21 research outputs found

    [Methylhydrazinium]2PbBr4, a ferroelectric hybrid organic-inorganic perovskite with multiple nonlinear optical outputs

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    An expansive library of structurally complex two-dimensional (2D) and three-dimensional (3D) lead halide perov-skites has emerged over the past decade, ļ¬nding applications in various aspects of photon management: photovoltaics, photo-detection, light emission, and nonlinear optics. Needless to say, the highest degree of structural plasticity enjoys the former group, oļ¬€ering a rich playground for modiļ¬cations of relevant optoelec-tronic parameters such as exciton energy. Structural tailorability is reļ¬‚ected in the ease of modiļ¬cation of the chemistry of the organic layers residing between inorganic slabs. In this vein, we show that the introduction of methylhydrazinium cation (MHy+, CH3NH2NH2+) into 2D perovskite gives a material with a record low separation of the inorganic layers (8.91 ƅ at 300 K). Optical studies showed that MHy2PbBr4 features the most red-shifted excitonic absorption among all known A2PbBr4 compounds as well as a small exciton binding energy of 99.9 meV. MHy2PbBr4 crystallizes in polar Pmn21 symmetry at room emperature (phase III) and at 351 K undergoes a phase transition to modulated Pmnm phase (II) followed by another phase transition at 371 K to Pmnm phase (I). The ferroelectric property of room-temperature phase III is inferred from switching of the pyrocurrent, dielectric measurements, and optical birefringence results. MHy2PbBr4 exhibits multiple nonlinear optical phenomena such as second-harmonic generation, third-harmonic generation, two-photon excited luminescence, and multiphoton excited luminescence. Analysis of MHy2PbBr4 single-crystal luminescence spectra obtained through linear and nonlinear optical excitation pathways indicates that free exciton emission is readily probed by the ultraviolet excitation, whereas crumpled exciton emission is detected under two- and multiphoton excitation conditions. Overall, our results demonstrate that incorporation of MHy+ into the organic layer is an emergent strategy for obtaining a 2D perovskite with polar character and multifunctional properties

    Dielectric properties and phase transition in SrBi2Nb2O9ā€“SrBi2Ta2O9 solid solution

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    Structural and dielectric properties of SrBi2(Nb1-xTx)(2)O-9 solid solution, (SBNT), with x = 0.0, 0.1, 0.2, 0.3, 0.5, 0.7, 1.0 were investigated. Crystal structure at room temperature was analyzed by X-ray diffraction study. SBNT exhibits an Aurivillius structure and the c lattice constant decreases with increasing concentration of Ta in the solution. The studies of real (epsilon') and imaginary (epsilon '') parts of permittivity as a function of temperature (20-500 degrees C) and frequency (0.1-100 kHz) were carried out, as well. The anomalies of epsilon'(T) and epsilon ''(T) associated with phase transition from the tetragonal paraelectric phase (PT) to the orthorhombic ferroelectric phase (F-O) were observed. They are strongly dependent on the Ta content in the solution. The grain structure and chemical composition were examined by a scanning electron microscope (SEM), with an energy dispersion spectrometer (EDS). The EDS analysis indicated a homogeneous distribution of all elements of ceramics within the grains. (C) 2009 Elsevier Ltd and Techna Group S.r.l. All rights reserved

    Impact of Fe doping on the electronic structure of SrTiO 3 thin films determined by resonant photoemission

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    Epitaxial thin films of Fe doped SrTiO3 have been studied by the use of resonant photoemission. This technique allowed us to identify contributions of the Fe and Ti originating electronic states to the valence band. Two valence states of iron Fe2+ and Fe3+, detected on the base of x-ray absorption studies spectra, appeared to form quite different contributions to the valence band of SrTiO3. The electronic states within the in-gap region can be attributed to Fe and Ti ions. The Fe2+ originating states which can be connected to the presence of oxygen vacancies form a broad band reaching binding energies of about 0.5 eV below the conduction band, while Fe3+ states form in the gap a sharp feature localized just above the top of the valence band. These structures were also confirmed by calculations performed with the use of the FP-LAPW/APW+lo method including Coulomb correlations within the d shell. It has been shown that Fe doping induced Ti originating states in the energy gap which can be related to the hybridization of Ti and Fe 3d orbitals

    Brillouin light scattering in niobium doped lead zirconate single crystal

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    Brillouin light scattering experiments were performed for lead zirconate single crystals doped with niobium. Special attention was paid to the elastic mode softening near phase transition temperatures. The results are compared with data obtained by Raman light scattering experiments. We observed that the interaction between acoustic and optic modes is responsible for symmetry breaking far above TC, leading to polar regions' appearance. No changes in the acoustic mode frequency and its damping are observed at TC, where Īµ(T) exhibits a maximum value. The absence of these changes and the central peak observed in Raman experiments suggest that the phase transition at TC is mainly of the orderā€“disorder type. The origin of other phase transitions is discussed as well

    The public sector's perspective on procuring public works projects - Comparing the views of practitioners in Hong Kong and Australia

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    Hong Kong has been one of the early jurisdictions to adopt Public Private Partnership (PPP) model for delivering large public infrastructure projects. The development of this procurement approach in Hong Kong has followed an intricate path. As such, it is believed that there are a number of areas which are interesting to unveil. As part of a comprehensive research study looking at implementing PPPs, interviews with experienced local industrial practitioners from the public sector were conducted to realize their perspective on the topic of procuring public works projects. Amongst these interviews, fourteen were launched government officials and advisers. The interview findings show that the majority of the Hong Kong and Australian interviewees had previously conducted some kind of research in the field of PPP. Both groups of interviewees agreed that ā€œPPPs gain private sectorā€™s added efficiency/expertise/management skillsā€ when compared to projects procured traditionally. Also, both groups of interviewees felt that projects best suited to use PPP are those that have an ā€œEconomic business caseā€. The interviewees believed that ā€œContractorā€™s performanceā€ could be used as key performance indicators for PPP projects. A large number of critical success factors were identified by the interviewees for PPP projects; two of these were similar for both groups of interviewees. These included ā€œProject objectives well definedā€ and ā€œPartnership spirit/commitment/trustā€. Finally it was found that in-house guidance materials were more common in the organizations of the Australian interviewees compared to the Hong Kong ones. This paper studies the views of the public sector towards the topic of PPPs in Hong Kong and Australia, which helps to answer some of the queries that both academics and the private sector in these jurisdictions are keen to know. As a result the private sector can be more prepared when negotiating with the public sector and realise their needs better, academics on the other hand are provided a wider perspective of this topic benefiting the research industry at large
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