New explanation for oxidation-induced Cr4+ formation in garnets

The demand for self-Q-switched lasers caused a growing interest in Cr4+:YAG materials. However, the main problem of Cr4+:YAG is an insufficient concentration of Cr4+ ions. This problem is complicated by the absence of the adequate model describing formation of Cr4+ ions. Analysis of the temperature dependence of Cr3+ luminescence in the Cr-doped YAG ceramics after different thermal treatments allows understanding the mechanism of Cr3+-Cr4+ valence transformation. Two types of Cr3+ centers with different spectroscopic properties were revealed in the Cr,Ca-doped YAG ceramics, namely, single Cr3+ ions, and Cr3+/ [CaC′…VO·] couples. Using the obtained spectroscopic data, a model of formation of Cr4+ ions in YAG lattice was proposed

Understanding the power of luminescence ratiometric thermal history indicators driven by phase transitions: the case of Eu3+ doped LaVO4

Finding thermal history phosphors with high sensitivity and a consistent readout is required for reliable thermal history determination with high temperature resolution. This work presents a new thermal history phosphor based on the luminescence of Eu3+ ions in LaVO4 to meet these requirements. As demonstrated, raising the annealing temperature causes a structural phase transition from a low-temperature tetragonal phase to a high-temperature single-stranded phase. The associated change in the local point symmetry of the crystallographic site occupied by Eu3+ ions result in a significant decrease in the emission intensity ratio of the 5D0 → 7F2 band relative to the 5D0 → 7F1 band, which enables the development of the ratiometric thermal history phosphor with the relative sensitivity of 0.38% °C−1 at 800 °C. Its applicative potential for thermal history readout was proved in the proof-of-concept experiment

Polyelectrolyte multilayer electrostatic gating of graphene field-effect transistors

We apply polyelectrolyte multilayer films by consecutive alternate adsorption of positively charged polyallylamine hydrochloride and negatively charged sodium polystyrene sulfonate to the surface of graphene field effect transistors. Oscillations in the Dirac voltage shift with alternating positive and negative layers clearly demonstrate the electrostatic gating effect in this simple model system. A simple electrostatic model accounts well for the sign and magnitude of the Dirac voltage shift. Using this system, we are able to create p-type or n-type graphene at will. This model serves as the basis for understanding the mechanism of charged polymer sensing using graphene devices, a potentially technologically important application of graphene in areas such as DNA sequencing, biomarker assays for cancer detection, and other protein sensing applications

Accurate Predictions of Forces in the Presence of Multivalent Ions by PoissonBoltzmann Theory

Forces between positively and negatively charged colloidal particles across aqueous salt solutions containing multivalent ions are measured directly with the atomic force microscope (AFM). The measurements are interpreted quantitatively with Poisson–Boltzmann (PB) theory. Thereby, the surface potentials and regulation properties of the particle surfaces are extracted from symmetric measurements between the same types of particles. This information is used to predict force profiles in the asymmetric situations involving different types of particles without any adjustable parameters. These predictions turn out to be very accurate, which demonstrates that the mean-field PB theory is reliable down to distances of about 5 nm. While various reports in the literature indicate that this theory should fail due to neglect of ion correlations, such effects seem important only at higher concentrations and smaller distances