Waetzig Gregory R, Depner Sean W, Asayesh-Ardakani Hasti, Cultrara Nicholas D, Shahbazian-Yassar Reza, Banerjee Sarbajit
Department of Chemistry and Department of Materials Science and Engineering , Texas A&M University , College Station , TX 77845-3012 , USA . Email:
Department of Chemistry , University at Buffalo , The State University of New York , Buffalo , New York 14260-3000 , USA.
Chem Sci. 2016 Aug 1;7(8):4930-4939. doi: 10.1039/c6sc01601d. Epub 2016 May 3.
There has been intense interest in stabilizing the tetragonal phase of HfO since it is predicted to outperform the thermodynamically stable lower-symmetry monoclinic phase for almost every application where HfO has found use by dint of its higher dielectric constant, bandgap, and hardness. However, the monoclinic phase is much more thermodynamically stable and the tetragonal phase of HfO is generally accessible only at temperatures above 1720 °C. Classical models comparing the competing influences of bulk free energy and specific surface energy predict that the tetragonal phase of HfO ought to be stable at ultra-small dimensions below 4 nm; however, these size regimes have been difficult to access in the absence of synthetic methods that yield well-defined and monodisperse nanocrystals with precise control over size. In this work, we have developed a modified non-hydrolytic condensation method to precisely control the size of HfO nanocrystals with low concentrations of dopants by suppressing the kinetics of particle growth by cross-condensation with less-reactive precursors. This synthetic method enables us to stabilize tetragonal HfO while evaluating ideas for critical size at which surface energy considerations surpass the bulk free energy stabilization. The phase assignment has been verified by atomic resolution high angle annular dark field images acquired for individual nanocrystals.
自从预测四方相的HfO在几乎所有因其较高介电常数、带隙和硬度而得到应用的场合中性能优于热力学稳定的低对称单斜相以来，人们对稳定HfO的四方相产生了浓厚兴趣。然而，单斜相在热力学上更稳定，HfO的四方相通常仅在高于1720℃的温度下才可获得。比较体自由能和比表面能竞争影响的经典模型预测，HfO的四方相在低于4nm的超小尺寸下应该是稳定的；然而，在缺乏能够精确控制尺寸以产生明确且单分散纳米晶体的合成方法的情况下，这些尺寸范围很难实现。在这项工作中，我们开发了一种改进的非水解缩合方法，通过与反应性较低的前驱体进行交叉缩合来抑制颗粒生长动力学，从而精确控制低掺杂浓度的HfO纳米晶体的尺寸。这种合成方法使我们能够在评估表面能考虑因素超过体自由能稳定作用的临界尺寸概念时稳定四方相的HfO。通过对单个纳米晶体获取的原子分辨率高角度环形暗场图像验证了相的归属。
