Ordered Particle Arrays via a Langmuir Transfer Process: Access to Any Two-Dimensional Bravais Lattice
Miriam Hummel, Christian Stelling, Bernd Kopera, Fabian Nutz, Matthias Karg, Markus Retsch, Stephan Förster:
Langmuir, 35, 973-979 (2019)
doi: 10.1021/acs.langmuir.8b03047 
link  

We demonstrate how to directly transform a close-packed hexagonal colloidal monolayer into non-close packed particle arrays of any two-dimensional symmetry at the air/water interface. This major advancement in the field of nanoparticle self-assembly is based on a simple one-dimensional stretching step in combination with the particle array orientation. Our method goes far beyond existing strategies and allows access to all possible 2D Bravais lattices. A key element of our work is the possibility to macroscopically stretch a particle array in a truly one-dimensional manner, which has not been possible up to now. We achieve this by stretching the nanoparticle array at an air/water interface during the transfer process. The degree of stretching is simply controlled by the wettability of the transfer substrate. To retain the symmetry of the transferred structure, the capillary forces upon drying have to be circumvented. We demonstrate two concepts based on thermal fixation for this. It allows for the first time to fabricate non-close packed, non-hexagonal colloidal monolayers on a macroscopic length scale.