Doping Rate Research Articles

Solid state synthesis and structural characterization of zinc titanates

Zinc titanate composite materials were synthesized via solid state sintering process using high-purity metal oxide powders (purity ∼99.99%). The titanium incorporation into ZnO matrix was investigated by X-ray diffraction which revealed the coexistence of spinel Zn2TiO4 and hexagonal ZnTiO3 with the ZnO wurtzite structures. No reflection peaks of rutile TiO2 phase were detected. The IR spectroscopy and Raman scattering spectroscopy were used to characterize the structural and chemical properties of the ZnO/TiO2 composites. The IR bands and vibrational modes of all crystalline phases were detected. The effect of TiO2 doping rates (x = 3, 5 and 7 wt%) on bands shifting, Raman intensity and structural quality was discussed.

Hybrid Structure White Organic Light Emitting Diode for Enhanced Efficiency by Varied Doping Rate.

Novel materials based on Zn(HPB)2 and Ir-complexes were synthesized as blue or red emitters, respectively. White organic light emitting diodes were fabricated using the Zn(HPB)2 as a blue emitting layer, Ir-complexes as a red emitting layer and Alq3 as a green emitting layer. The obtained experimental results, were based on white OLEDs fabricated using double emission layers of Zn(HPB)2 and Alq3:Ir-complexes. The doping rate of the Ir-complexes was varied at 0.4%, 0.6%, 0.8% and 1.0%. When the doping rate of the Alq3:Ir-complexes was 0.6%, a white emission was achieved. The Commission Internationale de l'Eclairage coordinates of the device's white emission were (0.316, 0.331) at an applied voltage of 10.75 V.

Structural and optical properties of the Fe-doped BaTiO3 thin films grown on LaAlO3 by polymer-assisted deposition technique

Fe-doped BaTiO3 thin films were grown on (001) oriented LaAlO3 substrates using a polymer-assisted deposition technique. The microstructural and chemical state characters of the films with the Fe doping range from 1 to 10 at% were systematically investigated by X-ray diffraction and X-ray photoelectron spectroscopy. Evolution of the preferred orientation and strain status of BaTiO3 films could be observed with increasing doping amount of Fe. Meanwhile, UV–Vis spectra of doped BaTiO3 thin films showed that the band gap could be strongly affected by the doping amount of Fe with the smallest value at the Fe doping rate of 3 at%, which is proposed to be associated with the inevitable oxygen vacancies and the substitution of Ti ions by the Fe dopants.

Tuning of Stepwise Neutral-Ionic Transitions by Acceptor Site Doping in Alternating Donor/Acceptor Chains.

The stepwise neutral-ionic (N-I) phase transition found in the alternating donor/acceptor (DA) chain [Ru2(2,3,5,6-F4PhCO2)4(DMDCNQI)]·2(p-xylene) (0; 2,3,5,6-F4PhCO2(-) = 2,3,5,6-tetrafluorobenzoate; DMDCNQI = 2,5-dimethyl-N,N'-dicyanoquinonediimine) was tuned by partly substituting the acceptor DMDCNQI with 2,5-dimethoxy-N,N'-dicyanoquinonediimine (DMeODCNQI), which displays a poorer electron affinity in an isostructural series. The site-doped series comprised [Ru2(2,3,5,6-F4PhCO2)4(DMDCNQI)1-x(DMeODCNQI)x]·2(p-xylene) for doping rates (x) = 0.05 (0.05-MeO), 0.10 (0.10-MeO), 0.15 (0.15-MeO), and 0.20 (0.20-MeO). The neutral chain [Ru2(2,3,5,6-F4PhCO2)4(DMeODCNQI)]·4(p-xylene) (1), which only contained DMeODCNQI, was also characterized. All site-doped compounds were isostructural to 0 except 1 despite their identical DA chain motif. Except at an x value of 0.20, they displayed a two-step N-I transition involving an intermediate phase. This transition occurred at high temperatures in 0 but shifted to lower temperatures in a parallel manner with increasing doping rate. Simultaneously, each transition broadened with increasing doping rate, leading to a convergence of two transitions at an x value approximating 0.2. Donor/acceptor-site-doping techniques present somewhat different impacts in terms of interchain Coulomb effects.

Local Doping of Semiconductor Crystals by Thermomigration

The article includes the analysis of the features related to local doping of silicon using electrically active doping agents by thermomigration of binary and ternary liquid zones as compared to doping by diffusion. The concentration range of doping by binary zone migration is found to be substantially narrower than that of doping by diffusion. Introduction of a third component to the liquid phase ena-bles expansion of the thermomigration doping range to the values exceeding the diffusion doping range by the same doping agent. For silicon crystals, this technological feature of thermomigration is produced by using GaxAl1-xSi and SnxAl1-xSi ternary zones. The crystal doping rate by thermomigration in techno-logically relevant situations was shown to exceed the rate of diffusion doping by orders of magnitude. The layers doped by thermomigration of stably moving liquid zones are structurally more perfect than diffusion layers.

Structural and optical properties of ZnO/TiO2 composites

This work investigates the structural and optical properties of mixed ZnO/TiO2 ceramics synthesized at 900 °C by the conventional solid state sintering method. We doped different weights of TiO2 powder (x = 1–7 wt%) to prepare the ceramic pellets. Then we studied the effect of TiO2 content on crystalline properties, grain size and residual stress using X-ray diffraction. XRD patterns revealed the alloying of hexagonal ZnTiO3 and spinel Zn2TiO4 phases as the doping rate x (wt%) increased. The growth of ZnO grains in wurtzite structure was favored along the (100) direction with coefficient texture 2.27 for a TiO2 amount x = 1wt%. Grain sizes were obtained in the range 33–73 nm. We used Williamson-Hall (W–H) analysis and size–strain plot (SSP) method to study the individual contributions of crystallite sizes and lattice strain on the peak broadening of the ZnO/TiO2 composites. Strain, stress and energy density values were calculated by assuming a uniform deformation model (UDM). We investigated the optical properties of the ceramics by Absorbance and Reflectance measurements at room temperature in the wavelength range 200–1000 nm. The results suggested that the addition of TiO2 to ZnO powder led to a high transparency in the visible range and two absorption edges in the UV region. Interestingly, it was seen slight enhancement of the band gap from 3.1 eV to 3.18 eV for the first edge while the second one was enhanced from 4.8 eV to 5.05 eV as the TiO2 doping concentration increased from 1 wt% to 7 wt% respectively.

The impact of doping rates on the morphologies of silver and gold nanowires grown in helium nanodroplets.

Silver and gold nanowires are grown within superfluid helium nanodroplets and investigated by high resolution electron microscopy after surface deposition. The wire morphologies depend on the rate of metal atom doping in the pickup sequence. While high doping rates result in a polycrystalline face-centered cubic nanowire structure, at lower doping rates the initial fivefold-symmetry seems to be preserved. An explanation for this observation is given by computer simulations, which allow the derivation of timescales for the nanowire growth process inside helium nanodroplets.

Effect of rapid thermal treatments on the physical properties of Cobalt Doped ZnO Films

ABSTRACTCobalt doped ZnO (ZnO:Co) films with a 1 at% doping rate have been successfully grown on (100) oriented p type Si substrates by radiofrequency magnetron sputtering. Post annealing treatments at 973 K for various short periods have been carried out and structural, optical and electrical properties of the films have been investigated. Upon rapid annealing, the dopant distribution in the film has been found homogeneous. The annealing improves the (002) texture of the film and the mean column width increases with the annealing duration from 60 nm up to 95 nm. The lattice parameter of the ZnO:Co films decreases upon annealing and approaches that of bulk ZnO. The photoluminescence (PL) study reveals that the Co2+ ions can be excited directly or through a transfer mechanism from the matrix. The PL intensity decreases with the annealing time suggesting a diffusion process of the dopant impeding the Co2+ emission. At last, the electrical conductivity reaches values compatible with potential electroluminescent applications of the ZnO:Co films.

Structural, electrical and mechanical properties of selenium doped thallium based high-temperature superconductors

In this study, highly-refined chemical powders were synthesized by having them ready in appropriate stoichiometric proportions with conventional solid state reaction method so that they would produce the superconductor TlPb0.3Sr2Ca1−xSexCu2Oy (x=0; 0.4; 0.6; 1.0). This study aims to understand effect of the selenium doping on the superconducting, structural and mechanical properties of the aforementioned superconducting material. The effect of the doping rates on the structural and electrical properties of the sample has been identified. Electrical characteristics of the TlPb0.3Sr2Ca1−xSexCu2Oy material were measured using standard four point probe method. Structural characteristics were examined with the powder X-ray diffractometer (XRD) and scanning electron microscope (SEM). Mechanical properties were analyzed with Vickers microhardness measurements on the sample surface. According to the results, it was observed that the reflection comes from the (00l) and parallel planes increased with Se doping. Particle size increases with increasing doping ratio. According to results of the mechanical measurements, all samples exhibit indentation size effect (ISE) behavior. Comparing the obtained results with theoretical studies, it was understood that Hays Kendall approach is the best method in determination of mechanical properties and analyzing microhardness of the materials.

Selectively enhanced up- and down-conversion emissions of Er3+ via Yb3+-Mn2+ dimer sensitizing in spinel MgGa2O4:Er3+,Yb3+,Mn2+

Selectively enhanced green upconversion (UC) and near-infrared (NIR) downconversion (DC) emissions of Er3+ by Yb3+-Mn2+ dimer sensitizing have been demonstrated in Er3+/Yb3+/Mn2+ tri-doped spinel MgGa2O4 upon excitation of 980nm laser diode (LD). By optimizing the doping rate of Mn2+, the intensity ratio of green to red UC emission intensities of Er3+ (G/R) could be enhanced about 10 times. Meanwhile, the 1.5μm NIR DC emission could also be increased about 3.6 times in this system. The DC and UC emission properties, UC mechanism, as well as the enhancing mechanism have been investigated and discussed in detail. A selectively enhancing mechanism based on Yb3+-Mn2+ dimer sensitizing is proposed to explain the selectively enhancing phenomena. This work could inspire the design of novel UC materials for various applications, such as temperature sensors and optical transmission, etc.

Asymmetry of superconductivity in hole- and electron-doped cuprates: Explanation within two-particle self-consistent analysis for the three-band model

In the hole-doped cuprate superconductors, the superconducting transition temperature $T_c$ exhibits a dome-like feature against the doping rate. By contrast, recent experiments reveal that $T_c$ in the electron-doped systems monotonically increases as the doping is reduced, at least up to a very small doping rate. Here we show that this asymmetry is reproduced by performing a two-particle self-consistent analysis for the three-band model of the CuO$_2$ plane. This is explained as a combined effect of the intrinsic electron-hole asymmetry in systems comprising Cu3$d$ and O2$p$ orbitals and the band-filling-dependent vertex correction.

Synthesis and Overall Photophysical Characterization of SiO2:(Ag/SiO2) Nanostructured Sonogel Hybrid Glasses

Bulk SiO 2-based inorganic–inorganic sonogel (SG) hybrid glasses were fabricated with Ag / SiO 2 supported metal nanoparticles (MNPs). The catalyst-free SG route was implemented to produce these optically active nanostructured composites by doping the liquid sol-phase with Ag / SiO 2 synthesized according to the deposition–precipitation method. As prepared Ag / SiO 2-MNPs exhibited particle diameters below 10 nm and homogeneous size distribution. The easy and homogeneous Ag / SiO 2 loading within the micro/mesoporous SiO 2-SG network has evidenced the guest–host chemical affinity of these systems. This fact allowed us to fabricate outstanding chemically, photo-physically and mechanically stable bulk hybrid monoliths with controllable geometry and doping rates, suitable for linear and nonlinear optical (NLO)–spectroscopic characterizations. Indeed, the hosting SG matrix provided an elevated thermal and mechanical stability protecting the reactive Ag nanoparticles from environment conditions, diminishing their tendency to from aggregates and, above all, preserving their pristine photophysical properties. Comprehensive morphological, structural, spectroscopic and NLO characterizations were performed on the obtained SiO 2:( Ag / SiO 2) hybrid composites. Results have shown that the nanocrystalline (NC) properties, multipolar nature and small sizes of the implemented Ag / SiO 2-nanoparticles, together with the bulk guest–host mechanical interactions, play a crucial role for the observation of outstanding spectroscopic and quadratic NLO properties of the developed hybrid systems.

Stability of Staggered Flux State in d-p Model Studied Using Variational Monte Carlo Method

The stability of a staggered flux (SF) or d-density wave state is studied in a d-p model using a variational Monte Carlo (VMC) method. This state possesses a local circular current and possibly causes the pseudogap phase in high-Tc cuprate superconductors. We introduce into the trial function a configuration dependent phase factor, which was recently shown to be indispensable to stabilize current-carrying states. We pay attention to the energy gain as a function of adjacent O-O hopping ▪ and Cu-site coulomb repulsion U. It is found the SF state becomes stable for hole doping rates of ▪ and ▪, owing to the reduction in kinetic energy, but seems fragile because the energy gain rapidly decays on doping holes for any ▪. The loop current tends to increase as ▪ increases.

Thermoelectric transport mechanism of Mg2Si0.4Sn0.6-yBiy prepared by low-temperature microwave reaction

According to Debye relaxation, the polarization of electric dipole can be accomplished in 10-10 s under the action of an alternating electromagnetic filed with a frequency of 2.45 GHz, so it is feasible to obtain nano powder by carrying out solid reaction in microwave at low temperature in a short time. In this work, the syntheses of Mg2Si0.4Sn0.6-yBiy (0 ≤ y ≤ 0.03) solid solution thermoelectric materials are successfully achieved by microwave-assisted solid state reaction at low temperature with MgH2 serving as one reactant instead of Mg, and their transportation mechanisms are studied based on the SPB (single parabolic band) model as well. The results indicate that the volatilization and oxidation of Mg can be suppressed effectively in this process. Fine stoichiometric product can be achieved with nano-lamellar structure with an interlayer spacing of about 100 nm by carrying out the reaction between MgH2 and Si, Sn in microwave at 400℃ in 15 min. The introduction of Bi dopant can increase carrier concentration and lattice distortion. With the cooperation between the nano lamellar structure and lattice distortion, the phone is scattered so effectively that the sample owns a lowest thermal conductivity, κmin of 1.36 W·m-1·K-1 at 550 K based on the fact that the phonon scattering is dominant in the heat transfer process. The calculated results show that the theoretical κmin is 0.93 W·m-1·K-1, which is lower than 1.36 W·m-1·K-1. Therefore, by further adjusting the process parameters and increasing the effective doping rate of Bi and the density of the lattice defects, it is expected to obtain lower thermal conductivity. The band convergence is also verified by increasing the density-of-state effective mass. The apparent increase in m* is due to a gradual increase in carrier concentration with increasing temperature. Despite the agreement between the data and the model, the irregular behavior between m* and temperature is a very strong indication and the electric transmission performance of the sample is likely to be influenced by the structure of the multi band structure. Owing possibly to the low reaction temperature, there are Bi precipitates at the grain boundary. In addition to the phonon scattering and the alloy scattering, the Bi segregation and the scattering of carrier by nano-lamellar structure make the carrier mobility of the sample slightly lower. The lower effective doping rate and complex band structure make the carrier concentration and density-of-state effective mass low coupled with the low carrier mobility, which leads to low material factor β with a ZT of 0.66 at 600 K consequently.

Flux Phase in Bilayer t–J Model

In order to study the time-reversal symmetry (${\cal T}$) breaking near a (110) surface of a high-$T_C$ cuprate YBCO, we consider the flux phase in a bilayer $t-J$ model. Although the stable solution in the bulk is the $d_{x^2-y-2}$-wave superconducting (SC) state, free energy of the flux phase is close to it, and thus the flux phase may occur when the SC order is disturbed by inhomogeneity, e.g., surface or impurity. It is found that, depending on the doping rate, the flux phase in which spontaneous magnetic fields in two layers have opposite signs may be stabilized. This may lead to a surface state with local ${\cal T}$ violation but without local magnetic field.

The electronic properties of doped single walled carbon nanotubes and carbon nanotube sensors

We present ab initio calculations on the band structure and density of states of single wall semiconducting carbon nanotubes with high degrees (up to 25%) of B, Si and N substitution. The doping process consists of two phases: different carbon nanotubes (CNTs) for a constant doping rate and different doping rates for the zigzag (8, 0) carbon nanotube. We analyze the doping dependence of nanotubes on the doping rate and the nanotube type. Using these results, we select the zigzag (8, 0) carbon nanotube for toxic gas sensor calculation and obtain the total and partial densities of states for CNT (8, 0). We have demonstrated that the CNT (8, 0) can be used as toxic gas sensors for CO and NO molecules, and it can partially detect Cl$_2$ toxic molecules but cannot detect H$_2$S. To overcome these restrictions, we created the B and N doped CNT (8, 0) and obtained the total and partial density of states for these structures. We also showed that B and N doped CNT (8, 0) can be used as toxic gas sensors for such molecules as CO, NO, Cl$_2$ and H$_2$S.

Metal–insulator transition in iron doped 2H-TaSe2: Suggestion of chiral unitary localization

We investigated the conduction of electrons in a quasi two-dimensional random magnetic field (RMF) system, namely iron-doped transition metal dichalcogenide 2H-FexTa1−xSe2. We observed a clear metal–insulator transition by increasing the Fe doping rate. The temperature dependence of the resistances is consistent with the unitary class of Anderson localization. However, the observed critical sheet resistance is between 2 and 19kΩ, which is inconsistent with the unitary class (2h/e2=51.6kΩ). We propose that chiral symmetry can be present in 2H-FexTa1−xSe2 because the magnetoresistance of the crystals is similar to that of dirty graphites.

Effects of Mg doping rate on physical properties of Mg and Al co-doped Zn1−x−0.02MgxAl0.02O transparent conducting oxide films prepared by rf magnetron sputtering

The transparent and conductive Mg–Al co-doped ZnMgAlO films with different Mg contents have been sputter grown on quartz substrates at room temperature. Modulation of the band-gap and conductivity was tried by varying the Mg doping rate from 5at.% to 20at.% at a fixed Al content of 2at.%. Optical characterization of the deposited ZnMgAlO films confirmed that the average optical transmittance in the visible region was ∼85% and the transparency of the near infra-red (NIR) region expanded to further IR region with increasing the Mg doping level, which can be profitable for the application where higher NIR transmittance is required. At a low Mg doping level of 5at.%, the best quality of electrical properties were achieved with the resistivity of 3.7×10−4Ωcm, the electron concentration of 7×1020/cm3, and the Hall mobility of 25.66cm2/Vs, whereas with further Mg doping up to 20at.%, the resistivity increased to 1.9×10−2Ωcm, which is accompanied with a degradation of crystal quality. Optical band-gap reached to 3.87eV at a high Mg doping level of 20at.%. Additional phonon modes at 275cm−1 and 648cm−1 found in the Raman spectra of the ZnMgAlO films were assigned to the host lattice defect vibrational complexes related with Al dopants and it was suggested that the broadening, weakening, and shift of phonon peaks with increasing the Mg doping level may be related with the enhanced lattice distortion in ZnO crystal and the reduced mass change induced by substitution of Mg in Zn sites.

Substitutional Doping Widens Silicene Gap

A numerical study of electronic transport properties of doped silicene is presented. By means of ab initio calculations, a self-consistent scattering potential is derived for boron, nitrogen, aluminum, and phosphorus substitutions in silicene, and the quantum-mechanical Landauer–Büttiker approach is used to evaluate the conductivities of doped silicene ribbons with various impurity concentrations. An individual defect introduces asymmetric electron–hole conductivities that depend on both the type of doping and the position of the foreign atom with respect to the edges. Quantum interference effects at zero temperature are modeled to show that randomly distributed defects over 1 μm long and realistically wide silicon nanoribbons widen the intrinsic electronic gap that arises from quantum confinement. Mobility gaps created at low doping rates may lead to greater efficiency in the design of new silicon-based devices, providing the ability for suitable control of silicon ribbons band gap.

Piezoelectric properties of graphene oxide: A first-principles computational study

Some highly ordered compounds of graphene oxide (GO), e.g., the so-called clamped and unzipped GO, are shown to have piezoelectric responses via first-principles density functional calculations. By applying an electric field perpendicular to the GO basal plane, the largest value of in-plane strain and strain piezoelectric coefficient, d31 are found to be 0.12% and 0.24 pm/V, respectively, which are comparable with those of some advanced piezoelectric materials. An in-depth molecular structural analysis reveals that the deformation of the oxygen doping regions in the clamped GO dominates its overall strain output, whereas the deformation of the regions without oxygen dopant in the unzipped GO determines its overall piezoelectric strain. This understanding explains the observed dependence of d31 on oxygen doping rate, i.e., higher oxygen concentration giving rise to a larger d31 in the clamped GO whereas leading to a reduced d31 in the unzipped GO. As the thinnest two-dimensional piezoelectric materials, GO has a great potential for a wide range of micro/nano-electromechanical system (MEMS/NEMS) actuators and sensors.