compound. The mechanism is explained in terms of the sub-nanostructure.

Soft actuators are gaining attention as flexible components for soft robotics. Responsive materials that respond to external stimuli such as light are often used as the driving part of the soft actuator. For example, thermo-responsive soft actuators are fabricated by bilayer polymer films of different thermal expansion coefficients. Heating of such bilayer polymer films by plasmon heating enables the soft actuators to be driven remotely by light. In the present study, we report the fabrication of plasmonic heating driven responsive soft actuators by using metal-organic framework (MOF) thin films as supports of plasmonic nanoparticles. The thin films of Cu₂(bpydc)₂ (bpydc = 2,2’-bipyridine-5,5’-dicarboxylate) of which metal ion adsorption capacity is high were fabricated on Cu₂(bpdc)₂ (bpdc = 4,4’-biphenyldicarboxylate) which grown on ceramics precursor, Cu(OH)₂. The actuating was operated by plasmon heating of Ag nanoparticles contained in MOF thin films inserted in between the two polymers, PDMS (poly(dimethylsiloxane)) and PVDC (poly(vinylidene chloride)). Two polymers have different thermal expansion coefficient so that they work as soft actuators by plasmon heating. Such fabrication process of soft actuators would pave the way for development of advanced devices in the field of soft robotics.

We investigated the changes in shape and extinction intensity of Au nanorod arrays by rapid thermal annealing (RTA). RTA was performed at 400~800°C for 3 min in an N₂ atmosphere using a halogen lamp as a heater. With increasing annealing temperature, the surface roughness of the rod decreases, the long axis of the rod shrinks while the short axis inflates, and finally some rods split. The wavelength and intensity of localized surface plasmon resonance varied with the rods shape. In particular, annealing at 800°C shifts the high-order (3λ/2) mode along the long axis from the near infrared to the visible region. Also surface lattice resonance is observed, showing that the periodicity of the array is retained even after annealing at 800°C.

Composites of DNA and gold nanoparticles are expected to be stimuli-responsive and photo-functional materials that can synergistically utilize both the stimuli-responsiveness derived from DNA and the optical properties derived from gold nanoparticles. However, conventional methods require the bottom-up synthesis of artificial DNA modified with functional groups such as thiols that can form chemical bonds with gold nanoparticles, which limits the flexible design of the resulting composite. Therefore, we conceived the idea of introducing a “linker” that can interact with both gold nanoparticles and the bases naturally exist in DNA. The introduction of such a linker allows naturally occurring DNA, which is abundant in nature and has long strand lengths, to utilize as the multi-functional material platform. In this work, we designed and synthesized a linker complex with disulfide group and platinum(II) ion to interact with gold nanoparticles and the bases of DNA, respectively. Furthermore, the interaction between gold nanoparticles and naturally occurring DNA via the platinum linker complex was confirmed using UV-visible absorption spectroscopy.