Simultaneous control of ionic and electronic conductivity in materials: thallium bromide case study.

Achieving simultaneous control of ionic and electronic conductivity in materials is one of the great challenges in solid state ionics. Since these properties are intertwined, optimizing one often results in degrading the other. In this Letter, we propose a method to limit ionic current without impacting the electronic properties of a general class of materials, based on codoping with oppositely charged ions. We describe a set of analyses, based on parameter-free quantum mechanical simulations, to assess the efficacy of the approach and determine optimal dopants. For illustration, we discuss the case of thallium bromide, a wide band gap ionic crystal whose promise as a room-temperature radiation detector has been hampered by ionic migration. We find that acceptors and donors bind strongly with the charged vacancies that mediate ionic transport, forming neutral complexes that render them immobile. Analysis of carrier recombination and scattering by the complexes allows the identification of specific dopants that do not degrade electronic transport in the crystal.

Si nanowires as sensors: choosing the right surface.

We show, using ab initio calculations based on density functional theory, that for hydrogen-passivated Si nanowires (SiNWs), the relative contribution of surface atoms to the band-edge states varies according to the way these surface atoms are bonded to the core ones. The largest influence occurs when these bonds are oriented along the wire's growth direction, which occurs either on the symmetric (001) 1 x 1 or the monohydrated (111) 1 x 1 surfaces. These results are obtained for wires grown along the [110] direction, with hexagonal cross-sections and facets corresponding to (111) and (001) planes, as observed experimentally. Their diameters range from about 10 to 35 A. On the basis of our results, we propose that particular facets should be more appropriate to be functionalized in order to build SiNW-based sensors. As an example, we have investigated the effect of NH2 adsorbed on some of these surfaces on the electronic structure of these wires.