Narrow Results

The resistive switching effect in TiO_2-x memory devices is thoroughly investigated in order to obtain optimum memory performance, in terms of switching ratio, uniformity of the switching characteristics and multilevel capability. As a result, various fabrication procedures were followed with the aim to enhance the above properties. The main body of our devices was TiO_2-x thin film, with constant thickness of 45 nm, while different approaches were implemented during the fabrication process, such as increasing the oxygen content during film growth, depositing top electrodes with different thicknesses, inserting a layer of nanocrystals (NCs) within the middle of the dielectric and using two various oxygen contents during film deposition in order to create a homobilayer structure. The common feature of these techniques is to control the regions where oxygen vacancy creation will take place in order to control the formation/rupture of the conducting filaments (CFs). Conductive-Atomic Force Microscopy (C-AFM) results support the localized nature of the switching effect. The gradual transition during the SET process, which allows precise CF control through proper external stimulus, as well as the self-rectification effect, which alleviates the sneakpath issue in crossbar architecture, are also important assets of our experimental approach.

A first-principle method is used to investigate the electronic structure and optical properties of MgO and MgO containing an oxygen vacancy. In the presence of the oxygen vacancy, a new electronic state appears in the band gap, which leads to additional peaks in the optical spectra. Furthermore, under applied pressure, the band gaps become larger, and the curves of optical properties including the dielectric function ε(ω) and absorption coefficient α(ω) shift towards higher energy. The knowledge of MgO and MgO with F center under high pressure may provide insight into their practical applications.

In this study, density-functional theory plane-wave pseudopotential method was employed to investigate several oxygen vacancies on TiO₂ anatase (101) surface. At first, a suitable defect-free slab model has been established by analyzing the surface energies and the atomic relaxations influenced by different technical parameters. The formation energies of different kinds of oxygen vacancies in the outermost layer have also been compared as well as the atomic displacement of the defective surfaces. It was found that the presence of bridging oxygen vacancy is more energetically favored and causes larger atomic displacement than other types of surface oxygen vacancies. The reactivity of oxygen vacancies has also been tested by both molecular and dissociated oxygen adsorption. Furthermore, we discussed the configurations and the electronic properties of O₂-adsorbed surface, and found that the appearance of oxygen adsorbate-induced states in the band gap can make the surface sensitive to visible light.

In order to investigate the effects of Ba doping BiFeO₃ on multiferroic properties, Bi_1-xBa_xFeO₃(0≤x≤1)(Ba_xBFO) ceramics were fabricated via rapid solid phase sintering method, and material's structures and electrical properties were investigated. The phase transitions from rhombohedral to pseudo-cubic (x = 10%) and then to tetragonal (x = 40%) were confirmed by X-ray diffraction investigation. Although the electrical conductivity of Ba_xBFO (x = 10%, 20% and 30%) ceramics was low, which is a similar trend to previous reports, an abnormal enhancement of electrical conductance was observed in Ba_xBFO (x = 1%, 3% and 5%) ceramics. Such as, the electrical conductivity of Ba_0.03BFO is calculated to be ~10⁶ Ω⋅cm that is five orders of magnitude higher than that of the BiFeO₃. This has been discussed and ascribed to more percent of oxygen vacancies and Fe²⁺ ions in Ba_xBFO ceramics, as confirmed by X-ray photoelectron spectroscopy investigation.

We employed ab initio calculations to study the effect of various defects on the structural, electronic and optical properties of anatase TiO₂. Single Cr doping at Ti sites introduced isolated Cr 3d states in the band gap of TiO₂. Charge compensated Cr doped model was introduced by simultaneously doping two Cr atoms at Ti sites along with one oxygen vacancy (Ti₁₄O₃₁Cr₂). It has been found that Ti₁₄O₃₁Cr₂ can narrow the band gap significantly and passivate gap states. Removal of the isolated Cr 3d states from the band gap and improved visible light absorption would effectively enhance the photocatalytic activity of Cr doped TiO₂. Our calculations provide reasonable explanation for the previously reported experimental findings.

Density functional theory (DFT) and generalized gradient approximation (GGA) have been employed to study origins of the intrinsic $n$-type electrical conductivity in the zinc oxide. Hubbard-like term has been introduced to provide a better description for the Zn 3$d$ electrons. Two intrinsic point defects, namely oxygen vacancy and hydrogen impurity, were taken into consideration. Results on conductivity are analyzed using density of states patterns for different configurations of defects. Microstructure and local magnetic moments are studied as well. The obtained results clearly indicate that oxygen vacancy does not and cannot be responsible for the intrinsic $n$-type electrical conductivity whereas inserted hydrogen atoms tend to lose its only valence electron, which in turn becomes a free electron contributing towards the $n$-type conductivity.

Ca${}_{1-3x/2}$Bi${}_x$Cu₃Ti₄O${}_{12}$ ($x$ = 0.0, 0.1, 0.2 and 0.3; BCCTO) ceramics were prepared by traditional solid-state sintering method. All samples had pure cubic perovskite-like structure. A drastic grain size reduction was observed with bismuth doping. Dielectric spectra showed two obvious relaxation steps corresponding to two series of peaks in the imaginary part of electric modulus spectra and dielectric loss spectra too. Activation energy fitting by electric modulus spectra reflected different conducting segments in BCCTO ceramics of grains and oxygen vacancies below room temperature. Normalized capacitance dependent of extra bias showed different voltage–capacitance coefficients at special frequencies that suggested multirelaxation mechanisms related with grain and oxygen vacancy. A positive capacitance curvature implies dipolar relaxations in CCTO grains. Whereas a negative curvature suggests oxygen-related relaxations at the interface.

To observe the effect of halogen-substitution on the Mn-O electron transfer of NiMn₂O₄, we calculated Mn-mixed-valence configuration (charge-disproportionation) and oxygen vacancy by the density functional theory (DFT). The results indicate that the halogen-p⁵ state induces the O-2p orbital splitting to create an oxygen vacancy in the VB (valence band: about −5 eV). The oxygen vacancy can capture an electron from Mn${}^{3+}$-3d⁵ orbital that makes the Mn${}^{3+}$-3d⁵ change to Mn${}^{4+}$-3d⁴ states (Mn-charge disproportionate), and providing many effective-hole (40.14 $\sim$ 96.72 × 10${}^{-31}$ kg). The halogen-p⁵-O-2p⁴ hybrid orbitals enhance the O-2p⁴-Mn-3d⁵ p-d hybrid orbital (about 19.18 electron). That increases the surface potential in Mn-O octahedron (for Cl-substituted: about 60 meV), the corresponding electron–electron interactions change from complex t${}_{2g}$ (O-2p⁴-Mn${}^{3+}$-3d${}^5)$ to complete $t_{2g}$ (O-2p⁴-Mn${}^{4+}$-3d${}^4)$-e${}_g$(O-2p⁴-halogen-p⁵) orbital. This study effectively analyzes the microscopic changes of the electron transfer caused by the small amount of doping, provides a theoretical basis for the design of NMO-based semiconductor material.

In this paper, we present the optical spectra of the ZrO₂ crystal containing oxygen vacancy based on the Density Functional Theory (DFT). The finite-size correction scheme (FNV) is employed to eliminate the artificial interactions and correct the defect formation energy of oxygen vacancies with three different charges (0, +1, +2). Besides, we use hybrid density functionals to relieve the band edge problem. Finally, we obtain the optical spectra for the F center and F${}^{+}$ center containing the electron–phonon coupling. The absorption peak of F center of threefold coordinate oxygen vacancy (V${}_{\mathrm{\text{O}}3}$) near 446 nm (2.78 eV) agrees well with the experimental value (2.83 eV), which can enhance the visible light photocatalytic ability of ZrO₂. The luminescence peak of the F${}^{+}$ center of fourfold coordinate oxygen vacancy (V${}_{\mathrm{\text{O}}4}$) is 561 nm (2.21 eV), which is close to the experimental value (2.5 eV).

The point-like defects known as E' centers are the most abundant natural defects in silicon dioxide (SiO₂) and have been identified as unpaired sp³ dangling bonds. Their importance stems from the fact that they deeply affect the quality of electronic and optical devices. For this reason, particular attention has been paid in their characterization since the early 1960s. In this work, we review theoretical and experimental results concerning these kinds of defects, focusing on the related charge and spin states. In particular, the defect known as E' in crystalline quartz and its analogous E'_γ in amorphous SiO₂, detected by electron spin resonance, are shown to be due to the Si dangling bonds that arise either upon removal or displacement of an oxygen atom in a SiO₂ network, accompanied by an asymmetric relaxation of the network.

Zn_1-xFe_xO (x = 0.04, 0.06, 0.08, 0.10, 0.12) thin films were grown on Si substrates using reactive magnetron sputtering. X-ray diffraction analyses show that the samples have wurtzite structures with the c-axis orientation. X-ray photoelectron spectroscopy results indicate that the Fe ions are in a +2 charge state in the films. Magnetization measurements indicate that room temperature ferromagnetism is present in films annealed in vacuum while films annealed in air were non-magnetic. The presence of oxygen vacancies in these films may mediate exchange coupling of the dopant ions, resulting in room temperature ferromagnetism.

Mechanism of resistance switching in heterostructure Au/LaMnO₃/SrNb_0.01Ti_0.99O₃ was investigated. In Au/LaMnO₃/SrNb_0.01Ti_0.99O₃ devices the LaMnO₃ films were fabricated under various oxygen pressures. The content of the oxygen vacancies has a significant impact on the resistance switching performance. We propose that the resistance switching characteristics of Au/LaMnO₃/SrNb_0.01Ti_0.99O₃ arise from the modulation of the Au/LaMnO₃ Schottky barrier due to the change of the oxygen vacancy concentration at Au/LaMnO₃ interface under the external electric field. The effect of the oxygen vacancy concentration on the resistance switching is explained based on the self-consistent calculation. Both the experimental and numerical results confirm the important role of the oxygen vacancies in the resistance switching behavior.

Single phase Bi_0.97Ba_0.03Fe_1-xTa_xO₃ ceramics with x = 0, 0.01, 0.03, 0.05 were synthesized by modified rapid sintering process method. The formation of rhombohedral perovskite-like structure was confirmed by X-ray diffraction investigation for all the samples. Dielectric and leakage current measurements indicated that the content of the oxygen vacancy in the samples decreased as a function of the substitution of Ta⁵⁺ ions. A distinct threshold switching behavior was observed in the leakage current density. The impedance measurements suggested that the grain effect made a major contribution to the resistance. The changes in dielectric, multiferroic properties and resistance modulation of the Ta⁵⁺ and Ba²⁺ co-doped BiFeO₃ ceramics could have a huge potential for material application.

In this study, we have studied the electronic structure and magnetic properties of oxygen vacancy on anatase TiO₂ (101) surface using density functional theory (DFT) calculations. The results show that only the vacancy of threefold-coordinated oxygen (O_3c) can introduce the magnetism on (101) surface. The spins induced by the O_3c vacancy will form a stable ferromagnetic state, and it can introduce a magnetic moment of 1.05 μ_B. Moreover, the magnetic moment mainly results from the d orbitals of three low-charge-state Ti ions adjacent to the O_3c vacancy. The O_3c vacancy on (101) surface can also result in a localized state of spin polarization in the bandgap, which is about 0.22 eV below the Fermi energy. Our findings imply that the oxygen vacancy on the surface may be responsible for the unexpected ferromagnetism in pristine TiO₂ film. The experimentally observed d⁰-ferromagnetism behavior in TiO₂ system is in good agreement with our calculated results.

The structural and electronic properties of neutral oxygen vacancies in (Mo + C)-doped anatase TiO₂ were investigated using frozen-core projector-augmented wave (PAW) method within GGA +U approximation. Six possible oxygen vacancy sites were considered in the present work. The results show that the octahedral vertex adjacent to Mo and opposite from C is the most stable position for oxygen vacancy based on the results of the formation energy. The Fermi level is located at above the bottom of the conduction band and a typical n-type metallic behavior occurs as a result of the oxygen vacancy appeared in (Mo + C) doped TiO₂.

${\mathrm{\text{La}}}_{0.1}{\mathrm{\text{Bi}}}_{0.9-x}{\mathrm{\text{Sr}}}_x{\mathrm{\text{FeO}}}_{(3-x/2)}({\mathrm{\text{LBFS}}}_{x}0\le x\le 0.8)$ ceramics were synthesized by solid-state reaction method, and their magnetic properties and conductive characteristics were investigated and discussed. It is found that La-doped ${\mathrm{\text{BiFeO}}}_3$ ceramic without ${\mathrm{\text{Sr}}}^{2+}$ doping is a rhombohedrally distorted perovskite structure, and with the increase of the ${\mathrm{\text{Sr}}}^{2+}$ concentration, the phase transits gradually from rhombohedral to pseudo cubic symmetry. Electrical and magnetic properties show strong dependence on ${\mathrm{\text{Sr}}}^{2+}$ dopant level. The measurement demonstrates that ${\mathrm{\text{LBFS}}}_x$ with 30% ${\mathrm{\text{Sr}}}^{2+}$ dopant exhibits the lowest values of the leakage current, dielectric constant $(\epsilon )$ and dielectric loss $(tan\delta )$. And the highest value of saturation magnetization of about 4.5 emu/g is observed in 60% Sr-doped ${\mathrm{\text{LBFS}}}_x$. An abnormal enhancement of the conductivity was observed in ${\mathrm{\text{LBFS}}}_x$ ceramics with $x$ = 0.60 and 0.70, and their magnitude of conductivity is nearly six orders larger than that of pure ${\mathrm{\text{LBFS}}}_x$.

A series of Ti${}_{1-x}$Co${}_x$O${}_{2-\delta }$ ($x$ = 0.01, 0.03, 0.05, 0.07) nanoparticles were synthesized by sol–gel method. The X-ray diffraction, transmission electron microscopy, Raman analysis and X-ray photoelectron spectroscopy ruled out the signatures of Ti${}^{3+}$, Co-clusters or any other oxides of Co. The ferromagnetic behavior was clearly observed at room temperature in doped samples with saturation magnetization $(M_s)$ of the order of 0.008–0.035 emu/g depending on doping concentrations. The saturation magnetization is found to be increased with the Co contents increasing from 1% to 7%. From the plot of the M–T curve, we obtain the $T_c$ as $\sim$515 K for 5% Co-doped TiO₂. Oxygen vacancies were detected from the photoluminescence (PL) measurement. Magnetic properties analyses and PL analyses showed that oxygen vacancies probably played a major role in ferromagnetism of the Ti${}_{1-x}$Co${}_x$O₂ system with Co substituting for Ti. The first-principles calculation was performed to investigate the magnetic properties of Co-doped TiO₂ nanoparticles. It can be found that the major magnetic moment is from the 3d electron of Co. The experiment results are consistent with the first-principles calculation. The ferromagnetism derived from the spin-split of O-2p and Co-3d electron states caused by p–d orbit hybridization.

Fe-doped TiO₂ crystals were successfully prepared using a sol–gel technique in reducing and oxidizing atmospheres. The effects of sintering atmosphere on phase transformation, oxygen vacancy concentration and photoabsorption behaviors were investigated. The results indicate that upon sintering in reducing atmosphere, Ti and Fe ion valences were decreased and highly Fe ions (12 mol%) were entirely dissolved into titania crystals, increasing oxygen vacancy concentration and leading to increased photoabsorption capability. In contrast, sintering in oxidizing atmosphere causes precipitation of the Fe₂O₃ phase, which is detrimental to the photoabsorption capability. The best photoabsorption performance is obtained by sintering 12 mol% Fe-doped TiO₂ in reducing atmosphere, resulting in an absorption edge of approximately 435 nm, which is much higher than that of undoped TiO₂ in the oxidizing atmospheres with the absorption edge 352 nm.

In this paper, titanium and tungsten films were coated on copper sheets, respectively, by plasma chemical vapor deposition (PCVD). The copper sheets before and after coating were, respectively, injected as charge into the low-density polyethylene (LDPE), the space charge accumulation in LDPE was tested by the thermally stimulated current (TSC), and the trap level of the dielectric was calculated by the TSC curve. The results show that the work function improves after the modification of the electrode material, the space charge accumulation in LDPE decreases, and the trap level of the polymer reduces. Because of the oxygen vacancies, the titanium coating electrode has significant suppression on the charge injection in LDPE under 30 kV/mm and the tungsten shows excellent inhibition under 70 kV/mm.

${\mathrm{\text{Sr}}}_{1-3x/2}{\mathrm{\text{La}}}_x{\mathrm{\text{TiO}}}_3$ (SLTO) ceramics ($x$ = 0.05, 0.10, 0.15) were prepared with giant dielectric constant by the traditional solid-state method at 1350${}^{\circ }$C. The temperature dependence of the dielectric constant was obtained at the temperature ranging from 29${}^{\circ }$C to 500${}^{\circ }$C and the frequency ranging from 2 kHz to 2 MHz. Two sets of relaxation peaks appear in the low temperature (region I) and the high temperature (region II), respectively. For region I, we conclude that the relaxation behavior is related to the oxygen vacancy migration. For region II, the two relaxation processes are caused by grain boundary for high frequency and Sr or Ti defects at grain interior for low frequency. With the doping amount reaching 0.15, the relaxation peaks disappear and become a common phase transition because of the aggravation of lattice distortion. These possible physical mechanisms of two sets of relaxation peaks are briefly discussed.