The Early Steps of Molecule-to-Material Conversion in Chemical Vapor Deposition (CVD): A Case Study
<p>Transition metal complexes with β-diketonate and diamine ligands are valuable precursors for the chemical vapor deposition (CVD) of metal oxide nanomaterials, but the metal-ligand bond cleavage mechanism on the growth surface is not clarified yet. We address this question by Density Functional Theory (DFT) and <i>ab initio</i> molecular dynamics (AIMD) in combination with the Bluemoon (BM) statistical sampling approach. AIMD simulations of the Zn β-diketonate-diamine complex Zn(hfa)<sub>2</sub>TMEDA (hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate; TMEDA = <i>N</i>,<i>N</i>,<i>N’</i>,<i>N’</i>-tetramethylethylenediamine) show that rolling diffusion of this precursor at 500 K on a hydroxylated silica slab leads to an octahedral-to-square pyramidal rearrangement of its molecular geometry. The free energy profile of the octahedral-to-square pyramidal conversion indicates that the process barrier (5.8 kcal/mol) is of the order of the thermal energy at the operating temperature. The formation of hydrogen bonds with surface hydroxyls plays a key role in aiding the cleavage of a Zn-O bond. In the square-pyramidal complex, the Zn center has a free coordination position, which might promote the interaction with incoming reagents on the deposition surface. These results provide a valuable atomistic insight on the molecule-to-material conversion process which, in perspective, might help to tailor by design the first nucleation stages of the target ZnO-based nanostructures.<b></b></p>