Extensive calculations based on density functional theory have been carried out to understand the origin of magnetism in undoped ZnO thin films as found in recent experiments. The observed magnetism is confirmed to be due to Zn, instead of O, vacancy. The main source of the magnetic moment, however, arises from the unpaired 2p electrons at O sites surrounding the Zn vacancy with each nearest-neighbor O atom carrying a magnetic moment ranging from 0.490 to 0.740 B. Moreover, the study of vacancy-vacancy interactions indicates that in the ground state, the magnetic moments induced by Zn vacancies prefer to ferromagnetically couple with the antiferromagnetic state lying 44 meV higher in energy. Since this is larger than the thermal energy at room temperature, the ferromagnetic state can be stable against thermal fluctuations. Calculations and discussions are also extended to ZnO nanowires that have larger surface to volume ratio. Here, the Zn vacancies are found to lead to the ferromagnetic state too. The present theoretical study not only demonstrates that ZnO samples can be magnetic even without transition-metal doping but also suggests that introducing Zn vacancy is a natural and an effective way to fabricate magnetic ZnO nanostructures. In addition, vacancy mediated magnetic ZnO nanostructures may have certain advantages over transition-metal doped systems in biomedical applications.

https://absimage.aps.org/image/MAR08/MWS_MAR08-2007-004197.pdf

Zinc Vacancy induced magnetism in ZnO thin films and nanowires

Recent advances on preparation and environmental applications of MOF-derived carbons in catalysis

Based on electronic structure and atomic size considerations, nitrogen has been regarded as the most suitable impurity for p-type doping in ZnO. However, numerous experimental efforts by many different groups have not resulted in stable and reproducible p-type material, casting doubt on the efficacy of nitrogen as a shallow acceptor. Based on advanced first-principles calculations we find that nitrogen is actually a deep acceptor, with an exceedingly high ionization energy of 1.3 eV, and hence cannot lead to hole conductivity in ZnO. In light of this result, we reexamine prior experiments on nitrogen doping of ZnO.

http://absimage.aps.org/image/MAR10/MWS_MAR10-2009-001476.pdf

Why nitrogen cannot lead to p-type conductivity in ZnO

Photoelectric detectors are the central part of modern photodetection systems with numerous commercial and scientific applications. p-Type semiconductor materials play important roles in optoelectronic devices. Photodetectors based on p-type semiconductor materials have attracted a great deal of attention in recent years because of their unique properties. Here, a comprehensive summary of the recent progress mainly on photodetectors based on inorganic p-type semiconductor materials is presented. Various structures, including photoconductors, phototransistors, homojunctions, heterojunctions, p-i-n junctions, and metal-semiconductor junctions of photodetectors based on inorganic p-type semiconductor materials, are discussed and summarized. Perspectives and an outlook, highlighting the promising future directions of this research field, are also given.

Photoelectric Detectors Based on Inorganic p-Type Semiconductor Materials.

We report here enhancement of ferromagnetism in pure ZnO upon thermal annealing with the ferromagnetic transition temperature Tc above room temperature. We observe a finite coercive field upto 300K and a finite thermoremanent magnetization upto 340K for the annealed sample. We propose that magnetic moments can form at anionic vacancy clusters. Ferromagnetism can occur due to either superexchange between vacancy clusters via isolated F+ centers, or through a limited electron delocalization between vacancy clusters. Isolated vacancy clusters or isolated F+ centers give rise to a strong paramagnetic like behaviour below 10K.

Enhancement of ferromagnetism upon thermal annealing in pure ZnO

A Durable Na0.56V2O5 Nanobelt Cathode Material Assisted by Hybrid Cationic Electrolyte for High-Performance Aqueous Zinc-Ion Batteries

Anionic defect-enriched ZnMn2O4 nanorods with boosting pseudocapacitance for high-efficient and durable Li/Na storage

A lithium and nitrogen codoping method has been employed to prepare p-type MgZnO films, and p-MgZnO/i-ZnO/n-ZnO structured light-emitting devices (LEDs) and photodetectors have been fabricated. The LEDs can work continuously for about 97 h under the injection of a 20 mA continuous current, which is the best value ever reported for ZnO-based LEDs. The performance of the photodetectors degrades little after several running cycles. The above results reveal the applicability of the p-MgZnO films in optoelectronic devices.

p-type doping of MgZnO films and their applications in optoelectronic devices.

A low temperature (10 K) photoluminescence study shows that green luminescence (GL) peaking at 2.47 eV and near band edge (NBE) emission at 3.23 eV are introduced in undoped ZnO grown by pulsed laser deposition (PLD) after 900 °C annealing. The NBE emission exhibiting blue shift with increasing temperature is assigned to the transitions of the donor–acceptor-pair (DAP)/free-electron-to-acceptor (FA). Positron annihilation spectroscopy (PAS) study shows that the introduction of the GL is correlated with the formation of the Zn vacancy-related defect (VZn). Comparing the transition energies of VZn obtained by the previous first principles calculation [Janotti and Van de Walle, Phys. Rev. B: Condens. Matter Mater. Phys., 2007, 76, 165202], the GL is associated with the transition from the conduction band to the e(−/2−) state of VZn and the DAP/FA emission involves the acceptor level e(0/−) of VZn.

/pdf/the-zn-vacancy-related-green-luminescence-and-donor-acceptor-u6j9wdbzb4.pdf

The Zn-vacancy related green luminescence and donor–acceptor pair emission in ZnO grown by pulsed laser deposition

As a novel sodium ion battery anode material, 3D pollen-scaffolded NiSe composite encapsulated by metal-organic framework-derived (MOF-derived) carbon shell is synthesized, denoted as P–NiSe@C. The results depict that well-crystallized NiSe particles were in-situ grown on 3D pollen framework, preventing the particles aggregation effectively. Meanwhile, the pyrolytic MOF-derived shell from 2-methylimidazole further strengthens the adhesion between NiSe particles and pollen skeleton, which would enhance the structural stability and mitigate the volumetric changes during sodium intercalation/deintercalation. Compared with the inferior sodium storage capability of raw NiSe, the P–NiSe@C electrode delivers a sustainably reversible capacity of 598.2 mAh g−1 (200 mA g−1 after 100 cycles) and excellent rate performance of 488.9 mAh g−1 even at a large current density of 2000 mA g−1. The P–NiSe@C electrode also has an impressive high-low temperature adaptability with durable and controllable capacities of 343.8–792.6 mAh g−1 from −5 to 70 °C.

Francis Chi-Chung Ling

Papers

A Durable Na0.56V2O5 Nanobelt Cathode Material Assisted by Hybrid Cationic Electrolyte for High-Performance Aqueous Zinc-Ion Batteries

Anionic defect-enriched ZnMn2O4 nanorods with boosting pseudocapacitance for high-efficient and durable Li/Na storage

p-type doping of MgZnO films and their applications in optoelectronic devices.

The Zn-vacancy related green luminescence and donor–acceptor pair emission in ZnO grown by pulsed laser deposition

3D pollen-scaffolded NiSe composite encapsulated by MOF-derived carbon shell as a high-low temperature anode for Na-ion storage