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

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 Zn 2+ 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.

Ligand‐Programmed Consecutive Symmetry Break(s) in Nanoparticle Based Materials Showing Emergent Phenomena: Transitioning from Sixfold to Threefold Symmetry in Anisotropic ZnO Colloids

“…[ 9 ] There are only few reports in literature, which show this is possible for Wurtzite‐based materials deposited on solid substrates. [ 42–54 ] In their study on MBE (molecular beam epitaxy) of GaN on GaN substrates Xie et al. showed that a hexagonal symmetric bilayer of the cation and the anion exhibits two types of alternating step edges: [ 42 ] Edge‐Type A with two dangling bonds and Type B with only one dangling bond (Scheme 1b).…”

Ligand-Programmed Consecutive Symmetry Break(s) in Nanoparticle Based Materials Showing Emergent Phenomena: Transitioning from Six-Fold to Three-Fold Symmetry in Anisotropic ZnO Colloids

“…[9] There are only few reports in literature, which show this is possible for Wurtzite-based materials deposited on solid substrates. [40][41][42][43][44][45][46][47][48][49][50][51][52] In their study on MBE (Molecular Beam Epitaxy) of GaN on GaN substrates Xie et al showed that a hexagonal symmetric bilayer of the cation and the anion exhibits two types of alternating step edges: [40] Edge-Type A with two dangling bonds and Type B with only one dangling bond (Scheme 1b). Thus, the two edge types are kinetically distinguishable.…”

Recent Progress on Two‐Dimensional Heterostructures for Catalytic, Optoelectronic, and Energy Applications

“…These 2D materials can be further classified into two groups, 2D layered materials and 2D non-layered materials. Wet chemical synthesis is a common route to produce nanosheets with non-layered parent materials such as ZnO, [27,28] CuI, [29] TiO 2 , [30] SnTe, [31] Pb 1-x Sn x Se, [32] Pb 1-x Sn x Te, [33] etc. Typical 2D layered materials include graphene, hexagonal boron nitride (h-BN), [16,17,18] transition metal dichalcogenides (TMDs), [19][20][21][22][23][24][25] and bismuth oxyhalide (BiOX, X = Cl, Br and I), [26] etc., with the electronic properties varied from semimetal to insulator.…”

“…In contrast, 2D non-layered materials cannot be simply achieved by exfoliation method due to the strong covalence bonds among crystal planes. Wet chemical synthesis is a common route to produce nanosheets with non-layered parent materials such as ZnO, [27,28] CuI, [29] TiO 2 , [30] SnTe, [31] Pb 1-x Sn x Se, [32] Pb 1-x Sn x Te, [33] etc. Very recently, molecular beam epitaxial (MBE) deposition method has also been employed to produce elemental 2D materials, which is expected to be absent in nature, such as graphene's cousins, silicene, [34][35][36][37][38][39][40][41][42][43][44][45][46][47] germanene, [5,[48][49][50][51][52] phosphorene, [53,54] stanene, [55,56,57] and so on.…”