Directed Assembly of Nanoparticles into Continuous Microstructures by Standing Surface Acoustic Waves

Directed-assembly by applying external fields can result in temporary alignment of nanoparticles (NPs) in a state of local equilibrium with complex spatial organization. However, electric, optical and magnetic fields usually require certain material characteristics (conductivity, transparency or magnetic susceptibility) that limit the span of possible applications and materials. One of the advantages of acoustic-based alignment techniques is that the only requirement for arrangement of particles is a difference in compressibility and density between the particle and the surrounding medium (acoustic contrast), which is true for most dispersed systems. Previous studies have shown how standing surface acoustic waves (SSAWs) can manipulate NPs. Here, the ability of SSAWs to direct NPs to produce permanent continuous and uniform microstructures is demonstrated for the first time. A radio-frequency signal that is applied to interdigital transducers fabricated on a piezoelectric wafer forms the SSAWs. Dispersed silver NPs (in a microfluidic channel) that are exposed to the SSAWs accumulate at the pressure nodes. The NPs are then sintered at room temperature by streaming chloride ions into the microchannel, resulting in stable conducting silver microstructures. This approach enables fabrication inside closed systems (such as microfluidic channels) even for opaque or optically distorted systems where laser-based assembly methods are inapplicable. This method could be conceptually implemented for directed-assembly of various nano-materials into continuous microstructures, and used for fabrication of microelectronic devices such as sensors and antennae.