Reaction of Trimethylaluminum with Water on Pt(111) and Pd(111) from 10^–5 to 10^–1 Millibar

Abstract

The reaction between adsorbed trimethylaluminum (TMA) and water was studied on Pt(111) and Pd(111) surfaces. Upon exposure to TMA at approximately 10 −5 mbar, C-and Al-containing species appeared on both surfaces, as observed by X-ray photoelectron spectroscopy (XPS). On both surfaces, the adsorbed Al oxidation state observed by XPS was closest to metallic. Density functional theory (DFT) calculations suggest that decomposition to methyl aluminum (Al-CH 3 ; "MMA") or atomic Al is thermodynamically favorable. The formation of a Pd−Al alloy was observed on Pd (111), but Pt−Al alloy formation was not observed on Pt(111). Following TMA adsorption, each surface was exposed to water vapor at 400°C either at a pressure of 7 × 10 −6 mbar (UHV-XPS) or at 0.1 mbar (in situ XPS). The substrate and water dosing conditions determined the ability of each surface to remove residual carbon: on Pt(111), carbon from the TMA precursor was removed from Pt(111) during 0.1 mbar water exposure at 400°C, whereas carbon was not removed after the 7 × 10 −6 mbar water exposure. On Pd(111), however, carbon-containing fragments of TMA were removed at both water pressures. XPS also revealed another effect of water dosing conditions: the as-deposited Al was only fully oxidized to Al 2 O 3 during water exposure at 0.1 mbar, whereas mixed hydroxidecontaining and metallic Al species persisted after exposure to water at 7 × 10 −6 mbar on both surfaces.

Atomic Layer Deposition of FeO on Pt(111) by Ferrocene Adsorption and Oxidation

“…The system, as described elsewhere, 38,39,41,42 consists of an UHV preparation chamber and UHV μ-metal analysis chamber with base pressures of 1 × 10 −9 mbar and 5 × 10 −11 mbar, respectively.…”

“…A detailed understanding of ferrocene adsorption and oxidation, as well as the formation of monolayer thick FeO islands on Pt(111), is developed here using a suite of Ultra High Vacuum (UHV) surface sensitive techniques, whose efficacy has been established in previous studies of vacuum ALD. [38][39][40][41][42] …”

Surface Chemistry of Trimethylaluminum on Pd(111) and Pt(111)

“…49 Although the influence of adsorbed species on the Pd 3d 5/2 peak position cannot be completely ruled out, the experimental facts are consistent with Al dissolution into the Pd bulk during heating, then segregation to the surface during O 2 exposure. The shift of Pd 3d due to carbon diffusion to the palladium bulk can be disregarded: during the ALD of TMA + H 2 O on Pd(111), 18 the Pd 3d moved to the metallic position at 335.1 eV as soon as most fraction of aluminum was oxidized while substantial carbon amount was still present. On Pt(111), Al coverage did not change during annealing at 623 K in UHV as shown in Figure 4, which proved that Al did not diffuse into the Pt bulk, and that Al did not evaporate.…”

“…We observed a +0.2 eV BE shift of the Pt 4f 7/2 peak after exposing Pt(111) to TMA using surface-sensitive synchrotron-based XPS, confirming the surface nature of the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 15 phenomenon. 18 The EAL of a Pt 4f photoelectron excited by Mg Kα X-ray radiation is approximately 10.5 Å, meaning that bulk layers dominate the signal and that surface changes would result in only a slight peak shift and broadening. Therefore, we conclude that no Pt-Al alloy formed, consistent with our synchrotron-based XPS results.…”

“…Pt(111) surfaces, 11,18 and adsorbed methylaluminum (MA, Al-CH 3,ads ) and metallic aluminum were found to be the most thermodynamically favorable TMA decomposition products. TMA adsorption and alumina overlayer growth mechanisms on Pd and Pt have been studied using quadrupole mass spectrometry (QMS) [10][11][19][20] and quartz crystal microbalance (QCM).…”