Pure and Oxidized Ag Substrate Influence on the Phase Transformation and Semiconducting Behaviors of Layered ZnO: A First-Principles Study

Abstract

Graphitic-like ZnO layers have been experimentally synthesized on metal substrates over the past few years. Nevertheless, the impact of metal substrates on the structural and electric properties of ZnO is still unclear. Utilizing first-principle calculations with van der Waals correction, we found that the phase transformation from graphitic-like to wurtzite structure occurs when the thickness of freestanding ZnO exceeds seven layers. With the presence of pure Ag(111) substrate, the critical transformation thickness decreases to two layers because of the depolarization effect originating from the charge transfer from Ag substrate to ZnO. Band structure analysis displays the semiconducting behaviors for the freestanding graphitic-like ZnO layers. On the pure Ag substrate, monolayer and bilayer ZnO is n-doped by the substrate and a metallic character of ZnO is observed. Importantly, the semiconducting behavior of ZnO layers is maintained when ZnO is in contact with oxidized Ag substrate because of less charge transfer between ZnO and Ag. The metal−semiconductor contact results in a Schottky barrier of 0.8 eV. The simulation findings indicate that the few-layered ZnO on oxidized Ag system possesses potential applications in optoelectronic devices.

Tunable Lattice Constant and Band Gap of Single- and Few-Layer ZnO

“…The polar wurtzite ZnO(0001)-Zn and ZnO(0001̅)-O surfaces are unstable, causing reconstructions that stabilize these surfaces. , However, in the ultrathin ZnO regime, a different stabilization mechanism is at work; where depolarization takes place via the formation of the unreconstructed planar sheets (Figure b). , Structurally, the bonding configuration of the Zn and O atoms in the ZnO layers changes from sp 3 tetrahedral in the wurtzite structure to sp 2 trigonal coordination in the planar structure. For free-standing ZnO, the critical thickness up to which the planar structure is stable was predicted computationally to be ∼8 layers, , while in the case of ZnO layers supported on a Ag(111) surface, the planar structure was calculated to be stable up to a bilayer . Experimentally, it was found that the ZnO layers on Ag(111) and Pd(111) adopt bulk-like structures at a thickness of 4 layers. , …”

“…For free-standing ZnO, the critical thickness up to which the planar structure is stable was predicted computationally to be ∼8 layers, 15,17 while in the case of ZnO layers supported on a Ag(111) surface, the planar structure was calculated to be stable up to a bilayer. 18 Experimentally, it was found that the ZnO layers on Ag(111) and Pd(111) adopt bulk-like structures at a thickness of 4 layers. 16,19 A material's band gap is another important property which can impact its application in photovoltaics, sensing, and photocatalysis.…”

“…22,23 For stacked layers of ZnO, the band gap was calculated to decrease as the number of layers increased, eventually approaching the bulk value. 18,24,25 Experimentally, for thicker ZnO films (>15 ML) grown on Au(111), the band gap was measured to be 2.8 eV. 26,27 However, a systematic measurement of the band gap modification of ZnO as a function of the number of ZnO layers in the few-layer regime is still lacking in the literature.…”

Tuning Photocatalytic Performance of Multilayer ZnO for Water Splitting by Biaxial Strain Composites

“…Furthermore, the calculated formation energies of nL ZnO are listed in Table 1 as inversely related to the number of layers. Many studies have successfully synthesized monolayer or few-layer ZnO [21,27]. therefore, these formation energies might indicate that several layers of ZnO were more stable and more comfortable to synthesize than monolayer ZnO.…”

“…In the case of ZnO, for instance, due to surface depolarization, ZnO films that are only a few atomic layers thick favor a stable graphite-like multilayer structure. Furthermore, when the number of layers increases, ZnO films begin transforming into the bulk structure at a certain critical thickness [21][22][23][24][25]. Theoretically, some previous first-principles calculation studies predicted the existence of graphite-like multilayer ZnO [9,26].…”