Atomically Flat {011} Facets and Phase Tunability of HfO₂ Nanocrystals via an Anhydrous Wet-Chemical Approach

The development of new nanomaterials in industry is driven by the everchanging socioeconomic trends in recent history. As for the IT sector, HfO₂ was introduced in 2007 as gate material in complementary metal oxide semiconductors (CMOS) and still is one of the state-of-the-art materials as of today. As high-κ-dielectric, hafnia has also risen awareness when Müller et al. found that it exhibits ferroelectric behaviour in thin films with a thickness of about 10 nm.[1] While nanocrystalline HfO₂ is usually processed via thin-film deposition techniques like atomic layer deposition (ALD) and chemical vapor deposition (CVD), wet-chemical syntheses in solution envisage three-dimensionally confined nanostructures that are not yet accessible for thin-films. Accordingly, we put our efforts in rationalizing the synthesis HfO₂nanocrystals (HfO₂-NC) in order to obtain novel hafnia material classes.<br/>Prior to our work, the scientific community developed numerous synthesis protocols for colloidal hafnia.[2] The results of Tang et al. stand out among them, due to the astonishingly monodisperse HfO₂-NCs with sizes below 5 nm in diameter. This sol-gel synthesis approach served as base for subsequent studies, specifically in terms of size effects and crystal twinning behavior. Still, the results of different groups were inconsistent regarding isotropically shaped nanocrystals with the metastable tetragonal (<i>P4₂/nmc</i>, 137) crystal structure and anisotropically grown monoclinic (space group <i>P2₁/c</i>, 14) nanorods, that were seemingly obtained under similar conditions. In our work, we resolve these ambiguities by showing that specific heating techniques are crucial for the crystal growth dynamics. Furthermore, we extended the scope of the synthesis by the formation of anisotropic nanoprisms with atomically well defined {011} facets. We present that these nanoprisms tend towards self-assembly alongside their preferred growing direction, which is perpendicular to the aforementioned {011} crystal planes. Contrary to the well-investigated HfO₂ nanorods, these particles showcase no twinning alongside their long-axis dimension, which makes them perfect single orientated nanocrystals with only small size deviations. The particle size distribution, morphology and crystallinity of our HfO₂-NCs was predominantly investigated by means of transmission electron microscopy (TEM), while the role of molecular capping agents was analyzed via X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA).<br/>Our findings elucidate the nature for morphology selective growth and phase formation of the well-established sol-gel synthesis. We strive to extend our findings towards other transition metal oxides, premised on the opportunities that our approach for HfO₂provides.<br/><br/>[1] J. Muller, T. S. Boscke, U. Schroder, S. Mueller, D. Brauhaus, U. Bottger, L. Frey and T. Mikolajick, <i>Nano lett.</i>, <b>2012</b>, 12, 4318–4323.<br/>[2] J. Tang, J. Fabbri, R. D. Robinson, Y. Zhu, I. P. Herman, M. L. Steigerwald and L. E. Brus, <i>Chem. </i><i>Mater</i>., <b>2004</b>, 16, 1336–1342.