We investigate possible mechanisms to induce electric polarization in layered organic-inorganic hybrids. Specifically, we investigate the structural phase transitions of PEA2MnCl4 (PEA = phenethylamine) using temperature dependent single-crystal X-ray diffraction analysis, including the symmetry analysis of the observed space groups. Our results show that PEA2MnCl4 transforms from a high-temperature centrosymmetric structure with space group I4/mmm to a low-temperature polar Pca21 phase via an intermediate phase with polar space group Aea2. We study the mechanism responsible for the I4/mmm to Aea2 polar phase transition and find that it is different from previously proposed mechanisms in similar systems. The transition is governed by the opening of a small dihedral angle between the phenyl ring planes of two adjacent PEA molecules, which consequently become crystallographically inequivalent in the Aea2 phase. This molecular rotation induces a significant difference in the lengths of the ethylammonium tails of the two molecules, which coordinate the inorganic layer asymmetrically and are consequently involved in different hydrogen bonding patterns. Consequently, the negatively charged chlorine octahedron that coordinates the Mn2+ cation deforms. This deformation moves the Mn2+ off-center along the out-of-plane-axis, contributing to the polar nature of the structure. Notably, the polar axis is out-of-plane with respect to the inorganic sheets. This is in contrast to other layered organic-inorganic hybrids as well as conventional layered perovskites, such as the Aurivillius phases, where in-plane polarization is observed. Our findings add to the understanding of possible mechanisms that can induce ferroelectric behavior in layered organic-inorganic hybrids.

We studied the atomic layer deposition (ALD) of titanium oxide (TiO2) using a newly-developed heteroleptic titanium precursor with a linked ligand. The titanium precursor, [2-(N-methylamino) 1-methyl ethyl cyclopentadienyl] bis(dimethylamino) titanium...

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The authors report the reaction mechanism of the initial fluorination process on the H-terminated Si and the OH-terminated SiO2 surfaces with HF, CF4, CHF3, NF3, and ClF3. The reaction process in which a fluorine atom in a gas molecule dissociates Si–OH or Si–H surface group to form Si-F bonds is modeled and simulated by density functional theory calculations using a slab surface model. The physisorption and the chemisorption of all gases on the SiO2 surface are exothermic. However, the activation energy for chemisorption varies depending on the molecule. HF demonstrates the lowest activation energy of 0.18 eV, while CF4 has the highest value of 6.32 eV. In the case of the Si surface, the physisorption and the chemisorption of all gases are also exothermic reactions. ClF3 and NF3 exhibit near zero activation energies of 0.02 and 0.04 eV, whereas CHF3 has the highest value of 2.33 eV. Their calculation results explain the mechanism of the vapor phase etching of Si and SiO2.