Colloidal particle, such as quantum dots and metal nanoparticles, which are becoming important devices for applications such as microelectronics, renewable energy, and medical / sensing applications. Unfortunately, standard photolithography methods using photons, focused ion beams, or electron beams can not pattern these particles on a solid substrate. Optical tweezers provide powerful performance to fully operate these particles, however, it is still challenging to fix the particles to the substrate. In addition, the high power operating state of optical tweezers (up to 100 mW / μm2) limits its application range.
Size of about tens of nanometers, the pitch on the slide on the single-digit nano-particles are used as plasma substrates. Low-power lasers whose power per square micron level is a few bits of milliwatts, and the wavelength can be tuned to match the plasma resonance wavelength of the nanoparticles, which is sufficient for patterning.
In order to form bubbles, a laser beam having a diameter of 2 μm was focused from the lower side of the plasma substrate to be focused on the colloidal particle solution, which was sandwiched between the substrate and the coverslips spaced from the substrate by 120 μm. As the plasma enhances the photothermal effect of water evaporation, the evaporated water vapor forms bubbles with a diameter as low as 1 μm at the top of the plasma substrate.
The Colloidal particle are then dragged to the microbubbles and trapped on the bubble / solution interface, and finally fixed to the substrate. When the laser power is turned off, the particles remain in their patterned position due to the enhanced substrate adhesion due to the thermal effect. Even after the substrate has been rinsed and dried, these particle patterns remain on the substrate, this feature makes the method suitable for manufacturing functional equipment. The laser beam as a pen, with the scanning laser beam and move the bubble, you can form a nanoparticle pattern, as shown in Figure 1.
Particle capture at the microbubble position is caused by a natural convection caused by a temperature gradient on the substrate, resulting from the Marangoni convection caused by the surface tension gradient along the microbubble surface. The in-plane drag force of the bubbles causes particles to be trapped when the particles contact the surface of the microbubbles, which is a phenomenon that can be quantified and predicted by mechanical equations. In fact, the temperature distribution of bubbles can also be predicted by computational fluid dynamics (CFD) simulation results.