Abstract
Transition metal complexes with β-diketonate and diamine ligands are valuable precursors for chemical vapor deposition (CVD) of metal oxide nanomaterials, but the metal-ligand bond dissociation mechanism on the growth surface is not yet clarified in detail. We address this question by density functional theory (DFT) and ab initio molecular dynamics (AIMD) in combination with the Blue Moon (BM) statistical sampling approach. AIMD simulations of the Zn β-diketonate-diamine complex Zn(hfa)2TMEDA (hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate; TMEDA = N,N,N′,N′-tetramethylethylenediamine), an amenable precursor for the CVD of ZnO nanosystems, 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 magnitude of the thermal energy at the operating temperature. The formation of hydrogen bonds with surface hydroxyl groups plays a key role in aiding the dissociation 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.
Highlights
The control and modulation of nanometer-level structures are of paramount importance in the fabrication of functional materials for advanced applications, encompassing gas sensing, energy, environmental sciences, and biomedical areas [1]
The exploratory ab initio molecular dynamics (AIMD) simulation at 500 K shows that the Zn(hfa)2TMEDA complex, initially adsorbed with an octahedral geometry, underwent diffusion on the heated substrate via a rolling motion triggered by thermal energy exchange with the substrate, in line with previous AIMD simulations in the 363–750 K temperature range [23]
The Zn-O bond dissociation was followed by a partial detachment of one hfa ligand, leading to the formation of a Zn(hfa)2TMEDA characterized by a penta-coordinated Zn center and a square-pyramidal geometry (See Movie S1, Supporting Information)
Summary
The control and modulation of nanometer-level structures are of paramount importance in the fabrication of functional materials for advanced applications, encompassing gas sensing, energy, environmental sciences, and biomedical areas [1]. The consequent extensive research efforts have recently triggered the introduction of nanoarchitectonics, a novel paradigm of materials science and technology on the nanoscale [2] involving the combination of nanotechnologies with other specific disciplines to produce systems with emerging functionalities [3]. In this regard, tailoring of structure, composition, and morphology offers unique opportunities for the applications of metal oxide nanomaterials, which are an endless source of functionalities thanks to the multitude of valence states/structures and the widely diversified chemical reactivity that they exhibit [4,5,6,7,8]. Important research progresses in this area are directly dependent on the attainment of additional insights into the involved reactive processes at the molecular scale, that can be successfully achieved by the combination of advanced experimental techniques and computational modeling [11,15,16,17,18]
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