Hafnium Sulfate Prenucleation Clusters and the Hf18 Polyoxometalate Red Herring

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• 作者：Rose E. Ruther ; Brenna M. Baker ; Jung-Ho Son ; William H. Casey ; May Nyman
• 刊名：Inorganic Chemistry
• 出版年：2014
• 出版时间：April 21, 2014
• 年：2014
• 卷：53
• 期：8
• 页码：4234-4242
• 全文大小：446K
• 年卷期：v.53,no.8(April 21, 2014)
• ISSN：1520-510X

文摘

In prior studies, aqueous Hf sulfate鈥損eroxide solutions were spin-coated, dehydrated, patterned by electron-beam lithography, ion-exchanged (OH^{鈥?/sup> for SO₄2鈥?/sup>), and finally converted to HfO₂ hard masks via annealing. The atomic-level details of the underlying aqueous chemistries of these processes are complex and yet to be understood. Yet a thorough understanding of this specific chemical system will inspire development of design rules for other aqueous-precursor-to-solid-state metal oxide systems. Often-observed crystallization of the Hf₁₈ polyoxometalate from aqueous Hf sulfate鈥損eroxide precursor solutions has led us to believe that Hf₁₈ may represent an important intermediate step in this process. However, via detailed solution studies described here (small-angle X-ray scattering, electrospray ionization mass spectrometry, and Raman spectroscopy), we ascertained that Hf₁₈ is in fact not a prenucleation cluster of Hf sulfate coatings. Rather, the Hf tetramers, pentamers, and hexamers that are the core building blocks of Hf₁₈ are robustly persistent over variable compositions and aging time of precursor solutions, and therefore they are likely the rudimentary building blocks of the deposited thin-film materials. These Hf clusters are capped and linked by sulfate and peroxide anions in solution, which probably prevents crystallization of Hf₁₈ during the rapid dehydration process of spin-coating. In fact, crystallization of Hf₁₈ from the amorphous gel coating would be detrimental to formation of a high-density conformal coating that we obtain from precursor solutions. Therefore, this study revealed that the well-known Hf₁₈ polyoxometalate is not likely to be an important intermediate in the thin-film process. However, its subunits are, confirming the universal importance of deriving information from the solid state, albeit judiciously and critically, to understand the solution state.}