Fabrication, optical and magnetic properties of the Fe doped Zn0.99Mn0.01S nanowires

The wurtzite Fe²⁺ doped Zn_0.99Mn_0.01S nanowires were synthesized by a simple hydrothermal process. The maximum concentration of the Fe²⁺ ions in the Zn_0.99Mn_0.01S nanowires was 7 %. The average diameter of the samples was in the range of 10–12 nm. The ZnS:Mn²⁺ nanowires exhibited the Mn^{2+ 4}T₁-⁶A₁ transition centered at 2.1 eV, while the ZnS:Mn²⁺Fe²⁺ nanowires exhibited the Fe²⁺ related transition centered at 2.55 eV. Moreover, the ZnS:Mn²⁺Fe²⁺ nanowires showed the room temperature ferromagnetism property, which can be attributed to the hybridization between the Fe’s, Mn’s d shell and the S’s p shell.     

Growth mechanism and room temperature ferromagnetism property of the Zn1−xCrxS nanobelts

In this paper, the wurtzite-type Zn_1?xCr_xS nanobelts (NBs) were obtained by a simple hydrothermal process with the ethanol amine as the oriented-assembly agent at 180 °C. The results showed that the Cr³⁺ ions substituted for the Zn²⁺ sites in the host zinc sulfide (ZnS), and the maximum concentration of the Cr³⁺ ions in the ZnS NBs was 4.80 %. The Zn_1?xCr_xS NBs were constructed by the nanowires, and the growth mechanism of the NBs was investigated in detail. The Zn_1?xCr_xS NBs showed the ferromagnetism property at room temperature and the magnetic saturation value was increased as the Cr³⁺ doped ratio increased.     

Electroluminescent properties of chemically synthesized zinc sulfide nanocrystals doped with manganese

High-field electroluminescence (EL) of chemically synthesized ZnS:Mn nanocrystals with a crystallite size of 4 nm was investigated using a device consisting of glass substrate/indium–tin oxide/ZnS:Mn NC emission layer/Al. For electric fields over ca. 1 MV/cm, the current was turned on, and orange EL was observed. The maximum luminance was 0.45 cd/m² at a DC voltage of 42 V. The EL spectrum comprises a single peak with a peak wavelength of 593 nm, which is ascribed to the ⁴T₁–⁶A₁ transitions of Mn²⁺ ions. The excitation mechanism of the Mn²⁺ ions is discussed according to a scheme of impact excitation by hot-electrons.     

Role of additives; sodium dodecyl sulphate and manganese chloride on morphology of Zn1−xMnxO nanoparticles and their photoluminescence properties

In the present study Zn_1−xMn_xO (x = 0, 0.05 and 0.1) nanoparticles (NPs) have been synthesised in aqueous solution phase at mild reaction temperature 100 °C in moderate alkaline medium (pH = 9.5), and the role of external additives; like sodium dodecyl sulphate and manganese chloride on the morphology and size of the products has been explored on the basis of transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared spectral analyses data. ZnO hexagonal nano-plates, core–shell like spherical/ellipsoidal Zn_0.95Mn_0.05O structures and thin sheets, thorn/needle mixed shaped Zn_0.9Mn_0.1O structures have been observed in TEM and SEM images. Zn(OH)₂ formed in moderate alkaline medium, converted to Zn(II) hydroxo complex ions on dissolution, which further recrystallizes to produce wurtzite ZnO at 100 °C. From XRD and EDX analysis, successful doping of Mn²⁺ ions at the Zn²⁺ sites in ZnO host has been proved. In the photoluminescence spectra, the observed blue shifts in NBE peaks and decrease of emissions intensity on Mn doping have thoroughly been discussed in the present investigation.     

Room-Temperature Ferromagnetism in Chemically Synthesized Ce0.97Cr0.03O2−δ Nanopowders

In this paper, we study structural and magnetic properties of undoped (CeO_2?δ ) and Cr-doped (Ce_0.97Cr_0.03O_2?δ ) cerium oxide nanopowders synthesized by a sol-gel-based method. We estimate the crystallite sizes calculated from X-ray diffraction measurements to be around 9.4±0.3 nm for both CeO_2?δ and Ce_0.97Cr_0.03O_2?δ samples. Raman measurements indicate that microstructural defects are generated when Cr substitutes the Ce in the CeO₂ crystal lattice. Magnetic measurements of the Ce_0.97Cr_0.03O_2?δ sample at 300 K indicate ferromagnetic behavior, with a coercive field of 34.27 Oe and a saturation magnetization of 5.8×10^?4 emu/g. We interpret the nature of room-temperature ferromagnetism by taking into account the exchange interaction between Cr³⁺ ions and oxygen vacancies in CeO₂.     

Electron-Spin-Resonance Study of Polycrystalline Zn1−x Cr x O Ceramics

The structural and magnetic properties of polycrystalline ceramics of Zn_1?x Cr _x O (x=0.01–0.10) annealed at 900 and 1200 °C were systematically investigated by means of X-ray diffractometer, electron spin resonance (ESR) spectroscopy, and a superconducting quantum interference device magnetometer. A coexistence of two structural phases of wurtzite-ZnO and spinel-ZnCr₂O₄ was found even in the samples with the lowest Cr-doping concentration of 1 at.%. Our experimental results have indicated that Cr ions are incorporated into the Zn site of the ZnO host lattice, and act as paramagnetic centers. Higher annealing temperature enhances the formation tendency of ZnCr₂O₄, and the proportion of Cr²⁺ relative to Cr³⁺ in ZnO. This results in the broadening of ESR spectral line. Dipole exchange interactions due to Cr³⁺–Cr³⁺, Cr³⁺–Cr²⁺, and Cr²⁺–Cr²⁺ pairs are assigned to be responsible for the ESR signals and paramagnetic behavior of Zn_1?x Cr _x O samples.     

Structural and optical properties of Cd1−xZnxS (0 ≤ x ≤ 0.3) nanoparticles

Cd_1?xZn_xS nanoparticles for Zn = 0–30 % were successfully synthesized by a conventional chemical co-precipitation method at room temperature. X-ray diffraction spectra confirmed the pure zinc blend cubic structure of undoped CdS; but Zn-doping on Cd–S matrix induced the mixed phases of cubic and hexagonal structure. The reduced crystal size, d-value, cell parameters and higher micro-strain at lower Zn concentration were due to the distortion produced by Zn²⁺ in Cd–S lattice. The enhancing diffraction intensity at lower Zn concentrations was due to the substitution of Zn²⁺ ions instead of Cd²⁺ ions whereas the reduced intensity after 20 % was due to the presence of Zn²⁺ ions both as substitutionally and interstitially in Cd–S lattice. The nominal stoichiometry and chemical purity was confirmed by energy dispersive X-ray analysis. The initial blue shift of energy gap from undoped CdS (3.75 eV) to Zn = 10 % (3.82 eV) was due to the size effect and also the incorporation of Zn²⁺ in the Cd–S lattice. The observed red shift of energy gap at higher Zn concentrations could be attributed to the improved crystallinity. The band gap tailoring was useful to design a suitable window material in fabrication for solar cells and other opto-electronic devices. The characteristic IR peaks around 617–619 cm^?1 and the reduced intensity by Zn-doping confirmed the presence of Zn in Cd–S lattice.     

Electron paramagnetic resonance and Mössbauer spectroscopy of transition metal ion doped mullite

Appreciable difference in the properties of undoped and oxide-doped mullite are observed. The oxidation state of cation, its concentration and the position of the mullite lattice occupied by it appear to be the responsible factors. Mullite has, therefore, been doped with four transition metal ions, Mn, Fe, Cr and Ti. With the help of EPR and Mössbauer spectroscopy (supplimented by X-ray diffractometry) the oxidation states of these ions and the mullite lattice sites where they enter has been investigated. It was observed that Mn ion was present in Mn²⁺ and Mn³⁺ states, the former remained as clusters and the latter occupied the octahedral sites in the mullite lattice. Only Fe³⁺ ion was detected and conclusive evidence was obtained for the entry of Fe³⁺ in the octahedral lattice position of mullite from the analysis of Mössbauer spectra with the help of a specially written computer programme. The Cr ion entered the mullite structure only in the Cr³⁺ state. The change in lattice parameters of Cr doped mullite were measured by the XRD technique. The results showed that the expansion of b-axis was more than that of the a-axis which supported the presence of Cr³⁺ ion in the octahedral site of mullite lattice. The absence of signal in the EPR spectra of Ti doped mullite suggested the presence of only Ti⁴⁺ (3d⁰) ion. Very low electrical resistivity of Ti doped mullite and close similarity between mullite and Al₂TiO₅ structures stood as evidence for incorporation of Ti⁴⁺ ion in the octahedral site of mullite lattice by replacing Al³⁺ ion.     

Tuning colossal magnetoresistance response at roomtemperature by La2/3+ySr1/3−yMn1−yCryO3

The special formula of La_2/3+ySr_1/3−yMn_1−yCr_yO₃ with [Mn³⁺]/([Mn⁴⁺] + [Cr³⁺]) ratio fixed at optimal proportion 2:1 was designed to tune colossal magnetoresistance (CMR) response around room temperature and test the possibility of ferromagnetic (FM) interaction of hetero-ionic coupling between Mn³⁺ and Cr³⁺. The polycrystalline bulk samples were fabricated by the traditional solid-state reaction method. The structural, magnetic, electrical transports and magnetoresistance (MR) properties were investigated. An enhancement of CMR at room temperature with appropriate content y has been observed, meanwhile, substituting Cr for Mn in manganites shows inefficiency in lowering ferromagnetic–paramagnetic (FM–PM) transition temperature T_c and metal–insulator (M–I) transition temperature T_p. Experimental results indicate that there might exist a FM coupling between Mn³⁺ and Cr³⁺.     

Band gap tailoring, structural and morphological properties of Zn0.98−xMn0.02CuxO (0 ≤ x ≤ 0.05) nanopowders by sol–gel method

Zn_0.98?xMn_0.02Cu_xO (0 ≤ x ≤ 0.05) nanopowders have been synthesized by sol–gel method. The synthesized nanopowders were characterized by powder X-ray diffraction, energy dispersive X-ray spectra, UV–visible spectrophotometer and Fourier transform infrared spectroscopy. The XRD measurement revealed that the prepared nanopowders have different microstructure without changing a hexagonal wurtzite structure. The calculated average crystallite size was decreased from 22.4 to 16.7 nm for Cu = 0–0.02 then gradually increased to 21.5 nm for Cu = 0.05 which were confirmed by SEM. The change in lattice parameters, shift in X-ray diffraction peaks and the change in energy gap revealed the substitution of Cu²⁺ ions into Zn–Mn–O lattice. The observed red shift of optical energy gap (E_g ≈ 0.27 eV) at lower concentrations (Cu ≤ 2 %) is explained by increasing charge carriers and Moss–Burstein effect meanwhile blue shift (E_g ≈ 0.56 eV) at higher Cu concentrations (Cu > 2 %) is explained in terms of the distortion of host lattice and generation of defects. The variation of crystallite size was discussed in terms of micro-strain.     

Structure and magnetic properties of (Co,Mn) co-doped ZnO diluted magnetic semiconductor nanoparticles

The ZnO, Zn_0.96Mn_0.04O, Zn_0.95Mn_0.04Co_0.01O, Zn_0.94Mn_0.04Co_0.02O and Zn_0.92Mn_0.04Co_0.04O nanoparticles were synthesized by simple chemical precipitation technique. The effects of co-doping on the structure and magnetic properties of these nanoparticles were studied. The X-rays diffraction (XRD) scans were performed in the 2θ range of 20°–80°. The XRD patterns, at 300 K, of all the pure and co-doped ZnO samples confirmed the formation of wurtzite-type structure. X-ray diffraction and transmission scanning electron microscope analysis indicated that the high spin Co²⁺ and Mn²⁺ ions were substituted for the Zn²⁺ ions at tetrahedral sites. The average size of the nanoparticles were increased from 17 to 24 nm with the increase of dopants concentration. Moreover, Energy Dispersive X-ray spectroscopy (EDX) confirmed the synthesis results. All Zn_0.96?xMn_0.04Co _x O (x?=?0.0, 0.1, 0.2 and 0.4) nanoparticles samples were observed to be paramagnetic below 300 K. However, a large increase in the magnetization was observed below 40 K. This behavior, along with the negative value of the Curie–Weiss constant obtained from the linear fit to the susceptibility data below room temperature, indicated the ferromagnetic nature of the samples. The origin of ferromagnetism is likely to be the intrinsic characteristics of the Co and Mn doped samples. The high magnetization was noted for the 1 wt% Co co-doped Mn–ZnO annealed samples as compared to other samples with Co concentration above and below this threshold concentration.     

Cu-doping effect on structure and magnetic properties of Fe-doped ZnO powders

Fe-doped and Cu, Fe co-doped ZnO diluted magnetic semiconductors powders were synthesized by sol–gel method. The x-ray diffraction (XRD) results showed that Zn_0.97−xFe_0.03Cu_xO (x ≤ 0.02) samples were single phase with the ZnO-like wurtzite structure. X-ray photoelectron spectroscopy (XPS) showed that Fe²⁺ and Fe³⁺ existed in Zn_0.97Fe_0.03O, while Fe²⁺, Fe³⁺and Cu⁺, Cu²⁺ were found in Zn_0.95Fe_0.03Cu_0.02O. Both Zn_0.97Fe_0.03O and Zn_0.95Fe_0.03Cu_0.02O exhibited ferromagnetic performance at room temperature. But the Cu incorporation reduced the saturation magnetization of Fe-doped ZnO diluted magnetic semiconductors.     

Room-Temperature Multiferroic Properties and Local Structures of the Mn-Doped and (Pb, Mn)-Codoped BiFeO3

The Mn-doped and the (Pb, Mn)-codoped BiFeO₃ polycrystalline samples are prepared by a sol–gel method. The X-ray diffraction patterns manifest that all samples are in single phase and a lattice structural transformation appears during doping process. Compared with Mn-doped BiFeO₃, the magnetic property of (Pb, Mn)-codoped BiFeO₃ is enhanced and ferroelectric polarization decreases. The results of XAFS indicate that the bond length of Fe–O shortens due to doping Mn ions into BiFeO₃, which results in the increase of magnetic property. Pb ion doping makes the valence state of Mn ion change, rather than Fe ion, and induces the occurrence of Mn⁴⁺–O–Fe³⁺ antiferromagnetic coupling, which decreases the magnetic property of the Bi_0.95Pb_0.05Fe_0.95Mn_0.05O₃ sample.     

Zinc-Doping Strategy on P2-Type Mn-Based Layered Oxide Cathode for High-Performance Potassium-ion Batteries

Mn-based layered oxide is extensively investigated as a promising cathode material for potassium-ion batteries due to its high theoretical capacity and natural abundance of manganese. However, the Jahn–Teller distortion caused by high-spin Mn³⁺(t_2g³e_g¹) destabilizes the host structure and reduces the cycling stability. Here, K_0.02Na_0.55Mn_0.70Ni_0.25Zn_0.05O₂ (denoted as KNMNO-Z) is reported to inhibit the Jahn–Teller effect and reduce the irreversible phase transition. Through the implementation of a Zn-doping strategy, higher Mn valence is achieved in the KNMNO-Z electrode, resulting in a reduction of Mn³⁺ amount and subsequently leading to an improvement in cyclic stability. Specifically, after 1000 cycles, a high retention rate of 97% is observed. Density functional theory calculations reveals that low-valence Zn²⁺ ions substituting the transition metal position of Mn regulated the electronic structure around the Mn O bonding, thereby alleviating the anisotropic coupling between oxidized O²⁻ and Mn⁴⁺ and improving the structural stability. K_0.02Na_0.55Mn_0.70Ni_0.25Zn_0.05O₂ provided an initial discharge capacity of 57 mAh g⁻¹ at 100 mA g⁻¹ and a decay rate of only 0.003% per cycle, indicating that the Zn-doped strategy is effective for developing high-performance Mn-based layered oxide cathode materials in PIBs.     

No ferromagnetic properties in polycrystalline Al-doped Zn0.97Mn0.03O diluted magnetic semiconductor

Zn_0.96Mn_0.03Al_0.01O powders have been synthesized using the co-precipitation technique. X-ray diffraction patterns show the typical würtzite structure with no additional peaks for all samples. Raman spectroscopy revealed that Mn²⁺ ions were substituted for Zn²⁺ into the ZnO matrix and that Al and Mn ions activate an additional vibration mode at 670 cm^− 1 which is specific to interstitial defects. The resistance measurements give a confirmation of the presence of free carriers. Nevertheless, magnetization measurements revealed only a mixture of paramagnetic and antiferromagnetic behaviors for all samples with no sign of ferromagnetism.     

DRIP-XII Conference 2007

Mn²⁺-doped ZnS nanoparticles were synthesized by a hydrothermal method. The reactive conditions are researched, such as the ratios of the reaction concentrations [S²⁻]/[Zn²⁺], the concentration of Mn²⁺ ions, the reaction temperatures and the pH value. The concentration doped-Mn is from 1 mol% to 20 mol% and the hydrothermal synthesis temperatures are from 70 °C to 110 °C. The structure and luminescent properties of ZnS:Mn nanoparticles are researched by using XRD, SEM and PL.     

Regular Arrays of (Zn,Mn)S Quantum Wires with Well-Defined Diameters in the Nanometer Range

Zn_1–x Mn _x S, with x varying between 0.01 and 0.30, were formed inside the ordered pore systems of different mesoporous SiO₂ matrices. Because of the highly ordered structure of the hosts, regular arrays of Zn_1–x Mn _x S quantum wires with lateral dimensions of 3 and 5.5 nm, respectively, separated by 2-nm SiO₂ barriers were obtained. The wires were characterized using photoluminescence (PL) and PL excitation (PLE) spectroscopy at liquid Helium temperatures. The PL of the wires is dominated by the ⁴T₁ ⁶A₁ internal transition of the Mn²⁺(3d⁵) ions. The corresponding PLE spectra show higher internal Mn transitions as well as the band to band transition. The energies of the internal Mn transitions are typical for Mn²⁺ on a cation site of (II,Mn)VI semiconductors. Because of the comparable bandgaps of the SiO₂ and the Zn_1–x Mn_xS as well as the small exciton Bohr radius in (Zn,Mn)S quantum confinement effects in the wires are less than about 150 meV.     

Applicability of the polymeric precursor method to the synthesis of nanometric single- and multi-layers of Zn1?xMnxO (x?=?0–0.3)

Polymeric precursor method (Pechini) was employed to fabricate single- and multilayers of Zn_1−x Mn _x O (x = 0–0.3) on glass substrates. X-ray diffraction measurements revealed that crystal structure of Zn_1−x Mn _x O multilayers is the typical hexagonal würzite structure of pristine ZnO. A reduced peak intensity and widened full width half maximum (FWHM) value of prominent peaks suggested that the Mn²⁺ ions have substituted the Zn²⁺ ion without changing the würzite structure of pristine ZnO up to Mn concentrations x ≤ 0.2. A distinct redshift of the absorption edge was observed as the Mn concentration x was increased. Additionally, the absorption edge was less sharp due, probably, to s–d and p–d interactions, which give rise to band gap bowing. Nevertheless, amorphous states appearing in the band gap as a consequence of reduced crystallinity may also be responsible for the shrinking of the band gap in this material. Interestingly, the field dependence of the magnetization showed typical paramagnetic behavior for all the chosen Mn concentrations with no evidence of ferromagnetic ordering. Probably, the absence of ferromagnetism in the studied Zn_1−x Mn _x O films is strongly related to defects (say Mn impurities at the interface between nano-crystallites) in ZnO due to partial substitution of host Zn ions by Mn ions.     

Microstructure and magnetic transition in Cr-substituted Mg–Zn spinel ferrite powders prepared via hydrothermal method

Mg–Zn ferrite powder specimens with the nominal composition Mg_0.5Zn_0.5Cr _x Fe_2?x O₄ (x = 0.0–1.0 with steps of 0.2) were synthesized via hydrothermal method. It is found that all the specimens exhibit a typical spinel structure, and the lattice parameter increases slightly with x, which confirms the substitution of Cr³⁺ for Fe³⁺. The average crystallite size first increases but then decreases with x, which is not the same as the results in previous reports on spinels. The particles in specimens are the aggregate of small nanoscale crystallites, and roughly aggregate more extensively with the increasing Cr content. With the increase of x, the saturation magnetization decreases rapidly, and it becomes more and more difficult for the specimens to magnetize to saturation. The increase of coercivity from 0.6 (x = 0.0) to 32.3 kA m^?1 (x = 1.0) shows a transition from a typical soft magnetic behavior to a hard magnetic behavior with the increase of Cr content, which was not reported before.     

Optical and EPR spectroscopy of Zn:Cr:ZnSe and Zn:Fe:ZnSe crystals

Optical and EPR characterization of Cr and Fe doped ZnSe crystals annealed in Zn vapor revealed a strong bleaching of the divalent state of transition metal ions. Photo induced EPR kinetics were studied in 20–80 K temperature range. Analysis of time-dependent data reveals Cr¹⁺ signal rise time decreases with increasing temperature. The non-exponential decay of Cr¹⁺ concentration were analyzed using Auger-type recombination process. The photoluminescence quantum yield of Cr²⁺ ions at ⁵E(D) → ⁵T₂(D) mid-IR transition excited via chromium ionization process was measured to be close to 100%.