Type of presentation: Oral

MS-7-O-2384 Respective influence of the organic and inorganic components on the optical properties of a hybrid layered molybdate: an EELS and DFT study

Lajaunie L.¹, Boucher F.¹, Dessapt R.¹, Moreau P.¹

¹Institut des Matériaux Jean Rouxel, (IMN) – Université de Nantes, CNRS, 2 rue de la Houssinère - BP 32229, 44322 Nantes Cedex 3, France

luc.lajaunie@cnrs-imn.fr

Introduction Investigation of photochromic materials has been extensive over the past decade due of their potential technological and marketable applications for high-density optical data storage and optical switching. Among them, polyoxometalate-based hybrid organic-inorganic materials exhibit remarkable solid-state photochromism at room temperature with reversible and multicolor possibilities, visible-light coloration and fast photoresponse compared to binary oxides. However the question concerning the respective influence of the organic and inorganic components on the optical properties is not clearly answered yet. On this aspect, valuable insights might be gained from a combination of Valence Electron Energy-Loss Spectroscopy and DFT calculations to properly determine and interpret dielectric properties.¹

Materials & Methods In this work, we report for the very first time VEELS experiment before photo-irradiation on (N,N’-H₂DMED)[Mo₅O₁₆] (N,N’-H₂DMED²⁺=N₂C₄H₁₄²⁺), here after labeled Mo₅O₁₆. This photochromic hybrid crystallized material is based on ^²/_∞[Mo₅O₁₆]^2- layers linked together via organoammonium chains (Fig. 1.a).² EELS experiments were performed at liquid-nitrogen temperature using a Hitachi HF2000 TEM (100 kV, cold-FEG). In addition, electronic band structures and optical properties were calculated at the DFT level with the WIEN2k and VASP codes. In particular, the macroscopic dynamic dielectric functions including local-field effects (LFE) were obtained from the inversion of the full microscopic dielectric matrices. To highlight the influence of the inorganic chains on the material properties, the results are compared to those obtained on α-MoO₃, which presents strong structural similarities with the inorganic framework of Mo₅O₁₆ (Fig. 1.b). ^1,2

Results The experimental and calculated EELS spectra of Mo₅O₁₆ and MoO₃ are compared in Fig. 2. An excellent overall agreement is observed and our calculations reproduce correctly the presence of the peak situated around 13 eV (labeled B) in the spectrum of MoO₃ and its absence in the spectrum of Mo₅O₁₆. This excellent agreement confirms the validity of our calculations and allows us to use them for further investigations. Total and partial DOS for MoO₃ and Mo₅O₁₆ are shown in Fig. 3. This figure highlights the lack of significant contributions directly linked to organic species in the band gap vicinity. This suggests that the low energy part of the dielectric function is mainly dominated by optical transitions involving the inorganic framework. Such influences on the optical properties will be discussed in details.

1. L. Lajaunie, F. Boucher, R. Dessapt and P. Moreau, Phys. Rev. B 81 115141 (2013)

2. R. Dessapt, D. Kervern, M. Bujoli-Doeuff et al., Inorg. Chem., 49, 11309 (2010)

Fig. 1: Figure 1. Representations of the Mo₅O₁₆ (a) and α-MoO₃ (b) structures. The [MoO₆] polyhedra are drawn in blue and the oxygen atoms of the ^²/_∞[Mo₅O₁₆]^2- layers implied in the hydrogen-bonding networks (dotted lines) are drawn in red.	Fig. 2: Figure 2. Comparison of the experimental VEELS spectra (thick black) of Mo₅O₁₆ and MoO₃ with the calculated ones including local-field effects (thin red).
Fig. 3: Figure 3. Electronic densities of states for Mo₅O₁₆ and MoO₃. The circles highlight the main contribution of the organic part of the hybrid compound.