Influence of oxygen vacancies on the structural,dielectric, and magnetic properties of (Mn,Co) co-doped ZnO nanostructures

We analysed the variation and effect of oxygen vacancies on the structural, dielectric and magnetic properties in case of Mn (4%) and Co (1, 2 and 4%) co-doped ZnO nanoparticles (NPs), synthesized by chemical precipitation route and annealed at 750 °C for 2 h. From the XRD, the calculated average crystallite size increased from15.30?±?0.73 nm to 16.71?±?012 nm, when Co content is increased from 1 to 4%. Enhancement of dopants (Mn, Co) introduced more and more oxygen vacancies to ZnO lattice confirmed from EDX and XPS. The high-temperature annealing leads to reduction of the dielectric properties due to enhancement in grain growth (large grain volume and lesser number of grain boundaries) with the incorporation of Co and Mn ions into the ZnO lattice. The electrical conductivity of the Mn doped and (Mn, Co) co-doped ZnO samples were enhanced due to increase in the volume of conducting grains and charge density (liberation of trapped charge carriers in oxygen vacancies and free charge carriers at higher frequencies). The Mn-doped and (Mn, Co) co-doped ZnO NPs show ferromagnetic (FM) behaviour. The saturation and remnant magnetizations (M_s and M_r) elevates from (0.235 to 1.489)?×?10^?2 and (0.12 to 0.27)?×?10^?2 emu/g while Coercivity (H_c) reduced from 97 to 36 Oe with enhancement in the concentration of dopants in ZnO matrix. Oxygen vacancies were found to be the main reason for room-temperature ferromagnetism (RTFM) in the doped and co-doped ZnO NPs. The results show that the enhanced dielectric and magnetic properties of Mn doped and (Mn, Co) co-doped ZnO is strongly correlated with the concentration of oxygen vacancies. The observed enhanced RTFM, dielectric properties and electrical conductivity makes TM doped ZnO nanoparticles suitable for spintronics, microelectronics and optoelectronics based applications.     

Physical and magnetic properties of (Co, Ag) doped ZnO nanoparticles

Undoped and (Co, Ag) co-doped ZnO nanostructure powders are synthesized by chemical precipitation method without using any capping agent and annealed in air ambient at 500 °C for 1 h. Here, the Ag concentration is fixed at 5 mol% and Co concentration is increased from 0 to 5 mol%. The X-ray diffraction studies reveal that undoped and doped ZnO powders consist of pure hexagonal structure and nano-sized crystallites. The novel Raman peak at 530 cm^?1 has corroborated with the Co doped ZnO nanoparticles. Moreover, the PL studies reveal that as the Co doping concentration increases and it enters into ZnO lattice as substitutional dopant, it leads to the increase of oxygen vacancies (Vo) and zinc interstitials (Zn_i). From the magnetization measurements, it is noticed that the co-doped ZnO nanostructures exhibit considerably robust ferromagnetism i.e. 4.29 emu g^?1 even at room temperature. These (Co, Ag) co-doped ZnO nanopowders can be used in the fabrication of spintronic and optoelectronic device applications.     

Presence of intrinsic defects and transition from diamagnetic to ferromagnetic state in Co<Superscript>2+</Superscript> ions doped ZnO nanoparticles

In this paper, we report the structural, morphological and magnetic properties of pure and Co²⁺ doped ZnO nanoparticles synthesized using sol–gel auto combustion method. The prepared nanoparticles were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area diffraction pattern (SAED), Fourier transform infrared spectroscopy (FTIR) and photoluminescence spectroscopy. The analysis of XRD pattern shows the single phase nature with a hexagonal wurtzite structure for the prepared nanoparticles. The average crystallite sizes of the prepared nanoparticles were found in the range 18–19 nm. SEM images showed that pure and Co²⁺ doped nanoparticles have different morphology. The shape of the prepared nanoparticles is approximately hexagonal shown by TEM image. SAED pattern also confirms the wurtzite structure with single crystalline nature. FTIR spectra showed the characteristic vibrations frequency band of Zn–O. Photo luminescence spectrum showed that two emission peaks, which are ascribed to near band edge transitions and broadened intensive green emission associated with oxygen-vacancy defects. The magnetic properties were measured by vibrating sample magnetometer (VSM) and superconducting quantum interference device with field dependant magnetization at 300 K and temperature dependant magnetization from 0 to 300 K. From VSM analysis, pure ZnO nanoparticles show diamagnetic behavior while Co²⁺ doped ZnO nanoparticles revealed ferromagnetic behaviour at room temperature. The significant changes in M–H loop from diamagnetic behavior to ferromagnetic behavior are due to the intrinsic defects such as oxygen vacancies (Vo) and zinc vacancies (Vz_n). The RTFM has been presented in terms of vacancies in the frame of bound magnetic polaron model.     

Exciting Dilute Magnetic Semiconductor: Copper-Doped ZnO

The present study is focused on the copper-doped ZnO system. Bulk copper-doped ZnO pellets were synthesized by a solid-state reaction technique and used as target material in pulsed laser deposition. Thin films were grown for different Cu doped pellets on sapphire substrates in vacuum (5×10^?5 mbar). Thin films having (002) plane of ZnO showed different oxidation states of dopants. M–H curves exhibited weak ferromagnetic signal for 1–3 % Cu doping but for 5 % Cu doped thin film sample showed the diamagnetic behavior. For deeper information, thin films were grown for 5 % Cu doped ZnO bulk pellet in different oxygen ambient pressures and analyzed. PL measurement at low temperature showed the emission peak in thin films samples due to acceptor-related transitions. XPS results show that copper exists in Cu²⁺ and Cu⁺¹ valence states in thin films and with increasing O₂ ambient pressure the valence-band maximum in films shifts towards higher binding energy. Furthermore, in lower oxygen ambient pressure (1×10^?2 mbar) thin films showed magnetic behavior but this vanished for the film grown at higher ambient pressures of oxygen (6×10^?2 mbar), which hints towards the decrease in donor defects.     

Behavior of dual ion beam sputtered MgZnO thin films for different oxygen partial pressure

Mg-doped ZnO (MgZnO) films were grown on p-Si (001) substrates by dual ion beam sputtering deposition system at a constant growth temperature of 600 °C for different oxygen partial pressure. The impact of oxygen partial pressure on the structural, electrical, elemental and morphological properties was thoroughly investigated. X-ray diffraction (XRD) spectra revealed that the deposited MgZnO films were polycrystalline in nature with preferred (002) crystal orientation. The peak of MgZnO (101) plane was reduced significantly as oxygen partial pressure was increased and disappeared completely at 80 and 100 % O₂. The maximum electron concentration was evaluated to be 5.79 × 10¹⁸ cm^?3 with resistivity of 0.116 Ω cm and electron mobility of 9.306 cm²/V s at room temperature, for MgZnO film grown with 20 % O₂. Raman spectra shows a broad peak at 434 cm^?1 corresponded to E ₂ ^high phonons mode of MgZnO wurtzite structure. The peak at 560 cm^?1 corresponded to the E₁ (LO) mode and was associated with oxygen deficiency in MgZnO films. Raman intensity at 560 cm^?1 reduced, on increasing oxygen partial pressure. A correlation between structural, electrical, elemental and morphological properties with oxygen partial pressure was also established.     

Structural, electrical and optical properties of Si doped ZnO films grown by atomic layer deposition

Si doped ZnO (SZO) films with various Si concentrations were deposited by atomic layer deposition at 300 °C using triethyzinc, tris(dimethylamino)silane and H₂O₂ as the precursors. The influences of Si doping concentration on structural, electrical and optical properties of ZnO films have been investigated. All the films exhibited a highly preferential c-axis orientation. A minimum resistivity of 9.2 × 10^?4 Ω cm, with a carrier concentration of 4.3 × 10²⁰ cm^?3 and a Hall mobility of 15.8 cm²/Vs, was obtained for SZO film prepared with the Si concentration of 2.1 at%. The increase of conductivity with Si doping was attributed to the presence of Si in +3 valence state acting as donor in ZnO and the increases of oxygen vacancies with Si concentration as proven by XPS measurements. The optical bandgap of SZO films initially increased from 3.25 to 3.55 eV with increasing of Si concentration to 2.1 at%, then decreased with further increase of Si concentration. The blue shift of band gap of SZO films with increasing carrier concentration can be explained by the Burstein-Moss (B-M) effects.     

Influence of cerium dopant on magnetic and dielectric properties of ZnO nanoparticles

Ce-doped ZnO nanoparticles (NPs) with different Ce doping concentrations (0, 0.96, 1.96, 2.52 and 3.12 at.% of Ce) were prepared by the chemical co-precipitation method. Energy-dispersive analysis of X-rays confirms the presence of Ce in Ce-doped ZnO nanoparticles. Raman spectra revealed the hexagonal wurtzite structure of pure and Ce-doped ZnO nanoparticles and presence of various defects. The photoluminescence spectra exhibited enhanced violet and blue emission peak intensities for 0.96 at.% of Ce, while broad band green emissions decreased with Ce content. Electron paramagnetic resonance (EPR) studies revealed the presence of oxygen vacancies (V _O), zinc vacancies (V _Zn) and Ce³⁺ ions in the prepared ZnO nanoparticles. VSM studies showed room temperature ferromagnetism (RTFM) in the Ce-doped ZnO NPs. The substituted Ce³⁺ions found to induce RTFM along with V _O, V _Zn in correlation with the results obtained from the EPR, PL and Raman studies. The variation of dielectric constants (ε _r), dielectric loss (ε″) and ac conductivity (σ _ac) as a function of frequency and Ce concentration is studied using ‘Maxwell–Wagner Model.’     

Improved performance of ZnO thin film solar cells by doping magnesium ions

This study investigates the effect of magnesium doping on the performance of nanocrystalline zinc oxide (ZnO) thin film solar cells. Mg_xZn_1?xO (x = 0.00, 0.05, 0.075 and 0.1 %) nanoparticles were prepared by simple precipitation method. The crystallinity and morphology of the photo-anode was characterized by X-ray diffraction analysis and scanning electron microscopy. The effect of various concentrations of magnesium ions on the structural and optical properties of ZnO nanoparticles was examined. Doping of magnesium ions helps the ZnO nanoparticles to grow larger in size with rod like surface morphologies. Solar cell with 0.075 % magnesium doped ZnO electrode exhibits an enhanced short-circuit current density of 3.36 mA/cm², open-circuit photo voltage of 0.79 V, fill factor of 0.73, and overall power conversion efficiency of 2.1 %.     

Low-temperature synthesis of ZnO nanoparticles for inverted polymer solar cell application

Zinc oxide (ZnO) nanoparticles were synthesized by a simple wet chemical method at low temperature. Morphologies, crystalline structure, and optical transmission of ZnO nanoparticles were investigated. The results showed that the average diameter of as-synthesized ZnO nanoparticles was about 4.9 nm, the nanoparticles were wurtzite-structured (hexagonal) ZnO and had optical band gap of 3.28 eV. Very high optical transmission (>80 %) in visible light region of ZnO nanoparticulate thin films was achieved. Furthermore, an inverted polymer solar cell consisted of ZnO nanoparticles and polymer were fabricated. The device exhibited an open circuit voltage (V_oc) of 0.50 V, a short circuit current density (J_sc) of 1.76 mA/cm², a fill-factor of 38 %, and a power conversion efficiency of 0.42 %.     

Influence of different sulfur sources on the phase formation of Cu<Subscript>2</Subscript>ZnSnS<Subscript>4</Subscript> (CZTS) nanoparticles (NPs)

Wurtzite (Wz) and kesterite (Ks) phases of Cu₂ZnSnS₄ (CZTS) nanoparticles (NPs) have been selectively synthesized via hot injection method using 1-octadecene (1-ODE) as solvent. The solvents, 1-dodecanethiol (1-DDT) and tert-dodecanethiol (t-DDT) were utilized to control the reactivity of metal precursors and to tune the desirable crystallographic phases. The phase purity of the as synthesized CZTS NPs was confirmed using X-ray diffraction results. TEM images indicate that the developed nanoparticles consist of a mixture of triangular shaped (height 20?±?3 nm, width 17?±?2 nm) and sphere shaped NPs (13.4?±?0.4 nm). These nanoparticles were formed due to the influence of thiols without any additional capping ligands. The band gap of as-synthesized CZTS NPs were calculated as 1.41 eV for wurtzite phase (Wz—1-DDT) and 1.47 eV for kesterite phase (Ks—t-DDT) from UV–Visible absorption results. CZTS thin films were prepared via spin coating and the electrical properties were analysed using Hall Effect measurements. Both the phases of CZTS films exhibit p-type conductivity. Wurtzite phase of CZTS has higher mobility (23.6 cm^?3) and carrier concentration (2.64?×?10¹⁷) compared to kesterite phase of CZTS films.     

Zn/ZnO core/shell nanoparticles synthesized by laser ablation in aqueous environment: Optical and structural characterizations

Zn/ZnO core/shell nanoparticles are synthesized by pulsed laser ablation (PLA) of Zn metal plate in the aqueous environment of sodium dodacyl sulfate (SDS). Solution of nanoparticles is found stable in the colloidal form for a long time, and is characterized by UV-visible absorption, transmission electron microscopy (TEM), photoluminescence (PL) and Raman spectroscopic techniques. UV-visible absorption spectrum has four peaks at 231, 275, 356, and 520 nm, which provides primary information about the synthesis of core-shell and elongated nanoparticles. TEM micrographs reveal that synthesized nanoparticles are monodispersed with three different average sizes and size distributions. Colloidal solution of nanoparticles has significant absorption in the green region, therefore, it absorbs 514·7 nm light of Ar⁺ laser and emits in the blue region centred at 350 and 375 nm, violet at 457 nm and green at 550 nm regions. Raman shift is observed at 300 cm⁻¹ with PL spectrum, which corresponds to ³E_2N and E_3L mode of vibrations of ZnO shell layer. Synthesis mechanism of Zn/ZnO core/shell nanoparticles is discussed.     

Electrical,optical and magnetoresistive behavior of nanostructured ZnO:Cu thin films deposited by sputtering

Copper doped ZnO (ZnO:Cu) nanostructured films with magnetoresistive behavior were produced by growing ZnO/Cu/ZnO arrays at room temperature (RT) by the sputtering technique on corning glass substrates. The arrays were made with two electrical insulating ZnO films of 50 and 105 nm, and a Cu film of 5 nm, both materials were deposited at RT by the RF- and DC-sputtering technique, respectively. The processing method involves two stages that proceed in the course of the growth process, the main one is originated by the non-equilibrium regime of the sputtering technique, and the second is the diffusion-redistribution of the intermediate Cu film towards the neighborhood ZnO layers aided by the nanocrystalline films character. The influence of applying an additional annealing stage to the arrays in N₂ atmosphere at 250 and 350 °C by periods of 30 min were studied. The resistivity of the ZnO:Cu films can be varied from 0.0034 to 2.83 Ω-cm, corresponding to electron concentrations of 1.12?×?10²¹ and 7.85?×?10¹⁷ cm^?3 with carrier mobility of 1.6 and 2.8 cm²/V s. Measured changes on the magnetoresistance behavior of the films at RT were of ?R?~?3% for annealed samples with electron concentration of 1.12?×?10²¹ cm^?3. The X-ray diffraction measurements show that the films are comprised of nanocrystallites with dimensions between 13 and 20 nm in size with preferred (002) orientation. The transmittance of the films in the visible region was of 83% with an optical band gap of ~?3.3 eV for the low-resistivity samples.     

Optical and magnetic properties of copper doped ZnO nanorods prepared by hydrothermal method

Surfactant free ZnO and Cu doped ZnO nanorods were synthesized by hydrothermal method. The formation of ZnO:Cu nanorods were confirmed by scanning electron microscopy, X-ray diffraction and Raman analysis. Diffuse reflectance spectroscopy results shows that band gap of ZnO nanorods shifts to red with increase of Cu content. The orange-red photoluminescent emission from ZnO nanorods originates from the oxygen vacancy or ZnO interstitial related defects. ZnO:Cu nanorods showed strong ferromagnetic behavior, however at higher doping percentage of Cu the ferromagnetic behavior was suppressed and paramagnetic nature was enhanced. The presence of non-polar E ₂ ^high and E ₂ ^low Raman modes in nanorods indicates that Cu doping didn’t change the wurtzite structure of ZnO.     

Structural and electrical properties of V-doped ZnO prepared by the solid state reaction

This paper reports the synthesis, crystal structure and electrical conductivity properties of vanadium (V)-doped zinc oxide (ZnO) powders (i.e. Zn_1?2X V _X O binary system, x = 0, 0.0025, 0.005, 0.0075 and in the range 0.01 ≤ x ≤ 0.15). I-phase samples, which were indexed as single phase with a hexagonal (wurtzite) structure in the V-doped ZnO binary system, were determined by X-ray diffraction (XRD). The limit solubility of V in the ZnO lattice at this temperature is 3 mol % at 950 °C. The impurity phase at 950 °C was determined as ZnV₂O₆ when compared with standart XRD data. The research focused on single I-phase ZnO samples which were synthesized at 950 °C because of the limit of the solubility range is widest at this temperature. It was observed that the lattice parameters a and c decreased with V doping concentration. The electrical conductivity of the pure ZnO and single I-phase samples were studied using the four-point probe dc method at temperatures between 100 and 950 °C in an air atmosphere. The electrical conductivity values of pure ZnO and 3 mol % V-doped ZnO samples at 100 °C were 2.75 × 10^?6 and 7.94 × 10^?5 Ω^?1 cm^?1, and at 950 °C they were 3.4 and 54.95 Ω^?1 cm^?1, respectively. In other words, the electrical conductivity increased with V doping concentration.     

Investigation of dual ion beam sputtered transparent conductive Ga-doped ZnO films

Ga-doped ZnO (GZO) transparent conducting films were deposited on sapphire (0001) substrates using dual ion beam sputtering deposition system. The impact of growth temperature on the structural, morphological, elemental, optical, and electrical properties was thoroughly investigated and reported. X-ray diffraction measurements explicitly confirmed that all GZO films had (002) preferred crystal orientation. The film deposited at 400 °C exhibited the narrowest full-width at half-maximum value of 0.24° for (002) crystalline plane and the lowest room temperature electrical resistivity of 4.11 × 10^?3 Ω cm. The Raman spectra demonstrated the vibrational modes at 576 and 650–670 cm^?1, associated with native oxygen vacancies and elemental Ga doping in ZnO lattice, respectively. All doped films showed an overall transmittance of above 95 % in the visible spectra. A correlation between structural, optical, elemental, and electrical properties with GZO growth temperature was established.     

Influence of magnetic ion doping on structural,optical, magnetic and hyperfine properties of nanocrystalline SnO<Subscript>2</Subscript> based dilute magnetic semiconductors

This article reports the structural, optical and magnetic properties of transition metal (Ni, Co, Mn and Fe) doped SnO₂ nanoparticles prepared by modified Pechini sol–gel method. From the X-ray diffraction studies, it is obvious that all the synthesized samples show a phase purity of rutile tetragonal crystal structure of SnO₂. The morphology was studied and the particle sizes were estimated from the field emission scanning electron microscopy. From photoluminescence spectra, we observed emission due to the presence of singly ionized oxygen vacancies. Raman spectroscopy shows dominant peaks at 644 and 782 cm^?1 which were ascribed to A_1g and B_2g modes of the rutile structure. Isomer shifting due to dopant addition and large quadrupole splitting due to surface defects were observed in Mössbauer spectra. All the samples show ferromagnetic ordering up to 1 T. The relatively stronger ferromagnetic nature in Fe and Co doped SnO₂ is due to the strong p–d exchange interaction. In case of Ni and Mn doped SnO₂ samples, the lack of carrier-mediated interaction due to its inherent semiconducting nature reduces the total magnetic moment observed in these samples. The exchange coupling depends on the dopant type and its concentration.     

Growth of ZnO nanorods on TiO2 nanoparticles films and their application to the electrode of dye-sensitized solar cells

A ZnO nanorods (NRs)/TiO₂ nanoparticles (NPs) film has been prepared by electrochemical deposition of ZnO NRs growth on P25 TiO₂ NPs film surfaces. It was found that ZnO NRs/TiO₂ NPs could significantly improve the efficiency of dye-sensitized solar cells owing to its relatively enhanced light-scattering capability and efficient charge transport efficiency. The overall energy-conversion efficiency (η) of 3.48 % was achieved by the formation of ZnO NRs/TiO₂ NPs film, which is 33 % higher than that formed by TiO₂ NPs alone (η = 2.62 %). The charge recombination behavior of cells was investigated by electrochemical impedance spectra, and the results showed that ZnO NRs/TiO₂ NPs film has the longer electron lifetime than TiO₂ NPs alone, which could facilitate the reduction of recombination processes and thus would promote the photocatalysis and solar cell performance.     

Effect of annealing temperature on the energy transfer in Eu-doped ZnO nanoparticles by chemical precipitation method

Eu-doped ZnO nanoparticles were synthesized by the chemical precipitation method and the annealing temperature effect on the structures and photoluminescence (PL) properties of the nanoparticles were briefly investigated. The X-ray diffraction and energy dispersive spectroscopy results indicated that the Eu³⁺ was successfully incorporated into the crystal lattice of ZnO host when the annealing temperature was fixed at 400 °C, but the Eu³⁺ ions were partly precipitated from the host with the annealing temperature increasing. The as-obtained ZnO: Eu nanocrystals composed of nanoparticles had an average size of 10 nm, and the valence states of europium ions in the nanocrystals were determined as tervalent. PL spectroscopy indicated that the characteristic red emissions of Eu³⁺ ions were attributed to the ⁵D₀ → ⁷F₀, ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions, respectively. Moreover, the annealing temperature was found to have effect on the red emission of Eu³⁺ ions. That is to say, the energy transfer in the doped nanocrystals could be adjusted by different annealing temperatures.     

P-type conductivity in doped and codoped ZnO thin films synthesized by RF magnetron sputtering

N-doped and Al–N codoped ZnO thin films with different volume ratios of N₂ reactive gas were deposited on plane glass substrates using the radio frequency magnetron sputtering method. The phase transition temperature and absorption edge of the ZnO powder were studied by differential scanning calorimetry at different heating rates and with Fourier transform infrared spectroscopy, respectively. The target used for the sputtering was synthesized using a palletize machine. It was sintered at 450 °C for 5 h. The X-ray diffraction results confirm that the thin films have wurtzite hexagonal structures with a very small distortion. The results indicate that the ZnO thin films have obviously enhanced transmittance of up to 80% on an average in the visible region. The Al–N codoped ZnO thin films exhibited the best p-type conductivity with a resistivity of 0.825 Ω-cm, a hole concentration of 6.55 × 10¹⁹ cm^?3, and a Hall mobility of 1.25 cm²/Vs. The p-type conductivity was observed after doping and codoping of the ZnO thin film.     

Effect of rapid thermal annealing on Zn/ZnO layers

Zn/ZnO layers were deposited on SiO₂/Si substrate by magnetron sputtering at room temperature, and then these layers were annealed at various temperatures from 200 to 400 °C in nitrogen atmosphere for 1 min. The structural and electrical properties of the Zn/ZnO layers before and after annealing are systematically investigated by X-ray diffraction, scanning electron microscopy, current–voltage measurement system, and Auger electron spectroscopy. Current–voltage measurements show that the Zn/ZnO layers exhibit an Ohmic contact behavior. It is shown that, initially, the specific contact resistivity decreases with the increase of the annealing temperature and reaches a minimum value of 9.76 × 10^?5 Ω cm² at an annealing temperature of 300 °C. However, with a further increase of the annealing temperature, the Ohmic contact behavior degrades. This phenomenon can be explained by considering the diffusion of zinc interstitials and oxygen vacancies. It is also shown that Zn-rich ZnO thin films can be obtained by annealing Zn on the surface of ZnO film and that good Ohmic contact between Zn and ZnO layers can be observed when the annealing temperature was 300 °C.