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- 저자 : Saouma, F.O. (검색결과 4 건)
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Saouma, Felix Ochieng,Park, Dae Young,Kim, Sung Hyuk,Jeong, Mun Seok,Jang, Joon Ik American Chemical Society 2017 Chemistry of materials Vol.29 No.16
<P>Hybrid organic inorganic lead halide perovskites have recorded unprecedented improvement in efficiency as fourth-generation photovoltaic materials. Recently, they have attracted enormous interest in nonlinear optics stemming basically from their excellent optoelectronic properties. Here, we investigate multiphoton absorption (MPA) in high-quality MAPbX(3) (MA = CH3NH3 and X = Cl, Br, I) bulk single crystals synthesized by an inverse temperature crystallization (ITC) method. The two-photon absorption (2PA) coefficients under picosecond pulse excitation are determined to be beta = 23 +/- 2 cm/GW and 9 +/- 1 cm/GW for MAPbI(3) and MAPbBr(3) at lambda = 1064 nm, and 13 +/- 2 cm/GW for MAPbCl(3) at lambda = 532 nm. The 2PA coefficients are comparable to those of conventional semiconductors having similar bandgaps and can be explained by a two-band model. Furthermore, we characterize the three-photon absorption behavior of MAPbCl(3) at lambda = 1064 nm, yielding gamma = 0.05 +/- 0.01 cm(3)/GW(2). The polarization dependence of MPA is also probed to experimentally estimate the degree of anisotropy. The hybrid perovskites are promising materials for nonlinear optical applications due to polarization dependent MPA response and subsequent strong photoluminescence emission, especially for the Br- and I-containing compounds.</P>
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Numerical modeling of heterogeneous material
W. Puatatsananon,V. Saouma,V. Slowik 사단법인 한국계산역학회 2008 Computers and Concrete, An International Journal Vol.5 No.3
Increasingly numerical (finite element) modeling of concrete hinges on our ability to develop a representative volume element with all its heterogeneity properly discretized. Yet, despite all the sophistication of the ensuing numerical models, the initial discretization has been for the most part simplistic. Whenever the heterogeneity of the concrete is to be accounted for, a mesh is often manually crafted through the arbitrary inclusion of the particles (aggregates and/or voids) in an ad-hoc manner. This paper develops a mathematical strategy to precisely address this limitation. Algorithms for the random generation and placement of elliptical (2D) or ellipsoid (3D) inclusions, with possibly radiating cracks, in a virtual concrete model are presented. Collision detection algorithms are extensively used.
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Ló,pez, C.A.,Bâ,ati, E.,Ferná,ndez-Dí,az, M.T.,Saouma, F.O.,Jang, J.I.,Alonso, J.A. Academic Press 2019 Journal of solid state chemistry Vol.276 No.-
<P><B>Abstract</B></P> <P>Bi<SUB>2</SUB>TeO<SUB>5</SUB> has been obtained in polycrystalline form via solid state reaction and structurally characterized from both synchrotron and neutron powder diffraction data, enabling the precise determination of the atomic positions. The crystal structure is defined in the acentric <I>Amb</I>2 space group, containing three crystallographically unequivalent Bi atoms. The position of the stereochemically active electron lone pairs of both Bi<SUP>3+</SUP> and Te<SUP>4+</SUP> ions is inferred to be opposite to the strongly covalent BiO and TeO chemical bonds; the global electron polarization arises from the non-compensated Bi<SUP>3+</SUP> lone electron pairs. Additionally, from difference Fourier maps between synchrotron and neutron diffraction data (X-N technique) it was possible to observe experimental evidence of the lone electron pair for Bi<SUP>3+</SUP>. Nonlinear optical measurements display highly active second harmonic generation response, comparable to that of reference AgGaSe<SUB>2</SUB>.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The crystal structure is defined in the non-centrosymmetric <I>Amb</I>2 space group. </LI> <LI> Fourier maps between synchrotron and neutron diffraction data (X-N technique). </LI> <LI> Experimental evidence of Bi<SUP>3+</SUP>lone electron pair from X-N study. </LI> <LI> Nonlinear optical measurements display highly active SHG response. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
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Phase Transitions of Formamidinium Lead Iodide Perovskite under Pressure
Jiang, Shaojie,Luan, Yiliang,Jang, Joon I.,Baikie, Tom,Huang, Xin,Li, Ruipeng,Saouma, Felix O.,Wang, Zhongwu,White, Timothy J.,Fang, Jiye American Chemical Society 2018 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.140 No.42
<P>The pressure-induced structural evolution of formamidinium-based perovskite FAPbI<SUB>3</SUB> was investigated using <I>in situ</I> synchrotron X-ray diffraction and laser-excited photoluminescence methods. Cubic α-FAPbI<SUB>3</SUB> (<I>Pm</I>3̅<I>m</I>) partially and irreversibly transformed to hexagonal δ-FAPbI<SUB>3</SUB> (<I>P</I>6<SUB>3</SUB><I>mc</I>) at a pressure less than 0.1 GPa. Structural transitions of α-FAPbI<SUB>3</SUB> followed the sequence of <I>Pm</I>3̅<I>m</I> → <I>P</I>4/<I>mbm</I> → <I>Im</I>3̅ → partial amorphous during compression to 6.59 GPa, whereas the δ-phase converted to an orthorhombic <I>Cmc</I>2<SUB>1</SUB> structure between 1.26 and 1.73 GPa. During decompression, FAPbI<SUB>3</SUB> recovered the <I>P</I>6<SUB>3</SUB><I>mc</I> structure of the δ-phase as a minor component (∼18 wt %) from 2.41-1.40 GPa and the <I>Pm</I>3̅<I>m</I> structure of the α-phase becomes dominant (∼82 wt %) at 0.10 GPa but with an increased fraction of δ-FAPbI<SUB>3</SUB>. The photoluminescence behaviors from both the α- and δ-forms were likely controlled by radiative recombination at the defect levels rather than band-edge emission during pressure cycling. FAPbI<SUB>3</SUB> polymorphism is exquisitely sensitive to pressure. While modest pressures can engineer FAPbI<SUB>3</SUB>-based photovoltaic devices, irreversible δ-phase crystallization may be a limiting factor and should be taken into account.</P> [FIG OMISSION]</BR>
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