Gas-Phase Reactions of Metal-tmhd Compounds—New Insights from Microreactor Studies Using Synchrotron Radiation

Metal-β-diketonate complexes are widely used as precursors for thin film deposition in the fields of catalysis, microelectronics and functional materials for energetic applications.¹ This is mainly because of their superior physical and chemical properties such as the volatility and inertness, which can be customized by changing the β-diketonate group attached to the respective metal center.²<br/>When used in deposition processes, often gas-phase reactions are the initial decomposition step, but gaseous intermediates can lead to unwanted film morphology and unsatisfactory purity or a depletion of the precursor by side reactions, and consequently a reduction in growth rate. A precise understanding of the decomposition mechanism, especially the sequence of bond dissociation steps, is important for improving and modelling such a processes.<br/>Consequently, the analysis of the initial stages of growth is important and requires sensitive analytical techniques with sufficiently low detection limits for elusive gas-phase species. Because of limitations in experimental techniques, it was until recently not possible to detect most of the postulated intermediate species; their temperature-dependent kinetics often remained unknown.³<br/>We have overcome some of these challenges and demonstrated for various metal-organic precursors, that by using a microreactor coupled to a very mild ionization source, aided by numerical simulation, we are capable to detect and characterize elusive species, especially metal-containing intermediates with short lifetimes below 50 μs.⁴<br/>Building up on previous work on acetylacetonate complexes, we present insights into the reactions of 2,2,6,6-tetramethyl-3,5-heptanedionate (tmhd) compounds. The vacuum pyrolysis of aluminium and zirconium 2,2,6,6-tetramethyl-3,5-heptanedionate, Al(tmhd)₃ and Zr(tmhd)₄ is investigated here. In brief, the precursor is sublimed, subsequently transported by helium carrier gas, and expanded through a pinhole into a resistively heated 1 mm inner diameter SiC-microreactor of 10 mm length. Species leaving the reactor are ionized by tuneable vacuum ultraviolet (VUV) synchrotron radiation, and characterized by imaging photoelectron photoion coincidence spectroscopy (i²PEPICO) and mass spectrometry at the Swiss Light Source. We recorded photoionization efficiency curves (PIE) and threshold photoelectron spectra (TPES) at photon energies of 6.3-11.5 eV, which give us direct evidence for the characterization of reactive intermediates and products.<br/>In the experiments, hydrocarbons, oxygenated and metal-containing species were detected and characterized unambiguously in the gas-phase at temperatures from 450-950 K, which provides insights in the underlying decomposition mechanisms. Most importantly, we detected and characterized metal-bis(diketo)acetylacetonate-H, M(C₁₁H₁₉O₂)(C₁₁H₁₈O₂) as major initial decomposition product in the gas-phase at temperatures above 650 K, which subsequently forms M(C₁₁H₁₉O₂)(C₁₀H₁₅O₂) by a methyl loss. Additionally, several hydrocarbons and oxygenated species were detected and assigned as decomposition species for the first time, i.e. H-tmhd (C₁₁H₂₀O₂), C₇H₁₁O₂, pinacolone (C₆H₁₂O), the tert-butyl radical (C₄H₉), which afford the formation of metal-containing intermediates. The temperature-dependent formation mechanisms of the assigned species will be discussed and compared to previous results on M(acac) precursors.<br/><br/>References<br/>1. A.L. Pellegrino, G. Lucchini, A. Speghini and G. Malandrino, <i>J. Mater. Res.</i>, 2020, <b>35</b>(21), 2950.<br/>2. G.G. Condorelli, G. Malandrino and I.L. Fragalà, <i>Coord. Chem. Rev.</i>, 2007, <b>251</b>(13-14), 1931.<br/>3. Y. Jiang, M. Liu, Y. Wang, H. Song, J. Gao and G. Meng, <i>J Phys Chem A</i>, 2006, <b>110</b>(50), 13479.<br/>4. S. Grimm, S.-J. Baik, P. Hemberger, A. Bodi, A.M. Kempf, T. Kasper and B. Atakan, <i>Phys Chem Chem Phys</i>, 2021, <b>23</b>(28), 15059.