Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
ACS Nano. 2013 Jul 23;7(7):6007-16. doi: 10.1021/nn4017108. Epub 2013 Jun 6.
Free-standing two-dimensional (2D) nanostructures, exemplified by graphene and semiconductor nanomembranes, exhibit exotic electrical and mechanical properties and have great potential in electronic applications where devices need to be flexible or conformal to nonplanar surfaces. Based on our previous development of a substrate-free synthesis of large-area, free-standing zinc hydroxy dodecylsulfate (ZHDS) hexagonal nanomembranes, herein, we report a spontaneous phase transformation of ZHDS nanomembranes under extended reaction time. The hexagonal ZHDS sheets transformed into rectangular single crystal nanomembranes with sizes of hundreds of micrometers. They contain long-range-ordered zinc vacancies that can be fitted into an orthorhombic superlattice. A surplus of dodecylsulfate ions and a deficit of Zn(2+) diffusion near the water surface are believed to be the factors that drive the phase transformation. The phase transformation starts with the formation of zinc vacancies at the topmost layer of the hexagonal hillock, and propagates along the spiral growth path of the initial hexagonal sheets, which bears a great resemblance to the classic "periodic slip process". Mechanical property characterization of ZHDS nanomembranes by nanoindentation shows they behave much like structural polymers mechanically due to the incorporation of surfactant molecules. We also developed a one-step exfoliation and dehydration method that converts ZHDS nanomembranes to ZnO nanosheets using n-butylamine. This work provides a further understanding of the growth and stability of ZnO-based nanomembranes, as well as advisory insight for the further development on solution-based synthesis of free-standing, single-crystalline 2D nanostructures.
独立的二维（2D）纳米结构，以石墨烯和半导体纳米膜为例，具有奇异的电学和力学性质，在需要设备具有柔韧性或可适应非平面表面的电子应用中具有巨大的潜力。基于我们之前开发的大面积、独立的锌羟十二烷基硫酸盐（ZHDS）六方纳米膜的无基底合成方法，本文报道了 ZHDS 纳米膜在延长反应时间下的自发相转变。六方 ZHDS 片层转变为具有数百微米尺寸的矩形单晶纳米膜。它们包含可以拟合为正交超晶格的长程有序锌空位。据信，在水面附近过量的十二烷基硫酸盐离子和 Zn(2+)扩散不足是驱动相转变的因素。相转变始于最顶层六方丘的锌空位形成，并沿初始六方片的螺旋生长路径传播，这与经典的“周期性滑移过程”非常相似。通过纳米压痕对 ZHDS 纳米膜的力学性能进行的特性分析表明，由于表面活性剂分子的掺入，它们在力学性能上非常类似于结构聚合物。我们还开发了一种一步剥离和脱水方法，使用正丁胺将 ZHDS 纳米膜转化为 ZnO 纳米片。这项工作进一步了解了基于 ZnO 的纳米膜的生长和稳定性，为基于溶液的独立单晶 2D 纳米结构的进一步发展提供了有益的见解。
