Thin films based on Prussian blue analogues: growth mechanism and physical properties

Abstract : In this work, we have investigated the nucleation and growth mechanisms and also the physicochemical properties of Prussian blue analogues (PBA) thin films of general formula of AxM[M'(CN)6]y.zH2O, where A is an alkali metal ion, and M and M' are divalent and trivalent transition cations. RbNiCr and RbCoFe films are grown from a solution containing all precursors on a well-defined functionalized Si(111) surface terminated by a densely packed monolayer terminated with pyridyl groups. A thickness dependent study, using X-ray reflectivity and AFM characterizations, demonstrates that the nucleation is instantaneous and that initial islands expand laterally until coalescence. Above a thickness of ca. 15 nm, the films are quasi continuous and grains grow in a layer by layer fashion. X-ray diffraction demonstrates that the columnar grains are crystalline from bottom to top and adopt the (002) orientation, with, however no in plane orientation. In the case of RbNiCr, a uniform composition Rb0.1Ni[Cr(CN)6]0.7.zH2O (measured by Rutherford Backscattering) is determined, which is consistent with that of corresponding colloidal nanoparticles. In the case of RbCoFe, an excess of Rb is found with respect to the particles of the same material. The magnetic characterization of RbNiCr layers (SQUID measurements) give a magnetic moment of 3.9 μB, in agreement with expectation. The Curie temperature increases from 30 K to 65 K with increasing film thickness. Tilted magnetization anisotropy is observed induced by the film texture and also by the columnar structure of grains which aspect ratio height / diameter is greater than unity. RbCoFe layers exhibit photo switching properties (XAS measurements at Co K edge) indicating the presence of a significant fraction of Co(III)-Fe(II) pairs (66 %). A preliminary study of heterostructures RbNiCr/RbCoFe is also presented.
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Lili Lu. Thin films based on Prussian blue analogues: growth mechanism and physical properties. Material chemistry. Ecole Polytechnique X, 2014. English. ⟨pastel-01058915⟩

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