In-House 2DCC Research Highlights 2018

In-House 2DCC Research Highlights 2018

Origami with regular paper can produce a limitless number of fascinating, beautiful and useful shapes, but we lack “fingers” at the nanoscale to guide the folding. Researchers in the Two-Dimensional Crystal Consortium at Penn State have conceived and simulated a novel way to “program” a folding pattern by patterning the nanoscale sheet with complementary regions of n-type and p-type doping. Such regions prefer to adhere to each other more than they like to adhere to themselves, and the boundaries between these regions define the fold lines of the final origami structure. Forcing the sheet through a small hole is enough to start the folding process, and controlling the shape and depth of such holes provides another means to influence the order in which the folds occur.

Monolayer 2D materials, particularly the family of 2D transition metal dichalcogenides, have been a focus of increasing interest due to their unique properties. The development of device technologies based on 2D chalcogenide has, however, been hampered by difficulties in synthesizing large area monolayer and few layer films. We developed a multi-step process involving nucleation, ripening and preferential lateral growth to achieve epitaxial WSe2 monolayers on sapphire by gas source chemical vapor deposition. This process enables control of the nucleation density of WSe2 on the sapphire and the lateral growth rate of domains resulting in coalescence of monolayer regions over wafer-scale areas with minimal multilayer growth. The process also provides insights into the fundamental kinetic and thermodynamic factors that impact monolayer growth.

When an atomically thin 2D material is suspended as a membrane so that adsorbed atoms can stick to both sides, these atoms can interact through the membrane and thus act as two coupled adsorbate systems, with new properties that are absent in either system alone. Computational modeling in the 2D Crystal Consortium predicts new patterns that emerge, such as an infinite staircase of fractional coverages of opposing sides with simple rational fractions being favored, a so-called “Devil’s Staircase.” Since adsorbate patterns are possible at different spatial scales, it is possible to obtain parallel staircases with different step heights, and even come arbitrarily close to violating a revered principle of thermodynamics, the Gibbs Phase Rule. These systems provide a new window into the behavior of atoms on surfaces.