Manganese(II) Molecular Sources for Plasma-Assisted CVD of Mn Oxides and Fluorides: From Precursors to Growth Process

A viable route to manganese-based materials of high technological interest is plasma-assisted chemical vapor deposition (PA-CVD), offering various degrees of freedom for the growth of high-purity nanostructures from suitable precursors. In this regard, fluorinated β-diketonate diamine Mn(II) complexes of general formula Mn(dik)2·TMEDA [TMEDA = N,N,N′,N′-tetramethylethylenediamine; Hdik = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (Hhfa), or 1,1,1-trifluoro-2,4-pentanedione (Htfa)] represent a valuable option in the quest of candidate molecular sources for PA-CVD environments. In this work, we investigate and highlight the chemico-physical properties of these compounds of importance for their use in PA-CVD processes, through the use of a comprehensive experimental–theoretical investigation. Preliminary PA-CVD validation shows the possibility of varying the Mn oxidation state, as well as the system chemical composition from MnF2 to MnO2, by simple modulations of the reaction atmosphere, paving the way to a suc...


Computational Section
Density functional theory (DFT) calculations for geometry optimizations (GO) were performed on the Mn(hfa) 2 •TMEDA and Mn(tfa) 2 •TMEDA complexes by using the PBE functional 1 augmented with Hirao long-range corrections. [2][3][4] Calculations were carried out with the Gaussian 09 (G09) code. 5 The Stuttgart-Dresden ECP pseudopotentials were used with basis sets of the Stuttgart-Dresden type for all atoms. 6 The basis sets for H, C, N, O, and F atoms were augmented with diffuse and polarization functions taken from the (D95++(d,p)) basis set, 7 already used for other M(hfa) 2 •TMEDA compounds (M= Fe, Co, Cu, Zn). [8][9][10][11][12][13] For both complexes, GO were performed by considering the following spin multiplicities: sextet, quartet, doublet. The highest-multiplicity spin state -namely, the sextet-was always energetically favored over the quartet and the doublet states by very large energy differences. Specifically, the calculated energy differences ∆E between sextet and doublet were 41.9 kcal×mol -1 , for Mn(hfa) 2 •TMEDA, and 44.1 kcal×mol -1 , for Mn(tfa) 2 •TMEDA, whereas the ∆E between sextet and quartet were 35.6 kcal×mol -1 , for Mn(hfa) 2 •TMEDA, and and 39.7 kcal×mol -1 , for Mn(tfa) 2 •TMEDA. Therefore, the spin state of Mn(dik) 2 •TMEDA was defined to be a sextet and all other calculations on the neutral complexes were performed in this multiplicity.
All optimized structures resulted to be energy minima (with all positive frequencies). Optimized geometries and vibrational frequencies were also calculated for the three possible isomers of Mn(tfa) 2 •TMEDA -iso1, iso2, iso3-shown in Figure 1. Relevant geometrical parameters of Mn(hfa) 2 •TMEDA and of the three Mn(tfa) 2 •TMEDA isomers are reported in Table S1, while calculated IR spectra for iso1, iso2, iso3 are shown in Figure S1. A constant shift factor of 0.963 was applied to the calculated vibrational frequencies in all reported simulated spectra.
The molecular ions [Mn(dik) 2 •TMEDA] +• were obtained by removing an electron from the Mn(dik) 2 •TMEDA compounds and performing GO with the above-described computational approach and the same combination of DFT functionals/basis sets used for the neutral complexes.
For both molecular ions, GO were performed by considering the following spin multiplicities: S3 quintet, triplet, singlet, and the lowest-energy spin state was found to be a quintet in view of the calculated energy differences. Specifically, the ∆E between quintet and singlet were: 48.  Table S4.
The first principles molecular dynamics simulations (FPMD) 14 of the two parent Mn(dik) 2 •TMEDA compounds were performed with the CPMD code 15 using the PBE approximation to DFT 1 with dispersion corrections, 16 plane wave (PW) basis sets, and ultrasoft pseudopotentials 17 for all atoms.
Periodic boundary conditions were adopted, using a cubic box of 2.0 nm length. The adopted PW cut-off was 25 and 200 Ry for wavefunction and electronic density representation, respectively.
The FPMD simulations were started from the minimum structures of Mn(dik) 2 •TMEDA obtained via GO. After equilibration, the room temperature behaviour of the two Mn(dik) 2 •TMEDA complexes (sextet spin state) was followed by 10 ps-long simulations in the canonical NVT ensemble using T = 25°C as a target temperature and Nose-Hoover chain thermostats. [18][19] A time step of 0.121 fs was used for trajectory integration, with an inertia parameter of 500 atomic units (au) for the electronic coefficients. 20 In addition, since such a FPMD approach could adequately describe complex phenomena, even at non-standard conditions, 21 for a broad variety of organicinorganic systems, [22][23] including also transition metals/metal cations, 24-26 we adopted it to explore the high-temperature dynamic behaviour of [Mn(dik) 2 •TMEDA] +• molecular ions.
For both systems, the 10-ps long FPMD simulations were performed, always in the canonical ensemble, using a target ionic temperature of 180°C. Moreover, in view of the relatively modest energy difference ∆E (triplet -quintet) calculated for these radical cations, two 10-ps long FPMD simulations were also performed for the two [Mn(dik) 2 •TMEDA] +• molecular ions in the triplet spin state. Relevant average geometrical parameters are reported in Table S5.

S4
In all cases, 5-10 ps-long equilibration runs were performed prior to the production FPMD simulations.
S7 Figure S2. 1 Table also contains the geometrical parameters of the minimum energy structures (quintet state) reported in Comment to Table S5. To facilitate comparison among the different spin states, this Table also contains the average geometrical parameters calculated from the FPMD of the two radical cations in the quintet state, reported in Table 2 in the main text.