Surface science approach to atomic layer deposition chemistry

Amir Gharachorlou, Purdue University

Abstract

Recently, atomic layer deposition (ALD) has been employed as a promising technique for the growth of solid thin films with high conformality on complex geometries such as heterogeneous catalysts and microelectronic devices. In this work, surface sensitive characterization techniques have been employed to investigate initial precursor-substrate reactions and ALD surface chemistry. To understand the nucleation and growth mechanisms of palladium nanoparticles on metal oxide supports by ALD, Pd nanoparticles were synthesized by thermal decomposition of palladium (II) hexafluoroacetylacetonate (Pd(hfac)2 )on a TiO2(110) surface. Pd(hfac)2 adsorption at room temperature is a self-limiting process on the TiO2(110) surface yielding Pd(hfac)ads and/or hfacads (hexafluoroacetylacetonate) species and partial hfac fragmentation. The removal of the hfac ligand and its fragments through thermal decomposition of surface species removes the nucleation inhibition of Pd nanoparticles previously observed for the Pd(hfac) 2 precursor on TiO2. Additional studies were devoted to understanding transition metal (Pd, Pt, Cu) interactions with trimethylaluminium (TMA) as an alumina ALD precursor. Over-coating of Pd, Pt and Cu nanoparticles with alumina films synthesized by TMA and water/O2 as precursors has been shown in the literature to prevent deactivation of catalyst by protecting them against particle instability (sintering). The surface chemistry of TMA was investigated on Pt(111), Pd(111) and Cu(111)/CuOads surfaces with regard to its application in preventing Pd, Pt and Cu nanoparticle sintering. Our results showed that TMA decomposes to form methylaluminum (MA) on both Pd(111) and Pt(111). Further dissociation of MA to aluminum and adsorbed methyl groups is limited to step sites. Similarly, the removal of Pt and Pd atoms from (111) terraces is hindered while surface vacancy formation is facilitated on Pt and Pd step edges. Pd steps are fully covered with fractal Pd-Al alloy islands that spread over terraces. Pd hydrogenates and removes carbonaceous species, freeing step sites for MA to Al decomposition and Pd step vacancies serve as nucleation sites for Pd-Al alloy formation. On Pt(111), TMA only decomposed to MA and methyl groups. No evidence of Pt-Al alloy formation was observed on the Pt surface. The Pt(111) surface was uniformly covered with MA and residual carbon species after TMA exposure. On the Pt(111) surface, the residual carbon species were hypothesized to block access to step sites preventing MA to Al dissociation and subsequent alloy formation. Unlike for Pt(111) and Pd(111), TMA does not interact with Cu(111). However, after exposing copper surface oxide (CuOads) to TMA, an alumina overlayer was detected as the TMA consumed the oxygen in CuO ads lattice and reduced the CuOads surface to Cu0 . After the first TMA half-cycle on CuOads, two dimensional alumina islands homogenously covered the surface. High resolution electron energy loss spectroscopy (HREELS) was used to investigate the relative occupancy of tetrahedrally and octahedrally coordinated Al (Aloct and Altet) after each alumina ALD half-cycle using TMA and O2 as precursors. TMA half-cycles produce alumina films which predominately consist of octahedral alumina (Al tet/Aloct ≈ 0.3) while O2 half-cycles favor formation of tetrahedral alumina (Altet/Aloct ≈ 0.5). O2 half-cycles remove the residual carbon species after TMA exposure and result in a carbon-free alumina film.

Degree

Ph.D.

Advisors

Ribeiro, Purdue University.

Subject Area

Chemical engineering|Materials science

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