Nano Lett. http://doi.org/bqrx (2016)

Two- and three-dimensional periodic arrays of nanomaterials can be exploited to fabricate sophisticated electronic devices or circuits. Engineering such assemblies into specific lattices is, however, challenging because a material's identity typically governs the structure that it will adopt. Now, Wooyoung Shim and colleagues from Yonsei University and Seoul National University in South Korea use low-frequency sound waves to manipulate micro- and nanomaterials into lattices with tunable parameters.

The team apply standing waves to ethanol suspensions of SiO2 particles or Cu nanowires spread over square silicon surfaces. The surface vibrates at its resonance frequency and the particles are carried by capillary waves in the liquid and repositioned in a symmetric grid-like pattern at the wave displacement antinodes. An enhanced coffee-ring effect — where suspended particles leave a ring-shaped structure on a surface once a liquid has evaporated — is induced at high temperature (∼60 °C) to ensure that the particles remain at the lattice points during the drying process. Altering the wave's frequency tunes the spacing between aggregates and the overall area of the lattice is limited only by the size of the surface. Depositing a transparent polymer layer on top of the patterned surface and repeating the process extends the assembly into three dimensions. While lattice spacing is currently restricted to millimetre and micrometre scales, exploring viscous liquids and high-frequency waves may eventually afford denser patterns.