# External Field Response and Applications of Metal Coated Hemispherical Janus Particles
## 1 Introduction  
A Janus particle is a particle having surface and shape with two or more different physical and chemical properties [1]. \"[PERSON]\" is named for the Roman god with two faces. As an essential feature of the Janus particle, anisotropy due to its asymmetric structure should be stressed. Because of this anisotropy, it possesses various physical and chemical functions. Janus particles are expected to be applied not only to basic science, such as biochemistry, physics, and colloid chemistry, but also to practical applications, such as drug delivery, electronic devices, optical biosensors, etc. Motility can be developed by giving driving force and controlling the orientation of particles. For example, the feature having self-propelled or external-field driven mobility [2; 3; 4; 5; 6] is expected to be applied to drug delivery. The feature that orientation is controllable can also be applied to display devices [7] such as electronic papers. Using the optically anisotropic particles that are fluorescent or transparent only on one side [8; 9; 10] as a probe, such applications are expected as tracers in microfluidics and optical biosensors for investigating rheology in living bodies. A typical method for preparing simple Janus particles is to arrange silica or polystyrene particles, so as to form a single layer on a substrate, followed by the deposition of metal on only one side of the particles by vacuum evaporation [11]. Since such particles have anisotropic optical properties, interesting researches on the interaction of light with the particles have been reported. Firstly, researches on active matters have been intensively reported. By irradiating the laser light on the particle, it actslike a self-driven particle that is powered by the light energy [12]. Applications towards optical tweezers have also been reported, as follows. Rotating particles, like a light mill, are reported [13]. Particles are not only trapped by a laser, but also they move up and down along the temperature gradient induced by strong absorption due to localized plasmon of the metal [14]. In studies on surface plasmons, color can also be changed by attaching gold nanoparticles to one side of polystyrene particles [15]. Recently, hetero-shaped (heteromorphic) Janus particles are also drawing attention [16; 17; 18]. For example, tile-shaped metal/Si Janus particles (Janus tile) function as variable concave mirrors with uniform alignment at the liquid/liquid interface [16]. In this way, Janus particles different in shape from a sphere can be fabricated by etching or other methods, but in many cases it is necessary to use advanced technology and large-scale equipment. There are few reports that hetero-shaped particles are coated with metals partially at the specific site on the surface. This is because it is difficult to arrange the hetero-shaped particles by controlling their orientation in a single layer, whereas it is easy to arrange the spherically shaped particles. Heteromorphic microparticles compose a unique crystal structure for their optical properties to be investigated as new photonic crystals [19; 20; 21; 22]. Some of them show electrophoresis [23]. The type of such functions and the magnitude of the response depend on the shape of the heteromorphic particle and its material distribution. Therefore, controlling the particle shape and the material distribution is important for developing the functional particles. Hemispherical particles, mushroom shaped particles, biconvex-lens shaped particles [24], dimple particles [25], snowman particles, raspberry particles [26], etc. have been so far fabricated. It is considered that further functions can be added and demonstrated if the heteromorphic particles that are thus produced can be site-selectively processed afterwards. We successfully arranged the asymmetric hemispherical particles unidirectionally in two dimensions on the substrate and deposited metal only on the equatorial plane [27]. We named it a metal coated hemispherical Janus particle.