Massive Vacancy Concentration Yields Strong Room-Temperature Ferromagnetism in Two-Dimensional ZnO

Abstract

Two-dimensional (2D) ZnO nanosheets with highly concentrated Zn vacancies (V Zn ) of up to approximately 33% were synthesized by ionic layer epitaxy at the water−toluene interface. This high cation vacancy concentration is unprecedented for ZnO and may provide unique opportunities to realize exotic properties not attainable in the conventional bulk form. After annealing, the nanosheets showed characteristic magnetic hysteresis with saturation magnetization of 57.2 emu/g at 5 K and 50.9 emu/g at room temperature. This value is 1 order of magnitude higher than other ZnO nanostructures and comparable to the conventional ferrimagnetic Fe 3 O 4 . Density functional theory calculations, with the support of experimental results, suggest that a high concentration of V Zn (approximately one-third of the Zn sites) can form spontaneously during synthesis when stabilized by H ions, and the formation of V Zn could be further facilitated by the presence of grain boundaries. It is essential to remove the H for the nanosheets to show ferromagnetism. The mechanisms identified for the origin of the high magnetism in ZnO nanosheets presents an intriguing example of a kinetically stabilized, non-equilibrium, highly defective 2D nanomaterial with a significantly enhanced physical property.

Harmonizing the Electronic Structures on BiOI with Active Oxygen Vacancies toward Facet‐Dependent Antibacterial Photodynamic Therapy

“…[ 10 ] For instance, ZnO nanosheets with highly concentrated Zn vacancies ( V Zn ) exhibit higher magnetism than other ZnO nanostructures and comparable to the conventional ferrimagnetic Fe 3 O 4 . [ 11 ] Interestingly, Wang et al reported a p‐type nonstoichiometric Ni 1− x O involving a semiconductor‐to‐insulator‐to‐metal transition with the formation of oxygen (anionic) vacancy defects, which provides a new understanding to defect effect. [ 12 ] Inspired by the results, it is of pivotal importance to harmonize the electronic structures combining the facet engineering and defect engineering.…”

“…[10] For instance, ZnO nanosheets with highly concentrated Zn vacancies (V Zn ) exhibit higher magnetism than other ZnO nanostructures and comparable to the conventional ferrimagnetic Fe 3 O 4 . [11] Interestingly, Wang et al reported a p-type nonstoichiometric Ni 1−x O involving a semiconductor-to-insulator-to-metal transition with the formation of oxygen (anionic) vacancy defects, which…”

Morphology-Dependent Room-Temperature Ferromagnetism in Undoped ZnO Nanostructures

“…ZnO HNWA has the highest M s is ~7.1 emu/g. Two-dimensional (2D) nanosheets/nanoplates have been synthesized by HYT [45,82,83] and ionic layer epitaxy (ILE) [41] in Figure 9. The average thickness of ZnO Nanoplates is estimated to be ~20 nm by the SEM images and XRD peak.…”

“…Two-dimensional ZnO nanosheets with high V Zn concentration have been grown by ILE [41] (Figure 9E,G). The nanosheets annealed in Ar at 400 • C for 1h show M s of 50.9 emu/g in Figure 9G.…”

“…Then, RTFM with a saturation magnetization up to 35.65 emu/g was also observed in undoped ZnO thin films (TFs) by pulsed laser deposition (PLD) and V Zn located at the surface or the interface of the samples could be the source of RTFM [40]. More importantly, various ZnO nanostructures with RTFM were obtained by various approaches, such as ionic layer epitaxy [41], polymer-assisted deposition (PAD) [42][43][44], electrochemical deposition method [45], sol-gel [36,46,47], ball milling (BM) [48][49][50], and PLD [51], etc.…”