Electron Induced Surface Reactions of HFeCo₃(CO)₁₂, a Bimetallic Precursor for Focused Electron Beam Induced Deposition (FEBID)

Abstract

The use of bimetallic precursors in focused electron beam induced deposition (FEBID) allows mixed metal nanostructures with well-defined metal ratios to be generated in a single step process. HFeCo3(CO)12 is an example of one such bimetallic precursor that has previously been shown to form deposits with unusually high metal content (>80%) as compared to that of typical FEBID deposits (<30% metal content). To better understand the elementary bond breaking steps involved in FEBID of HFeCo3(CO)12, we have employed a UHV surface science approach to study the effect of electron irradiation on nanometer thick films of HFeCo3(CO)12 molecules. Using a combination of in situ X-ray photoelectron spectroscopy and mass spectrometry, we observed that the initial step of electron induced HFeCo3(CO)12 dissociation is accompanied by desorption of ∼75% of the CO ligands from the precursor. A comparison with recent gas phase studies of HFeCo3(CO)12 indicates that this process is consistent with a dissociative ionization process, mediated by the secondary electrons produced by interaction of the primary beam with the substrate. The loss of CO ligands from HFeCo3(CO)12 in the initial dissociation step creates partially decarbonylated intermediates, HFeCo3(CO) )1x (x)avg. ≈ 3). During a typical FEBID process, further electron exposure or thermal reactions can further transform these intermediates. In our UHV surface science approach, the effect of these two processes can be studied in isolation and identified. Under the influence of further electron irradiation, XPS data reveals that the remaining CO ligands in the partially decarbonylated intermediates decompose to form residual carbon and iron oxides, suggesting that those CO ligands that desorbed in the initial step are lost predominantly from the Co atoms. However, annealing experiments demonstrate that CO ligands in the partially decarbonylated intermediates desorb under vacuum conditions at room temperature, leaving behind films that are free of almost any carbon or oxygen contaminants. This combination of efficient CO desorption during the initial dissociation step, followed by thermal CO desorption from the partially decarbonylated HFeCo3(CO) )1x (x)avg. ≈ 3) intermediates provide a rationale for the high metal contents observed in FEBID nanostructures created from HFeCo3(CO)12.