Towards the assembly of 2D tunable crystal patterns of spherical colloids

Entering an era of miniaturization prompted scientists to explore assembly strategies to assemble colloidal crystals for numerous applications, including photonic crystals, optical sensors, and new materials. Here, the manual rubbing assembly method is utilized to rapidly ($approx20$ seconds) attain monolayers comprising hexagonal closely packed (HCP) crystals of monodisperse dry powder spherical particles with a diameter ranging from 500 nm to 10 $mu$m on non-elastomeric substrates using a PDMS stamp. It is evident that the tribocharging-induced electrostatic attraction, particularly on relatively stiff substrates, and contact mechanics force between particles and substrate are critical contributors to attaining large-scale HCP structures on conductive and insulating substrates. Superior results were obtained with polystyrene and PMMA powder, while silica was only assembled in HCP structures on fluorocarbon-coated substrates within a zero-humidity environment. Furthermore, when PDMS or fluorocarbon are constituents of the system, crystal formation is promoted due to their tribocharging properties. At last, it is shown that crystal patterns can be tuned with great control on a chemically templated surface, which can be promising in microelectronics, bead-based assays, sensing and anti-counterfeiting applications.