Strain patterning and direct measurement of Poisson's ratio in nanoparticle monolayer sheets.

Close-packed monolayers self-assembled from ligated nanoparticles can form 10 nm thin sheets that stretch over micrometer-wide holes. Employing electron and focused ion beams, we show that one can locally tailor the strain in such sheets while they remain clamped around their perimeter, making it possible to imprint strain fields by design. Furthermore, using the nanoparticles themselves to track imposed strain gradients allows for the first direct measurement of Poisson's ratio in these two-dimensional materials.