Origin of ferromagnetism in Cu-doped ZnO

Coexistence of ferromagnetic-antiferromagnetic ground state, exchange bias effect and bandgap narrowing in Cr-doped ZnO nanocrystals derived by simple chemical method

A detailed investigation of the structural, optical and magnetic properties of Cr-doped ZnO nanostructures obtained via a simple chemical method has been carried out. The structural study using X-ray powder diffraction indicates the hexagonal wurtzite structure for undoped ZnO and ZnO doped with 1% Cr, whereas the appearance of a secondary cubic phase (ZnCr2O4) is witnessed with the incorporation of Cr content ≥3% in the ZnO compounds. Furthermore, the secondary phase is observed to increase systematically with the increase of the Cr concentration. Field emission scanning electron microscopy and high-resolution transmission electron microscopy studies indicate cuboid, hexagonal and rod-type structural morphology in all the nanocrystals. The presence of the cubic structure along with the hexagonal structure is further confirmed from the selected area electron diffraction pattern. Raman spectroscopy has been used to study the crystalline quality, defects and disorder present in the host lattice. UV-visible spectra were obtained to study the effect of Cr doping on the optical absorption and hence to determine the bandgap, and show a decrease in bandgap with increasing Cr concentration. PL spectra show near-band-edge emission along with visible emission, which decreases with a higher concentration of Cr-doped nanocrystals. X-ray photoelectron spectroscopy analysis indicates the incorporation of Cr3+ and Cr2+ ions in the ZnO lattice. Detailed magnetic studies reveal the co-existing ferromagnetic (FM) and antiferromagnetic (AFM) ground states, which result in the observation of an exchange bias (EB) effect in all the doped compounds. The observation of an EB effect arises from the coupling between the FM and AFM components in the Cr-containing ZnO nanocrystals, and provides a way to design new principles and materials platform that are useful for futuristic spintronic devices.

Structural, Morphological, Optical and Magnetic Studies of Cu-Doped ZnO Nanostructures

In the present work, Cu-doped ZnO nanostructures (Cu% = 0, 1, 5) have been prepared using microwave-assisted chemical route synthesis. The synthesized nanostructures were investigated through structural, morphological, optical, and magnetic characterizations. The results of the X-ray diffraction (XRD), high resolution transmission electron microscopy (HR-TEM), and selective area electron diffraction (SAED) patterns confirmed that all of the samples exhibit the single-phase polycrystalline hexagonal crystal structure. The XRD results infer a decrease in the lattice parameters (a/c) by increasing the Cu% doping into ZnO. The field emission scanning electron microscopy (FE-SEM) and energy dispersive x-ray (EDX) spectroscopic measurements revealed the formation of nanostructures, showing the major elemental presence of Zn and O in the samples. The photoluminescence (PL) spectra exhibited photoemission in the UV and blue-green regions. With the increase in the Cu%, the photoemission in the UV region is reduced, while it is enhanced in the blue-green region. Raman spectra of the Cu-doped ZnO nanostructures displayed a blue shift of the E2High mode and an increase in the peak intensity of E₁(LO), indicating the doping of Cu ion in the ZnO lattice. The dc magnetization measurements demonstrated the ferromagnetic behavior of all of the samples with an enhanced ferromagnetic character with increasing Cu%.

Modulation of intrinsic defects in vertically grown ZnO nanorods by ion implantation

Intrinsic defects created by chemically inert gas (Xe) ion implantation in vertically grown ZnO nanorods are studied by optical and X-ray absorption spectroscopy (XAS). The surface defects produced due to dynamic sputtering by ion beams control the fraction of O and Zn with ion fluence, which helps in tuning the optoelectronic properties. The forbidden Raman modes related to Zn interstitials and oxygen vacancies are observed because of the weak Fröhlich interaction, which arises due to disruption of the long-range lattice order. The evolution of the lattice disorder is identified by O K-edge and Zn K-edge scans of XAS. The hybridization strength between the O 2p and Zn 4p states increases with ion fluence and modulates the impact of intrinsic defects. The ion irradiation induced defects also construct intermediate defects bands which reduce the optical bandgap. Density functional theory (DFT) calculations are used to correlate the experimentally observed trend of bandgap narrowing with the origin of electronic states related to Zn interstitial and O vacancy defects within the forbidden energy gap in ZnO. Our finding can be beneficial to achieve enhanced conductivity in ZnO by accurately varying the intrinsic defects through ion irradiation, which may work as a tuning knob to control the optoelectronic properties of the system.

Fabrication of Ag-doped ZnO/PAN composite nanofibers by electrospinning: Photocatalytic and antiviral activities

Ag-doped ZnO nanoparticles (AZNs) were directly synthesized using sol-gel method to embed into polyacrylonitrile (PAN) nanofibers by electrospinning. The synthesized AZNs were optically and structurally characterized by UV-VIS spectroscopy, photoluminescence spectroscopy, high resolution HR-TEM and XRD. The photocatalytic activity of the AZNs was examined by photocatalytic degradation of methylene blue to correlate with their antiviral efficacy in PAN nanofibers fabricated via electrospinning technique. The PAN nanofibers containing AZNs were characterized using SEM and EDS. Finally, antiviral activity of AZNs/PAN nanofibers was investigated by using virus ϕx174 under visible light irradiation. As a result, the antiviral efficacy of nanofibers increased as the concentration of Ag in AZNs increased. The results show that better antiviral efficacy was obtained in AZNs/PAN nanofibers prepared with AZNs of higher photocatalytic performance.

Improving the Room-Temperature Ferromagnetism in ZnO and Low-Doped ZnO:Ag Films Using GLAD Sputtering

ZnO and doped ZnO films with non-ferromagnetic metal have been widely used as biosensor elements. In these studies, the electrochemical measurements are explored, though the electrical impedance of the system. In this sense, the ferromagnetic properties of the material can be used for multifunctionalization of the sensor element using external magnetic fields during the measurements. Within this context, we investigate the room-temperature ferromagnetism in pure ZnO and Ag-doped ZnO films presenting zigzag-like columnar geometry. Specifically, we focus on the films' structural and quasi-static magnetic properties and disclose that they evolve with the doping of low-Ag concentrations and the columnar geometry employed during the deposition. The magnetic characterization reveals ferromagnetic behavior at room temperature for all studied samples, including the pure ZnO one. By considering computational simulations, we address the origin of ferromagnetism in ZnO and Ag-doped ZnO and interpret our results in terms of the Zn vacancy dynamics, its substitution by an Ag atom in the site, and the influence of the columnar geometry on the magnetic properties of the films. Our findings bring to light an exciting way to induce/explore the room-temperature ferromagnetism of a non-ferromagnetic metal-doped semiconductor as a promising candidate for biosensor applications.

Computational studies of the electronic structure of copper-doped ZnO quantum dots

Copper-doped ZnO quantum dots (QDs) have attracted substantial interest. The electronic structure and optical and magnetic properties of Cu³⁺(d⁸)-, Cu²⁺(d⁹)-, and Cu⁺(d¹⁰)-doped ZnO QDs with sizes up to 1.5 nm are investigated using the GGA+U approximation, with the +U corrections applied to d (Zn), p(O), and d(Cu) orbitals. Taking +Us parameters, as optimized in previous bulk calculations, we obtain the correct band structure of ZnO QDs. Both the description of electronic structure and thermodynamic charge state transitions of Cu in ZnO QDs agree with the results of bulk calculations due to the strong localization of Cu defect energy levels. Atomic displacements around Cu are induced by strong Jahn-Teller distortion and affect Kohn-Sham energies and thermodynamic transition levels. The average bond length of Cu-O and the defect structure are crucial factors influencing the electronic properties of Cu in ZnO QDs. The analysis of the optical properties of Cu in ZnO QDs is reported. The GGA+U results, compared with the available experimental data, support Dingle's model [Phys. Rev. Lett. 23, 579 (1969)], in which the structured green luminescence observed in bulk and nanocrystals originates from the [(Cu⁺, hole) → Cu²⁺] transition. We also examine the magnetic interaction between the copper pair for two charge states: 0 and +2, and four positions relative to the center of QDs. Ferromagnetic interaction between ions is obtained for every investigated configuration. The magnitude of ferromagnetism increases for positive charge defects due to the strong hybridization of the d(Cu) and p(O) states.

Magnetic Response Dependence of ZnO Based Thin Films on Ag Doping and Processing Architecture

Multifunctional and multiresponsive thin films are playing an increasing role in modern technology. This work reports a study on the magnetic properties of ZnO and Ag-doped ZnO semiconducting films prepared with a zigzag-like columnar architecture and their correlation with the processing conditions. The films were grown through Glancing Angle Deposition (GLAD) co-sputtering technique to improve the induced ferromagnetism at room temperature. Structural and morphological characterizations have been performed and correlated with the paramagnetic resonance measurements, which demonstrate the existence of vacancies in both as-cast and annealed films. The magnetic measurements reveal changes in the magnetic order of both ZnO and Ag-doped ZnO films with increasing temperature, showing an evolution from a paramagnetic (at low temperature) to a diamagnetic behavior (at room temperature). Further, the room temperature magnetic properties indicate a ferromagnetic order even for the un-doped ZnO film. The results open new perspectives for the development of multifunctional ZnO semiconductors, the GLAD co-sputtering technique enables the control of the magnetic response, even in the un-doped semiconductor materials.

Increased static dielectric constant in ZnMnO and ZnCoO thin films with bound magnetic polarons

A novel small signal equivalent circuit model is proposed in the inversion regime of metal/(ZnO, ZnMnO, and ZnCoO) semiconductor/Si3N4 insulator/p-Si semiconductor (MSIS) structures to describe the distinctive nonlinear frequency dependent capacitance (C-F) and conductance (G-F) behaviour in the frequency range from 50 Hz to 1 MHz. We modelled the fully depleted ZnO thin films to extract the static dielectric constant (εr) of ZnO, ZnMnO, and ZnCoO. The extracted enhancement of static dielectric constant in magnetic n-type conducting ZnCoO (εr ≥ 13.0) and ZnMnO (εr ≥ 25.8) in comparison to unmagnetic ZnO (εr = 8.3-9.3) is related to the electrical polarizability of donor-type bound magnetic polarons (BMP) in the several hundred GHz range (120 GHz for CdMnTe). The formation of donor-BMP is enabled in n-type conducting, magnetic ZnO by the s-d exchange interaction between the electron spin of positively charged oxygen vacancies [Formula: see text] in the BMP center and the electron spins of substitutional Mn2+ and Co2+ ions in ZnMnO and ZnCoO, respectively. The BMP radius scales with the Bohr radius which is proportional to the static dielectric constant. Here we show how BMP overlap can be realized in magnetic n-ZnO by increasing its static dielectric constant and guide researchers in the field of transparent spintronics towards ferromagnetism in magnetic, n-ZnO.

Influence of the type of carrier on ferromagnetism in a Si semiconductor implanted with Cu ions

Silicon semiconductor samples implanted with Cu ions and samples co-implanted with Cu- and N-ions were prepared by MEVVA and the Kaufman technique. None of the samples showed evidence of secondary phases. The initially n-type Si matrix, when implanted with Cu ions, changed to a p-type semiconductor, and the Cu ions existed as local Cu2+ cations in the p-type environment. As a result, none of the Cu-implanted samples were ferromagnetic at room temperature. The co-implanted samples, on the other hand, showed room-temperature ferromagnetism because the introduction of N ions made the carrier type change from p-type to n-type which is favorable for the appearance of Cu2+. First principles calculations were applied to understand the experimental phenomena. The formation energy was reduced by implanting N ions, and was decreased effectively with the increase in ratio of N to Cu ions. The density of states and spin density of states indicated that the hybridization of s, p and d electrons induced ferromagnetism at 0 K. Particularly, we proposed possible exchange interactions between the Cu2+-N-(N4+)-Cu2+ ions to explain the ferromagnetism mechanism.

Doping of ZnO inorganic-organic nanohybrids with metal elements

We present a general and in-depth study of the effect of dopants in hybrid inorganic/organic ZnO/PAA (polyacrylic acid) nanocomposites. These dopants vary as much by their ionic size, as by their electronic valence and some of them have been used in ZnO due to their known magnetic and/or optical properties. The chemical nature of the dopants controls their ability to incorporate into ZnO crystal lattice. Three concentrations (0.1%, 1% and 5%) of dopants were studied in order to compare the effect of the concentration with the results obtained previously in the literature. Our results confirm in the first place the trend observed in the literature, that increase in dopant concentration leads to quenching of visible luminescence for ZnO nanocrystals obtained by very different processes. However, the degradation of photoluminescence quantum yield (PL QY) is not inevitable in our nanocomposites. At low doping concentration for some dopants with a small or comparable ionic radius than Zn²⁺, PL QY can be maintained or even improved, making it possible to tune the visible emission spectrum between 2.17 eV and 2.46 eV. This opens up the prospect of synthesizing phosphors without rare earth for white LEDs, whose spectrum can be tuned to render warm or cold white light, by a chemical synthesis process with a low environmental impact.

Doping effect and oxygen defects boost room temperature ferromagnetism of Co-doped ZnO nanoparticles: experimental and theoretical studies

Co-doped ZnO nanoparticles with different dosage concentrations were fabricated by a thermal decomposition method. The nanoparticles show a pure wurtzite structure without the formation of a secondary phase or Co clusters, in which Co ions present as Co2+ and occupy Zn2+ tetrahedral sites within the ZnO matrix. All the samples show ferromagnetic properties at room temperature with nonzero coercivity and remanence magnetization. Besides, the magnetic data is also fitted by the model of bound magnetic polarons (BMP). By increasing the Co2+ doping concentration, the saturation magnetization values of Co-doped ZnO nanoparticles increase first and then decreases, which is related to the variation tendency of oxygen defects on the surface and the number of BMPs. This phenomenon can be ascribed to the formation of defect-induced BMPs, in which ferromagnetic coupling occurs at lower Co2+ concentration and Co2+-O2--Co2+ antiferromagnetic coupling arises at higher Co2+ concentration. Air annealing experiments further demonstrate this result, in which the saturation magnetization of Co-doped ZnO nanoparticles is reduced after annealing in Air. The doping effect and oxygen defects on the magnetic ordering of Co-doped ZnO were calculated using density functional theory. The calculation results reveal that stable long-range magnetic ordering in Co-doped ZnO nanoparticles is mainly attributed to the localized spin moments from 3d electrons of Co2+ ions. Both the experimental and theoretical studies demonstrate that the ferromagnetism in Co-doped ZnO nanoparticles is originated from the combined effects of Co doping and oxygen vacancies. These results provide an experimental and theoretical view to understand the magnetic origination and tune the magnetic properties of diluted magnetic semiconductors, which is of great significance for spintronics.

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.