Nanometre-thick single-crystalline nanosheets grown at the water–air interface

Abstract

To date, the preparation of free-standing 2D nanomaterials has been largely limited to the exfoliation of van der Waals solids. The lack of a robust mechanism for the bottom-up synthesis of 2D nanomaterials from non-layered materials has become an obstacle to further explore the physical properties and advanced applications of 2D nanomaterials. Here we demonstrate that surfactant monolayers can serve as soft templates guiding the nucleation and growth of 2D nanomaterials in large area beyond the limitation of van der Waals solids. One- to 2-nm-thick, single-crystalline free-standing ZnO nanosheets with sizes up to tens of micrometres are synthesized at the water–air interface. In this process, the packing density of surfactant monolayers adapts to the sub-phase metal ions and guides the epitaxial growth of nanosheets. It is thus named adaptive ionic layer epitaxy (AILE). The electronic properties of ZnO nanosheets and AILE of other materials are also investigated.

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

“…This analysis supports the claim that by controlling the packing density of the surfactant monolayer one can digitally tune the nanosheet thickness with a resolution of one unit cell all the way down to the thickness of one unit cell. Since it was previously hypothesized that the growth of crystalline nanosheets is self-limited by the available precursor source provided by the Stern layer under the surfactant monolayer, 13 here the number of nanosheets per unit surface area, i.e. the nanosheet density, was also analyzed to test this hypothesis.…”

“…[13][14] In ILE, amphiphilic molecules (e.g., surfactants) self-assemble into a monolayer at the water-air interface, serving as a template to direct the nucleation and growth of a crystalline nanosheet underneath. Single-crystal ZnO nanosheet with ~2 nm thickness has been shown as a representative example of this unique 2D growth strategy.…”

Electronic properties of two-dimensional zinc oxide in hexagonal, (4,4)-tetragonal, and (4,8)-tetragonal structures by using Hybrid Functional calculation

“…For instance, the epitaxial growth of ZnO monolayer in hexagonal structure on graphene substrate was recently achieved and reported to have band gap up to 4 eV [4]. In addition, sub 100-nm-sized ZnO nanosheet in hexagonal wurtzite was also found to successfully grow in the solution synthesis using surfactant molecules as a facet at the water-air interface [5]. These then illuminate some possibilities in nanostructure-engineering to achieve ultrathin semiconductor with desired properties.…”

Memristive Behavior Enabled by Amorphous–Crystalline 2D Oxide Heterostructure

“…Recently, ionic layer epitaxy (ILE) was developed as a versatile solution‐based approach for growing large‐area oxide nanosheets on a liquid surface. [ 13 ] Atomically thin single‐crystalline ZnO nanosheets were synthesized with sizes over 10 µm, enabling the study of the memristor properties based on 2D metal oxides. In this work, we show extraordinary memristive behavior enabled by interfacing atomically thin single crystalline ZnO nanosheets with a few‐nm thick amorphous Al 2 O 3 layer.…”

“…Atomically thin single‐crystalline ZnO nanosheets were synthesized by ILE, [ 13,14 ] in which amphiphilic molecules (oleyl sulfate) self‐assembled into a monolayer at the water–air interface, serving as a template to direct the nucleation and growth of ZnO crystalline nanosheet underneath. The as‐synthesized ZnO nanosheet with a size up to ≈10 µm was then transferred onto a Si wafer with 50 nm thermal oxide layer for device fabrication.…”