Ruther Rose E, Baker Brenna M, Son Jung-Ho, Casey William H, Nyman May
Center for Sustainable Materials Chemistry and ‡Department of Chemistry, Oregon State University , Corvallis, Oregon 97331-4003, United States.
Inorg Chem. 2014 Apr 21;53(8):4234-42. doi: 10.1021/ic500375v. Epub 2014 Apr 2.
In prior studies, aqueous Hf sulfate-peroxide solutions were spin-coated, dehydrated, patterned by electron-beam lithography, ion-exchanged (OH(-) for SO4(2-)), 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-peroxide 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.
在先前的研究中，将硫酸铪 - 过氧化物水溶液旋涂、脱水，通过电子束光刻进行图案化，进行离子交换（用OH(-) 置换SO4(2-)），最后通过退火转化为HfO2硬掩膜。这些过程背后的水相化学的原子级细节很复杂，尚未被理解。然而，对这个特定化学体系的透彻理解将激发其他水相前驱体到固态金属氧化物体系设计规则的发展。经常从硫酸铪 - 过氧化物前驱体水溶液中观察到Hf18多金属氧酸盐的结晶，这使我们相信Hf18可能代表了这个过程中的一个重要中间步骤。然而，通过此处描述的详细溶液研究（小角X射线散射、电喷雾电离质谱和拉曼光谱），我们确定Hf18实际上并不是硫酸铪涂层的预成核簇。相反，作为Hf18核心构建单元的铪四聚体、五聚体和六聚体在前驱体溶液的不同组成和老化时间下都能稳定存在，因此它们可能是沉积薄膜材料的基本构建单元。这些铪簇在溶液中被硫酸根和过氧根阴离子封端并连接，这可能阻止了旋涂快速脱水过程中Hf18的结晶。事实上，从无定形凝胶涂层中结晶出Hf18会不利于我们从前驱体溶液中获得高密度保形涂层。因此，这项研究表明，著名的Hf18多金属氧酸盐不太可能是薄膜过程中的重要中间体。然而，它的亚基是重要的，这证实了从固态获取信息以理解溶液状态的普遍重要性，尽管要审慎且批判性地进行。
