;Mesoscopic liquids under confinement exhibit structural and dynamical behaviors distinct from those of bulk systems. A flat confinement boundary suppresses particle motion normal to the boundary, aligns nearby particles, and induces layering. Under sufficiently strong confinement and cooling, this layered order extends across the entire gap, and each layer favors triangular intralayer packing. The relative positions of adjacent layers then determine the three-dimensional steric structure. Although confinement-induced layering has been extensively studied, the detailed microstructure of tightly confined Yukawa solids far below the layering transition remains less understood.
Manipulating solid structures by growing a lattice on a designed surface is known as epitaxy. Conventional superlattices are formed by stacking thin crystalline layers with mismatched lattice constants and/or orientations, typically using patterned substrates or lattice-mismatched interfaces. This raises a fundamental question: can a structure-free flat boundary alone generate a self-assembled superlattice?
In this work, three-dimensional Yukawa solids tightly confined between two parallel, structure-free flat boundaries are investigated numerically over the screening-parameter range κ=r_w/λ_D=0.6–5. Cooling produces layers parallel to the boundaries, with the outermost layer exhibiting a higher packing density, a smaller mean intralayer lattice constant, and a larger spacing to the adjacent layer than the nearly uniform inner region. This boundary-induced asymmetry produces Moiré-patterned superlattices between the first two outermost layers. As κ increases, the system crosses over from large lattice-constant and arbitrary orientation mismatches at small κ, to partial alignment of one pair of lattice axes at intermediate κ, and finally to nearly uniform HCP/FCC-type packing without Moiré-patterned superlattice formation for κ≳4.
