Enhancing materials properties via targeted doping

In this thesis a series of theoretical studies aimed at enhancing the optical and electrical properties of selected oxide and hydride materials via defect incorporation is presented. Large-scale screening for useful defects was performed on two transparent tin-based oxide materials: a natively p-type tin monoxide and an intrinsically n-type tin dioxide. Novel dopant candidates that promise amplified charge-carrier generation if incorporated successfully were uncovered for both compounds. We further showed that some of these dopant elements are able to maintain the optical properties displayed by the bulk phases of the oxides. The two studies revealed the affinity of tin monoxide for both hole and electron free-carriers when doped appropriately, while tin dioxide was shown to be a strictly n-type conductor. The possibility of improving both optical and electronic attributes of tin-oxide materials further was investigated by exploring the interactions between impurity atoms and intrinsic defects of the host. Isovalent silicon doping in tin dioxide was shown to suppress absorption states arising from oxygen deficiencies, thus, presenting a novel path for improving optical properties in transparent conductive oxide materials. In tin monoxide, halogen interstitials were observed to bond with native tin vacancies ionizing them to higher charge states, which result in improved p-type carrier generation. Finally, acceptor doping was also considered in large band gap hydride materials under compression. Defects in ice, H$_{2}$O, and polyethylene, H$_{2}$C$_{n}$, were studied by identifying high-pressure phases that display covalent bonding and can, therefore, be successfully doped. The possibility of such doped phases displaying a superconducting transition was addressed and a transition temperature of 60 K in ice-X and a 35 K in a polymeric high-pressure phase of polyethylene was estimated

Particle-stabilized water droplets that sprout millimeter-scale tubes

A layer of colloidal particles will become irreversibly trapped at a fluid–fluid interface if they exhibit partial wettability with both fluid phases. This effect has been exploited to create Pickering emulsions, armored bubbles, and new materials of various kinds. When the interfaces are densely coated with particles, they behave like rigid elastic sheets with moduli that are proportional to the underlying interfacial tension. The interfaces are permeable, a characteristic that can, for example, lead to compositional ripening of Pickering emulsions Here we show that when particle-stabilized water droplets are created in a bath of toluene with ethanol, millimeter-scale tubes are observed to sprout from the top of the droplets. Growth is driven by the ethanol partitioning from the toluene into the water which leads to an internal overpressure. Vertical growth occurs over many minutes; finally the tube buckles when it can no longer support its own weight (Figure 1). There are several different growth modes controlled by the concentration of ethanol and of silica particles.[1] An alternative way to manipulate the system is by using a different alcohol, leading to insight on the role of the underlying three-fluid phase diagram. Our work paves the way for future studies of droplet growth because the liquid droplets and the interfacial properties can be independently studied. Please click Additional Files below to see the full abstract

Superconductivity in doped polyethylene at high pressure

In this work we study the pressure-dependent phase diagram of polyethylene (H2C)(x) from 50 to 200 GPa. Low-symmetry, organic polymeric phases, that are dynamically stable and thermodynamically competitive with elemental decomposition, are reported. Electronic structure calculations reveal that the band gap of the lowest energy polymeric phase decreases from 5.5 to 4.5 eV in the 50-200 GPa range, but metalization occurs only for pressures well above 500 GPa. The possibility of metalization via doping was also investigated, observing that it can be achieved through boron substitution at carbon sites. We report a sizable electron-phonon coupling (lambda similar or equal to 0.79) in this metallic phase, with an estimated superconducting transition temperature of about 35 K. However, a rather narrow domain of stability is found; most of the dopant elements render the polymeric phases unstable and induce amorphization. This suggests that doping under pressure, though presenting an alternative route to find high temperature superconductors, would be challenging to achieve experimentally