Analysis of TiO 2 Atomic Layer Deposition Surface Chemistry and Evidence of Propene Oligomerization using Surface-Enhanced Raman Spectroscopy

• Published in: Journal of the American Chemical Society 2019-01, Vol.141 (1), p.414-422
• Main Authors: Hackler, Ryan A, Kang, Gyeongwon, Schatz, George C, Stair, Peter C, Van Duyne, Richard P
• Format: Article
• Language: English
• Subjects: Chemistry
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.8b10689

Atomic layer deposition (ALD) of TiO was performed in tandem with in-situ surface-enhanced Raman spectroscopy (SERS) to monitor changes in the transient surface species across multiple ALD cycles. A self-assembled monolayer of 3-mercaptopropionic acid was used as a capture agent, to ensure that nucleation of the titanium precursor (titanium tetraisopropoxide [TTIP]) occurs. Comparisons between the Raman spectra of the neat precursor and the SER spectra of the 1 ALD cycle of TiO reveal typical ligand exchange chemistry taking place, with self-limiting behavior and intact isopropoxide ligands. However subsequent cycles show drastically different chemistry, with no isopropoxide ligands remaining at any point during the 2 and 3 cycles. Continuous exposure of either TTIP or isopropanol after the 1 cycle shows unlimited, chemical vapor deposition (CVD)-type growth. Comparisons with alternative precursors (aluminum isopropoxide, titanium tert-butoxide) and DFT calculations reveal that for the TTIP precursor, isolated TiO sites play a role in the dehydration of off-gassing isopropanol. The resulting propene then undergoes oligomerization into six-carbon olefins, before polymerizing into indistinguishable carbon products that accumulate on the surface. The emergence of the dehydration chemistry is expected to be exclusively the result of these isolated TiO sites, and as such is expected to occur on other surfaces where TiO ALD is feasible. This work showcases how seemingly innocuous ALD can evolve into a CVD process when the products can participate in various side reactions with newly made surface sites.