  Ionic layer epitaxy (ILE) has recently been developed as an effective
strategy to synthesize nanometer-thick 2D materials with a non-layered crystal
structure, such as ZnO. The packing density of the amphiphilic monolayer is
believed to be a key parameter that controls the nanosheet nucleation and
growth. In this work, we systematically investigated the growth behavior of
single-crystalline ZnO nanosheets templated at the water-air interface by an
anionic oleylsulfate monolayer with different packing densities. The
thicknesses of ZnO nanosheets were tuned from one unit cell to four unit cells,
and exhibited good correlation with the width of Zn2+ ion concentration zone
(the Stern layer) underneath the ionized surfactant monolayer. Further analysis
of the nanosheet sizes and density revealed that the nanosheet growth was
dominated by the steric hindrance from the surfactant monolayer at lower
surface pressure; while the nucleation density became the dominating factor at
higher surface pressure. The ZnO nanosheets exhibited a decreasing work
function as the thickness reduced to a few unit cells. This research validated
a critical hypothesis that the nanosheet growth is self-limited by the
formation of a double layer of ionic precursors. This work will open up a new
way towards controlled synthesis of novel 2D nanosheets from non-layered
materials with a thickness down to one unit cell.
