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Register a new userCharge Carrier Transport in Iron Pyrite Thin Films: Disorder Induced Variable Range Hopping
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The origin of p-type conductivity and the mechanism responsible for low carrier mobility was investigated in pyrite (FeS2) thin films. Temperature dependent resistivity measurements were performed on polycrystalline and nanostructured thin films prepared by three different methods. Films have a high hole density and low mobility regardless of the method used for their preparation. The charge transport mechanism is determined to be nearest neighbour hopping (NNH) at near room temperature with Mott-type variable range hopping (VRH) of holes via localized states occurring at lower temperatures. Density functional theory (DFT) predicts that sulfur vacancy induced localized defect states will be situated within the band gap with the charge remaining localized around the defect. The data indicate that the electronic properties including hopping transport in pyrite thin films can be correlated to sulfur vacancy related defect. The results provide insights on electronic properties of pyrite thin films and its implications for charge transport
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Many optoelectronic properties have been reported for lead halide perovskite polycrystalline films. However, ambiguities in the evaluation of these properties remain, especially for long-range lateral charge transport, where ionic conduction can complicate interpretation of data. Here we demonstrate a new technique to measure the long-range charge carrier mobility in such materials. We combine quasi-steady-state photo-conductivity measurements (electrical probe) with photo-induced transmission and reflection measurements (optical probe) to simultaneously evaluate the conductivity and charge carrier density. With this knowledge we determine the lateral mobility to be ~ 2 cm2/Vs for CH3NH3PbI3 (MAPbI3) polycrystalline perovskite films prepared from the acetonitrile/methylamine solvent system. Furthermore, we present significant differences in long-range charge carrier mobilities, from 2.2 to 0.2 cm2/Vs, between films of contemporary perovskite compositions prepared via different fabrication processes, including solution and vapour phase deposition techniques. Arguably, our work provides the first accurate evaluation of the long-range lateral charge carrier mobility in lead halide perovskite films, with charge carrier density in the range typically achieved under photovoltaic operation.
Iridate thin films are a prerequisite for any application utilizing their cooperative effects resulting from the interplay of stron spin-orbit coupling and electronic correlations. Here, heteroepitaxial Na2IrO3 thin films with excellent (001) out-of-plane crystalline orientation and well defined in-plane epitaxial relationship are presented on various oxide substrates. Resistivity is dominated by a three-dimensional variable range hopping mechanism in a large temperature range between 300 K and 40 K. Optical experiments show the onset of a small optical gap of about 200 meV and a splitting of the Ir 5d-t2g manifold. Positive magnetoresistance below 3 T and 25 K shows signatures of a weak antilocalization effect. This effect can be associated with surface states in a topological insulator and hence supports proposals for a topological insulator phase present in Na2IrO3.
The effects of space charges on hysteresis loops and field distributions in ferroelectrics have been investigated numerically using the phenomenological Landau-Ginzburg-Devonshire theory. Cases with the ferroelectric fully and partially depleted have been considered. In general, increasing the number of charged impurities results in a lowering of the polarization and coercive field values. Squarer loops were observed in the partially depleted cases and a method was proposed to identify fully depleted samples experimentally from dielectric and polarization measurements alone. Unusual field distributions found for higher dopant concentrations have some interesting implications for leakage mechanisms and limit the range of validity of usual semiconductor equations for carrier transport.
Experimental evidence for ferromagnetic ordering in isotropic atomically thin two-dimensional crystals has been missing until a bilayer Cr2Ge2Te6, and a three-atom thick monolayer CrI3 are shown to retain ferromagnetic ordering at finite temperatures. Here, we demonstrate successful isolation of a non-van der Waals type ultra-thin nanosheet of FeS2 derived from naturally occurring pyrite mineral (FeS2) by means of liquid-phase exfoliation. Structural characterizations imply that (111) oriented sheets are predominant and is supported theoretically by means of density functional theory surface energy calculations. Spin-polarized density theory calculations further predicted that (111) oriented three-atom thick pyrite sheet has a stable ferromagnetic ground state different from its diamagnetic bulk counterpart. This theoretical finding is evaluated experimentally employing low temperature superconducting quantum interference device measurements and observed an anomalous ferromagnetic kind of behavior.
We report ultrafast surface pump and interface probe experiments on photoexcited carrier transport across single crystal bismuth films on sapphire. The film thickness is sufficient to separate carrier dynamics from lattice heating and strain, allowing us to investigate the time-scales of momentum relaxation, heat transfer to the lattice and electron-hole recombination. The measured electron-hole ($e-h$) recombination time is 12--26 ps and ambipolar diffusivity is 18--40 cm$^{2}$/s for carrier excitation up to $sim 10^{19} text{cm}^{-3}$. By comparing the heating of the front and back sides of the film, we put lower limits on the rate of heat transfer to the lattice, and by observing the decay of the plasma at the back of the film, we estimate the timescale of electron-hole recombination. We interpret each of these timescales within a common framework of electron-phonon scattering and find qualitative agreement between the various relaxation times observed. We find that the carrier density is not determined by the $e-h$ plasma temperature after a few picoseconds. The diffusion and recombination become nonlinear with initial excitation $gtrsim 10^{20} text{cm}^{-3}$.
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