60th Annual Report on Research 2015

Introduction

Electron beam induced deposition (EBID) from organometallic precursors is a direct-write vacuum based deposition strategy with the ability to generate spatially and geometrically well-defined three-dimensional, metal-containing structures (Figure 1). Consequently, EBID is uniquely well positioned to create new catalysts across various length scales.  However, use of EBID to synthesize metal catalysts requires control over not only the size and shape, but also the chemical composition of the deposits. To date, EBID structures have been created from precursors designed for thermal processes, such as chemical vapor deposition (CVD).  However, ligands that dissociate cleanly during CVD are often decomposed by electron induced processes, affording high levels of organic contamination in EBID deposits, which will negatively impact applications.  This proposal is focused on the preparation and evaluation of new organometallic precursors designed specifically for EBID.

Figure 1.  Schematic representation of a size and shape selected metal-containing structure being deposited by electron beam induced deposition (EBID).

We are using mechanistic information obtained from modelling of the EBID process by complementary gas phase and UHV surface science studies (Figure 2) to provide information useful in formulating design strategies for EBID precursors.  In contrast to studies conducted in electron microscopes, where EBID deposits are created under steady state deposition conditions, the UHV surface science approach relies on studying the effect of electron irradiation on nanometer thick films of precursor molecules adsorbed onto chemically inert substrates at low temperatures. Surface analytical tools such as X-ray Photoelectron Spectroscopy (XPS) and Reflection Absorption Infrared Spectroscopy (RAIRS) can follow changes in the surface composition and bonding environment of the various elements within the precursor molecule, complemented by Mass Spectrometry (MS) which can detect the volatile species ejected from the film as a consequence of electron stimulated reactions.

Figure 2.  Schematic representation of the Ultra-High Vacuum Surface Science approach to study EBID precursors. 

Results and Discussion

The specific goal of the present study is to compare and contrast the behavior of different ligands that are often present in organometallic precursors used in EBID.  The fate of ligands in an organometallic precursor undergoing EBID can be expected to fall into two general categories:  (1) the ligands are ejected and pumped away into the gas phase as the precursor decomposes; or (2) the ligands are decomposed by electron beam irradiation following precursor decomposition and become incorporated into the deposit, contributing to contamination.  As a vehicle to study the behavior of different ligands, we have studied the electron-stimulated reactions of surface-bound η³-allyl ruthenium tricarbonyl bromide [(η³-C₃H₅)Ru(CO)₃Br)] and to a lesser extent η³-allyl ruthenium tricarbonyl chloride [η³-C₃H₅)Ru(CO)₃Cl)]. These complexes provide the opportunity to simultaneously evaluate the behavior of three different types of ligands in the same coordination sphere:  carbonyl (CO), η³-allyl (η³-C₃H₅) and halides (Br, Cl).  

XPS and MS data obtained after electron irradiation in UHV surface science experiments reveal that the surface reactions of adsorbed [(η³-C₃H₅)Ru(CO)₃Br] proceed in two stages. The initial step involves electron-stimulated precursor decomposition accompanied by the evolution of CO into the gas phase. However, under the influence of more prolonged electron beam irradiation, the film that forms as a result of (η³-C₃H₅)Ru(CO)₃Br decomposition loses Br atoms (step 2).  This is the first example of an organometallic precursor where any ligand desorption has occurred after precursor decomposition.  The overall effect of electron irradiation on (η³-C₃H₅)Ru(CO)₃Br is shown in Scheme 1.

Scheme 1:  Stage 1:  Electron Stimulated CO Desorption and (η³-C₃H₅) Decomposition from [(η³-C₃H₅)Ru(CO)₃Br].  Stage 2:  Electron stimulated desorption of halogens from the residual [(η³-C₃H₅)Ru(CO)₃Br] product from Stage 1.

Figure 3 shows Auger electron spectra (AES) for EBID deposits created on a Ag substrate under steady state deposition conditions.  Although the surface bound η³-allyl ruthenium tricarbonyl bromide [(η³-C₃H₅)Ru(CO)₃Br] molecules are decomposed by electron irradiation in a process that initially reduces the central metal (Ru) atoms and ejects CO ligands into the gas phase, the carbon atoms contained within the η³-allyl (η³-C₃H₅) ligand are incorporated into the metal-containing deposit that forms. In the second step that occurs for significantly larger electron doses, most of the bromine atoms are removed from the deposits via an electron-stimulated desorption process, analogous to a post-deposition electron-beam processing step. The electron-stimulated reactions of the organometallic precursors appear invariant to the nature of the halogen atom. Considered collectively, results from this investigation suggest that by using organometallic precursors that contain a small number of CO ligands and/or metal-halogen bonds, EBID could create deposits with high metal contents under precursor-limited deposition conditions.

Figure 3.  Auger electron spectra of EBID films created on an Ag substrate from (left) η³-C₃H₅Ru(CO)₃Br, and (right) η³-C₃H₅Ru(CO)₃Cl. In each case, the compound was deposited  (solid line) and then subjected to further electron irradiation (dashed line).  Deposition conditions were P_{η3-C3H5Ru(CO)3Br/Cl}  ~5x10^-7 Torr, incident beam energy = 3 keV, substrate current ~ 700 nA for a total electron dose of (a) (1) 1.18x10¹⁹ e^-/cm², (2) 2.06x10¹⁹ e^-/cm²,  and (b) (1) 1.18x10¹⁹ e^-/cm² and (2) 2.06x10¹⁹ e^-/cm².  All AES were normalized to the Ru/C peak.

Final Remarks

This project provided the preliminary data the PIs used to obtain industrial funding for related work on EBID precursors.  We are also using these results in proposals we are currently preparing for submission to federal agencies.  The work also provided excellent training for the graduate students who have participated in this collaborative project between PIs in different areas of chemistry at different universities.  Two of the graduate students who were involved obtained their PhDs during the grant period.  One is employed in the semiconductor industry and the other is an Assistant Professor at an undergraduate institution.