Electron induced surface reactions of (η⁵-C₅H₅)Fe(CO)₂Mn(CO)₅, a potential heterobimetallic precursor for focused electron beam induced deposition (FEBID)

Abstract

Electron-induced surface reactions of (η-CH)Fe(CO)Mn(CO) were explored in situ under ultra-high vacuum conditions using X-ray photoelectron spectroscopy and mass spectrometry. The initial step involves electron-stimulated decomposition of adsorbed (η-CH)Fe(CO)Mn(CO) molecules, accompanied by the desorption of an average of five CO ligands. A comparison with recent gas phase studies suggests that this precursor decomposition step occurs by a dissociative ionization (DI) process. Further electron irradiation decomposes the residual CO groups and (η-CH, Cp) ligand, in the absence of any ligand desorption. The decomposition of CO ligands leads to Mn oxidation, while electron stimulated Cp decomposition causes all of the associated carbon atoms to be retained in the deposit. The lack of any Fe oxidation is ascribed to either the presence of a protective carbonaceous matrix around the Fe atoms created by the decomposition of the Cp ligand, or to desorption of both CO ligands bound to Fe in the initial decomposition step. The selective oxidation of Mn in the absence of any Fe oxidation suggests that the fate of metal atoms in mixed-metal precursors for focused electron beam induced deposition (FEBID) will be sensitive to the nature and number of ligands in the immediate coordination sphere. In related studies, the composition of deposits created from (η-CH)Fe(CO)Mn(CO) under steady state deposition conditions, representative of those used to create nanostructures in electron microscopes, were measured and found to be qualitatively consistent with predictions from the UHV surface science studies.

Mechanism-based design of precursors for focused electron beam-induced deposition

“…In an effort to elucidate electronic structure effects, studies were carried out on Cp(CO) 2 FeMn(CO) 5 , a bimetallic precursor with a simpler structure. [105] Structurally, Cp(CO) 2 FeMn(CO) 5 is different than the clusters HFeCo 3 (CO) 12 and H 2 FeRu 3 (CO) 13 . In Cp(CO) 2 FeMn(CO) 5 , the metal centers are held together by a single unsupported metal-metal bond.…”

“…In surface science studies with Cp(CO) 2 FeMn(CO) 5 , initial electron-induced decomposition of thin films of the precursor resulted in facile CO desorption, producing a partially decarbonylated surface-bound intermediate. [105]…”

“…An interesting facet of the gas-phase electron-molecule interactions of Cp(CO) 2 FeMn(CO) 5 is the qualitative relationship between the DI studies carried out with 70 eV electrons [106] and the behavior of the precursor under UHV surface science conditions. [105] Because electron impact mass spectrometry (EI-MS) with incident electrons of similar energy is a standard characterization tool in synthesis laboratories, the DI (or EI-MS) behavior of potential precursors could be used for the pre-screening of precursors by synthetic chemists before more extensive studies of electron-molecule interactions under FEBID conditions are undertaken. The use of MS for the prescreening of precursors is a common practice in the CVD community, with the understanding that care during interpretation of the data is warranted because MS detects ionized species while thermal decomposition during CVD is more likely to produce neutral fragments.…”

“…In the gas-phase electron-molecule interaction studies, the highest intensity DI peaks ([CpFe] + , [CpFe(CO) 2 ] + , [CpFeMn(CO) 2 ] + , and [CpFeMn(CO)] + ) still contain the Cp ligand, [106] consistent with the persistence of the Cp carbons under electron irradiation in the UHV surface science experiments. [105] Significant intensity of peaks for various combinations of CO loss in the DI spectrum is reflected in loss of those ligands from the surface.…”

“…[29] Another caveat with EI-MS is that it can only measure the initial electron interactions with the precursor molecule, while secondary electron processes associated with adsorbed intermediates also play an important role in determining the ultimate composition of the deposit formed in FEBID. For example, in the case of Cp(CO) 2 FeMn(CO) 5 , the oxidation of Mn after the decomposition of CO on the surface [105] cannot, of course, be anticipated from the DI spectra. However, the critical point for pre-screening purposes is that the facile CO ligand loss pathways on the surface and the persistence of the Cp ligand can be seen in the DI.…”