CVD中分子到材料转化的早期步骤

Abstract: Transition metal complexes with 3-diketonate and diamine ligands are valuable precursorsfor chemical vapor deposition (CVD) of metal oxide nanomaterials, but the metal-ligand bonddissociation mechanism on the growth surface is not yet clarified in detail. We address this questionby density functional theory (DFT) and ab initio molecular dynamics (AIMD) in combination with theBlue Moon (BM) statistical sampling approach. AlMD simulations of the Zn p-diketonate-diaminecomplex Zn(hfa)2TMEDA (hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate; TMEDA = NNNNtetramethylethylenediamine), an amenable precursor for the CVD of ZnO nanosystems, show thatrolling diffusion of this precursor at 500 K on a hydroxylated silica slab leads to an octahedral-tosquare pyramidal rearrangement of its molecular geometry. The free energy profile of the octahedralto-square pyramidal conversion indicates that the process barrier (5.8 kcal/mol) is of the order ofmagnitude of the thermal energy at the operating temperature. The formation of hydrogen bondswith surface hydroxyl groups plays a key role in aiding the dissociation of a Zn-O bond. In thesquare-pyramidal complex, the Zn center has a free coordination position, which might promotethe interaction with incoming reagents on the deposition surface. These results provide a valuableatomistic insight on the molecule-to-material conversion process which, in perspective, might help totailor by design the first nucleation stages of the target ZnO-based nanostructures.

1. Introduction

The control and modulation of nanometer-level structures are of paramount impor-tance in the fabrication of functional materials for advanced applications, encompassing gassensing, energy environmental sciences, and biomedical areas (1]. The consequent exten-sive research efforts have recently triggered the introduction of nanoarchitectonics, a novelparadigm of materials science and technology on the nanoscale [2] involving the combina-tion of nanotechnologies with other specific disciplines to produce systems with emergingfunctionalities [3]. In this regard, tailoring of structure, composition, and morphologyoffers unique opportunities for the applications of metal oxide nanomaterials, which arean endless source of functionalities thanks to the multitude of valence states / structuresand the widely diversified chemical reactivity that they exhibit (4-81 The rational designof such systems is directly dependent on the availability of versatile preparation routesenabling their growth onto suitable substrates allowing, in turn, their direct integrationinto functional devices (9 . In the current tide of nanomaterial preparation routes, chemicavapor deposition (CVD)-related technologies hold a significant promise thanks to theirinherent flexibility, adaptability to large-scale processing, and possibility to yield a broad range of material features by a proper choice of the operational conditions. CVD processesare based on the nucleation and growth of the target materials on solid surfaces starting from suitable molecular precursors in the vapor phase, whose features significantlinfluence the subsequent molecule-to-material conversion (10-14 . mportant research progresses in this area are directly dependent on the attainment of additional insights into theinvolved reactive processes at the molecular scale, that can be successfully achieved by thecombination of advanced experimental techniques and computational modeling.

Over the last decade, we have focused our attention on the extensive investigation ofCVD precursors based on first-row transition metal oxides of general formula ML,TMEDAwhere L is a fluorine-containing 3-diketonate moiety (such as 1,1,1,5,5,5-hexafluoro-24pentanedionate, hfa) and TMEDA is NNNN-tetramethylethylenediamine. The joint useof both ligands yields the full saturation of metal coordination sphere, ultimately resultingin improved stability, volatility, and mass-transport behavior for CVD applications (19-21)In particular, we have evidenced, in the case of the Cu(hfa),TMEDA compound, a "rockand-roll over a hot floor motion consisting in a fast diffusion involving a vibrationalexcitation of metal-ligand bonds, paving the way to the subsequent precursor decompo-sition to copper oxide (CuyO, with x = 1, 2) nanomaterials (22). Such investigations havebeen partly extended to the homologous Zn(hfa)TMEDA (23] and Fe(hfa)TMEDA (24complexes, and the outcomes have revealed different high temperature behaviors forthe three systems, which might be responsible for diverse decomposition pathways onthe growth surface. In spite of these efforts, an open and hot challenge remains a thorough atomistic comprehension of the metal-ligand bond dissociation, related to the firststages of the molecule-to-material conversion in CVD processes. The understanding ofthis issue is indeed a crucial step towards the ultimate prediction of material proper-ties, which would indeed disclose general concepts to boost the development of novelfunctional nanosvstems.

On the basis of these results, the present research work aims at providing a stepforward in the investigation of these issues by combined calculations involving the densityfunctional theory (DFT) coupled with ab initio molecular dynamics (AIMD) (25]. It isworth noticing that, despite the growing interest in understanding via quantum chemicalapproaches the microscopic details of molecule-to-material conversion in CVD or ALIprocesses (26-33], most literature data refer to geometry optimization or transition statecalculations performed at 0 K. Whereas these studies contribute to shedding light on thebinding of the metal center with substrate atoms, the formation of metal-surface linkagesfor the target CVD precursors, where the metal center is completely surrounded by ligandsrequires the dissociation of at least one metal-ligand bond, and remains a key issue to befurther investigated.