Monovalent Dopant Research Articles

A predictive modeling study of the impact of chemical doping on the strength of a Ag/ZnO interface

Using first principles modeling, we predict how substitutional doping can influence the mechanical strength of a Ag/ZnO interface commonly found in various multilayer thin-film systems. Replacing Zn with a monovalent dopant strengthens the interface while dopants with a valence greater than two weaken it. Isovalent dopants have little effect. The results are explained in terms of charge transfer and hybridization effects at the interface. Although monovalent dopants are mechanically preferred, they do not inhibit Ag inter-diffusion in ZnO and could possibly cause chemical degradation of the interface. It is suggested that other dopants, such as Bi, avoid this issue by creating larger diffusion barriers while maintaining a relatively strong interface. The results indicate that complete control over the mechanics, kinetics, and chemistry of the interface requires a careful choice of dopant types and concentrations.

Electronic properties of doped and defective NiO: A quantum Monte Carlo study

NiO is a canonical Mott (or charge-transfer) insulator and, as such, is notoriously difficult to describe using density functional theory (DFT)--based electronic structure methods. Doped Mott insulators such as NiO are of interest for various applications but rigorous theoretical descriptions are lacking. Here, we use quantum Monte Carlo methods, which very accurately include electron-electron interactions, to examine energetics, charge structures, and spin structures of NiO with various point defects, such as vacancies and substitutional doping with potassium. The formation energy of a potassium dopant is significantly lower than that of a Ni vacancy, making potassium an attractive monovalent dopant for NiO. We compare our results with DFT results that include an on-site Hubbard $U$ ($\mathrm{DFT}\phantom{\rule{0.28em}{0ex}}+\phantom{\rule{0.28em}{0ex}}U$) to account for correlations and find relatively large discrepancies for defect formation energies as well as for charge and spin redistributions in the presence of point defects. Beyond fitting to a single property, it is unlikely that single-parameter tuning of the $\mathrm{DFT}\phantom{\rule{0.28em}{0ex}}+\phantom{\rule{0.28em}{0ex}}U$ will be able to obtain accurate accounts of complex properties in these materials. Responses that depend in subtle and complex ways on ground-state properties, such as charge and spin densities, are likely to contain quantitative and qualitative errors.

Synthesis and thermoluminescence characterization of MgB4O7:Gd,Li

Magnesium tetraborate (MTB) doped with rare earth elements were synthesized by solid state sintering technique. Among the different rare earth dopants studied in this phosphor, gadolinium doped phosphors resulted in a single intense dosimetric peak at 250 °C and this is the first report in rare earth-doped MgB4O7 with a glow peak above 200 °C Photoluminescence (PL) and thermoluminescence (TL) studies were performed with this phosphor after exposing the powder samples to ionizing radiation. Monovalent dopants, including Na, Li and Ag, were found to increase the TL sensitivity of the MgB4O7:Gd phosphor without a shift in the TL peak temperature. The TL emission spectra showed characteristic emission of the host lattice, which showed an increase on doping with rare earth or monovalent codopants. The TL sensitivity, dose response curve, and post-irradiation storage stability were studied for the possible use of this material in radiation dosimetry applications. The TL parameters, such as the activation energy, the frequency factor, and the order of kinetics were determined for the Gd-doped MgB4O7 phosphor. The phosphor was found to be reusable after a few cycles of irradiation and annealing. The post-irradiation storage stability studies showed that this near tissue-equivalent phosphor, which has a gamma sensitivity five times that of TLD-100, is suitable for medical dosimetry applications.

Defect and dopant properties of the α- and β-polymorphs of the Li3FeF6 lithium battery material

Li3FeF6 has attracted recent interest as a promising positive electrode for rechargeable lithium batteries. The defect chemistry and cation doping behaviour of the α- and β-polymorphs of Li3FeF6 have been investigated by advanced atomistic modelling techniques. Our simulations show good reproduction of both monoclinic (α) and orthorhombic (β) experimental structures, which are related to the cryolite crystal structure. The most favourable defect types are found to be the Li Frenkel and off-stoichiometry (Li-excess) disorder, suggesting that interstitial lithium is possible in the cryolite structure, and is important in rationalizing the lithium intercalation chemistry. Monovalent dopant substitution for Li and divalent substitution for Fe are energetically favourable, and could be a synthesis strategy to optimise the electrochemical behaviour of Li3FeF6.

Different incorporation of Cu+ and Cu2+ in lithium tetraborate single crystals

The incorporation site of Cu in the scintillator and thermoluminescent dosimeter material lithium tetraborate Li2B4O7:Cu has been shown to depend on the charge state of the dopant. As confirmed by a refined analysis, the EPR spectra of an unpaired hole can be ascribed to Cu2+ situated near the Li lattice site with C1 symmetry, in contrast to the higher (C2) overall symmetry reported earlier for Cu+ in its relaxed excited state, the latter suggesting interstitial incorporation for the monovalent dopant. Consequences for charge transfer processes involving copper ions are discussed.

Dopant complexes and their effect on optical and contact processes

We consider bound states of bipolarons with monovalent dopants, their transformation under illumination and by charge injection. These complexes provide effective traps for the minority carriers. On this basis we describe the photoluminescence quenching in LED, the stationary characteristics and hysteresis at the Schottky contacts.

Effect of dopants on ferroelectric properties of triglycine sulphate [(NH2CH2COOH)3. H2SO4] crystals

Abstract From the measurements on the dielectric constant at fixed temperature and under varying bias condition, it is observed that pure as well as Na doped TGS samples follow the same trend as reported by Hoshino et a1. An increase in the transition temperature (Tc) is observed with the increase in the concentration of the dopant. The decrease in the dielectric constant (∊) on either side of T is attributed to the dipole orientation. With the increase in the doping concentration, an increase in the electrical conductivity is observed at the Tc. Therefore, the decrease in the dielectric constant with the increase in the concentration of the monovalent dopant Na+ suggests that the dopant does not interfare with the strongly bonded zwitter ion of glycine but may he attributed to the splitting of anion pair formed by the structure (NH+ 3CH2 COO−)2.SO42-into NH+ 2 Na+.

Electronic structure of doped Sr 2VO 4

We report band structure calculations for the compound Sr 2VO 4. Calculations were also performed for the hypothetical compound, Tl 2VO 4, in the K 2NiF 4 structure in order to determine the valence of substitutional Tl in Sr 2VO 4. For pure Sr 2VO 4 we find a Stoner criterion, N( E F) I( E F ), of 2.21 implying a magnetic instability. This is consistent with the antiferromagnetic state found. experimentally. Rigid band calculations of the Stoner criterion indicate that the magnetic instability of Sr 2VO 4 may be removed by doping with either monovalent or trivalent ions the Sr sites. However, much smaller concentrations are required if monovalent dopants are used. The calculations for Tl 2VO 4 show that substitutional Tl is effectively monovalent on the Sr site and thus provides an alternative to doping with group I elements. We have evaluate the electron-phonon interaction parameters η and find that they have much larger values than in La 2CuO 4.

Activation of colour centres in MgF2 doped with LiF and NaF

The colour centres in MgF 2 can be activated by monovalent dopants which might form monovalent-dopant-ion-associated fluorine vacancies, thereby enhancing the efficiency of colour centre production. Experimental results showed that the efficiency of F centre production was proportional to the concentration of the doped monovalent ion. The density of F centres generated by γ rays was proportional to the square root of irradiation time at room temperature. The production efficiency of doped 4.8 wt. ppm lithium was four times that of undoped lithium at an absorbed energy of 10 16 MeV cm −3.

Summary Abstract: Effect of monovalent dopants on the electrical behavior of a-Se

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The basic atomic processes of corrosion I. Electronic conduction in MnO, CoO and NiO

Abstract We have used simple ionic models to study the lattice distortion and polarization about localized holes in MnO, CoO and NiO. Our results demonstrate a rather fine balance between factors favouring localized small polarons and band-state large polarons, but we find the correct trend consistent with the observation of small polarons in MnO and large polarons in CoO and NiO. Our calculations of the activation energy for hole hopping give results in good general agreement with the measured free hole mobility in MnO and the trapped small polaron motion in CoO and NiO. We can also make reasonable estimates of the binding of the hole to a monovalent dopant. These satisfactory conclusions seem to justify the neglect of the complications of Jahn-Teller, crystal field and covalency contributions in our models.