Understanding lattice defects to influence ferromagnetic order of ZnO nanoparticles by Ni, Cu, Ce ions

K. T, Mandal G, Chanda A, Vasundhara M. Structural, optical and magnetic properties of pure and 3d metal dopant-incorporated SnO2 nanoparticles

Dilute magnetic oxide semiconductors doped with transition metals have attracted significant attention both theoretically and experimentally due to their interesting and debatable magnetic behavior. In this work, we investigated the influence of Fe, Co and Ni doping on the structural, optical and magnetic properties of SnO₂ nanoparticles, which were produced via a simple sol–gel technique. Raman spectroscopy, XRD, XPS, TEM, FT-IR characterizations were performed to study the crystal structure and morphology of the pure and doped nanoparticles, which confirmed the tetragonal rutile structure of the SnO₂ nanoparticles. The XPS analysis revealed the incorporation of divalent dopant ions in the host matrix. The Raman plots indicated the generation of the cassiterite crystal structure, structural disorder and oxygen vacancies in the pure and doped SnO₂ nanoparticles. The UV-visible plots indicated a decrease in the bandgap for the doped SnO₂ nanoparticles because doping introduced defect levels in the band gap. The photoluminescence study showed the creation of oxygen vacancies due to the doping of different charge states of dopants. The magnetic study based on varying the temperature and field of magnetization revealed the diamagnetic nature of SnO₂ at 300 K and 5 K respectively, and the concurrence of ferromagnetic (FM) and paramagnetic (PM) nature in doped SnO₂ nanoparticles. The bound polaron model was used to explain the co-existence of FM and PM behavior in all the doped SnO₂ nanoparticles.

We focused on the systematic study of the effect of Fe, Co and Ni substitution on the structural, optical and magnetic properties of SnO₂ nanoparticles.

Effects of Copper Dopants on the Magnetic Property of Lightly Cu-Doped ZnO Nanocrystals

To explore the origin of magnetism, the effect of light Cu-doping on ferromagnetic and photoluminescence properties of ZnO nanocrystals was investigated. These Cu-doped ZnO nanocrystals were prepared using a facile solution method. The Cu²⁺ and Cu⁺ ions were incorporated into Zn sites, as revealed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). At the Cu concentration of 0.25 at.%, the saturated magnetization reached the maximum and then decreased with increasing Cu concentration. With increasing Cu concentration, the photoluminescence (PL) spectroscopy indicated the distribution of V_O⁺ and V_O⁺⁺ vacancies nearly unchanged. These results indicate that Cu ions can enhance the long-range ferromagnetic ordering at an ultralow concentration, but antiferromagnetic “Cu⁺-Vo-Cu²⁺” couples may also be generated, even at a very low Cu-doping concentration.

Lattice defect-formulated ferromagnetism and UV photo-response in pure and Nd, Sm substituted ZnO thin films

The induction of charge and spin in diluted magnetic semiconductor ZnO is explored for spintronic devices and its wide direct band gap (3.37 eV) and large exciton binding energy (60 meV) exhibit potential in UV photodetectors. We reported the ferromagnetic and optical properties of pure ZnO, Zn_0.97Nd_0.03O and Zn_0.97Sm_0.03O thin films. These thin films were synthesized by a metallo-organic decomposition method and annealed at 500 °C for 7 h. Rietveld refinement of the XRD data results in a wurtzite ZnO structure with Nd, Sm doping. The dopants and nanoparticle size are responsible for wurtzite structural deformation, inducing lattice strain effect, which may influence the band gap energy and high-T_C ferromagnetism of ZnO. The average size of ZnO nanoparticles with Nd, Sm doping is 10 nm, confirmed with atomic force microscopy. The Raman spectra confirm the wurtzite structure of ZnO with crystalline quality and lattice defect formation with dopant Nd, Sm ions. A near-band-edge emission due to band gap energy is evaluated with photoluminescence spectra, which also involved multiple visible emissions due to oxygen vacancies. The oxygen vacancies-mediated magnetic interactions impart room temperature ferromagnetism in pure ZnO which is enhanced with Nd, Sm doping. The electron paramagnetic resonance spectra revealed the effects of defects and unpaired electrons responsible for observed room temperature ferromagnetism. The zero field cooling and field cooling magnetic measurements include antiferromagnetic interactions without any spin-glass formation. The observed ferromagnetism also correlates with first principle calculations reported for Nd, Sm-doped ZnO and suggests long-range ferromagnetic ordering attributed to defect carriers. The Nd, Sm doping into ZnO thin films significantly enhances absorption in the UV region and suggests its usability for UV detectors. Under UV irradiation (λ = 325 nm), the value of photocurrent in Nd, Sm:ZnO thin films is highly enhanced for possible use in UV sensors.

Interface interactions and enhanced room temperature ferromagnetism of Ag@CeO2 nanostructures

Enhancement of room temperature ferromagnetism (RTFM) has been achieved with core-shell metal-oxide nanoparticles (Ag@CeO₂). To enhance the magnetic properties, interfacial charge transfer is achieved via the formation of a core-shell interface. Furthermore, by varying the shell thicknesses, additional control of the RTFM can be obtained. The Ag@CeO₂ core-shell nanoparticles are synthesized successfully via a two-step method. Ag nanoparticles (NPs) are first synthesized on a TiO₂ substrate by a thermally assisted photoreduction method, and then CeO₂ NPs are deposited on the surface of Ag NPs by chemical reduction. No surfactants or organic compounds are used during the synthesis. At the interface between the core and the shell, electron transfers from the Ag-p orbital to the Ag-d and Ce-f orbitals are evidenced by X-ray absorption spectroscopy and electron energy loss spectroscopy. Such interfacial charge transfer results in enhanced room temperature ferromagnetism in the Ag@CeO₂ core-shell NPs compared to the magnetism arising for bare Ag or CeO₂ NPs. This study suggests that tailoring the interface, the surface and their coupling in nanostructured metal-oxide core shell nanoparticles is an effective way to enhance their magnetic properties.