Unit Cell-Level Thickness Control of Single-Crystalline Zinc Oxide Nanosheets Enabled by Electrical Double Layer Confinement

TL;DR

This study demonstrates precise control over the thickness of single-crystalline ZnO nanosheets at the unit cell level by manipulating ionic double layer confinement during ionic layer epitaxy, enabling the synthesis of ultra-thin 2D materials.

Contribution

It introduces a method to tune ZnO nanosheet thickness from one to four unit cells via ionic double layer control, advancing 2D material synthesis techniques.

Findings

Thickness correlates with Stern layer width.

Growth dominated by steric hindrance at low pressure.

Work function decreases with reduced thickness.

Abstract

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.