Synthesis, electrical resistivity, thermo-electric power and magnetization of cubic ZnMnO3

Cubic ZnMnO3 powder in the form of well-crystalline nanoflakes have been synthesized at low temperatures from a nitrate precursor. The electrical properties of cubic ZuMnO(3) samples have been established by DC resistivity (rho) and thermo-electric power (Seebeck coefficient) measurements on a pressed pellet. The material exhibits insulator behavior with 0.7 eV acceptor ionization energy in the measured temperature range of 170-300 K. The thermo-electric power indicates a positive sign of the charge carriers. The obtained material exhibits a superparamagnetic signature with a blocking temperature of 9 K and the ZFC-FC splitting temperature of 15 K. (C) 2010 Elsevier Ltd. All rights reserved.

The development of high performance SnO2:F as TCOs for thin film silicon solar cells

Christophe Ballif, Laura Ding, Sylvain Nicolay

High performance transparent conducting oxides (TCOs) have significance for optimising photovoltaic (PV) performance. The efficiency of the resulting solar cells is dependent particularly on achieving high light scattering, low resistivity and low absorption (via low FCA). These properties have been targeted by systematic exploration of the atmospheric pressure chemical vapour deposition growth parameters, in particularly the deposition temperature and growth rate. APCVD processes are particularly suited to use in industry due to the high volume, continuous growth processes and fast growth rates achievable. Using the APCVD process, F-doped SnO2 has been deposited on glass using monobutyl tin trichloride with trifluoro-acetic acid as the dopant source. The deposited films were characterised for crystallinity, morphology (roughness), optical haze and electrical properties (via Hall measurement) to aid optimisation of material suitable for solar cells. Samples were then used in manufacture of single a-Si:H solar cells, which showed enhanced performance, in comparison to commercially available TCO CVD coated glasses, with high quantum efficiency yield. (C) 2012 Elsevier B.V. All rights reserved.

Elsevier2012

Study of bending reliability and electrical properties of platinum lines on flexible polyimide substrates

Danick Briand, Jérôme Courbat, Nico de Rooij, Francisco Molina Lopez

We have experimentally studied the variation in electrical resistance of flexible platinum lines patterned on polyimide foil when they are subjected to circular bending constraints. The lines were patterned by means of standard photolithography and sputtering deposition. Two different photolithography masks were used for comparative evaluation: an un-expensive transparency mask and a standard chromium mask. Measurements of the temperature coefficient of resistance (TCR) and time stability of the resistance have been acquired for lines bent down to 1.25 mm radius of curvature on a customized bending setup, showing good reliability results. The robustness of the lines has been also assessed by registering their change in resistance while bending at different radii of curvature. The lines showed reliability issues for radii of curvature below 1.25 mm, presenting a resistance variation of 19% for transparency mask-fabricated lines and 9% for chromium mask-fabricated lines. The worse reliability performances of transparency mask lines, compared to the chromium mask ones, was found to be due to their imperfect edges, which promoted the formation and propagation of cracks during bending. The results of the experiments in this work permitted to compare the performances of flexible conductive lines with, different geometry and fabricated with two different masks, establishing quantitative and qualitative bending limits for their appropriate operation in flexible electronics systems. (C) 2014 Elsevier Ltd. All rights reserved.

Elsevier2014

Pressure-tuned Transport Properties of Novel Iron-based Superconductors and Topological Insulators

Andrea Pisoni

The majority of interactions in solids strongly depend on the interatomic distances. The application of pressure changes the lattice parameters and modifies the electronic and the phononic energy spectra of a material avoiding some of the undesirable effects induced by chemical doping, like lattice disorder, impurity phases and phase separations. Layered materials are particularly affected by the application of pressure because their anisotropic structure eases the contraction of the lattice along specific crystallographic directions. In this thesis I study the effect of pressure on the transport properties of two new layered materials: L4Fe2As2Te(1-x)O4 (L = Pr, Sm, Gd) superconductors and beta-Bi4I4 topological insulator. The discovery of Fe-based superconductors (Fe-SCs) provided a new material base for studying the mechanism of high temperature superconductivity. These materials present unexpectedly high critical temperatures (Tc ) despite the presence of magnetic atoms. The parent compounds of Fe-SCs are semimetals with an antiferromagnetic ground state in which the itinerant electrons form a periodic modulation of spin density called spin density wave (SDW). By changing the structural properties or by adding/removing carriers the SDW order can be suppressed and superconductivity emerges. Our group recently succeeded in synthesizing a new family of Fe-SCs that presents Tc up to 45 K upon optimum chemical doping and substitution. I performed a systematic study of the upper critical field and the pressure-dependent electrical resistivity of single crystals of Pr4Fe2As2Te(1-x)O4 and Sm4Fe2As2Te(1-x)O(4-y)Fy . Hall coefficient and magnetoresistance of Pr4Fe2As2Te(1-x)O4 revealed electrons as the dominant type of charge carriers. The results can be successfully fitted by a two-band model that allows the estimation of the temperature dependent electron and hole densities and mobilities. In view of the exceptionally high structural anisotropy of this new class of Fe-SCs I investigated the electronic anisotropy of Sm4Fe2As2Te(1-x)O(4-y)Fy by means of microfabrication techniques. The results show a ratio between the out-of-plane and in-plane electrical resistivities that is the highest reported so far in Fe-SCs. Topological insulators (TIs) form another class of materials that is currently attracting a lot of attention both for fundamental physics and potential applications. In TIs metallic surface states coexist with an insulating bulk. Most of the TIs known so far are either three-dimensional strongly bonded bulk materials or layered van der Waals crystals. Beta-Bi4I4 is a material composed of quasi-one-dimensional molecular chains that was theoretically predicted, and only recently experimentally verified, to be a novel strong topological insulator. Due to its particular structure, beta-Bi4I4 offers the possibility to study the interplay between different ground states like topological insulating phase, charge-density-wave and superconductivity. Here I present, for the first time, a study of the thermoelectric properties of beta-Bi4I4 single crystals. Electrical resistivity, Seebeck coefficient, thermal conductivity and Hall coefficient measurements reveal a possible charge density-wave-order (CDW) at low temperature. According to resistivity and thermoelectric power measurements, pressure suppresses the CDW order. Resistivity measurements performed in a diamond anvil cell up to 20 GPa demonstrate that superconductivity is induced in beta-Bi4I4 for pressure above 10 GPa. Preliminary X-ray diffraction measurements by synchrotron radiation, performed on the sample recovered at ambient pressure after the experiment, reveal that a new Bi-I phase is formed at high pressure. The possibility that the topological insulator state could coexist with superconductivity in beta-Bi4I4 is certainly a fascinating option but it remains for now an open question.

EPFL2016

Pressure-tuned Transport Properties of Novel Iron-based Superconductors and Topological Insulators

Andrea Pisoni

EPFL2016